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LASER DRIVER FEEDBACK CIRCUIT

IP.com Disclosure Number: IPCOM000024722D
Original Publication Date: 1981-Oct-31
Included in the Prior Art Database: 2004-Apr-02
Document File: 6 page(s) / 414K

Publishing Venue

Xerox Disclosure Journal

Abstract

Electronic printing will play an increasing role in future office systems. Although several technologies exist to print marks on paper from an all electronic front end, laser technology is becoming very popular. Especially with the promise of cheap and available solid state IR laser diodes which can be directly modulated. Unfortunately, these lasers do have some drawbacks, for instance: 0) They have a very steep power vs. current curve and for this reason must be used in a constant current mode. (2) They are not very stable; the output power is a complicated function of temperature which in turn is related to current duty cycle, and this therefore, implies some form of feedback system. (3) They must be completely turned off due to fluorescence in the region between zero current and below lasting threshold, and hence, the possibility of background when used with a special alloy photoreceptor. This means the laser diode cannot be biased to some level at or just below lasing threshold. Since the laser cannot be modulated about a quiescent or Q point, the driver system must be digital and therefore, nonlinear. The problem is the laser output power is a linear function of current and a complicated function of temperature and must be used in a digital fashion. (4-) These lasers are very susceptible to damage due to reverse currents or overcurrents, even if these transients are less than 1.0 ns. (5) The total driver/feedback system must be reasonably cheap in order to realize all the benefits of using the laser diode. (6) Finally, the circuits must be physically small since they must fit in some sort of optical cavity.

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LASER DRIVER FEEDBACK CIRCUIT James W. Lannom

Proposed Classification U.S. Cl. 358/296 Int. Cl. H04n 1/22

Electronic printing will play an increasing role in future office systems. Although several technologies exist to print marks on paper from an all electronic front end, laser technology is becoming very popular. Especially with the promise of cheap and available solid state IR laser diodes which can be directly modulated. Unfortunately, these lasers do have some drawbacks, for instance: 0) They have a very steep power vs. current curve and for this reason must be used in a constant current mode. (2) They are not very stable; the output power is a complicated function of temperature which in turn is related to current duty cycle, and this therefore, implies some form of feedback system. (3) They must be completely turned off due to fluorescence in the region between zero current and below lasting threshold, and hence, the possibility of background when used with a special alloy photoreceptor. This means the laser diode cannot be biased to some level at or just below lasing threshold. Since the laser cannot be modulated about a quiescent or Q point, the driver system must be digital and therefore, nonlinear. The problem is the laser output power is a linear function of current and a complicated function of temperature and must be         used in a digital fashion. (4-) These         lasers         are         very susceptible to damage due to reverse currents or overcurrents, even if these transients are less than         1.0 ns.         (5) The total driver/feedback system         must be reasonably cheap in order to realize all the benefits of using the laser diode. (6) Finally, the circuits must be physically small since they must fit in some sort of optical cavity.

The circuits of Figure I and 2 are proposed for use in driving solid state laser diodes. Use thereof depends upon two assumptions: 0) that a reference signal is available from the laser to be used which is a linear function of the main beam; and (2) there is a trigger signal supplied by the imaging engine when the main beam is known to be on yet o.ut of the imaging area.

The driver subsystem is actually composed of two separate parts. The first part is, of course, the laser driver. Its main function is to convert a logic signal to drive the laser diode in the preferred constant current mode while meeting all the requirements listed above. But beyond that, the driver has local feedback to hold the laser at the latest current setting given by the feedback circuit. The other half is the feedback circuit which is used to hold the laser power stable over both the short term instability due to heating effects and the long term drift due to aging.

Figures IA and IB show simplified diagrams of the laser driver, Figure lA showing the current flow throughout the circuit when the laser is on and Figure IB showing the current flow when the laser is off. The laser driver is composed of basically four active parts, gate 5 and transistors 6, 7,...