JPH03296309A - Optical reception circuit - Google Patents

Abstract

PURPOSE:To make a mis-alarm rate down to a prescribed value or below and to allow the result to render remarkable services to the improvement of the performance of a laser range finder or the like by varying a multiplication factor of an avalanche photodiode(APD) in response to the level of a background optical power made incident in the APD, and controlling the factor automatically to make the S/N to an optimum value. CONSTITUTION:The noise level of the output of an amplifier 4 is detected by a noise level detection circuit 9 and the bias voltage of an avalanche photodiode(APD) 2 is controlled by a bias control circuit 10 and a DC power supply 7 so that the upper limit of the noise level is controlled to be a prescribed value. The APD 2 is controlled to have an optimum multiplication factor in response to the level of the background optical power made incident therein.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical receiving circuit for a laser distance measuring device or the like.

[Conventional technology]

In laser ranging devices and the like, avalanche photodiodes (hereinafter referred to as APDs) are often used as light receiving elements in optical receiving circuits due to their high sensitivity and high speed response. APD has a multiplication factor that depends on the bias voltage.

By the way, it is applied to laser distance measuring devices and the like. The main noise sources in an optical receiving circuit using an APD as a light receiving element are shot noise due to background light, dark current of the APD, and thermal noise due to amplifier input resistance.

In this case, the signal-to-noise ratio (hereinafter referred to as S/N ratio) of the received signal is expressed by the following equation.

Here, P is the signal optical power, Ps is the background optical power, ■, the APD dark current 1M is the APD multiplication factor, and η is the quantum efficiency.

e is the charge, hν is the photon energy, 2 is the excess noise figure of the APD, B is the bandwidth, RL is the load resistance (amplifier input resistance 1
h is the Boltzmann constant, T is the absolute temperature, and F is the noise figure of the amplifier.

As can be seen from equation (1), the value of the APD multiplication factor M has an optimal value M, 5, which maximizes the S/N ratio.

It is expressed by the following formula.

As can be seen from equation (2), the optimal multiplication factor changes depending on the level of background light power incident on the APD.

Further, the multiplication factor M of the APD changes depending on the bias voltage V, and the relationship between them is given by the following equation.

Here, V is a breakdown voltage and m is a constant.

FIG. 3 is a diagram showing an example of the configuration of a conventional optical receiving circuit.
In the figure, (1) is a DC power supply, (2) is an APD, and (3
) is the received light, (4) is the amplifier, (5) is the reference power supply, (6
) is a comparator.

A DC power supply (1) applies a fixed bias voltage to the APD (2). A P D (2) is the incident received light (3
) generates an output current proportional to the strength of the Amplifier (4)
converts the output current from the A P D (2) into a voltage, amplifies it, and outputs it to the comparator (6). Further, the reference power source (5) outputs a threshold voltage of a predetermined level considering the noise of the two-light receiver circuit to the comparator (6). The comparator (6) is
The output signal level from the amplifier (4) is compared with a threshold voltage level from the reference power supply (5), and a signal exceeding the threshold level is converted into a binary level.

It is sent as a received signal to the next stage signal processing circuit (circuit).

[Problem to be solved by the invention]

As described above, in conventional optical receiving circuits, a fixed bias voltage is applied to the APD, that is, the multiplication factor is set to a fixed value, so when the background light power incident on the APD increases, shot noise increases. However, there is a problem in that the S/N ratio decreases or the noise level exceeds the threshold level of the comparator, increasing the false alarm rate where it is erroneously detected as a received signal.

This invention was made to solve the above-mentioned problems, and the multiplication factor of the APD changes according to the level of background light power incident on the APD to maintain the S/N ratio at an optimal value. It is an object of the present invention to provide an optical receiving circuit that can maintain a false alarm rate below a certain value even when the level of background optical power increases.

[Means to solve the problem]

The optical receiving circuit according to the present invention detects the noise level of the output signal from the amplifier, and controls the APD so that the noise level caused by background light incident on the APD does not exceed a predetermined value.
The bias voltage of the APD is controlled to control the multiplication factor of the APD.

[Effect]

In the optical receiving circuit according to the present invention, the multiplication factor of APD is AP
varies depending on the level of background light power incident on D, and S/
The N ratio is automatically controlled to the optimum value, and the S/N ratio is kept at the optimum value, and the false alarm rate is maintained below a certain value.

〔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 an optical receiving circuit according to the present invention.

In the same figure, (7) applies a bias voltage to the APD and also applies this bias voltage to the bias control signal (8).
), (9) is a noise level detection circuit that detects the noise level of the output signal from the amplifier (4), and (10) is a noise level detection circuit that detects the noise level detected by the noise level detection circuit (9) at a predetermined value. This is a bias pressure control circuit that outputs a bias control signal (8) to a DC power supply (7) so as to control the bias voltage of the APD so that it does not exceed .

In FIG. 1, the same or equivalent parts as in FIG. 3 are given the same reference numerals.

The operations of the APD (2), amplifier (4), reference power supply (5), and comparator (6) are the same as in FIG.

By the way, the noise level of the received signal is expressed by the following equation from equation (1).

Furthermore, as mentioned above, the image magnification M of the APD includes the S of the received signal.
In order to maintain the best /N ratio, there is an optimum value M6111 depending on the level of background light power, which is given by Equation 1 (2).

The noise level Nook when the bias voltage of the APD is controlled to achieve the optimum multiplication factor Mas according to the level of background optical power is obtained from Equation 7 (2) and Equation (4),
Determined by the thermal noise caused by the input resistance of the amplifier.

It does not depend on the background light power level and remains constant.

At this time, the noise level of the output signal of the amplifier (4) is expressed by the following equation.

2 RL where G is the gain of the amplifier (4). Therefore, the noise level detection circuit (9) detects the noise level of the output of the amplifier (4).
1 bias control circuit (10) and DC power supply
In (7), if the bias voltage of APD (2) is controlled so that the upper limit of the noise level is controlled to the value expressed by equation (6), APD (2) will be adjusted to the level of the incident background light power. Accordingly, the multiplication factor is controlled to be the optimum multiplication factor.

Furthermore, when controlled as described above, the false alarm rate of the received signal will be maintained below a certain value because the noise level will be limited to a predetermined value even if the background optical power level increases. .

Also, as seen from Equation 9 (8), Ysap is the amplifier (
4), it is desirable to change the value that limits the upper limit of the noise level in accordance with the temperature of the amplifier (4).

FIG. 2 is a configuration diagram showing an embodiment of the optical receiving circuit in this case. In FIG. 2, the same or equivalent parts as in FIG. 1 are given the same reference numerals.

A temperature detector (11) is placed close to the amplifier (4) to detect the temperature of the amplifier (4), and a bias voltage control circuit (10) is configured to adjust the temperature in response to the temperature change of the amplifier (4). ,
By changing the value that limits the upper limit of the noise level, it is possible to always maintain an optimal state even with temperature fluctuations.

〔Effect of the invention〕

As described above, according to the present invention, the multiplication factor of APD is A
It changes depending on the level of background light power incident on the PD, and S
Since the N ratio is automatically controlled to the optimum value, the AP
Even if the level of background light power incident on D changes, the S/N ratio of the optical receiver circuit can be maintained at an optimal value, and the false alarm rate can be kept below a certain value, greatly contributing to improving the performance of laser distance measuring devices, etc. .

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of one embodiment of the optical receiving circuit according to the present invention, FIG. 2 is a diagram showing the configuration of another embodiment of the optical receiving circuit according to the present invention, and FIG. 3 is a diagram showing the configuration of a conventional optical receiving circuit. FIG. In the figure, (1) is a DC power supply, (2) is an avalanche photodiode, (3) is a received light, (4) is an amplifier, (5) is a reference power supply, (6) is a comparator, and (7) is a (8) is a bias control signal; (9) is a noise level detection circuit; (10) is a bias voltage control circuit; (11) is a temperature detector. Note that in the two figures, the same reference numerals indicate the same or equivalent parts. L.

Claims (4)

[Claims] (1) An avalanche photodiode that photoelectrically converts received light, a DC power supply that applies a bias voltage to the avalanche photodiode and can vary this bias voltage using a bias control signal, and an output current from the avalanche photodiode. an amplifier for converting and amplifying the voltage signal into a voltage signal; a noise level detection circuit for detecting the noise level of the output signal from the amplifier; and an output signal from the amplifier caused by background light incident on the avalanche photodiode. bias voltage control for outputting a bias control signal to the DC power supply so as to control the bias voltage of the avalanche photodiode so that the noise level of the avalanche photodiode does not exceed a predetermined value, and to control the multiplication factor of the avalanche photodiode; An optical receiving circuit comprising: a reference power source that generates a reference voltage; and a comparator that converts the difference between the output voltage of the reference power source and the output signal voltage from the amplifier into a binary signal. (2) The bias voltage control circuit sets a value that limits the upper limit of the noise level of the output signal from the amplifier to the multiplication factor of the avalanche photodiode at which the signal-to-noise ratio of the received signal becomes a substantially optimal value. 2. The optical receiver circuit according to claim 1, wherein the noise level of the output signal of the amplifier that is generated when a bias voltage that obtains the above is applied to the avalanche photodiode. (3) The bias voltage control circuit is characterized in that the value for controlling the upper limit of the noise level of the output signal from the amplifier is substantially determined by the following equation. ) The optical receiving circuit described in section 2. V_n=(x+2/x)^1^/^2(4kTBF/R
_L)^1^/2・R_L・G where V_n is the noise level of the output signal from the amplifier, x is the excess noise figure of the avalanche photodiode, k is the Boltzmann constant, T is the temperature of the amplifier, and B is the The bandwidth of the amplifier, F is the noise figure of the amplifier, R_L is the input resistance of the amplifier, and G is the gain of the amplifier. (4) The bias voltage control circuit is configured to change a value for controlling an upper limit of the noise level of the output signal from the amplifier in response to a temperature change in the amplifier. The optical receiving circuit according to paragraph (1).