Correction of shape distortions in laser beams apodized with circular serrated apertures.

Flat-top laser beams produced with apodizers comprising a circular serrated aperture and spatial filter are not optimal for propagation over long distances. Residual intensity fluctuations across the overall smooth profile at the apodizer exit significantly accelerate degradation of the beam at small Fresnel numbers. By solving the parabolic equation for uniform and Gaussian beams propagating through a serrated aperture apodizer, we show that a narrow opaque ring installed inside the serrated aperture can largely suppress unwanted diffraction effects and bring the output amplitude profile close to the flattened Gaussian function. With this correction, the usable propagation distance of the apodized beam can be extended to Fresnel numbers ${N_{F}} \approx 2\ldots 5$.

Remote helicopter-borne detector for searching of methane leaks.

Measurements of the content of various molecular impurities in the ambient air using helicopter- and aircraft-borne systems represent an extremely urgent challenge. In this respect, of special interest are the devices that that provide leakage monitoring in gas lines in order to prevent emergencies. In the paper results of the tunable diode laser-based instrument development and testing are presented.

UF6 enrichment measurements using TDLS techniques.

The objective of this work was investigation of possibility of tunable diode laser spectroscopy (TDLS) technique application for gaseous uranium hexafluoride (UF6) isotope measurement. Spectra of uranium hexafluoride gas mixture were investigated using two different Fourier Transform Spectrometers Vector 22 and Bruker 66v. Observed spectral features were identified and model spectra of different gas mixture components were developed. Optimal spectral range for measurements was determined near maximum of UF6 combination band nu1+nu3. Laboratory prototype of multi-channel instrument under consideration based on tunable diode lasers was built and algorithms were developed to measure gaseous UF6 isotopic ratios. Diode laser used operated at the wavelengths near lambda=7.68 microm. It was placed in a liquid nitrogen cooled cryostat. Three instrument channels were used for laser frequency calibration and spectra recording. Instrument was tested in measurements of real UF6 gas mixtures. Measurement accuracy was analyzed and error sources were identified. The root-mean-square random error in the 235U isotopic content is characterized by a spread of about 0.27% for quick measurements (at times less than 1 min) and 1% for periods of more than an hour. It was estimated that the measurement accuracy could be improved by at least an order of magnitude by minimizing the error sources.