Study On Marine CTD Detecting Technique Based On Optical Fiber Gratings

The conductivity, temperature and depth instruments (CTD), which are ocean monitoring sensors have been extensively used in fields of oceanographic survey, ocean resources investigation and development, ocean environment monitoring and military oceanography. Recently, the optical fiber sensors in marine monitoring have been aroused people’s interest for their advantages such as immunity to electromagnetic interference and multi-sensor operation in situ and remote monitoring. Besides the common advantages, the fiber grating sensors have higher sensitivity to environmental parameters and lower demodulation schemes cost. The fiber grating sensors are becoming a new technology for marine monitoring.In this thesis, the investigations of marine CTD detecting technique based on fiber gratings are carried out as follows:Firstly, the theory of fiber gratings is analyzed. The mode coupling theory for the FBG and LPG is analyzed and focused on the physical parameters, such as:the coupling efficiency, resonant wavelengths, bandwidth, dispersion and field equations, the coupling coefficient. The transmission spectrum of the LFG is also simulated and compared with the experimental results. The analysis and simulations provide theoretical guidance for the fabrication of fiber gratings.Secondly, the fabrication of the LPG written by the UV-laser mask is studied. The temperature, refraction and axial strain characteristics of the LPG are analyzed and tested. After compared the sensor characteristics of the H, GeB and thin-LPG, we concluded that the GeB-LPG is suited for temperature sensor, the thin-LPG is suited for refractive sensor, and the LPG is not suited for axial strain sensor. The sensitivity enhancement of the temperature and refractive are proposed using the HF etching, Polyimide and metal coating.Thirdly, the wavelength evolution of long-period fiber gratings in water environment is studied. In water environment, the wavelength shifts of the LPG written in H2-loaded fiber are still occurred even after annealing. By analysis, we found the reason is related to the hydrogen diffusion and the water molecule penetration. A water-mediated model that water molecules induce the second diffusion of the remaining-H2in the fiber and a diffusion-reaction mechanism that water molecules penetrating into fiber internal structures are proposed and are combined to explain the wavelength evolution process. Both the time calculations according to the model and the qualitative analysis according to the mechanism correspond well with the experimental results. The methods of changing the different species of fiber and metal coating are proposed for preventing water molecules and enhancement the stability.Fourthly, the marine CTD system based on fiber gratings is introduced. Form the angle of system, the design and fabrication of fiber gratings CTD system is studied including the high resolution modulation based on BaySpec analyzer, the temperature and refractive sensor based on the LPG and the pressure sensor based on the FBG. After that, the testing results of the CTD sensors using the high resolution modulation are proposed.Finally, the characteristics of the photo-thermal tuning DFB fiber laser are investigated. The wavelength fluctuation of fiber laser is often induced by the photo-thermal effect. When the photo-thermal light resource is980nm LD, the static and dynamic characteristics are studied. To improving the tuning performance, the905nm LD is induced as new photo-thermal light resource and the light system is changed and the static and dynamic characteristics are tested. This method gives a simple way of achieving both relatively high dynamic tuning range and high tuning speed.