Abstract: We report on the application of a compact and field-deployable instrument, based on a continuous-wave fiber-coupled Telecom external cavity diode laser, to measure the C-13/C-12 isotope ratio in CO2 from a wood-based combustion. Carbon dioxide, the most important greenhouse gas, is a major product of combustion. The measurements of the 13C/12C isotopic ratio in CO2 from combustion emission permit one to identify the CO, source and to study the temporal and spatial variations of pollution in the atmosphere. The average value of the (CO2)-C-13/(CO2)-C-12 ratio is found to be (1.1011 +/- 0.0024)%. The corresponding delta-value relative to PDB standard is (-20.17 +/- 2.14)parts per thousand, which is in good agreement with the typical value of (-25 +/- 2)parts per thousand for wood. Simultaneous monitoring of multiple species from gas emission has been performed using direct-absorption spectroscopy. The concentrations Of C2H2, CO, CO2 and H2O were determined on the basis of integrated absorbance measured by least-squares fitting a Voigt lineshape to experimental absorption spectra. (C) 2007 Elsevier Ltd. All rights reserved.
Abstract: 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 82.6 ps is re-radiated from the vapour and recorded up to 450 ps. 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. (c) 2008 Elsevier B.V. All rights reserved.
Abstract: We report the first demonstration of a terahertz photomixer made of ion-irradiated In0.53Ga0.47As lattice-matched to InP and fiber-optic coupled with the drive lasers. A continuous-wave radiation is generated at frequencies up to 0.8 THz by photomixing two continuous-wave laser diodes around 1.55 mu m. The measured 3dB-down bandwidth of 300 GHz yields a carrier lifetime of 0.53 ps, in agreement with the value of 0.41 ps measured in pump probe experiments. The detected signal is at the most 15 dB lower than the one obtained from similar photomixers fabricated from low-temperature-grown GaAs. (c) 2006 Optical Society of America.
Abstract: We report the first demonstration of a terahertz photomixer made of ion-irradiated In0.53Ga0.47As lattice-matched to InP and fiber-optic coupled with the 1.55 mu m wavelength drive lasers. The detected signal is at the most 15 dB lower than the one obtained from similar photomixers fabricated from low-temperature-grown GaAs.
Abstract: Continuous-wave terahertz spectroscopy by photomixing is applied to the analysis of mainstream cigarette smoke. Using the wide tunability of the source, spectral signatures of hydrogen cyanide (HCN), carbon monoxide (CO), formaldehyde (H2CO), and water (H2O) have been observed from 500 to 2400 GHz. The fine spectral purity allows direct concentration measurement from the pure rotational transitions of HCN and CO. The quantification of the measurement was validated by the means of a calibration gas containing CO. The potential of this technique for trace gas detection is demonstrated with an estimated detection limit of HCN equal to 9 parts in 10(6). (c) 2006 Optical Society of America.
Abstract: 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 approximate to 20 angstrom and R-min approximate to 200 angstrom. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: 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 X 10(-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.
Abstract: 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/mm(2) 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. (C) 2004 American Institute of Physics.
Abstract: 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 I 100 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. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: We report on permanent refractive-index changes as great as 6 X 10(-3) in standard fibers on irradiation with tightly focused femtosecond (120-fs) near-IR (lambda = 800-nm) pulses. The refractive-index increase was measured through changes in the transmission spectrum of an UV photowritten Bragg-grating-based Fabry-Perot interferometer. We depict the dependence of the index variation on exposure time. The changes in refractive index can be optimized after a few seconds of irradiation. The measured induced excess loss increases during near-IR exposure. (C) 2001 Optical Society of America.
Abstract: 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.
Abstract: 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. (C) 1998 American Institute of Physics.
Abstract: The second harmonic (SH) generation with a Ti:sapphire femtosecond laser system in a randomly oriented suspension of finely fragmented purple membranes of Halobacterium halobium has been studied experimentally. The existence of two interfering contributions to the SH signal due to second-and fourth-order nonlinear processes has been established. The phase and the value of the fourth-order electronic susceptibility (hyperpolarizability) relative to the second-order ones have been determined. The analysis of the experimental data shows the existence of a coherent contribution into the SH signal. (C) 1997 Elsevier Science B.V.
Abstract: 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. (C) 1997 American Institute of Physics.
Abstract: The generation of a femptosecond second harmonic in a bacteriorhodopsin 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 (chi((2)) and chi((4))). The magnitude and phase of the fourth-order electronic susceptibility chi((4)) of the medium with respect to chi((2)) determined. (C) 1996 American Institute of Physics.
Abstract: Bragg gratings have been written within four germanosilicate fibers either by a pulsed or by a continuous-wave exposure of each fiber to a coherent UV two-beam interference pattern. These gratings have been exposed under steady state conditions to gamma-ray doses as high as 10(4) Grays. The dose rates ranged between 10 Gy/h and 1.3 x 10(2) Gy/h. The transmission spectra of the fibers have been recorded during and after the Co-60 irradiation, near the grating Bragg wavelengths. Whereas the induced loss reached 600 dB/km near 1.3 mum, no significant change in the spectral characteristics of the gratings could be detected within the experimental accuracy, enabling their future use in a nuclear environment.
Abstract: 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 an 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.
Abstract: Bragg gratings written within GeO2 doped-silica-glass fibers were submitted to increasing temperature steps. Bragg wavelength permanent shifts induced by heating were measured at room temperature after each step using a tunable laser (accuracy of the measurement +/-3 10(-3) nm). Thus, hysteresis could be observed even after a temperature step as low as 80 degrees C from room temperature. Through other experiments, the temperatures of various gratings were progressively increased in stages. During each stage, the Bragg wavelength was measured as a function of the grating temperature. The thermal evolutions of the Bragg wavelength were shown to depend slightly upon the experimental conditions used for the photoinscription of the gratings. These two effects were minimized through a curing profess.
Abstract: The first demonstration of the feasibility of writing a moire grating into a germanosilicate fibre using double exposure by ultraviolet fringe patterns is reported. Using this technique a resonant filter response with a bandwidth of 0.2 nm centred around 1200 nm and with a finesse of 6 has been obtained.
Abstract: Bragg gratings have been written in Germanium doped core silica fibers by transversally exposing the fibers to fringe patterns produced by a pair of coherent U.V. pulsed beams combined inside a prism interferometer. The transmission spectra of the gratings have been recorded at various times of irradiation. The drop in transmission near resonance wavelength may be as large as 100%, depending on the experimental conditions of the writing experiment. The thermal annealing behavior of different gratings shows that the evolution of the fiber transmittivity recovery versus temperature depends upon the number and the energy of the laser pulses applied to form the grating and upon the nature of the fiber.
Abstract: The resonance wavelengths of Bragg gratings are shown to increase with irradiation time during their inscription if the core of a germanosilicate fibre is exposed to a 243 nm UV fringe pattern. Exposure of the gratings to a 243 nm UV monotonic illumination leads to a recovery in the transmittance of the filters. Whether this irradiation induces a shift, or not, in the Bragg wavelength of the filters depends on the history of the grating writing experiment.
