The core element of this experiment is the high-repetition rate diode-pumped Nd:YAG laser (Corona, Coherent Inc). The laser is continuously pumped but is acousto-optically Q-switched to produce 130 ns (FWHM) pulses at 10 kHz. The light is intra-cavity frequency doubled and produces 75 W of average power at 532 nm. This corresponds to about 7.5 mJ per pulse at 10 kHz.
The laser beam is focused into the test section by using a 300 mm focal length lens. The beam diameter was measured to be about 0.3 mm. An external photodiode (DET210, Thorlabs) is used to correct for variations in the laser pulse energy on a shot-by-shot basis.
Rayleigh scattered light is collected using custom-designed optics that consist of a pair of 150 mm diameter plano-convex lenses, one 50.8 mm diameter meniscus lens and one 50.8 mm diameter double-convex lens. The lens system was designed with ZEMAX and produced an aberration-limited blur-spot of less than 34 mm. The working f# was 2.4 and the magnification was 0.685.
A broadband hybrid cube beam splitter reflectes and transmits the split signal onto two different PMTs (R636-10, HAMAMATSU). An 8-channel high voltage power supply (8-channle high voltage power supply, EMCO) controlled by an analog D/A card (PCI-6703, National Instrument) is used to provide high negative voltages for the PMTs. Two 200 mm slits were placed in front of the PMTs to define the spatial resolution (i.e., length of the beam imaged). The slit width in the image plane corresponded to 300 mm in the object plane. These two slits were arranged such that the separation of probe volumes in the flow was 300 mm. The PMT and photodiode outputs were read by gated integrators (SR250, Stanford Research Systems) operated with gate widths of 300 ns. The integrated signals were synchronously sampled by a 12-bit A/D converter (AT-MIO-16E-1, National Instrument) at 10 kHz. The whole experimental system is synchronized by two digital pulse/delay generators (DG535, Stanford Research Systems).