Real time concentration monitoring of NO3, NO2, O3 and N2O5 for study of chemical kinetic process in smog chamber by open-path LED-IBBCEAS operating near 662 nm and QCL absorption spectroscopy at ~8 μm.

Elimination of residual amplitude modulation in wavelength modulated off-axis integrated cavity output spectroscopy allows achieving a noise equivalent absorption sensitivity of 4.5×10−12 cm−1/Hz1/2, yielding a 1σ detection limit of 5.7×109 OH radicals/cm3 at ~1435 nm.

Chemically reactive short-lived species play a crucial role in tropospheric processes affecting regional air quality and global climate change. Contrary to long-lived species (such as greenhouse gases), fast, accurate and precise monitoring changes in concentration of atmospheric short-lived species represents a real challenge due to their short life time (~1 s for OH radical) and very low concentration in the atmosphere (down to 106 molecules/cm3, corresponding to 0.1 pptv at standard temperature and pressure). We report on our recent progress in instrumentation developments for spectroscopic sensing of trace reactive species. Modern photonic sources such as quantum cascade laser (QCL), distributed feedback (DFB) diode laser, light emitting diode (LED), difference-frequency generation (DFG) parametric source are implemented in conjunction with highsensitivity spectroscopic measurement techniques for : (1) nitrous acid (HONO) monitoring by QCL-based long optical pathlength absorption spectroscopy and LED-based IBBCEAS (incoherent broadband cavity-enhanced absorption spectroscopy); (2) DFB laser-based hydroxyl free radical (OH) detection using WM-OA-ICOS (wavelength modulation off-axis integrated cavity output spectroscopy) and FRS (Faraday rotation spectroscopy), respectively; (3) nitrate radical (NO3) and nitrogen dioxide (NO2) simultaneous measurements with IBBCEAS approach. Applications in field observation and in smog chamber study will be presented.

We report on the development of an optical instrument based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) for simultaneous open-path measurements of nitrous acid (HONO) and nitrogen dioxide (NO2) in ambient air using a UV light emitting diode operating at ∼366 nm. Detection limits of ∼430 pptv for HONO and ∼1 ppbv for NO2 were achieved with an optimum acquisition time of 90 s, determined by an Allan variance analysis. Based on a 1.85 m long high optical finesse open-path cavity, the effective optical path length of 2.8 km was realized in aerosol-free samples or in an urban environment at modest aerosol levels. Such a kilometer long optical absorption is comparable to that achieved in the well established differential optical absorption spectroscopy (DOAS) technology while keeping the instrument very compact. Open-path detection configuration allows one to avoid absorption cell wall losses and sampling induced artifacts. The demonstrated sensitivity and specificity shows high potential of this cost-effective and compact infrastructure for future field applications with high spatial resolution.

A terahertz time domain spectroscopy experiment is used to study the coherent re-emission after exciting more than 60 energy rotational states of OCS molecules in gas phase. Due to the regular structure of the absorption spectrum of such linear molecules, a set of subsequent pulses separated by 82.6ps is re-radiated from the vapour and recorded up to 450ps. A model based on a linear response of the gas and by use of “Maxwell–Bloch” equations has permitted the re-emitted free induced decay to be investigated. Spectroscopic parameters, such as rotational constant, centrifugal distortion coefficient and relaxation times are responsible for the temporal shape and so can be evaluated. The influence of the optical thickness to access the relaxation times is discussed.

La methode de correlation a filtre de gaz permet de detecter un gaz cible au sein d'un melange. La pression dans les deux cellules joue des roles primordiaux sur la sensibilite ainsi que sur la selectivite. Cette methode a ete exploitee dans le domaine de l'infrarouge lointain. La source emettrice est une antenne a large ouverture dont le spectre s'etend jusqu'a environ 2 THz.

The photoinscription of standard telecommunications glass materials by femtosecond radiation is shown to induce significant mesoscopic structure. The scattering intensity for irradiated glasses is close to two orders of magnitude greater than that of unexposed material. Anomalous small-angle X-ray scattering (ASAXS) around the germanium K-edge for the silica and germanium doped silica regions of a fibre preform is used to demonstrate that identical structures are induced in both glass materials, with germanium displaying a capacity to isomorphically replace silicon in the case of the germanium doped silica. Analysis of measured scattering indicates that photo-inscribed features are produced at two distinct scales with typical radii of R≈20Å and Rmin≈200Å.

Introduction Since 1997, photoinscription of waveguides in a bulk glass has become a novel very promising application of femtosecond lasers [1]. Multiphoton absorption induces a local fusion of the glass in any chosen point of the bulk sample, probably accompanied by a densification of the material after its re-solidification. This process leads to a variation of the refractive index and allows a waveguide to be realised in the bulk glass interior and/or 3D devices. Little is known about the nature of photoinduced phenomena produced by ultrashort laser pulses. We started a systematic study of glass structure variations on mesoscopic, intermediateand shortrange scales using hard x-rays and neutrons.

Long-period gratings (LPGs) have been inscribed by femtosecond laser radiation into fiber optics with Ge-doped and pure silica cores. Grating parameters of 16-dB rejection and 0.3-dB insertion loss are obtained for a Ge-doped fiber core. Numerical calculation of the transmission characteristics yielded a good correlation between the measured and synthetic data showing that an induced refractive index change up to 3/spl times/10/sup -4/ can be achieved. Inscription of an LPG in pure silica fiber core is demonstrated and thought to be first achieved in this fiber type. Greater laser intensity is required in pure silica compared with Ge-doped cores to produce a refractive index change of similar magnitude.

The tomographic measurement of the residual stress profile in femtosecond-laser irradiated standard SMF-28 germanium-doped telecommunication fiber is demonstrated. The fiber is irradiated with weakly focused pulses to realize long-period fiber gratings. In the irradiated grating regions, an asymmetrical increase in axial core stress up to 6.2 kg/mm2 is found. The increase in stress is attributed to a densification of the irradiated glass matrix. The stress-induced anisotropic index distribution is calculated and related to the absolute index change in the irradiated regions.

We propose an experimental method for detecting molecules in the UV–visible range using ultrashort laser pulses. Two types of sources are used: a continuum generated by 200 kHz Ti:sapphire regenerative amplifier system extending from 320 to 1100 nm, and a near-gaussian femtosecond pulse (100 fs) generated by an optical parametric amplifier. Both broadband sources allow the real-time detection of the oxygen, the nitrogen dioxide NO2 and the water vapor bands. Moreover, the concentration of NO2 can be determined within the 10 ppb sensitivity range by using a specific nonlinear fit technique.

Permanent refractive index changes as large as 6.10-3 in standard fibers is obtained by means of irradiation with tightly focused femtosecond near-infrared pulses. The dependence of the index variation with the exposure time is depicted.

A new method for studying molecules with the chiral structure is proposed. The method is based on generation of the second optical harmonic (SH) upon reflection of light from the surface of solutions. It is shown that analysis of the polarization dependence of the s component of the generated SH allows one to detect chirality of molecules of the dissolved substance, to determine its relative value and sign, and to control the state of the surface of the sample under study. The advantage of this method is the fact that small amounts of substance are sufficient for its realization. Only the molecules inside the interaction volume with the size determined by the diameter of the waist of the excitation beam and the subwave thickness of the near-surface layer contribute to the SH signal.

The spectral dependence of the efficiency of generation of the forbidden (in the electric-dipole approximation) second harmonic (FSH) and its energy on the energy of the excitation pulses is investigated experimentally in a highly disperse suspension of purple membranes containing bacteriorhodopsin (BR) under excitation by femtosecond laser pulses into the single-photon absorption band of BR. The experimental data for the case of resonance excitation attest to an interference character of the interaction of optical nonlinearities of different orders in the process leading to the formation of the FSH signal.

We report on new applications of femtosecond laser pulses for studies of chiral media by nonlinear optical techniques.

The generation of the second harmonic of femtosecond laser pulses in a bacteriorhodopsin solution has been experimentally studied for various wavelengths and polarization states of radiation at the fundamental frequency. The polarization properties of the effect are analyzed under various experimental conditions. The nature and properties of the signal are treated as the manifestation of a superposition of nonlinear optical effects of various orders (the second and the fourth). The second-order effects can have both an electric-dipole and a magnetic-dipole or electric-quadrupole character. In analyzing fourth-order processes, besides the direct electric-dipole contribution, the possibility of the participation of cascade processes at second-and third-order nonlinearities is also allowed.

The generation of a femptosecond second harmonic in a bacterior-hodopsin solution is investigated. It is shown that the behavior of the harmonic can be explained by the coherent interference of several contributions from three-and five-wave mixing processes (χ(2) and χ(4)). The magnitude and phase of the fourth-order electronic susceptibility χ(4) of the medium with respect to χ(2) are determined.

Bragg gratings were written in a slice of an optical fibre preform using two interfering uv pulsed laser beams at 243 nm. Microstructural changes within the germanosilicate glass were studied by transmission electron microscopy (TEM) and infrared spectroscopy. TEM investigation shows that the glass is densified at the grating grooves. Infrared absorption spectra have been recorded on a germanosilicate glass sample before and after in irradiation by a fringeless uv pulsed beam. Our conclusion is that uv irradiation-induced bond breaking allows structural relaxation of the non-equilibrium glass network which tends to densify without any noticeable change in coordination.

A new kind of instrumentation based on Optical Fiber Bragg grating sensors is proposed for the main relevant monitoring needs in the electric power industry. The challenging metrological properties of these components are presented and their good resistance to (gamma) -ray irradiations experimentally proved.