JPS6218157A - Intermediate frequency stabilizing method for optical heterodyne detection communication - Google Patents

Abstract

PURPOSE:To stabilize the intermediate frequency by controlling the frequency of a local oscillating light source at a light receiving section independently of the presence of a signal light. CONSTITUTION:The 1st synthesized light 19 between a signal light 17 and a pilot signal light 14 and a local oscillation light 21 from the local oscillation source 31 driven by the 3rd bias current source 30 are synthesized by the 3rd synthesizer 22 at the optical reception section 20 and the 2nd obtained synthesized light 42 is subjected to heterodyne detection by the 2nd photodetector 23 and converted into an intermediate frequency signal 24. The signal 24 is amplified by the 2nd amplifier 25 and branched and inputted to the 1st and 2nd band pass filters 26, 32. The 1st band pass filter 26 has a characteristic passing only a pilot signal component beat spectrum and outputs the pilot signal 27. The frequency fluctuation of the signal 27 is detected by the 2nd frequency discriminator 28 and fed back negatively to a local oscillation light source 31 to stabilize the intermediate frequency. The stabilized beat spectrum is extracted by a filter 32, demodulated and a data signal is extracted by a discriminating and reproducing circuit 34.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an intermediate frequency stabilization method in optical communications, particularly in optical-terodyne detection communications.

[Prior art and its problems]

Optical heterodyne detection communication (coherent optical communication) has the advantage of being able to transmit over long distances because it has much higher reception sensitivity than direct detection communication methods that modulate the intensity of light (Saito, Yamamoto, Kimura, "Coherent Optical Fiber Transmission Modulation and Demodulation Technology - FSK Heterodyne Detection -" Telecommunications Corporation Research and Application Report Vol. 31, No. 12, 1982).

In this coherent optical communication, a photodetector receives the signal light sent from the optical transmitter and the light from the local oscillation light source built into the optical receiver. In this case, a beat that resonates with the frequency difference between the signal light and the local oscillation light appears as an intermediate frequency electric signal in the output of the photodetector, but a baseband signal can be obtained by demodulating this. However, in this method, when the frequency of the local oscillation light or signal light changes, fluctuations occur in the intermediate frequency, which deteriorates the reception characteristics. Therefore, for example, it is necessary to control the frequency of the local oscillation light source in synchronization with fluctuations in the center frequency of the signal light to stabilize the intermediate frequency.

Conventionally, this stabilization has been achieved by detecting fluctuations in the beat frequency of the signal light and the local oscillation light, and feeding the output back negatively to the oscillation frequency control system of the local oscillation light source. However, with this method, it is impossible to control the local oscillation light when the signal light is not being received. Moreover, the oscillation frequency of a local oscillation light source using a semiconductor laser, for example, is 10% due to the influence of temperature fluctuations when the control system is not working.
Since this causes a large fluctuation of ~20 GHz/° C., when the signal light is received again, the frequency of the local oscillation light often deviates from the appropriate value. As described above, conventionally, there has been no intermediate frequency stabilization method that can control the frequency of the local oscillation light source in the optical receiver regardless of the presence or absence of signal light.

[Purpose of the invention]

Therefore, an object of the present invention is to propose a method that can stabilize the intermediate frequency by controlling the frequency of a local oscillation light source in an optical receiver regardless of the presence or absence of signal light.

[Structure of the invention]

The present invention is an optical heterodyne system that combines signal light transmitted from an optical transmitter with local oscillation light in an optical receiver, converts the combined light into an electrical signal with a photodetector, and then extracts a demodulated signal output. In the detection communication method, the optical transmitter multiplexes pilot signal light having a frequency apart from the center frequency of the signal light by a certain frequency interval with the signal light, and the optical receiver transmits the multiplexed pilot signal light having a frequency apart from the center frequency of the signal light. It is characterized in that the beat frequency of the light and the pilot signal light is detected, and the oscillation frequency of the locally oscillated light is controlled so that the beat frequency becomes a predetermined constant value.

[Principle of the invention]

In the present invention, first, in an optical transmitter, a pilot signal light that is controlled to maintain a frequency separated by a fixed frequency interval from the center frequency of the signal light is multiplexed with the signal light and transmitted. At this time, the beat component due to the signal light and the beat component due to the pilot signal light appear in the intermediate frequency signal spectrum obtained by the optical receiver. Of these, the beat component due to the pilot signal is extracted with a bandpass filter, etc., and the frequency Fluctuations are detected using a frequency discriminator.

By negatively feeding back this detection signal as a signal for controlling the oscillation frequency of the local oscillation light source, stabilization of the intermediate frequency can be realized. In the above method, since the frequency of the locally oscillated light is controlled so that the difference frequency is constant with respect to the pilot signal light, the locally oscillated light can be stabilized regardless of the presence or absence of the signal light.

〔Example〕

FIG. 1 is a block diagram showing a first embodiment of the present invention;
The figure shows an intermediate frequency spectrum obtained by the optical receiver in the embodiment shown in FIG.

In the first embodiment, the present invention is applied to a 140 Mb/S phase shift keying (PSK) optical heterodyne detection communication system. In the optical transmitter 1, the first emitted light 15 emitted from one end face of the transmitting semiconductor laser 2 which is driven by the first bias current source 13 and performs single-axis moat oscillation is transmitted by the second bias current source 7. The monitor pilot signal light 50 from one end face of the pilot signal semiconductor laser 3 that is driven and oscillates in a single axis mode is combined by the first multiplexer 8 and heterodyne detected by the first photodetector 9. . Note that the wavelength of the two light sources used in the optical transmitter 1, that is, semiconductor lasers, is in the 1.5 μm band. A beat signal 10 corresponding to the frequency difference between the first emitted light 15 and the monitoring pilot signal light 50 is obtained from the output of the first photodetector 9, and after this is amplified by the first amplifier 11, The beat signal 10 is determined by the first frequency discriminator 12.
The frequency fluctuation component is extracted and superimposed on the current applied to the transmitting semiconductor laser 2 as the oscillation frequency control signal 18 of the transmitting semiconductor laser 2. As described above, the transmitting semiconductor laser 2 has a frequency equal to the discrimination center frequency of the first frequency discriminator 12 with respect to the monitor pilot signal light 50.
The frequencies are stabilized at frequencies separated by a frequency interval of 80 MHz.

On the other hand, the second emitted light 16 from the other end facet of the transmitting semiconductor laser 2 enters the optical phase modulator 4 driven by the pulse drive circuit 6 to which the data signal 51 is input, and is modulated at 140 Mb/S. PSK modulation is applied at different speeds. The PSK-modulated signal light 17 is then multiplexed with the pilot signal light 14 by the second multiplexer 5, and the first multiplexed light 1
9 through a single mode filter 35.

In the optical receiver 20, the signal light 17 and the pilot signal light 1
4 and the local oscillation light 2 from the local oscillation light source 31 driven by the third bias current source 30.
The second combined light 42 obtained thereby is subjected to heterodyne detection by the second photodetector 23 and converted into an electrical intermediate frequency signal 24. As shown in FIG. 2, the spectrum of the intermediate frequency signal 24 includes a beat spectrum 41 of the signal component and a beat spectrum 40 of the pilot signal component. These two beat spectra are the pilot signal light 1
4 and the signal light 17, that is, the first frequency discriminator 1
2, with a frequency spacing of 280 MHz. The intermediate frequency signal 24 is amplified by a second amplifier 25 and then branched to the first . A second bandpass filter 26.32 is input manually. The first pantono filter 26 is the pilot signal component beat spectrum 40
It has the characteristic of only passing through the signal, and outputs the pilot signal 27 at the point where it passes. The intermediate frequency is stabilized by detecting this frequency fluctuation of the pyro-node signal 27 with the second frequency discriminator 28 and feeding it back negatively to the local oscillation light source 31. The stabilized signal component beat spectrum 41 is extracted by the second bandpass filter 32, demodulated by the demodulation circuit 33, and then extracted by the identification and regeneration circuit 34.
A data signal 51 is taken out. In addition, Figure 2 shows the first
and the characteristics of the second 7<nd pass filter 26.32.

In this system, even when the transmitting semiconductor laser 2 is not oscillating, the local oscillation light source 31 is activated by the pyro-soto signal light 1.
4, and when transmitting the signal light 17 again, the transmitting semiconductor laser 2 is stabilized with respect to the monitoring pilot signal light 50, and then the optical phase modulator 4 starts driving and transmits the data. A signal 51 is transmitted. As a result, the optical receiver 20 is always in a state in which it can receive the data signal 51, and as a result, it is possible to correct the deviation of the intermediate frequency signal 24 from the set frequency immediately after starting reception of the signal light 17. Missing data signals 51 can be eliminated.

Furthermore, even in cases where the demodulation circuit 33 does not require a carrier wave regeneration circuit like the differential synchronous detection method, in the past it was necessary to perform carrier wave regeneration to detect beat frequency fluctuations, but in this embodiment, this is not necessary. Since this is not necessary, a new feature is obtained in that the system configuration of the receiving section is simplified.

FIG. 3 is a block diagram showing a second embodiment of the present invention, and FIG.
The diagram shows the spectrum of the beat signal obtained by the photodetector of the optical transmitter in the embodiment shown in FIG. 3, and FIG. 5 shows the spectrum of the intermediate frequency signal obtained by the optical receiver in the embodiment shown in FIG. 3. In FIG. 3, the same elements as those in FIG. 1 are designated by the same numbers.

In the second embodiment, the present invention is applied to a 100 Mb/S frequency shift keying (FSK) optical heterodyne detection communication system. In the optical transmitter 55,
The transmitting semiconductor laser 2 driven by the first bias current source 13 and oscillating in a single axis mode is directly FSK-modulated by a pulse current generator 36 that converts a data signal 51 into a current signal. Then, FSK-modulated first and second signal lights 37 .
38 is emitted. The first signal light 37 is multiplexed by the first multiplexer 8 with the monitoring pilot signal light 50 emitted from one end face of the pilot signal semiconductor laser 3 that oscillates in a single axis mode, and the first signal light 37 is The detector 9 performs heterodyne detection. Note that the wavelength of the two light sources used in the optical transmitter 55, that is, semiconductor lasers, is in the 1.5 μm band. A beat signal 10 corresponding to the frequency difference between the first signal light 37 and the monitoring pilot signal light 50 is obtained at the output of the first photodetector 9.

This beat signal 10 has a bimodal spectrum corresponding to marks and spaces as shown in FIG.

From this spectrum, only the spectrum corresponding to the mark is extracted by the low-pass filter 39 (Fig. 4 shows the characteristics of the low-pass filter 39). Next, the frequency fluctuation of the extracted spectrum is detected by the first frequency discriminator 12. This is detected and negatively fed back as the oscillation frequency control signal 18 of the transmitting semiconductor laser 2.

As described above, the oscillation frequency of the transmitting semiconductor laser 2 for the mark signal is 20, which is equal to the center frequency of the first frequency discriminator 12 for the monitoring pilot signal light 50.
The frequencies are stabilized at frequencies separated by a frequency interval of 0 MHz. On the other hand, the second signal light 38 emitted from the other end face of the transmitting semiconductor laser 2 is the pilot signal light 1 emitted from the other end face of the pilot signal semiconductor laser 3.
4 and a second multiplexer 5, and is transmitted as a first combined light 19 through a single mode fiber 35.

In the optical receiver 56 , the combined light 19 of the second signal light 38 and the pilot signal light 14 and the local oscillation light 21 from the local oscillation light source 31 driven by the third bias current source 30 are combined.
are multiplexed by the third multiplexer 22. This light is then heterodyne detected by the second photodetector 23 and converted into an electrical intermediate frequency signal 24. As shown in FIG. 5, the spectrum of this intermediate frequency signal includes a beat spectrum 41 of the signal component and a beat spectrum 40 of the pilot signal component. These two beat spectra have a frequency interval of 200 MHz, which is the frequency difference between the pilot signal light 14 and the mark signal of the second signal light 38, that is, the discrimination center frequency of the first frequency discriminator 12. The intermediate frequency signal 24 is amplified by a second amplifier 25 and then branched to the first . Second bandpass filter 2
It is manually operated at 6°32. The first bandpass filter 26 has a characteristic of passing only the pilot signal component beat spectrum 40 and outputs the pilot signal 27. The intermediate frequency is stabilized by detecting this frequency fluctuation of the pilot signal 27 with the second frequency discriminator 28 and feeding it back negatively to the local oscillation light source 31. The beat spectrum 41 of the stabilized signal component is extracted by the second bandpass filter 32, demodulated by the demodulation circuit 33, and then the data signal 51 is extracted by the identification and reproduction circuit 34.

Note that FIG. 5 shows the first and second band pass filters 26.
．． It shows 32 characteristics.

In this system as well, as in the first embodiment, even when the transmitting semiconductor laser 2 is not oscillating, the local oscillation light source 31 is stabilized by the pilot signal light 14 and transmits the second signal light 38 again. When transmitting semiconductor laser 2 is excited with an injection current corresponding to the mark, its oscillation frequency is stabilized with respect to the frequency of pilot signal light 14, and then injection of pulsed current is started to transmit signal 35. is being carried out. As a result, the receiving section 56 is always in a state where it can receive the second signal light 38, and as a result, immediately after starting reception of the signal light, the intermediate frequency signal 2
It is possible to eliminate the dropout of the data signal 51 that occurred during the period of correcting the deviation from the set frequency of No. 4.

Although two embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and it is of course possible to make various modifications and changes within the scope of the present invention.

For example, in the first embodiment, only a single semiconductor laser was used, but if high spectral purity is required, such as in PSK optical heterodyne detection, it is also possible to use a semiconductor radar with an external cavity or a helium-neon laser. be. In addition,
If the emitted light can only be extracted from one end surface, one emitted light may be demultiplexed into first and second emitted light 15.16.

Furthermore, in the first embodiment, it is also possible to generate the pilot signal light 14 from the transmission semiconductor laser 2 using an acousto-optic (A/○) modulator without using the pilot signal semiconductor laser 3. . That is, in this method, a part of the light emitted from the transmitting semiconductor laser 2 enters an A10 modulator driven at a constant frequency, undergoes frequency shifting, and becomes pilot signal light 14. However, in this case, the transmitting semiconductor laser 3
When the local oscillation light source 31 is not oscillating, the local oscillation light source 31 cannot be controlled or stabilized, but in PSK optical heterodyne detection communication using differential synchronous detection demodulation that does not require carrier wave regeneration, it is necessary to stabilize the intermediate frequency. This is effective because the carrier regeneration circuit can be omitted. Further, in this case, there is an advantage that a control system for the wavelength interval between the pilot signal semiconductor laser 3 and the transmission semiconductor laser 2 is not required.

In the second embodiment, the beat signal 10 appearing in the optical transmitter 1 is
In the spectrum and the intermediate frequency spectrum corresponding to the second signal light 38 obtained by the optical receiver 20, the low frequency peak is marked, but the mark may be a space.

1st. In the second embodiment, the first bandpass filter 26 is used to extract the pilot signal 27 in the receiving section, but this may be a low-pass filter having only the pilot signal 27 in its passband.

Also, 1st. In the second embodiment, the signal light 17 and the second signal light 38 are one, but the present invention can also be applied to wavelength division multiplex communication having a plurality of signal lights. In that case, the pilot signal light 14 can not only stabilize the local oscillation light source 31, which is the object of the present invention, but also serve as a wavelength reference to keep the wavelength of the signal light 17, 38 of each channel constant.

〔Effect of the invention〕

As described in detail above, in the present invention, the frequency of the local oscillation light can be stabilized in the optical receiver regardless of the presence or absence of the signal light, thereby eliminating data signal dropouts and stabilizing the system when receiving the signal light. It can be made to work. In addition, since there is no need to extract the carrier wave signal for stabilizing the frequency of the local oscillation light from the modulated signal, there is no need for a carrier wave regeneration circuit like in the case of PSK signals, making system configuration easier. Effects can also be obtained.

4. Brief explanation of the figures FIG. 1 is a block diagram showing the first embodiment of the present invention, FIG. 2 is a diagram showing the intermediate frequency signal spectrum in the first embodiment, and FIG. 3 is a diagram showing the second embodiment. A block diagram showing an example, FIG.
FIG. 5 is a diagram showing the beat signal spectrum obtained in the optical transmitter in the second embodiment. FIG. 5 is a diagram showing the intermediate frequency signal spectrum in the optical receiver in the second embodiment.

1.55... Optical transmitter 2... Semiconductor laser for transmission 3... Semiconductor laser for pilot signal 5 ・
...Second multiplexer 8 ...First multiplexer 9 ...First photodetector 10 ...Beat signal 11 ... ...First amplifier 12 ...First frequency discriminator 14 ...
... Pilot signal light 17 ... Signal light 19 ... Combined light 20.56 ... Optical receiving section 21 ... Local oscillation light 22 ...... Number 3 multiplexer 23...Second photodetector 24...Intermediate frequency signal 27...Pilot signal 28...Second frequency discriminator 31...
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Claims (1)

[Claims] (1) Optical heterodyne detection in which the signal light transmitted from the optical transmitter is combined with local oscillation light in the optical receiver, the combined light is converted into an electrical signal by a photodetector, and then a demodulated signal output is extracted. In the communication method, the optical transmitter multiplexes pilot signal light having a frequency apart from the center frequency of the signal light by a fixed frequency interval with the signal light, and the optical receiver transmits the locally oscillated light. and the pilot signal light, and controlling the oscillation frequency of the locally oscillated light so that the beat frequency becomes a predetermined constant value.