JP3807285B2

JP3807285B2 - Optical waveform evaluation apparatus, optical waveform evaluation method, optical waveform evaluation program, and recording medium recording optical waveform evaluation program

Info

Publication number: JP3807285B2
Application number: JP2001333398A
Authority: JP
Inventors: 誠二野極
Original assignee: Yokogawa Electric Corp
Current assignee: Yokogawa Electric Corp
Priority date: 2001-10-30
Filing date: 2001-10-30
Publication date: 2006-08-09
Anticipated expiration: 2021-10-30
Also published as: JP2003139619A

Description

【０００１】
【発明の属する技術分野】
本発明は、超高周波の被測定信号光のタイミングジッタを測定するための光波形評価装置、光波形評価方法、光波形評価プログラムおよび光波形評価プログラムを記録した記録媒体に関する。
【０００２】
【従来の技術】
図７は、従来のタイミングジッタの測定方法を示す図である。従来は、光波形評価装置を用いて図示したような被測定信号光の波形を取得した後に、波形の傾きが最も大きい部分に微小振幅区間Ａを設け、この微小振幅区間Ａに含まれる測定点の時間軸方向におけるばらつきの標準偏差τをタイミングジッタとしていた。
【０００３】
【発明が解決しようとする課題】
しかし、上記の従来技術には、次のような問題がある。すなわち、波形が完全な方形波ではなく、波形の傾きが有限の値をもつ場合に、微小振幅区間Ａの幅を広くすると、波形の傾きに起因する誤差が大きくなる。逆に、微小振幅区間Ａの幅を狭くすると、データ数が少なくなり、標準偏差を算出する際の誤差が大きくなる。データ数を増やすために、光波形評価装置で、被測定信号光の波形を複数回掃引すると、測定に時間がかかる。また、複数回掃引したデータどうしを重ね合わせる際のずれによって誤差が発生する可能性もでてくる。
【０００４】
本発明は、上記の問題を解決するためになされたもので、短時間で正確にタイミングジッタを測定することが可能な光波形評価装置、光波形評価方法、光波形評価プログラムおよび光波形評価プログラムを記録した記録媒体を提供するものである。
【０００５】
【課題を解決するための手段】
請求項１に記載の発明は、被測定信号光の繰り返し周波数のほぼ整数分の１の周波数のサンプリングパルス光を用いて、前記被測定信号光のタイミングジッタを測定する光波形評価装置であって、前記被測定信号光の振幅が最大時の半分となる半値の時点での振幅揺らぎを算出する半値振幅揺らぎ算出手段と、前記被測定信号光の振幅が最大時の半分となる半値の時点での波形の傾きを算出する半値傾き算出手段と、前記半値振幅揺らぎ算出手段が算出した半値の時点での振幅揺らぎを、前記半値傾き算出手段が算出した半値の時点での傾きで割ることによって、前記被測定信号光のタイミングジッタを算出するタイミングジッタ算出手段とを有することを特徴とする光波形評価装置である。
【０００６】
請求項２に記載の発明は、前記半値振幅揺らぎ算出手段は、前記被測定信号光の振幅が最大となるピーク値の時点での振幅揺らぎを算出するピーク値振幅揺らぎ算出手段と、前記被測定信号光の振幅が最小となるボトム値の時点での振幅揺らぎを算出するボトム値振幅揺らぎ算出手段と、前記ピーク値振幅揺らぎ算出手段が算出したピーク値の時点での振幅揺らぎから、前記ボトム値振幅揺らぎ算出手段が算出したボトム値の時点での振幅揺らぎを差し引くことによって、ピーク値の時点でのショット雑音を算出するピーク値ショット雑音算出手段と、このピーク値ショット雑音算出手段が算出したピーク値の時点でのショット雑音を２で割ることによって、半値の時点でのショット雑音を算出する半値ショット雑音算出手段とを有し、算出した半値の時点での振幅揺らぎから、前記半値ショット雑音算出手段が算出した半値の時点でのショット雑音と、前記ボトム値振幅揺らぎ算出手段が算出したボトム値の時点での振幅揺らぎが示す熱雑音とを差し引いた値を、雑音を含まない半値の時点での振幅揺らぎとして前記タイミングジッタ算出手段に送ることを特徴とする請求項１に記載の光波形評価装置である。
【０００７】
請求項３に記載の発明は、前記半値振幅揺らぎ算出手段、ピーク値振幅揺らぎ算出手段およびボトム値振幅揺らぎ算出手段が振幅揺らぎを算出する際に、前記サンプリングパルス光の周波数を前記被測定信号光の繰り返し周波数の整数分の１に完全に一致させる周波数同期手段を有することを特徴とする請求項２に記載の光波形評価装置である。
【０００８】
請求項４に記載の発明は、被測定信号光の繰り返し周波数のほぼ整数分の１の周波数のサンプリングパルス光を用いて、前記被測定信号光のタイミングジッタを測定する光波形評価方法であって、前記被測定信号光の振幅が最大時の半分となる半値の時点での振幅揺らぎを算出する半値振幅揺らぎ算出ステップと、前記被測定信号光の振幅が最大時の半分となる半値の時点での波形の傾きを算出する半値傾き算出ステップと、前記半値振幅揺らぎ算出ステップで算出した半値の時点での振幅揺らぎを、前記半値傾き算出ステップで算出した半値の時点での傾きで割ることによって、前記被測定信号光のタイミングジッタを算出するタイミングジッタ算出ステップとを有することを特徴とする光波形評価方法である。
【０００９】
請求項５に記載の発明は、被測定信号光の繰り返し周波数のほぼ整数分の１の周波数のサンプリングパルス光を用いて、前記被測定信号光のタイミングジッタを測定するための光波形評価プログラムであって、前記被測定信号光の振幅が最大時の半分となる半値の時点での振幅揺らぎを算出する半値振幅揺らぎ算出ステップと、前記被測定信号光の振幅が最大時の半分となる半値の時点での波形の傾きを算出する半値傾き算出ステップと、前記半値振幅揺らぎ算出ステップで算出した半値の時点での振幅揺らぎを、前記半値傾き算出ステップで算出した半値の時点での傾きで割ることによって、前記被測定信号光のタイミングジッタを算出するタイミングジッタ算出ステップとをコンピュータに実行させることを特徴とする光波形評価プログラムである。
【００１０】
請求項６に記載の発明は、被測定信号光の繰り返し周波数のほぼ整数分の１の周波数のサンプリングパルス光を用いて、前記被測定信号光のタイミングジッタを測定するための光波形評価プログラムを記録したコンピュータ読み取り可能な記録媒体であって、前記被測定信号光の振幅が最大時の半分となる半値の時点での振幅揺らぎを算出する半値振幅揺らぎ算出ステップと、前記被測定信号光の振幅が最大時の半分となる半値の時点での波形の傾きを算出する半値傾き算出ステップと、前記半値振幅揺らぎ算出ステップで算出した半値の時点での振幅揺らぎを、前記半値傾き算出ステップで算出した半値の時点での傾きで割ることによって、前記被測定信号光のタイミングジッタを算出するタイミングジッタ算出ステップとをコンピュータに実行させることを特徴とする光波形評価プログラムを記録した記録媒体である。
【００１１】
【発明の実施の形態】
図１は、本発明の一実施形態における光波形評価装置の構成を示すブロック図である。被測定信号光源１は、周期的に振幅が変化する被測定信号光を発生すると共に、発生する被測定信号光の振幅の変化に同期した繰り返し信号を出力するものとする。被測定信号光源１から出力された繰り返し信号は、周波数同期手段２に入力される。周波数同期手段２は、入力した繰り返し信号の周波数のほぼ整数分の１の周波数のサンプリング信号を出力する。周波数同期手段２から出力されたサンプリング信号は、サンプリングパルス光源３に入力される。サンプリングパルス光源３は、入力したサンプリング信号に同期して光の強度すなわち振幅が変化するサンプリングパルス光を発生する。
【００１２】
サンプリングパルス光源３が発生したサンプリングパルス光と、前記被測定信号光源１が発生した被測定信号光とは、合波器４に入射される。合波器４は、入射したサンプリングパルス光と被測定信号光とを合波し、合波光を出射する。合波器４が出射した合波光は、非線形光学結晶素子５に入射される。非線形光学結晶素子５は、非線形光学効果により、入射した合波光から和周波光を発生する。和周波光は、入射した合波光に含まれるサンプリングパルス光と被測定信号光との２つの光の強度の積に比例した強度と、２つの光の周波数の和に等しい周波数とを有する。非線形光学結晶素子５が発生した和周波光は、受光器６に入射される。受光器６は、入射した和周波光の強度に比例するアナログ信号（電気信号）を出力する。
【００１３】
前記周波数同期手段２から出力されたサンプリング信号は、移相器７にも入力される。移相器７は、入力したサンプリング信号を所定量だけ遅らせることによって位相を補正し、補正サンプリング信号を出力する。すなわち、移相器７は、サンプリングパルス光源３、合波器４、非線形光学結晶素子５および受光器６によって構成される系における遅れと同量の遅れを発生させる。
【００１４】
移相器７から出力された補正サンプリング信号と、前記受光器６から出力されたアナログ信号とは、Ａ／Ｄ変換器８に入力される。Ａ／Ｄ変換器８は、入力したアナログ信号を、補正サンプリング信号に同期したタイミングでサンプリングすることによりＡ／Ｄ変換（アナログ／デジタル変換）し、デジタル信号を出力する。Ａ／Ｄ変換器８から出力されたデジタル信号は、コンピュータ９に入力される。コンピュータ９は、入力したデジタル信号に基づいて、表示装置１０に被測定信号光の波形を表示させる。
【００１５】
図２は、周波数同期手段２の内部構成を示すブロック図である。周波数同期手段２内には、分周器２ａと、周波数加算手段２ｂと、移相器２ｃと、定電圧源２ｅとが設けられている。前記被測定信号光源１から出力された繰り返し信号は、分周器２ａに入力される。分周器２ａは、入力した繰り返し信号を分周し、入力した繰り返し信号の周波数の整数分の１の周波数の分周繰り返し信号を出力する。分周器２ａから出力された分周繰り返し信号は、周波数加算手段２ｂに入力される。周波数加算手段２ｂは、通常、入力した分周繰り返し信号の周波数にわずかな周波数を加算した信号を移相器２ｃへ出力する。なお、この周波数加算手段２ｂは、周波数を加算する動作を停止し、入力した分周繰り返し信号をそのままの形で出力することも可能であるものとする。定電圧源２ｅはコンピュータ９からのサンプリング位置制御信号により、ある一定の電圧値の信号を移相器２ｃに出力する。移相器２ｃは定電圧源２ｅからの信号の電圧値に従って周波数加算手段２ｂから出力された信号の位相をシフトしサンプリング信号として出力する。
【００１６】
なお、周波数加算手段２ｂの代わりに周波数減算手段を設け、分周繰り返し信号の周波数からわずかな周波数を減算した信号を移相器２ｃに出力してもよい。この場合にも、周波数減算手段は、周波数を減算する動作を停止し、入力した分周繰り返し信号をそのままの形で移相器２ｃに出力することが可能であるものとする。
【００１７】
図３は、周波数同期手段２の内部構成の別の例を示すブロック図である。この例では、周波数加算手段２ｂ、移相器２ｃおよび定電圧源２ｅの組み合わせの代わりに、移相器２ｃと、鋸波発生器２ｄとが設けられている。これ以外の構成は、図２に示した例と同様である。分周器２ａから出力された分周繰り返し信号は、移相器２ｃに入力される。移相器２ｃは、入力した分周繰り返し信号の位相をシフトさせ、サンプリング信号として出力する。鋸波発生器２ｄは、コンピュータ９からのサンプリング位置制御信号により鋸波の初期電圧をある一定の電圧に設定し、通常、鋸波を発生する。鋸波発生器２ｄが発生した鋸波は、移相器２ｃに入力される。移相器２ｃは、入力した鋸波の振幅の変化に応じて、分周繰り返し信号の位相のシフト量を変化させる。これにより、図２に示した構成から得られるサンプリング信号と同様のサンプリング信号が得られる。
【００１８】
なお、鋸波発生器２ｄは、鋸波以外に、直流波形すなわち一定の電圧を発生することも可能であるものとする。また、移相器２ｃは、鋸波発生器２ｄから直流波形すなわち一定の電圧を送られた場合には、分周繰り返し信号の位相をシフトさせる機能を停止するものとする。
【００１９】
図４は、被測定信号光の観測波形の模式図である。この図を用いて、前述したコンピュータ９の機能を説明する。コンピュータ９は、表示装置１０に被測定信号光の波形を表示させる波形表示手段と共に、半値振幅揺らぎ算出手段と、半値傾き算出手段と、タイミングジッタ算出手段とを有する。
【００２０】
半値振幅揺らぎ算出手段は、被測定信号光の振幅が最大時の半分となる半値の時点での振幅揺らぎの標準偏差σ_halfを算出する。半値傾き算出手段は、被測定信号光の振幅が最大時の半分となる半値の時点での波形の傾きｄＡ／ｄｔを算出する。タイミングジッタ算出手段は、半値振幅揺らぎ算出手段が算出した半値の時点での振幅揺らぎの標準偏差σ_halfを、半値傾き算出手段が算出した半値の時点での傾きｄＡ／ｄｔで割ることによって、被測定信号光のタイミングジッタを算出する。すなわち、タイミングジッタ算出手段は、下記の式（１）によってタイミングジッタJitterを算出する。
【数１】

【００２１】
図５は、表示装置１０に表示される被測定信号光の観測波形の一例を示す図である。この図を用いて、より正確にタイミングジッタを算出する方法を説明する。図示した観測波形の半値の時点での振幅揺らぎの標準偏差σ_halfには、受光器６が発生するショット雑音や熱雑音が含まれているので、これらの雑音を差し引くことによって、より正確にタイミングジッタを算出することができる。
【００２２】
このため、前記半値振幅揺らぎ算出手段内に、ピーク値振幅揺らぎ算出手段と、ボトム値振幅揺らぎ算出手段と、ピーク値ショット雑音算出手段と、半値ショット雑音算出手段とを設ける。
【００２３】
ピーク値振幅揺らぎ算出手段は、被測定信号光の振幅が最大となるピーク値の時点での振幅揺らぎの標準偏差σ_peakを算出する。ピーク値の時点での振幅揺らぎの標準偏差σ_peakは、ピーク値の時点で受光器６が発生するショット雑音と熱雑音との合計である合成雑音の量を表している。ボトム値振幅揺らぎ算出手段は、被測定信号光の振幅が最小となるボトム値の時点での振幅揺らぎの標準偏差σ_bottomを算出する。ボトム値の時点では受光器６に光が入射されていないので、ボトム値の時点での振幅揺らぎの標準偏差σ_bottomは、受光器６が発生する熱雑音のみの量を表している。
【００２４】
ピーク値ショット雑音算出手段は、ピーク値振幅揺らぎ算出手段が算出したピーク値の時点での振幅揺らぎの標準偏差σ_peakから、ボトム値振幅揺らぎ算出手段が算出したボトム値の時点での振幅揺らぎの標準偏差σ_bottomを差し引くことによって、ピーク値の時点でのショット雑音を算出する。ここで、受光器６が発生するショット雑音は、受光器６に入射される光の強度に比例する。そこで、半値ショット雑音算出手段は、ピーク値ショット雑音算出手段が算出したピーク値の時点でのショット雑音を２で割ることによって、半値の時点でのショット雑音を算出する。
【００２５】
そして、半値振幅揺らぎ算出手段は、算出した半値の時点での振幅揺らぎの標準偏差σ_halfから、半値ショット雑音算出手段が算出した半値の時点でのショット雑音と、ボトム値振幅揺らぎ算出手段が算出したボトム値の時点での振幅揺らぎの標準偏差σ_bottomが示す熱雑音とを差し引いた値を、雑音を含まない半値の時点での振幅揺らぎの標準偏差としてタイミングジッタ算出手段に送る。すなわち、タイミングジッタ算出手段は、下記の式（２）によって雑音成分を含まないタイミングジッタJitterを算出する。
【数２】

【００２６】
上記の波形表示手段、半値振幅揺らぎ算出手段、半値傾き算出手段およびタイミングジッタ算出手段は、コンピュータ９に内蔵されるハードウェアによって実現されるものであっても、ソフトウェアによって実現されるものであってもよい。「ソフトウェアによって実現される」とは、上記の各手段の機能を有する光波形評価プログラムをコンピュータ９内のメモリにロードして実行することにより、上記の各手段の機能が実現されることを意味する。
【００２７】
また、上記の各手段の機能を有する光波形評価プログラムをコンピュータ読み取り可能な記録媒体に記録して、この記録媒体に記録された光波形評価プログラムをコンピュータ９に読み込ませ、実行することにより、上記の各手段の機能を実現してもよい。なお、ここで言う「コンピュータ読み取り可能な記録媒体」とは、フレキシブルディスク、光磁気ディスク、ＣＤ−ＲＯＭ、ＣＤ−Ｒ、ＣＤ−ＲＷ等の可搬媒体や、コンピュータに内蔵されるハードディスク等の記憶装置のことである。
【００２８】
図６は、半値振幅揺らぎ算出手段、ピーク値振幅揺らぎ算出手段およびボトム値振幅揺らぎ算出手段が、それぞれ半値、ピーク値およびボトム値の時点での振幅揺らぎの標準偏差σ_half、σ_peakおよびσ_bottomを算出する際に取得するデータの一例を示す図である。この図における横軸はサンプリング数、縦軸は被測定信号光の波形の振幅である。このデータを取得する際には、サンプリングパルス光の周波数を被測定信号光の繰り返し周波数の整数分の１に完全に一致させ、被測定信号光の波形の半値、ピーク値またはボトム値のみを常にサンプリングするようにする。
【００２９】
このため、周波数同期手段２内では、被測定信号光源１から送られる繰り返し信号の分周のみを行い、周波数をずらすことは行わない。すなわち、周波数同期手段２の内部構成が図２に示したものである場合には、周波数加算手段２ｂにおける周波数を加算する動作を停止させ、入力した分周繰り返し信号をそのままの形で出力させ、出力された分周繰り返し信号の位相をコンピュータ９からのサンプリング位置制御信号により、定電圧源２ｅと移相器２ｃにて一定量シフトした信号をサンプリング信号として出力させる。また、周波数同期手段２の内部構成が図３に示したものである場合には、コンピュータ９からのサンプリング位置制御信号により、鋸波発生器２ｄから一定電圧の直流を発生させ、移相器２ｃの位相シフト量を一定値にし、分周繰り返し信号の位相を一定量シフトした信号をサンプリング信号として出力させる。これにより、周波数同期手段２は、被測定信号光源１から送られた繰り返し信号の周波数の整数分の１に完全に一致した周波数のサンプリング信号を出力する。また、半値、ピーク値、ボトム値の時点での振幅揺らぎのデータを取得するため、被測定信号光の半値、ピーク値、ボトム値を常にサンプリングするように、コンピュータ９はサンプリング位置制御信号で周波数同期手段２を制御し、周波数同期手段２内の移相器２ｃの位相シフト量を調節している。
【００３０】
なお、上記のデータを取得する際に、サンプリング数を変えることによって、タイミングジッタの周波数成分を解析することができる。すなわち、サンプリング数を少なくする（サンプリング期間を短くする）と、タイミングジッタの高周波成分のみが抽出され、サンプリング数を多くする（サンプリング期間を長くする）と、低周波成分も含めたタイミングジッタが抽出される。
【００３１】
【発明の効果】
本発明によれば、短時間で正確にタイミングジッタを測定することができる。
【図面の簡単な説明】
【図１】本発明の一実施形態における光波形評価装置の構成を示すブロック図である。
【図２】周波数同期手段２の内部構成を示すブロック図である。
【図３】周波数同期手段２の内部構成の別の例を示すブロック図である。
【図４】被測定信号光の観測波形の模式図である。
【図５】表示装置１０に表示される被測定信号光の観測波形の一例を示す図である。
【図６】半値振幅揺らぎ算出手段、ピーク値振幅揺らぎ算出手段およびボトム値振幅揺らぎ算出手段が、それぞれ半値、ピーク値およびボトム値の時点での振幅揺らぎの標準偏差σ_half、σ_peakおよびσ_bottomを算出する際に取得するデータの一例を示す図である。
【図７】従来のタイミングジッタの測定方法を示す図である。
【符号の説明】
１被測定信号光源２周波数同期手段
２ａ分周器２ｂ周波数加算手段
２ｃ移相器２ｄ鋸波発生器
２ｅ定電圧源
３サンプリングパルス光源４合波器
５非線形光学結晶素子６受光器
７移相器８Ａ／Ｄ変換器
９コンピュータ（半値振幅揺らぎ算出手段、半値傾き算出手段、タイミングジッタ算出手段）
１０表示装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical waveform evaluation apparatus, an optical waveform evaluation method, an optical waveform evaluation program, and a recording medium on which an optical waveform evaluation program is recorded for measuring timing jitter of signal light to be measured at an ultrahigh frequency.
[0002]
[Prior art]
FIG. 7 is a diagram illustrating a conventional method for measuring timing jitter. Conventionally, after acquiring the waveform of the signal light to be measured as shown in the figure using an optical waveform evaluation apparatus, a minute amplitude section A is provided in a portion where the slope of the waveform is the largest, and the measurement points included in this minute amplitude section A The standard deviation τ of the variation in the time axis direction is the timing jitter.
[0003]
[Problems to be solved by the invention]
However, the above prior art has the following problems. That is, when the waveform is not a perfect square wave and the waveform has a finite value, increasing the width of the minute amplitude section A increases the error due to the waveform. On the contrary, if the width of the minute amplitude section A is narrowed, the number of data decreases, and the error in calculating the standard deviation increases. In order to increase the number of data, if the waveform of the signal light under measurement is swept a plurality of times with the optical waveform evaluation apparatus, it takes time to measure. In addition, there is a possibility that an error may occur due to a shift in superimposing data swept several times.
[0004]
The present invention has been made to solve the above problems, and is an optical waveform evaluation apparatus, an optical waveform evaluation method, an optical waveform evaluation program, and an optical waveform evaluation program capable of measuring timing jitter accurately in a short time. Is provided.
[0005]
[Means for Solving the Problems]
The invention according to claim 1 is an optical waveform evaluation apparatus for measuring timing jitter of the signal light to be measured by using sampling pulse light having a frequency substantially equal to a fraction of the repetition frequency of the signal light to be measured. A half-value amplitude fluctuation calculating means for calculating an amplitude fluctuation at a half-value when the amplitude of the signal light under measurement is half of the maximum; and a half-value when the amplitude of the signal light under measurement is half of the maximum By dividing the amplitude fluctuation at the time of the half value calculated by the half value amplitude fluctuation calculating means by the inclination at the time of the half value calculated by the half value inclination calculating means, An optical waveform evaluation apparatus comprising timing jitter calculating means for calculating timing jitter of the signal light to be measured.
[0006]
The invention according to claim 2 is characterized in that the half-value amplitude fluctuation calculating means calculates a peak value amplitude fluctuation calculating means for calculating an amplitude fluctuation at a peak value at which the amplitude of the signal light to be measured becomes maximum, and the measured value. The bottom value amplitude fluctuation calculating means for calculating the amplitude fluctuation at the time of the bottom value at which the amplitude of the signal light is minimum, and the bottom value from the amplitude fluctuation at the time of the peak value calculated by the peak value amplitude fluctuation calculating means. By subtracting the amplitude fluctuation at the time of the bottom value calculated by the amplitude fluctuation calculating means, the peak value shot noise calculating means for calculating the shot noise at the time of the peak value, and the peak calculated by the peak value shot noise calculating means A half-value shot noise calculating means for calculating the shot noise at the half-value time by dividing the shot noise at the time of the value by 2 and calculating From the amplitude fluctuation at the half-value time, the shot noise at the half-value time calculated by the half-value shot noise calculation means, and the thermal noise indicated by the amplitude fluctuation at the time of the bottom value calculated by the bottom value amplitude fluctuation calculation means 2. The optical waveform evaluation apparatus according to claim 1, wherein a value obtained by subtracting is sent to the timing jitter calculation unit as amplitude fluctuation at a half-value point not including noise.
[0007]
According to a third aspect of the present invention, when the half-value amplitude fluctuation calculating unit, the peak value amplitude fluctuation calculating unit, and the bottom value amplitude fluctuation calculating unit calculate amplitude fluctuations, the frequency of the sampling pulse light is calculated as the signal light to be measured. 3. The optical waveform evaluation apparatus according to claim 2, further comprising a frequency synchronization means for making the frequency coincide completely with a fraction of an integer of the repetition frequency.
[0008]
The invention according to claim 4 is an optical waveform evaluation method for measuring timing jitter of the signal light under measurement using sampling pulse light having a frequency substantially equal to a fraction of the repetition frequency of the signal light under measurement. A half-value amplitude fluctuation calculating step for calculating an amplitude fluctuation at a half-value when the amplitude of the signal light under measurement is half of the maximum, and a half-value when the amplitude of the signal light under measurement is half of the maximum By dividing the amplitude fluctuation at the half-value time calculated in the half-value amplitude fluctuation calculation step by the half-value inclination calculation step for calculating the slope of the waveform, and the half-value inclination calculated at the half-value inclination calculation step, And a timing jitter calculating step for calculating timing jitter of the signal light to be measured.
[0009]
According to a fifth aspect of the present invention, there is provided an optical waveform evaluation program for measuring timing jitter of the signal light to be measured by using a sampling pulse light having a frequency substantially equal to a fraction of the repetition frequency of the signal light to be measured. A half-value amplitude fluctuation calculating step for calculating an amplitude fluctuation at a half value when the amplitude of the signal light under measurement is half of the maximum, and a half value where the amplitude of the signal light under measurement is half of the maximum Dividing the amplitude fluctuation at the half-value time calculated in the half-value amplitude fluctuation calculation step by the half-value inclination calculation step for calculating the waveform inclination at the time point and the half-value inclination calculated at the half-value inclination calculation step. And a timing jitter calculating step for calculating timing jitter of the signal light to be measured. Lamb.
[0010]
According to a sixth aspect of the present invention, there is provided an optical waveform evaluation program for measuring timing jitter of the signal light to be measured using a sampling pulse light having a frequency substantially equal to a fraction of the repetition frequency of the signal light to be measured. A recorded computer-readable recording medium, a half-value amplitude fluctuation calculating step for calculating an amplitude fluctuation at a half-value when the amplitude of the signal light under measurement is half the maximum, and the amplitude of the signal light under measurement The half-value slope calculation step for calculating the slope of the waveform at the half-value time at which the value is half of the maximum time, and the amplitude fluctuation at the half-value time calculated in the half-value amplitude fluctuation calculation step were calculated in the half-value slope calculation step. A timing jitter calculating step for calculating the timing jitter of the signal light under measurement by dividing by the slope at the time of half value; A recording medium on which is recorded an optical waveform evaluation program.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram showing a configuration of an optical waveform evaluation apparatus according to an embodiment of the present invention. The measured signal light source 1 generates a measured signal light whose amplitude periodically changes and outputs a repetitive signal synchronized with a change in the amplitude of the generated measured signal light. The repetitive signal output from the signal light source 1 to be measured is input to the frequency synchronization means 2. The frequency synchronization means 2 outputs a sampling signal having a frequency that is approximately 1 / integer of the frequency of the input repetitive signal. The sampling signal output from the frequency synchronization means 2 is input to the sampling pulse light source 3. The sampling pulse light source 3 generates sampling pulse light whose intensity, that is, amplitude, changes in synchronization with the input sampling signal.
[0012]
The sampling pulse light generated by the sampling pulse light source 3 and the signal light to be measured generated by the signal light source 1 to be measured are incident on the multiplexer 4. The multiplexer 4 combines the incident sampling pulse light and the signal light to be measured, and emits the combined light. The combined light emitted from the multiplexer 4 enters the nonlinear optical crystal element 5. The nonlinear optical crystal element 5 generates sum frequency light from the incident combined light by a nonlinear optical effect. The sum frequency light has an intensity proportional to the product of the two light intensities of the sampling pulse light and the signal light to be measured included in the incident combined light, and a frequency equal to the sum of the frequencies of the two lights. The sum frequency light generated by the nonlinear optical crystal element 5 enters the light receiver 6. The light receiver 6 outputs an analog signal (electric signal) proportional to the intensity of the incident sum frequency light.
[0013]
The sampling signal output from the frequency synchronization means 2 is also input to the phase shifter 7. The phase shifter 7 corrects the phase by delaying the input sampling signal by a predetermined amount, and outputs a corrected sampling signal. That is, the phase shifter 7 generates a delay equivalent to the delay in the system constituted by the sampling pulse light source 3, the multiplexer 4, the nonlinear optical crystal element 5, and the light receiver 6.
[0014]
The corrected sampling signal output from the phase shifter 7 and the analog signal output from the light receiver 6 are input to the A / D converter 8. The A / D converter 8 performs A / D conversion (analog / digital conversion) by sampling the input analog signal at a timing synchronized with the correction sampling signal, and outputs a digital signal. The digital signal output from the A / D converter 8 is input to the computer 9. The computer 9 displays the waveform of the signal light under measurement on the display device 10 based on the input digital signal.
[0015]
FIG. 2 is a block diagram showing the internal configuration of the frequency synchronization means 2. In the frequency synchronization means 2, a frequency divider 2a, a frequency addition means 2b, a phase shifter 2c, and a constant voltage source 2e are provided. The repetitive signal output from the signal light source 1 to be measured is input to the frequency divider 2a. The frequency divider 2a divides the input repetitive signal and outputs a frequency-divided repetitive signal having a frequency that is 1 / integer of the frequency of the input repetitive signal. The frequency division repetition signal output from the frequency divider 2a is input to the frequency adding means 2b. The frequency adding means 2b normally outputs a signal obtained by adding a slight frequency to the frequency of the inputted frequency division repetition signal to the phase shifter 2c. Note that the frequency adding means 2b can stop the operation of adding the frequencies and output the input frequency division repeat signal as it is. The constant voltage source 2e outputs a signal having a certain voltage value to the phase shifter 2c according to the sampling position control signal from the computer 9. The phase shifter 2c shifts the phase of the signal output from the frequency adding means 2b in accordance with the voltage value of the signal from the constant voltage source 2e and outputs it as a sampling signal.
[0016]
A frequency subtracting unit may be provided instead of the frequency adding unit 2b, and a signal obtained by subtracting a slight frequency from the frequency of the frequency division repetition signal may be output to the phase shifter 2c. Also in this case, it is assumed that the frequency subtracting means stops the operation of subtracting the frequency and can output the input frequency division repeat signal as it is to the phase shifter 2c.
[0017]
FIG. 3 is a block diagram showing another example of the internal configuration of the frequency synchronization means 2. In this example, a phase shifter 2c and a sawtooth generator 2d are provided instead of the combination of the frequency adding means 2b, the phase shifter 2c and the constant voltage source 2e. The other configuration is the same as the example shown in FIG. The frequency division repetition signal output from the frequency divider 2a is input to the phase shifter 2c. The phase shifter 2c shifts the phase of the input frequency division repetition signal and outputs it as a sampling signal. The sawtooth wave generator 2d sets the initial voltage of the sawtooth wave to a certain voltage in accordance with the sampling position control signal from the computer 9, and normally generates a sawtooth wave. The sawtooth wave generated by the sawtooth wave generator 2d is input to the phase shifter 2c. The phase shifter 2c changes the phase shift amount of the frequency division repetition signal in accordance with the change in the amplitude of the input sawtooth wave. Thereby, a sampling signal similar to the sampling signal obtained from the configuration shown in FIG. 2 is obtained.
[0018]
The sawtooth generator 2d can generate a direct current waveform, that is, a constant voltage in addition to the sawtooth wave. Further, the phase shifter 2c stops the function of shifting the phase of the frequency division repetition signal when a DC waveform, that is, a constant voltage is sent from the sawtooth generator 2d.
[0019]
FIG. 4 is a schematic diagram of the observed waveform of the signal light under measurement. The function of the computer 9 described above will be described with reference to this figure. The computer 9 includes a half-value amplitude fluctuation calculating unit, a half-value inclination calculating unit, and a timing jitter calculating unit, along with a waveform display unit that displays the waveform of the signal light to be measured on the display device 10.
[0020]
The half-value amplitude fluctuation calculating means calculates the standard deviation σ _half of the amplitude fluctuation at the time of the half value when the amplitude of the signal light to be measured is half of the maximum. The half-value slope calculating means calculates the slope dA / dt of the waveform at the half-value when the amplitude of the signal light under measurement is half the maximum. The timing jitter calculating means divides the standard deviation σ _half of the amplitude fluctuation at the half-value time calculated by the half-value amplitude fluctuation calculating means by the inclination dA / dt at the half-value time calculated by the half-value inclination calculating means. The timing jitter of the measurement signal light is calculated. That is, the timing jitter calculating means calculates the timing jitter Jitter by the following equation (1).
[Expression 1]

[0021]
FIG. 5 is a diagram illustrating an example of an observation waveform of the signal light under measurement displayed on the display device 10. A method for calculating timing jitter more accurately will be described with reference to FIG. Since the standard deviation σ _half of the amplitude fluctuation at the time of the half value of the observed waveform shown in the figure includes shot noise and thermal noise generated by the light receiver 6, more accurate timing can be obtained by subtracting these noises. Jitter can be calculated.
[0022]
For this reason, a peak value amplitude fluctuation calculating means, a bottom value amplitude fluctuation calculating means, a peak value shot noise calculating means, and a half value shot noise calculating means are provided in the half value amplitude fluctuation calculating means.
[0023]
The peak value amplitude fluctuation calculating means calculates a standard deviation σ _peak of amplitude fluctuation at the time of the peak value at which the amplitude of the signal light to be measured becomes maximum. The standard deviation σ _peak of amplitude fluctuation at the time of the peak value represents the amount of synthesized noise that is the sum of shot noise and thermal noise generated by the light receiver 6 at the time of the peak value. The bottom value amplitude fluctuation calculating means calculates the standard deviation σ _bottom of the amplitude fluctuation at the time of the bottom value at which the amplitude of the signal light to be measured becomes the minimum. Since no light is incident on the light receiver 6 at the time of the bottom value, the standard deviation σ _bottom of the amplitude fluctuation at the time of the bottom value represents the amount of only thermal noise generated by the light receiver 6.
[0024]
The peak value shot noise calculating means calculates the amplitude fluctuation at the time of the bottom value calculated by the bottom value amplitude fluctuation calculating means from the standard deviation σ _peak of the amplitude fluctuation at the time of the peak value calculated by the peak value amplitude fluctuation calculating means. By subtracting the standard deviation σ _bottom , the shot noise at the time of the peak value is calculated. Here, the shot noise generated by the light receiver 6 is proportional to the intensity of light incident on the light receiver 6. Therefore, the half-value shot noise calculation means calculates the shot noise at the half-value time by dividing the shot noise at the time of the peak value calculated by the peak value shot noise calculation means by two.
[0025]
Then, the half-value amplitude fluctuation calculating means calculates the shot noise at the half-value time calculated by the half-value shot noise calculating means and the bottom value amplitude fluctuation calculating means from the standard deviation σ _half of the amplitude fluctuation at the calculated half-value time. The value obtained by subtracting the thermal noise indicated by the standard deviation σ _bottom of the amplitude fluctuation at the time of the bottom value is sent to the timing jitter calculation means as the standard deviation of the amplitude fluctuation at the time of the half value not including noise. That is, the timing jitter calculating means calculates the timing jitter Jitter that does not include a noise component by the following equation (2).
[Expression 2]

[0026]
The waveform display means, half-value amplitude fluctuation calculation means, half-value inclination calculation means, and timing jitter calculation means described above are realized by software, even if they are realized by hardware built in the computer 9. Also good. “Realized by software” means that the function of each of the above means is realized by loading an optical waveform evaluation program having the function of each of the above means into a memory in the computer 9 and executing it. To do.
[0027]
Further, the optical waveform evaluation program having the functions of the respective means is recorded on a computer-readable recording medium, and the optical waveform evaluation program recorded on the recording medium is read by the computer 9 and executed. The functions of these means may be realized. The “computer-readable recording medium” here refers to a portable medium such as a flexible disk, a magneto-optical disk, a CD-ROM, a CD-R, a CD-RW, or a storage such as a hard disk built in the computer. It is a device.
[0028]
FIG. 6 shows that the half-value amplitude fluctuation calculating means, the peak value amplitude fluctuation calculating means, and the bottom value amplitude fluctuation calculating means are the standard deviations σ _half , σ _peak, and σ _bottom of the amplitude fluctuation at the time of half value, peak value, and bottom value, respectively. It is a figure which shows an example of the data acquired when calculating. In this figure, the horizontal axis represents the number of samplings, and the vertical axis represents the amplitude of the waveform of the signal light under measurement. When acquiring this data, the frequency of the sampling pulse light is completely matched to an integer of the repetition frequency of the signal light to be measured, and only the half value, peak value, or bottom value of the waveform of the signal light to be measured is always set. Try to sample.
[0029]
For this reason, in the frequency synchronization means 2, only the frequency division of the repetitive signal sent from the signal light source 1 to be measured is performed, and the frequency is not shifted. That is, when the internal configuration of the frequency synchronization means 2 is as shown in FIG. 2, the operation of adding the frequency in the frequency addition means 2b is stopped, and the input frequency division repeat signal is output as it is, Based on the sampling position control signal from the computer 9, the signal obtained by shifting the phase of the output frequency division repetition signal by a constant amount by the constant voltage source 2e and the phase shifter 2c is output as a sampling signal. When the internal structure of the frequency synchronization means 2 is as shown in FIG. 3, a constant voltage direct current is generated from the sawtooth generator 2d by the sampling position control signal from the computer 9, and the phase shifter 2c. And a signal obtained by shifting the phase of the frequency division repetition signal by a fixed amount is output as a sampling signal. As a result, the frequency synchronization means 2 outputs a sampling signal having a frequency that completely coincides with 1 / integer of the frequency of the repetitive signal transmitted from the signal light source 1 to be measured. In addition, in order to acquire amplitude fluctuation data at the time of the half value, peak value, and bottom value, the computer 9 uses the sampling position control signal to frequency the half value, peak value, and bottom value of the signal light to be measured. The synchronizing means 2 is controlled, and the phase shift amount of the phase shifter 2c in the frequency synchronizing means 2 is adjusted.
[0030]
Note that when acquiring the above data, the frequency component of timing jitter can be analyzed by changing the number of samplings. In other words, if the sampling number is reduced (sampling period is shortened), only the high frequency components of timing jitter are extracted, and if the sampling number is increased (sampling period is lengthened), timing jitter including low frequency components is extracted. Is done.
[0031]
【The invention's effect】
According to the present invention, timing jitter can be accurately measured in a short time.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an optical waveform evaluation apparatus according to an embodiment of the present invention.
FIG. 2 is a block diagram showing an internal configuration of frequency synchronization means 2;
FIG. 3 is a block diagram showing another example of the internal configuration of the frequency synchronization means 2;
FIG. 4 is a schematic diagram of an observation waveform of signal light under measurement.
FIG. 5 is a diagram showing an example of an observation waveform of signal light under measurement displayed on the display device 10;
FIG. 6 shows a half-value amplitude fluctuation calculation means, a peak value amplitude fluctuation calculation means, and a bottom value amplitude fluctuation calculation means, which are standard deviations σ _half , σ _peak, and σ _bottom of amplitude fluctuations at the time of half value, peak value, and bottom value, respectively. It is a figure which shows an example of the data acquired when calculating.
FIG. 7 is a diagram illustrating a conventional method for measuring timing jitter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Signal-to-be-measured light source 2 Frequency synchronization means 2a Frequency divider 2b Frequency addition means 2c Phase shifter 2d Saw wave generator 2e Constant voltage source 3 Sampling pulse light source 4 Multiplexer 5 Nonlinear optical crystal element 6 Light receiver 7 Phase shifter 8 A / D converter 9 Computer (half-value amplitude fluctuation calculating means, half-value inclination calculating means, timing jitter calculating means)
10 Display device

Claims

An optical waveform evaluation apparatus that measures timing jitter of the signal light under measurement using sampling pulse light having a frequency substantially equal to a fraction of the repetition frequency of the signal light under measurement,
Half-value amplitude fluctuation calculating means for calculating amplitude fluctuation at a half-value when the amplitude of the signal light to be measured is half of the maximum time;
A half-value inclination calculating means for calculating an inclination of the waveform at a half-value when the amplitude of the signal light to be measured is half of the maximum value;
Timing jitter for calculating the timing jitter of the signal light under measurement by dividing the amplitude fluctuation at the half-value time calculated by the half-value amplitude fluctuation calculating means by the inclination at the half-value time calculated by the half-value inclination calculating means An optical waveform evaluation apparatus comprising: a calculation means.

The half-value amplitude fluctuation calculating means includes
A peak value amplitude fluctuation calculating means for calculating an amplitude fluctuation at the time of a peak value at which the amplitude of the signal light to be measured becomes maximum;
Bottom value amplitude fluctuation calculating means for calculating amplitude fluctuation at the time of the bottom value at which the amplitude of the signal light to be measured is minimum;
A shot at the time of the peak value is obtained by subtracting the amplitude fluctuation at the time of the bottom value calculated by the bottom value amplitude fluctuation calculating means from the amplitude fluctuation at the time of the peak value calculated by the peak value amplitude fluctuation calculating means. Peak value shot noise calculating means for calculating noise;
The half value shot noise calculation means for calculating the shot noise at the half value time by dividing the shot noise at the time of the peak value calculated by the peak value shot noise calculation means by 2;
From the amplitude fluctuation at the half-value time calculated, the heat indicated by the shot noise at the half-value time calculated by the half-value shot noise calculation means and the amplitude fluctuation at the time of the bottom value calculated by the bottom value amplitude fluctuation calculation means 2. The optical waveform evaluation apparatus according to claim 1, wherein a value obtained by subtracting noise is sent to the timing jitter calculating means as amplitude fluctuation at a half value not including noise.

When the half value amplitude fluctuation calculating means, the peak value amplitude fluctuation calculating means, and the bottom value amplitude fluctuation calculating means calculate the amplitude fluctuation, the frequency of the sampling pulse light is set to 1 / integer of the repetition frequency of the signal light to be measured. The optical waveform evaluation apparatus according to claim 2, further comprising a frequency synchronization means for making them completely coincide with each other.

An optical waveform evaluation method for measuring timing jitter of the signal light under measurement using sampling pulse light having a frequency substantially equal to a fraction of the repetition frequency of the signal light under measurement,
A half-value amplitude fluctuation calculating step for calculating an amplitude fluctuation at a half-value when the amplitude of the signal light to be measured is half of the maximum value;
A half-value slope calculating step for calculating a slope of the waveform at a half-value when the amplitude of the signal light to be measured is half of the maximum value;
Timing jitter for calculating the timing jitter of the signal light under measurement by dividing the amplitude fluctuation at the half-value time calculated in the half-value amplitude fluctuation calculation step by the inclination at the half-value time calculated in the half-value inclination calculation step An optical waveform evaluation method comprising: a calculation step.

An optical waveform evaluation program for measuring timing jitter of the signal light under measurement using sampling pulse light having a frequency substantially equal to a fraction of the repetition frequency of the signal light under measurement,
A half-value amplitude fluctuation calculating step for calculating an amplitude fluctuation at a half-value when the amplitude of the signal light to be measured is half of the maximum value;
A half-value slope calculating step for calculating a slope of the waveform at a half-value when the amplitude of the signal light to be measured is half of the maximum value;
Timing jitter for calculating the timing jitter of the signal light under measurement by dividing the amplitude fluctuation at the half-value time calculated in the half-value amplitude fluctuation calculation step by the inclination at the half-value time calculated in the half-value inclination calculation step An optical waveform evaluation program that causes a computer to execute a calculation step.

A computer-readable recording medium on which an optical waveform evaluation program for measuring timing jitter of the signal light to be measured is recorded using a sampling pulse light having a frequency substantially equal to a fraction of the repetition frequency of the signal light to be measured. There,
A half-value amplitude fluctuation calculating step for calculating an amplitude fluctuation at a half-value when the amplitude of the signal light to be measured is half of the maximum value;
A half-value slope calculating step for calculating a slope of the waveform at a half-value when the amplitude of the signal light to be measured is half of the maximum value;
Timing jitter for calculating the timing jitter of the signal light under measurement by dividing the amplitude fluctuation at the half-value time calculated in the half-value amplitude fluctuation calculation step by the inclination at the half-value time calculated in the half-value inclination calculation step A recording medium having an optical waveform evaluation program recorded thereon, wherein the computer executes the calculation step.