A diode laser with an emission wavelength in the infrared region (832 nm) is used as the coherent source. The laser beam is coupled into a standard single mode glass fiber which is then split into two using a 2/2 fiber coupler. The two outputs from the coupler are used as the illumination ports of the ESPI system or one as the illumination port and the other as the reference port, depending on the required optical configuration.

The two output fiber ends of the illumination system are connected to the input ends of the phase modulators, which are made by wrapping a few turns of fiber around PZT cylinders. These phase modulators used for phase shifting and compensation. A CCD camera is used for imaging. The illumination system, phase modulators and the imaging system forms the optical head. This optical head is connected to the digital image processing card which is interfaced to a pentium processor.

The problems associated with the tension coiling of fiber on the PZT cylinder, mode hopping and wavelength instability due to specular reflections from fiber ends, ambient effects such as temperature and humidity on the phase modulators are studied and solved in novel ways. Finally, the developed system is used in different configurations for various applications like displacement and slope measurement and NDT of unidirectional Fiber Reinforced Plastic (FRP) specimens.