Volume 37, number 1 OPTICS COMMUNICATIONS 1 April 1981 DOPPLER-FREE OPTOGALVANIC SPECTROSCOPY USING AN INFRARED COLOR CENTER LASER ~ D.J. JACKSON l, H. GERHARDT 2 and T.W.H.~NSCH Department o f Physics, Stanford University, Stanford, California 94305, USA Received 23 December 1980 We have used a cw color center laser near 2.6 tzm to study highly excited states in helium and neon atoms by Doppler- free intermodulated optogalvanic spectroscopy in a hollow cathode discharge tube. For helium n = 4 to 6 transitions, the resolution was limited to about 320 MHz (FWHM) by Holtzmark broadening due to the presence of charged particles in the discharge. Lines as narrow as 60 MHz were observed for neon 3ss-SPlo. A color laser center has been used for nonlinear high resolution spectroscopy of excited helium and neon atoms in a DC gas discharge. Color center lasers have extended the arsenal of broadly tunable and highly monochromatic lasers to include near infrared wavelengths out to 3 tam [ 1]. This has opened up a wealth of possible spectroscopic applications at wavelengths which are convenient to study absorption lines from highly excited atomic levels or molecular absorption bands [2-4]. In an earlier work [2] a number of infrared transitions between sparsely populated states in helium were observed Doppler- limited for the first time by an optogalvanic detection scheme. Doppler-free measurements of transitions be- tween states of high principal quantum number can provide information on fine structure intervals and the interaction of the atoms within the discharge environ- ment. Therefore, we have explored the feasibility of Doppler-free experiments at infrared wavelengths in helium and neon and this paper describes the results and their implications for future experimental work. Four watts all lines from a Spectra Physics argon ion laser were used to pump a Spectra Physics 375 a Work supported by the National Science Foundation under Grant PHY80-10686, and U.S. Office of Naval Research under Contract N00014-78-C-0403. I Bell Laboratories Fellow. 2 Heisenberg Fellow. dye laser which is operated with rhodamine 6G dye and no internal cavity etalons. The dye laser wave- length was tuned to optimize the coupling of the laser light into the KC1 : Li crystal of a Burleigh FCL- 20 color center laser. A dye laser power of 750 mW produced 10 mW of infrared light near the 2.65 tzm wavelength. A reliable continuous, tunable single- mode output from the color center laser was only achieved after the insertion of an additional etalon which was solid and 5 mm thick. At the output of the color center laser, 10% of the infrared light is picked off and sent through a 150 MHz interferometer for frequency calibration. The finesse of the interferometer is 20; after careful align- ment of the laser light through the interferometer, we have established that the laser linewidth is 5 MI-lz or better. A 2.5 GHz single mode scan was obtained by apply- ing -500 V to 500 V from a high voltage power supply to the piezo driven folding mirror in the color center laser cavity. Another 10% of the beam is used to ge- nerate an error signal in a feedback loop which adjusts the etalon envelope function as the laser is tuned. This is achieved by modulating the etalon free spectral range at a frequency of 4.4 kHz. The tuning arm of the laser was flushed with dry nitrogen to reduce the ef- fects of mode hopping in the laser due to water vapor absorption. 23