Bessel beam assisted common-path spectral domain optical coherence phase microscopy

Abstract

In this work, the Bessel beam from the optical fiber negative axicon probe is utilized for spectral domain optical coherence phase microscopy (SDOCPM) based on common-path interferometry. An optical fiber negative axicon probe generating the Bessel beam is fabricated by the chemical etching at the distal end of a high germanium doped optical fiber in hydrofluoric acid (48% HF) under the effect of capillary action. This probe produces a non-diffracting Bessel beam having a long depth of focus (DOF), ~700μm and very small spot size. This is basically a lens-less approach for SDOCPM. The wrapped phase is retrieved in the spectral domain, afterwards, the unwrapping is done. The absolute optical path length (OPL) is estimated by the slope of the unwrapped phase which is further fitted by a least-square algorithm. The retrieved unwrapped phase is assumed as a reference to correct the wrapped phase obtained by the conventional direct phase extraction method. The phase fluctuation is evaluated to be ≤ 0.6 milliradian by a stationary coverslip of ~150μm and the corresponding displacement sensitivity in the free space is ~10 pm. The axial resolution and lateral resolution of our system in the air are found to be ~6.9μm. We successfully demonstrate the potential of our system for biological application by imaging the onion skin as a sample.