Wedge Fringe Interferograms

The Mach-Zehnder interferometer is not always used with the two interfering wavefronts parallel to each other, as in the infinite-fringe setting discussed above. There is a second mode in which the two interfering wavefronts have a small angle between them, introduced deliberately during alignment. Upon interference they produce an image consisting of bright and dark fringes, representing the loci of constructive and destructive interference respectively. These parallel and equally spaced fringes are referred to as wedge fringes. The spacing between the wedge fringes is a function of the tilt angle and the wavelength of the laser light used, and is given by

For small tilt angles, the above expression becomes

As is decreased to zero, the wedge fringes get farther apart, approaching the infinite-fringe pattern.

When a thermal or concentration field is introduced in the path of the test beam, the phase of the test wavefront gets distorted depending upon the nature of the disturbance introduced. Upon interference with the reference wavefront, it manifests itself as a change from straight and parallel to curved fringes. The two interference patterns are shown schematically in Figure 4.67 below.

Figure 4.67: Wedge Fringe pattern with and without the field of disturbance