JPS60190039A - Optical communication system - Google Patents

Abstract

PURPOSE:To obtain an analog optical communication system with low noise and distortion where the transmission quality is hardly deteriorated even at the long- distance transmission by branching an electric signal into at least >=two signals, allowing the said branch electric signal to modulate semiconductor light emitting elements having respectively different wavelengths and synthesizing each output of the said semiconductor light emitting element by means of an optical mixer or the like. CONSTITUTION:An original signal to be transmitted is inputted from an input terminal 1 and branched by an electric signal branching device 2. Each branch signal modulates semiconductor lasers having a respectively different oscillation wavelengths at electrooptic converting sections 3, 3'. After each modulated light is synthesized at an optical mixer 4, and the light is propagated through multi- mode optical fibers 5, 7. The propagated light is converted into an electric signal at an optoelectric converting section 8 and outputted to an output terminal 9. It is necessary for the semiconductor laser to be oscillated at different wavelengths respectively. It is possible by changing the mixed crystal ratio of the laser active layer. It is desirable for the interval of wavelength to be <=10nm taking the wavelength multiplex transmission into account.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Investigation] The present invention relates to improvements in communication systems using optical fibers and other optical transmission media, and particularly relates to optical communication systems suitable for transmitting analog signals.

[Background of the invention]

Conventionally, when transmitting an analog signal using an optical transmission medium such as an optical fiber, a method has been used in which a semiconductor laser is used as a light source and the semiconductor laser is directly modulated with the analog signal. Semiconductor lasers used for this type of communication include
One that oscillates in a single mode in both the longitudinal mode and the transverse mode is used. This is because, as a single laser, a single mode laser is the best in terms of noise characteristics and distortion characteristics.

However, when attempting to perform optical communication using this single mode laser and multimode optical fiber, noise and distortion called modal noise and modal distortion were newly generated, resulting in a phenomenon in which transmission quality was significantly degraded. This phenomenon is caused by the coherence of the semiconductor laser light and the propagation mode characteristics in the multimode fiber, and is explained as follows.

Single-mode laser oscillation light travels through a multimode fiber in various m-modes. Since there is an optical path difference between these modes, the light propagated by each mode interferes with each other at the fiber end face, producing an interference pattern called speckle. This speckle pattern fluctuates due to changes in the propagation mode due to fiber fluctuations, fluctuations in the semiconductor laser oscillation wavelength, and the like. If optical connector.

When a part of the speckle pattern is missing in a lens, a light receiver, etc., the fluctuation of the speckle pattern causes noise and distortion of the modulated light, and deteriorates the transmission quality.

The idea is that the best way to suppress this modal noise is to reduce the coherence of the laser.

A method has been developed that uses a semiconductor laser to oscillate in multiple modes. The main types include those that use a multi-mode laser, and those that convert a single mode laser into multiple modes by switching at a high frequency.

FIG. 1 shows the relationship between transmission quality and transmission distance when using this method.

When a single mode laser is used, noise increases significantly as the transmission distance becomes longer, as shown in C in the figure. In contrast, when using a multimode laser.

Or curve a when using high frequency superposition.

Like b, it hardly deteriorates. This is to avoid generating mortal noise.

However, if the required transmission quality is below the broken line d in FIG. 1, then either method is impossible if the transmission distance is long.

This is because a laser that oscillates in multiple modes has a different type of noise called mode partition noise, which is essential for a laser that oscillates in multiple modes.

As described above, when the transmission distance becomes long, it is impossible to perform high-quality transmission using conventional methods.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and provide an analog optical communication system with low noise and low distortion, which causes less deterioration in transmission quality even in long-distance transmission.

[Summary of the invention]

Comparing the advantages and disadvantages of multimode fiber transmission between a single mode laser and a multimode laser, the disadvantage of a single mode laser is modal noise. On the other hand, multi-mode lasers do not have the problem of modal noise, but multi-mode lasers have a problem with mode distribution noise.

From the above points, in order to perform high-quality long-distance transmission,
It is clear that it is sufficient to use a light source that oscillates in the multi-e mode and has no mode partition noise.

It is difficult to achieve these two conditions with a general multimode laser or a laser with superimposed crystal frequencies. However, by using a plurality of single mode lasers and causing them to oscillate at different wavelengths, a substantially multimode light source without mode distribution noise can be easily realized.

In the present invention, a plurality of (two or more) semiconductor lasers emitting 5 different wavelengths are modulated with the same electrical signal, and the modulated light emitted from the semiconductor lasers is combined by a light mixing means such as a light combiner. The combined light is transmitted through an optical transmission medium such as a multimode optical fiber and received. The received optical signal is photoelectrically converted by a light receiving element such as a photodiode to obtain an original signal.

According to the method of the present invention, since a light source that oscillates in multiple modes is used, the coherence of the light source is low, and problems of modal noise and modal distortion do not occur. Therefore, long-distance multimode fiber transmission is possible.

On the other hand, since each mode component of the light source is a single mode laser, there is no problem of mode distribution noise. Therefore, it has low noise as a light source.

As described above, according to the method of the present invention, long-distance transmission using a multimode fiber is possible with low noise and low distortion.

[Embodiments of the invention]

The present invention will be explained in detail below using examples.

FIG. 2 is an example of a transmission line configuration according to the present invention.

An original signal to be transmitted is input manually from an input terminal 1 and is branched by an electrical signal splitter 2. Each branch signal modulates a semiconductor laser having a different oscillation wavelength in the electro-optical converters 3 and 3'.

Each modulated light beam is combined in a light mixing section 4 and then propagated through a multimode optical fiber. 5 and 7 are multimode optical fibers, and 6 is an optical connector. The propagating light passes through the optical-to-electrical converter 8
The signal is converted into an electrical signal at the output terminal 9 and output to the output terminal 9.

In the 4th iIIIJ configuration, three or more electrical-optical converters are provided.
It is also possible to create multiple modes. In this case, there is an effect that the emitted light output increases and the C/N ratio becomes further advantageous.

In wood lamination, semiconductor lasers must oscillate at different wavelengths. This is possible by changing the mixed crystal ratio of the laser active layer. The wavelength interval is desirably 10 r+ rn or less when considering the case of wavelength division multiplexing transmission. Further, in consideration of the oscillation wavelength interval of a general multimode laser, a sufficient effect can be expected with a setting of 0.5 nrylW, degrees.

FIG. 3 is another embodiment of the invention. The electrically branched signal is sent to the input terminal 11. ．． 11', and the semiconductor laser is driven by the drive unit 12.12']:'1.13'
Modulate. These semiconductor lasers have the same active layer. 15 is an optical star coupler that combines two lights. The oscillation wavelengths of the two semiconductor lasers are controlled by humidity controllers 14 and 14'. This is due to the fact that the semiconductor laser oscillation wavelength has temperature dependence as shown in FIG. The temperature characteristics shown in Figure 4.

There is a region where the oscillation wavelength changes smoothly, such as spectral line S, for example, tl, and a region where it changes rapidly. In a rapidly changing region, for example tl, two or more modes alternately oscillate as shown in the spectrum line S2 at the top of the figure, resulting in instability and a lot of noise. In actual use, it is necessary to control the temperature to avoid such areas.

According to the method of this embodiment, the semiconductor laser can be used in a low noise region, and the two oscillation wavelengths can be set arbitrarily by controlling the temperature, which is the most effective method for suppressing modal noise. There is an efficiency in that the oscillation wavelengths can be spaced apart.

FIG. 5 shows another embodiment of the invention. In this example, the oscillation wavelengths of the two lasers are changed by shifting the laser resonator lengths. 21, 2 Bi is a laser active layer, 22.23
is an electrode, and 24 is an insulator.

According to this implementation, it is possible to easily integrate lasers with different wavelengths, so even though multiple lasers are required, they can be used in the same way as using a single element. Has characteristics.

In addition to the above, various modifications are possible in carrying out the present invention as a method for causing a semiconductor laser to oscillate at a different wavelength, such as a method using an external resonator, a method of applying pressure to the laser, etc.

In addition to the above-mentioned method of using an optical star coupler as a means of light synthesis, methods of using a prism or a diffraction grating,
Possible methods include creating a structure in which the laser active layers are placed close to each other. In the case of a structure in which the laser active layers are placed close to each other, the emitted light is condensed by, for example, one lens, and is made to enter the optical fiber.

So far, only the case where a completely single mode laser is used has been described. However, the approach according to the invention can be used even with lasers that are not completely single mode impaired. That is, even in a multimode laser, if the optical output of the main oscillation mode is sufficiently larger than the optical output of other oscillation modes, the mode distribution noise will be small, while the influence of the King-Dall noise will be large. In optical communication using such a signal, the system according to the present invention can be applied to a large effect.

Further, when a single mode fiber is used as the optical fiber/C fiber, the method of the present invention has the effect that a large amount of electric power can be input into the optical fiber.

Further, even when a light emitting diode is used as a semiconductor light emitting element, the method of the present invention has the effect that a large amount of light power can be input into an optical fiber.

In each of the above embodiments, analog optical communication was mainly dealt with. However, the problems of modal noise and mode distribution noise are similar problems in optical communication of digital signals. The optical communication system according to the present invention is
This is also an extremely effective method for solving the above problems in digital optical communications.

〔Effect of the invention〕

As described above, according to the present invention, a light source can be made into multiple modes without generating any mode distribution noise. That is, a light source with low noise and low coherence can be obtained. Due to the low coherence of the light source,
Even during multimode fiber transmission, modal noise, modal distortion, etc. do not occur, and high-quality transmission can be performed over long distances.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the correlation between transmission distance and noise of various conventional optical fiber transmission systems, Fig. 2 is a block diagram showing an example of a transmission line configuration according to the present invention, and Fig. 3 is a diagram showing a transmission line configuration example according to the present invention. A block diagram showing an embodiment of the electric-optical conversion section and the photosynthesis section, FIG. 4 is a temperature characteristic curve diagram of a semiconductor laser, and FIG.
The figure is a perspective view of essential parts showing another embodiment of the electrical-optical converter. 2... Electrical branching section, 3, 3'... Electrical-optical converting section, 4... Photosynthesizing section, 5, 7... Optical fiber, 6...
・Optical connector, 8...optical-electrical converter, 12.12'
... Semiconductor laser drive unit, 13.13'... Semiconductor laser, 14.14'... Temperature control unit, 15... Optical star cube Y 1 Figure Y4- Zuon Kan

Claims (1)

[Claims] 1. '1' In an optical transmission method in which a mind-body light emitting device is modulated with an electrical signal and the optical output of the semiconductor light emitting device is transmitted into an optical fiber, the electrical signal is modulated by at least two electrical signals. I”
-, the branched electric signals modulate semiconductor light emitting elements each having a different oscillation wavelength, the respective output lights of the semiconductor light emitting elements are combined by an optical mixer, etc., and the combined light is transmitted through an optical fiber. , an optical communication system characterized in that the transmitted optical signal is subjected to optical-to-electrical conversion by a light receiver. 2. W of patent claim 4, characterized in that each of 4 uses at least two temperature-controlled semiconductor light emitting devices;
1. The optical communication method described in item 1. 3. The optical communication system according to claim 1'A, characterized in that at least two or more different semiconductor lasers are used.