JP3996815B2

JP3996815B2 - Optical frequency synthesizer

Info

Publication number: JP3996815B2
Application number: JP2002240542A
Authority: JP
Inventors: 悦史山崎; 敏夫盛岡; 篤高田
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 2002-08-21
Filing date: 2002-08-21
Publication date: 2007-10-24
Anticipated expiration: 2022-08-21
Also published as: JP2004077979A

Description

【０００１】
【発明の属する技術分野】
本発明は、高確度に光周波数が制御された光を発生する光周波数シンセサイザに関する。
【０００２】
【従来の技術】
従来の光周波数可変光源は、半導体レーザ、グレーティング、ミラーがリットマン型に配置された構造を有する外部共振器型半導体レーザが主流であった。本構成では、ミラーの角度により発振光周波数を制御するので、発振光周波数の確度および安定性は低かった。
【０００３】
そこで、光源の発振光周波数を高確度かつ安定に制御するために、高確度かつ安定に光周波数が制御された光周波数参照光を用意し、その光周波数と光周波数可変光源の光周波数とを比較し、両者の差（ビート）が一定になるように光周波数可変光源の発振光周波数を制御する方法が提案されている。この方法では、光周波数参照光と同程度の光周波数確度と安定性が得られる。
【０００４】
【発明が解決しようとする課題】
ところで、光周波数可変光源の発振光周波数可変帯域は、ビートを検出する光検出手段の帯域によって制限されるので、光周波数参照光の光周波数近辺の数十ＧHz内にとどまる。
【０００５】
一方、大容量光通信において利用される帯域が数十ＴHzと広帯域化している現状では、上記の数十ＧHz程度の光周波数可変帯域は不十分なものである。
【０００６】
本発明は、数十ＴHzにわたる帯域において高確度かつ安定に光周波数が制御された光を発生することができる光周波数シンセサイザを提供することを目的とする。
【０００７】
【課題を解決するための手段】
第１の発明の光周波数シンセサイザは、発振光周波数が制御可能な光周波数可変光源と、基準光周波数ν₀の基準モードを中心に縦モード間隔ｆ_mのマルチモード光（光コム）を出力するマルチモード基準光源と、光周波数可変光源の出力光を２分岐し、その一方を光周波数シンセサイザの出力光として外部に出力する光分岐手段と、光分岐手段で分岐された光周波数可変光源の出力光とマルチモード基準光源の出力光を合波する光合波手段と、光合波手段の出力光を電気信号に変換し、光周波数可変光源の出力光とマルチモード基準光源の出力光（１つの縦モード）のビート光に対応するビート電気信号を出力する光検出手段と、ビート電気信号を入力し、光周波数可変光源の発振光周波数を測定し、その測定値ν_Xが目標光周波数ν_X0になるように、光周波数可変光源の発振光周波数を制御する光周波数制御部とを備え、光周波数制御部は、ビート電気信号を２分岐する電気信号分岐手段と、その一方のビート電気信号を入力して光周波数可変光源の発振光周波数ν _X を測定する光周波数計数回路と、測定値ν _X が目標光周波数ν _X0 になるように光周波数可変光源の発振光周波数を制御する制御信号を出力する光周波数制御信号生成部と、マルチモード基準光源のある１つの縦モードの光周波数ν _n と目標光周波数ν _X0 の差周波数｜ν _n −ν _X0 ｜の電気信号を出力する周波数シンセサイザと、電気信号分岐手段で分岐された他方のビート電気信号と周波数シンセサイザの出力電気信号をミキシングする電気信号ミキサと、電気信号ミキサの出力信号から低周波数成分を選択的に透過して光周波数可変光源に与える低域通過フィルタとを含む（請求項１）。
【００１０】
第２の発明の光周波数シンセサイザは、発振光周波数が制御可能な光周波数可変光源と、基準光周波数ν ₀ の基準モードを中心に縦モード間隔ｆ _m のマルチモード光（光コム）を出力するマルチモード基準光源と、光周波数可変光源の出力光を２分岐し、その一方を光周波数シンセサイザの出力光として外部に出力する光分岐手段と、光分岐手段で分岐された光周波数可変光源の出力光とマルチモード基準光源の出力光を合波する光合波手段と、光合波手段の出力光を電気信号に変換し、光周波数可変光源の出力光とマルチモード基準光源の出力光（１つの縦モード）のビート光に対応するビート電気信号を出力する光検出手段と、ビート電気信号を入力し、光周波数可変光源の発振光周波数を測定し、その測定値ν _X が目標光周波数ν _X0 になるように、光周波数可変光源の発振光周波数を制御する光周波数制御部とを備え、光周波数制御部は、ビート電気信号を２分岐する電気信号分岐手段と、その一方のビート電気信号を入力して光周波数可変光源の発振光周波数ν_Xを測定する光周波数計数回路と、出力周波数を制御できる周波数シンセサイザと、光周波数計数回路の測定値ν_Xと目標光周波数ν_X0を比較し、両者の差周波数｜ν_X0−ν_X｜が０まで減少するように周波数シンセサイザの出力周波数を掃引する周波数シンセサイザ出力周波数制御回路と、電気信号分岐手段で分岐された他方のビート電気信号と周波数シンセサイザの出力電気信号をミキシングする電気信号ミキサと、電気信号ミキサの出力信号から低周波数成分を選択的に透過して光周波数可変光源に与える低域通過フィルタとにより位相同期ループを構成し、光周波数可変光源の発振光周波数をオフセットロッキングする構成である（請求項２）。
【００１１】
光周波数制御部に用いる光周波数計数回路は、マルチモード基準光源の縦モード間隔ｆ_mを変化させ、光周波数可変光源の出力光の光周波数をシフトさせる。そして、マルチモード基準光源の縦モード間隔をｆ_mからｆ_m＋Δｆ_mに変化させたときに、縦モード間隔の変化分Δｆ_m（符号を含む）と、ビート電気信号の周波数ｆの変化分Δｆ（符号を含む）との比ｎ＝Δｆ／Δｆ_m（符号を含む）を計数する手段と、光周波数可変光源の出力光の光周波数ν_Xをシフトさせたときに、ビート電気信号の周波数ｆの変化分との比の符号Ｓ_T（正ならば１、負ならば−１）を判定する手段と、基準光周波数ν₀と、ｆ_mと、ｎと、Ｓ_Tに基づいて、光周波数可変光源の出力光の光周波数ν_Xを
ν_X＝ν₀−Ｓ_Tｎｆ_m＋Ｓ_Tｆ
により算出する手段とを含む（請求項３）。
【００１２】
また、別な光周波数計数回路は、マルチモード基準光源の縦モード間隔ｆ_mを変化させ、マルチモード基準光源の出力光の光周波数をシフトさせる。そして、マルチモード基準光源の縦モード間隔をｆ_mからｆ_m＋Δｆ_mに変化させたときに、縦モード間隔の変化分Δｆ_m（符号を含む）と、ビート電気信号の周波数ｆの変化分Δｆ（符号を含む）との比ｎ＝Δｆ／Δｆ_m（符号を含む）を計数する手段と、マルチモード基準光源の出力光の光周波数をシフトさせたときに、ビート電気信号の周波数ｆの変化分との比の符号Ｓ_M（正ならば１、負ならば−１）を判定する手段と、基準光周波数ν₀と、ｆ_mと、ｎと、Ｓ_Mに基づいて、光周波数可変光源の光周波数ν_Xを
ν_X＝ν₀＋Ｓ_Mｎｆ_m−Ｓ_Mｆ
により算出する手段とを含む（請求項４）。
【００１３】
第３の発明の光周波数シンセサイザは、発振光周波数が制御可能な光周波数可変光源と、基準光周波数ν ₀ の基準モードを中心に縦モード間隔ｆ _m のマルチモード光（光コム）を出力するマルチモード基準光源と、光周波数可変光源の出力光を２分岐し、その一方を光周波数シンセサイザの出力光として外部に出力する光分岐手段と、光分岐手段で分岐された光周波数可変光源の出力光とマルチモード基準光源の出力光を合波する光合波手段と、光合波手段の出力光を電気信号に変換し、光周波数可変光源の出力光とマルチモード基準光源の出力光（１つの縦モード）のビート光に対応するビート電気信号を出力する光検出手段と、ビート電気信号を入力し、光周波数可変光源の発振光周波数を測定し、その測定値ν _X が目標光周波数ν _X0 になるように、光周波数可変光源の発振光周波数を制御する光周波数制御部とを備え、マルチモード基準光源は、マルチモード光の縦モード間隔を変化させる手段と、マルチモード光の基準モードと光周波数基準とを一致させるフィードバック手段により、縦モード間隔の変化に係わらず基準モードの基準光周波数ν₀を一定に制御する手段を含む（請求項５）。
【００１４】
第３の発明のマルチモード基準光源のマルチモード光の縦モード間隔を変化させる手段として、マルチモード光を発生させるモード同期レーザと、モード同期レーザをモード同期周波数で駆動し、その駆動周波数を変化させてマルチモード光の縦モード間隔を変化させる駆動手段とを備え、さらに、モード同期レーザから出力されるマルチモード光を入力し、そのスペクトル幅を広げる光非線形媒質を備える（請求項６）。
【００１５】
第３の発明のマルチモード基準光源のマルチモード光の縦モード間隔を変化させる手段として、基準光周波数ν₀の単一モード光を出力する光源と、単一モード光を変調してマルチモード光を発生させる光変調手段と、光変調手段の駆動周波数を変化させてマルチモード光の縦モード間隔を変化させる駆動手段とを備え、さらに、光変調手段から出力されるマルチモード光を入力し、そのスペクトル幅を広げる光非線形媒質を備える（請求項７）。
【００１６】
【発明の実施の形態】
図１は、本発明の光周波数シンセサイザの基本構成を示す。
図において、光周波数可変光源１０は、発振光周波数が制御可能な構成である。マルチモード基準光源２０は、基準光周波数ν₀の基準モードを中心に縦モード間隔ｆ_mのマルチモード光（光コム）を発生させる。光周波数可変光源１０の出力光は光分岐手段３１で２分岐され、一方が光周波数シンセサイザの出力光として外部に出力されるとともに、他方が光合波手段３２に入力される。光合波手段３２は、光分岐手段３１で分岐された光周波数可変光源１０の出力光とマルチモード基準光源２０の出力光を合波する。光検出手段４０は、光合波手段３２の出力光を電気信号に変換し、光周波数可変光源１０の出力光とマルチモード基準光源２０の出力光（１つの縦モード）のビート光に対応するビート電気信号を出力する。光周波数制御部５０はこのビート電気信号を入力し、光周波数可変光源１０の発振光周波数ν_Xを測定し、目標光周波数ν_X0 になるように制御する。
【００１７】
なお、光周波数可変光源１０の発振光周波数の測定では、マルチモード基準光源２０の縦モード間隔ｆ_mを変化させ、光周波数可変光源１０またはマルチモード基準光源２０の出力光の光周波数をシフトさせる必要があるが、それぞれの構成について後述する（関連出願：特願２００２−２１５５９０）。
【００１８】
図２は、マルチモード基準光源２０の構成例を示す。
図において、マルチモード基準光源２０は、周波数ｆ_mの変調信号を出力する変調信号発生器２１と、その変調信号により駆動される能動モード同期レーザ２２を有する。能動モード同期レーザ２２は縦モード間隔ｆ_mの光コムを出力し、その出力光は光増幅器２３で増幅して非線形光ファイバ２４に入力され、非線形効果（スーパーコンティニウム発生等）によってスペクトル拡大し、スペクトルが数百ｎｍ（数十ＴHz）にわたる広帯域な光コムを発生させる構成である（参考文献：Electronics Letter, Vol.30, No.10, pp.790-791)。この光コムの縦モード間隔ｆ_mは、駆動する変調信号の周波数ｆ_mに応じて変化させることができる。
【００１９】
ここでは、１つの縦モードの光周波数を光周波数基準が有する基準光周波数ν₀に同期させる以下の構成をとり、縦モード間隔を変化させた場合でも基準モードの基準光周波数ν₀が変化しないようにする。能動モード同期レーザ２２から出力される縦モードの１つ（基準光周波数ν₀の基準モード）は、光カプラ２５、光バンドパスフィルタ２６を介して分波され、その吸収線の光周波数が校正された標準気体を有する光周波数基準２７を介して光検出回路２８に入力する。光発振周波数制御回路２９は、光検出回路２８の出力に応じて能動モード同期レーザ２２の発振周波数を制御し、基準モードの基準光周波数ν₀が気体吸収線に同期するように帰還ループが形成される。これにより、基準モードの基準光周波数ν₀を極めて安定に制御することができる。
【００２０】
なお、能動モード同期レーザ２２としては、半導体モード同期レーザ（ＭＬＬＤ）や、エルビウム添加光ファイバモード同期リングレーザ等が利用できる。このＭＬＬＤを利用する場合は、その実効共振器長によりモード同期周波数（縦モード間隔）が決定される。離調（駆動周波数の最適駆動周波数からの差分）は数パーセントが許容され、駆動周波数を数パーセント変化させても、パルス幅および出力パワー等に大きな変化はない。
【００２１】
また、能動モード同期レーザ２２に代えて、基準光周波数ν₀に同期した単一モード光を出力する光源と、変調信号により単一モード光を変調して光コムを発生させる光変調手段（例えは電気光学変調器）を用いることも可能である。なお、マルチモード基準光源の縦モード間隔ｆ_mは、光変調手段の変調信号周波数により決定されるので、その周波数を変化させることにより縦モード間隔ｆ_mを変化させることができる。
【００２２】
図３は、光周波数制御部５０の第１の構成例を示す。
図において、光周波数制御部５０は、光検出手段４０から出力されるビート電気信号を入力して光周波数可変光源１０の発振光周波数ν_Xを測定する光周波数計数回路５１と、この測定値ν_Xが目標光周波数ν_X0になるように光周波数可変光源１０の発振光周波数を制御する制御信号を出力する光周波数制御信号生成部５２とを有する。
【００２３】
図４は、光周波数制御部５０の第２の構成例を示す。
図において、光周波数制御部５０は、光検出手段４０から出力されるビート電気信号を２分岐する電気信号分岐手段５３と、その一方のビート電気信号を入力して光周波数可変光源１０の発振光周波数ν_Xを測定する光周波数計数回路５１と、この測定値ν_Xが目標光周波数ν_X0になるように光周波数可変光源１０の発振光周波数を制御する制御信号を出力する光周波数制御信号生成部５２と、マルチモード基準光源２０のある１つの縦モードの光周波数ν_nと目標光周波数ν_X0の差周波数｜ν_n−ν_X0｜の電気信号を出力する周波数シンセサイザ５４と、電気信号分岐手段５３で分岐された他方のビート電気信号と周波数シンセサイザ５４の出力電気信号をミキシングする電気信号ミキサ５５と、電気信号ミキサ５５の出力信号から低周波数成分を選択的に透過して光周波数可変光源１０に与える低域通過フィルタ（ＬＰＦ）５６とを有する。
【００２４】
図５は、光周波数制御部５０の第３の構成例を示す。
図において、光周波数制御部５０は、光検出手段４０から出力されるビート電気信号を２分岐する電気信号分岐手段５３と、その一方のビート電気信号を入力して光周波数可変光源１０の発振光周波数ν_Xを測定する光周波数計数回路５１と、出力周波数を制御できる周波数シンセサイザ５４’と、光周波数計数回路５１の測定値ν_Xと目標光周波数ν_X0を比較し、両者の差周波数｜ν_X0−ν_X｜が０まで減少するように周波数シンセサイザ５４’の出力周波数を掃引する周波数シンセサイザ出力周波数制御回路５７と、電気信号分岐手段５３で分岐された他方のビート電気信号と周波数シンセサイザ５４’の出力電気信号をミキシングする電気信号ミキサ５５と、電気信号ミキサ５５の出力信号から低周波数成分を選択的に透過して光周波数可変光源１０に与える低域通過フィルタ（ＬＰＦ）５６とを有する。この位相同期ループにより、光周波数可変光源１０の発振光周波数をオフセットロッキングすることができる。
【００２５】
以下、図３〜５における光周波数計数回路５１において、マルチモード基準光源２０の縦モード間隔の変化と、光周波数可変光源１０（またはマルチモード基準光源２０）の光周波数シフトの方向を観測することにより、ビート光を生成する縦モードを確定し、光周波数可変光源１０の発振光周波数ν_Xを一意に決定する方法について、図６および図７を参照して説明する。
【００２６】
まず、光周波数計数回路５１では、マルチモード基準光源２０の出力光（光コム）と光周波数可変光源１０の出力光のビート光を光電変換して得られるビート電気信号の周波数ｆを測定する。その測定には例えば周波数カウンタなどが用いられる。この周波数ｆは、光コムの１つの縦モード（光周波数ν_n）と光周波数可変光源１０の出力光（光周波数ν_X）の光周波数差に相当する（図６(a))。
【００２７】
次に、光周波数計数回路５１からの指示により、例えば図２に示すマルチモード基準光源２０の変調信号発生部２１の変調信号周波数を制御し、縦モード間隔ｆ_mをｆ_m＋Δｆ_mに変化させたときに、光周波数計数回路５１で測定される周波数ｆ＋Δｆを測定して周波数ｆの増大量Δｆを算出する。ここで、図６(b) に示すように、光周波数可変光源１０の出力光とビート光をつくる縦モードが、基準光周波数ν₀の基準モードから数えてｎ番目（ν_n）とすると、
｜Δｆ｜＝｜ｎΔｆ_m｜
の関係が存在する。このとき、Δｆ／Δｆ_mの符号も判別する。ちなみに図６の例ではΔｆ_mが正、Δｆが負であるので、Δｆ／Δｆ_mの符号は負となる。
【００２８】
次に、光周波数計数回路５１からの指示により、光周波数可変光源１０の出力光（光周波数ν_X）の光周波数をΔνだけ変化させたときに、測定される周波数ｆ、ｆ＋Δｆの変化方向からΔｆ／Δνの符号を判別する。なお、光周波数シフトを静的に与える場合も、正弦波的に与える場合も、Δｆ／Δνの符号判定は可能である。
【００２９】
これらの結果からν₀、ν_n、ν_Xの大小関係（図７ (a)〜(d) のいずれか）を判別し、その大小関係に基づいて光周波数可変光源１０の出力光の光周波数ν_Xを確定する。
【００３０】
図７(a) はν_n＜ν_X＜ν₀であり、Δｆ／Δｆ_m＞０、Δｆ／Δν＞０となる。図７(b) はν_X＜ν_n＜ν₀であり、Δｆ／Δｆ_m＜０、Δｆ／Δν＜０となる。いずれの場合もビート光を生成する縦モードの光周波数ν_nは、
ν_n＝ν₀−ｎｆ_m＝ν₀−ｆ_m｜Δｆ／Δｆ_m｜
となる。
【００３１】
図７(c) はν₀＜ν_n＜ν_Xであり、Δｆ／Δｆ_m＜０、Δｆ／Δν＞０となる。図７(d) はν₀＜ν_X＜ν_nであり、Δｆ／Δｆ_m＞０、Δｆ／Δν＜０となる。いずれの場合も
ν_n＝ν₀＋ｎｆ_m＝ν₀＋ｆ_m｜Δｆ／Δｆ_m｜
となる。
【００３２】
したがって、光周波数計数回路５１では、Δｆ／Δｆ_m＞０、Δｆ／Δν＞０であれば図７(a) の場合であり、光周波数可変光源１０の光周波数ν_Xは、
ν_X＝ν_n＋ｆ＝ν₀−ｆ_m｜Δｆ／Δｆ_m｜＋ｆ
として求める。
【００３３】
Δｆ／Δｆ_m＜０、Δｆ／Δν＜０であれば図７(b) の場合であり、光周波数可変光源１０の光周波数ν_Xは、
ν_X＝ν_n−ｆ＝ν₀−ｆ_m｜Δｆ／Δｆ_m｜−ｆ
として求める。
【００３４】
Δｆ／Δｆ_m＜０、Δｆ／Δν＞０であれば図７(c) の場合であり、光周波数可変光源１０の光周波数ν_Xは、
ν_X＝ν_n＋ｆ＝ν₀＋ｆ_m｜Δｆ／Δｆ_m｜＋ｆ
として求める。
【００３５】
Δｆ／Δｆ_m＞０、Δｆ／Δν＜０であれば図７(d) の場合であり、光周波数可変光源１０の光周波数ν_Xは、
ν_X＝ν_n−ｆ＝ν₀＋ｆ_m｜Δｆ／Δｆ_m｜−ｆ
として求める。
【００３６】
なお、以上の説明では、光周波数可変光源１０の出力光に光周波数シフトΔνを与える場合について示したが、マルチモード基準光源２０の出力光（光コム）に光周波数シフトΔνを与えるようにしてもよく、その場合には符号を逆にして考えればよい。マルチモード基準光源２０の出力光に光周波数シフトを与える方法としては、図２に示す構成の非線形光ファイバ２４の出力側に光周波数シフト手段を設ける構成、光増幅器２３の入力側に光周波数シフト手段を設ける構成、光周波数基準２７として用いる気体の温度または圧力を制御することにより光周波数基準２７が有する基準周波数をシフトさせる構成などを用いることができる。
【００３７】
図８は、本発明の光周波数シンセサイザの実施例構成を示す。本実施例構成は、図１に示す基本構成において、マルチモード基準光源２０として図２に示す構成、光周波数制御部５０として図４に示す構成を適用したものである。また、本実施例構成では、光周波数計数回路５１から出力される光周波数可変光源１０の光周波数シフト指示を光周波数制御信号生成部５２を介して与える構成としている。
【００３８】
また、ここでは光周波数可変光源１０として、半導体レーザ、グレーティング、ミラーがリットマン型に配置した外部共振器型構造の半導体レーザを用いる例を示す。本構成では、ミラー角度により発振光周波数の制御が可能であるので、光周波数制御信号生成部５２はミラー角度を制御するモータまたはピエゾ素子（ＰＺＴ）に対する制御信号を出力することにより発振光周波数を制御する。まず、所定の発振光周波数ν_X0の出力光が得られるように、ミラー角度と発振光周波数の関係に基づくある程度の精度でミラー角度が設定される。さらに、光周波数計数回路５１で測定された光周波数ν_Xが所定の光周波数ν_X0になるようにミラー角度を制御する。すなわち、モータまたはＰＺＴは、｜ν_X−ν_X0｜がほぼ０になるようにフィードバック制御される。
【００３９】
一方、周波数シンセサイザ５４は、周波数ｆ₀＝｜ν_n−ν_X0｜の電気信号を出力する。このとき、マルチモード基準光源２０の１つの縦モードの光周波数ν_nとしては、ｆ₀＝｜ν_n−ν_X0｜が最小になるものを選択すればよい。この周波数シンセサイザ５４の出力信号と、光検出手段４０から出力されるビート電気信号を電気信号ミキサ５５でミキシングし、その出力信号から低域通過フィルタ５６を用いて｜ｆ−ｆ₀｜の周波数成分を取り出す。この周波数成分は、光周波数可変光源１０の半導体レーザ注入電流の制御信号として用いられる。半導体レーザの注入電流を変化させることにより実効的共振長が変化し、光周波数可変光源１０の発振光周波数の制御が可能になる。以上の構成により、光ＰＬＬが実現され、出力光周波数の確度が非常に高く、かつ安定した光周波数シンセサイザを実現することができる。
【００４０】
次に、光周波数制御部５０として図５に示す構成を適用した場合の動作例について説明する。位相同期ループを動作させない状態で、所定の発振光周波数ν_X0の出力光が得られるように、ミラー角度と発振光周波数の関係に基づくある程度の精度でミラー角度が設定される。しかし、そこには数十ＧHz以下の誤差が生じるので、以下に示す方法で微調整する。
【００４１】
光周波数計数回路５１で測定された光周波数ν_Xと所定の光周波数ν_X0との誤差｜ν_X−ν_X0｜がｆ_m以下の場合、マルチモード基準光源２０のν_X0付近の縦モード周波数ν_nと測定値ν_Xの周波数差｜ν_X−ν_n｜の周波数の電気信号を周波数シンセサイザ５４’から出力し、その出力周波数｜ν_X−ν_n｜を｜ν_X0−ν_n｜に徐々に近づけていく。これにより、光周波数可変光源１０の出力光周波数はν_X0に固定される。その際、光周波数可変光源１０にＰＬＬ内の注入電流制御信号が過大にならないようにミラー角度を自動制御する機構を用いればよい。
【００４２】
一方、誤差｜ν_X−ν_X0｜がｆ_m以上の場合には、仮の目標周波数ν_X1，ν_X2，…を設定し、上記と同様の方法を用いてその仮の目標周波数を順々に達成することで、最終的に所定の光周波数ν_X0を実現する。この動作原理について図９を参照して具体的に説明する。所定の周波数ν_X0、測定値ν_X、仮の目標周波数ν_X1，ν_X2として、
ν_X0＝192.131000ＴHz、
ν_X＝192.085000ＴHz、
ν_X1＝192.111000ＴHz、
ν_X2＝192.091000ＴHz、
とし、仮の目標周波数としてｆ_m程度の間隔で設定する。
【００４３】
ここで、ν_X2＜ν_n-2＜ν_X1を満たすようなマルチモード基準光源２０の縦モード周波数ν_n-2とν_Xの周波数差｜ν_X−ν_n-2｜の周波数の電気信号を周波数シンセサイザ５４’から出力し、その周波数を｜ν_X−ν_n-2｜から｜ν_X2−ν_n-2｜まで掃引する。次に、周波数シンセサイザ５４’の出力周波数を｜ν_X2−ν_n-1｜とし、それを｜ν_X1−ν_n-1｜まで掃引する。以下同様に、最終的に所定の光周波数ν_X0を達成する。以上の構成により、出力光周波数の確度が非常に高く、かつ安定した光周波数シンセサイザを実現することができる。
【００４４】
【発明の効果】
以上説明したように、本発明は、ビート光を生成するマルチモード光の中の縦モードを確定し、光周波数可変光源の出力光の光周波数を一意に決定することができるので、数百ｎｍの広帯域に渡って光周波数可変光源の光周波数を高精度に測定することができる。この測定値に基づいて光周波数可変光源の光周波数を制御することにより、数十ＴHzにわたる帯域において高確度かつ安定に光周波数が制御された光を発生することができる。
【図面の簡単な説明】
【図１】本発明の光周波数シンセサイザの基本構成を示すブロック図。
【図２】マルチモード基準光源２０の構成例を示すブロック図。
【図３】光周波数制御部５０の第１の構成例を示すブロック図。
【図４】光周波数制御部５０の第２の構成例を示すブロック図。
【図５】光周波数制御部５０の第３の構成例を示すブロック図。
【図６】光周波数計数回路５１の動作原理を説明する図。
【図７】光周波数可変光源１０の光周波数の算出パターンを説明する図。
【図８】本発明の光周波数シンセサイザの実施例構成を示すブロック図。
【図９】図５の光周波数制御部５０を用いた場合の動作原理を説明する図。
【符号の説明】
１０光周波数可変光源
２０マルチモード基準光源
２１変調信号発生部
２２能動モード同期レーザ
２３光増幅器
２４非線形光ファイバ
２５光カプラ
２６光バンドパスフィルタ
２７光周波数基準
２８光検出回路
２９光発振周波数制御回路
３１光分岐手段
３２光合波手段
４０光検出手段
５０光周波数制御部
５１光周波数計数回路
５２光周波数制御信号生成部
５３電気信号分岐手段
５４周波数シンセサイザ
５５電気信号ミキサ
５６低域通過フィルタ（ＬＰＦ）
５７周波数シンセサイザ出力周波数制御回路[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical frequency synthesizer that generates light whose optical frequency is controlled with high accuracy.
[0002]
[Prior art]
The conventional optical frequency variable light source is mainly an external cavity semiconductor laser having a structure in which a semiconductor laser, a grating, and a mirror are arranged in a Littman type. In this configuration, since the oscillation light frequency is controlled by the angle of the mirror, the accuracy and stability of the oscillation light frequency is low.
[0003]
Therefore, in order to control the oscillation frequency of the light source with high accuracy and stability, an optical frequency reference light whose optical frequency is controlled with high accuracy and stability is prepared, and the optical frequency and the optical frequency of the optical frequency variable light source are determined. In comparison, a method has been proposed in which the oscillation optical frequency of the optical frequency variable light source is controlled so that the difference (beat) between the two becomes constant. In this method, optical frequency accuracy and stability comparable to those of the optical frequency reference light can be obtained.
[0004]
[Problems to be solved by the invention]
By the way, since the oscillation light frequency variable band of the optical frequency variable light source is limited by the band of the light detection means for detecting the beat, it remains within several tens of GHz near the optical frequency of the optical frequency reference light.
[0005]
On the other hand, in the present situation where the bandwidth used in large-capacity optical communication is widened to several tens of THz, the above-described optical frequency variable band of about several tens of GHz is insufficient.
[0006]
An object of the present invention is to provide an optical frequency synthesizer capable of generating light whose optical frequency is controlled with high accuracy and stability in a band over several tens of THz.
[0007]
[Means for Solving the Problems]
An optical frequency synthesizer according to a first aspect of the invention outputs an optical frequency variable light source capable of controlling an oscillation optical frequency and multimode light (optical comb) having a longitudinal mode interval f _m centering on a reference mode having a reference optical frequency ν _0. The output light of the multi-mode reference light source and the output light of the optical frequency variable light source is split into two, one of which is output to the outside as the output light of the optical frequency synthesizer, and the output of the optical frequency variable light source branched by the optical branch means Optical combining means for combining the light and the output light of the multi-mode reference light source, and the output light of the optical combining means is converted into an electrical signal, and the output light of the optical frequency variable light source and the output light of the multi-mode reference light source (one longitudinal light) Mode) that detects the beat electrical signal corresponding to the beat light, and inputs the beat electrical signal, measures the oscillation optical frequency of the optical frequency variable light source, and the measured value ν _X becomes the target optical frequency ν _X0 Become Sea urchin, and an optical frequency controller for controlling the oscillation optical frequency of the optical frequency variable light source, the optical frequency controller, and an electric signal branching means for two-branch a beat electrical signals, and inputs the beat electrical signals one of its An optical frequency counting circuit that measures the oscillation optical frequency ν _X of the optical frequency variable light source, and a light that outputs a control signal for controlling the oscillation optical frequency of the optical frequency variable light source so that the measured value ν _X becomes the target optical frequency ν _X0 A frequency control signal generation unit, a frequency synthesizer that outputs an electrical signal of a difference frequency | ν _n −ν _X0 | between the optical frequency ν _n of one longitudinal mode having a multimode reference light source and the target optical frequency ν _X0 , and an electrical signal An electric signal mixer that mixes the other beat electric signal branched by the branching means and the output electric signal of the frequency synthesizer, and selectively transmits low-frequency components from the output signal of the electric signal mixer. And a low pass filter applied to the wave number variable light source (Claim 1).
[0010]
An optical frequency synthesizer according to a second aspect of the invention outputs an optical frequency variable light source capable of controlling an oscillation optical frequency and multimode light (optical comb) having a longitudinal mode interval f _m centering on a reference mode having a reference optical frequency ν _0. The output light of the multi-mode reference light source and the output light of the optical frequency variable light source is split into two, one of which is output to the outside as the output light of the optical frequency synthesizer, and the output of the optical frequency variable light source branched by the optical branch means Optical combining means for combining the light and the output light of the multi-mode reference light source, and the output light of the optical combining means is converted into an electrical signal, and the output light of the optical frequency variable light source and the output light of the multi-mode reference light source (one longitudinal light) light detecting means for outputting a beat electrical signals corresponding to the optical beat mode), type a beat electrical signals, the oscillation optical frequency of the optical frequency variable light source is measured, the measured value [nu _X is the target optical frequency [nu _X0 Become Sea urchin, and an optical frequency controller for controlling the oscillation optical frequency of the optical frequency variable light source, the optical frequency controller, and an electric signal branching means for two-branch a beat electrical signals, and inputs the beat electrical signals one of its The optical frequency counter circuit that measures the oscillation optical frequency ν _X of the optical frequency variable light source, the frequency synthesizer that can control the output frequency, the measured value ν _X of the optical frequency counter circuit and the target optical frequency ν _X0 are compared, and the difference between the two A frequency synthesizer output frequency control circuit that sweeps the output frequency of the frequency synthesizer so that the frequency | ν _X0 −ν _X | decreases to 0, the other beat electric signal branched by the electric signal branching means, and the output electric power of the frequency synthesizer An electric signal mixer that mixes the signal and a low-pass signal that selectively transmits low-frequency components from the output signal of the electric signal mixer to the optical frequency variable light source Constitute a phase locked loop by the filter, the oscillation optical frequency of the optical frequency variable light source is configured to offset locking (claim 2).
[0011]
Optical frequency counter circuit used in the optical frequency controller changes the longitudinal mode interval f _m multimode reference light source shifts the optical frequency of the output light from the optical frequency variable light source. Then, the longitudinal mode spacing of the multimode reference light source is varied from f _m to f _m + Delta] f _m, the variation of the longitudinal mode spacing Delta] f _m (including the sign), change in Delta] f of the frequency f of the beat electrical signals The means for counting the ratio n = Δf / Δf _m (including the sign) to (including the sign) and the frequency f of the beat electrical signal when the optical frequency ν _X of the output light of the optical frequency variable light source is shifted. Based on the means for determining the sign S _T (1 if positive, −1 if negative) of the ratio of the change in the reference optical frequency ν ₀ , f _m , n, and S _T , the optical frequency the optical frequency [nu _X of the output light of the tunable light source _{_{_{ν X = ν 0 -S T nf}}} m + S T f
And means for calculating by ( claim 3 ).
[0012]
Another optical frequency counting circuit, the longitudinal mode interval f _m of multimode reference light source is varied, thereby shifting the optical frequency of the output light of the multimode reference light source. Then, the longitudinal mode spacing of the multimode reference light source is varied from f _m to f _m + Delta] f _m, the variation of the longitudinal mode spacing Delta] f _m (including the sign), change in Delta] f of the frequency f of the beat electrical signals A means for counting a ratio n = Δf / Δf _m (including a sign) with respect to (including a sign) and a change in the frequency f of the beat electric signal when the optical frequency of the output light of the multimode reference light source is shifted. Based on the means for determining the sign S _M (1 if positive, -1 if negative), the reference optical frequency ν ₀ , f _m , n, and S _M , the optical frequency variable light source the optical frequency _{_{_{ν X ν X = ν 0 +}}} S M nf m -S M f
And means for calculating by ( claim 4 ).
[0013]
Optical frequency synthesizer of the third invention, outputs an optical frequency variable light source capable controlled oscillation light frequency, around the reference mode of the reference light frequency [nu ₀ multimode optical longitudinal mode interval f _m (optical comb) The output light of the multi-mode reference light source and the output light of the optical frequency variable light source is split into two, one of which is output to the outside as the output light of the optical frequency synthesizer, and the output of the optical frequency variable light source branched by the optical branch means Optical combining means for combining the light and the output light of the multi-mode reference light source, and the output light of the optical combining means is converted into an electrical signal, and the output light of the optical frequency variable light source and the output light of the multi-mode reference light source (one longitudinal light) light detecting means for outputting a beat electrical signals corresponding to the optical beat mode), type a beat electrical signals, the oscillation optical frequency of the optical frequency variable light source is measured, the measured value [nu _X is the target optical frequency [nu _X0 Become Sea urchin, and an optical frequency controller for controlling the oscillation optical frequency of the optical frequency variable light source, a multimode reference light source includes a means for changing the longitudinal mode spacing of the multimode optical reference mode of the multimode light and the optical frequency reference the feedback means for matching the door, comprises means for controlling the reference beam frequency [nu ₀ reference mode regardless of changes in the longitudinal mode interval constant (claim 5).
[0014]
As means for changing the longitudinal mode interval of the multi-mode light of the multi -mode reference light source of the third invention, the mode-locked laser for generating the multi-mode light , the mode-locked laser is driven at the mode-locked frequency, and the drive frequency is changed. It is allowed and a driving means for changing the longitudinal mode spacing of the multi-mode light, furthermore, enter the multimode light output from the mode-locked laser, an optical nonlinear medium to broaden the spectral width (claim 6).
[0015]
Third as multimode reference multimode optical means for changing the longitudinal mode spacing of the light source of the invention, a light source for outputting a single mode light of the reference light frequency [nu _0, multimode light by modulating the single-mode optical a light modulation means for generating, by changing the driving frequency of the light modulation means and a driving means for changing the longitudinal mode spacing of the multi-mode light, furthermore, enter the multimode light output from the light modulation means, an optical nonlinear medium to broaden the spectral width (claim 7).
[0016]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a basic configuration of an optical frequency synthesizer of the present invention.
In the figure, the optical frequency variable light source 10 is configured to control the oscillation optical frequency. Multimode reference light source 20 is a multimode optical longitudinal mode interval f _m around the reference mode of the reference light frequency [nu ₀ to generate (optical comb). The output light of the optical frequency variable light source 10 is branched into two by the optical branching means 31, one of which is output to the outside as the output light of the optical frequency synthesizer, and the other is input to the optical multiplexing means 32. The optical multiplexing unit 32 combines the output light of the optical frequency variable light source 10 branched by the optical branching unit 31 and the output light of the multimode reference light source 20. The light detection means 40 converts the output light of the optical multiplexing means 32 into an electric signal, and beats corresponding to the beat light of the output light of the optical frequency variable light source 10 and the output light (one longitudinal mode) of the multimode reference light source 20. Outputs electrical signals. The optical frequency control unit 50 receives this beat electric signal, measures the oscillation optical frequency ν _X of the optical frequency variable light source 10, and controls the target optical frequency ν _X0 .
[0017]
In the measurement of the oscillation optical frequency of the optical frequency variable light source 10, the longitudinal mode interval f _m of multimode reference light source 20 is changed, shifts the optical frequency of the output light from the optical frequency variable light source 10 or multimode reference light source 20 Although necessary, each configuration will be described later (related application: Japanese Patent Application No. 2002-215590).
[0018]
FIG. 2 shows a configuration example of the multimode reference light source 20.
In the figure, the multi-mode reference light source 20 includes a modulation signal generator 21 for outputting a modulation signal of a frequency f _m, an active mode-locked laser 22 which is driven by the modulated signal. Actively mode-locked laser 22 outputs the optical comb of longitudinal mode interval f _m, the output light is amplified by the optical amplifier 23 is input to the nonlinear optical fiber 24, to expand the spectrum by nonlinear effects (supercontinuum generation, etc.) In this configuration, a broadband optical comb having a spectrum of several hundred nm (several tens of THz) is generated (reference: Electronics Letter, Vol. 30, No. 10, pp. 790-791). Longitudinal mode interval f _m of the optical comb can be varied in accordance with the frequency f _m of the modulation signal for driving.
[0019]
Here, take the configuration of the following synchronizing the optical frequencies of the single longitudinal mode to the reference optical frequency [nu ₀ having an optical frequency reference, the reference light frequency [nu ₀ reference mode even when changing the longitudinal mode spacing is not changed Like that. One of the longitudinal modes (reference mode of the reference optical frequency ν ₀ ) output from the active mode-locked laser 22 is demultiplexed via the optical coupler 25 and the optical bandpass filter 26, and the optical frequency of the absorption line is calibrated. Then, the light is input to the light detection circuit 28 through the optical frequency reference 27 having the standard gas. The optical oscillation frequency control circuit 29 controls the oscillation frequency of the active mode-locked laser 22 according to the output of the light detection circuit 28, and a feedback loop is formed so that the reference optical frequency ν ₀ of the reference mode is synchronized with the gas absorption line. Is done. Thereby, the reference optical frequency ν ₀ in the reference mode can be controlled extremely stably.
[0020]
As the active mode-locked laser 22, a semiconductor mode-locked laser (MLLD), an erbium-doped optical fiber mode-locked ring laser, or the like can be used. When this MLLD is used, the mode locking frequency (longitudinal mode interval) is determined by the effective resonator length. Detuning (difference from the optimum driving frequency of the driving frequency) is allowed to be several percent, and even if the driving frequency is changed by several percent, there is no significant change in pulse width, output power, and the like.
[0021]
Further, in place of the active mode-locked laser 22, a light source that outputs single-mode light synchronized with the reference optical frequency ν ₀ and an optical modulation unit that modulates the single-mode light with a modulation signal to generate an optical comb (for example, It is also possible to use an electro-optic modulator. The vertical mode interval f _m of multimode reference light source, since it is determined by the modulation signal frequency of the optical modulation means, it is possible to change the longitudinal mode interval f _m by changing its frequency.
[0022]
FIG. 3 shows a first configuration example of the optical frequency control unit 50.
In the figure, an optical frequency control unit 50 receives the beat electrical signal output from the light detection means 40 and measures the oscillation optical frequency ν _X of the optical frequency variable light source 10, and the measured value ν. An optical frequency control signal generation unit 52 that outputs a control signal for controlling the oscillation optical frequency of the optical frequency variable light source 10 so that _X becomes the target optical frequency ν _X0 .
[0023]
FIG. 4 shows a second configuration example of the optical frequency control unit 50.
In the figure, an optical frequency control unit 50 receives an electric signal branching unit 53 that splits a beat electric signal output from the light detection unit 40 into two, and one of the beat electric signals to receive the oscillation light of the optical frequency variable light source 10. An optical frequency counting circuit 51 that measures the frequency ν _X and an optical frequency control signal generation that outputs a control signal that controls the oscillation optical frequency of the optical frequency variable light source 10 so that the measured value ν _X becomes the target optical frequency ν _X0. Unit 52, frequency synthesizer 54 that outputs an electrical signal having a difference frequency | ν _n −ν _X0 | between the optical frequency ν _n of one longitudinal mode with the multimode reference light source 20 and the target optical frequency ν _X0 , and electrical signal branching An electric signal mixer 55 for mixing the other beat electric signal branched by the means 53 and the output electric signal of the frequency synthesizer 54, and a low frequency component from the output signal of the electric signal mixer 55 And selectively transmits supplies the optical frequency variable light source 10 and a low pass filter (LPF) 56.
[0024]
FIG. 5 shows a third configuration example of the optical frequency control unit 50.
In the figure, an optical frequency control unit 50 receives an electric signal branching unit 53 that splits a beat electric signal output from the light detection unit 40 into two, and one of the beat electric signals to receive the oscillation light of the optical frequency variable light source 10. The optical frequency counting circuit 51 that measures the frequency ν _X , the frequency synthesizer 54 ′ that can control the output frequency, the measured value ν _X of the optical frequency counting circuit 51 and the target optical frequency ν _X0 are compared, and the difference frequency between the two | ν _The frequency synthesizer output frequency control circuit 57 sweeps the output frequency of the frequency synthesizer 54 ′ so that _X0− ν _X | decreases to 0, and the other beat electric signal branched by the electric signal branching means 53 and the frequency synthesizer 54 ′. An electric signal mixer 55 for mixing the output electric signal of the optical signal, and a low frequency component selectively transmitted from the output signal of the electric signal mixer 55 to change the optical frequency It gives 10 and a low pass filter (LPF) 56. By this phase-locked loop, the oscillation optical frequency of the optical frequency variable light source 10 can be offset locked.
[0025]
Hereinafter, in the optical frequency counting circuit 51 in FIGS. 3 to 5, the change in the longitudinal mode interval of the multimode reference light source 20 and the direction of the optical frequency shift of the optical frequency variable light source 10 (or the multimode reference light source 20) are observed. A method for determining the longitudinal mode for generating the beat light and uniquely determining the oscillation light frequency ν _X of the optical frequency variable light source 10 will be described with reference to FIGS.
[0026]
First, the optical frequency counting circuit 51 measures the frequency f of the beat electric signal obtained by photoelectrically converting the output light (optical comb) of the multimode reference light source 20 and the beat light of the output light of the optical frequency variable light source 10. For example, a frequency counter is used for the measurement. This frequency f corresponds to the optical frequency difference between one longitudinal mode (optical frequency ν _n ) of the optical comb and the output light (optical frequency ν _X ) of the optical frequency variable light source 10 (FIG. 6 (a)).
[0027]
Then, by an instruction from the optical frequency counter circuit 51, for example, by controlling the modulating signal frequency of the modulation signal generator 21 of the multimode reference light source 20 shown in FIG. 2, by changing the longitudinal mode interval f _m to f _m + Delta] f _m The frequency f + Δf measured by the optical frequency counting circuit 51 is measured to calculate the increase amount Δf of the frequency f. Here, as shown in FIG. 6B, when the longitudinal mode for producing the output light and the beat light of the optical frequency variable light source 10 is nth (ν _n ) counting from the reference mode having the reference optical frequency ν ₀ ,
| Δf | = | nΔf _m |
The relationship exists. At this time, also determines the sign of Δf / Δf _m. Incidentally Delta] f _m is positive in the example of FIG. 6, since Delta] f is negative, the sign of Delta] f / Delta] f _m becomes negative.
[0028]
Next, when the optical frequency of the output light (optical frequency ν _X ) of the optical frequency variable light source 10 is changed by Δν in accordance with an instruction from the optical frequency counting circuit 51, the change direction of the measured frequencies f and f + Δf is measured. The sign of Δf / Δν is determined. Whether the optical frequency shift is given statically or sinusoidally, the sign of Δf / Δν can be determined.
[0029]
From these results, the magnitude relationship (any one of FIGS. 7A to 7D) of ν ₀ , ν _n , and ν _X is determined, and the optical frequency of the output light of the optical frequency variable light source 10 is determined based on the magnitude relationship. Confirm ν _X.
[0030]
In FIG. 7A, ν _n <ν _X <ν ₀ , and Δf / Δf _m > 0 and Δf / Δν> 0. In FIG. 7B, ν _X <ν _n <ν ₀ , and Δf / Δf _m <0 and Δf / Δν <0. In either case, the optical frequency ν _n of the longitudinal mode that generates the beat light is
ν _n = ν ₀ −nf _m = ν ₀ −f _m | Δf / Δf _m |
It becomes.
[0031]
In FIG. 7C, ν ₀ <ν _n <ν _X , and Δf / Δf _m <0 and Δf / Δν> 0. In FIG. 7D, ν ₀ <ν _X <ν _n , and Δf / Δf _m > 0 and Δf / Δν <0. In any case, ν _n = ν ₀ + nf _m = ν ₀ + f _m | Δf / Δf _m |
It becomes.
[0032]
Therefore, in the optical frequency counting circuit 51, when Δf / Δf _m > 0 and Δf / Δν> 0, the case of FIG. 7A is obtained, and the optical frequency ν _X of the optical frequency variable light source 10 is
ν _X = ν _n + f = ν ₀ −f _m | Δf / Δf _m | + f
Asking.
[0033]
If Δf / Δf _m <0 and Δf / Δν <0, the case of FIG. 7B is satisfied, and the optical frequency ν _X of the optical frequency variable light source 10 is
ν _X = ν _n −f = ν ₀ −f _m | Δf / Δf _m | −f
Asking.
[0034]
If Δf / Δf _m <0, Δf / Δν> 0, it is the case of FIG. 7C, and the optical frequency ν _X of the optical frequency variable light source 10 is
ν _X = ν _n + f = ν ₀ + f _m | Δf / Δf _m | + f
Asking.
[0035]
If Δf / Δf _m > 0 and Δf / Δν <0, it is the case of FIG. 7 (d), and the optical frequency ν _X of the optical frequency variable light source 10 is
ν _X = ν _n −f = ν ₀ + f _m | Δf / Δf _m | −f
Asking.
[0036]
In the above description, the optical frequency shift Δν is given to the output light of the optical frequency variable light source 10, but the optical frequency shift Δν is given to the output light (optical comb) of the multimode reference light source 20. In that case, the sign may be reversed. As a method of giving an optical frequency shift to the output light of the multi-mode reference light source 20, an optical frequency shift means is provided on the output side of the nonlinear optical fiber 24 having the configuration shown in FIG. 2, and an optical frequency shift is provided on the input side of the optical amplifier 23. A configuration in which means is provided, a configuration in which the reference frequency of the optical frequency reference 27 is shifted by controlling the temperature or pressure of the gas used as the optical frequency reference 27 can be used.
[0037]
FIG. 8 shows an embodiment of the optical frequency synthesizer of the present invention. The configuration of this embodiment is obtained by applying the configuration shown in FIG. 2 as the multimode reference light source 20 and the configuration shown in FIG. 4 as the optical frequency control unit 50 in the basic configuration shown in FIG. In the configuration of the present embodiment, an optical frequency shift instruction of the optical frequency variable light source 10 output from the optical frequency counting circuit 51 is given via the optical frequency control signal generation unit 52.
[0038]
Here, an example in which a semiconductor laser having an external resonator type structure in which a semiconductor laser, a grating, and a mirror are arranged in a Littman type is used as the optical frequency variable light source 10. In this configuration, since the oscillation optical frequency can be controlled by the mirror angle, the optical frequency control signal generation unit 52 outputs the control signal for the motor or the piezo element (PZT) that controls the mirror angle, thereby adjusting the oscillation optical frequency. Control. First, the mirror angle is set with a certain degree of accuracy based on the relationship between the mirror angle and the oscillation light frequency so that output light with a predetermined oscillation light frequency ν _X0 can be obtained. Further, the mirror angle is controlled so that the optical frequency ν _X measured by the optical frequency counting circuit 51 becomes the predetermined optical frequency ν _X0 . That is, the motor or PZT is feedback controlled so that | ν _X −ν _X0 |
[0039]
On the other hand, the frequency synthesizer 54 outputs an electrical signal having a frequency f ₀ = | ν _n −ν _X0 |. At this time, the optical frequency ν _n of one longitudinal mode of the multimode reference light source 20 may be selected so that f ₀ = | ν _n −ν _X0 | is minimized. The output signal of the frequency synthesizer 54 and the beat electric signal output from the light detection means 40 are mixed by the electric signal mixer 55, and the frequency component of | f−f ₀ | is generated from the output signal using the low-pass filter 56. Take out. This frequency component is used as a control signal for the semiconductor laser injection current of the optical frequency variable light source 10. By changing the injection current of the semiconductor laser, the effective resonance length changes, and the oscillation light frequency of the optical frequency variable light source 10 can be controlled. With the above configuration, an optical PLL is realized, and an optical frequency synthesizer with a very high accuracy of output optical frequency and a stable optical frequency can be realized.
[0040]
Next, an operation example when the configuration shown in FIG. 5 is applied as the optical frequency control unit 50 will be described. The mirror angle is set with a certain degree of accuracy based on the relationship between the mirror angle and the oscillation light frequency so that output light having a predetermined oscillation light frequency ν _X0 can be obtained without operating the phase locked loop. However, since an error of several tens of GHz or less occurs there, fine adjustment is performed by the following method.
[0041]
When the error | ν _X −ν _X0 | between the optical frequency ν _X measured by the optical frequency counting circuit 51 and the predetermined optical frequency ν _X0 is f _m or less, the longitudinal mode frequency near ν _{X0 of the} multimode reference light source 20 [nu _n and the frequency difference of the measured values ν _{_X} | ν _X -ν _n | of an electric signal having a frequency output from the frequency synthesizer 54 ', the output frequency | ν _X -ν _n | a | ν _X0 -ν _n | a Gradually approach. Thereby, the output optical frequency of the optical frequency variable light source 10 is fixed to ν _X0 . At that time, a mechanism for automatically controlling the mirror angle may be used in the optical frequency variable light source 10 so that the injection current control signal in the PLL does not become excessive.
[0042]
On the other hand, if the error | ν _X −ν _X0 | is greater than or equal to f _m , provisional target frequencies ν _X1 , ν _X2 ,... Are set, and the provisional target frequencies are sequentially set using the same method as described above. By achieving this, a predetermined optical frequency ν _X0 is finally realized. This operation principle will be specifically described with reference to FIG. As a predetermined frequency ν _X0 , measured value ν _X , and temporary target frequencies ν _X1 , ν _X2 ,
ν _X0 = 192.131000 THz,
ν _X = 192.085000 THz,
ν _X1 = 192.111000 THz,
ν _X2 = 192.091000 THz,
And then, set at intervals of about f _m as the target frequency tentative.
[0043]
Here, an electrical signal having a frequency difference | ν _X −ν _n−2 | the longitudinal mode frequency ν _n−2 and ν _X of the multimode reference light source 20 satisfying ν _X2 <ν _n−2 <ν _X1. Is output from the frequency synthesizer 54 ′, and the frequency is swept from | ν _X −ν _n−2 | to | ν _X2 −ν _n−2 |. Next, the output frequency of the frequency synthesizer 54 ′ is set to | ν _X2 −ν _n−1 |, and is swept up to | ν _X1 −ν _n−1 |. Similarly, the predetermined optical frequency ν _X0 is finally achieved. With the above configuration, it is possible to realize a stable optical frequency synthesizer with very high accuracy of output optical frequency.
[0044]
【The invention's effect】
As described above, the present invention can determine the longitudinal mode in the multi-mode light that generates the beat light, and can uniquely determine the optical frequency of the output light of the optical frequency variable light source. The optical frequency of the optical frequency variable light source can be measured with high accuracy over a wide band. By controlling the optical frequency of the optical frequency variable light source based on this measured value, it is possible to generate light whose optical frequency is controlled with high accuracy and stability in a band extending over several tens of THz.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a basic configuration of an optical frequency synthesizer of the present invention.
FIG. 2 is a block diagram showing a configuration example of a multimode reference light source 20;
3 is a block diagram showing a first configuration example of an optical frequency control unit 50. FIG.
4 is a block diagram showing a second configuration example of the optical frequency control unit 50. FIG.
5 is a block diagram showing a third configuration example of the optical frequency control unit 50. FIG.
6 is a diagram for explaining the operation principle of an optical frequency counting circuit 51. FIG.
FIG. 7 is a diagram for explaining a calculation pattern of an optical frequency of the optical frequency variable light source 10;
FIG. 8 is a block diagram showing the configuration of an embodiment of the optical frequency synthesizer of the present invention.
9 is a diagram for explaining an operation principle when the optical frequency control unit 50 of FIG. 5 is used.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Optical frequency variable light source 20 Multimode reference | standard light source 21 Modulation signal generation part 22 Active mode synchronous laser 23 Optical amplifier 24 Non-linear optical fiber 25 Optical coupler 26 Optical band pass filter 27 Optical frequency reference 28 Photodetection circuit 29 Optical oscillation frequency control circuit 31 Optical branching means 32 Optical multiplexing means 40 Optical detection means 50 Optical frequency control section 51 Optical frequency counting circuit 52 Optical frequency control signal generation section 53 Electric signal branching means 54 Frequency synthesizer 55 Electric signal mixer 56 Low-pass filter (LPF)
57 Frequency synthesizer output frequency control circuit

Claims

An optical frequency variable light source capable of controlling the oscillation optical frequency;
A multi-mode reference light source that outputs multi-mode light (optical comb) with a longitudinal mode interval f _m centering on a reference mode having a reference optical frequency ν ₀ ;
Optical branching means for branching the output light of the optical frequency variable light source into two and outputting one of them as output light of an optical frequency synthesizer;
Optical multiplexing means for combining the output light of the optical frequency variable light source branched by the optical branching means and the output light of the multimode reference light source;
Light that converts the output light of the optical multiplexing means into an electrical signal and outputs a beat electrical signal corresponding to the beat light of the output light of the optical frequency variable light source and the output light of the multimode reference light source (one longitudinal mode) Detection means;
Light that controls the oscillation light frequency of the optical frequency variable light source so that the beat electrical signal is input, the oscillation optical frequency of the optical frequency variable light source is measured, and the measured value ν _X becomes the target optical frequency ν _X0 A frequency control unit,
The optical frequency controller is
Electrical signal branching means for branching the beat electrical signal into two;
An optical frequency counting circuit that inputs one of the beat electrical signals and measures the oscillation optical frequency ν _X of the optical frequency variable light source;
An optical frequency control signal generating unit that outputs a control signal for controlling the oscillation optical frequency of the optical frequency variable light source so that the measured value ν _X becomes a target optical frequency ν _X0 ;
A frequency synthesizer that outputs an electrical signal having a difference frequency | ν _n −ν _X0 | between the optical frequency ν _n of one longitudinal mode having the multi-mode reference light source and the target optical frequency ν _X0 ; An electrical signal mixer for mixing the other beat electrical signal and the output electrical signal of the frequency synthesizer;
An optical frequency synthesizer, comprising: a low-pass filter that selectively transmits a low-frequency component from the output signal of the electric signal mixer and supplies the low-frequency component to the optical frequency variable light source.

An optical frequency variable light source capable of controlling the oscillation optical frequency;
A multi-mode reference light source that outputs multi-mode light (optical comb) with a longitudinal mode interval f _m centering on a reference mode having a reference optical frequency ν ₀ ;
Optical branching means for branching the output light of the optical frequency variable light source into two and outputting one of them as output light of an optical frequency synthesizer;
Optical multiplexing means for combining the output light of the optical frequency variable light source branched by the optical branching means and the output light of the multimode reference light source;
Light that converts the output light of the optical multiplexing means into an electrical signal and outputs a beat electrical signal corresponding to the beat light of the output light of the optical frequency variable light source and the output light of the multimode reference light source (one longitudinal mode) Detection means;
Light that controls the oscillation light frequency of the optical frequency variable light source so that the beat electrical signal is input, the oscillation optical frequency of the optical frequency variable light source is measured, and the measured value ν _X becomes the target optical frequency ν _X0 A frequency control unit,
The optical frequency controller is
Electrical signal branching means for branching the beat electrical signal into two;
An optical frequency counting circuit that inputs one of the beat electrical signals and measures the oscillation optical frequency ν _X of the optical frequency variable light source;
A frequency synthesizer that can control the output frequency,
A frequency synthesizer output that compares the measured value ν _X of the optical frequency counter circuit with the target optical frequency ν _X0 and sweeps the output frequency of the frequency synthesizer so that the difference frequency | ν _X0 −ν _X | A frequency control circuit;
An electric signal mixer for mixing the other beat electric signal branched by the electric signal branching means and the output electric signal of the frequency synthesizer;
A phase-locked loop is formed by a low-pass filter that selectively transmits low-frequency components from the output signal of the electric signal mixer and supplies the low-frequency components to the optical frequency variable light source, and offset locking the oscillation optical frequency of the optical frequency variable light source An optical frequency synthesizer characterized in that

The optical frequency counting circuit includes:
The longitudinal mode spacing of the multimode reference light source is varied from f _m to f _m + Delta] f _m, the variation of the longitudinal mode spacing Delta] f _m (including the sign), change in Delta] f of the frequency f of the beat electrical signal Means for counting the ratio n = Δf / Δf _m (including the sign) to (including the sign);
When the optical frequency ν _X of the output light of the optical frequency variable light source is shifted, the sign S _T (1 if positive, −1 if negative) of the ratio of the beat electric signal to the change in the frequency f. Means for determining;
And the reference optical frequency [nu _0, and f _m, and n, on the basis of S _T, the optical frequency [nu _X of the output light of the optical frequency variable light source _{_{_{ν X = ν 0 -S T nf}}} m + S T f
The optical frequency synthesizer according to claim 1 or 2 , characterized by comprising:

The optical frequency counting circuit includes:
The longitudinal mode spacing of the multimode reference light source is varied from f _m to f _m + Delta] f _m, the variation of the longitudinal mode spacing Delta] f _m (including the sign), change in Delta] f of the frequency f of the beat electrical signal Means for counting the ratio n = Δf / Δf _m (including the sign) to (including the sign);
When the optical frequency of the output light of the multi-mode reference light source is shifted, the sign S _M (1 if positive, −1 if negative) of the ratio to the change in the frequency f of the beat electrical signal is determined. Means,
And the reference optical frequency [nu _0, f and _m, and n, on the basis of the S _M, the optical frequency variable light source of the optical frequency _{_{_{ν X ν X = ν 0 +}}} S M nf m -S M f
Including the means for calculating by
Optical frequency synthesizer of claim 1 or claim 2, wherein the this.

An optical frequency variable light source capable of controlling the oscillation optical frequency;
A multi-mode reference light source that outputs multi-mode light (optical comb) with a longitudinal mode interval f _m centering on a reference mode having a reference optical frequency ν ₀ ;
Optical branching means for branching the output light of the optical frequency variable light source into two and outputting one of them as output light of an optical frequency synthesizer;
Optical multiplexing means for combining the output light of the optical frequency variable light source branched by the optical branching means and the output light of the multimode reference light source;
Light that converts the output light of the optical multiplexing means into an electrical signal and outputs a beat electrical signal corresponding to the beat light of the output light of the optical frequency variable light source and the output light of the multimode reference light source (one longitudinal mode) Detection means;
Light that controls the oscillation light frequency of the optical frequency variable light source so that the beat electrical signal is input, the oscillation optical frequency of the optical frequency variable light source is measured, and the measured value ν _X becomes the target optical frequency ν _X0 A frequency control unit,
The multi-mode reference light source is
Means for changing a longitudinal mode interval of the multimode light;
Characterized by comprising means for controlling the reference optical frequency ν ₀ of the reference mode to be constant regardless of a change in the longitudinal mode interval by feedback means for matching the reference mode of the multimode light with the optical frequency reference. Optical frequency synthesizer.

The multi-mode reference light source is
As means for changing the longitudinal mode interval of the multimode light,
A mode-locked laser that generates multimode light ; and
Driving the mode-locked laser at a mode-locked frequency, and changing the drive frequency to change the longitudinal mode interval of the multi-mode light ,
Further, the optical frequency synthesizer of claim 5, enter the multimode light output from the mode-locked laser, characterized by comprising an optical nonlinear medium to broaden the spectral width.

The multi-mode reference light source is
As means for changing the longitudinal mode interval of the multimode light,
A light source that outputs single-mode light having the reference light frequency ν ₀ ;
A light modulation means for generating multimode light and modulating the single-mode optical,
Drive means for changing the longitudinal mode interval of the multi-mode light by changing the drive frequency of the light modulation means ,
Further, the optical frequency synthesizer of claim 5, enter a is the multimode light output from said light modulating means, characterized by comprising an optical nonlinear medium to broaden the spectral width.