JPS63257341A - Method and equipment for optical homodyne detection optical communication - Google Patents

Abstract

PURPOSE:To avoid insertion loss of a modulator by applying binary frequency shift modulation to a transmission signal light and setting the center frequency of a low frequency component of the binary frequency shift modulation signal converted into an intermediate frequency band at a reception section to a frequency being 1/10 or below the signal transmission speed so as to demodulate only the low frequency component. CONSTITUTION:The information is sent by a signal light subjected to binary frequency shift modulation and converted into an intermediate frequency band by using a local oscillation light source 19 at a reception section. In this case, the center frequency of the low frequency component of the converted binary frequency shift modulation signal is set to the frequency being below 1/10 of the signal transmission speed and only the low frequency component is demodulated by an optical homodyne detection circuit 20. Thus, the loss by the insertion of an external modulator is excluded and since one of the signal subjected to binary frequency modulation is in homodyne detection, the reception sensitivity the same degree as that of the ASK homodyne system is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical communication method and apparatus for transmitting information by modulating light in accordance with an information signal.

(Prior Art) In recent years, the characteristics of semiconductor lasers have improved, and it has become possible to obtain semiconductor lasers that oscillate in a single-axis mode and have high spectral purity. By utilizing this, it has become possible to realize a coherent optical transmission method that uses information on the frequency and phase of light, and highly sensitive systems have become possible. In particular, the optical homodyne detection method, in which the baseband signal is directly regenerated from the optical signal in the optical detection circuit, has half the signal bandwidth compared to the optical heterodyne detection method, which regenerates the intermediate frequency signal from the optical signal, resulting in a 3 dB higher reception sensitivity. In addition, since the high cutoff frequency of the receiving circuit is at least 1/3 that of the optical heterodyne detection method, it has advantages such as easy design and manufacture of the receiving circuit and easy reduction of noise.

(Problem to be solved by the invention) However, amplitude modulation (ASK) and phase modulation (PSK) to which the homodyne detection method can be applied require the use of an external modulator, so direct modulation of a semiconductor laser, for example, is not possible. Compared to the possible frequency modulation optical heterogain detection method, it was not possible to avoid an increase in insertion loss and an increase in the number of components.

(Object of the Invention) An object of the present invention is to solve the above-mentioned problems and to propose an optical homodyne detection type communication device that has no modulator insertion loss.

(Means for Solving the Problem) The optical homodyne detection optical communication method and device according to the present invention transmits information using binary frequency shift keyed signal light, and transmits the information using a local oscillation light source in the receiving section. Convert to frequency band. At this time, the center frequency of the low frequency side component of the converted binary frequency shift keying signal is set to a frequency of 1 to 10 or less of the signal transmission speed, and only this low frequency component is demodulated by an optical homodyne type detection circuit. It is.

(Function) In the detection method according to the present invention, since the binary frequency modulated wave is obtained by directly modulating a semiconductor laser, for example, it is possible to eliminate the loss caused by inserting an external modulator. Moreover, since one of the binary frequency modulated signals is subjected to homodyne detection, reception sensitivity comparable to that of the ASK homodyne system can be obtained.

(Example) The first tooth is a block diagram showing a first example of the present invention. In this embodiment, the present invention is applied to FSK with a transmission capacity of 400 Mb/s.
It is applied to the system. The transmission side is similar to a normal optical frequency modulation system, and by directly modulating the signal light source 1 composed of a semiconductor laser with the transmission signal 2, the frequency difference between the signals 1 (mark) and 0 (space) is 3 GHz. A binary frequency modulated transmission light 3 is obtained.

The signal light thus obtained is transmitted through the optical fiber 4.

On the receiving side, a phase diversity method is adopted in order to avoid city deterioration of the signal waveform due to phase fluctuations between the signal light 3 and the local oscillation light 5, which is a particular problem in the homodyne detection method. The phase diversity method is Divis (
[A wideband homodyne receiver using phase diversity J ("A wid
eband homodyne receiver u
"singphase diversity", 11t
h European Conference onO
ptical Communication, Te
Chnical Digest, Vol, 1°198
5), so a detailed explanation will be omitted. First, the signal light 3 transmitted through the polarization-maintaining optical fiber 4 is transmitted through the optical 90° hybrid gold wave device 6 (leave).
W, R, Leeb) [90
Realization of 180° and 180° hybrids (Realiz
ation of90°and 180°hybrid
s for optical frequency
s”) AEU.

3! JP 203.1983) is combined with the local oscillation light 5. From this, the first. Second combined lights 7 and 8 are obtained. 1st. second combined light 7,
8 is the first. The first . It is amplified by a second amplifier 11.12. Then the first and second low-pass filters 13.1
At step 4, only the mark signal is extracted. This is the first. Second
After square law detection is performed by square law detectors 15 and 16, the two signals are combined in a linear synthesis circuit 17 to obtain a demodulated signal 18.

At this time, since the two signals are out of phase with each other by 90 degrees, by linearly combining them, a signal with a constant amplitude is demodulated without being affected by phase fluctuations between the local oscillation light and the signal light.

Note that the center frequency on the low frequency side of the signal extracted when performing phase diversity only needs to be about 1110 or less of the signal transmission speed, and in this embodiment, the automatic frequency control system 22 is used to stabilize it to 10 MHz or less. It has been converted into The beat spectrum width of the signal was 20 MHz, and the receiving sensitivity was stable at -45 dBm.

As described above, reception sensitivity comparable to that of the 400 Mb/S ASK phase diversity method can be obtained. Moreover, since the semiconductor laser is directly modulated, losses caused by inserting a modulator can also be avoided.

FIG. 2 shows a second embodiment of the invention.

The second example sets the transmission capacity to 1.2 Gb/s and has the same configuration as the first example to demonstrate the effectiveness of this system for increasing speed. However, the receiving system uses a 3-vote phase diversity system.

In this method, a 120° hybrid multiplexer 21 (scene (S)) is used.

``Optical fiber interferometer using 3x3 directional device - Analysis J (``0ptical
fiberinterferometers with
3X3 directional coupler
s-Analysis”), J, Appl, Phys.
,, 52. P3865.1981) is used, but it has the advantage of being easier to manufacture than the 90° hybrid multiplexer 6 used in the first embodiment. However, three optical homodyne detection systems 20 similar to those shown in FIG. 1 are required.

However, in the homodyne detection system, a baseband signal is obtained immediately after detection, so each receiving circuit can have a much narrower band than in the heterodyne detection system, and can be manufactured relatively easily.

In this embodiment, the cutoff frequency of the optical homodyne detection system 20 is set to about 900 MHz.

Various other modifications are possible to the present invention.

For example, placing a resonator reflector outside the semiconductor laser,
By using a light source with a narrow spectral width, such as an external cavity semiconductor laser or a monolithic external cavity semiconductor laser, the receiving system can be used in applications with strict requirements regarding the spectral width.
LL method (paper by Marion, “Digital fiber transmission using optical homodyne detection J”)
bre transmission using gopt
ical homodyne detection),
Electronics Letters.

20, p. 281.1984).

In addition, two or more signal lights with different modulation frequencies are sent simultaneously, the same number of local oscillation light sources are prepared, and the frequency of the local oscillation light sources is set so that the intermediate frequency on the low frequency side of each light is around OHz. A multiplex optical communication device is also conceivable.

(Effect of the invention) An optical homodyne detection communication device without large losses can be constructed.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a first embodiment of the invention, and FIG. 2 is a diagram showing a second embodiment of the invention. In the figure 1...Signal light source 2...Transmission signal 19...Local oscillation light source

Claims (1)

[Scope of Claims] 1. Information is transmitted in an optical transmitter using a transmission signal light modulated in accordance with an information signal, and a receiver demodulates the signal by homodyne-detecting the transmission signal light using a local oscillation light source. In the optical homodyne detection optical communication method, the transmission signal light is binary frequency shift modulated, and the center frequency of the lower frequency side component of the binary frequency shift modulation signal converted to an intermediate frequency band in the receiving section is used for signal transmission. An optical homodyne detection optical communication method characterized by setting a frequency to 1/10 or less of the speed and demodulating only this low frequency component. 2. A signal light source that has a function of emitting frequency-modulated light according to a transmission signal, and an optical fiber that transmits the signal light;
A local oscillation light source, a hybrid multiplexer that combines signal light and local oscillation light, a photodetector, amplifier, low-pass filter, and square law detector that converts the output light of each port from the multiplexer into electrical signals. , an optical homodyne detection circuit consisting of a linear synthesis circuit that synthesizes the electrical signals of each port,
An optical homodyne detection optical communication device consisting of an automatic frequency control system for local oscillation light.