Low-Cost Highly Sensitive Refractive Index Measurement Based on Deep Frequency Modulation Interferometry

A novel low-cost technique for refractive index (RI) measurement is proposed and experimentally demonstrated based on the concept of deep frequency modulation (DFM). By omitting electro-optic modulator from conventional DFM, the frequency modulation is realized by direct current modulation of a laser diode which leads to cost reduction. The flicker noise effect on the DFM performance, which is considered as a major obstacle of homodyne detection, has been considered in our numerical analysis; hence, a realistic analysis of the results is presented. Hereby, a modification in the DFM signal processing has been suggested to mitigate the flicker noise effect. Through proper selection of higher harmonics of the wideband detected DFM signal, the normalized error can be reduced more than 10 dB. We have implemented this method for a Michelson interferometer to measure RI of an aqueous solution. An interference signal is formed on the photodetector, which is then processed to measure RI by estimating the optical path length difference between the two interferometer arms. It is shown that this setup provides a resolution of better than 2.1 × 10^-5 in RI unit. The proposed cost-effective measurement technique can be readily integrated in applications, where high precision real-time measurements over a wide measurement range are required.