Abstract:
The present invention aims at providing a drive circuit and a drive method for a semiconductor laser module including an electro-absorption type optical modulator, capable of obtaining a stable optical output without controlling a module temperature to be constant. To this end, the drive circuit of the present invention detects a temperature in the semiconductor laser module comprising a semiconductor laser and an EA modulator, and based on the detected temperature, controls a drive current supplied to the semiconductor laser and a bias voltage and a modulated electric signal applied to the EA modulator, so that average power, an extinction ratio and an optical cross point of an optical signal output from the semiconductor laser module are held to be constant.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a drive circuit and a drive method of a semiconductor laser module that externally modulates carrier light output from a semiconductor laser using an electro-absorption type optical modulator to output the modulated light, in particular, to a drive control technique for compensating for temperature characteristics of the semiconductor laser and the electro-absorption type optical modulator.  
           [0003]    2. Description of the Related Art  
           [0004]    In various optical communication systems for transmitting optical signals over a long distance, there is sometimes used a semiconductor laser module in which a semiconductor laser and an external modulator of electro-absorption type are integrated with each other, since such a semiconductor laser module enables to increase a transmission distance compared with a semiconductor laser of direct modulation type. In the aforementioned semiconductor laser module, operation characteristics of the semiconductor laser and the electro-absorption type optical modulator (hereinafter referred to as EA modulator) each has dependence on temperature. Therefore, in order to hold optical output power in stable, it is necessary to stabilize a module temperature or to control a drive condition according to a temperature change.  
           [0005]    In a conventional drive control of a semiconductor laser module having an EA modulator, such a method is typical in that, for example, by using an electronic cooling device such as a Peltier device and a temperature detection device such as a thermistor integrated with each other in a module, a current to be supplied to the Peltier device is controlled so that a resistance value of the thermistor is maintained to be constant, to hold the temperature of a semiconductor laser and the EA modulator to be constant. In this case, the EA modulator is driven by a constant modulated electric signal and at the same time, a constant drive current or a drive current to make a back power monitor current constant is supplied to the semiconductor laser, thereby achieving a stable optical output. The drive control method described above, however, has a disadvantage that since the Peltier device needs to be driven, the power consumption is increased and also a package size of the module is made to be large.  
           [0006]    To cope with this problem, a semiconductor laser module using an EA modulator which is not provided with a Peltier device, has recently been under development. According to this semiconductor laser module, both the reduction of power consumption and the miniaturization can be achieved by eliminating the need of a Peltier device, although there is a need of stabilizing optical output power by controlling a bias voltage of the EA modulator or a drive current of the semiconductor laser according to a temperature change. Also, since this module is of an external modulation system using the EA modulator, it is possible to obtain an optical output having a small amount of chirping.  
           [0007]    As a conventional drive control technique applicable to such a semiconductor laser module without the Peltier device as described above, a drive circuit for an EA modulator is disclosed in Japanese Unexamined Patent Publication No. 11-119176. In this drive circuit, an anode voltage of the EA modulator is detected and according to the detection result, a bias voltage of the EA modulator is controlled, so that an applied voltage to the EA modulator is maintained to be constant even if a temperature change or a change with age occurs.  
           [0008]    However, in the case where the semiconductor laser module provided with EA modulator without the Peltier device is driven by applying the conventional control technique as described above, although the bias voltage of the EA modulator is controlled according to the temperature change and the like, since the EA modulator is driven by a constant modulated electric signal, an optical output waveform of the semiconductor laser module is considerably deteriorated.  
           [0009]    [0009]FIG. 16 is a diagram showing temperature dependence of operating characteristics of a typical EA modulator. In FIG. 16, each characteristic curve represents a relation between power Pf of an optical signal output from the EA modulator and an applied voltage Vea to the EA modulator at each of temperatures 0° C., 25° C. and 75° C. In FIG. 16, if a modulated electric signal of a waveform as shown in the lower left part is applied to drive the EA modulator, since the optical output power Pf is changed along each characteristic curve, an optical waveform at 0° C. output from the EA modulator is considerably deteriorated as compared with an optical waveform at 75° C. as shown in the upper right part. Even if the bias voltage is controlled according to the temperature change as in the conventional technique so that the EA modulator operates in a region where the characteristic curve has the large inclination and is changed substantially linearly, since the inclination or the distortion of the characteristic curve in each operating region differs from each other depending on the temperature, an extinction ratio or a duty of the optical output waveform is changed depending on temperature when the EA modulator is driven by the modulated electric signal in which a modulated amplitude and a cross point are set to be constant.  
           [0010]    Further, since a current generated by absorption of carrier light (hereinafter referred to as a photocurrent) flows through the EA modulator, there is caused a problem in that the applied voltage to the EA modulator deviates, if the photocurrent is changed when controlling a current source for drive controlling the EA modulator. Therefore, for the semiconductor laser module without the Peltier device, it is critically important to realize a drive control method that is not affected by a change in the photocurrent due to the electro-absorption effect, when the temperature change or the change with age is compensated by controlling the electrical drive signal of the EA modulator.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention has been accomplished in view of the aforementioned problems, and has an object to provide a drive circuit and a drive method for a semiconductor module provided with an EA modulator, capable of obtaining a stable optical output without a need of controlling a temperature of the module to be constant.  
           [0012]    In order to achieve the object described above, according to the present invention, there is provided a drive circuit for driving a semiconductor module including a semiconductor laser generating carrier light according to a drive current and an electro-absorption type optical modulator outputting an optical signal of which intensity is modulated by absorbing the carrier light output from the semiconductor laser according to the drive voltage, the drive circuit comprising: a laser drive unit; an optical modulator drive unit; a temperature detection unit; and a drive control unit. The laser drive unit supplies a drive current to the semiconductor laser. The optical modulator drive unit applies a drive voltage in which a modulated electric signal is superimposed on a bias voltage, to the electro-absorption type optical modulator. The temperature detection unit detects the ambient temperature of the semiconductor laser and the electro-absorption type optical modulator. The drive control unit controls the drive current supplied by the laser drive unit and the drive voltage applied by the optical modulator drive unit, based on the temperature detected by the temperature detection unit, so that average power, an extinction ratio and an optical cross point of the optical signal output from the semiconductor laser module are constant.  
           [0013]    In the drive circuit of this configuration, a temperature in the semiconductor laser module is detected by the temperature detection unit. Based on this detection result, the drive current supplied to the semiconductor laser is controlled by the laser drive unit, and at the same time, the drive voltage applied to the electro-absorption type optical modulator is controlled by the optical modulator drive unit. Thus, differently from the conventional technique, without the need of controlling the temperature of the module to be constant by a Peltier device or the like, the optical signal having constant average power, a constant extinction ratio and a constant cross point is output from the semiconductor laser module. In this way, it becomes possible to obtain an optical output stable to a temperature change.  
           [0014]    Further, the above drive circuit may be configured to include a photocurrent detection unit detecting a photocurrent generated as a result that the carrier light is absorbed by the electro-absorption type optical modulator, so that the drive current supplied by the laser drive unit and the drive voltage applied by the optical modulator drive unit are controlled by a drive control unit based on the temperature detected by the temperature detection unit and the photocurrent detected by the photocurrent detection unit, so as to hold the average power, the extinction ratio and the cross point of the optical signal output from the semiconductor laser module to be constant.  
           [0015]    With this configuration, the drive current of the semiconductor laser and the drive voltage of the electro-absorption type optical modulator are controlled according to a change in the photocurrent generated in the electro-absorption type optical modulator as well as the temperature change in the semiconductor laser module. Thus, it becomes to make the optical signal output from the semiconductor laser module more stable.  
           [0016]    The above and other objects, features and advantages will be made apparent from the detailed description below in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a functional block diagram showing a configuration of a first embodiment of a drive circuit of a semiconductor laser module according to the present invention.  
         [0018]    [0018]FIG. 2 is a diagram for explaining a method of controlling an applied voltage to an EA modulator in the first embodiment.  
         [0019]    [0019]FIG. 3 is a circuit diagram showing a specific example of the first embodiment.  
         [0020]    [0020]FIG. 4 is a circuit diagram showing another specific example of the first embodiment.  
         [0021]    [0021]FIG. 5 is a diagram showing specific examples of a DAC shown in FIG. 4.  
         [0022]    [0022]FIG. 6 is a diagram showing a specific example of a table stored in a ROM shown in FIG. 4.  
         [0023]    [0023]FIG. 7 is a graph showing an example in which a set value in the ROM table is obtained by a complement using a linear approximate expression.  
         [0024]    [0024]FIG. 8 is a graph showing an example in which the set value in the ROM table is obtained by a complement using an exponential approximate expression.  
         [0025]    [0025]FIG. 9 is a graph showing an example in which the set value in the ROM table is obtained by a complement using a polynomial approximate expression.  
         [0026]    [0026]FIG. 10 is a graph showing an example in which the set value in the ROM table is obtained by a complement using an inter-data linear approximation.  
         [0027]    [0027]FIG. 11 is a functional block diagram showing a configuration of a second embodiment of a drive circuit of a semiconductor laser module according to the present invention.  
         [0028]    [0028]FIG. 12 is a circuit diagram showing a specific example of the second embodiment.  
         [0029]    [0029]FIG. 13 is a diagram showing a simple model for explaining a relation between an AC component of photocurrent and an applied voltage to an EA modulator in the second embodiment.  
         [0030]    [0030]FIG. 14 is a conceptual diagram showing signal waveforms for explaining the relation between the AC component of photocurrent and the applied voltage to the EA modulator in the second embodiment.  
         [0031]    [0031]FIG. 15 is a circuit diagram showing another specific example of the second embodiment.  
         [0032]    [0032]FIG. 16 is a diagram showing temperature dependence of operating characteristics of a typical EA modulator. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Embodiments of the present invention will be described below with reference to the drawings.  
         [0034]    [0034]FIG. 1 is a functional block diagram showing a configuration of a first embodiment of a drive circuit for a semiconductor laser module according to the present invention.  
         [0035]    In FIG. 1, a semiconductor laser module  1  driven by a drive circuit  10  has a structure in which a semiconductor laser (LD)  2  as a light emitting element and an EA modulator  3  made of a semiconductor chip are integrated with each other. The semiconductor laser  2  generates carrier light (fixed light) of intensity corresponding to a drive current supplied by the drive circuit  10 . In this carrier light, forward emission light emitted from the front end face of the semiconductor laser  2  is input to the EA modulator  3 , and backward emission light emitted from the rear end face of the semiconductor laser  2  is input to a monitor photodiode (PD)  4  disposed in the module. A thermistor  5  disposed in the module is a temperature detecting element having a resistance value being changed according to the ambient temperature of the semiconductor laser  2  and the EA modulator  3 .  
         [0036]    The EA modulator  3  absorbs the carrier light according to the drive voltage applied by the drive circuit  10  to output intensity-modulated optical signals. The EA modulator  3  has a characteristic to supply a photocurrent corresponding to absorbance. Each cathode terminal of the semiconductor laser  2  and the EA modulator  3  is connected to a package of the semiconductor laser module  1 , and here, has the earth potential. A resistor  6  electrically connected in parallel to the EA modulator  3  is a terminal resistor for generating a voltage to be applied to the EA modulator  3  based on a signal supplied from the drive circuit  10 . An impedance value of the terminal resistor  6  to a microwave suitable for fast modulation is 50Ω, for example. Although not shown in the figure, the optical signals output from the EA modulator  3  are focused by a lens and coupled to an optical fiber, to thereby become an optical output from the semiconductor laser module  1 .  
         [0037]    The drive circuit  10  comprises, for example, as a configuration to drive control the semiconductor laser  2 , an LD current drive section  11 , a back power detecting section  12 , an APC reference section  13  and a comparison section  14 , and also comprises, as a configuration to drive control the EA modulator  3 , a bias voltage control section  15 , a modulated amplitude control section  16 , a cross point control section  17 , a modulated electric signal output section  18  and a DC terminator  19 . The drive circuit  10  further comprises a temperature detecting section  20  detecting a temperature in the semiconductor laser module  1  according to a resistance value of the thermistor  5 . A signal indicating this detection result at the temperature detecting section  20  is output to the APC reference section  13 , the bias voltage control section  15 , the modulated amplitude control section  16  and the cross point control section  17 , respectively.  
         [0038]    The LD current drive unit  10  generates a drive current to be supplied to the semiconductor laser  2 . This drive current is controlled according to an output voltage of the comparison section  14  comparing an output voltage of the back power detecting section  12  with an output voltage of the APC reference section  13 . The back power detecting section  12  converts a current flowing through the monitor PD  4  into a voltage according to the backward emission light of the semiconductor laser  2  to generate an output voltage corresponding to power of the backward emission light. Here, since a ratio between an optical output from the front end face and an optical output from the rear end face of the semiconductor laser  2  is constant, forward emission light power of the semiconductor laser  2  is detected indirectly by detecting the backward emission light power thereof.  
         [0039]    The APC reference section  13  is for outputting a reference voltage for an optical output power control loop of the semiconductor laser  2 . The reference voltage set by the APC reference section  13  is controlled according to an output signal from the temperature detecting section  20 , as described later. The comparison section  14  compares the output voltage of the back power detecting section  12  and the output voltage of the APC reference section  13  with each other, and outputs a voltage corresponding to a difference between those output voltages to the LD current drive section  11 .  
         [0040]    The bias voltage control section  15  generates a bias voltage to be applied to the EA modulator  3  through an inductor L 1 . This bias voltage is controlled according to the output signal from the temperature detecting section  20 . Impedance in an output portion of the bias voltage control section  15  is sufficiently high relative to a modulated signal band so that a modulated electric signal to be applied to the EA modulator  3  through a capacitor C 1  from the modulated electric signal output section  18  may not flow into the bias voltage control section  15 .  
         [0041]    The modulated amplitude control section  16  outputs to the modulated electric signal output section  18 , a signal for controlling amplitude of the modulated electric signal to be applied to the EA modulator  3 . The setting of modulated amplitude controlled by the modulated amplitude control section  16  is controlled according to the output signal from the temperature detecting section  20 . The cross point control section  17  outputs to the modulated electric signal output section  18 , a signal for controlling a cross point of the modulated electric signal to be applied to the EA modulator  3 . The setting of electric cross point controlled by the cross point control section  17  is also controlled according to the output signal from the temperature detecting section  20 .  
         [0042]    The modulated electric signal output section  18  generates a modulated electric signal having an amplitude and a cross point corresponding to the control signals from the modulated amplitude control section  16  and the cross point control section  17 , and applies the modulated electric signal thus generated to the EA modulator  3  through the capacitor C 1 . The DC terminator  19  is a DC terminal of the modulated electric signal output section  18 . Impedance of an output portion of the DC terminator  19  is also sufficiently high relative to the modulated signal band so that the modulated electric signal may not flow into the DC terminator  19 .  
         [0043]    Next, the description will be given of an operation of the drive circuit for the semiconductor laser module according to the first embodiment.  
         [0044]    In the semiconductor laser module  1  having the EA modulator described above but without a Peltier device, the carrier light is generated in the semiconductor laser  2  in accordance with the drive current supplied from the drive circuit  10  to be input to the EA modulator  3 . The carrier light is absorbed by the EA modulator  3  according to the voltage applied from the drive circuit  10  so that intensity-modulated optical signals are output from the EA modulator  3 . The ambient temperature of the semiconductor laser  2  and the EA modulator  3  driven by the drive circuit  10  in this way is detected by the temperature detecting section  20  according to the resistance value of the thermistor  5  in the module. A signal corresponding to the temperature thus detected by the temperature detecting section  20  is output from the temperature detecting section  20  to the APC reference section  13 , the bias voltage control section  15 , the modulated amplitude control section  16  and the cross point control section  17 , respectively.  
         [0045]    On the driving side of the semiconductor laser  2  in the drive circuit  10 , the output voltage of the APC reference section  13  being the reference voltage of the optical output power control loop is changed in accordance with the output signal from the temperature detecting section  20 , and this output voltage is applied to one of input terminals of the comparison section  14 . The other input terminal of the comparison section  14  is applied with the output voltage of the back power detecting section  12  corresponding to the backward emission light power of the semiconductor laser  2 . Thus, a difference of the output voltage from the back power detecting section  12  from the reference voltage from the APC reference section  13  is obtained, and an output voltage corresponding to the difference is applied to the LD current drive section  11 . In the LD current drive section  11 , a setting value of the drive current to be supplied to the EA modulator  3  from the semiconductor laser  2  is adjusted in accordance with the output voltage from the comparison section  14  so that the output voltage of the back power detecting section  12  coincides with the reference voltage. As a result, carrier light power applied from the semiconductor laser  2  to the EA modulator  3  is feedback controlled to be constant regardless of a temperature change in the module.  
         [0046]    On the driving side of the EA modulator  3  in the drive circuit  10 , on the other hand, an output voltage of the bias voltage control section  15  is controlled in accordance with the output signal from the temperature detecting section  20 , thereby optimizing a DC component (bias voltage) of the voltage to be applied to the EA modulator  3 . Also, output voltages of the modulated amplitude control section  16  and the cross point control section  17  are controlled, thereby optimizing the amplitude and cross point of the modulated electric signal being an AC component of the voltage to be applied to the EA modulator  3 .  
         [0047]    A method of controlling the voltage to be applied to the EA modulator  3  according to the temperature change in the module will be explained with reference to FIG. 2.  
         [0048]    An example in FIG. 2 schematically shows a state of controlling the voltage to be applied to the EA modulator  3  having a temperature characteristic similar to that of the typical EA modulator shown in FIG. 16 in the case where the temperature detected by the temperature detecting section  20  is changed from 75° C. to 25° C. Namely, in the case where the temperature detected by the temperature detecting section  20  is 75° C., an average voltage level (bias voltage) of an applied voltage Vea to the EA modulator  3  is set so that a low level (Von) and a high level (Voff) of the modulated electric signal are changed in a range of about 0V to 0.8V, as shown in the lower left side of FIG. 2, corresponding to a region where a curve indicating the temperature characteristic of the EA modulator  3  has a large inclination and is changed substantially linearly. At this time, the amplitude and cross point of the modulated electric signal are set, respectively, so that a desired extinction ratio and a desired optical cross point (optical waveform duty) can be obtained according to inclination and distortion of the characteristic curve in a driving region. By supplying the applied voltage having such a drive waveform to the EA modulator  3 , an optical signal having a waveform as shown on the upper right side of FIG. 2 is output from the EA modulator  3 .  
         [0049]    In the case where the temperature detected by the temperature detecting section  20  is 25° C., on the other hand, as shown in the lower central part of FIG. 2, the bias voltage of the EA modulator  3  is shifted to a high level according to a change in the characteristic curve of the EA modulator  3 , and at the same time, the amplitude and cross point of the modulated electric signal are adjusted so that the desired extinction ratio and the desired optical cross point can be obtained according to changes in the inclination and distortion of the characteristic curve. As a result, as shown in the upper right part of FIG. 2, in the case of the temperature of 25° C., the EA modulator  3  outputs an optical signal having the same average output power and waveform as those in the optical signal in the case of the temperature of 75° C. Although not shown in the figure, also in the case where the detected temperature is 0° C., the drive voltage in which the bias voltage, the modulated amplitude and the electric cross point are optimized according to the characteristic curve for 0° C. in the same manner as the above, is applied to the EA modulator  3 , so that an optical signal similar to those in the cases of the detection temperatures of 75° C. and 25° C. is output from the EA modulator  3 .  
         [0050]    As described above, the drive current of the semiconductor laser  2  and the drive voltage (the bias voltage, the modulated amplitude and the electrical cross point) of the EA modulator  3  are controlled according to the temperature detected by the temperature detecting section  20 . Thus, it becomes possible to control the average output power, the extinction ratio and the optical cross point of the optical signal output from the semiconductor laser module  1  to be constant, independently of the ambient temperature, without controlling the temperature of the module with a Peltier device or the like.  
         [0051]    Now, a specific example of the configuration of the drive circuit  10  shown in FIG. 1 will be described. The configuration of the drive circuit according to this invention, however, is not limited to the example described below.  
         [0052]    [0052]FIG. 3 is a circuit diagram showing a specific example of the drive circuit  10  shown in FIG. 1.  
         [0053]    In the configuration example of FIG. 3, the LD current drive section  11  comprises a transistor TR 1  with a collector terminal thereof connected to an anode terminal of the semiconductor laser  2  and an emitter terminal thereof connected through a resistor R1 to a negative supply voltage (−V) terminal, and a differential amplifier A 1  that is applied with the emitter voltage of the transistor TR 1  at one input terminal thereof to apply an output voltage thereof to a base terminal of the transistor TR 1 . The back power detecting section  12  comprises a resistor R2 with one end thereof connected to the monitor photodiode  4  of the semiconductor laser module  1  and the other end thereof grounded.  
         [0054]    The APC reference section  13  comprises resistors R3, R4 connected in series between a positive supply voltage (+V) terminal and an earth terminal, and a differential amplifier A 2  with one input terminal thereof applied with a voltage at a connection between the resistors R3, R4 and the other input terminal thereof applied with the output signal of the temperature detecting section  20 . The temperature detecting section  20  comprises a resistor R 21  with one end thereof connected to the thermistor  5  and the other end thereof grounded. The comparison section  14  comprises a differential amplifier A 3  with one input terminal thereof applied with a voltage at a connection between the monitor photodiode  4  and the resistor R2 and the other input terminal thereof applied with an output voltage of the differential amplifier A 2 .  
         [0055]    The bias voltage control section  15  comprises, here, resistors R 5 , R 6 , differential amplifiers A 4 , A 5  and a transistor TR 2 . The resistors R 5 , R 6  are connected in series between the positive supply voltage terminal and the earth terminal. The differential amplifier A 4  is applied with a voltage at a connection between the resistors R 5 , R 6  at one input terminal thereof and is applied with the output signal of the temperature detecting section  20  at the other input terminal thereof. The transistor TR 2  has an emitter terminal connected to the anode terminal of the EA modulator  3  through an inductor L 1  and a collector terminal connected to the negative supply voltage terminal. The differential amplifier A 5  is applied with an output voltage of the differential amplifier A 4  at one input terminal thereof and is applied with an emitter voltage of the transistor TR 2  at the other input terminal thereof, to apply an output voltage a base terminal of the transistor TR 2 .  
         [0056]    The amplitude modulation control unit  16  comprises, here, resistors R 7  to R 9 , differential amplifiers A 6 , A 7  and a field effect transistor FET 1 . The resistors R 7 , R 8  are connected in series between the positive supply voltage terminal and the earth terminal. The differential amplifier A 6  is applied with a voltage at a connection between the resistors R 7 , R 8  at one input terminal thereof and is applied with the output signal of the temperature detecting section  20  at the other input terminal thereof. The field effect transistor FET 1  is connected between one end of the resistor R 9  and source terminals of field effect transistors FET 2 , FET 3  constituting a differential pair of the modulated electric signal output section  18  described later, to operate as a current source. The other end of the resistor R 9  is connected to the negative supply voltage terminal. The differential amplifier A 7  is applied with a voltage at a connection between the field effect transistor FET 1  and the resistor R 9  at one input terminal thereof and is applied with an output voltage of the differential amplifier A 6  at the other input terminal thereof, to apply an output voltage to a gate terminal of the field effect transistor FET 1 .  
         [0057]    The cross point control section  17  comprises, here, resistors R 10  to R 17 , a differential amplifier A 8 , an amplifier a 1  and capacitors C 2 , C 3 . The resistors R 10 , R 11  are connected in series between the positive supply voltage terminal and the earth terminal, and the resistors R 12 , R 13  and the resistors R 14 , R 15 , are similarly connected in series between the positive supply voltage terminal and the earth terminal. The differential amplifier A 8  is applied with a voltage at a connection between the resistors R 10 , R 11  at one input terminal thereof, and is applied with the output signal of the temperature detecting section  20  at the other input terminal thereof. The amplifier al is for amplifying, up to required levels, a data signal and an inverted data signal applied to input terminals thereof through capacitors C 4 , C 5 , to output the amplified data signals, and each end of the resistors R 16 , R 17  is connected between each input terminal of the amplifier a 1  and the capacitors C 4 , C 5 , respectively. Each of the other terminals of the resistors R 16 , R 17  is grounded through the capacitors C 2 , C 3 , respectively. A voltage at a connection between the resistors R 14 , R 15  is applied to a connection between the resistor R 16  and the capacitor C 2 , while a voltage at a connection between the resistors R 12 , R 13  and an output voltage of the differential amplifier A 8  are applied to a connection between the resistor R 17  and the capacitor C 3 .  
         [0058]    The modulated electric signal output section  18  comprises an amplifier a 2 , the field effect transistors FET 1 , FET 2  described above and resistors R 18 , R 19 . The amplifier a 2  amplifies an output signal from the amplifier a 1  up to a required level, and applies an inverted output signal to a gate terminal of the field effect transistor FET 2 , and a non-inverted output signal to a gate terminal of the field effect transistor FET 3 . The resistor R 18  has one terminal connected to a drain terminal of the field effect transistor FET 2 , and the resistor R 19  has one terminal connected to a drain terminal of the field effect transistors FET 3 , and the other terminal thereof are grounded, respectively. Here, a voltage at a connection between the drain terminal of the field effect transistor FET 3  and the resistor R 19  is applied to the anode terminal of the EA modulator  3  through the capacitor C 1 . The DC terminator  19  comprises a resistor R 20  having one end connected to a signal line between the drain terminal of the field effect transistor FET 3  and the capacitor C 1  through an inductor L 2 , and the other end grounded.  
         [0059]    In the drive circuit  10  having such a specific configuration as described above, a voltage value at one end of the resistor R 21  is changed according to a change in the resistance value of the thermistor  5 , so that the temperature change in the semiconductor laser module  1  is detected. This voltage value is transmitted as the output signal of the temperature detecting section  20  to the APC reference section  13 , the bias voltage control section  15 , the modulated amplitude control section  16  and the cross point control section  17 .  
         [0060]    In the APC reference section  13 , a difference between the output voltage of the temperature detecting section  20  and the reference voltage set in accordance with the resistors R3, R 4 , is obtained by the differential amplifier A 2 , and the resultant differential voltage is multiplied by a multiplying factor, to be output to the differential amplifier A 3 . The differential amplifier A 3  compares the detection result of the back power detecting section  12  with the output voltage of the differential amplifier A 2  as a reference. According to the result of this comparison, a base voltage of the transistor TR 1  of the LD current drive section  11  is regulated, so that the drive current of the semiconductor laser  2  is feedback controlled.  
         [0061]    In the bias voltage control section  15 , a difference between the output voltage of the temperature detecting section  20  and the reference voltage set in accordance with the resistors R 5 , R 6 , is obtained by the differential amplifier A 4 . The resultant differential voltage is multiplied by a desired multiplying factor, and thereafter supplied to the differential amplifier A 5  so that a base voltage of the transistor TR 2 l is regulated. Thus, the bias voltage of the EA modulator  3  is controlled according to the temperature change. In the modulated amplitude control section  16 , a difference between the output voltage of the temperature detecting section  20  and the reference voltage set in accordance with the resistors R 7 , R 8  is obtained by the differential amplifier A 6 . The resultant differential voltage is multiplied by a desired multiplying factor, and thereafter supplied to the differential amplifier A 7  so that a gate voltage of the field effect transistor FET 1  is regulated. Thus, the amplitude of the modulated electric signal output from the modulated electric signal output section  18  is controlled according to the temperature change. Further, in the cross point control section  17 , a difference between the output voltage of the temperature detecting section  20  and the reference voltage set in accordance with the resistors R 10 , R 11 , is obtained by the differential amplifier A 8 . The resultant differential voltage is multiplied by a desired multiplying factor, and thereafter supplied to the connection between the resistor R 17  and the capacitor C 3 , so that the cross point of the modulated electric signal is controlled according to the temperature change.  
         [0062]    As described above, according to the specific example of the drive circuit  10  shown in FIG. 3, the drive current of the semiconductor laser  2  and the drive voltage of the EA modulator  3  can be controlled linearly with respect to the output signal of the temperature detecting section  20 .  
         [0063]    [0063]FIG. 4 is a circuit diagram showing another specific example of the drive circuit  10  of FIG. 1. The components same as those of the configuration example shown in FIG. 3 are denoted by the same reference numerals, and the description thereof is omitted.  
         [0064]    The configuration example shown in FIG. 4 is characterized to use a control LSI  21  provided with an analog-to-digital converter (ADC)  21 A, a ROM  21 B and digital-to-analog converters (DAC)  21 C 1  to  21 C 4 , to control the drive current of the semiconductor laser  2  and the drive voltage of the EA modulator  3 .  
         [0065]    In this control LSI  21 , first, an analog voltage value corresponding to the resistance value of the thermistor  5  output from the temperature detecting section  20  is converted into a digital value by the ADC  21 A. Then, in accordance with this digital value, addressing of a table stored in the ROM  21 B is performed, and a set value of each address is read out to be output to the corresponding DACs  21 C 1  to  21 C 4 , respectively. In each DAC, an analog voltage signal corresponding to the set value from the ROM  21 B is generated. Specifically, an output signal of the DAC  21 C, corresponding to the control of the drive current of the semiconductor laser  2  is supplied as the reference voltage in the differential amplifier A 3  of the comparison section  14 . Also, an output signal of the DAC  21 C 2  corresponding to the control of the bias voltage of the EA modulator  3  is supplied as the reference voltage in the differential amplifier AS of the bias voltage control section  15 , and an output signal of the DAC  21 C 3  corresponding to the control of the amplitude of the modulated electric signal is supplied as the reference voltage in the differential amplifier A 7  of the modulated amplitude control section  16 . Further, an output signal of the DAC  21 C 4  corresponding to the control of the cross point of the modulated electric signal is supplied to the connection between the capacitor C 3  and the resistor R 17  of the cross point control section  17 .  
         [0066]    In the control LSI  21 , if the DACs  21 C 1  to  21 C 4  outputs a voltage source output as shown in (A) of FIG. 5, for example, a control voltage output from each of the DACs  21 C 1  to  21 C 4  can be supplied as it is to each corresponding control unit. On the other hand, if the DACs  21 C 1  to  21 C 4  outputs a current source output as shown in (B) of FIG. 5, the control voltage to be supplied to each control unit may be obtained by connecting a resistor for voltage conversion to an output side of each DAC.  
         [0067]    [0067]FIG. 6 shows a specific example of the table stored in the ROM  21 B.  
         [0068]    As illustrated in FIG. 6, in the ROM table, there is recorded an appropriate set value (a value expressed in hexadecimal notation in this case) for each of the DACs  21 C 1  to  21 C 4  at each address corresponding to each digital value converted from the output voltage of the temperature detecting section  20  by the ADC  21 A. For this set value in the ROM table, an optimum value is determined in advance for each control by measuring a characteristic change in the semiconductor laser module  1  at each temperature. Also, in the case where temperature characteristic data of the semiconductor laser module  1  can be obtained for the setting of required temperatures (for example, 0° C., 25° C. and 75° C.), set values for other than the above-mentioned temperatures can be obtained by a complement using an approximate expression based on such data.  
         [0069]    For example, if the temperature characteristic data for three temperatures is obtained, assuming that T: temperature, and a, b, c, y: constants, it is possible to obtain a set value at a desired temperature T by a complement using a linear approximate expression =a·T+b as shown in FIG. 7, a complement using an exponential approximate expression =a·exp(b·T)+c as shown in FIG. 8, or a complement using a polynomial approximate expression =a·x y +b·x y−1 +. . . +c as shown in FIG. 9. Also, as shown in FIG. 10, the characteristics of adjacent data may be individually complemented using the linear approximate expression.  
         [0070]    Although the specific example of the complement has been shown for the case where the measurement points of the temperature characteristic data is three, the present invention is not limited thereto, but it becomes possible to obtain the set values with higher accuracy by using the data at many more measurement points. Also, it is possible to combine different complement methods by using, for example, the linear approximate expression for a given temperature range and the exponential approximate expression for another temperature range.  
         [0071]    As described above, in the specific example of the drive circuit  10  shown in FIG. 4, the drive current of the semiconductor laser  2  and the drive voltage of the EA modulator  3  are controlled in accordance with the set values in the ROM table. Thus, it becomes possible to perform a nonlinear control of the output signal of the temperature detecting section  20  as well as the linear control.  
         [0072]    In the specific example using the control LSI  21  described above, it is possible to adopt an application, for example, of a computation process by a CPU on a portion being controlled in analog by digitizing an input thereto.  
         [0073]    Next, a second embodiment of the drive circuit for the semiconductor laser module according to the present invention will be described.  
         [0074]    [0074]FIG. 11 is a functional block diagram showing a configuration of the drive circuit for the semiconductor laser module in the second embodiment. The components same as those of the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.  
         [0075]    In FIG. 11, the configuration of the semiconductor laser module driven by a drive circuit  10 ′ is basically the same as that of the first embodiment. In this second embodiment, instead of utilizing the monitor photodiode  4  in the semiconductor laser module  1  for controlling the optical output of the semiconductor laser  2  to be constant, detecting the photocurrent generated by the absorption of the carrier light in the EA modulator  3 , and based on a change in the photocurrent, the drive current of the semiconductor laser  2  is controlled and at the same time the modulated amplitude of the applied voltage to the EA modulator is controlled. Therefore, in this embodiment, it is possible to use a semiconductor laser module that is not provided with the monitor photodiode  4 .  
         [0076]    Specifically, the drive circuit  10 ′ comprises a bias current detecting section  22  detecting a DC current that is a combination of the photocurrent in the EA modulator  3  and the current flowing through the terminal resistor  6 . The bias current detecting section  22  converts a detection value of this DC current into a voltage value for a load value of the terminal resistor  6 , to output it to a DC photocurrent detecting section  23 . The DC photocurrent detection section  23  compares an output voltage of the bias voltage control section  15  similar to that of the first embodiment with an output voltage of the bias current detecting section  22 , and based on a comparison result, detects a DC component of the photocurrent flowing through the EA modulator  3 , to output a voltage signal corresponding to a detected value to the comparison section  14 .  
         [0077]    The comparison section  14  that is supplied with an output voltage of a DC photocurrent setting section  24  generating a voltage corresponding to a set value of the DC component of the photocurrent at the time of operation of the semiconductor laser module  1 , obtains a difference between an output voltages of the DC photocurrent detecting section  23  and the DC photocurrent setting section  24 , to supply, to the LD current drive section  11 , an output voltage multiplied by the difference.  
         [0078]    Also, the drive circuit  10 ′ comprises a high level detecting section  25  and a low level detecting section  26 , detecting a high-level voltage and a low-level voltage, respectively, of the modulated electric signal to be applied to the EA modulator through the capacitor C 1  from the modulated electric signal output section  18 . The detection results at the high level detecting section  25  and the low level detecting section  26  are sent to a peak detecting section  27 . The peak detecting section  27  obtains a difference between output voltages of the high level detecting section  25  and the low level detecting section  26 , and detects the amplitude of the modulated electric signal applied to the EA modulator  3 , to output a signal indicating the detection result to the AC photocurrent detecting section  28 .  
         [0079]    The AC photocurrent detecting section  28  that is supplied with an output voltage of the modulated amplitude monitor section  29  detecting the modulated amplitude set by the modulated amplitude control section  16  similar to that of the first embodiment, obtains a difference between an output voltage of the peak detecting section  27  and the output voltage of the modulated amplitude monitor section  29 , and detects an AC component of the photocurrent flowing through the EA modulator  3 , to output a voltage corresponding to that time to the comparison section  30 .  
         [0080]    The comparison section  30  that is supplied with an output voltage of the AC photocurrent setting section  31  generating a voltage corresponding to a set value of the AC component of the photocurrent at the time of operation of the semiconductor laser module  1 , obtains a difference between output voltages of the AC photocurrent detecting section  28  and the AC photocurrent setting section  31 , to supply an output voltage multiplied by the difference to the modulated amplitude control section  16 .  
         [0081]    The components other than the above described components of the drive circuit  10 ′ in the second embodiment are similar to the corresponding components of the drive circuit  10  in the first embodiment.  
         [0082]    A specific example of a configuration of the drive circuit  10 ′ shown in FIG. 11 will be described. The present invention, however, is not limited to the example of the drive circuit described below.  
         [0083]    [0083]FIG. 12 is a circuit diagram showing the specific example of the drive circuit  10 ′ described with reference to FIG. 11. The components the same as those in the configuration example of FIG. 3 are denoted by the same reference numerals, respectively, and the description thereof is omitted.  
         [0084]    In the configuration example shown in FIG. 12, the bias current detecting section  22  comprises a resistor R 22  inserted between the collector terminal of the transistor TR 2  and the negative supply voltage (−V) terminal of the bias voltage control section  15 , and a differential amplifier A 9  applied with voltages across the resistor R 22  at input terminals thereof. The DC photocurrent detecting section  23  comprises a differential amplifier A 10  applied with the emitter voltage of the transistor TR 2  of the bias voltage control section  15  at one input terminal thereof and an output voltage of the differential amplifier A 9  of the bias current detecting section  22  at the other input terminal thereof. The DC photocurrent setting section  24  comprises resistors R 23 , R 24  connected in series between the positive supply voltage (+V) terminal and the earth terminal. A voltage at a connection between the resistors R 23 , R 24  is applied to the one input terminal of the differential amplifier A 3  making up the comparison section  14 . The other input terminal of the differential amplifier A 3  of the comparison section  14  is applied with an output voltage of the differential amplifier A 10  of the DC photocurrent detecting section  23 .  
         [0085]    The high level detecting section  25  comprises a diode D 1 , a resistor R 25  and a capacitor C 6 . The diode D 1  has a cathode terminal connected to the signal line connected to the anode terminal of the EA modulator  3 , and an anode terminal connected to one input terminal of a differential amplifier A 11  making up the peak detecting section  27 . The resistor R 25  and the capacitor C 6  are connected in parallel between the anode terminal of the diode D 1  and the earth terminal. The low level detecting section  26  has a similar configuration to the high level detecting section  25 , and comprises a diode D 2 , a resistor R 26  and a capacitor C 7 . An anode terminal of the diode D 2  is connected to the other input terminal of the differential amplifier A 11  of the peak detecting section  27 .  
         [0086]    The AC photocurrent detecting section  28  comprises a differential amplifier A 12  applied with an output voltage of the differential amplifier A 11  of the peak detecting section  27  at one input terminal thereof. The modulated amplitude monitor section  29  comprises a resistor R 27  inserted between the source terminal of the FET 1  of the modulated amplitude control section  16  and the negative supply voltage terminal, and a differential amplifier A 1   3  applied with voltages across the resistor R 27  at input terminals thereof. An output voltage of the differential amplifier A 1   3  is applied to the other input terminal of the differential amplifier A 12  of the AC photocurrent detecting section  28 . The comparison section  30  comprises a differential amplifier A 14  applied with an output voltage of the differential amplifier A 12  of the AC photocurrent detecting section  28  at one input terminal thereof. The AC photocurrent setting section  31  comprises resistors R 28 , R 29  connected in series between the positive supply voltage terminal and the earth terminal. A voltage at a connection between the resistors R 28 , R 29  is applied to the other input terminal of the differential amplifier A 14  of the comparison section  30 . An output voltage of the differential amplifier A 14  of the comparator  30  is applied to the differential amplifier A 7  of the modulated amplitude control section  16 .  
         [0087]    Next, an operation of the second embodiment described above will be explained.  
         [0088]    First, the description will be given of a control operation performed to maintain the power of the forward emission light of the semiconductor laser  2  to be constant according to the DC component of the photocurrent generated in the EA modulator  3 .  
         [0089]    In the optical output constant control of the semiconductor laser  2 , the output voltage of the temperature detecting section  20  corresponding to the resistance value of the thermistor  5  is multiplied by the bias voltage control section  15 , and the bias voltage to be applied to the EA modulator  3  is controlled with respect to the value of the multiplied voltage. Here, the bias voltage applied to the EA modulator  3  is expressed by Vbias.  
         [0090]    A value Ibais of the current flowing into the bias voltage control section  15  at that time is a sum of the DC component Iphoto_dc of the photocurrent generated as a result that the carrier light is absorbed by the EA modulator  3  and a current Ir flowing through the terminal resistor  6 . In the bias current detecting section  22 , the current value Ibais is voltage converted by a monitor resistor or the like, to be detected, and the detection result is output to the DC photocurrent detecting section  23 . An output voltage V 22  of the bias current detecting section  22  is expressed by the following equation (1) with a voltage conversion gain as G 22 .  
           V   22 = G   22 ×( Ir+Iphoto   —   dc )   (1)  
         [0091]    In the DC photocurrent detecting section  23 , a difference between the output voltage Vbias from the bias voltage control section  15  and the output voltage V 22  from the bias current detecting section  22  is obtained, and the difference thus obtained is amplified to a required level. An output voltage V 23  of the DC photocurrent detecting section  23  is expressed by the following equation (2) with a differential amplification gain as G 23  and a resistance value of the terminal resistor  6  as R 6 .  
                     V                 23     =            G                 23   ×     (       V                 22     -     V                 bias       )                   =            G                 23   ×     {       G                 22   ×     (       l                 r     +   lphoto_dc     )       -     R                 6   ×   l                 r       }                     (   2   )                               
 
         [0092]    Assume that the resistance value R6 of the terminal resistor  6  is set to be equal to the voltage conversion gain G 22  of the bias current detecting section  22  (R6=G22). Then, the output voltage V 23  described above is given by the following equation (3).  
           V   23 = G   23 × G   22 × Iphoto   —   dc    (3)  
         [0093]    Accordingly, the output voltage V 23  of the DC photocurrent detecting section  23  is a value of multiplying the DC component of the photocurrent. Since the DC component of the photocurrent corresponds to the power of the carrier light input to the EA modulator  3  from the semiconductor laser  2 , it becomes possible to control the optical output power from the semiconductor laser  2  to be constant by feedback controlling the drive current of the semiconductor laser  2  in accordance with the output voltage V 23  of the DC photocurrent detecting section  23 . Here, in the comparison section  14 , a difference of the output voltage V 23  of the DC photocurrent detecting section  23  from a target value indicated by the output voltage of the DC photocurrent setting section  23  is obtained, and a control signal amplified with the difference is output to the LD current drive section  11  from the comparison section  14 . Thus, the drive current of the semiconductor laser  2  is feedback controlled so that the DC component of the photocurrent coincides with the target value.  
         [0094]    Next, the description will be given of a control operation for optimizing the amplitude of the modulated electric signal applied to the EA modulator  3  according to the AC component of the photocurrent generated in the EA modulator  3 .  
         [0095]    In the case where the amplitude of the modulated electric signal applied to the EA modulator  3  is set by a current source as shown in the configuration example of FIG. 12 above, voltages applied according to the photocurrent are different between the case where only the resistor  6  is connected to the modulation electrical output section  18  as a load and the case where the EA modulator  3  and the resistor  6  are connected in parallel to each other as loads. That is, since the photocurrent depends on the absorbance of the EA modulator  3 , an amount of the voltage shift due to the photocurrent is varied with the on/off state of the optical output. Therefore, the amplitude voltage of the modulated electric signal output from the modulated electric signal output section  18  deviates from the value set by the modulated amplitude control section  16 .  
         [0096]    Specifically, as shown by a simple model of FIG. 13, for example, an example is considered where it is assumed that the set value of a constant current source for determining the modulated amplitude is 80 mA, the photocurrent flowing when the optical output is off is 14 mA and the photocurrent flowing when the optical output is on is 6 mA. In this assumption, in the case where the load in which the EA modulator  3  and the resistor  6  are connected in parallel to each other is connected to the modulated electric signal output section  18 , a voltage of −1.65V is applied to the EA modulator  3  at a low level output (optical output is off) as shown in (A) of FIG. 13, and a voltage of 0.15V is applied to the EA modulator  3  when the output is at a high level output (optical output is on) as shown in (B) of FIG. 13. Therefore, the amplitude voltage of the modulated electric signal is 1.80V. On the other hand, a case is considered where only the resistor  6  is connected to the modulated electric signal output section  18 , a voltage of −2.0V is applied to the resistor  6  when the output is at a low level and the voltage of 0.0V is applied to the resistor  6  when the output is at a high level. Thus, the amplitude voltage of the modulated electric signal is 2.0V.  
         [0097]    It will be understood that a deviation amount of the amplitude voltage of the modulated electric signal is dependent on the amplitude ΔIphoto of the photocurrent as shown in FIG. 14, for example. That is, the amplitude voltage Vpp of the modulated electric signal of the case where only the resistor  6  is the load as shown in (A) of FIG. 14, is a difference between a high level voltage Von and a low level voltage Voff (Vpp=Von−Voff). On the contrary, in the case where both the EA modulator  3  and the resistor  6  are loads as shown in (B) of FIG. 14B, an amplitude voltage Vpp of the modulated electric signal can be expressed by the following equation (4) with a value of the photocurrent when the optical output is on: Iphoto_on, and a value of the photocurrent when the optical output is off: Iphoto_off.  
                   Vpp   =       (     Von   +                lphoto_on   ×   R                 6       )     -     (     Voff   +     lphoto_off   ×   R                 6       )                   =     Von   -   Voff   +       (     lphoto_on   -   lphoto_off     )     ×   R                 6                   =     Von   -   Voff   +     Δ                 lphoto   ×   R                 6                     (   4   )                               
 
         [0098]    By detecting the amplitude ΔIphoto of the photocurrent, therefore, it becomes possible to judge the amplitude voltage of the modulated electric signal actually applied to the EA modulator  3 .  
         [0099]    In the configuration example shown in FIG. 12, the high and low level voltage values of the modulated electric signal being applied to the EA modulator  3  are detected by the high level detecting section  25  and the low level detecting section  26 , respectively, and from each detection result, an amplitude voltage Vpp 1  is detected by the peak detecting section  27 . The amplitude voltage Vpp 1  detected by the peak detecting section  27  corresponds to the equation (4) described above. On the other hand, the modulated amplitude monitor section  29  detects an amplitude voltage Vpp 2  corresponding to the case where only the terminal resistance  6  is provided, without the EA modulator  3 . This amplitude voltage is Vpp 2 =Von−Voff.  
         [0100]    In the AC photocurrent detecting section  28 , a difference between the amplitude voltage Vpp 1  detected by the peak detecting section  27  and the amplitude voltage Vpp 2  detected by the modulated amplitude monitor section  29  is obtained, to thereby detect the amplitude ΔIphoto of the photocurrent. An output voltage V 28  of the AC photocurrent detecting section  28  is expressed by the following equation (5) with the differential amplification gain: G 28 .  
                     V                 28     =            G                 28   ×     (       Vpp                 1     -     Vpp                 2       )                   =            G                 28   ×   R                 6   ×   Δ                 l                 photo                   (   5   )                               
 
         [0101]    The output voltage V 28  of the AC photocurrent detecting section  28  is compared with the set value of the AC photocurrent by the comparator  30 , and the comparison result is transmitted to the modulated amplitude control section  16 . Thus, the amplitude of the modulated electric signal is feedback controlled so that the amplitude of the photocurrent becomes the target value.  
         [0102]    By the aforementioned series of control operation, it becomes possible to optimize the amplitude of the modulated electric signal applied to the EA modulator  3  according to the AC component of the photocurrent, to control an extinction ratio of the optical signal output from the EA modulator  3  to be constant.  
         [0103]    As described above, with the drive circuit  10 ′ in the second embodiment, by controlling the drive current of the semiconductor laser  2  according to the DC component of the photocurrent generated in the EA modulator  3 , and at the same time, controlling the amplitude of the modulated electric signal of the EA modulator  3  according to the AC component of the photocurrent, it becomes possible to control the average output power and the extinction ratio of the optical signal output from the semiconductor laser module  1  to be constant without the need of controlling the module temperature with a Peltier device or the like. Also, the bias voltage applied to the EA modulator  3  and the cross point of the modulated electric signal are controlled according to the temperature detected by the temperature detecting section  20 , in the same manner as in the first embodiment. Therefore, the waveform of the optical signal output from the semiconductor laser module  1  can also be controlled to be constant independently of the ambient temperature.  
         [0104]    In the second embodiment described above, as shown in the configuration example of FIG. 12, the specific example has been shown where the bias voltage control section  15  or the like is linearly controlled according to the output voltage of the temperature detecting section  20 . However, similarly to the configuration example shown in FIG. 4, the output voltage of the temperature detecting section  20  may be processed using the control LSI. A specific example of such a configuration is shown in FIG. 15. In the configuration example of FIG. 15, the output signal of the DAC  21 C 1  of the control LSI  21  is supplied to the differential amplifier A 5  of the bias voltage control section  15 , and the output signal of the DAC  21 C 2  is supplied to the cross point control section  17 .  
         [0105]    Also, in the second embodiment, the drive current of the semiconductor laser  2  and the modulated amplitude of the EA modulator  3  have been controlled according to the photocurrent generated in the EA modulator  3 . However, one of the drive current of the semiconductor laser  2  and the modulated amplitude of the EA modulator  3  may be controlled according to the photocurrent, and the other of them may be controlled, similarly to the first embodiment, according to the output signal of the temperature detecting section.