JPH098744A - Bias system for avalanche photodiode - Google Patents

Abstract

PURPOSE: To enable an operation with an optimum bias voltage regardlessly of the value of optical input power. CONSTITUTION: An optical signal received by an avalanche photodiode APD 31 is automatically gain controlled by an AGC amplifier 35 and becomes the output signal of a light receiver. The output signal is fed back, and the output signal of the light receiver is stabilized. At a Zener diode 37, the temperature of the APD 31 is detected and corresponding to the temperature, a voltage almost equal with a breakdown voltage Vbr is outputted from a first bias power source 42 and becomes a bias voltage VB of the APD 31. When the light input of the APD 31 is enlarged and the potential of the cathode of the APD 31 is made lower than the output voltage of a second bias power source 49 by voltage drop caused by a resistor 45, a diode 47 is conducted and the bias voltage VB of the APD 31 is applied from the second bias power source 49. The output voltage value of the second bias power source 49 and the value of a resistor 48 are selected so that the APD 31 can not be saturated and thermal destruction can be prevented from occurring by letting a large photoelectric current flow.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to light in a system having a wide variation range of received light intensity and a lot of background light other than light containing information used for an original signal, such as optical space communication used outdoors. The present invention relates to a biasing method for an avalanche photodiode used in a receiver.

[0002]

2. Description of the Related Art An avalanche photodiode (hereinafter abbreviated as APD) used for a receiver in optical communication is
There is a multiplication effect on the photovoltaic current, and its multiplication factor M is represented by the following equation as a function of the bias voltage VB, and varies in the range of 1 to several hundreds. M = 1 / {1- (VB / Vbr) ^N }

Here, Vbr is a breakdown voltage, and N is an experimentally determined constant, which is usually 1 or less, and this breakdown voltage Vbr changes with temperature.

To control the APD, the multiplication factor M is set to the optimum multiplication factor.
In addition to the mechanism to control to MB and the mechanism to stabilize the output, when the bias voltage VB exceeds the breakdown voltage Vbr, the dark current ID sharply increases and the S / N ratio deteriorates. Since it is destroyed, a mechanism for controlling the bias voltage VB so as not to exceed the breakdown voltage Vbr is required.

FIG. 7 is a block circuit diagram of a conventional example when the bias method of APD is used in an optical receiver. AP
The output of D1 is connected to a grounded resistor 2 and a coupling capacitor 3. The output of the coupling capacitor 3 is connected to the AGC amplifier 5 via the preamplifier 4,
A part of the output of is the output of the optical receiver as it is, and is also connected to the detector 6. The output of the detector 6 is connected to the AGC amplifier 5.

A zener diode 7 for temperature measurement, which is grounded, is provided in a package (not shown) of the APD 1 or in the vicinity thereof, and the output of the zener diode 7 is connected to a differential amplifier 8. Differential amplifier 8
A grounded reference voltage source 9 is also connected to the input of the.
The output of the differential amplifier 8 is connected to the adder 11 via the resistor 10, and the output of the adder 11 is connected to the bias power supply 12. A bias voltage reference voltage source 14 grounded via a resistor 13 is also connected to the input terminal of the adder 11. The output of the bias power source 12 is connected to the APD 1 via the resistor 15, and the grounded bias capacitor 16 is also connected to the input of the APD 1.

An optical signal received by the APD 1 is converted into a photocurrent and further converted into a voltage by a resistor 2 which is a load resistance of the APD 1. The signal converted into the voltage has the AC signal component of this voltage taken out by the coupling capacitor 3, and the preamplifier 4
Is input to The signal amplified by the preamplifier 4 is AGC
The gain is automatically controlled by the amplifier 5 and becomes the output signal of the optical receiver. Further, the output signal has its amplitude and the like detected by the detector 6 and is fed back to the AGC amplifier 5 to stabilize the output signal of the optical receiver. Further, the multiplication factor M is set to be close to the optimum multiplication factor MB with respect to the input optical signal level.

The Zener diode 7 detects the temperature of the APD 1, amplifies the potential difference between its terminal voltage and the reference voltage source 9 by the differential amplifier 8, and further, the reference voltage source 14 for the bias power source.
The voltage is added by the adder 11 and applied to the input terminal of the bias power supply 12. As a result, the bias power supply 12 outputs a voltage substantially equal to the breakdown voltage Vbr of the APD1 according to the temperature of the APD1. Bias power supply 12
The voltage output from the APD 1 is controlled by the voltage drop across the resistor 15 that changes according to the photocurrent flowing through the APD 1.
A bias voltage of 1 is VB. The bias capacitor 16 is for passing the AC signal component of the current flowing through the APD 1.

FIG. 8 shows that the breakdown voltage Vbr is 200V.
FIG. 6 is a graph showing the relationship between the signal light level and the multiplication factor M in the case where a 200 kΩ resistor 15 is connected to the input of the APD 1 for each background light level. The background light is natural light that is incident from the periphery of the APD 1 other than the signal light, and varies depending on weather conditions and installation environment. The dotted line shows the optimum multiplication factor MB that maximizes the S / N ratio of the output electric signal with respect to the power of the signal light level and the background light level of the optical signal received by the ADP1, and the multiplication factor M of the APD1 is It is controlled to be close to the optimum multiplication factor MB for the optical input.

FIG. 9 is a block circuit diagram of another conventional example, which is almost the same as the conventional example shown in FIG. 7, except that the diode 1 is provided between the bias power source 12 and the resistor 15.
7 is connected.

FIG. 10 is a graph showing the relationship between the signal light level and the amplification factor M for each background light level, and the dotted line shows the optimum multiplication factor MB. Due to the non-linearity of the current-voltage characteristic of the diode 17, the characteristic of ADP1 is closer to the optimum multiplication factor MB than the characteristic of ADP1 shown in FIG.

[0012]

However, in the above-mentioned conventional method, although a good control characteristic can be obtained under the use condition where the signal light level is 1 mW or less, the light beam propagating in the open free space is In the optical space communication using a communication path, the attenuation amount of the transmission path greatly changes depending on the weather conditions such as rain and fog. Also, since the transmission distance is wide from several thousand meters to several tens of meters, it is necessary for the optical receiver to have a wide dynamic range with respect to the input light level.

Therefore, by increasing the transmission distance and increasing the output of the optical transmitter so that it can be used even when the weather conditions are bad, the optical reception can be performed under the weather conditions where the atmospheric attenuation is small and when the transmission distance is short. If a large level of received light is input to the receiver and the dynamic range is not sufficiently large, the input to the optical receiver will be excessive and saturated, and the signal cannot be detected normally. Further, even if the signal light level is moderate, if the background light level is extremely strong light such as sunlight, the photoreceiver may saturate the photoreceiver current.

FIG. 11 is a block diagram of an experimental system of an optical transmitter and an optical receiver used for measuring the characteristics of the ADP with respect to a bias voltage. The optical transmitter 2 to which a signal source 21 is connected is shown in FIG.
The lens 23, the variable optical attenuator 24, the lens 25, and the ADP 1 shown in FIGS. 7 and 9 are sequentially arranged to face 2. The output of the ADP1 is connected to the grounded resistor 2, the coupling capacitor 3 and the bias voltage VB shown in FIGS. 7 and 9, and the output of the coupling capacitor 3 is connected to the output of the optical receiver via the preamplifier 4. Become.

FIG. 12 is a graph showing the output level of the signal with respect to the optical input power input to the APD1.
When the bias voltage VB is zero, the output saturates with an input of about 1.5 mW, and the output sharply decreases, but when the bias voltage VB is 1 V, the saturated optical input power increases to 5 mW.

In the conventional example shown in FIGS. 7 and 9, APD1 is used.
In order to bring the bias characteristics of to the characteristics of the optimum multiplication factor MB,
A resistor with a large resistance value is used for the resistor 15.
When the light input to D1 increases, the voltage drop due to the photocurrent also increases, and the bias voltage VB applied to APD1 increases.
Becomes smaller. For example, assuming that the output voltage of the bias power source 12 is 200 V, the value of the resistor 15 is 200 KΩ, and the sensitivity of the APD1 is 0.5 A / W, the photocurrent of the APD1 is 1 mA when the optical input power is 2 mW, and FIG. In the embodiment shown, the voltage effect of the resistor 15 reaches 200 V,
The bias voltage VB for PD1 becomes 0V.

At this time, from FIG. 12, the APD 1 is saturated with this optical input power and the output is lowered, and in the other conventional example shown in FIG.
Further, the light input to be saturated becomes smaller. As described above, the conventional APD bias method has a problem in that the APD is easily saturated in a region where the optical input power is large and the signal output is reduced.

The object of the present invention is to solve the above-mentioned problems,
Even when the optical input is large, the bias voltage of the APD is prevented from lowering and the output of the detection signal is prevented from lowering, and the APD always operates with the optimum bias voltage regardless of the input power values of the optical signal and the background light. It is to provide a bias method for an avalanche photodiode that can secure a dynamic range with respect to power.

[0019]

In order to achieve the above-mentioned object, the avalanche photodiode bias system according to the present invention comprises a first bias power source of a resistor or a series circuit of the resistor and at least one diode. And a second bias circuit in which a diode or a series circuit of the diode and a resistor is connected to a second bias power supply whose output voltage is lower than that of the first bias power supply, The first and second bias circuits are connected to the cathode of the avalanche photodiode directly or via a resistor, and the anode of the avalanche photodiode is connected to a reference potential directly or via a resistor. .

[0020]

In the bias system of the avalanche photodiode having the above-mentioned structure, in addition to the first bias power source,
A second bias power supply having an output voltage lower than that of the first bias power supply is provided, the bias voltage is supplied from the first bias power supply when the optical input power is small, and the first bias power supply when the optical input power is large. The voltage drop due to the bias voltage control resistor connected to the switch becomes large, and the bias voltage is supplied from the second bias power supply.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the embodiments shown in FIGS. FIG. 1 is a block circuit configuration diagram of the embodiment. The output of the APD 31 is connected to a grounded resistor 32 and a coupling capacitor 33. The output of the coupling capacitor 33 is connected to the AGC amplifier 35 via the preamplifier 34, and a part of the output of the AGC amplifier 35 is directly used as the output of the optical receiver and is also connected to the detector 36. The output of the detector 36 is connected to the AGC amplifier 35.

A grounded Zener diode 37 is provided in a package (not shown) of the APD 31 or in the vicinity thereof, and the output of the Zener diode 37 is connected to a differential amplifier 38. A grounded reference voltage source 39 is also connected to the input of the differential amplifier 38. The output of the differential amplifier 38 is connected to the adder 41 via the resistor 40, and the output of the adder 41 is the first bias power supply 4
Connected to 2. At the input end of the adder 41, a resistor 4
A bias voltage reference voltage source 44, which is grounded via 3, is also connected. The output of the first bias power supply 42 is connected to the APD 31 via a gain control resistor 45.
Bias capacitor 4 grounded to the input of APD 31
6 is also connected.

Further, a diode 47, a resistor 48, and a second bias power source 49 are sequentially connected to the cathode of the APD 31. The output voltage of the second bias power source 49 is set to be considerably smaller than the output voltage of the first bias power source 42, and the value of the resistor 48 is also considerably smaller than the value of the resistor 45.

The optical signal received by the APD 31 is the APD 3
It is converted into a photocurrent by 1, and further converted into a voltage by a resistor 32 which is a load resistance of the APD 31. The signal converted into the voltage has the AC signal component of this voltage extracted by the coupling capacitor 33 and input to the preamplifier 34. Preamplifier 34
The AGC amplifier 35 automatically controls the gain of the signal thus amplified, and becomes the output signal of the optical receiver. The amplitude of the output signal is detected by the detector 36, and the AGC amplifier 35
Is fed back to stabilize the output signal of the optical receiver. Further, the multiplication factor M is set to be close to the optimum multiplication factor MB with respect to the input optical signal level.

The Zener diode 37 detects the temperature of the APD 31, the potential difference between the terminal voltage of the APD 31 and the reference voltage source 39 is amplified by the differential amplifier 38, and the potential of the bias power source reference voltage source 44 is added by the adder 41. Then, the voltage is applied to the input terminal of the first bias power supply 42. This allows the AP
Breakdown voltage of APD31 depending on the temperature of D31
A voltage approximately equal to Vbr is output from the first bias power supply 42. The voltage output from the first bias power source 42 is controlled by the voltage drop of the resistor 45 that changes according to the photocurrent flowing through the APD 31, and becomes the bias voltage VB of the APD 31. The bias capacitor 46 is for passing the AC signal component of the current flowing through the APD 31.

When the light input to the APD 31 is small, the voltage drop due to the photocurrent of the resistor 45 is small and the potential of the cathode of the APD 31 is higher than the output voltage of the second bias power source 49, so that the diode 47 does not conduct. Therefore, the bias voltage VB is not supplied from the second bias power source 49.

The light input to the APD 31 becomes large, and the resistance 4
The voltage drop by 5 causes the potential of the cathode of the APD 31 to reach the second
When the output voltage of the bias power source 49 becomes lower than the output voltage of the bias power source 49, the diode 47 becomes conductive, and the bias voltage VB of the APD 31 is supplied from the second bias power source 49.

The output voltage value of the second bias power source 49 and the value of the resistor 48 are determined by the requirement for the magnitude of the optical input power that can be detected. The voltage obtained by subtracting the voltage drop due to the diode 47 is the APD for the required optical power.
The bias voltage VB is such that 31 does not saturate, and A
The value is selected so that a large photocurrent flows through the PD 31 and thermal destruction of the APD 31 does not occur. The resistor 48 may be omitted if there is no possibility that an excessive current will flow through the APD 31 depending on the output voltage of the second bias power source 49 and the value of the resistor 32.

FIG. 2 is a graph showing the output level of the signal with respect to the optical input power input to the APD 31, and the solid line indicates the resistance 45 of 200 KΩ and the resistance 4 of 1 KΩ in this embodiment.
The case where the second bias power source 49 of 8 and 12 V is connected is shown, and the dotted line shows the case of the conventional example. Comparing these, the optical input power at which the APD 31 is saturated and the signal level is lowered is about 10 times larger than in the case where the diode 47, the resistor 48, and the second bias power source 49 are not provided, and the optical input power is large. Wide dynamic range in the area.

In the present embodiment, the second bias power source 49 causes the bias voltage value to deviate from the optimum bias condition that maximizes the S / N ratio of the detection signal in a region where the optical input power is large, but this is the only condition. Since the S / N ratio of the signal is sufficiently large in the region where the optical input is large, it does not matter even if it deviates slightly from the optimum bias condition.

FIG. 3 is a block circuit configuration diagram of another embodiment, which has substantially the same configuration as that of FIG. 1, but a diode 50 is connected between the first bias power source 42 and the resistor 45. At this time, the operation in the region where the optical input power is large is the same as that of the embodiment shown in FIG. 1, but in the region where the optical input power is small, the optimum multiplication factor MB is larger than that in the embodiment shown in FIG.
It is possible to control the multiplication factor close to.

FIG. 4 is a block circuit configuration diagram in the case where a resistor 51 for current control is provided between the cathode of the APD 31 shown in FIGS. 1 and 3 and the resistor 45. When the resistance values of the resistors 45 and 51 are R1 and R2 respectively, and R1 >> R2,
The characteristics equivalent to those of the embodiments shown in FIGS. 1 and 3 can be obtained.

FIG. 5 is a block circuit configuration diagram in which a resistor 52 is provided at the cathode of the APD 31 instead of providing the resistor 32 at the anode of the APD 31 shown in FIGS.
FIG. 6 is also a block circuit configuration diagram in which a resistor 52 is provided at the cathode of the APD 31 instead of providing the resistor 32 at the anode of the APD 31 shown in FIG. The operation of bias is the same as that of the embodiments shown in FIGS. 1, 3 and 4 except that the way of extracting signals is different.

[0034]

As described above, the avalanche photodiode bias system according to the present invention controls the multiplication factor of the avalanche photodiode by using two types of bias power supplies having different voltages. When the input power is relatively small, the bias voltage of the avalanche photodiode is controlled by the first bias power supply and the resistance so as to give the optimum multiplication factor. When the optical input power is large, the second bias power supply controls the avalanche photodiode. In order to prevent the saturation of the avalanche photodiode by holding the bias voltage of, the optimum bias condition is maintained even in the system where the optical signal input level and the background light input level change greatly due to the usage environment such as spatial propagation optical communication. It is possible to control and secure a wide dynamic range.

[Brief description of drawings]

FIG. 1 is a block circuit configuration diagram of an embodiment.

FIG. 2 is a graph showing output response characteristics of an avalanche photodiode with respect to optical input power.

FIG. 3 is a block circuit configuration diagram of another embodiment.

FIG. 4 is a block circuit configuration diagram when a new resistor is provided at the cathode of the avalanche photodiode.

FIG. 5 is a block circuit configuration diagram in the case where a resistor of an avalanche photodiode is provided in the cathode.

FIG. 6 is a block circuit configuration diagram when a resistance of an avalanche photodiode is provided at a cathode.

FIG. 7 is a block circuit configuration diagram of a conventional example.

FIG. 8 is a graph showing the relationship between the signal light level and the optimum multiplication factor.

FIG. 9 is a block circuit configuration diagram of another conventional example.

FIG. 10 is a graph showing the relationship between the signal light level and the optimum multiplication factor.

FIG. 11 is a configuration diagram of an optical transmitter and an optical receiver.

FIG. 12 is a graph of output response characteristics of an avalanche photodiode with respect to optical input power.

[Explanation of symbols]

 31 Avalanche Photodiode 33 Coupling Capacitor 34 Preamplifier 35 AGC Amplifier 36 Detector 37 Zener Diode 38 Differential Amplifier 39 Reference Voltage Source 41 Adder 42 First Bias Power Supply 44 Bias Power Supply Reference Voltage Source 46 Bias Capacitor 47, 50 diode 49 second bias power supply

Claims (2)

[Claims] 1. A first bias circuit in which a resistor or a series circuit of the resistor and at least one diode is connected to a first bias power source, and a second bias circuit whose output voltage is lower than that of the first bias power source. A bias power supply is provided with a diode or a second bias circuit in which a series circuit of the diode and a resistor is connected, and the first and second bias circuits are connected to the cathode of the avalanche photodiode directly or through the resistor. A bias method for an avalanche photodiode, characterized in that the anode of the avalanche photodiode is connected to a reference potential directly or via a resistor. 2. The first bias power supply is capable of changing an output voltage according to a control input voltage,
The control input voltage terminal detects the temperature of the package of the avalanche photodiode or the vicinity thereof,
Based on the detection signal, the breakdown voltage of the avalanche photodiode is approximately equal to or is equal to the breakdown voltage of the avalanche photodiode at or near the temperature of the package detected by following a change in the breakdown voltage of the avalanche photodiode with temperature. The bias system for an avalanche photodiode according to claim 1, wherein a signal that outputs a voltage that does not exceed the threshold is provided.