JP3899996B2 - Optical waveguide, multi-wavelength light source, and tunable light source - Google Patents

Description

ここで、ａ＝ｗ／２（コア半幅）であり、Δ＝ｎ+ｎ_cladである。ｎ_cladは、クラッドの屈折率である。
【００２８】
従って、このコア２２ａ〜２２ｄ内に形成されたグレーティング２６ａ〜２６ｄを半導体レーザの外部共振器として利用した場合、図５のラインＬ１〜Ｌ４に示すように、いずれの発振波長についても発振波長の安定化が図られ、モードホップが生じるおそれが低減される。
【００２９】
合波器１６は、光導波路１０の出射端１４ｂ側に連続して設けられている。この合波器１６は、共通導波路２８とこの共通導波路２８から分岐する複数の分岐路３０ａ〜３０ｄとを含む合波回路を有している。この合波器１６は、複数の分岐路３０ａ〜３０ｄが、光導波路１４の複数のコア２２ａ〜２２ｄと光学的に結合されており、これらコア２２ａ〜２２ｄ内を伝搬する光を合波して共通導波路２８内を伝搬させる。
【００３０】
半導体光増幅器アレイ１８は、光増幅機能を有する半導体活性層を含む半導体光増幅器をアレイ状に設けたものである。この半導体光増幅器アレイ１８の光出射面１８ａには非反射コート（ＡＲコート）が施されており、この光出射面１８ａと対向する光反射面１８ｂには反射コートが施されている。この半導体光増幅器アレイ１８は、光出射面１８ａが光導波路１４の光入射面１４ａと対面するように溝３２内の基板１２上に配置され、複数のコア２２ａ〜２２ｄそれぞれと光学的な結合が図られている。なお、本実施形態に係る多波長光源１０では、半導体光増幅器アレイ１８の光反射面１８ｂと光導波路１４の各コア２２ａ〜２２ｄに設けられたグレーティング２６ａ〜２６ｄとにより、半導体レーザの共振器が構成されている。
【００３１】
モニタ用の受光器アレイ２０は、フォトダイオード等の受光器をアレイ状に設けたものである。この受光器アレイ２０は、半導体光増幅器アレイ１８の光反射面１８ｂから出射される光を受光してモニタする。この受光器アレイ２０は、受光面２０ａが半導体光増幅器アレイ１８の光反射面１８ｂと対面するように溝３２内の基板１２上に配置され、半導体光増幅器アレイ１８と光学的な結合が図られている。
【００３２】
次に、上記した構成の多波長光源の製造方法について説明する。
【００３３】
まず、基板（例えばシリコン基板）上に、化学的気相成長法（ＣＶＤ）あるいは火炎加水分解法（ＦＨＤ）により、アンダークラッド層を成膜し、さらにその上にコア層を成膜する。コア層の成膜時には、基板面内でＧｅ添加量の濃度勾配を設け、屈折率の勾配をつける。Ｇｅ添加量の濃度勾配は、ＦＨＤ法ではスス堆積時のバーナのトラバースとＧｅＣｌ₄投入量との関係を調整したり、あるいはスス堆積時の基板温度に傾斜を付けたりすることで実現できる。ＣＶＤ法では、コア層の成膜時にＧｅ添加量の濃度勾配を付けたい部分をチャンバの中央から離れた位置に置くなどすることで実現できる。
【００３４】
次に、フォトリソグラフィ技術と反応性イオンエッチング（ＲＩＥ）とにより、複数のコアと合波回路を加工する。このとき、使用するフォトマスクには、予め設計されたコア幅の複数のコア及び合波回路のパターンが書き込まれている。そして、これらの上に、ＣＶＤあるいはＦＨＤにより、オーバークラッド層を成膜する。
【００３５】
次に、上記の処理が施された基板上に高圧水素処理を施す。そして、位相格子を介して紫外光を照射し、複数のコアそれぞれに等しい周期のグレーティングを形成する。光源としては、コアに照射したときに屈折率を上昇させる紫外光を射出するものを用いる。例えば、コアの添加物（ここでは、Ｇｅ）を反応させる紫外光を出射するエキシマレーザなどを用いればよい。また、位相格子としては均一のパターン周期を有するものを用いる。このようにしてグレーティングを形成すれば、並設される複数のコアに対して紫外光を一度に照射することで、各コアに反射特性の異なるグレーティングを一括して形成することができる。従って、各コアに対して異なる干渉パターンの紫外光を個々に照射する必要がなくなり、製造の効率化が図られ生産性を向上させることができる。
【００３６】
次に、複数のコアの延伸方向と交差する方向に沿って、基板上の素子搭載領域に相当する部分にエッチングを施し、その領域のコア及びクラッドを取り除いて溝を形成する。そして、その溝内に半導体光増幅器アレイ及び受光器アレイを実装し、半導体光増幅器アレイと光導波路との間、及び半導体光増幅器アレイと受光器アレイとの間の光学的な結合を図る。
【００３７】
次に、本実施形態に係る光導波路１４、及びこれを備える多波長光源１０の作用及び効果について説明する。
【００３８】
本実施形態に係る多波長光源１０では、半導体光増幅器アレイ１８を駆動させると、半導体光増幅器アレイ１８の光反射面１８ｂと光導波路１４の複数のコア２２ａ〜２２ｄに形成されたグレーティング２６ａ〜２６ｄとを共振器として、所定のブラッグ波長λ_Bでレーザ発振が起こる。それぞれ異なるブラッグ波長で発振された光は、光導波路１４の各コア２２ａ〜２２ｄ内を伝搬し、合波器１６により合波されて、合波器１６の出射端１６ａ側から出射される。出射される光パワーは、受光器アレイ２０によりモニタすることで調整することができる。
【００３９】
このような構成の多波長光源１０が備える光導波路１４では、複数のコア２２ａ〜２２ｄそれぞれに等しい周期のグレーティング２６ａ〜２６ｄが形成されているため、異なる周期のグレーティングが形成されている場合と比べて、その製造が容易になり、生産性の向上を図ることが可能となる。また、コア２２ａ〜２２ｄそれぞれのコア幅Ｗと屈折率ｎとが変更されて、コア２２ａ〜２２ｄそれぞれの実効屈折率ｎ_effが異なるように設計されているため、コア幅Ｗのみが変更されて実効屈折率ｎ_effが異なるように設計された場合と比べて、グレーティング２６ａ〜２６ｄのブラック波長λ_Bとして利用可能な波長範囲を大きくすることができる。
【００４０】
図６は、コアの屈折率ｎを種々に変更したときの、コア幅Ｗと実効屈折率ｎ_effとの関係を示すグラフである。図６において、丸はコアの屈折率が１．４４５のとき、四角はコアの屈折率が１．４５のとき、三角はコアの屈折率が１．４５５のとき、星はコアの屈折率が１．４６のときをそれぞれ示している。図６に示すように、コアのコア幅Ｗが大きいほど実効屈折率ｎ_effは大きくなるが、その変動幅は小さい。これに対し、コア幅Ｗのみならずコアの屈折率ｎをも大きくすれば、実効屈折率ｎ_effの変動幅は大きくなり、グレーティングのブラック波長として利用可能な波長範囲を大きくすることができる。
【００４１】
また、この光導波路１４では、コア２２ａ〜２２ｄそれぞれのコア幅Ｗと屈折率ｎとは、シングルモード条件を満たすように設計されているため、半導体光増幅器アレイ１８の光反射面１８ｂとコア２２ａ〜２２ｄ内のグレーティング２６ａ〜２６ｄとを共振器とする半導体レーザからの出射光は、図５に示すように、いずれの発振波長においても発振波長が安定しており、モードホップが生じるおそれを低減することができる。
【００４２】
また、この光導波路１４では、コア２２ａ〜２２ｄそれぞれのコア幅Ｗと屈折率ｎとは、グレーティング２６ａ〜２６ｄそれぞれのブラック波長λ_Bにおける反射率を実質的に一定にするように設計されているため、グレーティング２６ａ〜２６ｄそれぞれのブラック波長λ_Bにおける光の反射率が実質的に均一化される。その結果、光の出射パワー、光の注入電力依存性をほぼ一定にすることが可能となる。
【００４３】
また、上記のような作用効果を奏しうる光導波路１４を用いて、複数波長の光を含む光を出射可能な多波長光源１０を、シンプルな構成で好適に構成することができる。
（第２実施形態）
次に、本発明の第２実施形態について説明する。なお、図２〜図６は、第２実施形態においても利用可能であるため、これらも参照して説明する。
【００４４】
図７は、本実施形態に係る波長可変光源５０の構成を示す図である。図７に示すように、波長可変光源５０は、同一基板５２上に設けられた、本実施形態に係る第１及び第２の光導波路５４，５６と、半導体光増幅器（半導体光増幅デバイス）５８と、温度制御手段６１と、を備えている。
【００４５】
第１及び第２の光導波路５４，５６は、積層型の平面導波路であり、図７に示すように、基板１２の上方に設けられた一のコア６０，６２を有している。コア６０，６２は、その延伸方向に沿って光を伝搬させるための導波経路であって、その周囲がコア６０，６２よりも屈折率の低いクラッド６４，６６により覆われている。これらのコア６０，６２は、例えば、石英（ＳｉＯ₂）中にゲルマニウム（Ｇｅ）を添加して形成されており、石英（ＳｉＯ₂）のみからなるクラッド６４，６６に対し高屈折率とされている。
【００４６】
第１及び第２の光導波路５４，５６のコア６０，６２は、コア６０，６２の延伸方向に沿ってコア幅Ｗと屈折率ｎとがステップ状に変更されて、実効屈折率ｎ_effが異なるように設計された複数のコアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄを有している。そして、コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄそれぞれに等しい周期のグレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄが形成されている。
【００４７】
ここで本実施形態に係る光導波路５４，５６では、コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄそれぞれのコア幅Ｗと屈折率ｎとは、グレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄそれぞれのブラック波長λ_Bにおける反射率を実質的に一定にするように設計されている。すなわち、図２に示す関係に従って、各コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄのコア幅Ｗと屈折率ｎとが共に変更されて、グレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄそれぞれのブラック波長λ_Bにおける反射率が実質的に一定になるように設計されている。従って、図３において実線で示すように、グレーティングそれぞれの光の反射率は、利用する波長範囲内におけるブラック波長の変化に対して変化が小さく、実質的に均一化されている。なお、図３において破線で示すように、コア幅Ｗのみを変更して実効屈折率ｎ_effを変化させた場合は、ブラック波長の変化に伴う反射率の変化を抑制することができず、反射率の均一化を図ることができない。
【００４８】
本実施形態に係る光導波路５４では、各コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄに形成されたグレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄにおけるブラッグ波長（光の回折波長）λ_Bは、第１の実施形態で説明したのと同様に（１）式で与えられ、コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄの実効屈折率ｎ_effに依存して変動する。
【００４９】
図４は、図２に示す関係に従って各コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄのコア幅Ｗと屈折率ｎとを共に変更し、コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄそれぞれの実効屈折率ｎ_effを変化させたときの、上記（１）式で求められるブラッグ波長λ_Bとコア幅Wとの関係を示すグラフである。本実施形態に係る光導波路５４，５６では、各コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄでコア幅Ｗと屈折率ｎとが変更されて、コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄそれぞれの実効屈折率ｎ_effが異なるように設計されているため、グレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄそれぞれの反射特性はそれぞれ異なり、異なるブラッグ波長λ_Bの光を反射させる。このコア幅Ｗと屈折率ｎとは、第１の光導波路５４と第２の光導波路５６との間でも変更されている。
【００５０】
従って、第１の光導波路５４の各コアセグメント部６０ａ〜６０ｄに形成されたグレーティング６８ａ〜６８ｄは、図８において実線で示すように、所定温度（例えば室温）において波長λ_1a、λ_2a、λ_3a、λ_4aで反射ピークを持つ。また第２の光導波路５６の各コアセグメント部６２ａ〜６２ｄに形成されたグレーティング７０ａ〜７０ｄは、図８において破線で示すように、所定温度（例えば室温）において波長λ_1b、λ_2b、λ_3b、λ_4bで反射ピークを持つ。なお、波長幅Δλ₁（＝λ_1b−λ_1a）、Δλ₂（＝λ_2b−λ_2a）、Δλ₃（＝λ_3b−λ_3a）、及びΔλ₄（＝λ_4b−λ_4a）は、それぞれ異なるものとされている。
【００５１】
また本実施形態に係る光導波路５４，５６では、コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄそれぞれのコア幅Ｗと屈折率ｎとは、シングルモード条件を満たすように設計されている。ここでシングルモード条件とは、導波路に基本モードしか伝搬しない条件のことであり、導波路のＶパラメータが、１次モードのカットオフＶ値Ｖ_cよりも小さくなる条件をいう。従って、このコアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄ内に形成されたグレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄを半導体レーザの外部共振器として利用した場合、図５のラインＬ１〜Ｌ４に示すように、いずれの発振波長においても発振波長の安定化が図られ、モードホップが生じるおそれが低減される。
【００５２】
半導体光増幅器５８は、光増幅機能を有する半導体活性層（図示しない）を含む。この半導体光増幅器５８の一対の光出射面５８ａ，５８ｂには非反射コート（ＡＲコート）が施されている。この半導体光増幅器５８は、一対の光出射面５８ａ，５８ｂそれぞれが第１及び第２の光導波路５４，５６の光入射面５４ａ，５６ａと対面するように、溝６８内の基板５２上において第１及び第２の光導波路５４，５６の間に配置され、第１及び第２の光導波路５４，５６のコア６０，６２と光学的な結合が図られている。なお、本実施形態に係る波長可変光源５０では、第１の光導波路５４のグレーティング６８ａ〜６８ｄのいずれかと、第２の光導波路５６のグレーティング７０ａ〜７０ｄのいずれかとにより、半導体レーザの共振器が構成されている。
【００５３】
温度制御手段６１は、図７に示すように、第１の光導波路５４上に設けられている。この温度制御手段６１は、第１の光導波路５４が備えるコア６０の温度を制御する。温度制御手段６１としては、例えばペルチエ素子、光導波路のオーバークラッド上に直接形成した薄膜ヒーターなどを用いることができる。この温度制御手段６１を利用してコア６０の温度を変更させることで、コア６０の実効屈折率ｎ_effが変更されるため、各コアセグメント部６０ａ〜６０ｄのグレーティング６８ａ〜６８ｄの光反射特性を変更させることができる。例えば、温度制御手段６１により第１の光導波路５４のコア６０の温度を上昇させることで、図８に示すように、グレーティング６８ａ〜６８ｄの光反射特性を矢印Ａの方向にシフトさせることができる。
【００５４】
次に、上記した構成の波長可変光源５０の製造方法について説明する。
【００５５】
まず、基板（例えばシリコン基板）上に、化学的気相成長法（ＣＶＤ）あるいは火炎加水分解法（ＦＨＤ）により、アンダークラッド層を成膜し、さらにその上にコア層を成膜する。コア層の成膜時には、基板面内でＧｅ添加量の濃度勾配を設け、屈折率の勾配をつける。
【００５６】
次に、フォトリソグラフィ技術と反応性イオンエッチング（ＲＩＥ）とにより、第１及び第２の光導波路それぞれのコアを加工する。このとき、使用するフォトマスクには、予め設計されたコア幅の複数のコアセグメント部のパターンが書き込まれている。そして、これらの上に、ＣＶＤあるいはＦＨＤにより、オーバークラッド層を成膜する。
【００５７】
次に、上記の処理が施された基板上に高圧水素処理を施す。そして、位相格子を介して紫外光を照射し、第１及び第２の光導波路の複数のコアセグメント部それぞれに等しい周期のグレーティングを形成する。このようにしてグレーティングを形成すれば、複数のコアセグメント部に対して紫外光を一度に照射することで、各コアセグメント部に反射特性の異なるグレーティングを一括して形成することができる。従って、各コアセグメント部に対して異なる干渉パターンの紫外光を個々に照射する必要がなくなり、製造の効率化が図られ生産性を向上させることができる。
【００５８】
次に、コアの延伸方向と交差する方向に沿って、基板上の素子搭載領域に相当する部分にエッチングを施し、その領域のコア及びクラッドを取り除いて溝を形成する。そして、その溝内に半導体光増幅器を実装し、半導体光増幅器と第１及び第２の光導波路の各コアとの光学的な結合を図る。その後、第１の光導波路上に温度制御手段を搭載する。
【００５９】
次に、本実施形態に係る光導波路５４，５６、及びこれを備える波長可変光源５０の作用及び効果について説明する。
【００６０】
本実施形態に係る波長可変光源５０では、半導体光増幅器５８を駆動させると、まず第１の光導波路５４のグレーティング６８ａによるブラッグ波長λ_1aと第２の光導波路５６のグレーティング７０ａによるブラッグ波長λ_1bが一致しているため、この波長λ_1bでレーザ発振が起こる。
【００６１】
次に、温度制御手段６１により、第１の光導波路５４のコア６０の温度を上昇させると、図８に示すように、各グレーティング６８ａ〜６８ｄの光反射特性は矢印Ａの方向にシフトする。そして、波長シフト量がΔλ₂となり、第１の光導波路５４のグレーティング６８ｂによるブラッグ波長λ_2aと第２の光導波路５６のグレーティング７０ｂによるブラッグ波長λ_2bとが一致したとき、この波長λ_2bでレーザ発振が起こる。
【００６２】
更に、温度制御手段６１により、第１の光導波路５４のコア６０の温度を上昇させると、各グレーティング６８ａ〜６８ｄの光反射特性は矢印Ａの方向に更にシフトし、波長シフト量がΔλ₃となって、第１の光導波路５４のグレーティング６８ｃによるブラッグ波長λ_3aと第２の光導波路５６のグレーティング７０ｃによるブラッグ波長λ_3aとが一致したとき、この波長λ_3bでレーザ発振が起こり、波長シフト量がΔλ₄となって、第１の光導波路５４のグレーティング６８ｄによるブラッグ波長λ_4aと第２の光導波路５６のグレーティング７０ｄによるブラッグ波長λ_4bとが一致したとき、この波長λ_4bでレーザ発振が起こる。このように、温度制御手段６１により第１の光導波路５４のコア６０の温度を制御することで、図９に示すように、広い波長範囲で発振波長を変更することができる。なお、波長シフト量Δλ₁、Δλ₂、Δλ₃、及びΔλ₄はそれぞれ異なっているため、波長λ_1b、λ_2b、λ_3b、及びλ_4bの光が同時に発振（多モード発振）するおそれはない。
【００６３】
このような構成の波長可変光源５０が備える光導波路５４，５６では、複数のコアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄそれぞれに等しい周期のグレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄが形成されているため、異なる周期のグレーティングが形成されている場合と比べて、その製造が容易になり、生産性の向上を図ることが可能となる。また、コア６０，６２の延伸方向に沿ってコア幅Ｗと屈折率ｎとが変更されて、実効屈折率ｎ_effが異なるように設計されているため、コア幅Ｗのみが変更されて実効屈折率ｎ_effが異なるように設計される場合と比べて、グレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄのブラック波長として利用可能な波長範囲を大きくすることができる。
【００６４】
またこの光導波路５４，５６では、コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄそれぞれのコア幅Ｗと屈折率ｎとは、シングルモード条件を満たすように設計されているため、第１及び第２の光導波路５４，５６のコアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄ内のグレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄを共振器とする半導体レーザからの出射光は、図５に示すように、いずれの発振波長においても発振波長が安定しており、モードホップが生じるおそれを低減することができる。
【００６５】
また、本実施形態に係る光導波路５４，５６では、コアセグメント部６０ａ〜６０ｄ，６２ａ〜６２ｄそれぞれのコア幅Ｗと屈折率ｎとは、グレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄそれぞれのブラック波長における反射率を実質的に一定にするように設計されているため、グレーティング６８ａ〜６８ｄ，７０ａ〜７０ｄそれぞれのブラック波長における光の反射率が実質的に均一化される。その結果、この光導波路５４，５６を備える波長可変光源５０によれば、いずれの発振波長の光でもパワーを略一定にすることが可能となる。
【００６６】
また、上記のような作用効果を奏しうる光導波路５４，５６を用いて、複数の波長の光を選択的に出射可能な波長可変光源５０を、シンプルな構成で好適に構成することができる。特に、石英系光導波路は、実効屈折率ｎ_effの温度依存性が小さいため、ブラッグ波長の温度依存性はｄλ_B／ｄＴ〜０．０１ｎｍ／℃であるが、この波長可変光源５０では上記のようにバーニア効果を利用しているため、変更できる波長の範囲が広い。
【００６７】
なお、本発明は上記した実施形態に限定されることなく、種々の変形が可能である。
【００６８】
例えば、上記した第１実施形態に係る光導波路１４では、図１に示すように並設された四本のコアを備えていたが、コアの数は複数であればその本数が三本以下または五本以上であってもよい。
【００６９】
また、上記した第２実施形態にかかる光導波路５４，５６では、図７に示すように、コアセグメント部の数を四つとしていたが、コアセグメント部の数は複数であればその数が三つ以下または五つ以上であってもよい。
【００７０】
【発明の効果】
本発明によれば、製造が容易で、且つグレーティングのブラック波長として利用可能な波長範囲が大きい光導波路、多波長光源、及び波長可変光源が提供される。
【図面の簡単な説明】
【図１】第１実施形態に係る光導波路を備える多波長光源の構成を示す図である。
【図２】グレーティングそれぞれのブラック波長における反射率を実質的に一定にするためのコア幅と屈折率との関係を示すグラフである。
【図３】実線は、コア幅と屈折率とを変更させたときのブラッグ波長と反射率との関係を示すグラフである。破線は、コア幅のみを変更させたときのブラッグ波長と反射率との関係を示すグラフである。
【図４】図２に示す関係に従って、コア幅と屈折率とを変更させたときのコア幅とブラッグ波長との関係を示すグラフである。
【図５】注入する電流と発振波長との関係を示すグラフである。
【図６】コアの屈折率を種々に変更したときの、コア幅と実効屈折率ｎ_effとの関係を示すグラフである。
【図７】第２実施形態に係る光導波路を備える波長可変光源の構成を示す図である。
【図８】波長と出射される光のパワーとの関係を示すグラフである。
【図９】温度と発振波長との関係を示すグラフである。
【符号の説明】
１０…多波長光源、１４…光導波路、１６…合波器、１８…半導体光増幅器アレイ、２２ａ〜２２ｄ…コア、２６ａ〜２６ｄ…グレーティング、５０…波長可変光源、５４…第１の光導波路、５６…第２の光導波路、５８…半導体光増幅器、６０，６２…コア、６０ａ〜６０ｄ，６２ａ〜６２ｄ…コアセグメント部、６１…温度制御手段、６８ａ〜６８ｄ，７０ａ〜７０ｄ…グレーティング。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical waveguide, a multi-wavelength light source including the same, and a tunable light source.
[0002]
[Prior art]
An optical waveguide including a core on which a grating having desired reflection characteristics is formed is preferably used as an external resonator of a semiconductor laser, for example.
[0003]
As such a conventional optical waveguide, for example, “1997 Annual Conference of the IEICE General Conference, C-3-160, pp. 345,“ Hybrid 4-wavelength laser using UV-induced grating and spot size conversion LD ” Takuya Tanaka et al., 1997. 3 (Reference 1) discloses an optical waveguide in which gratings having different periods are formed on a plurality of cores having the same effective refractive index.
[0004]
Further, for example, “Japanese Patent Laid-Open No. 10-90508” (Document 2) discloses an optical waveguide in which gratings having an equal period are formed on a plurality of cores having different effective refractive indexes.
[0005]
In these optical waveguides, light having different Bragg wavelengths can be selected and used by the grating formed in each of the plurality of cores.
[0006]
[Problems to be solved by the invention]
However, in the optical waveguide disclosed in the above-mentioned document 1, when forming gratings with different periods for each core, masking is performed so that the adjacent cores are not irradiated with ultraviolet light (UV light), and only the number of gratings. The phase grating had to be replaced and UV light irradiation had to be repeated. Therefore, not only the production efficiency is low, but also when hydrogen treatment is performed to increase the formation efficiency of the grating, etc., hydrogen will rapidly escape from the core with time, so that a large number of gratings can be formed well. Was very difficult. In addition, when each core is close to about several hundreds of μm or less, it is very difficult to mask the adjacent core so that the UV light is not irradiated when irradiating one core with UV light. there were.
[0007]
On the other hand, in the optical waveguide disclosed in the above-mentioned literature 2, since it is possible to form gratings having the same period on a plurality of cores at the same time, the manufacturing problem as in the optical waveguide disclosed in literature 1 is Although it can be eliminated, since only the core width (or core thickness) is changed in each of the multiple cores, the change in the effective refractive index of the core is small, and the Bragg wavelength in the grating formed in each of the multiple cores The available wavelength range was small.
[0008]
The present invention has been made to solve the above problems, and is an optical waveguide, a multi-wavelength light source, and a wavelength tunable light source that are easy to manufacture and have a large wavelength range that can be used as the black wavelength of a grating. The purpose is to provide.
[0009]
[Means for Solving the Problems]
  The optical waveguide according to the present invention includes a plurality of cores arranged in parallel, and the core width and the refractive index of each core are changed so that the effective refractive indexes of the cores are different from each other. Periodic grating is formedThe core width and the refractive index of each core are designed so that the optical waveguide satisfies the single mode condition, and the refractive index of each core can be changed by changing the Ge layer when forming the core layer on which each core is processed. It is characterized by providing a concentration gradient of the addition amount.
[0010]
  In this optical waveguide, since the gratings having the same period are formed in each of the plurality of cores, the manufacture thereof becomes easier as compared with the case where the gratings having different periods are formed. Also, because the core width and refractive index of each core are changed, and the effective refractive index of each core is designed to be different, so when only the core width is changed and the effective refractive index is designed to be different Compared to the above, the wavelength range usable as the black wavelength of the grating is large.In addition, the core width and refractive index of each core are designed so that the optical waveguide satisfies the single mode condition. For example, when the optical waveguide is used as an external resonator of a semiconductor laser, the oscillation wavelength is stabilized. As a result, the possibility of mode hops can be reduced.
[0012]
The optical waveguide according to the present invention may be characterized in that the core width and refractive index of each core are designed so that the reflectance at the black wavelength of each grating is substantially constant. In this way, the reflectance of light at the black wavelength of each grating is substantially uniformized.
[0013]
  The optical waveguide according to the present invention includes a single core, and the core has a plurality of core segments designed to have different effective refractive indexes by changing the core width and the refractive index along the extending direction of the core. Gratings with the same period are formed in each core segment part.The core width and refractive index of each core segment part are designed so that the optical waveguide satisfies the single mode condition, and the refractive index of each core segment part is changed by the core layer processed by each core segment part. This is done by providing a concentration gradient of the Ge addition amount during film formation.It is characterized by.

[0014]
  In this optical waveguide, since the gratings having the same period are formed in each of the plurality of core segment portions, the manufacture thereof becomes easier as compared with the case where the gratings having different periods are formed. In addition, since the core width and the refractive index are changed along the core extending direction so that the effective refractive index is different, only the core width is changed and the effective refractive index is different. Compared to the case, the usable wavelength range as the black wavelength of the grating is large.In addition, the core width and refractive index of each core are designed so that the optical waveguide satisfies the single mode condition. For example, when the optical waveguide is used as an external resonator of a semiconductor laser, the oscillation wavelength is stabilized. As a result, the possibility of mode hops can be reduced.
[0016]
In the optical waveguide according to the present invention, the core width and the refractive index of each of the core segment portions may be designed such that the reflectance at the black wavelength of each grating is substantially constant. In this way, the reflectance of light at the black wavelength of each grating is substantially uniformized.
[0017]
A multi-wavelength light source according to the present invention combines the above-described optical waveguide, a semiconductor optical amplification device optically coupled to a plurality of cores provided in the optical waveguide, and light propagating in the plurality of cores provided in the optical waveguide. And a multiplexer.
[0018]
Thus, a multi-wavelength light source capable of emitting light including light of a plurality of wavelengths can be suitably configured using the optical waveguide according to the present invention.
[0019]
The wavelength tunable light source according to the present invention includes the first and second optical waveguides, the semiconductor optical amplification device provided between the first and second optical waveguides, and the temperature of the core included in the first optical waveguide. Temperature control means for controlling the temperature. As described above, a tunable light source capable of selectively emitting light of a plurality of wavelengths can be suitably configured using the optical waveguide according to the present invention.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.
(First embodiment)
FIG. 1 is a diagram illustrating a configuration of a multi-wavelength light source according to the present embodiment. As shown in FIG. 1, the multi-wavelength light source 10 includes an optical waveguide 14, a multiplexer 16, a semiconductor optical amplifier array (semiconductor optical amplification device) 18, and the like, provided on the same substrate 12. And an optical receiver array 20 for monitoring.
[0021]
The optical waveguide 14 is a laminated planar waveguide, and has a plurality of cores 22a to 22d arranged in parallel above the substrate 12, as shown in FIG. The cores 22a to 22d are waveguide paths for propagating light along the extending direction, and the periphery thereof is covered with a clad 24 having a refractive index lower than that of the cores 22a to 22d. These cores 22a to 22d are made of, for example, quartz (SiO₂) In which germanium (Ge) is added, and quartz (SiO₂) Is a high refractive index.
[0022]
In these cores 22a to 22d, the core width W and the refractive index n are changed in each core, and the effective refractive index n of each of the cores 22a to 22d is changed._effAre designed to be different. The gratings 26a to 26d having the same period are formed in the cores 22a to 22d, respectively. These gratings 26 a to 26 d are provided on the inner side by a predetermined distance from the incident end 14 a side of the optical waveguide 14.
[0023]
Here, in the optical waveguide 14 according to the present embodiment, the core width W and the refractive index n of each of the cores 22a to 22d are designed to make the reflectance at the black wavelength of each of the gratings 26a to 26d substantially constant. ing. That is, according to the relationship shown in FIG. 2, the core width W and the refractive index n of each of the cores 22a to 22d are changed, and the black wavelengths λ of the gratings 26a to 26d are changed._BIt is designed so that the reflectance at is substantially constant. Therefore, as indicated by a solid line in FIG. 3, the reflectance of each of the gratings 26a to 26d is determined by the black wavelength λ within the wavelength range to be used._BThe change is small with respect to the change, and is substantially uniform. As shown by a broken line in FIG. 3, only the core width W is changed to change the effective refractive index n._effIs changed, the black wavelength λ_BIt is not possible to suppress the change in reflectivity due to the change in the thickness and to make the reflectivity uniform.
[0024]
In the optical waveguide 10 according to the present embodiment, the Bragg wavelength (light diffraction wavelength) λ in the gratings 26a to 26d formed in the cores 22a to 22d._BIs
λ_B= 2 · n_eff・ Λ (1)
Given in.
[0025]
Where n_effRepresents the effective refractive index in the grating, and Λ represents the period (formation pitch) of the grating. As shown in the above equation (1), the Bragg wavelength λ of the grating_BIs the effective refractive index n of the core_effFluctuates depending on.
[0026]
4 changes both the core width W and the refractive index n of each of the cores 22a to 22d according to the relationship shown in FIG. 2, and the effective refractive index n of each of the cores 22a to 22d._effBragg wavelength λ obtained by the above equation (1) when_BAnd a core width W. In the optical waveguide 10 according to the present embodiment, the core width W and the refractive index n are changed in each of the cores 22a to 22d, and the effective refractive index n of each of the cores 22a to 22d._effSince the gratings 26a to 26d have different reflection characteristics, the Bragg wavelengths λ are different._BTo reflect the light.
[0027]
Further, in the optical waveguide 10 according to the present embodiment, the core width W and the refractive index n of each of the cores 22a to 22d are designed so as to satisfy the single mode condition. Here, the single mode condition is a condition in which only the fundamental mode propagates in the waveguide, and the V parameter of the waveguide is the cutoff V value V of the primary mode._cWhich is smaller than The V parameter of the waveguide is expressed by the following equation.

Here, a = w / 2 (core half width) and Δ = n + n_cladIt is. n_cladIs the refractive index of the cladding.
[0028]
Therefore, when the gratings 26a to 26d formed in the cores 22a to 22d are used as external resonators of the semiconductor laser, as shown by lines L1 to L4 in FIG. The risk of mode hops is reduced.
[0029]
The multiplexer 16 is continuously provided on the emission end 14 b side of the optical waveguide 10. The multiplexer 16 has a multiplexing circuit including a common waveguide 28 and a plurality of branch paths 30 a to 30 d branched from the common waveguide 28. In the multiplexer 16, a plurality of branch paths 30 a to 30 d are optically coupled to a plurality of cores 22 a to 22 d of the optical waveguide 14, and light propagating in the cores 22 a to 22 d is multiplexed. It propagates in the common waveguide 28.
[0030]
The semiconductor optical amplifier array 18 is an array of semiconductor optical amplifiers including a semiconductor active layer having an optical amplification function. The light emitting surface 18a of the semiconductor optical amplifier array 18 is provided with a non-reflective coating (AR coating), and the light reflecting surface 18b opposite to the light emitting surface 18a is provided with a reflective coating. The semiconductor optical amplifier array 18 is disposed on the substrate 12 in the groove 32 so that the light emitting surface 18a faces the light incident surface 14a of the optical waveguide 14, and optical coupling with each of the plurality of cores 22a to 22d is performed. It is illustrated. In the multi-wavelength light source 10 according to the present embodiment, a semiconductor laser resonator is formed by the light reflecting surface 18b of the semiconductor optical amplifier array 18 and the gratings 26a to 26d provided on the cores 22a to 22d of the optical waveguide 14. It is configured.
[0031]
The light receiving array 20 for monitoring is provided with light receiving devices such as photodiodes in an array. The light receiver array 20 receives and monitors the light emitted from the light reflecting surface 18b of the semiconductor optical amplifier array 18. The light receiver array 20 is disposed on the substrate 12 in the groove 32 so that the light receiving surface 20a faces the light reflecting surface 18b of the semiconductor optical amplifier array 18, and is optically coupled to the semiconductor optical amplifier array 18. ing.
[0032]
Next, a manufacturing method of the multi-wavelength light source having the above configuration will be described.
[0033]
First, an under cladding layer is formed on a substrate (for example, a silicon substrate) by chemical vapor deposition (CVD) or flame hydrolysis (FHD), and a core layer is further formed thereon. At the time of forming the core layer, a concentration gradient of Ge addition amount is provided in the substrate surface to give a gradient of refractive index. In the FHD method, the concentration gradient of the Ge addition amount is determined by the burner traverse and GeCl during soot deposition._FourThis can be realized by adjusting the relationship with the input amount or by adding a gradient to the substrate temperature during soot deposition. The CVD method can be realized by placing a portion where a concentration gradient of the Ge addition amount is desired at the time of forming the core layer at a position away from the center of the chamber.
[0034]
Next, the plurality of cores and the combined circuit are processed by photolithography and reactive ion etching (RIE). At this time, a plurality of cores having a previously designed core width and a pattern of the multiplexing circuit are written in the photomask to be used. Then, an over clad layer is formed on these by CVD or FHD.
[0035]
Next, high-pressure hydrogen treatment is performed on the substrate subjected to the above-described treatment. Then, ultraviolet light is irradiated through the phase grating to form a grating having an equal period in each of the plurality of cores. A light source that emits ultraviolet light that increases the refractive index when irradiated onto the core is used. For example, an excimer laser that emits ultraviolet light that reacts with the core additive (here, Ge) may be used. A phase grating having a uniform pattern period is used. If the grating is formed in this way, a plurality of gratings having different reflection characteristics can be collectively formed on each core by irradiating a plurality of cores arranged side by side with ultraviolet light. Therefore, it is not necessary to individually irradiate each core with ultraviolet light having a different interference pattern, and the production efficiency can be improved and the productivity can be improved.
[0036]
Next, etching is performed on a portion corresponding to the element mounting region on the substrate along a direction intersecting the extending direction of the plurality of cores, and the core and the clad in that region are removed to form a groove. Then, a semiconductor optical amplifier array and a light receiver array are mounted in the groove, and an optical coupling between the semiconductor optical amplifier array and the optical waveguide and between the semiconductor optical amplifier array and the light receiver array is achieved.
[0037]
Next, operations and effects of the optical waveguide 14 according to the present embodiment and the multi-wavelength light source 10 including the same will be described.
[0038]
In the multi-wavelength light source 10 according to the present embodiment, when the semiconductor optical amplifier array 18 is driven, the gratings 26a to 26d formed on the light reflecting surface 18b of the semiconductor optical amplifier array 18 and the plurality of cores 22a to 22d of the optical waveguide 14. And a predetermined Bragg wavelength λ_BLaser oscillation occurs. Lights oscillated at different Bragg wavelengths propagate through the cores 22 a to 22 d of the optical waveguide 14, are multiplexed by the multiplexer 16, and are emitted from the emission end 16 a side of the multiplexer 16. The emitted optical power can be adjusted by monitoring with the light receiver array 20.
[0039]
In the optical waveguide 14 provided in the multi-wavelength light source 10 having such a configuration, since the gratings 26a to 26d having the same period are formed in the plurality of cores 22a to 22d, compared with the case where the gratings having different periods are formed. As a result, the manufacturing becomes easy and the productivity can be improved. Further, the core width W and the refractive index n of each of the cores 22a to 22d are changed, and the effective refractive index n of each of the cores 22a to 22d is changed._effAre designed to be different, only the core width W is changed and the effective refractive index n_effCompared to the case where the gratings 26a to 26d are designed to be different from each other,_BThe usable wavelength range can be increased.
[0040]
FIG. 6 shows the core width W and the effective refractive index n when the refractive index n of the core is variously changed._effIt is a graph which shows the relationship. In FIG. 6, the circle indicates the refractive index of the core is 1.445, the square indicates the refractive index of the core is 1.45, the triangle indicates the refractive index of the core is 1.455, and the star indicates the refractive index of the core. Each time is 1.46. As shown in FIG. 6, the effective refractive index n increases as the core width W of the core increases._effIs larger, but its fluctuation is small. On the other hand, if not only the core width W but also the refractive index n of the core is increased, the effective refractive index n_effThus, the fluctuation range of can be increased, and the wavelength range that can be used as the black wavelength of the grating can be increased.
[0041]
Further, in this optical waveguide 14, the core width W and the refractive index n of each of the cores 22a to 22d are designed to satisfy the single mode condition, so that the light reflecting surface 18b and the core 22a of the semiconductor optical amplifier array 18 are designed. As shown in FIG. 5, the emitted light from the semiconductor laser having the gratings 26a to 26d in .about.22d as a resonator has a stable oscillation wavelength at any oscillation wavelength and reduces the possibility of mode hops. can do.
[0042]
In this optical waveguide 14, the core width W and the refractive index n of each of the cores 22a to 22d are equal to the black wavelength λ of each of the gratings 26a to 26d._BIs designed to make the reflectance at substantially constant, the black wavelength λ of each of the gratings 26a to 26d._BThe light reflectivity at is substantially uniform. As a result, it is possible to make the light emission power and light injection power dependency almost constant.
[0043]
In addition, the multiwavelength light source 10 capable of emitting light including light of a plurality of wavelengths can be suitably configured with a simple configuration using the optical waveguide 14 that can exhibit the above-described effects.
(Second Embodiment)
Next, a second embodiment of the present invention will be described. 2 to 6 can also be used in the second embodiment, and will be described with reference to these figures.
[0044]
FIG. 7 is a diagram illustrating a configuration of the wavelength tunable light source 50 according to the present embodiment. As shown in FIG. 7, the wavelength tunable light source 50 includes first and second optical waveguides 54 and 56 and a semiconductor optical amplifier (semiconductor optical amplification device) 58 according to the present embodiment provided on the same substrate 52. And a temperature control means 61.
[0045]
The first and second optical waveguides 54 and 56 are stacked planar waveguides, and have one core 60 and 62 provided above the substrate 12 as shown in FIG. The cores 60 and 62 are waveguide paths for propagating light along the extending direction, and the periphery thereof is covered with claddings 64 and 66 having a refractive index lower than that of the cores 60 and 62. These cores 60 and 62 are made of, for example, quartz (SiO₂) In which germanium (Ge) is added, and quartz (SiO₂) Is a high refractive index.
[0046]
In the cores 60 and 62 of the first and second optical waveguides 54 and 56, the core width W and the refractive index n are changed stepwise along the extending direction of the cores 60 and 62, so that the effective refractive index n_effHave a plurality of core segment portions 60a to 60d and 62a to 62d designed to be different from each other. The gratings 68a to 68d and 70a to 70d having the same period are formed in the core segment portions 60a to 60d and 62a to 62d, respectively.
[0047]
Here, in the optical waveguides 54 and 56 according to the present embodiment, the core width W and the refractive index n of the core segment portions 60a to 60d and 62a to 62d are the black wavelengths λ of the gratings 68a to 68d and 70a to 70d, respectively._BIt is designed to make the reflectance at substantially constant. That is, according to the relationship shown in FIG. 2, the core width W and the refractive index n of each of the core segment portions 60a to 60d and 62a to 62d are changed, and the black wavelengths λ of the gratings 68a to 68d and 70a to 70d are changed._BIt is designed so that the reflectance at is substantially constant. Therefore, as indicated by the solid line in FIG. 3, the reflectance of the light of each grating is substantially uniform with little change with respect to the change of the black wavelength within the wavelength range to be used. As shown by a broken line in FIG. 3, only the core width W is changed to change the effective refractive index n._effIs changed, the change in reflectance due to the change in the black wavelength cannot be suppressed, and the reflectance cannot be made uniform.
[0048]
In the optical waveguide 54 according to the present embodiment, Bragg wavelengths (light diffraction wavelengths) λ in the gratings 68a to 68d and 70a to 70d formed in the core segment portions 60a to 60d and 62a to 62d, respectively._BIs given by the expression (1) as described in the first embodiment, and the effective refractive index n of the core segment portions 60a to 60d and 62a to 62d._effFluctuates depending on.
[0049]
4 changes both the core width W and the refractive index n of each of the core segment portions 60a to 60d and 62a to 62d according to the relationship shown in FIG. 2, and the effective refractive index of each of the core segment portions 60a to 60d and 62a to 62d. n_effBragg wavelength λ obtained by the above equation (1) when_B5 is a graph showing the relationship between the core width W and the core width W. In the optical waveguides 54 and 56 according to the present embodiment, the core width W and the refractive index n are changed in each of the core segment portions 60a to 60d and 62a to 62d, and the effective performance of each of the core segment portions 60a to 60d and 62a to 62d. Refractive index n_effSince the gratings 68a to 68d and 70a to 70d have different reflection characteristics, the Bragg wavelengths λ are different._BTo reflect the light. The core width W and the refractive index n are also changed between the first optical waveguide 54 and the second optical waveguide 56.
[0050]
Accordingly, the gratings 68a to 68d formed in the core segment portions 60a to 60d of the first optical waveguide 54 have a wavelength λ at a predetermined temperature (for example, room temperature) as shown by a solid line in FIG._1a, Λ_2a, Λ_3a, Λ_4aWith a reflection peak. Further, the gratings 70a to 70d formed in the core segment portions 62a to 62d of the second optical waveguide 56 have a wavelength λ at a predetermined temperature (for example, room temperature) as shown by a broken line in FIG._1b, Λ_2b, Λ_3b, Λ_4bWith a reflection peak. The wavelength width Δλ₁(= Λ_1b−λ_1a), Δλ₂(= Λ_2b−λ_2a), Δλ_Three(= Λ_3b−λ_3a), And Δλ_Four(= Λ_4b−λ_4a) Are different from each other.
[0051]
In the optical waveguides 54 and 56 according to the present embodiment, the core width W and the refractive index n of each of the core segment portions 60a to 60d and 62a to 62d are designed so as to satisfy the single mode condition. Here, the single mode condition is a condition in which only the fundamental mode propagates in the waveguide, and the V parameter of the waveguide is the cutoff V value V of the primary mode._cWhich is smaller than Accordingly, when the gratings 68a to 68d and 70a to 70d formed in the core segment portions 60a to 60d and 62a to 62d are used as external resonators of the semiconductor laser, as shown by lines L1 to L4 in FIG. At any oscillation wavelength, the oscillation wavelength is stabilized and the possibility of mode hops is reduced.
[0052]
The semiconductor optical amplifier 58 includes a semiconductor active layer (not shown) having an optical amplification function. A non-reflective coating (AR coating) is applied to the pair of light emitting surfaces 58 a and 58 b of the semiconductor optical amplifier 58. The semiconductor optical amplifier 58 includes a first light emitting surface 58a and a second light emitting surface 58b on the substrate 52 in the groove 68 so that the light incident surfaces 54a and 56a of the first and second optical waveguides 54 and 56 face each other. It is disposed between the first and second optical waveguides 54 and 56 and is optically coupled to the cores 60 and 62 of the first and second optical waveguides 54 and 56. In the wavelength tunable light source 50 according to the present embodiment, a semiconductor laser resonator is formed by any of the gratings 68a to 68d of the first optical waveguide 54 and any of the gratings 70a to 70d of the second optical waveguide 56. It is configured.
[0053]
The temperature control means 61 is provided on the first optical waveguide 54 as shown in FIG. The temperature control means 61 controls the temperature of the core 60 provided in the first optical waveguide 54. As the temperature control means 61, for example, a Peltier element, a thin film heater formed directly on the overclad of the optical waveguide, or the like can be used. The effective refractive index n of the core 60 is changed by changing the temperature of the core 60 using the temperature control means 61._effTherefore, the light reflection characteristics of the gratings 68a to 68d of the core segment portions 60a to 60d can be changed. For example, by increasing the temperature of the core 60 of the first optical waveguide 54 by the temperature control means 61, the light reflection characteristics of the gratings 68a to 68d can be shifted in the direction of arrow A as shown in FIG. .
[0054]
Next, a manufacturing method of the wavelength tunable light source 50 having the above configuration will be described.
[0055]
First, an under cladding layer is formed on a substrate (for example, a silicon substrate) by chemical vapor deposition (CVD) or flame hydrolysis (FHD), and a core layer is further formed thereon. At the time of forming the core layer, a concentration gradient of Ge addition amount is provided in the substrate surface to give a gradient of refractive index.
[0056]
Next, the cores of the first and second optical waveguides are processed by photolithography and reactive ion etching (RIE). At this time, a pattern of a plurality of core segment portions having a core width designed in advance is written on the photomask to be used. Then, an over clad layer is formed on these by CVD or FHD.
[0057]
Next, high-pressure hydrogen treatment is performed on the substrate subjected to the above-described treatment. Then, ultraviolet light is irradiated through the phase grating to form a grating having an equal period in each of the plurality of core segment portions of the first and second optical waveguides. If the grating is formed in this manner, a plurality of gratings having different reflection characteristics can be collectively formed on each core segment portion by irradiating the plurality of core segment portions with ultraviolet light at a time. Therefore, it is not necessary to individually irradiate each core segment portion with ultraviolet light having a different interference pattern, and the manufacturing efficiency can be improved and the productivity can be improved.
[0058]
Next, etching is performed on a portion corresponding to the element mounting region on the substrate along the direction intersecting the extending direction of the core, and the core and the clad in that region are removed to form a groove. Then, a semiconductor optical amplifier is mounted in the groove to achieve optical coupling between the semiconductor optical amplifier and each core of the first and second optical waveguides. Thereafter, temperature control means is mounted on the first optical waveguide.
[0059]
Next, operations and effects of the optical waveguides 54 and 56 according to the present embodiment and the wavelength tunable light source 50 including the same will be described.
[0060]
In the wavelength tunable light source 50 according to the present embodiment, when the semiconductor optical amplifier 58 is driven, first, the Bragg wavelength λ by the grating 68a of the first optical waveguide 54 is used._1aAnd the Bragg wavelength λ by the grating 70a of the second optical waveguide 56_1bSince this is the same, this wavelength λ_1bLaser oscillation occurs.
[0061]
Next, when the temperature of the core 60 of the first optical waveguide 54 is raised by the temperature control means 61, the light reflection characteristics of the gratings 68a to 68d are shifted in the direction of arrow A as shown in FIG. And the wavelength shift amount is Δλ₂And the Bragg wavelength λ by the grating 68b of the first optical waveguide 54_2aAnd the Bragg wavelength λ by the grating 70b of the second optical waveguide 56_2bThis wavelength λ_2bLaser oscillation occurs.
[0062]
Further, when the temperature of the core 60 of the first optical waveguide 54 is increased by the temperature control means 61, the light reflection characteristics of the gratings 68a to 68d are further shifted in the direction of arrow A, and the wavelength shift amount is Δλ._ThreeBecomes the Bragg wavelength λ by the grating 68c of the first optical waveguide 54._3aAnd the Bragg wavelength λ by the grating 70c of the second optical waveguide 56_3aThis wavelength λ_3bLaser oscillation occurs, and the wavelength shift amount is Δλ_FourAnd the Bragg wavelength λ by the grating 68d of the first optical waveguide 54_4aAnd the Bragg wavelength λ by the grating 70d of the second optical waveguide 56_4bThis wavelength λ_4bLaser oscillation occurs. Thus, by controlling the temperature of the core 60 of the first optical waveguide 54 by the temperature control means 61, the oscillation wavelength can be changed in a wide wavelength range as shown in FIG. The wavelength shift amount Δλ₁, Δλ₂, Δλ_Three, And Δλ_FourAre different, so the wavelength λ_1b, Λ_2b, Λ_3b, And λ_4bThere is no possibility that the light beams oscillate simultaneously (multimode oscillation).
[0063]
In the optical waveguides 54 and 56 provided in the wavelength tunable light source 50 having such a configuration, the gratings 68a to 68d and 70a to 70d having the same period are formed in the plurality of core segment portions 60a to 60d and 62a to 62d, respectively. Compared with the case where gratings with different periods are formed, the manufacture becomes easier and the productivity can be improved. Further, the core width W and the refractive index n are changed along the extending direction of the cores 60 and 62 so that the effective refractive index n_effAre designed to be different, only the core width W is changed and the effective refractive index n_effAs compared with the case where the gratings are designed to be different from each other, the wavelength range that can be used as the black wavelengths of the gratings 68a to 68d and 70a to 70d can be increased.
[0064]
In the optical waveguides 54 and 56, the core width W and the refractive index n of each of the core segment portions 60a to 60d and 62a to 62d are designed so as to satisfy the single mode condition. As shown in FIG. 5, the emission light from the semiconductor laser having the gratings 68a to 68d and 70a to 70d in the core segment portions 60a to 60d and 62a to 62d of the optical waveguides 54 and 56 as the resonator The oscillation wavelength is stable and the possibility of mode hops can be reduced.
[0065]
In the optical waveguides 54 and 56 according to the present embodiment, the core width W and the refractive index n of the core segment portions 60a to 60d and 62a to 62d are reflected at the black wavelengths of the gratings 68a to 68d and 70a to 70d, respectively. Since the ratio is designed to be substantially constant, the reflectances of light at the black wavelengths of the gratings 68a to 68d and 70a to 70d are substantially uniformized. As a result, according to the wavelength tunable light source 50 including the optical waveguides 54 and 56, it becomes possible to make the power substantially constant for light of any oscillation wavelength.
[0066]
Further, the variable wavelength light source 50 capable of selectively emitting light of a plurality of wavelengths can be suitably configured with a simple configuration by using the optical waveguides 54 and 56 capable of exhibiting the above-described effects. In particular, a quartz-based optical waveguide has an effective refractive index n_effThe temperature dependence of the Bragg wavelength is dλ_B/ DT to 0.01 nm / ° C. However, since the tunable light source 50 uses the vernier effect as described above, the range of wavelengths that can be changed is wide.
[0067]
The present invention is not limited to the above-described embodiment, and various modifications can be made.
[0068]
For example, the optical waveguide 14 according to the first embodiment described above includes four cores arranged side by side as shown in FIG. 1, but if the number of cores is plural, the number is three or less or Five or more may be sufficient.
[0069]
In the optical waveguides 54 and 56 according to the second embodiment described above, the number of core segment portions is four as shown in FIG. 7, but if the number of core segment portions is plural, the number is three. It may be less than or five or more.
[0070]
【The invention's effect】
According to the present invention, an optical waveguide, a multi-wavelength light source, and a wavelength tunable light source that are easy to manufacture and have a large wavelength range that can be used as a black wavelength of a grating are provided.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of a multi-wavelength light source including an optical waveguide according to a first embodiment.
FIG. 2 is a graph showing the relationship between the core width and the refractive index for making the reflectance at the black wavelength of each grating substantially constant.
FIG. 3 is a graph showing the relationship between the Bragg wavelength and the reflectance when the core width and the refractive index are changed. The broken line is a graph showing the relationship between the Bragg wavelength and the reflectance when only the core width is changed.
4 is a graph showing the relationship between the core width and the Bragg wavelength when the core width and the refractive index are changed according to the relationship shown in FIG.
FIG. 5 is a graph showing the relationship between injected current and oscillation wavelength.
FIG. 6 shows the core width and effective refractive index n when the refractive index of the core is variously changed._effIt is a graph which shows the relationship.
FIG. 7 is a diagram illustrating a configuration of a wavelength tunable light source including an optical waveguide according to a second embodiment.
FIG. 8 is a graph showing the relationship between the wavelength and the power of emitted light.
FIG. 9 is a graph showing the relationship between temperature and oscillation wavelength.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Multi-wavelength light source, 14 ... Optical waveguide, 16 ... Multiplexer, 18 ... Semiconductor optical amplifier array, 22a-22d ... Core, 26a-26d ... Grating, 50 ... Variable wavelength light source, 54 ... 1st optical waveguide, 56 ... second optical waveguide, 58 ... semiconductor optical amplifier, 60,62 ... core, 60a-60d, 62a-62d ... core segment, 61 ... temperature control means, 68a-68d, 70a-70d ... grating.

Claims (6)

With multiple cores,
The core width and refractive index of each of the cores are changed, and the effective refractive index of each of the cores is designed to be different,
A grating having an equal period is formed in each of the cores ,
The core width and refractive index of each of the cores are designed so that the optical waveguide satisfies the single mode condition,
The optical waveguide, wherein the refractive index of each of the cores is changed by providing a concentration gradient of Ge addition amount when forming a core layer for processing each of the cores .   The optical waveguide according to claim 1, wherein the core width and the refractive index of each of the cores are designed so that the reflectance at the black wavelength of each of the gratings is substantially constant. One core,
The core has a plurality of core segment portions designed such that the core width and the refractive index are changed along the extending direction of the core to change the effective refractive index,
A grating having an equal period is formed in each of the core segment parts ,
The core width and refractive index of each of the core segment parts are designed so that the optical waveguide satisfies the single mode condition,
The optical waveguide is characterized in that the refractive index of each of the core segment portions is changed by providing a concentration gradient of Ge addition amount when forming a core layer in which each of the core segment portions is processed . 4. The optical waveguide according to claim 3 , wherein the core width and the refractive index of each of the core segment portions are designed so that the reflectance at the black wavelength of each of the gratings is substantially constant. An optical waveguide according to claim 1;
A semiconductor optical amplification device optically coupled to the plurality of cores provided in the optical waveguide;
A multiplexer that multiplexes light propagating in the plurality of cores included in the optical waveguide;
A multi-wavelength light source comprising: First and second optical waveguides according to claim 3 ,
A semiconductor optical amplifying device provided between the first and second optical waveguides;
Temperature control means for controlling the temperature of the core provided in the first optical waveguide;
A tunable light source comprising:

Images