Iribas Haritz, Mariñelarena Jon, Feng Cheng, Urricelqui Javier, Schneider Thomas, Loayssa Alayn
Opt Express. 2017 Oct 30;25(22):27896-27912. doi: 10.1364/OE.25.027896.
We report on two previously unknown non-local effects that have been found to impair Brillouin optical time-domain analysis (BOTDA) sensors that deploy limited extinction ratio (ER) pump pulses. The first one originates in the increased depletion of the pedestal of the pump pulses by the amplified probe wave, which in turn entails a reduced amplification of the probe and a measurement distortion. The second effect is due to the interplay between the transient response of the erbium-doped fiber amplifiers (EDFA) that are normally deployed to amplify the pump and the pedestal of the pump pulses. The EDFA amplification modifies the pedestal that follows the pulses in such a way that it also leads to a distortion of the measured gain spectra after normalization. Both effects are shown to lead to non-local effects in the measurements that have similar characteristics to those induced by pump pulse depletion. In fact, the total depletion factor for calculations of the Brillouin frequency shift (BFS) error in BOTDA sensors is shown to be the addition of the depletion factors linked to the pump pulse as well as the pedestal. A theoretical model is developed to analyze both effects by numerical simulation. Furthermore, the effects are investigated experimentally in long-range BOTDA sensors. The pedestal depletion effect is shown to severely constrain the probe power as well as the minimum ER of the pulses that can be deployed in BOTDA sensors. For instance, it is shown that, in a long-range dual-probe BOTDA, an ER higher that 32-dB, which is above that provided by standard electro-optic modulators (EOM), is necessary to be able to deploy a probe power of -3 dBm, which is the theoretical limit for that type of sensors. Even more severe can be the limitation due to the depletion effect induced by the EDFA transient response. It is found that the impairments brought by this effect are independent of the probe power, hence setting an ultimate limit for the BOTDA sensor performance. Experimentally, a long-range BOTDA deploying a 26-dB ER EOM and a conventional EDFA is shown to exhibit a BFS error higher than 1 MHz even for very small probe power.
我们报告了两个此前未知的非局部效应，这些效应已被发现会损害采用有限消光比（ER）泵浦脉冲的布里渊光时域分析（BOTDA）传感器。第一个效应源于放大后的探测波对泵浦脉冲基座的损耗增加，这进而导致探测信号的放大减小以及测量失真。第二个效应是由于通常用于放大泵浦的掺铒光纤放大器（EDFA）的瞬态响应与泵浦脉冲基座之间的相互作用。EDFA放大以这样一种方式改变了跟随脉冲的基座，即归一化后也会导致测量增益谱的失真。这两种效应在测量中均会导致非局部效应，其特征与泵浦脉冲损耗所引起的效应相似。事实上，在计算BOTDA传感器的布里渊频移（BFS）误差时，总损耗因子显示为与泵浦脉冲以及基座相关的损耗因子之和。通过数值模拟建立了一个理论模型来分析这两种效应。此外，还在远程BOTDA传感器中对这些效应进行了实验研究。结果表明，基座损耗效应严重限制了可用于BOTDA传感器的探测功率以及脉冲的最小消光比。例如，结果表明，在远程双探测BOTDA中，要能够部署 - 3dBm的探测功率（这是该类型传感器的理论极限），消光比必须高于32dB，这高于标准电光调制器（EOM）所能提供的消光比。由于EDFA瞬态响应引起的损耗效应所导致的限制可能更为严重。结果发现，这种效应所带来的损伤与探测功率无关，因此为BOTDA传感器的性能设定了一个最终极限。实验表明，即使对于非常小的探测功率，采用26dB消光比的EOM和传统EDFA的远程BOTDA所呈现的BFS误差也高于1MHz。
