Bell Communications Research, Inc.

William E Stephens

Thomas C Banwell

The laser driver circuit of the invention stabilizes the low operating output power level of a laser (301) modulated by a digital input signal, at a fixed preset value L.sub.o, by automatically adjusting the bias current, I.sub.b, supplied to the laser to compensate for variations in the laser threshold due to temperature changes and aging. The circuit includes a fast photodetector (401) which is responsive to the modulated light output of the laser. The photodetector output is combined (306, 308) with a data sensitive reference signal (i.sub.r) and a preset level setting input signal (i.sub.o) which is equal to .eta. times the desired low operating output power level L.sub.o, where .eta. is the coupling coefficient of the photodetector. Reference signal i.sub.r varies in accordance with the input data between a zero value when the input is ZERO and a value equal to what is estimated to be the photodetector output due to the data modulation when the input is ONE. The combination of the photodetector output, the data sensitive reference signal i.sub.r, and the level setting input signal i.sub.o, is a difference signal proportional by a factor of .eta. to the difference between the desired low operating output level and the estimated operating output level. This difference signal is amplified by an operational transconductance amplifier (402) having a large multiplicand, A.sub.o, when the input data is ZERO and a substantially smaller multiplicand, A.sub.1, when the input data is ONE. The output of the amplifier drives an integrating low frequency error amplifier 309 which generates at its output the laser bias current, I.sub.b, which is driven to a value such that the low operating power level of the laser is equal to L.sub.o as determined by the level setting input signal i.sub.o.

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Basking Ridge, NJ, United States of America

William E Stephens