AN APPARATUS FOR DRIVING A SEMICONDUCTOR LASER DEVICE

Abstract: An apparatus for driving a semiconductor laser device (1) wherein the optical output of the laser device is maintained at a predetermined level by means of two feedback control loops (A and B). The apparatus comprises at least two D/A converters (10, 15). One of said D/A converters (10) is used for conducting a coarse adjustment of the optical output level, and another of the D/A converters (15) is used for conduct­ ing a fine adjustment of the optical output level.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: This invention relates to an apparatus for driving a semiconductor laser device. 2. Description of the prior art: When driving a semiconductor device to obtain a laser beam, a forward current I F is supplied to a pn junction in the laser device. The relationship between the forward current I F and the optical output P O of a semiconductor laser device is not linear. As the forward current I F supplied to a semiconductor laser device increases, the laser device begins to oscillate a laser beam at a certain level of the current (a threshold current I th ). As the forward current I F increases further, the laser output P O also increases. Figure 2 shows an example of the relationship between the forward current I F and the optical output P O of a semiconductor laser device. The level of a threshold current I th and the rate of change in an optical P O with respect to the change in a forward current I F are not constant but vary according to an ambient temperature or to the individual semiconductor laser device. Such a rate of change in an optical output P O is called a differential efficiency η. When a semiconductor laser diode is driven at an optical output of a constant level, the system shown in Fig. 3 is generally used. In this system, a semiconductor laser diode 1 is driven by a current source 4 which is controlled by the output of an amplifier 3. The optical output of the laser diode 1 is monitored by a photodiode 2, and is converted into voltage by a resistor 5. The voltage is applied to one input terminal of the amplifier 3. The system shown in Fig. 3 constitutes a negative feedback loop. A reference voltage V ref is applied to the other input terminal of the amplifier 3 so that the optical output of the laser diode 1 is controlled to have a predetermined level corresponding to the reference voltage V ref . The system shown in Fig. 3 is used for obtaining a constant optical output of a predetermined level. In certain kinds of application of a semiconductor laser diode, such as laser printers or optical communication, a semiconductor laser diode producing a constant optical output of a predetermined level is turned on and off at a high speed. Figure 4 shows an example of a system used for such an application. The system of Fig. 4 comprises, in addition to the components in the system of Fig. 3, an analog switch 7 and a capacitor 8 which constitute a sample-hold circuit. The system of Fig. 4 further comprises a high-speed switch 6 and a buffer amplifier 9. The operation of the system of Fig. 4 will be described. First, the switch 6 is set to the right position, and the switch 7 is turned on to obtain an optical output of a predetermined level corresponding to the reference voltage V ref . Thereafter, the switch 7 is turned off. At this stage, the voltage applied to the amplifier 9 is held at the voltage level of the capacitor 8 so that the driving current supplied to the laser diode 1 is maintained to be constant. Then, the switch 6 is turned off and on at a high speed, resulting in a high-speed switching of the optical output of a constant level. With the system shown in Fig. 4, it is difficult to maintain the optical output at a constant level for a long period of time because the voltage of the capacitor 8 is an analog value. The system illustrated in Fig. 5 has been proposed to solve this problem, i.e., the system can maintain the optical output at a constant level for a long period of time. The system of Fig. 5 is not provided with the sample-hold circuit (the analog switch 7 and capacitor 8) and the buffer amplifier 9 which are used in the system of Fig. 4, but comprises an up/down counter 11, a D/A converter 10, and an oscillator 12. In the system of Fig. 5, the amplifier 3 functions as a comparator. When the output of the comparator 3 is "HIGH", the up/down counter 11 counts up the output pulses of the oscillator 12. When the output of the comparator 3 is "LOW", in contrast, the counter 11 counts down the output pulses of the oscillator 12. The output of the counter 11 is converted into an analog value corresponding to the forward current I F by the D/A converter 10. When the switch 6 is set to the right position, the forward current I F from the current source 4 is supplied to the semiconductor laser diode 1 to drive the laser diode 1. The system of Fig. 5 constitutes a negative feedback loop so that the optical output of the laser diode 1 is regulated at a constant value corresponding to the reference voltage V ref . An error which corresponds to one pulse of the output of the oscillator 12 may occur in the level of the optical output. As digital codes are used in the feedback loop, the system of Fig. 5 can maintain the light output at a constant level for a long period of time. In the system of Fig. 5, however, the D/A converter 10 should have high resolution in order to maintain an optical output which is regulated to a constant level with high accuracy because the I F -P O characteristics of a semiconductor laser diode are not linear. The resolution of a D/A converter is the number of available analog levels output from the D/A converter. US-A-4 692 606, on which the preamble of claim 1 is based, discloses an apparatus for driving a semiconductor laser device to develop a predetermined constant optical output, said apparatus comprising means for monitoring an optical output of the laser device and generating a signal which is representative of the optical output; said apparatus further comprising at least two feedback loops, one of said feedback loops comprising: first comparing means for comparing said signal representative of the optical output with a first reference value and providing a first output signal, said first output signal depending on a comparison of said signal representative of the optical output with said first reference value; first counting means for developing a first digital signal for said first output signal being provided from said first comparing means; first D/A converting means for developing a first control signal in response to said first digital signal; and first current supply means for driving the laser device at a first optical output in response to said first control signal; and another of said feedback loops comprising second comparing means for comparing said signal representative of the optical output with a second reference value corresponding to the predetermined constant optical output and providing a second output signal, said second output signal depending on a comparison of said signal representative of the optical output with said second reference value; second counting means for developing a second digital signal for said second output signal being provided from said second comparing means; second D/A converting means for developing a second control signal in response to said second digital signal; and second current supply means for driving the laser device at the predetermined constant optical output in response to said second control signal. However, US-A-4 692 606 does not disclose that the error in the current supplied to the laser device may be reduced by the use of two feedback loops, while using a D/A converter of the same resolution. SUMMARY OF THE INVENTION According to the present invention an apparatus for driving a semiconductor laser device, according to the preamble of claim 1, is characterised in that a first value obtained by dividing a first output current of said first current supply means when said first D/A converting means is at full scale by the resolution of said second D/A converting means is greater than a second value obtained by dividing a second output current of said second current supply means when said second D/A converting means is at full scale by the resolution of said second D/A converting means. In a preferred embodiment, said first comparing means provides said first output signal when said signal representative of the optical output is less than said first reference value, and said second comparing means provides said second output signal when said signal representative of the optical output is less than said second reference value, said second reference value being more than said first reference value. In a preferred embodiment said first comparing means provides said first output signal when said signal representative of the optical output is less than said first reference value, said second comparing means provides said second output signal when said signal representative of the optical output is more than said second reference value, said second reference value being less than said first reference value. In a preferred embodiment wherein said second reference value is equal to said first reference value. In a further preferred embodiment said first comparing means and said second comparing means comprises a single common comparator. Thus, the invention described herein makes possible the objective of providing an apparatus for driving a semiconductor laser device which can control an optical output level with high accuracy. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is diagram showing an embodiment of the invention in which two negative feedback loops are formed. Figure 2 is a graph showing a relation between a forward current I F and an optical output level of a laser diode. Figure 3 is a diagram showing an example of a conventional apparatus. Figure 4 is a diagram showing another example of a conventional apparatus in which a sample-hold circuit is used and the optical output of a laser diode is switched at a high frequency. Figure 5 is a diagram showing a further example of a conventional apparatus in which a D/A converter is disposed in a feedback loop. Figure 6 is a graph showing a I F -P O characteristic of a laser diode and also a manner of controlling the forward current according to the embodiment shown in Fig. 1. Figure 7 is a diagram showing another embodiment of the invention. Figure 8 is a diagram showing a further embodiment of the invention. Figure 9 is a graph showing a I F -P O characteristic of a laser diode and also a manner of controlling the forward current according to the embodiment shown in Fig. 8. Figure 10 is a diagram showing a further embodiment of the invention. Figure 11 is a graph showing a I F -P O characteristic of a laser diode and also a manner of controlling the forward current according to the embodiment shown in Fig. 10. Figures 12(a) and 12(b) are respectively a graph showing a relation between digital codes and the forward current and a graph showing a relation between digital codes and the optical output of the laser diode, according to the conventional example shown in Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a block diagram of an embodiment of the invention. The system of Fig. 1 which drives a semiconductor laser diode 1 comprises a photodiode 2 for monitoring the optical output of the laser diode 1 , first and second comparators 3 and 14 , first and second up/down counters 11 and 16 , first and second D/A converters 10 and 15 , first and second current sources 4 and 13 which are controlled respectively by the outputs of the D/A converters 10 and 15 , a resistor 5 , and a high-speed switch 6 . A first reference voltage V ref1 is applied to one input terminal of the first comparator 3 , and a second reference voltage V ref2 is applied to one input terminal of the second comparator 14 . The first and second up/down counters 11 and 16 are controlled by the control signals S C1 and S C2 , respectively. The system has two negative feedback loops A and B . The first feedback loop A consists of the laser diode 1 , the photodiode 2 , the first comparator 3 , the first counter 11 , the first D/A converter 10 , the first current source 4 , and the switch 6 . The first feedback loop A conducts a coarse adjustment of the optical output P O . The second feedback loop B consists of the laser diode 1 , the photodiode 2 , the second comparator 14 , the second counter 16 , the second D/A converter 15 , the second current source 13 , and the switch 6 . The second feedback loop B conducts a fine adjustment of the optical output P O . In the system of Fig. 1, each of the two feedback loops A and B operates in approximately the same manner as the feedback loop used in the system of Fig. 5. But, the first reference voltage V ref1 is set to a level corresponding to an optical output P O1 which is slightly lower than a desired optical output P O2 , and the second reference voltage V ref2 is set to a level corresponding to the desired optical output P O2 . The operation of the system of Fig. 1 will be described with reference to Fig. 6. The counter 11 and 16 are reset by the control signals S C1 and S C2 so that the current sources 4 and 13 do not supply a forward current to the laser diode 1 . Then, the switch 6 is set to the right position. The first counter 11 is controlled to begin the count-up operation by the control signal S C1 to actuate the first feedback loop A . The output current I F1 ' of the first current source 4 increases toward the level of the driving current I F1 which can produce the optical output P O1 . After reaching the current level I F1 , the forward current I F1 ' comes to oscillate within the range of ±ΔI F1 (i.e., I F1 '=I F1 ±ΔI F1 ). The value ΔI F1 is obtained by dividing the output current of the first current source 4 when the first D/A converter 10 is at full scale by the resolution of the first D/A converter 10 . Namely, ΔI F1 is the current of the first current source 4 which corresponds to 1 LSB of the first D/A converter 10 . Then, the first counter 11 halts the operation of counting the pulses of the oscillator 12 , and maintains the output level of the value obtained at that time. The output current I F1 ' of the first current source 4 is kept at a constant value within the range of I F1 ±ΔI F1 . Thereafter, the second counter 16 is put into operation of counting up the pulses of the oscillator 12 to actuate the second feedback loop B . The driving current supplied to the laser diode 1 becomes a sum of the output current I F1 ' of the first current source 4 and the output current I F2 ' of the second current source 13 . The output current I F2 ' increases toward the level of an output current I F2 so that the optical output of the laser diode 1 reaches the desired output level P O2 which can be obtained by supplying a sum of the currents I F1 ' and I F2 to the laser diode 1 . As the current I F1 ' flowing in the feedback loop A is kept at a constant level (the final value of I F1 '), the current I F2 ' comes to oscillate within the range of ±ΔI F2 (i.e., I F2 '=I F2 ±ΔI F2 ). The value ΔI F2 is obtained by dividing the output current of the second current source 13 when the second D/A converter 15 is at full scale by the resolution of the second D/A converter 15 . Namely, ΔI F2 is the current of the second current source 13 which corresponds to 1 LSB of the second D/A converter 15 . Then, the second counter 16 halts the operation of counting the pulses of the oscillator 12 , and holds the value obtained at that time. The output current I F2 ' of the second current source 13 is kept at a constant value within the range of I F2 ±ΔI F2 . The degree of the deviation of the final driving current supplied to the laser diode 1 which is a sum of the output currents I F1 ' and I F2 ' from the objective value which is a sum of the currents I F1 ' and I F2 does not depend on the error of the first feedback loop A (i.e. ±ΔI F1 ), but depends only on the error of the second feedback loop B (i.e. ±ΔI F2 ). As mentioned above, the degree of the error ±ΔI F2 is proportional to the output current of the second current source 13 when the second D/A converter 15 is at full scale, and also to the reciprocal of the resolution of the second D/A converter 15 . The output current I F2 ' of the second current source 13 when the second D/A converter 15 is at full scale can be selected to be sufficiently smaller than the current I F1 . Consequently, compared to a system having one feedback loop in which a D/A converter having the same resolution is used, the error of the system of Fig. 1 which corresponds to ΔI F2 is very small, resulting in that the optical output P O can be controlled with high accuracy. The first reference voltage V ref1 may be set to be slightly higher than the voltage corresponding to the desired optical output P O2 so that the optical output of the laser diode 1 approaches the desired value from the side of higher values. In this case, the result is the same as that obtained in the system described above. Figure 7 shows a more specific embodiment of the invention. The system of Fig. 7 comprises a semiconductor laser diode 101 , a photodiode 102 , a resistor 105 , a high-speed switch 106 , first and second current sources 104 and 113 , first and second D/A converters 110 and 115 , first and second up/down counters 111 and 116 , and an oscillator 112 . In the system of Fig. 7, the first and second reference voltages V ref1 and V ref2 are supplied by the circuit which consists of a comparator 103 , a voltage source V S , resistors R₁ , R₂ and R₃ , and a transistor Q₁ . The system of Fig. 7 further comprises AND gates 117 to 120 , NOT gates 121 and 122 , and an OR gate 123 . The output of the comparator 103 is connected to the up/down terminal of each of the counters 111 and 116 . The output pulses of the oscillator 112 are supplied to the first and second counters 111 and 116 through the AND gates 119 and 120 , respectively. As shown in Fig. 7, control signals R and C which are sent from a control means (not shown) are supplied to the reset terminals of the first and second counters 111 and 116 , and the base of the transistor Q1 through the gates 117 to 122 . Switching pulses SW and the control signal C are supplied to input terminals of the OR gate 123 . The output of the OR gate 123 is supplied to the switch 106 . When the transistor Q1 is on, the first reference voltage V ref1 is applied to one input terminal of the comparator 103 . When the transistor Q1 is off, the second reference voltage V ref2 which is higher than the first reference voltage V ref1 is applied to one input terminal of the comparator 103 . The process of setting the optical output of the laser diode to a constant value comprises stages a to d. In the stages a to d, the control signal R is changed in the sequence of "1, 1, 0, and 0", and the control signal C is changed in the sequence of "0, 1, 1, and 0". In stage a, the control signal R is "1", and the control signal C is "0", whereby resetting both the first and second counters 111 and 116 and inhibiting the output pulses of the oscillator 112 from entering into the counters 111 and 116 . Hence, the laser diode 101 is not driven in stage a. In stage b, both the control signals R and C are "1". The transistor Q1 is turned on to apply the first reference voltage V ref1 to one input terminal of the comparator 103 . The output of the comparator 103 becomes high so that the first counter 111 starts the operation of counting up the pulses of the oscillator 112 . During stage b, the switch 106 is on. A first feedback loop consisting of the laser diode 101 , the comparator 103 , the first counter 111 , the first converter 110 , the first current source 104 , and the switch 106 begins to supply the forward current I F1 ' to the laser diode 101 , while increasing the forward current I F1 ' to the vicinity of the current I F1 which corresponds to the initial target value P O1 of the optical output. Then, the control signal R is changed to be "0" (stage c). In stage c, the pulses of the oscillator 112 are not supplied to the first counter 111 , and the first counter 111 halts the counting operation so that the first feedback loop maintains the supply of the forward current I F1 ' corresponding to the value of the first counter 111 which value has been obtained at the end of the stage b. The transistor Q1 is turned off to apply the second reference voltage V ref2 to one input terminal of the comparator 103 . The second counter 116 starts the operation of counting up the pulses of the oscillator 112 . A second feedback loop consisting of the laser diode 101 , the comparator 103 , the second counter 116 , the second converter 115 , the second current source 113 , and the switch 106 begins to supply the forward current I F2 ' to the laser diode 101 . In the same manner as the system of Fig. 1, the driving current of the laser diode 101 which is the sum of the currents I F1 ' and I F2 ' increases to the level at which the desired optical output P O2 is obtained. Thereafter, the control signal C is changed to be "0" (stage d). The second counter 116 halts the counting operation so that the second feedback loop maintains the supply of the forward current I F2 ' corresponding to the value of the second counter 116 which value has been obtained at the end of the stage c. Therefore, the laser diode 101 is driven by the sum of the currents I F1 ' and I F2 ' which corresponds to the desired optical output P O2 . The switch 106 is turned on and off in synchronization with the switching pulse SW so that the laser diode 101 which emits a laser beam of the desired level P O2 is turned on and off at a high speed. In this way, the system of Fig. 7 can control the optical output with high accuracy. The control logic and operation of the system of Fig. 7 are summarized in Table 1. Stage R C Counter 111 Counter 116 Q1 Switch 106 a 1 0 Reset Reset Off On or off according to SW b 1 1 Count Reset On On c 0 1 Halt Count Off On d 0 0 Halt Halt Off On or off according to SW Another embodiment of the invention is shown in Fig. 8. The arrangement of the system of Fig. 8 is substantially the same as that of the system of Fig. 7 except that a NAND gate 124 is provided instead of the AND gate 117 and that the control signal R is supplied to the set terminal of the second counter 116 . In the system of Fig. 8, the initial output level P O1 which is obtained by the forward current I F1 ' is set to be higher than the desired level P O2 of the optical output. When the set terminal of the counter 116 is HIGH, the counter 116 is set to have a value of the full scale. As shown in Fig. 9, the optical output approaches the desired value P O2 from the side of higher values. The operation of the system of Fig. 8 is summarized in Table 2. Stage R C Counter 111 Counter 116 Q1 Switch 106 a 1 0 Reset Set On On or off according to SW b 1 1 Count Set Off On c 0 1 Halt Count On On d 0 0 Halt Halt On On or off according to SW Figure 10 illustrates a further embodiment of the invention. According to the system of Fig. 10, an initial output level which is obtained by the first feedback loop (coarse adjustment) is set to a value which is the same as the desired (final) output level P O . Hence, the system of Fig. 10 does not require the components for changing the level of the reference voltage (i.e., the resistors R1 to R3 and the transistor Q1 ) which are used in the systems of Figs. 7 and 8. When the set terminal is HIGH, the counter 116 disposed in the second feedback loop (fine adjustment) is set to have a value of half of the full scale. The operation of the system of Fig. 10 is illustrated in Fig. 11 and is summarized in Table 3. Stage R C Counter 111 Counter 116 Switch 106 a 1 0 Reset 1/2 x full scale On or off according to SW b 1 1 Count 1/2 x full scale On c 0 1 Halt Count On d 0 0 Halt Halt On or off according to SW As seen from the above-mentioned description of the preferred embodiment, a D/A converter having low resolution which can be easily produced at a low cost can be used in an apparatus of the present invention. Therefore, an apparatus of the present invention can be easily manufactured at a low cost and is easily integrated. It will be apparent that many modifications can be made to the embodiments described above. Accordingly, it is not intended that the invention be limited to the embodiments described in detail above.

Claims: An apparatus for driving a semiconductor laser device (1) to develop a predetermined constant optical output, said apparatus comprising means (2) for monitoring an optical output of the laser device (1) and generating a signal which is representative of the optical output; said apparatus further comprising at least two feedback loops, one of said feedback loops comprising: first comparing means (3) for comparing said signal representative of the optical output with a first reference value and providing a first output signal, said first output signal depending on a comparison of said signal representative of the optical output with said first reference value; first counting means (11) for developing a first digital signal for said first output signal being provided from said first comparing means (3); first D/A converting means (10) for developing a first control signal in response to said first digital signal; and first current supply means (4) for driving the laser device (1) at a first optical output in response to said first control signal; and another of said feedback loops comprising: second comparing means (14) for comparing said signal representative of the optical output with a second reference value corresponding to the predetermined constant optical output and providing a second output signal, said second output signal depending on a comparison of said signal representative of the optical output with said second reference value; second counting means (16) for developing a second digital signal for said second output signal being provided from said second comparing means (14); second D/A converting means (15) for developing a second control signal in response to said second digital signal; and second current supply means (13) for driving the laser device (1) at the predetermined constant optical output in response to said second control signal; said apparatus being characterised in that a first value obtained by dividing a first output current of said first current supply means when said first D/A converting means is at full scale by the resolution of said first D/A converting means is greater than a second value obtained by dividing a second output current of said second current supply means when said second D/A converting means is at full scale by the resolution of said second D/A converting means. An apparatus according to claim 1, wherein said first comparing means (3) provides said first output signal when said signal representative of the optical output is less than said first reference value, and said second comparing means (14) provides said second output signal when said signal representative of the optical output is less than said second reference value, said second reference value being more than said first reference value. An apparatus according to claim 1, wherein said first comparing means (3) provides said first output signal when said signal representative of the optical output is less than said first reference value, said second comparing means (14) provides said second output signal when said signal representative of the optical output is more than said second reference value, said second reference value being less than said first reference value. An apparatus according to claim 1, wherein said second reference value is equal to said first reference value. An apparatus according to claim 2, 3 or 4, wherein said first comparing means (3) and said second comparing means (14) comprises a single common comparator.

Inventors: HATANAKA, KAZUOMI, OGOU, KENJI, OHNO, TAKASHI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: H01S3
IPC codes: H01S5, H05B33