JP2004020537A - Method and apparatus for measuring timing jitter of optical pulse - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and apparatus for measuring the timing jitter of optical pulse measurable with a photo detector or a signal analysis device. <P>SOLUTION: In the method for measuring the timing jitter of optical pulse, the optical pulse of high repetitive frequency is modulated in intensity with a signal of lower frequency than the high repetitive frequency to generate longitudinal mode signal light with higher-order modulation sideband wave light. The longitudinal mode signal light and the higher-order modulation sideband wave light are detected, and the timing jitter is obtained from the detection output. <P>COPYRIGHT: (C)2004,JPO

Description

により表される。ここで、Ｉ_０は各縦モードの強度に依存する定数、βは変調指数、Ｊ_ｎは第１種ベッセル関数を表す。式１より、ビート信号強度を最大にする変調指数の最適値を見積もると、２次、４次側帯波に対して２．２０、５．８７ラジアンになる。
確認のために行った実験で変調器に印可したＲＦ信号パワーはおよそ１８ｄＢｍで、増幅器の性能により制限されている。これにより２．０ラジアンの変調指数が得られ、２次側帯波に対しては最適値に近いが、４次側帯波に対してはかなり小さくなる。しかしながら、４次側帯波から生じる周波数１．７７６ＧＨｚのビート信号は、タイミングジッター計測に十分なＳＮ比を有していた。
【００１７】
前記時間領域復調法の一つの利点は、周波数領域だけでなく、時間領域データも得られることである。図６に７７．７７６　ＧＨｚのＯＴＤＭパルスに対するタイミングの３８０μｓに渡る時間変化を示す。サンプリング間隔は１９５ｎｓ、データポイント数は１９９６である。この間のＲＭＳジッターは０．７９ｐｓと見積もられ、パルス周期の６％に相当する。図７は前記図６に対応するヒストグラムで、タイミングの出現頻度を表している。各コラムの幅は１６０ｆｓである。図７のヒストグラムは正規分布に近く、観測時間の帯域内における揺らぎは白色雑音であることを表している。
図６に示すような時間領域の測定データから高速フーリエ変換を利用して、位相雑音パワースペクトル密度Ｓ_φ（ｆ）を９桁のフーリエ周波数に渡り計算した。図８の曲線Ａはシンセサイザ、曲線Ｂはレーザー出力である１９．４４４ＧＨｚの光パルス、曲線Ｄ、Ｃは周波数３８．８８８、７７．７７６ＧＨｚのＯＴＤＭパルスに対するパワースペクトル密度である。このような広い周波数範囲に渡る測定を一度に行うことはできないため、周波数スパンを変えて測定を行い、データを結合した。
【００１８】
前記位相雑音パワースペクトル密度Ｓ_φ（ｆ）から次式を利用して計算したＲＭＳジッターσ_Ｊを図８に示す。図８の曲線Ｅはシンセサイザ、曲線Ｆはレーザー出力である１９．４４４ＧＨｚの光パルス、曲線Ｇ、Ｈは周波数３８．８８８、７７．７７６ＧＨｚのＯＴＤＭパルスに対するＲＭＳジッターである。
【式２】

ここでｆ_ｌ、ｆ_ｈは積分の上限と下限を表し、σ_Ｊの値は固定のｆ_ｈ＝１８ＭＨｚに対して計算し、ｆ_ｌの関数としてプロットされている。曲線Ａ、Ｅは周波数１９．４４４ＧＨｚのシンセサイザに対応し、１９．０００ＧＨｚの位相同期発振器とミキサを用いて計測した。２．５ｍＨｚ〜１８ＭＨｚの帯域でＲＭＳジッターは３．１５ｐｓであり、主としてフーリエ周波数１００ｋＨｚ以下の雑音が寄与している。
【００１９】
曲線Ｂ、Ｆはレーザーの出力である１９．４４４ＧＨｚのパルスに対応し、図９の装置を用いて測定した。曲線ＡとＢを比較すると、１００ｋＨｚ以下の周波数域では両者はほとんど重なっており、レーザーがＲＦ信号によく追従していることを示している。しかしながら、１００ｋＨｚ以上の周波数域では両者に大きな差が見られる。印加するＲＦパワーを増大すると、１００ｋＨｚ以上の雑音は減少するが、２０ｄＢｍ（１００ｍＷ、０ｄＢｍ＝１ｍＷ）以上のパワーでは飽和する傾向が観測されている。曲線Ｆの右端の値からＲＭＳジッターは３．４２ｐｓと見積もられるが、シンセサイザの寄与分を除くと１．３３ｐｓになり、１００ｋＨｚ以上の雑音が原因である。
【００２０】
図８の曲線Ｃ、Ｄは周波数３８．８８８、７７．７７６ＧＨｚのＯＴＤＭパルスに対するパワースペクトル密度である。曲線Ｃ、Ｄ上のＳ_φ（ｆ）の値は曲線Ｂのそれぞれ４、１６倍になっており、繰り返し周波数の増大に対応している。また、曲線の形状がほぼ等しいことは、計測の正しさを表している。これらの結果より、パルス多重化の過程で付加される雑音は極めて小さく、元のレーザーパルスの雑音が支配的であることがわかる。曲線Ｇ、ＨはＣ、Ｄから計算されたＲＭＳジッターである。多重化によりパワースペクトル密度の値は増大するが、ジッター値はほぼ一定である。これは式２からわかるように、積分したパワースペクトル密度を繰り返し周波数で割っているためである。７７．７７６ＧＨｚのＯＴＤＭパルスに対するジッター値がわずかに増大しているのは、ビート信号のＳＮ比がやや低下したためである。周波数３８．８８８、７７．７７６ＧＨｚのＯＴＤＭパルスに対するジッター値は、３．４１、３．７４ｐｓと見積もられ、それぞれ繰り返し周期の１３．１％、２９．５％に相当する。ＯＴＤＭパルスのタイミングジッターを低減するためには、レーザー出力パルスにおけるフーリエ周波数１００ｋＨｚ以上の雑音成分を低減することが必要である。
【００２１】
上記実施例は、Ｍａｃｈ−Ｚｅｈｎｄｅｒ光強度変調器による電気光学的ハーモニックミキシングを利用して、繰り返し周波数３９、および７８ＧＨｚのＯＴＤＭパルスのタイミングジッター計測を行った。周波数変換された信号を時間領域復調法により解析し、フーリエ周波数２．５ｍＨｚ〜１８ＭＨｚの範囲で位相雑音パワースペクトル密度とＲＭＳジッターを計算した。周波数３９ＧＨｚ、および７８ＧＨｚのＯＴＤＭパルスに対するジッター値は、３．４１、３．７４ｐｓで、それぞれ繰り返し周期の１３．１％、２９．５％に相当する。実験においては、ビート信号のＳＮ比により、繰り返し周波数の上限は８０ＧＨｚ程度に制限された。しかしながら、この計測法は、変調周波数や変調度を増大することにより、より高い周波数のパルスへの適用が可能である。例えば、４０ＧＨｚ変調器により生成される４次測帯波を利用すれば、繰り返し周波数１６０ＧＨｚのＯＴＤＭパルスの計測が可能になる。
【００２２】
以上説明したように、本発明の実施例は、モード同期半導体レーザーから発生する繰り返し周波数１９．４４４ＧＨｚ、持続時間４．７ｐｓの光パルスを、４倍の光時分割多重により繰り返し７７．７７６ＧＨｚの光パルスに変換した。周波数１９．０００ＧＨｚのマイクロ波信号瀬（局部発振器）で駆動される導波路型強度変調器に光パルスを入射し、光パルスを構成する周波数間隔７７．７７６ＧＨｚの縦モード信号に対して、周波数間隔１９．０００ＧＨｚの変調側帯波を形成した。変調器から出力される光パルスを周波数帯域１２ＧＨｚの光検出器で受光すると、４次の変調側帯波と隣り合う縦モード信号との間のビート信号が周波数１．７７６ＧＨｚに生じ、中間周波信号に変換される。光パルスの繰り返し周波数をｆ_ｒｅｐ、局部発振器の周波数をｆ_ＬＯとすれば、中間周波信号の周波数はｆ_ＩＦ＝ｆ_ｒｅｐ−４＊ｆ_ＬＯで表される。周波数４０ＧＨｚで動作可能な導波路型変調器を用いれば、周波数１６０ＧＨｚの光パルスまで対応可能となる。
【００２３】
【発明の効果】
従来は、光パルスのタイミングジッターを計測するには、光パルス強度を電気信号に変換する必要があるため、繰り返し周波数の上限は光検出器や信号解析装置の帯域により制限されていた。これに対して、本発明では電気光学的な手法により、光パルス強度を低周波の中間周波信号に変換するため、光検出器や信号解析装置の帯域に制限されることなく、高い繰り返し周波数を持つ光パルスのジッター計測が可能になる。
【図面の簡単な説明】
【図１】本発明の光パルスのタイミングジッター計測方法およびそのための計測装置
【図２】光時分割多重による高繰り返し光パルスの生成説明図である。
【図３】ジッターの発生を説明する図である。
【図４】本発明の変調器から出力される光パルスのスペクトラム図である。
【図５】図１の装置に、光ファイバ増幅器３２，光時分割多重化装置３３、３４と、周波数同期システム（Ｒｂ原子発振器３９、周波数シンセサイザ４０、モード同期半導体レーザー３１からなる）を適用した装置の構成図である。
【図６】７７．７７６ＧＨｚのＯＴＤＭパルスに対するタイミングの３８０μｓに渡る時間変化を示す図である。
【図７】図６の時間変化に対応するヒストグラムである。タイミングの出現頻度を表している。
【図８】パワースペクトル密度およびタイミングジッターのフーリエ周波数変化を表す図である。
【図９】電気的なミキサーのブロック構成図である。
【符号の説明】
１１　　高繰り返しパルス光源
１２、３５　　光強度変調器
１３、３６　　光検出器
１４　　信号解析装置
１５　　マイクロ波信号源
２１　　パルス光源
２２、２３　　光ファイバ
２４、２５　　方向性結合器
３１　　モード同期半導体レーザー
３２　　光ファイバ増幅器
３３、３４　　光時分割多重化装置
３７　　ベクトル信号解析装置
３８　　位相同期発振器
３９　　Ｒｂ原子発振器
４０　　周波数シンセサイザ[0001]
TECHNICAL FIELD OF THE INVENTION
Stable light sources with small timing jitter are required in the technical fields that use highly repetitive ultrashort pulses, such as high bit rate optical fiber communication using optical time division multiplexing (OTDM) and optical sampling measurement. You. The present invention relates to a method for measuring timing jitter of an optical pulse used for measuring timing jitter of a highly repetitive optical pulse, and a measuring apparatus therefor.
[0002]
[Prior art]
Generation of a high repetition optical pulse signal is an important technical problem in applications such as optical fiber transmission of the optical time division multiplexing (OTDM) method and optical sampling measurement. In these applications, stringent conditions are imposed on the timing jitter as well as the pulse duration. For example, in OTDM transmission, the timing jitter needs to be suppressed to 10% or less of the bit rate, which corresponds to 630 fs jitter in 160 Gbit / s transmission. In ordinary timing jitter measurement, the intensity of a pulse is converted into an electric signal by a photodetector and analyzed, and the power spectrum density and jitter value of phase noise are calculated. For this reason, a wideband photodetector or analyzer is used for measuring an optical pulse having a repetition frequency of 10 GHz or more used for optical fiber communication or the like. The single band wave phase noise measurement method used there is a widely used method, but the frequency span and the dynamic range are limited, the separation of amplitude and phase noise is difficult, and the method such as passive mode-locked laser pulse is used. There are drawbacks such as errors that occur when applied to a non-stationary process.
[0003]
In order to solve these problems, the inventor has developed two time domain measurement methods called a time domain demodulation method and a time interval analysis method. The former is a measurement method in which an instantaneous phase is directly calculated from two orthogonal amplitude components, and the latter is a measurement method in which a time interval between pulses is counted using a counter having no idle time. These techniques are applied to a mode-locked semiconductor laser having a repetition frequency of 20 GHz, and the power spectral density and timing jitter are measured in a frequency range of 2.5 mHz to 18 MHz covering nine digits. Since the device used for these measurements cannot directly handle high-frequency signals having a frequency of 2.7 GHz or more, as shown in FIG. 9, a photodetector using a mixer (DBM) and a local oscillator (LO) is used. Is converted to an intermediate frequency (IF). FIG. 9 is a block diagram of the electric mixer. However, it has been difficult to apply this method to optical pulses having a repetition frequency of 50 GHz or more due to limitations on the band of the photodetector and the frequency of the microwave signal source.
[0004]
There is already an example in which the above-described single sideband phase noise measurement method is applied to an optical pulse exceeding 30 GHz using electro-optical mixing. In addition, a measurement method that converts a 36.6 GHz optical pulse intensity into a low-frequency IF signal by using a primary modulation sideband generated by a Mach-Zehnder optical intensity modulator and enables signal processing is also available. is there. However, for the same reason as described above, it has been difficult to apply this method to light pulses having a repetition frequency of 50 GHz or more.
[0005]
[Problems to be solved by the invention]
As described above, since the frequency band of the photodetector or the signal analyzer is limited to about 50 GHz, it has been difficult or impossible to measure the timing jitter for an optical pulse having a repetition frequency higher than that. That is, if the light intensity is converted into an electric signal, the signal can be converted into an intermediate frequency signal using a mixer, but it is difficult to obtain an electric signal corresponding to the light pulse intensity because the photodetector does not respond.
The present invention has been made in view of the above problems, and has as its object to provide a method for measuring timing jitter of an optical pulse that can be measured by a photodetector or a signal analyzer, and a measuring apparatus therefor.
[0006]
[Means for Solving the Problems]
The present invention employs the following solution in order to achieve the above object.
(1) In the optical pulse timing jitter measurement method, an optical pulse having a high repetition frequency is intensity-modulated with a signal having a frequency lower than the high repetition frequency to generate a longitudinal mode signal light having a higher-order modulation sideband light. Detecting the vertical mode signal light and the higher-order modulation sideband light, and obtaining a timing jitter from the detected output.
(2) In the optical pulse timing jitter measuring device, an optical pulse having a high repetition frequency is intensity-modulated with a signal having a frequency lower than the high repetition frequency to generate a longitudinal mode signal light having a high-order modulation sideband light. Detecting the vertical mode signal light and the higher-order modulation sideband light, and obtaining a timing jitter from the detected output.
(3) In the optical pulse timing jitter measuring apparatus, the high repetition pulse light is converted into a pulse light having a higher-order sideband by a local oscillator signal, and the difference between the higher-order sideband frequency and the longitudinal mode frequency is repeatedly repeated. Output as an intermediate-frequency light pulse reduced to each stage so as to be measurable compared to the sideband frequency, detect the intermediate-frequency light pulse and convert it into an electric signal, analyze the electric signal, and analyze the high-repetition pulse. It is characterized by seeking jitter.
(4) In a pulse timing jitter measuring device, a high repetition pulse light source that generates a longitudinal mode pulse light having a high repetition frequency, an optical intensity modulator, a photodetector, a signal analyzer, and a microwave signal of a local oscillator A high repetition pulse light of the high repetition pulse light source is converted into pulse light having a higher-order sideband by a local oscillator signal of the microwave signal source in an optical intensity modulator, and a higher-order sideband frequency is obtained. The difference between the frequency of the vertical mode and the longitudinal mode is output as a light pulse of an intermediate frequency reduced to each stage so as to be measurable compared to a high repetition sideband frequency, and the light pulse of the intermediate frequency is detected by a photodetector and And converting the electrical signal into a signal and analyzing the electrical signal with a signal analyzer to determine the jitter of the high repetition pulse.
[0007]
In the present invention, since a higher-order sideband is used, measurement can be performed on an optical pulse having a higher repetition frequency. At this time, for example, if a Mach-Zehnder light intensity modulator is used, the Mach-Zehnder light intensity modulator has a non-linear electro-optical conversion characteristic, so that the intensity of the microwave signal applied to the modulator is reduced. Giving a larger and deeper modulation results in a higher order band.
In the light intensity modulation and the light intensity modulator in the measurement method and the measurement device of the present invention, a high-intensity sideband is generated for a high-repetition light pulse in which the measurement frequency range of the target light cannot be directly detected. The modulation is applied to produce a modulation sideband. In the spectrum of the modulated optical pulse, a number of modulation sidebands are generated around a number of longitudinal modes arranged at an interval equal to the repetition frequency. The frequency difference between the higher-order sideband and the adjacent vertical mode can be made sufficiently small, can be detected by the photodetector, and the timing jitter can be measured.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following, following the description of the measurement principle and the device, the measurement results of the timing jitter for the OTDM pulses of 39 and 78 GHz are reported.
[0009]
(Example)
FIG. 2 is a diagram illustrating generation of a high repetition optical pulse by optical time division multiplexing.
The multiplexing apparatus shown in FIG. 2 includes two optical fibers 22 and 23 having different lengths corresponding to the amount of delay by 1/2 of the repetition frequency of the optical pulse (a) at both ends. It has a structure of being connected by the couplers 24 and 25.
The optical pulse (a) output from the pulse light source 21 is transmitted through the optical fiber line 23 by an optical time-division multiplexing device including an optical fiber 23, an optical fiber delay line 22, and directional couplers 24 and 25. The output optical pulse (a) and the optical pulse (a) having a phase delayed by a half of the repetition frequency of the optical pulse (a) via the optical fiber delay line 22 are coupled by the directional coupler 25, The repetition is converted into a double pulse train (b). If n stages of multiplexing devices are connected, the repetition is increased by 2n times.
FIG. 3 is a diagram for explaining the occurrence of jitter.
[0010]
As shown in FIG. 3, an optical pulse having such a high repetition frequency requires an ideal pulse without a timing jitter (solid line), but depending on the apparatus, a pulse with a jitter (dotted line) is shown in FIG. ), An out-of-phase output occurs.
In optical fiber communication, the appearance time of a pulse fluctuates due to jitter, affecting adjacent bits and causing a code error. Conventionally, the jitter cannot be measured because the repetition frequency interval of the light pulse exceeds the measurement range of the measuring instrument. The present invention relates to a measuring method for measuring the jitter irrespective of the interval of the repetition frequency and a measuring device therefor.
[0011]
FIG. 4 is a spectrum diagram of an optical pulse output from the modulator of the present invention.
As shown in FIG. 4, the basic solution of the present invention is to determine the jitter by measuring a frequency interval f _IF which is much smaller than a repetition frequency interval f _rep of an optical pulse and can be measured by a measuring device. Means. The frequency interval f _IF generates a higher-order modulation sideband by intensity-modulating the signal of the repetition frequency with a low-frequency signal, and calculates the difference between the frequency of the longitudinal mode signal and the sideband frequency more than the repetition frequency interval f _rep. It is characterized in that it is output to the measuring device after being made much smaller so that it can be measured by the measuring device.
[0012]
In FIG. 4, a thick vertical line represents a vertical mode of an optical pulse, and a thin vertical line represents a modulation sideband. The actual light pulse is composed of a number of longitudinal modes, but only two are displayed for convenience.
FIG. 1 is a block diagram of a timing jitter measuring apparatus for an optical pulse including electro-optical harmonic mixing according to the present invention, in which the features of the present invention are implemented. The timing jitter measuring apparatus shown in FIG. 1 includes a high repetition pulse light source 11 for generating an optical pulse having a repetition frequency f _rep , a microwave signal source 15 for generating a microwave signal having a frequency f _LO , and an optical pulse of the high repetition pulse light source 11. Is modulated by the microwave signal of the microwave signal source 15 to obtain a sideband optical pulse and a vertical band from the second harmonic to the n-th harmonic (n is arbitrarily selected, and practically, about n = 4). A light intensity modulator 12 for generating a mode light pulse, a photodetector 13 for detecting a repetition frequency f _rep of the longitudinal mode light pulse and an n-th microwave frequency n * f _LO from an output of the light intensity modulator 12, detecting a beat frequency of the difference between the repetition frequency f _rep and n following microwave frequency n * f _LO, it is composed of the signal analysis unit 14 for evaluating the jitter .
[0013]
The operation and measuring method of the measuring device in FIG. 1 are as follows.
The light pulse having the repetition frequency f _rep of the high repetition pulse light source 11 passes through a modulator driven by a sine wave signal having a frequency f _LO output from a microwave signal source 15 constituting a local oscillator. As a result, a modulated band wave is generated around each longitudinal mode signal of the optical pulse arranged at the interval of the frequency f _rep . Although the beat signal between the vertical mode signals cannot be directly detected due to the band limitation of the photodetector, the frequency difference between the n-th (n> 1) -order modulated band wave and the adjacent vertical mode signal is detected by the output of the photodetector. It is small enough to produce a low frequency beat signal. Therefore, using the high frequency signal of the frequency f _rep and the local signal of the frequency f _LO , the deviation between the frequency f _rep and the n-th microwave frequency n * f _LO is converted into the IF signal of the beat frequency. Become. This IF signal is converted to an intermediate frequency of a remarkably lower frequency that can be detected by the measuring device as compared with the repetition frequency of the pulse light source. The jitter of the repetitive pulse can be obtained by detecting the light pulse of the intermediate frequency, converting the light pulse into an electric signal, and analyzing the electric signal.
[0014]
This operation is similar to the harmonic mixer used in the microwave and millimeter wave regions. In order to perform measurement using high-order band waves, it is necessary to generate an IF signal having a high SN ratio, and deep modulation is required.
FIG. 5 shows an example in which an optical fiber amplifier 32, optical time-division multiplexers 33 and 34, and a frequency synchronization system (including an Rb atomic oscillator 39, a frequency synthesizer 40, and a mode-locked semiconductor laser 31) are applied to the apparatus of FIG. FIG. 2 is a configuration diagram of the device, for example, using a monolithic mode-locked semiconductor laser 31 having a wavelength of 1542 nm as a light source. The laser 31 includes a gain region, a saturable absorption region, and a passive waveguide, and passive mode locking is realized by applying a reverse bias voltage to the saturable absorption region. When a 19.444 GHz sine wave signal output from the frequency synthesizer 40 is applied in addition to the reverse bias, hybrid mode locking occurs, and the optical pulse output from the laser 31 synchronizes with the output of the synthesizer 40.
[0015]
After the optical pulse output from the monolithic mode-locked semiconductor laser 31 operating synchronously is amplified by the optical fiber amplifier 32, the optical pulse is amplified by the optical fiber type multiplexer of the optical time division multiplexers 33 and 34. Multiplexed to 888 or 77.776 GHz pulses. The multiplexer includes a directional coupler, a variable delay line, and a variable attenuator. The multiplexed pulse enters a light intensity modulator 35 driven by a phase-locked oscillator 38 having a frequency of 19.000 GHz. The single sideband phase noise of the phase locked oscillator 38 is -84, -104, -118, -119, and -128 dBc / Hz at frequency offsets of 0.1, 1, 10, 100, and 1000 kHz, respectively. The DC bias of the modulator 35 is adjusted so that the beat signal intensity becomes maximum. Output light from the modulator 35 is received by an InGaAs photodiode serving as a photodetector 36, and the output is subjected to demodulation and spectrum analysis by a vector signal analyzer 37. The photodiode has a frequency band of 12 GHz and does not directly respond to a light pulse of 19.444 GHz. A 10 MHz common reference signal is supplied from the Rb atomic oscillator 39 to all signal sources and measuring instruments. To separate the ground of the laser 31 from other measuring instruments, a reference signal to the frequency synthesizer 40 is supplied via an optical fiber isolator (not shown).
[0016]
For the OTDM pulses having the frequencies of 38.444 and 77.776 GHz, IF signals having the frequencies of 0.888 and 1.776 GHz were generated using the second- and fourth-order modulated band waves, respectively. The intensity I _{n of the} beat signal generated by the n-th band wave is expressed as follows when n is an even number:
(Equation 1)

Is represented by Here, I ₀ is a constant depending on the intensity of each longitudinal mode, β is a modulation index, and J _n is a Bessel function of the first type. When the optimum value of the modulation index that maximizes the beat signal strength is estimated from Expression 1, the modulation index is 2.20, 5.87 radians for the second and fourth sidebands.
The RF signal power applied to the modulator in the experiment performed for confirmation was approximately 18 dBm, which was limited by the performance of the amplifier. This gives a modulation index of 2.0 radians, which is close to the optimal value for the secondary sideband, but considerably smaller for the quaternary sideband. However, a beat signal having a frequency of 1.776 GHz generated from the fourth sideband had a sufficient SN ratio for timing jitter measurement.
[0017]
One advantage of the time domain demodulation method is that not only the frequency domain but also the time domain data can be obtained. FIG. 6 shows the time change over 380 μs of the timing for the OTDM pulse of 77.776 GHz. The sampling interval is 195 ns and the number of data points is 1996. The RMS jitter during this time is estimated to be 0.79 ps, which corresponds to 6% of the pulse period. FIG. 7 is a histogram corresponding to FIG. 6 and shows the frequency of occurrence of timing. The width of each column is 160 fs. The histogram in FIG. 7 is close to a normal distribution, and indicates that the fluctuation in the observation time band is white noise.
The phase noise power spectral density _Sφ (f) was calculated from the measured data in the time domain as shown in FIG. 6 using the fast Fourier transform over a nine-digit Fourier frequency. In FIG. 8, a curve A is a synthesizer, a curve B is a 19.444 GHz optical pulse which is a laser output, and curves D and C are power spectrum densities with respect to OTDM pulses of frequencies 38.888 and 77.776 GHz. Since measurement over such a wide frequency range cannot be performed at one time, the measurement was performed by changing the frequency span, and the data were combined.
[0018]
FIG. 8 shows the RMS jitter σ _J calculated from the phase noise power spectral density S _φ (f) using the following equation. 8, a curve E is a synthesizer, a curve F is a 19.444 GHz optical pulse which is a laser output, and curves G and H are RMS jitters with respect to an OTDM pulse having a frequency of 38.888 GHz and 77.776 GHz.
[Equation 2]

Here, f ₁ and f _h represent the upper and lower limits of the integration, and the value of σ _J is calculated for a fixed f _h = 18 MHz and plotted as a function of f ₁ . Curves A and E correspond to a synthesizer with a frequency of 19.444 GHz, and were measured using a 19.000 GHz phase-locked oscillator and a mixer. The RMS jitter is 3.15 ps in a band of 2.5 mHz to 18 MHz, and noise mainly at a Fourier frequency of 100 kHz or less contributes.
[0019]
Curves B and F correspond to a pulse of 19.444 GHz, which is the laser output, and were measured using the apparatus of FIG. When the curves A and B are compared, in the frequency range of 100 kHz or less, the two almost overlap, indicating that the laser follows the RF signal well. However, there is a large difference between the two in the frequency range of 100 kHz or more. When the applied RF power is increased, the noise at 100 kHz or more is reduced, but it tends to be saturated at a power of 20 dBm (100 mW, 0 dBm = 1 mW) or more. The RMS jitter is estimated to be 3.42 ps from the value at the right end of the curve F, but it is 1.33 ps excluding the contribution of the synthesizer, which is caused by noise of 100 kHz or more.
[0020]
Curves C and D in FIG. 8 are power spectral densities for OTDM pulses of frequencies 38.888 and 77.776 GHz. The value of the curve C, S on D φ _(f) is turned, respectively 4 and 16 times the curve B, and corresponds to the increase of the repetition frequency. The fact that the shapes of the curves are substantially equal indicates the correctness of the measurement. These results show that the noise added during the pulse multiplexing process is extremely small, and the noise of the original laser pulse is dominant. Curves G and H are RMS jitters calculated from C and D. The value of the power spectral density is increased by the multiplexing, but the jitter value is almost constant. This is because the integrated power spectrum density is divided by the repetition frequency, as can be seen from Equation 2. The reason why the jitter value with respect to the 77.776 GHz OTDM pulse is slightly increased is that the SN ratio of the beat signal is slightly lowered. The jitter values for OTDM pulses with frequencies 38.888 and 77.776 GHz are estimated to be 3.41 and 3.74 ps, which correspond to 13.1% and 29.5% of the repetition period, respectively. In order to reduce the timing jitter of the OTDM pulse, it is necessary to reduce a noise component having a Fourier frequency of 100 kHz or more in the laser output pulse.
[0021]
In the above embodiment, the timing jitter of the OTDM pulse having a repetition frequency of 39 and 78 GHz was measured using electro-optic harmonic mixing by a Mach-Zehnder light intensity modulator. The frequency-converted signal was analyzed by a time-domain demodulation method, and the phase noise power spectral density and RMS jitter were calculated in the Fourier frequency range of 2.5 MHz to 18 MHz. The jitter values for the 39 GHz and 78 GHz OTDM pulses are 3.41 and 3.74 ps, which correspond to 13.1% and 29.5% of the repetition period, respectively. In the experiment, the upper limit of the repetition frequency was limited to about 80 GHz due to the SN ratio of the beat signal. However, this measurement method can be applied to higher frequency pulses by increasing the modulation frequency and modulation degree. For example, if a fourth-order band wave generated by a 40 GHz modulator is used, an OTDM pulse having a repetition frequency of 160 GHz can be measured.
[0022]
As described above, in the embodiment of the present invention, an optical pulse having a repetition frequency of 19.444 GHz and a duration of 4.7 ps generated from a mode-locked semiconductor laser is repeated by four times optical time division multiplexing to generate a 77.776 GHz light. Converted to pulses. An optical pulse is incident on a waveguide-type intensity modulator driven by a microwave signal (local oscillator) having a frequency of 19.000 GHz, and a frequency interval of a longitudinal mode signal having a frequency interval of 77.776 GHz constituting the optical pulse is determined. A modulation sideband of 19.000 GHz was formed. When an optical pulse output from the modulator is received by a photodetector having a frequency band of 12 GHz, a beat signal between a fourth-order modulation sideband and an adjacent longitudinal mode signal is generated at a frequency of 1.776 GHz, and is output as an intermediate frequency signal. Is converted. Assuming that the repetition frequency of the optical pulse is f _rep and the frequency of the local oscillator is f _LO , the frequency of the intermediate frequency signal is represented by f _IF = f _rep −4 * f _LO . If a waveguide modulator operable at a frequency of 40 GHz is used, it is possible to cope with an optical pulse of a frequency of 160 GHz.
[0023]
【The invention's effect】
Conventionally, in order to measure the timing jitter of an optical pulse, it is necessary to convert the intensity of the optical pulse into an electric signal, so that the upper limit of the repetition frequency is limited by the bandwidth of a photodetector or a signal analyzer. On the other hand, in the present invention, since the light pulse intensity is converted into a low-frequency intermediate frequency signal by an electro-optical method, a high repetition frequency can be obtained without being limited to the band of the photodetector or the signal analyzer. It becomes possible to measure the jitter of the optical pulse.
[Brief description of the drawings]
FIG. 1 is a timing jitter measuring method of an optical pulse according to the present invention and a measuring apparatus therefor. FIG. 2 is an explanatory diagram of generation of a high repetition optical pulse by optical time division multiplexing.
FIG. 3 is a diagram illustrating generation of jitter.
FIG. 4 is a spectrum diagram of an optical pulse output from the modulator of the present invention.
FIG. 5 shows an optical fiber amplifier 32, optical time division multiplexers 33 and 34, and a frequency locked system (consisting of an Rb atomic oscillator 39, a frequency synthesizer 40, and a mode locked semiconductor laser 31) applied to the device of FIG. It is a block diagram of an apparatus.
FIG. 6 is a diagram showing a temporal change over 380 μs of a timing for an OTDM pulse of 77.776 GHz.
FIG. 7 is a histogram corresponding to the time change of FIG. 6; It represents the frequency of occurrence of timing.
FIG. 8 is a diagram illustrating Fourier frequency changes of power spectral density and timing jitter.
FIG. 9 is a block diagram of an electric mixer.
[Explanation of symbols]
Reference Signs List 11 High repetition pulse light source 12, 35 Light intensity modulator 13, 36 Photodetector 14 Signal analyzer 15 Microwave signal source 21 Pulse light source 22, 23 Optical fiber 24, 25 Directional coupler 31 Mode-locked semiconductor laser 32 Optical fiber Amplifiers 33 and 34 Optical time division multiplexer 37 Vector signal analyzer 38 Phase locked oscillator 39 Rb atomic oscillator 40 Frequency synthesizer

Claims (4)

The optical pulse having a high repetition frequency is intensity-modulated with a signal having a frequency lower than the high repetition frequency to generate a longitudinal mode signal light having a higher-order modulation sideband light, and the longitudinal mode signal light and the higher-order modulation are generated. A method for measuring timing jitter of an optical pulse, comprising detecting sideband light and obtaining timing jitter from the detected output. The optical pulse having a high repetition frequency is intensity-modulated with a signal having a frequency lower than the high repetition frequency to generate a longitudinal mode signal light having a higher-order modulation sideband light, and the longitudinal mode signal light and the higher-order modulation are generated. An apparatus for measuring timing jitter of an optical pulse, comprising detecting sideband light and obtaining timing jitter from the detected output. The high repetition pulse light is converted into a pulse light having a higher-order sideband by a local oscillator signal, and the difference between the higher-order sideband frequency and the longitudinal mode frequency is made much smaller than the high repetition frequency so that it can be measured. Outputting as an optical pulse of an intermediate frequency, detecting the optical pulse of the intermediate frequency, converting the optical pulse into an electric signal, and analyzing the electric signal to determine a jitter of the high repetition pulse; Measuring device. A high repetition pulse light source that generates a longitudinal mode pulse light with a high repetition frequency, a light intensity modulator, a photodetector, a signal analyzer, and a microwave signal source of a local oscillator, and the light intensity modulator The high repetition pulse light of the high repetition pulse light source is converted into pulse light having a higher-order sideband by the intermediate frequency signal of the microwave signal source, and the difference between the higher-order sideband frequency and the frequency of the longitudinal mode signal is repeatedly repeated. Output as an intermediate frequency optical pulse reduced to each stage to a degree that can be measured compared to the higher-order sideband frequency, the intermediate frequency optical pulse is detected by a photodetector and converted into an electric signal, and the electric signal is output. A jitter analyzer for the high repetition pulse, and analyzing the jitter of the high repetition pulse by using a signal analyzer.

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