Microwave Photonic Interrogation of a High-Speed and High-Resolution Multipoint Refractive Index Sensor

Microwave photonic interrogation of a high-speed and high-resolution multipoint refractive index (RI) sensor based on an Au-coated tilted fiber grating (Au-TFBG) array is proposed and experimentally demonstrated. Due to the surface plasmon resonance (SPR), the optical spectrum of an Au-TFBG has a dip in the spectral envelope. When an Au-TFBG is immersed in a solution and the RI of the solution changes, the location of the dip will shift, reflecting the change of the RI. For multipoint sensing, an Au-TFBG array with the array elements located at different locations is employed. However, due to the overlap of the spectra of the Au-TFBGs in the array, the dips cannot be precisely located by optical spectrum measurement. A solution is to convert the spectra to the time domain based on spectral shaping and wavelength-to-time (SS-WTT) mapping with the mapped temporal waveforms separated by different time delays. By using a digital signal processor (DSP), the sensing information can be extracted at a high speed and high resolution. The proposed approach is evaluated experimentally. Experimental results show that the sensor has high RI accuracies of 3.1×10⁻⁵, 2.5×10⁻⁵ and 2.6×10⁻⁵ refractive index unit (RIU) and a fast sensing speed of 11.75 kHz.