Laser Interferometry

The primary instrument used in the present work for temperature measurement in the fluid medium is the Mach-Zender interferometer. Figure 4.9 is a schematic drawing of the interferometer. The optical components present in it, namely the beam splitters BS1 and BS2 and mirrors M1 and M2, are oriented at an angle of 45 degrees with respect to the laser beam direction. The first beam splitters BS1 splits the incoming collimated beam into two equal parts- the transmitted and the reflected beams. The transmitted beam 2 is labeled as the test beam and the reflected beam 1 as the reference beam. The test beam passes through the physical region where the convection process is in progress. It is reflected by the mirror and recombines with the reference beam on the plane of the second beam splitter (BS2). The reference beam undergoes a reflection at mirror and passes through the reference medium unaltered and is superimposed with the test beam at, BS2. The two beams on superposition at the second beam splitter BS2, produce an interference pattern. This pattern contains the information of the variation of refractive index in the test region. For measurements in air, the reference medium is simply the ambient. For liquids, a compensation chamber is required to introduce an appropriate reference environment. The mirrors and beam splitters employed in the present configuration are of 150 mm diameter. The beam splitter has reflectivity and transimitivity. The mirrors are coated with pure silver and employ a silicon dioxide layer as a protective layer against oxidation.

Figure 4.9: Schematic drawing of the Mach-Zender interferometer.

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