A Hyperspectral Microscope Based on a Birefringent Ultrastable Common-Path Interferometer

Spectral microscopy is a method to acquire the spectrum for each point in the image of a sample. The most straightforward technique uses spectral filters to collect a sequence of images at a discrete number of spectral bands. A more complete spectral characterization is hyperspectral microscopy, which acquires the whole continuous spectrum of each point of the image. A powerful approach to this aim is Fourier-transform (FT) spectrometry [1, 2], in which an optical waveform is split by an interferometer in two delayed replicas, whose interference pattern is measured by a detector as a function of their delay. The FT of the resulting interferogram yields the continuous intensity spectrum of the waveform. The FT approach is able to retrieve in parallel the spectra for all pixels in a scene and is hence suited for wide-field microscopy, but it requires controlling the delay with sub-cycle precision, which is very difficult to achieve with Michelson and Mach-Zehnder interferometers. Here we introduce a hyperspectral microscope based on the FT approach and using a compact, highly stable common-path birefringent interferometer, a version of the Translating-Wedge-based Identical pulses eNcoding System (TWINS) [3, 4]. Figure 1(a) shows the schematic setup of the microscope. Light is collected by an infinity-corrected objective, it propagates in the interferometer and it is imaged on the 2D detector (14-bits, silicon monochrome CMOS camera) by a tube lens. The component P1 polarizes the input light at 45°. A and B are aBBO-birefringent blocks with crossed optical axes; block A is shaped in the form of two wedges, so that its total thickness can be changed by translating one of the wedges with a motorized stage. During propagation, the ordinary and extraordinary light projections accumulate a relative delay ranging from positive to negative values according to the relative thickness of A and B. P2 projects the replicas to the same polarization (45°), enabling interference. The spectral resolution of the interferometer is inversely proportional to the adjustable total phase delay. The largest position scan of our interferometer setup introduces a delay of ±250 fs at λ = 600 nm and corresponds to spectral resolution of 3 THz (~4 nm).