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A CW C02 wave guide laser for the optical pumping of far-infra-red molecular lasers

A CW C02 wave guide laser for the optical pumping of far-infra-red molecular lasers
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  LETTERE AL NLTOVO CIM]ENTO VOL. 28, N. 8 21 Giugno 1980 A CW CO 2 Wave Guide Laser for the Optical Pumping of Far Infra Red Molecular Lasers. N. IOLI, G. MORUZZI and F. STRUMIA Istituto di .~isiea dell Universith - .Pisa, Italia Gruppo Nazionale di Struttura della Materia del C.N.R. - .Pisa, Italia rieevuto il 22 Maggio 1980) The population inversion between roto-vibrational levels belonging to excited vibra- tional manifolds of several small molecules has been obtained by optical pumping with CO 2 and N20 lasers. This way laser emission has been obtained on hundreds of far-infra-red FIR) molecular lines 1), and they constitute the best and very often the only) sources of coherent radiation in the submillimeter and FIR spectral regions today, with powers in the range 1--100) mWCW. Pulsed emission up to lkW--1MW can also be obtained by using TEA CO 2 lasers as pump sources 2). The method is limited by the need of a fortuitous coincidence between a CO 2 laser line and an absorption transition of the candidate molecule. The tunability of a conventional, low-pressure, CW CO 2 laser is limited to the typical Doppler width in the 10 ~tm region, i.e. 60--80)MHz. Transitions with an offset of a few hundreds MHz can still be excited by a pulsed TEA laser because of the much higher power density available. Unfortunately a TEA laser is not suitable either for CW operation or for accurate, high-resolution spectroscopic investigations. Thus it was felt the need to increase the tunability of CW CO~ lasers, and the number of new FIR laser lines available is expected to be roughly proportional to the increase in the CO~ tunability. In the last few years the CO 2 wave guide laser has been extensively considered as a possible solution of the problem a.4). The working pressure for a dielectric wave guide with an internal diameter of about 0.1 cm is of the order of 80--150) Torr. The width of the lasing lines is increased by pressure broadening and the tunability can be improved in principle up to one order of magnitude. 1) T. Y. CHANG: Optical I)umping in gases, in Nonlinear Infrared Generation, edited by Y. R. SHEN Berlin, 1977). 2) T.A. DE TEMPLE: Pulsed optically pumped far infrared laser, in Infrared and Millimeter Waves, edited by ~i~. J. BUTTON, Vol. 1 New York, N.Y., 1979), p. 129. a) J. J. DEGNAN: A~gpl. Phys. (N. Y.), ll, 1 1976). 4) R.L. ABRAMS: IVaveguide gas lasers, in Laser Handbook, edited by M. L. STITCH, Vol. 3 Amster- dam, 1979), p. 41. 257  258 N. IOLI, G. MORUZZI and F. STRUMIA However, the need of single-line and single-mode operation with a resonable output power over the whole tuning range leads to contradictory requests on the practical design of the laser. Several possible solutions have been presented in the recent litera- ture, but no one has fully accomplished the expected performances. In particular no CW CO 2 wave guide laser, to our knowledge, has been successful in the excitation of FIR laser emission. The main problems to be solved can be summarized as follows: i) short cavities 10--60)cm) are needed for obtaining enough mode separa- tion to exploit fully the expected tunability, the use of an intracavity etalon has been proposed, but no satisfactory experimental results have been reported; ii) the active medium gain must be large enough to allow laser emission at the line wings; thus the wave guide has a minimum allowable length, and this is a limita- tion to point i); in principle the minimum length can be reduced by reducing the wave guide bore, however iii) the wave guide bore must be large enough to provide enough resolution at the dispersive element usually a 150 1/mm grating) which forces the single-line oper- ation. A compromise must be found according to the goals of the particular design. Powerful single-mode CW operation has been repetedly reported when the dispersive element has been replaced by a total-reflecting mirror 2,~). In this case, however, emission is usually limited to a few lines near the 10-/) 20) and line hopping occurs when the cavity length is tuned. The maximum reported power was 39.5 W 51 W/m) for a Be0 wave guide with 1.65 mm i.d. 8). Special materials, like Be0 and BN have been introduced in order to improve the cooling of the discharge gas. The use of a 150 1/mm grating has led to a large-frequency tuning for the 10-/) 20) line 7) and in general for the well-resolved lines of the/) branches 6). For weaker lines and for the less-resolved lines of the R branches, however, the frequency tuning remained much smaller than the mode spacing. Some improvement was obtained by using an intra- cavity lens 8.9). The beam spot on the grating is magnified by a factor 1.3 to 1.6, thus leading to a better resolution. However, in all the literature quoted above the reported output power did never exceed a few hundreds mW. This power is not sufficient to excite FIR laser emission. More power can be obtained by using a longer discharge and a larger wave guide bore the latter also avoids the need of an intracavity lens). Maximum output powers up to 3 W for the strongest lines have been reported very recently 10) for a BN wave guide 24 cm discharge length and 1.7 • 1.7)mm 2 square section), but the, successful excitation of FIR lasing is not reported. In this paper we describe a more powerful CW CO 2 wave guide laser giving a maxi- mum output power of 4--6.5)W on single-line and single-mode operation. We could observe FIR laser emission on several lines of CHaOH and CH3F pumped by this laser. We used cylindrical pyrex wave guides, trials were made with different internal diameters 2, 3 and 4 mm), while the external diameter was either 7 or 8 mm. The best ~) G. M. CARTER and S. MARCUS: Appl. Phys. Lett. 35 129 1979). s) R. L. ABRAMS: Appl. Phys. Left. 25, 304 1974). ~) M. LYSZYK, F. HERLEMONT and J. LEMAIRE: J. Phys. E 10, 1110 1977). a) ]=~. BENEDETTI, T. COLOI~IBO and F. STRUMIA: Letl. Nuovo Cimenlo 22, 167 1978). 9) A_. VAN LERBERGHE, S. AVRILLIER and CH. BORDI~: IEEE J. Quantum Electron. QE14, 481 1978). 10) D. E. EVANS, S. L. PRUNTY and M. C. SEXTON: Infrared Phys. 20 21 1980).  A CW CO 2 WAVE GUIDE LASER P0R THE OPTICAL PUMPING ETC. 2~9 results were given by the 3 mm i.d. wave guide, and only this 3 mm bore is used in our final version. The choice of pyrex seemed convenient because of i) the low cost and easy availability of the material, which allowed many trials with different dimensions, and it) the possibility of using a single tube of any length, thus avoiding the mismatch losses and the surface roughness of composite ceramic wave guide (like BeO or BN). The validity of our choice has been confirmed posteriori by the high efficiency attained in the conversion of electrical into optical energy: our maximum recorded efficiency of 11.3% equals the best results found in the available literature. A schematic drawing of the laser is shown in fig. 1. The active zone of the pyrex wave guide is 40 em long, divided into two discharges of 20 cm each. The wave guide is directly connected with two chambers containing the 90% reflecting ZnSe output mirror and the 150 1/mm grating, the use of Brewster windows is thus avoided. A ZnSe window on the grating chamber allows the exit of the zeroth-order radiation. The mechanical frame of the laser is supported by invar rods. The dimensions of all the laser parts have been carefully calculated in order to compensate the effects of thermal expansions on the cavity length, since maximum frequency stability is needed for spectroscopical investigations. The discharge current is either stabilized electronically for CW operation, or elec- tronically chopped when the laser is operating in long-pulse regime. A commercial refrigeration and a circulating pump operate the ethylene glycol closed-cycle cooling of the wave guide. The best output powers are obtained at temperatures of the coolant fluid of about -- 10 ~ The use of the refrigerator is needed when working with pyrex wave guides, since the thermal conductivity is much lower for pyrex than for BeO and BN. The laser was first operated with a total-reflecting gold-coated mirror replacing the diffraction grating. The maximum output power observed in these conditions was 17.5 W for a 3 mm i.d. wave guide. The optimum conditions were : average pressure ~ 80 Torr, discharge current 15 mA, potential drop for each discharge 5 kV, coolant fluid tem- perature ~-- 15 ~ Thus 150 W were supplied to the discharge and the electrlcal-to-optical energy conversion efficiency was 11.3%. For a comparison, the best published results are 8.3% (x0) and (10+12)% (11). In our ease the power per unit length of discharge is 44 W/m (pyrex), to be com- pared with 29 W/m (BN) of ref, (~0), 36 W/m (BN) of ref. (11) and 51 W/m (BeO) of ref. (s). Since the reflectivity of our output mirrcr was selected in view of its use with a diffraction grating on the other cavity end, it is most likely that some more power in the mirror-mirror configuration could be obtained by using a somewhat less-reflecting output mirror. From what reported above we conclude that the choice of the dielectric material used for the wave guide is not very important for a good efficiency of the laser. Single-line emission has been observed for many CO S lines after replacing the total- reflecting mirror with a 150 1/mm diffraction grating. Emission on all lines from R(4) to R(46) and from P(6) to P(44) in the 10 ~m branch, from R(6) to R(44) and from P(6) to P(46) in the 9 zm branch has been observed. Typical CW output powers are 6.5 W for the 10-P(20) line, 5 W for 10-P(14), 3.6 W for 10-P(6), 5.5 W for 9-/)(20), 4.5 W for 9-P(32), 4 W for 9-P(36) and 4 W for 9-R(14). This figures are the highest attained by a CW wave guide COz laser up to now, however they are still much too l~) A. PAPAYOANOU: IEEE J. Quantum Electron. QE 13, 27 1977).  2~0 N. IOLI, G. ~IORUZZI and F. STRUMIA ¢ o  A CW C0 2 WAVE GUIDE LASER FOR THE OPTICAL PUMPING ETC. 26 Fig. 2. - Far-field radiation pattern for the 9-P 22) line. The best Gaussian fit is shown by the dots. 90 7O 50 9 30 30 50 70 901 I I I I I PZT volt ge (a.u.) r 0.4 Fig. 3. - CO2 laser, 9-P 16) line: a) laser frequency vs resonator length, b) outpnt power vs resonator length. The frequency of the reference CO2 laser is tuned at the centre of the line.
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