JP3849019B2 - Method and apparatus for reducing timing jitter of optical pulse - Google Patents

Description

ここで、ｇ（ｔ）は個々のパルスの時間波形であり、δＴ_ｋがタイミングの変位を表す。δＴ_ｋがパルス周期に比べて小さい場合、Ｐ_in（ｔ）は１次までのテーラー展開で近似でき、さらに、δＴ_ｋを連続関数ＴＪ（ｔ）で置き換えることによって、次式（数式２）が得られる。
【数式２】

ここで、Ｐ_０（ｔ）はタイミングジッターのない理想的なパルス列を表すものとする。以上の導出は、前述した従来の文献（非特許文献５）の付録に記載されているので、ここでの詳細な説明は省略する。
【００２９】
マッハ・ツェンダー型干渉計の出力光強度は、２つの光路を伝搬する光パルス列の強度Ｐ _ｉｎ （ｔ）とＰ _ｉｎ （ｔ−τ）の和として表すことができる。これは、遅延時間がパルスのコヒーレンス時間に比べて十分に長いために、電場の干渉が生じないためである。光パルス列の強度Ｐ _ｉｎ （ｔ）とＰ _ｉｎ （ｔ−τ）に数式２を代入すると、マッハ・ツェンダー型干渉計の出力光強度は、数式３で表すことができる。ここで、光パルス列の周期性と、τ＝ｍ・Ｔより、Ｐ ₀ （ｔ）＝Ｐ ₀ （ｔ−τ）と、ｄＰ ₀ （ｔ）／ｄｔ＝ｄＰ ₀ （ｔ−τ）／ｄｔなることを利用した。
【数式３】

上記した数式１と数式３を比較すると、数式３におけるＪ（ｔ）が、数式４において［Ｊ（ｔ）＋Ｊ（ｔ−τ）］／２に置き換えられていることがわかる。干渉計を通過した光パルス列のタイミング変位をＪ_ｏｕｔ（ｔ）で表すことにすれば、Ｊ_ｏｕｔ（ｔ）は次式（数式４）で表される。
【数式４】

ここで、Ｊ（ｔ）、Ｊ_ｏｕｔ（ｔ）のフーリエ変換を、それぞれＪ（ω）、Ｊ_ｏｕｔ（ω）で表せば、数式４は次式（数式５）のように変換される。ただし、Ｆ｛Ｊ（ｔ）｝は関数Ｊ（ｔ）のフーリエ変換を表し、Ｆ｛Ｊ（ｔ−τ）｝＝Ｊ（ω）ｅｘｐ（−ｉωτ）の関係を利用した。
【数式５】

【００３０】
図２は、タイミングジッター低減の動作原理を説明する波形図である。前述したように、マッハ・ツェンダー型干渉計では、光路差のある２つの光路（光路ａおよび光路ｂ）を伝搬したパルスの強度波形Ｐ_in（ｔ−τ）／２、Ｐ_in（ｔ）／２が、方向性結合器により合成されて、干渉計から出力される。干渉計から射出される光パルスの強度波形Ｐ_out（ｔ）は、波形Ｐ_in（ｔ−τ）／２、Ｐ_in（ｔ）／２が合成されたものとなる。時刻（ｔ−τ）および時刻ｔにおいて、光パルスが時間軸において正、負方向に対照的に変位したとする。このとき、干渉計の出力では二つのパルス強度がインコヒーレントに加算され、中心に位置したパルスを形成する。このようなパルス位置の対称的な変位は、フーリエ周波数ｆが、ｆ＝ω／２π＝（ｍ−１／２）／τの変動成分に対して発生する。したがって、干渉計の遅延時間、すなわち光路差により決まる特定の周波数で、タイミング変位が相殺されることになる。
【００３１】
上述した説明および数式６から明らかなように、タイミングジッターが低減できる周波数は、干渉計の遅延時間に依存し、周波数帯域が制限される。この制限は、多重光路干渉計を用いることにより緩和できる。次に、このような本発明のタイミングジッター低減装置の変形例について説明する。
【００３２】
図３は、本発明の他の実施例のタイミングジッター低減装置を説明する図である。図３に示すタイミングジッター低減装置は、図１により説明したようなマッハ・ツェンダー型干渉計のタイミングジッター低減装置を単位装置１１，１２，…，１３として、この単位装置１１，１２，…，１３のＮ個を直列に接続した構造の干渉計となっている。各マッハ・ツェンダー型干渉計の単位装置１１，１２，…，１３は、それぞれに２つの方向性結合器１および２と、可変遅延線３と、可変光減衰器４を備えるものとなっている。それぞれの単位装置１１，１２，…，１３の干渉計の遅延時間を、τ_１、τ_２、…、τ_Ｎとすれば、雑音パワー抑圧度は次式で与えられることになる。
【数式７】

【００３３】
次に、別の変形例として、多重光路干渉計のリング干渉計を用いる場合の本発明のタイミングジッター低減装置の変形例について説明する。
【００３４】
図４は、本発明の別の実施例のタイミングジッター低減装置を説明する図である。図４に示す本発明の別の変形例に係るタイミングジッター低減装置は、光増幅器を含むリング干渉計である。図４において、２０は光ファイバ、２１は方向性結合器、２２は可変遅延線、２３は光増幅器である。方向性結合器２１、可変遅延線２２、光増幅器２３を光ファイバによりリング状に結合することにより、リング干渉計を構成する。方向性結合器２１によりリング内に入射する分岐して入射させる。分岐比を方向性結合器２１により設定される。このリング干渉計においては、入射パルス列はリング内を周回し、多重光路干渉計として機能する。パルス列の周回数を増大するために、方向性結合器２１による光の分岐と伝搬損失を補償するための光増幅器２３が設けられている。分岐比が１：１の方向性結合器を用いた場合の雑音パワー抑圧度は次式（数式８）で与えられる。
【数式８】

ここで、Ｇは、リング干渉計の一周回当たりの透過率を表し、リング干渉計自体がレーザー発振するのを防ぐため、Ｇ＜２に設定する必要がある。
【００３５】
次に、本発明に係るタイミングジッター低減装置の有効性を確認するための実験結果については説明する。この実験では、繰り返し周波数19.444ＧＨｚの光パルス列のタイミングジッターについて低減された実験結果が得られている。
【００３６】
図５は、実験に用いた装置の構成を示す図である。図５において、５０はルビジウム原子発振器、５１は周波数シンセサイザ、５２はモード動機半導体レーザー、５３はアイソレータ、５４はタイミングジッター低減装置であるマッハ・ツェンダー型の干渉計、５５は光検出器、５６はミキサ、５７は位相同期発振器、５８はベクトル信号解析装置である。この実験装置では、正確な時刻でのタイミングを測定するため、ルビジウム原子発振器５０の周波数１０ＭＨｚの発振信号を基本クロックとして、周波数シンセサイザ５１，位相同期発振器５７，ベクトル信号解析装置５８を動作させるようにしている。
【００３７】
光パルスを発光させるためのモード動機半導体レーザー５２としては、波長１５４２ｎｍのレーザー光を発光するモノリシック・モード同期半導体レーザーを用いた。モノリシック・モード同期半導体レーザーは、利得領域、可飽和吸収領域、受動導波路から構成され、可飽和吸収領域に逆バイアス電圧を印加することにより、受動モード同期で発光させる。逆バイアスを印加することに加えて、周波数シンセサイザ５１から出力される19.444ＧＨｚの正弦波信号を印加すると、ハイブリッドモード同期が起こり、モード動機半導体レーザー５２から出力される光パルスが周波数シンセサイザ５１の出力と同期する。
【００３８】
モード動機半導体レーザー５２の各縦モードのスペクトル線幅は２．２ＧＨｚで、コヒーレンス長はおよそ２２ｍｍである。ハイブリッドモード同期動作するモード動機半導体レーザー５２から出力される光パルスを、アイソレータ５３を通過させた後、マッハ・ツェンダー型の光ファイバ干渉計５４に入射させる。干渉計５４を透過した光パルスは、光検出器５５で受光させて電気信号に変換し、周波数19.444ＧＨzの信号成分を、ミキサ５６により周波数19.000ＧＨｚを発振する位相同期発振器５７を用いて、周波数４４４ＭＨｚの中間周波信号に変換する。ミキサ５６からの中間周波信号は、ベクトル信号解析装置５８に入力して、ベクトル信号解析装置５８により、時間領域復調法によるタイミング変位の検出と、高速フーリエ変換によるパワースペクトル密度の計算を行った。すべての信号源と測定器には、ルビジウム（Ｒｂ）原子発振器からの周波数１０ＭＨｚの発振信号を共通参照信号して供給して行った。
【００３９】
最初に、図１に示す構造のマッハ・ツェンダー型の干渉計を用いて、タイミングジッター低減の実験を行った。干渉計の光路差は約５ｍで、半導体レーザーのコヒーレンス長２２ｍｍに比べて十分大きい。数式６より雑音パワー抑圧度が最小となる最低のフーリエ周波数はおよそ２０ＭＨｚと見積もられるため、干渉計の遅延時間は、可変遅延線を用いて微調整した。
【００４０】
図６は、繰り返し周波数19.444ＧＨｚの光パルスに対するタイミングの３８μsに渡る時間変化を示す図である。サンプリング間隔は１９５ｎｓ、データポイント数は１９９６である。灰色の線は半導体レーザーからの出力、黒色の線は干渉計からの出力を表している。両者を比較すると、干渉計を伝搬した後に、タイミング変動が減少していることがわかる。
【００４１】
図７は、フーリエ周波数に対するタイミング雑音のパワースペクトル密度ＳＪ（ｆ）を示す図である。図７に示すように、タイミング雑音のパワースペクトル密度ＳＪ（ｆ）をみると、曲線Ａは半導体レーザーを駆動するシンセサイザのタイミング雑音を示しており、曲線Ｂは半導体レーザーの出力パルスのタイミング雑音を示している。曲線Ａと曲線Ｂには大きな差があり、半導体レーザーのシンセサイザへの同期が不十分であることを表している。また、曲線Ｃは、干渉計の出力パルスのタイミング雑音を表している。曲線Ａおよび曲線Ｃを比較すると、干渉計の出力パルスにおいて、フーリエ周波数５ＭＨｚ以上の雑音が減少していることがわかる。曲線Ｃの右端では、雑音パワー抑圧度が最小となる周波数に近く、ほぼ２桁の雑音低減が得られていることがわかる。
【００４２】
図８は、フーリエ周波数に対する雑音パワー抑圧度を示す図である。図８に示す曲線Ａおよび曲線Ｂは、それぞれ２段接続のマッハ・ツェンダー型干渉計、およびリング干渉計でえられた雑音パワー抑圧度を示している。黒色の曲線は理論値を示しており、その上に重ねて表示した変動波形（灰色の線）は測定値を表している。これらの両者はよく一致している。マッハ・ツェンダー型干渉計では、雑音抑圧度が正弦波状に変化するため、低周波数領域で雑音を低減することは困難である。このため、より広い周波数帯域で雑音低減を実現するためには、より大きな光路差を有する干渉計を用いることにより、また、さらには接続段数を増加することによって対応する。
【００４３】
また、前述したように、これに対してはリング干渉計を用いることにより対応するようにしてもよい。リング干渉計では、鋭い共振ピークが残るものの、広帯域に渡り、雑音を低減することが可能である。図８の上部側に示す曲線Ｃ、Ｄ、Ｅは、タイミング雑音のパワースペクトル密度ＳＪ（ｆ）から次式（数式９）を用いて計算したタイミングジッターσJである。
【数式９】

ここで、ｆ_ｈ、ｆ_ｌは積分の上限と下限の周波数を表し、タイミングジッターσ_Jの値はｆ_ｈ＝１８ＭＨｚに対して計算し、ｆ_ｌの関数としてプロットした。曲線Ｃは、半導体レーザーの出力パルスに対する結果であり、１００ｋＨｚ〜１８ＭＨｚの周波数帯域におけるジッターは１．８７ｐｓである。曲線Ｃより、タイミングジッターの大部分は、周波数１ＭＨｚ以上の雑音成分から生じていることが理解される。曲線Ｄは、２段接続のマッハ・ツェンダー型干渉計により得られた結果であり、曲線Ｅは、リング干渉計により得られた結果である。タイミングジッターは、それぞれ０．９９１ｐｓ（曲線Ｄ）、０．８７ｐｓ（曲線Ｅ）であり、モード同期半導体レーザーの出力パルスに対して、５３％、および４７％に減少している。
【００４４】
【発明の効果】
以上に説明したように、本発明のタイミングジッター低減装置によれば、半導体レーザー自体に電気的あるいは光学的な作用を必要とすることなく、半導体レーザーの出力光パルスのタイミングジッターを低減することができる。タイミングジッターの低減は、半導体レーザーの外部で行うため、半導体レーザーの構造や駆動回路を変更することなく、タイミングジッターを効果的に低減することができる。これにより、装置が大幅に単純化され、パッケージ化された素子などレーザーに直接作用することができない場合にも適用可能である。また、光ファイバを長距離伝搬してきた光パルスなど、レーザー光源が手元に無い場合のタイミングジッターを低減する場合にも有効である。
【図面の簡単な説明】
【図１】本発明の一実施例に係るタイミングジッター低減装置の構成を示す図である。
【図２】タイミングジッター低減の動作原理を説明する波形図である。
【図３】本発明の他の実施例のタイミングジッター低減装置を説明する図である。
【図４】本発明の別の実施例のタイミングジッター低減装置を説明する図である。
【図５】実験に用いた装置の構成を示す図である。
【図６】繰り返し周波数19.444ＧＨｚの光パルスに対するタイミングの３８μsに渡る時間変化を示す図である。
【図７】フーリエ周波数に対するタイミング雑音のパワースペクトル密度Ｓ_Ｊ（ｆ）を示す図である。
【図８】フーリエ周波数に対する雑音パワー抑圧度を示す図である。
【符号の説明】
１…第１方向性結合器
２…第２方向性結合器
３…可変遅延線
４…可変光減衰器
５…第１光路
６…第２光路
２０…光ファイバ
２１…方向性結合器
２２…可変遅延線
２３…光増幅器
５０…ルビジウム原子発振器
５１…周波数シンセサイザ
５２…モード動機半導体レーザー
５３…アイソレータ
５４…干渉計
５５…光検出器
５６…ミキサ
５７…位相同期発振器
５８…ベクトル信号解析装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical pulse timing jitter reduction method and an optical pulse timing jitter reduction apparatus that reduce timing jitter of a laser optical pulse train.
[0002]
[Prior art]
In the field of communication technology that uses pulse trains of ultra-short pulses that are repeated frequently, such as optical fiber communication at high bit rates using optical time division multiplexing (OTDM) transmission systems, and the technical field of optical sampling measurement, optical pulse trains Therefore, a stable semiconductor laser light source with low timing jitter is required.
[0003]
In application fields such as optical fiber communication using an optical time division multiplexing (OTDM) transmission system and optical sampling measurement, a technique for stably and accurately generating a high-frequency repeated optical pulse signal is an important technical issue. In these applications, severe conditions are imposed not only on pulse duration but also on pulse timing jitter. For example, when performing data communication by the optical time division multiplex transmission method, the code error rate is set to 10^-12In order to suppress to the following, it is necessary to suppress the timing jitter to 1 / 14.1 or less of the bit rate. This corresponds to a jitter of 443 fs (femtosecond) in a system with a transmission rate of 160 Gbit / s.
[0004]
The mode-locked semiconductor laser is expected to be a promising light source in the above-described application fields because it has features such as small size, low power consumption, and integration with other semiconductor elements. However, compared with other lasers such as a mode-locked fiber laser, the volume of the resonator is small, so that the influence of spontaneous emission light noise is large and the timing jitter is relatively large.
[0005]
In order to suppress the timing jitter of the mode-locked semiconductor laser, conventionally, (1) a method using an electro-optic phase locked loop, (2) a method using an external resonator structure, (3) subharmonic light injection is used. Various methods such as the method of (4), the method of utilizing optical feedback, and the like have been proposed.
[0006]
(1) A method using an electro-optic phase-locked loop is a method of comparing and detecting a phase difference between an optical pulse and a reference electrical signal, and electrically negatively controlling the semiconductor laser to reduce jitter (non-patent) Reference 1). (2) The method using the external resonator structure is a method of reducing the influence of spontaneous emission light noise by disposing a mirror or a diffraction grating outside the semiconductor laser and increasing the volume of the resonator ( (Refer nonpatent literature 2). (3) A method using subharmonic light injection is a method in which another stable pulse laser light source is prepared and the output light is injected into a semiconductor laser to cause synchronization and reduce jitter (Non-patent Document 3). See). (4) A method using optical feedback is a method in which a part of the output light pulse is fed back to the semiconductor laser itself and jitter is reduced by interference (see Non-Patent Document 4).
[0007]
[Non-Patent Document 1]
LA Buckman, JB Georges, J. Park, D. Vassilovski, JM Kahn, and KY Lau, "Stabilization of millimeter-wave frequencies from passively mode-locked semiconductor lasers using an optoelectronic phase-locked loop", IEEE Photon. Technol. ., 5, 1137 (1993).
[Non-Patent Document 2]
DJ Derickson, PA Morton, JE Bowers, and RL Thornton, "Comparison of timing itter in external and monolithic cavity mode-locked semiconductor lasers", Appl. Phys. Lett., 59, 3372 (1991) .; LA Jiang, ME Grein , EP Ippen, C. McNeilage, J. Searls, and H. Yokoyama, "Quantum-limited noise performance of a mode-locked laser diode", Opt. Lett., 27, 49 (2002).
[Non-Patent Document 3]
(3) 9. X. Wang, H. Yokoyama, and T. Shimizu, "Synchronized harmonic frequency mode-locking with laser diodes through optical pulse train injection", IEEE Photon. Technol. Lett., 8, 617 (1996). S. Arahira, S. Kutsuzawa, and Y. Ogawa, "Extreme timing jitter reduction of a passively mode-locked laser diode by optical pulse injection", IEEE J. Quantum Electron., 35, 1805 (1999) .; YJ Wen , D. Novak, and HF Liu, "Mode-locking of long cavity Fabry-Perot semiconductor laser at millimetre wave frequencies by subharmonic optical injection", Electron. Lett., 36, 879 (2000).
[Non-Patent Document 4]
P. Langlois. D. Gay, N. McCarthy, and M. Piche, "Noise reduction in a mode-locked semiconductor laser by coherent photon seeding", Opt. Lett., 23, 114 (1998) .; LA Jiang, KS Abedin, ME Grein, and EP Ippen, "Timing jitter reduction in modelocked semiconductor lasers with photon seeding", Appl. Phys. Lett., 80, 1707 (2002).
[Non-Patent Document 5]
D. von der Linde, "Characterization of the noise in continuously operating mode-locked lasers", Appl. Phys. B, 39, 201 (1986).
[0008]
[Problems to be solved by the invention]
By the way, any of the four methods described above can reduce the timing jitter of the mode-locked semiconductor laser. However, in order to use these methods, all of them are directly applied to the semiconductor laser. Since a mechanism for acting electrically and optically is necessary, it cannot be applied to a packaged semiconductor laser device. It is also quite difficult to apply.
[0009]
More specifically, in the method (1) using the electro-optical phase-locked loop, since the electric control is performed, the frequency band for reducing the timing jitter is limited to about 1 MHz or less. In the method (2) using the external resonator structure, since the structure of the laser is changed, there is a disadvantage that the repetition frequency of the light pulse is lowered and the apparatus becomes large. In the method (3) using the subharmonic light injection, it is necessary to prepare another stable pulse light source, which complicates the apparatus and increases its size. Regarding the method (4) using the optical feedback, since the apparatus becomes large and uses interference, it is easily affected by mechanical vibrations. There are various problems like this.
[0010]
The present invention has been made in order to solve the above-described problems, and an object of the present invention is, for example, a laser beam without acting electrically and optically on a mode-locked semiconductor laser. An object of the present invention is to provide an optical pulse timing jitter reduction method and an optical pulse timing jitter reduction apparatus that reduce timing jitter of a pulse train.
[0011]
[Means for Solving the Problems]
  In order to achieve the above object, the optical pulse timing jitter reduction method of the present invention is a Mach-Zehnder interferometer using an optical fiber in an optical path.For the optical pulse train output from the laser,Light propagation delay due to optical path differenceThe optical pulse trainIs set to be sufficiently longer than the coherence time and an integral multiple of the pulse period of the optical pulse train, and the Mach-Zehnder interferometer,SaidIn the optical pulse train that enters the optical pulse train and exits from the interferometer, the timing jitter is canceled in a frequency band depending on the delay time.
[0012]
  According to another aspect of the present invention, there is provided an optical pulse timing jitter reduction method in an optical fiber ring interferometer including an optical amplifier and a delay line in an optical path.For the optical pulse train output from the laser,Light propagation delay due to optical path differenceThe optical pulse trainIs set to be sufficiently longer than the coherence time, and the delay time is an integral multiple of the pulse period of the optical pulse train, and the optical fiber ring interferometerSaidIn the optical pulse train that enters the optical pulse train and exits from the interferometer, the timing jitter is canceled in a frequency band depending on the delay time.
[0013]
As an apparatus for carrying out such a method, the optical pulse timing jitter reduction apparatus according to the present invention is such that the optical propagation delay time caused by the optical path difference with respect to the optical pulse train output from the laser is the laser coherence time. A first optical path in which a delay line is set that is sufficiently longer than the delay time and is set to be an integral multiple of the period of the incident pulse, and an attenuation amount that attenuates when light propagates through the first optical path A second optical path in which an optical attenuator that provides an amount of attenuation corresponding to the first optical path is inserted, a first directional coupler that distributes incident light to the first optical path and the second optical path, and the first optical path And a second directional coupler that combines the light derived from the light and the light derived from the second optical path into outgoing light. The optical pulse timing jitter reduction apparatus of the present invention has a configuration in which the optical pulse timing jitter reduction apparatus configured as described above is used as a unit device, and a plurality of these interferometer devices are connected in series in multiple stages. Also good.
[0014]
In addition, the optical pulse timing jitter reduction apparatus of the present invention according to another feature is sufficiently longer than the coherence time of the laser, with respect to the optical pulse train output from the laser, the delay time of light propagation caused by the optical path difference, And a ring-shaped optical fiber optical path inserted with a delay line and an optical amplifier set so that the delay time is an integral multiple of the period of the incident pulse, and coupled to the optical fiber optical path for incident laser light And a directional coupler for deriving outgoing light. The optical pulse timing jitter reduction apparatus according to the present invention has a configuration in which the optical pulse timing jitter reduction apparatus configured as described above is used as a unit device, and a plurality of these interferometer devices are connected in series in multiple stages. Also good.
[0015]
The optical pulse timing jitter reduction method and optical pulse timing jitter reduction apparatus according to the present invention effectively reduces timing jitter when applied to a pulse train of an optical pulse of a mode-locked semiconductor laser, in which timing jitter occurs relatively frequently. it can.
[0016]
Further, when expressing the characteristics of the optical pulse timing jitter reduction method and optical pulse timing jitter reduction device according to the present invention in other words,
(1) In the optical pulse timing jitter reduction method according to the present invention, the optical propagation delay time caused by the optical path difference is sufficiently larger than the laser coherence time for the optical pulse train output from the mode-locked semiconductor laser. A Mach-Zehnder interferometer using an optical fiber which is long and has a delay time set to be an integral multiple of the period of the incident pulse is used. When the optical pulse train output from the mode-locked semiconductor laser is incident on the interferometer set in this way, the optical pulse train output from the directional coupler of the optical fiber interferometer is timed in a frequency band depending on the delay time. Jitter is offset.
[0017]
(2) The optical pulse timing jitter reduction apparatus according to the present invention is such that the delay time of light propagation caused by the optical path difference is sufficiently longer than the coherence time of the laser, and the delay time is an integral multiple of the period of the incident pulse. A fiber optic Mach-Zehnder interferometer set to be such that when an optical pulse train output from a mode-locked semiconductor laser is incident on the interferometer set in this way, the delay time at the output of the interferometer The timing jitter is canceled in a frequency band depending on the frequency.
[0018]
(3) Further, the optical pulse timing jitter reduction apparatus according to the present invention is such that the optical propagation delay time due to the optical path difference is sufficiently longer than the laser coherence time, and the delay time is equal to the period of the incident pulse. A multi-stage connection of a plurality of optical fiber Mach-Zehnder interferometers set to be an integral multiple is used. When the optical pulse train output from the mode-locked semiconductor laser is incident on the interferometers connected in series in this way, the timing jitter is canceled in the frequency band depending on the delay time at the output of the interferometer.
[0019]
(4) In the optical pulse timing jitter reduction apparatus, the delay time of light propagation due to the optical path difference is sufficiently longer than the coherence time of the laser, and the delay time is an integral multiple of the period of the incident pulse. An optical fiber ring interferometer that is set and includes an optical amplifier in the optical path is used. When an optical pulse train output from a mode-locked semiconductor laser is incident on such a ring interferometer, the timing jitter is canceled in the frequency band depending on the delay time at the output of the ring interferometer.
[0020]
Thus, according to the optical pulse timing jitter reduction method and optical pulse timing jitter reduction apparatus of the present invention, the timing jitter reduction is independent of the laser itself with respect to the optical pulses emitted from the laser. Therefore, the semiconductor laser itself requires no electrical or optical action. Therefore, it is possible to effectively reduce the timing jitter without changing the laser structure or the drive circuit. Further, since the Mach-Zehnder interferometer can be configured only with passive optical components, the configuration is extremely simple.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. In the following description, an experimental result in which the timing jitter of an optical pulse train having a repetition frequency of 19.444 GHz is reduced will be described together with the description of the operation principle and the device configuration.
[0022]
FIG. 1 is a diagram illustrating a configuration of a timing jitter reduction apparatus according to an embodiment of the present invention. In FIG. 1, 1 is a first directional coupler, 2 is a second directional coupler, 3 is a variable delay line, 4 is a variable optical attenuator, 5 is a first optical path, and 6 is a second optical path. The timing jitter reduction apparatus shown in FIG. 1 constitutes a Mach-Zehnder interferometer, and the first directional coupler 1 transmits an incident optical pulse into two optical paths (first optical path 5 and second optical path 6). ), And the optical pulse synthesized by the second directional coupler 2 is emitted while giving a delay time and an attenuation amount due to the optical path difference. In this case, the delay time is an integer of the period of the incident pulse and the delay time of light propagation due to the optical path difference is sufficiently longer than the coherence time of the laser with respect to the optical pulse train output from the laser. Set to double. The attenuation is set so that the intensity of the light pulse that has passed through the two optical paths is equal at the exit of the two optical paths. Therefore, in this case, an optical amplifier may be provided in the first optical path 5 without providing the variable optical attenuator 4 in the second optical path 6.
[0023]
When an optical pulse train having timing jitter is incident on the timing jitter reducing apparatus configured as described above, the timing jitter is reduced, and an optical pulse train with high timing accuracy is obtained. For example, a laser light pulse emitted from a modal semiconductor laser is used as an incident light pulse train.
[0024]
Laser light pulse P_inWhen (t) is incident on the first directional coupler 1 via the optical fiber, the first directional coupler 1 distributes the incident light and the first optical path 5 and the second optical path 6 of the optical fiber. To derive. The variable delay line 3 is inserted in the first optical path 1, and as described above, the delay time by the variable delay line 3 is the same as that of the first optical path and the second optical path with respect to the optical pulse train output from the laser. Since the delay time of light propagation caused by the optical path difference is set to be sufficiently longer than the coherence time of the laser and the delay time is set to be an integral multiple of the period of the incident pulse, The light incident on one optical path 5 is emitted and enters the second directional coupler 2. In addition, the variable optical attenuator 4 is inserted in the second optical path 6, and as described above, the variable attenuator 4 attenuates the optical attenuation when the light propagates in the first optical path. Is attenuated by the attenuation amount, and then the light incident on the second optical path 6 is emitted and enters the second directional coupler 2.
[0025]
Thereby, the intensities of the optical pulses that have passed through the two optical paths are equal at the exits of the two optical paths, and are derived from the respective exits of the first optical path 5 and the second optical path 6 after a delay time due to the optical path difference. The second directional coupler 2 synthesizes the light pulse derived from the first optical path 5 and the light pulse derived from the second optical path 6 to generate the emitted light P_out(T). As will be described later, the emission light has reduced timing jitter.
[0026]
Next, the operation principle will be described. The timing jitter reduction apparatus shown in FIG. 1 is a Mach-Zehnder interferometer having a structure in which two optical fibers having different optical path differences are coupled by directional couplers at both ends thereof. . Therefore, if the difference between the lengths of the two optical fibers for causing the optical path difference is ΔL, the delay time due to the optical path difference is represented by τ = n · ΔL / c. Here, n is the refractive index of the optical fiber used as the optical path, and c is the speed of light in vacuum. The delay time is sufficiently longer than the coherence time of the light pulse incident on the interferometer.
[0027]
  The repetition period of the incident light pulseTThen, the delay time of the variable delay line 3 arranged in the first optical path 5 (optical path a) of the interferometer is the period of the incident pulse.TAn integral multiple of τ = m ·TAdjust so thatHere, m is a positive integer.The variable optical attenuator 4 arranged in the second optical path 6 (optical path b) is adjusted so that the intensities of the optical pulse trains propagated through the two optical paths (optical path a and optical path b) are equal.
[0028]
The intensity of the optical pulse train incident on the interferometer can be expressed by the following formula (Formula 1).
[Formula 1]

Here, g (t) is a time waveform of each pulse, and δT_kRepresents the displacement of the timing. δT_kIs smaller than the pulse period, P_in(T) can be approximated by Taylor expansion up to first order, and δT_kIs replaced by the continuous function TJ (t), the following equation (Equation 2) is obtained.
[Formula 2]

Where P₀(T) represents an ideal pulse train without timing jitter. The above derivation is described in the appendix of the above-described conventional document (Non-Patent Document 5), and thus detailed description thereof is omitted here.
[0029]
  The output light intensity of the Mach-Zehnder interferometer is the intensity of the optical pulse train propagating in the two optical paths.P _in (T) and P _in (T-τ)Can be expressed as the sum ofTheThis is because the delay time is sufficiently longer than the coherence time of the pulse, so that electric field interference does not occur.Intensity P of optical pulse train _in (T) and P _in When Expression 2 is substituted into (t−τ), the output light intensity of the Mach-Zehnder interferometer can be expressed by Expression 3. Here, from the periodicity of the optical pulse train and τ = m · T, P ₀ (T) = P ₀ (T−τ) and dP ₀ (T) / dt = dP ₀ The fact that (t−τ) / dt is used.
[Formula 3]

Comparing Equation 1 and Equation 3 above,It can be seen that J (t) in Equation 3 is replaced with [J (t) + J (t−τ)] / 2 in Equation 4.Let J be the timing displacement of the optical pulse train that has passed through the interferometer._out(T) represents J_out(T) is expressed by the following formula (Formula 4).
[Formula 4]

Where J (t), J_outThe Fourier transform of (t) is expressed as J (ω), J_outIf expressed by (ω), Formula 4 is converted as the following Formula (Formula 5).However, F {J (t)} represents the Fourier transform of the function J (t), and the relationship of F {J (t−τ)} = J (ω) exp (−iωτ) was used.
[Formula 5]

[0030]
FIG. 2 is a waveform diagram for explaining the operation principle of timing jitter reduction. As described above, in the Mach-Zehnder interferometer, the intensity waveform P of the pulse propagated through two optical paths (optical path a and optical path b) having an optical path difference._in(T-τ) / 2, P_in(T) / 2 is synthesized by the directional coupler and output from the interferometer. Intensity waveform P of light pulse emitted from interferometer_out(T) is the waveform P_in(T-τ) / 2, P_in(T) / 2 is synthesized. Assume that at time (t−τ) and time t, the light pulse is displaced in the positive and negative directions on the time axis. At this time, two pulse intensities are added incoherently at the output of the interferometer to form a centrally located pulse. Such symmetrical displacement of the pulse position occurs with respect to the fluctuation component of the Fourier frequency f of f = ω / 2π = (m−1 / 2) / τ. Therefore, the timing displacement is canceled at a specific frequency determined by the delay time of the interferometer, that is, the optical path difference.
[0031]
As is apparent from the above description and Equation 6, the frequency at which the timing jitter can be reduced depends on the delay time of the interferometer, and the frequency band is limited. This limitation can be mitigated by using a multiple optical path interferometer. Next, a modified example of the timing jitter reducing apparatus of the present invention will be described.
[0032]
FIG. 3 is a diagram for explaining a timing jitter reducing apparatus according to another embodiment of the present invention. The timing jitter reduction apparatus shown in FIG. 3 has unit timing units 11, 12,..., 13 as Mach-Zehnder interferometer timing jitter reduction units as described with reference to FIG. The interferometer has a structure in which N pieces of N are connected in series. Each unit device 11, 12,..., 13 of each Mach-Zehnder interferometer includes two directional couplers 1 and 2, a variable delay line 3, and a variable optical attenuator 4. . The delay time of the interferometer of each unit device 11, 12,.₁, Τ₂, ..., τ_NIf so, the noise power suppression degree is given by the following equation.
[Formula 7]

[0033]
Next, as another modified example, a modified example of the timing jitter reducing apparatus of the present invention when a ring interferometer of a multiple optical path interferometer is used will be described.
[0034]
FIG. 4 is a diagram for explaining a timing jitter reducing apparatus according to another embodiment of the present invention. The timing jitter reduction apparatus according to another modification of the present invention shown in FIG. 4 is a ring interferometer including an optical amplifier. In FIG. 4, 20 is an optical fiber, 21 is a directional coupler, 22 is a variable delay line, and 23 is an optical amplifier. A ring interferometer is configured by coupling the directional coupler 21, the variable delay line 22, and the optical amplifier 23 in a ring shape with an optical fiber. A directional coupler 21 diverges and enters the ring. The branching ratio is set by the directional coupler 21. In this ring interferometer, the incident pulse train circulates in the ring and functions as a multiple optical path interferometer. In order to increase the number of circulations of the pulse train, an optical amplifier 23 is provided to compensate for light branching and propagation loss caused by the directional coupler 21. The noise power suppression degree when a directional coupler having a branching ratio of 1: 1 is used is given by the following equation (Equation 8).
[Formula 8]

Here, G represents the transmittance per round of the ring interferometer, and it is necessary to set G <2 in order to prevent the ring interferometer itself from lasing.
[0035]
Next, experimental results for confirming the effectiveness of the timing jitter reduction apparatus according to the present invention will be described. In this experiment, an experimental result in which the timing jitter of the optical pulse train having a repetition frequency of 19.444 GHz is reduced is obtained.
[0036]
FIG. 5 is a diagram showing the configuration of the apparatus used in the experiment. In FIG. 5, 50 is a rubidium atomic oscillator, 51 is a frequency synthesizer, 52 is a mode motive semiconductor laser, 53 is an isolator, 54 is a Mach-Zehnder type interferometer which is a timing jitter reduction device, 55 is a photodetector, 56 is A mixer, 57 is a phase-locked oscillator, and 58 is a vector signal analyzer. In this experimental apparatus, the frequency synthesizer 51, the phase-locked oscillator 57, and the vector signal analyzing apparatus 58 are operated using an oscillation signal having a frequency of 10 MHz of the rubidium atomic oscillator 50 as a basic clock in order to measure the timing at an accurate time. ing.
[0037]
As the mode motive semiconductor laser 52 for emitting light pulses, a monolithic mode-locked semiconductor laser that emits laser light having a wavelength of 1542 nm was used. A monolithic mode-locked semiconductor laser is composed of a gain region, a saturable absorption region, and a passive waveguide, and emits light in passive mode locking by applying a reverse bias voltage to the saturable absorption region. When a 19.444 GHz sine wave signal output from the frequency synthesizer 51 is applied in addition to applying a reverse bias, hybrid mode synchronization occurs, and an optical pulse output from the mode motive semiconductor laser 52 is output from the frequency synthesizer 51. Synchronize with.
[0038]
The spectral line width of each longitudinal mode of the mode motive semiconductor laser 52 is 2.2 GHz, and the coherence length is approximately 22 mm. The optical pulse output from the mode motive semiconductor laser 52 that operates in a hybrid mode synchronization is allowed to enter the Mach-Zehnder type optical fiber interferometer 54 after passing through the isolator 53. The optical pulse transmitted through the interferometer 54 is received by the photodetector 55 and converted into an electric signal, and a signal component having a frequency of 19.444 GHz is oscillated using a phase-locked oscillator 57 that oscillates a frequency of 19.000 GHz by the mixer 56. Convert to an intermediate frequency signal of 444 MHz. The intermediate frequency signal from the mixer 56 is input to the vector signal analysis device 58, and the vector signal analysis device 58 detects the timing displacement by the time domain demodulation method and calculates the power spectrum density by the fast Fourier transform. All signal sources and measuring instruments were supplied with an oscillation signal having a frequency of 10 MHz from a rubidium (Rb) atomic oscillator as a common reference signal.
[0039]
First, using a Mach-Zehnder type interferometer having the structure shown in FIG. The optical path difference of the interferometer is about 5 m, which is sufficiently larger than the coherence length of 22 mm of the semiconductor laser. Since the lowest Fourier frequency that minimizes the noise power suppression degree is estimated to be about 20 MHz from Equation 6, the delay time of the interferometer was finely adjusted using a variable delay line.
[0040]
FIG. 6 is a diagram showing a temporal change over 38 μs of timing with respect to an optical pulse having a repetition frequency of 19.444 GHz. The sampling interval is 195 ns and the number of data points is 1996. The gray line represents the output from the semiconductor laser, and the black line represents the output from the interferometer. Comparing both, it can be seen that the timing variation decreases after propagation through the interferometer.
[0041]
FIG. 7 is a diagram showing the power spectral density SJ (f) of timing noise with respect to the Fourier frequency. As shown in FIG. 7, when looking at the power spectral density SJ (f) of the timing noise, the curve A shows the timing noise of the synthesizer that drives the semiconductor laser, and the curve B shows the timing noise of the output pulse of the semiconductor laser. Show. There is a large difference between curve A and curve B, indicating that the synchronization of the semiconductor laser to the synthesizer is insufficient. Curve C represents the timing noise of the output pulse of the interferometer. Comparing curve A and curve C, it can be seen that noise with a Fourier frequency of 5 MHz or more is reduced in the output pulse of the interferometer. At the right end of the curve C, it can be seen that the noise reduction is close to the frequency at which the noise power suppression degree is minimum, and almost two-digit noise reduction is obtained.
[0042]
FIG. 8 is a diagram showing the noise power suppression degree with respect to the Fourier frequency. A curve A and a curve B shown in FIG. 8 indicate the noise power suppression degrees obtained by the Mach-Zehnder interferometer and the ring interferometer, respectively, connected in two stages. The black curve shows the theoretical value, and the fluctuation waveform (gray line) displayed superimposed on it represents the measured value. Both of these are in good agreement. In the Mach-Zehnder interferometer, the noise suppression degree changes in a sine wave shape, so it is difficult to reduce noise in a low frequency region. For this reason, in order to realize noise reduction in a wider frequency band, it is possible to use an interferometer having a larger optical path difference and further increase the number of connection stages.
[0043]
Further, as described above, this may be dealt with by using a ring interferometer. In a ring interferometer, although a sharp resonance peak remains, it is possible to reduce noise over a wide band. Curves C, D, and E shown on the upper side of FIG. 8 are timing jitters σJ calculated from the power spectrum density SJ (f) of the timing noise using the following formula (Formula 9).
[Formula 9]

Where f_h, F_lRepresents the upper and lower frequency limits of integration, and timing jitter σ_JThe value of is f_h= Calculated for 18 MHz, f_lPlotted as a function of. Curve C is the result for the output pulse of the semiconductor laser, and the jitter in the frequency band of 100 kHz to 18 MHz is 1.87 ps. From curve C, it is understood that most of the timing jitter is caused by noise components having a frequency of 1 MHz or more. A curve D is a result obtained by using a Mach-Zehnder interferometer having a two-stage connection, and a curve E is a result obtained by using a ring interferometer. The timing jitter is 0.991 ps (curve D) and 0.87 ps (curve E), respectively, and is reduced to 53% and 47% with respect to the output pulse of the mode-locked semiconductor laser.
[0044]
【The invention's effect】
As described above, according to the timing jitter reducing apparatus of the present invention, the timing jitter of the output light pulse of the semiconductor laser can be reduced without requiring an electrical or optical action on the semiconductor laser itself. it can. Since the timing jitter is reduced outside the semiconductor laser, the timing jitter can be effectively reduced without changing the structure or driving circuit of the semiconductor laser. This greatly simplifies the apparatus and is applicable when the packaged element cannot directly act on the laser. It is also effective in reducing timing jitter when a laser light source is not at hand, such as an optical pulse propagating through an optical fiber for a long distance.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of a timing jitter reduction apparatus according to an embodiment of the present invention.
FIG. 2 is a waveform diagram illustrating an operation principle of timing jitter reduction.
FIG. 3 is a diagram illustrating a timing jitter reduction apparatus according to another embodiment of the present invention.
FIG. 4 is a diagram illustrating a timing jitter reduction apparatus according to another embodiment of the present invention.
FIG. 5 is a diagram showing a configuration of an apparatus used in an experiment.
FIG. 6 is a diagram showing a temporal change over 38 μs of timing with respect to an optical pulse having a repetition frequency of 19.444 GHz.
FIG. 7: Power spectral density S of timing noise with respect to Fourier frequency_JIt is a figure which shows (f).
FIG. 8 is a diagram illustrating a noise power suppression degree with respect to a Fourier frequency.
[Explanation of symbols]
1 ... 1st directional coupler
2 ... Second directional coupler
3. Variable delay line
4 ... Variable optical attenuator
5 ... First optical path
6 ... Second optical path
20: Optical fiber
21 ... Directional coupler
22 ... Variable delay line
23: Optical amplifier
50 ... Rubidium atomic oscillator
51. Frequency synthesizer
52 ... Mode motive semiconductor laser
53 ... Isolator
54 ... Interferometer
55. Photodetector
56 ... Mixer
57 ... Phase-locked oscillator
58. Vector signal analyzer

Claims (8)

In a Mach-Zehnder interferometer using an optical fiber in the optical path,
With respect to the optical pulse train output from the laser, the delay time of light propagation due to the optical path difference is sufficiently longer than the coherence time of the optical pulse train , and the delay time is an integral multiple of the pulse period of the optical pulse train. set to, to the Mach-Zehnder interferometer, incident the optical pulse train in the optical pulse train emitted from the interferometer, the light pulses, characterized by offsetting the timing jitter in the frequency band that is dependent on the delay time Timing jitter reduction method. In an optical fiber ring interferometer including an optical amplifier and a delay line in the optical path,
With respect to the optical pulse train output from the laser, the delay time of light propagation due to the optical path difference is sufficiently longer than the coherence time of the optical pulse train , and the delay time is an integral multiple of the pulse period of the optical pulse train. set the incident said optical pulse train in the optical fiber ring interferometer, an optical pulse train emitted from the interferometer, the timing of light pulses, characterized by offsetting the timing jitter in the frequency band that is dependent on the delay time Jitter reduction method. For the optical pulse train output from the laser, the delay time of light propagation due to the optical path difference is set to be sufficiently longer than the coherence time of the laser, and the delay time is set to be an integral multiple of the period of the incident pulse. A first optical path with inserted delay lines;
A second optical path inserted with an optical attenuator that provides an amount of attenuation corresponding to the amount of attenuation attenuated when light propagates through the first optical path;
A first directional coupler for distributing incident light and leading it to the first optical path and the second optical path;
An optical pulse timing jitter reduction apparatus, comprising: a second directional coupler that combines light derived from the first optical path and light derived from the second optical path to generate outgoing light. For the optical pulse train output from the laser, the delay time of light propagation due to the optical path difference is set to be sufficiently longer than the coherence time of the laser, and the delay time is set to be an integral multiple of the period of the incident pulse. A first optical path with inserted delay lines;
A second optical path inserted with an optical attenuator that provides an amount of attenuation corresponding to the amount of attenuation attenuated when light propagates through the first optical path;
A first directional coupler for distributing incident light and leading it to the first optical path and the second optical path;
A plurality of interferometer devices each composed of a second directional coupler that combines light derived from the first optical path and light derived from the second optical path to generate outgoing light are connected in series in multiple stages. A device for reducing timing jitter of an optical pulse. For the optical pulse train output from the laser, the delay time of light propagation due to the optical path difference is set to be sufficiently longer than the coherence time of the laser, and the delay time is set to be an integral multiple of the period of the incident pulse. A ring-shaped optical fiber optical path into which a delay line and an optical amplifier are inserted, and
An optical pulse timing jitter reduction apparatus, comprising: a directional coupler coupled to the optical fiber optical path to receive laser light and to derive outgoing light. For the optical pulse train output from the laser, the delay time of light propagation due to the optical path difference is set to be sufficiently longer than the coherence time of the laser, and the delay time is set to be an integral multiple of the period of the incident pulse. A ring-shaped optical fiber optical path into which a delay line and an optical amplifier are inserted, and
A plurality of interferometer devices that are coupled to the optical fiber optical path and are configured to enter a laser beam and to emit outgoing light, and are connected in series in multiple stages. Timing jitter reduction device. 7. The optical pulse timing jitter reduction apparatus according to claim 3, wherein the optical pulse train is an optical pulse of a mode-locked semiconductor laser. 3. The optical pulse timing jitter reduction method according to claim 1, wherein the optical pulse train is an optical pulse of a mode-locked semiconductor laser.

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