Light receiving automatic gain control circuit

Details Light receiving automatic gain control circuit Light receiving automatic gain control circuit

1982-02-05

1985-01-22

Nelms, David C.

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

307360, 330138, H01J 4014

Patent

active

044954107

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to a light receiving circuit used for repeaters and terminals, etc., in an optical transmission system.
Repeaters are used in optical transmission systems in order to transmit light signals to the remote terminals. A repeater converts the received light signal into an electrical signal and converts it again into a light signal after waveform equalization and shaping, and finally transmits it to the next repeater. In the terminal station, on the other hand, the received light signal is converted into an electrical signal and thereafter demodulation is carried out.
Automatic gain control (AGC) is carried out at repeaters and terminals so that an electrical signal of constant level can be obtained even if the level of the received light signal changes. FIG. 1 is an ordinary light receiving circuit having an automatic gain control circuit. In this FIG, 11 is a light signal; 12 is an avalanche photo diode (APD); 13 is an amplifier; 14 is a level detector; 15 is the output terminal; 16 is an AGC circuit.
The following methods are currently proposed for executing AGC using the circuit shown in FIG. 1. when the light input level is doubled, M of the APD is reduced to a half (1/2). fixed for high input levels and with the gain of amplifier 13 being controlled. published unexamined Japanese patent application No. 53-58748. The gain amplifier 13 is changed step by step for high input levels and simultaneously M of the APD 12 is also controlled.
The methods listed above will contribute to providing an electrical signal of a constant level as an output even when the level of a received light signal changes. FIGS. 2 (1) to (3) show the relation between the light input level P, the multiplication factor M of the APD and the gain G of the amplifier in the methods (a) to (c) given above. In the same figures, the solid line indicates M, the broken line indicates G and the chained line indicates the optimum value of M, explained later. M.sub.min is the minimum value of the applicable multiplication factor of the APD. When M<M.sub.min, which is not practical, the response speed of APD becomes very low due to an increase of the capacitance.
On the other hand, the signal to noise ratio (SNR) of the output signal of amplifier 13 is basically given by the following equation.
The equation (1) teaches that SNR depends on P and M, and the optimum value M.sub.o for M gives the best SNR. From the equation (1), the dependency of M.sub.o on the light input level is expressed as follows.
Usually, x is selected to be a value ranging from 0.3 to 1.
The gain G of the amplifier corresponding to M.sub.o is automatically determined from the condition that the output of the amplifier is maintained at a constant level and the determined gain is the optimum value. Here, since there is a difference between the M which keeps an electrical signal at a constant level and said optimum value M.sub.o, the SNR deteriorates if only an electrical signal level is kept at a constant level. FIG. 3 (1), (2) and (3) show such conditions. Namely, the figures show the relation between light input and SNR corresponding to the methods (a), (b) and (c). The solid line in the figure shows the ideal values, while the broken lines 32, 33, 34 respectively show the values of the methods (a), (b) and (c). Explained below is the reason why the solid line 31 and broken lines 32, 33, and 34 cross at the point P.sub.0, and the solid line 31 and the broken lines 33 and 34 cross at the point P.sub.3. As shown in FIG. 2 (1), (2), (3), the value of M crosses the optimum value M.sub.o at the points P.sub. 0 and P.sub.3. At the point P.sub.0, M becomes equal to M.sub.o (M=M.sub.o) by the setting, while at the point P.sub.3, M becomes also equal to M.sub.o. In a method being employed currently in order to improve the SNR, a value of M is not lowered to M.sub.min but instead is always kept at a value which is larger than than the value M.sub.1 (>M.sub.min). Thereby, before the SNR deviates largely from the value SNR.sub.o for

REFERENCES:
patent: 4124825 (1978-11-01), Webb et al.
patent: 4236069 (1980-11-01), Laughlin
patent: 4399416 (1983-08-01), Gillespie

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