IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-14, NO. 1, JANUARY 1978 17 Frequency Stability and Stabilization oLa Chemical

JESPER MUNCH, MARC A. KOLPIN, AND JUDAH WIN@

Abamct-We have built a low-power CW W/DF chemical lawr, Hinchen and Freiberg [3] reported a free-running frequency designed to achieve high-frequencystability. Measurements are reported stability of *1 part in 10' per 150 ms by analyzing the laser which characterize the inrtantaneour rpectnl width of the laser output output with a Fabry-Perot, and a stability of several parts in to lsy than one put in 10" (Av < 1 IrHz) and the variations absdute in 10' by locking the laser to dip. Wang [4] reported a frequency of w emhion with time to four puts in IO'O(AV = t2O ~r~z) Lamb's per 0.1 ma. Two expedments to actively rtabilize the loser frequency free-running laser frequency deviation of 30 MHz per 100 ms, are reported. In one expedment the laser waa locked to a hififinesse using the beat spectrum of two longitudinal modes in the Fabry-Perot to five puts in lo9 (Au = t250 IrHz) for many minutes. laser. In the other experiment one lPser WY locked to another using hetero- In the present paper, we will report the results of our mea- dyne beat to 1.7 puts lo9 (Av = t85 IrHz). The atabi- surements of the frequency deviations and stabilization of a Uzation experiments were limited by the feedback loops used. low-power CW chemical laser. These measurements show im- provements of nearly an order of magnitude over the above results. I. INTRODUCTION frequency stabilized chemical laser is a very useful tool 11. LASER DESIGN A in many aspects of laser research and applications, in- The general problem of frequency stabilization of a laser has cluding chemical laser diagnostics, spectroscopy, and laser been summarized by Bimbaum [SI. The fractional variation radar. A laser radar using a chemical laser amplifier is espe- in frequency of the output of a laser Av/v depends on the cially attractive due to the excellent atmospheric transmission fractional change in the optical length of the laser : in the DF laser band. The present work was motivated by such an application, where a frequency stabilized laser is re- -_-Av- AL ~ -An quired to serve as a driver for the amplifier and as a local vLn oscillator for the receiver. To be useful for some defensa where L is the length of the resonator and n the index of applications, the laser radar must have a reference laser with a refraction. Passive stabilization is accomplished by minimizing minimum frequency stability of one part in lo9 (total fre- variations in L and n through careful engineering, the stability quency excursion of 100 kHz) for periods of several milli- achievable by this method being limited mainly by vibration seconds, or longer. The objective of the present work is to and thermal expansion of the resonator, and by density fluc- demonstrate that a CW chemical laser can be stabilized to tuations in the medium. Active stabilization on the other hand Au/v = for periods of time equal to or greater than 1 ms. is accomplished by detecting a change in the frequency, and Prior to the present work, the chemical laser had been ob- subsequently adjusting L or n to remove that change. The served to operate on a single transition [l] but there were no stability achievable in this case depends on the minimum fre- measurements of the frequency stability characteristics of the quency change detectable, and the ability of the accompanying laser. Concurrent with the initial stages of the present effort, feedback loop to correct the change as quickly as it occurs. several authors reported measurements of the frequency char- In frequency stabilizing a laser one can follow one of two acteristics and active stabilization of chemical . Eng and approaches. One method relies on 'improving the passive Spears [2] reported a relative stabilization of Au = *SO0 kHz stability to the best practical level achievable, and then use a (Le., a total Au/u = one part in 10') on theP1(6) transition in comparatively simple feedback loop to remove whatever minor HF by using a heterodyne lock to the second harmonic of a instabilities and slow drifts remain. This method has been very stabilized CO laser, and an absolute stability of *7 parts in 10'. successfully applied to the COz laser [6]. The other method places the full burden of stabilization on the feedback loop, re- Manuscript received July 12,1977; revised September 16,1977. This quiring a high -bandwidth in the loop to remove all fre- work was supported in part under BMDATC Contract DASC60-75-C- 0046, MI. D. Schenk coatract monitor, and in part under TRW IR&D quency variations. This method has been very successful for funds. dye lasers [7] , [8]. Due to the uncertainties of the chemical J. Munch and M. A. Kolpin are with TRW DSSC, Redondo Beach, laser frequency characteristics, we chose the first method for CA 90278. J. Levine is with the Time and Frequency Division, National Bureau the initial careful laser design, and later applied more effort to of Standards, Boulder, CO 80302. the feedback loop.

001 8-91!?7/78/01OO-0017$00.75 0 1978 IEEE 18 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-14, NO. 1, JANUARY 1978

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?- : Fig. 1. Schematic of laser flow channel. Dimensions of the gain region are 5 mm x 6 mm X 100 mm.

The laser concept selected was that of Shirley et ul. [9] With the multiline configuration, simultaneous lasing on four which is a purely chemical device. This concept was selected to six transitions at a maximum outcoupled power of 1.4 W because control of density fluctuations within the lasing me- was observed. Single-line lasing on more than a dozen P,(J) dium is a fluid mechanical problem only. In other designs [l] andP2(J) lines with a maximum outcoupled power of 120 mW it was felt that the noisy electrical F2 dissociation schemes on P,(7) was observed using a Bausch and Lomb replica grating. would add unnecessary complications. The laser flow channel was carefully designed to ensure a 111. FREQUENCYCHARACTERISTICS quiet, laminar, subsonic flow. It consists of three regions, the The laser was constrained to operate on a single frequency mixing chamber, the flow channel, and the exhaust region (single longitudinal and transverse mode) by using a 0.5-m illustrated in Fig. 1. In the mixing chamber, fluorine and nitric long resonator and an intracavity aperture. By applying a saw- oxide react in the presence of helium diluent to produce free tooth voltage to the PZT crystal on the laser, the output fluorine atoms. The resulting mixture is accelerated down a power as a function of frequency was observed, verifying laminar flow channel to the lasing region where hydrogen (or single-mode operation, and displaying a prominent Lamb's deuterium) is injected to react with the free fluorine, thereby dip (Fig. 2). Homodyne spectroscopy of the laser output, producing the lasing inversion. Calcium fluoride windows, performed by spectral analysis of the output of a fast InAs mounted at Brewster's angle, couple the gain medium to an detector illuminated by the laser showed a spectrum with external optical resonator. A nitrogen purge is provided to detectable signal only below 400 Hz, and with a FWHM of less protect the windows from the fluorine. Typical operating than 100 Hz. The spectrum agreed well with the slow, 2 per- conditions are: 3.9 mmol/s Fz, 0.1 mmol/s NO, 0.7 mmol/s cent amplitude variations in the laser output intensity, and H2,9.7 mmol/s He, and 1 .O mmol/s N2. At a distance 37 mm confirmed that the laser was operating on only one indepen- downstream from the lasing zone, the temperature is 750 K dent optical frequency. This does not necessarily mean that md the pressure is 0.7 kPa (5 torr). the spectral content of the emission is a single frequency, From the introductory remarks, it is