JP2641592B2 - Optical receiving circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiving circuit used in an optical communication system or the like, and more particularly, to an improvement in a control method of a light receiving element such as a gain of a light receiving element. It is.

[Conventional technology]

Conventionally, photodetectors with multiplication factor control (hereinafter referred to as APD)
2 is generally used in an optical receiving circuit using the APD1. The control method is such that the output amplitude of the variable gain amplifier 3 is detected by the multiplication factor control circuit 4 and the multiplication factor of the APD 1 is negatively fed back. On the other hand, the gain control circuit 5 also detects the output amplitude of the variable gain amplifier 3 and performs negative feedback control so as to have a constant amplitude. That is, in this method, when the optical input level is small, the gain of the variable gain amplifier 3 is fixed to the maximum gain,
The output amplitude is kept constant by controlling the multiplication factor of APD1. Also,
When the optical input level is large, the gain of the APD 1 is fixed to the minimum value, and the gain of the variable gain amplifier 3 is controlled to keep the output amplitude constant. FIG. 4 shows the relationship between the optical input level and the gain of the APD and the gain of the variable gain amplifier.

As a reference describing this kind of technology, for example, "Optical Communication Handbook", published by Kagaku Shimbun, August 1984,
705-.

[Problems to be solved by the invention]

In the above prior art, when the light input level is small, there is a problem that the multiplication factor of the APD deviates from the optimum value and the S / N ratio deteriorates.

An object of the present invention is to solve the above-mentioned problems in the prior art, to maintain the APD gain at an optimum value even when the optical input level fluctuates, to obtain a good S / N ratio, and to improve the light receiving sensitivity. An object of the present invention is to provide an optical receiving circuit having a control method that can be improved.

[Means for solving the problem]

In order to achieve the above object, in the present invention, the gain control circuit detects the output amplitude of the preamplifier, and sets an
The multiplication factor of D is fixed, and when the optical input level is large, the control circuit performs a negative feedback control on the APD to reduce the multiplication factor so that the output amplitude of the preamplifier does not exceed a predetermined value.

[Action]

In the optical receiving circuit, the amplification factor control circuit always detects the output amplitude of the preamplifier and controls the APD so that it does not exceed a certain value.
The negative feedback control of the cathode voltage applied to is performed. Further, the gain control circuit detects the output amplitude of the variable gain amplifier and performs negative feedback control on the gain of the variable gain amplifier so that the gain becomes a constant value. Thereby, the output amplitude of the optical receiving circuit is kept constant even if the optical input level goes up and down. In this case, when the optical input level is low, the APD noise is suppressed and the minimum light receiving sensitivity is improved, and when the optical input level is high, distortion due to excessive output to the preamplifier and excessive input to the gain enable amplifier are The accompanying distortion can be prevented, and the maximum light receiving sensitivity is also improved.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIG. The present embodiment includes an APD 1 for converting an optical input signal into a current signal, a preamplifier 2 for converting and amplifying the current signal into a voltage signal and outputting the same, and a variable gain for amplifying or attenuating and outputting the output signal. An amplifier 3, a gain control circuit 4 for detecting the output amplitude of the preamplifier 2 and generating a cathode voltage for determining the gain of the APD 1, and a gain for detecting the output amplitude of the variable gain amplifier 3 and controlling the gain. And a control circuit 5. Of these, the APD 1, the preamplifier 2, and the gain control circuit 4 constitute a first negative feedback loop. That is,
When the optical input level is small and the output amplitude of the preamplifier 2 is equal to or less than the maximum allowable input amplitude of the variable gain amplifier 3, the gain of the APD 1 is fixed to a value at which the S / N ratio is maximized. When the level is large, the output amplitude of the preamplifier 2 is fixed to the maximum allowable input amplitude of the variable gain amplifier 3,
Negative feedback control of the cathode voltage of APD1. On the other hand, a second negative feedback loop is formed by the variable gain amplifier 3 and the gain control circuit 5, and controls the gain of the variable gain amplifier 3 so that the output amplitude of the variable gain amplifier 3 is always constant. In this configuration, when the optical input level is low, the variable gain amplifier 3
Is controlled, and when the optical input level is large, only the multiplication factor of APD1 is controlled. FIG. 3 shows the relationship between the optical input level and the gain of the APD 1 and the gain of the variable gain amplifier 3.

If the noise of the preamplifier is sufficiently small, the S / N ratio that determines the most important performance of the optical receiving circuit is almost determined by the multiplication factor of the APD, and there is an optimum value. When the multiplication factor is set so as to be the optimum value when the light input level is small, the relationship between the light input level and the S / N ratio is as shown in FIG.

As described above, according to the present embodiment, the AP
Since the multiplication factor of D does not deviate from the optimum value, the S / N ratio is improved, and the minimum light receiving sensitivity is improved. Also, when the optical input level is large, the preamplifier output does not exceed a certain value,
Since the waveform distortion due to the excessive output of the preamplifier and the distortion due to the maximum input of the variable gain amplifier, which limit the maximum light receiving sensitivity in the conventional circuit, are eliminated, the maximum light receiving sensitivity is improved.

In the above embodiment, when the optical input level is small, the maximum value of the multiplication factor of the APD is assumed to be constant. However, when the noise of the preamplifier cannot be ignored, the optimum value depends on the optical input level.
Therefore, the APD must be adjusted according to the preamplifier noise in advance.
If the gain is determined and the gain is controlled so as to have a dependency on the output amplitude of the preamplifier, a better S / N ratio can be obtained.

It is also known that the APD multiplication factor has temperature dependence. Therefore, if a temperature sensor is provided in the multiplication factor control circuit and control is performed to compensate for the temperature dependency of the APD,
Better properties are obtained.

〔The invention's effect〕

According to the present invention, when the light input level is small, the multiplication factor of the light receiving element is fixed, so that the multiplication factor of the light receiving element is maintained at the optimum value even if the light input level fluctuates. When the sensitivity is improved and the optical input level is large, the preamplifier and variable gain amplifier are saturated by reducing the multiplication factor of the light receiving element so that the output amplitude of the preamplifier does not exceed a certain value. And the maximum light receiving sensitivity is improved. Furthermore, since the multiplication factor of the light receiving element is controlled by the output amplitude of the preamplifier, there is an advantage that it does not depend on the mark ratio of the signal, that is, the code type, and the two negative feedback loops operate independently. Therefore, there is an advantage that an uneasy operation is not performed at the switching point of the loop.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention, FIG. 2 is a block diagram of a conventional example, FIG. 3 is a characteristic diagram of the circuit of FIG. 1, and FIG.
5 is a characteristic diagram of the circuit shown in FIG. 5, and FIG.
FIG. 4 is a diagram showing characteristics of N ratio in comparison. DESCRIPTION OF SYMBOLS 1... Light receiving element (APD) 2... Preamplifier 3... Variable gain amplifier 4.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Reference number in the agency FI Technical display location H04B 10/28

Claims (1)

(57) [Claims] 1. A light receiving element capable of controlling a multiplication factor, a preamplifier for converting and amplifying an output current signal of the light receiving element into a voltage signal, and a variable gain amplifier for amplifying or attenuating the output signal of the preamplifier. A gain control circuit for controlling the gain so as to maintain the output signal amplitude of the variable gain amplifier at a constant value and a gain control circuit for controlling the gain of the light receiving element; The control circuit detects the output signal amplitude of the preamplifier, and fixes the multiplication factor of the light receiving element when the optical input level is small and the output amplitude of the preamplifier is equal to or less than a predetermined value, and when the optical input level is large. The optical receiving circuit is characterized in that it is a control circuit of a system for reducing the multiplication factor of the light receiving element by performing feedback control so that the output amplitude of the preamplifier does not exceed a predetermined value.