JPS63136863A - Highly sensitive optical receiving system - Google Patents

Abstract

PURPOSE:To reduce the influence of the noise of a photodetector and to attain a highly sensitive optical reception by setting an identification reference voltage in an identification circuit in accordance with an optical input level. CONSTITUTION:An optical receiver which amplifies the output signal of the photodetector 1 having an amplifying function through an amplifier 2 and which identifies it in the identification circuit so as to reproduce data is provided with an identification reference voltage control operator 6. It is inputted with a signal showing the size of the amplification factor of the photodetector 1 and a signal showing the size of the gain of the amplifier 2, and it generates the identification reference voltage of the identification circuit 4 which is optimum for reducing the influence of the noise of the photodetector 1 predicted from said signals. Even if the photodetecting level falls, the identification reference voltage is optimized, whereby identification errors can be reduced and therefore light signals can be received with high sensitivity.

Description

[Detailed Description of the Invention] [Summary] In an optical receiver using a light-receiving element having a multiplication effect such as an APD, by setting the discrimination reference voltage in the discriminator according to the optical input level, Enables highly sensitive optical reception by reducing the influence of noise on the light receiving element.

[Industrial application field]

The present invention provides a high-sensitivity optical receiver that automatically controls the identification reference voltage of a discriminator in an optical receiver that uses a multiplier element such as an APD (avalanche photo diode) as a light-receiving element. It is related to the method.

In an optical receiving circuit using an APD as a light receiving element,
The data is recovered from the input optical signal by identifying the received signal using the identification reference voltage in the discriminator.
In this case, if the discrimination reference voltage value is not appropriate, the number of erroneous discriminations due to the influence of Chicot noise will increase, so it is desired to always make the discrimination reference voltage uniform. - [Prior Art] Fig. 3 shows a block diagram of an optical receiving circuit (optical module) using a conventional APD. In the figure, 1 indicates a light receiving element consisting of an APD, and 2 indicates an AP
an amplifier having an AGC function that amplifies the output signal of Dl;
3 is a multiplication factor control circuit for controlling the multiplication factor of the light receiving element 1; 4 is a discriminator for identifying the output signal of the amplifier 2; and 5 is a discrimination reference voltage regulator.

The light receiving element 1 receives an input optical signal and generates an electric signal corresponding thereto. Amplifier 2 has automatic gain control (AGC)
It has a function of changing its gain according to the input signal level and outputting a constant output signal level. The multiplication factor control circuit 3 controls the multiplication factor M of the light receiving element 1 by changing the bias of the light receiving element I according to this voltage level when the output voltage of the amplifier 2 becomes below a certain value. In a state below the automatic gain control limit of the amplifier 2, the output signal level of the light receiving element 1 is kept constant. The discriminator 4 discriminates the input signal from the amplifier 2 based on the externally applied discrimination reference voltage vth.
Regenerate data and clock. The discrimination reference voltage vth is externally set to a common value by adjusting the discrimination reference voltage regulator 5 made of, for example, a variable resistor.

[Problem that the invention seeks to solve]

In the conventional optical receiving circuit shown in FIG. 3, as the level of light received by the light receiving element 1 decreases, the gain of the amplifier 2 and the multiplication factor of the light receiving element 1 increase, and therefore the noise level increases.

When the received light level is low and therefore the noise level in the discriminator is high, it is necessary to change the discrimination reference voltage accordingly in order to avoid misidentification. Since the reference voltage vth is set externally, the identification reference voltage vth is fixedly determined, and it is not possible to change the identification reference voltage vth according to the input level to make it compatible.

[Means for solving problems]

The present invention is an attempt to solve the problems of the prior art, and has the principle configuration shown in FIG. The optical receiver is then equipped with an identification reference voltage control calculator 6 for identifying and reproducing data using the identifier 4.

The identification reference voltage control calculator 6 receives a signal indicating the magnitude of the multiplication factor of the light-receiving element 1 and a signal indicating the magnitude of the gain of the amplifier 2, and calculates the predicted value of the light-receiving element 1 from these signals.
The discrimination reference voltage of the discriminator 4 is generated most effectively to reduce the influence of noise.

[For production]

The noise component in the output of the APD consists of three components: ■ thermal noise ■ shot noise due to the dark current of APD 1 ■ shot noise due to the amplified dark current. Among these, the shot noise due to the amplified dark current is Varies depending on the received light level.

Therefore, by suppressing the shot noise caused by the amplified dark current, it is possible to reduce the noise level when the received light level becomes small.

In this case, the amplified dark current at the output of the APD can be expressed by the following equation.

I 52=2 e I oM2FB=2 e I oM
2"χB where e: Electron charge M: Current multiplication factor F' A P D over 111 tifa (F = M'
) B: Frequency band χ: Multiplication noise coefficient io: Dark current when M=1 In the above equation, only the multiplication factor M changes depending on the received light level. The value of M can be known from the bias value for the APD at that time. Therefore, by knowing the multiplication factor M, it is possible to predict the shot noise due to the amplified dark current mentioned above in the APD output,
Furthermore, if you know the gain of the AGC amplifier connected to the APD, you can also predict the shot noise due to the amplified dark current at the output of the AGC amplifier, and use these to determine the appropriate discrimination reference voltage to reduce the effect of shot noise. can be determined.

In the present invention, an identification reference voltage control calculator 6 is provided, and a signal indicating the magnitude of the multiplication factor of the light receiving element l and an amplifier 2 are provided.
The light receiving element 1 predicted from the signal indicating the magnitude of the gain of
A discrimination reference voltage for the discriminator 4 that is most effective for suppressing noise is generated, and by using this reference voltage, the discriminator 4 discriminates the signal obtained by amplifying the output signal of the light-receiving element 1 via the amplifier 2, and reads the data. Since the information is reproduced, erroneous identification due to the influence of shot noise can be reduced.

〔Example〕

FIG. 2 shows one embodiment of the present invention, and FIG.
The same parts as in the figure are indicated by the same numbers, and 6 is an identification reference voltage control calculator.

In FIG. 2, a light receiving element 1 generates an electrical signal in response to an input optical signal, and an amplifier 2 performs automatic gain control in accordance with the input signal level to generate an output signal with a constant level. The multiplication factor control circuit 3 controls the multiplication factor M of the light receiving element 1 by changing the bias of the light receiving element 1 according to the output voltage level of the amplifier 2 when the gain is below the automatic gain control limit of the amplifier 2. It acts to keep the output signal level of element 1 constant.

The identification reference voltage control calculator 6 outputs a signal indicating the magnitude of the gain of the amplifier 2, that is, the AGC control voltage of the amplifier 2, and a signal indicating the magnitude of the multiplication factor M of the light receiving element 1, that is, the amplifier 2 when the gain is below a predetermined level. , and calculates and outputs a discrimination reference voltage vth suitable for suppressing expected shot noise. The discriminator 4 uses the discrimination reference voltage vth to discriminate the input signal from the amplifier 2, and reproduces data and a clock.

Therefore, in the case of the present invention, even when the received light level is low and shot noise due to amplified dark current is increased, the discrimination reference voltage of the discriminator that discriminates the output signal is standardized, so the influence of noise is reduced. Thus, deterioration of the error rate in the identification output can be prevented.

〔Effect of the invention〕

As explained above, according to the present invention, in an optical receiver using a light-receiving element having a multiplication effect such as an APD, even in a state where the received light level has decreased, it is possible to make identification errors by making the identification reference voltage common. Therefore, it is possible to receive optical signals with high sensitivity.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of the present invention, FIG. 2 is a diagram showing the configuration of an embodiment of the present invention, and FIG. 3 is a diagram showing an example of the configuration of a conventional optical receiver. ■... Light receiving element 2 - Amplifier 3 - Multiplication factor control circuit 4 - Discriminator 5 - Discrimination reference voltage regulator 6 - Discrimination reference voltage control calculator Diagram Ml showing the configuration of the original W1 of the present invention Figure 3 shows an example of the configuration of a conventional optical receiver circuit.

Claims (1)

[Claims] An amplifier (
In an optical receiver that reproduces data by amplifying it through 2) and identifying it with a discriminator (4), a signal indicating the magnitude of the multiplication factor of the light receiving element (1) and a signal of the amplifier (2) are used. a signal indicating the magnitude of the gain, and a discriminator for generating a discrimination reference voltage for the discriminator (4) most suitable for reducing the influence of noise on the light receiving element (1) expected from these signals. A highly sensitive optical reception system characterized by comprising a reference voltage control calculator (6).