Patent Application
20050224697
1. Field of the Invention
The present invention relates to an light-receiving circuit for an optical communication, especially for the light-receiving circuit using an avalanche photodiode as a light-receiving device.
2. Related Prior Art
The avalanche photodiode (APD) is often used as a light-receiving device for a faint optical signal because the APD enables to gain carriers from a single photon entered therein. An index called as the M-value is well known, which denotes the multiplication factor how many carriers does the APD generates from a single photon. The M-value strongly depends on, nearly nonlinear to the bias condition VAPD applied to the APD.
Further increasing the bias VAPD and exceeding 27V, the M-value becomes larger than unity, where the APD generates a plurality of carriers for one photon, namely, the region is called as the APD region. In this APD region, the M-value shows strong dependence on the bias condition VAPD.
When the APD is operates in a fixed bias condition, for example, the bias is fixed to 40V in
In a conventional light-receiving circuit for the APD, a resistor is serially connected to the APD to expand a dynamic range of the APD. The resistor controls the bias VAPD applied to the APD by a current feedback thereof. When the input light has a great magnitude and the APD generates a large current, the bias thereto is lowered by a voltage drop at the serially connected resistor, thus current feedback operation is realized.
However, such current feedback operation by the serially connected resistor is only for the condition that the APD receives the large optical input. For is small and faint optical input, the serially connected resistor shows no function to the APD.
Therefore, one object of the present invention is to provide a light-receiving circuit for the APD, which enables to enhance the dynamic range thereof.
According to one aspect of the present invention, an light-receiving circuit includes a light-receiving device, a bias supply, a reference resistor and a feedback control circuit. The light-receiving device is preferably an avalanche photodiode and receives an optical signal with a predetermined transmission speed. The bias supply provides a bias voltage to the light-receiving device. The reference resistor detects a signal current generated by the light-receiving device. The feedback control circuit receives the signal current detected by the reference resistor and controls the bias supply such that the signal current detected by the reference resistor is maintained to be a predetermined magnitude.
The bias supply may include a high voltage source and a voltage control circuit serially connected to the high voltage source. The feedback control circuit may adjust the bias voltage provided to the light-receiving device via the voltage control circuit.
The light-receiving circuit may further comprise a current mirror circuit, which has one input port connected to the output of the bias supply and two output ports. One of two output ports is connected to the light-receiving device, while the other of two output ports is connected to the reference resistor, whereby the current flowing the reference resistor is equivalent to the current generated by the light-receiving device.
The feedback control circuit may has a time constant greater than the predetermined speed to stabilize the feedback operation thereof The light-receiving device may be a PIN photodiode instead of the avalanche photodiode, a cathode of which is connected to the bias supply.
Next, preferred embodiments of the present invention will be described as referring to accompanying drawings.
The light-receiving circuit 1 comprises an avalanche photodiode (APD) 11, a high-voltage source 12, a pre-amplifier 13, a current-mirror circuit 14, a voltage control circuit 15, a feedback controlling circuit 16 and a sensing resistor RREF.
The high-voltage source 12, the voltage control circuit 15, the current mirror circuit 14, the APD and the pre-amplifier 13 are serially connected in this order, namely, the cathode of the APD is connected to one of the current path of the current mirror circuit 14, and the anode of the APD is connected to the pre-amplifier 13.
The pre-amplifier 13 includes an inverting amplifier 13a and feedback impedance 13b connected between the input and the output of the inverting amplifier 13a.
The current mirror circuit 14 has one input port 14a and two output ports 14b and 14c. Between the input port 14a and one of the output ports 14b is provides a pnp-type transistor Q21 whose collector and the base are short circuited, while between the input port 14a and the other output port 14c is provided another pnp-type transistor Q22. Resistors R21 and R22 are connected between the input port 14a and the emitter of the transistor Q21 and that of the transistorQ22, respectively. In this current mirror circuit, when performance of transistors Q21 and Q22 are equivalent to each other, currents output from each output ports 14b and 14c are determined by a ration of each resistors R21 and R22. In the case that the resistance of resistors R21 and R22 are identical, the current output from the output port 14b is equal to the current from the output port 14c. Accordingly, a current signal IAPD that corresponds to the optical signal received by the APD 11 is equal to the current flowed from the output ports 14b of the current mirror circuit 14. At the same time, the current IREF flowed from the other port 14c of the current mirror circuit 14 can be related to the signal current IAPD.
The voltage control circuit 15 includes an npn-type transistor Q1, where a voltage between the collector and the emitter thereof is controlled by a signal input to the base. Therefore, when a high-voltage VH for the APD is applied to the collector of the transistor Q1, a voltage output from the emitter of the transistor Q1, which is practically applied to the APD, can be adjusted by the control signal applied to the base of the transistor Q1.
The feedback controlling circuit 16 includes a comparator 16a, a reference signal VREF, three resistors R1 to R3, a capacitor C1 and a transistor Q3. The comparator 16a compares a voltage generated in the reference resistor RREF by the current IREF with the reference signal VREF, and transmits the result of comparison to the transistor Q3. The resistor R1 and the capacitance, they are connected between the comparator 16a and the transistor Q3 and constitute a low-pass filter, set a large time constant for the closed loop formed by the voltage control circuit 15, the current mirror circuit 16 and the feedback controlling circuit, thereby stabilizing the closed loop and prohibiting the response of the closed loop to the optical signal input to the APD 11. In the case that the time constant of the closed loop is small such that the closed loop is capable of responding the optical signal, the current signal generated by the APD becomes small because the bias voltage supplied to the APD 11 generated by the closed loop may compensate the amplitude of the optical signal from moment to moment.
Next, operation of the receiving circuit will be described in detail.
Receiving the optical signal into the APD 11, the APD generates corresponding current signal IAPD. Due to the operation of the current mirror circuit 14 described above, a reference current IREF equivalent to the signal current IAPD is output from the another output port 14c.
The comparator 16b of the feedback controlling circuit compares a voltage generated in the reference resistor RREF due to the reference current IREF, namely IREF×RREF, to the reference signal VREF.
When the derived voltage, IREF×RREF, is smaller then the reference signal VREF, namely, the signal current generated by the APD 11 is smaller than a defined value, the output of the comparator 16b is set to low level. Therefore, the transistor Q3 turns off, the collector of the transistor Q3 is nearly equal to the supply voltage Vcc, which appears in the output of the feedback controlling circuit 16c. Accordingly, the transistor Q1 that receives the output 16c of the feedback controlling circuit to the base thereof turns on and the high-voltage VH is directly carried to the current mirror circuit 14 nearly as it is, thereby biasing the APD 11 with the high-voltage VH.
In the case that the bias voltage of the APD 11 is high, the multiplication factor thereof also keeps high, and the large current is generated. Then, the reference current IREF becomes large, the input of the comparator that is the voltage between the reference resistor RREF increases and exceeds the reference signal VREF, and the output of the comparator 16b turns to the high level. The transistor Q3 that receives the output of the comparator 16v turns on, and the collector of which is lowered, whereby the voltage between the collector and the emitter of the transistor Q1, the vase of which receives the collector level of the transistor Q3, increases and the output of the voltage control circuit decreases.
The feedback loop thus described controls the reference current IREF, which is equivalent to the signal current IAPD, equal to a current calculated by the reference signal VREF divided by the reference resistor RREF, VREF/RREF. One example of the feedback control is that the resistance of the reference resistor RREF, the reference signal VREF, resistors R11 and R12 are 1.5 kΩ, 1.5 V, 10 kΩ and 10 kΩ, respectively, and the transistors Q11 and Q12 have the same specification, then the feedback control starts at the signal current of 1 mA, and due to thus feedback control, the signal current IAPD does not exceed 1 mA.
Increasing the optical input and reaching about −7 dBm, the APD generates about 1 mA as the signal current IAPD under the bias voltage of about 54 V and the feedback controlling starts its operation. At this bias condition, the multiplication factor of the APD may be estimated as about 5. Further increasing the optical input, the feedback control may operate so as to decrease the bias voltage to the APD, which is equivalent to reduce the multiplication factor thereof, and the bias voltage becomes about 30 V at the optical input of −3 dBm. Since the high-voltage VH is 55 V, the difference of 25 V between the high-voltage and the practically applied bias voltage to the APD 11 is consumed by the transistor Q1 of the voltage control circuit 16.
Still further increasing the optical input and amounting to 0 dBm, the feedback control sets the bias voltage to the APD equal to about 15V, and sets it about 11V at the optical input of +3 dBm. For such optical input, the average signal current of 1 mA for the APD 11 may be maintained.
In the first embodiment described above, the feedback control operates so as to maintain the average signal current to be 1 mA. As shown in
A circuit that escapes from the noise is shown in VH is equivalently reduced to a ratio of the internal resistance of the APD 11 to the resistance of the resistor R4. That is, denoting the internal resistance of the APD 11 as RAPD, the fluctuation
VAPD of the bias voltage to the APD 11 is:
VAPD=VHV•RAPD/(RAPD+R4).
The case that the resistance of the resistor R4 is 10 kΩ will be described below.
When no optical signal is input, the output of the feedback control circuit 16 is set to the high level because of no signal current generated by the APD 11. The transistor Q1 of the voltage control circuit turns on and the high-voltage VH from the high-voltage source is applied to the APD 11. Therefore, the APD is biased at 55 V. Increasing the optical input, the APD 11 generates a signal current IAPD and twice of the signal current will flow through the resistor R4 due to the operation of the current mirror circuit.
Reaching the signal current IPAD of the APD 11 to be 1 mA, the feedback control becomes active. In this occasion, the voltage drop at the resistor R4 becomes 20 V because twice of the signal current IAPD is flowing therethrough, whereby the APD 11 is applied by 35 V as the bias voltage. Referring to
In the case that the feedback control starts at the bias voltage of 55 V, which is same as that of the first embodiment, the high voltage source VH may be raised to 75 V. It is applicable to connect a Zener diode in parallel to the resistor R4, when the resistor R4 with greater resistance is used to cramp the resistor R4. Alternatively, the resistor R4 may be inserted between the current mirror circuit 14 and the APD 12.
Although preferred embodiments thus described are directed to the avalanche photodiode (APD), the present invention will be also applicable not only to a PIN-photodiode but also a photodiode having a general configuration. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.
