We describe a simple and passive nanosecond-long laser-pulse stretcher using multiple optical ring cavities. We present a model of the pulse-stretching process for an arbitrary number of optical ring cavities. This new model explicitly includes the effects of cavity delay time, beam-splitter reflectivity, total number of optical cavities, and describes the effects of spatial profile sensitivity. Using the model, we optimize the design of a pulse stretcher for use in a spontaneous Raman-scattering excitation system that avoids laser-induced plasma spark problems. From the optimized design, we then experimentally demonstrate and verify the model with a three-cavity pulse-stretcher system that converts a 1000-mJ, 8.4-ns-long input laser pulse into an approximately 75-ns-long (FWHM) output laser pulse with a peak power reduction of 0.10× and an 83% efficiency.

Laser pulse-stretching with multiple optical ring cavities.

A method or apparatus for controlling the output pulse of a Q-switched laser. The intensity of laser light within the resonator of a Q-switch laser is sensed. A reference pulse related to a desired laser output pulse shape is compared with the sensed intensity. A variation signal corresponding to differences between the sensed light intensity and the reference pulse is fed to a Pockels cell or Q-switch within the resonator. The Q-switch responds to the variation signal by altering the resonator laser light polarization to maintain the Q-switch laser's output pulse in the desired shape. The reference pulse may be dynamically altered to change the shape of the output pulse.

Method and apparatus to dynamically control the resonator gain of a laser

An optical pulse stretcher and a mathematical algorithm for the detailed calculation of its design and performance is disclosed. The optical pulse stretcher has a plurality of optical cavities, having multiple optical reflectors such that an optical path length in each of the optical cavities is different. The optical pulse stretcher also has a plurality of beam splitters, each of which intercepts a portion of an input optical beam and diverts the portion into one of the plurality of optical cavities. The input optical beam is stretched and a power of an output beam is reduced after passing through the optical pulse stretcher and the placement of the plurality of optical cavities and beam splitters is optimized through a model that takes into account optical beam divergence and alignment in the pluralities of the optical cavities. The optical pulse stretcher system can also function as a high-repetition-rate (MHz) laser pulse generator, making it suitable for use as a stroboscopic light source for high speed ballistic projectile imaging studies, or it can be used for high speed flow diagnostics using a laser light sheet with digital particle imaging velocimetry. The optical pulse stretcher system can also be implemented using fiber optic components to realize a rugged and compact optical system that is alignment free and easy to use.

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Temporal laser pulse manipulation using multiple optical ring-cavities

The ability to transport large quantities of heat by nucleate boiling is reduced for mixtures compared to their pure components. To study the heat and mass transfer during the boiling process, a one-dimensional Raman spectroscopy setup was applied to measure, for the first time to the best of our knowledge, the composition and temperature gradients in the vicinity of single boiling bubbles of a mixture of acetone and isopropanol. A novel evaluation strategy allowed the simultaneous detection of the mixture composition and temperature from the CH and the OH Raman spectra. The composition gradients arise because of rectification effects, which cause unlike compositions in the gas and liquid phases. Heat transfer from the boiling bubble to the surrounding liquid induces temperature gradients. High temporal and spatial resolution measurements using an imaging spectrograph and an electron multiplying charge-coupled device camera revealed large composition and temperature gradients with similar shapes. The accuracy of the composition results was validated by a species conservation calculation. Copyright © 2010 John Wiley & Sons, Ltd.

Simultaneous determination of the composition and temperature gradients in the vicinity of boiling bubbles in liquid binary mixtures using one-dimensional Raman measurements

The usual solid-state laser systems with phase-conjugating elements produce Q-switch pulses of some 10 ns pulse width, which for many applications is too short. In the following study an additive stimulated Brillouin scattering (SBS) cell is introduced in the cavity to compensate for this Q-switch behavior by nonlinearly increasing intracavity losses resulting from the direct output coupling of the Stokes wave generated in this additive cell. For direct output coupling this study takes advantage of polarizing optical elements in the laser cavity. With this arrangement an averaged pulse duration of approximately 300 ns is realized compared with the 60-ns pulse duration that is realized when working without an additional SBS cell. The experimental results are compared with numerical calculations based on modified rate equations for the active medium and a two-wave-mixing formalism for the nonlinear interaction with the SBS media.

Pulse lengthening by intracavity stimulated Brillouin scattering in a Q-switched, phase-conjugated Nd:YAG laser oscillator.

In testing a holgraphic particle track recording system for the Fermilab 15-ft bubble chamber, it was shown that the peak power of Q-switched laser pulses (~50-ns duration) at the required energy gave rise to boiling during the chamber expansion A pulse stretching technique is described which was developed to reduce the peak power Applied to a ruby laser (oscillator and three amplifiers) with a maximum Q-switched output of 30 J, pulses of up to 100-μs duration with coherence up to and exceeding 11 m at 25 μs were produced These pulses were amplified to ~5 J without shape degradation The considerably increased coherence length will find applications in many fields of pulsed holography, and its use with fiber optics is particularity promising

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R. Michaels

Papers

Pulse stretching in a Q-switched ruby laser for bubble chamber holography.