Bessel beam interferometry and measuring refractive index of small volume liquid sample

Abstract

Optical interferometry is widely used for various sensing applications. In general, it uses conventional Gaussian beam to generate interference pattern. Recent literature has shown that one can use non-diffracting Bessel beam to generate interference pattern and can exploit the non-diffracting property of the beam to overcome some sample-to-source distance related issues. This article demonstrates the use of a packaged optical fiber negative axicon probe generating quality Bessel beam for common-path Bessel beam interferometry to measure the refractive index of a small volume liquid sample. The refractive index measuring device consists of a broadband Superluminescent diode (SLD) source, a circulator, and a spectrometer. The packaged probe is connected to a broadband SLD source through an optical circulator, and a drop of microliter liquid sample is placed on the packaged probe's head. The reflected light from the glass-liquid interface couples to the axicon probe and interferes with the reference beam generated at the axicon's air- glass interface. The interference spectra are recorded in a spectrometer through the circulator. The interference spectrum is further processed in MATLAB by applying fast Fourier transform (FFT) to calculate power from the respective interface and solve the Fresnel equation for refractive index measurement. The refractive index of sucrose solutions is measured for a small sample volume ~ 2 µL at different concentrations for testing the suitability of the design. We have also measured the refractive index of bovine serum albumin (BSA) solution. This sensing platform shows promising applications in biomedicine for monitoring of various bio sample concentrations. It can work on remote sample and with longer working distance.