Bessel-Beam Interferometry for Turbidity With Refractive Index, Attenuation Coefficient, and Loss

This letter demonstrates a quasi-Bessel beam optical interferometry and fast Fourier transform (FFT) to estimate turbidity of liquid samples in terms of the refractive index (RI), optical attenuation coefficient, and loss. The device comprises a broadband superluminescent diode (SLD) source and a packaged deep-seated optical fiber negative axicon (DSNA) probe connected to the SLD source guiding light to the liquid sample trapped in between DSNA probe head and a glass slide. It generates a low-coherence interference spectrum as recorded in a spectrometer via an optical circulator. The FFT of the said spectrum gives power spectrum corresponding to the respective interfaces of the trapped sample. The power spectrum is used wisely to estimate turbidity in terms of RI, optical attenuation, and optical loss. The design is able to measure as low as 5 NTU (nephelometric turbidity units). This sensing platform may be useful in environmental protection, pharma, and processing industries.