Cw stimulated Raman gain spectroscopy of the ν1 fundamental of methane

13CH4/12CH4 sensing using Raman spectroscopy

The paper presents a technique for measuring the concentration of 13CH4 in natural methane using Raman spectroscopy. The peak positions and the relative scattering cross-sections of the Q-branches for the most intense vibrational bands of 13CH4 are determined. Features of the 13CH4/12CH4 ratio measurement methods using Q-branches of the ν1 and ν3 bands were considered. It was shown that the 13CH4/12CH4 ratio can be determined by simulation of the ν3 bands of these molecules without the use of experimental spectra. In our experiments the measurement error of δ13C value was 10 ‰ using the 100-s exposure spectrum at a gas pressure close to 1 atm recorded on the developed Raman spectrometer. In addition, the Raman spectra of alkanes (up to n-hexane) in the range of 2850-3050 cm-1 at a resolution of 0.4 cm-1 are presented, and their integrated intensities in the ranges of the characteristic bands of 13CH4 and 12CH4 are provided. The data obtained make it possible to expand the capabilities of Raman gas analyzers in the mud gas logging industry.

Origin of continuous curves and dotted spots in laser transverse modes with geometric structures

The transverse structures of geometric modes in degenerate laser resonators with large astigmatism are systematically investigated by starting from the Gaussian wave packet formulation with ray-wave connections. The overlapping degree D between Gaussian wave packets is derived as an analytic expression that manifests the critical overlapping with D=1. The transverse patterns are confirmed to display a structure of continuous curves or dotted spots, depending on D>1 or D<1, respectively. A thorough comparison between experimental results and numerical calculations is performed to validate the theoretical analysis.

Exploring the self-mode locking and vortex structures of nonplanar elliptical modes in selectively end-pumped Nd:YVO4 lasers: manifestation of large fractional orbital angular momentum

An end-pumped Nd:YVO₄ laser under selective pumping is used to excite lasing modes with transverse patterns performed to exhibit the characteristics of multiple spots arranged on elliptical features near degenerate cavities. The spatial distribution of elliptical lasing modes is clearly revealed to be localized on the nonplanar ray orbits, so-called nonplanar elliptical modes, which possess large fractional orbital angular momentum. Moreover, temporal dynamics for the output emission of nonplanar elliptical modes are verified to obtain self-mode-locked operation. We further numerically manifest not only the influence of radial-asymmetry distributions on the vortex structures of nonplanar elliptical modes, but also the vector field of transverse lasing modes altered with twisting phase structures in the propagation direction.

A Time-Encoded Technique for fibre-based hyperspectral broadband stimulated Raman microscopy

Raman sensing and microscopy are among the most specific optical technologies to identify the chemical compounds of unknown samples, and to enable label-free biomedical imaging. Here we present a method for stimulated Raman scattering spectroscopy and imaging with a time-encoded (TICO) Raman concept. We use continuous wave, rapidly wavelength-swept probe lasers and combine them with a short-duty-cycle actively modulated pump laser. Hence, we achieve high stimulated Raman gain signal levels, while still benefitting from the narrow linewidth and low noise of continuous wave operation. Our all-fibre TICO-Raman setup uses a Fourier domain mode-locked laser source to achieve a unique combination of high speed, broad spectral coverage (750–3,150 cm⁻¹) and high resolution (0.5 cm⁻¹). The Raman information is directly encoded and acquired in time. We demonstrate quantitative chemical analysis of a solvent mixture and hyperspectral Raman microscopy with molecular contrast of plant cells.

Raman microscopes suffer from the compromise between speed and spectral information and are often unsuited for fibre beam delivery. Karpf et al. overcome these limitations using continuous-wave rapidly wavelength-swept probe lasers and a short-duty-cycle actively modulated pump laser in an all-fibre setup.

Sensitive H(2) detection by use of thermal-lens Raman spectroscopy without a tunable laser

A new nonlinear Raman spectroscopy technique for trace-gas detection was proposed and demonstrated. The technique involved the use of a thermal-lens detection scheme to monitor thermal emission from the stimulated Raman process. We termed this technique thermal-lens Raman spectroscopy, and it was combined with a novel scheme involving a nonlinear Raman spectroscopy without a tunable laser. This technique was applied to detecting trace hydrogen molecules in the atmosphere by use of a pulsed Nd:YAG laser and a continuous-wave He-Ne probe laser. A detection limit of 9 parts in 10(6) was attained.

The Stimulated Raman Spectrum of Symmetric 13C Cyanogen, 13C2N2

The Raman spectrum of cyanogen 13C214N2 has been investigated at nearly Doppler resolution by means of a stimulated Raman technique. The regions around the bandheads of the nu1 (2260 cm-1) and nu2 (835 cm-1) vibrations have been recorded. Besides the fundamentals, hot bands arising from v5 = 1-3 and v4 = 1 have been observed. A number of strong perturbations have been detected. The spectra have been analyzed and rotational constants for the excited states have been obtained. Computer simulations of the Raman contours have been carried out as a test of the assignments. A few weak DeltaJ = -2 lines have also been observed in the nu1 region.

Stable propagation of non-Gaussian beams in a multiple-pass cell

To apply annular output beams emitted from an unstable resonator to a multiple-pass cell (MPC) for Raman conversion, we studied the mode-matching condition of non-Gaussian beams to a MPC using beam propagation analysis based on Laguerre-Gaussian functions. During transits of the MPC, the radial profile of an annular beam changes between annular and Airy patterns. Although such behavior indicates that it is impossible to achieve complete mode matching of an annular beam, we found a quasi-mode-matching condition under which the variation of beam size was minimized. The above theoretical analysis was verified experimentally using a CO(2) laser beam prepared for a para-hydrogen Raman laser.

Evaluation of Nd:YAG-pumped Raman shifter as a broad-spectrum light source

We have examined the utility of a gas-filled, Nd:YAG-laser-pumped Raman shifter as a possible broad-spectrum light source. Six to nine new output frequencies with pulse energies above 1 µJ are produced when a pure-hydrogen or pure-methane Raman shifter is pumped with 40 mJ of secondharmonic, 20 mJ of third-harmonic, or 11 mJ of fourth-harmonic pump pulse energy. Optimum output occurs at pressures of approximately 10 atm for the pure-gas experiments. We also report the output frequencies and pulse energies of a mixed hydrogen-methane Raman shifter pumped by 20 mJ of the third harmonic of the laser for various proportions of the two gases at pressures up to nearly 20 atm. Depending on composition and pressure, over a dozen new output lines with pulse energies over 1 µJ can be produced. We discuss the nonlinear processes involved, the optimum operating conditions, and the suitability of the source for our application of groundwater monitoring.

Multipass cell for molecular beam absorption spectroscopy

We report a two-spherical mirror multipass cell which allows efficient coupling of a laser beam to a collimated molecular beam at the center of the cell. With near-concentric alignment and the laser beam coupled into and out of the cell at the edges of opposite mirrors, an approximately parabolic pattern of reflections is observed on each mirror. Aligned for forty to fifty passes, the cell is used for direct absorption spectroscopy of jet-cooled acetylene, propyne, and 1-butyne in the 3-microm region. Sub-Doppler linewidths as narrow as 12 MHz have been recorded.

Picosecond coherent anti-Stokes Raman spectroscopy of molecules in free jet expansions

By using time-resolved coherent Raman scattering, closely spaced vibrational resonances are simultaneously excited and subsequently monitored over long, nanosecond time intervals. Coherent superposition of molecular transitions leads to dramatic oscillations of the picosecond anti-Stokes scattering signals. Experimental data are presented for the nu(1) mode of CH(4) in supersonic expansions at low rotational temperatures (T(ROT) >/= 25). Our timedomain data are in good agreement with spectroscopic results. The corresponding frequency resolution of less, similar3 x10(-3) cm(-1) suggests that this method of Fourier-transform Raman spectroscopy may prove useful in high-resolution molecular spectroscopy.

High-resolution continuous-wave coherent anti-Stokes Raman spectroscopy in a supersonic jet

We have obtained high-resolution cw coherent anti-Stokes Raman spectra of the nu(1) Q branch of methane in an underexpanded supersonic jet at temperatures as low as 31.5 K and pressures below 2 Torr.

Multiple-pass Raman gain cell

The application of a multiple-pass Herriott cell to stimulated Raman scattering (SRS) is evaluated and demonstrated. The cell combines a long optical path length with periodic refocusing to enhance Raman gain. This technique is especially useful for reducing SRS threshold pump power in the IR where the Raman gain is low. This paper presents an analysis and design procedure for a multiple-pass Raman gain cell and gives experimental results for SRS in H(2) gas.

Gaseous trace analysis using pulsed photoacoustic Raman spectroscopy

A new method for trace analysis of gases, based on the pulsed photoacoustic Raman spectroscopy (PARS) technique, is described. The method has been applied to the analysis of mixtures of CH(4) in N(2), CO(2) in N(2), and N(2)O in N(2) at concentrations near 1 ppm. The apparatus used is described in some detail. Means of improving the method's sensitivity as well as sensitivity-limiting processes are evaluated. The analytical capabilities of this technique are compared with both direct (IR) absorption and other Raman techniques such as CARS and stimulated Raman gain spectroscopy (SRGS).