A selfstabilized 3.5μm waveguide He–Xe laserP.W. Smith and P.J. Maloney Citation: Applied Physics Letters 22, 667 (1973); doi: 10.1063/1.1654547 View online: http://dx.doi.org/10.1063/1.1654547 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/22/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in He–Xe laser magnetometry J. Appl. Phys. 83, 4994 (1998); 10.1063/1.367305 Longitudinal mode selfstabilization in semiconductor lasers J. Appl. Phys. 53, 4631 (1982); 10.1063/1.331290 Infrared–microwave double resonance spectroscopy of DCCCHO using the 3.51 μm He–Xe laser J. Chem. Phys. 72, 4089 (1980); 10.1063/1.439637 Extension of absolute frequency measurements to 148 THz: Frequencies of the 2.0 and 3.5μm Xe laser Appl. Phys. Lett. 26, 510 (1975); 10.1063/1.88238 OBSERVATION OF AN ENHANCED LAMB DIP WITH A PURE Xe GAIN CELL INSIDE A 3.51μ He–XeLASER Appl. Phys. Lett. 17, 120 (1970); 10.1063/1.1653330

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A self-stabilized 3.5-J,Lm waveguide He-Xe laser

P.W. Smith and P.J. Maloney Bell Telephone Laboratories, Holmdel, New Jersey 07733 (Received 19 March 1973;in final form 16 April 1973)

By building a He-Xe laser within a hollow dielectric waveguide and by using a single isotope of Xe, we have been able to attain gains of 1000 dB/m. The dispersion associated with these large gains significantly modifies the "empty-resonator" mode spacing when such a medium is placed within a laser resonator. We describe experiments with a short single-frequency waveguide He-Xe laser in which the gain medium pulls the oscillating frequenCies so that the laser always oscillates within ± (1/100) (c/2L) of the center of the Xe net-gain curve, regardless of the physical length of the laser resonator.

Waveguide gas lasers were first proposed by Marcatili and Schmeltzer1 and were first demonstrated with the 6328-A. He-Ne laser by Smith. 2 Basically, such a laser uses a smooth straight dielectric capillary tube both to guide the laser light and to confine the gas discharge. These lasers operate at higher gas pressures than normal gas lasers, and may have correspondingly higher gains per unit length. 2,3 The experiments we will describe below show that the waveguide He-Xe laser not only exhibits the expected high gains per unit length, but also operates at very much higher Xe pressures than conventional He-Xe lasers. This feature may mean that gas clean-up effects are much less severe for wave­guide He-Xe lasers, but we did not investigate this.

In this letter we describe a technique to obtain laser oscillation within a narrow frequency band. This techni­que utilizes the frequency-pulling effects due to the large dispersion associated with the high gain. Bennett4

and, more recently, Garside5 and Casperson and Yariv6 have described the pulling of the "empty-resona­tor" resonance frequencies by the dispersion of the gain medium. For the case where the gain medium is not appreciably saturated, the frequency pulling near line center can be described by the equation6

.1v = (c/2L){1 + (3r1, (1)

where .1v is the longitudinal mode spacing in the pres­ence of the laser medium, c is the vacuum velOCity of light, L is the length of the resonator, and (3, the diS­persion parameter, is given by the equation,

(3=Kl....E&:.. L .1vG'

(2)

where 1 is the length of the gain medium, g is the peak small-Signal incremental gain, and .1vG is the fullwidth at half-maximum of the gain curve. K is a numerical factor. If the shape of the gain curve is determined pri­marily by Doppler broadening (Gaussian), K = rr-3/

Z

(In2)1/2. If the shape of the gain curve is determined by gas collision effects (Lorentzian), K = ,,-1. For values of (3 appreciably greater than unity, we see from Eq. (1) that the resonator mode spacing will be appreciably less than the empty-resonator value of c/2L. '

Our self-stabilized waveguide He-Xe laser uses also the mode competition that results from operating at a high enough pressure to have the gain curve partially homo­geneously broadened. Under these conditions it has previously been observed with the 6328-A. He-Ne laser that only the resonator mode closest to line center will oscillate and that, as the resonator length is varied,

667 Appl. Phys. Lett., Vol. 22, No. 12, 15 June 1973

the oscillating mode switches so that the maximum frequency excursion from line center is ± i.1v.

Experiments were performed with specially straightened and selected sections of 250-/-Lm-i.d. Pyrex capillary tubing, which were tested for optical wave guiding prop­erties before being used to make the laser tubes. Be­cause of the vary high gains involved, gain measure­ments were made on short tubes of 2. 5-cm length. The gain was measured by passing a chopped Single-fre­quency beam from a conventional 3. 5-j..Lm He3 :Xe136 laser through the amplifier tube and by using phase­sensitive detection to measure the increase in the signal when the discharge in the amplifier tube was turned on. The results were corrected for the focusing effects of the discharge by scanning the small-area detector over the output beam and by computing the total intergated intensity. Figure 1 shows the results for three mix­tures of He3 :Xe136. The maximum gain occurred at the lowest discharge current for which a stable discharge

860

820

780

E ..... 740 In "CI

~ z 700 « (!)

660

620

580 :3 4 5 6 7

TOTAL PRESSURE IN TORR

FIG. 1. Experimentally determined plots of gain vs pressure for various mixtures of Hes :Xe 136. For clarity the experi­mental points are not shown. The scatter was ± 10 dB for 3 : 1 and 5: 1 curves, and ± 30 dB for the 1.5: 1 curve.

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668 P.W. Smith and P.J. Maloney: Self·stabilized 3.5-/lm-waveguide He-Xe laser 668

(a)

PIEZO ELECTRIC CERAMIC

WAVEGUIDE LASER

PIEZO ELECTRIC CERAMIC

23 MHz --t I+-

A EMPTY RESONATOR

6.V ~FREQUENCY SHIFT OF 800 MHz

&1 = 800 -1 = 34 to' 23

(b)

FIG. 2. (a) Schematic diagram of the apparatus used to make dispersion pulling measurements. (b) Typical measurement and computation of fJ parameter.

could be maintained. This was typically 0.35 mA, at a voltage drop of -1000 V across the tube. A reduction in gain of about 30% was found when naturally occurring Xe was substituted for Xe136

• By using a combination of rf and dc excitation, a peak gain of 1000 dB/m was measured for a 3: 1 mixture of He3

: Xe136 at a total pres­sure of 5. 9 Torr. For various reasons, only dc excita­tion was used for the experiments desCribed below.

Mode-pulling experiments were performed with the ap­paratus shown in Fig. 2(a). Because of the term l/L in Eq. (2), we see that, to obtain large mode pulling, it is necessary to have the laser tube as long as possible for a given resonator length. If the laser tube is too long, however, the amplified spontaneous emission will saturate the gain, and smaller mode-pulling effects will occur. Because the Brewster-angle windows occupy a certain amount of unused resonator space, the maxi­mum mode pulling should be found with the longest laser tube consistent with the second criterion above. It was computed that a 5-cm length of discharge was the longest that could be used with the gain of 1000 dB/m before amplified spontaneous emission causes apprecia­ble saturation, and thus this length of laser tube was used for the experiments described below. The mirrors both had a radius of curvature of 1. 5 cm and a reflec­tivity of 80%. The total resonator length was 7.5 cm.

To measure f3, the resonator length could be changed a known amount by varying the voltage applied to the piezo­electric ceramic translator on which one of the laser mirrors was mounted. An absolute calibration could be made by measuring the change in voltage necessary to change the resonator length by ~/2, 1. e., to tune the laser through a complete resonator period. The output frequency could be observed with a slowly scanned interferometer, and the output was recorded on a chart recorder. In order to prevent feedback from the scan-

Appl. Phys. Lett .• Vol. 22. No. 12. 15 June 1973

ning interferometer back into the laser resonator, a confocal scanning interferometer was used with off-axis excitation as shown in Fig. 2(a). A measurement of f3 was made by sCanning the scanning interferometer through the laser frequency, then changing the laser resonator length a known amount by applying a voltage step ~ V as shown in Fig. 2(a), and observing the change in frequency of the laser output as the scanning inter­ferometer again scans over the laser output. The fre­quency scale of the scanning interferometer output can be calibrated from traCing successive resonances of the scanning interferometer with a Single fixed frequen­cy input and measuring the interferometer length with a ruler.

Figure 2(b) shows a typical result. The output frequency was observed to shift only 23 MHz when the resonator ,frequency was changed by 800 MHz, which indicates a f3 of 34. From the derivation of Eq. (2) it is clear that the maximum frequency pulling occurs when the gain is unsaturated, Le., at low power levels. Loss was in­troduced into the laser resonator both by closing down an aperture within the resonator and by inserting a sheet of lossy material. A maximum f3 of 50 was ob­served at low power levels (- O. 5 J.LW in the resonator) with a 3: 1 mixture of He3 :Xe136 at a total pressure of 4 Torr. It was also observed that under these conditions only the single laser mode closest to line center would oscillate, and mode switching would occur as the laser was tuned away from line center. Thus we have achieved a Single-frequency laser oscillator which, without any external electronics, would always oscillate within ±rAn-(c/2L) of the center of the net gain curve.

It should be pointed out that because of the focusing ef­fects of the laser medium7 (which are frequency depen­dent) the absolute value of the mean laser frequency will depend on the laser power level and the size of aperture in the laser resonator. These effects can be minimized by using a uniform thin sheet of lossy mate­rial to introduce loss into the laser resonator.

Wang8 has measured the colliSion -broadening param­eters for He-Xe collisions. He finds the collision­broadening contribution to the 3. 5-J.Lm line due to He-Xe colliSions to be9

~Vcoll = (3. 7 + 19P) MHz (3)

where P is the total pressure in Torr. For our experi­ments at 4 Torr, the colliSion linewidth of - 80 MHz is comparable to the Doppler linewidth (110 MHz).

Let us compute f3 for our experiments from Eq. (2). We will use g= 840 dB/m (see Fig. 1), ~vG= 190 MHz (the sum of the GaUSSian and Lorentzian linewidths), and K= O. 234 (the average of the Lorentzian and Gaussian values). We obtain f3=48 to be in very good agreement with the experimentally measured value of f3= 50.

Because of the short length of the laser resonator, the frequency stability of our laser was only ± 20 MHz [=±Ifur(c/2L)].10 In order to decrease c/2L and thus increase the frequency stability of our laser, we must use a longer resonator. The length of the gain medium must also be increased so that f3, which is proportional to l/L [see Eq. (2)], will not be reduced. This can be

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669 P.W. Smith and P.J. Maloney: Self·stabilized 3.5-/lm-waveguide He·Xe laser 669

done by using a number of short laser tubes (- 5 cm) with attenuators between them to prevent saturation of the output by amplified spontaneous emission.

Casperson and Yariv6 have pointed out that, for a high­gain laser with f3> 3. 51, it should be possible to observe three frequencies, each having the same number of half-wavelengths of light within the resonator. Although some broadening of the scanning interferometer trace was observed as the laser was tuned across the gain curve, no clear-cut indication of three-frequency opera­tion was observed. This may be due to the fact that in these narrow capillary tubes a stable discharge could not be sustained at gas pressures lower than 2 Torr, and from Eq. (3) we see that at these pressures the homogeneous linewidth is still large enough to cause mode suppression over a large fraction of the gain bandwidth.

Appl. Phys. lett., Vol. 22, No. 12, 15 June 1973

lE.A.J. MarcatiliandR.A. Schmeltzer, BellSyst. Tech. J. 43, 1783 (1964).

2p. W. Smith, Appl. Phys. Lett. 19, 132 (1971). 3T.J. Bridges, E. G. Burkhardt, and P. W. Smith, Appl. Phys. Lett. 20, 403 (1972).

4W.R. Bennett, Jr., Phys. Rev. 126, 580 (1962). 5B.K. Garside, IEEE J. Quantum Electron. QE-5, 97 (1969). 6Lee Casperson and Amnon Yariv, Appl. Phys. Lett. 17, 259 (1970).

7See , for example, L. W. Casperson and A, Yariv, Appl. Cpt. 11, 462 (1972).

8S. C. Wang, Ph. D. thesis (Stanford University, 1971) (unpublished) .

9Wang measured primarily the effect of He-Xe collisions. With the relatively large proportion of Xe in our lasers, resonant Xe-Xe collisions may appreciably increase the pressure broadening.

10With a thermally stabilized resonator this frequency stability could be improved to ± 0.1 MHz or Ix 10-8.

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