The recoil-induced spectral doubling of the CH 4 saturated absorption peaks at 3.39 nm has been clearly resolved. A working resolving power of 8 xio 10 (1 kHz half width at halfmaximum), achieved with aberration-compensated parabolic optics of 32 cm diam, was available to study the height ratio and splitting of the recoil peaks. The resolved Dopplergenerated level crossings are not recoil doubled and give improved hfs information. We derive new, exact equations relating the atomic natural frequency to laboratory resonance frequencies.As predicted by Kol'chenko, Rautian, and Sokolovskii, 1 momentum exchange between the radiation field and a quantum absorber gives rise to spectral doubling in saturation spectroscopy. The smallness of that frequency splitting may provide an ultimate natural challenge to the techniques of saturated-absorption spectroscopy. The CH 4 transition at 3.39 jum is a favorable case for this investigation in view of the small mass and high transition frequency. On the other hand, structure within the linewidth can only be regarded as detrimental to the otherwise very attractive prospects for an optical frequency standard. With suitable attention to the laser frequency stability and to the various spectral broadening mechanisms, 2 it has become possible to resolve 3 the two components clearly for the first time and to begin investigation of their pressure and power dependence.We begin by recalling the physical origin of the recoil effect. We then mention the improvements of the laser spectrometer which led to this new level of resolution, and consider the several residual broadening mechanisms that limited the working resolution. Finally, we report improved frequency intervals for the CH 4 hyperfine spectrum.An essential feature of sub-Doppler spectroscopy is the necessity to consider momentum conservation between the light and the absorbing molecules, as well as energy conservation: In fact the usual Doppler broadening is just a manifestation of momentum conservation. We also know that the resolution limit in sub-Doppler spectroscopy depends directly on how precisely the light momentum is defined. 2 * 4 A second manifestation of momentum conservation-not so familiar in optical spectroscopy-is the recoil effect. Although the recoil energy can be obtained from a nonrelativistic approach, a complete and consistent formulation including both the recoil and the relativistic Doppler effect can be developed by making a relativistic energy and momentum balance. In that way we can obtain the resonance condition for light of frequency a? and wave vector 5 applied to the two-level atom of transition frequency QO 0 = 2UI/ Q . The atom's mass M may conveniently be referenced midway between the two atomic levels. For an absorber having a velocity v a in its lower state, one derives the absorption resonance condition (*> _(l-Vg 2 /c 2 ) 1/2 ( 1 o) 0 l-k«v a /co \l-e where e =fiu) 0 /2Mc 2 per state, For emission from the upthe resonance condition is 6 2 /c 2 ) 1/2 -E • V b /(A) (£)•where v b is the velocity...

High-sensitivity real-time remote detection of methane in air with a 1.66-microm distributed-feedback diode laser operating at room temperature is demonstrated by laboratory simulations. The laser current was modulated at a high frequency of ~5 MHz, and the laser-center frequency was locked onto a methane-absorption line. The laser light directed toward the probed region was received after one-way transmission or further reflection from a topographic target. The methane absorption was detected by the second-harmonic component in the optical-power variation. The minimum-detectable concentration-path-length product in the transmission scheme was 0.3 part in 10(6) m for a signal averaging time of 1.3 s. In the reflection scheme, the amount of methane could be measured from the ratio of the fundamental and second-harmonic signal intensities independently of the received power.

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