Progress in optically pumped CO transfer lasers

Volume 18, n u m b e r 1

OPTICS COMMUNI('ATIONS

in the infrared. Here, we report using optical p u m p i n g with a hydrogen halide laser followed by vibrational energy transfer in the p u m p e d gas to obtain laser oscillation near 16 u m ~ a wavelength region o f interest because of its possible applicability to laser isotope separation of actinides [ 1 ]. COLLISION

l l ~ ~

I

v=l

0220

4.1 ~.m

0000

v: 0

~

CO 2

HBr

Fig. 1. S u m m a r y of physical processes involved in s i x t e e n m i c r o m e t e r CO 2 laser. Levels designed by 1 and II are the Fermi m i x e d C O 2 l e v e l s [10 0 0 , 0 2 0 0 ] l a n d [10 0 0, O2 0 0 ] i i , respectively. Fig. 1 illustrates the physical principles of the 16 p m laser. The o u t p u t in the v = 1 ~ u = 0 band from a high energy HBr TEA laser is used to excite a cooled, low pressure m i x t u r e of HBr and CO 2 gases. The HBt in the gas m i x t u r e resonantly absorbs this radiation and coUisionally transfers the e n e r g y via vibrational energy exchange to the 0001 m o d e o f CO 2. A 9.6 ~ m pulse from a CO 2 TEA laser stimulates the energy into the Fermi m i x e d [1000, 0200] II level. Sixteen micrometer laser

12-i, tl

i--m :,.~ -VACUUM

M°'ECE'L\ t ~.--~-Jl

SYSTEM =

I

!

/

1/

sox

[ 1 ] P. Robinson, Proc. New York Acad. Science (2 April 1975, to be published).

PROGRESS IN OPTICALLY PUMPED CO T R A N S F E R LASERS * T.F. DEUTSCH and H. K I L D A L

~-9 A n u m b e r of optically p u m p e d lasers using energy transfer from CO e x c i t e d by a frequency doubled CO 2 TEA laser have recently been demonstrated. The lasing molecules included OCS, C2H 2, CS2, N 2 0 and CO 2 [1 ]. The optically p u m p e d transfer laser was originally d e m o n s t r a t e d using an HBr TEA laser to p u m p

,i : ,[ MONOCHROMATOR k--

" ",-re

.i-/ COi LASER

WITH GRATING

Fig. 2. A p p a r a t u s used for s i x t e e n m i c r o m e t e r CO 2 laser.

124

Reference,

Lincoln Laboratory, Massachusetts Institute o1 Technology, Lexington, Massachusetts 02173, USA

BEAMSPLITTER

II M1 , I

k/ VACUUM

oscillation then occurs between the [ 100¢), 0200111 level and the lowest excited bending mode. A fourteen micrometer laser can be generated in an analogous way by using a 10.6/~m pulse to populate the [1000, 02001i level. The most important aspects of the experimental apparatus are summarized in fig. 2. We note the following additional details. The 9.6 p m stimulating pulse, which was at the wavelength of a single v i b r a t i o n - r o t a t i o n a l line in the P-branch of the 0001 - [1000, 02001ii band, contained 80 mjoules ol energy and was 3 psec in duration. The HBr laser had a total . I o u t p u t energy of 100 m J, with approximately ~ of the power occurring in the desired v = l -, v = 0 lines. In geneTal, the sample cell was cooled to - 1 8 0 ° C ; however, under sonre conditions wall temperatures as high as 0°C could be tolerated. Spectral m e a s u r e m e n t s of the laser o u t p u t showed that it included P, Q, and R lines. The relative strength of each line depended on the cell gas pressure, and instantaneous optical cavity length. In particular, at low pressures (1 tort) the lines which oscillated were those whose upper rotational level coincided with the lower rotational level of the 9.6 ~zm stimulating pulse; however, at higher pressures (5 torr) unconnected transitions were observed. These and other spectral observations m a y be explained by the competition between the stimulated rate of p u m p i n g of a particular rotational sub-level o f the [ 1000, 0200]1i state and rotational relaxation out of that sub-level. Although only comparatively low gas pressures have been used, the results of the spectral and time resolved measurements on the laser pulse indicate that upward scaling of the laser pressure is possible. Tire use of other hydrogen halide optical p u m p i n g systems such as DF laser excitation of a DF-CO 2 gas m i x t u r e to obtain laser oscillation at 16 u m and other infrared wavelengths wilt also be discussed.

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HANC}LING

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BOFz LENS /

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* This work was s p o n s o r e d b y the D e p a r t m e n t of the Air Force and the U.S. E n e r g y Research and D e v e l o p m e n t Administration u n d e r a s u b c o n t r a c t f r o m the Los Alamos Scientific

Laboratory.

LASERS 11 CO 2 with subsequent energy transfer into N 2 0 [2] ; more recently an HBr TEA laser was used to p u m p an H B r - C O 2 transfer system and obtain CO 2 laser action at 14 and 16 ~zm [3]. In the CO based transfer systems, progress has been made in three areas: the discovery oI new molecular systems, the development of tuning techniques, and parametric studies of a high pressure CO 2 system. The CO molecule is ideal for storage of vibrational energy because of its exceedingly slow vibration to translation transfer rate of 1.9 X 10 3 t o r r - I sec_l [4]. The second harmonic of the P(24) line of the 9.6 ~zm CO 2 band falls within 0.003 cm -1 of the CO 0 ~ 1 P(14) transition and is efficiently absorbed. Resonant energy transfer from the CO molecule makes it possible to vibrationally excite a number of molecules without requiring an exact frequency match between the pump laser and the active molecule. With CO2, for example, the mismatch is 206 c m - 1 . In the experiments performed to date an uncoated CdGeAs 2 crystal [5}, cooled to 77 K reduce the absorption, was used to produce up to 26 mJ of second harmonic energy at an external efficiency of 8%. The TEA laser beam, which was reduced to a diameter of 4 mm by a telescope, filled only a portion of the 7 X 13 mm cross section of the crystal. The results of current efforts to obtain second harmonic energies in excess of 100 mJ by using large aperture (~ 1 cm 2) a.r. coated crystals will be described. Such energies should make possible continuously tunable operation of the high pressure CO 2 lasers described below. Laser action has been obtained in SiH4-CO mixes at pressures up to 35 torr. This is the first reported laser action in Sill 4 and it is also the first time an optically pumped energy transfer laser has been demonstrated for other than a linear molecule. A total of 6 laser lines have been observed in the 7.90 to 7.99 ~m region. The maximum output energy was 0.03 mJ at an efficiency of 0.6c~. Efforts to obtain lasing action on the analogous molecules GeH 4 amd CD 4 were not successful. In the case of CD 4 rapid self deactivation by nearly resonant vibrational to vibrational relaxation processes may be responsible for the failure to achieve lasing. A grating tuned OCS laser providing over 80 lines between 8.19 and 8.46/am was developed. Laser action in the 8.6 ~m region was obtained using O 13CS; the threshold is higher and the output lower than with the normal isotope, probably because of an increased self deactivation of the upper laser level. ttigh pressure operation of optically p u m p e d molecular lasers has the potential of continuous tunability. We have examined the high pressure limit for a number of systems. In a 4.3 cm long cell the pressure limit for the C2H 2 C O - H e system was 610 tort with an optimum mixing ratio of 1 : 1 : 5 . For the O C S - C O - - H e system the o p t i m u m mixing ratio was 1 : 1 : 7 and the high pressure limit was 420 tort. In the CO 2 - C O - He system, laser action was obtained up to the 16 atm pressure limit imposed by the cell windows, with thresholds as low as 2.1 mJ and output energies up to 0.25 mJ for a 9 : 1 : 57 C O 2 - C O - H e mixture. At 16 atm the rotational lines of CO 2 are sufficiently broadened to allow continuous

N4 tuning. The use of helium rather than CO 2 for pressure broadening has certain advantages, since it pressure broadens CO 2 about ~ as much as CO 2 itself, but deactivates the CO 2 upper level only ¼ as rapidly. The three component gas system thus allows the absorption of tile pump laser, the density of the active medium, and the pressure broadening to be controlled somewhat independently. In order to be useful as tunable sources, the high pressure CO 2 laser will need to incorporate a grating, an etalon, and apertures for mode selection in an external mirror cavity. The additional window losses and the increased build-up times in an external cavity raised the 16 a tm threshold to ~ 11 m J; grating tunable operation is estimated to require at least twice this energy and should be possible with the expected improvements in doubling technology. Such advances in second harmonic generation should make possible a high repetition rate, continuously tunable CO 2 laser useful for high resolution spectroscopy. References [1 ] H. Kildal and T.F. Deutsch, Appl. Phys. Letters 27 (1975) 500. [2] T.Y. Chang and O.R. Wood. Appl. Phys. Letters 24 (1974) IB2. [3] R.M. Osgood, (to be published). [4] M.A. Kovacs and M.E. Mack, Appl. Phys. Letters 20 (1972) 487. [5] H. Kildal and J.C. Mikkelsen, Opt. Commun. 10 (1974) 306.

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I N F R A R E D M O L E C U L A R L A S E R S P U M P E D BY E - V ENERGY TRANSFER A.B. PETERSEN, C. WITTIG and S.R. LEONE

UniversiO, of Southern California, Los Angeles. California 9000 7, USA Recent experiments have shown that gas-phase collisions between electronically excited atoms and small molecules can lead to fast, efficient, and specific vibrational excitation of the molecules. This process is referred to as electronic-vibrational (E--V) energy transfer. We have investigated this phenomenon with respect to its application as a pumping mechanism for ir molecular lasers [ 1 ]. In particular, we have used Br atoms in the 2P1/~2 state, hereafter referred to as Br*, produced by photodissociation of Br 2. The excitation of these atoms (3685 cm -1 ) is comparable to the vibrational levels of many small molecules in which only one or two quanta are excited. Our experimental apparatus consists basically of a laser tube filled with Br 2 vapor, He diluent, and the lasing molecule and a linear flashlamp, both 1.5 in long. Using this equipment, we have investigated a munbcr of pulsed molecular laser systems. Detailed results have been obtained for the cases of CO 2, N20. HCN, NO and H20. E - V pumping of CO 2 leads to stimulated emission on the usual 10.4 and 9.4 um bands, as well as new transitions at 4.3 $ Research supported by the United States E.P,.D.A.

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