External frequency modulation of a laser source for non-incremental interferometric measurements

Interferometric techniques for absolute measurements require stable and precise frequency modulation of the laser source. A method is proposed herein whereby frequency modulation is induced externally to a semiconductor laser source by an integrated-optic phase modulator. When the electro-optic modulation depth is low, the optical power spectrum of the modulated signal consists of a central frequency with two sinusoidally swept sidebands; when this signal is applied to an unbalanced interferometer, it induces a phase-shift, from which measurements of the absolute unbalance are unobtainable. The feasibility of the method is shown for measurements over 3.5 m distances.