Compact and stable real-time dual-wavelength digital holographic microscopy with a long-working distance objective.

We propose a compact dual wavelength digital holographic Microscopy (DHM) based on a long working distance objective, which enabling quantitative phase imaging of opaque samples with extended measurement range in one shot. The compactness of the configuration is achieved by constructing a miniature modified Michelson interferometer between the objective and the sample, and as a result it provides higher temporal stability than conventional dual wavelength DHM. In the setup, the propagation directions of two reference beams of different wavelengths can be independently adjusted, and thus two off axis interferograms having orthogonal fringe directions can be simultaneously captured through a monochrome CCD camera. The unambiguous vertical measurement range in optical path length is extended to 8.338 μm, the length of a synthetic wavelength, by selecting two wavelengths with a gap of 52 nm. The capability of the proposed setup is demonstrated with measurements of a standard 1.8 μm height step as well as a moving micro staircase structure.