Programmable spectral interferometric microscopy

Abstract

A programmable optical sensor is proposed based on spectrally programmable heterodyne optical interferometric confocal microscopy implemented via an ultrastable in-line acousto-optic tunable filter (AOTF) based interferometer using double anisotropic acousto-optic Bragg diffraction. The design uses a tunable laser as the light source and anisotropic diffractions in an AOTF to generate two near-collinear orthogonal linear polarization and slightly displaced beams that both pass via the test sample to deliver highly sensitive sample birefringence or material optical retardation measurements. A spherical lens is used to form focused spots for high resolution confocal spatial sampling of the test object. Thus the instrument also forms a classic interferometric confocal microscope via the use of single mode fiber optics for the receive light. The laser and AOTF tuning allows birefringence measurements taken at different wavelengths, one at a time with minimal interwavelength crosstalk. Experimental demonstration of the instrument is achieved using a 1550nm center 100nm band tunable laser and variable birefringence liquid crystal and fixed retardance birefringent materials, all showing accurate retardation measurements to within a 0.5° rf phase accuracy. The instrument shows a fiber-in to fiber-out loss of 8.5dB. An alternate design via a transmissive beam generation design provides collinear co-located beams on the sample plane for superaccurate measurements.