The Enhanced Rayleigh Distributed Optical Fiber Sensing Systems Integrated With Weak FBGs

Since distributed fiber sensors based on Rayleigh scattering can be used to measure vibration, strain and transmission loss continuously along long sensing fiber, they can be applied in many applications, such as the loss monitoring of communication link, structure health monitoring of large structure building and facilities, perimeter security monitoring, marine resource exploration, disaster prevention and mitigation and national security. Compared to distributed fiber sensors based on Brillion and Raman scattering, the signal of the distributed fiber sensor based on Rayleigh scattering is much higher while the sensing principle and the signal detection are much easier to be realized. Therefore, the distributed fiber sensor based on Rayleigh scattering has advantages on sensing distance, sensitivity and response speed compared to other distributed fiber sensors. Nevertheless, because the Rayleigh scattering in single mode fiber (SMF) is very weak, the signal to noise ratio (SNR) of these distributed fiber sensors based on Rayleigh scattering is still limited. Thus, the performance such as the sensing range, sensitivity and spatial resolution of the Rayleigh scattering based sensing system are restricted. In addition, distributed fiber sensors based on Rayleigh scattering do not offer good performance for some sensing applications, such as strain and temperature detection, which limits the application of the distributed fiber sensor based on Rayleigh scattering.The SNR of the fiber sensing systems based on fiber Bragg grating (FBG) are relative higher than the SNR of the fiber sensing systems based on Rayleigh scattering. Moreover, the sensing function of FBG is also more abundant. However, due to the slow response speed of the system based on FBG, these sensing systems are hard to be used for vibrations sensing, especially vibrations with high frequency. In addition, the sensors based FBG cannot be used to achieve truly distributed fiber sensing. In order to meet the requirements of applications, achieving distributed fiber sensing system with higher SNR and more functions is very important. Therefore, in this thesis, on the basis of studying the reflected spectrum of FBGs, FBGs are designed and fabricated to improve the SNR and function of the distributed fiber sensing system based on Rayleigh scattering. Then the integration scheme of distributed fiber sensor based on Rayleigh scattering and FBGs is proposed and studied to improve the SNR of the distributed fiber sensors based on Rayleigh scattering. Moreover, integrate the Rayleigh scattering based distributed fiber sensing function and FBGs based quasi-distributed fiber sensing function, a function enhanced distributed fiber sensing system is also proposed. The main works of this thesis are as follows:1) Firstly, the spectra of FBG and LPG are analyzed based on the mode coupling theory. Then, the influence of UV light intensity, exposure time and length of gratings to the parameters of FBG, such as reflectivity, wavelength and spectral width are discussed using numerical simulation. All of these help to fabricate WFBGs with wide and narrow spectral width and LPGs. After that, the system and steps of fabricating WFBGs and LPGs are also studied. The performance of the fabricated WFBGs and LPGs are also measured and analyzed in the third chapter.2) We propose and demonstrate an SNR enhanced distributed sensing system based on WFBGs associated POTDR which can be used for load and vibration sensing with improved sensitivity. Moreover, the proposed system is able to perform the true distributed sensing function. Firstly, the influence of pulse width and interval of WFBGs are analyzed using numerical simulation. Then the pulse width and WFBG interval is suggested to be matched, which makes the SNR improvement is achieved continuously and uniformly along the whole sensing fiber. The simulation results show that it is possible to integrate more than 4472 WFBGs in the system when the reflectivity of WFBGs is less than 10-5. At last, an experimental setup is proposed to certify the improvement. Using WFBGs with 1.1 × 10-5 reflectivity and 8 nm spectral width, the SNR of POTDR is improved by 11 dB. Meanwhile, the sensitivity of load and vibration measurement are improved by 10.7 dB and 9 dB respectively.3) A multifunctional fiber sensing system integrated with WFBGs assisted OTDR and long period fiber grating (LPG) is proposed for distributed and key-positions monitoring. The strain, temperature experienced by each WFBG can be monitored by using the OTDR technology through detecting the intensity change of the reflected light passing through a LPG based linear filter, which transform the wavelength shift to the intensity change. The strain and temperature sensitivities of 4.1 × 10-4/με and 5.9× 10-3/℃ are achieved respectively. Moreover, the distributed fiber sensing can also be achieved by detecting the filtered back-Rayleigh scattering from the sensing fiber without WFBGs. Therefore, multiple sensing ways such as distributed fiber sensing and key-positions monitoring are achieved simultaneously. In addition, due to the fast response speed of WFBGs and LPG based sensing system, vibration especially multiple vibrations sensing can also be achieved. Therefore, dynamic and static events sensing can be achieved simultaneously. All of these greatly expand the sensing functions of single fiber sensing system.