JP3610530B2 - Wavelength multiplexed signal light generator - Google Patents

Description

と表せる。ここで、波長λ₀±Δλ／2の４光波混合光が発生するためには、励起用光パルスと４光波混合光との時間的な重なりが必要で、それは両者の群遅延差τが励起用光パルスのパルス幅Δt₀(ps：半値全幅）以下であるという条件
Δt₀≧τ＝(lnＧ₀／16)｜ｄＤ(λ₀)／ｄλ｜・Δλ²／(｜γ｜Ｐ） …(5)
で表される。したがって、分散スロープの条件として
｜ｄＤ(λ₀)／ｄλ｜≦16（｜γ｜ＰΔt₀)／(Δλ²lnＧ₀) …(6)
が得られる。ここで、ＰΔt0は励起用光パルスのエネルギーＥにほぼ等しいので、光導波路の分散スロープに関する条件式
｜ｄＤ(λ₀)／ｄλ｜≦16（｜γ｜Ｅ)／(Δλ² ln Ｇ ₀ ) …(7)
が得られる。
【００２０】
次に、光導波路が波長λ₀ の励起用光パルスから波長λ₀±Δλ／2の４光波混合光を発生させるためには、少なくとも式(1) で与えられる利得帯域（∝Ω_S）がΔλ／４以上でなければならない。したがって、
Ω_S ＝（２｜γ｜Ｐ／｜β₂｜)^1/2≧２πｃΔλ／(４λ₀ ²） …(8)
でなければならない。ここで、β₂ とＤ(λ)の関係式
｜β₂｜＝λ²｜Ｄ(λ)｜／（２πｃ） …(9)
を用いて、光導波路の分散に関する条件式
｜Ｄ(λ₀)｜≦(16／πｃ)・λ₀ ²｜γ｜Ｐ／(Δλ)²…(10)
が求まる。式(10)は、少なくとも式(3) で規定される長さln Ｇ ₀ ／(２｜γ｜Ｐ）にわたって満たされなければならない。
【００２１】
なお、光導波路中の励起用光パルスのピークパワーＰと、光導波路に入力される励起用光パルスのピークパワーＰ_０ とは、励起用光パルスの波形変化等のために一般に異なる。これらは補正係数αを用いて、Ｐ＝αＰ_０ と表すことができる。
アレイ導波路型光合分波器１２は、図１に示すように、入力用導波路アレイ３１、入力側コンケイブスラブ導波路３２、アレイ導波路（隣接する導波路がΔＬの光路長差を有する）３３、出力側コンケイブスラブ導波路３４、出力用導波路アレイ３５により構成される。入力ポートｉ１から入力用導波路アレイ３１に入射された光は、入力側コンケイブスラブ導波路３２において回折により広がり、その回折面と垂直に配置されたアレイ導波路３３に導かれる。アレイ導波路３３は、各導波路が導波路長差ΔＬで順次長くなっているので、各導波路を伝搬して出力側コンケイブスラブ導波路３４に到達した光には導波路長差ΔＬに対応する位相差が生じている。この位相差は波長（光周波数）により異なるので、出力側コンケイブスラブ導波路３４のレンズ効果で出力用導波路アレイ３５の入力端に集光する際に、波長（光周波数）ごとに異なる位置に集光する。したがって、各出力ポートには異なる光周波数の光が取り出され、光分波器として機能することになる。また、同様に光合波器として機能する。
【００２２】
このアレイ導波路型光合分波器１２の入出力関係は、図３に示すように入力ポートの位置を１つずらすと、透過中心波長と出力ポートの対応関係が１チャネルずつ巡回的にずれる。たとえば、入出力ポート数を１６とし、入力ポートｉ１にコヒーレント白色光が入力されたときに、出力ポートｏ１〜ｏ１６に波長λ_１ 〜λ_１６が分波されるとすると、入力ポートｉ４に入力されたコヒーレント白色光は、出力ポートｏ１，ｏ２，…，ｏ１６に波長λ_４，λ_５，…，λ_１６，λ_１，λ_２，λ_３ が分波される。このとき、各波長のバンド幅およびチャネル間隔は、アレイ導波路型光合分波器１２の導波路パラメータにより決定され、各出力ポートに分波される光パルスのパルス幅はアレイ導波路型光合分波器１２のバンド幅より決定されるトランスフォームリミットパルスとなる。
【００２３】
図１に示す構成では、入力ポートｉ１にコヒーレント白色光が入力されると、出力ポートｏ１，ｏ２，…，ｏＮに波長λ_１，λ_２，…， λ_Ｎ のコヒーレントな光パルスがそれぞれ出力される。このうち、出力ポートｏ２，ｏ３，…，ｏＮに出力される波長λ_２，λ_３，…， λ_Ｎ の光パルスは、対応する光変調器１３−２，１３−３，…，１３−Ｎで変調信号光となり、それぞれ入力ポートｉ２，ｉ３，…，ｉＮに戻される。これらの変調信号光はアレイ導波路型光合分波器１２で合波され、出力ポートｏ１から波長多重信号光として一括して出力される。なお、出力ポートｏ１に出力される波長λ_１ の光パルスはアレイ導波路型光合分波器１２を１回通過しただけであるので、無変調であるとともに挿入損失が他の波長の信号光と比べて小さい。すなわち、出力ポートｏ１に取り出される波長多重信号光には、光パワーが大きい波長λ_１ の光パルス（無変調光）が含まれることになる。
【００２４】
このように、本実施例ではアレイ導波路型光合分波器１２の所定の入出力ポートを光変調器１３を介してループ状に接続することにより、光分波器と光合波器の機能を１つのアレイ導波路型光合分波器で実現させることができる。これにより、光分波器と光合波器を別々に構成した場合（特願平５−２２１２５９号の図７の構成）の特性の違いによる影響を回避することができる。
【００２５】
１つの出力ポートから波長多重信号光が出力されるためのアレイ導波路型光合分波器１２の入出力ポートの接続条件は、
ｔ−１＝ ｍｏｄ〔（ｓ−１）＋（ｑ−ｒ）＋Ｎ，Ｎ〕 …（１１）
となる（石田，高橋，“ Ｌｏｓｓ−ｉｍｂａｌａｎｃｅ ｅｑｕａｌｉｓａｔｉｏｎ ｏｆ ａｒｒａｙｅｄ−ｗａｖｅｇｕｉｄｅ ｇｒａｔｉｎｇ ａｄｄ−ｄｒｏｐ ｍｕｌｔｉｐｌｅｘｅｒ”， Ｅｌｅｃｔｒｏｎ． Ｌｅｔｔ．，３０， ｐｐ．１１６０−１１６２， １９９４）。ここで、ｑ，ｒは、コヒーレント白色光の入力ポート番号（ｉｑ）と波長多重信号光の出力ポート番号（ｏｒ）に対応する。ｔ，ｓは、接続される入力ポート番号（ｉｔ）と出力ポート番号（ｏｓ）に対応する。 ｍｏｄ〔Ａ，Ｂ〕は、ＡをＢで割った余りである。
【００２６】
（第２実施例−請求項１に対応）
図４は、本発明の波長多重信号光発生装置の第２実施例の構成を示す。
本実施例は、（１１）式における入出力ポート番号の差ｔ−ｓの絶対値が、Ｎ／２となるようにオフセットを与えて接続したものである（ ｍｏｄ〔（ｑ−ｒ）＋Ｎ，Ｎ〕＝Ｎ／２）。ここでは、入出力ポート数Ｎを１６とし、コヒーレント白色光源１１から出力されたコヒーレント白色光がアレイ導波路型光合分波器１２の入力ポートｉ４に入力され、出力ポートｏ１２から波長多重信号光が出力される様子を示す。出力ポートｏ１，ｏ２，…，ｏ１６には、波長λ_４，λ_５，…， λ_１６， λ_１，λ_２，λ_３ のコヒーレントな光パルスがそれぞれ出力される。このうち、出力ポートｏ１，ｏ２，…，ｏ１１，ｏ１３，…，ｏ１６に出力される波長λ_４，λ_５，…，λ_１４， λ_１６，…， λ_３ の光パルスは、対応する光変調器１３−９，１３−１０，…，１３−１６，１３−１，…，１３−３，１３−５，…，１３−８を介して入力ポートｉ９，ｉ１０，…，ｉ１６，ｉ１，…，ｉ３，ｉ５，…，ｉ８に戻される。
【００２７】
これらの変調信号光はアレイ導波路型光合分波器１２で再び合波され、出力ポートｏ１２から波長多重信号光として一括して出力される。なお、出力ポートｏ１２に出力される波長λ_１５の光パルスはアレイ導波路型光合分波器１２を１回通過しただけであるので、無変調であるとともに挿入損失が他の波長の信号光と比べて小さい。すなわち、出力ポートｏ１２に取り出される波長多重信号光には、光パワーが大きい波長λ_１５の光パルス（無変調光）が含まれることになる。
【００２８】
第１実施例に対する本実施例の利点は、波長多重信号光の各波長の光パワーのばらつきが低減されることである。その理由を以下に示す。一般にアレイ導波路型光合分波器１２の各ポートの損失は、中心のポートを対称にして釣鐘型になっており、中心のポートの損失が最低であり、端のポートの損失が最大となる。したがって、同一番号の入出力ポートを接続する第１実施例の構成では、光がアレイ導波路型光合分波器１２を２回通過する際にポートによる損失の違いが倍加する。一方、第２実施例の構成では、Ｎ／２だけずらした番号の入出力ポートを接続しているので、損失のばらつきが平均化されて波長多重信号光の波長による光パワーのばらつきが小さくなる。
【００２９】
（第３実施例−請求項２に対応）
図５は、本発明の波長多重信号光発生装置の第３実施例の構成を示す。
図において、コヒーレント白色光源１１から出力されたコヒーレント白色光は、アレイ導波路型光合分波器１２の１つの入力ポートｉ１から入力される。出力ポートｏ１〜ｏＮはそれぞれ隣接するものが光変調器１３−１〜１３−Ｎ／２を介して１対１に接続される。
【００３０】
入力ポートｉ１にコヒーレント白色光が入力されると、出力ポートｏ１，ｏ２，…，ｏＮに波長λ_１，λ_２，…， λ_Ｎ のコヒーレントな光パルスがそれぞれ出力される。ここで、出力ポートｏ１の波長λ_１ の光パルスは光変調器１３−１を介して出力ポートｏ２に入力され、出力ポートｏ２の波長λ_２ の光パルスは光変調器１３−１を介して出力ポートｏ１に入力される。他のペアとなっている出力ポート間でも同様である。奇数番号の波長λ_１，λ_３，λ_５，…，λ_Ｎ−１ の変調信号光は、出力ポートｏ２，ｏ４，…，ｏＮから入力されてアレイ導波路型光合分波器１２で合波され、入力ポートｉＮから波長多重信号光として一括して出力される。また、偶数番号の波長λ_２，λ_４，λ_６，…，λ_Ｎ の変調信号光は、出力ポートｏ１，ｏ３，…，ｏＮ−１から入力されてアレイ導波路型光合分波器１２で合波され、入力ポートｉ２から波長多重信号光として一括して出力される。
【００３１】
本実施例では、例えば光変調器１３−１で波長λ_１ の光パルスと波長λ_２ の光パルスが同時に変調され、それぞれ異なるポートに出力されることになる。すなわち、２つの波長群（λ_１，λ_３，λ_５，…，λ_Ｎ−１ ）と（λ_２，λ_４，λ_６，…，λ_Ｎ ）は同じ信号成分を有する波長多重信号光となる。また、本実施例では独立にＮ／２個の信号しか変調できないが、波長多重信号光の波長チャネル間隔を第１実施例および第２実施例に比べて２倍にすることができる。また、各波長がすべてアレイ導波路型光合分波器１２を２回通過するので、第１実施例および第２実施例のように高光パワーの無変調光が波長多重信号光に合波されることがなくなる。
【００３２】
（第４実施例−請求項２に対応）
図６は、本発明の波長多重信号光発生装置の第４実施例の構成を示す。
本実施例では、入出力ポート数Ｎを１６とし、コヒーレント白色光源１１から出力されたコヒーレント白色光がアレイ導波路型光合分波器１２の入力ポートｉ４に入力される。出力ポートｏ１とｏ９，ｏ２とｏ１０，…，ｏ８とｏ１６はそれぞれ光変調器１３−９〜１３−１６を介して１対１に接続される。
【００３３】
入力ポートｉ４にコヒーレント白色光が入力されると、出力ポートｏ１，ｏ２，…，ｏ１６に波長λ_４，λ_５，…， λ_１６，λ_１，λ_２，λ_３ のコヒーレントな光パルスがそれぞれ出力される。ここで、出力ポートｏ１の波長λ_４ の光パルスは光変調器１３−９を介して出力ポートｏ９に入力され、出力ポートｏ９の波長λ_１２の光パルスは光変調器１３−９を介して出力ポートｏ１に入力される。他のペアとなっている出力ポート間でも同様である。これらの変調信号光はアレイ導波路型光合分波器１２で合波され、入力ポートｉ１２から波長多重信号光として一括して出力される。
【００３４】
１つの出力ポートから波長多重信号光が出力されるためのアレイ導波路型光合分波器１２の入出力ポートの接続条件は、
ｍｏｄ〔ｒ−ｑ＋Ｎ，Ｎ〕＝ ｍｏｄ〔ｔ−ｓ＋Ｎ，Ｎ〕 …（１２）
または
ｍｏｄ〔ｑ−ｒ＋Ｎ，Ｎ〕＝ ｍｏｄ〔ｔ−ｓ＋Ｎ，Ｎ〕 …（１３）
となる。ここで、ｑ，ｒは、コヒーレント白色光の入力ポート番号（ｉｑ）と波長多重信号光の入力ポート番号（ｉｒ）に対応する。ｔ，ｓは、接続される出力ポート番号（ｏｔ）と出力ポート番号（ｏｓ）に対応する。 ｍｏｄ〔Ａ，Ｂ〕は、ＡをＢで割った余りである。
【００３５】
本実施例では、例えば光変調器１３−９で波長λ_４ の光パルスと波長λ_１２の光パルスが同時に変調され、ともに入力ポートｉ１２から出力されることになる。すなわち、２つの波長群（λ_４，λ_５，…， λ_１１）と（λ_１２，…， λ_１６，λ_１，λ_２，λ_３）は同じ信号成分を有する波長多重信号光となる。したがって、その一方の波長多重信号光を用いるか、２系列の波長多重信号光として扱う放送型伝送に用いることができる。また、本実施例では、特にｔ−ｓの絶対値がＮ／２＝８になるように設定しており、第２実施例と同様にアレイ導波路型光合分波器１２の損失分布の影響を最小限に抑えることができる。
【００３６】
（第５実施例−請求項３に対応）
図７は、本発明の波長多重信号光発生装置の第５実施例の構成を示す。
図において、コヒーレント白色光源１１から出力された超広帯域のコヒーレント白色光は１×Ｎ光カプラ１４でＮ分岐され、それぞれ異なる所定の波長を選択する光バンドパスフィルタ１５−１〜１５−Ｎに入力される。各光バンドパスフィルタから出力される光パルスはそれぞれ対応する光変調器１３−１〜１３−Ｎで変調される。各信号光はＮ×１光カプラ１６で合波され、波長多重信号光として出力される。
【００３７】
本実施例における１×Ｎ光カプラ１４と光バンドパスフィルタ１５−１〜１５−Ｎは、上述した実施例におけるアレイ導波路型光合分波器１２に相当し、波長λ_１ 〜λ_Ｎ のコヒーレントな光パルスを得る光分波器として機能する。各波長のバンド幅および波長チャネル間隔は、各光バンドパスフィルタのフィルタ特性により決定される。
【００３８】
【発明の効果】
以上説明したように、本発明の波長多重信号光発生装置に用いられるコヒーレント白色光源は、複雑なスペクトル構造をもたず、一様で連続的なスペクトルを有し、超広帯域（２００ｎｍ程度）で高いコヒーレンスを有し、さらにＧＨｚの繰り返し周波数の白色パルスを低励起パワーで発生させることができる。
【００３９】
この白色パルスをアレイ導波路型光合分波器または波長選択手段に入力することにより、複数の異なる波長成分を有する光パルス群とし、これらを独立に変調して合波することにより波長多重信号光を発生させることができる。複数の異なる波長成分を有する光パルスのパルス幅、波長チャネル間隔は、用いるアレイ導波路型光合分波器または波長選択手段の特性により任意に選択することができる。これにより、 １００Ｇｂｉｔ／ｓ 〜１Ｔｂｉｔ／ｓ の波長多重超短パルス通信に不可欠な波長多重信号光発生装置を実現することができる。
【図面の簡単な説明】
【図１】本発明の波長多重信号光発生装置の第１実施例の構成を示す図。
【図２】コヒーレント白色光源１１の構成を示す図。
【図３】アレイ導波路型光合分波器１２の入出力関係を示す図。
【図４】本発明の波長多重信号光発生装置の第２実施例の構成を示す図。
【図５】本発明の波長多重信号光発生装置の第３実施例の構成を示す図。
【図６】本発明の波長多重信号光発生装置の第４実施例の構成を示す図。
【図７】本発明の波長多重信号光発生装置の第５実施例の構成を示す図。
【図８】モード同期ファイバリングパルスレーザの構成を示す図。
【符号の説明】
１１ コヒーレント白色光源
１２ アレイ導波路型光合分波器
１３ 光変調器
１４ １×Ｎ光カプラ
１５ 光バンドパスフィルタ
１６ Ｎ×１光カプラ
２１ 励起用パルス光源
２２ 非線形光学媒質
２３ バンド除去フィルタ
３１ 入力用導波路アレイ
３２ 入力側コンケイブスラブ導波路
３３ アレイ導波路
３４ 出力側コンケイブスラブ導波路
３５ 出力用導波路アレイ[0001]
[Industrial application fields]
The present invention relates to a wavelength-multiplexed signal light generator that simultaneously generates signal light having two or more wavelengths, and multiplexes and outputs the signal light.
[0002]
[Prior art]
In future optical communication systems, wavelength division multiplexing (WDM) transmission systems are promising as a method for dramatically increasing communication capacity. In this method, a signal is superimposed and transmitted on a plurality of different optical carrier frequencies (wavelengths), and the total transmission capacity increases by the number of optical carriers.
[0003]
By the way, the wavelength division multiplexing transmission system requires a plurality of light sources having different wavelengths. Currently, as such light sources, there are laser arrays in which about 20 DFB lasers that continuously oscillate at different wavelengths are integrated, and mode-locked fiber ring pulse lasers that simultaneously oscillate at about 4 wavelengths.
FIG. 8 shows the configuration of a mode-locked fiber ring pulse laser. In the figure, 1 is an optical modulator, 2 is a birefringent medium, 3 is a polarizer, and 4 is a gain medium such as an optical fiber amplifier. The optical pulse output from the optical modulator 1 has a ring configuration that is input to the optical modulator 1 via the birefringence medium 2, the polarizer 3, and the gain medium 4, and the output optical pulse from the optical modulator 1. Is taken out. In this configuration, the birefringent medium 2 becomes the birefringent region 5, and the other region becomes the single polarization region 6. Number of wavelengths oscillated, become ^the 2 ^N of the number of birefringent region 5 as N, wavelength spacing is determined by the polarization dispersion of the birefringent medium 2.
[0004]
[Problems to be solved by the invention]
In the mode-locked fiber ring pulse laser, the oscillation wavelength is limited by the band of the gain medium 4, and it is difficult to freely select the oscillation wavelength of the optical pulse and to obtain an ultrashort pulse of picosecond or less.
Conventionally, by filtering the output light of a broadband white noise light source such as LED, SLD, semiconductor laser amplifier, and spontaneous emission noise (ASE) of an optical fiber amplifier using an optical multiplexer / demultiplexer, a plurality of Multi-wavelength light sources that extract wavelength components have been proposed. However, in principle, since the coherence of the light source is low, it has been difficult to generate broadband multi-wavelength signal light having a good S / N ratio and higher than gigahertz.
[0005]
The present invention combines a coherent white light source that generates an ultra-wideband coherent white pulse and an optical multiplexer / demultiplexer, and enables wavelength-division-multiplexed signal light that can generate an ultrashort pulse signal of a plurality of wavelengths in a wide wavelength range. An object is to provide a generator.
[0006]
[Means for Solving the Problems]
Wavelength-multiplexed signal light generating device of the present invention includes a pumping pulse light source for outputting excitation light pulses of the wavelength lambda _0, the wavelength range Δλ a predetermined length including the wavelength lambda _0, the dispersion slope, the dispersion, A coherent white light source including a nonlinear optical medium that inputs an excitation light pulse and outputs ultra-wideband coherent white light.
[0007]
The coherent white light source, an arrayed waveguide type optical multiplexer / demultiplexer having N input ports and N output ports, and an optical pulse demultiplexed to N-1 output ports are modulated. N-1 optical modulators that respectively input modulated signal light to N-1 corresponding input ports, and input coherent white light to one input port of the arrayed waveguide optical multiplexer / demultiplexer, Wavelength multiplexed signal light is output from one output port.
[0008]
Further, the coherent white light source, an arrayed waveguide type optical multiplexer / demultiplexer having N input ports and N output ports, and a predetermined two output ports, respectively, are arranged on one output port. And N / 2 optical modulators for modulating the modulated optical pulses and inputting the modulated signal light to the other output port, respectively, and coherent to a predetermined input port of the arrayed waveguide optical multiplexer / demultiplexer White light is input, and wavelength division multiplexed signal light is output from a predetermined input port.
[0009]
Further, the coherent white light source, an optical branching unit for N-branching coherent white light, N wavelength selecting units for selecting different predetermined wavelengths from the N-branched coherent white light, and each wavelength selecting unit N optical modulators for individually modulating the output light and optical coupling means for outputting the wavelength multiplexed signal light by combining the output lights of the respective optical modulators.
[0010]
[Action]
The coherent white light source of the wavelength multiplexed signal light generation apparatus of the present invention uses self-phase modulation and cross-phase modulation of excitation light pulses in a nonlinear optical medium, and highly efficient and coherent optical parametric four-wave mixing over a wide wavelength range. Thus, it is possible to generate ultra-wideband coherent white light having uniform spectral intensity and high coherence (coherence) in the wavelength range of Δλ on both sides of the center wavelength λ ₀ of the excitation light pulse.
[0011]
By inputting the coherent white light to one input port of the arrayed waveguide type optical multiplexer / demultiplexer, optical pulse groups having a plurality of different wavelength components are output to different output ports. Here, these optical pulse groups are obtained by connecting the N-1 output ports of the arrayed waveguide optical multiplexer / demultiplexer to the N-1 input ports via the optical modulator in a one-to-one relationship. Can be independently modulated, input again to the arrayed waveguide optical multiplexer / demultiplexer, and can be combined and output from one output port as wavelength multiplexed signal light.
[0012]
In addition, by connecting predetermined output ports of an arrayed waveguide optical multiplexer / demultiplexer in a one-to-one relationship via an optical modulator, an optical pulse group having a plurality of different wavelength components is modulated independently. In addition, the signals can be input again to the arrayed waveguide type optical multiplexer / demultiplexer, and can be output as wavelength multiplexed signal light from one input port. Further, the optical pulse group having a plurality of different wavelength components is output by N-branching the coherent white light and selecting a different predetermined wavelength from each coherent white light. This optical pulse group can be independently modulated by each optical modulator, input to the optical coupling means, and combined to be output as wavelength multiplexed signal light.
[0013]
【Example】
(First embodiment-corresponding to claim 1)
FIG. 1 shows the configuration of a first embodiment of a wavelength multiplexed signal light generator according to the present invention.
In the figure, the ultra-wideband coherent white light output from the coherent white light source 11 is input from one input port i 1 of the arrayed waveguide optical multiplexer / demultiplexer 12. The remaining N-1 input ports i2 to iN are connected to the output ports o2 to oN on a one-to-one basis via the optical modulators 13-2 to 13-N, respectively. Wavelength multiplexed signal light is output from the output port o1 of the arrayed waveguide optical multiplexer / demultiplexer 12.
[0014]
FIG. 2 shows the configuration of the coherent white light source 11.
In the figure, the coherent white light source 11 includes an excitation pulse light source 21 that outputs an excitation light pulse having a center wavelength λ ₀ , and a nonlinear optical medium 22 having a predetermined dispersion slope and dispersion. In addition, you may provide the band removal filter 23 which removes the optical pulse for excitation as needed.
[0015]
Here, the operation of the coherent white light source 11 in the nonlinear optical medium 22 will be described.
The excitation light pulse input into the nonlinear optical medium 22 has its spectral width spread symmetrically on both sides of the excitation wavelength by self-phase modulation. When the spectrum spread due to self-phase modulation begins to overlap the four-wave mixing band, the spectrum begins to coherently spread to the gain bandwidth along the gain curve of the four-wave mixing using the spectrum component spread by the self-phase modulation as a seed. . Then, when the pump light power gradually decreases and the gain bandwidth proportional to the square root of the pump light power starts to decrease, the spectrum broadening stops at that point. The coherent white light spectrum generated at this time becomes continuous and uniform because it gradually spreads on both sides of the excitation wavelength, as is apparent from the generation process. In addition, the coherence of the generated white pulse is determined by the coherence of the excitation light pulse when the four-wave mixed light is generated. If a high-coherence pulse is used as the excitation light pulse, the coherence of the white light increases.
[0016]
Furthermore, self-phase modulation that generates a spectral component that becomes a seed for four-wave mixing occurs at low excitation power, and the necessary excitation power for four-wave mixing that gives a constant gain band decreases in proportion to the dispersion, so low dispersion If the nonlinear optical medium 22 is used, the excitation power can be reduced.
Hereinafter, provisions regarding the dispersion slope and the magnitude of dispersion of an optical waveguide (single mode optical fiber) used as the nonlinear optical medium 22 are derived.
[0017]
From the theory of four-wave mixing, the optical angular frequency difference between the pumping light (wavelength λ ₀ ) and the four-wave mixing light (wavelength λ _f ) giving the maximum gain Ω _S (= 2πc | 1 / λ ₀ −1 / λ _f | : C is the speed of light in vacuum) is the third-order nonlinear coefficient of the optical waveguide γ (= 2πn ₂ / (λA _eff ) [1 / (W km)]: n ₂ is the nonlinear refractive index of the optical waveguide [m ² / W], A _eff is the effective area of the optical waveguide [m ² ]), the peak power of the excitation light pulse in the optical waveguide is P [W], and the dispersion of the optical waveguide is β ₂ [ps ² / km]. ,
Ω _S = (2 | γ | P / | β ₂ |) ^1/2 [THz] (1)
It becomes. When the length of the optical waveguide is L [km], the gain G ₀ of the four-wave mixed light intensity at Ω _S is
G ₀ = exp (2 | γ | PL) (2)
Given in.
[0018]
Here, in order for white light to be generated continuously, it is necessary to have a gain at Ω _S , and therefore, G _{0 in} equation (2) must be greater than 1. Note that G ₀ is preferably 2 or more. From this condition, the optical waveguide length L _C required for white light generation is L _C = ln G ₀ / (2 | γ | P) (3)
It is obtained.
[0019]
Incidentally, the wavelength lambda ₀ the group delay difference τ in the light waveguide length L _C of the excitation light pulse and the wavelength lambda ₀ 4-wave mixing light of ± [Delta] [lambda] / 2 of [ps], the absolute value of the dispersion slope | dD (lambda) / Dλ | [ps / nm ² / km] approximately

It can be expressed. Here, in order to generate the four-wave mixed light having the wavelength λ ₀ ± Δλ / 2, it is necessary to overlap the excitation light pulse with the four-wave mixed light in terms of time. pulse width Δt ₀ of use light pulse: the condition that it (ps full width at half maximum) following _{Δt 0 ≧ τ = (lnG 0} /16) | dD (λ 0) / dλ | · Δλ 2 / (| γ | P) ... (Five)
It is represented by Therefore, the condition of the dispersion slope is | dD (λ ₀ ) / dλ | ≦ 16 (| γ | PΔt ₀ ) / (Δλ ² lnG ₀ ) (6)
Is obtained. Here, Pderutati0 than INO substantially equal to the energy E of the excitation light pulse, condition concerning the dispersion slope of the optical waveguide _{| dD (λ 0) / dλ} | ≦ 16 (| γ | E) / (Δλ 2 ln G ₀ )… (7)
Is obtained.
[0020]
Then, to the optical waveguide to generate four-wave mixed light of wavelength λ ₀ ± Δλ / 2 from the excitation light pulse of the wavelength lambda ₀ is given by at least the formula (1) gain bandwidth (Arufaomega _S) is Must be greater than Δλ / 4. Therefore,
Ω _S = (2 | γ | P / | β ₂ |) ^1/2 ≧ 2πcΔλ / (4λ ₀ ² ) (8)
Must. Here, the relational expression of β ₂ and D (λ) | β ₂ | = λ ² | D (λ) | / (2πc) (9)
The conditional expression regarding dispersion of the optical waveguide | D (λ ₀ ) | ≦ (16 / πc) · λ ₀ ² | γ | P / (Δλ) ² (10)
Is obtained. Equation (10) must be satisfied over at least the length ln G ₀ / (2 | γ | P) defined by equation (3).
[0021]
Note that the peak power P of the excitation optical pulses in the optical waveguide, the peak power P ₀ of the excitation light pulse input to the optical waveguide, generally different for waveforms changes in the excitation optical pulses. These can be expressed as P = αP ₀ using the correction coefficient α.
As shown in FIG. 1, the arrayed waveguide type optical multiplexer / demultiplexer 12 includes an input waveguide array 31, an input side concave slab waveguide 32, and an arrayed waveguide (adjacent waveguides have an optical path length difference of ΔL). 33, an output side concave slab waveguide 34, and an output waveguide array 35. The light incident on the input waveguide array 31 from the input port i1 spreads by diffraction in the input side concave slab waveguide 32, and is guided to the arrayed waveguide 33 arranged perpendicular to the diffraction surface. In the arrayed waveguide 33, each waveguide is sequentially lengthened with a waveguide length difference ΔL. Therefore, light that propagates through each waveguide and reaches the output side concave slab waveguide 34 corresponds to the waveguide length difference ΔL. A phase difference occurs. Since this phase difference varies depending on the wavelength (optical frequency), when the light is condensed on the input end of the output waveguide array 35 by the lens effect of the output side concave slab waveguide 34, the phase difference is different at each wavelength (optical frequency). Condensate. Therefore, light of different optical frequencies is extracted from each output port and functions as an optical demultiplexer. Similarly, it functions as an optical multiplexer.
[0022]
As shown in FIG. 3, the input / output relationship of the arrayed waveguide multiplexer / demultiplexer 12 is such that the correspondence between the transmission center wavelength and the output port is cyclically shifted by one channel when the position of the input port is shifted by one. For example, when the number of input / output ports is 16, and when the coherent white light is input to the input port i1, and the wavelengths λ ₁ to λ ₁₆ are demultiplexed to the output ports o1 to o16, they are input to the input port i4. In the coherent white light, the wavelengths λ ₄ , λ ₅ ,..., Λ ₁₆ , λ ₁ , λ _2, λ ₃ are demultiplexed to the output ports o 1, o 2,. At this time, the bandwidth and channel spacing of each wavelength are determined by the waveguide parameters of the arrayed waveguide optical multiplexer / demultiplexer 12, and the pulse width of the optical pulse demultiplexed to each output port is determined by the arrayed waveguide optical multiplexing / demultiplexing. The transform limit pulse is determined from the bandwidth of the waver 12.
[0023]
In the configuration shown in FIG. 1, when coherent white light is input to the input port i1, coherent light pulses having wavelengths λ _1, λ ₂ ,..., Λ _N are output to the output ports o1, o2,. The Among these, the optical pulses of wavelengths λ _2, λ ₃ ,..., Λ _N output to the output ports o2, o3,..., ON are the corresponding optical modulators 13-2, 13-3,. The modulated signal light is returned to the input ports i2, i3,. These modulated signal lights are multiplexed by the arrayed waveguide type optical multiplexer / demultiplexer 12, and are collectively output as wavelength multiplexed signal light from the output port o1. Since the optical pulse having the wavelength λ ₁ output to the output port o ₁ has only passed through the arrayed waveguide optical multiplexer / demultiplexer 12 once, it is unmodulated and has an insertion loss of signal light of other wavelengths. Smaller than that. That is, the wavelength multiplexed signal light extracted to the output port o1 includes an optical pulse (unmodulated light) having a wavelength λ ₁ having a large optical power.
[0024]
As described above, in this embodiment, predetermined input / output ports of the arrayed waveguide type optical multiplexer / demultiplexer 12 are connected in a loop through the optical modulator 13 so that the functions of the optical demultiplexer and the optical multiplexer are increased. It can be realized by one arrayed waveguide type optical multiplexer / demultiplexer. As a result, it is possible to avoid the influence due to the difference in characteristics when the optical demultiplexer and the optical multiplexer are configured separately (configuration of FIG. 7 of Japanese Patent Application No. 5-221259).
[0025]
The connection condition of the input / output ports of the arrayed waveguide type optical multiplexer / demultiplexer 12 for outputting the wavelength multiplexed signal light from one output port is as follows:
t−1 = mod [(s−1) + (q−r) + N, N] (11)
(Ishida, Takahashi, “Loss-imbalance evaluation of arrayed-waveguided adding add-drop multiplexer”, Electron. Lett., 30, pp. 1160-1162, 1994). Here, q and r correspond to the input port number (iq) of coherent white light and the output port number (or) of wavelength multiplexed signal light. t and s correspond to the input port number (it) and the output port number (os) to be connected. mod [A, B] is the remainder of A divided by B.
[0026]
(Second embodiment-corresponding to claim 1)
FIG. 4 shows the configuration of a second embodiment of the wavelength multiplexed signal light generator according to the present invention.
In the present embodiment, the input / output port number difference ts in equation (11) is connected with an offset so that the absolute value is N / 2 (mod [(q−r) + N, N] = N / 2). Here, the number of input / output ports N is 16, coherent white light output from the coherent white light source 11 is input to the input port i4 of the arrayed waveguide optical multiplexer / demultiplexer 12, and wavelength multiplexed signal light is output from the output port o12. It shows how it is output. The output ports o1, o2,..., O16 output coherent optical pulses having wavelengths λ ₄ , λ ₅ ,..., Λ ₁₆ , λ ₁ , λ ₂ , λ ₃ , respectively. Among these, the optical pulses of the wavelengths λ _4, λ ₅ ,..., Λ ₁₄ , λ ₁₆ ,..., Λ ₃ output to the output ports o1, o2,. , 13-16, 13-1, ..., 13-3, 13-5, ..., 13-8 through the input ports i9, i10, ..., i16, i1, ... , I3, i5,..., I8.
[0027]
These modulated signal lights are multiplexed again by the arrayed waveguide type optical multiplexer / demultiplexer 12, and are collectively output as wavelength multiplexed signal light from the output port o12. Note that the optical pulse having the wavelength λ ₁₅ output to the output port o12 has only passed through the arrayed waveguide optical multiplexer / demultiplexer 12 once, so that it is unmodulated and has an insertion loss of signal light of other wavelengths. Smaller than that. That is, the wavelength-multiplexed signal light extracted to the output port o12 includes an optical pulse (unmodulated light) having a wavelength λ ₁₅ with high optical power.
[0028]
The advantage of this embodiment over the first embodiment is that variations in the optical power of each wavelength of the wavelength multiplexed signal light are reduced. The reason is as follows. In general, the loss of each port of the arrayed waveguide type optical multiplexer / demultiplexer 12 is a bell shape with the center port symmetrical, the loss of the center port is the lowest, and the loss of the end port is the maximum. . Therefore, in the configuration of the first embodiment in which input / output ports having the same number are connected, the difference in loss due to the ports is doubled when light passes through the arrayed waveguide optical multiplexer / demultiplexer 12 twice. On the other hand, in the configuration of the second embodiment, since the input / output ports with numbers shifted by N / 2 are connected, the variation in loss is averaged and the variation in optical power due to the wavelength of the wavelength multiplexed signal light is reduced. .
[0029]
(Third embodiment-corresponding to claim 2)
FIG. 5 shows the configuration of a third embodiment of the wavelength multiplexed signal light generator according to the present invention.
In the figure, the coherent white light output from the coherent white light source 11 is input from one input port i 1 of the arrayed waveguide optical multiplexer / demultiplexer 12. The adjacent output ports o1 to oN are connected one-to-one via the optical modulators 13-1 to 13-N / 2.
[0030]
When coherent white light is input to the input port i1, coherent light pulses having wavelengths λ _1, λ ₂ ,..., Λ _N are output to the output ports o1, o2,. Here, the wavelength lambda ₁ of the optical pulse of the output port o1 is input to the output port o2 via the optical modulator 13-1, the wavelength lambda ₂ of the optical pulses of the output port o2 via the optical modulator 13-1 Input to output port o1. The same applies to the output ports in other pairs. .., ΛN ₋₁ modulated signal light is input from the output ports o2, o4,..., ON and multiplexed by the arrayed waveguide type optical multiplexer / demultiplexer 12. The odd-numbered wavelengths λ ₁ , λ ₃ , λ ₅ ,. Then, the signals are collectively output from the input port iN as wavelength multiplexed signal light. Also, even-numbered wavelengths λ ₂ , λ ₄ , λ ₆ ,..., Λ _N modulated signal light is input from the output ports o 1, o 3,. The signals are multiplexed and output collectively from the input port i2 as wavelength multiplexed signal light.
[0031]
In this embodiment, for example, an optical pulse and a wavelength lambda ₂ of the light pulses of the wavelength lambda ₁ in the optical modulator 13-1 is modulated at the same time, to be output to different ports. That is, the two wavelength groups (λ ₁ , λ ₃ , λ ₅ ,..., Λ _N−1 ) and (λ ₂ , λ ₄ , λ ₆ ,..., Λ _N ) are wavelength multiplexed signal light having the same signal component. Become. In this embodiment, only N / 2 signals can be modulated independently, but the wavelength channel interval of the wavelength multiplexed signal light can be doubled compared to the first and second embodiments. Since each wavelength passes through the arrayed waveguide type optical multiplexer / demultiplexer 12 twice, unmodulated light with high optical power is multiplexed into the wavelength multiplexed signal light as in the first and second embodiments. Nothing will happen.
[0032]
(Fourth embodiment-corresponding to claim 2)
FIG. 6 shows the configuration of a fourth embodiment of the wavelength multiplexed signal light generating apparatus of the present invention.
In the present embodiment, the number N of input / output ports is 16, and the coherent white light output from the coherent white light source 11 is input to the input port i4 of the arrayed waveguide optical multiplexer / demultiplexer 12. The output ports o1 and o9, o2 and o10,..., O8 and o16 are connected one to one via optical modulators 13-9 to 13-16, respectively.
[0033]
When coherent white light is input to the input port i4, output port o1, o2, ..., wavelength lambda ₄ in _{o16, λ 5, ..., λ} 16, λ 1, λ 2, coherent light pulses of lambda _3, respectively Is output. Here, the optical pulse with the wavelength λ ₄ of the output port o1 is input to the output port o9 via the optical modulator 13-9, and the optical pulse with the wavelength λ ₁₂ of the output port o9 is input via the optical modulator 13-9. Input to output port o1. The same applies to the output ports in other pairs. These modulated signal lights are multiplexed by the arrayed waveguide type optical multiplexer / demultiplexer 12, and are collectively output as wavelength multiplexed signal light from the input port i12.
[0034]
The connection condition of the input / output ports of the arrayed waveguide type optical multiplexer / demultiplexer 12 for outputting the wavelength multiplexed signal light from one output port is as follows:
mod [r-q + N, N] = mod [ts + N, N] (12)
Or mod [q−r + N, N] = mod [ts−N + N, N] (13)
It becomes. Here, q and r correspond to the input port number (iq) of coherent white light and the input port number (ir) of wavelength multiplexed signal light. t and s correspond to the output port number (ot) and the output port number (os) to be connected. mod [A, B] is the remainder of A divided by B.
[0035]
In this embodiment, for example, an optical pulse of the optical pulse and a wavelength lambda ₁₂ wavelength lambda ₄ in the optical modulator 13-9 is modulated at the same time, so that both output from the input port i12. That is, the two wavelength groups (λ _4, λ ₅ ,..., Λ ₁₁ ) and (λ _12, ..., Λ ₁₆ , λ _1, λ ₂ , λ ₃ ) are wavelength multiplexed signal lights having the same signal component. Therefore, one of the wavelength multiplexed signal lights can be used, or it can be used for broadcast-type transmission handled as two series of wavelength multiplexed signal lights. In this embodiment, the absolute value of ts is set to be N / 2 = 8, and the influence of the loss distribution of the arrayed waveguide optical multiplexer / demultiplexer 12 is the same as in the second embodiment. Can be minimized.
[0036]
(Fifth embodiment-corresponding to claim 3)
FIG. 7 shows the configuration of the fifth embodiment of the wavelength division multiplexing signal light generating apparatus of the present invention.
In the figure, the ultra-wideband coherent white light output from the coherent white light source 11 is N-branched by a 1 × N optical coupler 14 and input to optical bandpass filters 15-1 to 15-N that select different predetermined wavelengths. Is done. The optical pulses output from each optical bandpass filter are modulated by the corresponding optical modulators 13-1 to 13-N. Each signal light is multiplexed by the N × 1 optical coupler 16 and output as wavelength multiplexed signal light.
[0037]
The 1 × N optical coupler 14 and the optical bandpass filters 15-1 to 15- _{N in} the present embodiment correspond to the arrayed waveguide optical multiplexer / demultiplexer 12 in the above-described embodiment, and coherent with wavelengths λ _{1 to} λ _N. It functions as an optical demultiplexer that obtains an optical pulse. The bandwidth and wavelength channel spacing of each wavelength are determined by the filter characteristics of each optical bandpass filter.
[0038]
【The invention's effect】
As described above, the coherent white light source used in the wavelength multiplexed signal light generation apparatus of the present invention does not have a complicated spectral structure, has a uniform and continuous spectrum, and has an ultra-wideband (about 200 nm). A white pulse having high coherence and a repetition frequency of GHz can be generated with low excitation power.
[0039]
By inputting this white pulse into an arrayed waveguide type optical multiplexer / demultiplexer or wavelength selection means, an optical pulse group having a plurality of different wavelength components is formed, and these are independently modulated and combined to be wavelength multiplexed signal light. Can be generated. The pulse width and wavelength channel spacing of optical pulses having a plurality of different wavelength components can be arbitrarily selected according to the characteristics of the arrayed waveguide optical multiplexer / demultiplexer or wavelength selection means used. As a result, it is possible to realize a wavelength multiplexed signal light generator indispensable for wavelength multiplexed ultrashort pulse communication of 100 Gbit / s to 1 Tbit / s.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a first embodiment of a wavelength multiplexed signal light generator according to the present invention.
FIG. 2 is a diagram showing a configuration of a coherent white light source 11;
3 is a diagram showing the input / output relationship of an arrayed waveguide optical multiplexer / demultiplexer 12. FIG.
FIG. 4 is a diagram showing a configuration of a second embodiment of a wavelength multiplexed signal light generating apparatus according to the present invention.
FIG. 5 is a diagram showing the configuration of a third embodiment of the wavelength division multiplexing signal light generating apparatus of the present invention.
FIG. 6 is a diagram showing the configuration of a fourth embodiment of the wavelength division multiplexing signal light generating apparatus of the present invention.
FIG. 7 is a diagram showing the configuration of a fifth embodiment of the wavelength division multiplexing signal light generator according to the present invention.
FIG. 8 is a diagram showing a configuration of a mode-locked fiber ring pulse laser.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 Coherent white light source 12 Array waveguide type optical multiplexer / demultiplexer 13 Optical modulator 14 1 × N optical coupler 15 Optical band pass filter 16 N × 1 optical coupler 21 Excitation pulse light source 22 Non-linear optical medium 23 Band elimination filter 31 For input Waveguide array 32 Input side concave slab waveguide 33 Array waveguide 34 Output side concave slab waveguide 35 Output waveguide array

Claims (3)

A pump pulse light source that outputs a pumping light pulse of wavelength λ ₀ , a third-order nonlinear coefficient γ, an energy per pulse of the pumping light pulse E, a peak power of the pumping light pulse P, and a four-wave mixing When the gain of light intensity is G ₀ (G ₀ is 2 or more) and the speed of light in vacuum is c, the length is in the wavelength range Δλ equal to the spectral width of the coherent white light generated around the wavelength λ _0.
ln G ₀ / (2 | γ | P) [km]
Above, the absolute value of the dispersion slope is
16 (| γ | E) / (Δλ ² ln G ₀ ) [ps / nm ² / km]
And the absolute value of the dispersion at wavelength λ ₀ is
(16 / πc) · λ ₀ ² | γ | P / (Δλ) ² [ps / nm / km]
A non-linear optical medium for inputting the pumping optical pulse, and self-phase modulation and cross-phase modulation of the pumping optical pulse in the non-linear optical medium, and a highly efficient and coherent optical parametric over a wide wavelength range. A coherent white light source that generates ultra-wideband coherent white light with uniform spectral intensity and high coherence on both sides of the excitation light wavelength using four-wave mixing;
An arrayed waveguide type optical multiplexer / demultiplexer having N input ports and N output ports;
N− that modulates the optical pulse demultiplexed into N−1 output ports of the arrayed waveguide multiplexer / demultiplexer, and inputs the modulated signal light of each wavelength to N−1 corresponding input ports. One optical modulator,
The coherent white light is input to one input port of the arrayed waveguide optical multiplexer / demultiplexer, and wavelength multiplexed signal light obtained by multiplexing the modulated signal light of each wavelength is output from one output port. A wavelength-division multiplexed signal light generator. A coherent white light source according to claim 1;
An arrayed waveguide type optical multiplexer / demultiplexer having N input ports and N output ports;
The arrayed waveguide type optical multiplexer / demultiplexer is arranged between two predetermined output ports, respectively, modulates an optical pulse demultiplexed to one output port, and inputs the modulated signal light to the other output port, respectively. N / 2 optical modulators,
The coherent white light is input to a predetermined input port of the arrayed waveguide optical multiplexer / demultiplexer, and wavelength multiplexed signal light obtained by multiplexing the modulated signal light of each wavelength is output from the predetermined input port. A wavelength-division multiplexed signal light generator. A coherent white light source according to claim 1;
A light branching means for N-branching coherent white light output from the coherent white light source;
N wavelength selection means for selecting different predetermined wavelengths from the N-branched coherent white light,
N optical modulators for individually modulating the output light of each wavelength selection means;
A wavelength-multiplexed signal light generating apparatus, comprising: an optical coupling means for multiplexing the output light of each of the optical modulators and outputting wavelength-multiplexed signal light.

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