JPS62122344A - Optical transmission system - Google Patents

Abstract

PURPOSE:To expand the degree of freedom of an optical wavelength split multiplex transmission system by using a frequency conversion system to apply frequency conversion to a signal before it is inputted to an optical transmission section in advance so as to almost avoid the electric crosstalk between signals. CONSTITUTION:A frequency converter 16 applies the frequency conversion to a signal frequency conversion inputted from an information signal input terminal 10 into the frequency of f3-f2. That is, a signal is subjected to the frequency-conversion so as not to cause an electric interference between the signals inputted to optical transmission sections 3, 4 thereby obtaining a suffi cient isolation. An optical signal in a wavelength lambda1 is led to an optical recep tion section 6 and an optical signal in a wavelength lambda2 is led to an optical reception section 7 while being branched respectively. A signal passing through a band pass filter 18 is frequency-converted by the frequency of an oscillator 15 at the frequency converter 16 and the information signal of the frequency component f2 is outputted at a terminal 13. The signal is converted into an optical signal while electric isolation is applied in advance and the optical isolation is applied between optical signals by optical wavelength multiplex.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a system for transmitting a plurality of information signals in the same direction or in both directions using a plurality of different wavelengths using a single optical fiber.

[Background of the invention]

4MHz band color TV using a single optical fiber
2. Description of the Related Art Various methods are being considered for bidirectionally transmitting signals, digital signals, and the like by wavelength multiplexing.

Figure 1 is an example. This is 3-wave multiplex transmission,
9.10 is a 4MHz-TV analog video signal input terminal,
11 is a digital signal output terminal, 12.13 is 4MHz
-TV analog video signal output terminal; 14 is a digital signal input terminal; 3°4.8 is the wavelength λ1. λ
2. an optical transmitter having a semiconductor light emitting element of λ8, 6, 7.
5 are wavelengths λ1. λ2. 2-1.2-2 is an optical multiplexer/demultiplexer; an optical receiver having a light receiving element for receiving an optical signal of λ8;

1 is an optical fiber. In optical wavelength division multiplexing transmission as described above, wavelengths λ1. λ2. The wavelength interval of λ3 depends on the system configuration such as transmission signal, transmission direction, transmission distance, wavelength fluctuation of semiconductor light emitting element (initial wavelength setting accuracy, temperature fluctuation, bias current deterioration, spectrum fluctuation, etc.), cutoff characteristics of optical multiplexer/demultiplexer, When determining the characteristics of optical components such as return loss,
It is selected to satisfy the required amount of optical crosstalk attenuation under those conditions. For example, when using the characteristics of current optical components such as semiconductor light emitting devices and optical multiplexers and demultiplexers, the minimum wavelength interval is approximately 80 nm for unidirectional transmission in the short wavelength band, approximately 50 nm for bidirectional transmission, and Regarding wavelength bands, the wavelength interval is limited to about 140 nm for unidirectional transmission and about 90 nm for bidirectional transmission, so currently semiconductor light emitting devices must be used with great selection. However, it has become extremely expensive. Furthermore, since the optical multiplexer/demultiplexer is required to have attenuation of 40 dB or more in the stopband, it is necessary to use a plurality of interference film filters stacked one on top of the other. However, it is very expensive, and when stacking each sheet, it is necessary to give each sheet a slight angle of inclination, which increases assembly and adjustment costs.Furthermore, insertion loss in the passband increases. Therefore, there were problems such as the transmission distance being shortened.

[Purpose of the invention]

An object of the present invention is to provide an optical transmission system that solves the problems of the conventional system. In other words, it combines electrical crosstalk improvement using a frequency conversion method and crosstalk improvement using an optical wavelength division multiplexing transmission method.

The object of the present invention is to provide a transmission method that has an extremely simple configuration, low cost, and performs high quality information transmission.

[Summary of the invention]

Conventionally, optical wavelength division multiplexing has been used in optical communication as a method for simultaneously transmitting various different types of signals such as analog signals and digital signals, but in order to reduce crosstalk between these signals, wavelength spacing and optical The use of duplexers, semiconductor light-emitting elements, etc. must be carefully selected, which limits expandability and makes it difficult to reduce costs.

In the present invention, before inputting the signals to the optical transmitter, the signals are frequency-converted to shift each signal to a different frequency region, so that there is almost no electrical crosstalk between the signals. be. Therefore, the degree of freedom of the optical wavelength division multiplexing transmission system is expanded. In other words, since the wavelength interval of the semiconductor light emitting elements may be narrower than in the conventional method, wavelength fluctuations of each semiconductor light emitting device can be tolerated more than in the conventional method, and the cost increase due to conventional sorting can be reduced. I can do it. Furthermore, since the stopband characteristics of the optical multiplexer/demultiplexer can be relaxed, the number of interference film filters can be reduced and the configuration can be simplified, making it possible to significantly reduce costs.

[Embodiments of the invention]

FIG. 2 shows an embodiment of the optical transmission system of the present invention. This is the case of two-wavelength multiplexed unidirectional transmission. In this figure, parts with the same symbols as in FIG. 1 have the same functions. 15 has a frequency Cf5=ft> higher than the signal frequency component (fl) input from the information signal input terminal 9.
ft, for example f8=10 Ml(z, f x=
4 MHz) oscillator.

16 is a frequency converter, which converts signal frequency components (f z, f *<fa) input from the information signal input terminal 10
The frequency is converted to the frequency of fs fx.

That is, one of the signals input to the optical transmitters 3 and 4 is frequency-converted so that no electrical interference occurs between the signals, and sufficient isolation is obtained. The wavelengths λ1 and λ2 are set to be different from each other so that optical interference does not occur between the optical signals output from the optical transmitters 3 and 4. 1
9 is an optical multiplexer. 2-0 is an optical demultiplexer, and the wavelength λ
The optical signal of 1 is sent to the optical receiver 6, and the optical signal of wavelength λ2 is demultiplexed to the optical receiver 7. 17 is a low-pass electric filter, and the information signal of the frequency component (jl) is demultiplexed and respected. Only pass through. Reference numeral 18 denotes a band-pass electric filter, which passes only the signal having the frequency component of fs fx among the signals photoelectrically converted by the optical receiver 7 . This bandpass filter 1
The signal passed through 8 is sent to oscillator 15 by frequency converter 16.
The information signal of the frequency component (fz) is outputted to the terminal 13. In this configuration, the optical signals are converted into optical signals with electrical isolation provided in advance, and optical isolation is provided between the optical signals by optical wavelength multiplexing, so that a sufficiently large amount of crosstalk attenuation can be achieved on the receiving side. Conversely, the number of interference film filters can be reduced because the optical demultiplexer does not need to have a large amount of stopband attenuation. Moreover, the wavelength λ1. The wavelength interval of the λ2 semiconductor light emitting device does not have to be strictly limited as in the conventional case, and even if the wavelength interval becomes somewhat narrow, it is not a problem because electrical isolation is sufficiently large. Therefore, the wavelength selection of the semiconductor light emitting device is eased, making it possible to reduce costs.

FIG. 3 shows another embodiment of the optical transmission system of the present invention, which is a case of two-way multiplexed two-way transmission. In the same figure, the second
Components with the same reference numerals as in the figure have the same functions. The number of interference film filters in the optical demultiplexer 20-1 and 20-2 can be reduced, allowing for a simpler configuration and lower cost. Furthermore, since the interval between the wavelengths λ1 and λ2 may be narrower than in the conventional configuration, it is possible to suppress high costs by selecting semiconductor light emitting elements as in the conventional case. Furthermore, by reducing the number of interference film filters, it becomes possible to extend the distance.

FIG. 4 shows an embodiment in the case of three-wavelength multiplexed bidirectional transmission. In this figure, parts having the same reference numerals as in FIGS. 1 to 3 have the same functions. 22 is the frequency fδ
(fb<fs), and the information signal input terminal 14
This signal has a higher frequency than the frequency component (f4) of the signal. 23 is a band pass filter that passes only the frequency component (fs-fa). That is, the signal (f4) input from the information input signal terminal 14 is sent to the frequency converter 24.
The signal is converted into a frequency component of (fs fs) and sent to the optical transmitter 8. The output signal of the optical receiver 5 is input to a band pass filter 238. Bandpass filter 23
Then, only the signal of the frequency component (fIS-f4) is passed and input to the frequency converter 24. The frequency is then converted by the frequency converter 24, and an information signal having a frequency component of f4 is extracted at 11. However, 21-1
．． 21-2 is an optical multiplexer/demultiplexer.

FIG. 5 shows another embodiment of the optical transmission system of the present invention. This includes an optical transmitter 3 having semiconductor light emitting elements of almost the same wavelength,
This is a case of unidirectional transmission using 3'. In this case, only one optical receiving section 6 is required, and signals are selected and extracted from the output of the optical receiving section 6 using a low-pass filter 17 and a band-pass filter 18, respectively.

In the configurations shown in FIGS. 2 to 5, the wavelength λ.

j'z+ λS is a short wavelength band, λl and λ2 are short wavelength bands, and λ8 is a long wavelength band, λ1 and λ8 are short wavelength bands, and λ2 is a long wavelength band, λl.

One type of combination is possible, such as a configuration in which both fz and λ3 are in the long wavelength band. These combinations are designed to simplify the configuration of the optical multiplexer/demultiplexer and reduce costs.
Selected for long distance. Further, in the above embodiment, the information sent to the information input terminals 9, 10, and 14 is limited, but the present invention is not limited to this. for example,
Both signals may be analog signals, digital signals, or a mixture of analog signals and digital signals. The number of wavelengths multiplexed is also possible to be any number from 2 to more. The optical fiber 1 may be either a multimode optical fiber or a single mode optical fiber. The semiconductor light emitting device may be either a light emitting diode or a semiconductor laser. [Effects of the Invention] The present invention has the following effects.

(1) Conventionally, optical wavelength division multiplexing has been used as a method for simultaneously transmitting various different types of signals such as analog signals and digital signals, but in order to reduce crosstalk between these signals, wavelength spacing, optical It was necessary to carefully select and use wave devices, semiconductor light emitting devices, etc., which limited expandability. However, in the present invention, before inputting the signal to the optical transmitter, the signal is frequency-converted using a frequency conversion method.
Since there is almost no electrical crosstalk between each signal, the degree of freedom of the optical wavelength division multiplex transmission system can be expanded. For example, semiconductor light-emitting devices can tolerate wavelength fluctuations better than before, so there is no need to select and use them as in the past, which makes it possible to reduce costs.

(2) Since the stopband characteristics of the optical multiplexer/demultiplexer can also be relaxed, the number of interference film filters can be reduced and the configuration can be simplified, making it possible to significantly reduce costs. Furthermore, the reduction in insertion loss in the passband makes it possible to extend the distance.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a conventional optical transmission system, and FIGS. 2 to 5 are examples of the optical transmission system of the present invention. l...Optical fiber, 2-1.2-2.21-1°21
-2... Optical multiplexer/demultiplexer, 3, 4.8... Optical transmitter, 5
．． 6.7... Optical receiver, 9, 10.14... Information signal input terminal, 11, 12, 13... Information signal output terminal, 15.22... Oscillator, 16.24... Frequency converter, 17...Reduced pass filter, 18.23...
Bandpass filter, 19... optical multiplexer, 20, 20-
1.20-2... Optical demultiplexer.

Claims (1)

[Claims] 1. When a plurality of information signals are converted into optical signals and then transmitted unidirectionally or bidirectionally using a single optical fiber, the frequency regions of the respective information signals do not overlap. An optical transmission method that converts the frequency into optical signals and then transmits them. 2. The optical transmission system according to item 1, characterized in that the wavelengths of the optical signals are made different when converting each information signal into each optical signal. 3. The optical transmission system according to items 1 and 2, wherein each information signal is an analog signal, a digital signal, or a mixture of these signals.