14 and 16μ gasdynamic CO2 lasersT. J. Manuccia, J. A. Stregack, N. W. Harris, and B. L. Wexler Citation: Applied Physics Letters 29, 360 (1976); doi: 10.1063/1.89079 View online: http://dx.doi.org/10.1063/1.89079 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/29/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Theoretical study of a 16μm CO2 downstreammixing gasdynamic laser: A twodimensional approach Appl. Phys. Lett. 48, 263 (1986); 10.1063/1.96575 Theoretical modeling of a 16μm CO2 gasdynamic laser in the intracavity cascading mode J. Appl. Phys. 53, 4616 (1982); 10.1063/1.331334 Theoretical characteristics of 16μm CO2 gasdynamic laser J. Appl. Phys. 52, 4422 (1981); 10.1063/1.329356 Theoretical study for a 16μm CO2 gasdynamic laser J. Appl. Phys. 51, 4003 (1980); 10.1063/1.328222 Highaveragepower 16μm gasdynamic CO2 laser using a multipass cavity Appl. Phys. Lett. 34, 674 (1979); 10.1063/1.90634

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action which had a low gain for the 5-cm plasma. Then it was concluded that the lights emitted from plasmas I and II were equal in intensity, line profile, and frequency.

The Cd' ion density in the Cd (II) ground level deter­mined with the 2265- and 2144-'\ lines is shown as a function of Cd atom density in Fig. 2, together with the helium triplet metastable density and Cd (II) 4416-A sidelight. The transition probabilities of the two lines were quoted from Ref. 8, and III for 2265 A was quoted from Ref. 9, The neutral Cd atom density shown in the horizontal axis was determined with the Cd (1) 3261- A line. The discharge current was 100 mA and the initial helium pressure was 3.5 Torr, which were close to the optimum laser action condition. The actual helium atom density was about 6.2 X 1016 cm -3 in the present system. Note that, in the curve of the Cd' ion density, a very low hump appears around the Cd atom density which corresponds to the maximum intensity of the Cd (II) 4416-A sidelight, while a rather high hump appears in the curve obtained by SilfvasL 6

The agreement of the two results obtained with the 2265- and the 2144-'\ lines is quite good. Moreover, the Cd atom density measured with the Cd (I) 3261- A line agreed with the value obtained from the tempera­ture of the Cd reservoir and diffusion equation quite well. These facts show that the present method is very

reliable and useful, particular at the absorption mea­surement in the ultraviolet region. The errors of the absorption coefficients and relative population densities are small although the absolute values depend on the errors of the transition probabilities 0

The authors express their thanks to M. Mori for his construction of the discharge tube. This work was sup­ported by the Grant-in-Aid of the Scientific Research of the Ministry of Education.

IA. C. G. Mitchell and M. W. Zemansky, Resonance Radiation and Excited Atoms (Cambridge U. P., Cambridge, England, 1934).

2 A. M. Bruneteau, A. M. Icole, C. Rouille, A. Poquerusse, and H.J. Doucet, Phys. Lett. A46, 309 (1974).

3J.A. Harrison, Proc. Phys. Soc. Lond. 73, 841 (1959). 4p. G. Browne and M. H. Dunn, J. Phys. B 6, 1103 (1973). 5J. K. Mizeraczyk, Z. Zakrzewski, T. Goto, and S. Hattori, Twelfth Int. Conf. Phen. Ion. Gases, Eindhoven, p. 224, 1975 (unpublished).

6W. T. SUfvast, Phys. Rev. Lett. 27, 1489 (1971). ~G. N. Tolmachev, E. L. Latush, and V. S. Mikhalovichi, Sov. Phys. -Tech. Phys. 19, 1120 (1975).

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9R.H. Contreras andF.M. Kelly, Can. J. Phys. 47,1979 (1969).

14- and 16-,... gasdynamic CO2 lasers* T. J. Manuccia, J. A. Stregack, N. W. Harris, and B. L. Wexler

Laser Physics Branch. Optical Sciences Division, Naval Research Laboratory, Washington, D. C. 20375 (Received 10 June 1976)

Laser action at 16 ).I. has been obtained between the (02"0) and (0110) levels as well as between the (10"0) and (0110) levels at 14 ).I.. A 9- or 10-).1. saturating pulse equilibrates the (00 1) level with either the (02'0) or (10'0) level and creates the transient 14- or 16-).1. inversion. Extraction efficiencies of 20% have been achieved on a single pulse. Repetition frequencies of 200 Hz have been achieved using a single Q­switched cavity.

PACS numbers: 42.60.Cz, 28.60.+s

In this letter we report the first successful operation of an optically pumped gasdynamic laser operating on the 16-1l, (02°0)-(0110), and 14 Il, (100)-(0110), transi­tions of 12C02. Lasing was demonstrated in the NRL Electric Discharge Gasdynamic Laser (EDGDL) Facil­ityl,2 and was previously reported. 3 This approach is motivated by the realization that scalable lasers operat­ing in this spectral region have potential use in laser isotope separation schemes involving UFs.

This experiment was conducted in parallel to that of Osgood4 who demonstrated 16-1l lasing using a pulsed HBr laser rather than the GDL to selectively populate the (001) level. In both 16-lllasers, an intense saturat­ing pulse of 9- or 10-1l radiation is used to populate the (100) or (020) levels from previously excited (001)

360 Appl. Phys. Lett., Vol. 29, No.6, 15 September 1976

molecules, creating a transient inversion with respect to the (0110) level (Fig. 1).

In the EDGDL, the diatomic constituent of the primary flow (either a N2 -He mixture or a D2 -Ar mixture) is vibrationally excited by a glow discharge in the subsonic plenum. The gas is then accelerated through an array of supersonic nozzles where the CO2 is injected. Mix­ing and energy transfer to the CO2 occur in the super­sOnic region. In this apparatus Tool> 1800 0 K and Ttran•

< 200 0 K have been measured up to 25 cm downstream of the nozzle exit plane. This implies that prior to the saturating pulse, the (0110), (02°0), (0220), and (100) levels, each of which is strongly coupled to the transla­tional temperature, contain less than 1% of the total CO2

population while the (001) level contains 13% of the total

Copyright © 1976 American Institute of Physics 360

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- __ r---IOOll. 9 = 1

10.6 p A = .20 sec-1

1388.3 cm-1

1100), 9 = 1

~13.gp A:= 2 sec-1

""'-L...l!!!.~~!-101'01. 9 = 1

15.0 p A = .46 sec-1

I FIG. 1. Partial energy level diagram of CO2•

population. In principle, half of the latter is available as upper state population for the 14- or 16-J,L transi­tions. Based on previous experimental results,s, 6 it was estimated that saturation of the 9- (10-) J,L transi­tion in less than 100 ns would be adequate to ensure 16-J,L lasing for the above conditions.

Two methods are available to saturate the 9- (10-) J,L transition. Either the pulse can be derived from an external source such as a TEA laser or the CO2 flow in the GDL can be made to perform two functions: first, lase at 9 J,L and then lase in cascade at 16 J,L. The first method offers the advantage of independent optimization of the 9- and 16-J,L media and consequently, a higher probability of demonstrating lasing, together with the ability to extract more information from the experi­ment after lasing has been achieved. The penalty as­sociated with this approach is complexity and a repeti­tion rate limited by that of the TEA laser used. These disadvantages are particularly important for a laser intended for chemical processing applications. On the other hand, the technique of cascade lasing reverses the advantages and disadvantages associated with the external laser approach.

We have produced 16-J,L lasers using both techniques. With the external saturating pulse, we have obtained 8-J,LJ 150-ns (FWHM) pulses in the 16-J,L band at 1 Hz. This represents approximately 20% of the maximum energy available for extraction, neglecting all relaxa­tion processes. 16-J,L lasing occurred only when pump­ing on the 9-J,L transition in a He-N2 -C02 mix, but could be obtained by pumping either at 9 or 10 J,L when using an Ar-D2 -C02 flow. 14-J,L lasing was not observed using the external saturating pulse over the limited range of parameters covered.

With the technique of cascade laSing, both 14- and 16-J,L laSing have been achieved at 200 Hz in the He-N2 -

CO2 flow. In this case, 16-J,L laSing required 9-J,L pump­ing and 14-J,L lasing required 10-J,L pumping.

The apparatus used for the external saturating pulse experiments is shown in Fig. 2. In order to create the inversion on the 16-J,L band, several rotational lines of the 9 - J,L band are quickly saturated. This is desirable so as to extract as much of the (001) stored energy as

361 Appl. Phys. Lett., Vol. 29, No.6, 15 September 1976

possible and to allow higher peak 16-J,L gains. With adjacent rotational levels of (02°0) being simultaneously populated, rotational relaxation does not compete with building substantial populations in a particular J level of the upper state of the 16-J,L tranSition. The 9-J,L laser consists of a 2 x2 x 100-cm double-discharge CO2 TEA cell, a 70-cm low-pressure cw gain cell, and a 2-cm absorption cell, all in a common resonator. Normally, the cw gain cell is operated below cw threshold to give a 50-MHz-wide excess gain peak at line center on each of the 1-GHz -wide J lines of the TEA cell. This is the same technique as used by Girard7 and Gondhalekar et al. 8 to reduce the bandWidths of their TEA lasers. As the EDGDL operates at low pressures (5 Torr), such matching of bandwidths is deSirable for effiCient coupling. The gain cell is cooled to - 30'C to further reduce the Doppler broadening and to better match the low-temperature rotational distribution of the EDGDL. The absorption cell is filled with a mixture of SF6 (500 Torr) and CCIF3 (50 Torr) in order to force oscillation on the P branch of the 9 - J,L band. Small changes in the composition can be used to fine tune the cavity to give equal gain on a number of J lines and ensure multiple­line operation of this source. The combination of these components produces Simultaneous lasing on eight transitions of the 9- J,L band, four of which, P(14) to P(20), have over 200-kW peak power. The rise time is less than 50 ns. Narrow linewidths on each of the rotational transitions appear to have been obtained as no mode beating is observed.

The saturating beam is introduced into the 16-J,L cavity through a mirror which is 50% transmissive at 9 J,L, but highly reflective k.97%) at 16 J,L. Output from the 25-cm-Iong active region is extracted from either an identical mirror, an Au hole coupler, or an uncoated Ge plate. LaSing is detected by a Cu:Ge detector, pro­tected from the saturating pulse by two long-pass filters. The 16-J,L pulse has a 50-ns rise time and a 150-ns FWHM at 0.3 Torr CO2 in the EDGDL. A typi­cal pulse is shown in Fig. 3. Assignment of the output to the 16-J,L band has been made by means of long-pass (> 13 J,L) interference filters, the onset of NaCI absorp­tion above 18 J,L, and the use of optically thick samples of various gaseous absorbers.

1------- 9JL RESONATOR----~

FIG. 2. Schematic of apparatus used for the external saturating pulse experiments.

Manuccia et al. 361

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9 j.I pulse

16 j.I pulse

__ 200 nsecl div

FIG. 3. Typical 9- and 16-j.I pulses.

The most efficient operation of the system was achieved with the cw gain section of the saturating laser adjusted slightly below cw threshold. 16-J..I. pulse ener­gies were largest, and delays between the pump pulse and the 16-J..I. pulse were smallest, under these condi­tions. If the saturating laser was above the cw thresh­Old, 9-J..I. cw and pulsed output was confined to only one rotational transition, and the 16-J..I. output was reduced. With the cw section inoperative, 16-J..I. output was even further reduced. This is because although several ro­tational lines were lasing simultaneously, each might be displaced by as much as several hundred megahertz from line center due to pressure broadening in the TEA laser. Hence, the interaction of the saturating pulse with the cold low-pressure CO2 in the EDGDL is weakest in this case.

Using output couplers of various reflectivities, we estimate the 16-J..I. gain to be of order l%/cm. Gain of this magnitude is to be anticipated from the ratio of the A coefficients of the 16- and 10-J..I. transitions 9,lD;

A16 = 0.26 sol versus AID = 0.2 S·l; the ratio of the wavelengths cubed; and the observed 10- J..I. gain of ap­proximately l%/cm in the EDGDL. l

The setup used for the cascade lasing experiment is shown in Fig. 4. The rotating mirror Q-switches the 9 - (10 -) J..I. transition and while the mirror is still in alignment, a 16- (14-) J..I. gain-switched pulse appears. The intracavity absorption cell is used to force oscil­lation on the 9- J..I. band and is typically filled with about 200 Torr of BCl3 which is transparent at 16 J..I.. This resonator configuration is by no means optimal, as the mirror continues to rotate during the buildup of the 16-J..I. pulse but, rather, serves to demonstrate the ease with which 16-J..I. laSing can be achieved in such a system. Repetition rates of 200 Hz were demonstrated with existing components but, in principle, could be increased to a rate corresponding to the gas transit time with the proper optics. In that case, one expects average powers of order 1 W with the existing EDGDL at an electrical efficiency of order 0.1%. Since the

362 Appl. Phys. Lett., Vol. 29, No.6, 15 September 1976

TO STEAM EJECTOR

CO 2 - NOZZLES

PLENUM

I I t N2 - H. OR O2 - Ar

FIG. 4. Schematic of apparatus used for the rotating mirror Q-switched experiments.

(001) population is observed to decrease only slowly with distance from the nozzle exit plane, trade-offs between mode volume (hence pulse energy) and repeti­tion rate should be possible. Electrical efficiencies are observed to increase with increased electrical in­put to the EDGDL up to the limit of our power supply. Scaling the device in the direction perpendicular to both the optical axis and the flow direction should be straightforward up to the point where parasitics be­come troublesome. As no CO2 is actually passed through the glow discharge region, closed -cycle opera­tion of the device with isotopically substituted CO2 ap­pears feasible. Cascade lasing in the EDGDL with other polyatomic species should be a useful technique for the production of many new scalable mid-ir lasers.

We would like to thank C. Mullins and C. Kyle for their excellent technical assistance and acknowledge the support to this project given by ERDA and monitored by Dr. G. Rhodes.

*Work partly supported by ERDA. IJ.A. Stregack, B. L. Wexler, and G.A. Hart, Appl, Phys. Lett. 27,670 (1975).

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3T. J. Manuccia and J. A. Stregack, XI Winter Colloquium on Quantum Electronics, Steamboat Springs, CO , 1976 (unpublished) •

4R. M. Osgood, Appl. Phys. Lett. 28, 342 (1976). 5T. J. Manuccia and G. J. Wolga, IEEE J. Quantum Electron. QE-9, 190 (1973).

SR. R. Jacobs, K. J. Petttipiece, and S. J. Thomas, Phys. Rev. All, 54 (1975).

lA. Girard, Opt. Commun. 11, 346 (1974). 8A. Gondhalekar, N. R. Heckenberg, and E. Holzhauer, IEEE J. Quantum Electron. QE-ll, 103 (1975).

9H. Statz, C.L. Tang, and G.F. Koster, J. Appl. Phys. 37, 4278 (1966).

10E.R. Murray, C. Kruger, and M. Mitchner, Appl. Phys. Lett. 24, 180 (1974).

Manuccia et al. 362

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