Spectral broadening of picosecond pulses forming dispersive shock waves in optical fibers.

We investigate analytically, numerically, and experimentally the spectral broadening of pulses that undergo the formation of dispersive shocks, addressing in particular pulses in the range of tens of ps generated via electro-optic modulation of a continuous-wave laser. We give an analytical estimate of the maximal spectral extension and show that super-Gaussian waveforms favor the generation of flat-topped spectra. We also show that the weak residual background of the modulator produces undesired spectral ripples. Spectral measurements confirm our estimates and agree well with numerical integration of the nonlinear Schrödinger equation.

Mid-infrared optical frequency combs at 2.5 µm based on crystalline microresonators.

The mid-infrared spectral range (λ~2-20 µm) is of particular importance as many molecules exhibit strong vibrational fingerprints in this region. Optical frequency combs--broadband optical sources consisting of equally spaced and mutually coherent sharp lines--are creating new opportunities for advanced spectroscopy. Here we demonstrate a novel approach to create mid-infrared optical frequency combs via four-wave mixing in a continuous-wave pumped ultra-high Q crystalline microresonator made of magnesium fluoride. Careful choice of the resonator material and design made it possible to generate a broadband, low-phase noise Kerr comb at λ=2.5 µm spanning 200 nm (≈10 THz) with a line spacing of 100 GHz. With its distinguishing features of compactness, efficient conversion, large mode spacing and high power per comb line, this novel frequency comb source holds promise for new approaches to molecular spectroscopy and is suitable to be extended further into the mid-infrared.

Adaptive dual-comb spectroscopy in the green region.

Dual-comb spectroscopy is extended to the visible spectral range with two short-pulse frequency-doubled free-running ytterbium-doped fiber lasers. When the spectrum is shifted to other domains by nonlinear frequency conversion, tracking the relative fluctuations of the femtosecond oscillators at their fundamental wavelength automatically produces the correction signal needed for the recording of distortion-free spectra. The dense rovibronic spectrum of iodine around 19,240 cm(-1) is recorded within 12 ms at Doppler-limited resolution.

Raman-induced Kerr-effect dual-comb spectroscopy.

We report on the first (to our knowledge) demonstration of nonlinear dual-frequency-comb spectroscopy. In multi-heterodyne femtosecond Raman-induced Kerr-effect spectroscopy, the Raman gain resulting from the coherent excitation of molecular vibrations by a spectrally narrow pump is imprinted onto the femtosecond laser frequency comb probe spectrum. The birefringence signal induced by the nonlinear interaction of these beams and the sample is heterodyned against a frequency comb local oscillator with a repetition frequency slightly different from that of the comb probe. Such time-domain interference provides multiplex access to the phase and amplitude Raman spectra over a broad spectral bandwidth within a short measurement time.

Continuous-wave 1.55 microm diode-pumped surface emitting semiconductor laser for broadband multiplex spectroscopy.

A room-temperature-operating vertical external cavity surface emitting laser is applied around 1550 nm to intracavity laser absorption spectroscopy analyzed by time-resolved Fourier-transform interferometry. At an equivalent path length of 15 km, the high-resolution spectrum of the semiconductor disk laser emission covers 17 nm simultaneously. A noise-equivalent absorption coefficient at 1 s averaging equal to 1.5 x 10(-10) cm(-1)Hz(-1/2) per spectral element is reported for 65 km, the longest path length employed.

Infrared frequency combs and supercontinua for multiplex high sensitivity spectroscopy.

An infrared high-brightness light source based on supercontinuum generation through a SF6 photonic crystal fiber seeded by a Cr(4+):YAG femtosecond oscillator is developed for high resolution multiplex spectroscopy in the 1.5 µm region. Moreover, a multiplex high resolution approach based on a Cr(4+):YAG frequency comb enables to probe large spectral domains, with simultaneous sensitive measurement of the absorption and the dispersion associated with all individual spectral features.

Sensitive multiplex spectroscopy in the molecular fingerprint 2.4 mum region with a Cr(2+):ZnSe femtosecond laser.

An ultrashort-pulse Cr(2+):ZnSe laser is a novel broadband source for sensitive high resolution molecular spectroscopy. A 130-fs pulse allows covering of up to 380 cm(-1) spectral domain around 2.4 mum which is analyzed simultaneously with a 0.12 cm(-1) (3.6 GHz) resolution by a Fouriertransform spectrometer. Recorded in 13 s, from 70-cm length absorption around 4150 cm(-1), acetylene and ammonia spectra exhibit a 3800 signal-tonoise ratio and a 2.4.10(-7) cm(-1).Hz(-1/2) noise equivalent absorption coefficient at one second averaging per spectral element, suggesting a 0.2 ppbv detection level for HF molecule. With the widely practiced classical tungsten lamp source instead of the laser, identical spectra would have taken more than one hour.

Emission spectra of HCN/HNC in the 2-5 microm range of astrophysical interest.

Emission from H12C14N observed with a Fourier transform spectrometer from a radio-frequency excited plasma is reported in the 2400-3400 cm(-1) spectral range of astrophysical interest. The molecular constants, for 21 vibration-rotation bands are given, as well as estimates of the first-order Herman-Wallis coefficients for 11 bands. These constants are derived from about 900 observed transitions in HCN, and are used to generate a sequential linelist of about 1400 calculated line positions, within a standard deviation equal to 3 x 10(-4) cm(-1). The relative intensities of the observed lines are also reported, as well as those for the nu1 band of H14N12C, at 3650 cm(-1), simultaneously observed from the same plasma.

High-information time-resolved Fourier transform spectroscopy at work.

The first instrumental setup, to our knowledge, that is capable of recording in a few hours the time-resolved Fourier transform (TRFT) interferograms of gas-phase spectra that cover several thousands of inverse centimeters with spectral- and time-resolution limits that are equal, at best, to 2.5 x 10(-3) cm(-1) and 2 ns, respectively, is reported. It was developed on the stepping-mode Connes-type interferometer of the Laboratoire de Photophysique Moléculaire Université de Paris Sud. Also, for the first time, to our knowledge, these high-resolution TRFT spectra, illustrated with the Doppler-limited emission spectra of the N(2) transitions (B-A) and (B'-B) between 5500 and 11 000 cm(-1) and of the atomic Ar lines between 1800 and 4000 cm(-1), are recorded in the infrared spectral range. To obtain identical results that have the same signal-to-noise ratio, we should have increased the recording time of our unique previous high-information TRFT spectra by approximately 50,000. In other words, one hour is now long enough to obtain what would previously have required six years to record.

High-resolution multimodulation Fourier-transform spectroscopy.

High-resolution double-modulation Fourier-transform (FT) spectroscopy (FTS) is demonstrated for what is, to our knowledge, the first time. Two high-resolution FT interferograms are simultaneously recorded. The first one is nonselective and contains all the spectral information from the observed source, and the other one is made of the samples that are sensitive to only a specific source modulation. General formulations and practical recording procedures are given for phase- and intensity-modulated spectra. The advantage of selectivity is illustrated by velocity-modulated emission spectra of the Dv = 1 sequence of the Doppler-shifted ArH(+) ion. It is also shown that for a source perturbation of small amplitude, only the product of intensity x shift can be retrieved from the selective line shapes obtained in a phase-modulated laser or FT spectra. Thanks to the multimodulation FTS approach, the intensity and the shift of the transitions are measured in a single experiment.

Absolute Wavenumbers and Self-Induced Pressure Lineshift Coefficients for the 3-0 Vibration-Rotation Band of 12C16O

The absolute wavenumbers for 36 lines of the 3-0 band of 12C16O are measured from P(17) at 6271 cm-1 to R(19) at 6405 cm-1, with an uncertainty of about +/-3 x 10(-5) cm-1 (about +/-1 MHz). The experimental positions are compared with the best predicted positions from recent Dunham coefficients. Self-induced pressure lineshifts are determined and reach, at most, about -8 x 10(-3) cm-1 atm-1. Copyright 1997 Academic Press. Copyright 1997Academic Press