JPH0522075A - Bias circuit for avalanche photodiode - Google Patents

Abstract

PURPOSE:To unnecessitate control and to enable a stable operation over a wide temperature range by connecting a constant current circuit and a constant voltage circuit through a switching means to an avalanche photodiode (APD). CONSTITUTION:A bias circuit 1 for the APD is equipped with a constant current circuit 20 and a constant voltage circuit 30, and the constant current circuit 20 and the constant voltage circuit 30 are connected through a switching means 40 to the APD 10. When optical input signal power does not satisfy a set value arbitrarily decided in advance, the switching means 40 is operated to connect the constant current circuit 20 and the APD 10. Therefore, a constant current bias is impressed to the APD 10 and at that time, the merit of using the low current bias is effectively utilized. Namely, the circuit of feedback control or temperature compensation is unnecessitated, an excess breakdown current caused by the dispersion of elements or a temperature change or the considerable deviation of an amplification factor M is eliminated, and control is unnecessitated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel avalanche photo diode (hereinafter referred to as APD) bias circuit in an optical receiver, which does not require adjustment.
The present invention relates to a bias circuit for an APD that operates stably in a wide temperature range without temperature compensation, has little deterioration in reception performance due to aging, and has a wide dynamic range.

[0002]

2. Description of the Related Art APDs are used for high-sensitivity optical receivers because they have a function of multiplying a received light current Ip. The multiplication factor M changes depending on the bias voltage V applied to the APD.
In order to obtain high sensitivity, the bias voltage is increased to set it near the breakdown voltage Vb of the APD, the multiplication factor M is increased and the optimum multiplication factor MOPT is set. However, the multiplication factor M
Under a high bias voltage such that A becomes large, the change of the multiplication factor M with respect to the change of the bias voltage is sharp.
In order to maintain PD at the optimum multiplication factor MOPT and realize high sensitivity, it is generally necessary to accurately control the applied bias voltage V. Further, the characteristics of the APD depend on the temperature, and the multiplication factor M corresponding to the breakdown voltage and the bias voltage.
Will also change with temperature. Therefore, AP
Bias circuits in D often require temperature compensation means. Furthermore, the properties of APDs change over time.

Conventionally, a constant voltage bias circuit or a constant current bias circuit has been adopted as the bias circuit of the optical receiving device using the APD having the above-mentioned properties. The constant voltage bias circuit and the constant current bias circuit will be described below.

FIG. 2 shows an example of a conventional constant voltage bias circuit. In the constant voltage bias circuit 2, the constant voltage circuit 30 is connected to the APD 10, and the voltage control circuit 90 is connected to the constant voltage circuit 30. A high voltage is supplied from the high voltage power supply 50 to the constant voltage circuit 30 to drive the bias voltage of the APD at a constant voltage. The reception output of the APD 10 is passed through a preamplifier 80 and an AGC (Automatic Gain Con).
control) amplifier 81, and the output of the AGC amplifier 81 is taken out as the output of the constant voltage bias circuit 2. The output signal amplitude is kept constant by the action of the AGC amplifier 81. Further, the AGC amplifier 81 detects the input signal level, and the result is input to the voltage control circuit 90. The voltage control circuit 90 feedback controls the multiplication factor M of the APD 10 according to the input signal level. At this time, the bias voltage is determined so that the multiplication factor M becomes a preset target value in accordance with the change in the optical input power. Further, a temperature detection circuit 100 is provided, and the temperature information obtained here is sent to the voltage control circuit 90. The voltage control circuit 90 controls the bias voltage so that the multiplication factor M reaches the target value regardless of the change in the characteristics of the APD 10 due to the temperature change.

As described above, since the bias circuit 2 of FIG. 2 using the constant voltage circuit is fed back so that the multiplication factor M is optimized corresponding to the power of the optical input signal, the high receiving sensitivity is obtained. And a wide dynamic range can be achieved at the same time. In addition, the temperature detection circuit 100 and the voltage control circuit 90
The internal temperature compensating circuit compensates the bias voltage according to the ambient temperature and can maintain the receiving performance of the optical receiver regardless of the temperature change.

Next, an example of a conventional constant current bias circuit will be described with reference to FIG. In the constant current bias circuit 3, the high voltage power supply 50 is connected to the constant current circuit 20, and the constant current circuit 20 is connected to the APD 10. A capacitor 60 is connected to the APD 10 to reduce the impedance in the high frequency region. The constant current circuit 20 keeps the average current Is flowing through the APD 10 constant. A
The optical signal output of the PD 10 is amplified by the preamplifier 80. The current output value Is of the constant current circuit 20 is set according to the following equation so that M = MOPT at the target minimum received power PMIN.

Here, Ip is a light receiving current, and S is a receiving sensitivity.

Is = MOPT.S.PMIN When there is no light input, the APD 10 is in a break down state. When the target minimum received power PMIN is input,
M = MOPT. As the optical power received increases, A
Since the impedance of the PD 10 decreases, the APD bias voltage decreases, and the multiplication factor M decreases in inverse proportion to the optical input power. When the light receiving current Ip = Is, M = 1. Average current I of the APD 10 with respect to changes in optical input power
Since s is constant, if the mark rate of the optical signal is constant,
The amplitude of the light receiving signal current does not change, either. Therefore, the AGC operation is automatically performed. Ambient temperature changes and APD1
Even if the characteristics such as the breakdown voltage of 0 change,
The average current value flowing through the APD 10 is always kept constant.
Therefore, unless the dark current becomes particularly large, the multiplication factor M does not change due to the temperature change.

As described above, the APD using the constant current circuit
Since the bias circuit 3 does not require feedback control, the circuit is simplified. Since the multiplication factor M is determined only by the bias current value Is regardless of the change of the APD characteristic due to the temperature change, the temperature compensating circuit is not required, and the circuit can be further simplified. Also, no AGC amplifier circuit is required. Since the influence of the error in the current value of the constant current circuit on the circuit characteristics is relatively small, it is not necessary to set the current value precisely and the circuit can be adjusted. in addition,
Even if the characteristics of the APD, such as the breakdown voltage, change over time, the influence on the reception performance is small.

[0010]

In the constant voltage bias circuit 2, the change of the multiplication factor M with respect to the change of the bias voltage is sharp under a high bias voltage. Therefore, in order to stabilize the multiplication factor M, The bias voltage must be controlled accurately. For this, it is indispensable to make adjustments in order to cope with variations in APD and other elements. Further, since the APD depends on the temperature, temperature compensation is necessary. However, since the temperature compensation is performed in a closed loop, it is necessary to adjust according to the variation in the temperature characteristic of the APD in order to perform the accurate temperature compensation. There is. Especially, the multiplication factor M of APD
In a region where the input optical signal is high, that is, in a region where the optical input signal is small, the change of the multiplication factor M due to the change of the applied bias voltage is abrupt, so that extremely accurate voltage control is required. Is extremely difficult.
Further, the APD characteristics change with time, and it is difficult to change the voltage in response to the change with time in order to keep the multiplication factor M stable. For the above reasons, it is difficult to stably apply the bias in the region where the optical input signal is small by the bias circuit 2 of the APD using the constant voltage circuit.

On the other hand, in the constant current bias circuit 3,
Unlike the constant voltage bias circuit 2, it operates stably without adjustment, can be realized by a very simple circuit, and does not require an AGC circuit. However, the constant current bias circuit 3 has a drawback that the dynamic range of the optical input power described below is narrow. That is, when the multiplication factor M is less than 2, the response speed of photoelectric conversion is drastically reduced. Therefore, under high optical input power such that the multiplication factor M is less than 2, optical reception of high-speed signals is not possible. It will be impossible. Further, when the constant current bias circuit 3 is used, the multiplication factor M changes in inverse proportion to the optical input power, so that the variation of the multiplication factor M with respect to the change of the optical input power is large under a low optical input power. Therefore, if the upper limit of the multiplication factor M is set to 40, for example, the multiplication factor M is 2 to 40.
It means that it is possible to receive in the range of. The dynamic range of the optical input power corresponding to this range is about 13 dB, which is extremely narrow. The constant current bias circuit 3 has a drawback that the dynamic range of optical input power is narrow,
The main cause of this drawback is that optical reception of high-speed signals becomes impossible when the optical input power is high. In this way, the constant current bias circuit 3 has a problem in the region where the optical input signal is large, contrary to the constant voltage bias circuit 2.

Therefore, an object of the present invention is to solve the above problems, to perform stable operation in a wide temperature range without requiring adjustment, without temperature compensation, with little deterioration in receiving performance due to aging, and with a wide dynamic range. To provide a bias circuit for the.

[0013]

In order to achieve the above object, the first aspect of the present invention is to provide a constant current circuit and a constant voltage circuit in a bias circuit of an APD in an optical receiver for converting an optical signal into an electric signal. The constant current circuit and the constant voltage circuit are connected to the APD via the switching means.

A second aspect of the invention is to connect the constant current circuit and the APD to apply a bias when the switching means does not reach a preset value arbitrarily determined in advance.
When the optical input signal power is above the set value, the constant voltage circuit and A
It was configured to be connected to the PD and biased.

Further, in the third invention, the switching means is constituted by a diode.

The fourth invention includes a resistor between the constant voltage circuit and the avalanche photodiode.

[0017]

With the above structure, the switching means operates when the optical input signal power does not reach the preset value arbitrarily determined in advance,
The constant current circuit and the APD are connected. Therefore, the constant current bias is applied to the APD. When a constant current bias is applied to the APD, the advantages of using a low current bias will be taken advantage of. That is, there is no need for a feedback control or temperature compensation circuit, and an excessive break-down current or multiplication factor M caused by element variations or temperature changes.
There is no significant shift in the number, and no adjustment is required. Further, since the constant current circuit does not require high accuracy, it can be realized with a simple circuit configuration. Furthermore, it is not significantly affected by the secular change in the characteristics of APD.

On the other hand, when the optical input signal power is equal to or greater than the preset value arbitrarily determined, the switching means operates to connect the constant voltage circuit and the APD. Therefore, the constant voltage bias is applied to the APD. When a constant voltage bias is applied to the APD, the advantages of using a low voltage bias will be taken advantage of. That is, since the multiplication factor M can be kept substantially constant regardless of the change in the optical input power, the optical receiving device operates even if the optical input power is large to some extent, and the dynamic range is expanded. Further, in a region where the multiplication factor M is relatively small, such as 2 to 3, the change in the multiplication factor M with respect to the change in the bias voltage is small, so that the setting of the bias voltage does not require high accuracy. Therefore, the constant voltage circuit can be easily constructed.

Further, the switching means is simple because it is constituted by a diode.

As described above, an optical receiver having a wide dynamic range can be constructed by using a low current circuit having a simple structure and a constant voltage circuit having a simple structure.
This optical receiving device requires no adjustment, is stable with respect to temperature, and has little influence of aging of the characteristics of the APD.

[0021]

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

As shown in FIG. 1, the optical receiving device 1 for converting an optical signal into an electric signal using the APD 10 includes an APD 1
Constant current circuit 20 and A for applying constant current bias to 0
Constant voltage circuit 3 for applying constant voltage bias to PD10
0 and are provided. Both the constant current circuit 20 and the constant voltage circuit 30 are composed of extremely simple circuits, which are generally known in the past. In the optical receiver 1,
A high voltage generating circuit 50 for generating a high voltage is provided, and its outputs are connected to the constant current circuit 20 and the constant voltage circuit 30, respectively, and are configured to supply power. The output of the constant current circuit 20 is directly connected to the constant current side terminal 42 of the switching means 40, while the output of the constant voltage circuit 30 is connected to the constant voltage side terminal 43 of the switching means 40 via the resistor 70.

In this embodiment, the switching means 40 is composed of only the diode 41. The constant current side terminal 42 of the switching means 40 is directly connected to the output terminal 44 of the switching means 40. The constant voltage side terminal 43 of the switching means 40 is connected to the anode side of the diode 41, and the cathode side of the diode 41 is connected to the output terminal 44 of the switching means 40. That is, from the voltage of the constant current side terminal 42 to the constant voltage side terminal 4
The diode 41 is configured to conduct only when the voltage of 3 is high.

The output terminal 44 of the switching means 40 is the APD 10
Is connected to the cathode side of the capacitor and is connected to a capacitor 60 for reducing the impedance in the high frequency region. AP
The anode side of D10 is grounded via a resistor 61 for limiting the current and is also input to a preamplifier 80 for amplifying a small signal obtained from the APD10. The output of the preamplifier 80 is input to an AGC amplifier 81 for automatically adjusting the amplification factor and transmitting a signal having an appropriate amplitude to the circuit in the subsequent stage. The output of the AGC amplifier 81 is the optical receiver 1.
Will be output.

In the resistor 70, the diode 41 is used.
Is provided between the constant voltage circuit 30 and the APD 10 in order to supply the output of the constant voltage circuit 30 to the APD 10 when the power is turned on.

Here, the voltage between the bias circuit side of the APD 10 and the ground is V, and the voltage output of the constant voltage circuit is Vs.
And

Next, the operation of the embodiment will be described.

First, when there is no light input to the APD 10, the voltage V becomes the highest and the APD 10 is in the break down state. Further, the voltage V is the voltage output Vs of the constant voltage circuit 30.
Since it is higher, a diode 41 that constitutes the switching means 40 is used.
Is blocked and a constant current bias is applied to the APD 10 from the constant current circuit 20. When an optical input is input to the APD 10, the capacitor 60 lowers the impedance in the high frequency region, so that the current changes according to the optical input signal. However, since the average current value is controlled to a constant value by the constant current circuit 20, the APD
The multiplication factor M of 10 is expressed by the following equation.

M = Is / Ip = Is / (P · S) where Is is the bias current, Ip is the average light receiving current, and S
Is the light receiving sensitivity, and P is the average light receiving power.

As the optical input power increases, the average received light power P increases, the multiplication factor M decreases in inverse proportion to this, and the voltage V also gradually decreases. During this time, the amplitude of the output signal of the APD 10 is constant. It is constant even if the ambient temperature changes and the characteristics of the APD 10 change. In this way, when the optical input power is small, the constant current bias is applied from the constant current circuit 20, and the output signal of the APD 10 is stably obtained.

Next, when the optical input power further increases and the average received power P increases, the multiplication factor M decreases and the voltage V decreases. When the voltage V becomes equal to or lower than the voltage output Vs of the constant voltage circuit, the diode 41 forming the switching means 40 becomes conductive and the voltage V is constantly applied via the resistor 70. That is, a constant voltage bias is applied to the APD 10. After that, the voltage V is kept constant even when the optical input is increased, and thus the multiplication factor M is also kept constant. Since the multiplication factor M is not remarkably reduced, the response speed of photoelectric conversion does not suddenly decrease. Further, since the constant voltage bias voltage is sufficiently small, stable operation can be obtained even if the accuracy of the constant voltage circuit 30 is not high. At this time, the amplitude of the output signal of the APD 10 increases substantially in proportion to the amplitude of the optical input, so that the AGC amplifier 81 operates and the output of the optical receiver 1 is stably obtained. In this way, a constant voltage bias is applied from the constant voltage circuit 30 when the optical input power is large, and the output signal of the APD 10 is stably obtained.

When an excessive optical input is input, the current is limited by the grounded resistor 61 to protect the APD 10 from damage due to overcurrent.

As described above, according to the present invention, it is possible to configure the bias circuit of the APD having both the advantages of the constant voltage bias circuit and the advantages of the constant current bias circuit. Further, the switching means provided for switching between the constant voltage bias circuit and the constant current bias circuit is realized by only one diode. That is, no adjustment is required,
A bias circuit for an APD that operates stably in a wide temperature range without temperature compensation, has little deterioration in receiving performance due to aging, and has a wide dynamic range can be realized with a simple circuit. Such an APD bias circuit is particularly effective in a digital optical signal receiver in which the mark ratio of the optical input signal is constant.

[0034]

The present invention exhibits the following excellent effects.

(1) No adjustment is necessary.

(2) Stable operation over a wide temperature range.

(3) The influence of aging of APD is small.

(4) Wide dynamic range.

(5) No high precision parts are required.

(6) The circuit structure is simple.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional constant voltage bias circuit.

FIG. 3 is a circuit diagram of a conventional constant current bias circuit.

[Explanation of symbols]

1 Optical receiver 10 Avalanche Photodiode (APD) 20 constant current circuit 30 constant voltage circuit 40 switching means 41 diode 70 resistor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04B 10/06 8426-5K H04B 9/00 Y

Claims (4)

[Claims] 1. A bias circuit of an avalanche photo diode in an optical receiving device for converting an optical signal into an electric signal, comprising a constant current circuit and a constant voltage circuit, and these constant current circuit and constant voltage circuit are the above-mentioned. A bias circuit for an avalanche photo diode, which is connected to the avalanche photo diode through a switching means. 2. The switching means connects the constant current circuit and an avalanche photodiode to bias the optical input signal power when the optical input signal power is less than a preset value, and the optical input signal power is set to the preset value. 2. A bias circuit for an avalanche photo diode according to claim 1, wherein in the above case, the constant voltage circuit and the avalanche photo diode are connected to each other for biasing. 3. A bias circuit for an avalanche photodiode according to claim 1, wherein the switching means is composed of a diode. 4. A bias circuit for an avalanche photodiode according to claim 1, further comprising a resistor between the constant voltage circuit and the avalanche photodiode.