Abstract:
In a laser diode driving method, a bias current set about the light emission threshold current of a laser diode, and a pulse current for causing the laser diode to emit light are adjusted in accordance with the ambient temperature. The laser diode is driven by a current prepared by superposing the bias current and the pulse current, thereby controlling the optical output and extinction ratio of the laser diode at a constant level. A laser diode driving circuit is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a laser diode driving method and circuit for controlling an optical output and extinction ratio at a constant level in correspondence with deterioration of the laser diode with time. 
     An example of a laser diode driving circuit used in an optical transmission system or the like is disclosed in Japanese Patent Laid-Open No. 2-308584 in which the optical output from the laser diode is controlled at a constant level regardless of the ambient temperature. 
     FIG. 5 shows the arrangement of a conventional laser diode driving circuit for controlling the optical output from the laser diode. 
     A laser diode LD is driven by a current prepared by superposing a driving current Id and a bias current Is. The driving current Id is a pulse current based on transmission data, whereas the bias current Is is a base current for causing the LD to emit light by induced emission. A temperature sensor  110  generates a voltage corresponding to the ambient temperature and outputs the voltage as an analog temperature signal representing the ambient temperature to an A/D converter  120 . The A/D converter  120  converts the input temperature signal into a digital signal and outputs the digital signal to a memory  150 . 
     The memory  150  uses input digital signal as an address signal to read out digital data stored at the corresponding address from the memory  150  and output the data to a D/A converter  130 . The D/A converter  130  converts the input digital data into an analog signal and outputs the analog signal to a current controller  140 . The current controller  140  controls an emitter current Is common to transistors Q 1  and Q 2  in accordance with the analog signal from the D/A converter  130 . 
     The operation of the laser diode driving circuit will be described. 
     A pre-bias signal is applied to the base of the transistor Q 1 . Assume that the state in which the pre-bias signal voltage is higher than a reference voltage (−VR) is a disable state. In the disable state, the transistor Q 1  is turned on, the transistor Q 2  is turned off, and the laser diode LD is not driven. Assume that the state in which the pre-bias signal voltage is lower than the reference voltage (−VR) is an enable state. In the enable state, the transistor Q 1  is turned off, the transistor Q 2  is turned on, and the laser diode LD is driven by a current, i.e., a current which changes between Is and Is+Id, prepared by superposing the driving current Id and the bias current Is generated by the current controller  140 . 
     In the memory  150 , data about the bias current corresponding to the ambient temperature is stored. When data obtained by digitally converting a temperature signal representing the ambient temperature is input from the A/D converter  120  to the address line of the memory  150 , the memory  150  outputs data about the bias current corresponding to the ambient temperature to the data line. 
     The D/A converter  130  D/A-converts the bias current data output to the data line, and outputs the analog signal to the current controller  140 . The current controller  140  controls the emitter current of the transistors Q 1  and Q 2  in accordance with the analog signal output from the D/A converter  130 . 
     In the laser diode driving circuit, the emitter current Is is adjusted in correspondence with the ambient temperature. That is, when the ambient temperature changes, data on the address line changes, and data about a new bias current appears on the data line. The D/A converter  130  D/A-converts the data on the data line, and the current controller  140  converts the signal output from the D/A converter  130  into a current. 
     At this time, if the pre-bias signal changes to the enable state, the laser diode LD is driven by a current prepared by superposing the driving current Id on the new bias current Is adjusted in correspondence with the ambient temperature. 
     The laser diode driving circuit employs a feed forward controller. Since the circuit performs control for only optical output fluctuation conditions set in advance, it cannot control the optical output from the laser diode in correspondence with optical output fluctuation conditions other than ambient temperature fluctuations. For this reason, e.g., when the laser diode deteriorates with time to decrease the optical output, the optical output may be smaller than its lower limit defined in the optical transmission system. 
     In the laser diode driving circuit, light emission may delay, the extinction ratio may decrease, and the quality of the transmission system may degrade because no consideration is given to temperature fluctuations in differential quantum efficiency of the laser diode. That is, the optical output is controlled at a constant level by changing only the bias current without changing the driving current in correspondence with the ambient temperature. 
     FIGS. 1A and 1B show the current vs. optical output characteristics of a general laser diode. FIG. 1A shows current vs. optical output characteristics when the bias current and the driving current are ideally distributed. FIG. 1B shows current vs. optical output characteristics when the driving current is kept constant. 
     In FIGS. 1A and 1B, t 1 , t 2 , and t 3  (t 1 &lt;t 2 &lt;t 3 ) represent ambient temperatures; Isn, Idn, and Ithn (n=1, 2, 3), the bias current, the driving current, and the light emission threshold current of the laser diode; and Po, the optical output. The laser diode driving circuit controls the optical output Po at a constant level at the respective temperatures. The laser diode has such a characteristic that both the bias current Is and driving current Id required to obtain a constant optical output Po increase along with an increase in ambient temperature. As shown in FIG. 1A, it is ideal for efficiently driving the laser diode that the bias current Is is set about the light emission threshold current Ith of the laser diode, and the driving current Id is superposed on the bias current Is to keep the optical output constant. 
     In the laser diode driving circuit in FIG. 5 for controlling the optical output at a constant level by changing the bias current Is while keeping the driving current Id constant, if the bias current Is 2  and the driving current Id 2  are optimum at an ambient temperature t 2 , but the ambient temperature decreases to t 1 , the set value Is 1  of the bias current becomes smaller than the light emission threshold current Ith 1  to delay light emission. If the ambient temperature increases from t 2  to t 3 , the set value Is 3  of the bias current exceeds the light emission threshold current Ith 3  to decrease the extinction ratio, failing to obtain a reliable extinction state. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a laser diode driving method and circuit capable of controlling an optical output and extinction ratio at a constant level against an optical output fluctuation factor such as deterioration of the laser diode with time that cannot be set in advance. 
     It is another object of the present invention to provide a laser diode driving method and circuit capable of controlling the optical output and extinction ratio of the laser diode at a constant level regardless of the ambient temperature. 
     To achieve the above objects, there is provided a laser diode driving method comprising the steps of adjusting, in accordance with an ambient temperature, a bias current set about a light emission threshold current of a laser diode, and a pulse current for causing the laser diode to emit light, and driving the laser diode by a current prepared by superposing the bias current and the pulse current, thereby controlling an optical output and extinction ratio of the laser diode at a constant level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are graphs each showing the current vs. optical output characteristics of a laser diode; 
     FIG. 2 is a block diagram showing the arrangement of a laser diode driving circuit according to the first embodiment of the present invention; 
     FIG. 3 is a block diagram showing the arrangement of a laser diode driving circuit according to the second embodiment of the present invention; 
     FIG. 4 is a block diagram showing the arrangement of a laser diode driving circuit according to the third embodiment of the present invention; and 
     FIG. 5 is a block diagram showing the arrangement of a conventional laser diode driving circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
     First Embodiment 
     FIG. 2 shows the arrangement of a laser diode driving circuit according to the first embodiment of the present invention. The laser diode driving circuit of the first embodiment comprises a temperature sensor  10 , A/D converters  20  and  21 , D/A converters  30  to  32 , current controllers  40  to  42 , a memory  50 , an average detector  57 , an inverting amplifier  65 , a low-pass filter (LPF)  70 , a switch  75 , an LD module  80  constituted by a laser diode  81  and a monitor photodiode (to be referred to as a monitor PD hereinafter)  82  for detecting the optical output from the laser diode  81 , and transistors Q 1  and Q 2 . 
     In the laser diode driving circuit, the A/D converter  21 , the D/A converter  32 , the current controller  42 , the memory  50 , the average detector  57 , the inverting amplifier  65 , the LPF  70 , and the switch  75  constitute a laser diode time deterioration compensation circuit  1 . The average detector  57 , the inverting amplifier  65 , and the LPF  70  constitute a time deterioration state detection means. 
     The laser diode  81  and monitor PD  82  constituting the LD module  80  are formed on the same substrate by the same process. 
     The temperature sensor  10  generates a voltage in correspondence with the ambient temperature and outputs the voltage as a temperature signal to the A/D converter  20 . The A/D converter  20  A/D-converts the temperature signal and outputs the analog signal as an address to the memory  50 . 
     Data about the pulse current, the bias current, and a light-receiving current Ir of the monitor PD  82  before deterioration is stored in the memory  50  at each address corresponding to the ambient temperature. The values of the pulse current and bias current stored in the memory  50  are determined to keep the optical output and extinction ratio of the laser diode  81  constant against fluctuations in ambient temperature. 
     The memory  50  outputs pulse current data and bias current data stored at the address designated by the A/D converter  20  to the D/A converters  30  and  31 , respectively. The D/A converters  30  and  31  respectively D/A-convert the digital data output from the memory  50  and output the analog signals to the current controllers  40  and  41 . 
     The current controller  40  adjusts a constant current Iac common to the transistors Q 1  and Q 2  in accordance with the analog signal from the D/A converter  30 . The current controller  41  adjusts a constant current Idc in accordance with the analog signal from the D/A converter  31 . 
     The detailed arrangement of the laser diode time deterioration compensation circuit  1  will be explained. The light-receiving current Ir of the monitor PD  82  is input to the average detector  57 . The average detector  57  detects the average of the light-receiving current Ir, converts it into a voltage signal, and outputs the voltage signal to the A/D converter  21  and the inverting input terminal of the inverting amplifier  65 . 
     The A/D converter  21  A/D-converts the input voltage from the average detector  57 . The data digitally converted by the A/D converter  21  is output to the memory  50  via the switch  75 . 
     In the memory  50 , the data input from the A/D converter  21  is stored at an address corresponding to the temperature detected by the temperature sensor  10 . This data serves as a reference voltage in compensating the optical output upon deterioration of the laser diode. 
     The memory  50  outputs the reference voltage data to the D/A converter  32  via the switch  75 . The D/A converter  32  D/A-converts the data and outputs the analog signal to the non-inverting input terminal of the inverting amplifier  65 . 
     The inverting amplifier  65  uses the voltage input from the D/A converter  32  to the non-inverting input terminal as a reference voltage to invert and amplify the voltage input from the average detector  57  to the inverting input terminal and output the voltage to the LPF  70 . 
     The LPF  70  smoothes the inverted/amplified voltage and outputs the resultant voltage to the current controller  42 . The current controller  42  adjusts a constant current Iapc in accordance with the output signal from the LPF  70 . 
     The operation of the laser diode driving circuit according to the first embodiment will be explained with reference to a numerical example. 
     The temperature sensor  10  detects an ambient temperature of −40 to +115° C., converts the detected temperature into a voltage of 0 to 2 V, and outputs the voltage. The A/D converter  20  converts the voltage output from the temperature sensor  10  into 7-bit digital data and outputs the data as an address to the memory  50 . 
     The memory  50  outputs to the D/A converter  30  7-bit pulse current data stored at an address output from the A/D converter  20 , and outputs to the D/A converter  31  5-bit bias current data stored at the same address. 
     The D/A converter  30  D/A-converts the input digital data and outputs the analog signal to the current controller  40 . Similarly, the D/A converter  31  D/A-converts the input digital data and outputs the analog signal to the current controller  41 . 
     The current controller  40  adjusts the constant current Iac common to the transistors Q 1  and Q 2  between 0 mA and 70 mA in accordance with the analog signal from the D/A converter  30 . 
     An input signal to the base of the transistor Q 2  is a signal based on transmission data, and an inverted input signal to the base of the transistor Q 1  is a signal obtained by inverting the input signal. 
     When the input signal is at high level, the transistor Q 2  is turned on, the transistor Q 1  is turned off, and the constant current Iac flows through the laser diode  81 . When the input signal is at low level, the transistor Q 1  is turned on, the transistor Q 2  is turned off, and no constant current Iac flows through the laser diode  81 . That is, the constant current Iac drives the laser diode  81  as a pulse current based on transmission data. 
     The current controller  41  adjusts the constant current Idc between 0 mA and 50 mA in accordance with the analog signal from the D/A converter  31 . The constant current Idc directly flows through the laser diode  81  and drives it as a bias current. 
     The operation of the laser diode time deterioration compensation circuit  1  will be explained. The light-receiving current Ir of the monitor PD  82  is input to the average detector  57 . The average detector  57  detects the average of the light-receiving current Ir, converts it into a voltage of 0 to 550 mV, and outputs the voltage to the A/D converter  21  and the inverting input terminal of the inverting amplifier  65 . 
     The A/D converter  21  converts the voltage output from the average detector  57  into 5-bit digital data. The data digitally converted by the A/D converter  21  is output to the memory  50  via the switch  75 . 
     In the memory  50 , the data input from the A/D converter  21  is stored at an address corresponding to the temperature detected by the temperature sensor  10 . This data is used as an initial output value of the monitor PD, i.e., reference voltage data before deterioration of the laser diode  81 . 
     The memory  50  outputs the reference voltage data to the D/A converter  32  via the switch  75 . The D/A converter  32  converts the reference voltage data into a voltage of 0 to 550 mV and outputs the voltage to the non-inverting input terminal of the inverting amplifier  65 . 
     The switch  75  connects either the A/D converter  21  or the D/A converter  32  to the memory  50 . The switch  75  switches to the A/D converter  21  side ({circle around (2)}) in writing the reference voltage data in the memory  50 , and to the D/A converter  32  side ({circle around (0)}) in reading out the reference voltage data from the memory  50 . 
     The inverting amplifier  65  uses the voltage input from the D/A converter  32  to the non-inverting input terminal as a reference voltage to invert and amplify the voltage input from the average detector  57  to the inverting input terminal and output the voltage to the LPF  70 . 
     The LPF  70  smoothes the inverted/amplified voltage and outputs the resultant voltage to the current controller  42 . The current controller  42  adjusts the constant current Iapc between 0 mA and 30 mA in accordance with the output signal from the LPF  70 . The constant current Iapc directly flows through the laser diode  81  and drives it as a bias current, similarly to the constant current Idc. That is, the current controller  42  adjusts the constant current Iapc in correspondence with a decrease in light-receiving current Ir of the monitor PD  82  caused by deterioration of the laser diode  81  over time. A decrease in bias current caused by deterioration of the laser diode  81  over time is compensated up to 30 mA by the constant current Iapc. 
     According to the first embodiment, the optical output fluctuating upon deterioration of the laser diode  81  over time is controlled at a constant level. More specifically, data about the light-receiving current Ir of the monitor PD  82  when the laser diode  81  before deterioration outputs a desired light power is stored in the memory  50  for each temperature. The initial data stored in the memory  50  is used as a reference value. A decrease in light-receiving current Ir of the monitor PD  82  is detected as a decrease in optical output from the laser diode  81  on the basis of the reference value, and feedback control for increasing the bias current is performed. Accordingly, the optical output can be compensated by an amount corresponding to deterioration of the laser diode  81 , and the optical output can be controlled at a constant level. 
     The measured data of the light-receiving current Ir is used as a reference value, which includes variations in an individual laser diode and another device. Therefore, the optical output and the extinction ratio can be controlled without any special adjustment for the time deterioration compensation circuit  1 . 
     Not only the bias current but also the pulse current can be independently controlled in correspondence with the ambient temperature. More specifically, the bias current and the pulse current are stored in the memory  50  at an address corresponding to the ambient temperature. The bias current and pulse current corresponding to the ambient temperature drive the laser diode  81 . In this case, since the bias current and the pulse current can be independently controlled, not only the optical output but also the extinction ratio can be controlled at a constant level in correspondence with the ambient temperature by storing the bias current and the pulse current in the memory  50  in accordance with the current vs. optical output characteristics of each laser diode  81  for each temperature. As a result, the laser diode driving circuit can drive the laser diode suitably for the transmission system without any light emission delay and any failure to obtain a reliable extinction state due to a decrease in extinction ratio. 
     Second Embodiment 
     The second embodiment of the present invention will be described below with reference to FIG.  3 . 
     FIG. 3 shows the arrangement of a laser diode driving circuit according to the second embodiment of the present invention. The laser diode driving circuit of the second embodiment comprises a preamplifier  55  and a peak detector  60  instead of the average detector  57  in the first embodiment shown in FIG.  2 . Since the remaining arrangement is the same as in the first embodiment shown in FIG. 2, the same reference numerals as in FIG. 2 denote the same parts, and a description thereof will be omitted. 
     A light-receiving current Ir of a monitor PD  82  is input to the preamplifier  55  and converted into a voltage signal. The voltage signal is output to the peak detector  60 . The peak detector  60  detects the peak voltage of the voltage signal output from the preamplifier  55 , and outputs the detection voltage to an A/D converter  21  and the inverting input terminal of an inverting amplifier  65 . The remaining operation is the same as in the first embodiment. 
     In the first embodiment, deterioration of the laser diode  81  over time is compensated based on the average of the light-receiving current Ir of the monitor PD  82 . In the second embodiment, deterioration of the laser diode  81  over time is compensated based on the peak value of the light-receiving current Ir of the monitor PD  82 . 
     In the first embodiment, when a burst signal is used as transmission data, the average of the light-receiving current Ir of the monitor PD  82  is very low, and fluctuations in light-receiving current Ir of the monitor PD  82  are difficult to detect. For this reason, deterioration of the laser diode  81  cannot be determined. 
     In the second embodiment, however, since the peak value of the light-receiving current Ir of the monitor PD  82  is detected, even if a burst signal is used as transmission data, deterioration of the laser diode  81  over time can be determined, and a decrease in optical output can be compensated in correspondence with a decrease in peak value. 
     Third Embodiment 
     The third embodiment of the present invention will be described with reference to FIG.  4 . 
     FIG. 4 shows the arrangement of a laser diode driving circuit according to the third embodiment of the present invention. The laser diode driving circuit of the third embodiment has the same constituent elements as in the second embodiment shown in FIG. 3 except that a current controller  42  controls two different constant currents, i.e., a constant current Iapc 1  and a constant current Iapc 2 . Since the remaining arrangement is the same as in the second embodiment shown in FIG. 3, the same reference numerals as in FIG. 3 denote the same parts, and a description thereof will be omitted. 
     The constant current Iapc 1  serves as a bias current directly flowing through a laser diode  81 , whereas the constant current Iapc 2  serves as a pulse current controlled by an input signal. 
     More specifically, in the third embodiment, both the bias current and the pulse current are compensated in accordance with deterioration of the laser diode  81  over time. The optical output and the extinction ratio can be controlled at a constant level in correspondence with deterioration of the laser diode  81  over time. 
     Fourth Embodiment 
     The third embodiment adopts the preamplifier  55  and the peak detector  60 . However, in the arrangement of the third embodiment shown in FIG. 4, an average detector  57  may replace the preamplifier  55  and the peak detector  60 , and deterioration of a laser diode  81  over time may be compensated based on the average of the light-receiving current Ir of a monitor PD  82 , similarly to the first embodiment.