In this paper, a reflective surface plasmon resonance (SPR) sensor based on multimode fiber (MMF)-hollow core fiber (HCF) structure is proposed for simultaneous measurement of temperature and humidity. Along the HCF, two different films, i.e., polyvinyl alcohol (PVA) film and polydimethylsiloxane (PDMS) film are coated on the outer surface to generate two SPR resonances at different wavelengths and achieve the simultaneous measurement function. In order to get a better sensing performance, the length ratio of PVA film and PDMS film, and the PVA coating times are optimized. The experimental results show that the humidity sensitivity of the sensor is 0.6351 nm/% RH in the relative humidity (RH) range of 48-84% RH, and the temperature sensitivity of the sensor is 1.791 nm/°C in the temperature measurement range of 25-80 °C. The sensor has great potential applications in monitoring the environment temperature and humidity changes.

Due to the problems of low absorptivity and the narrow bandwidth of existing broadband absorbers, a perfect broadband tunable absorber with high absorptivity based on double-layer graphene is proposed in this study with 99% high absorptivity as the reference standard, and its theoretical analysis is performed. The designed absorber consists of complete graphene, upper dielectric material, patterned graphene, lower dielectric material and the gold substrate. Through numerical calculation, the designed absorbing structure can achieve high absorption rate of more than 99% in the range of 2.54–3.70 THz, and has a continuous bandwidth of 1.16 THz. In addition, the absorption range is 52%–100% by changing the Fermi level of the graphene layer. The perfect absorber is symmetrical in rotation center and insensitive to incident angle and polarization. It can be widely used in terahertz switch, modulator design, power acquisition and other related fields.

In this article, a surface plasmon resonance (SPR) sensor based on a stepped side-polished plastic optical fiber (POF) is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. A differentiated Au-film thickness was deposited on the stepped polished region, which can enlarge the wavelength space for the SPR peaks of RI and temperature sensing channels and reduce their crosstalk. The polydimethylsiloxane (PDMS) film was fabricated on the polished region with a deeper polished depth and a thicker Au-film thickness for temperature sensing. The stepped polished structure can obtain a better resonance peak visibility for the temperature sensing. The experimental results show that the RI sensitivity of the sensor is 1759 nm/RIU in the RI range of 1.335–1.37 and the temperature sensitivity is −0.82 nm/°C in the temperature range of 20 °C–80 °C. The probe has the advantages of easy fabrication and simple structure, which has potential application prospect in the field of biochemical sensing.

As a fully distributed sensor, the phase-sensitive optical time domain reflectometer (φ-OTDR) has attracted remarkable attention in real-time vibration detection. We present a dual-channel φ-OTDR (DC-φ-OTDR), formed by two single-channel φ-OTDRs (SC-φ-OTDR), to monitor running vehicles on a highway. In the double-channel system, an improved algorithm (will be referred to as the CDM&V) is proposed to alleviate the strong dependence of vibration detection on the differential step as in the widely used conventional differential method (CDM). The DC-φ-OTDR is first tested over campus road before applying it to locate moving vehicles on the highway. For comparison purposes, both the DC-φ-OTDR and SC-φ-OTDR are used to monitor the vehicles with respective signal processing methods of the CDM and CDM&V. The experimental results at campus show that the dual-path scheme can undoubtedly reduce vibration misjudgment relative to the single one due to the very small possibility of false measurements occurred simultaneously at the same location in both channels. In signal demodulation, the CDM&V greatly relaxes the constraints on the differencing interval for identifying the vehicle-caused vibration. With a step size of 5 or lower, the CDM fails to locate the running vehicle at z=~8.5 km, but the CDM&V successfully demonstrates the feasible capability of locating the vibration. With an increase in the differential interval, both the CDM and CDM&V are able to detect the vibration signal, but with the latter showing a much better noise suppression performance and hence a larger SNR. Importantly, in comparison with the SC-φ-OTDR system, the DC-φ-OTDR exhibits a considerable enhanced SNR for the detection signal regardless of which processing algorithm (i.e., CDM, CDM&V) is used. The vehicle locations positioned by the DC-φ-OTDR are confirmed by the monitoring cameras.

Terahertz (THz) technologies have achieved great progress in the past few decades. Developing active devices to control the THz waves is the frontier of THz applications. In this paper, a new scheme of two-bit THz encoder is proposed. Different from the present THz modulators whose spectra at different bands are varied simultaneously, our encoder can realize the individually efficient modulation of every channel. The encoder comprises the double-sided graphene-based metamaterials, in which the graphene structures on each side are connected to the external electrodes individually. The well-designed metamaterials on the front and back sides determine the resonances at two different bands (0.20 THz and 0.33 THz) separately. Through simulating the performance of this device by changing the conductivities of the graphene on each side independently, we demonstrate two-bit encoding realized by the dual-band modulation of transmission amplitude with electronic control, and the modulation depth can reach as high as 79.6%. Our encoder can promote the development of multifunctional and integrated devices, such as frequency division multiplexers and logical circuitry, which will contribute to THz communications.

In order to reduce computational redundancy, and to recognize the perturbation signals quickly and efficiently, this paper proposed a novel recognition method for Φ-OTDR system based on affinity propagation clustering. Variational mode decomposition (VMD) are utilized to accurately extract the perturbation signal characteristic information buried in the vibration signals. Moreover, in order to improve the efficiency of clustering algorithm, a novel perturbation signal selection technique was developed in this work, whose effectiveness was experimentally verified. The proposed novel recognition method is then applied to the perturbation signal detection in Φ-OTDR system. The obtained results demonstrated that the proposed method is reliable and accurately identify different perturbation categories of signals.