OTDR基本原理知识(20页)解读.pptVIP

OTDR 基本原理 光纤固有参数 Rayleigh Backscattering(瑞利散射) Fresnel Reflections(菲涅耳反射) Rayleigh Backscattering(瑞利散射) Rayleigh Backscattering(瑞利散射) Fresnel Reflections(菲涅耳反射) OTDR 方块图 计算故障或事件点在光纤上的位置 L = vT/2 或 L = cT/(2 ? IOR) L = 到故障或事件点的距离 T = 反射脉冲在双路由上的总时延 IOR = c/v (IOR：折射率) c = 真空中的光速 = 3 x 108 米/秒 v = 给定媒介中的光速 典型的测试结果 脉冲宽度 更大的脉冲宽度: 更大的功率, 即更大的动态范围 更小的脉冲宽度: 更好的分辨率, 但动态范围减小, 需要更多的平均。 脉冲宽度和分辨率 精度 长度 时钟精度 偏移误差标记位置 IOR（折射率）误差 损耗 线性度 dB/dB 激光波长 盲区 – 菲涅耳反射 依赖于: 脉冲宽度/ 开关 / 接收器带宽 发射连接器回损/ 清洁度 噪声和平均 LSA(最小二乘法)/2PA(两点法) 2PA: 在两个标记数据点之间简单地画一条线 LSA(最小二乘法)/2PA(两点法) LSA: 对多个点进行最小二乘法的数学统计计算得到的直线 LSA(最小二乘法)/2PA(两点法) 端到端损耗， 采用2PA LSA(最小二乘法)/2PA(两点法) 端到端损耗，采用LSA产生误差 接续或菲涅耳反射会产生误差 LSA(最小二乘法)/2PA(两点法) 接续损耗,采用2个标记的2PA产生误差 LSA(最小二乘法)/2PA(两点法) 接续损耗 , 在损耗点之前和之后的直线采用2PA也产生误差 标记位置决定结果 LSA(最小二乘法)/2PA(两点法) 接续损耗, LSA – 正确的方法 If LSA is used with two markers over a trace which includes splices or Fresnel reflections, the best straight line fit will not give the most reliable results. When measuring end to end loss of the fiber which includes Fresnel reflections or splices, 2PA is the better method. The line is drawn which minimises the sum of the squares of the distances of all the data points between the markers from the line. This is very effective when the data is noisy. Slide * 采用折衷办法 100ns 20m 100.10-9x2.108=20m 2PTR=两点分辨率 PW=脉冲宽度 BW=检波器带宽 c=光速 n=光纤折射率 例. 如果实际的IOR=1.467 和1.477 时 那麽在20km长度上的误差是138m IOR不可能是准确的常数 The OTDR works like an optical Radar. It sends a pulse, then waits and detects the backscattered and reflected light from that pulse. The source and detector are generally temperature controlled, and a range of techniques exist to effect the switching or coupling of the optical paths. The OTDR trace is a graph of received light intensity plotted against time delay. Time delay is converted to distance via the refractive index of the glass (IOR). The backscatter slope gives the fiber loss, splices appear as drops, co