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.
The reference wave phase was modulated with a sinusoidal vibrating mirror attached to a Piezoelectric Transducer (PZT),
the integration was performed by a CCD, and the charge storage period of the CCD image sensor was one-quarter period
of the sinusoidal phase modulation. With the frequency- synchronous detection technique, four images (four frames of
interference pattern) were recorded during one period of the phase modulation. In order to obtain the optimum
modulation parameter, the values of amplitude and phase of the sinusoidal phase modulation were determined by
considering the measurement error caused by the additive noise contained in the detected values. The PZT oscillation
was controlled by a closed loop control system based on PID controller. An ideal discrete digital sine function at 50Hz
with adjustable amplitude was used to adjust the vibrating of PZT, and a digital phase shift techniques was used to adjust
vibrating phase of PZT so that the phase of the modulation could reach their optimum values. The CCD detector was
triggered with software at 200Hz. Based on work above a small coherent signal masked by the preponderant incoherent
background with a CCD detector was obtained.