JPH11220443A - Optical transmission / reception method - Google Patents

Abstract

(57) [Summary] [Problem] To reduce the restrictions due to the frequency characteristics of a Mach-Zehnder interferometer and to perform differential phase modulation-direct detection (DPSK-D)
An optical transmission / reception system that can utilize the advantage of D) is provided. A transmission / reception system for coherent optical communication in which differentially encoded phase modulated light is input and demodulated, wherein N
The input signal of the RZ code is put into a space when the signal is 0,
In the case of signal 1, 1 bit is equally divided into 2n (n is a natural number)
In the first two equally divided slots, one slot is used as a mark, the other slot is used as a space, and the remaining 2n-2 slots are converted so that marks and spaces are repeated in the same order. An encoder 13 that encodes the signal and a phase-modulated light that is phase-modulated in accordance with the signal encoded by the encoder 13 are branched into two, one signal light is delayed by 1 / (2n) bits, and both signals are interfered. And a Mach-Zehnder interferometer 14 for converting the light into intensity-modulated light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of coherent optical communication, and more particularly to differential phase modulation-direct detection (hereinafter referred to as "D").
The present invention relates to a transmission / reception system for coherent optical communication suitable for use in a PSK-DD system.

[0002]

2. Description of the Related Art The DPSK-DD system has higher light receiving sensitivity than the intensity modulation-direct detection (IM-DD) system.
Practical use is expected as an optical communication system suitable for long-distance optical transmission. In addition, since the differential receiving method using the balanced photodetector is used, the threshold value for identifying the signal can always be maintained at the center of the signal amplitude regardless of the intensity of the input light. Because of these advantages, practical applications are expected in recent years as transmission / reception systems corresponding to systems that process optical signals as they are, such as optical ATM (asynchronous transfer mode) switches.

FIG. 3 shows the principle of the conventional DPSK-DD system. In FIG. 3, 31 is a laser light source, 32 is a phase modulator, 33 is an encoder that converts an input signal into an NRZ-I code (non-return-to-zero reverse code), and 34 is an intensity-modulated light that reads the phase difference of the signal. A Mach-Zehnder (hereinafter referred to as MZ) interferometer for converting to the above, and 35 represents a balanced light receiver. FIG. 4 shows an input signal, an NRZ-I coded signal, a phase modulation signal φ _t , a phase difference φ _t −φ _tT , and an output signal. Here, t represents time, and T represents a bit period. The input signal (NRZ code) is input to the NRZ-I encoder 33.
By being converted to NRZ-I coding, the phase of the laser beam input from the laser light source 31 is modulated to the phase modulation signal phi _t in the phase modulator 32. This signal is the MZ interferometer 3
After the signal light is split into two at 4 and one of the signal lights is delayed by 1 bit, the phase difference between the signal lights when both signal lights interfere with each other is φ _t
−φ _tT . The phase difference is a ternary code, and φ _t −φ _tT
= Π, -π, the same output port of the MZ interferometer 34
_{When t− φtT} = 0, the signal is output to another output port. When these are received by the balanced photodetector 35 and the difference is calculated, an NRZ (non-return to zero) signal is reproduced as an output signal. Such an optical communication system is disclosed in, for example, JP-A-63-52.
No. 530, "Receiver for Coherent Optical Communication".

[0004]

Although the MZ interferometer is generally used as a frequency filter, the DPSK-DD
In the system, the phase difference of the input phase modulated light is divided into two, and one of them is delayed, and then the two are interfered with each other to detect the phase difference of the phase modulated light. Used as a detector. At this time, it is necessary to match the frequency of the light source with the optimum frequency at which the on / off ratio of the intensity-modulated light output from the MZ interferometer is maximized.

[0005] The signal amplitude intensity I (f) at one output of the MZ interferometer 34 is represented by f, the light source frequency, the delay time T, and the optical electric field input to the MZ interferometer E (t) = A exp [j
(2πft + θ (t))]

[0006]

(Equation 1)

Can be expressed as Here, A and θ indicate the amplitude and phase of the signal light, respectively. In this equation, Δ
The frequency at which the output signal amplitude becomes maximum when θ is modulated at 0 and π is when πfT = mπ (m is a natural number). The frequency f of the light source drifts due to thermal fluctuations or the like, and the intensity-modulated light output from the MZ interferometer is turned on.
When the frequency deviates from the optimal frequency at which the OFF ratio becomes the maximum, or when the refractive index of the optical waveguide changes due to heat fluctuation or external pressure applied to the MZ interferometer and the frequency characteristic of the MZ interferometer shifts, π ( f ₀ + δf) T = mπ + π · δf · T and M
The signal amplitude I (f) output from the Z interferometer deteriorates. Here, the optimum frequency at which the on / off ratio of the intensity-modulated light output from the MZ interferometer is maximum is f ₀ , and the difference between this frequency and the frequency of the light source is δf. As an example, communication at a signal speed of 10 Gbit / s (1 bit delay time is T = 10
Assuming that δf is 2 GHz ( ⁻¹⁰ seconds), the amplitude of the signal input to the balanced photodetector is reduced by about 5.1 dB, and the communication quality is significantly deteriorated.

The most important problem in actual communication due to the shift of the frequency transmission characteristic is the polarization dependence of the MZ interferometer. This is due to the fact that when the MZ interferometer is made of a planar lightwave circuit (PLC), the effective refractive index of the TE wave and the TM wave differs due to the stress of the PLC substrate. Difference is several GHz
Extend to It is technically difficult to make the MZ interferometer independent of the polarization, and it is very difficult in terms of cost to configure the entire system with a polarization maintaining system.

The present invention has been made to solve the above-mentioned problems, and alleviates the restrictions imposed by the frequency characteristics of the MZ interferometer.
It is an object of the present invention to provide an optical transmission / reception system that can take advantage of the above.

[0010]

According to a first aspect of the present invention, there is provided a transmission / reception system for coherent optical communication in which differentially encoded phase modulated light is inputted and demodulated. In the case of signal 0, the input signal is divided into spaces, and in the case of signal 1, one bit is equally divided into 2n (n is a natural number), and one of the first two slots is marked with a mark. One slot is used as a space, and the remaining 2n-2 slots are encoded in such a manner that the mark and space are converted in such a manner that the mark and the space are repeated in order, and a phase is determined according to a signal encoded by the encoder. A Mach-Zehnder interferometer that splits the modulated phase-modulated light into two, delays one of the signal lights by 1 / (2n) bits, interferes both signals, and converts the signals into intensity-modulated light. Features.

According to a second aspect of the present invention, in the optical transmission / reception system according to the first aspect, when n = 1, the encoder converts an input signal of an NRZ code into an RZ code. The Mach-Zehnder interferometer splits the phase-modulated light phase-modulated according to the signal RZ-encoded by the encoder into two, and splits one of the signal lights into 1 /
It is characterized in that the signals are delayed by two bits, and both signals are made to interfere with each other to be converted into intensity-modulated light.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the principle of the present invention will be described with reference to FIG. 1 which is a block diagram of the present invention. In FIG. 1, 11 is a laser light source, 12 is a phase modulator, 13 is NRZ
In the case of the signal 0, the input signal is divided into 2n equal parts (n is a natural number) for a signal 0 and a 1 bit for a signal 1, and one of the first two slots equally divided. To the mark, the other slot to the space,
An encoder for converting the remaining 2n-2 slots so that marks and spaces are repeated in the same order, 14
Is an MZ interferometer that is connected to the phase modulator 12 via optical transmission means, reads the phase difference of the signal sent from the phase modulator 12 and converts it into intensity-modulated light, and 15 is a balanced type. Represents a light receiver. In the case of the conventional DPSK-DD system, the MZ interferometer delays one bit, and the NRZ code of the intensity modulation is reproduced by causing bits adjacent to one bit to interfere with each other. Using a short MZ interferometer to delay 1 / (2n) bits,
The NRZ signal is reproduced by using the 2n intensity-modulated lights generated by the interference as one bit.

In the DPSK-DD system, the amount of decrease in the amplitude of the optical signal caused by the drift of the light source frequency or the change in the frequency characteristic of the MZ interferometer depends on the frequency of the light source and the ON / OFF of the intensity-modulated light output from the MZ interferometer. The difference δf from the optimum frequency at which the off ratio becomes the maximum, and the delay time T of the MZ interferometer
Is determined by the product of

In the present system, unlike the conventional system, since the MZ interferometer delays 1 / (2n) bits, the time delay is 1 / (2n) times that of the MZ interferometer used in the conventional system. Is used. Where MZ interferometer
The signal amplitude intensity I (f) at one output of 34 is represented by f (f) for the frequency of the light source, T for the delay time, and E (t) = A exp [j (2πft + θ (t)) for the electric field input to the MZ interferometer. ]

[0015]

(Equation 2)

## EQU1 ## Here, A and θ indicate the amplitude and phase of the signal light, respectively, and the optimal frequencies at which the on / off ratio of the intensity-modulated light output from the MZ interferometer is maximum are f ₀ and f ₀ . The difference from the frequency of the light source was δf, and m was a natural number. The expansion of δf is caused by the frequency drift of the light source or the polarization dependence of the MZ interferometer. As can be seen from this equation, the MZ interferometer with a short delay time is applied to reduce the MZ. It is possible to suppress a decrease in the amplitude of the output signal from the interferometer and to suppress a deterioration in communication quality due to these causes.

FIG. 2 shows an example of a coherent optical communication system as an embodiment when n = 1 in the optical transmission and reception system according to the present invention shown in FIG. In FIG. 2, reference numeral 20 denotes a transmitter according to the present invention, reference numeral 21 denotes a laser light source, reference numeral 22 denotes a phase modulator, reference numeral 23 denotes an input signal encoded by NRZ, a signal 0 denotes a space, and a signal 1 denotes a space. An encoder that divides one bit into two and converts the first slot into a mark and the subsequent slot into a space, ie, RZ
(Return to zero) Encoder, 24 is an optical communication network, 25 is a receiver according to the present invention, 26 is a branching input signal, delays one branching signal by 1/2 bit, interferes both signals, and modulates intensity. An MZ interferometer for producing light, 27 is a balanced light receiver, 28 is an amplifier, and 29 is a low frequency filter.

The NRZ signal input to the RZ encoder 23 is converted to an RZ signal, and drives the phase modulator 22 composed of, for example, an optical waveguide formed of LiNbO ₃ . The phase modulator 22 phase-modulates the laser light input from the laser light source 21 into an RZ code.

The signal light modulated by the phase modulator 22 is
The signal is input to the receiver 25 via the optical communication network 24. MZ interferometer
26 splits the input signal light, delays the split signal on one side by 干 渉 bit, and interferes both signals to generate intensity-modulated light. The converted intensity-modulated light is output to different ports of the MZ interferometer 26 in the case of a mark and in the case of a space, respectively, and is converted into an electric signal by the balanced light receiver 27. Thereafter, the signal is amplified by an amplifier 28, shaped by passing through a low-frequency filter 29, and output as an NRZ code.

FIG. 5 shows the input signal, the RZ encoded signal,
The phase modulation signal φ _t , the phase difference φ _t −φ _{t− T / 2} , and the output signal. Input signal (NRZ code) is converted into an RZ code by RZ encoder 23 performs phase modulation as the signal phi _t. Phase difference when the signal interferes after receiving 1/2 bit delay MZ interferometer 26 becomes φ _t -φ _{tT / 2.}
The phase difference is a ternary code, and φ _t −φ _{tT / 2} = π, −π is M
To the same output port of the Z interferometer 26, φ _t −φ _{tT / 2} = 0 is output to another output port. When these are received by the balanced photodetector 27 and the difference is obtained, NR is obtained as an output signal.
The Z signal is reproduced.

In the present system, since interference is caused by a 1/2 bit delay instead of a 1 bit delay as in the conventional system, the receiver 25
The MZ interferometer 26 applied inside is the same as that of the conventional system.
An MZ interferometer giving a delay time of / 2 is used. The amount of decrease in the amplitude of the optical signal caused by the drift of the light source frequency or the change in the frequency characteristic of the MZ interferometer depends on the frequency of the light source and the MZ.
It is determined by the product of the difference δf from the optimal frequency at which the on / off ratio of the intensity-modulated light output from the interferometer becomes the maximum and the delay time T of the MZ interferometer. Can be doubled.
Therefore, the tolerance on the accuracy on the frequency axis is large,
That is, it is strong against frequency fluctuations of the light source and changes in frequency transmission characteristics due to the polarization dependence of the MZ interferometer. Therefore, as compared with the conventional method, it is more resistant to frequency fluctuations of the light source and changes in frequency transmission characteristics due to the polarization dependence of the MZ interferometer, and is less likely to generate a code error at the time of reception.

[0022]

As described above, according to the present invention,
It is possible to reduce the deterioration of the reception characteristic caused by the frequency fluctuation of the light source and the shift of the frequency transmission characteristic of the MZ interferometer, which are problems in the conventional DPSK-DD system. Until now, when implementing the DPSK-DD system, the MZ interferometer was made polarization-independent and the entire system was polarized due to the shift in frequency characteristics caused by the polarization dependence of the MZ interferometer. Although measures such as a configuration using a wave maintaining system have been devised, by applying the present invention, it is possible to alleviate restrictions due to the frequency characteristics of the MZ interferometer, and DPSK can be performed without using the above-described measures. -The advantage of DD can be utilized.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating an example of a coherent optical communication system using the present invention.

FIG. 3 is a principle diagram of a transceiver in a conventional DPSK-DD system.

FIG. 4 is a diagram illustrating encoding of a conventional DPSK-DD scheme.

FIG. 5 is a diagram illustrating encoding in an embodiment of the present scheme.

[Explanation of symbols]

 11, 21 Laser light source 12, 22 Phase modulator 13 Encoder 14 MZ interferometer (1 / (2n) bit delay) 15 Balanced light receiver 20 Transmitter 23 RZ encoder 24 Optical communication network 25 Receiver 26 MZ interferometer (1 / 2 bit delay) 27 Balanced receiver 28 Amplifier 29 Low frequency filter

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04L 27/22

Claims (2)

[Claims] In a transmission / reception system for coherent optical communication in which differentially encoded phase modulated light is input and demodulated, an input signal of an NRZ code is input to a space when signal 0 and to an input signal when signal 1 Divides 1 bit into 2n equal parts (n is a natural number), marks one of the first two slots as a mark, the other as a space, and the other 2n-2 slots in the same order. An encoder that encodes by converting the mark and space so as to repeat in order; and a phase-modulated light that is phase-modulated according to a signal encoded by the encoder into two, and one signal light is divided by 1 /
(2n) An optical transmission / reception system for coherent optical communication, comprising: a Mach-Zehnder interferometer that delays a bit, interferes both signals, and converts the signals into intensity-modulated light. 2. The optical transmission / reception system according to claim 1, wherein, when n = 1, the encoder converts an input signal of an NRZ code into an RZ code, and the Mach-Zehnder interferometer includes: The phase-modulated light phase-modulated in accordance with the signal RZ-encoded by the encoder is branched into two, one signal light is delayed by 1/2 bit, and the two signals are interfered to be converted into intensity-modulated light. The optical transmission / reception system for coherent optical communication according to claim 1, wherein: