Method for obtaining a symmetric power tuningcurve for a single-mode continuous-wave CO2 laser

Mangal M. Nagarkar and Pradeep K. Gupta

We propose and demonstrate a technique for obtaining a symmetric power tuning curve for a single-modecontinuous-wave CO2 laser regardless of the operating power level. © 1997 Optical Society of America

Key words: CO2 laser, off-line center tuning, power tuning curve.

Single-mode line-tunable CO2 lasers are used exten-sively for pumping molecular gas lasers and for spec-troscopic applications. Off-line center tunability ofthe laser frequency is often desirable for both appli-cations.1,2 Laser frequency tuning across the gainprofile can be achieved by changing the cavity length.However the output versus cavity length curve for asingle-mode cw CO2 laser is generally asymmetric,which makes off-line center tuning difficult on oneside of the center frequency.3 The asymmetry hasbeen shown to arise because of the heating of intra-cavity optical components in the presence of a laserbeam and the consequent changes in the resonatorpath length.3 The asymmetry can be minimized byuse of optical components with a smallertemperature-induced path-length change. How-ever, it is important to emphasize that, for givenintracavity optical components, an increase in oper-ating power level will lead to a larger rise in temper-ature and hence a more asymmetric power tuningcurve.

In this paper we show that a symmetric powertuning curve, regardless of operating power level, canbe obtained by sealing the cw CO2 laser dischargesection with windows having opposite sign for thetemperature-induced change in optical path length~for example, one ZnSe window and one KCl window!.With an appropriate choice for the thickness of thetwo windows ~which depends on the thermal charac-teristics of the windows and their absorption coeffi-cient at the operating laser wavelength! the nettemperature-induced change in resonator path

The authors are with the Laser Programme, Centre for Ad-vanced Technology, D Block, P.O. Box CAT, Indore 452 013, India.

Received 1 August 1996.0003-6935y97y184099-03$10.00y0© 1997 Optical Society of America

length can be made insignificant, leading to a sym-metric power tuning curve regardless of the operat-ing power levels.

The cw CO2 laser used for this study comprised awater-cooled discharge tube of 60-cm length and12-mm internal diameter. The optical cavity wasformed by a 150-groovesymm grating blazed at 10.6mm and a 10-m radius of curvature ZnSe mirror with;10% output coupling. The output coupler wasmounted on an annular piezoelectric transducer~PZT! tube that contracted when the applied voltagewas increased. The laser tube was sealed at bothends by appropriate windows as discussed below.The cavity elements were mounted on Invar rods andthe laser was operated in a single longitudinal andtransverse mode. The gas mixture used in the ex-periment was CO2:N2:He in the ratio of 1:1:8 at anoperating pressure of 15 Torr.

Figure 1 shows typical laser power tuning curvesthat were obtained by tuning the resonator modefrequency across the gain profile by varying the ap-plied voltage to the PZT in steps of 50 V. The curveswere recorded at two operating power levels of ;1and ;2 W with the laser discharge tube sealed atboth ends by ~i! antireflection–antireflection-coatedZnSe windows ;3 mm thick ~curves marked by filledcircles! and ~ii! with polished uncoated KCl windows;3.5 mm thick ~curves marked by open circles!. Thesame 3-mm-thick ZnSe output coupler was used inboth cases.

The asymmetry in the power tuning curve seen inFig. 1 arises because of the heating of intracavitywindows and output coupler in the presence of thelaser beam.3 Briefly, the change in the cavity length~DL! resulting from a change in temperature ~DT! ofthe intracavity optical components leads to a change~Dn! in the laser oscillation frequency given by Dn 52n~DLyL!, where L is the optical path length of the

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cavity and DL is given by3

DL 5 (pF~np 2 1!gplp 1 lpSdn

dTDpGDTp.

Here np, gp, and lp are the index of refraction, linearcoefficient of thermal expansion, and the distance tra-versed by the beam through the pth material; andDTp is the change in temperature of the pth material.The latter depends on the operating power level of thelaser and the thermal characteristics as well as theabsorption coefficient of the material at the laserwavelength. This temperature-dependent contribu-tion Dn adds in phase to the change in frequencyeffected by a change in PZT voltage on one side of thecenter frequency leading to a regenerative feedbackeffect and is out of phase on the opposite side. Thisleads to the observed asymmetry in the power tuningcurve.

For ZnSe both the path length and index of refrac-tion increase with temperature and thus contributeto an increase in optical path length with tempera-ture. In contrast for KCl, the refractive index de-creases with increased temperature. This leads to anet reduction in optical path length, the magnitude ofwhich increases with increased temperature. Theopposite sign for the temperature-dependent path-length change in KCl and ZnSe explains the fact thatthe asymmetry for these is on the opposite side of thecenter frequency as can be seen in Fig. 1, where onecan also observe the increase in asymmetry of thepower tuning curve with an increase in operatingpower level.

Since the change in optical path length is oppositein sign for ZnSe and KCl windows, sealing one endof the cw CO2 laser discharge tube with a ZnSewindow and the other end with a KCl windowof appropriate thickness can make the net

Fig. 1. Output power versus PZT voltage. Curves marked byfilled circles were obtained with the cw discharge section sealed atboth ends with antireflection–antireflection-coated ZnSe windowsand curves marked by open circles were obtained with the cwdischarge section sealed at both ends with KCl windows.

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temperature-dependent path-length change insig-nificant. To implement this, it is necessary to es-timate the temperature-dependent change inoptical path length of the different intracavity com-ponents at the operating power level of the laser.This can be estimated from the hysteresis betweenthe forward and reverse scans of the power profileas discussed in Ref. 3. For our laser, at a powerlevel of 2 W the temperature-dependent path-length change was estimated to be 1.255 mm whenthe cw discharge section was sealed by a ZnSe win-dow at both ends. The corresponding values forthe laser when the discharge section was sealedwith KCl windows of different thicknesses were alsodetermined. Since the expansion of the ZnSe out-put coupler caused by heating leads to a reductionin resonator length, the measured path-lengthchange gives the values for 2DLZnSe 2 DLOyP couplerand 2DLKCl 2 DLOyP coupler from which one can esti-mate and minimize DLZnSe 1 DLKCl 2 DLOyP coupler, thepath length that results when the tube is sealed atone end with a ZnSe window and at the other with aKCl window.

The filled circles in Fig. 2 represent the powertuning curves obtained with a proper choice of KClwindow at two power levels, ;1 and ;2 W. Thepower tuning curves are symmetric at both operat-ing power levels, suggesting the absence of a nettemperature-induced path-length change. Theopen circles in Fig. 2 represent the power tuningcurve obtained when an additional KCl window wasmounted in the cavity at the Brewster angle. Thecurve in this case is again asymmetric, confirmingthat an appropriate choice of thickness of intracav-ity components is necessary to obtain a symmetricpower tuning curve independent of the operatingpower levels. This also suggests that, when anexact choice of window thickness is not practical, a

Fig. 2. Output power versus PZT voltage. Curves marked byfilled circles were obtained with the cw discharge section sealed atone end with an antireflection–antireflection-coated ZnSe windowand the other end with a KCl window. Curves marked by opencircles were obtained with an additional KCl window mountedinside the cavity at the Brewster angle.

change in the orientation of an intracavity plate canprovide a fine control of the net temperature-induced path-length change.

The authors thank S. C. Mehendale and I. Mukho-padhyay for a critical reading of the manuscript andS. Chatterjee for the polished KCl windows that weused in the experiment.

References1. See, for example, S. Profeti, A. Dilieto, P. Minguzzi, and M.

Tonelli, “Light-shifts in a two level molecular system observedby CO2 lasers,” Opt. Commun. 67, 425–430 ~1988!.

2. See, for example, S. Jacobson, “Optically pumped far infraredlasers,” Infrared Phys. 29, 853–874 ~1989!; R. G. Harrison andP. K. Gupta, “Optically pumped mid-infrared molecular gaslasers,” in Infrared and Millimeter Waves, Vol. 7: CoherentSources and Applications, Part II, K. J. Button, ed. ~Academic,New York, 1983!, pp. 43–163.

3. Vas Dev, U. Nundy, N. S. Shikarkhane, P. K. Gupta, and U. K.Chatterjee, “Thermal effect of intracavity optical components onthe power tuning curve in a CW CO2 laser,” Opt. Commun. 57,107–110 ~1986!.

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