A relatively simple design of a terahertz (THz) polarization splitter based on an asymmetric dual-suspended-core fiber is proposed. One core is formed by two intersecting rectangular dielectric strips with dissimilar thickness, whereas the other is a round solid core suspended by crossed dielectric strips with the same thickness. The distance between the cores can be adjusted to ensure a short splitting length and low transmission loss. A THz polarization splitter with a length of 1.27 cm is realized with a low transmission loss of 0.53 and 0.67 dB for the x- and y-polarization modes, respectively. An extinction ratio of about −20 dB and a broad bandwidth of 0.046 THz are demonstrated.
Metal coating can provide fiber Bragg grating (FBG) effective protection and improve the FBG sensor’s performance in harsh environments. However, the metalizing can impact the optical properties of the FBG. As a result, the sensor accuracy of FBG will lower. The reason for the effect is the stress produced in the metalizing. In this study, we coated the FBG by the electroless-electroplating method. The common changes of the FBG’s spectrum after the metalizing were apparent side-lobes and reduced peak loss, and the hysteresis error for temperature sensor was more than 3.5°C. To reduce the deformation of the FBG’s spectrum in the metalizing, we analyzed the origin of stress, calculated the FBG’s refractive index perturbation due to the thermal stress, and simulated the spectrums of FBG under different stress distributions. On the basis of this, we developed a stress control technology. After using the technology, the optical properties of the metal-coated fiber Bragg gratings have been retained, and their hysteresis errors for temperature sensors were less than 1.5°C. The results showed good repeatability. The study can be used for regulating metalizing processes, and for providing valuable information for metalizing other optical fiber devices.
Proc. SPIE. 8418, 6th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Design, Manufacturing, and Testing of Smart Structures, Micro- and Nano-Optical Devices, and Systems
This paper reports a thick nickel coating for CO<sub>2</sub> laser-induced long- period fiber grating (LPFG) by an electrolesselectroplating method. The thickness of the metal coating is more than 150 micrometer. As well as affording effective protection, the thick metal coating can give the LPFG enough stiffness to overcome its cross-sensitivity between bend and other measurements. In our metallization, electroless Ni-P was deposited on a bare LPFG at 86°C. We observed degradation with broadened spectrum and lessened peak value after the LPFG was electroless plated and was cooled down to room temperature. The degradation may be caused by the new metal coating instead of air and stress. Degradation was also observed in the later electroplating nickel which was induced by the stress. The mechanisms of the stress, such as thermal stress, film growing, hydrogen, and excess energy, were studied. To reduce the degradation, we took optimal plating, such as reducing the cooling speed after electroless plating, higher and stable electroplating temperature, mixing timely and proper electrodes distribution. Under the optimal condition, we got a metallized LPFG whose 3-dB bandwidth was 3.942nm, peak loss was -15.389dB, resonant wavelength was 1547.354nm, and external diameter was 0.425mm. Following temperature sensor experiments showed the metal coated LPFG presented high temperature sensitivity from 10°C to 80°C. Its temperature sensitivity was 44.9 pm/°C, and R-square was 0.9977.