The calibration of the path-length imbalance in optical fibre ESPI systems employing source-wavelength modulation

A technique is presented that allows simple and potentially automated determination of the optical path-length imbalance in ESPI systems that utilize source-wavelength modulation to obtain phase shifting. The method is based on modulating the emission wavelength of the laser synchronously with the framing rate of the CCD/image processing board and recording the average speckle intensity of the image. The injection current is increased until a , or multiple of , change in the phase has been achieved. Repeating this measurement for two different optical path-length imbalances of known difference allows determination of the laser diode coefficient and the optical path-length imbalance without the use of additional spectrometers.