JP3757622B2 - Laser diode driver - Google Patents

Description

次に、２度目の自動光量制御において、変調電流Ｉ1-2 は、変調電流Ｉ1-1 からバイアス電流Ｉ2-1 を差し引いて、以下の式（４）のように設定される。
【００８２】

また、バイアス電流Ｉ2-2 は、目標駆動電流から、ディジタル信号入力Ｄｉｎ＝５により設定される変調電流５／１５・Ｉ1-2 を差し引いて、以下の式（５）のように設定される。
【００８３】

以降、同様にして自動光量制御動作を繰り返すことにより、変調電流Ｉ１，ｎは目標とする変調電流Ｉｓに、バイアス電流Ｉ２，ｎは目標とするバイアス電流Ｉｂに、それぞれ近づいていく。
【００８４】
次に、変調動作期間について説明する。変調動作期間では、スイッチ手段１４Ａを開くことにより、コンデンサ１５Ａに蓄積された駆動電流制御電圧ＶＣ１により変調電流が発生し、該変調電流の電流値は信号入力端子３Ａ、３Ｂ、３Ｃ、３Ｄより入力されるデジタル信号値Ｄｉｎにより１６段階に変調される。また、スイッチ手段１４Ｂを開くことにより、コンデンサ１５Ｂに蓄積されたバイアス電流制御電圧ＶＣ２によりバイアス電流が発生する。このバイアス電流と前記変調後の変調電流とが重畳され、重畳された電流によりレーザダイオード１が発光駆動される。
【００８５】
さて、ここで、誤差増幅器１２及び位相補償手段２３の動作を説明する。図３には、誤差増幅器１２を構成するＣＭＯＳ演算増幅器の回路構成が示されている。この図３において、ＩＮ−は反転入力端子、ＩＮ＋は非反転入力端子を示し、Ｖ０は電源に接続された定電流源トランジスタの電流値を決めるバイアス電圧である。位相補償手段２３はこの回路構成の場合、出力電圧ＯＵＴと内部制御電圧Ｎ０の間に接続される。図１と対比すると、出力電圧ＯＵＴは端子８Ａから取り出され、内部制御電圧Ｎ０は端子８Ｅから取り出される。
【００８６】
図３の誤差増幅器１２において、非反転入力端子ＩＮ＋からＰＭＯＳ１２Ａに入力された基準電圧ＶＲと、図１のフィードバックループＲＰを経由して反転入力端子ＩＮ−よりＰＭＯＳ１２Ｂに入力された出力電圧ＯＵＴとが比較され、ここで例えば出力電圧ＯＵＴが基準電圧ＶＲよりも大きければ、内部制御電圧Ｎ０は内部電圧Ｎ１よりも大きくなり、ＮＭＯＳ１２Ｃにより出力電圧ＯＵＴが下げられる。このようにして出力電圧ＯＵＴが基準電圧ＶＲに等しくなるように制御される。
【００８７】
次に、位相補償について説明する。
【００８８】
一般的に演算増幅器の発振条件は以下の２つである。
【００８９】
(1) フィードバック内の利得が１以上であること。
【００９０】
(2) フィードバック内を１回りした時の位相のずれが３６０°以上であること。
【００９１】
本実施形態のレーザダイオード駆動装置３５では、負帰還（負のフィードバック）をかけており、この構成だけで１８０°の遅れが発生するため、演算増幅器の発振を回避するための余裕分は１８０°となる。
【００９２】
図３の誤差増幅器１２は、差動増幅器１２Ｓと出力段１２Ｔの２段で構成されているので、１段目の遅れに２段目の遅れが重畳される。これをボード線図上に表すと図６（Ａ）、（Ｂ）のようになる。このうち図６（Ａ）では、特性Ａ１が１段目のゲインの周波数特性を、特性Ａ２が２段目のゲインの周波数特性を、それぞれ示しており、演算増幅器全体のゲインの周波数特性は太い破線のような周波数特性となる。また、演算増幅器全体の位相の周波数特性は図６（Ｂ）の太い破線のような周波数特性となる。なお、周波数ｆ１は１段目のポールの位置を、周波数ｆ２は２段目のポールの位置を、それぞれ示している。
【００９３】
特性Ａ１、Ａ２においてゲインはポール以降、（−２０ｄｂ／ｄｅｃ）の傾きで低下するので、周波数ｆ２以上では、演算増幅器全体のゲインは図６（Ａ）の太い破線の特性より明らかなように数十ｄｂとなり、演算増幅器全体の位相遅れも図６（Ｂ）の太い破線の特性より明らかなように１３５°以上なので、演算増幅器が発振する。
【００９４】
ここで、本実施形態では、位相遅れを改善するために容量の大きなコンデンサ２６Ａ、２６Ｂにより位相を大きく進ませている。これにより、１段目のポールの位置を低周波側ｆ１’にずらすことにより、演算増幅器全体のゲインは図６（Ａ）の太い実線のような周波数特性となり、周波数ｆ２（２段目のポールの位置）でゲインを０ｄｂにすることができ、発振を回避することができる。
【００９５】
具体的に図１のレーザダイオード駆動装置３５において、第１の自動光量制御動作では、スイッチ手段２５Ａを閉じ、スイッチ手段２５Ｂを開くことにより、抵抗２４、スイッチ手段２５Ａのオン抵抗及びコンデンサ２６Ａによって上述した位相補償を行う。そして、第１の自動光量制御動作が終了した時、サンプルホールド回路１３Ａを構成するスイッチ手段１４Ａを開くのと同じタイミングでスイッチ手段２５Ａを開くことにより、第１の自動光量制御動作で位相補償を行っていた時の誤差増幅器１２の内部電圧がコンデンサ２６Ａに保持される。
【００９６】
次に第２の自動光量制御動作では、スイッチ手段２５Ａを開き、スイッチ手段２５Ｂを閉じることにより、抵抗２４、スイッチ手段２５Ｂのオン抵抗及びコンデンサ２６Ｂによって上述した位相補償を行う。そして、第２の自動光量制御動作が終了した時、サンプルホールド回路１３Ｂを構成するスイッチ手段１４Ｂを開くのと同じタイミングでスイッチ手段２５Ｂを開くことにより、第２の自動光量制御動作で位相補償を行っていた時の誤差増幅器１２の内部電圧がコンデンサ２６Ｂに保持される。
【００９７】
その後、再度第１の自動光量制御動作の実行時になると、スイッチ手段２５Ａが再度閉じられるが、このときコンデンサ２６Ａには、前回の第１の自動光量制御動作時の内部電圧が既に保持されているので、コンデンサ２６Ａに改めて位相補償容量と誤差増幅器１２の内部電圧に応じた電荷のチャージ等を行う必要が無く、第１の自動光量制御動作に要する時間を短縮することができる。また、誤差増幅器１２の出力電圧の変動を小さくすることができ、安定的な第１の自動光量制御動作への移行を図ることができる。
【００９８】
また、その後、第２の自動光量制御動作に移行したときも、上記と同様に、コンデンサ２６Ｂには、前回の第２の自動光量制御動作時の内部電圧が既に保持されているので、コンデンサ２６Ｂに改めて位相補償容量と誤差増幅器１２の内部電圧に応じた電荷のチャージ等を行う必要が無く、第２の自動光量制御動作に要する時間を短縮できると共に、誤差増幅器１２の出力電圧の変動を小さくして安定的な第２の自動光量制御動作への移行を図ることができる。
【００９９】
なお、上記実施形態では、誤差増幅器１２が図３のようにＣＭＯＳ演算増幅器の基本回路により構成され、位相補償手段２３が誤差増幅器１２の出力段の入力Ｎ０と出力ＯＵＴとの間に設けられた例を説明したが、本発明は、上記以外の構成の誤差増幅器にも適用可能であり、位相補償手段の設置場所も限定されるものではない。
【０１００】
例えば、図７に示すように、ゲインの重畳により誤差増幅器全体のゲインを増加させるべく、差動増幅器９０Ａ、シフト段９０Ｂ及び出力段９０Ｃの３段で構成され、位相補償手段２３がシフト段９０Ｂの出力Ｓ０と出力段９０Ｃの入力Ｎ０との間に設けられた誤差増幅器９０に対しても、本発明は適用可能である。
【０１０１】
［第２実施形態］
次に、第２実施形態として、第１実施形態のレーザダイオード駆動装置３５（図１）を更に安定化するために、変調動作期間において、誤差増幅器１２の入出力電圧を一定にするための回路を付与したレーザダイオード駆動装置を説明する。
【０１０２】
図２に示すように、本実施形態に係るレーザダイオード駆動装置３５Ｓでは、バッファ手段１６Ａの出力側がスイッチ手段２１Ｄを介して、バッファ手段１６Ｂの出力側がスイッチ手段２１Ｅを介して、それぞれ誤差増幅器１２の非反転入力端子に接続されており、誤差増幅器１２の出力側がスイッチ手段２１Ｃを介して該誤差増幅器１２の反転入力端子に接続されている。
【０１０３】
また、位相補償手段２３には、位相補償回路として、抵抗２４、スイッチ手段２５Ｃのオン抵抗及びコンデンサ２６Ｃから成る回路と、抵抗２４、スイッチ手段２５Ｄのオン抵抗及びコンデンサ２６Ｄから成る回路とが追加されている。
【０１０４】
なお、レーザダイオード駆動装置３５Ｓでは、第１の自動光量制御動作、変調動作、第２の自動光量制御動作、変調動作が順に繰り返し実行される。ここで、便宜上、第１の自動光量制御動作の直後の変調動作を第１の変調動作と、第２の自動光量制御動作の直後の変調動作を第２の変調動作と、それぞれ称する。
【０１０５】
このようなレーザダイオード駆動装置３５Ｓにおいて、第１の自動光量制御動作時には、スイッチ手段１４Ａ、スイッチ手段２１Ａ、２１Ｂ、及びスイッチ手段２５Ａが閉じ、スイッチ手段１４Ｂ、スイッチ手段２１Ｃ、２１Ｄ、２１Ｅ、及びスイッチ手段２５Ｂ、２５Ｃ、２５Ｄが開く。第１の自動光量制御におけるフィードバックループの動作は図１の場合と同じである。また、位相補償手段２３による誤差増幅器１２の位相補償は、スイッチ手段２５Ａを介してコンデンサ２６Ａを用いて行われる。
【０１０６】
そして、レーザダイオード１の出力パワーが目標とする最大光量で安定になり、第１の自動光量制御動作から第１の変調動作へ切り替わると、まず、スイッチ手段１４Ａ、２５Ａを開く。その後、スイッチ手段２１Ａ、２１Ｂを開くと共に、スイッチ手段２１Ｃ、２１Ｅ、２５Ｃを閉じる。第１の変調動作では、サンプルホールド回路１３Ａ、１３Ｂに充電された変調電流制御電圧、バイアス電流制御電圧を用いてレーザダイオード１を発光駆動する。誤差増幅器１２はボルテージ・フォロワ構成となり、スイッチ手段２１Ｅを介して該誤差増幅器１２に入力されるバイアス電流制御電圧が該誤差増幅器１２から出力される状態に設定される。また、誤差増幅器１２の位相補償は、スイッチ手段２５Ｃを介してコンデンサ２６Ｃを用いて行われる。
【０１０７】
上記で、スイッチ手段１４Ａ、２５Ａをその他のスイッチ手段に先んじて開くことにより、他のスイッチ手段の開閉により生じる制御電圧の変動、及びフィードバックループの切断による誤差増幅器１２の内部電圧の変動によるコンデンサ１５Ａ、２６Ａの保持電圧の変動を低減することができる。
【０１０８】
そして、第１の変調動作から第２の自動光量制御動作へ切り替わった時、スイッチ手段１４Ｂ、２１Ａ、２１Ｂ、２５Ｂを閉じ、スイッチ手段１４Ａ、２１Ｃ、２１Ｅ、２５Ｃを開く。第２の自動光量制御におけるフィードバックループの動作は図１（第１実施形態）の場合と同じである。また、位相補償手段２３による誤差増幅器１２の位相補償は、スイッチ手段２５Ｂを介してコンデンサ２６Ｂを用いて行われる。
【０１０９】
さらに、レーザダイオード１の出力パワーが目標とする光量で安定になり、第２の自動光量制御動作から第２の変調動作へ切り替わると、まず、スイッチ手段１４Ｂ、２５Ｂを開く。その後、スイッチ手段２１Ａ、２１Ｂを開くと共に、スイッチ手段２１Ｃ、２１Ｄ、２５Ｄを閉じる。第２の変調動作でも、駆動電流を変調出力する動作に関しては第１の変調動作と同様である。誤差増幅器１２はボルテージ・フォロワ構成となり、スイッチ手段２１Ｄを介して該誤差増幅器１２に入力される変調電流制御電圧が該誤差増幅器１２から出力される状態に設定される。また、誤差増幅器１２の位相補償は、スイッチ手段２５Ｄを介してコンデンサ２６Ｄを用いて行われる。
【０１１０】
上記で、スイッチ手段１４Ｂ、２５Ｂをその他のスイッチ手段に先んじて開くことにより、他のスイッチ手段の開閉により生じる制御電圧の変動、及びフィードバックループの切断による誤差増幅器１２の内部電圧の変動によるコンデンサ１５Ｂ、２６Ｂの保持電圧の変動を低減することができる。
【０１１１】
以上のシーケンスにより、位相補償手段２３のコンデンサについて、第１及び第２の自動光量制御動作期間、もしくは第１及び第２の変調期間でスイッチ手段２５Ａ、２５Ｂ、２５Ｃ、２５Ｄにより個別に接続を切り替えることにより、各動作の切り替わり時において位相補償手段２３のコンデンサを再充電する必要がなくなる。これにより、自動光量制御動作に要する時間を大幅に短縮することができる。
【０１１２】
その上、第１の変調動作時に、スイッチ手段２１Ｅを介して誤差増幅器１２に入力されるバイアス電流制御電圧が該誤差増幅器１２から出力される状態に設定されているので、第１の変調動作から第２の自動光量制御動作に移行したときに、誤差増幅器１２の出力電圧が連続的な電圧値で推移することとなる。また、第２の変調動作時に、スイッチ手段２１Ｄを介して誤差増幅器１２に入力される変調電流制御電圧が該誤差増幅器１２から出力される状態に設定されているので、第２の変調動作から第１の自動光量制御動作に移行したときに、誤差増幅器１２の出力電圧が連続的な電圧値で推移することとなる。
【０１１３】
これにより、変調動作から自動光量制御動作に移行した場合について、誤差増幅器１２の出力電圧の変動を低減し、自動光量制御動作に要する時間をさらに短縮することができる。
【０１１４】
ところで、上記第１、第２実施形態で説明した図１及び図２に示す回路は、１つの誤差増幅器を共用してバイアス電流と変調電流とを自動的に調整する回路の一例であり、この他にも１つの誤差増幅器を共用してバイアス電流と変調電流とを自動的に調整する回路が知られている。本発明は、このように１つの誤差増幅器を共用して複数の電流値（例えば、バイアス電流値と変調電流値）を順番に自動的に調整する回路に対して適用可能であり、上記と同様の効果を得ることができる。
【０１１５】
また、図１及び図２において、鎖線２７で囲った回路が集積化の対象となる回路に相当する。但し、位相補償手段２３は、誤差増幅器１２に内蔵しても良いし、誤差増幅器１２に外部接続して構成しても良い。ここでは、外部接続による外付け部品の増加やアッセンブリを考慮した場合、位相補償手段２３の抵抗、スイッチ手段、コンデンサを全て内蔵した方が好ましい。ところが、コンデンサは通常ポリシリコンもしくは金属配線で酸化膜を挟み込むことにより作成されるので、上記のようにコンデンサを内蔵した場合、単位面積当たりの保持容量が小さくなり、トランジスタなどの他の機能素子に比較して集積化率を著しく低下させることがある。このためコンデンサで保持するべき電気容量が大きい場合には、コンデンサを外付けにした方が好ましいケースもある。
【０１１６】
図１には、位相補償手段２３の抵抗、スイッチ手段、コンデンサを全て外付けとした場合の例を示しており、図２には、コンデンサのみを外付けとして抵抗及びスイッチ手段を内蔵させた場合の例を示している。コンデンサについては、受光器の負荷抵抗の大きさ及びフィードバックループ内の系の違いに応じてフィードバックループの周波数特性が変化するので、要求される機器の仕様に応じて、容量値を調整する必要がある。図２のようにコンデンサのみを外付けとしスイッチ手段を内蔵させれば、該コンデンサの容量値の切替えを行うスイッチ手段をスイッチング制御する制御回路も誤差増幅器１２に内蔵させることができ、実装する際の部品点数を削減できる、という利点が有る。
【０１１７】
［第３実施形態］
以下、図面を参照して、第３実施形態を説明する。図８には、単一のフィードバックループを用いてレーザダイオード１０１の光量を単一の目標レベルに調整するレーザダイオード駆動装置１００の構成が示されている。
【０１１８】
このレーザダイオード駆動装置１００には、基準電圧発生回路１０８が設けられており、この基準電圧発生回路１０８は可変抵抗器１０９により電源電圧を分圧し所望の基準電圧ＶＲを得ている。この基準電圧ＶＲに応じた基準信号はバッファ手段１１０Ｂ、スイッチ手段１２０Ｂを介して誤差増幅器１１１の非反転入力端子に入力される。
【０１１９】
また、レーザダイオード駆動装置１００にはレーザダイオード１０１が設けられており、このレーザダイオード１０１が出力するレーザ光は光検出器１０２により受光される。該光検出器１０２が出力する光モニタ電流は可変抵抗器１１９により電流−電圧変換され、変換で得られたモニタ電圧ＶＭに応じたモニタ信号が、サンプルホールド回路１０５、バッファ手段１１０Ａ、スイッチ手段１２０Ａを介して誤差増幅器１１１の反転入力端子に入力される。上記サンプルホールド回路１０５は、一端が可変抵抗器１１９の出力端に接続されたスイッチ手段１０６と、該スイッチ手段１０６の他端に接続され且つ接地されたコンデンサ１０７とにより構成されている。
【０１２０】
誤差増幅器１１１では、基準電圧ＶＲとモニタ電圧ＶＭとで誤差増幅が行われ、誤差増幅後の信号（誤差増幅信号）はサンプルホールド回路１１２に入力される。サンプルホールド回路１１２は、一端が誤差増幅器１１１の出力端に接続されたスイッチ手段１１３と、該スイッチ手段１１３の他端に接続され且つ接地されたコンデンサ１１４とにより構成されている。
【０１２１】
スイッチ手段１１３が閉じている場合、誤差増幅器１１１からの誤差増幅信号はバッファ手段１１５の非反転入力端子に入力される。このバッファ手段１１５の出力側は、該バッファ手段１１５の反転入力端子に接続されていると共に、スイッチ手段１２０Ｄを介して誤差増幅器１１１の非反転入力端子に接続されている。また、バッファ手段１１５の出力側は駆動電流出力回路１１７の入力端に接続されている。なお、誤差増幅器１１１の出力側は、スイッチ手段１２０Ｃを介して該誤差増幅器１１１の反転入力端子に接続されている。
【０１２２】
駆動電流出力回路１１７の構成は、第１実施形態の駆動電流出力回路１７Ａの構成（図４）と同様であり、信号入力回路１１６の構成は、第１実施形態の信号入力回路１９の構成（図５）と同様であるので、説明を省略する。
【０１２３】
駆動電流出力回路１１７で発生する駆動電流は、自動光量制御期間において、上記モニタ電圧ＶＭが基準電圧ＶＲに一致するよう調整することで任意のレーザ光量に相当するよう設定される。一方、変調期間においては、駆動電流は信号入力端子１０３Ａ、１０３Ｂ、１０３Ｃ、１０３Ｄから入力される強度変調データ信号に応じて１６段階に強度変調され、バイアス電流出力回路１１８により供給されるバイアス電流と電流加算されて、加算後の駆動電流によってレーザダイオード１が発光駆動される。
【０１２４】
次に、スイッチ手段の切替えにより選択される各動作モードについてそれぞれ説明する。なお、スイッチ手段１２０Ａ、１２０Ｂ、１２０Ｃ、１２０Ｄ、１０６、１１３の開閉動作は、図示しない制御装置が出力するスイッチ信号により制御される。
【０１２５】
最初に、自動光量制御動作について説明する。
【０１２６】
自動光量制御動作は、スイッチ手段１２０Ａ、１２０Ｂ、１０６、１１３を閉じ、スイッチ手段１２０Ｃ、１２０Ｄを開くことにより選択される。光検出器１０２はレーザダイオード１０１が出力する光出力を受光して、それに準じた光電流を出力する。この光電流を可変抵抗器１１９により電流−電圧変換し、そのモニタ電圧ＶＭは、スイッチ手段１０６、バッファ手段１１０Ａ、スイッチ手段１２０Ａを介して誤差増幅器１１１の反転入力端子に入力される。一方、基準電圧発生回路１０８で発生した基準電圧ＶＲは、バッファ手段１１０Ｂ、スイッチ手段１２０Ｂを介して誤差増幅器１１１の非反転入力端子に入力される。
【０１２７】
誤差増幅器１１１は、モニタ電圧ＶＭと基準電圧ＶＲとを比較し、その差に応じた電圧ＶＳを出力し、スイッチ手段１１３を介して、コンデンサ１１４を充電する。コンデンサ１１４に充電される電圧ＶＣはバッファ手段１１５を介して、レーザ駆動電流出力回路１１７に入力され、以下のようにして電流源を流れる電流を制御する。
【０１２８】
例えば、バッファ手段１１５の出力電圧ＶＣ’は、図４に示されるレーザ駆動電流出力回路の端子４２に入力され、信号入力回路１９により、スイッチ信号群２０Ａをすべて「Ｈ」、スイッチ信号群２０Ｂをすべて「Ｌ」にすることで、差動スイッチ手段４１Ａ、４１Ｂ、４１Ｃ、４１Ｄを介して、電流源４０Ａ、４０Ｂ、４０Ｃ、４０Ｄを流れる電流がすべてレーザダイオード１０１（図８）を流れるように制御する。
【０１２９】
実際にレーザダイオード１０１には、駆動電流出力回路１１７が出力する電流とバイアス電流出力回路１１８が出力する電流とを重畳した電流が流れ、該重畳した電流によりレーザダイオード１０１が駆動され発光する。この出力光は光検出器１０２により受光され、負帰還することにより、レーザダイオード１０１の光量が、目標とする光量に調整される。なお、目標とする光量は、基準電圧発生回路１０８が発生する基準電圧ＶＲを調整するか、もしくは電流−電圧変換回路を構成する可変抵抗器１１９の抵抗値を調整することにより、調整可能とされている。レーザダイオード１０１を流れる駆動電流が安定になったところで、スイッチ１１３が開放され、その時の駆動電流制御電圧ＶＣがコンデンサ１１４に保持される。
【０１３０】
自動光量制御動作から変調動作に移行する場合には、スイッチ手段１２０Ａ、１２０Ｂ、１２０Ｃ、１２０Ｄが切断することにより生じる誤差増幅器１１１の出力電圧変動の影響を防ぐべく、スイッチ手段１１３をスイッチ手段１２０Ａ、１２０Ｂ、１２０Ｃ、１２０Ｄよりも先に開放することで、駆動電流制御電圧ＶＣをコンデンサ１１４に保持することが望ましい。
【０１３１】
また、スイッチ手段１１３と同時にサンプルホールド回路１０５のスイッチ手段１０６が開放され、自動光量制御動作終了時に光検出器１０２が出力していたモニタ電圧ＶＭがコンデンサ１０７に保持される。スイッチ手段１１３と同時にサンプルホールド回路１０５のスイッチ手段１０６を開放することで、モニタ電圧ＶＭの変動を防ぐことができる。
【０１３２】
次に、変調動作について説明する。変調動作は、コンデンサ１１４に保持された駆動電流制御電圧ＶＣを用いて以下のように行われる。この制御電圧ＶＣはバッファ手段１１５を介して駆動電流出力回路１１７に入力され、強度変調信号１０３Ａ、１０３Ｂ、１０３Ｃ、１０３Ｄ、およびパルス幅変調信号１０４の外部制御信号により、１６段階の変調電流に変換され、前記バイアス電流と加算されて、レーザダイオード１０１に流れる駆動電流を自在に変調する。
【０１３３】
この変調動作を行う期間において、スイッチ手段１２０Ａ、１２０Ｂ、１０６、１１３がそれぞれ開き、スイッチ手段１２０Ｃとスイッチ手段１２０Ｄがそれぞれ閉じる。そのため誤差増幅器１１１は、バッファ手段１１５が出力する制御電圧ＶＣ’を入力とするボルテージ・フォロワ回路として動作する。即ち、バッファ手段１１５が出力する制御電圧ＶＣがスイッチ手段１２０Ｄを介して誤差増幅器１１１に入力され、スイッチ手段１２０Ｃを閉じたことで、制御電圧ＶＣが誤差増幅器１１１から出力される状態に設定される。また、サンプルホールド回路１０５のコンデンサ１０７には、目標とするモニタ電圧ＶＭに略一致する電圧が保持される。
【０１３４】
そして、変調動作から自動光量制御動作に移行したとき、コンデンサ１０７には、目標とするモニタ電圧ＶＭに略一致する電圧が保持されているため、例えば自動光量制御動作に入る直前にレーザダイオード１０１が消灯もしくは低光量で発光していて、光検出器１０２から出力される光電流が少ない場合においても、誤差増幅器１１１に入力されるモニタ電圧は目標とするモニタ電圧ＶＭに略一致する電圧となり、目標とするモニタ電圧ＶＭとの電圧差が非常に少ないので、モニタ電圧の立ち上がりの遅れを小さくすることができる。また、コンデンサ１０７に蓄積されたモニタ電圧ＶＭ１がスイッチ手段１０５Ａを介して光検出器１０２と可変抵抗器１１９の接続端の電圧をチャージするので、長い収束時間を必要とする光モニタ電流−モニタ電圧変換部の収束を早める効果がある。
【０１３５】
図１１（Ａ）〜（Ｃ）はレーザダイオード駆動装置１００の動作を説明するための波形図である。図１１（Ａ）〜（Ｃ）において期間Ｐ２、Ｐ４は自動光量制御動作期間を示し、期間Ｐ１、Ｐ３は変調動作期間を示している。
【０１３６】
自動光量制御動作期間Ｐ２から変調動作期間Ｐ３に移行する際、図１１（Ａ）のモニタ電圧波形に示されるように、自動光量制御により安定化した光検出器１０２のモニタ信号がサンプルホールド回路１０５により保持され、変調動作期間Ｐ３でも引続き出力される。これにより、変調動作期間Ｐ３から自動光量制御動作期間Ｐ４に移行する場合において、コンデンサ１０７の保持電圧ＶＭがスイッチ手段１０６を介して、光検出器１０２と可変抵抗器１１９の接続端の電圧を保持電圧ＶＭでチャージし、かつ、誤差増幅器１１１に入力されるモニタ電圧が目標とするモニタ電圧ＶＭに略一致するので、モニタ電圧の立ち上がりの遅れを防止できる。
【０１３７】
また、サンプルホールド回路１１２が保持した制御電圧ＶＣを入力としボルテージ・フォロワ構成となった誤差増幅器１１１の出力（図１１（Ｂ））は、自動光量制御動作期間Ｐ２から変調動作期間Ｐ３にかけて制御電圧ＶＣのまま維持される。
【０１３８】
従って、変調動作期間Ｐ３から自動光量制御動作期間Ｐ４に移行する際の誤差増幅器１１１の入出力端子電圧の変動量が少なくなるので、レーザ光量が目標光量に高速に整定されることとなり、かつモニタ電圧の立ち上がりの遅れによるレーザ駆動電流のオーバーシュートを大幅に低減することができる。
【０１３９】
また、図１１に示す変調動作期間Ｐ１から自動光量制御動作期間Ｐ２への移行は、図１１（Ｃ）より明らかなように、レーザダイオード１０１を自動光量制御期間Ｐ２の直前に最大光量で発光させた場合であるが、サンプルホールド回路１０５により前回の自動光量制御動作時のモニタ電圧ＶＭが変調動作時においても保持されているので、レーザダイオード１０１の出力光量の大小にかかわらず、誤差増幅器１１１の出力電圧の変動を低減することができる。
【０１４０】
［第４実施形態］
次に、第４実施形態として、レーザダイオードを駆動するための駆動電流出力回路が複数設けられたレーザダイオード駆動装置を説明する。なお、上記レーザダイオード駆動装置としては、光ディスク駆動装置のように、再生光量と書き込み光量の異なる２種類のレーザダイオード出力光を必要とし、各駆動電流出力回路が異なる光出力を出力するレーザダイオード駆動装置等が挙げられる。
【０１４１】
図９に示すレーザダイオード駆動装置１００Ｓは、前述した図８のレーザダイオード駆動装置１００と同様な構成であるが、このレーザダイオード駆動装置１００Ｓには、２つの駆動電流出力回路１１７、１１７Ｓが設けられており、このうち駆動電流出力回路１１７で発生する駆動電流Ｉ１を設定するために、基準電圧発生回路１０８Ａ、サンプルホールド回路１１２Ａ、１０５Ａが設けられ、駆動電流出力回路１１７Ｓで発生する駆動電流Ｉ２を設定するために、基準電圧発生回路１０８Ｂ、サンプルホールド回路１１２Ｂ、１０５Ｂが設けられている。
【０１４２】
即ち、レーザダイオード駆動装置１００Ｓでは、駆動電流出力回路１１７で発生する駆動電流Ｉ１を設定する第１の自動光量制御動作と、駆動電流出力回路１１７Ｓで発生する駆動電流Ｉ２を設定する第２の自動光量制御動作とが行われる。
【０１４３】
第１の自動光量設定動作では、スイッチ手段１２０Ａ、１２０Ｂ、１０６Ａ、１１３Ａ、１２２Ａ、１２３Ａを閉じ、それ以外のスイッチ手段を開くことにより、モニタ電圧ＶＭと基準電圧発生回路１０８Ａが発生する基準電圧ＶＲ１とが誤差増幅器１１１により比較され、可変抵抗器１１９の抵抗値と上記基準電圧ＶＲ１とで規定されるレーザ光量が負帰還フィードバック制御により設定される。
【０１４４】
この時、誤差増幅器１１１の出力電圧がサンプルホールド回路１１２Ａのコンデンサ１１４Ａに充電されると共に、光検出器１０２に係るモニタ出力電圧がサンプルホールド回路１０５Ａのコンデンサ１０７Ａに充電される。
【０１４５】
そして、レーザダイオード１０１の光量が第１の目標光量で安定した時に、スイッチ手段１０６Ａ、１１３Ａをそれぞれ開き、第１の目標光量に対応する光検出器１０２のモニタ電圧ＶＭ１をサンプルホールド回路１０５Ａに保持し、第１の目標光量に対応するレーザ光量が得られる制御電圧ＶＣ１をサンプルホールド回路１１２Ａに保持する。
【０１４６】
第２の自動光量制御動作でも上記同様に、スイッチ手段１２０Ａ、１２０Ｂ、１０６Ｂ、１１３Ｂ、１２２Ｂ、１２３Ｂを閉じ、それ以外のスイッチ手段を開くことにより、モニタ電圧ＶＭと基準電圧発生回路１０８Ｂが発生する基準電圧ＶＲ２とが誤差増幅器１１１により比較され、可変抵抗器１１９の抵抗値と上記基準電圧ＶＲ２とで規定されるレーザ光量が負帰還フィードバック制御により設定される。
【０１４７】
この時、誤差増幅器１１１の出力電圧がサンプルホールド回路１１２Ｂのコンデンサ１１４Ｂに充電されると共に、光検出器１０２に係るモニタ出力電圧がサンプルホールド回路１０５Ｂのコンデンサ１０７Ｂに充電される。
【０１４８】
そして、レーザダイオード１０１の光量が第２の目標光量で安定した時に、スイッチ手段１０６Ｂ、１１３Ｂをそれぞれ開き、第２の目標光量に対応する光検出器１０２のモニタ電圧ＶＭ２をサンプルホールド回路１０５Ｂに保持し、第２の目標光量に対応するレーザ光量が得られる制御電圧ＶＣ２をサンプルホールド回路１１２Ｂに保持する。
【０１４９】
変調期間中の駆動電流は、第１の自動光量制御動作で得られた制御電圧ＶＣ１を用いてレーザ駆動電流出力回路１１７が出力する駆動電流Ｉｓ１、もしくは第２の自動光量制御動作で得られた制御電圧ＶＣ２を用いてレーザ駆動電流出力回路１１７Ｓが出力する駆動電流Ｉｓ２のいずれか一方を選択してレーザダイオード１０１を駆動する場合と、両者の合成電流（Ｉｓ１＋Ｉｓ２）でレーザダイオード１０１を駆動する場合がある。
【０１５０】
変調動作では、スイッチ手段１２０Ｃを閉じると共に、スイッチ手段１２０Ｄ１またはスイッチ手段１２０Ｄ２のいずれか一方を閉じ、これら以外のスイッチ手段を開く。
【０１５１】
ここで、変調動作後に第１の自動光量制御動作が行われるケースであれば、スイッチ手段１２０Ｃ、１２０Ｄ１を閉じ、第１の自動光量制御動作直前の変調動作期間において、誤差増幅器１１１の出力電圧を、サンプルホールド回路１１２Ａに保持された電圧（＝第１の目標光量に対応する制御電圧ＶＣ１）に略一致させておく。
【０１５２】
そして、第１の自動光量制御動作時になると、スイッチ手段１０６Ａ、１２２Ａを閉じ、スイッチ手段１０６Ｂ、１２２Ｂを開くことにより、バッファ手段１１０Ａの入力電圧は、第１の自動光量制御動作での目標光量に対応するモニタ電圧ＶＭ１に略一致した電圧となり、モニタ電圧の立ち上がりの遅れを小さくすることができる。また、コンデンサ１０７に蓄積されたモニタ電圧ＶＭ１がスイッチ手段１０５Ａを介して光検出器１０２と可変抵抗器１１９の接続端の電圧をチャージするので、長い収束時間を必要とする光モニタ電流−モニタ電圧変換部の収束を早める効果がある。
【０１５３】
これにより、変調動作期間の終了時のレーザ光量が低く、それに対応してモニタ電圧が小さい場合においても、第１の自動光量制御動作開始時では、バッファ手段１１０Ａ、スイッチ手段１２０Ａを介して誤差増幅器１１１の反転入力端子に入力される電圧は、前回の第１の自動光量制御動作での目標光量に対応するモニタ電圧ＶＭ１に略一致した電圧となり、スイッチ手段１２３Ａを閉じることにより選択される基準電圧ＶＲ１と略一致することになるので、第１の自動光量制御動作に要する時間を大幅に短縮できる。
【０１５４】
また、変調動作後に第２の自動光量制御動作が行われるケースについても、同様に動作することで、同様の効果を得ることができる。
【０１５５】
このように、誤差増幅器１１１の反転入力端子に入力されるモニタ電圧の収束性が悪いことにより引き起こされる誤差増幅器１１１の出力電圧の変動を大幅に低減でき、モニタ電圧の収束時間の短縮とオーバーシュートの低減に大きな効果がある。
【０１５６】
［第５実施形態］
次に、第５実施形態として、図１０を用いて、第４実施形態のレーザダイオード駆動装置１００Ｓ（図９）の自動光量制御期間を更に短縮するために好適なレーザダイオード駆動装置の回路を説明する。
【０１５７】
前述した図９に示す回路構成では、変調動作から自動光量制御動作に切り替わる瞬間にスイッチ手段１０６Ａ、１０６Ｂを閉じて、光検出器１０２の出力端と保持コンデンサ１０７Ａ、１０７Ｂの端子とを接続するために、自動光量制御動作を開始した瞬間のモニタ電圧が若干不安定になるケースがあったが、図１０に示す回路は、このモニタ電圧を安定化するものである。
【０１５８】
図１０の回路の動作は、スイッチ信号１８３Ａ、１８３Ｂ、１８３Ｃ、１８３Ｄにより選択されるスイッチ手段１８２Ａ、１８２Ｂ、１８２Ｃ、１８２Ｄの切替えによって決定される。
【０１５９】
第１の自動光量制御動作時には、スイッチ手段１８２Ａ、１８２Ｂ、１８２Ｄは開き、スイッチ手段１８２Ｃは閉じる。レーザダイオードの出力光は光検出器１０２で受光され、可変抵抗器１１９により発生するモニタ電圧ＶＭ１は、スイッチ手段１８２Ｃを介してコンデンサ１７１にサンプリングされる。
【０１６０】
一方、演算増幅器１８４によりＭＯＳトランジスタ１８６のゲート電圧が制御され抵抗１８５とＭＯＳトランジスタ１８６との接続端子１９０の電圧はモニタ電圧ＶＭ１に一致する。また、ＭＯＳトランジスタ１８６と抵抗１８７との接続端子１９１は図９の電圧端子ＶＭ、バッファ手段１１０Ａを介してスイッチ手段１２０Ａに接続されており、接続端子１９１の電圧ＶＭ１’は誤差増幅器１１１に入力される。レーザダイオードの出力光が目標値に達したときにスイッチ手段１８２Ｃは切断され、コンデンサ１７１に第１の自動光量制御期間の最終モニタ電圧ＶＭ１が保持される。
【０１６１】
次の第１の自動光量制御動作の直前の変調期間では、スイッチ手段１８２Ａ、１８２Ｂ、１８２Ｃが開き、スイッチ手段１８２Ｄが閉じることにより、コンデンサ１７１に保持されている最終モニタ電圧ＶＭ１が、演算増幅器１８１Ａから出力され、スイッチ手段１８２Ｄを介して演算増幅器１８４に入力される。演算増幅器１８４によりＭＯＳトランジスタ１８６のゲート電圧が制御されるので、抵抗１８５とＭＯＳトランジスタ１８６との接続端子１９０の電圧はモニタ信号ＶＭ１に一致し、ＭＯＳトランジスタ１８６と抵抗１８７との接続端子１９１の電圧は前記モニタ信号ＶＭ１’に一致する。
【０１６２】
そして、変調動作から第１の自動光量制御動作に移行する際に、再度スイッチ手段１８２Ｄが開き、代わってスイッチ手段１８２Ｃが閉じる。この時、図９におけるモニタ電圧端子ＶＭから見たモニタ電圧の変動は、図９に示す回路構成の場合よりも図１０に示す回路構成の場合の方が少なくなる（詳細は以下に記載）。このため、オーバーシュートを更に低減することが可能である。
以下、具体的に、変調動作時にレーザダイオードが消灯している状態から自動光量制御動作に移行したケースを例にとって、図９、図１０の各回路でのモニタ電圧の変動を説明する。
【０１６３】
図９の回路の場合、変調動作でレーザダイオード１０１が消灯しているとき、光検出器１０２に流れる電流（ＰＤ電流）も０であるので、可変抵抗器１１９と光検出器１０２との接続端は５Ｖ（＝電源電圧）となり、この状態から自動光量制御動作へと移行する。自動光量制御動作時のＰＤ電流を０．１ｍＡ、可変抵抗器１１９の抵抗値を１０ｋΩとすると、最終的にモニタ電圧ＶＭは４Ｖ（＝５−１０ｋΩ×０．１ｍＡ）に整定する。
【０１６４】
ここで、コンデンサ１０７Ａの容量を１ｎＦとし、充電すべき光検出器１０２の容量と寄生容量の和を１０ｐＦと仮定すれば、５Ｖと４Ｖとの１Ｖの電位差を充電した場合、０．０１Ｖ（＝１Ｖ×１０ｐＦ／１ｎＦ）の誤差が生じる。即ち、光検出器１０２のカソード部の電圧（＝可変抵抗器１１９と光検出器１０２との接続端の電圧）は、変調動作時の５Ｖから、自動光量制御動作への切り替わり時に４．０１Ｖとなり、その後自動光量制御動作により４Ｖに収束する。
【０１６５】
一方、図１０の回路では、変調動作時には、例えば、ボルテージフォロワ構成の演算増幅器１８１Ａとスイッチ手段１８２Ｄにより可変抵抗器１１９と光検出器１０２との接続端に、コンデンサ１７１に保持された電圧（＝前回の自動光量制御動作時の最終モニタ電圧ＶＭ１）が出力されるので、光検出器１０２に流れる電流（ＰＤ電流）が０であっても、光検出器１０２のカソード部の電圧（＝可変抵抗器１１９と光検出器１０２との接続端の電圧）は上記最終モニタ電圧ＶＭ１で一定となる。即ち、光検出器１０２のカソード部の電圧は、変調動作時、自動光量制御動作への切り替わり時、自動光量制御動作時を通して、４Ｖでほぼ一定に維持される。
【０１６６】
このように図９におけるモニタ電圧端子ＶＭから見たモニタ電圧の変動は、図９に示す回路構成の場合よりも図１０に示す回路構成の場合の方が少なくなる。
【０１６７】
なお、第２の自動光量制御動作についても、上記と同様にスイッチ手段１８２Ａ、１８２Ｂを開閉することにより、同様の効果を得ることができる。
【０１６８】
また、図１０に示す回路１８９を取り去ることにより、図１０に示す回路を図８の回路に対応させることができる。
【０１６９】
【発明の効果】
以上説明したように、請求項１記載の発明によれば、再度光量制御期間になったとき、位相補償手段には既に前回の光量制御期間の終了時での誤差増幅手段の内部電圧が保持されているので、改めて位相補償容量と誤差増幅手段の内部電圧に応じた電荷の充電を行う必要が無く、光量制御動作に要する時間を短縮することができると共に、誤差増幅手段の出力電圧の変動を小さくして安定的な光量制御動作への移行を図ることができる。
【０１７０】
また、請求項２記載の発明によれば、複数の基準値の各々に関する光量制御期間において、光量制御動作に要する時間の短縮及び安定的な光量制御動作への移行を図ることができる。
【図面の簡単な説明】
【図１】 第１実施形態に係るレーザダイオード駆動装置の構成を示すブロック図である。
【図２】 第２実施形態に係るレーザダイオード駆動装置の構成を示すブロック図である。
【図３】 誤差増幅器の回路構成例を示す概略図である。
【図４】 駆動電流出力回路の回路構成例を示す概略図である。
【図５】 信号入力回路の回路構成例を示す概略図である。
【図６】 （Ａ）は周波数−ゲイン特性を示すボード線図であり、（Ｂ）は周波数−位相特性を示すボード線図である。
【図７】 誤差増幅器の他の回路構成例を示す概略図である。
【図８】 第３実施形態に係るレーザダイオード駆動装置の構成を示すブロック図である。
【図９】 第４実施形態に係るレーザダイオード駆動装置の構成を示すブロック図である。
【図１０】 第５実施形態に係るレーザダイオード駆動装置の要部の構成を示すブロック図である。
【図１１】 第３実施形態に係るレーザダイオード駆動装置の動作を説明するためのグラフであり、（Ａ）はモニタ電圧ＶＭの推移を、（Ｂ）は誤差増幅器の出力電圧ＶＨの推移を、（Ｃ）はレーザ駆動電流Ｉの推移を、それぞれ示すグラフである。
【図１２】 レーザダイオードの光量を異なる複数の目標レベルに調整する従来のレーザダイオード駆動装置の構成を示すブロック図である。
【図１３】 レーザダイオードの光量を単一の目標レベルに調整する従来のレーザダイオード駆動装置の構成を示すブロック図である。
【図１４】 図１３のレーザダイオード駆動装置の動作を説明するためのグラフであり、（Ａ）はモニタ電圧ＶＭの推移を、（Ｂ）は誤差増幅器の出力電圧ＶＨの推移を、（Ｃ）はレーザ駆動電流Ｉの推移を、それぞれ示すグラフである。
【図１５】 特開平９−４５９８０号公報のレーザ駆動回路の概略構成図である。
【図１６】 特開昭６２−２４３１３７号公報のレーザ駆動回路の概略構成図である。
【符号の説明】
１、１０１ レーザダイオード
２、１０２ 光検出器
１２、１１１ 誤差増幅器
１３Ａ、１３Ｂ、１０５、１１２ サンプルホールド回路
１４Ａ、１４Ｂ、２１Ａ〜２１Ｅ、２５Ａ〜２５Ｄ スイッチ手段
１０６、１１３、１２０Ａ〜１２０Ｄ スイッチ手段
１５Ａ、１５Ｂ、２６Ａ〜２６Ｄ、１０７、１１４ コンデンサ
１７Ａ、１７Ｂ、１１７ 駆動電流出力回路
１９、１１６ 信号入力回路
２３ 位相補償手段
３５、３５Ｓ、１００、１００Ｓ レーザダイオード駆動装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a laser diode driving device that drives a laser diode, and more particularly to a laser diode driving device that performs an automatic light amount control operation at high speed.
[0002]
[Prior art and problems to be solved by the invention]
Conventionally, in a laser diode driving device that drives a laser diode, the optical output of the laser diode is converted into a monitor voltage using a photodetector such as a photodiode during a period called automatic light quantity control operation, and the converted monitor The drive current control voltage for obtaining the target light amount as the light amount of the laser diode is obtained by comparing the voltage and the reference voltage corresponding to the target light amount by the error detector. Further, during the modulation period in which the laser diode is modulated based on an external input signal, the light output of the laser diode is modulated using the drive current control voltage set by the automatic light quantity control operation.
[0003]
Incidentally, FIG. 12 shows a laser diode driving device 200 that adjusts the light amount of the laser diode 201 to a plurality of different target levels using a plurality of feedback loops. In this laser diode driving device 200, in order to automatically adjust two types of currents, a bias current fixedly supplied in accordance with a threshold current of the laser diode 201 and a modulation current modulated in accordance with an input signal, respectively. The two feedback loops are selectively controlled using the switch means 214A and 214B.
[0004]
Here, each operation mode selected by switching of the switch means 214A and 214B will be described with reference to FIG.
[0005]
The first automatic light amount control operation for setting the maximum light amount of the laser diode 201 shown in FIG. 12 is executed by closing the switch unit 214A and opening the switch unit 214B. At this time, the optical monitor current output from the photodetector 202 is subjected to current-voltage conversion by the variable resistor 222, and the monitor voltage VM obtained by the conversion is applied to the inverting input terminal of the error detector 212 through the buffer means 211A. At the same time, the reference voltage VR1 generated by the variable resistor 210A constituting the reference voltage generation circuit 209A is input to the non-inverting input terminal of the error detector 212 via the switch means 230A and the buffer means 211B. The error voltage VC1 output from the error detector 212 by comparing the voltages at both ends is input to the drive current output circuit 217A via the switch means 214A and the buffer means 216A, and is converted into a current by the drive current output circuit 217A. The converted current is superimposed on the bias current output from the drive current output circuit 217B, and the laser diode 201 is driven by the superimposed combined current. When the output light of the laser diode 201 becomes stable at the desired maximum light amount, the switch means 214A constituting the sample and hold circuit 213A is opened, and the modulation current control voltage VC1 for setting the drive current corresponding to the maximum light amount. Is held in the capacitor 215A.
[0006]
Next, a second automatic light amount control operation for setting a bias current corresponding to the threshold current of the laser diode 201 will be described. The second automatic light quantity control operation is executed by closing the switch unit 214B and opening the switch unit 214A. At this time, the optical monitor current output from the photodetector 202 is subjected to current-voltage conversion by the variable resistor 222, and the monitor voltage obtained by the conversion is input to the inverting input terminal of the error detector 212 through the buffer unit 211A. At the same time, the reference voltage VR2 generated by the variable resistor 210B constituting the reference voltage generation circuit 209B is input to the non-inverting input terminal of the error detector 212 via the switch means 230B and the buffer means 211B. The error voltage VC2 output from the error detector 212 by comparing the voltages at both ends is input to the drive current output circuit 217B via the switch means 214B and the buffer means 216B, and is converted into a current by the drive current output circuit 217B. The converted current is superimposed on the modulation current output from the drive current output circuit 217A, and the laser diode 201 is driven by the superimposed combined current. When the output light of the laser diode 201 becomes stable at a desired light quantity, the switch means 214B constituting the sample hold circuit 213B is opened, and a bias current control voltage VC2 for setting a drive current corresponding to the desired light quantity. Is held in the capacitor 215B.
[0007]
By repeatedly performing the first automatic light amount control operation and the second automatic light amount control operation, the bias current controlled by the bias current control voltage VC2 held by the sample hold circuit 213B becomes the threshold current of the laser diode 201. It is gradually approximated. On the other hand, the modulation current controlled by the modulation current control voltage VC1 held by the sample hold circuit 213A is approximated to a current corresponding to the maximum light amount.
[0008]
By the way, the error detector 212 composed of an operational amplifier is provided with a phase compensation circuit 223 composed of, for example, a series circuit of a resistor 224 and a capacitor 225 as shown in FIG. Is adjusted in accordance with the phase delay and loop gain of a negative feedback loop including a laser diode 201, a monitor light receiver 202, and a resistor 222 that converts the light output of the light receiver into a voltage. Such a phase compensation circuit 223 performs phase compensation of the error detector 212.
[0009]
As a technique for providing a phase compensation circuit in a negative feedback control system as described above, Japanese Patent Application Laid-Open No. 5-113802 detects an operating state of a drive target by a detection unit, and the detection output is compared with a reference value by a comparison unit. An error signal is obtained by comparison, and when a negative feedback control is performed by applying the error signal to the control unit, a time control unit corresponding to a phase compensation circuit is provided between the comparison unit and the control unit, and the time A technology that stabilizes negative feedback during automatic adjustment even when the time constant is set to a sufficiently large value in a negative feedback control system by inputting an error signal whose time characteristic has been changed by the control unit to the control unit. It is disclosed.
[0010]
However, in the case of the laser drive current output circuit 200 shown in FIG. 12, the output voltage of the error detector 212 is a voltage corresponding to the maximum light amount when the first automatic light amount control operation is switched to the second automatic light amount control operation. Since it is VC1 and the target voltage to be settled is the voltage VC2 corresponding to the minimum light quantity, it takes a long time for the output voltage of the error detector 212 to settle from the voltage VC1 to the voltage VC2. In particular, regarding the phase compensation unit 223 of the error detector 212, since the target feedback loop includes a light-current conversion path and has a long time constant, several hundred pF to several hundreds pF are included in the phase compensation unit for stabilization. It must have nF phase compensation capacitance.
[0011]
When the phase compensation capacity increases in this way, the phase compensation capacity has to be charged in order to switch from the voltage VC1 to the voltage VC2, and in order to settle down to the target control state, the second automatic light quantity control requires several μm. A settling time of seconds to tens of microseconds is required. The technique disclosed in Japanese Patent Application Laid-Open No. 5-113802 has a similar problem that it takes a lot of settling time because the time constant is set to a sufficiently large value in the negative feedback control system.
[0012]
In addition, when the modulation operation is switched to the first automatic light amount control operation, if the laser diode 201 emits light with a low light amount or is turned off by the modulation operation, the error detector 212 is connected to the non-inverting input terminal. Since the input voltage is significantly lower than the input voltage to the inverting input terminal, the output voltage of the error detector 212 may swing to the power supply voltage side. Therefore, when switching to the first automatic light amount control operation, it takes a long time to set the voltage close to the power supply voltage to the target control voltage VC1, and the output voltage of the error detector 212 is changed to the power supply voltage. If it is swung out to the side, an excessive current may flow to the laser diode 201 from the current output circuits 217A, 217B.
[0013]
Incidentally, the laser diode driving apparatus that uses a single footback loop to adjust the light amount of the laser diode to a single target level has the same problem as described above.
[0014]
FIG. 13 is a circuit block diagram of a laser diode driving device that adjusts the light quantity of the laser diode to a single target level. In FIG. 13, the automatic light quantity control operation is performed by closing the switch means 270A, 270B, 263 and opening the switch means 270C, 270D. At this time, the optical monitor current output from the photodetector 252 is converted into current-voltage by the variable resistor 269, and the monitor voltage VM is applied to the inverting input terminal of the error detector 261 via the buffer means 260A and the switch means 270A, respectively. The reference voltage VR generated by the variable resistor 259 constituting the reference voltage generation circuit 258 is input to the non-inverting input terminal of the error detector 261 via the buffer means 260B and the switch means 270B. Then, the error detector 261 compares the monitor voltage VM with the reference voltage VR. The error voltage output from the error detector 261 is converted into a laser drive current by the drive current output circuit 267 input via the switch means 263 and the buffer means 265, and the laser diode 251 emits light with the drive current added to the bias current. . The light amount of the laser diode 251 is adjusted to the target light amount by adjusting the monitor voltage VM by the variable resistor 269 or adjusting the reference voltage VR1 generated by the reference voltage generation circuit 258. When the light amount of the laser diode 251 is stabilized at a desired light amount, the switch means 263 is opened, and the control voltage VC at that time is held in the capacitor 264.
[0015]
However, when the laser diode is extinguished or emits light with a low light amount immediately before the start of automatic light amount control, the light monitor current output from the photodetector is as small as several tens of μA to several hundred μA, and the load resistance converts this into voltage Since it is necessary to increase the resistance value of the detector, a phase delay occurs due to the CR time constant, the automatic light quantity control operation is started, and when the laser diode emits light at the maximum light quantity, it converges to the corresponding monitor voltage. The time will be longer. Therefore, the monitor voltage input to the inverting input terminal of the error detector 261 shown in FIG. 13 is lower than the reference voltage VR, and the control voltage for driving the laser diode 251 is increased. Driving currents flow and automatic light control time becomes a factor.
[0016]
FIGS. 14A, 14B, and 14C are waveform diagrams for explaining this. 14A, 14B, and 14C, periods P1 and P3 indicate modulation periods, periods P2 and P4 indicate automatic light quantity control periods, respectively, and the waveform in FIG. 14B, the waveform in FIG. 14B shows the control voltage VH output from the sample hold circuit 262, and the waveform in FIG. 14C shows the drive current I flowing through the laser diode 251.
[0017]
When shifting from the modulation operation period P3 to the automatic light amount control operation period P4, the monitor voltage VM varies in response to the output of the laser diode 251, and thus the laser diode 251 is turned off immediately before the automatic light amount control operation period P4. In this case, as indicated by P4 in FIG. 14A, since it takes time for the monitor voltage VM to rise even if the automatic light quantity control operation is started, the output voltage VH of the error detector 261 is set to the reference voltage. By comparison with VR, the light amount increases in the direction of increasing, and as shown in FIG. 14C, the laser drive current I also increases beyond the target drive current. For this reason, the time required to complete the automatic light quantity control operation also becomes longer.
[0018]
In order to solve this, a method of causing the laser diode to emit light with the maximum light amount just before the automatic light amount control operation is conceivable. If the laser diode 251 emits light with the maximum light amount immediately before the automatic light amount control operation period P2 as indicated by P1 in FIG. 14A, the fluctuation of the output voltage VH of the error detector 261 can be reduced. As shown in FIG. 14C, the fluctuation of the laser drive current I can also be reduced.
[0019]
However, the fact that the laser diode emits light with the maximum light amount before starting the automatic light amount control operation is always incorporated, the time for the light monitor voltage to settle always must be provided before the automatic light amount control operation. Accordingly, the modulation operation period is shortened.
[0020]
As described above, Japanese Patent Application Laid-Open No. 9-45980 discloses a technique for receiving output light of a laser diode by a photodiode and holding a voltage output corresponding to the received light amount by a sample hold circuit. A buffer amplifier is configured by an operational amplifier in a period, and a provisional drive voltage is input to maintain the state of the operational amplifier in an active state, and the provisional drive voltage is close to the final target value of the drive voltage in the laser power adjustment period A technique for shortening the time until the comparison output becomes a light emission start level and shifts to a normal loop response operation in the laser power adjustment period by setting the level is disclosed.
[0021]
Japanese Laid-Open Patent Publication No. 62-243137 discloses a control system that feeds back an output of a laser diode that generates a laser beam for recording data on an optical storage medium and controls the output of the laser diode element to a predetermined value. Discloses a technique for preventing an excessive driving current from being supplied to the laser diode by fixing the feedback amount to a value corresponding to the predetermined value before the recording operation.
[0022]
In the technique described in Japanese Patent Laid-Open No. 9-45980 among the above, specifically, the output (laser diode control voltage) of the comparator 292 of FIG. 15 constituted by an operational amplifier and the temporary drive voltage (sample hold circuit 290). The output of the photodiode 294 (the output of the sample hold circuit 290) is input to the comparator 292, and the output of the comparator 292 and the output of the photodiode 294 are made to coincide with each other. It has become. However, in practice, the comparator 292 performs the automatic light quantity control operation so that the output of the photodiode 294 and the reference voltage E _RR Therefore, the output of the comparator 292 does not match the laser diode control voltage at the end of the automatic light quantity control operation. Therefore, highly accurate drive control is difficult. On the other hand, the technique described in Japanese Patent Laid-Open No. Sho 62-243137 also has the same problems because the configurations of the sample hold circuit 296 and the comparator 298 in FIG. 16 are the same as described above.
[0023]
The present invention has been made to solve the above-mentioned problems, and when shifting from a modulation operation to an automatic light amount adjustment operation, or from an automatic light amount adjustment operation based on a predetermined light amount reference value to another light amount reference value. Provided a laser diode driving device capable of reducing the fluctuation of the output voltage of the error amplifier when shifting to the automatic light amount adjustment operation based on the above, and reducing the time required for stable operation transfer and automatic light amount adjustment operation The purpose is to do.
[0024]
[Means for Solving the Problems]
In order to achieve the above object, a laser diode driving device according to claim 1, wherein the laser diode driving device receives laser light from the laser diode and outputs a detection value corresponding to the amount of received light, and output by the light receiving detection device. An error amplifying means for comparing and amplifying the detected value with a predetermined reference value; a holding means for holding an output value after error amplification by the error amplifying means; and the output value held by the holding means And a driving means for driving the laser diode based on the above, a drive current value of the laser diode when the detected value of the light emission amount of the laser diode becomes equal to the predetermined reference value is set During the light quantity control period for performing the phase compensation, the voltage is electrically connected to a predetermined location in the error amplifying means and the internal voltage of the error amplifying means is set. Characterized in that a phase compensation means for lifting.
[0025]
In the laser diode driving apparatus according to the first aspect, the light reception detection means receives the laser light from the laser diode, outputs a detection value corresponding to the received light amount, and the output detection value is input to the error amplification means. . The error amplifying means compares the detected value with a predetermined reference value to amplify the error, and the holding means holds the output value after the error amplification. As the holding means here, for example, a capacitor can be adopted, and the output voltage after error amplification output from the error amplifying means may be held in the capacitor.
[0026]
And a drive means drives a laser diode based on the hold | maintained output value. For example, the drive means converts the output voltage held in the capacitor into a drive current for the laser diode, and supplies the obtained drive current to the laser diode, thereby driving the laser diode.
[0027]
In such a laser diode drive device, a light amount control period is provided for setting a drive current value of the laser diode when the detected value of the light emission amount of the laser diode becomes equal to a predetermined reference value. During this light amount control period, the phase compensation means is electrically connected to a predetermined location in the error amplification means to perform phase compensation and hold the internal voltage of the error amplification means.
[0028]
Therefore, at the end of the light amount control period, the internal voltage of the error amplifying unit immediately before the end of the light amount control period is held in the phase compensation unit. After that, for example, when the light amount control period is reached again after a modulation period for modulating laser light based on image data, the phase compensation means is electrically connected to a predetermined location in the error amplification means. The compensating means holds the internal voltage of the error amplifying means at the end of the previous light amount control period.
[0029]
Preferably, the error amplifying means is a switching means that is turned on during the light amount control period, and is electrically connected to a predetermined location in the error amplifying means via the switching means to perform phase compensation and an internal voltage of the error amplifying means. It is preferable to include a compensation means that holds
[0030]
In this case, in the light amount control period, the switching unit is turned on, and the compensation unit is electrically connected to a predetermined location in the error amplification unit to perform phase compensation. As this compensation means, for example, a CR circuit constituted by a capacitor and a resistor connected in series may be adopted, or it may be constituted only by a capacitor. In the first phase compensation, the capacitor is charged with the charge of the phase compensation capacitance. When the light amount control period ends, the switching unit is turned off, and the compensation unit holds the internal voltage of the error amplifying unit at the end of the light amount control period. Thereafter, when the light quantity control period is reached again, the switching means is turned on, and the compensating means is electrically connected to a predetermined location in the error amplifying means. At this time, the compensation means already holds the internal voltage of the error amplifying means at the end of the previous light amount control period.
[0031]
Thus, at the start of the light quantity control period, the phase compensation means holds the internal voltage of the error amplification means at the end of the previous light quantity control period. There is no need to charge according to the charge. As a result, the time required for the light amount control operation can be shortened, and the shift to the stable light amount control operation can be achieved by reducing the fluctuation of the output voltage of the error amplifying means.
[0032]
As the predetermined location in the error amplifying means, for example, the interval between the input and output of the output stage of the error amplifying means can be adopted. In this case, the voltage of the output stage immediately before the end of the light amount control period is held as the internal voltage. However, in the present invention, as shown in FIG. 7, for example, an error amplifier composed of three stages of a differential amplifier 90A, a shift stage 90B, and an output stage 90C in order to increase the gain of the entire error amplifier by gain superposition. The phase compensation means 23 may be provided between the output S0 of the shift stage 90B and the input N0 of the output stage 90C. Thus, the application range of the present invention is not limited by the configuration of the error amplifier and the connection location of the phase compensation means.
[0033]
Incidentally, a plurality of reference values may be provided as reference values for the light emission amount of the laser diode. As the plurality of reference values, for example, the maximum light amount and the minimum light amount of the laser diode (arbitrary light amount between the light amount corresponding to the threshold current and the maximum light amount) are often adopted.
[0034]
The laser diode driving device according to claim 2 is a laser diode driving device in which a plurality of reference values are provided as reference values for the light emission amount of the laser diode. The laser diode driving device according to claim 2 receives the laser beam from the laser diode and outputs a detection value corresponding to the amount of received light, and the detection value output by the light reception detection unit and a predetermined value. Based on the error amplification means for comparing and amplifying each of the plurality of reference values, holding means for holding the output value after error amplification by the error amplification means, and the output value held by the holding means And a driving means for driving the laser diode. Each reference for setting a driving current value of the laser diode when a detected value of the light emission amount of the laser diode is equal to each reference value. During a light amount control period for each value, phase compensation is performed by being electrically connected to a predetermined location in the error amplifying means, and the internal voltage of the error amplifying means is held. Characterized in that a phase compensation means for each reference value.
[0035]
In the laser diode driving device according to claim 2, as in the laser diode driving device according to claim 1, the phase compensation means corresponding to the reference value is provided in the first light amount control period for one reference value. It is electrically connected to a predetermined location in the error amplification means to perform phase compensation and hold the internal voltage of the error amplification means.
[0036]
After that, when the light quantity control period relating to another reference value or the modulation period for modulating laser light based on image data, for example, is entered again into the first light quantity control period, the phase compensation means corresponding to the reference value performs error amplification. It is electrically connected to a predetermined location in the means. At this time, the phase compensation means already holds the internal voltage of the error amplification means at the end of the previous light amount control period.
[0037]
Preferably, the error amplifying means is electrically connected to a predetermined location in the error amplifying means via the switching means for each reference value that is turned on during the light amount control period for each reference value and the switching means for each reference value. And a compensation means for each reference value for performing phase compensation and holding the internal voltage of the error amplifying means.
[0038]
In this case, when the first light quantity control period for one reference value is reached, the switching means corresponding to the reference value is turned on, and the compensation means corresponding to the reference value is electrically connected to a predetermined location in the error amplifying means. Phase compensation. In the first phase compensation, the compensation means (capacitor or the like) is charged with the charge of the phase compensation capacitance. When the first light amount control period ends, the switching unit is turned off, and the compensation unit holds the internal voltage of the error amplifying unit at the end of the first light amount control period. After that, when the light quantity control period relating to another reference value, for example, the modulation period for modulating laser light based on image data, and the first light quantity control period again, the switching means is turned on, and the compensation means is error amplified. It is electrically connected to a predetermined location in the means. At this time, the compensation means already holds the internal voltage of the error amplifying means at the end of the previous light amount control period.
[0039]
As described above, when the first light quantity control period starts again, the phase compensation means already holds the internal voltage of the error amplifying means at the end of the previous light quantity control period. There is no need to charge the capacitor according to the capacity and the internal voltage of the error amplifying means, the time required for the light intensity control operation can be shortened, and the fluctuation of the output voltage of the error amplifying means can be reduced to stabilize the light quantity. Transition to control operation can be achieved. Similarly, regarding the light amount control period related to other reference values, it is possible to shorten the time required for the light amount control operation and shift to a stable light amount control operation.
[0054]
DETAILED DESCRIPTION OF THE INVENTION
[First Embodiment]
A first embodiment according to the first and second aspects of the present invention will be described below with reference to the drawings.
[0055]
FIG. 1 shows a configuration of a laser diode driving device 35 that adjusts the light amount of the laser diode 1 to a plurality of different target levels for each feedback loop using a plurality of feedback loops.
[0056]
The laser diode driving device 35 is provided with two reference voltage generation circuits 9A and 9B. The reference voltage generation circuit 9A divides the power supply voltage by the variable resistor 10A to obtain a desired reference voltage VR1. The generation circuit 9B divides the power supply voltage by the variable resistor 10B to obtain a desired reference voltage VR2 (note that the reference voltages VR1 and VR2 are collectively referred to as the reference voltage VR). One of the reference voltages VR1 and VR2 is selected by opening / closing the switch means 30A and 30B, and a reference signal corresponding to the selected reference voltage is input to the non-inverting input terminal of the error amplifier 12 via the buffer means 11B. .
[0057]
The laser diode driving device 35 is provided with a laser diode 1, and laser light output from the laser diode 1 is received by the photodetector 2. The optical monitor current output from the photodetector 2 is subjected to current-voltage conversion by the variable resistor 22, and a monitor signal corresponding to the monitor voltage VM obtained by the conversion is supplied to the inverting input terminal of the error amplifier 12 via the buffer means 11A. Is input.
[0058]
In the error amplifier 12, error amplification is performed with the selected reference voltage and the monitor voltage VM, and a signal after error amplification (hereinafter referred to as an error amplification signal) is input to the sample hold circuits 13A and 13B. The sample and hold circuit 13A is composed of a switch means 14A having one end connected to the output terminal of the error amplifier 12, and a capacitor 15A connected to the other end of the switch means 14A and grounded. When closed, the error amplification signal from the error amplifier 12 is input to the drive current output circuit 17A via the buffer means 16A.
[0059]
The sample hold circuit 13B is configured in the same manner as the sample hold circuit 13A. When the switch means 14B in the sample hold circuit 13B is closed, the error amplification signal from the error amplifier 12 is driven through the buffer means 16B. Input to the output circuit 17B.
[0060]
The drive current I1 generated by the drive current output circuit 17A and the drive current I2 generated by the drive current output circuit 17B are adjusted by adjusting the monitor voltage VM to the reference voltage VR in different automatic light amount control periods. An arbitrary laser light amount is set. On the other hand, during the modulation period, the drive current I1 is intensity-modulated in 16 steps according to the intensity-modulated data signal input from the signal input terminals 3A, 3B, 3C, and 3D, and the current is added to the other drive current I2. The laser diode 1 is driven to emit light by the drive current I after the addition.
[0061]
Here, the configuration of the drive current output circuit 17A (FIG. 4) and the configuration of the signal input circuit 19 (FIG. 5) will be described. In FIG. 5, input signals IN1, IN2, IN3, and IN4 correspond to signals input from the input terminals 3A, 3B, 3C, and 3D of FIGS. 1 and 4 to the signal input circuit 19, respectively. This corresponds to a pulse width modulation signal input from the input terminal 4 to the signal input circuit 19. Further, the normal rotation signals OUT1, OUT2, OUT3, and OUT4 correspond to the differential switch signal group 20A in FIGS. 1 and 4, and the inverted signals OUT1 ′, OUT2 ′, OUT3 ′, and OUT4 ′ are in FIGS. Corresponds to the differential switch signal group 20B.
[0062]
In the signal input circuit 19 shown in FIG. 5, the input intensity modulation signal IN1 is input to the AND circuit 19C via the inverters 19A and 19B. The input pulse width modulation signal PWM is also input to the AND circuit 19C via the inverters 19J and 19K. A signal representing a logical product result of the intensity modulation signal IN1 and the pulse width modulation signal PWM by the logical product circuit 19C is inverted at the output terminal of the logical product circuit 19C, and then the normal rotation signal OUT1 through the inverters 19D, 19E, and 19F. Is output as The signal representing the logical product result is inverted at the output terminal of the logical product circuit 19C, and then output as the inverted signal OUT1 ′ via the inverters 19D, 19G, 19H, and 19I.
[0063]
Similarly, the other intensity modulation signals IN2, IN3, and IN4 are ANDed with the pulse width modulation signal PWM, and a normal rotation signal and an inverted signal corresponding to the logical product result are output. Further, the normal rotation signal and the inverted signal that are output are inverted from each other.
The drive current output circuit 17A shown in FIG. 4 is provided with transistor pairs 41A, 41B, 41C and 41D composed of a pair of MOS transistors. The drain of the left transistor in FIG. The drain of the right transistor connected to the diode 1 is the power supply V _DD It is connected to the.
[0064]
Each signal of the differential switch signal group 20A is input in a one-to-one correspondence to the gate of the left transistor, and each signal of the differential switch signal group 20B is input in a one-to-one correspondence to the gate of the right transistor. .
[0065]
The source of each transistor of the transistor pair 41A is grounded via the current source transistor 40A and the resistor R. Similarly, the transistor pairs 41B, 41C, and 41D are connected to the current source transistors 40B, 40C, and 40D, respectively.
[0066]
The transistor ratio of these current source transistors 40A, 40B, 40C, and 40D = (gate width / gate length) is configured to be a ratio of 8: 4: 2: 1, and is turned on / off for each current source transistor. By performing the off control, the current value can be switched in 16 stages including the case where all the current source transistors are turned off (when no current flows).
[0067]
For example, when only OUT1 and OUT4 are ON in the differential switch signal group 20A (only OUT2 ′ and OUT3 ′ are ON in the differential switch signal group 20B at this time), the transistor pair 41A and 41D has the left side in FIG. The transistor is turned on, and the transistor on the right side in FIG. 4 is turned on in the transistor pair 41B and 41C. At this time, the combined current of the current set by the current source transistors 40A and 40D and the bias current set by the drive current output circuit 17B is supplied to the laser diode 1. That is, the laser diode 1 is driven by the combined current of the current of the 9th (= 8 + 1) stage and the bias current among the 16 stages.
[0068]
The drive current control voltage 42 corresponds to the output voltage VC1 of the sample hold circuit 13A of FIG. 1, and the current controlled by the drive current control voltage 42 and set by the current source transistors 40A, 40B, 40C, and 40D is differential. The modulated current is modulated in 16 stages by the switch signals 20A and 20B, and the modulated current is added to the bias current set by the drive current output circuit 17B, and the laser diode 1 is driven by the added current.
[0069]
In the laser diode driving device 35, a first automatic light amount control operation for setting the maximum light amount of the laser diode 1 and a second automatic light amount control operation for setting a bias current corresponding to the threshold current of the laser diode 1 are alternately performed. The voltage of the two feedback loops is controlled by using one error amplifier 12, so that the output light of the laser diode 1 is set to the target light amount. In the circuit of FIG. 1, a switch is used to automatically adjust two types of currents: a bias current that is fixedly supplied according to the threshold current of the laser diode 1 and a modulation current that is modulated according to an input signal. The two feedback loops are selectively controlled using the means. The drive current output circuit 17A outputs a modulated current obtained by intensity-modulating a current corresponding to the desired maximum light amount by an external signal input. The drive current output circuit 17 </ b> B has a function of constantly supplying a bias current determined from the threshold current of the laser diode 1 to the laser diode 1. This bias current serves to increase the lighting speed of the laser diode.
[0070]
Although the details will be described later, the error amplifier 12 of the laser diode driving device 35 is constituted by a basic circuit of a CMOS operational amplifier shown in FIG. 3, and as shown in FIG. Means 23 is connected. This phase compensation means 23 includes, as a phase compensation circuit, a circuit comprising a resistor 24, an on-resistance of a switch means 25A and a capacitor 26A, and a circuit comprising a resistor 24, an on-resistance of a switch means 25B and a capacitor 26B. Switching is performed by opening and closing the switch means 25A and 25B. As the phase compensation circuit, a single capacitor may be employed instead of the CR circuit composed of the resistor and the capacitor as described above.
[0071]
In FIG. 1, reference voltage generation circuits 9A and 9B divide a power supply voltage by variable resistors 10A and 10B to obtain a plurality of desired reference voltages VR. The reference voltage generation circuits 9A and 9B A digital-analog conversion circuit (DA converter) may be used so that the generated reference voltage value can be automatically controlled by a CPU or the like.
[0072]
Further, a circuit similar to the signal input circuit 19 shown in FIG. 5 may be provided in the drive current output circuit 17B, and the drive current output from the drive current output circuit 17B may be modulated stepwise.
[0073]
Next, each operation mode of the first automatic light amount control operation, the second automatic light amount control operation, and the modulation operation will be described. The opening / closing operation of the switch means 14A, 14B, 30A, 30B, 25A, 25B is controlled by a switch signal output from a control device (not shown).
[0074]
First, the first automatic light quantity control operation will be described. In FIG. 1, the first automatic light amount control operation for setting the maximum light amount of the laser diode 1 is executed by closing the switch means 14A and opening the switch means 14B. At this time, the optical monitor current output from the photodetector 2 is subjected to current-voltage conversion by the variable resistor 22, and the monitor voltage VM obtained by the conversion is input to the inverting input terminal of the error amplifier 12 through the buffer means 11A. At the same time, the reference voltage VR1 generated by the variable resistor 10A constituting the reference voltage generating circuit 9A is input to the non-inverting input terminal of the error amplifier 12 via the switch means 30A and the buffer means 11B. The error voltage VC1 output from the error amplifier 12 by comparing the voltages at both ends is input to the drive current output circuit 17A via the switch means 14A and the buffer means 16A, and is converted into a current by the drive current output circuit 17A. The converted current is superimposed on the bias current output from the drive current output circuit 17B, and the laser diode 1 is driven by the superimposed combined current. When the output light of the laser diode 1 becomes stable at the desired maximum light amount, the switch means 14A constituting the sample hold circuit 13A is opened, and the modulation current control voltage VC1 for setting the drive current corresponding to the maximum light amount. Is held in the capacitor 15A. In this case, the bias current output from the drive current output circuit 17B has already been held in the sample hold circuit 13B by the second automatic light quantity control operation described below, and the current value is set by the bias current control voltage. Is set.
[0075]
Next, a second automatic light amount control operation for setting a bias current corresponding to the threshold current of the laser diode 1 will be described. The second automatic light quantity control operation is executed by closing the switch unit 14B and opening the switch unit 14A. At this time, the optical monitor current output from the photodetector 2 is subjected to current-voltage conversion by the variable resistor 22, and the monitor voltage obtained by the conversion is input to the inverting input terminal of the error amplifier 12 through the buffer means 11A. At the same time, the reference voltage VR2 generated by the variable resistor 10B constituting the reference voltage generation circuit 9B is input to the non-inverting input terminal of the error amplifier 12 via the switch means 30B and the buffer means 11B. The error voltage VC2 output from the error amplifier 12 by comparing the voltages at both ends is input to the drive current output circuit 17B via the switch means 14B and the buffer means 16B, and is converted into a current by the drive current output circuit 17B. The converted current is superimposed on the modulation current output from the drive current output circuit 17A, and the laser diode 1 is driven by the superimposed combined current. When the output light of the laser diode 1 becomes stable at a desired light amount, the switch means 14B constituting the sample hold circuit 13B is opened, and a modulation current control voltage VC2 for setting a drive current corresponding to the desired light amount. Is held in the capacitor 15B.
[0076]
In this case, the modulation current output from the drive current output circuit 17A is such that the digital signal value Din input from the signal input terminals 3A, 3B, 3C, and 3D is set to an integer of 1 to 14, and the reference voltage generation circuit 9 The generated reference voltage VR2 is set according to the digital signal value Din as follows.
[0077]
For example, when Din is set to 10, the voltage VR2 generated by the reference voltage generation circuit 9B is based on the reference voltage VR1 generated at the maximum value Din (max) by the reference voltage generation circuit 9A that defines the maximum light amount. (1) is set.
[0078]
VR2 = VR1 × 10 / Din (max) (1)
By repeatedly performing the first automatic light amount control operation and the second automatic light amount control operation as described above, the bias current controlled by the bias current control voltage VC2 held by the sample hold circuit 13B can be controlled by the laser diode 1. It is gradually approximated to the threshold current. On the other hand, the modulation current controlled by the modulation current control voltage VC1 held by the sample hold circuit 13A is approximated to a current corresponding to the maximum light amount.
[0079]
For example, a case will be described in which the automatic light amount control operation is repeated with the output current I1 = 0 of the drive current output circuit 17A and the output current I2 = 0 of the drive current output circuit 17B as the initial state. Here, the target maximum modulation current is Is, the target bias current is Ib, and the digital input signal Din = 5 from the signal input terminals 3A, 3B, 3C, and 3D in the second automatic light amount control. In the first first automatic light quantity control operation, since the bias current I2 = 0, the modulation current I1-1 is set as shown in the following equation (2).
[0080]
I1-1 = (Is + Ib) (2)
In the second automatic light quantity control, the bias current I2-1 is obtained by subtracting the modulation current 5/15 · I1-1 set by the digital signal input Din = 5 from the target drive current (Ib + Is / 3). It is set as equation (3).
[0081]

Next, in the second automatic light amount control, the modulation current I1-2 is set as the following formula (4) by subtracting the bias current I2-1 from the modulation current I1-1.
[0082]

The bias current I2-2 is set as shown in the following equation (5) by subtracting the modulation current 5/15 · I1-2 set by the digital signal input Din = 5 from the target drive current.
[0083]

Thereafter, by repeating the automatic light amount control operation in the same manner, the modulation currents I1 and n approach the target modulation current Is, and the bias currents I2 and n approach the target bias current Ib.
[0084]
Next, the modulation operation period will be described. During the modulation operation period, by opening the switch means 14A, a modulation current is generated by the drive current control voltage VC1 stored in the capacitor 15A, and the current value of the modulation current is input from the signal input terminals 3A, 3B, 3C, 3D. The digital signal value Din is modulated in 16 steps. Further, by opening the switch means 14B, a bias current is generated by the bias current control voltage VC2 stored in the capacitor 15B. This bias current and the modulated current after modulation are superimposed, and the laser diode 1 is driven to emit light by the superimposed current.
[0085]
Now, operations of the error amplifier 12 and the phase compensation unit 23 will be described. FIG. 3 shows a circuit configuration of a CMOS operational amplifier constituting the error amplifier 12. In FIG. 3, IN- indicates an inverting input terminal, IN + indicates a non-inverting input terminal, and V0 is a bias voltage that determines a current value of a constant current source transistor connected to a power source. In the case of this circuit configuration, the phase compensation means 23 is connected between the output voltage OUT and the internal control voltage N0. In contrast to FIG. 1, the output voltage OUT is taken out from the terminal 8A, and the internal control voltage N0 is taken out from the terminal 8E.
[0086]
In the error amplifier 12 of FIG. 3, the reference voltage VR input to the PMOS 12A from the non-inverting input terminal IN + and the output voltage OUT input to the PMOS 12B from the inverting input terminal IN− via the feedback loop RP of FIG. For example, if the output voltage OUT is larger than the reference voltage VR, the internal control voltage N0 becomes larger than the internal voltage N1, and the output voltage OUT is lowered by the NMOS 12C. In this way, the output voltage OUT is controlled to be equal to the reference voltage VR.
[0087]
Next, phase compensation will be described.
[0088]
In general, the oscillation conditions of an operational amplifier are the following two.
[0089]
(1) The gain in the feedback is 1 or more.
[0090]
(2) The phase shift when making one turn in the feedback is 360 ° or more.
[0091]
In the laser diode drive device 35 of the present embodiment, negative feedback (negative feedback) is applied, and a delay of 180 ° occurs only with this configuration, so that the margin for avoiding oscillation of the operational amplifier is 180 °. It becomes.
[0092]
Since the error amplifier 12 of FIG. 3 includes two stages, that is, a differential amplifier 12S and an output stage 12T, the delay in the second stage is superimposed on the delay in the first stage. This is represented on the Bode diagram as shown in FIGS. 6 (A) and 6 (B). In FIG. 6A, the characteristic A1 indicates the frequency characteristic of the first stage gain, and the characteristic A2 indicates the frequency characteristic of the second stage gain. The frequency characteristic of the gain of the entire operational amplifier is thick. The frequency characteristic is as shown by the broken line. Further, the frequency characteristic of the phase of the entire operational amplifier is a frequency characteristic as indicated by a thick broken line in FIG. The frequency f1 indicates the position of the first-stage pole, and the frequency f2 indicates the position of the second-stage pole.
[0093]
In the characteristics A1 and A2, the gain decreases with a slope of (−20 db / dec) after the pole. Therefore, at the frequency f2 or higher, the gain of the entire operational amplifier is a number as apparent from the characteristics of the thick broken line in FIG. Since the phase delay of the entire operational amplifier is 135 ° or more as apparent from the characteristic of the thick broken line in FIG. 6B, the operational amplifier oscillates.
[0094]
Here, in this embodiment, in order to improve the phase delay, the phase is greatly advanced by the capacitors 26A and 26B having large capacitance. Thus, by shifting the position of the first stage pole to the low frequency side f1 ′, the gain of the entire operational amplifier becomes a frequency characteristic as shown by the thick solid line in FIG. 6A, and the frequency f2 (the second stage pole) ), The gain can be set to 0 db, and oscillation can be avoided.
[0095]
Specifically, in the laser diode driving device 35 of FIG. 1, in the first automatic light quantity control operation, the switch means 25A is closed and the switch means 25B is opened, whereby the resistor 24, the ON resistance of the switch means 25A and the capacitor 26A are used. Phase compensation. When the first automatic light quantity control operation is completed, phase compensation is performed in the first automatic light quantity control operation by opening the switch means 25A at the same timing as opening the switch means 14A constituting the sample hold circuit 13A. The internal voltage of the error amplifier 12 at the time of the operation is held in the capacitor 26A.
[0096]
Next, in the second automatic light quantity control operation, the phase compensation described above is performed by the resistor 24, the ON resistance of the switch means 25B and the capacitor 26B by opening the switch means 25A and closing the switch means 25B. Then, when the second automatic light quantity control operation is completed, the phase compensation is performed in the second automatic light quantity control operation by opening the switch means 25B at the same timing as opening the switch means 14B constituting the sample hold circuit 13B. The internal voltage of the error amplifier 12 at the time of the operation is held in the capacitor 26B.
[0097]
Thereafter, when the first automatic light amount control operation is executed again, the switch means 25A is closed again. At this time, the capacitor 26A already holds the internal voltage at the time of the previous first automatic light amount control operation. Therefore, there is no need to recharge the capacitor 26A according to the phase compensation capacitance and the internal voltage of the error amplifier 12, and the time required for the first automatic light quantity control operation can be shortened. In addition, fluctuations in the output voltage of the error amplifier 12 can be reduced, and a transition to a stable first automatic light amount control operation can be achieved.
[0098]
After that, when the operation shifts to the second automatic light amount control operation, the capacitor 26B already holds the internal voltage at the previous second automatic light amount control operation as described above. Thus, there is no need to charge the phase compensation capacitor and the internal voltage of the error amplifier 12 again, the time required for the second automatic light quantity control operation can be shortened, and the fluctuation of the output voltage of the error amplifier 12 can be reduced. Thus, it is possible to shift to a stable second automatic light amount control operation.
[0099]
In the above embodiment, the error amplifier 12 is constituted by a basic circuit of a CMOS operational amplifier as shown in FIG. 3, and the phase compensation means 23 is provided between the input N0 and the output OUT of the output stage of the error amplifier 12. Although an example has been described, the present invention can also be applied to error amplifiers having configurations other than those described above, and the installation location of the phase compensation means is not limited.
[0100]
For example, as shown in FIG. 7, in order to increase the gain of the entire error amplifier by gain superposition, the differential amplifier 90A, a shift stage 90B, and an output stage 90C are configured in three stages, and the phase compensation unit 23 is configured as a shift stage 90B. The present invention can also be applied to the error amplifier 90 provided between the output S0 of the output and the input N0 of the output stage 90C.
[0101]
[Second Embodiment]
Next, as a second embodiment, a circuit for making the input / output voltage of the error amplifier 12 constant during the modulation operation period in order to further stabilize the laser diode driving device 35 (FIG. 1) of the first embodiment. A laser diode driving device to which is given will be described.
[0102]
As shown in FIG. 2, in the laser diode driving device 35S according to the present embodiment, the output side of the buffer means 16A is connected to the error amplifier 12 via the switch means 21D, and the output side of the buffer means 16B is connected to the error amplifier 12 via the switch means 21E. The error amplifier 12 is connected to the non-inverting input terminal, and the output side of the error amplifier 12 is connected to the inverting input terminal of the error amplifier 12 via the switch means 21C.
[0103]
Further, the phase compensation means 23 is added with a circuit comprising a resistor 24, an on-resistance of a switch means 25C and a capacitor 26C, and a circuit comprising a resistor 24, an on-resistance of a switch means 25D and a capacitor 26D as a phase compensation circuit. ing.
[0104]
In the laser diode driving device 35S, the first automatic light amount control operation, the modulation operation, the second automatic light amount control operation, and the modulation operation are repeatedly executed in order. Here, for the sake of convenience, the modulation operation immediately after the first automatic light amount control operation is referred to as a first modulation operation, and the modulation operation immediately after the second automatic light amount control operation is referred to as a second modulation operation.
[0105]
In such a laser diode driving device 35S, during the first automatic light quantity control operation, the switch means 14A, the switch means 21A, 21B, and the switch means 25A are closed, and the switch means 14B, the switch means 21C, 21D, 21E, and the switch The means 25B, 25C, 25D are opened. The operation of the feedback loop in the first automatic light quantity control is the same as that in FIG. Further, the phase compensation of the error amplifier 12 by the phase compensation means 23 is performed using the capacitor 26A via the switch means 25A.
[0106]
When the output power of the laser diode 1 becomes stable at the target maximum light amount and the first automatic light amount control operation is switched to the first modulation operation, first, the switch means 14A and 25A are opened. Thereafter, the switch means 21A, 21B are opened, and the switch means 21C, 21E, 25C are closed. In the first modulation operation, the laser diode 1 is driven to emit light using the modulation current control voltage and the bias current control voltage charged in the sample hold circuits 13A and 13B. The error amplifier 12 has a voltage follower configuration, and is set to a state in which the bias current control voltage input to the error amplifier 12 via the switch means 21E is output from the error amplifier 12. Further, the phase compensation of the error amplifier 12 is performed using the capacitor 26C via the switch means 25C.
[0107]
As described above, by opening the switch means 14A and 25A prior to the other switch means, the capacitor 15A is caused by fluctuations in the control voltage caused by opening and closing of the other switch means and fluctuations in the internal voltage of the error amplifier 12 due to disconnection of the feedback loop. , The variation in the holding voltage of 26A can be reduced.
[0108]
When the first modulation operation is switched to the second automatic light amount control operation, the switch means 14B, 21A, 21B, and 25B are closed, and the switch means 14A, 21C, 21E, and 25C are opened. The operation of the feedback loop in the second automatic light quantity control is the same as that in FIG. 1 (first embodiment). The phase compensation of the error amplifier 12 by the phase compensation means 23 is performed using the capacitor 26B via the switch means 25B.
[0109]
Further, when the output power of the laser diode 1 becomes stable at the target light amount and the second automatic light amount control operation is switched to the second modulation operation, the switch means 14B and 25B are first opened. Thereafter, the switch means 21A, 21B are opened and the switch means 21C, 21D, 25D are closed. Even in the second modulation operation, the operation for modulating and outputting the drive current is the same as the first modulation operation. The error amplifier 12 has a voltage follower configuration, and is set to a state in which the modulation current control voltage input to the error amplifier 12 via the switch means 21D is output from the error amplifier 12. Further, the phase compensation of the error amplifier 12 is performed using the capacitor 26D via the switch means 25D.
[0110]
As described above, by opening the switch means 14B and 25B prior to the other switch means, the capacitor 15B is caused by fluctuations in the control voltage caused by opening and closing of the other switch means and fluctuations in the internal voltage of the error amplifier 12 due to disconnection of the feedback loop. , 26B can be reduced.
[0111]
With the above sequence, the connection of the capacitor of the phase compensation unit 23 is individually switched by the switch units 25A, 25B, 25C, and 25D during the first and second automatic light amount control operation periods or the first and second modulation periods. As a result, it is not necessary to recharge the capacitor of the phase compensation means 23 when each operation is switched. As a result, the time required for the automatic light quantity control operation can be greatly reduced.
[0112]
In addition, since the bias current control voltage input to the error amplifier 12 via the switch means 21E is set to be output from the error amplifier 12 during the first modulation operation, the first modulation operation starts. When the operation shifts to the second automatic light amount control operation, the output voltage of the error amplifier 12 changes at a continuous voltage value. Further, since the modulation current control voltage input to the error amplifier 12 via the switch means 21D is set to be output from the error amplifier 12 during the second modulation operation, the second modulation operation starts from the second modulation operation. When the automatic light quantity control operation 1 is started, the output voltage of the error amplifier 12 changes at a continuous voltage value.
[0113]
As a result, in the case of shifting from the modulation operation to the automatic light amount control operation, the fluctuation of the output voltage of the error amplifier 12 can be reduced and the time required for the automatic light amount control operation can be further shortened.
[0114]
The circuits shown in FIGS. 1 and 2 described in the first and second embodiments are examples of a circuit that automatically adjusts the bias current and the modulation current by sharing one error amplifier. In addition, a circuit that automatically adjusts the bias current and the modulation current by sharing one error amplifier is known. The present invention can be applied to a circuit that automatically adjusts a plurality of current values (for example, a bias current value and a modulation current value) in order by sharing one error amplifier as described above. The effect of can be obtained.
[0115]
1 and 2, the circuit surrounded by the chain line 27 corresponds to a circuit to be integrated. However, the phase compensation means 23 may be built in the error amplifier 12 or may be configured to be externally connected to the error amplifier 12. Here, in consideration of an increase in external parts and assembly due to external connection, it is preferable to incorporate all of the resistance of the phase compensation means 23, the switching means, and the capacitor. However, a capacitor is usually made by sandwiching an oxide film with polysilicon or metal wiring. Therefore, when a capacitor is built in as described above, the holding capacity per unit area is reduced, and other functional elements such as transistors are used. In comparison, the integration rate may be significantly reduced. For this reason, when the electric capacity to be held by the capacitor is large, it may be preferable to attach the capacitor externally.
[0116]
FIG. 1 shows an example in which the resistance, switching means, and capacitor of the phase compensation means 23 are all externally attached. FIG. 2 shows the case where only the capacitor is externally attached and the resistance and switching means are incorporated. An example is shown. For capacitors, the frequency characteristics of the feedback loop change depending on the magnitude of the load resistance of the receiver and the difference in the system in the feedback loop, so it is necessary to adjust the capacitance value according to the required equipment specifications. is there. As shown in FIG. 2, if only a capacitor is externally attached and a switch means is incorporated, a control circuit for switching control of the switch means for switching the capacitance value of the capacitor can also be incorporated in the error amplifier 12 for mounting. There is an advantage that the number of parts can be reduced.
[0117]
[Third Embodiment]
Referring to the drawings below The second Three embodiments will be described. FIG. 8 shows a configuration of a laser diode driving apparatus 100 that adjusts the light amount of the laser diode 101 to a single target level using a single feedback loop.
[0118]
The laser diode driving apparatus 100 is provided with a reference voltage generation circuit 108. The reference voltage generation circuit 108 divides a power supply voltage by a variable resistor 109 to obtain a desired reference voltage VR. A reference signal corresponding to the reference voltage VR is input to the non-inverting input terminal of the error amplifier 111 via the buffer means 110B and the switch means 120B.
[0119]
Further, the laser diode driving apparatus 100 is provided with a laser diode 101, and laser light output from the laser diode 101 is received by the photodetector 102. The optical monitor current output from the photodetector 102 is subjected to current-voltage conversion by the variable resistor 119, and monitor signals corresponding to the monitor voltage VM obtained by the conversion are sample hold circuit 105, buffer means 110A, switch means 120A. To the inverting input terminal of the error amplifier 111. The sample and hold circuit 105 includes a switch means 106 having one end connected to the output terminal of the variable resistor 119 and a capacitor 107 connected to the other end of the switch means 106 and grounded.
[0120]
In the error amplifier 111, error amplification is performed using the reference voltage VR and the monitor voltage VM, and a signal after error amplification (error amplified signal) is input to the sample hold circuit 112. The sample and hold circuit 112 includes a switch unit 113 having one end connected to the output end of the error amplifier 111 and a capacitor 114 connected to the other end of the switch unit 113 and grounded.
[0121]
When the switch unit 113 is closed, the error amplification signal from the error amplifier 111 is input to the non-inverting input terminal of the buffer unit 115. The output side of the buffer means 115 is connected to the inverting input terminal of the buffer means 115 and is connected to the non-inverting input terminal of the error amplifier 111 via the switch means 120D. The output side of the buffer means 115 is connected to the input terminal of the drive current output circuit 117. The output side of the error amplifier 111 is connected to the inverting input terminal of the error amplifier 111 via the switch means 120C.
[0122]
The configuration of the drive current output circuit 117 is the same as the configuration of the drive current output circuit 17A of the first embodiment (FIG. 4), and the configuration of the signal input circuit 116 is the configuration of the signal input circuit 19 of the first embodiment ( Since it is the same as FIG. 5), description is abbreviate | omitted.
[0123]
The drive current generated in the drive current output circuit 117 is set to correspond to an arbitrary laser light amount by adjusting the monitor voltage VM to coincide with the reference voltage VR in the automatic light amount control period. On the other hand, in the modulation period, the drive current is intensity-modulated in 16 steps according to the intensity-modulated data signal input from the signal input terminals 103A, 103B, 103C, and 103D, and the bias current supplied from the bias current output circuit 118 The current is added, and the laser diode 1 is driven to emit light by the drive current after the addition.
[0124]
Next, each operation mode selected by switching the switch means will be described. The opening / closing operation of the switch means 120A, 120B, 120C, 120D, 106, 113 is controlled by a switch signal output by a control device (not shown).
[0125]
First, the automatic light quantity control operation will be described.
[0126]
The automatic light quantity control operation is selected by closing the switch means 120A, 120B, 106, 113 and opening the switch means 120C, 120D. The photodetector 102 receives the light output output from the laser diode 101 and outputs a photocurrent corresponding to the light output. This photocurrent is converted from current to voltage by the variable resistor 119, and the monitor voltage VM is input to the inverting input terminal of the error amplifier 111 via the switch means 106, the buffer means 110A, and the switch means 120A. On the other hand, the reference voltage VR generated by the reference voltage generation circuit 108 is input to the non-inverting input terminal of the error amplifier 111 via the buffer means 110B and the switch means 120B.
[0127]
The error amplifier 111 compares the monitor voltage VM with the reference voltage VR, outputs a voltage VS corresponding to the difference, and charges the capacitor 114 via the switch means 113. The voltage VC charged in the capacitor 114 is input to the laser drive current output circuit 117 via the buffer means 115, and the current flowing through the current source is controlled as follows.
[0128]
For example, the output voltage VC ′ of the buffer means 115 is input to the terminal 42 of the laser drive current output circuit shown in FIG. 4, and the switch signal group 20A is all set to “H” by the signal input circuit 19, and the switch signal group 20B is set. By setting all to “L”, the current flowing through the current sources 40A, 40B, 40C, and 40D is controlled to flow through the laser diode 101 (FIG. 8) via the differential switch means 41A, 41B, 41C, and 41D. To do.
[0129]
Actually, a current obtained by superimposing the current output from the drive current output circuit 117 and the current output from the bias current output circuit 118 flows through the laser diode 101, and the laser diode 101 is driven by the superimposed current to emit light. This output light is received by the photodetector 102 and negatively fed back, whereby the light quantity of the laser diode 101 is adjusted to the target light quantity. The target light quantity can be adjusted by adjusting the reference voltage VR generated by the reference voltage generation circuit 108 or by adjusting the resistance value of the variable resistor 119 constituting the current-voltage conversion circuit. ing. When the drive current flowing through the laser diode 101 becomes stable, the switch 113 is opened, and the drive current control voltage VC at that time is held in the capacitor 114.
[0130]
When shifting from the automatic light amount control operation to the modulation operation, the switch means 113 is switched to the switch means 120A, in order to prevent the influence of the output voltage fluctuation of the error amplifier 111 caused by the switch means 120A, 120B, 120C, 120D being cut off. It is desirable to hold the drive current control voltage VC in the capacitor 114 by opening it before 120B, 120C, 120D.
[0131]
Simultaneously with the switch means 113, the switch means 106 of the sample hold circuit 105 is opened, and the monitor voltage VM output from the photodetector 102 at the end of the automatic light quantity control operation is held in the capacitor 107. By opening the switch means 106 of the sample hold circuit 105 simultaneously with the switch means 113, fluctuations in the monitor voltage VM can be prevented.
[0132]
Next, the modulation operation will be described. The modulation operation is performed as follows using the drive current control voltage VC held in the capacitor 114. This control voltage VC is input to the drive current output circuit 117 via the buffer means 115, and converted into 16 steps of modulation current by the external control signals of the intensity modulation signals 103A, 103B, 103C, 103D and the pulse width modulation signal 104. Then, the drive current flowing through the laser diode 101 is freely modulated by being added to the bias current.
[0133]
During the period during which this modulation operation is performed, the switch means 120A, 120B, 106, 113 are opened, and the switch means 120C and the switch means 120D are closed. Therefore, the error amplifier 111 operates as a voltage follower circuit that receives the control voltage VC ′ output from the buffer unit 115. That is, the control voltage VC output from the buffer unit 115 is input to the error amplifier 111 via the switch unit 120D, and the control unit VC is set to a state in which the control voltage VC is output from the error amplifier 111 by closing the switch unit 120C. . The capacitor 107 of the sample and hold circuit 105 holds a voltage that substantially matches the target monitor voltage VM.
[0134]
When the shift from the modulation operation to the automatic light amount control operation is performed, the capacitor 107 holds a voltage that substantially matches the target monitor voltage VM. Even when the light is turned off or the light is emitted with a low light quantity and the photocurrent output from the photodetector 102 is small, the monitor voltage input to the error amplifier 111 is a voltage that substantially matches the target monitor voltage VM. Since the voltage difference with the monitor voltage VM is very small, the delay in the rise of the monitor voltage can be reduced. Further, since the monitor voltage VM1 stored in the capacitor 107 charges the voltage at the connection end of the photodetector 102 and the variable resistor 119 via the switch means 105A, the optical monitor current-monitor voltage requiring a long convergence time. This has the effect of accelerating the convergence of the conversion unit.
[0135]
FIGS. 11A to 11C are waveform diagrams for explaining the operation of the laser diode driving apparatus 100. FIG. In FIGS. 11A to 11C, periods P2 and P4 indicate automatic light quantity control operation periods, and periods P1 and P3 indicate modulation operation periods.
[0136]
When shifting from the automatic light quantity control operation period P2 to the modulation operation period P3, the monitor signal of the photodetector 102 stabilized by the automatic light quantity control is supplied to the sample and hold circuit 105 as shown in the monitor voltage waveform of FIG. And is continuously output even during the modulation operation period P3. Thus, when the modulation operation period P3 shifts to the automatic light amount control operation period P4, the holding voltage VM of the capacitor 107 holds the voltage at the connection end of the photodetector 102 and the variable resistor 119 via the switch means 106. Since the monitor voltage charged with the voltage VM and the monitor voltage input to the error amplifier 111 substantially matches the target monitor voltage VM, it is possible to prevent a delay in the rise of the monitor voltage.
[0137]
Further, the output (FIG. 11B) of the error amplifier 111 having the voltage follower configuration with the control voltage VC held by the sample hold circuit 112 as an input is the control voltage from the automatic light quantity control operation period P2 to the modulation operation period P3. It is maintained as VC.
[0138]
Accordingly, since the fluctuation amount of the input / output terminal voltage of the error amplifier 111 when shifting from the modulation operation period P3 to the automatic light amount control operation period P4 is reduced, the laser light amount is quickly set to the target light amount, and the monitor The overshoot of the laser driving current due to the delay of the voltage rise can be greatly reduced.
[0139]
In addition, the transition from the modulation operation period P1 to the automatic light quantity control operation period P2 shown in FIG. 11 causes the laser diode 101 to emit light with the maximum light quantity immediately before the automatic light quantity control period P2, as is apparent from FIG. In this case, since the monitor voltage VM at the time of the previous automatic light amount control operation is held by the sample hold circuit 105 even during the modulation operation, regardless of the output light amount of the laser diode 101, the error amplifier 111 The fluctuation of the output voltage can be reduced.
[0140]
[Fourth Embodiment]
Next, as a fourth embodiment, a laser diode driving device provided with a plurality of driving current output circuits for driving a laser diode. Place explain. As the laser diode driving device, as in the optical disk driving device, two types of laser diode output lights having different reproduction light amounts and writing light amounts are required, and each driving current output circuit outputs different light outputs. Examples thereof include an apparatus.
[0141]
The laser diode driving device 100S shown in FIG. 9 has the same configuration as the laser diode driving device 100 of FIG. 8 described above, but the laser diode driving device 100S is provided with two drive current output circuits 117 and 117S. Among them, in order to set the drive current I1 generated by the drive current output circuit 117, a reference voltage generation circuit 108A and sample hold circuits 112A and 105A are provided, and the drive current I2 generated by the drive current output circuit 117S is obtained. In order to set, a reference voltage generation circuit 108B and sample hold circuits 112B and 105B are provided.
[0142]
That is, in the laser diode driving apparatus 100S, the first automatic light amount control operation for setting the driving current I1 generated by the driving current output circuit 117 and the second automatic light amount setting for setting the driving current I2 generated by the driving current output circuit 117S. A light amount control operation is performed.
[0143]
In the first automatic light quantity setting operation, the switch means 120A, 120B, 106A, 113A, 122A, 123A are closed, and the other switch means are opened, whereby the monitor voltage VM and the reference voltage VR1 generated by the reference voltage generation circuit 108A. Are compared by the error amplifier 111, and the amount of laser light defined by the resistance value of the variable resistor 119 and the reference voltage VR1 is set by negative feedback feedback control.
[0144]
At this time, the output voltage of the error amplifier 111 is charged in the capacitor 114A of the sample and hold circuit 112A, and the monitor output voltage related to the photodetector 102 is charged in the capacitor 107A of the sample and hold circuit 105A.
[0145]
When the light amount of the laser diode 101 is stabilized at the first target light amount, the switch means 106A and 113A are opened, and the monitor voltage VM1 of the photodetector 102 corresponding to the first target light amount is held in the sample hold circuit 105A. Then, the control voltage VC1 for obtaining the laser light amount corresponding to the first target light amount is held in the sample hold circuit 112A.
[0146]
In the second automatic light quantity control operation, the monitor means VM and the reference voltage generation circuit 108B are generated by closing the switch means 120A, 120B, 106B, 113B, 122B, 123B and opening the other switch means in the same manner as described above. The reference voltage VR2 is compared with the error amplifier 111, and the laser light quantity defined by the resistance value of the variable resistor 119 and the reference voltage VR2 is set by negative feedback feedback control.
[0147]
At this time, the output voltage of the error amplifier 111 is charged in the capacitor 114B of the sample hold circuit 112B, and the monitor output voltage related to the photodetector 102 is charged in the capacitor 107B of the sample hold circuit 105B.
[0148]
When the light quantity of the laser diode 101 is stabilized at the second target light quantity, the switch means 106B and 113B are opened, and the monitor voltage VM2 of the photodetector 102 corresponding to the second target light quantity is held in the sample hold circuit 105B. Then, the control voltage VC2 for obtaining the laser light amount corresponding to the second target light amount is held in the sample hold circuit 112B.
[0149]
The drive current during the modulation period is obtained by the drive current Is1 output from the laser drive current output circuit 117 using the control voltage VC1 obtained by the first automatic light quantity control operation, or by the second automatic light quantity control operation. When one of the drive currents Is2 output from the laser drive current output circuit 117S is selected using the control voltage VC2 and the laser diode 101 is driven, and when the laser diode 101 is driven with the combined current (Is1 + Is2) There is.
[0150]
In the modulation operation, the switch unit 120C is closed, one of the switch unit 120D1 and the switch unit 120D2 is closed, and the other switch units are opened.
[0151]
In this case, if the first automatic light amount control operation is performed after the modulation operation, the switch means 120C and 120D1 are closed, and the output voltage of the error amplifier 111 is changed during the modulation operation period immediately before the first automatic light amount control operation. The voltage held in the sample-and-hold circuit 112A (= the control voltage VC1 corresponding to the first target light amount) is made to substantially match.
[0152]
At the time of the first automatic light amount control operation, the switch means 106A, 122A are closed and the switch means 106B, 122B are opened, so that the input voltage of the buffer means 110A becomes the target light amount in the first automatic light amount control operation. The voltage substantially matches the corresponding monitor voltage VM1, and the delay in rising of the monitor voltage can be reduced. Further, since the monitor voltage VM1 stored in the capacitor 107 charges the voltage at the connection end of the photodetector 102 and the variable resistor 119 via the switch means 105A, the optical monitor current-monitor voltage requiring a long convergence time. This has the effect of accelerating the convergence of the conversion unit.
[0153]
Thereby, even when the laser light quantity at the end of the modulation operation period is low and the monitor voltage is small correspondingly, at the start of the first automatic light quantity control operation, the error amplifier is connected via the buffer means 110A and the switch means 120A. The voltage input to the inverting input terminal 111 is a voltage that substantially matches the monitor voltage VM1 corresponding to the target light amount in the previous first automatic light amount control operation, and is selected by closing the switch means 123A. Since it substantially coincides with VR1, the time required for the first automatic light quantity control operation can be greatly shortened.
[0154]
In addition, the same effect can be obtained by performing the same operation in the case where the second automatic light amount control operation is performed after the modulation operation.
[0155]
As described above, the fluctuation of the output voltage of the error amplifier 111 caused by the poor convergence of the monitor voltage input to the inverting input terminal of the error amplifier 111 can be greatly reduced, and the convergence time and overshoot of the monitor voltage can be shortened. There is a great effect in reducing the above.
[0156]
[Fifth Embodiment]
Next, as a fifth embodiment, a circuit of a laser diode driving device suitable for further shortening the automatic light quantity control period of the laser diode driving device 100S (FIG. 9) of the fourth embodiment will be described with reference to FIG. To do.
[0157]
In the circuit configuration shown in FIG. 9, the switch means 106A and 106B are closed at the moment of switching from the modulation operation to the automatic light amount control operation, and the output terminal of the photodetector 102 and the terminals of the holding capacitors 107A and 107B are connected. In some cases, the monitor voltage at the moment when the automatic light quantity control operation is started becomes slightly unstable. The circuit shown in FIG. 10 stabilizes the monitor voltage.
[0158]
The operation of the circuit of FIG. 10 is determined by switching of the switch means 182A, 182B, 182C, 182D selected by the switch signals 183A, 183B, 183C, 183D.
[0159]
During the first automatic light quantity control operation, the switch means 182A, 182B, 182D are opened and the switch means 182C is closed. The output light of the laser diode is received by the photodetector 102, and the monitor voltage VM1 generated by the variable resistor 119 is sampled by the capacitor 171 via the switch means 182C.
[0160]
On the other hand, the gate voltage of the MOS transistor 186 is controlled by the operational amplifier 184, and the voltage at the connection terminal 190 between the resistor 185 and the MOS transistor 186 matches the monitor voltage VM1. The connection terminal 191 between the MOS transistor 186 and the resistor 187 is connected to the switch means 120A via the voltage terminal VM and the buffer means 110A of FIG. 9, and the voltage VM1 ′ at the connection terminal 191 is input to the error amplifier 111. The When the output light of the laser diode reaches the target value, the switch unit 182C is disconnected, and the final monitor voltage VM1 in the first automatic light quantity control period is held in the capacitor 171.
[0161]
In the modulation period immediately before the next first automatic light quantity control operation, the switch means 182A, 182B, 182C are opened and the switch means 182D is closed, whereby the final monitor voltage VM1 held in the capacitor 171 is changed to the operational amplifier 181A. Is input to the operational amplifier 184 via the switch means 182D. Since the operational amplifier 184 controls the gate voltage of the MOS transistor 186, the voltage at the connection terminal 190 between the resistor 185 and the MOS transistor 186 matches the monitor signal VM1, and the voltage at the connection terminal 191 between the MOS transistor 186 and the resistor 187. Corresponds to the monitor signal VM1 ′.
[0162]
Then, when shifting from the modulation operation to the first automatic light amount control operation, the switch unit 182D is opened again, and the switch unit 182C is closed instead. At this time, the fluctuation of the monitor voltage viewed from the monitor voltage terminal VM in FIG. 9 is smaller in the circuit configuration shown in FIG. 10 than in the circuit configuration shown in FIG. 9 (details will be described below). For this reason, it is possible to further reduce the overshoot.
Hereinafter, the fluctuation of the monitor voltage in each circuit of FIG. 9 and FIG. 10 will be specifically described by taking as an example a case where the laser diode is turned off during the modulation operation to shift to the automatic light amount control operation.
[0163]
In the case of the circuit of FIG. 9, when the laser diode 101 is turned off by the modulation operation, the current (PD current) flowing through the photodetector 102 is also zero, so that the connection end of the variable resistor 119 and the photodetector 102 Becomes 5 V (= power supply voltage), and the state shifts from this state to the automatic light quantity control operation. When the PD current during the automatic light quantity control operation is 0.1 mA and the resistance value of the variable resistor 119 is 10 kΩ, the monitor voltage VM is finally set to 4 V (= 5-10 kΩ × 0.1 mA).
[0164]
Here, assuming that the capacitance of the capacitor 107A is 1 nF and the sum of the capacitance of the photodetector 102 to be charged and the parasitic capacitance is 10 pF, when a potential difference of 1 V between 5 V and 4 V is charged, 0.01 V (= An error of 1 V × 10 pF / 1 nF occurs. That is, the voltage at the cathode portion of the photodetector 102 (= the voltage at the connection end of the variable resistor 119 and the photodetector 102) becomes 4.01V when switching from 5V during the modulation operation to the automatic light amount control operation. Then, it converges to 4V by the automatic light quantity control operation.
[0165]
On the other hand, in the circuit of FIG. 10, at the time of the modulation operation, for example, the voltage (== Since the final monitor voltage VM1) during the previous automatic light quantity control operation is output, even if the current (PD current) flowing through the photodetector 102 is zero, the voltage (= variable resistance) of the cathode portion of the photodetector 102 The voltage at the connection end of the detector 119 and the photodetector 102 becomes constant at the final monitor voltage VM1. That is, the voltage at the cathode portion of the photodetector 102 is maintained substantially constant at 4 V throughout the modulation operation, the switching to the automatic light amount control operation, and the automatic light amount control operation.
[0166]
As described above, the variation in the monitor voltage viewed from the monitor voltage terminal VM in FIG. 9 is smaller in the circuit configuration shown in FIG. 10 than in the circuit configuration shown in FIG.
[0167]
In the second automatic light amount control operation, the same effect can be obtained by opening and closing the switch means 182A and 182B in the same manner as described above.
[0168]
Further, by removing the circuit 189 shown in FIG. 10, the circuit shown in FIG. 10 can correspond to the circuit shown in FIG.
[0169]
【The invention's effect】
As described above, according to the first aspect of the present invention, when the light quantity control period comes again, the phase compensation means already holds the internal voltage of the error amplifying means at the end of the previous light quantity control period. Therefore, there is no need to recharge the electric charge according to the phase compensation capacitor and the internal voltage of the error amplifying means, the time required for the light amount control operation can be shortened, and the fluctuation of the output voltage of the error amplifying means can be reduced. It is possible to make a shift to a stable light amount control operation with a small size.
[0170]
According to the second aspect of the present invention, it is possible to shorten the time required for the light amount control operation and shift to a stable light amount control operation in the light amount control period for each of the plurality of reference values.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a laser diode driving apparatus according to a first embodiment.
FIG. 2 is a block diagram showing a configuration of a laser diode driving apparatus according to a second embodiment.
FIG. 3 is a schematic diagram illustrating a circuit configuration example of an error amplifier.
FIG. 4 is a schematic diagram illustrating a circuit configuration example of a drive current output circuit.
FIG. 5 is a schematic diagram illustrating a circuit configuration example of a signal input circuit.
6A is a Bode diagram showing frequency-gain characteristics, and FIG. 6B is a Bode diagram showing frequency-phase characteristics.
FIG. 7 is a schematic diagram illustrating another circuit configuration example of the error amplifier.
FIG. 8 is a block diagram showing a configuration of a laser diode driving apparatus according to a third embodiment.
FIG. 9 is a block diagram showing a configuration of a laser diode driving apparatus according to a fourth embodiment.
FIG. 10 is a block diagram showing a configuration of a main part of a laser diode driving device according to a fifth embodiment.
11 is a graph for explaining the operation of the laser diode driving apparatus according to the third embodiment, in which (A) shows the transition of the monitor voltage VM, (B) shows the transition of the output voltage VH of the error amplifier, (C) is a graph which shows transition of the laser drive current I, respectively.
FIG. 12 is a block diagram showing a configuration of a conventional laser diode driving device that adjusts the light amount of a laser diode to a plurality of different target levels.
FIG. 13 is a block diagram showing a configuration of a conventional laser diode driving device that adjusts the light amount of a laser diode to a single target level.
14 is a graph for explaining the operation of the laser diode driving device of FIG. 13, in which (A) shows the transition of the monitor voltage VM, (B) shows the transition of the output voltage VH of the error amplifier, and (C). These are graphs showing the transition of the laser drive current I, respectively.
FIG. 15 is a schematic configuration diagram of a laser driving circuit disclosed in Japanese Patent Laid-Open No. 9-45980.
FIG. 16 is a schematic configuration diagram of a laser driving circuit disclosed in Japanese Patent Laid-Open No. 62-243137.
[Explanation of symbols]
1, 101 Laser diode
2,102 Photodetector
12, 111 Error amplifier
13A, 13B, 105, 112 Sample hold circuit
14A, 14B, 21A-21E, 25A-25D Switch means
106, 113, 120A to 120D switch means
15A, 15B, 26A-26D, 107, 114 Capacitor
17A, 17B, 117 Drive current output circuit
19, 116 Signal input circuit
23 Phase compensation means
35, 35S, 100, 100S Laser diode driving device

Claims (2)

A light receiving detection means for receiving laser light from a laser diode and outputting a detection value according to the amount of light received;
An error amplifying means for comparing and amplifying an error by comparing a detection value output by the light receiving detection means with a predetermined reference value;
Holding means for holding the output value after error amplification by the error amplification means;
Driving means for driving the laser diode based on the output value held by the holding means;
In a laser diode driving device having:
Electrically connected to a predetermined location in the error amplifying means during the light amount control period for setting the drive current value of the laser diode when the detected value of the light emission amount of the laser diode becomes equal to the predetermined reference value And a phase compensation means for performing phase compensation and holding the internal voltage of the error amplifying means. A light receiving detection means for receiving laser light from a laser diode and outputting a detection value according to the amount of light received;
An error amplification means for comparing and amplifying the detection value output by the light receiving detection means and each of a plurality of predetermined reference values;
Holding means for holding the output value after error amplification by the error amplification means;
Driving means for driving the laser diode based on the output value held by the holding means;
In a laser diode driving device having:
In a light amount control period for each reference value for setting the drive current value of the laser diode when the detected value of the light emission amount of the laser diode becomes equal to each reference value, electric power is supplied to a predetermined location in the error amplifying means. And a phase compensation means for each reference value for performing phase compensation and holding the internal voltage of the error amplification means.

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