A bi-functional coated tapered Long Period Fiber Grating LPFG as a gas and a temperature sensor is proposed. Obtains a sensor with a high sensitivity is still a matter of challenge. In this work, a taper LPFG is used for sensing purposes. Besides, in order to increase the sensor performance, numbers of Cr and Al layers are coated on the taper region. In this work, different concentrations of four gases Ar, NH3, CH4 and N2 are applied for temperature range (25℃ up to 80℃). however, the experimental results illustrate that the transmission loss is reduce which means a significant interaction between sensing medium and cladding mode is occurred successfully which leads to higher sensitivity. Nevertheless, from the main findings, the temperature sensitivities are 2.9 pm/℃, 4.9 pm/℃, 4.5 pm/℃ and 3.1 pm/℃ for 4% concentrations of Ar, NH3, CH4 and N2 respectively. In addition, the gases sensitives are 0.213 nm%-1, 0.41 nm%-1, 0.39 nm%-1, 0.201 nm%-1 with limit of detection LoD of 0.061%, 0.024%, 0.032% and 0.073% for Ar, NH3, CH4 and N2 respectively. Additionally, the sensor shows a good repeatability, recoverability and stability. In terms of stability, the sensor shows a stable value of wavelength shift under different values of gas concentration and temperatures which was estimated 1.55 µm. Besides, it shows a fast responsivity with 53 s and 60 s as response time and recovery time respectively. Interestingly, with less than 0.06 nm of standard deviation, the signal returned to its original resonance wavelength which confirms that this sensor has a good recoverability. From these results, a coated taper LPFG can be a promising candidate for medical and industrial applications.

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A low temperature sensor based on etched Long Period Fiber Grating LPFG is proposed and demonstrated. A chemically etched LPFG sensor coated with (Indium In, Aluminum Al, Silver Ag, Palladium Pd and Titanium Ti) embedded within a build low temperature setup. The sensor investigation was carried out under temperature range of 20℃ to -150℃; and the resonance wavelength shift was collected with different cooling rates of (10, 15, 20, 25℃/min) in order to investigate the effect of cooling rates on the sensor performance. However, the experimental results show that 10 mm LPFG sensor with grating period of 400 µm offer temperature sensitivity of 1.5 times, 2 times, 2.5 times, 3 times and 3.5 times higher than bare LPFG for Ti-coated LPFG, Pd-coated LPFG, Ag-coated LPFG, Al-coated LPFG and In-coated LPFG respectively. The maximum measuring error is less than ±0.5℃, which confirms the effectively using of LPFG sensor in cryogenic application. Moreover, the overall resonance wavelength shifts are 1.213 nm, 1.532 nm, 1.935 nm, 2.015 nm, 2.397 nm and 2.671 for bare LPFG, Ti-coated LPFG, Pd-coated LPFG, Ag-coated LPFG, Al-coated LPFG and In-coated LPFG sensors respectively. According to the cooling rates, the results illustrate that as cooling rate increase, the sensor sensitivity decrease due to the sensor response. For more investigations, simulation work is carried out using MATLAB and the sensor shows a good agreement results between experimental and simulation measurements. It is worth to mention that the main findings will essentially contribute to choose suitable materials for coating LPFG for low temperature sensing purposes and will increase the existing knowledge about optical fiber sensor applications.

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