JPH01245725A - Optical reception circuit - Google Patents

Abstract

PURPOSE:To operate normally an alarm function even if an APD bias voltage is set higher in the presence of a signal by detecting a current flowing through an avalanche photodiode(APD) and the presence or absence of a transmission signal and controlling the APD bias voltage low in the absence of signal and high in the presence of signal. CONSTITUTION:A current detection circuit 20 generates an output when a current flowing to the APD 1 exceeds a prescribed value in a region where a dark current increases rapidly. A transmission signal detection circuit 30 generates when an optical transmission signal of a prescribed level or over is inputted to the APD 1. A control circuit 40 acts like raising an output voltage of a high voltage generating circuit 50 and increasing a bias voltage of the APD 1 through the combination of both outputs only when the optical transmission signal is sent at a prescribed level or over. In the case of no optical input to the APD 1, the APD 1 receives a low bias voltage. When an output is given from the APD 1 by the input of the optical transmission signal, the bias voltage of the APD 1 is increased, and in the absence of the optical transmission signal, the dark current is increased rapidly, and an output appears from the current detection circuit 20, an output voltage of the high voltage generating circuit 50 is dropped to lower the bias voltage.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides an optical receiving circuit using an avalanche photodiode (APD) as a light receiving element, particularly a circuit that prevents malfunction of an alarm circuit for determining a decrease in the amount of incident light. It is related to.

[Conventional technology] FIG. 5 shows an example of the configuration of a conventional optical receiving circuit.

Resistors 2 and 3 with resistance values R1 and R2 are connected in series to an avalanche photodiode (APD) 1, and the alarm circuit calculates the voltage drop (Io , the comparator 5 compares it with the threshold voltage V tht . The load resistor 2 is grounded via a capacitor 4 connected in parallel to the resistor 3 in terms of AC. When the amount of light Pr entering the APDI drops to the minimum reception level at which the signal after photoelectric conversion cannot be identified, the voltage drop across the resistor 2 becomes less than the threshold voltage V th1, so the comparator 5 operates and outputs an alarm. v1 switches from logic "0" to rlJ. This operation determines whether the level of the amount of incident light Pr has decreased.

Note that the signal component from the APDI is extracted from the terminal of the resistor 3 via the capacitor 6 and the waveform shaping circuit 7, but if the amount of incident light Pr has decreased to the above-mentioned minimum reception level, for example under abnormal conditions. Measures are taken to distinguish this signal from a normal signal.

[Problems to be Solved by the Invention] Figure 6 shows the APD bias voltage VA applied across the APD.
The relationship between PD and dark current flowing through the APD when there is no incident light amount Pr is shown. In the case of APD, since the dark current I n passing through the high electric field region undergoes avalanche multiplication,
The APD bias voltage VAPD increases to a certain value V[+
The closer to R, the more rapidly the dark current In of the APD tends to increase.

If the APD bias voltage VAP11 is set high near the above value VOR, even if the incident light amount Pr has decreased beyond the identification level and an alarm signal should be output, the noise current is increasing and the resistor 3 The voltage across the voltage exceeds the threshold voltage Vth1, and the comparator 5 does not operate. Therefore, even if the amount of incident light Pr decreases, no alarm signal will be output.

Normally, in order to operate the alarm circuit normally, the APD bias voltage VAP11 must be lowered sufficiently. However, from the perspective of transmission characteristics, the APD bias voltage V
A higher Apn is advantageous because the multiplication factor increases.

The purpose of the present invention is to eliminate the drawbacks of the prior art described above, and to
It is an object of the present invention to provide a novel optical receiving circuit that can normally operate an alarm circuit even if the bias voltage VAPD of the PD is set near the above-mentioned VIIR.

[Solving the Problem] The optical receiving circuit of the present invention includes an avalanche photodiode and detects when the amount of current flowing through the avalanche photodiode exceeds a certain value in a region where dark current rapidly increases. a transmission signal detection circuit that detects the presence or absence of an optical transmission signal of a certain level or higher; a high voltage generation circuit with a variable output voltage for driving the avalanche photodiode; and the current amount detection circuit. Control that adjusts the output voltage of the high voltage generation circuit based on a combination of the output signal of the circuit and the output signal of the transmission signal detection circuit, and increases the output voltage of the high voltage generation circuit only when there is an optical transmission signal of a certain level or higher. A circuit is provided, and the alarm signal is extracted using the output of the transmission signal detection circuit.

[Function] The current amount detection circuit generates an output when the amount of current flowing through the APD exceeds a certain constant value in a region where dark current rapidly increases. Further, the transmission signal detection circuit generates an output when the optical transmission signal is input to the APD at a certain level or higher. The control circuit receives both outputs and operates to increase the output voltage of the high voltage generation circuit and increase the drive voltage of the APD only when the optical transmission signal is sent at a certain level or higher.

Therefore, when there is no optical input to the APD, there is no output from the current amount detection circuit or transmission signal detection circuit, and the APD
The driving voltage of the PD becomes a low bias voltage. When an output is generated from the optical transmission signal, the output voltage of the high voltage generation circuit is increased by the control circuit, and the bias voltage of the APD is increased. When the optical transmission signal disappears, the dark current rapidly increases and an output appears in the current amount detection circuit, and the control circuit lowers the output voltage of the optical voltage generation circuit, lowering the bias voltage of the APD.

In this way, by detecting the amount of current flowing through the APD and the presence or absence of a transmission signal, and controlling the APD bias voltage to be low when there is no signal and high when there is a signal, the APD bias voltage can be set high when there is a signal. Also, the alarm function that determines whether the reception level is below the minimum reception level operates normally.

[Example] FIG. 1 shows an embodiment of the optical receiving circuit of the present invention.

The optical receiving circuit shown in FIG. 1 includes an avalanche photodiode (APD) 1, a current amount detection circuit 20 that detects when the amount of current Io flowing through the APDI exceeds a certain value.
A transmission signal detection circuit 30 that detects the presence or absence of a transmission signal converted into an optical-electrical signal by APDI, a high voltage generation circuit 50 for driving the APDI, and an output voltage vDc of this high voltage generation circuit 50 as a current amount. Output signal x1 of detection circuit 20
and a control circuit 40 that controls the output signal x2 of the transmission signal detection circuit 30 depending on the combination thereof.

APDI has its sword connected to a terminal of bias power supply voltage Vnc, its anode grounded via a series circuit of resistors 2 and 3, and resistor 2 grounded via a capacitor 4 in an alternating current manner.

The current amount detection circuit 20 has a configuration in which a comparator 5 compares the voltage drop generated across the resistor 3 due to the current flowing through the APDI with a threshold voltage Vth1.

The difference from FIG. 1 is that the voltage across the resistor 3 is applied to the non-inverting input terminal of the comparator 5 instead of the inverting input terminal.

The signal component from APD 1 is taken out from the terminal of resistor 2 via capacitor 6 and waveform shaping circuit 7, as in the case of FIG.
and is outputted to the signal output terminal 16 via the waveform shaping circuit 8. Even a weak signal below the signal identification level is output to this signal output terminal 16.

The transmission signal detection circuit 30 includes a peak detection circuit 9 connected 17 to the waveform shaping circuit 7 via a capacitor 15, and a comparator 10 having a non-inverting input terminal connected to the peak detection circuit 9. The peak value of the signal component from the waveform shaping circuit 7 is detected by the peak detection circuit 9, and compared with the threshold voltage Vtt++, which is the signal identification level, by the comparator 10. If it is greater than the level, the comparator 10 outputs a logic "1".

The alarm signal for detecting a decrease in the amount of incident light is the signal X2. In other words, it can be extracted using the output of the comparator 10. For example, even though the output signal x2 is not generated, the waveform shaping circuit 8
When a signal output is obtained from , the signal is below the minimum reception level, so the negation of the output x1 of the comparator 10 can be treated as an alarm signal.

The control circuit 40 receives the output signal x of the current amount detection circuit 20.
1, that is, the output of the comparator 5, and the output signal x2 of the transmission signal detection circuit 30, that is, the output of the comparator 10. In this embodiment, the control circuit 40 controls the output x of the comparator 5
A NOT circuit 11 that takes the negation of 1, and an AND circuit 12 that takes the output of the NOT circuit 11 and the output x2 of the comparator 10 by two people.
It is made up of.

The high voltage generation circuit 50 has a control input terminal connected to the control circuit 4.
0 and whose output terminal is connected to the terminal of the power supply voltage (applied voltage) Vnc of APDI. The DC voltage generator 13 changes its output voltage Vnc according to a control command from the control circuit 40, and raises or lowers the voltage Vnc applied to the APDI. Specifically, when the output of the AND circuit 12 is logic "1", the applied voltage V nc of APDI is increased to Vn+, and when the output is logic "0", the applied voltage vI]c is decreased to 02.

FIG. 2 shows a current amount detection circuit 20 and a transmission signal detection circuit 3.
Which combination of signals X1 and X2 are input from 0 to the control circuit 40, the output signal Y generated from the control circuit 40 by that combination, and the output voltage vDc of the high voltage generation circuit 50 controlled by this output signal Y shows the relationship between "xl" in the input field is logic "'0'" when the current IAPD flowing through APD 1 is less than the set value I3 (voltage setting is Vtht) of comparator 5, and logic "'0'" when it exceeds the set value I3.
It becomes 1 pa. Further, "X2" in the input field becomes logic "0" when there is no transmission signal, and becomes logic "1'" when there is a transmission signal. "vL" in the output column indicates that a low potential voltage Vo+ is output from the high voltage generation circuit 50, and "■H" indicates that a high potential voltage V112 is output.

FIG. 3 shows the relationship between the voltage V[]C applied to the APD bias circuit and the bias voltage VAPD applied across the APD. - Dot-dashed line A is a curve with optical input,
Solid line B is a curve when there is no optical input.

In FIG. 3, r V n + J is the applied voltage (VH) when applying a higher bias voltage to APD 1, and the APD terminal voltage when there is optical input and when there is no optical input,
In other words, within the range where a difference occurs in the APD bias voltage ■ APD,
It is set relatively high. When the applied voltage Voc is this higher voltage VDI (VH), when the amount of incident light Pr exists, the APD bias voltage VAPD is lower than the curve A.
becomes vl, and when there is no incident light amount Pr, curve A becomes smaller than curve B.
PD bias voltage VAPD becomes v2. rVnzJ is an applied voltage (VL) when a lower bias voltage is applied to APDl, and is set to a value comparable to that of the conventional art within a range in which a decrease in the amount of incident light can be determined. When the applied voltage oc is the lower voltage D2 (VL) and the amount of incident light Pr is zero, the APD bias voltage VAPD becomes vO according to the curve B.

Figure 4 shows the voltage between the APD terminals, that is, the bias voltage VAPD.
The relationship between IAP[+] and current IAP[+ flowing through APDI is shown. The high bias voltage V2°vl and low bias voltage Vo set in FIG. 3 mean that the APD current IAP[+ is 11. This corresponds to points I2 and Ioc7). "
v3'' is the threshold of the comparator 5 that determines the operating point of the current amount detection circuit 20, which is the boundary of whether or not the amount of incident light is buried in the noise current level, that is, the level switching point of the current IAPD flowing through APDI. The value voltage is Vth1. This setting value ■3 (Vthl) is
In the APD curve c, the midpoint Q of the region where IAPll rapidly increases, that is, the point where the APD current IAPD becomes I3, is set, and the high bias voltage ■ described in FIG.
2. ■Make sure that 1 comes. In other words, the higher applied voltage Vn+ (VH) explained in FIG. The setting is made so that the high bias voltage V2 when there is no light intensity is applied.

Next, the operation will be explained. For convenience of explanation, the second
In the figure, the voltage Vnc applied to APDI is the higher voltage V[1'.
Assume that it is I (VH).

When the amount of incident light Pr disappears from the above state (X2=0),
Moving from the curve A to the curve B in FIG. 3, the APD bias voltage VAPD rises from the high bias voltage v1 to an even higher voltage 2. Therefore, the amount of current API flowing through the APD is
Along the curve C in Figure 4, there is a sudden increase from work 1 to work 2,
The set value exceeds 3. In other words, the voltage drop across the resistor R3 increases and exceeds the set value V3 (Vtht). Therefore, the comparator 5 operates, and the logic 11111 is output as the output signal x1 of the comparator 5 (X1=1, X2=O;
Figure 2 (row ■).

The output signal X1=1 is inverted by the NOT circuit 11 in the control circuit 40 and applied to the AND circuit 12, so that
The output signal Y of the circuit 12 becomes logic "O" regardless of the content of the signal x2 from the transmission signal detection circuit 30. In response, the high voltage generation circuit 50 output voltage, that is, the APD applied voltage Voc is the third
The higher voltage shown in the figure ■ The lower voltage V from the old (VH)
D2 (VL). APD bias voltage■＾p
.

becomes the low bias voltage vO from v2 in Fig. 3, and the APD
Correspondingly, the current IAPD also decreases from 2 in FIG. 4 to a current value 0 which is smaller than the set value I3. 1. Therefore, the output signal x1 of the comparator 5 returns to logic "0".

In the end, X 1 = “0” + X 2 = “O”
(') Output the entire state (line ■ in Figure 2).

In this way, while there is no optical input, a low bias voltage VO is applied to APDI, and the APD current IAPD is set to a small value Io.
Therefore, if a signal is not transmitted and it is due to a decrease in the amount of light, it can be detected without being buried in noise current.

Next, the above X1 = “0”, X2 = “O” c7
), consider the case where the amount of incident light Pr increases. At this time, the lower applied voltage VD2 is applied to APDI, and the bias voltage VAPD is even lower than the low bias voltage vO when there is no optical input, so the output signal x1 of the current amount detection circuit 20 is " It remains O゛.

Now, when a signal is transmitted with a normal amount of incident light, the peak value of the transmission signal detected by the peak detection circuit 9 of the transmission signal detection circuit 30 exceeds the threshold value Vth2 of the comparator 10. Therefore, the logic "1" is output as the output signal x2 of the comparator 10 (X1=O, X2=1; second
Figure ■ row). When the control circuit 40 receives this, the output signal Y of the AND circuit 12 becomes logic "1", and the output voltage of the photovoltage generating circuit 50, that is, the applied voltage Vnc, becomes the logic "1" again, as shown in row (2) of FIG. The voltage increases to the higher voltage Vn+, which causes a high bias voltage ■1 to be applied to APDI.
The multiplication rate of DI increases. Even when the bias voltage V1 is high, the current IAPD flowing through the APD is curve C in Figure 4.
, the set value I3 (Vth
t), so Xl remains "o".

Due to the above operation, when there is no incident light amount Pr or when the transmitted signal level is weak, the voltage Voc applied to APDl decreases, and only when the signal is transmitted at a discernible level, the applied voltage DC increases, the alarm can be activated normally and the APDI can be operated at a high multiplication factor.

In the embodiment shown in FIG. 1, the output signal of the comparator 5 is amplified by an inverting amplifier and a certain inverting circuit 11, but instead, the input signal to the non-inverting input terminal of the comparator 5 is amplified by an inverting amplifier. It is also possible to input the signal to the comparator 5 after amplification. The Y signal that controls the high voltage generation circuit 50 is ANDed with the NOT circuit.
Although it is created by combining circuits, it can also be created by combining other logic circuits.

[Effects of the Invention] As described above, according to the present invention, even if the APDI bias voltage VAP [1, that is, the applied voltage Dc is set high, the applied voltage Voc changes according to the amount of incident light.
Since the bias voltage is low when there is no amount of incident light, alarm malfunctions due to noise current do not occur.

Furthermore, since the APD bias voltage can be set higher than before when a transmission signal exists normally, the APD can operate at a high multiplication factor, and the characteristics of the optical receiving circuit can be improved.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the optical receiving circuit of the present invention, and FIG.
The figure shows the input/output relationship of the control of the high voltage generation circuit.
Figure 3 is a diagram showing the relationship between the voltage applied to the APD bias circuit and the APD terminal voltage, and Figure 4 is a diagram showing the relationship between the voltage applied to the APD bias circuit and the APD terminal voltage.
A diagram showing the relationship with the current flowing through D, Figure 5 is a diagram showing an example of a conventional optical receiver circuit, and Figure 6 is an example of an AP when there is no amount of incident light.
FIG. 3 is a diagram showing the relationship between the D terminal voltage and the current flowing through the APD. In the figure, 1 is an avalanche photodiode (APD)
, 2.3 is a resistor, 4 is a capacitor, 5 is a comparator, 6 is a capacitor, 7 and 8 are waveform shaping circuits, 9 is a peak detection circuit, 10 is a comparator, 11 is a NOT circuit, 12 is an AND circuit, 13 is a DC voltage generator, 14.15 is a capacitor, 2
0 is a current amount detection circuit, 30 is a transmission signal detection circuit, 40 is a control circuit, and 50 is a high voltage generation circuit.

Claims (1)

[Claims] 1. An avalanche photodiode, a current detection circuit that detects when the amount of current flowing through the avalanche photodiode exceeds a certain value in a region where dark current rapidly increases, and an optical transmission signal that exceeds a certain level. A transmission signal detection circuit that detects the presence or absence of a transmission signal, a high voltage generation circuit whose output voltage is variable for driving an avalanche photodiode, and a combination of the output signal of the current amount detection circuit and the output signal of the transmission signal detection circuit. A control circuit is provided that adjusts the output voltage of the high voltage generation circuit and increases the output voltage of the high voltage generation circuit only when there is an optical transmission signal of a certain level or higher, and the output of the transmission signal detection circuit is used. An optical receiving circuit characterized by extracting an alarm signal.