Thin Cladding Long Period Fiber Grating Sensors

Prof. Rao, the supervisor invented the fabrication method of long-period fiber gratings (LPFGs) in the single-side of fiber by high-frequency CO₂ laser pulses in 2000. In the past several years, the fabrication method, formation mechanism, based characteristics and applications have been investigated and gained some prominent achievements. In this thesis, the method of etching the fiber cladding first and then writing the LPFG is reported. This kind of LPFG with thin cladding is not only very sensitive to ambient refractive index, but also more asymmetric in refractive index distribution on the cross-section of the LPFG. And the characteristics of these LPFGs have been studied by experiments. The main work is provided as follows:1. In this paper, for the first time to our knowledge, we report LPFGs written by high-frequency CO₂ laser pulses on fibers with thinned claddings. The distribution of refractive index on the cross-section of such a kind of LPFGs is explained too. By using the etching method of the LPFG, we can find out that the method of etching the fiber cladding first and then writing the LPFG is more effective in terms of adjusting the resonant wavelength of the LPFG over that of writing the LPFG first and then etching the fiber cladding. Furthermore, it is found that these two kinds of LPFGs with thinned claddings are more sensitive to ambient refractive index than a normal LPFG unetched.2. The basal characteristics of the LPFGs written by high-frequency CO₂ laser pulses on fibers with thinned claddings are demonstrated in this paper for the first time. It came out that the characteristics of temperature,strain,distortion and bend change only a little when comparing the cases of writing LPFGs first then etching and un-etched. The resonant wavelength of the LPFG shifts linearly with temperature with sensitivity of 0.1 n m·℃^-1. The amplitude of the loss peak is hardly affected by temperature change. The strain-sensitivity of the LPFG is smaller, and a resonant wavelength sensitivity of -0.14nm/ mεand an amplitude sensitivity of 0.18dB/mεare obtained. It is demonstrated that the torsion characteristic of resonant wavelength for the LPFG depends strongly on twist direction. That is, the resonant wavelength shifts linearly toward the longer wavelength as the LPFG is twisted clockwise, whereas it shifts linearly toward the shorter wavelength as the LPFG is twisted anti-clockwise with a sensitivity of 0.04nm/(rad·m^-1). It is discovered that the bend-sensitivity of the LPFG depends strongly on curved direction.3. The LPFG applied as chemic sensor is not selective and is not effective in the case of ambient refractive index is bigger than that of cladding. LPFG with thin film sensors can overcome the problem. In this thesis, the carbon nanotube film is coated on three types of LPFG and it is discovered that their resonant wavelength shift a little toward the shorter wavelength, leading to the possibility of gas measurement with enough sensitivity.