Mach-Zehnder Interferometer

The Mach-Zehnder interferometer is a popular instrument used in experimental studies of heat transfer in fluids. The heated test cell is located in the path of the test beam 2 shown in the Figure 4.6. Quantitative experiments are possible when the temperature field is two dimensional in the cross-sectional plane and uniform parallel to the beam direction. Otherwise the test beam averages the temperature field as it traverses the test cell and only qualitative information can be obtained. The reference beam 1 passes through a region identical to the test cell except that the fluid here is at a uniform temperature. The spatial filter expands the laser beam which is subsequently made parallel using a convex lens. The spatial filter along with the lens constitutes the collimating arrangement of the interferometer.

The initial (geometric) path lengths of beams 1 and 2 are nearly equal except for possible angular misalignment of the mirrors and the beam splitters. As these are rotated the interferometer approaches a state of complete alignment. The initial fringe pattern corresponds to wedge fringes whose spacing increases as the alignment of the interferometer is improved (Figure 4.7). The alignment referred here is the parallelism of the optical components - two mirrors and two beam splitters. The best initial setting corresponds to the optical components being strictly parallel when the image is a uniformly bright field. In practice, two fringes will span the full field-or-view. This position of the interferometer is called the infinite-fringe setting. Every point in the reference beam and the test beam have the same path length when measured from the pinhole of the spatial filter (the virtual origin of the coordinate system). A thermal disturbance now placed in the path of the test beam will produce a fringe pattern where each fringe is an isotherm.

Figure 4.7: Fringes due to Misalignment of the Optics in a M-Z Interferometer.