Fourier Analysis Applied on MZI Transmission Spectrum for Refractive Index Measurement

An approach for surrounding refractive index (SRI) measurement is proposed and demonstrated using all fiber Mach-Zehnder interferometer (MZI) formed by fusion splicing a short section of thin-core fiber between two sections of single-mode fibers with lateral offsets. The transmission spectrum of the MZI is dominantly formed by the superposition of two cladding modes separately interference with the core mode, which correspond to two frequency components in its fast Fourier transform (FFT) spectrum, respectively. In this letter, the two frequency components are extracted to take the inverse FFT, respectively, thus we can obtain two dual-mode interference (DMI) patterns between the cladding modes and the core mode. As the SRI changes, the shifts of the two DMI patterns with SRI will be observed. Through this way, we can eliminate the influence of multiple interferences and improve the accuracy of the sensor. The approach can also be utilized with other modal interferometric sensors.