| Popis: |
Short term laser frequency stability is an important parameter affecting target velocityresolution of a coherent laser radar (ladar) system. This paper presents CO2 laser designconsiderations which affect frequency stability, measures the frequency sensitivity of thelaser to various stimuli, and evaluates performance of an actively stabilized, unmodulatedoutput CO2 laser. A frequency modulated acousto -optic modulator is used to produce a con-trol beam which is passed through a low pressure SF6 gas cell and the laser is locked tothe resultant saturated absorption peak. This investigation demonstrated laser frequencyinstability for periods up to 50 msec of less than +500 Hz when the piezoelectric trans-ducer (PZT) drive voltage was off, and about +900 Hz when the laser was free running withvoltage applied to the PZT. When a 90 dB, 200 Hz acoustic disturbance was applied, thefrequency instability was less than +4.3 kHz and +2.8 kHz when the laser was free runningand locked, respectively.IntroductionThe evolution of high velocity resolution CO2 laser radars from the laboratory to air-borne platforms for two -dimensional wind field measurements) and, perhaps, to space plat-forms for global wind measurements2,3 necessitates investigations of techniques to main-tain high frequency stability operation of lasers in dynamic environments. Specifically,for space platforms tens of kilohertz frequency stability must be maintained for tens ofmilliseconds to achieve submeter per second wind velocity resolution. This paper willpresent the results of an initial investigation evaluating a technique capable of providingthis level of laser frequency stability in dynamic environments.Excellent CO2 laser frequency stability has been demonstrated in laboratories using thefollowing techniques: Passive stabilization,4 -6 locking two lasers together by maintain-ing a constant difference frequency between them,6 locking the laser to the gain profileof the active medium,7 locking the laser to a reference cavity,7 locking the laser to theLamb dip of a low pressure active medium,8 and locking the laser to the saturated absorp-tion peaks in the following low pressure gases: CO2,9 SF6'10 -11 and 0304. 11-12 Using CO2assures that the absorption wavelength is within the laser gain profile; whereas, for othergases wavelength coincidences must be sought. The frequency stability achieved with eachof these methods is listed in Table 1.Table 1. CO2 Laser Frequency Stability Achieved With Different Stabilization MethodsSTABILIZATION SHORT TERM LONG TERMMETHOD STABILITY STABILITY REFERENCEPassive, Sealed 9 kHz (30 msec) |
