Optical time-domain reflectometer (OTDR) is used to characterize fiber optic links by identifying and localizing various refractive and reflective events such as breaks, splices, and connectors, and measuring insertion/return loss and fiber length. Essentially, OTDR inserts a pulsed signal into the fiber, from which a small portion that is commonly referred to as Rayleigh backscatter, is continuously reflected back with appropriate delays of the reflections expressed as the power loss versus distance, by conveniently scaling the time axis. Specifically, for long-distance events visibility and measurement accuracy, the crucial OTDR attribute is dynamic range, which determines how far downstream the fiber can the strongest transmitted optical pulse reach. As many older-generation but still operable OTDR units have insufficient dynamic range to test the far-end of longer fibers, we propose a simple and cost-effective solution to reactivate such an OTDR by inserting a low-noise high-gain optical preamplifier in front of it to lower the noise figure and thereby the noise floor. Accordingly, we developed an appropriate dynamic range and distance span extension model which provided the exemplar prediction values of 30 dB and 75 km, respectively, for the fiber under test at 1550 nm. These values were found to closely match the dynamic range and distance span extensions obtained for the same values of the relevant parameters of interest by the preliminary practical OTDR measurements conducted with the front-end EDFA optical amplifier, relative to the measurements with the OTDR alone. This preliminary verifies that the proposed concept enables a significantly longer distance span than the OTDR alone. We believe that the preliminary results reported here could serve as a hint and a framework for a more comprehensive test strategy in terms of both test diversification and repeating rate, which can be implemented in a network operator environment or professional lab.

中文翻译：

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通过外部前端光前置放大器扩展 OTDR 距离跨度

光时域反射计 (OTDR) 用于通过识别和定位各种折射和反射事件（例如断裂、熔接和连接器）并测量插入/回波损耗和光纤长度来表征光纤链路。本质上，OTDR 将脉冲信号插入到光纤中，其中一小部分通常称为瑞利背向散射，通过方便地缩放时间轴，以适当的反射延迟（表示为功率损耗与距离的关系）连续反射回来。具体来说，对于远距离事件的可见性和测量精度，关键的 OTDR 属性是动态范围，它决定了光纤下游最强传输的光脉冲可以到达多远。由于许多老一代但仍可操作的 OTDR 单元的动态范围不足以测试较长光纤的远端，因此我们提出了一种简单且经济高效的解决方案，通过将低噪声高增益光学前置放大器插入以降低噪声系数，从而降低本底噪声。因此，我们开发了一个适当的动态范围和距离跨度扩展模型，该模型分别为 1550 nm 的被测光纤提供了 30 dB 和 75 km 的示例预测值。发现这些值与使用前端 EDFA 光放大器进行的初步实际 OTDR 测量（相对于单独使用 OTDR 的测量）为相关相关参数的相同值获得的动态范围和距离跨度扩展非常匹配。这初步验证了所提出的概念能够实现比单独的 OTDR 显着更长的距离跨度。我们认为这里报告的初步结果可以作为一个提示和框架，在测试多样化和重复率方面更全面的测试策略，可以在网络运营商环境或专业实验室中实施。