Laser stabilization technique

Claim 2

In combination with a laser for providing a beam of monochromatic coherent radiation in response to a discharge current, a control circuit for stabilizing said beam at a predetermined set point energy level, said control circuit comprising in combination detector means coupled to said laser for providing a first signal having a voltage which tends to track any variations in the energy level of said beam, said detector means including a photodetecting diode positioned to intercept said coherent light beam for providing a current proportional to the energy level of said beam, and a buffer amplifier responsive to the current from said photodetecting diode for providing said first signal, said buffer amplifier including an operational amplifier having an inverting input, a non-inverting input and an output, comparator means including a differential amplifier having one input coupled to receive said first signal and another input held at a reference voltage corresponding to said set point energy level of said beam for comparing said first signal against said reference voltage level to provide an error signal having a voltage proportional to any difference between the energy level of said beam and said set point level, integrating means including a capacitor coupled to said comparator means to provide a feedback signal having a level proportional to a time-average value of said error signal, means coupled between said integrating means and said laser for applying said feedback signal to said laser for adjusting said discharge current to thereby stabilize said beam at said set point level, means for biasing the non-inverting input of said operational amplifier at said reference voltage level, a first feedback loop including a resistor connected from the output to the inverting input of said operational amplifier, means for biasing the inverting input of said operational amplifier at a predetermined level, and a diode connected to said inverting input for neutralizing the dark current through said photodetecting diode so that the dark current is prevented from affecting the feedback signal.

Claim 3

The control circuit of claim 1 further including a second feedback loop for said amplifier; said second loop including a second transistor having a collector-emitter circuit connected between the output and the inverting input of said amplifier in parallel with said first feedback loop and a base electrode biased to cause said second transistor to switch into conduction when the output of said amplifier reaches a predetermined pre-saturation level, whereby saturation of said amplifier is avoided.

Claim 4

The control circuit according to claim 2 wherein said differential amplifier includes bypass means connected to said one input for bleeding off any transient currents accompanying said first signal.

Claim 5

The control circuit according to claim 3 wherein said bypass means includes a transistor.

Claim 6

The control circuit according to claim 3 wherein said bypass means iNcludes a diode connected to the output of said differential amplifier, said diode being poled to block any such transient currents.

Claim 7

In combination with a laser for providing a beam of monochromatic coherent radiation in response to a discharge current, a control circuit for stabilizing said beam at a predetermined set point energy level, said control circuit comprising in combination detector means coupled to said laser for providing a first signal having a voltage which tends to track any variations in the energy level of said beam, said detector means including photodetecting means positioned to intercept said coherent light beam for providing a current proportional to the energy level of said beam, and a buffer amplifier responsive to the current from said photodetecting means for providing said first signal, comparator means including a differential amplifier having one input coupled to receive said first signal and another input held at a reference voltage corresponding to said set point energy level of said beam for comparing said first signal against said reference voltage level to provide an error signal having a voltage proportional to any difference between the energy level of said beam and said set point level, integrating means including a capacitor coupled to said comparator means to provide a feedback signal having a level proportional to a time-averaged value of said error signal, and means coupled between said integrating means and said laser for applying said laser for adjusting said discharge current to thereby stabilize said beam at said set point level, said means for applying said feedback signal to said laser including means for setting the discharge current of said laser at a nominal level below a predetermined threshold level at which said laser is activated to emit said light beam, and means for algebraically combining the time averaged error signal with the nominal discharge current for said laser to thereby stabilize the energy level of said beam, said combining means including a first transistor having a collector-emitter circuit connected to provide a nominal load impedance for series current flow of said discharge current, and a second transistor having a collector-emitter circuit connected in parallel with at least a portion of the collector-emitter circuit of said first transistor, said second transistor being coupled to said integrating means to respond to said time-averaged error signal for varying the impedance to said current flow as a function of said time-averaged error signal.

Claim 8

In combination with a laser for providing a beam of monochromatic coherent radiation in response to a discharge current, a control circuit for stabilizing said beam at a predetermined set point energy level, said control circuit comprising in combination detector means coupled to said laser for providing a first signal having a voltage which tends to track any variations in the energy level of said beam, said detector means including photodetecting means positioned to intercept said coherent light beam for providing a current proportional to the energy level of said beam, and a buffer amplifier responsive to the current from said photodetecting means for providing said first signal, comparator means including a differential amplifier having one input coupled to receive said first signal and another input held at a reference voltage corresponding to said set point energy level of said beam for comparing said first signal against said reference voltage level to provide an error signal having a voltage proportional to any difference between the energy level of said beam and said set point level, integrating means including a capacitor coupled to said comparator means to provide a feedback signal having a level proportional to a time-averaged value of said error signal, and means coupled between said integrating means and said laser for applying said laser for adjusting said discharge current to thereby stabilize said beam at said set point level, said means for applying said feedback signal to said laser including means for setting the discharge current of said laser at a nominal level below a predetermined threshold level at which said laser is activated to emit said light beam, and means for algebraically combining the time averaged error signal with the nominal discharge current for said laser to thereby stabilize the energy level of said beam, said combining means including a first transistor having a collector-emitter circuit connected to provide a nominal load impedance for series current flow of said discharge current, and a second transistor having a collector-emitter circuit connected in parallel with at least a portion of the collector-emitter circuit of said first transistor, said second transistor being coupled to said integrating means to respond to said time-averaged error signal for varying the impedance to said current flow as a function of said time-averaged error signal.

Claim 9

The control circuit of claim 6 further including modulating means coupled to said second transistor for switching said second transistor between a conductive and a non-conductive state to thereby modulate the energy level of said beam.