JPS635633A - Optical multivalued communication system - Google Patents

Abstract

PURPOSE:To attain optical multivalued communications without being affected by the nonlinear modulation characteristics of semiconductor lasers and to expand transmission capacity by providing plural modulation light generation means outputting light subjected to binary intensity modulations at different amplitudes and a light multiplexing means multiplexing modulated light. CONSTITUTION:The semiconductor lasers 3 and 4 have approximately the the same oscillation wavelengths and intensity-modulated by modulation currents 1 and 2. After the timings of the modulated currents are synchronized, they are poured into the semiconductor lasers 3 and 4, from which the modulated light beams 9 and 10 with an amplitude ratio of 1 vs 2, 8Gb/S and NRZ codes come out. Said beams 9 and 10 are made incident on optical fibers 12 and 13 through a lens 5, multiplexed by a multiplexer 6 with a branching ratio of 1 vs 1 so as to acquire the synchronization of their timings, and transmitted to an optical fiber 7. A signal light 8 obtained in such a way ends up a quaternary NRZ code at a transmission speed 16 Gd/S.

Description

[Detailed Description of the Invention] [Prior Art] Currently, efforts are being made to increase the capacity of optical communication systems using semiconductor lasers. High-speed modulation experiments are being conducted (Takano et al., ``Basic Experiments on 8Gb/S Optical Modulation'', 1985, Institute of Electronics and Communication Engineers). However, using current electric circuits, semiconductor lasers, optical fibers, etc. (There is a limit in terms of the response speed of each device), and new methods or device proposals and development will be required.

An amplitude modulation multi-level code communication system is one effective means for realizing a large capacity in a communication system where the transmission capacity is limited by the response speed of the device. In this multi-level code communication system, for example, when two binary codes are converted into one four-level code, twice the capacity can be transmitted at the same modulation rate. Therefore, by using a quaternary code, it is possible to double the transmission capacity using a device that has the same response speed as that required for binary code transmission.

Furthermore, if the number of multilevels is further increased to four or more, the transmission capacity can be dramatically increased.

By the way, when directly modulating a light source with a multilevel code, the light source is required to have linearity in output light level with respect to a modulation signal. However, since normal semiconductor lasers have nonlinearity,
Until now, optical multilevel communication systems have been studied using light-emitting diodes, which have a slow response speed but good linearity.

In other words, in high-speed transmission systems using semiconductor lasers, currently,
Only binary codes are used, and if the light source is directly modulated by a multi-level modulation signal such as that used in conventional light-emitting diode drives, the relaxation oscillation of the semiconductor laser and the nonlinearity of the output level with respect to the injection current will occur. However, the advantages of multi-level code transmission could not be obtained because the influence of the modulated light is noticeable and causes waveform deterioration. No method has been proposed so far that can avoid the effects of this nonlinearity in multilevel code transmission.

[Problem that the invention seeks to solve]

The present invention proposes an optical multilevel communication system using a semiconductor laser that avoids nonlinearity in the output light level of a light source with respect to a modulated signal.

It is configured to include a plurality of modulated light generating means for outputting modulated light that is binary intensity modulated in width, and an optical multiplexing means for multiplexing the modulated light.

[Effect]

Each of the modulated lights before being combined is produced, for example, by directly modulating a semiconductor laser with a binary code.

When modulating with a binary code, the nonlinearity of the semiconductor laser does not appear significantly. Therefore, the influence of nonlinearity does not appear on the multilevel signal light obtained after multiplexing.

For example, when producing a four-value signal light using this method,
It is sufficient to modulate two semiconductor lasers so that the amplitude ratio of output modulated light is 1:2, and combine the modulated lights with the modulation phases matched.

〔Example〕

FIG. 1 shows the configuration of a first embodiment of the present invention.

The embodiment shown in FIG. 1 is an example in which the invention is applied to an optical four-level communication system. The semiconductor laser 3 and the semiconductor laser 4 have substantially the same oscillation wavelength, and are intensity-modulated by a modulation current 1 and a modulation current 2, respectively. The waveforms of the modulation current 1 and the modulation current 2 are two series of binary NRZ codes with an amplitude ratio of 1:2, as shown in the eye pattern in Fig. 1, and the bit rate of each series is 8Gb. /S. These modulated currents are injected into the semiconductor lasers 3 and 4 after their mutual timings are synchronized. As a result, from the semiconductor laser 3 and the semiconductor laser 4, the mll&f ratio is 1:2.8 Gb/S, the NRZ code O modulated light 9 and the modulated light 1
0 is emitted. After passing through the lens 5, these modulated lights are input into optical fibers 12 and 13, respectively. The modulated light 9 and the modulated light 10 that have entered the optical fibers 12 and 13 are combined by a multiplexer 6 with a branching ratio of 1:1 so that the mutual modulation timings are synchronized, and the optical fibers 12 and 13 are combined by a multiplexer 6 with a branching ratio of 1:1. be included. The signal light 8 thus obtained is shown in the eye pattern shown in FIG. K, transmission speed 1f 16 Gb/So 4 value N RZ code.

By using the 16 Gb/S 4-level NRZ signal light obtained in this way for transmission, it is possible to eliminate the effects of nonlinearity of the light source to areas that have not been possible due to band limitations of devices, etc. It is possible to increase transmission capacity without receiving any damage.

Note that the high-speed four-value identification circuit used in the receiver can be realized with the same configuration as that used in the conventional optical multi-value communication system.

FIG. 2 shows the configuration of an embodiment of the present invention.

The embodiment shown in FIG. 2 is a modification of the embodiment shown in FIG. 1 for short-range communication. Since the modulated light 9 and the modulated light 10 have substantially the same wavelength, if both have the same polarization component, beat noise of these two lights will appear in the receiver in short-range communication. In the embodiment shown in FIG. 2, a polarization adjuster 11 is added to prevent the generation of this beat noise, and the polarization direction of the modulated light 9 is made perpendicular to the polarization direction of the second modulated light 1°. It has the function of

Various other modifications are possible to the embodiments of the present invention. For example, by further increasing the number of modulation light sources, 8-value or 16-value
A value signal light can also be generated.

Further, it is also possible to use an unused boat among the two output boats of the multiplexer 6 as an output terminal for the monitor light for adjusting the amplitude of the modulated light.

If fiber dispersion can be ignored, there is no restriction on the wavelength of each light source. In this case, beat noise can be reduced by setting each wavelength to a different value.

〔Effect of the invention〕

As described in detail above, when the present invention is used, it becomes possible to perform optical multilevel communication without being affected by the nonlinear modulation characteristics of semiconductor lasers, and as a result, it becomes possible to perform optical multi-level communication, which has not been possible until now. This has the effect that the transmission capacity can be expanded to an area that was not previously available, and also has the effect that multilevel modulated light can be obtained using only conventional binary code modulation technology.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention, and FIG. 2 is a block diagram showing a second embodiment of the present invention. 1.2...Modulation current, 3,4...Semiconductor laser, 5...Lens, 6...Multiplexer, ? , 12.13...optical fiber, 8...
...Signal light, 9.10...Modulated light, 11...
...Polarization adjuster. 3. 4 2”yo* 84 L -su゛ 3
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Claims (1)

[Claims] An optical multilevel communication system comprising: a plurality of modulated light generating means that output modulated light that has been binary intensity modulated with different amplitudes; and an optical multiplexing means that multiplexes the modulated light.