An apparatus for driving a semiconductor laser device

Abstract: An apparatus for driving a semiconductor laser device (1) comprises monitoring means (2) for monitoring the optical output of the laser device (1) and generating a signal which is representative of the optical output, and a feedback loop to keep the optical output of the laser device (1) at a predetermined value. The feedback loop comprises: comparing means (3) for comparing the signal from the monitoring means with a reference value and providing an output signal; counting means (11) for developing a digital signal in response to the output signal provided from the comparing means (3); a D/A converter (10a) for developing a control signal in response to the digital signal; and current supply means (4) for driving the laser device (1) at the predetermined optical output in response to the control signal from the D/A converter. The D/A converter (10a) has a non-linear characteristic.

Description: 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 laser 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 1 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 output P O with respect to the changes 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. When a semiconductor laser diode is driven at an optical output of a constant level, the system shown in Fig. 2 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. 2 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. 2 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 3 shows an example of a system used for such an application. The system of Fig. 3 comprises, in addition to the components in the system of Fig. 2, an analog switch 7 and a capacitor 8 which constitute a sample-hold circuit. The system of Fig. 3 further comprises a high-speed switch 6 and a buffer amplifier 9 . The operation of the system of Fig. 3 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. 3, 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. 4 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. 4 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. 3, but comprises an up/down counter 11 , a D/A converter 10 , and an oscillator 12 . In the system of Fig. 4, 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. 4 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. One problem is the fact that the I F -P O characeristics of a semiconductor laser diode are such that, if a foward current I F is less than the threshold current I th , laser oscillation does not occur. Accordingly, if the forward current I F is less than the threshold current I th , it cannot be used for control in driving the laser diode. For this reason, the quantization error of the optical output P O is greater than the quantization error of the forward current I F , resulting in less accuracy in controlling the optical output P O . The above-mentioned problem will be discussed in more detail. Figure 5 (a) shows a relation between an input (digital codes) and an output (forward current I F ) of D/A converter which are usually used in a conventional system such as shown in Fig. 4. Figure 5 (b) shows a relation between optical outputs P O of laser diode and digital codes in the conventional system of Fig. 4 which employs a D/A converter having the linear characteristics shown in Fig. 5 (a). The digital codes corresponding to values lower than the threshold current I th of a laser diode do not contribute to the optical output of the laser diode. As shown in Fig. 5 (b), therefore, the range of the codes which are effective in practical use is restricted, resulting in a reduced effective resolution of the D/A converter 10. In Fig. 5 (b), "W q " indicates a quantization width of the optical output P O . CA-B-1 210 070 discloses a controller for supplying a bias current (I) to a laser used in a digital data transmission system. The controller measures the second derivative of the laser output power (L) with respect to the bias current. The controller sets the bias current at a value corresponding to a peak in d²L/dI², as this value of the bias current maximises the ratio of the laser output power for data "1" to the laser output power for data "0". CA-B-1 210 070 does not disclose a controller which can operate a laser device at a predetermined optical output. According to the present invention there is provided 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 and a feedback loop comprising:- comparing means for comparing said signal representative of the optical output with a reference value and providing an output signal, said output signal depending on a comparison of said signal representative of the optical output with said reference value; counting means for developing a digital signal for said output signal being provided from said comparing means; D/A converting means for developing a control signal in response to said digital signal; and current supply means for driving the laser device at said predetermined optical output in response to said first control signal; said apparatus being characterised in that said D/A converting means has a non-linear characteristic. According to the present invention there is further provided 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 and a feedback loop comprising:- comparing means for comparing said signal representative of the optical output with a reference value and providing an output signal, said output signal depending on a comparison of said signal representative of the optical output with said reference value; counting means for developing a digital signal for said output signal being provided from said comparing means; D/A converting means for developing a control signal in response to said digital signal; and current supply means for driving the laser device at said predetermined optical output in response to said first control signal; said apparatus being characterised in that said D/A converting means provides an offset output. 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. 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 accompany drawings as follows: Figure 1 is a graph showing a relation between a forward current I F and an optical output level of a laser diode. Figure 2 is a diagram showing an example of a conventional apparatus. Figure 3 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 4 is a diagram showing a further example of a conventional apparatus in which a D/A converter is disposed in a feedback loop. Figures 5 ( a ) and 5( 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. 4. Figure 6 is a diagram showing one embodiment of the invention. Figures 7 ( a ) and 7 ( b ) are graphs similar to Figs. 5(a) and 5 (b), according to the embodiment shown in Fig. 6. Figure 8 is a diagram showing a further embodiment of the invention. Figures 9 ( a ) and 9 ( c ) are respectively a graph showing an example of 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 embodiment shown in Fig. 8. Figures 9 ( b ) and 9 ( d ) are respectively a graph showing another example of 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 embodiment shown in Fig. 8. Figure 10 is a diagram showing a further embodiment of the invention. Figures 11 ( a ) and 11( b ) are graphs showing relations between digital codes and the optical output of the laser diode, according to the embodiment shown in Fig. 10. Figure 6 shows an embodiment of the invention. The configuration of the system shown in Fig. 6 is similar to that of the system of Fig. 4, but employs a D/A converter 10a having non-linear characteristics instead of the converter 10 in the system of Fig. 4. The relation between an input (digital codes) and an output (forward current I F ) of the D/A converter 10a in this embodiment is shown in Fig. 7( a ). In the range wherein the forward current I F is smaller than the threshold current I th , the quantization width W₁ is wide, and in the range wherein the forward current I F is larger than the threshold current I th , the quantization width W₂ is narrow. Figure 7 ( b ) shows a relation between optical output P O and the digital codes of the D/A converter 10a in this embodiment. Because of the non-linear characteristics of the converter 10a , the number of the digital codes which correspond to the current levels lower than the threshold current I th is small, compared to that of the conventional system which is shown in Fig. 5. In other words, the number of the codes which are effective in the control of the optical output of the laser diode is large even when the D/A converter 10a has the same number of digital codes as that of the D/A converter 10 of Fig. 4, resulting in an improved effective resolution of the D/A converter 10a and also in a very accurate control of the optical output. Figure 8 shows a further embodiment of the invention. The configuration of the system shown in Fig. 8 is similar to that of the system of Fig. 6, but employs a D/A converter 10b having a characteristic that an offset current of a certain level flows when the digital code is zero. In an example of the characteristic shown in Fig. 9 ( a ), the offset current I OS is greater than the threshold current I th . In another example shown in Fig. 9 ( b ), the offset current I OS is smaller than the threshold current I th . The relation between an input (digital code) and an output (forward current I F ) of the D/A converter 10b having the characteristics of Fig. 9 ( a ) is shown in Fig. 9 ( c ), and that of the D/A converter 10b having the characteristics of Fig. 9 ( b ) is shown in Fig. 9( d ). There is no digital codes in the range lower than the threshold current I th (Fig. 9 ( a )), or there is a small number of digital codes in the range lower than the threshold current I th (Fig. 9 ( b )). Namely, the number of the codes which are effective in the control of the optical output of the laser diode is large even when the D/A converter 10b has the same number of digital codes as that of the D/A converter 10 of Fig. 4, resulting in an improved effective resolution of the D/A converter 10b and also in a very accurate control of the optical output. Figure 10 shows a further embodiment of the invention. The configuration of the system shown in Fig. 10 is similar to that of the system of Fig. 4 except that a second current source 4a is provided. The current source 4a supplies an offset current I OS of a certain level to the laser diode 1 , i.e., the laser diode 1 is driven by the sum of the forward current I F and the offset current I OS . Figures 11 ( a ) and 11 ( b ) show the relations between digital codes of the D/A converter 10 and optical outputs P O of the laser diode 1 when the offset current I OS is greater or smaller than the threshold current I th , respectively. When the offset current I OS is smaller than the threshold current I th , the number of the codes which are ineffective in the control of the optical output of the laser diode 1 is small (Fig. 11 ( b )). When the offset current I OS is greater than the threshold current I th , there is no code which is ineffective in the control of the optical output of the laser diode 1 (Fig. 11( a )). Consequently, the quantization widths W₃ (Fig. 11 ( a )) or W₄ (Fig. 11( b )) in the system of Fig. 10 is narrow, compared to those obtained by the system of Fig. 4 even when the D/A converters 10 used in both the systems have the same resolution characteristics, resulting in an improved effective resolution of the D/A converter 10 and also in a very accurate control of the optical output. As seen from the above-mentioned description of the preferred embodiments, 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 a low cost and is easily integrated. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention. Accordingly, it is not intended that the scope of the invention be limited to the detailed description as set forth above, but rather by the scope of the appended claims.

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 and a feedback loop comprising:- comparing means (3) for comparing said signal representative of the optical output with a reference value and providing an output signal, said output signal depending on a comparison of said signal representative of the optical output with said reference value; counting means (11) for developing a digital signal for said output signal being provided from said comparing means (3); D/A converting means (10a) for developing a control signal in response to said digital signal; and current supply means (4) for driving the laser device (1) at said predetermined optical output in response to said control signal; said apparatus being characterised in that said D/A converting means (10a) has a non-linear characteristic. 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 and a feedback loop comprising:- comparing means (3) for comparing said signal representative of the optical output with a reference value and providing an output signal, said output signal depending on a comparison of said signal representative of the optical output with said reference value; counting means (11) for developing a digital signal for said output signal being provided from said comparing means (3); D/A converting means (10b) for developing a control signal in response to said digital signal; and current supply means (4) for driving the laser device (1) at said predetermined optical output in response to said control signal; said apparatus being characterised in that said D/A converting means (10b) provides an offset output. 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 and a feedback loop comprising:- comparing means (3) for comparing said signal representative of the optical output with a reference value and providing an output signal, said output signal depending on a comparison of said signal representative of the optical output with said reference value; counting means (11) for developing a digital signal for said output signal being provided from said comparing means (3); D/A converting means (10) for developing a control signal in response to said digital signal; and first current supply means (4) for driving the laser device (1) at said predetermined optical output in response to said control signal; said apparatus being characterised in that it further comprises second current supply means (4a) for supplying current to said laser device (1), said second current supply means (4a) not being within said feedback loop.

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