长周期光纤光栅模式与耦合的研究

长周期光纤光栅是在光纤纤芯中沿轴向形成折射率周期性调制的带阻型光纤器件。与光纤布拉格光栅不同，它表现为前向传播的纤芯基模与同向传播的各阶次包层模式在特定波长的耦合。研究了纤芯基模、包层模式（HE1t/EH1t）及其有效折射率随波长的变化，研究了纤芯基模与包层模式（HE1t/EH1t）的耦合系数。研究表明，纤芯基模与一阶低次包层模式的有效折射率随波长增大而减小，纤芯基模与HE1t产生的耦合系数远大于与EH1t的耦合系数，并且包层模式次数较低时，耦合系数随次数的升高而增加。这与以前的研究结果有所不同。

Modes and Couplings of Long Period Fiber Grating

A long period fiber grating (LPFG) is a band rejection fiber device with periodical retractive index modulation formed along the axis of the fiber core. Different with a fiber Bragg grating (FBG), it couples light from the forward propagating fundamental core mode into co propagating cladding modes at discrete wavelengths. Dependences of effective indices for fundamental core mode and cladding modes (HE 1l /EH 1l ) upon wavelength, and the coupling constants between fundamental core mode and cladding modes (HE 1l /EH 1l ) are studied. It is found that effective refractive indices for the core mode and cladding modes of LPFG both decrease with wavelength. Coupling constant formed with HE 1l mode is much greater than that with EH 1l mode. Also coupling constants increase with the order of cladding modes at the lowest few ones. These differ with the conclusions drawn before.