Coherent light generators – Particular component circuitry – Optical pumping
Patent
1996-12-04
1998-06-02
Scott, Jr., Leon
Coherent light generators
Particular component circuitry
Optical pumping
372 31, H01S 300
Patent
active
057612315
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention refers to the field of electronic reproduction technology and is directed to a method and to a circuit arrangement for regulating the luminous power of a laser diode in the optoelectronic recording element of a recording device, also referred to as a recorder or exposer, for the point-by-point and line-by-line recording of information on a recording material.
Such a recording element is essentially composed of a light source for generating a brightness-modulated light beam, of a drive circuit charged with an image signal for the light source, and of an objective arranged on the optical axis of the light source for focussing the light beam onto the recording material. The image signal contains the information to be recorded. The light beam modulated in brightness by the image signal undertakes the point-by-point and line-by-line exposure of the recording material, for example a film. The light power emitted by the laser diode is defined by a driver current that is generated in a drive circuit dependent on the image signal.
For exposing two-level black/white information (line information), the laser diode operates in switched mode wherein the light output by the laser diode is switched on and off by the two-level driver current or, respectively, by the two-level image signal (image data). In order to achieve a high recording quality, the level of the emitted light must be as constant as possible in the on-time intervals of the laser diode. Based on its very nature, a laser diode does not meet the demand for a constant light level in the on-time intervals since the emitted light power of a laser diode is temperature-dependent.
A method for regulating the luminous power of a laser diode is disclosed, for example, by GB Letters Patent 21 01 841. The light emitted by the laser diode is measured with a light detector and the measured signal generated by the light detector is forwarded as an actual luminous power value to a control circuit for the driver current of the laser diode via a feedback branch. In the control circuit, the actual luminous power value is compared to a predetermined rated luminous power value, and the corresponding repetitive errors are stored in a sample-and-hold circuit with the assistance of a sample clock sequence, being stored line-by-line in sample intervals that lie in front of the recording intervals required for the line-by-line recording. The stored repetitive errors are then supplied as correction values to a regulator that correspondingly regulates the driver current for the laser diode in the sample intervals.
In the known method, the temperature-dependent luminous power of the laser diode is only regulated in the sample intervals preceding the recording intervals but not in the recording intervals themselves. Disadvantageously, brief-duration fluctuations of the luminous power during the exposure within the recording intervals can therefore not be corrected, so that an optimum recording quality is not achieved Another disadvantage is comprised therein that the regulator must generate the entire driver current when the laser diode is switched on, a high control gain being required for this. Given a high control gain, however, there is the risk of hunting, so that fluctuations in luminous intensity that influence the recording quality can occur.
A method is also already known for regulating the luminous power of a laser diode wherein the driver current for the laser diode is composed of the regulator current generated in the regulator and of a variable bias. The bias shifts the operating point of the laser diode into the proximity of the current threshold at which the laser effect begins. The regulation then ensues in the linear characteristics range of the laser diode. When a constant bias is impressed on the laser diode, the luminous power output by the laser diode does not drop to zero with the laser diode shut off due to the operating point set by the bias since a light emission already occurs below the current threshold. This effect
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Hofmann Klaus
Ofenloch Thomas Philipp Jakob
Jr. Leon Scott
Linotype-Hell AG
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