L波段掺铒光纤放大器的增益平坦滤波器设计

EDFA的增益平坦化是WDM系统中的重要问题．用成本低、插损小的光纤光栅实现该功能是一项有吸引力的方案，采用剥层法设计了基于啁啾光栅的增益平坦滤波器。基于时间因果律的剥层算法将光纤光栅看成一个分离的模型，由一系列长度为△的复反射器所组成，每个反射器的后端耦合系数都可由它的前端耦合系数递归地求出，从而能快速、精确地反演出光栅的耦合系数函数。啁啾光栅的目标反射谱由理想的增益平坦滤波器透射谱获得，利用与反射谱群时延有关的常数α可控制光栅的长度，α取值为0．0024cm。时，对应的光栅长度为3．5cm。用剥层法反演出耦合系数函数后，又通过解Riccati方程模拟了合成光栅的透射谱。数值模拟结果显示理想透射谱与合成光栅透射谱之间的峰峰值误差小于0．1dB．并且在工作带宽范围内，透射谱群时延的变化量小于0．6ps，表明该滤波器对系统没有额外的色散影响。

Design on Gain-Flattening Filters of L-Band EDFAs

Gain flattening of EDFA (erbium-doped fiber amplifier) is a critical issue for WDM (wavelength-division-multiplex) system, and cost efficiency and insertion loss make in-fiber gratings very attractive candidates for this application. A chirp fiber grating to flatten the gain spectrum of L-EDFAs by using layer-peeling method is designed. This algorithm, based on the law of causality, considers fiber Bragg gratings as a discrete model, which divides the entire grating into a series of discrete, complex reflectors with a distance Δ between the adjacent reflectors, the coupling coefficient at the back of a reflector can be derived from one at the front of this reflector recursively, so coupling coefficient function can be extracted rapidly and accurately. The target reflection spectrum of chirp gratings is derived from the ideal transmission spectrum of gain-flattening filters. A constant, α, is utilized to control the length of chirp gratings, which is relative with group time delay of reflection spectrums. When α is equal to 0.0024 cm2, the grating has a short length of 3.5 cm. After extracting coupling coefficients of the chirp grating using layer-peeling algorithm, transmission spectrum of the synthesized grating by solving Riccati equation is simulated, numerical analysis indicates that peak-to-peak error function is below 0.1 dB between ideal and realistic transmission spectrums. The variation of group time delay in transmission spectrum is less than 0.6 ps over entire operation bandwidth; therefore, the grating has no impact of additional chromatic dispersion on system performance.