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Owned Patent for Publication in the STAR National Aeronautics and Space Administration ‘Washmgton. 0 C 20546 TO : CASI/Thelma Mosley FROM : GP/Jean Riordan SUBJECT: NASA-Owned Patent for Publication in the STAR The enclosed NASA patent is presented for abstracting and publication in the STAR. Issue Date: --4a Patent Number: q/aKf7v9 NASA Case Number: Me/977a-/ Government or Contractor Inventor: Thank you. ean Riordan Enclosure (1) I11111 11111111 Ill 11111 11111 11111 lllll1111111111111111111111ll1ll111111111ll US005 128949A United States Patent [ii] Patent Number: Barnes [45] Date of Patent: Jul. 7, 1992 METHOD AND CIRCUIT FOR IEEE Journal of Quantum Electronics, vol. QE-17, CONTROLLING THE EVOLUTION TIME No. 9 (Sep. 1981), pp. 1656. INTERVAL OF A LASER OUTPUT PULSE Primary Examiner-William L. Sikes Inventor: Norman P. Barnes, Tabb, Va. Assistanr Examiner-Robert E. Wise Assignee: The United States of America as Attorney, Agent, or Firm-Harold W. Adams; George F. represented by the Administrator of Helfrich the National Aeronautics and Space Administration, Washington, D.C. 1571 ABSTRACT Appl. No.: 359,4450 The invention, a method and circuit for controlling the evolution time interval of a laser output pulse, com- Filed: May 31, 1989 prises means for pumping a laser medium 13 in a resona- Int. ................................................ HOlS 3/10 tor 10 that includes a Q-switch 22 and polarizer 26 that US. Cl. ......................................... 372/25; 372/30 act in combination to control the loss in the resonator Field of Search .................................... 372/25, 30 10. A photodiode 11 senses the resulting fluorescence 12 which is applied to a two level Q-switch driver 27 the References Cited output of which selectively adjusts the level of loss in PUBLICATIONS the Q-switch 22 and polarizer 26 from high to interme- Koechner, Solid State Laser Engineering, pp. 426-43 1, diate to substantially zero loss states to control the evo- Jan. 1976. lution time interval of the resulting laser output pulse. Optics Communications, vol. 65, No. 4, (Feb. 1988), pp. 283-286. 13 Claims, 2 Drawing Sheets 16 U.S. Patent July 7, 1992 Sheet 1 of 2 5,128,949 U.S. Patent July 7, 1992 Sheet 2 of 2 5.128,949 2 pumping level is too low, the laser may never come to METHOD AND CIRCUIT FOR CONTROLLING threshold and lasing preventing altogether. In addition, THE EVOLUTIOA’ TIME INTERVAL OF A LASER this method introduces an indeterminacy or jitter into OUTPUT PULSE the time at which the laser output pulse occurs. 5 As the output pulse evolution time interval depends ORIGIN OF THE INVENTION on the level of the pump, the time at which the pulse The invention described herein was made by an em- occurs may be set by the pumping level rather than an ployee of the US. Government and may be manufac- external clock. While external clocks are known to be, tured and used by or for the Government for govern- highly accurate, the level of pumping is usually not mental purposes without the payment of any royalties 10 accurately known or controllable. Thus, though any thereon or therefor. resulting jitter may not be much, probably on the order of a microsecond or so, it may cause problems if precise CROSS-REFERENCE TO RELATED ranging is to be performed. APPLICATIONS This application is related to copending application 15 OBJECT AND SUMMARY OF THE INVENTION Ser. No. 359,459 filed May 31, 1989, entitled “Method An object of the invention is to provide a method and and Circuit for Shaping Laser Pulses” now US. Pat. circuit for controlling the evolution time interval of a No. 4,959,838. laser output pulse and more particularly to such a BACKGROUND OF THE INVENTION method and circuit for regulating the spectral band- 20 width of laser pulses used for making spectral measure- 1. Field of the Invention ment. The invention is a method of and circuit for control- In accordance with the invention, the level of a pump ling the evolution time interval of a laser output pulse is sensed and the loss level is regulated in the laser. By and more particularly to such a method and circuit for regulating the loss level in relation to the pump, the producing narrow spectral bandwidths for use in mak- 25 pulse evolution time interval may be regulated more ing precise spectral measurements. accurately. This is achieved by making the pulse evolu- 2. Description of the Prior Art tion time interval, and thus the number of round trips, A recurring problem confronting lasers used for pre- independent of the level of the pump at least through cise spectral measurements is the need for a narrow first order. Thus improving the performance of the laser linewidth laser. A known approach to this problem is 30 while allowing the time of switching to be set by an the use of a self-injection locking lasers in which a laser external clock rather than by the level of the pump. is operated in a high loss condition during the initial portion of the pulse evolution time interval. By operat- The method and circuit in accordance with this in- ing in a high loss condition, the resulting laser output vention in general includes means for achieving the controlled, two level switching a Q-switch to pro- pulse will have made many round trips through a laser 35 of resonator. On each round trip, line narrowing elements vide a narrow line width laser pulse without a severe operate on the radiation in the laser pulse to succes- loss in efficiency. Means are provided for varying the sively narrow its spectral linewidth. Thus, by increasing switching voltage of the two level Q-switching system the loss, and thus the number of round trips, the line- to allow for varying levels of gain in an active laser or width of the laser becomes narrower. 40 medium in a laser resonator. Timing means permit vary- Once a small laser pulse has evolved in a high loss ing the timing of the two level Q-switching action to resonator, the loss is switched in a single step or level achieve the best performance based on both the result- action to a low loss state. In the low loss state, the ma- ing spectral line width and laser output energy. jority of the energy is extracted in an efficient manner BRIEF DESCRIPTION OF THE DRAWINGS while retaining the narrow spectral linewidth of the 45 initial pulse. Typically, a Q-switch, either an electro-op- The above and numerous other objects and advan- tic or an acoustic-optic model, is used to effect switch- tages of the invention will become apparent from the ing to a low loss condition. following Detailed Description when read in view of While this method of single level switching is opera- the appended drawings wherein: ble, the time at which the laser is switched directly from 50 FIG. 1 is an electrical schematic of a preferred em- a high loss condition to a low loss condition is a critical bodiment of the invention including electro-optical limitation. If the single level of switching occurs too means for sensing the pumping level of a laser device early, insufficient round trips will have been completed and a two level driver for a Q-switch; and leaving a wider than desired Iinewidth. Thus the output FIG. 2 is an electrical schematic of an alternative pulse evolution time interval is too short. 55 embodiment of the invention illustrated in FIG. 1 in- In the known method, this effect is counteracted cluding means for sensing and using a laser output to through the use of an optical sensor to detect the level control the switching action of the two level driver for of the output laser radiation from an output element the Q-switch. such an output mirror. When the output laser radia- as BRIEF DESCRIPTION OF THE PREFERRED tion builds to a sufficient level, such an optical sensor 60 detects this level and switches the laser to a low loss EMBODIMENT condition in a single level switching action. Although In many laser resonator systems which may include this is an improvement, it does not directly regulate the self-injection locking lasers, spectral line width is deter- number of round trips required to achieve the desired mined by the physical characteristics of the laser me- spectral linewidth. 65 dium. In such arrangements, a conventional Q-switch in Thus, if the pumping level is too high, a laser output combination with a polarizer is either in a high loss or a pulse will evolve in the resonator quickly, thus counter- low loss state determined by the voltage applied to the acting the effect of the line narrowing elements. If the Q-switch. In the high loss state, laser oscillation or laser 5,128,949 3 4 action is prevented by the high loss associated with the situation to an intermediate loss situation. In this config- Q-switch and polarizer. When the Q-switch is switched uration, the round trip gain is relatively low. That is, the in a single switching action or level directly to the low gain employed by the nascent laser pulse in the active loss state. a laser pulse evolves. The ensuing pulse evo- laser 13 medium is offset to a large degree by the losses lution time interval is determined by the gain of the 5 produced by the Q-switch 22. In this situation, many active laser medium and the loss in the laser system. round trips are required for the laser to evolve. As such, While the laser pulse is evolving, the spectral line the spectral line width is narrow.
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