Phase-shifting interferometry by an open-loop voltage controlled laser diode

The paper presents a simple and straightforward procedure for determining the phase of an interferogram by exploiting the specific behavior of laser diodes—i.e., the small variation of the operating wavelength attainable by varying the forward current of the diode. The proposed procedure was developed by using a laser diode for feeding a quasi-balanced Michelson interferometer, by which the optimal operating conditions were investigated—i.e., the proper unbalancing and the proper variation of the supply voltage of the diode. The selected operating conditions were then used for evaluating the phase-steps by a simple algorithm developed on purpose for the present application. The phase measurements carried out by the proposed method have shown a very good accordance with those obtained by a conventional temporal phase-shifting procedure performed by a high-end commercial PZT actuator with quasi-nanometric accuracy.