Patent Publication Number: US-2015071318-A1

Title: Driving circuit of a laser diode and driving method of a laser diode

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/874,369, filed on Sep. 6, 2013 and entitled “Dual Closed Loop,” the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a driving circuit of a laser diode and a driving method of a laser diode, and particularly to a driving circuit of a laser diode and a driving method of a laser diode that have dual feedback loop for adjusting current to make the laser diode maintain a fixed extinction ratio under different operation temperatures. 
     2. Description of the Prior Art 
     Please refer to  FIG. 1 .  FIG. 1  is a diagram illustrating relationships between output powers, input currents, and operation temperatures of a laser diode. As shown in  FIG. 1 , if the operation temperature of the laser diode is 25° C., when the input current is a bias current IBIAS 1 , the bias current IBIAS 1  can make the laser diode output an output power P 0 , and when the input current is a sum of the bias current IBIAS 1  and a modulation current IMOD 1 , the sum of the bias current IBIAS 1  and the modulation current IMOD 1  can make the laser diode output an output power P 1 , wherein the output power P 1  corresponds to a logic value ┌1┐ of a light signal and the output power P 0  corresponds to a logic value ┌0┐ of the light signal, and an average value of the output power P 0  and the output power P 1  is an average power PAVE. If the operation temperature of the laser diode is 85° C., when the input current is a bias current IBIAS 2 , the bias current IBIAS 2  can make the laser diode output the output power P 0 , and when the input current is a sum of the bias current IBIAS 2  and a modulation current IMOD 2 , the sum of the bias current IBIAS 2  and the modulation current IMOD 2  can make the laser diode output the output power P 1 . 
     As shown in  FIG. 1 , because a slope of a characteristic curve of the laser diode under the operation temperature (25° C.) is greater than the slope of the characteristic curve of the laser diode under the operation temperature (85° C.), the bias current IBIAS 2  needs to be greater than the bias current IBIAS 1  and the modulation current IMOD 2  also needs to be greater than the modulation current IMOD 1  to maintain an extinction ratio (P 1 /P 0 ) of the laser diode. For solving the above mentioned problem, the prior art adjusts the bias currents and the modulation currents of the laser diode under different operation temperatures according to a lookup table, wherein the lookup table records relationships between the operation temperatures, the bias currents and the modulation currents. Thus, the prior art may need a large number of memories to store the lookup table, resulting in cost being increased. In addition, another prior art provides a single loop automatic power control to fix an average output power of the laser diode. Although the single loop automatic power control can fix the average output power of the laser diode, the single loop automatic power control cannot make the extinction ratio of the laser diode unchangeable. Therefore, the above mentioned prior arts are not good choices for the laser diode when the laser diode operates under different temperature. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a driving method of a laser diode. The driving method includes setting a bias current, a modulation current, a first target corresponding to a predetermined average power, and a second target corresponding to a predetermined average modulation power; executing a first adjusting current step group, wherein the first adjusting current step group includes driving a laser diode according to the bias current and the modulation current; generating a first monitor value corresponding to an average power of the laser diode according to light emitted by the laser diode; comparing the first monitor value with the first target; and adjusting the bias current or maintaining the bias current according to a first comparison result; generating a temporary modulation current according to the modulation current; executing a second adjusting current step group, wherein the second adjusting current step group includes driving the laser diode according to the bias current and the temporary modulation current; generating a second monitor value corresponding to an average modulation power of the laser diode according to the light emitted by the laser diode; comparing the second monitor value with the second target; and adjusting the modulation current or maintaining the modulation current according to a second comparison result; and executing the first adjusting current step group again. 
     Another embodiment of the present invention provides a driving method of a laser diode. The driving method includes setting a bias current, a modulation current, a first target corresponding to a predetermined average power, and a second target corresponding to a predetermined average modulation power; repeatedly executing a first adjusting current step group a first predetermined times, wherein the first adjusting current step group includes driving a laser diode according to the bias current and the modulation current; generating a first monitor value corresponding to an average power of the laser diode according to light emitted by the laser diode; comparing the first monitor value with the first target; and adjusting the bias current or maintaining the bias current according to a first comparison result; generating a temporary modulation current according to the modulation current; repeatedly executing a second adjusting current step group a second predetermined times, wherein the second adjusting current step group includes driving the laser diode according to the bias current and the temporary modulation current; generating a second monitor value corresponding to an average modulation power of the laser diode according to the light emitted by the laser diode; comparing the second monitor value with the second target; and adjusting the modulation current or maintaining the modulation current according to a second comparison result; and executing the first adjusting current step group again the first predetermined times. 
     Another embodiment of the present invention provides a driving circuit of a laser diode. The driving circuit includes a driving unit, a power generation unit, a comparison unit, a first current generation module, and a second current generation module. The driving unit is used for driving a laser diode according to a bias current, a modulation current, and a first driving signal, or according to the bias current, a temporary modulation current, and a second driving signal, or according to the bias current and the modulation current, or according to the bias current and the temporary modulation current. The monitor unit is used for generating a first monitor value corresponding to an average power of the laser diode and a second monitor value corresponding to an average modulation power of the laser diode according to light emitted by the laser diode. The comparison unit is used for comparing the first monitor value with a first target corresponding to a predetermined average power to generate a first comparison result, and comparing the second monitor value with a second target corresponding to a predetermined average modulation power to generate a second comparison result. The first current generation module is used for executing a first corresponding operation on the bias current according to the first comparison result. The second current generation module is used for generating the temporary modulation current according to the modulation current, and executing a second corresponding operation on the modulation current according to the second comparison result. 
     The present invention provides a driving circuit of a laser diode and a driving method of a laser diode. The driving circuit and the driving method utilize a first current generation module of the driving circuit and an first target to adjust a bias current driving the laser diode, and utilize a second current generation module of the driving circuit and a second target to adjust a modulation current driving the laser diode. Therefore, compared to the prior art, the present invention has advantages as follows: first, because the present invention has a feedback loop corresponding to the first current generation module adjusting the bias current and a feedback loop corresponding to the second current generation module adjusting the modulation current, the present invention does not need an additional memory; and second, because the present invention has the feedback loop corresponding to the first current generation module adjusting the bias current and the feedback loop corresponding to the second current generation module adjusting the modulation current, the present invention can make the laser diode maintain a fixed extinction ratio under different operation temperatures. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating relationships between output powers, input currents, and operation temperatures of a laser diode. 
         FIG. 2  is a diagram illustrating a driving circuit of a laser diode according to a first embodiment. 
         FIG. 3A  and  FIG. 3B  are flowcharts illustrating a driving method of a laser diode according to a second embodiment. 
         FIG. 4  is a diagram illustrating the bias current, the modulation current, the temporary modulation current, a first monitor value, a second monitor value, and corresponding output powers. 
         FIG. 5A  and  FIG. 5B  are flowcharts illustrating a driving method of a laser diode according to a third embodiment. 
         FIG. 6  is a diagram illustrating the bias current, the modulation current, the temporary modulation current, a first monitor value, a second monitor value, and the corresponding output powers. 
         FIG. 7  is a diagram illustrating a driving circuit of a laser diode according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 2 .  FIG. 2  is a diagram illustrating a driving circuit  200  of a laser diode according to a first embodiment. As shown in  FIG. 2 , the driving circuit  200  includes a driving unit  202 , a monitor unit  204 , a comparison unit  206 , a first current generation module  208 , and a second current generation module  210 . As shown in  FIG. 2 , the comparison unit  206  is coupled to the monitor unit  204 , wherein the comparison unit  206  includes a first comparator  2062  and a second comparator  2064 . The first current generation module  208  is coupled between the first comparator  2062  and the driving unit  202 , wherein the first current generation module  208  includes a first flip-flop  2082 , a first digital filter  2084 , a first counter  2086 , and a first digital-to-analog converter  2088 . The second current generation module  210  is coupled between the second comparator  2064  and the driving unit  202 , wherein the second current generation module  210  includes a second flip-flop  2102 , a second digital filter  2104 , a second counter  2106 , a second digital-to-analog converter  2108 , and a temporary modulation current generator  2110 , and the temporary modulation current generator  2110  includes a multiplier  21102 , an adder  21104 , and a switch  21106 . 
     Please refer to  FIGS. 2 ,  3 A,  3 B,  4 .  FIG. 3A  and  FIG. 3B  are flowcharts illustrating a driving method of a laser diode according to a second embodiment, and  FIG. 4  is a diagram illustrating the bias current, the modulation current, the temporary modulation current, a first monitor value, a second monitor value, and output powers. The method in  FIG. 3A  and  FIG. 3B  is illustrated using the driving circuit  200  in  FIG. 2 . Detailed steps are as follows: 
     Step  300 : Start. 
     Step  302 : A user sets a bias current IB, a modulation current IM, a first target PAVT corresponding to a predetermined average power, and a second target PMT corresponding to a predetermined average modulation power. 
     Step  304 : The driving unit  202  drives a laser diode  214  according to the bias current IB and the modulation current IM. 
     Step  306 : The monitor unit  204  generates a first monitor value PAV corresponding to an average power of the laser diode  214  when the laser diode  214  is driven by the bias current IB and the modulation current IM according to light emitted by the laser diode  214 . 
     Step  308 : The first comparator  2062  of the comparison unit  206  compares the first monitor value PAV with the first target PAVT to generate a first comparison result. 
     Step  310 : The first current generation module  208  executes a first corresponding operation on the bias current IB according to the first comparison result. 
     Step  312 : The temporary modulation current generator  2110  of the second current generation module  210  generates a temporary modulation current IMT according to the modulation current IM. 
     Step  314 : The driving unit  202  drives the laser diode  214  according to the bias current IB and the temporary modulation current IMT. 
     Step  316 : The monitor unit  204  generates a second monitor value PMV corresponding to an average modulation power of the laser diode  214  when the laser diode  214  is driven by the bias current IB and the temporary modulation current IMT according to the light emitted by the laser diode  214 . 
     Step  318 : The second comparator  2064  of the comparison unit  206  compares the second monitor value PMV with the second target PMT to generate a second comparison result. 
     Step  320 : The second current generation module  210  executes a second corresponding operation on the modulation current IM according to the second comparison result, go to Step  304 . 
     As shown in  FIGS. 2 ,  4 , in Step  302 , the user can first set an initial value (e.g. 3 mA) of the bias current IB, an initial value (e.g. 5 mA) of the modulation current IM, an initial value of the first target PAVT corresponding to the predetermined average power (e.g. 9 mW), and an initial value of the second target PMT corresponding to the predetermined average modulation power (e.g. 9.6 mW). As shown in  FIGS. 2 ,  4 , in Step  304 , during a period T1, the driving unit  202  drives the laser diode  214  according to the bias current IB (e.g. 3 mA) and the modulation current IM (e.g. 5 mA). But, in another embodiment of the present invention, the driving unit  202  drives the laser diode  214  according to the bias current IB (e.g. 3 mA), the modulation current IM (e.g. 5 mA), and a first driving signal A 0 , wherein the first driving signal A 0  is a burst mode driving signal. But, the present invention is not limited to the first driving signal A 0  being a burst mode driving signal, that is, the first driving signal A 0  can also be a continuous mode driving signal. In addition, as shown in  FIG. 4 , when the driving unit  202  drives the laser diode  214  according to the bias current IB (e.g. 3 mA) and the modulation current IM (e.g. 5 mA), the laser diode  214  can emit an output power P 1  (e.g. 8 mW) and an output power P 0  (e.g. 3 mW). As shown in  FIGS. 2 ,  4 , in Step  306 , the monitor unit  204  can generate the first monitor value PAV (wherein the first monitor value PAV can be a current value or a voltage value) according to the light emitted by the laser diode  214 , wherein the first monitor value PAV corresponds to the average power (e.g. 5.5 mW) of the laser diode  214  when the laser diode  214  is driven by the bias current IB and the modulation current IM. In Step  308 , the first comparator  2062  of the comparison unit  206  compares the first monitor value PAV (corresponding to the average power (e.g. 5.5 mW)) with the first target PAVT (corresponding to power 9 mW) to generate the first comparison result (that is, the first monitor value PAV is less than the first target PAVT). In addition, as shown in  FIG. 2 , the second comparator  2064  of the comparison unit  206  can also compare the first monitor value PAV with the second target PMT in fact. But, because clocks CLKB, CLKB′ are disabled, the second comparison result outputted by the second comparator  2064  is neglected. As shown in  FIG. 4 , in Step  310 , when clocks CLKA, CLKA′ are enabled and the clocks CLKB, CLKB′ are disabled, the first comparison result generated by the first comparator  2062  can pass the first flip-flop  2082  and be filtered by the first digital filter  2084 , wherein the first flip-flop  2082  is used for storing the first comparison result generated by the first comparator  2062 . Because the first monitor value PAV is less than the first target PAVT, an output generated by the first digital filter  2084  can make the first counter  2086  count upward. Because the first counter  2086  counts upward, the first digital-to-analog converter  2088  increases the bias current IB (e.g. 3 mA) to generate a new bias current IB (e.g. 4 mA). In Step  312 , when the first digital-to-analog converter  2088  generates the bias current IB (e.g. 4 mA), the switch  21106  of the temporary modulation current generator  2110  is turned on, so the temporary modulation current generator  2110  can utilize the multiplier  21102  and the adder  21104  to generate the temporary modulation current IMT according to the modulation current IM (e.g. 5 mA), wherein the temporary modulation current IMT is a sum of the modulation current IM (e.g. 5 mA) and a product of the modulation current IM (e.g. 5 mA) and a predetermined value (e.g. 0.2), that is, the temporary modulation current IMT is 5+0.2×5=6 mA. 
     As shown in  FIGS. 2 ,  4 , in Step  314 , during a period T2, the driving unit  202  drives the laser diode  214  according to the bias current IB (e.g. 4 mA) and the temporary modulation current IMT (e.g. 6 mA). But, in another embodiment of the present invention, the driving unit  202  drives the laser diode  214  according to the bias current IB (e.g. 4 mA), the temporary modulation current IM (e.g. 6 mA), and a second driving signal B 0 , wherein the second driving signal B 0  is a burst mode driving signal. But, the present invention is not limited to the second driving signal B 0  being a burst mode driving signal, that is, the second driving signal B 0  can also be a continuous mode driving signal. In addition, as shown in  FIG. 4 , when the driving unit  202  drives the laser diode  214  according to the bias current IB (e.g. 4 mA) and the temporary modulation current IMT (e.g. 6 mA), the laser diode  214  can emit the output power P 1  (e.g. 10 mW) and the output power P 0  (e.g. 4 mW). As shown in  FIGS. 2 ,  4 , in Step  316 , the monitor unit  204  can generate the second monitor value PMV (wherein the second monitor value PMV can be a current value or a voltage value) according to the light emitted by the laser diode  214 , wherein the second monitor value PMV corresponds to the average modulation power (e.g. 7 mW) of the laser diode  214  when the laser diode  214  is driven by the bias current IB (e.g. 4 mA) and the temporary modulation current IMT (e.g. 6 mA). In Step  318 , the second comparator  2064  of the comparison unit  206  compares the second monitor value PMV (corresponding to the average modulation power (e.g. 7 mW)) with the second target PMT (corresponding to power 9.6 mW) to generate the second comparison result (that is, the second monitor value PMV is less than the second target PMT). As shown in  FIG. 4 , in Step  320 , when the clocks CLKA, CLKA′ are disabled and the clocks CLKB, CLKB′ are enabled, the second comparison result generated by the second comparator  2064  can pass the second flip-flop  2102  and be filtered by the second digital filter  2104 , wherein the second flip-flop  2102  is used for storing the second comparison result generated by the second comparator  2064 . Because the second monitor value PMV is less than the second target PMT, an output generated by the second digital filter  2104  can make the second counter  2106  count upward. Because the second counter  2106  counts upward, the second digital-to-analog converter  2108  increases the modulation current IMT (e.g. 5 mA) to generate a new modulation current IM (e.g. 6 mA). In addition, after the second current generation module  210  generates the modulation current IM (e.g. 6 mA), because operational principles of the driving circuit  200  during a period T3, a period T4, and a period T5 are the same as those of the driving circuit  200  during the period T1 and the period T2, further description thereof is omitted for simplicity. Further, in another embodiment of the present invention, the period T3, the period T4, and the period T5 corresponds to a first driving signal A 1 , a second driving signal B 1 , and a first driving signal A 2 , respectively. 
     As shown in  FIGS. 2 ,  4 , after the period T5, during a period T6, in Step  314  and Step  316 , when the driving unit  202  drives the laser diode  214  according to the bias current IB (e.g. 6 mA) and the temporary modulation current IMT (7+0.2×7=8.4 mA), the monitor unit  204  can generate the second monitor value PMV according to the light emitted by the laser diode  214 , wherein the second monitor value PMV corresponds to the average modulation power (e.g. 10.2 mW) of the laser diode  214  when the laser diode  214  is driven by the bias current IB (e.g. 6 mA) and the temporary modulation current IMT (e.g. 8.4 mA). In Step  318 , the second comparator  2064  of the comparison unit  206  compares the second monitor value PMV (corresponding to the average modulation power (e.g. 10.2 mW)) with the second target (corresponding to power 9.6 mW) to generate the second comparison result (that is, the second monitor value PMV is greater than the second target PMT). As shown in  FIG. 4 , in Step  320 , when the clocks CLKA, CLKA′ are disabled and the clocks CLKB, CLKB′ are enabled, the second comparison result generated by the second comparator  2064  can pass the second flip-flop  2102  and be filtered by the second digital filter  2104 . Because the second monitor value PMV is greater than the second target PMT, the output generated by the second digital filter  2104  can make the second counter  2106  count downward. Because the second counter  2106  counts downward, the second digital-to-analog converter  2108  decreases the modulation current IMT (e.g. 7 mA) to generate a new modulation current IM (e.g. 6 mA). 
     As shown in  FIGS. 2 ,  4 , after the period T6, during a period T7, in Step  304  and Step  306 , the driving unit  202  drives the laser diode  214  according to the bias current IB (e.g. 6 mA) and the modulation current IM (e.g. 6 mA), the monitor unit  204  can generate the first monitor value PAV according to the light emitted by the laser diode  214 , wherein the first monitor value PAV corresponds to the average power (e.g. 9 mW) of the laser diode  214  when the laser diode  214  is driven by the bias current IB (e.g. 6 mA) and the modulation current IM (e.g. 6 mA). In Step  308 , the first comparator  2062  of the comparison unit  206  compares the first monitor value PAV (corresponding to the average power (e.g. 9 mW)) with the first target PAVT (corresponding to power 9 mW) to generate the first comparison result (that is, the first monitor value PAV is equal to the first target PAVT). As shown in  FIG. 4 , in Step  310 , when the clocks CLKA, CLKA′ are enabled and the clocks CLKB, CLKB′ are disabled, the first comparison result generated by the first comparator  2062  can pass the first flip-flop  2082  and be filtered by the first digital filter  2084 . However, because the first monitor value PAV is equal to the first target PAVT, the output generated by the first digital filter  2084  can make the first counter  2086  maintain a current count. Because the first counter  2086  maintains the current count, the first digital-to-analog converter  2088  maintains to output the current bias current IB (e.g. 6 mA). 
     As shown in  FIGS. 2 ,  4 , after the period T7, during a period T8, in Step  314  and Step  316 , when the driving unit  202  drives the laser diode  214  according to the bias current IB (e.g. 6 mA) and the temporary modulation current IMT (6+0.2×6=7.2 mA), the monitor unit  204  can generate the second monitor value PMV according to the light emitted by the laser diode  214 , wherein the second monitor value PMV corresponds to the average modulation power (e.g. 9.6 mW) of the laser diode  214  when the laser diode  214  is driven by the bias current IB (e.g. 6 mA) and the temporary modulation current IMT (e.g. 7.2 mA). In Step  318 , the second comparator  2064  of the comparison unit  206  compares second monitor value PMV (corresponding to the average modulation power (e.g. 9.6 mW) with the second target PMT (corresponding to power 9.6 mW) to generate the second comparison result (that is, the second monitor value PMV is equal to the second target PMT). As shown in  FIG. 4 , in Step  320 , when the clocks CLKA, CLKA′ are disabled and the clocks CLKB, CLKB′ are enabled, because the second monitor value PMV is equal to the second target PMT and the clocks CLKB, CLKB′ are enabled, the second comparison result generated by the second comparator  2064  can pass the second flip-flop  2102  and be filtered by the second digital filter  2104 . However, because the second monitor value PMV is equal to the second target PMT, the output generated by the second digital filter  2104  can make the second counter  2106  maintains a current count. Because the second counter  2106  maintains the current count, the second digital-to-analog converter  2108  maintains to output the current modulation current IM (e.g. 6 mA). In addition, after the second digital-to-analog converter  2108  maintains to output the current modulation current IM (e.g. 6 mA), because operational principles of the driving circuit  200  during a period T9 and a period T10 are the same as those of the driving circuit  200  during the period T7 and the period T8, further description thereof is omitted for simplicity. Further, in another embodiment of the present invention, the period T9 and the period T10 corresponds to a first driving signal A 4  and a second driving signal B 4 , respectively. 
     Please refer to  FIGS. 2 ,  5 A,  5 B,  6 .  FIG. 5A  and  FIG. 5B  are flowcharts illustrating a driving method of a laser diode according to a third embodiment, and  FIG. 6  is a diagram illustrating the bias current, the modulation current, the temporary modulation current, the average power, the modulation power, and the output powers. The method in  FIG. 5A  and  FIG. 5B  is illustrated using the driving circuit  200  in  FIG. 2 . Detailed steps are as follows: 
     Step  500 : Start. 
     Step  502 : The user sets a bias current IB, a modulation current IM, a first target PAVT corresponding to a predetermined average power, and a second target PMT corresponding to a predetermined average modulation power. 
     Step  504 : The driving unit  202  drives the laser diode  214  according to the bias current IB and the modulation current IM. 
     Step  506 : The monitor unit  204  generates a first monitor value PAV corresponding to an average power of the laser diode  214  when the laser diode  214  is driven by the bias current IB and the modulation current IM according to light emitted by the laser diode  214 . 
     Step  508 : The first comparator  2062  of the comparison unit  206  compares the first monitor value PAV with the first target PAVT to generate a first comparison result. 
     Step  510 : The first current generation module  208  executes a first corresponding operation on the bias current IB according to the first comparison result, and the first counter  2086  of the first current generation module  208  accumulates a comparison number executed by the first comparator  2062 . 
     Step  512 : If the comparison number accumulated by the first counter  2086  is equal to a first predetermined value; if yes, go to Step  514 ; if no, go to Step  504 . 
     Step  514 : The temporary modulation current generator  2110  of the second current generation module  210  generates a temporary modulation current IMT according to the modulation current IM. 
     Step  516 : The driving unit  202  drives the laser diode  214  according to the bias current IB and the temporary modulation current IMT. 
     Step  518 : The monitor unit  204  generates a second monitor value PMV corresponding to an average modulation power of the laser diode  214  when the laser diode  214  is driven by the bias current IB and the temporary modulation current IMT according to the light emitted by the laser diode  214 . 
     Step  520 : The second comparator  2064  of the comparison unit  206  compares the second monitor value PMV with the second target PMT to generate a second comparison result. 
     Step  522 : The second current generation module  210  executes a second corresponding operation on the modulation current IM according to the second comparison result, and the second counter  2106  of the second current generation module  210  accumulates a comparison number executed by the second comparator  2064 . 
     Step  524 : If the comparison number accumulated by the second counter  2106  is equal to a second predetermined value; if yes, go to Step  504 ; if no, go to Step  506 . 
     A difference between the embodiment in  FIG. 5A  and  FIG. 5B  and the embodiment in  FIG. 3A  and  FIG. 3B  is that a second adjusting current step group (Step  516 - 524 ) is executed after a number of a first adjusting current step group (Step  504 - 512 ) being repeatedly executed is equal to the first predetermined value, and the first adjusting current step group (Step  504 - 512 ) is executed again after a number of the second adjusting current step group (Step  516 - 524 ) being repeatedly executed is equal to the second predetermined value, wherein the first predetermined value and the second predetermined value can be positive integers. In addition, as shown in  FIG. 6 , operational principles of the driving circuit  200  during periods T1-T8 are the same as those of the driving circuit  200  during the periods T1-T10 in  FIG. 3A  and  FIG. 3B , so further description thereof is omitted for simplicity. 
     Please refer to  FIG. 7 .  FIG. 7  is a diagram illustrating a driving circuit  700  of a laser diode according to a fourth embodiment. As shown in  FIG. 7 , differences between the driving circuit  700  and the driving circuit  200  are that the driving circuit  700  integrates the first comparator  2062  and the second comparator  2064  of the driving circuit  200  into a comparator  706 , and integrates the first digital filter  2084  and the second digital filter  2104  of the driving circuit  200  into a filter  708 ; a first current generation module  710  includes a first flip-flop  2082 , a first counter  2086 , and a first digital-to-analog converter  2088 ; and a second current generation module  712  includes a second flip-flop  2102 , a second counter  2106 , a second digital-to-analog converter  2108 , and a temporary modulation current generator  2110 . In addition, operational principles of the driving circuit  700  are the same as those of the driving circuit  200 , so further description thereof is omitted for simplicity. 
     To sum up, the driving circuit of a laser diode and the driving method of a laser diode utilize the first current generation module and the first target to adjust the bias current driving the laser diode, and utilize the second current generation module and the second target to adjust the modulation current driving the laser diode. Therefore, compared to the prior art, the present invention has advantages as follows: first, because the present invention has a feedback loop corresponding to the first current generation module adjusting the bias current and a feedback loop corresponding to the second current generation module adjusting the modulation current, the present invention does not need an additional memory; and second, because the present invention has the feedback loop corresponding to the first current generation module adjusting the bias current and the feedback loop corresponding to the second current generation module adjusting the modulation current, the present invention can make the laser diode maintain a fixed extinction ratio under different operation temperatures. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.