Effects Of The Coating To Properties Of LPFG Written By High-frequence CO₂ Laser

Long-period fiber gratings(LPFGs),as an important member of the optical fiber gratings, has been highlighted by the peoples because of its outstanding advantages on the measuring of temperature, strain,refractive index and many other physical parameters. Due to such advantages as corrosion resistance, high temperature resistance and immunity to electromagnetic interference and so on, long-period fiber gratings generally is used in the site that environment is harsh and complex.So their mechanical strength should have a high level.But the actual situation is that in order to improve the efficiency of writing of LPFGs,the coating which is a protection layer outside the fiber usually need to be stripped when inscribing the optical fiber gtatings.Thus the mechanical strength of LPFGs inscribed greatly be reduced. In this thesis,we will investigate how to write long-period fiber gratings in coated optical fiber by high-frequence CO2 laser through the experiments.Then we will research their sensing characteristics of temperature,strain and refractive index,so as to provide some useful references for the fabrication of sensors based in long-period fiber gratings in practical applications.The conclusions is provided as follows:1. The writing methods,history of development of long-period fiber gratings and the applications of long-period fiber gratings in the fields of communication and sensing were briefly discussed. Then the related equations of long-period fiber gratings coupled-mode theory were derived.And the physical meanings of parameters involved in the theory were also explained.2. The writing method and mechanisms of long-period fiber gratings inscribed by high-frequency CO2 pulse laser were introduced. Then two kinds of long-period fiber gratings were written out by high-frequence CO2 laser.And the effects of coating to the writing process of long-period fiber gratings were comparatively studed. The experimental results showed that the existence of the coating would play a role of buffer to laser radiation.So it should need more times to write long-period fiber gratings with coating.3. Experiments of anneling and temperature sensing of long-period fiber gratings written as the method above were implemented.Then the effects of the coating to temperature characteristics of long-period fiber gratings written by the highfrequency CO2 pulse laser were analyzed. The experimental results showed that the coating would affect the heat conduction to the fiber core and cladding.Not only that,because of the material difference between coating and the core/cladding, it could bring in a amount of strain.But it had no effect to the temperature sensitivity(0.1 nm / ℃).4.The sensing characteristics of the liquid concentration was implemented experimently. Then the influences of the coating of long-period fiber gratings on the sensing characteristics were analyzed. The experimental results showed that the existence of the coating could increase the radius of the fiber cladding, and it reduced the refractive index sensitivity of the long-period fiber gratings.5. The axial strain sensing characteristics experiment was also implemented. Effects of the coating on the axial strain sensing characteristics were discussed. Experimental results showed that with a same stretch distance, the strain of coated long-period fiber gratings was smaller than that of uncoated.And as a result,the strain sensitivity(-1.2921 nm/mε) of coated long-period fiber gratings was larger than that of uncoated(-0.7907 nm/mε).In short,the long-period fiber gratings written in coated fiber has a better sensing characteristics of temperature and strain.