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提出了一种光纤横截面折射率变化呈旋转非对称变化的长周期光纤光栅(R-LPFG)结构,并利用多层圆波导理论和横截面折射率离散分析方法,结合模式耦合方程组和数值求解方法理论分析了这种光栅的模式耦合特征.理论分析表明R-LPFG纤芯基模主要与一阶非对称包层模发生耦合,当光栅旋转度逐渐变大时,R-LPFG基模会与一阶非对称包层模的奇模和偶模同时发生耦合,这就会使原来单一的谐振峰逐渐分裂成双峰,这是常规光栅类型所不具有的透射谱特征.由于R-LPFG的双峰来自同一对耦合模式,它们对温度的响应很相似,因此可利用双峰间距来进行无需温度补偿的扭曲、应变等物理量的测量.最后利用高频CO2激光脉冲写入法制作了这种光栅,并实验研究了这类光栅的传输谱演变特征,实验结果和理论分析一致.可以预见,这类光栅在光纤传感或通信中将具有较大的潜在应用价值.
A long period fiber grating (R-LPFG) structure is proposed, in which the refractive index of the optical fiber changes in a rotationally asymmetric manner, and the multilayer circular waveguide theory and the cross-sectional refractive index discrete analysis method are used in combination with the mode coupling equations and numerical values Theoretical analysis shows that the fundamental mode of R-LPFG core is mainly coupled with the first-order asymmetric cladding mode. When the degree of grating rotation becomes larger, the R-LPFG fundamental mode Coupling with odd-mode and even-mode first-order asymmetric cladding modes causes the original single resonance peak to gradually split into bimodals, which is characteristic of the transmission spectrum that conventional grating types do not have.As R-LPFG Of the bimodal from the same pair of coupling modes, their response to temperature is very similar, so the use of bimodal spacing can be without temperature compensation for distortion, strain and other physical measurements .Finally, the use of high-frequency CO2 laser pulse writing method produced this The experimental results are in good agreement with the theoretical analysis.It is foreseeable that such a grating will have a large potential application value in optical fiber sensing or communication .