JPS61212127A - Apd bias control circuit - Google Patents

Abstract

PURPOSE:To always operate an APD at the operating point of a high current multiplication factor and also stably by monitoring the noise level of an out- band in the output terminal of a pre-amplifier connected to the output of the APD and controlling the bias voltage of the APD, when said level exceeds some reference value. CONSTITUTION:The APD 1 receives an optical signal and converts it to an electric signal and the converted electric signal is amplified by the pre-amplifier 2 and also amplified by an amplifier 3 and outputted. To the output of the pre-amplifier 2, a band pass filter 4 is connected, and the frequency component of the outside of a working band is extracted from in the electric signal. The extracted frequency component of the outside of the working band is converted to a DC voltage by a detector 5. This DC voltage V is inputted to a comparator 6 and compared with a prescribed voltage value V0. If the DC voltage is larger than the prescribed voltage value V0, a bias voltage is lowered by executing feedback. On the other hand, if the DC voltage is smaller than the prescribed voltage value V0, the bias voltage is not changed.

Description

[Detailed description of the invention] [Summary] Avalanche photodiode (hereinafter referred to as APD)
A stable high-voltage bias is supplied to the APD, and the bias is controlled so as to always operate within the APD specifications and with a high current multiplication factor.

[Industrial application field]

The present invention provides A
This invention relates to a circuit that controls the maximum bias voltage of a PD.

Since the bias voltage supplied to a conventional APD is operated close to the specified upper limit of the APD in order to increase the current multiplication factor, there is a risk that the APD may be destroyed due to temperature characteristics or the like.

Therefore, this improvement has long been strongly desired.

[Conventional technology]

As is well known, in optical communication systems, AP is used as a light receiving element.
D is passing. APD has the disadvantage of requiring a high bias voltage, but due to the current multiplication effect, the S/
There is an advantage that N can be improved.

FIG. 2 is a diagram illustrating the relationship between the bias voltage of the APD, the current multiplication factor a, and the dark current S.

As shown in the figure, when the bias voltage increases, the current multiplication factor a
However, at the same time, the dark current also increases. Note that the dark current is a current that is output when there is no optical input to the APD, and it becomes noise.

Therefore, when applying a bias voltage to the APD, it is common to increase the bias voltage and use it in a state where the current multiplication factor a is large.

However, there is an upper limit to the bias voltage applied to the APD, and exceeding this upper limit may lead to destruction of the APD.

This upper limit is called the breakdown voltage (2), and the breakdown voltage (1 day) is defined as the voltage at which the dark current becomes 100 μA.

Since the breakdown voltage (1) is fixed at the maximum bias voltage V, which is quite close to 8, various problems occur.

That is, the breakdown voltage (18) of the APD changes due to a change in the temperature of the APD. For example, when the temperature rises, the dark current changes to characteristic C (moves to the right) as shown by the dotted line. Therefore, the breakdown voltage V, moves to the right and becomes 8°.

Furthermore, when the temperature decreases, the dark current changes to the characteristic d shown by the dotted line (moves to the left).

Therefore, if it is currently used with a fixed bias, for example, as the temperature rises, the breakdown voltage V will increase (but on the other hand, the current multiplication factor a will decrease).

On the other hand, as the temperature rises, the current multiplication factor (a) becomes high (but on the other hand, the breakdown voltage (2) decreases, which may lead to destruction of the APD.

There is also a method of installing a limiter on the current flowing to the APD,
This method has the disadvantage that distortion characteristics deteriorate.

[Problem that the invention seeks to solve]

An object of the present invention is to eliminate the drawbacks of the above-mentioned conventional system and to control the maximum bias voltage of the APD by a feedback circuit to eliminate the influence of variations in the temperature characteristics of the APD element.

[Means for solving problems]

As shown in Fig. 1, at the output end of the preamplifier 2 connected to the output of the APDI, out-of-band noise is extracted by a bandpass filter 4, this noise level is converted to a DC voltage by a detector 5, and then this noise level is converted to DC voltage by a detector 5. This is achieved by adjusting the bias voltage of APD 1 when the DC voltage of APD 1 exceeds a reference value, thereby stably operating at an operating point with a constant high current multiplication factor.

[Effect]

According to the present invention, the out-of-band noise level at the output terminal of the preamplifier 2 connected to the output of the APDI is monitored, and when the out-of-band noise level exceeds a reference value, the bias voltage of the APDI is controlled to keep the APDl constant. The effect is that stable operation can be achieved even at an operating point with a high current multiplication factor.

〔Example〕

FIG. 1 is a diagram showing one embodiment of an APD bias control circuit according to the present invention.

In the figure, 1 is an APD, 2 is a preamplifier, 3 is an amplifier, 4 is a bandpass filter, 5 is a detector, and 6 is a comparator.

The present invention will be described in detail below with reference to the drawings.

The APDI receives an optical signal and converts it into an electrical signal, and the converted electrical signal is amplified by a preamplifier 2, further amplified by an amplifier 3, and output.

In the present invention, a bandpass filter 4 is connected to the output of the preamplifier 2 to extract frequency components outside the used band from the electrical signal.

The extracted frequency components outside the used band are converted into DC voltage by the detector 5.

Input this DC voltage V to the comparator 6 to obtain the specified voltage value ■. Compare with.

If the DC voltage (2) is larger than the specified voltage value v0, it is fed back and the bias voltage is lowered.

If the DC voltage (■) is smaller than the specified voltage value V0, the bias voltage is not changed.

In this case, the specified voltage value (■) is set to a value smaller than the input DC voltage of the detector 5, which corresponds to the amount of noise generated just before the breakdown voltage VI. Set.

〔Effect of the invention〕

As explained in detail above, according to the present invention, the maximum bias voltage of the APD is controlled by the feedback circuit, and the maximum bias voltage of the APD is controlled by the feedback circuit.
This has the great effect of eliminating the influence of variations in the temperature characteristics of the D elements.

[Brief explanation of drawings]

FIG. 1 is a diagram showing one embodiment of an APD bias control circuit according to the present invention. FIG. 2 is a diagram illustrating the relationship between the bias voltage of the APD, the current multiplication factor a, and the dark current S. In the figure, 1 is an APD, 2 is a preamplifier, 3 is an amplifier, 4 is a bandpass filter, 5 is a detector, and 6 is a comparator.

Claims (1)

[Claims] In an optical-to-electrical conversion circuit using an avalanche photodiode 1, a preamplifier 2 for amplifying an electrical signal output from the avalanche photodiode 1, connected to the output of the preamplifier 2; a bandpass filter 4 for extracting noise outside the required band; a detector 5 for converting the output of the bandpass filter 4 into a DC voltage; a comparator 6 for comparing the DC voltage output from the detector 5 with a reference voltage; An APD bias control circuit comprising means for controlling the bias of the avalanche photodiode using the output of a comparator 6.