In this paper, a sensor for simultaneous measurement of seawater temperature and depth using polarization-maintaining fiber Bragg grating (PM-FBG) and extrinsic Fabry-Perot interferometer (EFPI) is presented. The PM-FBG and the EFPI are cascaded into the sensor. Epoxy resin is used to increase the temperature sensitivity of PM-FBG while an aluminum sleeve is used to reduce the pressure sensitivity of PM-FBG. And the EFPI is formed by the pigtail of the PM-FBG and the stainless steel diaphragm at the end of the aluminum sleeve. The new sensor design, fabrication, packaging, and measurement are addressed. The performance of the sensor for the concurrent measurement of pressure and temperature is investigated by analyzing the experimental data, and the characteristic curves for the measurement of the sensor are presented. Experimental results show that the temperature sensitivity is about 25.18 pm/°C (left peak) and 24.69 pm/°C (right peak) and the pressure sensitivity is about 35.78 um/MPa.
For a semi-conductive thin film and substrate system, it is difficult to obtain the geometric thickness and optical constants directly from reflectance value, if the materials of interest have finite thickness, absorption and dispersion. To simplify this, the model of Forouhi-Bloomer (F-B) is adopted to express reflectance merely with wavelength and several parameters which have definite values. Then the genetic algorithm (GA) is introduced to optimize and fit these parameters on data of reflection index in detected wave ranges. By comparing with a traditional optimization method the fitting error of GA is smaller. Finally, a sample with a thin film of amorphous Si coated on a crystal Si is analyzed. Thus, a novel method is presented to characterize the optical property of the film coated on a semi-conductive substrate, merely basing on the reflection index data.
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