JPH01258514A - Light receiving circuit - Google Patents

Abstract

PURPOSE:To easily detect an input disconnection by detecting a current to flow at a light receiving element, and when the detected value becomes the reference value or above specified beforehand, deciding that the input shut-off of a light signal is obtained. CONSTITUTION:A light signal 11 is received by an avalanche photo diode 1, converted to an electric signal, inputted through a current detecting circuit 9 to a pre-amplifying circuit 2, a prescribed amplifying is executed and thereafter, the signal is outputted from a terminal 12 through a gain variable amplifying circuit 3 and an equalizing circuit 4. On the other hand, the output of the equalizing circuit 4 is inputted through a peak detecting circuit 5 and direct current amplifying circuits 6 and 7 to a direct current voltage converting circuit 8. In this case, since the gain of the amplifying circuits 6 and 7 is set to the sufficiently large value, when a light input is absent, the amplifying circuit 3 becomes a maximum gain, the converting circuit 8 becomes the maximum voltage and the current to flow at the diode 1 also becomes maximum. Consequently, the current is detected by the current detecting circuit 9 and an alarm signal is outputted.

Description

[Detailed description of the invention] [Industrial application field] The present invention is utilized in the field of optical communications.

The present invention relates to an optical receiving circuit, and more particularly to an optical receiving circuit equipped with optical input disconnection detection means.

〔overview〕 .

The present invention provides an optical receiving circuit including a means for controlling a bias voltage of a light receiving element that receives an optical signal according to a reception level of the optical signal, in which a current flowing through the light receiving element is detected, and the detected value is determined in advance. When the optical signal exceeds the set reference value, an input disconnection alarm signal indicating that the input of the optical signal has been disconnected is outputted, so that the input disconnection alarm signal can be easily outputted.

[Conventional technology]

In general, in optical digital communications, an optical receiver circuit detects the peak value of the output of an equalization circuit and controls the gain of a gain control amplifier in order to keep the input amplitude to the identification circuit for signal reproduction constant. (electrical AGC) and a method (FULL-AGC) in which the bias voltage of an avalanche photodiode, which is a light receiving element, is controlled to control the multiplication factor of the avalanche photodiode (FULL-AGC) are used together.

In addition, the identification circuit requires a clock that determines the identification point, which is achieved by extracting the timing component (sine wave) from the output of the equalization circuit using a band-pass filter and converting it into a clock using an IJ mitter circuit. I am using it. The peak value of the timing component that is the output of the bandpass filter decreases as the optical input signal decreases.

Conventionally, the peak value of the '7' timing component was detected using the above property, and when the level fell below a reference value, an optical input cutoff alarm signal was output.

[Problem that the invention seeks to solve]

In the conventional optical input disconnection detection means described above, even if the optical input is constant, it fluctuates depending on the mark rate of the signal, and the peak value is detected when the optical input level is small, so it is very difficult to set the identification point for issuing an alarm. However, there were drawbacks that made it difficult to understand.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical receiver circuit that can easily output an input disconnection alarm signal by eliminating the above-mentioned drawbacks.

[Means for solving problems]

The present invention provides an optical receiving circuit comprising a light receiving element that receives an optical signal and a bias control means that controls a bias voltage of the light receiving element according to a light reception level of the optical signal.
current detection means for detecting the current flowing through the light receiving element;
The present invention is characterized in that it includes input disconnection alarm output means for outputting an input disconnection alarm signal indicating input disconnection of the optical signal when the output voltage of the current detection means is equal to or higher than a predetermined reference voltage.

[Effect]

The bias voltage of a light-receiving element, such as an avalanche photodiode, is controlled by a bias control means so that it increases as the light reception level of the optical signal decreases, and the multiplication factor of the avalanche photodiode increases, and the level of its electrical photodetection current remains constant. It will become as follows. That is, when the input of the optical signal is cut off, the light reception level of the optical signal becomes zero, and the bias voltage increases accordingly, reaching the breakdown voltage of the avalanche photodiode, V, and the current flowing through the avalanche photodiode reaches its maximum. reach the value.

Therefore, the current detection means detects the current of the avalanche photodiode as a voltage appearing across the insertion resistor, and the alarm output means converts this detected voltage value to a reference voltage V set slightly lower than the breakdown voltage Va. . By outputting an input disconnection alarm signal when the input disconnection alarm signal is greater than , it becomes possible to easily set and output the input disconnection alarm signal.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing details of a current detection circuit and a comparison circuit thereof.

According to FIG. 1, this embodiment includes an avalanche photodiode 1 as a light-receiving element that receives an optical signal 11, and a bias voltage control means that controls the bias voltage of the avalanche photodiode 1 according to the reception level of the optical signal 11. In an optical receiving circuit including a value detection circuit 5, a DC amplification circuit 6, a DC amplification circuit 7, and a DC voltage conversion circuit 8, a current detection circuit 9 as a current detection means for detecting the current flowing in the avalanche photodiode 1, and this current When the output voltage of the detection circuit 9 is lower than the reference voltage V0 given in advance from the reference voltage terminal 14, an input disconnection alarm signal 13a representing input disconnection of the optical signal 11 is output from the alarm output terminal 13. It includes a comparison circuit 10 as an output means. Note that 2 is a preamplifier circuit, 3 is a variable gain amplifier circuit, 4 is an equalization circuit, and 12 is an equalization output terminal to which an equalized signal 12a is output. The output of the DC amplifier 6 is the variable gain amplifier circuit 3.
is also connected to.

According to FIG. 2, the DC detection circuit 9 includes a resistor R1, which has one end connected to the output of the avalanche photodiode 1 and the other end connected to the preamplifier circuit 20, and a resistor R1 connected in series between one end of the resistor R1 and ground. resistors R2 and R3 and resistor R
Resistors R4 and R5 connected in series between the other end of 1 and ground
and a capacitor C2 connected between the connection point of resistors R4 and R5 and ground, the positive phase input terminal is grounded via capacitor C1, and the negative phase input terminal is connected to resistors R4 and R5 via resistor R7. 1 and an operational amplifier 15 whose output terminal is connected to the negative phase input terminal via a resistor R6 and connected to one of the input terminals of the comparator circuit 10.

Further, the comparison circuit 10 has a positive phase input terminal connected to the current detection circuit 9.
, an operational amplifier 16 having a negative phase input terminal connected to the reference voltage terminal 14 and an output connected to the alarm output terminal 13 .

A feature of the present invention is that, in FIG. 1, a current detection circuit 9 and a comparison circuit 10, an example of which is shown in FIG. 2, are provided.

Next, the operation of this embodiment will be explained.

First, the operation of the entire circuit will be explained. The optical signal 11 is received by the avalanche photodiode 1, converted into an electrical signal, inputted to the preamplifier circuit 2 via the current detection circuit 9, and after a predetermined amplification, inputted to the variable gain amplifier circuit 3. Ru. The variable gain amplifier circuit amplifies the manually input electric signal according to the output of the DC amplifier circuit 6 and outputs it to the equalization circuit 4. The equalization circuit 4 performs predetermined equalization and then outputs it from the equalization output terminal 12 as an equalized signal 12a.

On the other hand, the peak value of the amplitude of the output of the equalization circuit 4 is detected by the peak value detection circuit 5, and its fluctuation is amplified by the first DC amplifier circuit 6 to control the gain of the variable gain amplifier circuit 3. The second DC amplifier circuit 7 amplifies and controls the output voltage of the DC voltage conversion circuit 8, thereby controlling the multiplication factor of the avalanche photodiode 1. Since the gains of the DC amplification circuit 6 and the DC amplification circuit 7 are set to sufficiently large values, the output of the equalization circuit 4 is maintained at a constant value even when the optical input varies. Therefore, when there is no optical input, the variable gain amplifier circuit 3 has the maximum gain, and the output of the DC voltage conversion circuit 8 also has the maximum voltage.

Therefore, the bias voltage applied to the avalanche photodiode 1 reaches the breakdown voltage of the avalanche photodiode, and the current flowing through the avalanche photodiode 1 rapidly increases (about 100 μA). This is a sufficiently large value compared to the current at the minimum light reception level (500 μ8 to 1 μA).

The current detection circuit 9 is connected to this avalanche photodiode 1.
The comparator circuit 10 outputs an input disconnection alarm signal at the current value when the breakdown voltage is reached.

Next, the operations of current detection circuit 9 and comparison circuit 10 will be explained in more detail.

A voltage proportional to the current flowing through the avalanche photodiode 1 appears across the resistor R1, and after being level-shifted by the resistors R2 and R3 and the resistors R1 and R5,
An inverting amplifier circuit composed of an operational amplifier 15 and resistors R6 and R7 converts the voltage into a voltage proportional to the current flowing through the avalanche photodiode 1.

The output of the current detection circuit 9 is the reference voltage V of the comparison circuit 10 because the voltage increases as the current of the avalanche photodiode I increases. If is set to be slightly lower than the output voltage of the current detection circuit 9 corresponding to the current flowing through the avalanche photodiode 1 when the breakdown voltage of the avalanche photodiode 1 is reached, the output of the comparison circuit 10 is inverted when the optical input is cut off. The optical input disconnection is detected and outputted from the alarm output terminal 13 as an input disconnection alarm signal 13a.

〔Effect of the invention〕

As described above, the present invention detects a sudden increase in the current of a light emitting element (avalanche photodiode) due to a cutoff of optical input and outputs an input cutoff alarm signal, so that adjustment is easy. Further, there is an effect that it does not depend on fluctuations in the mark rate of the received optical signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention. FIG. 2 is a circuit diagram showing details of the current detection circuit and comparison circuit. ■... Avalanche photodiode, 2... Preamplifier circuit, 3... Variable gain amplifier circuit, 4... Equalization circuit, 5... Peak value detection circuit, 6.7... DC amplification Circuit, ... DC voltage conversion circuit, 9... Current detection circuit,
DESCRIPTION OF SYMBOLS 10... Comparison circuit, 11... Optical signal, 12... Equalization output terminal, 12a... Equalization signal, 13... Alarm output terminal, 13a... Input disconnection alarm signal, 14.
...Reference voltage terminal, 15.16...Operation amplifier, C1
, C2... Capacitor, R1-R7... Resistor, V. ...Reference voltage. '・, 4.6゜Example (configuration of current detection circuit comparison circuit) Fig. 2

Claims (1)

[Claims] 1. A light receiving element (1) that receives an optical signal (11), and bias control means (5 to 8) that controls a bias voltage of the light receiving element in accordance with a light receiving level of the optical signal. In the light receiving circuit, current detection means (9) detects a current flowing through the light receiving element.
), and input disconnection alarm output means (10) for outputting an input disconnection alarm signal indicating input disconnection of the optical signal when the output voltage of the current detection means is higher than a predetermined reference voltage. optical receiving circuit.