JPH028294B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical communication system using a coherent light source.

In recent years, interest in so-called coherent optical communication has increased due to improvements in the coherence of semiconductor lasers and reductions in loss in long-wavelength optical fibers.
It is thought that a new technological field will be formed by combining it with IC technology.

One of the advantages of coherent optical communication is that it can utilize an extremely wide frequency range. However, unless a method is developed for transmitting and receiving data without directly handling this wide frequency range in electrical circuits, it will not be possible to take full advantage of this advantage. That is, the frequencies that can be handled by electrical circuits are at most gigahertz, and it is difficult to completely handle frequencies as high as terahertz in the optical region.

The present invention focuses on the recent dramatic improvement in the coherence of semiconductor lasers, and by frequency modulating the carrier laser light with an electrical signal and transmitting it,
The object of the present invention is to provide an optical communication system that can effectively utilize the broadband properties of the optical domain without requiring a broadband electrical circuit.

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

FIG. 1 is a schematic diagram of an embodiment of the present invention. This example has a baseband bandwidth of 30MHz ~
It shows a method for long-distance transmission of high-definition moving images that reach 60MHz.

First, a high-definition image electric signal introduced into the input terminal 1 on the transmitting side A is applied to the wavelength-tunable carrier laser oscillator 3 via the drive electric circuit section 2, and modulates its oscillation frequency, that is, the carrier laser frequency. and create an optical FM signal. this light
The FM signal is guided to a 3 dB directional optical coupler 5 together with a pilot laser beam from a pilot laser oscillator 4 whose oscillation frequency is sufficiently stabilized. The photodetector 6 combines the optical FM signal extracted from one output port of the optical coupler 5 with the pilot signal.
The average oscillation frequency of the carrier laser oscillator 3 has a certain frequency difference with the oscillation frequency of the pilot laser oscillator 4. Control as follows. Then, the combined light is extracted from the other port of the 3dB directional optical coupler 5.
The FM signal and the pilot laser beam are transmitted to the receiving side B via the optical fiber cable 8.

On the other hand, on the receiving side B, the received light and the local laser light from the wavelength-tunable local laser oscillator 9 are combined by a 3 dB directional optical coupler 10. The photodetector 11 detects the average difference frequency between the received light extracted from one output port of the optical coupler 10 and the local laser light, and detects the oscillation of the local laser oscillator 9 via the AFC processing section 12. The frequency is maintained at a constant frequency difference with respect to the frequency of the pilot laser beam included in the received light, and is made to match the average frequency (center frequency) of the carrier laser beam.

The received light and the pilot laser light taken out from the other output port of the 3 dB directional optical coupler 10 are guided to the optical discriminator 13 after phase face matching. The optical discriminator 13 is constituted by a narrow band optical filter consisting of a Fabry-Perot resonator or the like having frequency characteristics as shown in FIG. Here, the center frequency c of the carrier laser beam is placed at the center of the slope of the frequency characteristic of the optical discremator 13 made of a narrow band optical filter. Therefore, the carrier laser light is transmitted to this optical discremator 1 along with the local laser light.
3 and guided to a photodetector 14, where it is subjected to homodyne optical detection. The output of the photodetector 14 is taken out as a demodulated electrical signal to an output terminal 16 through a receiving circuit section 15. Pilot laser light frequency
p is placed at the cutoff frequency of the optical discriminator 13 and removed. In this way, the pilot
By removing the laser beam frequency fp component, occurrence of interference due to a beat between the pilot laser beam frequency fp component and the carrier laser beam frequency fc component is prevented. In some cases, in order to remove residual portions of the pilot laser beam,
A low pass filter may be included in the receiving circuit 15. Also, if necessary, from the receiving circuit section 15
The AGC information is extracted, and the local laser oscillator 9 outputs the local laser oscillator 9 using the AGC control unit 17.
An AGC function may be added to control the intensity of the laser beam.

As explained above, according to the present invention, the frequency of the carrier laser light is modulated by an electrical signal and transmitted, so the broadband property of the laser light can be effectively utilized without requiring a broadband electric circuit. This makes it possible to take full advantage of the inherent advantages of coherent optical communication.

Furthermore, in the present invention, the optical discriminator is constructed by an optical narrowband filter such as a Fabry-Perot resonator, and the center frequency of the carrier laser beam is arranged at the slope of the frequency characteristic of this narrowband filter. By placing the frequency of the pilot laser beam at the cutoff frequency of the bandpass filter, the optical discriminator simultaneously discriminates the frequency of the optical FM signal and removes the frequency component of the pilot laser beam. Therefore, the pilot
No special optical elements are required to remove the frequency components of the laser beam, which simplifies the configuration of the device and reduces optical loss.Furthermore, the baseband filter in the receiving circuit is free from the effects of the frequency components of the pilot laser beam. Since there is no need to take this into account, there is an advantage that the design becomes easier.

In addition, in the example, an optical FM method that does not use premodulation is described, but optical FM methods that involve premodulation such as pulse encoding etc.
The present invention can be similarly applied to the FM method. Furthermore, it goes without saying that a pre-equalization technique on the transmitting side can be applied to compensate for the nonlinearity of the optical discriminator.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention, and FIG.
The figure is a frequency characteristic diagram of the optical discriminator in the same embodiment. A...Transmission side, B...Reception side, 1...Electric signal input terminal, 2...Drive electric circuit section, 3...Carrier laser oscillator, 4...Pilot laser oscillator, 5, 10...3dB Directional optical coupler, 6,1
1, 14... Photodetector, 7, 12... AFC processing section, 8... Optical fiber cable, 9... Local laser oscillator, 13... Optical disc liminator, 15... Receiving circuit section, 16 ...Demodulated electric signal output terminal, 17...AGC control section.

Claims (1)

[Claims] 1 On the transmitting side, the frequency of the carrier laser beam is modulated with an electrical signal, combined with a pilot laser beam whose frequency is a predetermined distance from the frequency of the carrier laser beam, and transmitted via an optical fiber cable, and on the receiving side, the frequency of the carrier laser beam is modulated with an electrical signal. Using the frequency of the pilot laser beam as a reference, the frequency of the local laser beam is tuned to the frequency of the carrier laser beam in the received light, and the local laser beam and the received light are passed through an optical discriminator to a photodetector. In the optical communication method, the optical discriminator is configured by an optical narrow band filter, and the center frequency of the carrier laser beam is set at the center of the slope of the frequency characteristic of the filter, and An optical communication system characterized in that the frequencies of the pilot laser beams are arranged at the cutoff frequencies of the filters.