JPS6139775B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Currently, high-output semiconductor lasers are generally used for intensity modulated long-distance optical transmission. Semiconductor lasers have poor linearity in current vs. optical output characteristics, and when performing optical frequency multiplexing, problems arise in the occurrence of beat disturbances due to cross modulation and intermodulation due to the nonlinearity. Moreover, the linearity of the current vs. optical output characteristic of the semiconductor laser is affected by the lot, and as a result, the number of multiplexed channels is restricted. Currently, the number of transmission channels is determined by the number of semiconductor laser lots, making system design extremely difficult. Situations arise where you have to wait until you get something good.

The present invention solves the above problems and provides an excellent transmission system that is not affected by variations in the characteristics of semiconductor lasers and is therefore very useful for multiplex transmission. One embodiment will be described below based on the drawings.

FIG. 1 shows an example of a device using this system, in which a plurality of mixed carrier waves are input to terminal 1. This signal is demultiplexed by a demultiplexer 2 composed of a bandpass waveform, and the demultiplexed carrier signals are divided into wavelengths of the emission spectrum λ ₁ , λ ₂ . . . λ _o
The light is input to a different semiconductor laser 3 and electrical-to-optical conversion is performed. Here, the intensity-modulated optical output signals are combined through dedicated fibers 4, and input to a light combiner 5 using a grating, a dielectric multilayer film, or the like. Since the center wavelength of the light passing region of the optical combiner 5 coincides with the oscillation spectrum wavelengths λ ₁ , λ _{2 ,} . Each intensity-modulated optical signal is sent out from the output of the optical combiner 5 through one transmission fiber 6. The transmitted optical signal is sent to an optical demultiplexer 7 which has the same configuration as the optical combiner 5 but has only the input and output terminals connected in reverse.
is input. Since this optical input signal is intensity-modulated by semiconductor lasers having different oscillation wavelengths, it is demultiplexed by the optical demultiplexer 7 and optical-to-electrical conversion is performed by the respective light receiving elements 9. The optical signal transmitted up to this point is twice or three times the total fundamental wave due to the nonlinearity of the current vs. optical output characteristic of the semiconductor laser.
2nd and 3rd order beat distortion occurs, deteriorating the quality of the signal. The harmonic distortion of the distorted electric signal converted by the light receiving element is removed by the mixer 10 which is constituted by a band pass wave generator. Here, the mixer 10 is set to the center frequency of the carrier wave similarly to the branching filter 2. Then, an electrical signal of only the fundamental wave is obtained at the terminal 11, respectively. In this way, the harmonic components generated by the light emitting elements and the like are removed by the electrical system, so the structure can be very simple.

Next, an example will be shown in which the above invention is used in a system.
For example, to explain the case of optical multiplex transmission of VHF band television signals, the current television frequency arrangement in Japan is as shown in Figure 2. When a signal with such a frequency arrangement is applied to a non-linear element, in the case of the Kanto channel, low band CH1 and CH3
The secondary beat due to the sum of the video carriers causes a beat of 1.25MHz with the video carrier of CH8, and
and twice the video carrier of CH3 are CH6 and CH3, respectively.
It generates a beat of 0.75MHz and 0.25MHz with the video carrier of CH10. This beat disturbance becomes a striped pattern on the television screen, which is very unpleasant. The apparatus of this embodiment is shown in FIG. 3. First, a television signal input from an antenna 20 is split into a low band and a high band by a splitter 21, and a semiconductor laser is driven by separate amplifiers 22 and 23, respectively. Amplified until possible. Then, the television signal split into low band and high band is applied to the semiconductor lasers 24 and 25, each having a different oscillation spectrum wavelength, and is intensity-modulated. The optical output signals whose intensity is modulated by the oscillation spectrum wavelength are transmitted through fibers 26 and 27 coupled to semiconductor lasers 24 and 25, respectively.
The signals are input to the optical combiner 28, mixed, and transmitted using the transmission fiber 29. In the optical receiver, the transmitted optical signal is demultiplexed into a low band signal and a high band signal by an optical demultiplexer 30, and outputted to fibers 31 and 32, respectively. Optical signals containing harmonic distortion components are optical-to-electrically converted by the light receiving elements 33 and 34 coupled to the respective fibers 31 and 32, and then sent to the respective amplifiers 3.
5 and 36, harmonic distortion components are removed and mixed by a low-band and high-band wave generator 37. Then, only the fundamental wave component is input to the television 38.

FIG. 4 shows spectral waveforms corresponding to various parts of the system shown in FIG. 3. In the spectrum waveform shown in the upper part of the figure, 40 is the antenna 20
41 is the low band signal branched by 21, 42 is the high band signal, 4
3 is a transmission signal distorted by the non-linearity of the current vs. light output characteristics of the semiconductor lasers 24 and 25 and mixed by the optical combiner 28; 44 is a low band signal demultiplexed by an optical demultiplexer 30; 45 is a high band signal; 46
is the television signal waveform mixed by the waver 37. In the above embodiment, the distortion caused by the non-linearity of the current versus optical output characteristic of the semiconductor laser is removed by passing the signal through the wave generator 37, and only the fundamental components of both the high band and low band signals can be regenerated. Therefore, optical multiplex transmission of television signals becomes possible regardless of the nonlinearity of the semiconductor laser.

As described above, according to the present invention, it is possible to regenerate the original signal regardless of the characteristics of each individual semiconductor laser in response to distortion caused by the nonlinear operating characteristics of the semiconductor laser during optical multiplex transmission. Therefore, it is possible to provide an excellent optical multiplex transmission system that can realize optical multiplex transmission with a simple configuration.

[Brief explanation of the drawing]

Fig. 1 is a basic configuration diagram for realizing the optical multiplex transmission system of the present invention, Fig. 2 is a frequency arrangement diagram of a television signal, and Fig. 3 is a configuration diagram showing an embodiment for transmitting a television signal. , FIG. 4 is a transmission spectrum diagram of the main part shown corresponding to FIG. 3. 1...Terminal, 2...Demultiplexer, 3...Semiconductor laser, 4...Fiber, 5...Optical combiner, 6...Transmission fiber, 7...Optical demultiplexer, 8...Fiber,
9... Light receiving element, 10... Mixer.

Claims (1)

[Claims] 1. A frequency-multiplexed carrier signal is input to a demultiplexing means, each demultiplexed output is made into an optical output by a plurality of light emitting elements having different emission spectrum wavelengths, and the plurality of optical outputs are optically combined. and guide it to a transmission means, input it to an optical demultiplexer on the other side of the transmission means to produce a demultiplexed optical output, and extract this optical output as an electrical output by opening the same number of light receiving elements as the light emitting elements, An optical multiplex transmission system characterized in that the electrical output is passed through a plurality of corresponding band-pass transducers and then mixed to obtain the general transmission signal.