Laser Source

A 35 mW, continuous wave He-Ne laser is employed as the coherent light source for the interferometer. This laser is sturdy in construction, economical, and stable in operation. The original laser beam is of 2 mm diameter. A spatial filter along with a convex lens or a concave mirror is required to expand the beam to any convenient size. In the present study, the expanded beam diameter is 70 mm. The spatial filter is a lens-pinhole arrangement with two adjustable screws. The distance between the pin hole and have to be adjusted so that the small laser beam is focused on the pin-hole and the outgoing beam is expanded. The specifications of the laser used in the present work are given in Table 1.

CCD Camera

A CCD (charged couple device) camera (Pulnix, model: T5 565) of spatial resolution of 512 X 512 pixels has been used to capture the interferometric images. The fringes formed at the second beam splitter of the Mach-Zehnder interferometer are projected on to a screen. The selection of the screen material and collection of interferometric images from it is an important step. This is because the clarity in the images will reduce the uncertainty in the subsequent calculations. For this purpose,tracing sheet has been used to display the interferometric images in the present work. The attenuation of the laser beam through the thickness of the paper of the paper was found to be small. It was observed that two screens used together with one of them physically perturbed led to distinctly clear images on the video monitor. This effect can be understood as a spatial averaging of the images over the two screens which suppresses noise by superposition and makes the fringe pattern clear.

The CCD camera is connected to a PC-based image processing system through an 8 bit A/D card (Pip 1024 Matrox). The fringe pattern is stored in an integer matrix from with intensities varying between 0 to 255 (the gray scale), where 0 indicates black and 255 indicates white. With the present setup the image acquisition speed is at video rates , namely 50 frames per second. The digital output of the CCD camera is projected to a high-resolution video monitor to visualize, and focus the fringe patterns.