Abstract

The HOLOMAR collaboratio¹ have designed and build an underwater holographic camera specrifically designed to record marine plankton to a maxnnum depth of 100m betow sea level. The camera records holograms usmg both the in-line and the off-axis techniques simultaneously The laser is a specially designed frequency doubled Nd-YAG laser, using a passive Q-switch and LBO doubler It produces a single pulse with an energy of 700 mJ and duration of less than 10 ns The oscillator-amplifier design results in a single longitudinal mode with a measured coherence length of over 2m The output can be split, with continuously variable intensity, into two parallel light paths The low intensity path is further split 50/50 into two paths, each of which is expanded to a diameter of 92 mm using a Galilean telescope The output from one telescope provides the in-air reference beam for the off-axis hologram, and is incident on the hologram at an angle of 60° to the normal The other beam is emitted into the water volume via a λ/10 optical flat window to form an in-line hologram The in-line path records a water column of 500 mm long by 92 mm diameter The centre-line of the in-line optics is approximately 400 mm from the front face of the housing. The major part of the laser energy is routed via a series of mirrors into three specially designed plastic light guides, each of which provide side and back illumination of the volume of water (of order 10⁵ cm³) in front of the off-axis hotogram window (useful diameter 230 mm) The optical path is compensated via a movable prism The camera contains two plate magazines each with a capacity of twenty 4”×4” plates

© 2000 IEEE