Abstract:
The present light-receiving circuit includes a light-receiving device, typically an avalanche photodiode (APD), a bias supply, a reference resistor and a feedback control circuit. The APD receives an optical signal with a predetermined transmission speed. The bias supply provides a bias voltage to the APD. The reference resistor detects a signal current generated by the APD. 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.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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.  
         [0003]     2. Related Prior Art  
         [0004]     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 V APD  applied to the APD.  
         [0005]      FIG. 5  shows an example of the behavior of the M-value to the bias V APD . When the V APD  is smaller than 11V, the APD generates nearly no carrier, namely, even if the signal light enters the APD, no corresponding electrical signal can be obtained. Exceeding the bias V APD  over 11V, the APD generates one carrier for one photon entering the APD, namely, the M-value is nearly equal to 1. This region, where the M-value is unity, is called as the photodiode region.  
         [0006]     Further increasing the bias V APD  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 V APD .  
         [0007]     When the APD is operates in a fixed bias condition, for example, the bias is fixed to 40V in  FIG. 5 , the obtained signal from the APD is large because the M-value in this region is about 2.5, which enables to design the electronic circuit which is subsequently connected to the APD and receives the large output from the APD. However, for an optical signal with relatively great magnitude, the subsequent electronic circuit may saturate because the M-value of the APD is maintained to be about 2.5 and the APD outputs an greater electronic signal.  
         [0008]     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 V APD  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.  
         [0009]     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.  
       SUMMARY OF THE INVENTION  
       [0010]     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.  
         [0011]     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.  
         [0012]     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.  
         [0013]     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.  
         [0014]     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. 
     
    
     BRIEF DESCRIPTION OF THE INVENTION  
       [0015]      FIG. 1  is the light-receiving circuit according to the first embodiment of the present invention;  
         [0016]      FIG. 2  shows an optical response of the avalanche photodiode to the applied bias voltage;  
         [0017]      FIG. 3  shown a bias condition of the avalanche photodiode in which the avalanche photodiode gives a predetermined photo current;  
         [0018]      FIG. 4  is the light-receiving circuit according to the second embodiment of the present invention;  
         [0019]      FIG. 5  shows a multiplication factor M of the avalanche photo diode against the bias voltage. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     Next, preferred embodiments of the present invention will be described as referring to accompanying drawings.  
       First Embodiment  
       [0021]      FIG. 1  is an light-receiving circuit according to the first embodiment of the present invention.  
         [0022]     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 R REF .  
         [0023]     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 .  
         [0024]     The pre-amplifier  13  includes an inverting amplifier  13   a  and feedback impedance  13   b  connected between the input and the output of the inverting amplifier  13   a.    
         [0025]     The current mirror circuit  14  has one input port  14   a  and two output ports  14   b  and  14   c.  Between the input port  14   a  and one of the output ports  14   b  is provides a pnp-type transistor Q 21  whose collector and the base are short circuited, while between the input port  14   a  and the other output port  14   c  is provided another pnp-type transistor Q 22 . Resistors R 21  and R 22  are connected between the input port  14   a  and the emitter of the transistor Q 21  and that of the transistorQ 22 , respectively. In this current mirror circuit, when performance of transistors Q 21  and Q 22  are equivalent to each other, currents output from each output ports  14   b  and  14   c  are determined by a ration of each resistors R 21  and R 22 . In the case that the resistance of resistors R 21  and R 22  are identical, the current output from the output port  14   b  is equal to the current from the output port  14 c. Accordingly, a current signal I APD  that corresponds to the optical signal received by the APD  11  is equal to the current flowed from the output ports  14   b  of the current mirror circuit  14 . At the same time, the current I REF  flowed from the other port  14   c  of the current mirror circuit  14  can be related to the signal current I APD .  
         [0026]     The voltage control circuit  15  includes an npn-type transistor Q 1 , where a voltage between the collector and the emitter thereof is controlled by a signal input to the base. Therefore, when a high-voltage V H  for the APD is applied to the collector of the transistor Q 1 , a voltage output from the emitter of the transistor Q 1 , which is practically applied to the APD, can be adjusted by the control signal applied to the base of the transistor Q 1 .  
         [0027]     The feedback controlling circuit  16  includes a comparator  16   a,  a reference signal V REF , three resistors R 1  to R 3 , a capacitor C 1  and a transistor Q 3 . The comparator  16   a  compares a voltage generated in the reference resistor R REF  by the current I REF  with the reference signal V REF , and transmits the result of comparison to the transistor Q 3 . The resistor R 1  and the capacitance, they are connected between the comparator  16   a  and the transistor Q 3  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.  
         [0028]     Next, operation of the receiving circuit will be described in detail.  
         [0029]     Receiving the optical signal into the APD  11 , the APD generates corresponding current signal I APD . Due to the operation of the current mirror circuit  14  described above, a reference current I REF  equivalent to the signal current I APD  is output from the another output port  14   c.    
         [0030]     The comparator  16   b  of the feedback controlling circuit compares a voltage generated in the reference resistor R REF  due to the reference current I REF , namely I REF ×R REF , to the reference signal V REF .  
         [0031]     When the derived voltage, I REF ×R REF , is smaller then the reference signal V REF , namely, the signal current generated by the APD  11  is smaller than a defined value, the output of the comparator  16   b  is set to low level. Therefore, the transistor Q 3  turns off, the collector of the transistor Q 3  is nearly equal to the supply voltage Vcc, which appears in the output of the feedback controlling circuit  16   c.  Accordingly, the transistor Q 1  that receives the output  16   c  of the feedback controlling circuit to the base thereof turns on and the high-voltage V H  is directly carried to the current mirror circuit  14  nearly as it is, thereby biasing the APD  11  with the high-voltage V H .  
         [0032]     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 I REF  becomes large, the input of the comparator that is the voltage between the reference resistor R REF  increases and exceeds the reference signal V REF , and the output of the comparator  16   b  turns to the high level. The transistor Q 3  that receives the output of the comparator  16   v  turns on, and the collector of which is lowered, whereby the voltage between the collector and the emitter of the transistor Q 1 , the vase of which receives the collector level of the transistor Q 3 , increases and the output of the voltage control circuit decreases.  
         [0033]     The feedback loop thus described controls the reference current I REF , which is equivalent to the signal current I APD , equal to a current calculated by the reference signal V REF  divided by the reference resistor R REF , V REF /R REF . One example of the feedback control is that the resistance of the reference resistor R REF , the reference signal V REF , resistors R 11  and R 12  are 1.5 kΩ, 1.5 V, 10 kΩ and 10 kΩ, respectively, and the transistors Q 11  and Q 12  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 I APD  does not exceed 1 mA.  
         [0034]      FIG. 2  is an output current spectrum of the APD for the optical input. When 55 V is applied for the bias voltage V HV  and no optical input, because of no signal current is generated, the transistor Q 1  of the voltage control circuit completely tuns on. Therefore, the APD  11  is biased about 54 V, which is the high-voltage V H  reduced by the voltage drop (about 0.8 V to 1.0 V in the present case) at the transistor Q 21  of the current mirror circuit  14 .  
         [0035]     Increasing the optical input and reaching about −7 dBm, the APD generates about 1 mA as the signal current I APD  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 V H  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 Q 1  of the voltage control circuit  16 .  
         [0036]     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.  
       Second Embodiment  
       [0037]     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  FIG. 2 , the condition that the signal current is 1 mA is sensitive to change of the bias voltage, namely, a ∂(I APD )/∂(V APD ) in  FIG. 2  is large at the point where the signal current is 1 mA. The circuit is susceptible to a noise included in the applied bias V APD .  
         [0038]     A circuit that escapes from the noise is shown in  FIG. 3 , in which a resistor R 4  is inserted between the high-voltage source  12  and the voltage control circuit  15 . By inserting the resistor R 4 , the fluctuation of the high-voltage          V H  is equivalently reduced to a ratio of the internal resistance of the APD  11  to the resistance of the resistor R 4 . That is, denoting the internal resistance of the APD  11  as R APD , the fluctuation          V APD  of the bias voltage to the APD  11  is: 
 
           V   APD   =V   HV   •R   APD /( R   APD   +R   4 ). 
 
         [0039]     The case that the resistance of the resistor R 4  is 10 kΩ will be described below.  
         [0040]     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 Q 1  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 I APD  and twice of the signal current will flow through the resistor R 4  due to the operation of the current mirror circuit.  
         [0041]     Reaching the signal current I PAD  of the APD  11  to be 1 mA, the feedback control becomes active. In this occasion, the voltage drop at the resistor R 4  becomes 20 V because twice of the signal current I APD  is flowing therethrough, whereby the APD  11  is applied by 35 V as the bias voltage. Referring to  FIG. 2 , when the APD generates the signal current of 1 mA under the bias voltage of 35 V, the optical input is about −4 dBm. By inserting the resistor R 4  between the high-voltage source  12  and the voltage control circuit  15 , a starting condition of the feedback control shifts from −7 dBm to −4 dBm.  
         [0042]     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 V H  may be raised to 75 V. It is applicable to connect a Zener diode in parallel to the resistor R 4 , when the resistor R 4  with greater resistance is used to cramp the resistor R 4 . Alternatively, the resistor R 4  may be inserted between the current mirror circuit  14  and the APD  12 .  
         [0043]     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.