JPH0440025A - Optical input disconnection detecting circuit - Google Patents

Abstract

PURPOSE:To stably detect the disconnection of an optical input without increasing the noise of an avalanche photodiode (APD) by switching a variable gain amplifying means and an APD bias supplying means corresponding to whether a control amount exceeds a prescribed value or not so as to variably operate one means and to fix the other means. CONSTITUTION:When the input level of an optical signal is low, the control amount to be outputted from a comparative control means 5 is enlarged and a control switching means 6 fixes the output of an APD bias supplying means 2 and variably operates a variable gain amplifying means 3. On the other hand, when the input level of the optical signal is high, the output of the variable gain amplifying means 3 is fixed and the APD bias supplying means 2 is variably operated. Thus, since a bias voltage is fixed at a constant value when the input level of the optical signal is low, the shot noise is prevented from being increased due to the amplification factor becoming extremely large.

Description

[Detailed Description of the Invention] [Summary] An optical signal attached to an optical signal receiving circuit that performs photoelectric conversion using an avalanche photodiode (APD) and amplifies the output of the avalanche photodiode by AGC control. Regarding the optical input disconnection detection circuit, the aim is to provide an optical input disconnection detection circuit that can stably detect optical input disconnection without increasing shot noise of the APD. - A photodiode (APD), an APD bias supply means for supplying a variable bias voltage to the avalanche photodiode, and a variable gain amplifier that has a variable gain and amplifies the electrical signal output from the avalanche photodiode. means, a peak detecting means for detecting a peak level of the output of the variable gain amplifying means, and comparing the peak level detected by the peak detecting means with a predetermined threshold, and determining the peak level and the threshold. a comparison control means for outputting a control amount according to a difference between the control amount and the control amount; and one of the variable gain amplification means and the APD bias supply means is variably operated depending on whether the control amount exceeds a predetermined value. , control switching means for switching to fix the other; window means for detecting whether the output level of the variable gain amplification means is within a predetermined range between a high level and a low level; and counting means for detecting the frequency with which the window means detects that the frequency is within a predetermined range between the low level and the variable gain amplification means and the APD bias supply means, respectively. , is configured to be performed in accordance with the control amount.

[Industrial application field]

The present invention is an avalanche photodiode (APD)
The present invention relates to an optical signal disconnection detection circuit attached to an optical signal receiving circuit that performs photoelectric conversion using an avalanche photodiode and amplifies the output of the avalanche photodiode using AGC control.

In optical communications, avalanche photodiodes are used to convert optical signals into electrical signals. Furthermore, in terms of system operation, it is necessary to determine the presence or absence of optical signal input. However, due to the characteristics of the avalanche photodiode, it has been difficult for conventional optical signal receiving circuits to detect interruption of optical signal input.

[Prior art and problems to be solved by the invention] No. 5
The figure shows a schematic configuration of an optical signal receiving circuit. That is, the optical signal receiving circuit consists of an APD section and an electric signal amplification section, and the gain of the APD section is changed by changing the bias voltage, and the gain of the electric signal amplification section is also independently variable. Generally, the intensity of an optical signal is not constant, but by adaptively changing the gains in the APD section and the electrical signal amplification section, an electrical signal with a constant amplitude is obtained.

FIG. 6 shows the relationship between optical signal input level and gain in the APD section and electrical signal amplification section in a conventional optical signal receiving circuit. As shown in FIG. 6, in the conventional optical signal receiving circuit, on the low level side of the optical signal input, the gain of the electrical signal amplifying section is kept constant and the APD
By controlling the gain (APD multiplication factor) of the APD section, the above-mentioned electric signal with a constant amplitude is obtained. On the high level side of the optical signal, the gain of the APD section (APD multiplication factor) is kept constant and the electric signal is By controlling the gain of the signal amplification section, the above-mentioned electric signal of constant amplitude is obtained.

On the other hand, conventional methods for detecting optical input disconnection have been to detect a decrease in the timing component reproduced from the optical input signal, but since the timing circuit has a high gain, oscillation is likely to occur, making disconnection detection difficult. There was a problem with instability. Therefore, it has been considered to perform disconnection detection using a window identification circuit that detects whether the output level of the electrical signal amplification section is within a predetermined range between a high level and a low level. For example, when performing photoelectric conversion using a PIN diode, when the optical input signal is cut off, the level distribution of the output signal of the electrical signal amplification section no longer has peaks at high and low levels, and Since the frequency in the middle range becomes large, the above-mentioned disconnection detection method using the window identification circuit is effective. However, when performing photoelectric conversion using an avalanche photodiode, the input level of the optical signal is low (
As the multiplication factor of the APD increases and shot noise increases, the level distribution of the output signal of the electrical signal amplification section will concentrate at a high level even if the optical input signal is cut off, as shown in Figure 7. (as shown in Figure 8,
There has been a problem in that the increase in frequency in the intermediate range between high and low levels (compared to normal optical signal input) is no longer noticeable.

The present invention has been made in view of the above problems.
It is an object of the present invention to provide an optical input interruption detection circuit that can stably detect optical input interruption without increasing the shot noise of D.

[Means to solve the problem]

FIG. 1 is a basic configuration diagram of the present invention. In Figure 1,
1 is an avalanche photodiode (APD), 2
3 is an APD bias supply means, 3 is a variable gain amplification means, and 4 is an APD bias supply means.
5 is a peak detection means, 5 is a comparison control means, 6 is a control switching means, 7 is a window means, and 8 is a counting means.

The avalanche photodiode (APD) 1 receives an optical signal and converts it into an electrical signal.

APD bias supply means 2 supplies a variable bias voltage to the avalanche photodiode 1.

The variable gain amplification means 3 has a variable gain and amplifies the electrical signal output from the avalanche photodiode 1.

The peak detection means 4 detects the peak level of the output of the variable gain amplification means 3.

The comparison control means 5 compares the peak level detected by the peak detection means 4 with a predetermined threshold, and outputs a control amount according to the difference between the peak level and the threshold.

The control switching means 6 switches between the variable gain amplification means 3 and the A depending on whether the control amount exceeds a predetermined value.
One of the PD bias supply means 2 is operated variably and the other is switched to be fixed.

The window means 7 detects whether the output level of the variable gain amplification means 3 is within a predetermined range between a high level and a low level.

The counting means 8 detects the frequency with which the window means 7 detects that the level is within a predetermined range between the high level and the low level.

Variable operations in the variable gain amplification means 3 and the APD bias supply means 2 are performed in accordance with the control amounts, respectively.

[For production]

According to the present invention, when the optical signal input level is low, the difference between the output level of the peak detection means 4 and the predetermined threshold is large, so that the control amount output from the comparison control means 5 becomes large. The control switching means 6 fixes the output of the APD bias supply means 2 and causes the variable gain amplification means 3 to operate variably.

The variable gain amplification means 3 changes the gain according to the above-mentioned control amount, and controls the output so that the peak level thereof is constant. When the optical signal input level is high, since the difference between the output level of the peak detection means 4 and the predetermined threshold is small, the control amount output from the comparison control means 5 becomes small, and the control switching means 6 The output of the variable gain amplification means 3 is fixed, and the APD bias supply means 2 is operated variably. The APD bias supply means 2 changes the output bias voltage according to the above-mentioned control amount, and controls the output so that the peak level thereof is constant.

In this way, when the optical signal input level is low, the bias voltage supplied to the APD is fixed at a constant value, and the multiplication factor does not become abnormally large and increase shot noise.

〔Example〕

FIG. 2 is a diagram showing the configuration of an embodiment of the present invention.

In FIG. 2, 11 is an APD, 12 is a preamplifier, 1
3 is a variable gain amplifier, 14 is a DC/DC converter, 1
5, 18, 20 and 21 are comparison circuits, 16 is a peak detection circuit, and 17 is V A G. Generation circuit, 19 is VA
Pn control circuit, 22 and 23 are D flip-flop circuits, 24 is an AND circuit, 25 is a counter, and 2
6 is a timer.

The optical signal is converted into an electrical signal by the APD II, amplified by the preamplifier 12, and then amplified by the variable gain amplifier 13 and supplied to the fourth circuit and the timing extraction circuit.

The bias voltage of APDII is supplied from the DC/DCC/equation converter 14 according to the control voltage output from the vapn control circuit 19. Further, the gain of the variable gain amplifier 13 is V
It is changed according to the control voltage generated by the AGc generation circuit 17.

The peak detection circuit 16 detects the peak level of the output of the variable gain amplifier 13, and the detected peak level is compared with the reference voltage V^ in the comparison circuit 18.
8 generates a control voltage according to the difference between the detected peak level and the reference voltage VA. The output of the comparison circuit 18 is supplied to the V A G C generation circuit 17 and the VAPD control circuit 19 . VAo. Generation circuit 17 and VA
During each variable operation, the Pf1 control circuit 19 sends a control signal corresponding to the control voltage output from the comparator circuit 18 to the variable gain amplifier 13 and the DC/DCC/equation
Supply to 4.

The comparison circuit 15 compares the output level of the V Ar, c generation circuit 17 with the reference voltage VB, and determines that the output level of the AGc generation circuit 17 is equal to the reference voltage v! When it is less than 1, VA
By fixing the output of the pn control circuit 19, the VAGC generation circuit 1
7 is operated variably. Also, vA... When the output level of the generating circuit 17 exceeds the reference voltage V, V A G
By fixing the output of the C generation circuit 17, the vApn control circuit 1
9 is operated variably.

FIG. 3 shows how the gains in APD II and variable gain amplifier 13 are controlled by the switching operation as described above. That is, when the optical signal input level is low, the difference between the output level of the peak detection circuit 16 and the reference voltage VA is large, so the control voltage output from the comparison circuit 18 becomes large, and the output of the comparison circuit 15 becomes: v A
By fixing the output of the po control circuit 19, V A G C
The generating circuit 17 is operated variably. The variable gain amplifier 13 changes its gain in accordance with the output of the V A g c generation circuit 17, and is controlled to keep the peak level of the output constant. When the optical signal input level is high, the difference between the output level of the peak detection circuit 16 and the reference voltage ■^ is small, so the control voltage output from the comparison circuit 18 becomes small, and the output of the comparison circuit 15 becomes VAG . The output of the generation circuit 17 is fixed, and the VAP and control circuit 19 are operated variably. DC/DCC/Formula 14 is the above VAP
The bias voltage supplied to APDI 1 is changed in accordance with the output of control circuit 19, and the multiplication factor of APD II is changed to control the peak level of the output to be constant.

In this way, when the optical signal power level is low, the bias voltage supplied to the APD is fixed at a constant value, and the multiplication factor does not become abnormally large and increase shot noise. Therefore, when the optical input signal is cut off in the above configuration, the distribution of the output signal level of the variable gain amplifier 13 becomes as shown in FIG. (in comparison with time)
The increase in frequency in the range intermediate between high and low levels becomes significant.

The comparator circuit 20 in FIG. 2 converts the output signal of the variable gain amplifier 13 into a reference voltage V, h+Δ, which is higher by a predetermined width Δ than Vth, which is equal to the intermediate level between the high level and the low level.
(See FIG. 4), when the output signal of the variable gain amplifier 13 is lower than the reference voltage V+Δ, it outputs "1", and when it is higher, it outputs "0". On the other hand, the comparator circuit 21 converts the output signal of the variable gain amplifier 13 to a reference voltage Vth-Δ (see FIG. ), when the output signal of the variable gain amplifier 13 is higher than the reference voltage Vzh-Δ, it outputs "1", and when it is lower, it outputs "0". The outputs of the comparison circuits 20 and 21 are latched in the D flip-flop 1 circuits 22 and 23, respectively, in each cycle, and the outputs of the D flip-flop circuits 22 and 23 are logically ANDed in the AND circuit 24, and then the counter 2
5 counts the number of cycles in which the AND circuit 24 is "1" for a predetermined time set in the timer 26. In this way, the frequency at which the output signal of the variable gain amplifier 13 changes from the voltage Vth-Δ, which is between the high level and the low level, to Vth+Δ is detected. In the counter 25, when the count at a predetermined time exceeds a predetermined value, a signal disconnection signal is output.

〔Effect of the invention〕

According to the present invention, optical input interruption can be stably detected without increasing shot noise of the APD.

[Brief explanation of drawings]

FIG. 1 is a basic configuration diagram of the present invention, FIG. 2 is a configuration diagram of an embodiment of the present invention, and FIG. 3 is an optical signal input level in the APD and electric signal amplification circuit in the optical signal receiving circuit of the present invention. FIG. 4 is a diagram showing the distribution of the output signal level when optical input is cut off in the optical signal receiving circuit of the present invention, and FIG. 5 is a diagram showing the schematic configuration of the optical signal receiving circuit. , FIG. 6 is a diagram showing the relationship between optical signal input level and gain in the APD section and electrical signal amplification section in the conventional optical signal receiving circuit, and FIG. 7 is a diagram showing the relationship between the optical signal input level and the gain using the conventional APD. A diagram showing the distribution of the output signal level when the optical input is cut off in the receiving circuit, and FIG. 8 shows the distribution of the output signal level when the optical signal is normally input in the AGC control optical signal receiving circuit using the conventional APD. It is a diagram. [Explanation of symbols] 1... Avalanche photodiode (APD),
2... APD bias supply means, 3... Variable gain amplification means, 4... Peak detection means, 5... Comparison control means, 6... Control switching means, 7... Winding means, ...Counting means, 11...APD, 12...Preamplifier, 13...Variable gain amplifier, 14...DC
/DC converter, 15.18, 20.21... Comparison circuit, 16... Peak detection circuit, 17... V A
G (Generation circuit, 19... V A P D control circuit,
22.23...D flip-flop circuit, 24...
AND circuit, 25...counter, 26...timer. Gain diagram Diagram diagram showing the output signal level distribution when optical signal input is interrupted Diagram diagram showing the output signal level distribution when optical signal input is normal Diagram diagram

Claims (1)

[Claims] An avalanche photodiode (APD) (1) that inputs an optical signal and converts it into an electrical signal, and an APD bias supply means (1) that supplies a variable bias voltage to the avalanche photodiode (1). 2), variable gain amplification means (3) with variable gain, which amplifies the electrical signal output from the avalanche photodiode (1); and a peak level of the output of the variable gain amplification means (3). A peak detection means (4) for detecting, and a control according to the difference between the peak level and the threshold by comparing the peak level detected by the peak detection means (4) with a predetermined threshold. Comparison control means (5
), and depending on whether or not the controlled amount exceeds a predetermined value, one of the variable gain amplification means (3) and the APD bias supply means (2) is operated variably, and the other is switched to be fixed. control switching means (6); window means (7) for detecting whether the output level of the variable gain amplification means (3) is within a predetermined range between a high level and a low level; and a high level. and a counting means (8) for detecting the frequency at which the window means (7) detects that the frequency is within a predetermined range intermediate between the variable gain amplification means (3) and the low level. An optical input interruption detection circuit, wherein the variable operation in the APD bias supply means (2) is performed in accordance with the control amount.