Method and circuit arrangement for driving a laser diode

The invention relates to a process and circuitry for controlling a laser diode. To compensate for the temperature-dependent drop in the light intensity produced by the laser diode, the laser diode is controlled by a voltage generator with an adjustable internal resistance. A rise in the driver current of the laser diode as the temperature rises is set by the adjustable internal resistance in such a way that the rise in the light intensity produced determined by the rise in the driver current compensates for the temperature-dependent fall in the light intensity.

BACKGROUND OF THE INVENTION 
The invention refers to the fields of technology herein laser diodes are 
utilized, for example to reproduction technology, and is directed to a 
method and to a circuit arrangement for driving a laser diode in a 
recording device such as a laser printer or laser recorder. 
In such a recording device, the light output by the laser diode and 
modulated with the information to be recorded is shaped into a recording 
beam with optical means and is deflected point-by-point and line-byline 
across a recording medium with a deflection system during recording. 
In most cases, a constant current source is employed for driving the laser 
diode. The constant current source generates a constant driver current 
that is modulated by an image signal that contains the information to be 
recorded and is supplied to the laser diode. 
The laser diode works in switched mode fashion for recording line 
information, wherein the driver current modulated by a two-level image 
signal switches the laser diode on and off. 
In order to achieve a high recording quality in the recording of line 
information, the laser diode must switch quickly and the light level 
output by the laser diode must be constant in the on-time intervals in 
order to achieve a uniform illumination. 
Based on its very nature, the laser diode does not satisfy the demand for a 
constant light level in the on-time intervals. 
The output light power is temperature dependent, namely such that the light 
power decreases with rising operating temperature of the laser diode. 
The gradual rise in the operating temperature in the substrate of the laser 
diode and the drop of the output light power caused as a result thereof 
can in fact be compensated by regulating the housing temperature; in a 
modulation or, switched mode, however, the laser diode still has a dynamic 
temperature effect. The cause of the dynamic temperature affect is the 
temperature change of the laser transition in the chip dependent on the 
modulation or, on the image signal that controls the modulation. Due to 
this temperature dependency of the laser transition, a temperature 
difference between substrate and laser transition continues to exist even 
when the housing temperature is regulated and an internal temperature 
compensation process is the consequence. Given a laser diode working in 
switched mode, this inner temperature compensation process leads to a 
variation of the output light power dependent on the image signal in such 
a way that the output light power rises above the nominal level in the 
respective turn-on time and then only gradually reaches the nominal level 
within the individual on-time intervals. Due to this effect, a disturbing 
lag effect that has a considerable influence on the recording quality 
arises on the recording medium when recording line information. 
Various measures for correcting the light power output of a laser diode are 
already known. 
For example, it has already been proposed to measure the light power output 
by the laser diode within the respective time interval required for the 
recording of a line and to control the light power via the driver current 
dependent on the measured result. 
GB Patent 21 01 851 likewise already discloses that the output light power 
be measured with a photodiode (monitor diode) integrated in the laser 
diode within or outside the time interval required for the recording of a 
line, to calculate correction values from the respectively measured light 
power, to intermediately store the correction values line-byline in 
sample-and-hold circuits, and to control the light power via the driver 
current dependent on the stored correction values. 
A regulation for correcting the light power output has the disadvantage 
that it is involved and that stability problems arise due to the control 
loops, and the switching speed of the laser diodes is reduced. 
It is therefore an object of the invention to specify a method and a 
circuit arrangement for driving a laser diode with which a simple 
correction of the output light power and an improved switching behavior 
are achieved. 
According to the method of the invention for driving a laser diode, the 
laser diode is charged by a driver current which defines an output light 
power, the output light power being dependent upon and dropping with a 
rise of internal temperature of the laser diode. For compensating the 
temperature-dependent drop in light power, the laser diode is driven by a 
voltage generator having a variable internal resistance R.sub.i. The 
internal resistance is calculated according to the equation: 
##EQU1## 
wherein .sigma. is the efficiency of the laser diode, T.sub.KU is a 
temperature coefficient based on an on-state voltage of the laser diode, 
T.sub.kp is a temperature coefficient based on a power of the laser diode, 
and R.sub.Di is an internal resistance of the laser diode, a rise of the 
driver current given rising temperature of the laser diode being limited 
by the internal resistance R.sub.i such that a rise in the output light 
power caused by a rise of the driver current compensates the 
temperature-dependent drop in light power. 
As a result of these techniques, it is particularly a correction of the 
disturbing lag effect that can be achieved given employment of the laser 
diodes in recording devices without deteriorating the switching speed and 
a good recording quality given high recording speed can be achieved 
overall. 
The invention shall be set forth in greater detail below with reference to 
FIGS. 1 through 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a fundamental exemplary embodiment a circuit arrangement for 
driving a laser diode. The circuit arrangement 1 for driving a laser diode 
2 is essentially composed of a voltage source 3 having a variable internal 
resistance R,. In switched mode, the voltage source 3 is controlled by a 
two-level image signal B which modulates the generator voltage U.sub.O of 
the voltage source 3 dependent on the information to be recorded. During 
continuous mode operation, the voltage source 3 outputs a constant 
generator voltage U.sub.O. 
The real laser diode 2 is represented in the equivalent circuit diagram as 
a series circuit composed of an ideal laser diode 2' and of the internal 
resistance R.sub.Di of the real laser diode. The generator voltage U.sub.O 
generates a driver current I.sub.T for the laser diode 2, this driver 
current I.sub.T being dependent on the generator voltage U.sub.O, on the 
on-state voltage U.sub.D of the laser diode 2' and the internal 
resistances R.sub.1 and R.sub.01 according to equation (1). 
##EQU2## 
The light power P output by the laser diode 2 is proportional to the driver 
current I.sub.T. A variation of the driver current .DELTA.T.sub.T leads to 
a variation of the output light power .DELTA.P according to equation 2. 
##EQU3## 
In equation (2), ".sigma." is the efficiency of the laser diode 2 that, 
for example, amounts to 0.3 through 1 mW/mA. 
Due to the temperature behavior of the laser diode 2 set forth in the 
introduction to the specification, the output light power P and the 
on-state voltage U.sub.D of the laser diode 2 drop with rising temperature 
T. 
A temperature change .DELTA.T leads to a modification of the output light 
power .DELTA.P.sub.U according to equation (3). 
##EQU4## 
In equation (3), T.sub.KP =dP/dT is the temperature coefficient referred 
to the power which and for example, can be -0.4 through -0.7 mw/.degree.C. 
A temperature-dependent change in the on-state voltage U.sub.D Of the laser 
diode 2 leads to a rise in the driver current I.sub.T due to the inventive 
drive of the laser diode 1 with a voltage source 3 according to equation 
(1). By varying the on-state voltage .DELTA.U.sub.D given a temperature 
change .DELTA.T, a variation of the driver current .DELTA.T.sub.t derives 
according to equation (4). 
##EQU5## 
In equation (5), T.sub.KU =dU.sub.D /dT is the temperature coefficient 
referred to the voltage which, for example, is -1.7 through -1.3 
mW/.degree.C. 
Due to the type of drive for the laser diode, the rise in the driver 
current IT due to the temperature-dependent decrease in the on-state 
voltage UD can be set such according to the idea of the invention with the 
variable internal resistance R.sub.1 such that the temperature-dependent 
drop of the light power P is corrected, whereby a constant nominal level 
is advantageously achieved in continuous mode operation and in the on-time 
intervals of switched mode. 
A compensation of the temperature-dependent drop in light power is achieved 
when .DELTA.P.sub.1 =-.DELTA.P.sub.U applies. With equation (4), the 
internal resistance R.sub.1 required for the compensation can be 
calculated therefrom according to equation (5) as follows: 
##EQU6## 
In practice, the calculated internal resistance R, can be positive or 
negative, so that the drive of the laser diode 2 must occur with a voltage 
source having positive or having negative internal resistance. Exemplary 
embodiments of a voltage source having positive and negative internal 
resistance are shown and described in FIG. 3. 
In diagrams FIGS. 2a), 2b), and 2c) for the switched mode, the 
chronological curves of the image signal B (diagram 2a), of the driver 
current I.sub.T (diagram 26 and the light power P output by the laser 
diode 2 (diagram 2c) are shown on the basis of the temperature 
compensation process at the laser transition set forth in the introduction 
to the specification given traditional drive of the laser diode. Diagram 
2c) shows that the output light power P respectively rises above the 
nominal power level (100%) at the turn-on time and only gradually reaches 
the nominal power level within the respective on-time intervals, as a 
result whereof the disturbing lag effect already likewise mentioned in the 
introduction to the specification arises on the recording medium. 
The result achieved by the inventive drive of the laser diode 2 is shown in 
diagram 2e) and 2f) . 
Diagram 2d shows the driver current I*.sub.T that occurs 2f voltage drive 
of the laser diode and diagram 2f shows the chronological curve of the 
corrected light power P*. 
FIG. 3a shows an exemplary embodiment of a voltage source having a 
variable, positive internal resistance R.sub.i that is composed of a wired 
operational amplifier 4. 
FIG. 3b shows an exemplary embodiment of a voltage source having a 
variable, negative internal resistance R.sub.i that differs from the 
exemplary embodiment shown in FIG. 3a in that the voltage source shown in 
FIG. 3b is followed by a circuit 5 that simulates a negative internal 
resistance. 
Although various minor changes and modifications might be proposed by those 
skilled in the art, it will be understood that I wish to include within 
the claims of the patent warranted hereon all such changes and 
modifications as reasonably come within my contribution to the art.