JP2011182198A - Optical phase noise suppression circuit, phase fluctuation detection circuit, and phase fluctuation detection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical phase noise suppression circuit for reducing optical phase fluctuation, and to suppress signal transmission characteristic degradation of an optical DPSK transmission system. <P>SOLUTION: In this optical phase noise suppression circuit, a transmitted DPSK optical signal is branched; monitor signals differentially received by two delay interferometers having phase differences of π/2 and 0 are generated; the two differentially-received monitor signals are mixed by a mixing means; a branched main signal of the DPSK optical signal is subjected to phase modulation by a phase modulation means by using the mixed signal. Then, the phase fluctuation of the main signal is canceled by subjecting a coefficient of -1 to phase modulation because the two mixed monitor signals are proportional to the phase fluctuation of the main signal. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical phase noise suppression circuit, a phase fluctuation detection circuit, and a phase fluctuation detection method, and more particularly to optical phase noise suppression for detecting and suppressing phase fluctuations that occur during fiber transmission in an optical fiber transmission system. The present invention relates to a circuit, a phase fluctuation detection circuit, and a phase fluctuation detection method.

  Optical fiber communication is widely used as a technology for realizing long-distance and large-capacity communication. There are various types of optical communication, but differential phase shift keying (DPSK) modulation / demodulation methods have higher reception sensitivity than conventional intensity modulation / direct detection methods, and are more resistant to optical nonlinear effects in fiber transmission lines. There are advantages such as, and research and development is actively promoted.

  In the optical DPSK system, bit information is transmitted using a phase difference of light between adjacent time slots. That is, from the transmitter, phase modulated signal light is transmitted with the optical phase difference of the adjacent slot being 0 when the bit is “0” and the optical phase difference of the adjacent slot being π when the bit is “1”.

  The receiver then demodulates the bit value by measuring the phase difference between the adjacent slots by an interferometer configured to cause the light in the adjacent slot to interfere (referred to as a 1-bit delay interferometer).

  As a conventional technique, a method using gain saturation of optical parametric amplification has been studied as a means for suppressing transmission characteristic degradation caused by fluctuations in the light intensity of a single-channel phase-modulated signal light itself (for example, non-patented). Reference 1).

  Optical parametric amplification is a phenomenon in which signal light that satisfies a specific wavelength relationship is amplified when high-power pump light is incident on an optical fiber with high optical nonlinearity (referred to as a highly nonlinear fiber). When the input optical power is increased, the output optical power increases linearly as long as the input optical power is low, but the slope gradually decreases, and when it reaches a peak at a certain input level, it starts to decrease thereafter. .

  By utilizing this input / output characteristic, it is possible to suppress fluctuations in the light intensity of the signal light. That is, the input signal light power is set to a level at which the output reaches a peak. Then, since the slope of the output light power is zero near the peak, even if the input light intensity fluctuates, the light power is output at a constant level. Thereby, it is possible to obtain signal light in which fluctuation of light intensity is suppressed.

  If such a light intensity fluctuation suppressing device is installed at a required place of the transmission line, the signal light in which the light intensity fluctuation is suppressed propagates through the optical fiber. Then, conversion from intensity fluctuations to phase fluctuations due to the optical Kerr effect does not occur, and the above-described deterioration in transmission characteristics can be suppressed.

Masayuki Matsumoto and Kenichi Sanuki, "Performance improvement of DPSK signal transmission by a phase-preserving amplitude limiter," Optic Express, vol. 15, no. 13, pp. 8094-8103 (2007).

  The light intensity fluctuation suppression method by the optical parametric amplification can suppress the light intensity fluctuation, but cannot suppress the phase fluctuation caused by the mutual phase modulation generated between the signal lights of a plurality of channels.

  The present invention has been made in view of the above points, and an object thereof is to provide an optical phase noise suppression circuit, a phase fluctuation detection circuit, and a phase fluctuation detection method that compensate for phase fluctuations generated during fiber transmission.

  FIG. 1 is a principle configuration diagram of the present invention.

The present invention (Claim 1) is an optical phase noise suppression circuit used in an optical fiber transmission system for transmitting an optical differential phase shift keying (DPSK) signal to a receiving device via an optical fiber transmission line,
First optical branching means 11 for branching a part of the transmitted DPSK signal light;
A second optical branching unit 21 that further splits the signal light branched by the first optical branching unit 11 into two parts;
A third optical branching unit 221 that splits one of the signal lights branched by the second optical branching unit 21 into the first and second optical paths;
Time delay means 22 for providing a time difference equal to the bit slot time of the DPSK signal between the first optical path and the second optical path;
First phase setting means 222 for setting the propagation phase difference between the first optical path and the second optical path to π / 2,
A first optical coupler 223 having 2 × 2 input / output terminals and having first and second optical paths connected to each input terminal;
First and second light detecting means 41.42 connected to output terminals of the first optical coupler 223, respectively;
First signal differential synthesizing means 43 for differentially synthesizing outputs from the first and second light detecting means 41 and 42;
A fourth optical branching means 321 for branching the other signal light branched by the second optical branching means 21 into the third and fourth optical paths;
Time delay means 32 for providing a time difference equal to the bit slot time of the DPSK signal between the third optical path and the fourth optical path;
Second phase setting means 322 that sets the propagation phase difference between the third optical path and the fourth optical path to 0;
A second optical coupler 323 having 2 × 2 input / output terminals and having third and fourth optical paths connected to each input terminal;
Third and fourth light detection means 51 and 52 connected to the output terminals of the second optical coupler 323, respectively;
Second signal differential synthesizing means 53 for differentially synthesizing outputs from the third and fourth light detecting means 51 and 52;
Signal mixing means 25 for outputting a multiplication signal of outputs from the first and second signal differential combining means 43 and 53;
An optical phase modulation means 13 for modulating the phase of the signal light different from the signal light input to the second optical branch means 21 among the outputs from the first optical branch means 11;
A driving means for driving the optical phase modulation means 13 based on an output signal from the signal mixing means 25;
Timing adjusting means 12 for adjusting the timing of the signal light input to the optical phase modulator 13 and the drive signal.

The present invention (Claim 2) is a phase fluctuation detection circuit used in an optical fiber transmission system for transmitting an optical differential phase shift keying (DPSK) signal to a receiving device through an optical fiber transmission line,
First optical branching means for branching the input signal light into two;
A second optical branching unit that splits one of the signal beams branched by the first optical branching unit into two first and second optical paths;
Time delay means for providing a time difference between the first optical path and the second optical path equal to the bit slot time of the DPSK signal;
Phase setting means for setting a propagation phase difference between the first optical path and the second optical path to π / 2;
A first optical coupler having 2 × 2 input / output terminals and having first and second optical paths connected to each input terminal;
First and second light detection means respectively connected to the output terminals of the first optical coupler;
First signal differential combining means for differentially combining outputs from the first and second light detecting means;
A fourth optical branching unit that splits the other signal light branched by the second optical branching unit into the third and fourth optical paths;
Time delay means 32 for providing a time difference equal to the bit slot time of the DPSK signal between the third optical path and the fourth optical path;
Phase setting means for setting the propagation phase difference between the third optical path and the fourth optical path to 0;
A second optical coupler having a 2 × 2 input / output terminal and having a third and a fourth optical path connected to each input terminal;
Third and fourth photodetecting means respectively connected to the output terminals of the second optical coupler;
Second signal differential combining means for differentially combining outputs from the third and fourth light detecting means;
Signal mixing means for outputting a multiplication signal of outputs from the first and second signal differential combining means.

The present invention (Claim 3) is a phase fluctuation detection method used in an optical fiber transmission system for transmitting an optical differential phase shift keying (DPSK) signal to a receiving device via an optical fiber transmission line,
The first to fourth optical branching means, the second optical branching means, the time delaying means, the first and second phase setting means, and 2 × 2 input / output terminals are provided. A first optical coupler to which two optical paths are connected; first and second photodetecting means connected to output terminals of the first optical coupler; first and second signal differential combining means; A second optical coupler having 2 × 2 input / output terminals, with the third and fourth optical paths connected to each input terminal, and a third and a second optical coupler connected to the output terminals of the second optical coupler, respectively; In the circuit having the fourth light detection means and the signal mixing means,
Branching the signal light input in the first optical branching means;
In the second optical branching means, one signal light branched by the first optical branching means is branched into two into the first and second optical paths,
In the first phase setting means, the propagation phase difference between the first optical path and the second optical path is π / 2, and in the time delay means, the first optical path is between the first optical path and the second optical path. Gives a time difference equal to the bit slot time of the DPSK signal,
In the first optical coupler, the signal light of the first optical path and the second optical path are combined and output to the first and second light detection means,
In the first and second light detection means, the combined signal light is detected and output to the first signal differential synthesis circuit,
In the first signal differential synthesis circuit, the outputs from the first and second light detection means are differentially synthesized and output to the signal mixing means,
In the fourth optical branching means, the other signal light branched by the second optical branching means is branched into two into the third and fourth optical paths,
In the phase setting means on the third optical path, the propagation phase difference between the third optical path and the fourth optical path is set to 0,
In the time delay means, a time difference equal to the bit slot time of the DPSK signal is given between the third optical path and the fourth optical system,
The signal light of the third optical path and the fourth optical path is input to the second optical coupler,
The second optical coupler combines the signal light of the third optical path and the fourth optical path, and outputs the combined signal light to the third and fourth light detection means,
In the third and fourth light detection means, the input signal light is detected and output to the second signal differential combining means,
In the second signal differential combining means, the outputs from the third and fourth light detecting means are differentially combined and output to the signal mixing means,
The signal mixing means multiplies the output signals from the first and second signal differential combining means and outputs a multiplication signal.

  As described above, according to the present invention, a DPSK optical signal is branched, and a differential signal is received by two delay interferometers having a phase difference of π / 2 and 0 to produce a monitor signal. Since the two monitor signals that are mixed are proportional to the phase fluctuation of the main signal by phase-modulating the main signal with the mixed signal, phase modulation is performed by multiplying the coefficient by −1. The phase fluctuation of the main signal can be canceled.

It is a principle block diagram of this invention. It is a block diagram of the optical phase noise suppression circuit and phase fluctuation detection circuit in one embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 2 shows a configuration of an optical phase noise suppression circuit and a phase fluctuation detection circuit according to an embodiment of the present invention.

  The configuration shown in FIG. 1 includes a phase fluctuation detection circuit 1 (monitor signal) including an optical coupler 21, delay interferometers 22, 32, photodetectors 41, 42, 51, 52, differential synthesizers 43, 53, and a mixing circuit 25. And an optical phase noise suppression circuit in which an optical coupler 11, a delay adjustment circuit 12, and a phase modulator 13 are added thereto.

  It is assumed that DPSK signal light with phase fluctuation is input to the optical coupler 11 from the left in FIG. This signal photoelectric field is

と表される。Ａは振幅、θsは所望の信号位相、δθは所定値からの位相変位である。ここでは、入力信号光の振幅Ａは、分散補償などの手段により一定になっているものとする。

It is expressed. A is the amplitude, θs is the desired signal phase, and δθ is the phase displacement from the predetermined value. Here, the amplitude A of the input signal light is assumed to be constant by means such as dispersion compensation.

  A part of the input signal light is branched by the optical coupler 11, and one part is input to the optical phase modulator 13 through the time delay adjustment circuit 12 as the main signal light. The other is input as monitor signal light to an optical coupler 21 of a phase fluctuation detection circuit 1 (monitor signal detection circuit) described below.

  The monitor signal light branched from the input signal light by the optical coupler 11 is further branched into two by the optical coupler 21, and one is input to the delay interferometer A22 and the other is input to the delay interferometer B32.

  In the delay interferometer A22, the input light is branched into two by the optical coupler 221, and after being given a time delay by the π / 2 delay circuit 222, it is multiplexed again by the optical coupler 223 having 2 × 2 input / output terminals. Is done. Here, in the phase difference setting circuit 222, a time delay equal to the bit slot interval of the transmission signal is given. The propagation phase difference between the two paths is set to π / 2. As a result, in the multiplexing coupler 223, two light waves in adjacent bit slots interfere with each other with the addition of the phase difference π / 2. An output photoelectric field from the multiplexing coupler 223 is expressed as follows.

Ｅ₁₁、Ｅ₁₂はそれぞれ後段の光検出器４１，４２へと出力される光電場である。各表式において、第１項は短経路を経た出力光成分、第２項は長経路を経た出力光成分を表している。また、τは長経路で与えられる遅延時間、Ａ_mは遅延干渉計Ａ２２への入力光振幅である。

E ₁₁ and E ₁₂ are photoelectric fields output to the photodetectors 41 and 42 in the subsequent stage, respectively. In each expression, the first term represents the output light component through the short path, and the second term represents the output light component through the long path. Further, tau is the delay time given by the long path, A _m is the input light amplitude of the delay interferometer A22.

  The output light from the delay interferometer A22 is input to the photodetectors 41 and 42, and the light intensity is converted into an electrical signal and output. The output signals from the photodetectors 41 and 42 are expressed as follows.

Ｉ₁₁、Ｉ₁₂はそれぞれ光検出器４１，４２から出力される電気信号、ηは光強度から電気信号への変換係数である。

I ₁₁ and I ₁₂ are electric signals output from the photodetectors 41 and 42, respectively, and η is a conversion coefficient from light intensity to an electric signal.

  Output signals from the photodetectors 41 and 42 are differentially combined by a differential synthesizer 43. As a result, a signal represented by the following equation is output and output to the mixing circuit 25.

ところで光DPSK伝送では、

By the way, in optical DPSK transmission,

である。これを上式（４）に適用すると、

It is. Applying this to equation (4) above,

となる。さらに、位相変位の時間変化は十分小さいとすると、上式は次のように近似される。

It becomes. Further, assuming that the time change of the phase displacement is sufficiently small, the above equation is approximated as follows.

さて、光カップラ２１で分岐されたモニタ信号光の他方は、遅延干渉計３２に入力される。遅延干渉計Ｂ３２の構成は基本的には上記遅延干渉計Ａ２２と同様であるが、位相設定回路３２２で２経路の伝播遅延位相差が０に設定されている点だけが異なっている。信号の流れを上記と同様に辿っていくと、以下のように表される。

The other of the monitor signal light branched by the optical coupler 21 is input to the delay interferometer 32. The configuration of the delay interferometer B32 is basically the same as that of the delay interferometer A22, except that the propagation delay phase difference between the two paths is set to 0 by the phase setting circuit 322. If the signal flow is traced in the same manner as described above, it is expressed as follows.

  The output photoelectric field from the delay interferometer B32 is

各光検出器５１，５２からの電気信号出力は、

The electrical signal output from each photodetector 51, 52 is

差動合成器５３の出力は、

The output of the differential synthesizer 53 is

を適用すると、

Apply

位相変位の時間変化量は十分小さいとすると、

If the amount of phase change over time is sufficiently small,

次に、遅延干渉計Ａ２２、光検出器４１，４２を経て出力された差動合成器４３の出力Ｓ_m1と遅延干渉計Ｂ３２、光検出気５１，５２を経て出力された差動合成器５３の出力Ｓ_m2はミキシング回路２５に入力される。ミキシング回路２５からは、両者Ｓ_m1、Ｓ_m2が掛け算された信号Ｓ_ｍが出力される。すなわち、次式で表されるモニタ信号Ｓ_ｍが出力される。

Next, the output S _m1 of the differential synthesizer 43 output through the delay interferometer A22 and the photodetectors 41 and 42, and the differential synthesizer 53 output through the delay interferometer B32 and the photodetectors 51 and 52. The output S _m2 is input to the mixing circuit 25. From the mixing circuit 25, the signal _{S m} where both S _m1, S _{m @ 2} is multiplied is outputted. That is, the monitor signal S _m which is expressed by the following equation is output.

これに式（６）（１１）を代入すると、次式となる。

Substituting equations (6) and (11) into this gives the following equation.

上式は、θ_s(t)−θ_s（t−τ）＝０またはπのいずれであっても、位相変位の時間変化量に比例したモニタ信号が出力されることを示している。

The above equation indicates that a monitor signal proportional to the amount of time change in phase displacement is output regardless of whether θ _s (t) −θ _s (t−τ) = 0 or π.

  The monitor signal represented by Expression (13) is applied to the phase modulator 13 to which the main signal light branched by the optical coupler 11 is input. Here, the time when the monitor signal is applied by the time delay adjustment circuit 12 in the previous stage of the phase modulator 13 coincides with the time when the time slot of the main signal light corresponding to the time change of the phase displacement of the monitor signal is input. It shall be. Then, the signal light represented by the following expression is output from the phase modulator 13.

ｋは、印加電気信号から位相変調へ変換係数を含めた比例定数である。

k is a proportionality constant including a conversion coefficient from an applied electric signal to phase modulation.

  Here, various circuit parameters are set so that k = −1. Then, equation (14) becomes

となる。上式は、時刻ｔでの位相変位＝時刻（ｔ−τ）での位相変位、となることを示している。この関係はどの時刻についても成り立つので、位相変位δθは常に一定に保たれるということである。

It becomes. The above equation shows that phase displacement at time t = phase displacement at time (t−τ). Since this relationship holds at any time, the phase displacement δθ is always kept constant.

  If the phase displacement is constant, it can be said that the signal light has no phase fluctuation. That is, with the above configuration, signal light with reduced phase fluctuation can be obtained.

  The present invention is not limited to the above-described embodiment, and various modifications and applications can be made within the scope of the claims.

  The present invention is applicable to an optical fiber transmission system that transmits a DPSK signal to a receiving device via an optical fiber transmission line.

DESCRIPTION OF SYMBOLS 1 Phase fluctuation detection circuit 11 1st optical branch means, optical coupler 12 Timing adjustment means, Time delay adjustment circuit 13 Optical phase modulation means, Phase modulator 21 Second optical branch means, Optical coupler 22 Delay interferometer A
25 Signal mixing means, mixing circuit 32 Delay interferometer B
41 1st light detection means, light detector 42 2nd light detection means, light detector 43 1st signal differential combination means, differential combiner 51 3rd light detection means, light detector 52 4th Photodetection means, photodetector 53 second signal differential combining means, differential combiner 221 third optical branching means, optical coupler 222 first phase setting means, π / 2 phase setting circuit 223 first Optical coupler, 2 × 2 combined optical coupler 321 Fourth optical branching means, optical coupler 322 Second phase setting means, 0 phase setting circuit 323 Second optical coupler, 2 × 2 optical coupler

Claims (3)

An optical phase noise suppression circuit used in an optical fiber transmission system for transmitting an optical differential phase shift keying (DPSK) signal to a receiving device via an optical fiber transmission line,
First optical branching means for branching a part of the transmitted DPSK signal light;
Second optical branching means for further branching the signal light branched by the first optical branching means into two;
Third optical branching means for branching one of the signal lights branched by the second optical branching means into first and second optical paths;
Time delay means for providing a time difference between the first optical path and the second optical path equal to the bit slot time of the DPSK signal;
Phase setting means for setting a propagation phase difference between the first optical path and the second optical path to π / 2;
A first optical coupler having a 2 × 2 input / output terminal and having the first and second optical paths connected to each input terminal;
First and second light detection means respectively connected to output terminals of the first optical coupler;
First signal differential synthesizing means for differentially synthesizing outputs from the first and second light detecting means;
A fourth optical branching unit that splits the other signal light branched by the second optical branching unit into a third and a fourth optical path;
Time delay means for providing a bit slot time difference of the DPSK signal between the third optical path and the fourth optical path;
Phase setting means for setting a propagation phase difference between the third optical path and the fourth optical path to 0;
A second optical coupler having a 2 × 2 input / output terminal, wherein the third and fourth optical paths are connected to each input terminal;
Third and fourth light detection means respectively connected to the output terminals of the second optical coupler;
Second signal differential combining means for differentially combining outputs from the third and fourth light detecting means;
Signal mixing means for outputting multiplication signals of outputs from the first and second signal differential combining means;
Optical phase modulation means for modulating the phase of signal light different from the signal light input to the second optical branching means among the outputs from the first optical branching means;
Driving means for driving the optical phase modulation means based on an output signal from the signal mixing means;
Means for adjusting the timing of the signal light and the drive signal input to the optical phase modulator;
An optical phase noise suppression circuit comprising: A phase fluctuation detection circuit used in an optical fiber transmission system for transmitting an optical differential phase shift keying (DPSK) signal to a receiving device via an optical fiber transmission line,
First optical branching means for branching the input signal light into two;
Second optical branching means for branching one of the signal lights branched by the first optical branching means into first and second optical paths;
Time delay means for providing a time difference between the first optical path and the second optical path equal to the bit slot time of the DPSK signal;
Phase setting means for setting a propagation phase difference between the first optical path and the second optical path to π / 2;
A first optical coupler having a 2 × 2 input / output terminal and having the first and second optical paths connected to each input terminal;
First and second light detection means respectively connected to output terminals of the first optical coupler;
First signal differential synthesizing means for differentially synthesizing outputs from the first and second light detecting means;
A fourth optical branching unit that splits the other signal light branched by the second optical branching unit into a third and a fourth optical path;
Time delay means for providing a time difference equal to the bit slot time of the DPSK signal between the third optical path and the fourth optical path;
Phase setting means for setting a propagation phase difference between the third optical path and the fourth optical path to 0;
A second optical coupler having a 2 × 2 input / output terminal, wherein the third and fourth optical paths are connected to each input terminal;
Third and fourth light detection means respectively connected to the output terminals of the second optical coupler;
Second signal differential combining means for differentially combining outputs from the third and fourth light detecting means;
Signal mixing means for outputting multiplication signals of outputs from the first and second signal differential combining means;
A phase fluctuation detection circuit comprising: A phase fluctuation detection method used in an optical fiber transmission system for transmitting an optical differential phase shift keying (DPSK) signal to a receiving device via an optical fiber transmission line,
First to fourth optical branching means, second optical branching means, time delay means, first and second phase setting means, 2 × 2 input / output terminals, and the first and A first optical coupler to which a second optical path is connected; first and second photodetecting means connected to an output terminal of the first optical coupler; and first and second signal differential combining means. A second optical coupler having 2 × 2 input / output terminals and having third and fourth optical paths connected to each input terminal; and a third optical coupler connected to the output terminal of the second optical coupler. And a circuit having a fourth light detection means and a signal mixing means,
Branching the signal light input in the first optical branching means;
In the second light branching means, one of the signal lights branched by the first light branching means is branched into two first and second optical paths,
In the first phase setting means, a propagation phase difference between the first optical path and the second optical path is π / 2, and in the time delay means, the first optical path and the second light are set. Giving a time difference to the path equal to the bit slot time of the DPSK signal,
In the first optical coupler, the signal light of the first optical path and the second optical path are combined and output to the first and second light detection means,
In the first and second light detection means, the combined signal light is detected and output to the first signal differential synthesis circuit,
In the first signal differential synthesis circuit, the outputs from the first and second light detection means are differentially synthesized and output to the signal mixing means,
In the fourth light branching means, the other signal light branched by the second light branching means is branched into two into the third and fourth optical paths,
In the phase setting means on the third optical path, the propagation phase difference between the third optical path and the fourth optical path is set to 0,
In the time delay means, a time difference equal to the bit slot time of the DPSK signal is given between the third optical path and the fourth optical path,
The signal light of the third optical path and the fourth optical path is input to the second optical coupler,
The second optical coupler combines the signal light of the third optical path and the fourth optical path, and outputs the combined signal light to the third and fourth light detection means;
In the third and fourth light detection means, the input signal light is detected and output to the second signal differential combining means,
In the second signal differential synthesizing means, the outputs from the third and fourth light detecting means are differentially synthesized and output to the signal mixing means,
A phase fluctuation detection method characterized in that said signal mixing means multiplies output signals from said first and second signal differential synthesizing means and outputs a multiplication signal.

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