CF4 and NOCl molecular lasers operating in the 16μm regionJoe J. Tiee and Curt Wittig Citation: Applied Physics Letters 30, 420 (1977); doi: 10.1063/1.89427 View online: http://dx.doi.org/10.1063/1.89427 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/30/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Operation of cleavedcoupledcavity Pbsalt diode lasers in the 4–5μm spectral region Appl. Phys. Lett. 44, 377 (1984); 10.1063/1.94780 16μm laser oscillation in propyne Appl. Phys. Lett. 39, 6 (1981); 10.1063/1.92519 Highenergy optically pumped molecular lasers in the 13 and 16μm regions Rev. Sci. Instrum. 50, 958 (1979); 10.1063/1.1135977 Optically pumped molecular lasers in the 11–17μm region J. Appl. Phys. 49, 61 (1978); 10.1063/1.324368 MOLECULAR LASER ACTION IN NITRIC OXIDE BY PHOTODISSOCIATION OF NOCl Appl. Phys. Lett. 9, 94 (1966); 10.1063/1.1754659

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CF4 and NOCI molecular lasers operating in the 16-,...m region *

Joe J. Tiee and Curt Wittig

Electrical Engineering Department. University of Southern California. University Park. Los Angeles. California 90007 (Received 22 November 1976; accepted for publication 15 February 1977)

We have obtained stimulated emission in the 16-/Lm region from CF4 and NOCI with an optical pumping scheme which uses a single CO2 TEA laser as the excitation source. Measured laser energies from CF4 and NOCI are 4 and 3 mI. respectively. which represents an order-of-magnitude improvement over exisiting molecular lasers oscillating in this region. There appear to be no fundamental limitations to scaling these lasers to energies ~ I 1. These lasers offer considerable promise for the laser isotope separation of uranium.

PACS numbers: 42.55.Hq. 28.6O.+s

The possibility of laser isotope separation (LIS) has spurred a rather substantial effort toward the genera­tion of molecular lasers operating in the wavelength regions of UF6 absorption, particularly the region near 16 /Jffi.l-6 In spite of considerable effort directed to­ward this problem from an array of highly competent engineers and SCientists, this problem has proved to be most formidable. In this paper, we report results of experiments which are aimed at generating line­tunable molecular laser emiSSion in the 16- /Jffi wave­length region, at pulsed energies which are acceptable for performing photochemical experiments. Using a Simple optical pumping technique, which employs a commercial CO2 TEA laser, we have obtained 4 and 3 mJ per pulse from CF4 and NOCI, respectively. in the 16-J.J.m region. This represents an order-of-magnitude improvement over the present state of the art, and there are no fundamental limitations to scaling such devices up to pulsed energies'" 1 J, using present CO2

laser technology. In a nondispersive laser cavity, we have observed stimulated emissions at 615 cm- l in the case of CF4 , and 598.5 and 588.5 cm-l in the case of NOCI. Since the emiSSion occurs between discrete vibrational levels, it will be possible to tune the emis­sion over rotational components of the vibration-rota­tion (VR) bands.

The experimental technique used for the generation of the far-infrared (FIR) laser emissions is optical pumping using a line-tunable CO2 TEA laser. A sche­matic drawing of the experimental technique is shown in Fig. 1. Many variations of the arrangement shown in Fig. 1 were used during the work reported here; nevertheless, the basic philosophical approach was preserved. The output from the TEA laser (nominally

FILTERS

FREOUENCY SELECTION, DETECTION, AND RECORDING APPARATUS

FIG. 1. Schematic drawing of the experimental arrangement. M1 and M2 are mirrors and BS is a beamsplitter.

420 Applied Physics Letters, Vol. 30, No.8, 15 April 1977

O. 5-1. 0 J, 50-150 nsec FWHM, multi mode ) is direct­ed through a cell which contains the appropriate molec­ular vapor(s). Two cell designs were used, the only essential difference being the lengths (350 and 90 cm) and the methods of cooling the cells (LN2 boil off for the 350-cm cell and various slushes for the 90-cm cell). The minimum achievable cell temperatures using LN2 boil off (350-cm cell) and various slushes (90-cm cell) are 155 and 180 OK, respectively. Details of the experiment will be reported in a future publication.

T 10"10

HORIZONTAL ,4 VERTICAL, 5

~

I I I I I

1080 lOGO ENERGy (em-')

1200 I I

1000 800 ENERGY (em-')

HORIZONTAL ,2 VERTICAL, 5 ~

NOCI

1 ) I I I

960 900 ENERGY (em-')

I I

1200 1000 800 ENERGy (em-')

600

600

FIG. 2. ir spectrophotometer traces showing transmitted intensity through a 10-cm cell versus frequency for CF4 (top) and NOCI (bottom). The pressures of CF4 and NOel are 100 and 20 Torr, respectively. Absorption regions which overlap available CO2 laser lines are shown with expanded scales.

Copyright © 1977 American Institute of Physics 420

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It is important to cool the medium, since the lower laser level is close to the ground state (435 and 332 cm-l for CF4 and NOCI, respectively). This places some restriction on the choice of laser species, but not a serious one. There are many Freons which absorb CO2 laser radiation and can be cooled to -150 OK while maintaining several Torr of vapor pressure. At 150 OK, the fraction of CF4 molecules in the lower laser state is only 1. 6%.

ir absorption spectra of CF4 and NOCI, obtained with a Beckman 4220 spectrophotometer, are shown in Fig. 2. The absorptions which overlap the CO2 laser spec­trum are not strong by any standard. It should be em­phasized, however, that a weak absorption coefficient at the pump laser frequency is not a fundamental limita­tion. By changing other parameters, such as absorp­tion length, gas pressure, pump pulse duration, etc., it is still possible to obtain high conversion efficiency, even in the case of a weak absorber.

In separate experiments, using a low-power line­tunable CO2 probe laser, we have measured absorption coefficients, near the laser line center, for laser lines which fall within the envelopes shown in Fig. 2. This is necessary, since CF4 and NOCI are simple molecules with resolvable VR structure, and the unresolved spec­tra shown in Fig. 2 only suggest that a true coincidence may be possible. For the case of CF4 , only (001)-(020) R(12) is absorbed appreCiably, and the measured ab­sorption coefficient is 1. 1 X 10-4 cm-l Torr- l • For the case of NOCI, (001)-(100) P(34) is absorbed apprecia­bly, and the measured absorption coefficient is 3.5 X 10-4 cm- t Torr-t • Notice that the observed FIR laser transitions also lie within regions of absorption as shown in Fig. 2. Again, this is consistent with the picture of sharp resolvable absorption lines rather than the broad features shown in Fig. 2. Since the CF4 and NOCI vapors are maintained at low temperatures in the present experiments, the sharpness of the absorption features is even more pronounced.

Detection is accomplished with photon drag, Ge:Cu, and pyroelectric detectors. FIR energies are mea­sured with a. pyroelectric detector which has been calibrated against a thermopile (Hadron mOdel 101) at CO2 laser wavelengths. The fastest response time of the FIR detector system (Ge:Cu loaded with 50 0) is

~ .-r-r-'-"-r-'~-'-'r-~ ::> a.. - ___ __ _ _ )-

is 1-+-+--+--+-+-Ml\.-4~'--I--+---j 1\

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I .'0

400 nsec

FIG. 3. CF4 laser output (arbitrary units) versus time. The short pulse occurring immediately after t = 0 is due to frequency-doubled CO2 laser radiation which is partially trans­mitted through the interference filter. This doubling occurs randomly on various surfaces in the optical path and is a con­venient reference. At higher CF4 laser energies, the pulse narrows to :::s100 nsec; at lower CF4 laser energies, the pulse lengthens to as much as 0.5 j.lsec.

421 Appl. Phys. Lett., Vol. 30, No.8, 15 April 1977

6 (ii ~

Z 5 ::>

ai ~ 4

~ 3f-a. ~ ::J o 2 0:: W Vl <t If-..J

..... I u I I 11\

160 200 ~240 TEMPERATURE (OK)

FIG. 4. CF4 laser output (arbitrary units) versus temperature. These data were taken using the 90-cm cell with a pulsed CO2 laser energy of 600 mJ, 150-nsec FWHM, oscillating on the (001)-(020) R(12) transition.

- 70 nsec. Thus, it is not possible to follow rapid FIR laser fluctuations such as those observed on the partial­ly mode-locked CO2 pump pulse when a photon drag detector is used. Frequencies are measured with a O. 25-m Jarrell-Ash monochromator. The monochroma­tor is calibrated with CO2 laser lines and is accurate to ± 1 cm-!,

Stimulated emission from CF4 was observed over a wide range of conditions. They included variation in cavity length, output coupling, operating pressure, and cell temperature. Maximum output of 4 mJ was ob­tained with the 350-cm cell maintained at 155 OK with 3.5 Torr of CF4, Under these conditions, stimulated emission was superfluorescent (M1 removed, M2 = 10-m-radius Au mirror), and the emission frequency was 615 ± 1 cm- l

• Even in the absence of superfluorescence (smaller cell and/or higher cell temperatures), we observed only this laser frequency in a non-frequency­dispersive cavity.

The time behavior of the CF4 laser emission, as mea­sured with the Ge:Cu detector, is shown in Fig. 3. The delay between the CO2 pump pulse and the FIR pulse is not atypical of optically pumped lasers, 7 Pulse shape, duration, and delay fluctuate from pulse to pulse, Pulses of maximum energy are also of the shortest duration (FWHM "" 100 nsec) and have the shortest delay time ("" 100 nsec). Under optimum conditions, the amount of CO2 laser energy deposited in the medium is estimated to be 150 mJ. This corresponds to a con­version efficiencyB of almost 3%. Based on our own experiences, we feel that with improved optical cou­pling, cell design, and CO2 laser performance, con­version of CO2 laser energy to the FIR laser energy can exceed 10%. Thus a CF4 laser of - 1 % total electri­cal efficiency operating in the 16-/.LIl1 region appears quite feasible.

Using the gO-cm cell and various slushes, it was possible to measure the effect of temperature on the laser output (Fig. 4). These data indicate the dramatic effect of changing the lower laser level thermal popula­tion. We observed little effect of temperature variation on the absorption of the CO2 laser. With the 350-cm cell, the variation of CF4 laser output with temperature

J.J. Tiee and C. Wittig 421

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showed the same monotonic increase with decreasing temperature but the variation was less pronounced than with the 90-cm cell.

In the NOCI laser experiments, we obtained results which were qualitatively similar to the case of CF4 with the following important distinct features. (1) The NOCllaser typically operated at lower pressures than the CF4 laser. The operating range was 0.2-2.0 Torr, with optimum laser energy obtained at 1. 0-1, 5 Torr, This is in contrast to the CF4 laser where oscillation was obtained over the range 0.5-9.0 Torr with optimum energy obtained at 3.5 Torr. (2) Conversion efficiency was higher in the case of NOCI than CF4 , The CO2

laser only produced 40% as much energy at P(34) for pumping NOCI as it did on R(12) for pumping CF4 , Since we were able to deposit the same fraction of laser energy in the medium, the conversion of deposited CO2

energy to NOCllaser energy is ,., lOst. (3) Optimum operating temperature is 220 oK (NOCI freezes at 208.5°K), (4) Laser emission on both P- and R-branch transitions, at 588.5 ± 1. 0 and 598.5 ± 1. 0 cm-t, respec­tively, were observed. Within the resolution of our measurements, these transitions share a common upper state. The R-branch transition was stronger, indicating the important role of accidental coincidences between the laser frequency and unexcited molecules, (5) The NOCI laser has a much lower pump threshold than the CF4 laser.

Obviously, all these features cannot be attributed to anyone factor, and will only be explained when more information concerning the dynamic processes involved are made available.

In conclusion, we have demonstrated that efficient high-power laser sources in the 16-J..L1l1 region can be obtained by optically pumping suitable molecular gases

422 Appl. Phys. Lett., Vol. 30, No.8, 15 April 1977

with a conventional CO2 TEA laser, This technique is of fundamental simplicity and with present CO2 laser technology these lasers can be easily scaled to energies ~ 1 J, To emphasize this aspect, we should like to point out that a 200-J CO2 laser, optically pumping a cell, 30 cm in diameter by 350 cm, will deposit precisely the same amount of energy per unit volume as in our pres­ent configuration (in which we have obtained 4 mJ from CF4), resulting in 750 mJ of 16 /lllliaser output. By increaSing the length of the tube to 10-100 m, we believe that much higher conversion efficienCies can be achieved, although at the expense of spatial coherence. Thus it is our firm conviction that there exist no junddmental limitations to scaling the CF4 laser to energies ~ 1 J. Experiments where a dispersive ele­ment such as a grating is introduced in the cavity and where high-pressure buffer gases are added to the laser medium are presently under investigation. These ex­periments should provide information concerning the tuning range of these lasers.

*Research supported by the U. S. Energy Research and Development Administration.

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3M.I. Buchwald, C.R. Jones, H.R. Fetterman, and H.R. Schlossberg, Appl. Phys. Lett. 29, 300 (1976).

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6W.E. Barch, H.R. Fetterman, and H.R. Schlossberg, Opt. Commun. 15, 358 (1975).

7T.A. De Temple, T.K. Plant, and P.D. Coleman, Appl. Phys. Lett. 22, 644 (1973).

8This conversion effiCiency is defined as the ratio of the 16-Mm laser energy to the amount of CO 2 laser energy absorbed by the gas.

J.J. Tiee and C. Wittig 422

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