Fig. 1. Experimental setup.
(TOP VIEW) The two polarization components of a linearly polarized HeNe laser beam are split by a Wallaston prism into two angles to yield two beams which are collimated by a lens and turned by a mirror down onto a liquid phantom. The reflected light from the phantom surface returns the two beams back through the Wallaston prism, off the beamsplitter, through a polarizer at 45 degrees to equally sample both components of the two reflected polarized beams, through an interference filter to reject any Nd:YAG laser radiation (532 nm), and focused by a lens through a pinhole to reach a photodiode detector. The two beams interfere at the photodiode yielding a signal which is sensitive to the differential total pathlength of photons after reflection off the phantom surface.
(SIDE VIEW) The two beams are reflected by the phantom surface.
(END VIEW) Laser irradiation was delivered via a mirror down through the clear water. The adjustable mirror varied the site of laser irradiation of the absorbing gel.