Frequency-stabilized cavity ring-down spectrometer for high-sensitivity measurements of water vapor concentration

We present a portable spectrometer that uses the frequency-stabilized cavity ring-down spectroscopy technique capable of high-precision measurements of trace water vapor concentration. Measuring one of the strongest rovibrational transitions in the ν₁+ν₃ water vapor combination band near ˜ν=7181.156 cm^-1, we compare spectroscopic and thermodynamic determinations of trace water vapor in N₂, and find systematic differences attributable to water vapor background effects and/or uncertainties in line intensities. We also compare the frequency-stabilized ring-down method with other cavity ring-down approaches that are based on unstabilized probe lasers and unstabilized ring-down cavities. We show that for the determination of water vapor concentration, the frequency-stabilized cavity ring-down method has the minimum measurement uncertainty of these techniques. The minimum noise-equivalent absorption coefficient of the spectrometer was 1.2×10^-10 cm^-1 Hz^-1/2, which further corresponds to a minimum detectable water vapor mole fraction equal to 0.7×10^-9 for an absorption spectrum of 10 minutes duration. PACS 33.20.-t; 33.70.Jg; 33.70.Fd; 42.62.Fi     

Low-uncertainty H2O line intensities for the 930-nm region

We measured room-temperature H₂O spectra of the 3ν polyad within the wave number range 10 603 cm⁻¹ to 10 852 cm⁻¹ using frequency-stabilized cavity ring-down spectroscopy. More than 70 transitions, divisible into two groups based on their relative intensities (strong and weak) were investigated. For the 23 lines comprising the set of strong transitions, the water vapor sample was prepared as a dilute mixture in a steady flow of N₂ carrier gas, and the measurement of the water vapor sample number density was linked to primary methods of humidity generation. For these strong transitions, the measured line intensities were within 1% of previous measurements. The line intensity measurements of the weaker H₂O transitions, were made on samples of static pure water vapor, and these results were systematically greater than previous measurements by approximately 20%.     

High-precision pressure shifting measurement technique using frequency-stabilized cavity ring-down spectroscopy

We describe a high-precision method for measuring pressure shifting of absorption lines. The technique involves the acquisition of high-resolution spectra using a cavity ring-down spectrometer whose length is continuously locked to a frequency-stabilized reference laser over a range of sample pressures. We discuss a relatively large correction arising from the pressure-dependence of dispersion in the cavity modes, and we demonstrate pressure shifting measurements in air for transitions in the ¹⁶O₂A-band. Pressure shifts in the range -0.011 to are reported. We measured relative positions of line centers to within 70 kHz and determined pressure shifting coefficients over a 5 kPa pressure range with relative uncertainties approximately equal to 1.0%, which constitutes a five-fold improvement over previous measurements.     

The hyperfine and isotope structure of the Cd intercombination line – revisited

Careful analysis of the intercombination 5¹S₀–5³P₁ line of the ¹¹³Cd isotope with two hfs components and was carried out. The hyperfine splitting of this line was determined to uncertainty less than 10^-3 cm^-1 using neon-perturbed Doppler limited spectra.     

Cavity ring-down spectroscopy of the oxygen B-band with absolute frequency reference to the optical frequency comb

Absolute positions of several oxygen B-band lines were measured with the Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer. The frequency axis of spectra was linked to the optical frequency comb. Achieved uncertainties of line positions are between 0.9 and 2.9 MHz. Self-pressure shifts coefficients are also reported.     

Analysis of the dynamics of high intensity Gaussian laser beams in nonlinear de-focusing Kerr media

An approximate analytical approach is presented for analyzing the evolution of the radial intensity profile of initially Gaussian laser beams propagating in nonlinear de-focusing Kerr media. Explicit and accurate solutions are obtained describing the first stage of the evolution of high intensity beams in which the beam profile evolves into an almost flat-topped distribution, subsequently leading to the well known phenomena of wave breaking and concomitant intensity ring formation.     

Eigenvalues of the Zakharov-Shabat scattering problem for real symmetric pulses

The classical problem of determining the solitons generated from symmetric real initial conditions in the nonlinear Schr?dinger equation is revisited. The corresponding Zakharov-Shabat scattering problem is solved for real and symmetric double-humped rectangular initial pulse forms. It is found that such real symmetric pulses may generate eigenvalues with nonzero real parts corresponding to separating soliton pulse pairs. Moreover, it is found that the classical formula relating the number of eigenvalues to the area of the pulse is not always correct.     

Vector solitons in (2 + 1) dimensions

We address the problem of the existence and stability of vector spatial solitons formed by two incoherently interacting optical beams in bulk Kerr and saturable media. We identify families of (2+1)-dimensional two-mode self-trapped beams, with and without a topological charge, and describe their properties analytically and numerically.     

Laser-induced fluorescence study of the influence of N <Emphasis Type="Bold">2</Emphasis> and CH <Emphasis Type="Bold">4</Emphasis> on the <Emphasis Type="Bold">114</Emphasis>Cd intercombination line

Using a laser-induced fluorescence method a detailed analysis of profiles of the ¹¹⁴Cd 326.1 nm line perturbed by N₂ and CH₄ was performed which revealed deviations from the ordinary Voigt profile. These deviations are shown to be consistent with fits of experimental profiles to an asymmetric Voigt profile. Coefficients of the pressure broadening, shift and collision-time asymmetry are determined and compared with those calculated for van der Waals interaction potential. Received 29 November 2001 / Received in final form 12 July 2002 Published online 4 February 2003