A switching multifunctional metasurface with wideband absorption, cross-polarization conversion, and wideband line-to-circle in the terahertz frequency band is designed based on the phase transition characteristic of vanadium dioxide (VO2). The simulation results show that when the VO2 is in the metallic state, the structure has the function of broadband absorption; the absorption rate reaches more than 90% in the range of 3.32 to 8.04 THz, and it has the characteristics of wide-angle incidence and polarization insensitivity. When the VO2 is in the insulated state, the designed metasurface can realize cross-polarization conversion and broadband line to circular polarization conversion. The linear polarization of 1.85 to 2.76 THz is transformed into cross-polarization, the polarization conversion efficiency is >90 % , and the ellipticity is >90 % for 1.61 to 1.75 THz and 3.06 to 8.37 THz ranges, which can convert line polarization to circular polarization and maintain good efficiency in the polarization conversion function within acceptable incidence and polarization angles. The proposed optical metasurface has great potential in terahertz fields, such as stealth technology, polarization conversion, and radar communication.
This paper demonstrates a portable optical sensor for human gait monitoring. The device is based on a smartphone and POF sensor specifically designed for use in shoe insoles. The monitoring of multiple sensors by a single smart device is achieved by image segmentation based on Voronoi tessellation, as this work describes in detail. Experimental tests performed with the sensor have demonstrated its ability to provide information on spatial and temporal parameters of gait as well as pressure at different plantar loci.
An ultrawideband metamaterial perfect absorber based on vanadium dioxide is proposed. It achieves >95 % absorption of vertically incident electromagnetic waves in the range of 3.50 to 10 THz. The absorption intensity can be dynamically adjusted in the range of 0.2% to 99.98% by varying the conductivity of VO2. The mechanism of ultrawideband perfect absorption is interpreted using electric field distribution analysis and impedance-matching theory. The absorption rate related to the structural parameters of the absorber is investigated by numerical simulation. Finally, its polarization angle-insensitive and incidence angle-insensitive properties are demonstrated. This proposed absorber has potential applications in optical switching, electromagnetic stealth, and sensing applications.
In our work, the temperature sensing properties and intrinsic mechanism based on a bismuth–erbium co-doped optical fiber (BEDF) were explored. Through temperature sensing experiments, we found that when the 980-nm pump laser was used, the fluorescence intensity ratio (FIR) at 1560 and 1435 nm showed a good linear relationship at different temperatures and its sensitivity reached 0.1151 dB/°C, the accuracy was 0.2°C, and the R2 of the FIR curve is about 0.9923. At the same time, we also proposed a detection algorithm to judge the working state of the sensor. By changing the BEDF coating material while the fiber is being fabricated, the temperature measurement range can be further improved. The optical fiber temperature sensor will have a broader range of applications.
We propose a fiber optical cell catapult that is bird beak-shaped fiber cone optical tweezers that trap cells, then push them to the fiber tip via the evanescent fields on the side surface of the fiber cone, and finally eject them in a particular direction. The intensity distribution of the light field and the optical force of the fiber catapult are calculated by the finite element method. Moreover, an experimental study of the fiber catapult is given using yeast cells.
A macrobending plastic optical fiber (POF) with a multi-notched structure was presented for displacement sensing. The multi-notched structure of the POF was fabricated by an imprinted process. The effects of the structural parameters on the displacement sensing performance were studied experimentally. By changing the multi-notched structure parameters, the displacement sensing performance of the POF probe could be optimized. The repeatability and the hysteresis of the sensor probe were also tested. Experimental results showed that the repeatability and hysteresis of the sensor were good, and a sensitivity of 3.58%/mm was obtained. The sensor is a low cost solution for displacement measurement, which is easy fabrication, simple structure, and intensity modulation at visible wavelength.
A core-offset hetero structured optical fiber was fabricated and proposed for surface plasmon resonance (SPR) sensor. The hetero structure was constituted by a multimode fiber (MMF), core-offset single mode fiber (SMF), and MMF, which are spliced together by a fusion splicer. The effect of the size of core-offset on the RI sensing performance was studied experimentally. The results show that compared with the traditional MMF-SMF-MMF structure, the sensitivity of the proposed structure can be improved, and the highest sensitivity can reach to 6760.17 nm/RIU when the core-offset is 9 μm. The sensor is easy to fabricate, simple structure.
Linear-to-circular polarization converters are widely used in optical and microwave systems, but the polarization devices of traditional materials are untunable, and devices made of graphene materials can overcome this disadvantage. A circular polarization converter based on graphene metasurface is designed, whose properties are tunable over a broad range at terahertz frequencies. With appropriate structural parameters, simulations show that the axial ratio of reflected electromagnetic wave of the proposed device is lower than 3 dB in the frequency band of 2.25 to 2.475 THz, which means the linearly incident polarization can be converted to the circular polarization wave. The proposed design can also work when the electromagnetic wave is oblique incidence up to 40 deg with a high polarization conversion ratio. Moreover, the operating frequency band can be arbitrarily adjusted by applying a bias voltage.
A low cost and easily fabricated plastic optical fiber (POF) displacement sensor is presented. The sensor is based on the macrobending POF with a V-groove structure fabricated by a simple die-press-print method, which is easy to implement and effectively reduces the complexity of the fabrication process. The intensity modulation method is adopted for displacement sensing, which lowers the sensor system’s cost and complexity. Experiments are carried out to investigate the influence of the structural parameters on the displacement sensing performance and the proposed POF probe is optimized by changing the structure parameters. Results showed that when the V-groove structure depth is 200 μm, the length is 22 mm, the angle is 60 deg, the pitch is 2 mm, and the macrobending radius of the POF probe is 15 mm, the highest sensitivity could reach to 3.19 × 10 − 2 / mm with the measurement range of 18 mm.
The plastic optical fiber (POF) with a multi-notched structure was used for liquid level measurement. The multi-notched structure was fabricated on the POFs by a die-press-print method. When the notched structure was immerged by the liquid, the transmitted light power of the POF probe could be changed. So, this can be used as a liquid level sensor. The influence of the structure parameters on the sensor performances was investigated experimentally. Experimental results show that the sensitivity can reach to 0.0457/mm with a resolution of 1 mm, and the sensor resolution is flexible. The sensor is simple structure and easy fabrication, and it is a low cost solution for the liquid level measurement.
A multi-mode plastic optical fiber (POF) with a long period grating (LPG) was proposed for a refractive index (RI) sensing probe. The LPG was fabricated on the surface of the POF by a simple die-press-print method using a commercial available thread rod as the mould. The RI sensing performances for straight and U-shaped POFs with LPGs were studied. It is found that the straight RI sensing probe with LPG structure was not sensitive enough for RI measurement. After bending the straight POF probes with LPGs into U-shaped probes, the RI sensing performance was improved markedly. By altering the structural parameters, the RI sensing performances of the U-shaped POF probes with LPGs were optimized, a sensitivity of 1130%/RIU with a resolution of 8.44×10-4 in the RI range of 1.33-1.41 was obtained. The probe is a low cost solution for RI sensing purpose, which has the features of simple structure, easy fabrication, compact size and intensity modulation at visible wavelengths.
Copper-doped planar glass waveguides were prepared by thermal ion exchange technique in molten mixture of copper sulfate and sodium sulfate (molar ratio of 50:50%) at temperatures of 565-605°C and different diffusion times of 15-30 minutes. The effective indices were measured by means of prism-coupled technique, and the refractive index profiles of the waveguides were reconstructed by IWKB method. The photoluminescence (PL) of the waveguides were studied at room temperature. The broad blue-green emission bands centered 510 nm (at the excitation wavelength of 286 nm) were observed on the waveguides. The refractive index profiles and the blue-green luminescence of the waveguides were strongly influenced by the prepared and annealed processes.
The temperature dependence of a refractive index (RI) sensing probe based on a U-shape tapered plastic optical fiber (POF) was investigated experimentally. The changes in light propagation loss in the probe induced by temperature are of the same order of magnitude as those induced by measured RI changes. The temperature dependence loss and temperature dependence RI deviation of the sensing probe were measured (at the wavelength of 635 nm) in temperature of 10-60 °C. By extracting pure temperature dependence of the sensing probe alone, the influence of temperature to the sensor was characterized.
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