International Journal of Infrared and Millimeter Waves, Vol. 4, No. 1, 1983

NEW CH3OH LASER LINES PUMPED WITH A FINE-TUNED HIGH-POWER CO2-TEA LASER

Pierre Bernard* and Jerald R. Izatt**

Universitd Laval Laboratoire de Recherches en Optique et Laser (LROL)

Ddpartment de Physique Qudbec, Canada, GIK 7P4

Received October 19, 1982

Joule-level pulses from an etalon tuned C02-TEA laser with an - 4GHz intraline tuning range have been used to pump a large number of new CH_0H FIR laser lines. FIR wavelength

3 measurements, pump laser offsets which produced FIR laser lines and pump laser offsets at which CH30H absorption lines were observed are reported. Identzfications are pro- posed for most of the absorption lines, and an attempt to identify the FIR laser lines is described.

Key words: far-infrared laser, fine-tuned C02-TEA laser, CH30H spectroscopy

Introduction

Optical pumping with a C02 laser has produced far in- frared (FIR) laser action in some 50 different molecules. Of these, methanol has so far proved to be the most pro- lific source of FIR laser lines, with more than 150 lines between 34 and 1217~m having been pumped by at

*Present address: Institut de Recherches d'Hydro~ Quebec, 1800, Mont~e Ste-Julie, C.P. i000, Varennes, Canada, J0L 2PO

**Present address: University of Alabama, Department of Physics and Astronomy, Tuscaloosa, AL 35486

21

0195-9271/83/01004)021503.00/0 .7,1983 Plenum Publishing Corporation

22 Bernard and lzal!

least 45 different infrared transitions (i). A signifi- cant by-product of the ongoing development of the methanol laser is the production of new spectroscopic data. Combi- nation of FIR laser data with other spectroscopic measure- ments has already led to an improved model for the C-0 stretch band (2), and the laser data may also prove to be important for modelling the as yet poorly understood structure of the excited torsional-rotational states of this band, Each time a new optical pumping technique is applied to the CHAOH laser, and in particular, whenever 3 the interline tunlng range of the C02 laser is increased so as to make more transitions in the rich CH30H spectrum accessible to pumping, new FIR laser lines are observed and new spectroscopic data are generated. We report here the first use of a new C0^-TEA laser tuning technique which produces ~iJ pulsesZwith a spectral width of - 250 MHz and a tuning range of approximately • aboutthe cen- tral frenquencies of the strong C02 laser lines. This represents a 16-fold increase in tuning range as compared to a waveguide laser (i), as well as an order of magnitude increase in the output energy normally available from eta- lon tuned TEA lasers. Since the optimum operating pressure for a CH30H laser pumped with high power TEA laser pulses is 3-4 torr, the width (FWHM) of the absorption lines is

i00 MHz (3). As a consequence there is only a relative- ly small discrepancy between the spectral width of the laser emission and that of the absorption line to be pump- ed, and the pumping can be very efficient. By way of con- trast, the emission bandwidth of a conventional grating tuned TEA laser may be 40 times the absorption line width, so that efficient energy transfer is impossible. Using the new fine-tuned TEA laser, we have observed a CH30H FIR

laser output with each of the pump lines from 9P6 to 9P42 and from 9R4 to 9R38, inclusively. Casual observation in- dicates that pumping with at least 23 different lines in the C02 10.6~m band also produces a FIR laser output. In this paper, we report pump frequency offsets and wave- lengths for 34 of the new FIR lines, which were obtained by pumping with 12 P-branch lines in the C02 9.4Dm band.

Experiment

The experimental setup is shown in Fig. in The eta- lon tuned C02-TEA laser has been described in detail and its performance analyzed elsewhere (4). It uses a fold- ed ring cawity configuration with the beam making two

New CHjOH Laser Lines 23

slightly off-axis passes through the gain section, so that essentially the same gain is realized as in a standard Fabry-Perot configuration. The extra-cavity mirror M es- tablishes a preferred direction of circulation inside the cavity, and the etalon is placed so as to receive only radiation reflected frdm the output coupler C. In these experiments the reflectivity of the output coupler was 36%.

SMM LASER

Figure i: Experimental setup. FP: Fabry-Perot interferometer; D: Golay cell; BC: boxcar integrator; REC: recorder; IRS: infrared spectrometer; E: etalon; C: output coupler; M: plane mirror; TC: temperature control unit; AC: propylene attenuation cell; PAl:C02 photoacoustic cell; PA2: CH3OH photoacoustic cell; AMP: amplifier.

24 Bernard and Izatt

Consequently~ the etalon was exposed to only a small frac- tion of the power load it would have received were it mounted in a standard Fabry-Perot cavity and thus exposed to the the full intracavity power on one face and the coupler reflected power on the other~ On the strongest lines used in these experiments the laser produced ~ 1.7J pulses when

operated in the Fabry-Perot configuration~ With the eta- lon mounted as in Fig~ i, the output on these same lines

was ~ 1.4J, and no etalon damage was observed~ The Ge eta- lon~ which had a free spectral range of 5~36 GHz and a

finesse of 3~5, could be tuned either by rotating it about an axis parallel to its faces or by changing its tempera- ture. In the latter case the tuning coefficient was

2.5GHz/~C and frequency stabilization within • 25 MHz could easily be maintained~ The laser output usually con- tained two, but occasionally three longitudinal modes with an intermode spacing of 125 MHz.

After passing through a propylene filled cell, where it could be attenuated as required, the C0~ laser beam

z was split into two parts. About 4% of the energy was directed into two sequential photoacoustic cells, and the rest was used to pump the FIR laser~ One of the photo- acoustic cells contained ~ i0 torr of C0~, and absorption

z lines measured with this cell were used to calibrate the

C02 laser wavelength scale~ The second photoacoustic cell was filled with ~ 2 torr of CH~0H so that the position of the methanol absorption lines could be measured on this same wavelength scale~ The energy transmitted by the

photoacoustic cells was detected by a joulemeter which was calibrated so as to give a direct reading of the energy

at the entrance of the FIR laser~

The FIR laser was a rectangular guided wave resonator 2.5cm x 1.2cm in cross-section and i~i meter long. Hole

coupled mirrors were used on both ends~ The input end was closed with a Brewster's angle NaCI window and the exit end with a quartz window~ A Golay cell was used for FIR detection and the wavelengths were measured with a Fabry-Perot interferometer~

In a typical measurement sequence, the C02-TEA laser was grating tuned to one of its lines and the etalon temp- erature was then varied by about 12~ in order to scan the laser frequency through about 5 etalon periods. The joule- meter was used to record the laser output throughout the

New CH~OH Laser Lines 25

a)

b)

c)

d)

l I I I -2 -I 0 I

Az/(GHz)

Figure 2. Signals recorded as the etalon tuned C02-TEA laser was scanned over the tuning range associated with the 9P12 line. a) FIR laser output; b) CH~0h absorption; c) CO~-TEA laser outpuT; d) C02 absorption.-

26 Bernard and lzatt

scan. The signal from the C02 photoacoustic cell was also recorded on the same scale, and the resulting series of repeated absorption maxima, being separated by the etalon's free spectral range, served as fiduciary marks for the frequency calibration. A typical joulemeter scan over one laser output peak is shown in Fig. 2c, and the correspon- ding signal from C02 photoacoustic cell is shown in Fig. 2d. During the same frequency scan the FIR laser output

and the CH~0H absorption spectrum produced by the second photoacous~ic cell were also recorded. These spectra are illustrated in parts a and b of Fig. 2. Note that an ab- sorption dip also appears in the laser output spectrum of part c due to the fact that the joulemeter was located be- hind the photoacoustic cells. Finally, the etalon tempera- ture was stabilized at a convenient value; the etalon was then rotated to the position which produced the desired FIR laser line, and the wavelength of the latter was meas- ured.

Results and Discussion

The results are summarized in Table I. The offsets from the center frequency of the pump laser lines for which FIR laser outputs were observed are given in the second column. Similarly, the offsets for which a CH30H absorption line was observed are listed in the fourth column. When these two offsets agree to within the experi- mental error of • 80 MHz, the FIR emission is assumed to result from pumping of the corresponding absorption line and the two are placed on the same horizontal line in the table. Note that absorption lines and laser emissions are not always paired in this way; i.e., strong FIR laser emission was sometimes observed for a pump laser offset which produced no detectable CH~0H absorption, and vice versa. A striking example is s~own in Fig~ 3.

The observation that there is often no correlation between the frequency offsets associated with the FIR laser lines and the absorption lines strongly suggests that the dynamics of the pulsed CH~0H laser can produce FIR gain even when no direct pumpifig path is evident. Several other observations also point to the importance of dynamic effects. Note first that C02-TEA laser pumping rarely pro- duces the same FIR laser lines that are produced by cw C02 laser pumping. Secondly, we have made preliminary measure-

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New CH~OH Laser Lines 31

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J lines at corresponding pump laser offsets. The C0~-TEA laser was operating on theZl0P20 line. a) FIR laser output; b) CH30H absorption~

32 Beznard and Izs~t

ments which show that there is wide variation in the threshold and saturation behavior of different FIR laser lines pumped with the TEA laser. The FIR lines studied to date fall into three distinct categories i) lines whose strength displays essentially linear pump energy dependence from zero up to the highest pump energies we used ( ~ 0~6J in 50 nsec pulses); 2) lines which start off linearly but show strong saturation effects; 3) lines which display a well-defined pump energy threshold which is followed by a linear growth region and subsequent saturation. For these latter lines the saturation is less rapid than for the lines in category 2~ In all three cases FIR laser power is produced over a CH~0H pressure range extending up to

J 6 tort, and it peaks between 3 and 4 torr. However,

in marked contrast to the other two types, category 3 lines display a well-defined pressure threshold at i~5 to 2 torr. We have also shown elsewhere that collisional transfer between torsional-rotational levels in a i00

mtorr, quasi-cw, CH30H laser is important, and that it can lead to the preferential population enhancement of specific levels (3). Such effects might also be expected to occur in our pulsed laser which was operated at a pressure some 30 times greater. Finally, at these high pump power densities multiphoton effects may also be important. Taken together, these observations seem to indicate that complex, line-specific, dynamic modelling will be required in order to explain the operation of the pulsed CH30H laser.

The FIR laser wavelengths given in the third column of Table I were measured with a precision of • 0.1%, except in a few instances which are indicated explicitly in the table. Note that at some offsets multiple FIR laser lines were produced. The relative intensities shown have not been normalized to the pump power nor adjusted in any other way, and they should therefore not be regarded as anything more than crude estimates~ Use of a metallic waveguide laser precluded meaningful measurement of the relative polarization of the pump beam and SMM laser output.

The absorption line identifications given in Table I were arrived at by the following procedure. The

t~ torsional-rotational levels of the v vibrational state of the C-0 stretch mode are given by the formula (5),

New CH~OH Laser Lines 33

E(nTK, J) v v v 2 BnT K - DnT K he ~vib + J(X + i) j2(j + l)

v j3(j + 1)3 + WnTKV • v HnTK nTK, J (i)

0 1 Here v = 0, and v ~ is the central frequency of

vib iD the first ~xclted state o~ the v~ vibrational mode; i.e.

- v ~ ' i033~9 cm . B is an effective rotation constant that depends on K and also on the internal rotational state designated by the quantum numbers n and r. D v represents the effect of centrifugal stretching on the rotation of the whole molecule and also includes a small asymmetry shift which depends on n, T and K. W v is the J-independent

part of the internal and whole-molecule rotational energy. v S represents the K-type asymmetry splitting, and H ms an

v empirical correction factor which becomes importan~ for large Jo Henningsen has tabulated the quantities B~TK,

0 0 1 1 1 and 1 D0T K, W0T K, B0T K, DOTK, WOTK, H0T K for 0 i K ! i0 and

T = 1,2,3 (5). Also, H~ ~ z = 0. Using the tabulated values and the appropriate selec[ion rules (Av = i, AK = 0 and AJ = 0, • i) together with eq. (i) we calculated all of the possible absorption line frequencies. The identifi- cations were made by direct comparison of the measured frequencies with these calculated values.

In order to verify the accuracy of our absorption frequency measurements, 18 of the lines were compared with much higher resolution diode laser measurements made by Sattler et al (6,7). The mean discrepancy between the two sets of measurements is 120 MHz, which is very satis- fying in light of the 250 MHz width of the TEA laser out- put. In addition to the three absorption lines listed for the 9P24 C0~ laser line in Table I, we also observed g a weak fourth llne very near the center frequency of the laser line. This line is not present in the diode laser spectrum. Since the smallest offset for the other three lines is - 390 MHz, this weak line is probably responsi- ble for the SMM laser lines pumped with a cw C02 laser operating on 9P24 (8).

The X-levels which appear in Table I are members of a frequently postulated but as yet unidentified ro-vibra- tional manifold, many of whose states are quasi-degenerate with levels of the ~8 band (2,9,10). The perturbation

34 Bernard and Izatt

appears to be sufficiently strong for K = 5 to require that the series (J, 025) C-0 and (J, 025) X be considered as hy-

brid doublets. The upper and lower doublet members for this case are designated respectively by the superscripts

u and i. Sattler et al give an empirical formula for the doublet separation which was deduced from their laser diode spectra (6). We used this formula in our calcula- tions. Another nuance in the energy level notation of Table I is the use of + and - superscripts to denote the individual members of the K-type doublets associated with the term S v in eq. (i).

Further caluclations at the same level of precision

were made in an attempt to identify the new FIR laser lines. Various pumping schemes which have led to the successful identification of other FIR laser lines were tried, but they produced no identifications for the new lines. It has already been noted that in many cases a FIR laser line was not accompanied by an absorption line at the same pump laser offset. In other cases where an absorption line was indeed observed, our calculations predicted no absorption line close enough to be within the limits of experimental accuracy. In still other

cases, an identifiable absorption line occurs at the same offset as a FIR laser line, but the frequency of the

latter is very different from the caluclated values of all the expected transitions. In no case were we con- fronted with a choice between two or more transitions, any one of which would satisfy the constraints imposed by the measurements. In only one case was it possible to identify both an absorption line and FIR laser emission that corresponded to the same pump laser offset. This occured for the 205.5~m line pumped with 9P18, but this

line had been observed previously with C02 waveguide laser pumping, and our calculations served only to confirm

the existing identification (2).

Although it is likely that some of the shorter FIR laser lines result from transitions between n = 1 and n = 0 states, we were able to identify none of them nor any of the absorption lines with such transitions, per- haps due to the limited accuracy with which the molecular

constants for the excited torsional state are known.

New CH3OH Laser Lines 35

Conclusion

A large number of FIR laser lines can be produced by

pumping CH_0H with a high power C02-TEA laser. We have 3

~emonstrated that these FIR emissions can be selectively and efficiently excited by etalon tuning within the line- width of the TEA laser. We have shown also that simple techniques can be used in conjunction with the fine-tuned TEA laser to measure CH30H absorption line frequencies with sufficient accuracy to permit the identification of most of them. If required, the spectral resolution can be enhanced by using an etalon with sufficient finesse to assure SLM operation of the TEA laser~ Our measurements of pump offsets and FIR wavelengths, which were also made with good precision, did not result in the identification of any of the FIR laser lines. This failure does not appear to result from a need for still greater precision in the measurement, but rather from an incorrect or in- complete conception of the gain mechanisms which are domi- nant under these experimental conditions.

This work was supported in part by funds from the National Science and Engineering Research Council of Canada~

36 Bernard and lzatt

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3. P. Bernard, J~ R. Izatt (to be published).

4.

5.

P. Bernard, P. Mathieu, J. R. Izatt, Opt. Commun., 37_7, 285 (1981).

J. O. Henningsen, IEEE J. Quant. Electron., ~E-13, 435 (1977).

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J. P. Sattler, T. L. Worchesky and W. A. Riessler, Infrared Physics, 18, 521 (1978).

J. P. Sattler, W. A. Riessler and T. L. Worchesky, Infrared Physics, 19, 217 (1979).

F. R. Peterson, K. M. Evensen, D. A. Jennings and A. Scalabrini, IEEE J. Quant. Electron., QE-16, 319 (1980).

E. Arimondo, IEEE J. Quant. Electron., QEFI5, 1081 (1979).

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R. M. Lees, M. A. Walton, J. O. Henningsen, J. Mol. Spect., 88, 90 (1981).

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