JPH05167535A - Optical communication method - Google Patents

Abstract

PURPOSE:To suppress the deterioration in the linearity in the coherent light communication. CONSTITUTION:A transmission section 30 sends a signal light and a non- modulation light with a different frequency through one optical fiber while applying optical multiplex to them. A reception section 40 uses an optical heterodyne receiver 10 to extract a 1st IF signal 13 corresponding to the signal light and a 2nd IF signal 14 corresponding to the non-modulation light and an analog multiplier circuit 15 multiplies the 1st IF signal and the 2nd IF signal and an output signal is demodulated to extract information.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an in-fiber nonlinear distortion suppression method used for long-distance coherent optical fiber communication by multistage optical amplification relay.

[0002]

2. Description of the Related Art Optical fiber amplifiers bring about the possibility of long-distance multi-stage repeater transmission optical communication systems reaching several thousand km, and coherent optical communication is said to be an advantageous method in such long-distance transmission systems. The reason is that the coherent method has high reception sensitivity,
This is because it has the advantage that the influence of chromatic dispersion on the transmission line can be easily compensated for in the IF band of the receiver.

[0003]

Recently, however, it has been pointed out that the non-linear optical effect in the fiber limits the transmission distance of the coherent system (the optical fiber in the "shade" long-distance coherent optical communication system). Increase in signal beam width due to internal nonlinear phenomenon "199
1st Annual IEICE Fall Conference B-622). This non-linear optical effect means that the spontaneous emission noise generated from the optical repeater and the intensity modulation component of the signal light itself are Ke in the fiber.
This is a phenomenon that the reception sensitivity is deteriorated by being converted into phase noise of signal light by the rr effect.

An object of the present invention is to provide a new optical communication method for suppressing deterioration due to this non-linear phenomenon in coherent optical communication.

[0005]

According to a first aspect of the present invention, there is provided an optical communication method in which a signal light and unmodulated light having a different frequency from that of the signal light are optically multiplexed and transmitted in one optical fiber, and an optical heterodyne receiver is used to transmit the signal light. The first IF signal corresponding to the signal light and the second IF signal corresponding to the non-modulated light are taken out, these first IF signal and the second IF signal are multiplied by the analog multiplication circuit, and this output signal is obtained. The feature is that information is extracted by demodulation.

In the optical communication method of the second invention, the signal light and the non-modulated light having different frequencies are orthogonalized to each other in polarization, and then optically multiplexed and transmitted in one optical fiber. The first I corresponding to the signal light by the optical heterodyne receiver
The F signal is extracted, and the second IF signal corresponding to the unmodulated light is extracted by the second optical heterodyne receiver,
Information is taken out by multiplying the first IF signal and the second IF signal by an analog multiplication circuit and demodulating the output signal.

[0007]

The principle of suppressing the phase noise of the signal light increased by the in-fiber nonlinear optical effect according to the present invention will be described below. On the transmitting side, unmodulated light (hereinafter referred to as pilot light) whose spectrum line width is about the same as that of the carrier wave of the signal light and whose frequency is shifted to the range where crosstalk does not occur with respect to the signal light modulated by the data signal Call)
Is transmitted after being multiplexed with the signal light. Several tens of kilometers on the transmission line
Optical amplification repeaters are installed with a certain interval, and the transmitted signal light and pilot light are collectively amplified and sent out to a cascaded transmission line. At this time, spontaneous emission noise is also output from the optical repeater. Therefore, in the fiber connected after the optical repeater, in addition to the signal light and pilot light,
There will be spontaneous emission light. This spontaneous emission light is noise of light intensity and causes the Kerr effect in the optical fiber. That is, the refractive index in the fiber is fluctuated by the light intensity noise, and as a result, the phase of the signal light and the pilot light fluctuates as shown in the following equation (Yamamoto et al. Analysis of AM Conversion Noise by Phase Modulation "1991 IEICE Autumn Meeting SB
-8-4).

[0008]

[Equation 1]

As a result, phase noises having mutual correlation are added to the phases of the signal light and the pilot light after transmission. When these lights are received by the optical heterodyne receiver, the first IF signal for the signal light and the second IF signal for the pilot light can be extracted separately.
Since the first and second IF signals also have phase noises that are correlated with each other, when these IF signals are multiplied using an analog multiplication circuit (mixer), the phase noise added in the transmission line is added. It is possible to extract the third IF signal, which is only canceled. By demodulating the third IF signal, the signal can be read without being affected by the phase noise.

Incidentally, the unnecessary intensity modulation component of the signal light itself also causes the Kerr effect, but if the present invention is used, deterioration is caused by the same principle as in the case of spontaneous emission optical noise generated by an optical amplifier. It can be reduced.

According to a first aspect of the present invention, one optical heterodyne receiver simultaneously receives signal light and pilot light, and electrically separates and extracts the first IF signal and the second IF signal in the IF band. And has a characteristic that the configuration is simple.

The second invention is a method for suppressing four-wave mixing of the signal light and the pilot light incident on the fiber. That is, when the powers of the signal light and the pilot light are strong, four-wave mixing occurs in the fiber due to the nonlinear optical effect, and crosstalk occurs. As a method of suppressing this phenomenon, it is effective to make the polarizations of the signal light and the pilot light orthogonal to each other. On the receiving side, different receivers are used to receive two lights whose polarizations are orthogonal to each other. The second invention is also effective when the signal light is modulated at high speed and there is no margin in the band of the receiver, and it is difficult to receive the signal light and pilot light collectively.

[0013]

First, an embodiment of the first invention will be described. In this embodiment, the modulation / demodulation method is 2.5 Gb / s CPFSK heterodyne method, the transmission path is a relay interval of 100 km, and the transmission distance is 10
The first invention is applied to a multistage relay transmission system of 000 km. FIG. 1 shows the multistage optical amplification repeater system.

The transmission unit 30 is provided with a signal light source 1 and a pilot light source 2 having a spectrum line width of 1 MHz when no modulation is performed. The signal light from the first signal light source 1 directly CPFSK-modulated by the data signal is the second pilot light source 2
After being multiplexed by the multiplexer 3 with the unmodulated pilot light emitted from, it is incident on the booster optical amplifier 5 and the frequency interval stabilizing circuit 4. The frequency interval stabilizing circuit 4 detects the beats of the signal light and the pilot light, and the frequency of this beat is 5
The bias currents of the signal light source 1 and the pilot light source 2 are controlled so that the frequency becomes GHz. As a result, the frequency interval between the signal light and the pilot light is stabilized at 5 GHz. The booster optical amplifier 5 outputs +7 dBm signal light and 0 dBm pilot light. This light is input to the transmission path 50 having a repeater interval of 100 km and a transmission distance of 10,000 km. A zero-dispersion fiber 7 and a repeater 6 including an Er ion-doped optical fiber amplifier are used for the transmission line 50.
Attenuating the attenuation of 52 μm signal light and pilot light
It is possible to transmit 000 km. The gain of the output of each repeater 6 is adjusted so that it becomes signal light +7 dBm pilot light 0 dBm.

The transmitted light that has reached the receiving section 40 passes through the polarization controller 8 and is then combined with the local oscillation light having a spectral line width of 1 MHz and converted into an electric signal by the heterodyne receiver 10. An IF signal as shown in FIG. 2 appears at the output of the heterodyne receiver 10. These IF signals carry phase noise generated in the transmission path, and the data signal cannot be demodulated as they are, so the following processing is performed. The pass characteristics of the first band-pass filter 11 and the second band-pass filter are as shown in FIG. 2, and the first IF signal 13 and the second IF signal 14 can be separated and extracted. The first IF signal 13 is branched and then input to the frequency discriminator 18 and the mixer 15. The second IF signal is branched and then input to the polarization control circuit 19 and the mixer 15. The output of the mixer 15 has a third center frequency of 5 GHz from which phase noise generated during transmission is removed.
IF signal appears.

FIG. 3 shows the spectrum of the third IF signal. When this third IF signal is input to the differential detection circuit 16, the data signal is demodulated. Excessive noise components are removed from the data signal demodulated by the 2.5 GHz low-pass filter 17. The frequency discriminator 18 has a first I
When the F signal 13 exists on the high frequency side with 5 GHz as a boundary, a + voltage is output, and when it exists on the low frequency side, a − voltage is output. Since this output is fed back to the bias current of the local oscillation light source 9, the first IF signal 13 is stabilized at 5 GHz. Along with this, the second IF signal 14 is 10 GHz.
Will be stabilized. The polarization control circuit 19 is composed of a power meter, an A / D converter, a CPU, and a D / A converter, and always detects the power level of the second IF signal 14 so that this level is always the maximum. It controls the polarization controller 8. The polarization controller 8 can electrically control the polarization of the signal light and the pilot light by applying lateral pressure to the metal-coated fiber using PZT. As a result, the polarizations of the signal light and the pilot light can always be matched with the polarization of the local oscillation light. From the above, by using the first invention,
High-speed data of 2.5 Gb / s can be stably transmitted for 10,000 km.

Next, an embodiment of the second invention will be described with reference to FIG. This embodiment is based on the second invention of 5 Gb / s 1
This is applied to a 0000 km multistage relay transmission system. The differences between the operation of each part and the first embodiment will be described in detail below.

The 5 Gb / s CPFSK signal light emitted from the signal light source 1 of the transmitter 50 is polarization-multiplexed with the pilot light by the polarization multiplexing circuit 21. The polarization multiplexing circuit 21 outputs signal light and pilot light whose polarizations are orthogonal to each other. This light is transmitted after being amplified by the booster optical amplifier 5. The powers of the signal light and the pilot light at the output of the booster optical amplifier 5 are set to +10 dBm and +7 dBm, respectively. Even if such high power light is transmitted, the four light wave mixing does not occur in the zero-dispersion fiber 7 of the transmission line because the polarizations of the signal light and the pilot light are orthogonal to each other. The signal light and the pilot light multiplexed by the multiplexer 3 are incident on the frequency interval stabilizing circuit 4, and the frequency interval between the signal light and the pilot light is stabilized as in the embodiment of FIG. In this embodiment, this frequency interval is set to 10 GHz. The transmission line is the same as in the embodiment of FIG. The power of the signal light and pilot light at the output end of each repeater 6 is +10 dBm.
The gain is adjusted to be +7 dBm.

The transmitted signal light and pilot light are incident on the polarization controller 8 of the receiver 60, and the post-polarization demultiplexing circuit 22.
Separated by. The separated signal light and pilot light are the first
In the heterodyne receiver 10 and the second heterodyne receiver 20, the local oscillation light is multiplexed and received. The first IF signal 13 is output to the output of the first heterodyne receiver 10. The second IF signal 14 is output to the output of the second heterodyne receiver 20.

FIG. 5 shows the spectra of these IF signals. The first IF signal 13 and the second IF signal 14 are multiplied in the mixer 15. The third IF signal shown in FIG. 6 appears at the output of the mixer. This is the delay detection circuit 1
By inputting to 6, the 5 Gb / s data signal is demodulated. The polarization control of the signal light and the pilot light is performed based on the second IF signal 14 as in the embodiment of FIG.
As described above, by using the second invention, it is possible to suppress the effects of four-wave mixing and phase noise that occur on the transmission path, and it is possible to suppress a 5 Gb / s high-speed signal to 10,000 km.
Stable transmission is possible.

When the optical fiber of the transmission line is usually a dispersion fiber, the influence of four-wave mixing is unlikely to occur, and therefore, in the embodiment of FIG. 4, it may not be necessary to make the polarizations of the signal light and the pilot light orthogonal. In the embodiment, the polarization control of the first IF signal 13 and the polarization control of the second IF signal 14 are performed for frequency stabilization of the local oscillation light source 9, but there is no problem even if the combination is reversed. ..

The present invention is not limited to the CPFSK signal, and the same effect can be obtained for the ASK, PSK and QPSK signals. In the embodiment, the wavelength dispersion of the transmission line is almost zero.
Therefore, the equalizer was not used, but if the reception sensitivity deteriorates due to dispersion, the first optical heterodyne receiver 1
A chromatic dispersion equalizer is inserted immediately after the 0 and second optical heterodyne receivers 20. In the embodiment shown in FIG. 4, it is possible to further widen the frequency interval between the signal light and the pilot light, and in this case, the local oscillation light source 9 has one optical heterodyne receiver.
Mounted one by one. In the embodiment, a light source different from the signal light source 1 is used for the pilot light, but the signal light is split into two and A / O is used.
It is also possible to use a modulator or the like to shift the frequency of one light to obtain pilot light. In this case, it is necessary to use an external modulation method for modulating the signal light.

[0023]

As described above in detail, the use of the present invention suppresses the influence of the increase in the phase noise of the signal light caused by the in-fiber nonlinear optical effect of the ultra-long-distance coherent optical amplification repeater transmission system. It becomes possible.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the first invention.

FIG. 2 is a diagram showing first and second IF signal spectra of the embodiment of FIG.

FIG. 3 is a diagram showing a third IF signal spectrum of the embodiment of FIG.

FIG. 4 is a diagram for explaining an embodiment of the second invention.

5 is a diagram showing first and second IF signal spectra of the embodiment of FIG.

6 is a diagram showing a third IF signal spectrum of the embodiment of FIG. 4. FIG.

[Explanation of symbols]

 1 signal light source 2 pilot light source 3 multiplexer 4 frequency interval stabilizing circuit 5 booster optical amplifier 6 repeater 7 zero-dispersion fiber 8 polarization controller 9 local oscillation light source 10 first optical heterodyne receiver 11 first bandpass Filter 12 Second bandpass filter 13 First IF signal 14 Second IF signal 15 Mixer 16 Delay detection circuit 17 Low-pass filter 18 Frequency discriminator 19 Polarization control circuit 20 Second optical heterodyne receiver 21 Polarization multiplexing Circuit 22 Polarization separation circuit 30,50 Transmitter 40,60 Receiver

Claims (2)

[Claims] 1. A signal light and unmodulated light having a frequency different from that of the signal light are optically multiplexed and transmitted in one optical fiber, and an optical heterodyne receiver is used to transmit the first IF signal corresponding to the signal light and the unmodulated signal. Take out the second IF signal corresponding to the light,
An optical communication method characterized in that the first IF signal and the second IF signal are multiplied by an analog multiplication circuit and information is extracted by demodulating the output signal. 2. A signal light and non-modulated light having a frequency different from that of the signal light are orthogonalized to each other in polarization, and then optically multiplexed and transmitted to one optical fiber, and the first optical heterodyne receiver transmits the signal light. , A second IF signal corresponding to the non-modulated light is extracted by a second optical heterodyne receiver, and the first IF signal and the second IF signal are converted to an analog multiplication circuit. The optical communication method is characterized in that the information is taken out by multiplying by and demodulating the output signal.