High resolution spectroscopy of narrow band giant pulse lasers: time-dependent frequency shifts in ruby

Abstract

Both plane and defocused spherical Fabry-Perot interferometers, with a limiting resolution of less than 10 MHz, have been employed to study the output spectral brightness of narrow band giant pulse ruby lasers. Rotating prism and Pockels cellQ-switched systems have been compared and the mode selection properties of resonant reflectors demonstrated. The advantages of uniform pumping of the laser rod, when combined with a dye solution isolator, are illustrated by interferograms showing the direct spectroscopic detection of pure transverse mode structure in giant pulse lasers. To elucidate apparent discrepancies between time integrated interferograms and pulse envelope beat patterns, time resolved spectroscopy with a fast image tube streaking camera was carried out. Time and intensity dependent blue and red frequency shifts were detected and measured and rapid (< 10 ns) transverse changes of the active region of the ruby were revealed by streaked photographs of the beam cross-section. These results are qualitatively accounted for in terms of the combined effects of rapid bleaching of the central region of an inhomogeneous distribution of population inversion, and nonlinear anomalous dispersion in the ruby. Beam spreading or narrowing with the preferential growth of higher or lower frequency transverse modes respectively then results. Disagreement between theoretical and measured durations of laser giant pulses is thus explained. Beat patterns are shown to be inadequate for the determination of mode structures. Such frequency shifts (about 200 MHz for a 35 ns 5 MW pulse) seriously limit the time integrated spectral brightness obtainable from giant pulse ruby lasers. A method of reducing them to increase spectral brightness is briefly discussed.