JP3571539B2

JP3571539B2 - Optical pulse generator

Info

Publication number: JP3571539B2
Application number: JP24283098A
Authority: JP
Inventors: 悦橋本; 篤高田
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 1998-08-28
Filing date: 1998-08-28
Publication date: 2004-09-29
Anticipated expiration: 2018-08-28
Also published as: JP2000077768A

Description

【０００１】
【発明の属する技術分野】
本発明は、光サンプリングなどに用いる、繰り返し周波数可変にして低位相雑音でパルス幅の狭い光パルス発生装置に関するものである。
【０００２】
【従来の技術】
光サンプリング光源として望まれる性能は、いろいろな周波数の被測定信号に対応できるように繰り返し周波数が可変で、かつ、分周比（被測定信号の周波数とサンプリング周波数との比）Ｎ（整数）が大きくとれるよう数１０ＭＨｚ以下の繰り返し周波数でのパルス発生が可能であること、高い時間分解能のためにパルス幅が１ｐｓ以下と狭いこと、更に、タイミングジッタ（ジッタとは高速の時間的なふらつきのことである）が小さいことが望まれる。
【０００３】
従来の簡易で小型な光パルス発生装置として、（１）半導体モード同期レーザ装置、（２）連続光源から発生した連続光を光変調器で切り出して光パルス列を発生する装置、（３）ゲインスイッチ半導体レーザをパルス信号で駆動して光パルス列を発生する装置の３つが挙げられる。
【０００４】
【発明が解決しようとする課題】
本発明者は、前記の従来技術を検討した結果、以下の問題点を見い出した。
前記（１）のモード同期レーザでは、繰り返し周波数ｆ_ｒｅｐが、ｆ_ｒｅｐ＝ｃ／２ｎＬ（ｃ：光速ｎ：屈折率Ｌ：共振器長）と共振器長Ｌで決まってしまうため外部信号への同調性が乏しい。また、この式から数１０ＭＨｚ以下の繰り返し周波数の場合、共振器長Ｌが数ｍ以上必要になり機械的不安定性が増加し、装置規模も長大化するため現実的なパルス発生法ではない。
【０００５】
前記（２）の装置では、連続光を光変調した光パルス列は、外部参照信号への同期は容易であるが、そのパルス幅が光変調器の変調帯域幅と光変調器の駆動電気信号の帯域で制限される。駆動信号生成回路と光変調器の帯域幅は高々５０ＧＨｚなので、パルス幅が数１０ｐｓ以下のパルス発生は困難である。また、駆動信号生成回路ではタイミングジッタが付加される。
【０００６】
前記（３）のゲインスイッチ半導体レーザを利用した装置では、比較的容易に１０ｐｓ程度のパルス幅をもつ光パルス列を発生できるが、ゲインスイッチパルス発生の過程で比較的大きなジッタが加わる。
【０００７】
したがって、前記（２）、（３）共に、測定用光源として用いるには大きなタイミングジッタと広いパルス幅が問題となっている。パルス幅に関しては、光ファイバを用いた線形／非線形光パルス圧縮技術により１ｐｓ以下に狭くすることが可能である。しかし、圧縮には数１０ｍ以上の光パルス圧縮用ファイバを用いるので、ファイバの周辺温度が数度（℃）変化するだけで遅延量変化が起こり、光パルス圧縮後の光パルス列の位相が大きく変化し、ワンダ（低速の時間的なふらつきのことである）が大きくなるという問題があった。
【０００８】
本発明の目的は、繰り返し周波数が数１００ｋＨｚ〜数１０ＭＨｚ、パルス幅が１ｐｓ以下の、タイミングジッタ及びワンダが共に小さい光パルス列を発生させることが可能な技術を提供することにある。
【０００９】
本発明の前記ならびにその他の目的と新規な特徴は、本明細書の記述及び添付図面によって明らかにする。
【００１０】
【課題を解決するための手段】
本願において開示される発明のうち代表的なものの概要を簡単に説明すれば、以下のとおりである。
【００１１】
（１）外部参照信号に同期した光パルス列を発生する光パルス発生装置において、電圧制御発振器と、該電圧制御発振器により駆動される光パルス発生手段と、該光パルス発生手段より発生する光パルスの一部を分岐する光分岐手段と、該光分岐手段により分岐された光パルス列を受光する受光器と、該受光器の出力信号と外部参照信号の位相誤差を検出し、その誤差信号を前記電圧制御発振器に負帰還する位相比較回路と、該位相比較回路と電圧制御発振器の間に誤差信号の低周波成分を通過させるループフィルタとを具備し、前記光パルス発生手段は、利得スイッチにより光パルス列を発生する半導体レーザを含むことを特徴とする。
【００１２】
（２）外部参照信号に同期した光パルス列を発生する光パルス発生装置において、外部参照信号により駆動される光パルス発生手段と、該光パルス発生手段より発生する光パルスの一部を分岐する光分岐手段と、該光分岐手段により分岐された光パルス列を受光する受光器と、該受光器の出力信号と外部参照信号の位相誤差を検出する位相比較回路と、該位相比較回路より出力される誤差信号を時間積分し、光パルス発生手段に負帰還する積分回路とを具備し、前記光パルス発生手段は、利得スイッチにより光パルス列を発生する半導体レーザを含むことを特徴とする。
【００１３】
（３）外部参照信号に同期した光パルス列を発生する光パルス発生装置において、第１の電圧制御発振器と、該第１の電圧制御発振器により駆動される光パルス発生手段と、該光パルス発生手段より発生する光パルスの一部を分岐する第１の光分岐手段と、該第１の光分岐手段により分岐された光パルス列を受光する第１の受光器と、該第１の受光器の出力信号と外部参照信号の位相誤差を検出する第１の位相比較回路と、該第１の位相比較回路と第１の電圧制御発振器の間に誤差信号の低周波成分を通過させるループフィルタと、前記出力光パルス列のパルス幅を狭窄化する光パルス圧縮手段と、該光パルス圧縮手段の出力の一部を分岐する第２の光分岐手段と、該第２の光分岐手段により分岐された光を受光する第２の受光器と、該第２の受光器の出力信号と外部参照信号の位相誤差を検出する第２の位相比較回路と、該第２の位相比較回路より出力される誤差信号の必要帯域を通過する第２のループフィルタと、該第２のループフィルタを通過した信号を入力とする第２の電圧制御発振器とを備え、該第２の電圧制御発振器の出力を第１の位相比較回路の入力とするものである。
【００１４】
（４）外部参照信号に同期した光パルス列を発生する光パルス発生装置において、電圧制御発振器と、該電圧制御発振器により駆動される光パルス発生手段と、該光パルス発生手段より発生する光パルスの一部を分岐する第１の光分岐手段と、該第１の光分岐手段により分岐された光パルス列を受光する第１の受光器と、該第１の受光器の出力信号と外部参照信号の位相誤差を検出する第１の位相比較回路と、該第１の位相比較回路と第１の電圧制御発振器の間に誤差信号の低周波成分を通過させるループフィルタと、前記出力光パルス列のパルス幅を狭窄化する光パルス圧縮手段と、光パルス圧縮手段の出力の一部を分岐する第２の光分岐手段と、該第２の光分岐手段により分岐された光を受光する第２の受光器と、該第２の受光器の出力信号と外部参照信号の位相誤差を検出する第２の位相比較回路と、該第２の位相比較回路より出力される誤差信号を時間積分する積分回路と、該積分回路出力によって駆動され、外部参照信号を入力とする位相シフタとを備え、該位相シフタの出力を第１の位相比較回路の入力とするものである。
【００１５】
（５）外部参照信号に同期した光パルス列を発生する光パルス発生装置において、外部参照信号により駆動される光パルス発生手段と、該光パルス発生手段より発生する光パルスの一部を分岐する第１の光分岐手段と、該第１光分岐手段により分岐された光パルス列を受光する第１の受光器と、該第１の受光器の出力信号と外部参照信号の位相誤差を検出する第１の位相比較回路と、該第１の位相比較回路より出力される誤差信号を時間積分し、光パルス発生手段に負帰還する第１の積分回路と、前記出力光パルス列のパルス幅を狭窄化する光パルス圧縮手段と、該光パルス圧縮手段の出力の一部を分岐する第２の光分岐手段と、該第２の光分岐手段により分岐された光を受光する第２の受光器と、該第２の受光器の出力信号と外部参照信号の位相誤差を検出する第２の位相比較回路と、該第２の位相比較回路より出力される誤差信号を時間積分する第２の積分回路と、該第２の積分回路出力によって駆動され、外部参照信号を入力とする位相シフタとを備え、該位相シフタの出力を第１の位相比較回路の入力とするものである。
【００１６】
（６）外部参照信号に同期した光パルス列を発生する光パルス発生装置において、電圧制御発振器と、該電圧制御発振器により駆動される光パルス発生手段と、該光パルス発生手段より発生する光パルスの一部を分岐する第１の光分岐手段と、該第１の光分岐手段により分岐された光パルス列を受光する第１の受光器と、該第１の受光器の出力信号と外部参照信号の位相誤差を検出する第１の位相比較回路と、該第１の位相比較回路と第１の電圧制御発振器の間に誤差信号の低周波成分を通過させるループフィルタと、前記出力光パルス列のパルス幅を狭窄化する光パルス圧縮手段と、光パルス圧縮手段の出力の一部を分岐する第２の光分岐手段と、該第２の光分岐手段により分岐された光を受光する第２の受光器と、該第２の受光器の出力信号と外部参照信号の位相誤差を検出する第２の位相比較回路と、該第２の位相比較回路より出力される誤差信号の必要帯域を通過する第２のループフィルタと、該第２のループフィルタを通過した信号を入力とする電圧制御発振器とを備え、該電圧制御発振器の出力を第１の位相比較回路の入力とするものである。
【００１７】
（７）前記（３）乃至（６）のいずれか１つに記載の光パルス発生装置において、前記光パルス発生手段は、利得スイッチにより光パルス列を発生する半導体レーザを含むことを特徴とする。
【００１８】
【発明の実施の形態】
以下、図面を参照して本発明の実施形態（実施例）を詳細に説明する。
【００１９】
なお、実施形態（実施例）を説明するための全図において、同一機能を有するものは同一符号を付け、その繰り返しの説明は省略する。
【００２０】
（実施形態１）
図１は本発明の実施形態１の概略構成を示すブロック構成図である。
本発明の実施形態１の光パルス発生装置は、図１に示すように、電圧制御発振器（以下、単にＶＣＯと称する）１と、このＶＣＯ１により駆動される光パルス発生手段２と、光パルス発生手段２より発生する光パルス列の一部を分岐する光分岐手段３と、この光分岐手段３により分岐された光パルス列を受光する受光器４と、この受光器４の出力信号と外部参照信号の位相誤差を検出し、その誤差信号をＶＣＯ１に負帰還する位相比較器５と、この位相比較回路５と前記ＶＣＯ１の間に誤差信号の低周波成分を通過させるループフィルタ６から構成される。
【００２１】
前記光パルス発生手段２は、ＶＣＯ１の出力信号により駆動される。この出力光パルスの平均繰り返し周波数は、ＶＣＯ１のそれと同じである。ところが、そのタイミングジッタは、ＶＣＯ１の出力信号のそれと比較して一般に大きい。本発明では、光パルス発生手段２で加わるタイミングジッタの増加分を位相同期回路（ＰＬＬ）で補償する。すなわち、光パルス発生手段２の出力パルスの一部を光分岐し、分岐した光パルスを受光器４で光電変換する。その光電変換された信号と、基準となる外部参照信号との位相誤差を位相比較器５で抽出して、ループフィルタ６へ出力する。ループフィルタ６では誤差信号の低周波数成分のみを通過させて、ＶＣＯ１へ負帰還させることにより、出力パルス列の外部参照信号に対するタイミングジッタが減少するように光パルス発生手段２を制御する。
【００２２】
（実施形態２）
図２は本発明の実施形態２の概略構成を示すブロック構成図である。
本発明の実施形態２の光パルス発生装置は、図２に示すように、外部参照信号に同期した光パルス列を発生する光パルス発生装置において、外部参照信号により駆動される光パルス発生手段２と、この光パルス発生手段２より発生する光パルスの一部を分岐する光分岐手段３と、光分岐手段３により分岐された光パルス列を受光する受光器４と、この受光器４の出力信号と外部参照信号の位相誤差を検出する位相比較器５と、この位相比較器５より出力される誤差信号を時間積分し、光パルス発生手段２に負帰還する積分回路１００と、積分回路１００から出力信号と外部参照信号とを重畳させる加算手段１０２から構成され、加算手段１０２の出力信号により光パルス発生手段２を制御する。
【００２３】
光パルス発生手段２は直接外部参照信号により駆動されるため、出力光パルスの平均繰り返し周波数は、外部参照信号のそれと同じである。ところが、その光パルス発生手段２においてタイミングジッタが付加されるため、その出力光信号のタイミングジッタは、外部参照信号と比較して一般に大きい。本発明では、光パルス発生手段２で加わるタイミングジッタの増加分に対応する信号を光パルス発生手段２に負帰還することにより、出力パルスのタイミングジッタ増加を抑制圧する。
【００２４】
すなわち、光パルス発生手段２の出力パルスの一部を光分岐し、分岐した光パルスを受光器４で光電変換する。この光電変換された信号と、基準となる外部参照信号との位相誤差を位相比較器５で抽出して、積分回路１００へ出力する。積分回路１００では位相誤差信号の累積誤差である積分値を光パルス発生手段２へ負帰還させる。光パルス発生手段２では積分回路１００より出力された信号に対応して出力パルスの位相（時間的位置）が変化し、累積誤差を解消するため、出力パルス列の外部参照信号に対するタイミングジッタが減少するのである。
【００２５】
（実施形態３）
図３は本発明の実施形態３の概略構成を示すブロック構成図である。
本発明の実施形態３の光パルス発生装置は、図３に示すように、前記実施形態１の同様の構成である、ＶＣＯ１と、このＶＣＯ１により駆動される光パルス発生手段２と、光パルス信号の一部を分岐する第１の光分岐手段３と、この光分岐手段３により分岐された光パルス列を受光する第１の受光器４と、この受光器４の出力信号と外部参照信号の位相誤差を検出し、その誤差信号をＶＣＯ１に負帰還する第１の位相比較回路５と、この位相比較回路５と前記ＶＣＯ１の間に誤差信号の低周波成分を通過させるループフィルタ６からなる構成に、
出力光パルス列のパルス幅を狭窄化する光パルス圧縮手段７と、この光パルス圧縮手段７の出力の一部を分岐する第２の光分岐手段８と、第２の光分岐手段８により分岐された光を受光する第２の受光器９と、この受光器９の出力信号と外部参照信号の位相誤差を検出する第２の位相比較器１０と、この第２の位相比較回路の出力する誤差信号を時間積分する積分回路１０１と、この積分回路１０１からの出力によって制御され、外部参照信号を入力とする位相シフタ１２を追加し、この位相シフタ１２の出力を第１の位相比較器５の入力とするものである。本実施形態３では、前記光パルス発生手段２から出力した光パルス列を、更に短い時間幅の光パルスを得るために光ファイバパルス圧縮器を用いて狭窄化した場合にでも、外部参照信号に対するタイミングジッタの小さい光パルス列を得ることが可能な構成である。すなわち、一般的にパルス幅を狭窄化するために光パルス圧縮手段７として光フアイバパルス圧縮器が用いられるが、本発明は、この光パルス圧縮手段７の時間遅延変動をも補償するためのものである。
【００２６】
前記光パルス圧縮手段７を通過した光パルスは第２の光分岐手段８で分岐され、この分岐出力は第２の受光器９により光電変換される。電気信号に変換された光パルス列信号と、基準となる外部参照信号との位相誤差を第２の位相比較器１０で抽出し、これを積分回路１０１により時間積分することで位相誤差信号の累積誤差を得る。この累積誤差を外部参照信号を入力とする位相シフタ１２の制御信号として負帰還し、第１の位相比較器５に入力する。この結果、光パルス発生手段２から出力される光パルス列の位相は光パルス圧縮手段７で生じた位相の累積誤差を解消するように移相されるので、光パルス圧縮後の光パルス列の位相は外部参照信号と一致するのである。また、光パルス発生手段２によるタイミングジッタの増加に対しては、第１の光分岐手段３、第１の受光器４、第１の位相比較器５、ループフィルタ６から得られる光パルス列信号と参照信号との間の誤差信号を使って、実施形態１の発明と同様にタイミングジッタが減少するよう位相同期ループ制御する。
【００２７】
本実施形態３の構成は、第２の位相比較器１０の出力誤差信号を時間積分して、累積位相誤差を求めて位相シフタによりジッタ抑圧することができる。図３ではジッタを抑圧する第１のループに関して、前記実施形態１（図１）の構成である位相同期ループ構成を描いてあるが、前記実施形態２（図２）の光パルス発生手段に加える信号に位相制御信号を重畳する構成を採用してもよい。
【００２８】
（実施形態４）
図４は本発明の実施形態４の概略構成を示すブロック構成図である。
本発明の実施形態４の光パルス発生装置は、図４に示すように、前記実施形態１と同様の構成である、ＶＣＯ１と、このＶＣＯ１により駆動される光パルス発生手段２と、光パルス信号の一部を分岐する第１の光分岐手段３と、この光分岐手段３により分岐された光パルス列を受光する第１の受光器４と、この受光器４の出力信号と外部参照信号の位相誤差を検出し、その誤差信号をＶＣＯ１に負帰還する第１の位相比較回路５と、誤差信号の低周波成分を通過させるループフィルタ６からなる構成に、
出力光パルス列のパルス幅を狭窄化する光パルス圧縮手段７と、光パルス圧縮手段７の出力の一部を分岐する第２の光分岐手段８と、この光分岐手段８により分岐された光を受光する第２の受光器９と、この受光器９の出力信号と外部参照信号の位相誤差を検出する第２の位相比較回路１０と、この第２の位相比較器１０から出力する誤差信号の必要帯域を通過する第２のループフィルタ６１と、この通過した信号を入力とする第２のＶＣＯ１１を追加し、この第２のＶＣＯ１１の出力を第１の位相比較器５の入力としたものである。
【００２９】
本実施形態４では、光パルス発生手段２から出力した光パルス列を、更に短い時間幅の光パルスを得るために光ファイバパルス圧縮器を用いて狭窄化した場合にでも、外部参照信号に対するタイミングジッタの小さい光パルス列を得ることが可能な構成である。すなわち、一般的にパルス幅を狭窄化するために光パルス圧縮手段７として光フアイバパルス圧縮器が用いられるが、本発明は、この光パルス圧縮手段７の時間遅延変動をも補償するためのものである。
【００３０】
光パルス圧縮手段７を通過した光パルスは第２の光分岐手段８で分岐され、分岐出力は第２の受光器９により光電変換される。電気信号に変換された光パルス列信号と、基準となる外部参照信号との位相誤差を第２の位相比較器１０で抽出し、第２のループフィルタ６１へ出力する。ループフィルタ６１では位相誤差信号の低周波成分のみを通過させて、第２のＶＣＯ１１へ負帰還する。よって、第２のＶＣＯ１１からの出力周波数は時間遅延変動を解消するように制御される。この第２のＶＣＯ１１からの出力を第１の位相比較器５に入力する。この結果、光パルス発生手段２から出力される光パルス列の位相は光パルス圧縮手段７で生じた位相変化を相殺するので、光パルス圧縮後の光パルス列の位相は外部参照信号と一致するのである。また、光パルス発生手段２によるタイミングジッタの増加に対しては、第１の光分岐手段３、第１の受光器４、第１の位相比較器５、ループフィルタ６から得られる光パルス列信号と外部参照信号との間の誤差信号を使って、前記実施形態１と同様にタイミングジッタが減少するよう位相同期ループ制御する。図４ではジッタを抑圧する第１のループに関して、前記実施形態１の構成である位相同期ループ構成を描いてあるが、実施形態２の光パルス発生手段に加える信号に位相制御信号を重畳する構成を採用してもよい。
【００３１】
（実施例１）
図５は本発明の実施形態３に係わる実施例１の光パルス発生装置の概略構成を示すブロック構成図である。
本発明の実施例１の光パルス発生装置は、前記実施形態３の光パルス発生装置を具現化したものであり、ゲインスイッチ半導体レーザを光パルス発生源とし、パルス圧縮ファイバによってパルス幅を狭窄することにより、繰り返し周波数が数１０ＭＨｚ、パルス幅が１ｐｓ以下の低ジッタ光パルスを発生するための装置である。図５に示すように、光パルス発生手段２は、約２０ｐｓ程度のパルス幅をもつ、ゲインスイッチされるＤＦＢ半導体レーザ２０、このレーザに適当な直流（ＤＣ）バイアス電圧を与えるバイアス電源２１、数ＭＨｚの正弦波入力を時間幅が約２００ｐｓと短いスパイク波に変換してレーザを駆動するパルサ２２で構成される。ＶＣＯ１は同調範囲として１ＭＨｚ〜１０ＭＨｚが可能である。
【００３２】
３１は発振周波数設定用電圧源で、ＶＣＯ１の平均繰り返し周波数が参照信号とほぼ同じになるように設定するためのものである。第１の光分岐手段３及び第２の光分岐手段８として光カプラを用いる。光信号を電気信号に変換する受光器４、電気信号に変換された光パルス信号と外部参照信号の位相を比較する位相比較器５、及びループフィルタ６によりレーザのジッタを少なくするための位相同期ループが構成される。光パルス圧縮手段７としては、例えばソリトン圧縮効果により２０ｐｓ程度のパルス幅を１ｐｓ以下のパルス幅に圧縮する光パルス圧縮ファイバを用い、全長は約１ｋｍである。
【００３３】
光信号を電気信号に変換する受光器９、電気信号に変換された光パルス信号と外部参照信号の位相を比較して誤差信号を得る位相比較器１０、誤差信号を時間積分して位相変化量を求める積分回路１０１、位相変化量に応じた位相変化を外部参照信号に与える位相シフタ１２は前記実施形態３と同じものである。３０は外部参照信号を与える参照信号源である。
【００３４】
ここでは、繰り返し周波数１０ＭＨｚ程度の光パルスを発生する場合について説明する。始めにゲインスイッチから発生する光パルスのジッタを抑える位相同期ループの効果について述べる。
発振周波数設定用電圧源３１で１０ＭＨｚ近傍に繰り返し周波数を設定されたＶＣＯ１の出力により、ゲインスイッチ半導体レーザ２０からの繰り返し周波数約１０ＭＨｚの光パルスが発生する。位相同期ループ制御を行わない場合、レーザ２０やＶＣＯ１やパルサ２２及びゲインスイッチ半導体レーザのパルス発生過程で生じる、繰り返し周波数の半分以下（〜５ＭＨｚ）に広がる位相雑音により、この光パルスは数ｐｓ〜１０数ｐｓのジッタを持つ。この光パルスを受光器４で電気信号に変換し、位相比較器５で参照信号と比較すれば、直流（ＤＣ）〜５ＭＨｚに広がる誤差信号が得られる。誤差信号以外の不要な信号はカットオフ周波数が５ＭＨｚ以下のバントパス機能を有するループフィルタ６で取り除き、誤差信号だけを増幅してＶＣＯ１の入力電圧に負帰還する。その結果、ＶＣＯ１の瞬時周波数が誤差信号を小さくするように変化するので、光パルスのジッタが参照信号と同程度まで減少する。制御周波数帯域幅はＶＣＯ１の応答周波数特性、ループ遅延量により制限されるが、ＶＣＯ１の応答周波数特性として５ＭＨｚ、ループ遅延量として５ｎｓは容易に実現できるためジッタの周波数成分の全帯域にわたった制御が可能であり、十分にジッタを抑圧することができる。このときの抑圧されたジッタは外部参照信号のジッタレベルまで抑圧される。
【００３５】
次に、パルス圧縮の際のワンダを位相シフタ１２で抑える機構について述べる。レーザ２０から出力したパルス幅が２０ｐｓ程度の光パルスは長さ約１ｋｍのパルス圧縮ファイバ（７）により、１ｐｓ以下にパルス幅が圧縮される。圧縮ファイバの周辺温度が±５℃の範囲でゆっくりと変化すると、ファイバの全長も温度変化に伴う熱膨張のために±５ｃｍ程度変化する。このため光パルス出力の位相が±０．６°の範囲で変化する。この光パルス出力の一部を光カプラ８で分岐し、これを受光器９で電気信号に変換し、位相比較器１０で参照信号源３０から出た参照信号と比較し、ここで求まる位相誤差を積分回路１１で時間積分することで、参照信号に対する位相変化量を求める。
【００３６】
この位相変化量を１０ＭＨｚ信号に対して±０．６°以上の位相制御が可能な位相シフタ１２に制御信号として負帰還する。位相シフタ１２に入力された参照信号源３０からの参照信号は、圧縮ファイバによる位相の変化を相殺するように位相を制御されて出力される。この位相を制御された参照信号を先に説明した位相同期ループの参照信号とするので、ゲインスイッチ半導体レーザ２０から発生する光パルス列の位相も、圧縮ファイバによる位相変化を減少するように制御される。この結果、光パルス列出力は参照信号源３０に同期することになるので、本発明の目的の一つである圧縮ファイバに起因するワンダの抑圧が実現する。位相シフタは温度変化の速度程度で追随すれば十分であり、絶対遅延量±５ｃｍ程度の位相シフタは容易に実現できる。また、作用の章で述べたように積分回路と位相シフタの代わりにループフィルタと低雑音ＶＣＯを用いても同様のワンダ抑圧が可能である。
【００３７】
（実施例２）
本発明の実施形態２に係わる実施例２は、前記実施例１のパルス発生手段２のジッタ抑圧のためのＰＬＬの代わりに、積分回路とパルス位相制御を用いた例である。
ここでは、図６に示すように、位相比較器５の出力信号である位相誤差信号を時間積分回路１００へ入力することにより累積誤差に対応した信号が得られる。この信号をゲインスイッチ半導体レーザ２０のバイアス電流に負帰還する構成を採用している。ゲインスイッチ半導体レーザは、一般に閾値電流よりも低い電流でバイアスされており、その範囲で、バイアス電流が大きいとパルサからのスパイク上電流印可時点からの光パルス発生の時間遅延量が小さくなり、バイアス電流が小さいと遅延量が大きくなる。従って、累積遅延量を相殺するように積分回路１００からの信号をバイアス電流に重畳することにより光パルス発生手段２で生じるタイミングジッタを抑圧することができる。ゲインスイッチ半導体レーザ２０のバイアス電流変化に対する応答は十分に速いので、前記実施例１と同じく、ジッタの存在する全周波数範囲（直流から繰り返し周波数の半分まで）にわたって制御帯域幅内に含めることが可能であり、十分なジッタ抑圧が可能である。
【００３８】
以上、本発明を実施形態及び実施例に基づき具体的に説明したが、本発明は前記実施形態及び実施例に限定されるものではなく、その要旨を逸脱しない範囲において種々変更し得ることはいうまでもない。
【００３９】
【発明の効果】
以上のように、本発明によれば、サンプリング光源に適した同調範囲が広く、繰り返し周波数が数１０ＭＨｚで、かつ、パルス幅が１ｐｓ程度の低ジッタ光パルス列を発生する装置を提供することができる。
【図面の簡単な説明】
【図１】本発明の実施形態１の光パルス発生装置の概略構成を示すブロック構成図である。
【図２】本発明の実施形態２の光パルス発生装置の概略構成を示すブロック構成図である。
【図３】本発明の実施形態３の光パルス発生装置の概略構成を示すブロック構成図である。
【図４】本発明の実施形態４の光パルス発生装置の概略構成を示すブロック構成図である。
【図５】本発明の実施形態３に係わる実施例１の光パルス発生装置の概略構成を示すブロック構成図である。
【図６】本発明の実施形態２に係わる実施例２の光パルス発生装置の概略構成を示すブロック構成図である。
【符号の説明】
１，１１…電圧制御発振器（ＶＣＯ）、２…光パルス発生手段、
３，８…光分岐手段（光カプラ）、４，９…受光器、５，１０…位相比較器、
６，６１…ループフィルタ、７…光パルス圧縮手段（光パルス気縮ファイバ）、１２…位相シフタ、／第１のループフィルタ、２０…ゲインスイッチ半導体レーザ、２１…バイアス電源、２２…パルサ、３０…参照信号源、３１…発振周波数設定用電圧源、１００，１０１…積分回路、１０２…加算手段。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical pulse generator used for optical sampling and the like, which has a variable repetition frequency and low phase noise and a narrow pulse width.
[0002]
[Prior art]
The performance desired as an optical sampling light source is that the repetition frequency is variable so as to be able to cope with signals under measurement of various frequencies, and the frequency division ratio (the ratio between the frequency of the signal under measurement and the sampling frequency) N (integer) Pulse generation at repetition frequencies of several tens of MHz or less is possible, and pulse width is as narrow as 1 ps or less for high time resolution. In addition, timing jitter (jitter is high-speed temporal fluctuation) Is desired to be small.
[0003]
As conventional simple and compact optical pulse generators, (1) a semiconductor mode-locked laser device, (2) an apparatus for generating an optical pulse train by cutting out continuous light generated from a continuous light source by an optical modulator, and (3) a gain switch There are three devices that generate a light pulse train by driving a semiconductor laser with a pulse signal.
[0004]
[Problems to be solved by the invention]
The present inventor has found the following problems as a result of studying the above-described conventional technology.
In the mode-locked laser of the above (1), the repetition frequency f_repIs f_rep= C / 2nL (c: speed of light n: refractive index L: resonator length) and resonator length L, so that the tunability to an external signal is poor. Further, from this equation, when the repetition frequency is several tens MHz or less, the resonator length L is required to be several meters or more, mechanical instability increases, and the scale of the device increases.
[0005]
In the device (2), the optical pulse train obtained by optically modulating the continuous light can be easily synchronized with an external reference signal, but the pulse width is determined by the modulation bandwidth of the optical modulator and the drive electric signal of the optical modulator. Limited by bandwidth. Since the bandwidth of the drive signal generation circuit and the optical modulator is at most 50 GHz, it is difficult to generate a pulse having a pulse width of several tens ps or less. In the drive signal generation circuit, timing jitter is added.
[0006]
In the device using the gain switch semiconductor laser of the above (3), an optical pulse train having a pulse width of about 10 ps can be generated relatively easily, but relatively large jitter is added in the process of generating the gain switch pulse.
[0007]
Therefore, both (2) and (3) have problems of large timing jitter and wide pulse width when used as a measurement light source. The pulse width can be reduced to 1 ps or less by a linear / nonlinear optical pulse compression technique using an optical fiber. However, since an optical pulse compression fiber of several tens of meters or more is used for the compression, the delay amount changes only when the temperature around the fiber changes by a few degrees (° C.), and the phase of the optical pulse train after the optical pulse compression greatly changes. However, there is a problem that wander (which means low-speed temporal fluctuation) increases.
[0008]
An object of the present invention is to provide a technique capable of generating an optical pulse train having a repetition frequency of several hundreds of kHz to several tens of MHz and a pulse width of 1 ps or less and having both small timing jitter and small wander.
[0009]
The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.
[0010]
[Means for Solving the Problems]
The outline of a representative invention among the inventions disclosed in the present application will be briefly described as follows.
[0011]
(1) In an optical pulse generator for generating an optical pulse train synchronized with an external reference signal, a voltage controlled oscillator, an optical pulse generating means driven by the voltage controlled oscillator, and an optical pulse generated by the optical pulse generating means. An optical branching means for branching a part, a light receiver for receiving the optical pulse train branched by the light branching means, a phase error between an output signal of the light receiver and an external reference signal, and the error signal is converted to the voltage. A phase comparison circuit that performs negative feedback to the control oscillator; and a loop filter that passes a low-frequency component of the error signal between the phase comparison circuit and the voltage control oscillator.The optical pulse generating means includes a semiconductor laser which generates an optical pulse train by a gain switch.
[0012]
(2) In an optical pulse generator for generating an optical pulse train synchronized with an external reference signal, an optical pulse generating means driven by the external reference signal, and light for branching a part of the optical pulse generated by the optical pulse generating means Branching means, a light receiver for receiving the optical pulse train branched by the light branching means, a phase comparator for detecting a phase error between an output signal of the light receiver and an external reference signal, and an output from the phase comparator An integration circuit for integrating the error signal over time and negatively feeding back the optical pulse generation means.The optical pulse generating means includes a semiconductor laser which generates an optical pulse train by a gain switch.
[0013]
(3) In an optical pulse generator for generating an optical pulse train synchronized with an external reference signal, a first voltage controlled oscillator, an optical pulse generating means driven by the first voltage controlled oscillator, and an optical pulse generating means First light branching means for branching a part of the light pulse generated by the first light branching means, a first light receiving device for receiving the light pulse train branched by the first light branching means, and an output of the first light receiving device A first phase comparison circuit for detecting a phase error between the signal and the external reference signal, a loop filter for passing a low frequency component of the error signal between the first phase comparison circuit and the first voltage controlled oscillator, An optical pulse compression means for narrowing the pulse width of the output optical pulse train, a second optical branching means for branching a part of the output of the optical pulse compression means, and a light branched by the second optical branching means. A second light receiver for receiving light; A second phase comparison circuit that detects a phase error between the output signal of the light receiver and the external reference signal, a second loop filter that passes a required band of an error signal output from the second phase comparison circuit, A second voltage-controlled oscillator that receives a signal that has passed through the second loop filter as an input, and uses an output of the second voltage-controlled oscillator as an input of the first phase comparison circuit.
[0014]
(4) In an optical pulse generator for generating an optical pulse train synchronized with an external reference signal, a voltage controlled oscillator, an optical pulse generating means driven by the voltage controlled oscillator, and an optical pulse generated by the optical pulse generating means A first optical branching unit for branching a part, a first optical receiver for receiving the optical pulse train branched by the first optical branching unit, and an output signal of the first optical receiver and an external reference signal. A first phase comparison circuit for detecting a phase error, a loop filter for passing a low frequency component of an error signal between the first phase comparison circuit and the first voltage controlled oscillator, and a pulse width of the output optical pulse train Pulse compressing means for narrowing the wavelength, a second optical branching means for branching a part of the output of the optical pulse compressing means, and a second light receiving device for receiving the light branched by the second optical branching means And the output of the second receiver A second phase comparison circuit for detecting a phase error between the signal and the external reference signal, an integration circuit for time-integrating the error signal output from the second phase comparison circuit, and an external reference driven by the output of the integration circuit. A phase shifter to which a signal is input, and an output of the phase shifter to be an input of the first phase comparison circuit.
[0015]
(5) In an optical pulse generator for generating an optical pulse train synchronized with an external reference signal, an optical pulse generator driven by the external reference signal and a part of a part of an optical pulse generated by the optical pulse generator are branched. A first optical branching unit, a first optical receiver for receiving the optical pulse train branched by the first optical branching unit, and a first optical detector for detecting a phase error between an output signal of the first optical receiver and an external reference signal. , A first integration circuit that integrates the error signal output from the first phase comparison circuit with time, and negatively feeds back to the optical pulse generation means, and narrows the pulse width of the output optical pulse train. An optical pulse compressing unit, a second optical branching unit that branches a part of the output of the optical pulse compressing unit, a second light receiving device that receives the light branched by the second optical branching unit, Output signal of second receiver and external reference signal A second phase comparison circuit for detecting a phase error, a second integration circuit for time-integrating an error signal output from the second phase comparison circuit, and an external reference driven by the output of the second integration circuit A phase shifter to which a signal is input, and an output of the phase shifter to be an input of the first phase comparison circuit.
[0016]
(6) In an optical pulse generator for generating an optical pulse train synchronized with an external reference signal, a voltage controlled oscillator, an optical pulse generating means driven by the voltage controlled oscillator, and an optical pulse generated by the optical pulse generating means. A first optical branching unit for branching a part, a first optical receiver for receiving the optical pulse train branched by the first optical branching unit, and an output signal of the first optical receiver and an external reference signal. A first phase comparison circuit for detecting a phase error, a loop filter for passing a low frequency component of an error signal between the first phase comparison circuit and the first voltage controlled oscillator, and a pulse width of the output optical pulse train Pulse compressing means for narrowing the wavelength, a second optical branching means for branching a part of the output of the optical pulse compressing means, and a second light receiving device for receiving the light branched by the second optical branching means And the output of the second receiver A second phase comparison circuit for detecting a phase error between the signal and the external reference signal, a second loop filter passing a required band of an error signal output from the second phase comparison circuit, and the second loop A voltage-controlled oscillator having a signal passed through the filter as an input, and having an output of the voltage-controlled oscillator as an input of the first phase comparison circuit.
[0017]
(7)In the optical pulse generator according to any one of (3) to (6),The optical pulse generating means includes a semiconductor laser that generates an optical pulse train by a gain switch.Characterized by.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment (example) of the present invention will be described in detail with reference to the drawings.
[0019]
In all the drawings for describing the embodiments (examples), those having the same functions are denoted by the same reference numerals, and their repeated description will be omitted.
[0020]
(Embodiment 1)
FIG. 1 is a block diagram showing a schematic configuration of the first embodiment of the present invention.
As shown in FIG. 1, an optical pulse generator according to a first embodiment of the present invention includes a voltage controlled oscillator (hereinafter, simply referred to as a VCO) 1, an optical pulse generator 2 driven by the VCO 1, an optical pulse generator, An optical branching unit 3 for branching a part of the optical pulse train generated by the unit 2; a light receiver 4 for receiving the optical pulse train branched by the light branching unit 3; an output signal of the light receiver 4 and an external reference signal It comprises a phase comparator 5 for detecting a phase error and negatively feeding back the error signal to the VCO 1, and a loop filter 6 for passing a low frequency component of the error signal between the phase comparator 5 and the VCO 1.
[0021]
The light pulse generator 2 is driven by an output signal of the VCO 1. The average repetition frequency of this output light pulse is the same as that of VCO1. However, the timing jitter is generally larger than that of the output signal of the VCO 1. In the present invention, the increase in the timing jitter applied by the optical pulse generation means 2 is compensated by a phase locked loop (PLL). That is, a part of the output pulse of the optical pulse generating means 2 is optically branched, and the optical pulse is photoelectrically converted by the optical receiver 4. A phase error between the photoelectrically converted signal and a reference external reference signal is extracted by a phase comparator 5 and output to a loop filter 6. The loop filter 6 passes only the low frequency component of the error signal and performs negative feedback on the VCO 1 to control the optical pulse generation means 2 so that the timing jitter of the output pulse train with respect to the external reference signal is reduced.
[0022]
(Embodiment 2)
FIG. 2 is a block diagram showing a schematic configuration of the second embodiment of the present invention.
As shown in FIG. 2, an optical pulse generator according to a second embodiment of the present invention includes an optical pulse generator that generates an optical pulse train synchronized with an external reference signal. An optical splitter 3 for splitting a part of the optical pulse generated by the optical pulse generator 2, a light receiver 4 for receiving the optical pulse train split by the light splitter 3, and an output signal of the light receiver 4. A phase comparator 5 for detecting a phase error of the external reference signal, an integration circuit 100 for integrating the error signal output from the phase comparator 5 with time, and negatively feeding back to the optical pulse generating means 2, and an output from the integration circuit 100 It comprises an adding means 102 for superimposing a signal and an external reference signal, and controls the light pulse generating means 2 by an output signal of the adding means 102.
[0023]
Since the optical pulse generating means 2 is directly driven by the external reference signal, the average repetition frequency of the output optical pulse is the same as that of the external reference signal. However, since timing jitter is added in the optical pulse generating means 2, the timing jitter of the output optical signal is generally larger than that of the external reference signal. In the present invention, the signal corresponding to the increase in the timing jitter applied by the optical pulse generating means 2 is negatively fed back to the optical pulse generating means 2 to suppress the increase in the timing jitter of the output pulse.
[0024]
That is, a part of the output pulse of the optical pulse generating means 2 is optically branched, and the optical pulse is photoelectrically converted by the optical receiver 4. A phase error between the photoelectrically converted signal and a reference external reference signal is extracted by the phase comparator 5 and output to the integration circuit 100. In the integrating circuit 100, the integrated value, which is the accumulated error of the phase error signal, is negatively fed back to the optical pulse generating means 2. In the optical pulse generation means 2, the phase (time position) of the output pulse changes in accordance with the signal output from the integration circuit 100, and the accumulated error is eliminated, so that the timing jitter of the output pulse train with respect to the external reference signal is reduced. It is.
[0025]
(Embodiment 3)
FIG. 3 is a block diagram showing a schematic configuration of the third embodiment of the present invention.
As shown in FIG. 3, an optical pulse generator according to a third embodiment of the present invention has a VCO 1 having the same configuration as that of the first embodiment, an optical pulse generator 2 driven by the VCO 1, and an optical pulse signal. , A first light receiving unit 4 for receiving the optical pulse train branched by the light branching unit 3, and a phase of an output signal of the light receiving unit 4 and an external reference signal. A first phase comparison circuit 5 for detecting an error and negatively feeding back the error signal to the VCO 1 and a loop filter 6 for passing a low frequency component of the error signal between the phase comparison circuit 5 and the VCO 1 ,
The optical pulse compression means 7 for narrowing the pulse width of the output optical pulse train, the second optical branching means 8 for branching a part of the output of the optical pulse compression means 7, and the second optical branching means 8 A second light receiver 9 for receiving the reflected light, a second phase comparator 10 for detecting a phase error between an output signal of the light receiver 9 and an external reference signal, and an error output from the second phase comparison circuit. An integrating circuit 101 for integrating the signal with time and a phase shifter 12 controlled by an output from the integrating circuit 101 and having an external reference signal as an input are added. The output of the phase shifter 12 is used as the output of the first phase comparator 5. Input. In the third embodiment, even when the optical pulse train output from the optical pulse generating means 2 is narrowed using an optical fiber pulse compressor in order to obtain an optical pulse having a shorter time width, the timing with respect to the external reference signal is reduced. This is a configuration capable of obtaining an optical pulse train with small jitter. That is, in general, an optical fiber pulse compressor is used as the optical pulse compression means 7 for narrowing the pulse width. However, the present invention is intended to compensate for the time delay fluctuation of the optical pulse compression means 7. It is.
[0026]
The light pulse that has passed through the light pulse compression means 7 is split by a second light splitting means 8, and the split output is photoelectrically converted by a second light receiver 9. The phase error between the optical pulse train signal converted into the electric signal and the external reference signal serving as a reference is extracted by the second phase comparator 10, and this is time-integrated by the integration circuit 101 to obtain the accumulated error of the phase error signal. Get. The accumulated error is negatively fed back as a control signal of the phase shifter 12 to which the external reference signal is input, and is input to the first phase comparator 5. As a result, the phase of the optical pulse train output from the optical pulse generating means 2 is shifted so as to eliminate the accumulated error of the phase generated by the optical pulse compressing means 7, so that the phase of the optical pulse train after the optical pulse compression becomes It matches the external reference signal. In addition, with respect to the increase in the timing jitter caused by the optical pulse generating means 2, the optical pulse train signal obtained from the first optical branching means 3, the first optical receiver 4, the first phase comparator 5, and the loop filter 6 is used. Using the error signal between the reference signal and the reference signal, the phase locked loop is controlled so as to reduce the timing jitter as in the first embodiment.
[0027]
In the configuration of the third embodiment, the output error signal of the second phase comparator 10 is time-integrated, the accumulated phase error is obtained, and the jitter can be suppressed by the phase shifter. FIG. 3 illustrates a phase locked loop configuration which is the configuration of the first embodiment (FIG. 1) with respect to the first loop for suppressing jitter, but is added to the optical pulse generating means of the second embodiment (FIG. 2). A configuration in which a phase control signal is superimposed on a signal may be employed.
[0028]
(Embodiment 4)
FIG. 4 is a block diagram showing a schematic configuration of the fourth embodiment of the present invention.
As shown in FIG. 4, an optical pulse generator according to a fourth embodiment of the present invention has a VCO 1, an optical pulse generator 2 driven by the VCO 1, and an optical pulse signal , A first light receiving unit 4 for receiving the optical pulse train branched by the light branching unit 3, and a phase of an output signal of the light receiving unit 4 and an external reference signal. A first phase comparison circuit 5 that detects an error and negatively feeds back the error signal to the VCO 1 and a loop filter 6 that passes a low-frequency component of the error signal,
An optical pulse compression unit 7 for narrowing the pulse width of the output optical pulse train, a second optical branching unit 8 for branching a part of the output of the optical pulse compression unit 7, and a light branched by the optical branching unit 8. A second photodetector 9 for receiving light, a second phase comparison circuit 10 for detecting a phase error between an output signal of the photodetector 9 and an external reference signal, and an error signal output from the second phase comparator 10 A second loop filter 61 that passes a required band and a second VCO 11 that receives the passed signal are added, and the output of the second VCO 11 is used as an input of the first phase comparator 5. is there.
[0029]
In the fourth embodiment, even when the optical pulse train output from the optical pulse generator 2 is narrowed using an optical fiber pulse compressor to obtain an optical pulse having a shorter time width, the timing jitter with respect to the external reference signal is reduced. This is a configuration capable of obtaining an optical pulse train with a small value. That is, in general, an optical fiber pulse compressor is used as the optical pulse compression means 7 for narrowing the pulse width. However, the present invention is intended to compensate for the time delay fluctuation of the optical pulse compression means 7. It is.
[0030]
The light pulse that has passed through the light pulse compression means 7 is split by the second light splitting means 8, and the split output is photoelectrically converted by the second light receiver 9. The phase error between the optical pulse train signal converted into the electric signal and the external reference signal serving as a reference is extracted by the second phase comparator 10 and output to the second loop filter 61. In the loop filter 61, only the low frequency component of the phase error signal is passed and negative feedback is provided to the second VCO 11. Therefore, the output frequency from the second VCO 11 is controlled so as to eliminate the time delay fluctuation. The output from the second VCO 11 is input to the first phase comparator 5. As a result, the phase of the optical pulse train output from the optical pulse generating means 2 cancels the phase change generated by the optical pulse compressing means 7, so that the phase of the optical pulse train after the optical pulse compression matches the external reference signal. . In addition, with respect to the increase in the timing jitter caused by the optical pulse generating means 2, the optical pulse train signal obtained from the first optical branching means 3, the first optical receiver 4, the first phase comparator 5, and the loop filter 6 is used. As in the first embodiment, the phase locked loop control is performed so as to reduce the timing jitter by using the error signal with the external reference signal. FIG. 4 illustrates a phase locked loop configuration as the configuration of the first embodiment with respect to the first loop for suppressing jitter, but a configuration in which a phase control signal is superimposed on a signal to be applied to the optical pulse generating means of the second embodiment. May be adopted.
[0031]
(Example 1)
FIG. 5 is a block diagram showing a schematic configuration of the optical pulse generator of Example 1 according to Embodiment 3 of the present invention.
The optical pulse generator according to the first embodiment of the present invention is an embodiment of the optical pulse generator according to the third embodiment. The optical pulse generator is an optical pulse generation source, and the pulse width is narrowed by a pulse compression fiber. This is an apparatus for generating a low-jitter optical pulse having a repetition frequency of several tens of MHz and a pulse width of 1 ps or less. As shown in FIG. 5, the optical pulse generating means 2 includes a gain-switched DFB semiconductor laser 20 having a pulse width of about 20 ps, a bias power supply 21 for applying an appropriate direct current (DC) bias voltage to this laser, The pulsar 22 is configured to convert a sine wave input of MHz into a spike wave having a short time width of about 200 ps and drive the laser. The VCO 1 can have a tuning range of 1 MHz to 10 MHz.
[0032]
Reference numeral 31 denotes an oscillation frequency setting voltage source for setting the average repetition frequency of the VCO 1 to be substantially the same as the reference signal. Optical couplers are used as the first optical branching means 3 and the second optical branching means 8. A photodetector 4 for converting an optical signal into an electric signal, a phase comparator 5 for comparing the phase of an optical pulse signal converted to an electric signal with an external reference signal, and a phase synchronization for reducing laser jitter by a loop filter 6. A loop is formed. As the optical pulse compression means 7, for example, an optical pulse compression fiber for compressing a pulse width of about 20 ps to a pulse width of 1 ps or less by a soliton compression effect is used, and has a total length of about 1 km.
[0033]
A photodetector 9 for converting an optical signal into an electric signal, a phase comparator 10 for comparing an optical pulse signal converted into an electric signal with a phase of an external reference signal to obtain an error signal, and a phase change amount by time-integrating the error signal And the phase shifter 12 that applies a phase change according to the phase change amount to the external reference signal are the same as those in the third embodiment. Reference numeral 30 denotes a reference signal source for providing an external reference signal.
[0034]
Here, a case where an optical pulse having a repetition frequency of about 10 MHz is generated will be described. First, the effect of the phase locked loop for suppressing the jitter of the optical pulse generated from the gain switch will be described.
An optical pulse having a repetition frequency of about 10 MHz is generated from the gain switch semiconductor laser 20 by the output of the VCO 1 whose repetition frequency is set to about 10 MHz by the oscillation frequency setting voltage source 31. If the phase-locked loop control is not performed, this optical pulse will be several ps to less than half the repetition frequency (up to 5 MHz) generated in the pulse generation process of the laser 20, VCO 1, pulser 22, and gain switch semiconductor laser. It has a jitter of more than 10 ps. When this light pulse is converted into an electric signal by the light receiver 4 and compared with a reference signal by the phase comparator 5, an error signal extending from direct current (DC) to 5 MHz can be obtained. Unnecessary signals other than the error signal are removed by a loop filter 6 having a band-pass function with a cutoff frequency of 5 MHz or less, and only the error signal is amplified and negatively fed back to the input voltage of the VCO 1. As a result, since the instantaneous frequency of the VCO 1 changes so as to reduce the error signal, the jitter of the optical pulse is reduced to the same level as the reference signal. Although the control frequency bandwidth is limited by the response frequency characteristic of the VCO 1 and the loop delay amount, the response frequency characteristic of the VCO 1 can be easily set to 5 MHz and the loop delay amount can be easily set to 5 ns. And jitter can be sufficiently suppressed. The suppressed jitter at this time is suppressed to the jitter level of the external reference signal.
[0035]
Next, a mechanism for suppressing wander during pulse compression by the phase shifter 12 will be described. The optical pulse output from the laser 20 and having a pulse width of about 20 ps is compressed to a pulse width of 1 ps or less by a pulse compression fiber (7) having a length of about 1 km. If the temperature around the compressed fiber changes slowly within the range of ± 5 ° C., the total length of the fiber also changes by about ± 5 cm due to thermal expansion accompanying the temperature change. Therefore, the phase of the optical pulse output changes within a range of ± 0.6 °. A part of this optical pulse output is branched by an optical coupler 8, converted into an electric signal by a light receiver 9, compared with a reference signal output from a reference signal source 30 by a phase comparator 10, and a phase error calculated here is obtained. Is time-integrated by the integration circuit 11 to obtain a phase change amount with respect to the reference signal.
[0036]
The amount of this phase change is negatively fed back as a control signal to the phase shifter 12 capable of controlling the phase by ± 0.6 ° or more with respect to the 10 MHz signal. The reference signal from the reference signal source 30 input to the phase shifter 12 is output with its phase controlled so as to cancel the change in phase due to the compression fiber. Since the reference signal whose phase is controlled is used as the reference signal of the above-described phase locked loop, the phase of the optical pulse train generated from the gain switch semiconductor laser 20 is also controlled so as to reduce the phase change due to the compression fiber. . As a result, since the output of the optical pulse train is synchronized with the reference signal source 30, the suppression of wander caused by the compression fiber, which is one of the objects of the present invention, is realized. It is sufficient for the phase shifter to follow at the speed of temperature change, and a phase shifter with an absolute delay amount of about ± 5 cm can be easily realized. Also, as described in the section on operation, similar wander suppression can be achieved by using a loop filter and a low-noise VCO instead of the integrating circuit and the phase shifter.
[0037]
(Example 2)
Example 2 according to Example 2 of the present invention is an example in which an integrating circuit and pulse phase control are used instead of the PLL for jitter suppression of the pulse generation means 2 of Example 1 described above.
Here, as shown in FIG. 6, a signal corresponding to the accumulated error is obtained by inputting a phase error signal, which is an output signal of the phase comparator 5, to the time integration circuit 100. A configuration in which this signal is negatively fed back to the bias current of the gain switch semiconductor laser 20 is adopted. The gain switch semiconductor laser is generally biased at a current lower than the threshold current.In this range, if the bias current is large, the amount of time delay of light pulse generation from the time of application of the current on the spike from the pulser becomes small, and When the current is small, the delay amount increases. Therefore, by superimposing the signal from the integration circuit 100 on the bias current so as to cancel the accumulated delay amount, it is possible to suppress the timing jitter generated in the optical pulse generator 2. Since the response of the gain switch semiconductor laser 20 to the change in the bias current is sufficiently fast, the gain switch semiconductor laser 20 can be included in the control bandwidth over the entire frequency range where jitter exists (from DC to half the repetition frequency), as in the first embodiment. Thus, sufficient jitter suppression is possible.
[0038]
As described above, the present invention has been specifically described based on the embodiments and the examples. However, the present invention is not limited to the embodiments and the examples, and various changes can be made without departing from the gist of the present invention. Not even.
[0039]
【The invention's effect】
As described above, according to the present invention, it is possible to provide an apparatus that generates a low-jitter optical pulse train having a wide tuning range suitable for a sampling light source, a repetition frequency of several tens of MHz, and a pulse width of about 1 ps. .
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration of an optical pulse generator according to a first embodiment of the present invention.
FIG. 2 is a block diagram showing a schematic configuration of an optical pulse generator according to a second embodiment of the present invention.
FIG. 3 is a block diagram illustrating a schematic configuration of an optical pulse generator according to a third embodiment of the present invention.
FIG. 4 is a block diagram showing a schematic configuration of an optical pulse generator according to a fourth embodiment of the present invention.
FIG. 5 is a block diagram showing a schematic configuration of an optical pulse generator of Example 1 according to Embodiment 3 of the present invention.
FIG. 6 is a block diagram showing a schematic configuration of an optical pulse generator of Example 2 according to Embodiment 2 of the present invention.
[Explanation of symbols]
1, 11: voltage controlled oscillator (VCO), 2: optical pulse generating means,
3, 8 ... optical branching means (optical coupler), 4, 9 ... light receiver, 5, 10 ... phase comparator,
6, 61: loop filter, 7: optical pulse compression means (optical pulse compression fiber), 12: phase shifter, first loop filter, 20: gain switch semiconductor laser, 21: bias power supply, 22: pulser, 30 ... Reference signal source, 31 oscillation voltage setting voltage source, 100, 101 integrating circuit, 102 addition means.

Claims

An optical pulse generator for generating an optical pulse train synchronized with an external reference signal, comprising: a voltage controlled oscillator; an optical pulse generating means driven by the voltage controlled oscillator; and a part of an optical pulse generated by the optical pulse generating means. A light branching means for branching, a light receiver for receiving the optical pulse train branched by the light branching means, a phase error between an output signal of the light receiver and an external reference signal, and the error signal is sent to the voltage controlled oscillator. A phase comparison circuit that performs negative feedback; and a loop filter that passes a low-frequency component of an error signal between the phase comparison circuit and the voltage-controlled oscillator. The optical pulse generation unit generates an optical pulse train by a gain switch. An optical pulse generator including a semiconductor laser .

In an optical pulse generator for generating an optical pulse train synchronized with an external reference signal, an optical pulse generating means driven by the external reference signal, and an optical branching means for splitting a part of an optical pulse generated by the optical pulse generating means, A light receiver for receiving the optical pulse train branched by the light branching unit, a phase comparator for detecting a phase error between an output signal of the light receiver and an external reference signal, and an error signal output from the phase comparator. An optical pulse generating apparatus, comprising: an integrating circuit that performs time integration and negatively feeds back to an optical pulse generating means, wherein the optical pulse generating means includes a semiconductor laser that generates an optical pulse train by a gain switch .

An optical pulse generator for generating an optical pulse train synchronized with an external reference signal, a first voltage controlled oscillator, an optical pulse generating means driven by the first voltage controlled oscillator, and an optical pulse generated by the optical pulse generating means A first optical branching unit for branching a part of the optical pulse, a first optical receiver for receiving the optical pulse train branched by the first optical branching unit, and an output signal of the first optical receiver and an external signal A first phase comparison circuit for detecting a phase error of a reference signal; a loop filter for passing a low-frequency component of an error signal between the first phase comparison circuit and a first voltage-controlled oscillator; Optical pulse compressing means for narrowing the pulse width of the optical pulse compressing means, second optical splitting means for splitting a part of the output of the optical pulse compressing means, and a second optical splitting means for receiving the light split by the second optical splitting means. Two light receivers and the second light receiver A second phase comparison circuit for detecting a phase error between the output signal of the second phase comparison circuit and the external reference signal, a second loop filter passing a required band of the error signal output from the second phase comparison circuit, A second voltage-controlled oscillator having a signal passed through the loop filter as an input, wherein an output of the second voltage-controlled oscillator is used as an input of a first phase comparison circuit. .

An optical pulse generator for generating an optical pulse train synchronized with an external reference signal, comprising: a voltage controlled oscillator; an optical pulse generating means driven by the voltage controlled oscillator; and a part of an optical pulse generated by the optical pulse generating means. First optical branching means for branching, a first light receiver for receiving the optical pulse train branched by the first light branching means, and a phase error between an output signal of the first light receiver and an external reference signal. A first phase comparison circuit for detecting, a loop filter for passing a low frequency component of an error signal between the first phase comparison circuit and the first voltage controlled oscillator, and a narrowing of a pulse width of the output optical pulse train An optical pulse compressing means, a second optical branching means for branching a part of the output of the optical pulse compressing means, a second light receiving device for receiving the light branched by the second optical branching means, The output signal of the second receiver and A second phase comparison circuit for detecting a phase error of the reference signal, an integration circuit for time-integrating an error signal output from the second phase comparison circuit, and an external reference signal driven by the output of the integration circuit. An optical pulse generator, comprising: a phase shifter as an input, wherein an output of the phase shifter is used as an input of a first phase comparison circuit.

In an optical pulse generator for generating an optical pulse train synchronized with an external reference signal, an optical pulse generating means driven by the external reference signal, and a first light for branching a part of the optical pulse generated by the optical pulse generating means Branching means, a first light receiver for receiving the optical pulse train branched by the first light branching means, and a first phase comparison for detecting a phase error between an output signal of the first light receiver and an external reference signal Circuit, a first integration circuit for integrating the error signal output from the first phase comparison circuit with time, and performing negative feedback to the optical pulse generation means, and an optical pulse compression for narrowing the pulse width of the output optical pulse train. Means, a second light branching means for branching a part of the output of the light pulse compression means, a second light receiving device for receiving the light branched by the second light branching means, Phase of the output signal of the receiver and the external reference signal A second phase comparison circuit for detecting a difference, a second integration circuit for time-integrating an error signal output from the second phase comparison circuit, and an external reference signal driven by an output of the second integration circuit An optical pulse generator, comprising: a phase shifter having an input of the phase shifter; and an output of the phase shifter being an input of a first phase comparison circuit.

An optical pulse generator for generating an optical pulse train synchronized with an external reference signal, comprising: a voltage controlled oscillator; an optical pulse generating means driven by the voltage controlled oscillator; and a part of an optical pulse generated by the optical pulse generating means. First optical branching means for branching, a first light receiver for receiving the optical pulse train branched by the first light branching means, and a phase error between an output signal of the first light receiver and an external reference signal. A first phase comparison circuit for detecting, a loop filter for passing a low frequency component of an error signal between the first phase comparison circuit and the first voltage controlled oscillator, and narrowing a pulse width of the output optical pulse train An optical pulse compressing means, a second optical branching means for branching a part of the output of the optical pulse compressing means, a second light receiving device for receiving the light branched by the second optical branching means, The output signal of the second receiver and A second phase comparison circuit for detecting a phase error of the reference signal, a second loop filter passing a required band of an error signal output from the second phase comparison circuit, and a second loop filter. An optical pulse generator, comprising: a voltage-controlled oscillator that receives a passed signal as an input, and using an output of the voltage-controlled oscillator as an input of a first phase comparison circuit.

7. The optical pulse generator according to claim 3, wherein the optical pulse generator includes a semiconductor laser that generates an optical pulse train by a gain switch.