JPH0422253B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical communication method, and particularly to a communication method using optical heterodyne and optical homodyne detection.

In general, communication methods that use optical heterodyne and optical homodyne detection have the great advantage of increasing optical reception sensitivity by 10 to 100 times or more compared to communication methods that use conventional optical direct detection. It is expected to be an effective communication method for trunk line systems.

Among communication methods that use optical heterodyne or optical homodyne detection, the ASK heterodyne detection method that uses amplitude information has the advantage that the system can be configured easily because the requirements for the spectral width and frequency stability of the light source are relatively loose. be. However, the intensity of semiconductor lasers, which are expected to be used as light sources, can be directly modulated by changing the injection current, but the oscillation frequency changes as the injection current changes, so optical heterodyne is Also, communication methods using optical homodyne detection require the use of an external modulator. However, since the external modulator has insertion loss, it has the disadvantage that the transmitted light power level is reduced by that amount.

On the other hand, communication methods that use optical heterodyne or optical homodyne detection use frequency information.
The FSK heterodyne detection method can directly measure the frequency of the light source if a semiconductor laser is used as the light source, and it also has advantages such as being able to theoretically improve reception sensitivity by 3 dB compared to the ASK heterodyne detection method. However, in the case of the FSSK heterodyne detection method, there are strict requirements for the spectral width and frequency stability of the light source, and current semiconductor lasers do not necessarily meet these requirements, which has the disadvantage of deteriorating reception sensitivity.

The purpose of the present invention is to solve these conventional drawbacks, to provide a novel optical heterodyne/homodyne communication that has loose requirements on the spectral width of the light source, allows direct modulation of the light source, and provides reception sensitivity equal to or higher than that of the ASK heterodyne detection method. The purpose is to provide a method.

In the optical heterodyne/homodyne communication method of the present invention, an optical transmitter transmits signal light modulated by an information signal, an optical multiplexer combines the transmitted signal light with local oscillation light, and the optical multiplexer combines the transmitted signal light with local oscillation light. In an optical heterodyne/homodyne communication method in which light is received by an optical receiver and a demodulated signal output is extracted, the signal light is modulated in the optical transmitter by binary frequency modulation, and the optical receiver uses two modulated frequencies. The feature is that only one of the components is extracted and demodulated.

Next, the present invention will be explained in detail with reference to Examples.

FIG. 1 is a block diagram for explaining one embodiment of the present invention, FIG. 2 is a frequency characteristic diagram for explaining the principle of the present invention, and FIG. 3 is a signal waveform diagram of each part. First, the transmission light source 1 is subjected to binary frequency modulation using a modulation signal 3 from a signal generator 2. As a result, the frequency-modulated signal light 4 propagates through the optical transmission line 5 and is then multiplexed with the local oscillation light 8 which is the output of the local oscillation light source 7 by the optical multiplexer 6 . This combined light 9 is detected by a photodetector 10 in an optical receiving section, and a band-limited receiving circuit 11 obtains a heterodyne detection output 12 of only one of the two frequency components of the transmitted light. A demodulated output 14 is obtained by detecting this signal using a short envelope detector 13.

In this embodiment, a semiconductor laser that oscillates in a single axis mode is used as the transmitting light source 1. By slightly changing the injection current of this semiconductor laser, binary frequency modulation of 100 Mb/S was performed between two frequencies corresponding to the signal code "1" and the signal code "0". The amplitude of the injection current at this time is
At 5mA, the frequency deviation was 3GHz. As the local oscillation source 7, a semiconductor laser was used whose oscillation frequency was controlled by an AFC circuit so that the oscillation frequency was shifted by 400 MHz from the oscillation frequency of the transmitting semiconductor laser when the sign of the signal was "1". Signal light 4 propagated through a single mode optical fiber serving as an optical transmission line 5 is multiplexed with local oscillation light 8 by an optical multiplexer 6 using a half mirror. Here, an avalanche photodiode was used as the photodetector 10, and the combined light 9 was subjected to heterodyne detection. The output of the avalanche photodiode at this time is a beat output with center frequencies of 400 MHz and 3.4 GHz as shown in FIG. 2a, and its signal waveform is as shown in FIG. 3a. At this time, the component of the signal with code "1" is a beat output with a center frequency of 400 MHz, and the component of the signal with code "0" is a beat output with a center frequency of 3.4 GHz. Here, as the receiving circuit 11, the second
As shown in figure b, the band is from 100MHz to 700M
Hz limit, as shown in Figure 2c, only the beat output corresponding to the sign “1” of the signal is extracted, and the beat output corresponding to the sign “0” of the signal is not attenuated. It is received and not taken out.
The signal waveform at this time is as shown in FIG. 3b, and by detecting this with the envelope detector 13, a demodulated signal output shown in FIG. 3c is obtained.

As described above, the feature of the optical heterodyne/homodyne communication method of the present invention is that one of the information sent as frequency information is extracted as amplitude or intensity information. Therefore, if the frequency shift of the transmitted signal is large, the requirement for a narrow spectrum width required by the FSK heterodyne detection method can be avoided, and the system can be easily configured. The optical reception sensitivity at this time is almost the same as that of ASK heterodyne detection, but
Since the optical heterodyne/homodyne communication method of the present invention allows direct modulation of the light source, there is no need to use an external modulator that causes insertion loss, and a system with high transmission power can be constructed. In the above example, since the injection current was changed by 5 mA by binary frequency modulation of the semiconductor laser, the semiconductor laser also received some intensity modulation correspondingly. This is the second
This corresponds to a slight difference in the magnitude of the beat output with a center frequency of 400 MHz and the beat output with a center frequency of 3.4 GHz in the figures and Figure 3, but in the present invention, only one beat output is used for demodulation, so both beat outputs It does not matter if the sizes of the two are different.

In the present invention, various modifications can be made in addition to the above-described embodiments.

As the transmitting light source 1, in addition to a semiconductor laser,
It is also possible to use a gas laser such as a He-Ne laser or an external mirror type semiconductor laser, and apply frequency modulation by changing the resonator length. Furthermore, the transmission light source 1 can be made by combining a gas laser or a solid-state laser with a frequency modulator such as an acousto-optic modulator. As the optical transmission line 5, in addition to optical fibers, spatial propagation or other optical waveguides can also be considered. As the optical multiplexing section 6, it is also possible to use a fiber coupler or a diffraction grating in addition to a half mirror. As the local oscillation light source 7, various lasers such as a semiconductor laser, a gas laser, a solid laser, etc. can be used, and as the photodetector 10, a photodiode, a photomultiplier tube, etc. can be used. Further, when performing heterodyne detection, it is also possible to use a synchronous detector using a certain reference frequency instead of the envelope detector 13 as the electrical detector. Furthermore, homodyne detection is performed by matching the oscillation frequency of the local oscillation light source 7 with one frequency component of the signal light from the transmission light source 1, and the signal is demodulated from the amplitude or intensity information of the baseband signal. It is also possible.

[Brief explanation of drawings]

Fig. 1 is a block diagram for explaining one embodiment of the present invention, Fig. 2 a, b, and c are frequency characteristic diagrams for explaining the principle of the present invention, and Fig. 3 a, b, and c are respective parts. FIG. In the figure, 1... transmission light source, 2... signal generator, 3... modulated signal, 4... signal light, 5... optical transmission line, 6... optical multiplexer, 7... local oscillation light source,
8...local oscillation light, 9...combined light, 10...photodetector, 11...receiving circuit, 12...heterodyne detection output, 13...envelope detector, 14...demodulation output.

Claims (1)

[Claims] 1. An optical transmitter transmits signal light modulated by an information signal, an optical multiplexer multiplexes the transmitted signal light with local oscillation light, and an optical receiver receives the multiplexed light. In an optical heterodyne/homodyne communication system that extracts a demodulated signal output, the signal light is modulated in the optical transmitter by binary frequency modulation, and the optical receiver modulates one of the two modulated frequency components. An optical heterodyne/homodyne communication method characterized by extracting and demodulating only the components.