Zhou Zichao, Chen Chen, Lu Ping, Mihailov Stephen, Chen Liang, Bao Xiaoyi
Department of Physics, University of Ottawa, 25 Templeton Street, Ottawa, ON K1N 6N5, Canada.
National Research Council Canada, Ottawa, ON K1A 0R6, Canada.
Sensors (Basel). 2020 Oct 26;20(21):6071. doi: 10.3390/s20216071.
Random fiber gratings (RFGs) have shown great potential applications in fiber sensing and random fiber lasers. However, a quantitative relationship between the degree of randomness of the RFG and its spectral response has never been analyzed. In this paper, two RFGs with different degrees of randomness are first characterized experimentally by optical frequency domain reflectometry (OFDR). Experimental results show that the high degree of randomness leads to low backscattering strength of the grating and strong strength fluctuations in the spatial domain. The local spectral response of the grating exhibits multiple peaks and a large peak wavelength variation range when its degree of randomness is high. The linewidth of its fine spectrum structures shows scaling behavior with the grating length. In order to find a quantitative relationship between the degree of randomness and spectrum property of RFG, entropy was introduced to describe the degree of randomness induced by period variation of the sub-grating. Simulation results showed that the average reflectivity of the RFG in dB scale decreased linearly with increased sub-grating entropy, when the measured wavelength range was smaller than the peak wavelength variation range of the sub-grating. The peak reflectivity of the RFG was determined by κ2LΔP (where κ is the coupling coefficient, L is the grating length, ΔP is period variation range of the sub-grating) rather than κL when ΔP is larger than 8 nm in the spatial domain. The experimental results agree well with the simulation results, which helps to optimize the RFG manufacturing processes for future applications in random fiber lasers and sensors.
随机光纤光栅（RFG）在光纤传感和随机光纤激光器中已展现出巨大的潜在应用价值。然而，RFG的随机性程度与其光谱响应之间的定量关系从未被分析过。本文首先通过光频域反射法（OFDR）对两个具有不同随机性程度的RFG进行了实验表征。实验结果表明，高随机性程度会导致光栅的背向散射强度较低，且在空间域中强度波动较大。当光栅的随机性程度较高时，其局部光谱响应呈现出多个峰值，且峰值波长变化范围较大。其精细光谱结构的线宽随光栅长度呈现出标度行为。为了找到RFG的随机性程度与光谱特性之间的定量关系，引入了熵来描述由子光栅周期变化引起的随机性程度。模拟结果表明，当测量波长范围小于子光栅的峰值波长变化范围时，以分贝为单位的RFG平均反射率随子光栅熵的增加呈线性下降。当在空间域中ΔP大于8 nm时，RFG的峰值反射率由κ2LΔP（其中κ为耦合系数，L为光栅长度，ΔP为子光栅的周期变化范围）决定，而非κL。实验结果与模拟结果吻合良好，这有助于优化RFG制造工艺，以便在未来的随机光纤激光器和传感器中应用。
