Operating and control system for lasers useful in bar code scanners

Abstract

In order to set the output optical power of a laser diode precisely and under digital control without expensive digital to analog conversion devices, a digital controller provides a sequence of pulses. A capacitor is charged by these pulses to produce a control voltage which controls the current through the laser diode via an operational amplifier driver stage. The output power obtained from the laser as detected by a photodetector, which is optically coupled to the laser and may be packaged therewith, is monitored by the computer so as to determine the rate at which laser power changes and indirectly at the rate at which the capacitor charges. This rate may vary depending upon environmental conditions and manufacturing tolerances. Based upon the charging rate, a pulse is generated and used to either provide additional charge or to charge the capacitor to the voltage corresponding to the desired optical power output. Thus, relatively low cost capacitors and other components which may have values of capacitance, resistance and the like, which vary from component to component and with temperature, aging and other environmental effects may be used in providing a digitally controlled and selected laser optical output. In a bar code scanner the operating pulses may be generated at the start of each scan or at the start of a sequence of pulses in a pulse train utilized to locate bar codes prior to initiation of scanning.