Tunable laser-diode digital holographic interferometry without measuring wavelength differences

Multiple-wavelength holographic interferometry is an effective technique to generate a contour map of a diffusely reflecting object surface in the speckle field. A synthetic wavelength can be well generated by changing the current of laser diodes. A small synthetic wavelength needs a wide tunable external-cavity laser diode. A step object profile with discontinuous structure can be measured by using a synthetic wavelength to be sufficiently large that is derived from a known step height measured by vernier calipers but not wavelength measurements. The tunable laser-diode holographic interferometry is verified by experimental results.