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DEIGN ASPECTS AND PARAMETRIC CHARACI•ER•SATIONOF A NT'TTAL RARE GAS LASR .4

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CONTENTS Page No

I INTRODUCTION ...................................................................................................................... 1

2 LASER SYSTEM ........................................................................................................................ IS3 EXPERIM ENTAL LAYOUT ........................................................................................................ I

4 CONTINUOUS W AVE OUTFUT ............................ * ............................................................. 1

5 THE 2 sun ATM OSPHERIC W INDOW ................................................................................. 2

5.1 He-Xe 2.026 !un line ..................................................................................................... 2

* 5.2 He-Ar 2.061 and 2.207 im lines ...................................................................................... 2

ACKNOW LEDGEM ENTS ........................................................................................................ 3

TABLES

I Continuous w ave output ......................................................................................................... 2

FIGURES

1 Experimental layout.......................................................................................................... 4

2 Laser system ............................................................................................................................ 5

3(a) Power of He-Xe 2.026 pm line versus current ........................................................................ 73(b) Power of He-Xe 2.026 pm line versus He:Xe ratio ................................................................. 7

4(a) Power of He-Ar 2.061 pm Line versus current ........................................................................ 84(b) Power of He-Ar 2.061 pun line versus He:Ar ratio ................................................................. 8

5(s) Power of He-Ar 2207 m line versus current, ........................................................................ 9

5(b) Power of He-Ar 2.207 pm line versus He:Ar ratio ................................................................. 9

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1 INTLODUCTION

A neutral rare gas laser Was been built, and used to produce continuous wave output Mt severalwavelengths in the mid- infra-red regicn. Some of these lines are of particular interest because of their

0 location in tl..ý atmospheric window around 2 gm.

The later design is described, and results are presented of a parametric study of its operatingcharacteristics. Mtixtures of He-Ar, He-Kr and He-Xe were used, with total gas pressures in the range11 -35 torr. Excitation of the gas was acheved by a hollow cathode longitudinal discharge, with amaximum current of 110 mA. The uutput power at each wavelength was monitored as a function of the

5 discharge parameters (total pressure, partial pressuie ratio, and current), and the output couplerreflectivity, to determine the optimum working regines.

2 LASER SYSTEM

3 The laser cavity is built around two 0.75" diameter, 1.7 m long invar rods, connected by aluminiumblocks on which the optical components are mounted. The rear mirror is gold-plated copper, and thereis the choice of three output couplers with reflectivities optimised for the

(i) 1.8 - 2.7 gm,

(ii) 3-Sj/mand

(Uii) 2.7- 4.8 p• regions (Laser Optics, Inc.).

The output couplers are 25 nun ZnSe concave mirrors with a 15 m radius curvature on the inside face.This configuration means that the cavity is a stable resonator. The external surfaces areanti-reflection coated. The Pyrex discharge tube is 1.3 m Iong, with a nominal internal diameter of6.6 mm. A water-cooled jacket around the tube removes excess heat.

The gases used in the laser (He, Ar, Kr >99.999%, Xe >99.995% purity) are delivered to the tube via amoisture trap and 7 pm particulate filters. Fine control is maintained over the pressure of the gases bycalibrated needl ' valves. The total pressure is measured by a capacitance pressura gauge(MKS Baratron type 122A). Ballast resistors of 5 kQ and 7.5 kO are in series with the power supplyM(rdustek Pty Ltd) at the cathode and anode ends of the tube, respectively.

3 EXPERIMENTAL LAYOUT

The experiratental layout is shown schematically in Figure 1, and a photograph of the apparatus isshown in Figire 2. The laser raliation was dispersed by a 0.25 m focal length monochromator(Oriel 77200), and detected by a power meter (Laser Precision Corp., RKP-360). Correction was madefor the 9pectral response of the monochrrmator. An XT-compatible computer controlled the current andrecorded the power.

4 CONTINUOUS WAVE OUTPUT

Table I tOw lins which have been observed to lase in continuous wave mode. The powers are those-uder optinm conditions, and transitions are given in Racah notation. The atmosphert- absorption

como€Nmn wun calculated with the Lowtran 6 model.

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Table I Continuous wave output.

Wavelength Power Atmo' AeuicS Cass Transition I~%1absorptionGajes ira]sition coefficient [km"1 ]

i e-"e 2.02.6 -Md3/211 - 6p[3/211 13.7 0.21

2.651 5,13/211 -6p[1/21o 1.2 26.43.366 5a5/212 - 6r(3/211 6.4 0.60

3.434 7p[5 / 212 - 7s[3/211 4.0 0.23

3.507 5"17/213 - 6p!5/2]2 30.0 0.19

3.650 7p(1/211 - 7s[3/2]2 0.5 0.16

He-Kr 2.523 U41/211 - Sp[3/212 0.8 7.17

3.066 6p01/211 - 6s[3/212 4.1 2.00

He-Ar 2.061 3d13/212 - 4 p'13/ 212 0.1 0.21

2.207 34[11/211 - 4 p'13 / 212 0.3 0.16

2.397 3d[11/210 - 4p'[1/21I 0.3 0.28

S THE 2 gm ATMOSPHERL WINDOW

O, the laser lines detected, three were chosen for detailed study because of their location in the 2 ;matmospheric window, viz. the He-Xe 2.026 pm line, and the He-Ar lines at 2.061 and 2.207 sun.

5.1 He-Xe 2.026 gim line

The output power of this line Is shown as r function of discharge conditions at a total pressure of11 tort in Figures 3a and b. Figure 3a shows that, for all HeXe ratios, the power increasesmonotonically with curzent up to the madximum of 110 mA. In Figure 3b, power is plotted versusHe-Xe partial pressure ratio for a range of currents. For each current there is an optimum He.Xeratio. The higher the current, the larger the optimum He-Xe ratio ie, the leaner the mixture isin Xe. The maximum power of 13.7 mW ii obtained at 110 mA and He:Xe = 80:1. This line wyilllase with higher total gas pressures, but with lower power. (11 torr is the lowest pressure thatwill sustain a stable discharge.)

5.2 He-Ar 2.061 and 2.207 jim lines

Figures 4 and 5 show the output of the 2.061 and 2.207;zn lines, respectively, as functions of thedischarge conditions at 35 tort. These lines have significantly different characteristics to the Xeline, and lase over a much narrower range of conditions. This is particularly so in the case of the2.061 I.Lm line, which only lases between 35 and 70 mA (Fig. 4a). The 2.207 gm line lasesbetween 15 and 55 mA, with the optimum value being dependent on the He:Ar ratio. Figures 4band Sb show power versus partial pressure ratio. For each line, the optimum value of I e:Ar is

-denr cut on the current Both lire* require gas mixtures which are rich In Ar, and will not laseP 6.5:1.

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'UNCLASSIFIED ERVO'/03 -D

0 ACKNOWLEDGEMENTS

The author gratefully acknowledges the contributions of the foelowing,: Dr Shane Brunker for fruitfuldiscussions regarding this work- Robert Ros.iter for writing the data acquisition software; FredButtignol and John Wheatley for their assistance with the high voltage system; and Norm Jeffrey and

* John Bridgman for the construction of various mechanical components.

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r-U1 MU 7. No. of Refs. I1DESIGN ASPECTS AND PARAMETRIC Document T'.tl AbstractCHARACTERISATION OF ANEUTRAL RARE GAS LASER S (Sect C (Confi) R (Rbt) U (Unclass)

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Gas lasers Design 1709Wavelengths La-ser outputs

16. Abstract

A neutral rare gas laser has been built, and used to produce continuous wave output at severalwavelengths in the mid-infra-red region. The laser design is described, and results are presented"of a parametric study of its operating characteristics. Mixtures of He-Ar, He-Kr, and He-Xe wereused. The output power at each wavelength was monitored as a function of the dischargeparameters, to determine the optimum working regime&

This General Document Is a paper presented at the 8th Conference of the Australian OpticalSociety, held at the University of Sydney, NSW, in February 1993.

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