Compact and portable low-coherence interferometer with off-axis geometry for quantitative phase microscopy and nanoscopy

Abstract

We present a simple-to-align, highly-portable interferometer, which is able to capture wide-field, off-axis interference patterns from transparent samples under low-coherence illumination. This small-dimensions and low-cost device can be connected to the output of a transmission microscope illuminated by a low-coherence source and measure sub-nanometric optical thickness changes in a label-free manner. In contrast to our previously published design, the τ interferometer, the new design is able to fully operate in an off-axis holographic geometry, where the interference fringes have high spatial frequency, and the interference area is limited only by the coherence length of the source, and thus it enables to easily obtain high-quality quantitative images of static and dynamic samples. We present several applications for the new design including nondestructive optical testing of transparent microscopic elements with nanometric thickness and live-cell imaging.

Doubling the field of view in off-axis low-coherence interferometric imaging

“…One of the beams is filtered by pinhole P and reflected by mirror M, where the pinhole transfers only a very small bandwidth around the zero spatial frequency, and thus creates a reference beam by effectively erasing the sample information. 6,7 Next, the reference beam is optically Fourier transformed back to the camera plane by lens L2, where lenses L1 and L2 are positioned in a 4f lens configuration (i.e., the distance between the image plane in the output of the imaging system and L1 is equal to the lens focal length, the distance between L1 and L2 is equal to sum of their focal lengths, and the distance between lens L2 and the camera sensor is equal to the focal length of L2). The second beam that propagates from BS1 is split again by beam splitter BS2, and both sample beams are reflected by two retro-reflectors, RR1 and RR2, …”

“…[1][2][3][4][5][6][7] Although it is easier to obtain interference with a highly coherent source, using such a source in interferometric and holographic imaging in general, and in IPM in particular, significantly reduces the image quality due to parasitic interferences and coherent noise. [7][8][9][10][11][12] To overcome this problem, low-coherence light sources are employed. However, to obtain interference using these sources, meticulous alignment between the optical paths of the two beams is required.…”

“…20 This feature has been used in the past for other applications, which include solving the phase unwrapping problem, 9,21 recording ultrafast events using suitable temporal encoding, [22][23][24] and obtaining super-resolution capabilities by synthetically increasing the numerical aperture (NA) of the imaging system. [25][26][27] Although the IDIA principle can be implemented in various holographic and interferometric imaging systems, we implemented this technique by modifying the compact and portable interferometric module recently developed in our group, 6,7 which can be connected directly to the digital camera to turn it into a powerful off-axis interferometric camera with a wider FOV.…”

Label-Free Quantitative Imaging of Sperm for In-Vitro Fertilization Using Interferometric Phase MicroscopyIVF, IMSI, ICSI, IPM, Holography, Imaging, Sperm

“…These modules, such as the TAU interferometer [29], are capable of adapting existing microscopes for use as holographic microscopes.…”

“…The unit itself is a small box and contains very few optical elements, making production of this unit very cost-effective [29].…”