LASER DRIVER FEEDBACK CIRCUIT
Original Publication Date: 1981-Oct-31
Included in the Prior Art Database: 2004-Apr-02
Xerox Disclosure Journal
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.