JPS623625B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an optical receiving circuit that receives an optical pulse signal;
In particular, it relates to an optical receiving circuit using an avalanche photo diode (abbreviated as APD).

Conventional optical pulse receiving circuits often use an APD, which has an amplification effect, as an optical-to-electrical conversion element. APD is used with reverse bias,
By changing the bias voltage, the multiplication factor M is changed. However, when the value of M is increased, it corresponds to M.
Since noise generated from inside the APD is also multiplied, a large multiplication factor M does not necessarily improve the signal-to-noise ratio S/M, and there is a certain value of M that makes S/M the best. Also, as will be explained in detail later, the APD bias voltage vs. multiplication factor M characteristic changes greatly with temperature, so in order to achieve the best S/M regardless of temperature, it is necessary to needs to be constant.

Further, it is desirable that the output signal of the optical receiving circuit has a constant amplitude regardless of the magnitude of the input optical signal for the operation of the electric circuit thereafter. For this reason, it is necessary to change the multiplication factor M of the APD in accordance with the magnitude of the input optical signal to keep the output of the optical receiving circuit constant.

In order to satisfy the above two requirements, conventionally,
As will be explained in detail later, the amplitude of the output of the optical receiver circuit is detected by a peak detection circuit, the detected value is compared with a reference voltage, and the reverse bias voltage of the APD, that is, M, is determined from the error voltage. The size of
A method is used in which the output of the optical receiver circuit is kept constant regardless of changes due to the temperature of the APD.

FIG. 1 is a block diagram of the conventional optical receiver circuit explained above, in which 1 is an APD, 2 is an amplification circuit, 3 is a peak detection circuit, 4 is a DC amplification circuit, and 5 is a block diagram that shows a signal depending on the input voltage. DC/DC conversion circuit with variable output voltage,
Reference numeral 6 represents an optical input terminal, 7 represents a receiving circuit output terminal, and 8 represents a reference voltage terminal. In the circuit shown in FIG. 1, the reference voltage (V _REF ) is applied to the terminal 8 after detecting the peak value of the output in the amplifier circuit 3.
The error voltage is calculated by the DC/DC converter 5.
Convert to APD bias voltage and APD multiplication factor M
It operates so as to change the amplitude of the output terminal 7 and keep the amplitude of the output terminal 7 constant.

The present invention uses a circuit configuration that maintains the amplitude of the receiving circuit output constant without configuring a large control loop including an amplification circuit as in the conventional system.

If the code of the optical transmission line is a DC-balanced code, the DC component of the code is constant regardless of the type of information. APD has the function of converting the input optical signal into a current, so when a DC balanced optical pulse train is input as input light, the APD's output current changes depending on the amplitude of the input optical signal. changes. It is clear that the APD output current and the amplitude of the optical pulse do not have a one-to-one correspondence because the mark rate of the code changes in a code that is not DC balanced. From the above, if the APD bias power source is a constant current source, the magnitude of the input optical signal or
Even if the APD characteristics change due to temperature, the APD
The output current (output amplitude) is constant, that is, the output of the receiving circuit is constant, and the function is equivalent to that of the conventional system.

FIG. 2 is a diagram showing an outline of the configuration of the optical receiving circuit according to the present invention, in which 11 is an APD, 12 is an amplification circuit, 13 is a constant current power supply, 14 is an optical input terminal, and 15 is a receiving circuit output. Represents a terminal. In this configuration, the APD output current is kept constant by the 13 constant current sources, so the amplitude of the APD output signal remains constant regardless of changes in APD characteristics due to input optical signal amplitude and temperature.

FIG. 3 is a diagram showing an example of a detailed configuration of the receiving circuit according to the present invention shown in FIG. 2, in which 21 is an APD;
22 is a signal amplification circuit, 23 and 24 are DC amplification circuits, and 25 is a circuit that changes the output voltage depending on the input voltage.
26 is a resistor, 27 is an optical input terminal, 28 is an output terminal, and 29 is a reference voltage input terminal. In this embodiment, the current flowing through the APD 21 is converted into a voltage by the resistor 26 and detected, and the detected voltage is amplified by the DC amplifier circuit 23, and the error is the difference voltage between the voltage of the amplified signal and the reference voltage V _REF . The signal is amplified by the DC amplifier circuit 24 and the DC/DC conversion circuit 25 is operated to keep the current flowing through the APD 21 constant.

As explained above, by using a constant current source as the bias power source of the APD, the configuration of the optical receiving circuit can be simplified.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the configuration of a conventional optical receiver circuit, Figure 2 is a diagram showing an outline of the configuration of an embodiment of the present invention, and Figure 3 is a detailed diagram of the configuration of the circuit according to the present invention shown in Figure 2. FIG. Symbol explanation: 11 is APD, 12 is amplifier circuit,
13 is a constant current power supply, 21 is an APD, 22 is a signal amplification circuit, 23 and 24 are a DC amplification circuit, and 25 is a
In the DC/DC conversion circuit, 26 represents a resistor.

Claims (1)

[Claims] 1. An optical repeater transmission line using a DC balanced transmission line code, consisting of an avalanche photo diode that converts an optical pulse train into an electric pulse train, and a constant current source that reverse biases the avalanche photo diode. An optical receiver circuit featuring: