Short-wavelength far-infrared laser cavity yielding new laser emissions in CD3OH

A significantly improved far-infrared laser has been used to generate optically pumped laser emissions from 26 to 150μm for CD3OH. Using an X–V-pumping geometry, several new laser emissions have been found for CD3OH. In addition, an increase in power, by factors from 10 to 1000, for many of the previously known shorter-wavelength laser lines, below 100μm, has been observed. Frequency measurements for several lines have also been performed and have been reported to a fractional uncertainty up to ±2×10−7, permitting the spectroscopic assignment of the laser transition. One of the frequencymeasured lines, 44.256μm observed using the 10R34 pump, has confirmed the assignment of the previously reported FIR emission (n, K; J) = (1, 7; 20)→ (0, 8; 20)A in the ground vibrational state. PACS: 42.60.By; 42.60.Da; 42.62.Fi Following the initial observation of far-infrared (FIR) laser emissions from methanol (CH3OH) by Chang et al. [1], methanol and its isotopes have produced more than 2000 FIR laser lines when optically pumped by cw and pulsed CO2 lasers [2, 3]. CD3OH alone contributes more than 400 laser lines in the wavelength range 21.7–3030.0μm [2–4]. The richness of FIR emissions is principally due to the excellent overlap between the CD3-deformation region (1068.2 cm−1) and the C−O vibrational band (984.4 cm−1) with the 9and 10-μm CO2 laser bands, respectively [5, 6]. In this paper, we report the discovery of new optically pumped FIR laser emissions from CD3OH. Using a new FIR cavity and pumping scheme, designed for wavelengths below 100μm, substantial increases in output power for many previously observed emissions [7–15] have been found. Frequency measurements on several new lines have also been performed. ∗Corresponding author. (Fax: +1-608-785-8403, E-mail: [email protected]) 1 Experimental details The experimental setup for the optically pumped molecular laser (OPML) system is partially shown in Fig. 1. The CO2 pump laser is 2-m long and includes a partially ribbed cavity surrounded by a water-cooled jacket. The Pyrex-glass tube has an inner diameter of 18 mm and contains five equally spaced glass ribs whose inner diameters increase from 16.5 to 17.5 mm. By introducing glass ribs into the laser cavity many wall-bounce modes are eliminated, thereby forcing an open-structure mode and increasing the effective resolution of the grating [16]. The laser uses the zeroth-order output coupling from a 150-line/mm grating with 3% output coupling in zero order. The CO2 laser is capable of producing ≈ 275 lines, including lines on the regular-, hotand sequence-band emissions. A high-reflectivity gold-coated 20-m-radius mirror is used on the other end. Both the 9and 10-μm branches exhibit high-J lines out to 9R(58), 9P(60), 10R(58) and 10P(60). Regular laser lines reach powers up to 30 W with the hotand sequence-band emissions reaching 10 W [17]. The CO2 laser radiation is focused into the FIR cavity with a 12-m-radius-of-curvature gold-coated concave mirror, externally mounted on the far (fixed-mirror) end of the Fig. 1. Side view of the optically pumped molecular laser system