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.