Abstract:
A drive device for a light emitting component includes a reference source for generating a power stipulation signal that stipulates a desired power. A correction device compensates for a temperature-dictated measurement error of the photodetector by modifying, in a temperature-dependent manner, the power stipulation signal generated by the reference source. A regulating device is connected to the reference source and generates a regulating signal that regulates the light power of the light emitting component to minimize the deviation between the actual light power and the desired light power. This configuration avoids monitor tracking errors of a monitor diode used to measure the actual light power.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     In drive devices for light-emitting components, for example lasers, as is known, detectors (e.g. monitor photodiodes) are present which measure the light power of the light-emitting component and enable the light power to be regulated. 
     In monitor photodiodes integrated in laser diodes, various causes give rise to so-called “monitor tracking errors”. These “monitor tracking errors” are based on temperature-dependent inaccuracies or measurement errors of the monitor diode, which should actually measure the coupled-in optical power of the laser diode correctly. In the case of an edge-emitting laser, for example, a “monitor tracking error” may be based on the fact that the optical power at the front mirror of the laser and the optical power at the rear mirror of the laser are not proportional—in a temperature-dependent manner. In the case of a surface-emitting laser (VCSEL laser), a “monitor tracking error” may also be caused by a mode-selective and thus temperature-dependent coupling between the laser and its monitor photodiode. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of specifying a drive device for a light-emitting component in which fluctuations in the output power of the light-emitting component on account of measurement errors of the assigned photodetector, in particular on account of “monitor tracking errors”, are avoided. 
     Accordingly, the invention provides a drive device having a reference source, which generates a power stipulation signal stipulating a desired light power. Moreover, the drive device has a photodetector for measuring the respective actual light power. A regulating device is connected to the photodetector and to the reference source. The regulating device generates a regulating signal, which regulates the light power, for the light-emitting component. In addition, the drive device according to the invention has a correction device, which compensates for temperature-dictated measurement errors of the photodetector by modifying, in a temperature-dependent manner, the power stipulation signal generated by the reference source. 
     An essential advantage of the drive device according to the invention is to be seen in the fact that this drive device can be realized with relatively simple and cost-effective components, since only the power stipulation signal generated by the reference source has to be modified in a temperature-dependent manner in order to compensate for the temperature-dictated measurement errors. 
     A further essential advantage of the drive device according to the invention is that the hitherto customary components for light regulation, that is to say the regulating device and the photodetector, can continue to be used unchanged; these components do not have to be modified since, according to the invention, only the desired light power or the power stipulation signal is changed in a temperature-dependent manner. 
     Digital components are particularly cost-effective, so that it is regarded as advantageous if the correction device is formed at least partly by digital components. 
     In accordance with one advantageous refinement of the drive device, the correction device has a memory. Correction values for the temperature-dependent modification of the power stipulation signal are stored in the memory. 
     Preferably, the correction device has a control device, which, with a temperature sensor, measures the temperature of the monitor diode or a temperature proportional thereto and then reads from the memory that correction value which is respectively assigned to the measured temperature value. 
     The correction values and the assigned temperature levels or temperature ranges may be stored preferably in table form in the memory. The table may preferably be configured as a “look-up table”. 
     The control device of the correction device is preferably formed by a controller module, in particular by a microprocessor. 
     In order to ensure that the temperature response or the compensation regulation can be changed arbitrarily by the user or can be set externally, it is regarded as advantageous if the memory and thus the memory values contained therein are arbitrarily programmable. In such a case, it is possible to compensate for tracking errors with arbitrary temperature characteristic curves; the compensation can thus be adapted to the optical components respectively used—thus e.g. to the respective laser and the respectively assigned monitor diode. 
     Furthermore, the correction device preferably has a digital-to-analog converter connected downstream of the control device. This digital-to-analog converter (D/A converter) forms an analog modification signal from the correction value read from the memory by the control device or an auxiliary correction value derived therefrom by the control device. The power stipulation signal of the reference source is modified using the modification signal. 
     In the context of a further advantageous refinement, the control device has an analog adder, which adds the power stipulation signal of the reference source and the modification signal of the D/A converter. The adder may be formed by an operational amplifier circuit, for example. 
     As has already been explained in the introduction, “monitor tracking” errors occur particularly in the case of laser diodes, so that it is regarded as advantageous if the drive device is used for driving a laser as the light-emitting component. The photodetector for detecting the light power of the laser is then preferably a monitor diode of the laser. 
     The invention furthermore relates to a method for driving a light-emitting component. 
     In order to be able to carry out such a method without a high outlay and using simple components, the invention provides for a desired light power to be stipulated and for the actual light power to be measured by means of a photodetector. The light power of the light-emitting component is regulated in such a way that the deviation between desired light power and the measured actual light power becomes minimal. A temperature-dictated measurement error of the photodetector is compensated for by virtue of the desired light power being modified in a temperature-dependent manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary embodiment of a drive device according to the invention, and 
         FIG. 2  shows an exemplary embodiment of a correction device of the kind that can be used in the drive device according to the invention as shown in FIG.  1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  reveals a drive device  10  for a laser diode  20 . The drive device  10  has a reference source  30 , which generates a power stipulation signal UREF 1  stipulating a desired light power. 
     The drive device  10  furthermore has a monitor diode  40 , which is suitable as photodetector for measuring the actual light power of the laser diode  20 . 
     The monitor diode  40  is connected to an input E 50   a  of a regulating device  50  embodied as a BIAS regulator. The regulating device  50  generates a regulating signal that controls the light power of the laser diode  20 —for example a laser current Il—for the laser  20 . Furthermore, the regulating device  50  is connected by its further input E 50   b  to the reference source  30  via a correction device  60 . 
     The task of the correction device  60  is to modify the power stipulation signal UREF 1  of the reference source  30 , to be precise in such a way that a temperature-dictated measurement error of the monitor diode  40  is compensated for. For this purpose, the correction device  60  generates a modified power stipulation signal UREF 2  from the power stipulation signal UREF 1  stipulated by the reference source  30 . 
     The modified power stipulation signal UREF 2  passes to the further input E 50   b  of the regulating device  50  and is processed by the regulating device  50 . The task of the regulating device  50  is to set the laser current Il for the laser diode  20 , taking account of the modified power stipulation signal UREF 2  present on the input side and the actual light power measurement signal Imeas supplied by the monitor diode  40 , in such a way that the deviation between the actual light power and the desired light power stipulated by the modified power stipulation signal UREF 2  becomes minimal. 
     In order to generate the laser current Il, the regulating device  50  has an operational amplifier  510 , whose “inverting” input is connected to a variable resistor RBIAS. A voltage which is proportional to the current Imeas flowing through the monitor diode  40  is thus present at the “inverting” input of the operational amplifier  510 . 
     By means of the variable resistor RBIAS, the laser current Il can be preset “by hand” or in user-specific fashion. 
     The “noninverting” input of the operational amplifier  510  is connected to the further input E 50   b  of the regulating device  50  and thus has the modified power stipulation signal UREF 2  of the correction device  60  applied to it. 
     On the output side, the operational amplifier  510  is connected to a base terminal of a transistor whose emitter terminal is grounded and whose collector terminal forms the terminal for the laser diode  20 . The output voltage of the operational amplifier  510  is “buffered” by a capacitance CBIAS. 
     At its input E 60 , the correction device  60  has an analog adder  600 , whose output forms the output A 60  of the correction device  60 . The analog adder  600  is additionally equipped with a control input S 600  connected to an output A 610  of a digital/analog converter (D/A converter)  610 . On the input side, the D/A converter  610  is connected to a control device  620  connected to a temperature sensor  630  and a freely programmable memory (RAM module)  640 . 
     The drive device in accordance with  FIG. 1  is operated as follows: 
     The reference source  30  generates the power stipulation signal UREF 1  stipulating the desired light power of the laser diode  20 . This power stipulation signal UREF 1  is modified by the correction device  60 , the modified power stipulation signal UREF 2  being generated. The modified power stipulation signal UREF 2  passes to the regulating device  50 , which drives the laser diode  20  by means of the laser current Il in such a way that the laser diode  20  emits a light power corresponding to the modified power stipulation signal UREF 2 . 
     The light power of the laser diode  20  is measured by means of the monitor diode  40 , which forwards a measurement signal Imeas corresponding to the measured actual light power to the regulating device  50 . The operational amplifier  510  within the regulating device  50  then readjusts the laser current Il in such a way that the actual light power measured by the monitor diode  40  corresponds to the desired light power stipulated by the modified power stipulation signal UREF 2 . 
     If the monitor diode  40  were an “ideal” monitor diode having no temperature-dictated measurement error whatsoever, then a modification of the power stipulation signal UREF 1  would be unnecessary. In reality, however, monitor diodes such as the monitor diode  40  have so-called “monitor tracking errors”; these are temperature-dependent measurement errors. On account of these measurement errors, the actual light power measured by the monitor diode  40  does not correspond to the real actual light power of the laser diode  20 . A regulating error of the regulating device  50  thus occurs, so that the laser current I 1  is no longer set correctly by the regulating device  50 . 
     In order to avoid this temperature-dictated measurement error of the monitor diode  40 , the correction device  60  modifies the power stipulation signal UREF 1  generated by the reference source  30  to form the modified power stipulation signal UREF 2 . This is done as follows: 
     By means of the temperature sensor  630 , the control device  620  measures the temperature respectively prevailing at the monitor diode  40 , or a temperature proportional thereto. Depending on the temperature measurement value T measured by means of the temperature sensor  630 , the control device  620  reads from the memory  640  a correction value (K(T)) appropriate for the respective temperature measurement value T. For this purpose, correction values together with the assigned temperature levels or temperature ranges are stored in table form in the memory  640 . This table forms a so-called “look-up table”. The look-up table may contain “delta values” for n different temperature levels, for example, which values modify the original power stipulation signal UREF 1  of the reference source  30  “additively” or “subtractively”. The “look-up table” may be constructed for example in such a way that the memory addresses of the memory cells of the memory  640  in each case correspond to a temperature or a temperature measurement value T; the content of the memory cells then specifies the assigned correction value K(T). The number of temperature levels results from the number of memory cells implemented and thus from the number of available address bits (e.g. 128 memory cells given 7 bits). 
     Once the control device  620  has read the correction value K(T) associated with the respective temperature measurement value T from the memory  640 , it transfers said correction value to the D/A converter  610 . The D/A converter  610  generates from this an analog modification signal Imod and transfers the latter to the analog adder  600 . The analog adder  600  uses the analog modification signal Imod to generate the modified power stipulation signal UREF 2  from the power stipulation signal UREF 1  present on the input side by means of addition. 
     The functioning of the analog adder  600  is illustrated in detail in FIG.  2 .  FIG. 2  reveals the reference source  30  connected to an input E 600  of the analog adder  600 . Also shown is the D/A converter  610 , which is connected to the control input S 600  of the analog adder  600  and feeds in the modification signal Imod. 
     The analog adder  600  has an operational amplifer  610 , whose “noninverting” input is connected to the reference source  30 . The output of the operational amplifier  610  is connected to the “inverting” input of the operational amplifier and, in addition, to one terminal of a resistor R. The other terminal of the resistor R forms the output of the adder and thus the output A 60  of the correction device  60 . A current source  650  is additionally connected to the other terminal of the resistor R. The current source  650  generates a current Imod′ corresponding to the analog modification signal Imod of the D/A converter  610 . 
     A positive or negative analog modification signal Imod generates a positive or negative current flow Imod&#39; through the current source  650  and thus a voltage drop UREF 2 -UREF 1  across the resistor R. This “positive” or “negative” voltage drop—depending on the direction of the current Imod′—is added to the reference voltage UREF 1 . In other words, the modified power stipulation signal UREF 2  results in accordance with:
 
 U REF 2 = U REF 1 + I mod′* R 
 
where the current direction of the current Imod′ depends on the respective sign of the analog modification signal Imod of the digital-to-analog converter  610 .