High-Speed Holocinematography Of Cavitation Bubbles

Abstract

AbstractTwo methods of high speed holocinematography are described using spatial or_spatial fre-quency multiplexing for image separation. The experimental devices operate in the range oflo 000 to 20 00o holograms per second with a total number of holographic frames of four toeight. Spatial multiplexing is achieved by a rotating disk with apertures placed directly infront of the holographic plate. Spatial frequency multiplexing is obtained with the aid ofa special acousto -optic beam splitter and deflector unit. Objects investigated by both meth-ods are bubbles moving in acoustically generated cavitation bubble fields as well as bubblesformed after laser- induced breakdown in water.IntroductionFast moving objects like bullets or cavitation bubbles are usually studied by means ofhigh speed cinematography(1,2). Several devices like rotating drum, rotating mirror or imageconverter cameras are commercially available for this purpose. An essential drawback of allphotographic methods lies in their limited depth of focus. This becomes quite obvious whenmany tiny objects distributed in a large volume are to be recorded.A method to overcome the problems connected with the limited depth of field is hologra-phy. Whereas holography with continuous wave lasers is a familiar technique used in manylaboratories, pulsed -laser holography and especially multiple pulsed -laser holography as re-quired for high speed holocinematography is still a field of research. Until now a real ho-lographic equivalent to the rotating drum or rotating mirror camera has not yet been deve-loped because of the great technical difficulties involved.The present paper, as others in this field(3,4), is a step towards this final aim. Twoholographic recording techniques and experimental devices are described capable to take se-quences of four to eight holograms with a hologram framing rate of lo to 2o kHz. The deviceshave been developed in connection with problems in cavitation physics where the knowledgeof the dynamics of bubble fields in three dimensioiis is of considerable interest.The performance of both experimental arrangements is demonstrated by reconstructions ofhologram sequences taken of laser -generated and acoustically generated bubbles.General ConsiderationsIn holocinematography several holograms are taken successively in short time intervals.To expose the holographic plate a series of intense coherent light pulses is required, usu-ally delivered by a multiply Q- switched solid state laser. If high framing rates of say 10to 20 kHz are needed it is almost impossible to change or even move the holographic platebetween successive exposures because the interference pattern would be blurred. Thus theinformation from all single exposures has to be stored on the same plate. There are two dif-ferent methods of multiplexing which provide separation of reconstructed images. Each singlehologram can either be recorded on different portions of the holographic plate (spatial mul-tiplexing) or on the whole plate but with different directions of the reference beam (spa-tial frequency multiplexing). The speed with which the area of exposure (one holographicframe) can be changed on the plate or the reference beam can be switched into a new direc-tion limits the framing rate of the actual device.Multiplexing by spatial division is very simple. A common holographic set -up can be used,where object and reference beam cover the whole holographic plate. Different areas of theplate are exposed by placing a rotating disk with apertures directly in front of it. Whenthe speed of revolution of the disk is properly chosen each light pulse of the series illu-minates a separate portion of the holographic plate. If a sequence of n holograms is to betaken, the fraction 1/n of the whole area of the plate is available for each pulse. In this