Preliminary Development of a High-Speed 3-D Laser Induced Fluorescence Technique

Abstract

A 3-D density measurement technique is being developed based on the laser induced fluorescence (LIF) of acetone vapor seeded into the flow field. High speed 3-D imaging of the acetone fluorescence can be achieved using a MHz rate pulse burst laser system, a galvanometric scanning mirror and a high-speed camera, which allow for acquisition of 3-D data in tens of microseconds. This system has been demonstrated for flow visualization and is currently being adapted for 3-D LIF of acetone. One requirement for this technique is significant enhancement of the energy output of the pulse burst laser and fourth harmonic conversion to 266 nm. The modification of the pulse burst laser system through the addition of two more amplifier stages and a fourth harmonic generator is described. Preliminary results show the capability to produce individual pulses 15 nsec in duration with energy in excess of 100 mJ/pulse at 1064 nm. The measurements were taken at reduced power setting with an overall system gain of 2 x 10 8 ; higher pulse energies are anticipated in the future. Second harmonic output (532 nm) was achieved using a Type II KTP crystal with conversion efficiency near 50%. Fourth harmonic output (266 nm) was achieved by frequency doubling the 532 nm light with either a Type I KDP (10% efficiency) or Type I BBO (<1 % efficiency) crystal. Significantly higher conversion efficiencies are expected in the future with better alignment and conditioning of the laser beam prior to entering the crystal, particularly in the case of BBO, which has a small angular bandwidth, but relatively large non-linear coefficient.