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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-interval bimodal filter characteristics of the PS-LPFG at PMTP are studied. LPFG work in the PMTP, the core mode and the high cladding mode coupling loss peak 3dB bandwidth up to 288nm above the mid-point of this LPFG into a single π phase shift in the central wavelength of the two stop bands The interval is up to 388nm, which is much larger than the bimodal interval of the low-order clad mode-coupled π-phase shift LPFG. When M (M> 1) π-phase shifts are uniformly introduced into LPFG, the bimodal spacing increases with the increase of M, and the center wavelength loss is 0 when M is odd and the center wavelength loss with M is even M increases and decreases. The increase of the refractive index and the thickness of the film will cause the two stopband peaks to move toward the short wavelength and increase the bimodal interval. The increase of grating length reduces the bimodal interval while changing the peak loss of bimodal.