Abstract:
An apparatus is provided with a biasing unit supplying a bias signal to a first avalanche photodiode and a second avalanche photodiode, the bias signal having a first level and a second level, the first level causing a multiplying effect and the second level causing a non-multiplying effect for the first and the second avalanche photodiodes; a smoothing unit smoothing the bias signal biasing the first avalanche photodiode into a smoothed bias signal; a current detecting unit detecting photocurrent from the second avalanche photodiode, with a multiplying current value corresponding to the multiplying bias level and a non-multiplying current value corresponding to the non-multiplying bias level; a multiplication factor calculating unit calculating a multiplication factor of the second avalanche photodiode based on the multiplying current value and the non-multiplying current value; and a control unit controlling the first level of the bias signal based on the calculated multiplication factor.

Description:
[0001]    The present invention claims foreign priority to Japanese application 2006-332728, filed Dec. 11, 2006, which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a bias control circuit of an avalanche photodiode (APD) and particularly to a bias control circuit of avalanche photodiode for controlling a multiplication factor of the avalanche photodiode to a constant value. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    An avalanche photodiode is a photodiode utilizing avalanche breakdown of carrier under a higher bias level and as a multiplication factor. APD is used in a receiving element of an optical transmitting apparatus to realize high sensitivity reception. The multiplication factor of APD is controlled with a bias level but such bias level must be controlled in accordance with change of environment temperature because it changes with temperature. 
         [0004]      FIG. 11  is a block diagram of a bias control circuit of APD. An APD bias unit  111  of the bias control circuit of APD shown in this figure applies a bias to APD  101  of an optical receiving unit  100  via a resistor R 101 . A signal light is inputted to a transmission line  121  to APD  101  of the optical receiving unit  100  and thereby a photocurrent flows in accordance with a signal light. A transfer impedance amplifier (TIA)  102  converts a photocurrent flowing in APD  101  to an electric signal and then outputs the electric signal to a circuit in the subsequent stage. 
         [0005]    APD  101  changes a multiplication factor thereof in accordance with temperature. Therefore, A bias control circuit of APD monitors ambient temperature with a temperature sensor  112  and an APD bias control unit  113  feed-forwardly controls a bias level of the APD bias unit  111  in order to keep constant the multiplication factor of APD  101 . 
         [0006]    The APD bias control unit  113  controls, for example, a bias level for temperature on the basis of attached data provided from a manufacturer of APD  101 . The data attached to APD  101  shows, for example, a breakdown level at 25° C. and a primary temperature slope. 
         [0007]    Following is an explanation of a known bias level control circuit for controlling a multiplication factor of APD to a constant value, as shown in Japanese Unexamined Patent Application Publication JP2005-93834. This circuit receives a signal light with a light receiving region having the multiplication function (limited region Ps at the center of the APD) and a light receiving region having no multiplication function (region Pm surrounded with a ring at the external circumference formed to surround the region Ps). A TIA output is branched to HPF (High Pass Filter) and LPF (Low Pass Filter), a monitor signal is extracted received by the light receiving region having no multiplication function is extracted by calculating output information of HPF and LPF, and the multiplication factor is controlled to a constant value with reference to this monitor signal. 
         [0008]    However, the data attached to APD shows a certain difference from the actual data of APD. Therefore, it has a problem that a certain deviation is generated in the control to keep a constant multiplication factor. 
         [0009]      FIG. 12  shows an example of the data attached to APD. The characteristic  131  shown in  FIG. 12  indicates a breakdown level (VB) for temperature of APD. In the attached data, the VB of APD is indicated with a linear curve as shown by the characteristic  131 . 
         [0010]    However, the VB of the actual APD is deviated from the characteristic  131  in the attached data as shown by the characteristic  132 . Therefore, deviation  133  between the characteristics  131  and  132  causes deviation in control of bias level and thereby deviation is also generated in the constant control of the multiplication factor for temperature change. 
         [0011]    Moreover, the circuit in JP2005-93834 also has a problem that when a DC (Direct Current) element of input light from the transmission line increases, a monitor signal is deteriorated in accuracy and control of multiplication factor is disabled. In addition, when only a DC light is inputted, output information of HPF cannot be attained and control of multiplication factor is also disabled. 
         [0012]      FIG. 13  explains operations of the bias control circuit when the DC element of signal light increases and when the signal light is a DC light. Input signal light in the first row indicates the signal light inputted from the transmission line. APD signal in the second row indicates a current of an electric signal flowing when the signal light is received. 
         [0013]    Is designates a current element of an electric signal received with the region Ps, while Im, a current element of an electric signal (monitor signal) received with the region Pm. Ino designates a noise element included in both current elements. 
         [0014]    Ps and Pm correspond to a first light receiving region Ps and a second light receiving region Pm in FIG. 1 of JP2005-93834. Also, Is, Im, and Ino correspond to Is, Im, Ino in FIG. 4 of JP2005-93834. 
         [0015]    Column (1.) of  FIG. 13  shows a case when a signal light with a good S/N, as shown in the first row, is inputted to the APD. In this case, by receiving the signal light, an electric signal with current of (Is+Ino+Im) having the waveform shown in the second row flows into the APD. 
         [0016]    In HPF, a current element of Is+Ino is detected from the electric signal with current of Is+Ino+Im. As the electric signal for current element Im has a low frequency, the current element of Im is shut off. From LPF, an electric signal with current of Is+Ino+Im is outputted. Accordingly, the Im element can be extracted through subtraction of current element of Is+Ino output from the HPF. The multiplication factor of APD is expressed as M=(Is+Ino)/Im. 
         [0017]    Column (2.) of  FIG. 13  shows a case when a signal light including a large amount of DC element and has a bad S/N ratio is inputted to APD. In this case, by receiving the signal light, an electric signal with current of (Is+Ino+Im+Idc), the waveform as shown in the second row, flows into the APD. 
         [0018]    Idc is current element of a DC element. In HPF, an electric signal with current of Is+Ino is detected from the electric signal with current of Is+Ino+Im+Idc. From the LPF, an electric signal with current of Is+Ino+Im+Idc is outputted and a current element of Im+Idc can be obtained through subtraction of the current element of Is+Ino output from the HPF. The multiplication factor M of APD is expressed as M (Is+Ino)/(Im+Idc), M is controlled to a small value due to the influence of Idc. 
         [0019]    Column (3.) of  FIG. 13  shows a case when a signal light of only DC element, as shown in the first row, is inputted to APD. In this case, by receiving the signal light, a current (Ino+Im+Idc) having the waveform shown in the second row flows into the APD. 
         [0020]    In the HPF, only Ino element is detected because there is no Is element. An electric signal with current of Ino+Im+Idc is outputted from the LPF and a current element of Im+Idc is obtained through subtraction of current element Ino output from the HPF. The multiplication factor M of APD is expressed as M=Ino/(Im+Idc). When the signal light has only DC element, the multiplication factor M cannot be controlled because the multiplication factor is not expressed to be related to a signal element (Is), and cannot be monitored. 
         [0021]    As explained above, when an input signal light includes a large amount of DC element, or an input signal light has only DC element, (Im+Idc) and the multiplication factor M can not be controlled accurately. 
       SUMMARY 
       [0022]    According to an aspect of an embodiment, an apparatus is provided with a biasing unit supplying a bias signal to a first avalanche photodiode and a second avalanche photodiode, the bias signal having a first level and a second level, the first level causing a multiplying effect and the second level causing a non-multiplying effect for the first and the second avalanche photodiodes; a smoothing unit smoothing the bias signal biasing the first avalanche photodiode into a smoothed bias signal; a current detecting unit detecting photocurrent from the second avalanche photodiode, with a multiplying current value corresponding to the multiplying bias level and a non-multiplying current value corresponding to the non-multiplying bias level; a multiplication factor calculating unit calculating a multiplication factor of the second avalanche photodiode based on the multiplying current value and the non-multiplying current value; and a control unit controlling the first level of the bias signal based on the calculated multiplication factor. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic diagram showing an APD bias control circuit. 
           [0024]      FIG. 2  is a diagram showing an APD bias control circuit of the first embodiment. 
           [0025]      FIG. 3  is a diagram for explaining relationship between bias level and APD multiplication factor. 
           [0026]      FIG. 4  is a diagram for explaining relationship between APD multiplication factor and light intensity of APD input signal light. 
           [0027]      FIG. 5  is a diagram showing details of smoothing circuit. 
           [0028]      FIG. 6  is a diagram showing error of multiplication factor due to the smoothing of bias level. 
           [0029]      FIG. 7  is a diagram for explaining operations of bias level control circuit when DC element of input signal light increases and when input signal light has only DC element. 
           [0030]      FIG. 8  is a diagram showing an APD bias control circuit of the second embodiment. 
           [0031]      FIG. 9  is a diagram showing an example of preset multiplication factor table. 
           [0032]      FIG. 10  is a flowchart showing bias level control. 
           [0033]      FIG. 11  is a block diagram of APD bias control circuit. 
           [0034]      FIG. 12  is a diagram showing an example of attached data of APD. 
           [0035]      FIG. 13  is a diagram for explaining operation of bias level control circuit when DC element of input signal light increases and when the signal light has only DC element. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0036]    Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0037]      FIG. 1  is a schematic diagram of an APD bias control circuit. As shown in  FIG. 1 , the bias control circuit includes a biasing unit  1 , a smoothing unit  2 , a first APD  3 , a second APD  4 , a current detecting unit  5 , a multiplication factor calculating unit  6 , a control unit  7 , and a TIA  9 . The first APD  3 , second APD  4 , and TIA  9  compose an optical receiving unit. 
         [0038]    The biasing unit  1  outputs bias with a multiplied bias level Va, which has a multiplying effect, and bias with a non-multiplied bias level Vb, which has no multiplying effect, to the first APD  3  and the second APD  4 . The multiplied bias level Va is a level having a multiplication factor larger than 1 and the non-multiplied bias level Vb is a level having a multiplication factor 1. 
         [0039]    The smoothing unit  2  outputs a smoothed electronic signal by smoothing the bias with the multiplied bias level Va and the non-multiplied bias level Vb, outputted from the biasing unit  1 . The smoothing unit  2  is, for example, an LPF which smoothes an electronic signal with the multiplied bias level Va. 
         [0040]    The first APD  3  is biased by the smoothed bias with the multiplied bias level Va, outputted from the smoothing unit  2 . The second APD  4  is biased by the bias with multiplied level Va and the non-multiplied level Vb, outputted from the biasing unit  1 . The both first APD  3  and the second APD  4  receive a signal light from the transmission line  8  and have an identical characteristic. 
         [0041]    When the second APD  4  is biased by the multiplied level Va, the current detecting unit  5  detects a multiplied current of Ia. When the second APD  4  is biased by the non-multiplied level Vb, the current detecting unit  5  detects a non-multiplied current lb. 
         [0042]    The multiplication factor calculating unit  6  calculates a multiplication factor (M) of the second APD  4  from the multiplied current of Ia and the non-multiplied current of Ib, which are detected by the current detecting unit S. The multiplication factor (M) is calculated by dividing the multiplied current of Ia, a current corresponding to the multiplication factor, with the non-multiplied current of lb. 
         [0043]    The control unit  7  controls the multiplied bias level Va from the biasing unit  1  on the basis of the multiplication factor, which is calculated with the multiplication factor calculating unit  6 , and the preset the multiplication factor. 
         [0044]    When the multiplication factor calculated with the multiplication factor calculating unit  6  is equal to or less than the preset multiplication factor, the multiplied bias level Va is controlled to become larger. When the multiplication factor calculated with the multiplication factor calculating unit  6  is larger than the preset multiplication factor, the multiplied bias level Va is controlled to become smaller. As explained above, the multiplication factor of the first APD  3  is controlled to become equal to the present multiplication factor by controlling the multiplied bias level Va. 
         [0045]    The current-to-voltage converter  9  converts a current value of the electric signal flowing due to reception of signal light of the first APD  3  into a voltage level and outputs an electric signal with the converted level to a subsequent circuit. 
         [0046]    As explained above, the APD bias control circuit calculates the multiplication factor of the first APD  3  using the second APD  4 , which has an identical characteristic to that of the first APD  3 . In this case, the bias control circuit of APD impresses the multiplied bias level Va having the multiplying effect and the non-multiplied bias level Vb having no multiplying effect to the second APD and obtains the multiplication factor of the first APD from the multiplied current Ia and the non-multiplied current Ib due to reception of the signal light of the second APD  4 . 
         [0047]    when the multiplication factor is 1, the multiplication factors of the first and second APDs do not depend on temperature. As the current Ib corresponds to a multiplication factor of 1, which do not change for temperature, and the current Ia corresponds to a multiplication factor of M, which changes for temperature, a multiplication factor M is calculated by M=Ia/Ib. The multiplication factor calculating unit  6  calculates M by M=Ia/Ib, and calculated M changes in accordance to an ambient temperature change. 
         [0048]    By calculating M by M=Ia/Ib, the bias control circuit  10  can control a multiplication factor of the first APD  3  corresponding to ambient temperature change. 
         [0049]    Moreover, as the calculation do not related to the DC element level of the input signal, the multiplication factor of the first APD  3  can be obtained and controlled accurately. That is, even when the signal light includes DC element or the signal light has only DC element, the biasing condition of second APD  4  is not changed, with the multiplied bias level Va having the multiplying effect and the non-multiplied bias level Vb having the multiplication factor 1, and thus a multiplied current Ia and a non-multiplied current Ib flows into the second APD  4 . 
         [0050]      FIG. 2  is a diagram showing an APD bias control circuit. The APD bias control circuit of APD includes an APD bias unit  11 , a smoothing circuit  12 , a current monitor circuit  13 , an M calculating unit  14 , an M setting unit  15 , an M comparing unit  16 , and an optical receiving unit  20 . 
         [0051]    The optical receiving unit  20  has an APD  21 , a monitor APD  22 , and a TIA  23 . A signal light is inputted to the APD  21  and the monitor APD  22  from a transmission line  31 . The APD  21  and monitor APD  22  have an identical breakdown level (VB) and breakdown characteristic. The APDs with identical characteristic can be obtained by utilizing the APDs formed adjacently in a same wafer or the APDs in a same lot. TIA  23  converts a current value of an electric signal flowing due to reception of the signal light of APD  21  into an electric signal with corresponding voltage level and then outputs the electric signal to a circuit in a subsequent stage. 
         [0052]    The APD bias unit  11  outputs a bias with its level modulated by two levels of V 1  and VM, for example, by pulse electric signal as shown in  FIG. 2 . The bias with levels of V 1  and VM is outputted to the smoothing circuit  12  and current monitor circuit  13 . 
         [0053]    The frequency of the modulated bias outputted from the APD bias unit  11  is set to the frequency sufficiently lower than the frequency of signal light (main signal). For example, when the signal light received by APD  21  is 10 Gb/s, the frequency of the modulated bias is set to 1 MHz. The reason to make the bias modulating frequency sufficiently lower is because if the frequency of the bias modulation is not sufficiently lower than the frequency of the signal light, the modulated bias for monitor APD  22  diffuses to the side of APD  21  and results in deterioration in receiving sensitivity and ALM characteristic. 
         [0054]    The bias level V 1  (having a multiplication factor of 1) causes the APD  21  and the monitor APD  22  to have no multiplying effect, and the bias level VM (having a multiplication factor of M) causes the APD  21  and the monitor APD  22  to have a multiplying effect. 
         [0055]    The smoothing circuit  12  smoothes the bias with levels of V 1  and VM, outputted from the APD bias unit  11  and biases the APD  21  with smoothed level of VM, as shown in  FIG. 2 . Accordingly, APD  21  has a multiplication factor M. 
         [0056]    The current monitor circuit  13  biases the APD  22  with levels of V 1  and VM, just same as the levels outputted from the APD bias unit  11 . When biased by the level of V 1 , the monitor APD  22  has a multiplication factor 1 and when biased by the level of VM, it has a multiplication factor M. Therefore, when biased by the level of VM, the monitor APD  22  has the multiplication factor M just like APD  21  does. 
         [0057]    APD  22  also inputs a signal light from the transmission line  31 , just as APD  21 . Therefore, by receiving a signal light, when biased by the level of V 1  an electric signal with current I 1  flows into the monitor circuit  13  and when biased by the level of VM, an electric signal with current IM flows into the current monitor circuit  13 . For example, as shown in  FIG. 2 , an electric pulse signal with monitor current I 1  and IM flows into the current monitor circuit  13 . The current monitor circuit  13  monitors this electric pulse signal and outputs this current value to the M calculating unit  14 . 
         [0058]    By receiving the signal light, the amount of a current flowing into APD  21  is identical to the amount of a current IM flowing into the monitor APD  22 , because the identical bias level VM is supplied. Since the monitor APD  22  has the characteristic identical to that of APD  21  as explained above, when a current flowing into APD  21  changes depending on temperature, the current IM of monitor APD  22  also changes. 
         [0059]    The M calculating unit  14  calculates the multiplication factor of the monitor APD  22  by the current values of I 1  and IM, which are monitored by the current monitor circuit  13 . IM is a current value for the bias level VM, I 1  is a current value for the bias level V 1  with multiplication factor 1. Therefore, the multiplication factor M can be calculated by M=IM/I 1 . Since APE  21  is biased by the level of VM, the same level with the monitor APD  22 , the multiplication factor of monitor APE  22  calculated with the M calculating unit  14  also equals to the multiplication factor of APD  21 . The multiplication factor calculated with the M calculating unit  14  is outputted to the M comparing unit  16 . 
         [0060]    The multiplication factor for APE  21  is set to the M setting unit  15  and outputted to the M comparing unit  16 . 
         [0061]    The M comparing unit  16  compares the multiplication factor calculated with the M calculating unit  14  with the multiplication factor set to the M setting unit  15 . The M comparing unit  16  controls the bias level VM of the APD bias unit  11  on the basis of the comparison result. 
         [0062]    For example, when the multiplication factor of the M calculating unit  14  (actually measured multiplication factor of monitor APD  22 ) becomes larger than that being set to the M setting unit  15  due to temperature change, the M comparing unit  16  controls the bias level VM of the APD bias unit  11  to become smaller. 
         [0063]    Also, when the multiplication factor of the M calculating unit  14  becomes smaller than that being set to the M setting unit  15  due to temperature change, the M comparing unit  16  controls the bias level VM of the APD bias unit  11  to become larger. 
         [0064]    Accordingly, the multiplication factor of the APD  21  is controlled to be equal to that set to the M setting unit  15  and kept constant. 
         [0065]    For example the M calculating unit  14  and the M comparing unit  16  can be a CPU (Central Processing Unit) and the M comparing unit  15  can be a memory. 
         [0066]      FIG. 3  explains relationship between bias level and multiplication factor.  FIG. 3  shows the relationship between bias levels supplied to APD and multiplication factors at different temperatures of 10° C., 25° C., 70° C. and 85° C. 
         [0067]    As shown in  FIG. 3 , temperature dependence of multiplication factor is extremely small within the range where the multiplication factor is around 1. The multiplication factor of APD becomes 1 irrespective of temperature when the bias level is within a range from about 4V to about 11V. When the multiplication factor is increased, temperature dependence of multiplication factor increases and results in multiplication factor fluctuation. For example, in order to set the multiplication factor of APD to 10 the bias level must be set to about 24V for 10° C. and to about 26V for 25°. 
         [0068]    To attain the multiplication factor 1, the APD bias unit  11  biases by the level of V 1  and V 1  can be within the range from 4V to 11V, such as 7.5V. 
         [0069]    On the other hand, the bias level of VM is controlled with the M comparing unit  11  to provide the multiplication factor of the monitor APD  22  equal to that set by the M setting unit  15 . Here, the multiplication factor of the monitor APD  22  can be obtained with the following equation (1) from current values of I 1  and IM of an electric signal flowing into the monitor APD  22 . 
         [0000]        M=IM/I 1  (1) 
         [0070]    The current value of I 1  corresponding to the multiplication factor 1 (bias level of V 1 ) does not depend on temperature. The current value of IM corresponding to the multiplication factor M (bias level VM) depends on temperature. Accordingly, the M comparing unit  16  can control the bias level of VM on the basis of the actually measured multiplication factor of the monitor APD  22  calculated by the equation (1). 
         [0071]    Since the bias control circuit of APD shown in  FIG. 2  controls the bias level based on the multiplication factor of the monitor APD  22  having the characteristic identical to that of APD  21 , the bias level control is made without use of the attached data of APD  21  and the multiplication factor of APD  21  can be controlled adequately even when ambient temperature changes. 
         [0072]    Moreover, since the multiplication factor of the APD  21  can be obtained from the equation (1), which only relates to the modulated bias levels of V 1  and VM that are applied to the monitor APD  22  and the current values of I 1  and IM of the electronic signal that flow into the APD  21 , obtaining the multiplication factor of the APD  21  is not affected by the DC element of the input signal light and possible even when DC element of signal light increases and when only DC light is inputted to the APD  21 . 
         [0073]      FIG. 4  explains relationship between multiplication factor of APD and optical intensity of the signal light. In  FIG. 4 , there are shown two sets of current values flowing into the monitor APD  22 , for signal lights with different optical intensity. The one is with current values of I 11  and IM 11  and the other is with current values of I 12  and IM 12 . When the bias levels of V 1  and VM are constant, a ratio of the current values of I 11  and IM 11  is identical to that of the current values of I 12  and IM 12  even if optical intensities of the signal lights are different. In other words, even if optical intensities of signal lights are different, multiplication factor does not change if the bias levels of V 1  and VM are constant. That is because if optical intensity of signal light changes, both current values of I 11  and IM 11  also change in accordance with such change and a ratio thereof does not change. 
         [0074]      FIG. 5  shows details of the smoothing circuit. FIG.  5  also shows the APD bias unit  11 , the APD  21 , and the TIA  23  shown in  FIG. 2  together with the smoothing circuit  12 . The smoothing circuit  12  can be an LPF RC circuit with a resistor R 1  and a capacitor C 1 . 
         [0075]    Frequency of the pulse signals with bias levels of V 1  and VM outputted from the APD bias unit I 1  is 1 MHz and a cut-off frequency of the smoothing circuit  12  is set to about 50 Hz in order to smooth the pulse signal to the bias level of VM. 
         [0076]    In order to remove pulse element of a pulse signal as much as possible and smooth the signal to the bias level of VM, it is required to increase an RC constant. Taking account of a mounting area of the devices or dielectric strength voltage, it is preferable to have a resistor R 1  with about 3 kΩ and a capacitor C 1  with about 1 μg F. 
         [0077]    When the pulse signal is smoothed by the smoothing circuit  12 , the level of the smoothed signal changes with a gradual discharge curve along an exp(−t/RC). For R 1 =3 kΩ and C=1 μg F, exp (−t/RC) is 0.9997 and variation of bias level of VM by the smoothing circuit  12  is suppressed to within about ±0.03. That means variation of the multiplication factor by the smoothing circuit  12  may be limited to about 0.07%, following equation (2). Thus, the bias level VM is applied to APD  12  within the level where influence of the smoothing circuit  12  can be neglected. 
         [0000]        M= 1{1−( VAPD/VB ) n }  (2) 
         [0078]    VAPD denotes a bias level of APD, VB denotes a breakdown voltage of APD, and n denotes a fitting coefficient (n=1.2). 
         [0079]      FIG. 6  shows influence to a multiplication factor by smoothing the bias level. In  FIG. 6 , the horizontal axis indicates a target multiplication factor and the vertical axis indicates a multiplication factor of the APD  21  when VM is smoothed with the smoothing circuit  12 . 
         [0080]    It is preferable that a multiplication factor using the smoothing circuit matches a target multiplication factor. For example, when the target multiplication factor on the horizontal axis is 10, it is preferable that the multiplication factor of the vertical axis is also 10. In other words, when the bias levels of V 1  and VM are smoothed with the smoothing circuit  12 , multiplication factor of APD  21  is preferable to be 10). 
         [0081]    As explained above, when the resistor R 1  is 3 kΩ and capacitor C 1  is 1 μF in the smoothing circuit  12 , variation of the bias level applied to APD  21  is about ±0.03% and variation of multiplication factor due to the variation of the bias level VM is about ±0.07%. 
         [0082]      FIG. 6  shows relationships between the target multiplication value and multiplication value using the smoothing circuit, in conditions with an ideal condition, +0.03% change of the bias level VM, and −0.03% change of the bias level VM. As shown in the figure, it may be obvious that even when the bias levels of V 1  and VM applied to APD  21  are smoothed with the smoothing circuit  12 , with a resistor R 1  of 3 kΩ and a capacitor C 1  of  1 M F, the multiplication factor is almost overlapped on the ideal multiplication factor. 
         [0083]      FIG. 7  explains operations of the bias level control circuit when DC element of the signal light increases and when the signal light has only DC light element. An input signal light in the first row shows a signal light inputted from the transmission line. An APD signal in the second row shows a current flowing into the monitor APD  22  due to reception of the signal light. The current IM indicates the current flowing due to reception of the signal light when the bias level VM of the monitor APD  22  is applied, and the current I 1  indicates the current flowing due to reception of the signal light when the bias level V 1  of the monitor APD  22  is applied. 
         [0084]    In a case when a signal light as shown in column (1.) of  FIG. 7  is inputted to the monitor APD  22 , a current with a waveform shown in the second row of APD signal flows due to reception of the signal light in the monitor APD  22 . 
         [0085]    The current I 1  flows when the bias level is V 1  as explained above. The bias level of V 1  results in the multiplication factor 1 for the monitor APD  22 . Thus, the current I 1  corresponds to the multiplication factor 1 and does not depend on temperature. 
         [0086]    On the other hand, when the current IM flows when the bias level is VM, the multiplication factor is M and the multiplication factor changes in accordance with temperature change of the monitor APD  22 . 
         [0087]    Accordingly, the multiplication factor M (M=IM/I 1 ) obtained by dividing the current value IM with the current value I 1  indicates the actual multiplication factor (actually measured multiplication factor of the monitor APD  22 ) changing in accordance with temperature change of the monitor APD  22 . This monitor APD  22  has the characteristic identical to that of APD  21  and the actually measured multiplication factor obtained from the monitor APD  22  indicates the actual multiplication factor of APD  21 . Therefore, the multiplication factor of APD  21  can be controlled properly even for ambient temperature change, by controlling the actually measured multiplication factor of the monitor APD  22  to become equal to that preset by the M setting unit  15 . 
         [0088]    In the case when a signal light including considerable amount of DC element is inputted to the monitor APD  22  as shown in (2.) of  FIG. 7 , the multiplication factor M (M=IM/I 1 ) can be obtained from the current I 1  flows in when the bias level of V 1  is applied and the current IN flows in when the bias level of VM is applied without relation in amount of the DC element included in the light. 
         [0089]    In the case when the signal light has only DC element as shown in column (3.) of  FIG. 7  is inputted to the monitor APD  22 , the multiplication factor M (M=IM/I 1 ) can be obtained from the currents I 1  and IM, because the current I 1  flows in when the bias level of V 1  is applied and the current IM flows in when the bias level of VM is applied. 
         [0090]    As explained above, the bias control circuit for of APD calculates the multiplication factor of APD  21  using the monitor APD  22  having the characteristic identical to that of APD  21 . In this case, the bias control circuit of APD obtains the multiplication factor of APD  21  from the current IM corresponding to the multiplication factor M and the current I 1  corresponding to the multiplication factor 1 flowing into the monitor APD  22  by applying, to the monitor APD  22 . The bias level of VM results in the multiplication factor to M and the bias level V 1  results in the multiplication factor to 1. 
         [0091]    The multiplication factor of APD  21  and the monitor APD  22  when the multiplication factor is 1 does not depend on temperature as is explained in regard to  FIG. 3 . Accordingly, the M calculating unit  14  calculates the multiplication factor (M=IM/I 1 ) depending on ambient temperature change from the current I 1  corresponding to the multiplication factor 1 not depending on temperature and the current IM corresponding to the multiplication factor M depending on temperature. 
         [0092]    Accordingly, the bias control circuit of APD can control the multiplication factor of APD  21  depending on ambient temperature change. 
         [0093]    Moreover, even when the signal light include DC element and when the signal light has only of DC element, the current IM flows corresponding to the multiplication factor M and the current I 1  flows corresponding to the multiplication  1  into the monitor APP  22  because the bias levels of VM and V 1  are applied to the monitor APP  22 . 
         [0094]    Therefore, even when the signal light includes DC element and when the signal light has only DC element, the multiplication factor (M=IM/I 1 ) of APD  21  can be obtained and the multiplication factor of APD  21  can be controlled adequately. 
         [0095]    In addition, any deviation is not generated between the attached data of APD  21  and the actual multiplication factor of APD  21  and therefore any influence is not applied to deterioration of receiving sensitivity characteristic and generation of ALM or the like by controlling the multiplication factor of APD  21  through actual measurement of the monitor APP  22  having the characteristic identical to that of APD  21 . 
         [0096]    Next, a second embodiment will be explained in detail with reference to the accompanying figures. In the second embodiment, the multiplication factor to be set is changed in accordance with optical intensity of the signal light. For example, if optical intensity of signal light is low, the multiplication factor to be set is increased and if optical intensity of signal light is high, the multiplication factor to be set is decreased. 
         [0097]      FIG. 8  shows an APD bias control circuit of the second embodiment. The elements like that of  FIG. 2  are designated by the like reference numerals and explanation of these elements is omitted here. 
         [0098]    In  FIG. 8 , the power monitor unit  41  detects optical intensity of the signal light received by the monitor APD  22  on the basis of the current I 1  monitored with the current monitor circuit  13 . Here, as the APD  21  and the monitor APD  22  receive the same signal light, the power monitor unit also detects the intensity of the signal light received by the APD  21   
         [0099]    The M setting unit  42  in  FIG. 8  includes the preset multiplication factor table storing the multiplication factor to be set for optical intensity of the input signal light. The M setting unit  42  acquires the multiplication factor corresponding to optical intensity detected with the power monitor unit  41  on the basis of the preset multiplication table and outputs the acquired multiplication factor to the M comparing unit  16 . 
         [0100]      FIG. 9  shows an example of the preset multiplication table. The horizontal axis indicates optical intensity of the input signal light of the APD  21  or the monitor APD  22 , while the vertical axis indicates the multiplication factor to be set. 
         [0101]    For example, when the power monitor unit  41  has detected an optical intensity of the input signal light of APD  21  as −22 dBm based on the current I 1 , the M setting unit  42  acquires the multiplication factor 10 as the factor to be set from the preset multiplication factor table of the figure. The M setting unit  16  compares the actually measured multiplication factor outputted from the M calculating unit  14  with the multiplication factor outputted from the M setting unit  42  and controls the bias level VM of the APD bias unit  11  based on the comparison result. 
         [0102]    The power monitor unit  41  calculates optical intensity Pin (AVR.)  from the current I 1  based on the following equation (3). 
         [0000]        I   APD(AVR.) ={( e ·λ)/( h·c )}×η× M ×Pin (AVR.)   (3) 
         [0103]    Here, I APD(AVR.)  [A] is a mean value of photocurrent flowing into APD  21  (monitor APD  22 ) (mean value of the current I 1  detected with the power monitor APD  41 ), e [C] is prime charge (1.60 E-19), λ[m] is input wavelength of APD  21  (1.5GE-0.6), h[J*s] is Planck constant (6.63 E-34), c[m/s] is velocity of light(3.00 E+08) η is quantum efficiency of APD  21  (0.6), M is multiplication factor (1) of APD  21 , Pin (AVR.)  [W] is mean value of input optical power (optical intensity) of APD. 
         [0104]      FIG. 10  is a flowchart showing the bias level control. The bias control circuit of APD shown in  FIG. 8  controls the bias level VM in accordance with the following steps. 
         [0105]    [Step S 1  ] The current monitor circuit  13  inputs a monitor current signal (currents I 1 , IN) flowing due to reception of the signal light of the monitor APD  22 . 
         [0106]    [Step S 2 ] The current monitor circuit  13  extracts the currents I 1 , IM from the input monitor current signal. 
         [0107]    [Step S 3 ] The power monitor circuit  41  calculates an optical intensity of the signal light received by APD  21  based on the current I 1  extracted with the current monitor circuit  13 . In more concrete, the power monitor unit  41  calculates optical intensity of signal light using the equation (3) explained above. 
         [0108]    [Step S 4 ] The M setting unit  42  acquires the multiplication factor (preset multiplication factor) corresponding to the optical intensity calculated by the power monitor unit  41 , for example, with reference to the preset multiplication factor table shown in  FIG. 9 . 
         [0109]    [Step S 5 ] The M calculating unit  14  calculates the actually measured multiplication factor of the monitor APD  22  from the currents I/, IM extracted by the current monitor circuit  13 . 
         [0110]    [Step S 6 ] The M comparing unit  16  compares the actually measured multiplication factor of the monitor APD  22  calculated by the M calculating unit  14  with the preset multiplication factor acquired by the M setting unit  42 . 
         [0111]    [Step S 7 ] The comparing unit  11  controls the bias level VM of the APD bias unit  11  on the basis of the comparison result. For example, when the actually measured multiplication factor is equal to the preset multiplication factor or less, the M comparing unit  16  controls the bias level VM to increase and when the actually measured multiplication factor is larger than the preset multiplication factor, it controls the bias level VM to decrease. 
         [0112]    As explained above, the bias control circuit of APD acquires the multiplication factor to be set in accordance with the input signal light. Therefore, the multiplication factor can be set in accordance with the input signal light. 
         [0113]    Moreover, in the related art, a circuit for monitoring optical intensity has been required to conduct compensation for conversion of the multiplication factor. However, the bias control circuit of APD shown in  FIG. 8  can obtain optical intensity without conversion of the multiplication factor, because optical intensity is calculated from the current I 1  having the multiplication factor 1. 
         [0114]    The present invention is not limited to the illustrated examples described above. The present invention can be variably changed without departing from the spirit and scope of the invention, as a matter of course.