JPS6027458B2 - Optical multiplex relay transmission system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a wavelength division multiplexing optical transmission system for transmitting modulated lights of a plurality of different wavelengths through one optical transmission line (fiber).

Optical wavelength division multiplexing communication, which increases the transmission capacity in one transmission line, is an effective method when the transmission line price is high or when the transmission capacity is limited due to the band limit of the transmission line.

Also, if the upstream and downstream signals are transmitted by wavelength multiplexing, 1
Bidirectional transmission is possible using a real transmission line. FIG. 1 shows a configuration diagram of wavelength multiplexed optical communication.

The output lights of the electro-optical converters 51, 52, and 53, each of which modulates the optical intensity of a plurality of lights of different wavelengths with electrical signals, are combined by the optical coupler 4 and sent to one transmission line 1. The optical multiplexed signal that has passed through the transmission line is separated into wavelengths by a demultiplexer 2, and then enters optical-to-electrical converters 61, 62, and 63, where the optical signal is demodulated into an electrical signal by a light receiving element. The intermediate repeater 7 performs regenerative repeating or non-regenerative repeating of the demodulated electrical signal, and then modulates the optical intensity of wave light sources with different wavelengths and transmits the modulated signals through the coupler 4 onto one transmission line. 31, 32 in Figure 1
, 33 represents an optical repeater which repeats the optical input signal after photoelectric conversion, modulates the light source again, and outputs modulated light. The main causes of transmission loss in fibers used as optical transmission lines are absorption loss and scattering loss due to the fiber material.

Both are highly dependent on wavelength. Scattering loss is inversely proportional to the fourth power of wavelength. For example, if the wavelength becomes 10% longer, the loss increases by 2 dB'. The absorption loss is greatly influenced by the fiber material, and the loss is high near the wavelength of 9500A.
Among the factors that determine the fiber coverage, refractive index dispersion is also wavelength dependent. At wavelengths around 13,000 A, there is no significant band deterioration due to refractive index dispersion, and at shorter wavelengths, the band becomes narrower as the wavelength becomes shorter. A PIN photodiode or an amparanche photodiode (abbreviated as APD) as a photoelectric conversion element for optical communication also has different wavelength characteristics of quantum efficiency depending on the material used. For example, a photodetector made of silicon has the highest quantum efficiency near a wavelength of 8000A, and the quantum efficiency decreases by 4.0 at wavelengths longer or shorter than 8000A. As described above, the loss and bandwidth of the transmission path differ depending on the wavelength, and the characteristics of the light receiving element are also wavelength dependent.

Therefore, in wavelength division multiplexing optical communication in which a plurality of signals are transmitted using a plurality of lights of different wavelengths as carrier waves, a difference occurs in the quality of each received signal. When transmitting the same type of signal at each wavelength, the transmission quality at the wavelength with the greatest deterioration is designed to be the desired quality, and the wavelength with the least deterioration results in excessive quality or excess margin, resulting in poor transmission path utilization efficiency. The purpose of the present invention is to equalize the transmission quality of a plurality of signals transmitted and received by an optical wavelength division multiplexing method.According to the present invention, a wavelength division multiplexing optical communication method that utilizes the transmission characteristics of a transmission path without waste is realized. can be provided.

The present invention is characterized in that, in optical wavelength division multiplexing communication, a light emitting source with a wavelength that causes a large loss in a fiber, a light receiver, and a light receiver with a different wavelength are used to reduce the loss.

Light emitting sources include Caesar diodes (hereinafter abbreviated as LD) and light emitting diodes (hereinafter abbreviated as LED).

Since the output light of the LD is coherent and the light emitting portion is small, it can be focused on a small area by the lens system. Therefore, the light from the LD can be efficiently coupled to the core portion (with a diameter of several hundred micrometers or less) which is the light transmitting portion at the center of the fiber.
On the other hand, LEDs have a large light emitting area and are not coherent light, so it is difficult to focus the light into a small size even using a lens system, and the efficiency of coupling LED light to the core is 4. Furthermore, since the light emitting power is greater in the LO, the optical power input to the fiber is greater when the LD is used as the light source. Usually L
The optical power coupled into the fiber when an ED is used is 1 or more lower than that of an LD. Furthermore, both LEDs and LDs are elements that convert current into optical power, and the temperature change in the electrical-optical conversion characteristics is greater in LDs. For LEDs, temperature compensation can be achieved with a simple circuit, but for LDs, a complicated control circuit for temperature compensation is required. LED is also better in terms of lifespan.
longer. Next, a comparison will be made between a Pm photo diode (hereinafter abbreviated as PIN) and an avalanche photo diode (hereinafter abbreviated as APD) used as a light receiver.

Since the APD increases the current converted from the optical power due to the avalanche multiplication effect, the S/N required for relaying can be ensured even when the input optical power is small. APD is 100 to several 10
It is used as a reverse bias at 0V, and the degree of avalanche multiplication varies greatly depending on the temperature. On the other hand, since Pm does not have an avalanche multiplication effect, a sufficient S/N ratio cannot be obtained when a small amount of light is input. However, when the reverse bias voltage is 100 V or less, there is little temperature change in the optical turtle conversion characteristic. As described above, since the LD has a large output optical power, it is suitable as a light source for wavelengths that have a large loss in the fiber.

When an LED is used as a light source with a wavelength that causes less loss in the fiber, the circuit can be simplified because it can be operated with a simple temperature compensation circuit. Although an APD is suitable as a light-receiving element for wavelengths with large losses, it requires a high-voltage reverse bias circuit and a high-power temperature compensation circuit. Since the PIN simplifies the circuit, it is advantageous to use it in a low-loss section. Second
An example is shown in the figure.

Here, an example will be shown in which light of four different wavelengths is transmitted through one fiber. 33 1, 33 3 is LD, 3 32
, 3 34 is an LED, and the wavelength is around 8000A.
The length increases in the order of 31, 332, 333, and 334.

Therefore, the loss in the fiber becomes smaller in this order. L.D.
, the optical output from the LED is coupled to one fiber 1 by a coupler 4, and the fiber output is split into wavelengths by a splitter 2, and is sent to 331' and 311, 332 to 312, and 333 to 3.
13 and 334 are separately coupled to 314, respectively. 31
1,312 represents an APD, and 313,314 represent a PIN.

LD and APD are used for the wavelengths with the greatest loss in the fiber, and LEDs are used in the wavelengths with the least loss in the fiber.
, using a PIN. For other wavelengths, LD, PIN, LED, and APD are used. That is, a light-emitting element and a light-receiving element that match the characteristics of the transmission path are used. By adopting a configuration that uses a PIN and an LED, the circuit for compensating temperature characteristics is simplified and the circuit scale is reduced. As is clear from the above explanation, if optical wavelength multiplexing communication is performed by appropriately selecting the type of light receiving element and light emitting element for each wavelength, a highly reliable system with a small circuit scale can be constructed as a whole. .

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[Brief explanation of drawings]

Figure 1 is a configuration diagram of a wavelength multiplexing optical communication system that uses optical carrier waves of four different wavelengths in one optical transmission line, and Figure 2 shows a light receiving element and a light emitting element matched to the wavelength of the modulated light of the present invention. An example of a relay method using . 331 and 333 are LDs, 332 and 334 are LEDs, 4 is a coupler, 1 is a fiber, 2 is a duplexer, 3 1 1, 3
1 2 is APD, 3 1 3, 3 1 4 is PIN. Figure 1 Figure 2

Claims (1)

[Claims] 1. In a system in which multiple signals are optically multiplexed onto one transmission path using multiple modulated lights with different wavelengths and then relayed, different signals are used to match the transmission loss characteristics of the transmission path with respect to the wavelength of the modulated light. An optical multiplex relay transmission system characterized in that a light emitting source or a different light receiver is selected and assigned to a plurality of the modulated lights.