CH<Subscript>3</Subscript>OH optically pumped by a <Superscript>13</Superscript>CO<Subscript>2</Subscript> laser: new laser lines and assignments

We report 12 new THz (far-infrared) laser lines from methanol (CH₃OH), ranging from 58.1 μm (5.2 THz) to 624.6 μm (0.5 THz). A ¹³CO₂ laser of wide tunability (110 MHz) has been used for optical pumping, allowing access to previously unexplored spectral regions. Optoacoustic absorption spectra were used as a guide to search for new THz laser lines, which have been characterized in wavelength, polarization, offset, relative intensity, and optimum operation pressure. For 20 laser lines previously observed, we have measured the absorption offset with respect to the ¹³CO₂ laser line center. PACS 33.20.Ea; 33.20.Vq; 33.80.-b     

Investigation of 13CH3OH methanol: new laser lines and assignments

We have reinvestigated ¹³CH₃OH as a source of far-infrared (FIR) laser emission using a CO₂ laser as a pumping source. Thirty new FIR laser lines in the range 36.5 μm to 202.6 μm were observed and characterized. Five of them have wavelengths between 36.5 and 75 μm and have sufficient intensity to be used in LMR spectroscopy. Using Fourier-transform spectroscopic data in the infrared (IR) and FIR regions we have determined the assignment for 10 FIR laser transitions and predict nine frequencies for laser lines which have yet to be observed. Received: 17 July 2000 / Published online: 6 December 2000     

Optically pumped 13CD3I: new TeraHertz laser transitions

We investigated the ¹³CD₃I isotopomer of methyl iodide as a source of TeraHertz laser radiation using the optical pumping technique. We used a pulsed waveguide CO₂ laser as the pump laser and an open Fabry–Pérot cavity for new laser line generation. We discovered 18 new laser lines with wavelengths ranging from 308.4 to 1132.7 μm, plus two lines previously assigned to ¹²CD₃I. All of the lines were characterized in wavelength, pump frequency offset, optimum pressure of operation, and relative polarization and intensity.     

Femtosecond fiber laser based methane optical clock

An optical clock based on an Er³⁺ fiber femtosecond laser and a two-mode He–Ne/CH₄ optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er³⁺ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH₄ laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent connection between the optical frequency of the He–Ne/CH₄ standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1×10⁻¹² at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He–Ne/CH₄ optical standard is dominant.     

CW laser performance of Yb and Er,Yb doped tungstates

 Room temperature cw laser action of Yb³⁺-doped KY(WO₄)₂ and KGd(WO₄)₂ crystals at 1.025 μm and Er, Yb : KY(WO₄)₂ at 1.54 μm has been demonstrated under pumping by both Ti-sapphire laser and InGaAs laser diodes. A slope efficiency of Yb-lasers up to 78% has been obtained. Received: 19 June 1996     

Infrared multiple photon decomposition of CHBrF2 induced by a free-electron laser

2 . The laser generates an intense infrared macropulse with a duration of 17 μs; the macropulse consists of a train of 380 micropulses, each of which has a duration of a few picoseconds. The fluence of a macropulse was estimated to be about 16 Jcm^-2 at a beam waist. Peak wavelengths were set in the range of 9–10 μm. The macropulse induced the IRMPD of 1 and 5 Torr CHBrF₂; most of molecules in the focal region seemed to decompose at a wavelength of 9.3 μm. The mechanism is the initial decomposition of CHBrF₂ to CF₂ and HBr, followed by the dimerization of CF₂ to form C₂F₄. The decomposition was found to be isotopically selective at 9.7 μm; the final product C₂F₄ had a ¹³C atomic fraction of 6%. Th e addition of CO₂ to CHBrF₂ significantly decreased the yield of C₂F₄. vibrationally excited CHBrF₂ molecules produced by laser pulses were efficiently deactivated by CO₂ molecules. Received: 7 October 1996     

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 150 μm 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 100 μm, 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 frequency-measured lines, 44.256 μm 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. Received: 26 October 2000 / Published online: 7 February 2001     

New far infrared laser lines obtained by optically pumping13CD3OD

We have obtained laser action on 34 far infrared lines for the first time in fully deuterated methyl alcohol with the¹³C isotope (¹³CD₃OD), and we have measured the frequency of 13 lines. The molecule was pumped by a cw CO₂ laser. We have measured the wavelength, the relative polarization, the relative intensity of most lines, the frequency, and the CO₂ pump frequency offset of the strongest lines. The new lines are distributed in the wavelength region from 75.27 m to 464.7 m.Supported in part by a joint grant with the U.S. National Science Foundation grant # INT 80-19014 and the Brazilian Conselho Nacional de Pesquisas (CNPq).     

Infrared laser emission in two-photon pumped sodium vapour

We report infrared laser emission in the region of 3 to 5 μm from sodium vapour optically pumped by a pulsed dye laser with wavelengths ranging from 585 to 610nm. Twophoton excitation processes are believed to be responsible for the primary excitation. Both molecular transitions (4 to 5 μm) between high lying states, and atomic transitions (5² S _1/2−4² P _3/2,1/2 at 3.41 μm) have been identified.     

High-sensitivity detection of NO2 using a 740 nm semiconductor diode laser

 An AlGaAs diode laser was used to detect NO₂ absorption lines belonging to the (0 0 0)–(2 13 1) vibrational band, within the X˜² A ₁ electronic ground state, at 739 nm. A simple absorption spectrometer based on wavelength-modulation spectroscopy with second-harmonic detection was developed. The minimum detectable pressure of pure NO₂ was 0.1 μbar with 2 m absorption path-length, corresponding to an absorbance of 10^-6. High-sensitivity detection of NO₂ was also performed in the presence of N₂ and air at different total pressures: The effects on the detection limit of our apparatus were accurately investigated. The minimum NO₂ concentration at 500 mbar of air was measured to be 2 ppm. Received: 11 June 1996 / Revised version: 11 October 1996     

New FIR laser lines from CD3OD optically pumped by a CO2 waveguide laser

In this work we report new FIR laser lines from CD₃OD optically pumped by a CO₂ waveguide laser. The wide tunability of this laser (290 MHz) makes it possible to pump absorption lines with large frequency offset relative to the CO₂ laser line center, which are not possible by using conventional CO₂ lasers. As a consequence 19 new laser lines have been discovered, ranging from 38.0 m to 455.2 m in wavelength. For all lines, precise frequency offset measurements between the CO₂ line center and the center of the absorber CD₃OD line were performed using the transferred Lamb-dip technique. We also present direct Doppler-free offset measurements of infrared absorption, obtained within the FIR laser cavity itself, using optoacoustic detection.Work supported by FAPESP, CNPq, FAP-Brasil and CNR-Italy     

Discovery and frequency measurement of short-wavelength far-infrared laser emissions from optically pumped <Superscript>13</Superscript>CD<Subscript>3</Subscript>OH and CHD<Subscript>2</Subscript>OH

A three-laser heterodyne system was used to measure the frequencies of twelve previously observed far-infrared laser emissions from the partially deuterated methanol isotopologues ¹³CD₃OH and CHD₂OH. Two laser emissions, a 53.773 μm line from ¹³CD₃OH and a 74.939 μm line from CHD₂OH, have also been discovered and frequency measured. The CO₂ pump laser offset frequency was measured with respect to its center frequency for twenty-four FIR laser emissions from CH₃OH, ¹³CD₃OH and CHD₂OH. PACS 07.57.Hm; 42.55.Lt; 42.62.Eh     

Synchronous pulsed infrared detection in AgGaSe2 crystal using 1.318 μm pump

2 laser by nonlinear upconversion, is reported for the first time in AgGaSe₂ crystal under near noncritical phase-matching, pumped by an electro-optically Q-switched Nd:YAG laser operated at 1.318 μm. The capability for low-level infrared signal detection at room temperature with a fast response was also studied. Received: 15 December 1996/Revised version: 20 February 1997     

New far infrared laser lines from 13CH2F2 and frequency measurements

We report new FIR laser lines from ¹³CH₂F₂ molecules optically pumped by a waveguide CO₂ laser. The increased tunability (300 MHz) with respect to a conventional CO₂ laser allows the pumping of ¹³CH₂F₂ vibrational transitions of large offset. 34 new laser lines have been discovered, ranging from 113.1 m to 491.4 m in wavelength, thus increasing the number of known FIR laser lines from this important molecule to 99. For all the new lines and many (36) of those known previously, precise offset measurements through the transferred Lamb-dip technique were performed. The frequency of six new laser lines was also directly measured by heterodyne detection with known laser lines.     

Discovery and frequency measurement of far-infrared laser emissions generated by optically pumped CH2DOH

A recently improved three-laser heterodyne system was used to frequency measure ten previously observed optically pumped far-infrared (FIR) laser emissions from the partially deuterated methanol isotopologue CH₂DOH. Also, a 64.0 μm FIR emission generated by the 9P32 line of the carbon dioxide (CO₂) laser was discovered and frequency measured. These newly measured frequencies have fractional uncertainties on the order of ±2×10^-7 and correspond to laser wavelengths ranging from 42.6 to 152.7 μm. The offset frequency for the CO₂ pump laser was measured for twenty-two CH₂DOH FIR laser emissions. PACS 07.57.Hm; 42.55Lt; 42.62.Eh     

A hollow cathode carbon atom laser

Near-infrared laser action of atomic carbon was obtained using a hollow cathode discharge tube. Besides the known 1.4543 μm and 1.0691 μm laser lines, cw laser operation at 1.0683 μm, 0.9658 μm, and 0.9406 μm wavelength was observed for the first time. The source of carbon was due to dissociation of CO, CO₂ or CH₄ added to the He buffer gas.     

New short-wavelength laser emissions from partially deuterated methanol isotopes

The partially deuterated isotopes of methanol, CH₂DOH and CHD₂OH, have been reinvestigated as sources of far-infrared (FIR) laser emissions using an optically pumped molecular laser (OPML) system recently designed for wavelengths below 150 μm. With this system, 10 new FIR laser emissions from these isotopes ranging from 32.8 to 174.6 μm have been discovered. This includes the shortest known OPML emission from CHD₂OH, at 32.8 μm. These lines are reported with their operating pressure, polarizations relative to the CO₂ pump laser and wavelengths, measured to ±0.5 μm. In addition, polarizations for three previously observed FIR laser lines from CHD₂OH were measured for the first time. This paper is dedicated to the memory of Dr. K.M. Evenson, a pioneer in the field for his role in the development of optically pumped molecular lasers and their use in laser frequency measurements and the laser magnetic resonance technique. His scientific expertise, guidance, mentoring and friendship will be greatly missed. Received: 27 March 2002 / Published online: 8 May 2002     

Mid-infrared trace gas detection using continuous-wave difference frequency generation in periodically poled RbTiOAsO4

A tunable mid-infrared continuous-wave (cw) spectroscopic source in the 3.4–4.5 μm region is reported, based on difference frequency generation (DFG) in a quasi-phase-matched periodically poled RbTiOAsO₄ (PPRTA) crystal, DFG power levels of 10 μW were generated at approximately 4 μm in a 20-mm long PPRTA crystal by mixing two cw single-frequency Ti:Al₂O₃ lasers operating near 713 nm and 871 nm, respectively, using a laser pump power of 300 mW. A quasi-phase-matched infrared wavelength-tuning bandwidth (FWHM) of ∼12 cm^-1 and a temperature tuning rate of 1.02 cm^-1/°C were achieved. Experimental details regarding the feasibility of trace gas detection based on absorption spectroscopy of CO₂ in ambient air using this DFG radiation source are also described. Received: 23 October 2000 / Revised version: 22 January 2001 / Published online: 27 April 2001     

Very high resolution CO laser spectrometer and first sub-Doppler line-shape studies near 60 THz (5 μm)

2 laser standards. Using this technique, we can tune the CO laser frequency with absolute frequency control within the gain profile of each laser transition. The frequency uncertainty is smaller than 15 kHz, corresponding to Δν/ν=2.5×10^-10. Moreover, we obtain a reduction of the CO laser linewidth by a factor of 2 down to 65 kHz, corresponding to a spectral resolution of δν/ν=1×10^-9. With this outstanding accuracy and resolution we studied the shape of saturation dips in rovibrational lines of CO and carbonyl sulfide (OCS) at low pressure (<5 Pa). The self-pressure-broadening rate of CO was found to be γ_c=+83(7) kHz/Pa in this pressure region. This value is about four times higher than values resulting from previous measurements at much higher pressures. To our knowledge the measurements described here are the first line-shape studies with sub-Doppler resolution in the 5 μm spectral region. Received: 4 November 1996     

Telecom-grade fiber laser-based difference-frequency generation and ppb-level detection of benzene vapor in air around 3 μm

We report on the development of a field deployable compact laser instrument tunable over ∼232 cm⁻¹ from 3.16 to 3.41 μm (2932.5–3164.5 cm⁻¹) for chemical species monitoring at the ppb-level. The laser instrument is based on widely tunable continuous-wave difference-frequency generation (DFG), pumped by two telecom-grade fiber lasers. DFG power of ∼0.3 mW near 3.3 μm with a spectral purity of ∼3.3 MHz was achieved by using moderate pumping powers: 408 mW at 1062 nm and 636 mW at 1570 nm. Spectroscopic performance of the developed DFG-based instrument was evaluated with direct absorption spectra of ethylene at 3.23 μm (∼3094.31 cm⁻¹). Absorption spectra of vapor-phase benzene near 3.28 μm (∼3043.82 cm⁻¹) were recorded with Doppler-limited resolution. Line intensities of the most intense absorption lines of the ν ₁₂ band near 3043.8 cm⁻¹ were determined to support development of sensitive mid-infrared trace gas detection of benzene vapor in the atmosphere. Detection of benzene vapor in air at different concentration levels has been performed for the first time using multi-pass cell enhanced direct absorption spectroscopy at ∼3.28 μm with a minimum detectable concentration of 50 ppb (1σ).