Thick metal coating long-period fiber grating

This paper reports a thick nickel coating for CO2 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.