Narrow-band KrF Excimer Laser Research Articles

OH Measurement in Propane Diffusion Flames by a Laser Induced Fluorescence Using a KrF Excimer Laser.

Laser induced fluorescence was applied for the measurement of OH concentration in the propane diffusion flames. A tunable, narrow band KrF excimer laser was used to excite the A(ν'=3)←X(ν=0) band of OH radical. An emission spectrum of the induced light from the flames was measured by a spectroscope. An OH fluorescence component was picked up from the emission spectrum in order to obtain the accurate distribution of OH fluorescence in the flames. We call this method for spectrum-LIF (S-LIF). The spectrum of the induced light from the diffusion flames was compared with that of the premixed flames. Although the spectrum had only OH fluorescence components in the case of premixed flames, high intensity emission from the yellow flame appeared on the LlF image in the case of diffusion flames. To remove the high intensity emission from the LIF image, off resonance image was deducted from the OH resonance image as the background image. Using this deduction method, a planer laser induced fluorescence (P-LIF) was tried to measure the distribution of OH fluorescence. OH fluorescence distribution measured by P-LIF was compared with that of S-LIF. As the result, the P-LIF image qualitatively agreed with the data of S-LIF.

Premixed Methane-Air Flame Spectra Measurements Using UV Raman Scattering

A spontaneous vibrational Raman scattering technique is developed for simultaneous measurements of temperature and multi-species (C02 O2, CO, N2, CH4, H20, H2, and OH) concentrations in premixed methane - air flames. The Raman spectra and spectral background are obtained using a narrowband tunable KrF excimer laser working at 248 nm and a spectrometer combined with an intensified CCO camera. Optimal tuning of the narrowband KrF excimer laser (248.632 nm) for minimization of OH A2Σ_X2П and 02 B3Σu- X3Σg­fluorescence interference is determined from fluorescence excitation spectra. Laser emission spectra are made to ensure that the optimum wavelength of the laser is within a good locking region. Natural fluorescence spectra (without laser excitation) are used to determine the gate time of the ICCO camera. Measured Raman spectra demonstrate that single-pulse concentration and temperature measurements, with only minimal fluorescence interference, are possible for turbulent premixed methane-air flames. A time-averaged temperature measurement technique is presented that matches the theoretical spectra to the measured N2 Stokes Raman spectrum. Methodologies for simultaneous temperature, major species, and OH concentration measurements in turbulent premixed flames are discussed.

Fast wavelength switching of narrow-band excimer lasers

A novel system was developed, which allows one to switch the wavelength of a narrow-band excimer laser between two successive light pulses at a repetition rate of at least 250 Hz. This is realized by a periodically driven piezo actuator, which is attached to the diffraction grating of the narrow-band KrF excimer laser. The achieved position accuracy of the grating leads to a wavelength reproducibility of ±0.2 pm, which allows one to apply this system to laser spectroscopic investigations like LIF or LIPF of OH in flames. Using the fast wavelength switching system background reduced concentration and temperature fields in flames can be measured within one sequence. Some possible realized and planned applications like the measurement of gas temperature, the diagnostic of turbulent combustion processes, and the investigation of combustion processes under microgravity are discussed.

Atomic vapor filter for two-dimensional Rayleigh imaging experiments with a narrow-band KrF excimer laser

An optical transition of atomic iron can be used to build a two-dimensional absorption filter that operates in the tuning range of a narrow-band KrF excimer laser (0.05-cm(-1) bandwidth). Iron atoms with a number density of 2 × 10(13) cm(-3) are produced by thermal dissociation of iron pentacarbonyl [Fe(CO)(5)]. With this filter it is possible to discriminate Mie and surface-scattered light against Rayleigh scattering. For Rayleigh scattering from the exhaust gas of a methane-air flame at a measured temperature of 1970 K, the ratio of transmission for Rayleigh scattering to transmission for Mie scattering is 16.8. Single-shot scattering from the exhaust gas of a high-pressure burner is presented as an application.

KrF-Laser-Induced Fiber Bragg Gratings: Formation Characteristics and Cutback Measurement

The formation characteristics of fiber Bragg gratings were studied by using the side-writing holographic method with a 248 nm injection-locked narrow-band KrF excimer laser. Type I gratings with reflectivity as high as 99% were formed in hydrogen-loaded fibers by multiple-pulse exposure. Type II gratings with the maximum reflectivity of 90% were also fabricated. The characterization of fiber gratings by the cutback method is demonstrated. Using this method, we determined the effective grating length and calculated the magnitude of photoinduced index change. Chirp characteristics of the gratings were also studied.

Spatially resolved multispecies and temperature analysis in hydrogen flames

We report on spatially resolved simultaneous measurements of temperature and majority species concentrations along a line segment in a premixed laminar H(2)-air flame. The results are obtained from Raman and Rayleigh scattering by using a narrow-band KrF excimer laser and a spectrally and spatially resolving detector system that consists of a high-throughput spectrometer and a gated, intensified, two-dimensional CCD camera. The data presented here are integrated over 100 laser shots. Absolute density profiles of N(2), O(2), H(2)O, and H(2), as well as temperature profiles at various heights through the flame, are presented. A discussion of the required calibration procedures and a summary of the necessary spectroscopic background are also included in this paper.

Raman scattering measurements in flames using a tunable KrF laser

Using a narrow-band tunable KrF excimer laser as a spontaneous vibrational Raman scattering source, we demonstrate that single-pulse concentration and temperature measurements, with only minimal fluorescence interference, are possible for all major species (O(2), N(2), H(2)O, and H(2)) at all stoichiometries (fuel-lean to fuel-rich) of H(2)-air flames. Photon-statistics-limited precisions in these instantaneous and spatially resolved single-pulse measurements are typically 5%, which are based on the relative standard deviations of single-pulse probability distributions. Optimal tuning of the narrow-band KrF excimer laser (248.623 nm) for the minimization of OH A(2)Sigma-X(2)II and O(2)B(3)Sigma(u)(-)-X(3)Sigma(g)(-) fluorescence interference is determined from fluorescence excitation spectra. In addition to the single-pulse N(2) Stokes/anti-Stokes ratio temperature measurement technique, a time-averaged temperature measurement technique ispresented that matches the N(2) Stokes Raman spectrum to theoretical spectra by using a single intermediate sta frequency to account for near-resonance enhancement. Raman flame spectra in CH(4)-air flames are presented that have good signal-to-noise characteristics and show promise for single-pulse UV Raman measurements in hydrocarbon flames.

Beam quality measurement of a narrow band KrF excimer laser with stimulated Brillouin scattering phase conjugate double-pass amplifier at 100 Hz operation

A narrow band KrF excimer laser device which consists of an oscillator and a double-pass amplifier with a stimulated Brillouin scattering phase conjugation (SBS-PC) mirror (double-pass self-compensation scheme) was operated with the constant reflectivity of phase conjugation at 100 Hz repetition rate, by vibrating the SBS cell. The dependence of the beam quality on the operating frequency was evaluated for every shot by using a beam profiler. The beam divergence was less than 100 μrad and the fluctuation at the far-field peak position was less than 10 μrad at 100 Hz with the spectral bandwidth less than 0.2 cm−1.

UV raman spectroscopy of H2-air flames excited with a narrowband KrF laser

Raman spectra of H2 and H2O in flames excited by a narrowband KrF excimer laser are reported. Observations are made over a porous-plug, flat-flame burner reacting H2 in air, fuel-rich with nitrogen dilution to control the temperature, and with a H2 diffusion flame. Measurements made from UV Raman spectra show good agreement with measurements made by other means, both for gas temperature and relative major species concentrations. Laser-induced fluorescence interferences arising from OH and O2 are observed in emission near the Raman spectra. These interferences do not preclude Raman measurements, however.