光纤光学-4-4.pptVIP

2000.3 SG 13 /GII presentation 光纤光栅 (FBGs) 光纤布拉格光栅 An in-fibre Bragg grating is constructed by varying the refractive index of the core lengthwise along the fibre. Light of the specified wavelength traveling along the fibre is reflected from the grating back in the direction from which it came. Wavelengths which are not selected are passed through with little or no attenuation. 基本结构 工作原理 resonant wavelengths are reflected back toward the source non-resonant wavelengths are transmitted through the device without loss. The centre wavelength is given by: Principle of Operation 结构参数 The grating period is the distance between modulations of the refractive index in the grating. The grating length. The “modulation depth” , determined by the RI contrast within the grating. The RI contrast profile. 性能参数 Centre Wavelength This is the wavelength at the centre of the gratings reflection band. Bandwidth This is the width of the reflection band and specifies the range of wavelengths reflected. Reflectance Peak This is a measure of the proportion of incident light reflected at the centre wavelength. 反射谱 切趾光栅（变迹光栅） Apodisation is a process of tapering the strength of the grating at either end so that the apparent RI change is gradual rather than abrupt. The reflection band of an apodised grating is shown in part (c) of the figure. 啁啾光栅 A “chirp” is where you get a variation in the period of the grating (and hence a variation in its response to different wavelengths) along the length of the grating: vary the period of the grating or vary the average RI of the grating. 啁啾切趾光栅 a good 100% reflection over the whole reflected band. 级联光栅 you can write many different FBGs into the same section of fibre - one on top of the other. Each grating will then respond quite separately and independently to light of its own resonant wavelength. 闪耀光栅 长周期光栅 Most FBGs are constructed as first order gratings. That is the grating period is the same as the centre wavelength of the reflection