Real-time contour fringes obtained with a variable synthetic wavelength from a single diode laser

In this work, we present a method to generate digital speckle contour fringes by tuning a red diode laser with a single external cavity. In the cavity, light is reflected by a diffraction grating and re-injected into the emitter. A proper alignment of the cavity provides dual emission of the laser, thus generating a synthetic wavelength λS. The resulting image of the studied object appears covered with the usual high spatial frequency speckle pattern modulated by a low-frequency interferogram of contour interval λS/2 which describes the object surface shape. Changes in the separation between the two laser emissions correspond to an extended range of synthetic wavelengths ranging from tens of micrometers to some millimeters. In the experimental section we demonstrate the potential of this technique by changing the contour interval of the interferogram according to the object’s shape and to the desired measurement precision. An analytical expression relating the interferogram intensity with the Fourier transform of the laser output intensity was obtained, and possible applications of this result for wavefront shaping are discussed.