| Long period fiber grating (LPFG) is a new optical passive device appeared in recent years. LPFG can couple the core-mode propagating in the core of the fiber into the cladding-modes. Due to the cladding-modes' transmitting loss, LPFG works as a notch filter. LPFG has many unique advantages, such as all-fiber-structure, small size, no back-reflection, and can be easily integrated or connected with other optical communication devices. LPFG has many important application areas both in fiber communications and fiber sensing systems. For this, LPFG becomes a research focus of people. In this paper we have analyzed the characteristics of LPFG's transmission spectra by coupled-mode theory, and we have investigated the tuning of resonant wavelength both theoretically and experimentally. The main parts in this work are as follows:(1) Based on coupled-mode theory, the electric field's distributing of core-mode and cladding-modes and the coupled-mode equations between them are introduced. And the LPFG's parameters are calculated.(2) The characteristics of LPFG's transmission spectra are analyzed by simulation. Through the curve of resonant wavelength versus period, we present a new method to analyze the shift of resonant wavelength of LPFG during UV laser writing without solving the coupled-mode equations.(3) The relation-ships between the resonant wavelength and the refractive index out of cladding-layer are researched. We find out that it's good way to tune the resonant wavelength by changing the refractive index out of cladding-layer.(4) In experiment, we produce acrylate-based polymer with a negative refractive-temperature index as LPFG's surrounding-layer material. For the polymer- recoated LPFG, the tuning range of resonant wavelength is as large as 60nm and the sensitivity is excess 1nm/℃.
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