New SMMW Laser Transitions Optically Pumped by a Tunable C 02 Waveguide Laser

Optical pumping of a submillimeter wave (SMMW) laser with a relatively compact RF-excited CW CO 2 laser is described. The increased frequency tunability of the waveguide pump laser has resulted in new low threshold SMMW emissions in C 2 H 2 F 2 , CDF 3 , and CD 2 F 2 by pumping into absorption lines which are beyond the tuning range of a conventional CO 2 laser. Frequency offsets and some assignments obtained with the aid of a tunable diode laser heterodyne spectrometer are reported.


INTRODUCTION
PTICAL pumping of vibrational-rotational transitions of polar molecules has produced over a thousand discrete frequencies in the 40 pm-2 mm range [ 11.A more compact submillimeter wave (SMMW) optically pumped source is highly desirable and would more easily permit applications of these lasers outside the laboratory.In this paper we report the results of using an RF-excited CO, waveguide laser to pump several SMMW laser gases.The advantage of combining a CO, waveguide pump laser with an SMMW waveguide laser to produce a more compact source was first discussed by Hodges and Hartwick [2] .Their initial effort, however, was not pursued because of technical difficulties of low pump power and poor stability [3] .Techniques in RF excitation of CO, waveguide lasers [4], [5] appear particularly attractive for use in compact systems because of 1) the reported improved performance in power and efficiency, 2) the potential for long-lived sealedoff operation attributed to cathode elimination, and 3) the replacement of the high-voltage power supply by the more compact lower voltage R F power supply.
Besides the reduced size, the CO, waveguide laser has a greater frequency tunability.This desirable feature permits optimal pumping of already known SMMW laser lines with large infrared (IR) absorption frequency displacements from the COz line center [ 6 ] , [7].Additionally, many new laser lines should be expected [8] , [9].The operating pressures of the optically pumped lasers are low enough that the IR absorption transition at frequency vo is Doppler broadened with a linewidth Av,(FWHM) = 7.162 X lo-' ( -: ) ' I2 vo MHz (1) for molecular weight M at temperature T ( K ) .The calculated linewidths (FWHM) for several laser gases as listed in  range from 98 MHz for the relatively light NH, molecule to 29 MHz for the heavier CH31 molecule.It can be seen that there is a more critical need for a good coincidence between the pump and absorption frequencies in pumping the heavier molecular gases.
The collision-broadened linewidth for a CO,/N,/He laser can be obtained from where f , is the fraction of gas x , and P and Tare the pressure and temperature of the gas.The total bandwidth of oscillation depends on the small-signal gain go(vo) at line center, the mirror reflectivities r1 and Y,, and length L through the expression [ 101 Degnan [lo] has shown that, for a given cavity length, optimization of parameters for maximum output power at line center results in a sacrifice of maximum tunability.Most CO, lasers are optimized for optimum power near line center and not for maximum bandwidth.For the waveguide lasers, which have larger gain at the higher operating pressures, the requirements for a large oscillating bandwidth and sufficient power for optical pumping can be met by increasing the discharge length.However, in the absence of a mode-selecting optical element internal to the resonator, the maximum tunability is limited by the length to the longitudinal mode spacing of c / 2 L .Greater than 1 GHz tuning on a single line of a CO, waveguide laser was reported a number of years ago [ 1 1 1 .The U.S. Government work not protected by U.S. copyright output power, however, was limited to 80 mW.Today, there still exist technological problems in simultaneously obtaining wide-bandwidth single-mode operation and sufficient power for optical pumping [6], [7].The required power also leads to a tradeoff of the available bandwidth for other applications such as high resolution saturation spectroscopy [ 121.
The power (Ppump) necessary for pumping an SMMW laser depends on the pump threshold for a particular laser gas and the SMMW power (PSMMW) desired.For a low-loss SMMW resonator, CW pump thresholds of less than 1 W have been observed in a number of laser gases.The maximum theoretical conversion of CO, power into SMMW power is given by [ 11 Assuming a CO, laser power of 5 W, which is well above threshold for many optically pumped SMMW laser lines and also obtainable from an RF-excited waveguide laser, one calculates a maximum theoretical SMMW power of 250-25 mW for the 0.1-1 mm wavelength (A) range.Although for certain cases conversion efficiencies up to 30 percent of the theoretical limit have been obtained [13], more realistically one might obtain 25-2.5 mW for 5 W pump power at discrete frequencies in the 0.1-1 mm range.

EXPERIMENTAL DETAILS
The CO, waveguide laser used in this experiment is an RFexcited Laakmann Electro-optics Model RF-4400D with a grating and bimorph bender pyroelectric mirror mount for line and frequency selection.
When freshly filled with gas, the laser produces 6-7 W CW power on the strongest lines.The laser is sealed off at a pressure of 115 torr, nearly five times that for our conventional pump laser.
A homogeneous linewidth of 860 MHz is calculated from (2).Assuming a gain of 1.8 percent/cm based on a similar device [5], we estimate the calculated oscillation bandwidth to be near 1700 MHz from (3).The maximum frequency tunability of the laser is limited by the 45 cm length to the free spectral range of 330 MHz.Some reduction in the tuning range from this value should be expected because of the anomalous index of refraction of the medium [12].Beat signals out to approximately 150 MHz were obtained in the 1OP region by heterodyning the waveguide laser against a stable low-pressure COz laser set at line center.Heterodyne measurements could not be performed in the 1OR region because the reference laser was confined to operation in the 1OP region; however, in the 1OR region more difficulty with hopping to adjacent lines was encountered when the cavity length of the waveguide laser was tuned.This can be attributed to the closer spacing in frequency of the 1OR lines and the small spot size, resulting in somewhat poor grating resolution in a waveguide laser.
The SMMW resonator is an unoptimized hole-coupled resonator with a 2.54 cm diameter and a 90 cm long metallic waveguide.The input radiation injection hole is 1.5 mm in diameter and the SMMW output hole is 3.5 mm.Further details [14] and some preliminary results obtained with the waveguide pump laser [8] have been reported.The diode laser heterodyne spectrometer used to obtain additional spectroscopic information has been described elsewhere [ 151 .
RESULTS AND DISCUSSION Our first efforts using the waveguide pump laser have been to pump several gases already known to be good SMMW laser sources.These include 1,l-difluoroethylene (C,H,F,) [16], deuterated methylene fluoride (CD,F,) [ 171 , and deuterated fluoroform (CDF,) [14].These gases absorb well in the 10 p n R and P regions, where our waveguide laser has its maximum power.C2H,F, provides an excellent demonstration of the advantage of using a waveguide laser to pump into strong absorptions which are beyond the tuning range of a conventional CO, laser.Of particular interest here is the CO, 10P22 laser line which already is known to pump several SMMW laser lines.Koo and Claspy [18] observed that the absorption coefficient of C2H,F, remains constant as the 10P22 CO, laser line is tuned +50 MHz from line center.2(a) for the tuning range of our waveguide laser.The calculated SMMW transition strengths for the strong absorption lines at -11 7 and +87 MHz are seen in Fig. 2(b) to also be quite substantial.
Our original efforts to pump 1,l-difluoroethylene with the CO, 10P22 waveguide laser line led to oscillation on two new laser lines corresponding to the absorptions at frequency offsets -11 7 and + 86 MHz.The measured wavelengths (Table 111) agree with the calculated values within our estimated experimental accuracy of k0.2 percent.Despite the low pump threshold of less than 1 W, these lines have not been previously reported with conventional pump lasers.In order to obtain oscillation of the previously reported 890pm lines with the waveguide laser, the optimum cavity position had to be found with a 20 W conventional CO, laser.A frequency scan [Fig.3(a)] across the CO, waveguide laser tuning profile shows the positions of the absorptions for the two new lines and one of the old 890 pm lines.Fig. 3(b) was obtained with about 50 percent less pump power, in which case the  The symmetric top molecule CDF3 was selected for pumping with the waveguide laser because of the rich IR absorption spectrum and the relatively small IR Doppler widths.The gas also appears to be a promising radiation source at several wavelengths past 500 pm where there are few strong lines.Previously, 27 laser lines were observed from this gas when the pump was a conventional C 0 2 laser with a maximum of about 14 W CW coupled into the SMMW cavity [14].Optical pumping of CDF3 with the waveguide pump laser resulted in the observation of 11 measurable SMMW lines.Five of these lines (Table IV) had not previously been observed with the conventional pump laser.As was expected, several of the strongest previously observed CDF, lines (658.5, 488, 1008, and 532.4pm) were observed.It was more impressive that some "weaker" lines would oscillate with the relatively low power of the CW waveguide laser.The 388 pm line pumped by the 10R32 line operated only in the chopped mode of the conventional C 0 2 laser, but operated true CW in the identical SMMW resonator with 1 W pump power from the waveguide laser.Similarly, the 1264 pm line pumped by the 10R16 line had a much lower threshold with the waveguide laser.These observations can easily be understood on the basis of the relatively large absorption frequency offsets for these particular pump lines and the small IR Doppler widths for this gas (approximately one half those for CH3F or CH30H).
The approximate frequency displacements for the IR absorption transitions were determined by observing the SMMW output as the C02 laser frequency was tuned.The frequency offsets listed in Table IV were then determined more accurately from the tunable diode laser heterodyne spectrum.Several specific examples in CDF3 are described below.In general, the "strong" previously observed lines were found to have absorptions with relatively small frequency offsets.On the other hand, previously observed lines giving improved power output with the waveguide pump were found to have larger offsets for the absorption frequency.
As an example of this case, the tuning profile for the 10R16 C02 line  the frequency offset of the absorption transition was determined to be -65 MHz (k6).In the final case of the five newly discovered CDF3 lines, all of the corresponding absorption transitions were found to be greater than 75 MHz from the CO, line center.The transferred Lamb dip from the absorption at -104 MHz relative to the 10R 18 line is seen on the SMMW 459.4 pm line [Fig.5(a)] .The corresponding tunable laser diode spectrum appears in Fig. 5(b).
A knowledge of the SMMW relative polarizations aids considerably in the assignment of the laser transitions.Unfortunately, in this experiment where a low-loss metal waveguide was used, we were unable to determine the preferred polarization with certainty.The measured value of SMMW wavelength (within kO.2 percent) is sufficiently accurate to determine J (but not K ) unambiguously.In some cases, where the absorptions are @branch transitions, we were able to make assignments on the basis of the diode laser spectral pattern along with the SMMW data.For example, the general features of the CDF, RQ3 spectrum [Fig.4(b)] near the 10R16 line were first identified from a Fourier transform IR spectrum.The fact that the absorption located at -65 MHz produces 1260 pm SMMW radiation enables us to assign that absorption as RQ(12, 3) and also to assign the other J values in this region.
The assignments for the 10R 16 and ) Fig. 3. Upper trace: C02 waveguide laser tuning profile for 10P22 line.
Lower trace: SMMW output with cavity optimized for 890 pm line.C2H2F2 pressure = 60 mtorr.Frequency axis is not strictly linear, (a) maximum C 0 2 laser power = 6 W (-50 percent into cavity), (b) maximum C 0 2 laser power = 3 W (-50 percent into cavity).aRelative to C02 line center.bQualitative strength of IR absorption relative to nearby lines as observed with a particular diode laser mode; S-strong, M-medium, W-CCDF3 pressure = 80 mtorr; relative strength of 100 E 0.5 mW Scientech meter reading.dNotation x Y ( J , K ) , where "r) = P, Q, R * :

83
: } = -1, 0, +1, where J, K refer to lower level.eDoes not contain K correction.molecule, difluoromethane, reported so far.However, in comparison with CH2F2, CD2F2 has a better overlap of IR absorption transitions with the C02 laser 10 pm lines, where more pump power is available in this experiment.CD2F2, an asymmetric top molecule with K = -0.891,has the feature that six of the nine normal modes have fundamental frequencies in the 900-1200 cm-' region [all.CD2Fz should exhibit a fast V-V rate because of the closeness of these levels, and a fast V-T rate similar to that for CHzF2.Therefore, CDzF2 would be expected to be an efficient CW laser molecule, provided that optical pumping into optimal transitions is obtained.The wavelengths of 16 different SMMW lines were measured when pumped with the COz waveguide laser; 10 of these are new lines (Table V> not previously observed with the 50 W conventional pump laser.The fact that 16 true CW lines would oscillate with a maximum of only 2.5 W coupled into the SMMW cavity demonstrates the exceptional nature of this gas.The IR frequency offsets were found from the diode laser spectra and the transferred Lamb dip measurements.Similar to the case of pumping CDF3, the newly discovered lines were found to have larger IR frequency offsets than the previously observed lines.A few additional SMMW lines, which were too marginally pumped for an accurate wavelength determination and consequently, not included in the tables, were observed in CD,F, and CDF,.An example of this case for the 10R38 pump line is shown in Fig. 6.The SMMW line (207.8pm)corresponding to the IR absorption at offset of +9 MHz has been previously observed [17] ; however, the SMMW output corresponding to the stronger IR absorption transition at -100 MHz is a new relatively longer wavelength transition.With slightly greater pump power, undoubtedly this line and additional SMMW lines would be well above threshold.No attempt has been made to assign the CD,F, lines because of the complicated nature of the 10 pm spectrum.

SUMMARY
We have demonstrated that a waveguide laser can pump into many new low threshold transitions because of the greater frequency tunability of the pump laser.Pumping three previously known molecular gases has produced 17 new CW laser transitions and lowered the pump threshold for some previously observed lines which have sizable pump frequency offsets.Power levels of greater than 1 mW have been observed for a number of these lines in an unoptimized cavity.Application of the waveguide pump laser to other gases should produce many new low threshold lines.aRelative to C02 line center.bQualitative strength of IR absorption relative to nearby lines as observed with a particular diode 'CD2F2 pressure 1 5 0 mtorr; relative strength of 100 = 0.5 mW Scientech meter reading.laser mode; S-strong, M-medium, W-weak.The main difficulties experienced with this system were related to frequency drift and mode control of the pump laser.The thermal stability of the C 0 2 waveguide laser would improve considerably with a more stable structure employing Invar rods.Advances in CQ2 laser waveguide technology, including the development of low-loss internal mode selectors such as tunable etalons, should permit wider tunability and make the combined C02/SMM waveguide laser a very useful source in the future.
(upper trace) and corresponding SMMW output (lower trace) are shown in Fig. 4(a).F.rom the diode laser spectrum [Fig.4!b)] , a) C2H2F2 IR transition strengths as a function of line-center frequency relative to 10P22 C02 laser line.Arrows indicate ug IR pump transitions of previously observed lines.(b) Corresponding SMMW transition strengths.Arrows indicate the previously reported v9 directly pumped transitions.
-weak.10R22 pump transitions are consistent with laser Stark spec-when pumped with UP to 35 W power from a conventional troscopy results recently reported by Ibisch et al. [2O] .A C 0 2 laser [ 171 .Ten additional lines obtained with the wave- more detailed analysis of the diode laser data is in progress, guide pump laser are reported here.CD2F2 was selected for along with a determination of rotational constants.pumping with the waveguide laser because it is the deuterated CD2F2 has recently been reported to produce 38 laser lines version of the most efficient optically pumped SMMW laser IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL.QE-18, NO. 1 , JANUARY 1982 (b) Fig. 4. (a) Upper trace: COz waveguide tuning profile for 10R16 line.Frequency axis is not strictly linear.Lower trace: corresponding SMMW output at 1264 pm; CDF, pressure = 65 mtorr.(b) Upper trace: diode laser absorption spectrum of CDF, in region of 10R16 C02 laser line.Lower trace: channel spectrum of a germanium bar etalon for relative frequency calibration.The positions of the absorptions relative to 10R16 C 0 2 laser line at 973.2885 cm-l were determined by heterodyning the C02 and diode lasers.The numbers indicate the value of the rotational quantum number J for the R Q 3 ( J ) absorption lines.
(a) Upper trace: COz waveguide laser tuning profile for 10R18 line.Lower trace: CDF, laser output at 459.4 pm; pressure = 60 mtorr.(b) Upper trace: oscilloscope presentation of diode laser absorption spectrum of CDF, in vicinity of 10R18 COz laser line.Lower trace: heterodyne markers at +lo0 MHz relative to 10R18 C02 laser line at 974.6219 cm-l.

TABLE I CALCULATED IR DOPPLER WIDTHS (AvD) FOR SEVERAL SMMW LASER GASES AT SPECIFIC PUMP TRANSITION FREQUENCIES (~0). T = 300 K Molecule M
Sattler et al.

TABLE I1 CZHZFZ
'Observed this work (see Table 111).dPreviously observed [ 151 .890 pm line would not oscillate.The dips in the SMMW output represent a saturation effect due to a decrease in pump absorption at the Doppler-broadened line centers (transferred Lamb dip), as previously explained [ 191 .

TABLE V
CD2F2 LASER RESULTS FOR WAVEGUIDE PUMP LASER