Active energy control for diode pumped laser systems using pulsewidth modulation

• US 5,291,505 A
• Filed: 01/21/1993
• Issued: 03/01/1994
• Est. Priority Date: 01/21/1993
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Nielsen, Keith E.
• Primary Examiner(s): Scott, Jr., Leon
• Application Number:
  US08/006,557
• Time in Patent Office: 404 Days
• Field of Search: 372/38, 372/25, 372/10, 372/29, 372/26
• US Class Current: 372/38.03
• CPC Class Codes: H01S 3/094076 Pulsed or modulated pumping H01S3/1024 takes precedence
• CPC Class Codes: H01S 3/0941 of a laser diode
• CPC Class Codes: H01S 3/10046 Pulse repetition rate control H01S3/11 takes precedence
• CPC Class Codes: H01S 3/115 using intracavity electro-optic devices
• CPC Class Codes: H01S 3/1312 by controlling the optical pumping

Abstract

An active energy control circuit and method employing pulsewidth modulation for use with diode pumped laser systems. The active energy control circuit and method employs optical energy feedback sensing and output control of the electrical pulsewidth to pump diodes of the laser system. This creates an active energy control loop for maintaining diode pumped laser output energy at a desired level over different pulse repetition frequencies, time, and environmental conditions. The diode pumped laser output energy is controlled by varying the pulsewidth of the output energy pulse while maintaining a current amplitude set point. By using a pulsewidth-modulation-based control system, the current applied to the pump diodes is regulated at an efficient set point below their damage level, and the pulsewidth is adjusted to maintain a desired output optical energy of the laser system. The pulse width modulated output energy control scheme provided by the present invention is appropriate for all diode pumped laser applications including laser rangefinders, laser designators, laser radar systems, laser welding systems, and other low and high power laser systems.

Claims

1. An active energy control circuit for use with a diode pumped laser system comprising a laser cavity, a laser rod, a Q-switch, a Q-switch firing pulse circuit, and a current controlled pulse drive circuit coupled to a plurality of pump diodes, and wherein the Q-switch generates an output energy pulse in response to a firing signal provided by the Q-switch firing pulse circuit, said control circuit comprising:

optical to electrical sampling means for sampling the output energy of the diode pumped laser system and for converting the sampled output energy into a digitized energy signal;

a controller comprising;

pulse duration logic for setting a desired initial pulse duration of the output energy pulse of the diode pumped laser system;

threshold logic coupled to the optical to electrical sampling means for comparing the digitized energy signal comprising the sampled output energy of the laser system to a predetermined energy threshold, and for providing output signals that are adapted to increase or decrease the pulse duration of the pulse provided by the current controlled pulse drive circuit depending upon whether the sampled output energy is below or above the predetermined energy threshold, respectively;

pulse duration output logic coupled to the current controlled pulse drive circuit for providing an output signal that is adapted to set the pulse duration of the output energy pulse in the pulse drive circuit;

laser firing logic coupled to the pulse duration output logic and to the Q-switch firing pulse circuit for causing the firing pulse circuit to generate the firing signal that is adapted to switch the Q-switch and generate the output energy pulse from the laser system; and

whereby the controller is adapted to maintain the output energy of the diode pumped laser system at a desired level.

2. The control circuit of claim 1 wherein the optical to electrical sampling means comprises:

a beamsplitter adapted to sample a portion of the output energy of the laser output pulse;

an attenuator/diffuser adapted to attenuate and diffuse the laser output pulse to a predetermined and uniform level;

an energy detector for detecting the attenuated laser output pulse and convert it to an electrical pulse;

a pulse stretching circuit for electrically stretching the length of the pulse to provide a stretched pulse;

analog to digital converter for converting the stretched pulse into a digital signal for processing by the controller; and

a sample control circuit adapted to transfer the digital signal from the analog to digital converter to the controller.

3. An active energy control circuit for use with a diode pumped laser system comprising a laser cavity, a laser rod, a Q-switch, a Q-switch pulse circuit, and a current controlled pulse drive circuit coupled to a plurality of pump diodes, said control circuit comprising:

optical to electrical sampling means for sampling the output energy of the diode pumped laser system and for converting the sampled output energy into a digitized energy signal;

a controller comprising;

repetition rate logic for setting a repetition rate of the laser system;

pulse duration logic for setting a desired initial pulse duration of the output energy pulse of the pump diodes;

threshold logic coupled to the optical to electrical sampling means for comparing the digitized energy signal comprising the sampled output energy of the laser system to a predetermined energy threshold, and for providing output signals that are adapted to increase or decrease the pulse duration of the pulse provided by the current controlled pulse drive circuit depending upon whether the sampled output energy is below or above the predetermined energy threshold, respectively;

pulse duration output logic coupled to the laser firing logic and to the current controlled pulse drive circuit for providing an output signal that is adapted to set the pulse duration of the output energy pulse in the pulse drive circuit;

laser firing logic coupled to the pulse duration output logic and to the Q-switch firing pulse circuit for causing the firing pulse circuit to generate the firing signal that is adapted to switch the Q-switch and generate the output energy pulse from the laser system; and

whereby the controller is adapted to maintain the output energy of the diode pumped laser system at a desired level.

4. The control circuit of claim 3 wherein the optical to electrical sampling means comprises:

a beamsplitter adapted to sample a portion of the output energy of the laser output pulse;

an attenuator/diffuser adapted to attenuate and diffuse the laser output pulse to a predetermined and uniform level;

an energy detector for detecting the attenuated laser output pulse and convert it to an electrical pulse;

a pulse stretching circuit for electrically stretching the length of the pulse to provide a stretched pulse;

analog to digital converter for converting the stretched pulse into a digital signal for processing by the controller; and

a sample control circuit adapted to transfer the digital signal from the analog to digital converter to the controller.

5. A method for use with a diode pumped laser system comprising a laser cavity, a laser rod, a Q-switch, a Q-switch pulse circuit, and a current controlled pulse drive circuit coupled to a plurality of pump diodes, that is adapted to provide for active energy control of the laser system, said method comprising the steps of:

setting a desired initial pulse duration value for the output energy pulse of the pump diodes;

sampling the output energy of the diode pumped laser system;

converting the sampled output energy into a digitized output energy signal;

comparing the digitized output energy signal to a predetermined energy threshold;

providing a control signal indicative of increasing or decreasing the pulse duration value of the pump diode output energy pulse depending upon whether the sampled output energy is below or above the predetermined energy threshold, respectively;

generating a new pump diode output energy pulse based upon the increased or decreased pulse duration value of the control signal; and

generating a new laser energy pulse from the energy stored in the laser rod from the prior pump diode pulse, which produces a corresponding increase or decrease in the energy of the new laser energy pulse;

whereby the output energy of the diode pumped laser system is maintained at a desired level.

6. The method of claim 5 wherein the step of generating a new laser output energy pulse comprises the steps of:

coupling the control signal to the current controlled pulse drive circuit; and

switching the Q-switch in response to the control signal.

7. The method of claim 5 wherein the step of generating a new laser output energy pulse comprises the steps of:

setting a new pulse duration value in the current controlled pulse drive circuit based upon the increased or decreased pulse duration value of the control signal that is adapted to increase or decrease the pulsewidth output of the laser pump diodes; and

switching the Q-switch in response to the control signal.