Characterization of waveguides, beam coupling and propagation for simulated Raman conversion

Abstract

The Raman conversion of a 10 μm CO2 laser beam in parahydrogen is an integral part of the 235 UF6 laser isotope enrichment process. The nonlinear Raman process depends strongly on the input laser power density. Lasers providing such high power ( > 2 J in 80 ns pulsed) in a Gaussian mode are complex and sensitive items. To achieve high power ( > 150 mJ in 40 ns pulsed), high repetition rate ( > 1 kHz) 16 μm output, two beam confinement schemes filled with cold (77 K) parahydrogen were investigated: A novel multimirror multipass (35 passes) Raman cell and a hollow metal waveguide. A rate equation model was developed and verified using experimental results obtained from the conventional multipass (25 passes) Raman experiments. The parameters investigated for the waveguide confinement scheme are: ■ the manufacture of minimum loss metal waveguides with an inner germanium layer, ■ the effective coupling of the free space laser mode (TEM00) to the minimum loss HE11 waveguide mode, ■ the power transmission (experimental) through the waveguide, and ■ the effect the waveguide has on the input laser beam polarization. The actual (non-ideal) laser beam propagation parameters differ from the ideal Gaussian parameters (ro 0 and z0). It is characterized by three parameters: W0 (minimum beam radius or "waist"), Zo (position of the "waist" along the propagation axis), and the beam quality parameter (M2 ). Knowing these parameters, the master-oscillator-power-amplifier chain can be optimized for maximum amplifier extraction with as little possible interference from the finite apertures on the propagation path. The characterization of the high repetition rate CO2 oscillator includes the following: ■ the size and position of the intracavity aperture for optimum output beam quality, and ■ the effect of gas lifetime and high pulse rates on the output laser beam quality. Some experimental results obtained from 16 μm line width measurements, Raman threshold measurements (for 10P20 and 10R18 lines), and the influence of the beam quality (M2 ) on the 16 μm output will be given.