JP3553909B2 - Semiconductor laser and manufacturing method thereof - Google Patents

Description

ここでＰ₀は所望の光出力、Ｓ´は共振器内光子数、Ｌは共振器長、Ｒｆ、Ｒｒはそれぞれ光出力の出射側端面、モニタ側端面の反射率、ｈはプランク定数、νは振動数、Ｖｇは群速度
なる式を満足する反射率Ｒｆ、Ｒｒになるように、前記ストライプの両端の端面に誘電体膜が設けられることにより、使用機種に応じて定まる読取り用の低出力で可干渉性が極小値に設定されている。
【００１４】
本発明による半導体レーザの他の形態は、ストライプ構造の発光領域を有する活性層と、該活性層を挟んで両側に設けられるクラッド層と、前記発光領域のストライプの両端に形成され、該発光領域で発生した光の一部を放射するとともに残部を反射して前記活性層内で増幅させる端面とを有する光ピックアップの光源用の半導体レーザであって、前記活性層に印加される電流の変化により変化する前記端面の一方から放射する光出力が、少なくとも２５ｍＷ以下の範囲で電流の増加に対して前記光出力が低下する現象が生じないように、かつ、横方向の屈折率差が４ . ２×１０ ^-3 〜５×１０ ^-3 に形成されると共に、使用機種に応じて定まる読取り用の低出力で可干渉性が極小になるように、前記端面に光反射膜がその反射率を調整して設けられている。
【００１５】
ここに横方向の屈折率差とは、活性層において活性層の上または下側に設けられる電流制限層のストライプ溝に対応する部分（発光領域）とその外側の部分との屈折率の差をいい、光吸収損失や電流値の違いにより屈折率が異なってくる。
【００１６】
本発明の半導体レーザの製法は、（ａ）少なくとも活性層の両側にクラッド層を有し、ストライプ構造の発光領域を有するとともに、発光端面に誘電体膜を設けない状態で屈折率導波構造でありながら、読取り用の低出力で可干渉性が極小値を有する半導体ベアチップを作製し、（ｂ）前記極小値のときの光子数を維持しながら前記可干渉性の極小値が読取り用の所望の出力に移動するように、前記半導体ベアチップの光の出射端面およびその反対側のモニタ端面にそれぞれ誘電体膜を設けることを特徴とする。
【００１７】
本発明によれば低出力のところで可干渉性が小さくなるように形成されているため、読取り用の低出力では干渉が小さく、戻り光による誘起ノイズの発生が充分に抑えられ、低ノイズで正確に記録情報を読み取ることができる。
【００１８】
一方、電流を増加させ、光出力が増加するばあいに、少なくとも２５ｍＷ以下の出力ではキンクが生じないように形成されているため、書込み用に必要な２０ｍＷ以上の高出力が確実にえられ、正確に書き込むことができる。
【００１９】
すなわち、本発明によればキンクが生じにくく高出力がえられ易い屈折率導波構造において、横方向の屈折率差を小さくして利得導波構造に近づいけているため、高出力では屈折率導波構造の性質を維持してキンクが生じにくく高出力がえられる。一方、低出力においては利得導波構造の性質も現れ、可干渉性が低下し、戻り光誘起ノイズの発生を充分に抑制できる。
【００２０】
また、本発明の製法によれば、発光端面に誘電体膜を設けない状態で屈折率導波を基本とする発光の光出力の範囲に可干渉性が極小値を有する半導体ベアチップを作製し、その極小値のときの光子数を維持しながら前記可干渉性の極小値が所望の出力に移動するように、前記半導体ベアチップの光の出射端面およびその反対側のモニタ端面にそれぞれ誘電体膜を設けているため、使用機種に応じて読取り用の光出力が異なっても、その機種に応じた光出力において、容易に可干渉性を極小にすることができ、読取時の必要な出力において、とくにノイズを低減させることができる。
【００２１】
【発明の実施の形態】
つぎに、本発明の半導体レーザおよびその製法について説明する。図１は本発明による半導体レーザの一実施形態を示すチップ構造の断面説明図である。
【００２２】
図１において、１はｎ側電極、２はｎ型ＧａＡｓ基板、３はｎ型Ａｌ_ｘ Ｇａ_１−ｘ Ａｓ（０．４５≦ｘ≦０．７５、たとえばｘ＝０．６）からなる下クラッド層（たとえば２μｍ）、４はｎ型、ｐ型またはノンドープのＡｌ_ｙ Ｇａ_１−ｙ Ａｓ（０≦ｙ≦０．２２、たとえばｙ＝０．１５）からなる活性層（たとえば５０ｎｍ）、５はｐ型Ａｌ_ｘ Ｇａ_１−ｘ Ａｓからなる上部第１クラッド層（たとえば３７０ｎｍ）、６はｎ型ＧａＡｓからなる電流制限層（たとえば２８０ｎｍ）、７は、ｎ型Ａｌ_ｚ Ｇａ_１−ｚ Ａｓ（０．０５≦ｚ≦０．２２、たとえばｚ＝０．１５）からなる蒸発防止層（たとえば７０ｎｍ）、８はｐ型Ａｌ_ｘ Ｇａ_１−ｘ Ａｓからなる上部第２クラッド層（たとえば１．６μｍ）、９はｐ型ＧａＡｓからなるキャップ層（たとえば２μｍ）、１０はｐ側電極をそれぞれ示している。また、ストライプ幅Ｓはたとえば３．９μｍである。
【００２３】
ｎ型ドーパントとしてはＳｉを使用し、ｐ型ドーパントとしてはＢｅを使用したが、ｎ型としてはＳｎも使用可能であり、ｐ型として、Ｚｎも使用可能である。また前記構造では、ｎ型基板を使用したが、ｐ型基板でもよく、このばあい前記説明のｎ型とｐ型を全て逆にすればよい。
【００２４】
この構造はＧａＡｓ電流制限層による光吸収損失によって、光をストライプ内に閉じこめる複素屈折率導波構造であるが、このような構造で安定した単一横モードをうるために、ｐ型Ａｌ_ｘ Ｇａ_１−ｘ Ａｓからなる上部第１クラッド層５の膜厚は通常３００ｎｍ前後にコントロールされている。
【００２５】
前述のように、本発明者らは光磁気ディスクの読取り用の低出力では低ノイズとなり誤りなく正確に読み取られるようにするとともに、書込み時には書込みのために必要な高出力がえられる半導体レーザをうるために鋭意検討を重ねた結果つぎのことを見出した。すなわち、たとえば前述の上部第１クラッド層５の膜厚を通常の３００ｎｍから３７０ｎｍ程度に厚くすることにより、横方向の屈折率差が小さくなり、出射端面に誘電体膜を設けない（ノンコーティング）ベアチップの状態で、たとえば１〜３ｍＷの低出力で可干渉性の極小値がえられ、光の放射端面の反射率を調整することにより可干渉性の極小値が現れる光の出力を調整できること、ベアチップの状態で可干渉性が極小になる出力値が１〜３ｍＷになるように半導体レーザチップを作製することにより少なくとも２５ｍＷ以下の出力においてはキンクが生じないで、高出力がえられること、を見出した。
【００２６】
すなわち、本発明の半導体レーザは、ストライプ構造の発光領域を有する活性層と、該活性層を挟んで両側に設けられるクラッド層と、前記発光領域のストライプの両端に形成され、該発光領域で発生した光の一部を放射するとともに残部を反射して前記活性層内で増幅させる端面とを有する半導体レーザであって、前記端面に誘電体膜が設けられない状態で、可干渉性がほぼ極小値をとるときの前記共振器内での光子数を維持しながら、後述する共振器内光子数Ｓ′と光出力Ｐ_０との関係式を満足するように、誘電体膜が前記両端面に設けられることにより、所望の出力で可干渉性が極小値に設定されているものである。
【００２７】
まず、前述の上部第１クラッド層５の膜厚を変えて可干渉性とキンクの発生する出力との関係について調べた。前述のように利得導波構造の採用はとくにキンクの低下という高出力半導体レーザとしては致命的な欠陥をもつことになるのであるが、屈折率導波構造において横方向の屈折率差を小さくしていく（すなわち、上部第１クラッド層５の膜厚を厚くすることなどにより利得導波構造に近づける）と、屈折率導波構造でありながら利得導波的な特性も合せもつようになる。利得導波構造に近づけると自然放出光の寄与が大きくなり可干渉性が低下するが、それにしたがってキンクも低下する。図２は光の出射面の端面コーティングを施していない半導体レーザベアチップ（以下、単にチップという）において、上部第１クラッド層５の膜厚を変えたときの可干渉性の指標となる明瞭度（ビジビリティ）とキンクとの相関をとったものである。ここで明瞭度の指標として図３に示したように、光路差が０の干渉スペクトルから３本目の干渉ピーク（以下、γ_３ という）の大きさを光路差が０のときの干渉ピークを１としたときの値で示し、光出力が３ｍＷのときの明瞭度により相関をみた。図２から、γ_３ が低下するにしたがって、急激にキンクが低下することがわかる。とくにγ_３ ＜０．９において急激なキンクの低下およびばらつきがみられるので、２０ｍＷ以上の光出力をうるには０．８以上のγ_３ が必要である。しかしながら、従来、γ_３ が０．８以上であるような半導体レーザは、可干渉性が高いので、充分な戻り光誘起ノイズの抑制が困難であった。
【００２８】
本発明者らは図２のなかでキンクが現れる出力が２５ｍＷ以上のチップ（図２に示すように、出力の低い順に試料ａ〜ｅという）について、さらにノイズ特性およびスペクトル特性を詳細に調べ、つぎのような結果をえた。なお、ａ、ｂは上部第１クラッド層５の膜厚が３８０ｎｍ、ｃは３５０ｎｍ、ｄ、ｅは３００ｎｍで、Ａの群は３８０ｎｍ、Ｂの群は４００ｎｍで、他の各層の膜厚は前述の例示のものを使用した。Ａの群はａ、ｂと同じであるが、もれ電流量のバラツキなど製造条件のバラツキと考えられる。
【００２９】
図４〜５は、試料ａ〜ｅ（図２にａ〜ｅで示した試料）の各々に関して端面コーティングを施さないばあいと出射側端面の反射率ＲｆがＲｆ＝１０％、１７％となるように端面コーティングを施したばあいの戻り光誘起ノイズの光出力依存性を示したものである。もう一方の放射端面であるモニタ端面の反射率Ｒｒは７５％で一定とした。このとき、戻り光誘起ノイズの指標として、式（１）で表されるノイズパラメータＬＮ、すなわち、各光出力において２００ｋＨｚ以下の最大ＡＣ成分（ノイズ成分）のピーク−ピーク値を規格化するために、３ｍＷ光出力のＤＣ値との比の対数を用いた。２００ｋＨｚ以下にて調査したのは光ディスクにおいては、しばしばこの帯域の低周波ノイズが問題視されるためである。
【００３０】
ＬＮ＝ｌｏｇ（（ＡＣ成分）／（３ｍＷＤＣ成分）×１００）…（１）
図４〜５の（ａ）〜（ｅ）は、試料のａ〜ｅに対応する。図４〜５よりａ〜ｅのうちａ、ｂ、ｃは端面コート後の戻り光誘起ノイズが、Ｒｆ＝１０％においては５〜６ｍＷにおいて極小となり、Ｒｆ＝１７％においては３〜４ｍＷにおいて極小となっており、出射側端面の反射率Ｒｆを大きくすると、ノイズの極小値をとる光出力が低出力側に移動している。
【００３１】
すなわち、放射端面に誘電体膜を設けないノンコーティングの状態では反射率が３０％程度で、ノイズの極小値は１〜３ｍＷに現れるのが、反射率を下げることにより、ノイズが極小となる出力は大きい方に移る。これに伴いキンクの現れる出力もさらに大きい方に移り、高出力の帯域では異常なく高出力がえられる。
【００３２】
一方、試料ｄ、ｅはこのような傾向は見られない。若干Ｒｆ＝１０％よりは１７％の方が戻り光とレーザ共振器との結合が少々弱まるためノイズが良好になる傾向にあるが、試料ａ〜ｃの極小値に比べて明らかにノイズレベルが高い。これは図５からも明らかなように、端面に誘電体膜を設けないノンコーティングの状態でもノイズの極小値が現れておらず、従来の屈折率導波構造と同じであることを示している。
【００３３】
また図６〜７は図４〜５の（ａ）〜（ｅ）と同じ試料の可干渉性光出力依存性を示したものである。
【００３４】
試料ａ、ｂ、ｃと試料ｄ、ｅとの違いは、図６〜７の可干渉性の光出力依存性にも見られる。とくに大きな違いは、試料ｄ、ｅではγ_３ は単調増加しているが、試料ａ、ｂ、ｃでは１ｍＷから３ｍＷのあいだの約２ｍＷ程度でγ_３ が極値をとっていることに特徴がある。
【００３５】
以上の結果から、試料ａ、ｂ、ｃはそれぞれ端面コーティングが施されていない段階ではしきい値付近（すなわち光出力１ｍＷ以下にてγ_３ が低くなっている部分）を除けば、光出力２ｍＷ程度において可干渉性が最も低下する。すなわち、最も戻り光誘起ノイズが抑制される。しかも端面反射率を変化させることによって、可干渉性が極小値をとる光出力が変化する。そのため、最も低ノイズになる部分を端面反射率のコントロールによって、任意の光出力に移動させることができるということが言える。
【００３６】
これらの結果は、つぎのように考えれば、理論的にも成り立つ。明瞭度（ビジビリティ）をフーリエ変換したものがレーザ発振スペクトルであるから、スペクトル線幅が太いほど可干渉性が低くなる。スペクトル線幅Δνは次式で与えられる（たとえば、ジェンズ ブウス（Ｊｅｎｓ Ｂｕｕｓ）による「シングル フリクェンシー セミコンダクタ レーザーズ（Ｓｉｎｇｌｅ ＦｒｅｑｅｎｃｙＳｃｍｉｃｏｎｄｕｃｔｏｒ Ｌａｓｅｒｓ）」、エス ピー アイ イー オプティカル エンジニアリング プレス（ＳＰＩＥ ＯＰＴＩＣＡＬ ＥＮＧＩＮＥＥＲＩＮＧ ＰＲＥＳＳ）、１９９０年、６１頁参照）。
【００３７】
Δν＝Ｒ（１＋α^２ ）／（４πＳ′）
ここでＲは自然放出光の寄与を表す定数、Ｓ′はレーザ共振器内の光子数、αは線幅増大係数と呼ばれる係数である。すなわち、線幅は共振機内光子数の関数である。
【００３８】
一方、共振器内光子数Ｓ′と光出力Ｐ_０ との関係は
【００３９】
【数３】

【００４０】
となる。ここでＬは共振器長、Ｒｆ、Ｒｒはそれぞれ光出力の出射側端面、モニタ側端面の反射率、ｈはプランク定数、νは振動数、Ｖｇは群速度である。この式から、端面コートを施していない段階での光出力と同じ共振器内光子数を与える端面コート後の光出力との関係を表わしたものが図８である。図８において、実線はＲｆ＝１０％、Ｒｒ＝７５％のばあい、破線はＲｆ＝１７％、Ｒｒ＝７５％のばあいを示す。図８には前述の試料ａのＲｆ＝１０％、試料ｂのＲｆ＝１０％、１７％、試料ｃのＲｆ＝１７％のデータをプロットしたが、計算値とほぼ一致することがわかる。
【００４１】
以上をまとめれば、低出力時に低ノイズ特性を示しながら高出力まで単一横モード発振可能な半導体レーザをうるための製法はつぎのようになる。
【００４２】
まず、屈折率導波構造の半導体レーザチップにおいて、用途に応じた高出力動作が可能なキンクレベルを保つ範囲において、横方向屈折率差をできるだけ小さくする。図２に見られるように、横方向屈折率差を小さくすると（すなわち可干渉性を下げていくと）キンクの低下を伴う。したがって、屈折率差をあまり小さくできないため、自然放出の寄与は少なく、たとえば３ｍＷ程度にまで光出力を大きくすると、共振器内光子数の増加に伴って可干渉性はかなり高くなってしまうが、たとえば１〜３ｍＷの約２ｍＷ近辺において可干渉性が一旦小さくなるかまたは極値をとるような振る舞いをするようになる。
【００４３】
このようなチップに端面コートを施して、目的とする高出力半導体レーザをうるように端面反射率を調整するのであるが、その実施方法はつぎのようになる。
【００４４】
たとえば、光出力３〜４ｍＷ近辺で低ノイズとなるようにするには、図４〜５に見られるようにＲｆ＝１７％、Ｒｒ＝７５％とすればよく、４〜５ｍＷで低ノイズとなるようにするには、Ｒｆ＝１０％、Ｒｒ＝７５％とすればよい。ここではＲｒ＝７５％と一定にしてＲｆを変化させたが、Ｒｒ＝７５％に限定されるものではなくて、前述のように、共振器内光子数が端面コーティングを施していない段階での２ｍＷ出力時と同等になるように設計すればよい。ただし、高出力時の信頼性確保のためにＲｆ≦３０％、Ｒｆ≧５０％の範囲で設計するのが望ましい。
【００４５】
前述の、たとえば光出力が２〜７ｍＷの範囲で可干渉性の極小値を有し、かつ、前記光出力が少なくとも２５ｍＷ以下の範囲でキンクが生じないようにする手段として、前記実施例では上部第１クラッド層の膜厚を厚くして電流制限層を遠ざけることにより横方向の屈折率差を小さくし、利得導波構造に近づけた。しかし、屈折率導波構造を利得導波構造の性質に近づけるためには、すなわち横方向の屈折率差を決定する要因としてはこれ以外に活性層の膜厚、クラッド層と活性層の屈折率差、ストライプ幅、電流制限層の膜厚、もれ電流量（結晶成長条件に大きく依存する）などがある。
【００４６】
たとえば活性層の厚さは厚くする程、横方向の屈折率差を小さくすることができ、クラッド層の屈折率を活性層の屈折率より小さくする程、横方向の屈折率差を小さくすることができる。電流制限層を薄くすると屈折率差は周期的に変化し始める。もれ電流量を小さくする程プラズマ効果により横方向の屈折率差を小さくすることができる。もれ電流量は結晶成長条件に多く依存するため、ある程度製造上のプロセスでバラツキが発生しコントロールしきれない面があるものの、各半導体層の材料や膜厚は製造時の選択により自由に制御できる。またストライプ幅が３μｍ以下では同様にプラズマ効果で屈折率差は小さくなる。しかしこれらの各要素を組合わせることにより、それぞれの要素の寄与の集合として現れるため、前記の光出力が、たとえば２〜７ｍＷの範囲で可干渉性の極小値を有し、かつ、前記光出力が少なくとも２５ｍＷ以下の範囲でキンクが生じないようにする手段としては無限の組合わせが生じうる。
【００４７】
たとえば、前述の例では半導体材料として活性層がＡｌ_ｙ Ｇａ_１−ｙ Ａｓのｙ＝０．１５を５０ｎｍ、上部第１クラッド層がＡｌ_ｘ Ｇａ_１−ｘ Ａｓのｘを０．６としたときに、上部第１クラッド層の厚さが３５０〜３８０ｎｍであれば前記要件を満たすことを示している。このときの横方向の屈折率差は電流による屈折率の変化を無視して計算により求めると屈折率差Δηは
Δη＝（β_０ −Ｒｅａｌβ′）／ｋ_０ …（２）
ここでβ_０ はストライプ内での共振器方向の伝播係数、β′はストライプ
外部での共振器方向の伝播係数、ｋ_０ は真空での波数
で求められ、４．２×１０^−３〜５×１０^−３の範囲となる。
【００４８】
一般に誘電体導波路を共振器方向（図１でｚ軸方向）に伝搬するＴＥモード（半導体レーザで伝搬する光はＴＥモードである）を表わす電場の方程式は
【００４９】
【数４】

【００５０】
ここで、Ｅｙは電場のｙ方向成分、ｎ（ｘ）は屈折率
【００５１】
【数５】

【００５２】
であり、βはこの方程式を満たす固有値であたえられるｚ方向の伝搬係数である。ストライプ内部のある１点ｙ_１ でｙ軸（図１参照）に垂直に切った点での誘電体がｚ軸方向およびｙ軸方向に一様に無限にあると仮定して式（３）より求めた伝搬係数βをβ_０ 、ストライプ外部のある１点ｙ_２ でｙ軸に垂直に切った点での誘電体がｚ軸方向およびｙ軸方向に一様に無限にあると仮定して求めた伝搬係数βをβ′（このばあい電流制限層による吸収の影響でβ′は複素数となる）とおけばストライプ内外での実効屈折率差Δηは式（２）となる。
【００５３】
β_０ −Ｒｅａｌβ′は、活性層厚が薄いほど大きく活性層とクラッド層のＡｌ組成差が大きいほど小さく、また活性層と電流制限層の距離が近いほど大きくなる。
【００５４】
したがって、活性層および上部第１クラッド層の半導体材料から定まる屈折率および厚さから計算により求められる屈折率差が４．２×１０^−３〜５×１０^−３の範囲になるように製造することにより前記手段が達成される。
【００５５】
このような計算による屈折率差がえられる半導体層の組成や厚さの関係の一部を表１に示すように、上部第１クラッド層の組成（ｘの値）、活性層の厚さに応じて上部第１クラッド層の必要な厚さが設定される。表１は組合せの極く一部を例示したもので、たとえば上部第１クラッド層Ａｌ_ｘ Ｇａ_１−ｘ ＡｓのＡｌの組成が０．４５≦ｘ≦０．７５、活性層Ａｌ_ｙ Ｇａ_１−ｙ ＡｓのＡｌの組成が０≦ｙ≦０．２２、蒸発防止層Ａｌ_ｚ Ｇａ_１−ｚ ＡｓのＡｌの組成が０．０５≦ｚ≦０．２２の範囲で様々な組合わせが可能である。
【００５６】
【表１】

【００５７】
この上部第１クラッド層の組成に対する活性層の厚さをパラメータとした上部第１クラッド層の膜厚と屈折率差との関係を図９に示す。図９（ａ）は上部第１クラッド層のＡｌの組成がｘ＝０．６のばあいで、図９（ｂ）は上部第１クラッド層のＡｌの組成がｘ＝０．５のばあいである。
【００５８】
またこの関係はＡｌＧａＡｓ系半導体のみならず、ＡｌＧａＩｎＰ系およびＩｎＧａＡｓＰ系の半導体を使用したばあいでも各層の屈折率および膜厚を調整することにより前述の理論的屈折率差がえられ、たとえば２〜７ｍＷの低出力でノイズが小さく、少なくとも２５ｍＷ以下の出力まではキンクが生じない半導体レーザがえられる。
【００５９】
さらに前記実施例においては、結晶成長法に後述するようにＭＢＥ法を用いたセルフアライン構造であるＳＡＭ構造について説明したが、ＳＡＭ構造の他にもＬＰＥ法によるＣＳＰ構造やＶＳＩＳ構造またはＭＯＣＶＤ法によるＳＡＳ構造などの複素屈折率導波構造の他、特開平５−１６０５０３号公報に開示されている実屈折率導波構造の半導体レーザにおいても同様に低出力で低ノイズになり、キンクが生じない高出力がえられる半導体レーザにすることができる。
【００６０】
前述の光出射端面に設ける誘電体膜としては、Ａｌ_２ Ｏ_３ の単層膜やＳｉ_３ Ｎ_４ などの単層膜を使用することができ、膜厚を変えることにより、反射率を変えることができる。しかし、これらの単層膜で使用する限り、反射率Ｒｆは常に５０％より小さくなる。一方モニタ側端面は前述のＡｌ_２ Ｏ_３ ／アモルファスＳｉ（以下、α−Ｓｉという）をそれぞれλ／４の厚さに設けた多層膜のほか、Ｓｉ_３ Ｎ_４ ／α−Ｓｉ、Ｓｉ_３ Ｎ_４ ／ＳｉＯ_２ 、Ａｌ_２ Ｏ_３ ／ＳｉＯ_２ などをそれぞれλ／４の厚さだけ設けた多層膜でもよい。
【００６１】
この端面の反射率は誘電体膜の厚さを変えることにより任意に調整でき、光放射端面の反射率は前述のように、使用される機種の読取用出力に応じて該出力で可干渉性が極小になるように調整されるが、出射側端面は３０％以下で、またモニタ側端面の反射率は５０％以上、さらに好ましくは７０〜９０％の反射率になるように調整される。モニタ側端面の反射率が小さいと光出射端面からの放射光が少なくなり、反射率が大きすぎるとモニタ出力が小さくなりすぎるからである。
【００６２】
実施例１
つぎに、具体的な実施例により本発明の半導体レーザについてさらに詳細に説明する。図１に示す構造の半導体レーザを製造する方法として、ＳＡＭ（Ｓｅｌｆ Ａｌｉｇｎｅｄ Ｓｔｒｕｃｔｕｒｅ ｂｙ ＭＢＥ）構造（田中治夫による「ＭＢＥによるセルフアライン構造型レーザ」、ジャパニーズ ジャーナル オブ アプライド フィジックス（Ｊａｐａｎｅｓｅ Ｊｏｕｒｎａｌ ｏｆ Ａｐｐｌｉｅｄ Ｐｈｙｓｉｃｓ）２４巻、８９頁、１９８５年）を用いた。すなわち、第一回目のＭＢＥ（分子線エピタキシー）法により、ｎ型ＧａＡｓ基板２上にｎ型Ａｌ_ｘ Ｇａ_１−ｘ Ａｓ（ｘ＝０．６）からなる下クラッド層３を２μｍ、ｎ型Ａｌ_ｙ Ｇａ_１−ｙ Ａｓ（ｙ＝０．１５）からなる活性層４を５０ｎｍ、ｐ型Ａｌ_ｘ Ｇａ_１−ｘ Ａｓ（ｘ＝０．６）からなる上部第１クラッド層５を３７０ｎｍ、ｎ型ＧａＡｓからなる電流制限層６を２８０ｎｍ、ｎ型Ａｌ_ｚ Ｇａ_１−ｚ Ａｓ（ｚ＝０．１５）からなる蒸発防止層７を７０ｎｍ、それぞれ順次結晶成長させ、最後に保護膜としてノンドープＧａＡｓ４０ｎｍを成長させた。
【００６３】
これを、ＭＢＥチャンバー内から取り出し、エッチング工程により、この第１回目の成長層の表面から、電流制限層６が１００ｎｍ程度残る程度の深さまで幅３．９μｍのストライプ溝を掘った。この１００ｎｍ程度残す理由は、上部第１クラッド層５を空気に触れさせないためである。
【００６４】
このようにしてストライプ溝を掘った基板をＭＢＥ装置に入れ、まずヒ素分子線を照射しながら基板を加熱させる。このとき、ストライプ溝の底に残っていたＧａＡｓと第１回成長で最後に成長させたＧａＡｓ保護膜が蒸発し、清浄なｐ型Ａｌ_ｘ Ｇａ_１−ｘ Ａｓ（ｘ＝０．６）からなる上部第１クラッド層５および、ｎ型Ａｌ_ｚ Ｇａ_１−ｚ Ａｓ（ｚ＝０．１５）からなる蒸発防止層７が露出する。そののち、第２回目のＭＢＥ成長としてｐ型Ａｌ_ｘ Ｇａ_１−ｘ Ａｓ（ｘ＝０．６）からなる上部第２クラッド層８を１． ６μｍ、ｐ型ＧａＡｓキャップ層９を２μｍ結晶成長させたのちに、上下の電極１、１０を形成し、基板を切断して各チップに分離した。このとき、共振器長を３５０μｍとした。
【００６５】
このようにしてえられたチップの電流−光出力特性を図１０（ａ）に、光の放射角による相対光強度を図１０（ｂ）に示す。なお、図１０（ｂ）において、それぞれＣは活性層に対し垂直方向の放射角、Ｄは平行の放射角に対する相対光強度である。また、γ_３ の光出力依存性を図１１に示す。
【００６６】
このチップに端面コートとして、出射側端面にはＡｌ_２ Ｏ_３ を０．３２λ／η_１ （η_１ はＡｌ_２ Ｏ_３ の屈折率）の厚さ、モニター側端面にはＡｌ_２ Ｏ_３ をλ／η_１ の厚さ、α−Ｓｉをλ／４η_２ （η_２ はα−Ｓｉの屈折率）の厚さだけ順次スパッタリングにより積層させた。そして光の出射側端面の反射率はＲｆ＝１１％、モニター側端面の反射率＝Ｒｒ＝７７％をえた。このときの電流と光出力の特性を図１２に、γ_３ の光出力依存性を図１３に示す。すなわち、端面コート前には２５ｍＷ近辺にあったキンクも３５ｍＷまで見られず、端面コート前には２ｍＷ近辺にあったγ_３ の極小位置が、端面コート後では５ｍＷ近辺に移動しているのがわかる。これらの結果から、端面コーティングにより、５ｍＷ近辺にノイズの極小値が現れ、３０ｍＷ以上の高出力でもキンクが生じなかった。またこの素子の戻り光５％でのＲＩＮ（相対強度雑音）の光出力依存性を図１４に示す。５ｍＷにおいては１２５ｄｂ／Ｈｚ以下にあり、充分実用レベルである。また非点隔差は約７μｍ（ナイフェッジ法にて測定）とこれも充分実用レベルにあった。
【００６７】
【発明の効果】
本発明によれば、
（１）高出力動作までの単一横モードが可能な屈折率導波構造でありながら、たとえば２〜７ｍＷの低出力動作において、戻り光誘起ノイズが抑制され、かつ、少なくとも２５ｍＷ以下でキンクが現れず、高出力がえられる半導体レーザをうることができる。
【００６８】
（２）前記した、ミニディスク、および光磁気ディスクのピックアップ用光源などに使われる半導体レーザに要求される他特性（低楕円率、低非点隔差）を従来どおりに満たすことができる。
【００６９】
（３）チップ構造は１種のままで、読取り出力の異なる機種に対応して、必要な読取り出力で戻り光誘起ノイズを極小にすることができる。
【図面の簡単な説明】
【図１】本発明による半導体レーザの一実施形態の断面説明図である。
【図２】端面に誘電体膜を設けないベアチップの出力３ｍＷのときの明瞭度（干渉性）とキンクの関係を示す図である。
【図３】本発明の干渉性を示す明瞭度の説明図である。
【図４】光出射面の反射率による光出力とノイズパラメータとの関係を示す図である。
【図５】光出射面の反射率による光出力とノイズパラメータとの関係を示す図である。
【図６】光出射面の反射率による光出力と明瞭度（干渉性）との関係を示す図である。
【図７】光出射面の反射率による光出力と明瞭度（干渉性）との関係を示す図である。
【図８】光出射面の誘電体膜のコーティング前の出力とコーティング後の出力との関係を示す図である。
【図９】活性層の膜厚をパラメータとして下クラッド層の膜厚と計算値による屈折率差との関係を示す図である。
【図１０】実施例１の半導体レーザの端面コーティングを行なわない状態の特性で（ａ）は駆動電流と光出力との関係を示す図、（ｂ）は光放射角度と相対強度の関係を示す図である。
【図１１】実施例１の半導体レーザの端面コーティングを行なわない状態の光出力と明瞭度との関係を示す図である。
【図１２】実施例１の半導体レーザの光出射面に端面コーティングを行って反射率を１１％としたときの駆動電流と光出力との関係を示す図である。
【図１３】実施例１の半導体レーザの光出射面に端面コーティングを行って反射率を１１％としたときの光出力と明瞭度（干渉性）の関係を示す図である。
【図１４】実施例１の半導体レーザの光出射面に端面コーティングを行って反射率を１１％としたときの光出力と相対強度雑音の関係を示す図である。
【符号の説明】
３ 下クラッド層
４ 活性層
５ 上部第１クラッド層
６ 電流制限層
８ 上部第２クラッド層[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a semiconductor laser having small return light-induced noise and a method for manufacturing the same. More specifically, the present invention relates to a semiconductor laser used as a light source for pickup of a recordable optical disk such as a mini disk and a magneto-optical disk, and particularly to a high-output semiconductor laser and a method of manufacturing the same.
[0002]
[Prior art]
Recently, as a light source for reading and writing on a mini disk or a magneto-optical disk, a semiconductor laser having an optical output of 3 to 5 mW and a low noise output and a high output of 20 mW or more (usually 30 to 40 mW) has been developed. It has been requested.
[0003]
The initial characteristics required for such a semiconductor laser include (1) single transverse mode oscillation up to high-power operation, (2) low astigmatic difference, (3) low ellipticity, and (4) low noise. can give.
[0004]
Among them, (1) Single transverse mode oscillation up to high output operation (that is, a phenomenon in which the optical output does not monotonously increase with the increase in current until the target high output operation, and the output decreases at a certain current (hereinafter, referred to as the following) In order to satisfy (2) low astigmatism, a refractive index waveguide structure is adopted as a chip structure of a semiconductor laser, and a low reflection coating is provided on an end face of a light emitting side. For example, a method of applying the same. This is because a single transverse mode and a low astigmatic difference are easily obtained in the refractive index waveguide structure. Further, if the emission side end face is made to have a low reflectance, the light extraction efficiency is increased, and in addition to the effect of further increasing the kink level, the reliability can be improved.
[0005]
Also, in order to obtain a low ellipticity, the current injection width of the chip is narrowed to increase the spread angle of light emission in the direction parallel to the active layer, or the active layer is made thinner to reduce the thickness in the direction perpendicular to the active layer. Means have been taken to reduce the radiation spread angle. All of these will make the difference in the refractive index in the lateral direction (the direction parallel to the active layer) larger, and will not obstruct the above required characteristics (1) and (2) (for example, see "Semiconductor by Ryoichi Ito et al." Laser, Baifukan, 1989, pp. 50-60 or 108-110).
[0006]
[Problems to be solved by the invention]
However, a refractive index waveguide structure usually facilitates single longitudinal mode oscillation even in a low output operation (for example, an optical output of about 3 mW) when reading information from a disk such as a magneto-optical disk. Will be higher. It is known that such a semiconductor laser having high coherence easily generates return light-induced noise, and cannot satisfy the low noise of (4). That is, the laser light reflected from the disk plate and returned again enters the semiconductor laser, so that the laser oscillation becomes unstable.
[0007]
On the other hand, conventionally, a method has been adopted in which a high-frequency current of several hundred MHz is superimposed on a laser drive current to multiplex longitudinal modes, thereby reducing coherence and reducing return light-induced noise. "Semiconductor Laser" by Ryoichi Ito et al., Baifukan, 1989, pp. 314-315). However, this method requires a high-frequency superimposing circuit in addition to a normal semiconductor laser driving circuit, and thus has problems such as an increase in package size, high power consumption, and high cost. Furthermore, since high-frequency electromagnetic noise is generated, there is a problem that it causes serious problems in handling electronic devices such as computers.
[0008]
On the other hand, as means for reducing coherence without using high-frequency superposition, there are methods of (1) using a gain waveguide structure and (2) causing self-excited oscillation. There is a problem that it is apt to be unstable and an astigmatic difference is increased. Further, in the case of (2), it is difficult to obtain a single transverse mode stably until a high output operation, and it is easy to cause a kink. It is substantially difficult to obtain an optical output of 20 mW or more.
[0009]
That is, there is a problem that a conventional semiconductor laser capable of obtaining a high output for writing has a high noise at a low output for reading and a laser having a low output and a small noise cannot obtain a high output for writing.
[0010]
An object of the present invention is to provide a semiconductor laser capable of sufficiently suppressing the generation of return light-induced noise in a low output operation (for example, around 2 to 7 mW) and obtaining a high output.
[0011]
Another object of the present invention is to provide a method of manufacturing a semiconductor laser in which the noise is minimized in the reading output of a model in which the semiconductor laser is used.
[0012]
[Means for Solving the Problems]
The present inventors have developed a light source having both writing and reading functions, such as a mini disk or a magneto-optical disk, that is, a high output of 20 mW or more for writing and a low noise of about 2 to 7 mW for reading. As a result of intensive studies to obtain a semiconductor laser having an output power of, a semiconductor laser having a refractive index waveguide structure having a high coherence and a high output can reduce the difference in the refractive index in the lateral direction (ie, the gain). (Close to the waveguide structure), there is a portion (minimum value) where the coherence decreases in the range of 1 to 3 mW while maintaining high output, and it is possible to reduce the return light-induced noise in the vicinity thereof, and furthermore, the coherence is reduced. It has been found that the optical output of a semiconductor laser at which the minimum value of (1) is generated can be freely adjusted in the range of 2 to 7 mW by changing the reflectivity of the end face which is the light emitting surface of the semiconductor laser. As a result, a semiconductor laser bare chip having a low output of 1 to 3 mW and a minimum value of coherence was produced, and the reflectivity of both end surfaces of the chip was adjusted by a dielectric film to thereby obtain the position of the minimum value of coherence. Can be adjusted to the vicinity of a desired read output, and a semiconductor laser which reduces coherence at the read output, that is, reduces return induced noise, and emits high output light for writing has been obtained. .
[0013]
The semiconductor laser according to the present invention includes an active layer having a light emitting region having a stripe structure, cladding layers provided on both sides of the active layer, and light generated at the both ends of the stripe of the light emitting region and generated in the light emitting region. And an end face that radiates a part of the active layer and reflects the remainder to amplify in the active layer.For light source of optical pickupA semiconductor laser,
In the state where the dielectric film is not provided on the end face, the coherence is formed to have a minimum value at a low output for reading while having a refractive index waveguide structure, and the coherence has a minimum value. The number of photons S 'in the resonator at the time and the desired low power P for reading₀And
(Equation 2)

Where P₀Is the desired light output, S 'is the number of photons in the resonator, L is the resonator length, Rf and Rr are the reflectivity of the light output side end face and the monitor side end face, h is Planck's constant, ν is the frequency, Vg is the group velocity
By providing a dielectric film on both end faces of the stripe so that the reflectances Rf and Rr satisfy the following expression, the coherence is minimized at a low output for reading determined according to the type of use. Is set.
[0014]
Another aspect of the semiconductor laser according to the present invention is an active layer having a light emitting region of a stripe structure, cladding layers provided on both sides of the active layer, and formed at both ends of a stripe of the light emitting region. An end face that emits a part of the light generated in the above and reflects the remainder to amplify in the active layer.For light source of optical pickupA semiconductor laser, wherein a light output radiated from one of the end faces, which changes due to a change in current applied to the active layer, has a phenomenon in which the light output decreases with an increase in current in a range of at least 25 mW or less. Not to occur, andLateral refractive index difference is 4 . 2 × 10 ^-3 ~ 5 × 10 ^-3 Along withIn order to minimize coherence at low output for reading determined according to the model used,ToA light reflecting film is provided with its reflectance adjusted.
[0015]
Here, the difference in the refractive index in the horizontal direction refers to the difference in the refractive index between a portion (light emitting region) corresponding to the stripe groove of the current limiting layer provided above or below the active layer in the active layer and a portion outside the portion. In other words, the refractive index varies depending on the difference in light absorption loss and current value.
[0016]
The method of manufacturing a semiconductor laser according to the present invention comprises: (a) a cladding layer on at least both sides of an active layer, a light emitting region having a stripe structure, and a refractive index waveguide structure having no dielectric film on a light emitting end face.IsA semiconductor bare chip having a low output and low coherence having a minimum value for reading is manufactured, and (b) the minimum value of the coherence is reduced to a desired value for reading while maintaining the number of photons at the minimum value. A dielectric film is provided on each of the light emitting end face of the semiconductor bare chip and the monitor end face on the opposite side so as to move to the output.
[0017]
According to the present invention, since the coherence is formed to be small at the low output, the interference is small at the low output for reading, the generation of the induced noise due to the return light is sufficiently suppressed, and the low noise and accurate The recorded information can be read.
[0018]
On the other hand, when the current is increased and the light output is increased, at least 25 mW or less of the output is formed so as not to cause a kink, so that a high output of 20 mW or more required for writing can be obtained with certainty. Can be written accurately.
[0019]
That is, according to the present invention, in the refractive index waveguide structure in which kink is less likely to occur and a high output is easily obtained, the difference in the refractive index in the lateral direction is reduced to approach the gain waveguide structure. Kink is less likely to occur while maintaining the properties of the waveguide structure, and a high output is obtained. On the other hand, at low output, the properties of the gain waveguide structure also appear, the coherence is reduced, and the generation of return light-induced noise can be sufficiently suppressed.
[0020]
Further, according to the manufacturing method of the present invention, a semiconductor bare chip having a minimum value of coherence in a range of light output of light emission based on refractive index waveguide in a state where a dielectric film is not provided on a light emitting end face, A dielectric film is formed on the light emitting end face of the semiconductor bare chip and the monitor end face on the opposite side thereof so that the coherent minimal value moves to a desired output while maintaining the number of photons at the minimal value. Even if the optical output for reading differs depending on the model used, the coherence can be easily minimized in the optical output according to the model, and the required output during reading can be reduced. In particular, noise can be reduced.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the semiconductor laser of the present invention and a method for manufacturing the same will be described. FIG. 1 is an explanatory sectional view of a chip structure showing one embodiment of a semiconductor laser according to the present invention.
[0022]
In FIG. 1, 1 is an n-side electrode, 2 is an n-type GaAs substrate, 3 is n-type Al_xGa_1-xLower cladding layer (for example, 2 μm) made of As (0.45 ≦ x ≦ 0.75, for example, x = 0.6), 4 is n-type, p-type or non-doped Al_yGa_1-yAn active layer (for example, 50 nm) made of As (0 ≦ y ≦ 0.22, for example, y = 0.15), 5 is p-type Al_xGa_1-xAn upper first cladding layer (for example, 370 nm) made of As, 6 is a current limiting layer (for example, 280 nm) made of n-type GaAs, and 7 is an n-type Al_zGa_1-zAn evaporation preventing layer (for example, 70 nm) made of As (0.05 ≦ z ≦ 0.22, for example, z = 0.15);_xGa_1-xAn upper second cladding layer (eg, 1.6 μm) made of As, 9 is a cap layer (eg, 2 μm) made of p-type GaAs, and 10 is a p-side electrode. The stripe width S is, for example, 3.9 μm.
[0023]
Although Si was used as the n-type dopant and Be was used as the p-type dopant, Sn can be used as the n-type dopant, and Zn can also be used as the p-type. In the above structure, an n-type substrate is used, but a p-type substrate may be used. In this case, all of the n-type and p-type described above may be reversed.
[0024]
This structure is a complex refractive index waveguide structure in which light is confined within a stripe by light absorption loss due to the GaAs current limiting layer. To obtain a stable single transverse mode with such a structure, p-type Al is used._xGa_1-xThe thickness of the upper first cladding layer 5 made of As is normally controlled to be around 300 nm.
[0025]
As described above, the present inventors have developed a semiconductor laser that has low noise at low output for reading a magneto-optical disk so that it can be read accurately without error, and at the time of writing, has a high output necessary for writing. As a result of diligent studies to find out, we found the following. That is, for example, by increasing the film thickness of the above-mentioned upper first cladding layer 5 from the usual 300 nm to about 370 nm, the difference in the refractive index in the horizontal direction is reduced, and no dielectric film is provided on the emission end face (non-coating). In the state of the bare chip, a minimum value of coherence is obtained at a low output of, for example, 1 to 3 mW, and the output of light at which a minimum value of coherence appears can be adjusted by adjusting the reflectance of the light emitting end face; By producing a semiconductor laser chip such that the output value at which the coherence becomes minimal in the bare chip state becomes 1 to 3 mW, at least an output of 25 mW or less does not cause kink and a high output is obtained. I found it.
[0026]
That is, the semiconductor laser of the present invention includes an active layer having a light emitting region having a stripe structure, cladding layers provided on both sides of the active layer, and formed at both ends of a stripe of the light emitting region, and generated in the light emitting region. A semiconductor laser having an end face that emits a portion of the light and reflects the remainder to amplify the light in the active layer, wherein the coherence is substantially minimized in a state where the dielectric film is not provided on the end face. While maintaining the number of photons in the resonator at the time of taking the value, the number of photons in the resonator S ′ and the light output P described later are maintained.₀By providing the dielectric films on the both end surfaces so as to satisfy the relational expression, coherence is set to a minimum value at a desired output.
[0027]
First, the relationship between the coherence and the output at which kink occurs was examined by changing the thickness of the upper first cladding layer 5 described above. As described above, the adoption of the gain waveguide structure has a fatal defect as a high-power semiconductor laser, in particular, a reduction in kink.However, in the refractive index waveguide structure, the difference in the refractive index in the lateral direction is reduced. When the thickness of the upper first cladding layer 5 is increased (e.g., by increasing the thickness of the upper first cladding layer 5, etc.), gain guiding characteristics can be obtained while having a refractive index guiding structure. When approaching the gain waveguide structure, the contribution of the spontaneous emission light increases and the coherence decreases, but the kink also decreases accordingly. FIG. 2 shows the clarity (indicator of coherence) as an index of coherence when the thickness of the upper first cladding layer 5 is changed in a semiconductor laser bare chip (hereinafter simply referred to as a chip) in which the end surface of the light emitting surface is not coated. Visibility) and kink. Here, as shown in FIG. 3 as an index of clarity, a third interference peak (hereinafter referred to as γ) from an interference spectrum with an optical path difference of 0 is shown.₃Is expressed as a value when the interference peak when the optical path difference is 0 is 1 and the correlation is seen by the clarity when the optical output is 3 mW. From FIG. 2, γ₃It can be seen that the kink sharply decreases as the value decreases. Especially γ₃At <0.9, a sharp decrease and variation in kink are observed. Therefore, to obtain an optical output of 20 mW or more, a γ of 0.8 or more is required.₃is necessary. However, conventionally, γ₃Is 0.8 or more, the coherence is high, and it is difficult to sufficiently suppress the return light-induced noise.
[0028]
The present inventors further examined the noise characteristics and the spectral characteristics of chips having kink appearing in FIG. 2 having an output of 25 mW or more (samples a to e in ascending order of output as shown in FIG. 2). The following results were obtained. In addition, a and b indicate that the thickness of the upper first cladding layer 5 is 380 nm, c is 350 nm, d and e are 300 nm, the group A is 380 nm, the group B is 400 nm, and the thicknesses of the other layers are as described above. Was used. The group A is the same as a and b, but is considered to be a variation in manufacturing conditions such as a variation in leakage current.
[0029]
FIGS. 4 and 5 show the reflectances Rf of the emission side end faces Rf = 10% and 17% when the end faces are not coated with respect to each of the samples a to e (samples indicated by ae in FIG. 2). This shows the optical output dependence of the return light induced noise when the end face coating is applied as described above. The reflectance Rr of the monitor end face, which is the other radiation end face, was kept constant at 75%. At this time, as an index of the return light-induced noise, a noise parameter LN represented by Expression (1), that is, a peak-peak value of a maximum AC component (noise component) of 200 kHz or less at each optical output is standardized. The logarithm of the ratio of the 3 mW light output to the DC value was used. The reason for the investigation at 200 kHz or lower is that low frequency noise in this band is often regarded as a problem in optical disks.
[0030]
LN = log ((AC component) / (3 mWDC component) × 100) (1)
4A to 5E correspond to a to e of the sample. 4 to 5, out of a to e, a, b, and c indicate that the return light induced noise after the end face coating is minimum at 5 to 6 mW when Rf = 10%, and minimum at 3 to 4 mW when Rf = 17%. When the reflectance Rf of the end face on the emission side is increased, the light output having the minimum value of the noise moves to the low output side.
[0031]
That is, in a non-coating state in which no dielectric film is provided on the radiation end face, the reflectivity is about 30%, and the minimum value of noise appears at 1 to 3 mW. Moves to the larger one. Accordingly, the output at which the kink appears also shifts to a higher output, and a high output is obtained without any abnormality in a high output band.
[0032]
On the other hand, samples d and e do not show such a tendency. 17% is slightly weaker than Rf = 10% and the coupling between the return light and the laser resonator is slightly weakened, so that the noise tends to be good. However, the noise level is clearly higher than the minimum values of the samples a to c. high. As is clear from FIG. 5, even in the non-coating state where the dielectric film is not provided on the end face, the minimum value of noise does not appear, indicating that the structure is the same as the conventional refractive index waveguide structure. .
[0033]
FIGS. 6 and 7 show the coherent light output dependence of the same sample as FIGS. 4 (a) to (e).
[0034]
The difference between the samples a, b, and c and the samples d and e is also seen in the dependence of the coherence on the light output in FIGS. The most significant difference is that in samples d and e, γ₃Is monotonically increasing, but for samples a, b, and c, γ is about 2 mW between 1 mW and 3 mW.₃Is characterized by having an extreme value.
[0035]
From the above results, the samples a, b, and c were close to the threshold value at the stage where the end face coating was not applied (that is, γ at the light output of 1 mW or less)₃Excluding the part where the light output is low), the coherence is the lowest at an optical output of about 2 mW. That is, return light-induced noise is most suppressed. Moreover, by changing the end face reflectance, the light output at which the coherence takes a minimum value changes. Therefore, it can be said that the portion having the lowest noise can be moved to an arbitrary light output by controlling the end face reflectance.
[0036]
These results are theoretically valid, if considered as follows. Since the laser oscillation spectrum is obtained by Fourier transforming the clarity (visibility), the larger the spectral line width, the lower the coherence. The spectral linewidth Δν is given by See page 61).
[0037]
Δν = R (1 + α²) / (4πS ')
Here, R is a constant representing the contribution of spontaneous emission light, S 'is the number of photons in the laser resonator, and α is a coefficient called a line width increase coefficient. That is, the line width is a function of the number of photons in the resonator.
[0038]
On the other hand, the number of photons in the resonator S ′ and the light output P₀Relationship with
[0039]
(Equation 3)

[0040]
It becomes. Here, L is the resonator length, Rf and Rr are the reflectivities of the light output side end face and the monitor side end face, h is Planck's constant, ν is the frequency, and Vg is the group velocity. FIG. 8 shows the relationship between the light output at the stage where the end face coating is not applied and the light output after the end face coating that gives the same number of photons in the resonator. In FIG. 8, the solid line shows the case where Rf = 10% and Rr = 75%, and the broken line shows the case where Rf = 17% and Rr = 75%. FIG. 8 plots the data of Rf = 10% for sample a, Rf = 10% and 17% for sample b, and Rf = 17% for sample c, and it can be seen that the values almost match the calculated values.
[0041]
In summary, the manufacturing method for obtaining a semiconductor laser capable of oscillating in a single transverse mode up to high output while exhibiting low noise characteristics at low output is as follows.
[0042]
First, in a semiconductor laser chip having a refractive index waveguide structure, the difference in the refractive index in the lateral direction is reduced as much as possible within a range in which a kink level capable of performing a high-output operation according to an application is maintained. As can be seen in FIG. 2, reducing the lateral index difference (i.e., decreasing coherence) is accompanied by a reduction in kink. Therefore, since the difference in the refractive index cannot be reduced so much, the contribution of spontaneous emission is small. For example, if the optical output is increased to about 3 mW, the coherence becomes considerably high with the increase in the number of photons in the resonator. For example, in the vicinity of about 2 mW of 1 to 3 mW, the coherence once becomes small or the coherence takes an extreme value.
[0043]
The end face coat is applied to such a chip, and the end face reflectivity is adjusted so as to obtain a desired high-power semiconductor laser. The method of implementing the method is as follows.
[0044]
For example, in order to reduce the noise near the optical output of 3 to 4 mW, it is sufficient to set Rf = 17% and Rr = 75% as shown in FIGS. In this case, Rf = 10% and Rr = 75%. Here, Rf was changed while keeping Rr = 75%. However, the value is not limited to Rr = 75%. What is necessary is just to design so that it may become equivalent to 2 mW output. However, it is desirable to design in the range of Rf ≦ 30% and Rf ≧ 50% in order to ensure reliability at the time of high output.
[0045]
In the above-mentioned embodiment, as means for preventing the occurrence of kink when the optical output has a minimum value of coherence in the range of 2 to 7 mW and the optical output is at least 25 mW or less, for example, By increasing the thickness of the first cladding layer and moving the current limiting layer away from the current limiting layer, the difference in the refractive index in the lateral direction was reduced, and the structure became closer to the gain waveguide structure. However, in order to bring the refractive index waveguide structure closer to the property of the gain waveguide structure, that is, the factors that determine the difference in the refractive index in the lateral direction are, in addition to this, the thickness of the active layer and the refractive index of the cladding layer and the active layer. There are a difference, a stripe width, a film thickness of a current limiting layer, a leakage current amount (which greatly depends on crystal growth conditions), and the like.
[0046]
For example, as the thickness of the active layer increases, the difference in the refractive index in the lateral direction can be reduced. As the refractive index of the cladding layer is smaller than the refractive index of the active layer, the difference in the refractive index in the lateral direction is reduced. Can be. When the current limiting layer is thinned, the refractive index difference starts to change periodically. As the amount of leakage current decreases, the difference in the refractive index in the horizontal direction can be reduced due to the plasma effect. Since the amount of leakage current depends largely on the crystal growth conditions, there are some aspects that cannot be controlled due to variations in the manufacturing process to some extent, but the material and thickness of each semiconductor layer can be freely controlled by selection during manufacturing. it can. When the stripe width is 3 μm or less, the difference in refractive index is similarly reduced due to the plasma effect. However, by combining these elements, they appear as a set of contributions of the respective elements, so that the light output has a minimum coherence in the range of, for example, 2 to 7 mW, and As a means for preventing kink in a range of at least 25 mW or less, an infinite number of combinations may be generated.
[0047]
For example, in the above example, the active layer is Al_yGa_1-yAs of y = 0.15 is 50 nm, the upper first cladding layer is Al_xGa_1-xThe above requirement is satisfied when the thickness of the upper first cladding layer is 350 to 380 nm when x of As is 0.6. The difference in the refractive index in the horizontal direction at this time is obtained by calculation ignoring the change in the refractive index due to the current.
Δη = (β₀−Realβ ′) / k₀… (2)
Where β₀Is the propagation coefficient in the resonator direction in the stripe, β 'is the stripe
External propagation coefficient in the resonator direction, k₀Is the wave number in vacuum
4.2 × 10^-3~ 5 × 10^-3Range.
[0048]
In general, an electric field equation representing a TE mode (light propagated by a semiconductor laser is a TE mode) propagating through a dielectric waveguide in a resonator direction (z-axis direction in FIG. 1) is
[0049]
(Equation 4)

[0050]
Here, Ey is the y-direction component of the electric field, and n (x) is the refractive index.
[0051]
(Equation 5)

[0052]
And β is a propagation coefficient in the z direction given an eigenvalue satisfying this equation. One point y inside the stripe₁Assuming that the dielectric at a point cut perpendicular to the y-axis (see FIG. 1) is uniformly infinite in the z-axis direction and the y-axis direction, the propagation coefficient β obtained from the equation (3) is β₀, One point outside the stripe y₂The propagation coefficient β obtained by assuming that the dielectric at the point cut perpendicular to the y-axis is uniformly infinite in the z-axis direction and the y-axis direction is β ′ (in this case, the absorption coefficient of the current limiting layer is (Β ′ becomes a complex number due to the influence), and the effective refractive index difference Δη inside and outside the stripe is given by equation (2).
[0053]
β₀-Real β 'increases as the thickness of the active layer decreases, and decreases as the Al composition difference between the active layer and the cladding layer increases, and increases as the distance between the active layer and the current limiting layer decreases.
[0054]
Therefore, the refractive index difference calculated from the refractive index and thickness determined from the semiconductor material of the active layer and the upper first cladding layer is 4.2 × 10^-3~ 5 × 10^-3The above means is achieved by manufacturing so as to fall within the range of
[0055]
As shown in Table 1, a part of the relationship between the composition and the thickness of the semiconductor layer from which the refractive index difference can be obtained by such a calculation, the composition (value of x) of the upper first cladding layer and the thickness of the active layer The required thickness of the upper first cladding layer is set accordingly. Table 1 illustrates only a part of the combination. For example, the upper first cladding layer Al_xGa_1-xThe composition of Al in As is 0.45 ≦ x ≦ 0.75, and the active layer Al_yGa_1-yAs composition of Al of 0 ≦ y ≦ 0.22, evaporation prevention layer Al_zGa_1-zVarious combinations are possible when the Al composition of As is in the range of 0.05 ≦ z ≦ 0.22.
[0056]
[Table 1]

[0057]
FIG. 9 shows the relationship between the thickness of the upper first cladding layer and the refractive index difference with the parameter of the thickness of the active layer relative to the composition of the upper first cladding layer. FIG. 9A shows the case where the Al composition of the upper first cladding layer is x = 0.6, and FIG. 9B shows the case where the Al composition of the upper first cladding layer is x = 0.5. It is.
[0058]
In addition, this relationship can be obtained by adjusting the refractive index and the film thickness of each layer even when using AlGaInP-based and InGaAsP-based semiconductors as well as AlGaAs-based semiconductors. A semiconductor laser with a low output of 7 mW and low noise and no kink at least up to an output of 25 mW or less can be obtained.
[0059]
Further, in the above embodiment, the SAM structure which is a self-aligned structure using the MBE method as described later in the crystal growth method has been described. However, in addition to the SAM structure, the CSP structure by the LPE method, the VSIS structure, or the MOCVD method. In addition to a complex refractive index waveguide structure such as a SAS structure, a semiconductor laser having a real refractive index waveguide structure disclosed in Japanese Patent Application Laid-Open No. 5-160503 similarly has low output and low noise, and does not generate kink. A semiconductor laser with high output can be obtained.
[0060]
As the dielectric film provided on the above-mentioned light emitting end face, Al₂O₃Single layer film or Si₃N₄For example, a single-layer film can be used, and the reflectance can be changed by changing the film thickness. However, as long as these single-layer films are used, the reflectance Rf is always smaller than 50%. On the other hand, the monitor side₂O₃/ Amorphous Si (hereinafter referred to as α-Si), each having a thickness of λ / 4,₃N₄/ Α-Si, Si₃N₄/ SiO₂, Al₂O₃/ SiO₂And the like may be a multilayer film provided with a thickness of λ / 4.
[0061]
The reflectivity of this end face can be arbitrarily adjusted by changing the thickness of the dielectric film, and the reflectivity of the light emitting end face is, as described above, coherent at the output according to the reading output of the model used. Is adjusted so that the reflectance on the emission side end face is 30% or less, and the reflectance on the monitor side end face is 50% or more, more preferably 70 to 90%. This is because if the reflectance of the monitor side end face is small, the amount of light emitted from the light emitting end face decreases, and if the reflectivity is too large, the monitor output becomes too small.
[0062]
Example 1
Next, the semiconductor laser of the present invention will be described in more detail with reference to specific examples. As a method of manufacturing the semiconductor laser having the structure shown in FIG. 1, a self-aligned structure by MBE (SAM) structure (“a self-aligned structure type laser using MBE” by Haruo Tanaka), Japanese Journal of Applied Physics 24 Vol. 89, 1985). That is, the first MBE (Molecular Beam Epitaxy) method is used to form n-type Al on the n-type GaAs substrate 2._xGa_1-x2 μm of lower cladding layer 3 made of As (x = 0.6), n-type Al_yGa_1-yAn active layer 4 made of As (y = 0.15) is made 50 nm thick and p-type Al_xGa_1-xThe upper first cladding layer 5 made of As (x = 0.6) is 370 nm, the current limiting layer 6 made of n-type GaAs is 280 nm, and the n-type Al_zGa_1-zAn evaporation prevention layer 7 made of As (z = 0.15) was crystal-grown in order of 70 nm each, and finally a non-doped GaAs 40 nm was grown as a protective film.
[0063]
This was taken out of the MBE chamber, and a 3.9 μm-wide stripe groove was dug from the surface of the first growth layer to a depth such that the current limiting layer 6 remained about 100 nm by an etching step. The reason for leaving about 100 nm is to prevent the upper first cladding layer 5 from being exposed to air.
[0064]
The substrate in which the stripe groove is dug in this way is put into an MBE apparatus, and the substrate is first heated while irradiating an arsenic molecular beam. At this time, the GaAs remaining at the bottom of the stripe groove and the GaAs protective film finally grown in the first growth evaporate, resulting in clean p-type Al._xGa_1-xUpper first cladding layer 5 made of As (x = 0.6) and n-type Al_zGa_1-zThe evaporation preventing layer 7 made of As (z = 0.15) is exposed. After that, the p-type Al_xGa_1-xThe upper second cladding layer 8 made of As (x = 0.6) is formed as follows. After growing a 6 μm p-type GaAs cap layer 9 with a 2 μm crystal, upper and lower electrodes 1 and 10 were formed, and the substrate was cut into chips. At this time, the resonator length was set to 350 μm.
[0065]
FIG. 10A shows the current-light output characteristics of the chip thus obtained, and FIG. 10B shows the relative light intensity depending on the light emission angle. In FIG. 10B, C is a radiation angle in a direction perpendicular to the active layer, and D is a relative light intensity with respect to a parallel radiation angle. Also, γ₃Is shown in FIG.
[0066]
As an end face coat on this chip, the output side end face is made of Al₂O₃To 0.32λ / η₁(Η₁Is Al₂O₃Refractive index), and the monitor side end face has Al₂O₃To λ / η₁And α-Si is λ / 4η₂(Η₂(Refractive index of α-Si). The reflectivity of the end face on the light emission side was Rf = 11%, and the reflectivity of the end face on the monitor side was Rr = 77%. FIG. 12 shows the current and optical output characteristics at this time.₃Is shown in FIG. That is, no kink which was near 25 mW before the end face coat was observed up to 35 mW, and γ which was near 2 mW before the end face coat.₃It can be seen that the minimum position of (1) has moved to around 5 mW after the end face coating. From these results, the minimum value of the noise appeared near 5 mW due to the end face coating, and no kink occurred even at a high output of 30 mW or more. FIG. 14 shows the light output dependency of RIN (relative intensity noise) at 5% return light of this element. At 5 mW, it is 125 db / Hz or less, which is a sufficiently practical level. The astigmatic difference was about 7 μm (measured by the Nifej method), which was also at a sufficiently practical level.
[0067]
【The invention's effect】
According to the present invention,
(1) Despite having a refractive index waveguide structure capable of a single transverse mode up to a high output operation, in a low output operation of, for example, 2 to 7 mW, return light induced noise is suppressed, and a kink is formed at least at 25 mW or less. A semiconductor laser which does not appear and can obtain high output can be obtained.
[0068]
(2) Other characteristics (low ellipticity and low astigmatic difference) required for the semiconductor laser used for the pickup light source of the mini disk and the magneto-optical disk can be satisfied as before.
[0069]
(3) The return light-induced noise can be minimized at a required read output in accordance with a model having a different read output while keeping the same chip structure.
[Brief description of the drawings]
FIG. 1 is an explanatory sectional view of an embodiment of a semiconductor laser according to the present invention.
FIG. 2 is a diagram showing a relationship between clarity (interference) and kink at a power of 3 mW of a bare chip having no dielectric film on an end face.
FIG. 3 is an explanatory diagram of clarity indicating the coherence of the present invention.
FIG. 4 is a diagram illustrating a relationship between a light output and a noise parameter based on the reflectance of a light exit surface.
FIG. 5 is a diagram showing a relationship between a light output based on a reflectance of a light exit surface and a noise parameter.
FIG. 6 is a diagram showing a relationship between light output and clarity (coherence) based on the reflectance of the light exit surface.
FIG. 7 is a diagram showing the relationship between light output and clarity (coherence) based on the reflectance of the light exit surface.
FIG. 8 is a diagram showing a relationship between an output before coating a dielectric film on a light emitting surface and an output after coating.
FIG. 9 is a diagram showing the relationship between the thickness of the lower cladding layer and the refractive index difference based on a calculated value, using the thickness of the active layer as a parameter.
10A and 10B show characteristics of the semiconductor laser of Example 1 in a state where the end face coating is not performed, wherein FIG. 10A shows a relationship between a driving current and a light output, and FIG. FIG.
FIG. 11 is a diagram showing a relationship between optical output and clarity of the semiconductor laser of Example 1 in a state where end face coating is not performed.
FIG. 12 is a diagram illustrating a relationship between a drive current and a light output when the light emitting surface of the semiconductor laser according to the first embodiment is coated with an end face to have a reflectance of 11%.
FIG. 13 is a diagram showing the relationship between light output and clarity (interference) when the light emitting surface of the semiconductor laser of Example 1 is coated with an end face to have a reflectance of 11%.
FIG. 14 is a diagram showing the relationship between the light output and the relative intensity noise when the light emitting surface of the semiconductor laser of Example 1 is coated with an end face to have a reflectance of 11%.
[Explanation of symbols]
3 Lower cladding layer
4 Active layer
5 Upper first cladding layer
6 Current limiting layer
8 Upper second cladding layer

Claims (3)

An active layer having a light emitting region of a stripe structure, cladding layers provided on both sides of the active layer, and formed at both ends of a stripe of the light emitting region, and emits a part of light generated in the light emitting region. A semiconductor laser for a light source of an optical pickup having an end face that reflects the remaining portion and amplifies in the active layer,
In the state where the dielectric film is not provided on the end face, the coherence is formed to have a minimum value at a low output for reading while having a refractive index waveguide structure, and the coherence has a minimum value. The number of photons S ′ in the resonator at the time and the desired low output P ₀ for reading are

Here, P ₀ is the desired light output, S ′ is the number of photons in the resonator, L is the resonator length, Rf and Rr are the reflectivity of the light output side end face and the monitor side end face, h is Planck's constant, ν Is a frequency, and Vg is a dielectric film provided on both end faces of the stripe so that the reflectances Rf and Rr satisfy the formula of group velocity. Semiconductor laser whose coherence is set to a minimum value. An active layer having a light emitting region of a stripe structure, cladding layers provided on both sides of the active layer, and formed at both ends of a stripe of the light emitting region, and emits a part of light generated in the light emitting region. A semiconductor laser for a light source of an optical pickup having an end surface for reflecting the remaining portion and amplifying the light in the active layer, wherein a light output radiated from one of the end surfaces changed by a change in current applied to the active layer. but, as the light output with respect to an increase in current in the range of at least 25mW does not occur a phenomenon that decreases and the refractive index difference in the lateral direction to ^{4. 2 × 10 -3 ~5 ×} 10 -3 A semiconductor laser formed and provided with a light reflection film on its end face with its reflectance adjusted so that coherence is minimized at a low output for reading determined according to the type of use. (A) It has a cladding layer on at least both sides of the active layer, has a light emitting region of a stripe structure, and has a refractive index waveguide structure without a dielectric film on a light emitting end face, but has a low output for reading. Produce a semiconductor bare chip with minimal coherence,
(B) a light emitting end face of the semiconductor bare chip and a monitor end face on the opposite side thereof so that the coherent minimal value moves to a desired output for reading while maintaining the number of photons at the minimal value. Wherein a dielectric film is provided on each of the semiconductor lasers.

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