High-Speed and High-Resolution Microwave Photonic Interrogation of a Fiber-Optic Refractometer With Plasmonic Spectral Comb

Abstract

Microwave photonic interrogation of a high-speed and high-resolution refractive index (RI) sensor based on a tilted fiber Bragg grating with surface plasmon resonance (TFBG-SPR) is proposed and experimentally demonstrated. Instead of demodulating the wavelength shift or intensity change of a TFBG-SPR spectrum in the optical domain, we convert the TFBG-SPR spectrum to the time domain based on spectral shaping (SS) and wavelength-to-time (WTT) mapping and use a digital signal processor (DSP) to extract the RI information at a high speed and high resolution. In the experiment, when an Au-coated TFBG is immersed in a solution, the TFBG-SPR spectrum will produce a dip. When the RI changes, the location of the dip in the TFBG-SPR spectrum will shift, which is a function of the RI. By passing a broadband frequency-chirped optical pulse generated by a frequency swept laser source to the TFBG-SPR and detecting the optical pulse at the output of the TFBG-SPR at a photodetector (PD), due to SS-WTT mapping, a temporal waveform with its shape identical to the optical spectrum is produced. A DSP is then used to extract the SPR envelope information from the temporal waveform. By monitoring the changes of the SPR envelope, the RI information is interrogated with a high resolution of 1.123 × 10−6 RIU at a high speed over 20 kHz.