Tomoo Fujioka

Theoretical simulation of electron-beam-excited xenon-chloride (XeCl) lasers

By developing a comprehensive computer code for e -beam excited XeCl lasers, we studied mainly the effect of Ar and Ne diluents on the performance characteristics of XeCl lasers. According to the analysis of the XeCl* formation process, the XeCl* relaxation process, and the 308 nm absorption process, it is found that the XeCl* formation efficiency is determined mainly by the rate of the charge transfer process (from Ar+ and Ne+ diluent ions to Xe+); in other words, by the difference between ionic potentials of Xe and the diluent gas used. The extraction efficiency is found to be decided mainly by the quenching rate of a three-body reaction for a short-pulse (55 ns) and a high-excitation-rate (∼ 3 MW/cm3) pumping, and by the absorption process for a long-pulse (500 ns) and a low-excitation-rate (∼ 0.2 MW/cm3) pumping. However, note that no appreciable difference in the intrinsic efficiency is found between the Ar/Xe/HCl and Ne/Xe/HCl mixtures. We also analyzed the dependence of the intrinsic XeCl laser efficiency on the pumping pulse width and excitation rate for Ar/Xe/HCl and Ne/Xe/HCl mixtures. As a result, the same intrinsic efficiencies are obtainable for both Ar- and Ne-based mixtures although the optimum operating conditions are slightly different. The maximum intrinsic efficiency of 5 percent is obtainable both for the Ar/Xe/HCl mixture at 3 atm and with 1.5 MW/cm3, 200 ns (FWHM) pumping and for the Ne/Xe/HCl mixture at 4 atm and with 2 MW/cm3, 200 ns (FWHM) pumping.

Output Power Enhancement of a Chemical Oxygen-Iodine Laser by Predissociated Iodine Injection.

Output power enhancement of a chemical oxygen-iodine laser (COIL) by an injection of predissociated iodine was studied. Iodine molecules were dissociated into atoms by the microwave discharge prior to injection. It was determined that predissociation caused a negative effect on the output power enhancement when this technique was applied to a conventional supersonic COIL. Model calculations revealed that the existence of atomic iodine at the plenum caused the dissipation of stored energy. It was demonstrated that decreasing the mixing point pressure was crucial to obtain output power enhancement by the predissociation technique. For this purpose, a low-pressure transonic mixing scheme with a grid nozzle array was developed. A 9% enhancement of output power was demonstrated.

Two-dimensional simulation of an unstable resonator with a stable core

An optical resonator simulation code based on the idea of a partially coherent optical field has been developed and used to optimize the design parameters of an unstable resonator with a stable core. The resonator was intended for use with low-gain, large-bore lasers, such as the chemical oxygen-iodine laser (COIL). First the design parameters of the resonator were optimized by the simulation code; then a set of mirrors was fabricated for a small-scale COIL. A 14-W output with M(2) = 29 was obtained. The experimentally obtained results were in good agreement with calculations.

Critical Power of Stimulated Brillouin Scattering in Multimode Optical Fibers.

A set of coupled-wave equations for stimulated Brillouin scattering (SBS) in multimode optical fibers excited by single-mode laser light is derived. The analytical solution of the critical pump power is also derived. The solution explains the experimental results well.

Yb fiber laser pumped mid-IR source based on difference frequency generation and its application to ammonia detection.

A Yb fiber laser pumped cw narrow-linewidth tunable mid-IR source based on a difference frequency generation (DFG) in a periodically poled LiNbO3 (PPLN) crystal for trace gas detection was demonstrated. A high power Yb fiber laser and a distributed feedback (DFB) laser diode were used as DFG pump sources. This source generated mid-IR at 3 microns with a powers of ~2.5 microW and a spectral linewidth of less than 30 MHz. A frequency tuning range of 300 GHz (10 cm-1) was obtained by varying the current and temperature of the DFB laser diode. A high-resolution NH3 absorption Doppler-broadened spectrum at 3295.4 cm-1 (3.0345 microns) was obtained at a cell pressure of 27 Pa from which a detection sensitivity of 24 ppm m was estimated.

Pulse propagation in the amplifier of a high-power iodine laser

Pulse propagations in an amplifier in the case of the photodissociation iodine laser are investigated with the use of semiclassical treatments, especially when the input pulse fed into the amplifier has a width comparable to or shorter than the transverse relaxation time of the amplifier medium. In our semiclassical equations for pulse propagation in an iodine amplifier, the populations of all hyperfine levels [2P 1/2 ( F= 3, 2 ),2P 3/2 ( F = 4, 3, 2, 1 )], the polarizations of optically allowed transitions ( \DeltaF = 0, \pm 1 ), and the Doppler detuning effects were taken into consideration. Based on the numerical integrations of the semiclassical equations, the propagated pulse shape and the population distributions of the hyperfine levels are discussed. From our calculations, it has been made clear that bandwidth limitations of pulses in the range of pulsewidths of 0.1 ns and below can be avoided by making use of the nonlinear amplification that results from the coherent interaction between the propagated pulse field and the amplifier medium.

Performance Characteristics of Narrow Linewidth Fiber Laser Pumped Mid-IR Difference Frequency Mixing Light Source for Methane Detection

A high-power, narrow-linewidth Yb fiber laser with a fiber Bragg grating (FBG) pumped difference frequency generation (DFG) in a periodically poled lithium niobate (PPLN) crystal was investigated in detail. A mid-IR power of approximately 2.3 microW at 3.3 micrometers with a slope efficiency of 0.85 mW/W2 was achieved. A Doppler-broadened absorption spectrum of CH4 at 3038.497 cm-1 (3.2911 micrometers) was obtained with a 0.1-m long-gas cell at a pressure of 133 Pa. The linewidth of the DFG source was evaluated to be less than 96 MHz from the observed spectral linewidth. Real-time monitoring of CH4 (approximately 1.78 ppm) in ambient air in a multipass cell which has an optical path length of 10 m was also demonstrated.

Numerical simulation of the w-axicon type optical resonator for coaxial slab CO2 lasers

The numerical simulation of the optical resonator for a coaxial slab CO2 laser is conducted. The resonator consists of a toric mirror, a w-axicon, and a plane output coupler. To hold the two slabs coaxially, struts exist in the annular gap region. The effects of the struts on the output power and beam quality for the given resonator are discussed. It is found that varying the vertex of the toric mirror can control the oscillation mode between the E00 fundamental mode and the E01 first azimuthal mode. It is revealed that the resonator loss of the fundamental mode can be smaller than the first azimuthal mode by shifting the vertex inward, and this results in single-mode oscillation at the E00 fundamental mode. On the other hand, the maximum output is obtained under conditions where the resonator oscillates at the E01 mode, because the nodes of the mode coincide with the position of the struts. Loss measurement of each eigenmode well describes the aforementioned phenomena.

S1–S0 Electronic spectrum of jet-cooled m-aminophenol

The S1 and S0 states of m-aminophenol have been investigated using laser induced fluorescence and dispersed fluorescence spectroscopy in a supersonic jet. The dispersed fluorescence spectra, obtained by exciting the bands at 34 109 and 34 469 cm−1, show the same vibronic structure, which suggests the coexistence of rotational isomers in m-aminophenol. A quantum chemical calculation also supports the coexistence of rotational isomers. From the relative intensities in the spectrum and the calculated stabilization energies of isomers, the bands are assigned to the origin of the cis- and trans-isomers, respectively. The dispersed fluorescence spectra obtained by exciting the S1 vibronic bands were analyzed by comparing with the calculated vibrational frequencies and IR and Raman spectra. From the analysis, the S1 vibronic bands have been assigned. It was found that a one-to-one correspondence between the S1 and S0 vibrations is broken, and vibrational mixing due to Fermi resonance or the Duschinsky effect is suggested.

Development of a Mist Singlet Oxygen Generator.

The singlet oxygen generator (SOG) generates singlet oxygen for a chemical oxygen iodine laser (COIL), using the gas-liquid reaction between basic hydrogen peroxide (BHP) and Cl2 gas. The Jet-SOG has been widely used, wherein jet BHP from small orifices reacts with Cl2 gas, and the BHP utilization is less than 1% in a single pass through the reaction zone. To improve BHP utilization, the reaction surface with Cl2 gas should be increased, and the droplet diameter of BHP should be decreased. In this study, two types of mist generators were tested for the SOG, with which 65-µm- and 15-µm-diameter droplets were generated. In the 65 µm mist generator, BHP utilization was 22.5% at the Cl2 flow rate of 8.3 mmol/s, and in the 15 µm mist generator, BHP utilization was 41.5% at the Cl2 flow rate of 9.0 mmol/s, that is, BHP utilization of the new SOG, Mist-SOG, markedly exceeded that of the conventional Jet-SOG.