JPH05129857A - Gain control method for avalanche photodiode - Google Patents

Abstract

PURPOSE:To keep an avalanche photodiode at an optimum gain (maximum gain) regardless of the change of a surrounding temperature or the change of each characteristic. CONSTITUTION:This method is provided with a DC DC converter 3 to impress a reverse bias to an avalanche photodiode 1, comparator A provided at the output of an amplifier 10 of the diode 1 reference voltage generating means A' to set the noise level of a maximum S/N to this comparator A, and MPU 14 to decide a controlled voltage to the DC/DC converter 3 based on the output of the comparator A, while stopping photodetection, a set voltage value is compared with the noise level of the avalanche photodiode 1, and the controlled voltage is increased/decreased so as to make a bias voltage maximum in a range so that the noise level can not exceed the set voltage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gain control method for an avalanche photodiode, which keeps the gain of an avalanche photodiode light receiving portion at an optimum (maximum) regardless of changes in ambient temperature and variations in characteristics of the avalanche photodiode.

[0002]

2. Description of the Related Art Avalanche photodiodes (hereinafter referred to as AP
D) performs the amplification of carriers generated by light by utilizing the electron avalanche phenomenon of a diode to which a reverse bias is applied, and has both a high multiplication factor for incident light and a high-speed response characteristic. For this reason, it is often used for optical fiber communication and for detecting weak optical signals.

[0003] The multiplication factor M of the APD is a break voltage of the diode V _b, when the bias voltage is V,

[0004]

[Equation 1]

It is shown by the simple function As is clear from the above equation, in order to obtain a high multiplication factor M, it is best to use the bias voltage V near the break voltage _Vb , and it is usually used in that way.

However, the break voltage V _{b of the} APD is
For example, silicon has a temperature coefficient of about +0.5 V / ° C., and at a constant bias voltage, the multiplication factor varies with changes in the ambient temperature. Therefore, in order to keep the multiplication factor constant, it is necessary to change the bias voltage V according to the change in the break voltage V _b .

Therefore, conventionally, as shown in FIG. 6, an AGC circuit 2 is provided in the light receiving circuit, the high voltage control power supply 3 is controlled by the AGC circuit 2, and the bias voltage V applied to the APD 1 is changed and output. A method of keeping the level constant, or a temperature compensation circuit 6 using a diode 5 or a varistor having the same temperature coefficient of forward voltage as shown in FIG.
Similar to the above, a method is used in which the high voltage power supply 3 is controlled to correct the bias voltage V to reduce the gain variation due to temperature.

[0008]

However, both of the above methods are applied to, for example, a receiver or an optical device for measuring a distance or velocity using an optical pulse to which a burst-shaped optical pulse having a wide dynamic range is intermittently incident. When the pulse is incident on the optical fiber and is used for the photodetector for measuring the temperature distribution of the fiber, the following problems occur in each method.

First, in the case of using the AGC circuit, the AG
When the time constant of the C circuit is set to be larger than the pulse width of the burst input, the gain becomes maximum during the light receiving pause period when there is no burst input, and follows the burst input having a wide dynamic range received after the light receiving pause period. Can not. Therefore, if the time constant is set smaller than the pulse width of the burst input, the output waveform is distorted due to a gain change during the burst wave input. Further, the gain is also temperature, AP
It constantly changes due to variations in the characteristics of D.

Next, in the method of reducing the fluctuation of the gain by the temperature compensation circuit, the gain is fixed, so that the dynamic range may be insufficient due to the intensity of the incident light.
Also, the constants of the components used in the temperature compensation circuit are APD
Adjustment work is required because it must be changed due to variations in individual characteristics.

Therefore, an object of the present invention is to pay attention to variations in the characteristics of the APD, ambient temperature, and changes in the APD.
The maximum gain of PD is to be obtained.

[0012]

In order to solve the above problems, a reverse bias voltage is applied to an avalanche photodiode by a bias voltage control power supply whose output voltage is varied by a control input, and the avalanche photodiode is intermittently injected. First comparing means for comparing the light receiving portion output with a first set voltage output from the first reference voltage generating means to a light receiving portion for outputting a light receiving input through an amplifier;
And a control unit that determines a control input to the bias voltage control power supply based on the output of the comparison unit, and repeatedly compares the set voltage and the light receiving unit output for each light receiving suspension period, and includes the light receiving unit output. The control input is increased or decreased so that the bias voltage is maximized within a range in which the noise component does not exceed the set voltage level.

Further, at this time, the light receiving section is provided with second comparing means for comparing the output of the light receiving section during the light reception with the second set voltage output from the second reference voltage generating means,
It is also possible to use a method of decreasing the bias voltage so that the output of the light receiving unit is equal to or lower than the second set voltage when the output of the light receiving unit exceeds the set voltage.

The first set voltage is preferably set to a voltage which is about 2 to 10 times the noise level when the APD is not broken down.

Further, the second set voltage is set to a voltage slightly smaller than the saturation voltage of the amplifier for amplifying the output of the APD, for example.

[0016]

With such a gain control method for the APD, the APD
The optimum multiplication factor that maximizes the S / N ratio (maximum gain)
Alternatively, set an arbitrary multiplication factor.

That is, as the bias voltage of the APD increases, the multiplication factor also increases, and impulse noise appears at the output at a certain bias voltage. This noise increases rapidly with increasing bias voltage.
The noise is mainly due to shot noise in the opto-electrical conversion of the APD and gain fluctuation in current multiplication,
If the multiplication factor is made excessively large, the noise also increases, and the noise is increased beyond the signal power to reduce the SN ratio.

Therefore, in this control method, the noise level is detected, this noise level is compared with a noise level preset so as to obtain the optimum multiplication factor that maximizes the SN ratio, and the noise level set is The bias voltage is controlled so as to bring the detected noise levels closer to each other, and the APD is controlled to the optimum multiplication factor regardless of variations in characteristics. At this time, the APD can be set to an arbitrary multiplication factor by changing the setting level.

The detection of the noise level and the control of the multiplication factor are repeated during the light-reception pause period during the reception of no light into the APD, and the multiplication factor is set again. The influence of the APD on the optimum multiplication factor is corrected, and the optimum multiplication factor of the APD is maintained so that the gain is always maximized.

Further, in the device provided with the second comparing means and the second reference voltage generating means, when the saturation of the amplifier is detected, the bias voltage is lowered to lower the multiplication factor of the APD to lower the incident light sensitivity. Widen the dynamic range of incident light.

[0021]

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

As shown in FIG. 1, the optical receiver of this embodiment is
The DC-DC converter 3 is connected to the cathode of the APD 1 as a bias voltage control power supply, and the light receiving unit R in which the amplifier 10 is connected to the anode of the APD 1 and the amplifier 10 output of the light receiving unit R are connected to the amplifier 10 output and the reference. The comparators A and B as comparing means for comparing the set voltages V _A and V _{B of the} voltage generating means A ′ and B ′ with the comparators A and B, and DC-via the D / A converter 13 It comprises a DC converter 3 and a microprocessor (hereinafter referred to as MPU) 14 as a control means connected to the DC converter 3.

The DC-DC converter 3 of the light receiving section R is for generating a high voltage for reverse biasing the APD 1 from a low voltage source in the light receiver, and the generated voltage is a DC voltage applied to the control input. It is variable.

Further, the amplifier 10 has a current-voltage conversion circuit, converts the photocurrent output from the APD 1 into a voltage, amplifies and outputs the voltage.

As the comparator A, for example, a comparator IC for single power source operation is used, the output of the amplifier 10 is connected to the inverting input of the comparator A, and the reference voltage generating means A'is connected to the non-inverting input. In the present embodiment, the reference voltage generating means A'sets its output voltage to be about 2 to 10 times the noise level when the APD 1 is not broken down. The output of the comparator A is connected to the I / O input of the MPU 14 or the data bus via the latch 15, and the output falls when the output of the amplifier 10 exceeds the set voltage V _A of the reference voltage generating means A ′. (Negative logic). The falling edge is latched and read by the MPU 14.

Similar to the comparator A, the comparator B has the inverting input connected to the output of the amplifier 10 and the non-inverting input connected to the reference voltage generating means B '. In the present embodiment, the reference voltage generating means B ′ is set to a set voltage V _B which is slightly smaller than the saturated output voltage of the amplifier 10. The output of the comparator B is connected to, for example, the data bus or the interrupt terminal of the MPU 14, and when the output of the amplifier 10 exceeds the set voltage V _B , the output falls,
The MPU 14 is notified of the saturation of the amplifier 10.

The MPU 14 includes the comparators A and B.
Output data is increased / decreased and output according to the built-in processing program based on the output from. This data is converted into an analog output by the D / A converter 13 and is converted into DC-DC.
It is applied to the control input of the converter 3 and controls the output voltage.

This embodiment is configured as described above. Now, the present control method when this photodetector is used in, for example, a distance measuring system using an optical burst wave will be described with reference to the flowchart of FIG.

The distance measuring system is one in which a laser pulse burst wave is intermittently emitted from a laser oscillator to the object to be measured, the reflection time of the reflected light is measured, and the distance from the measurement time to the object is calculated. In some cases, the reflection angle of the reflected light is measured and the distance to the object is calculated by so-called triangulation. In any case, the reflected light from the object is detected by the light receiver.

First, prior to the emission of the measurement laser pulse burst wave from the measuring device, the MPU 14 of the photodetector starts the bias voltage setting process (“process” 100 and subsequent “process” are omitted).

The MPU 14 outputs the voltage data to the D / A converter 13, increases the output data of the DC-DC converter 3 in Xv steps (101), reads the output of the comparator A each time it increases, and the comparator A reads the output. The bias voltage V is gradually increased until the output of (10) falls (10
2).

At this time, as the bias voltage V is increased, an impulse noise N appears as shown in FIG. 3A, and this noise N is shown in FIG. 3B as the bias voltage increases. As described above, the voltage rapidly increases and exceeds the set voltage V _a of the comparator A. This state will be referred to as “yield” here for convenience. The MPU 14, which has detected such a “breakdown” state, gradually reduces the bias voltage V in Yv (Y <X) steps (103), and reduces the bias voltage to a level at which the “breakdown” is not detected. That is, the bias voltage V is lowered until the output of the comparator A always outputs a high level (104). At this time,
Bias voltage V at the optimum gain when Y = 0.1 to 2V
Is preferably 0.1 to 2 V lower than the bias voltage V at breakdown.

At this time, if there is no laser pulse emission command (105), the above process is repeated (101 to 1).
05). On the other hand, if there is a laser pulse emission command (105), the reflected light of the emitted laser pulse (106) is received (107). That is, the received optical burst wave is converted into a photocurrent by the APD 1, converted into a voltage by the amplifier 10 in the next stage, and then amplified and output. This output is compared with the set voltage _Vb by the comparator B (108), and the saturation of the amplifier 10 is determined.

At this time, the saturation of the amplifier 10, that is,
When the comparator B falls (108), the MPU14
Lowers the bias voltage in Zv steps (10
9) The laser pulse is emitted again (106), and the bias voltage V is applied until the output of the comparator B becomes high level.
Lower.

On the other hand, when the output of the amplifier 10 is not saturated during light reception, that is, when the output of the comparator B is in the high level state, the light reception is completed (110) (11).
2). After this, in the case of continuing the measurement, the above processing (101 to
Repeat 111).

As a result of performing the gain control in this way,
For example, when the InGaAs-APD is controlled according to this flowchart, the gain-ambient temperature characteristic can maintain the gain substantially constant over a relatively wide range of the ambient temperature as shown in FIG.

That is, in the APD 1, the gain characteristic has a large temperature dependency, and the break voltage also largely changes with temperature as shown in FIG. 5, and the break voltage is different even at the same temperature due to variations in individual characteristics of the APD 1. When the setting of the bias voltage V is repeated during the light receiving pause period by the above method, variations in ambient temperature and characteristics are corrected and A
PD1 can always be controlled to the optimum gain (gain maximum).

Further, by subtracting a constant value from the bias voltage of the set voltage, it is possible to obtain a constant arbitrary gain for a plurality of light receivers regardless of the type and characteristics of the APD.

Further, in principle, in this method, the noise of the photo detector output becomes Vav or less, so that the noise management of the circuit becomes easy.

The gain control method of the present invention is not only applied to a photodetector for distance and speed measurement using an optical burst wave, but the above method may be used, for example, by providing an interval period during communication to obtain an optical fiber. It can be considered to be used as a light receiver for communication.

[0041]

The present invention has the following effects by the above method.

The optimum gain (maximum gain) point of the APD can be obtained carelessly with respect to variations in the characteristics of the APD, ambient temperature, and changes in the ambient temperature.

The output noise of the light receiver is set to the set voltage Vav.
It can be managed below.

The APD gain is, for example, MP in the control means.
When U is used, the gain can be easily changed by rewriting the program, and the dynamic range of incident light can be increased.

If the light receiver is equipped with an MPU, the circuit can be constructed at low cost simply by adding a D / A converter, a latch, a comparator, and the like.

In particular, this method is effective when used in a photodetector that measures a distance and a velocity using an optical burst wave.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment.

FIG. 2 is a flowchart of an embodiment.

[Fig. 3] Action diagram

[Fig. 4] Same as above

[FIG. 5] Same as above

FIG. 6 is a block diagram showing a conventional example.

FIG. 7 is a block diagram showing a conventional example.

[Explanation of symbols]

1 APD 3 DC-DC converter 10 Amplifier 13 D / A converter 14 MPU A 1st comparator A'1st reference voltage generation means B 2nd comparator B'2nd reference voltage generation means R Light receiving part _{VA A} 1 set voltage (“breakdown level”) V _B 2nd set voltage (saturation level of amplifier)

Claims (2)

[Claims] 1. A light receiving section for applying a reverse bias voltage to an avalanche photodiode by a bias voltage control power supply whose output voltage is varied by a control input, and outputting a light receiving input intermittently incident on the avalanche photodiode through an amplifier. A first comparing means for comparing the output of the light receiving section with a first set voltage output from the first reference voltage generating means; and a control input to the bias voltage control power supply based on the output of the comparing means. And a control unit for determining the bias voltage within a range in which the noise component contained in the light receiving unit output does not exceed the set voltage level. A gain control method for an avalanche photodiode, which is characterized by increasing or decreasing a control input so as to maximize the gain. 2. A gain control method for an avalanche photodiode according to claim 1, wherein the light receiving section is provided with an output while the light receiving section is receiving light and a second set voltage output from the second reference voltage generating means. An avalanche photodiode comprising a second comparing means for comparing, wherein when the output of the light receiving unit exceeds the second set voltage, the bias voltage is reduced and the output of the light receiving unit is set to the set voltage or less. Gain control method.