Laser diode supply circuit

Claim 1

A laser diode supply circuit, comprising: first and second voltage busses; a laser diode, having an optical power output related to current flow therethrough, connected in a first series circuit path with collector and emitter terminals of a first transistor between said first and second voltage busses, there being substantially no resistance in said first series path between the first and second voltage busses; a second transistor having collector and emitter terminals connected in a second series path with a collector load resistance between the first and second voltage busses; means coupling said second series path with a base terminal of said first transistor; a light sensitive device optically coupled with said laser diode and electrically connected with a resistance in a third series path between the first and second voltage busses; and means coupling said third series path with a base terminal of said second transistor such that in operation a control voltage is applied to said base terminal which is a proportional part of the voltage difference between said first and second voltage busses, said proportional part being determined by the optical power output of said laser diode; and wherein said second transistor is switched into conduction when control voltage exceeds an intrinsic threshold voltage value of said second transistor means to provide a feedback control of the current in said first series path and thereby to stabilize the optical power of said laser diode against a variation in voltage difference between said first and second voltage busses.