JPS6149530A - Optical heterodyne communicating method - Google Patents

Abstract

PURPOSE:To output a demodulated signal extracted from a composite signal and to reduce the influence of noise in a high frequency component by extracting a low modulation frequency part and a high modulation frequency part independently from an IF signal from an optical reception part and composing the composite signal of them while the intensity ratio of detection outputs is varied. CONSTITUTION:Signal light 4 is sent from an optical transmission line 5 to an optical multiplexing part 6 after binary frequency modulation is imposed by the transmitting light source 1 of the system with a modulating signal 3 from a signal generator 2. This multiplexing part 6 multiplexes the light with local oscillation light 8, the composite light 9 is detected by a photodetector 10, and a receiving circuit 11 outputs the double-peak IF signal 12 which has a peak value at a low frequency and a high frequency. This signal 12 is applied to envelope detectors 15 and 16 through an LPF13 and a BPF14 to detect signals of the low frequency and high frequency. Their detection outputs 17 and 18 are applied to a power mixer 19 and the composition ratio is adjusted according to the noise ratio of the detection outputs 17 and 18 to reduce the influence of a noise in the high-frequency component, thereby outputting a demodulation output 20.

Description

Detailed Description of the Invention (3-1) Technical field to which the invention pertains The present invention relates to optical communication methods, particularly to the use of semiconductor lasers by improving the performance of optical heterogain detection semiconductor lasers, and in particular by improving single-axis mode oscillation characteristics. Even in optical fiber communication, it has become possible to realize coherent communication methods that transmit signals by adding signals to the phase and frequency of light waves, such as optical heterogain/homodyne detection communication methods using frequency shift key ink or phase shift key ink. . In particular, since the oscillation frequency of semiconductor lasers can be directly modulated by changing the magnitude of the injected current, the optical heterodyne communication method using frequency shift key ink has a simple configuration and is considered to be an effective communication method.

(3-3) Disadvantages of Prior Art Among the communication methods using optical heterodyne detection, the optical heterodyne detection method using frequency shift key ink using frequency information allows direct frequency modulation of the light source if a semiconductor laser is used as the light source. In addition to being simple, this method also has the advantage that it is theoretically possible to improve reception sensitivity by 3 dB compared to the intensity shift key ink detection method. This optical heterogain detection method using frequency shift key ink was published by Nis Saito et al. in the American magazine IBEE Journal of Q.
ua-ntum Electronics) Volume 19, No. 2 (published February 1983), page 180
SNA and Error Rate Evaluation Fount Optit Force Rouet 7 SC Heterodyne Detection System Using Semiconductor Lasers J
(S/N and error rate
application for an optical FSK-
heterodyne detection system
using semiconductor 1as
ers). In this paper, a semiconductor laser with 200
After direct frequency modulation at a signal speed of Mb/s and multiplexing of the signal light and local oscillation light, the light is received by an optical receiver and a demodulated signal output is extracted using a frequency discriminator.

Generally, optical heterodyne detection using frequency shift key ink requires a light source to have a narrow spectrum width and high frequency stability, but current semiconductor lasers do not necessarily meet these requirements. Therefore, in the system of the above-mentioned paper as well, the spectrum broadening of the semiconductor laser causes deterioration of the signal-to-noise ratio in the baseband, which worsens the code error rate characteristic.

To solve this problem, the frequency shift can be increased to eliminate the effect of spectrum broadening of the conductor laser, but in this case, an optical receiver with a wide band is required. but,
In the optical receiver, noise increases as the frequency increases, so the signal-to-noise ratio of the modulated frequency component of the high frequency deteriorates, making it impossible to increase reception sensitivity when demodulating using a frequency discriminator.

On the other hand, as an optical heterogain/homodyne detection method using frequency shift key ink that has loose requirements on the spectral width of the light source and allows direct modulation of the light source, the rD method proposed by Shufu et al.
"Characteristics of optical heterodyne single filter detection method for F8 using FB-LD" (1981 National Conference of the Institute of Electronics and Communication Engineers, 2612). When frequency modulating the signal light, the larger the frequency shift, the more binary frequency modulation is performed. This method extracts only the components of the light source and demodulates them, so that the spectral spread of the light source is not affected by There is.

However, since this method uses only half the components of the modulated signal during demodulation, it has the disadvantage that the reception sensitivity is 3 dB lower than the original binary frequency modulation method.

(3-4) Purpose of the Invention The purpose of the present invention is to reduce the influence of noise of high frequency components in the optical receiver when demodulating a binary or higher frequency modulated signal, and to obtain high reception sensitivity. The purpose of the present invention is to provide an optical heterodyne communication system.

(3-5) Structure of the Invention In the optical heterogain communication method of the present invention, an optical transmitter transmits signal light modulated into an information signal, and an optical multiplexer converts the transmitted signal light into local oscillation light. Combined,
This multiplexed light is converted into an intermediate frequency signal at the optical receiver,
extracting a demodulated signal from the intermediate frequency signal in a demodulation section;
In the optical heterogain communication method, the modulation of the signal light in the optical transmission section is performed by binary or higher frequency modulation, and the demodulation section independently extracts and detects the modulated frequency components in the intermediate frequency signal, and The present invention is characterized in that the intensity ratio of each detected signal is changed according to the signal-to-noise ratio of the detected signal, and the synthesized signal is extracted as a demodulated output.

(3-6) Principle of the Invention As mentioned above, in order to directly frequency modulate a semiconductor laser so that the spectrum expansion of the semiconductor laser is not affected by The spectrum of the frequency signal is considerably spread, and an optical receiver with a wide band is required.

However, in the optical receiving section, noise increases as the frequency increases, so the signal-to-noise ratio of high modulation frequency components deteriorates.

On the other hand, in the method of extracting only the low frequency component that has been binary frequency modulated in the demodulation section, the spectrum expansion of the semiconductor laser is less susceptible to the influence of shading, but the receiving sensitivity is 3 dB lower than the original binary frequency modulation method. Therefore, in the present invention, the low modulation frequency component and the high modulation frequency component of the intermediate frequency signal are independently extracted and detected, and the intensity ratio of each detection output is changed and synthesized, and the result is extracted as a demodulated output. Reduce the influence of component noise and obtain high reception sensitivity.

(,3-7) Example Next, the present invention will be explained with reference to Examples.

FIG. 1 is a block diagram for explaining a first embodiment of the present invention, and FIG. 2 is a diagram showing frequency characteristics 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. Thereby, 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 light of the local oscillation light source 7 in the optical multiplexing section 6 . This multiplexed light 9 is detected by a photodetector 10 in the optical receiving section, and then sent to a receiving circuit 11, which has a bimodal peak at a low frequency and a high frequency as shown in FIG. An intermediate frequency signal 12 of values is retrieved. This is divided into two and applied to a low-pass filter 13 and a band-pass filter 14 to obtain intermediate frequency signals each having one peak of different frequencies. These outputs are each detected by envelope detectors 15 and 16, and depending on the signal-to-noise ratio of the detected outputs 17 and 18,
A power combiner 19 adjusts the combination ratio to obtain a decoded output 20.

As the transmission light source 1, a semiconductor laser that oscillates in a single axis mode is used, and the injected current is slightly changed, (/r7.'i
Binary frequency modulation of Mb/s was performed. The fluctuation width of the injected current at this time was 4 mA, and the frequency deviation was IGHz. A single mode optical fiber was used as the optical transmission line 5, and a -7 mirror was used as the optical multiplexer 6. Photodetector 1
0, an avalanche photodiode was used, and the combined light 9 was subjected to heterodyne detection. The band of the receiving circuit 11 is 2G) (z), and the intermediate frequency signal 12 has a frequency band of 2G) (z).
), we obtained a bimodal beat output with peaks at 500 MHz and 1.5 GHz.

When the intermediate frequency signal 12 is divided into two equal parts and one is passed through a low-pass filter 13 with a cutoff frequency of IGHz whose frequency characteristics are shown in FIG. 2(b), the output waveform is as shown in FIG. 2(c).
) is a beat output with a center frequency of 500 MHz corresponding to the signal code "1".

By detecting this with the envelope detector 15, a first detection output 17 was obtained.

Similarly, the intermediate frequency signal 12 is filtered using a band-pass filter 14 having frequency characteristics as shown in FIG. 2(d), which corresponds to the signal code "0" as shown in FIG. 2(e). Take out the beat output with a center frequency of 1.5 GHz,
Detection was performed by an envelope detector 16, and a second detection output 18 having a polarity opposite to that of the first detection output was obtained.

The signal-to-noise ratio of the first detection output 17 was 27 dB. On the other hand, since circuit noise is large in the high frequency region of the receiving circuit, the signal-to-noise ratio of the second detection output 18 was 21 dB. Considering the signal-to-noise characteristics of the second detection outputs 17 and 18, the power combiner 19 attenuates the second detection outputs 18 by about 6 dE and differentially outputs them from the first detection outputs 17. In addition, a demodulated signal 20 was obtained.

In this case, the reception sensitivity of each envelope detector 15j is
The receiving sensitivity was improved by about 1 dB compared to when the output from 16 was added differentially with 1=1.

FIG. 3 is a block diagram for explaining the second embodiment of the present invention, and FIG. 4 is a diagram showing frequency characteristics of each part. In this embodiment, instead of using a wideband receiving circuit, the intermediate frequency signal 21 obtained by the photodetector 10 is separated into a low frequency component and a high frequency component, and matched at the center frequency of the beat of each intermediate frequency signal. By using a low frequency amplifier 25 and a high frequency amplifier 26, each intermediate frequency signal is independently extracted and demodulated.

In this embodiment, a semiconductor laser that oscillates in a back-axis mode is used as the transmitting light source 1, and binary frequency modulation of 450 Mb/s is performed by slightly changing the injection current. At this time, the amplitude of the injection current is 6 mA, and the frequency deviation is 1.6.
It was GHz.

The combined light 9 of the signal light 4 and the local oscillation light 8 is detected by a photodetector 1.
Heterodyne detection was performed using 0. At this time, the intermediate frequency signal 21 was a bimodal beat output as shown in FIG. 4(a), and had a wide band of about 3.2 GHz. This intermediate frequency signal 21 is converted using a directional coupler 22 which operates as a 3 dB coupler at 1.5 GHz as shown in FIG. 4(b), and each corresponds to the signal code "1" shown in FIG. 4(c). The low-frequency beat output 23 with a center frequency of 700 MHz and the signal code shown in FIG. 4(d)"
The center frequency corresponding to 0" was separated into high frequency beat outputs 24 of 2.3 GHz.The directional coupler 2 used here
Reference numeral 2 is a broadband phase-inverted hybrid ring composed of microstrip lines. Directional coupler 22
The low frequency beat output 23 and high frequency beat output 24 separated by the low frequency amplifier 25 are matched at a center frequency of 700 MHz, respectively, and the center frequency is 2.3 GHz.
The signal is amplified using a Z-matched high frequency amplifier 26, and detected using envelope detectors 15 and 16. At this time, the signal-to-noise ratio of the first detection output 17 and the second detection output 18 is 2.
3dB? It was 19 dB. In the power combiner, the second
The first detection output 17 is attenuated by 3 dB.
A good demodulated signal 20 was obtained.

In addition to the above-described embodiments, various modifications can be made to the present invention. As the transmission 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 blunt semiconductor laser, and to apply frequency modulation by changing the resonator length. be. 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 be considered. Optical multiplexing section 6
In addition to a half mirror, it is also possible to use a fiber coupler or a diffraction grating. As the local oscillation light source 7, various lasers such as a gas laser and a solid-state laser can be used in addition to a semiconductor laser, 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 in place of the envelope detectors 15 and 16 as the electrical detector.

It is also possible to perform detection by combining the oscillation frequency of the local oscillation light source 7 with one frequency component of the signal light from the transmitting light source 1, and demodulate the signal from the amplitude or intensity information of the baseband signal. It is possible.

Furthermore, it is also possible to use a PIN-FFiT type optical receiver in which the photodetector 10 and the receiving circuit 11 are configured in a hybrid manner to reduce noise in the optical receiving section. Further, in the first embodiment, a band-pass filter 14 is used, but a high-pass filter, a resonator, etc. may also be used. In the second embodiment, a directional coupler 2 is used to separate the two beat outputs.
2 is used, however, a duplexer using a ring resonator or the like may also be used.

Further, although the embodiment is an example of a binary frequency shift key ink method, the method of the present invention can be applied to a multi-value frequency shift key ink method such as a four-value method in the same way as the binary frequency shift key ink method.

(3-8) Effects of the Invention As described in detail above, in the present invention, in an optical heterogain communication method that performs binary or higher frequency modulation, the low modulation frequency component and the high modulation frequency component of the intermediate frequency signal from the optical receiver By extracting and detecting them independently, changing the intensity ratio of each detection output, and combining them to extract the demodulated output, the influence of noise of high modulation frequency components generated in the optical receiver can be reduced, and high reception can be achieved. Sensitivity can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram for explaining the first embodiment of the present invention, and FIG. 2 (a) l (b) # (c) j (
d) j (e) is a frequency characteristic diagram for explaining the first embodiment, FIG. 3 is a plot diagram for explaining the second embodiment of the present invention, and FIG. 4 (a) t ( b) y (c
)y(d) is a frequency characteristic diagram for explaining the second embodiment. 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--Reception circuit, 12--Intermediate frequency signal, 13
．． 14...filter, medicine 15.16...
Envelope detector, 17t18...Detection output, 19...
Power combiner, 20... Demodulation output, 21... Intermediate frequency signal, 22... Directional coupler, 23... Low frequency
Big wave beat output, 24... High frequency beat output, 25... Low frequency amplifier, 26... High frequency amplifier,
27 28...filter. 71-2 Figure

Claims (1)

[Claims] The optical transmitter transmits a signal light modulated by an information signal, the optical multiplexer multiplexes the transmitted signal light with local oscillation light, and the optical receiver intermediates the multiplexed light. In an optical heterodyne communication method in which the intermediate frequency signal is converted into a frequency signal and a demodulated signal is extracted from the intermediate frequency signal in a demodulation section,
Modulation of the signal light in the optical transmitter is performed by binary or higher frequency modulation, and in the demodulator, each modulated frequency component in the intermediate frequency signal is independently extracted and detected, and the signal of each detected signal is An optical heterodyne communication method characterized by changing the intensity ratio of each detected signal according to the noise ratio, synthesizing the signal, and then extracting the signal as a demodulated output.