镀膜相移长周期光纤光栅大间隔双峰滤波特性

Large-Separation Dual-Peak Filtering Characteristics of Phase-Shifted Long-Period Fiber Gratings with Film Coatings

研究了镀膜相移长周期光纤光栅(PS-LPFG)工作于相位匹配转折点(PMTP)时的大间隔双峰滤波特性。LPFG工作于PMTP时,纤芯模与高次包层模耦合产生的损耗峰3dB带宽可达288nm以上,在此LPFG中点引入单个π相移时在中心波长两侧出现的两个阻带峰间隔达388nm,远大于低次包层模耦合π相移LPFG的双峰间隔。在LPFG中均匀地引入M(M>1)个π相移时,双峰间隔随M的增大而增大,M为奇数时,中心波长损耗为0;M为偶数时,中心波长损耗随M的增大而减小。薄膜折射率与厚度的增加都将使两个阻带峰向短波长方向移动并增大双峰间隔。光栅长度的增大在改变双峰峰值损耗的同时使双峰间隔逐渐减小。

The large-separation dual-peak filtering characteristics of phase-shifted long-period fiber gratings （PS- LPFG） with film coatings are studied. When an LPFG is designed to work at the phase-matching turning point （PMTP）, the coupling between core mode and a higher-order cladding mode produces a single resonant peak whose bandwidth reaches 288 nm and above. The introduction of a phase-shift into the center of this LPFG gives rise to two band-rejection peaks locating at the two sides of the central wavelength. The separation between these two peaks reaches 388 nm which is much larger than that of the PS-LPFG based on the coupling between core mode and a lower- order cladding mode. When the LPFG is devised uniformly by introducing M （M〉1） π phase shifts, the separation increases with the increase of M. The loss at central wavelength is constant at zero when M is odd, whereas the loss at central wavelength decreases with the increase of M when M is even. The increase of film thickness and refractive index causes the two peaks to shift towards short wavelength and increases the separations. Furthermore, the increment of grating length changes the peak loss of dual peaks and decreases the separation.