IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 46, NO. 2, APRIL 1997 149

The Influence of Ne Isotopes on the SingleMode Operation of He-Ne and He-Ne/ILasers

Stefka S. Cartaleva, Yordanka V. Dancheva, and Vladislav P. Gerginov

Abstract—When using He-Ne lasers in our experiments, weobserved that two adjacent longitudinal modes with the samepolarization operate simultaneously with steady output powerfor each mode when the frequency spacing between them isseveral times lower than the homogeneous linewidth of the lasertransition. In the case of a mixture of 20Ne and 22Ne isotopes, ananalytical expression for the output power is obtained and cal-culated for parameters close to the experimental conditions. Thetheoretical predictions are in agreement with the experimentalresults. The theory also shows that in the case of pure neon isotopethe competition between the longitudinal modes is stronger thanin the case of neon isotopes mixture. It is proposed to improvethe single mode selection efficiency for He-Ne and He-Ne/I2 lasersand to enhance their single-mode output power using pure neonisotopes.

Index Terms—Frequency stabilized lasers, gas laser, laser inter-ferometry, laser metrology, laser mode competition, laser modeselection, single-frequency lasers.

I. INTRODUCTION

T HE CONVENTIONAL single-longitudinal-mode He-Nelaser and He-Ne laser containing an internal iodine cell

(He-Ne/I laser) are widely used in the fields of laser inter-ferometry and metrology. In the short-cavity He-Ne lasersmonomode operation is obtained without additional opticalelements. However, their output power is usually less than1 mW. The realization of monomode operation in the He-Ne/I laser requires an excellent quality of optical elements.This kind of laser is very sensitive to the laser cavity length.Their output power is about 100W.

Later development of monomode He-Ne and He-Ne/Ilasers has resulted in an efficient single-longitudinal-modeselection realized by increasing the homogeneous part of thelaser transition broadening [1]–[3]. For a He-Ne laser, anoutput power from (1 to 25) mW was achieved without usingany optical selective elements [4], and for a He-Ne/Ilaser,the output power was increased to 2 mW.

It is well known [5] that if the homogeneous linewidthof the laser transition is smaller than the frequency spacingbetween the longitudinal modes, they will oscillate simultane-ously. When the linewidth becomes larger than the frequencydifference between the adjacent modes a competition betweenthem starts, leading to an unstable mode operation or even

Manuscript received June 20, 1996; revised October 1, 1996. This workwas supported by the Bulgarian National Science Fund under Grant F-439.

The authors are with the Institute of Electronics, Bulgarian Academy ofSciences, Boul, 1784 Sofia, Bulgaria.

Publisher Item Identifier S 0018-9456(97)01602-1.

to suppression of all but one mode. However, in the earlystate of our development of high-power monomode He-Nelasers which do not contain optical elements for single modeselection, it was observed that several longitudinal modesoperate simultaneously, stable in time, without competitioneven when the frequency spacing between them is severaltimes lower than the homogeneous linewidth of the lasertransition.

In this paper, we present an explanation of this phenomenonand a proposal to use it for further enhancement of the singlemode selection efficiency and the output power in both typesof lasers.

II. EXPERIMENTAL RESULTS

In the He-Ne laser (laser tube: 0.58 m long, 1.85 mm borediameter; optical cavity: two spherical mirrors with radius ofcurvature m placed at a distance m) a naturalmixture of Ne isotopes was used. The laser contains externalmirrors and a Brewster-angle laser tube which determinesonly one polarization of the laser oscillation. In order toexplain the lack of mode competition in the case of largehomogeneous linewidth, an intermediate regime of operationwas experimentally investigated. When the cavity length waschanged by , in one part of the tuning intervalthe laser oscillated in a single longitudinal mode, and in theremaining part of the interval it oscillated in two modes. Thisspecial operation was realized by appropriate choice of the gasmixture pressure and cavity loss.

Using a Spectra-Physics 450-30 optical spectrum analyzer,the following behavior was observed [Fig. 1(a)]. If at pointthe laser starts monomode oscillation, on increasing the fre-quency of the laser cavity modes (i.e., ), the monomodeoscillation will continue up to point . When the oscillatingmode is at point two-mode oscillation starts. A new,second, mode is born at point . On further increase of thefrequency of the cavity modes the “old” mode is scanningin the interval and the “new” one in the interval .During this scan the “old” mode is decreasing while the “new”one is continuously increasing its power. The “old” modestops oscillating at point . It must be pointed out that thiscontinuous flow of the power from one adjacent mode to theother is observed when the mode spacing

MHz is much narrower than the homogeneous linewidthMHz of the laser transition. Hence, despite the

strong overlap of Bennett’s holes, the two modes oscillatesimultaneously without noticeable competition.

0018–9456/97$10.00 1997 IEEE

150 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 46, NO. 2, APRIL 1997

Fig. 1. (a) Scheme of the frequency tuning intervals and (b) exper-imental output power dependence on cavity length change—solid line(p = 720 Pa, Ne-natural mixture). Computed output power depen-dence on cavity length change—dashed line(��L = 540 MHz,f = 4:6%; Gm = 6%; 20Ne=22Ne = 10):

In Fig. 1(b) (solid curve) the experimental output powerdependence on the cavity length change is shown. In theinterval – the laser operates in a single longitudinal mode.It is seen that the output power increases. The mode is scanningin the interval [Fig. 1(a)]. In the interval – laseroperates in two modes, where the output power is the sum ofthe output power of the “old” mode scanning in the interval

[Fig. 1(a)] and the “new” one scanning in the interval. At the point the single-mode operation starts again.

In our previous work [6], it was shown that in the case ofa natural mixture of isotopes, the frequency at the maximumof the unsaturated gain is lower than the frequency of theoutput power maximum (Fig. 2). The laser starts single-modeoperation at the closest to the unsaturated gain maximumcavity mode and the single-mode frequency tuning is in theinterval (1–2). Hence, in the interval – in Fig. 1(b) theoutput power increases with increasing the frequency.

III. T HEORETICAL CONSIDERATIONS

To explain the experimentally observed stable simultaneousoperation of the two adjacent longitudinal modes, the Bennett’shole-burning model [5] was expanded for a two mode oper-ation and a two neon isotope mixture. For the output powerof the laser we obtained:

(1)

where is a constant, and are unsaturated gain profilesfor the two neon isotopes given by:

(2)

where for Ne and for Ne are theFWHM and the frequency of the maxima of the Dopplerprofiles. is the “new” mode frequency and is the “old”mode frequency. Hence, MHz.and are the depths of the holes made by the two modesin the Ne gain profile. For the two-mode operation, therequirement is and [5]. The values of

Fig. 2. Computed unsaturated gain (dashed curve) and output power (solidcurve) versus frequency. The frequency at the maximum of the unsaturatedgain is taken as zero. Experimentally observed single-frequency tuning isin the interval (1–2). Gas mixture pressurep = 1 kPa corresponding to��L = 750 MHz, f = 3%; 20Ne=22Ne= 10—natural mixture of isotopes.

and are obtained by solving the equations:

(3)

where

is the homogeneous linewidth and is the resonatorloss. For the frequency difference between maxima of the twoisotope gain profiles the value MHzis taken [5].

IV. RESULTS AND DISCUSSIONS

Using (1), the output power dependence on the cavity lengthchange was computed (Fig. 1(b), dashed curve) for a naturalmixture of Ne and Ne isotopes (i.e. Ne Neand compared with the experimental one. It can be seen

CARTALEVA et al.: INFLUENCE OF Ne ISOTOPES 151

Fig. 3. Curve 1—combined absorption of R(127) and P(33) lines ofI2;

curves 2 and 3—gain profiles of20Ne and22Ne; respectively in the case ofnatural mixture.

that experimental and theoretical dependencies are in verygood agreement. The output powers of the two modes arealso calculated separately, and theoretical considerations show(in agreement with the experimental results) that, startingfrom point , the “old” mode decreases its power and stopsoperating at point . The “new” mode increases its powerfrom point to point , operating simultaneously with the“old” mode.

If the second isotope, for exampleNe, is excluded fromthe consideration, under the same conditions (homogeneouslinewidth, unsaturated gain, and cavity loss) the laser operatesin one longitudinal mode during the entire scanning intervalas a result of a strong mode competition. Consequently,the experimentally observed lack of competition between thelongitudinal modes in the case of strong overlapping of theBennett’s holes can be explained by the influence of the secondisotope.

Calculations were also performed for a He-Ne/Ilaser.Equations (1)–(3) were used where instead of a constant loss

we introduced variable with frequency cavity losswhich includes the cavity loss and Iabsorption. I absorptionprofile (Fig. 3) is calculated using experimentally measuredabsorption profiles of the R(127) and P(33) lines (11-5 and6-3 vibrational bands, respectively) of theelectronic I transition [7], [8]. The frequency position ofR(127) and P(33) are taken from [9].

For both lasers, a monomode operation can be achieved byincreasing the homogeneous broadening or at a certain homo-geneous linewidth by increasing the cavity loss. Calculationswere performed of the lowest loss which can be used at agiven homogeneous linewidth for monomode operation to beachieved (Fig. 4). It can be seen that at a certain homogeneouslinewidth the single mode selection for both kinds of lasers,He-Ne and He-Ne/I is more effective (i.e. obtained at lowercavity loss) when pure Ne is used.

V. CONCLUSIONS

For He-Ne and He-Ne/I lasers, an enhancement of thesingle-mode selection efficiency is proposed using pure neon

Fig. 4. Homogeneous broadening dependence on cavity loss whensingle-mode operation starts. Curves: 1, He-Ne(nat.mixture); 2,He-Ne(nat.mixture)/I2; 3, He-20Ne/I2; 4, He-20Ne:

isotopes. A simple, (2 to 3 mW) frequency-stabilized He-Ne laser, would be useful for two- or three-dimensionalmeasurements in laser interferometry in place of more than oneconventional frequency stabilized He-Ne laser. If pureNe isused in the conventional He-Ne/laser, this will make thelaser less critical to the cavity length and will improve itsoutput power.

REFERENCES

[1] S. S. Cartaleva, S. V. Gateva, and G. V. Kolarov, “Simple single-frequency He-Ne laser,”Appl. Phys., vol. B40, p. 153, 1986.

[2] L. Shang-Yi, X. Jian-Ning, and C. Da-Guang, “Theoretical analysis ofa single-frequency He-Ne laser,”Chin. J. Lasers, vol. 13, p. 392, 1986.

[3] S. S. Cartaleva and S. V. Gateva, “Output power enhancement andfrequency tuning peculiarities of 632.8 nm monomode He-Ne/I2 laser,”Appl. Phys., vol. B54, p. 307, 1992.

[4] , “Multiline, single longitudinal mode helium-neon laser,” in1stGeneral Conf. Balkan Phys. Union, Thessaloniki, Greece, 1991, p. 929.

[5] W. R. Bennett, Jr.,Physics of Gas Lasers, W. R. Bennett, Ed. NewYork: Gordon and Breach, 1977.

[6] S. S. Cartaleva, Y. Dancheva, S. V. Gateva, and V. P. Gerginov,“Frequency tuning peculiarities of enhanced power monomode He-Nelasers,”Opt. Quantum Electron., vol. 28, p. 359, 1996.

[7] G. R. Hanes, J. Lappierre, P. R. Bunker, and C. Shotton, “Nuclear hyper-fine structure in the electronic spectrum of127I2 by saturated absorptionspectroscopy, and comparison with theory”,J. Mol. Spectrosc., vol. 39,p. 506, 1971.

[8] A. Brillet and P. Cerez, “Quantitative description of the saturatedabsorption signal in iodine stabilized He-Ne lasers”,Metrologia, vol.13, p. 137, 1977.

[9] S. Gersternkorn and P. Luc, “Absolute iodine (I2) standards measuredby means of Fourier transform spectroscopy,”Rev. Phys. Appl., vol. 14,p. 791, 1979.

Stefka S. Cartaleva received the M.Sc. degreein optics and spectroscopy in 1968 and the Ph.D.degree in 1973, both from the Moscow State Uni-versity, Russia.

In 1974 she joined the Quantum Electronics Divi-sion, Institute of Electronics, Bulgarian Academy ofSciences, Sofia, Bulgaria, where she has been As-sociate Professor from 1991. Her research interestsinclude mode selection and frequency stabilizationin gas and diode lasers, optically pumped dimerlasers, linear and nonlinear laser spectroscopy.

Dr. Cartaleva is a member of the Bulgarian chapter of SPIE.

152 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 46, NO. 2, APRIL 1997

Yordanka V. Dancheva received the M.Sc. degreein quantum electronics and laser technique fromSofia University, Sofia, Bulgaria, in 1992. She iscurrently pursuing the Ph.D. degree at the Instituteof Electronics, Bulgarian Academy of Sciences,Sofia, where she is a research assistant.

Her research interests are in the fields of molecu-lar and atomic physics, gas lasers, diode lasers, op-tical feedback and its applications to high-resolutionspectroscopy.

Vladislav P. Gerginov received the M.Sc. degreein quantum electronics and laser technique fromSofia University, Sofia, Bulgaria, in 1995. He iscurrently pursuing the Ph.D. degree in the field ofhigh-resolution spectroscopy with diode lasers atthe Institute of Electronics, Bulgarian Academy ofSciences, Sofia.

His main fields of interest are gas lasers, single-frequency lasers, diode lasers, and wavelength mea-surements.