|Title:||Polarization-maintaining photonic crystal fiber based sensors and fiber Bragg gratings sensor system|
|Subject:||Hong Kong Polytechnic University -- Dissertations.|
Optical fiber detectors.
|Department:||Department of Electrical Engineering|
|Pages:||158 p. : ill. (some col.) ; 30 cm.|
|Abstract:||This thesis describes the investigation of fibre optic sensor technologies that focus on the development of fibre sensing elements based on polarization-maintaining photonic crystal fibre (PM-PCF) and fibre Bragg gratings (FBGs), and their multiplexing techniques. Various multiplexing techniques of PM-PCF based Sagnac interferometers have been investigated. A novel scheme to realize high-speed FBG sensor interrogation by using dispersion-compensating module, and a new wavelength-division multiplexing technique to achieve long-distance FBG sensing over 75 km using broadband light sources were investigated. In addition, we also studied a time-division multiplexing scheme for FBG sensors based on pulse modulated semiconductor optical amplifier (SOA) ring cavity. A fibre optic polarimetric torsion sensor based on PM-PCF with high torsion sensitivity and low sensitivity to temperature was developed. The torsion sensitivity of the proposed sensor is ~ 0.014/o within a linear twist range from 30o to 70o It is highly repeatable over 90-degree twist in both clockwise and counterclockwise directions. The experimental results were in excellent agreement with predicted results. PM-PCF as sensing elements in Sagnac configuration for pressure and high temperature measurements were also investigated. Measurement results show that high pressure sensitivity of 3.42 nm/MPa, and high temperature measurement up to 1160 oC with good linearity of about 11 pm/oC in the range of 400 oC - 800 oC were achieved. Their performance was compared with temperature sensors based on FBGs written with UV irradiation or with femtosecond laser. The results show that PM-PCF based Sagnac interferometer is a good candidate for long-term high temperature measurements because of the lack of a doped core that would diffuse to the cladding and eventually render the fibre non-singlemode. Three multiplexing techniques of PM-PCF sensors in Sagnac configuration were proposed. One technique employed coarse wavelength division multiplexer (CWDM) to separate each Sagnac interferometer in different wavelength bands. The other two schemes were based on connecting several Sagnac interferometers in series along a single fibre and by using couplers to multiplex the sensors in parallel. The CWDM multiplexing technique is simple to implement, whereas the other two techniques require post measurement data processing. Two main challenges of FBG sensor systems are high-speed FBG sampling beyond the kHz range and long-distance interrogation. We have developed a novel ultra-fast FBG interrogation system based on dispersion-compensation fibre that converts wavelength shift to time difference, permitting FBG interrogation up to 2.44 MSamples/second. Two different approaches for long-distance FBG sensing were investigated. By cascading three broadband light sources with different wavelength bands along a fibre link, a 75-km sensing distance with an impressive high effective dynamic range of 60 dB was achieved. The second scheme used a pulse modulated SOA ring cavity demonstrated time-division multiplexing of 6 FBG sensors along a 40-km optical fibre with better than 20 dB optical signal-to-noise ratio.|
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