Recently, improving the sensing performance of refractive index sensors by using of the weak far-field radiation and strong local field enhancement properties of toroidal dipole resonances has been intensively studied. Transmission/reflection spectra with significant narrow linewidth resonance has a vital effect on improving the sensing performance. However, narrower linewidth always leads to smaller modulation depth of the resonance which hinders the sensing performance to be improved for experiments. In this paper, we design an ultrathin all-dielectric asymmetric X-type metasurface array where an extremely narrow linewidth and high modulation depth of transmission resonance in the near-infrared has been demonstrated with Mie lattice resonance formed by the coupling of the toroidal dipole with Rayleigh Anomalous diffraction. With optimized structure parameters, a transmission dip with a full width at half-maximum as narrow as 0.061 nm and a modulation depth as high as 99.24% is achieved at the wavelength of 943.33 nm with a corresponding Q factor of 15464. According to the analysis of the displacement current distributions and the scattered powers in far field at the resonant and non-resonant wavelengths, it is confirmed that the narrow linewidth resonance is originated from the coupling of the toroidal dipole with Rayleigh Anomalous diffraction. A sensitivity and a figure of merit of 321 nm/RIU and 5262 RIU-1 are numerically demonstrated respectively for a refractive index sensor based on the all-dielectric asymmetric X-type metasurface array.
Under realistic scenarios, more fiber Bragg gratings (FBGs) are always expected to be multiplexed in one sensor array to share the expensive optical components and electrical devices. However, either the sensing number or the interrogation frequency is limited in previous works due to the huge amount of data generated from large-scale sensing arrays. This paper presents a field-programmable gate array (FPGA)-based dynamic wavelength interrogation system for thousands of identical FBGs. With the advantages of parallel controlling and pipeline processing, FPGA can accelerate the data-processing rate of the wavelength interrogation, realizing a continuous-running and real-time sensing system. The signal-processing system precisely synchronizes the generation of interrogation pulses, the acquisition of reflected signals, and the processing of the wavelength-related data, making the interrogation frequency fundamentally limited by the round-trip time of light pulses traveling in the fiber. Multiple sensing arrays can be independently carried out simultaneously, affecting hardly the interrogation frequency. Experimental results show that over 4000 FBGs with a 3-m spatial resolution in four channels are interrogated with a 150-Hz sensing frequency, 3-nm dynamic range, and ±5.9-pm sensing precision, greatly improving the interrogation frequency while ensuring the multiplexing number.
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