基于取样光纤布拉格光栅的全光纤拉曼测温分光系统设计及优化

为实现大气温度全天时和高精度主动遥感探测, 转动拉曼测温激光雷达的分光系统需要滤除强烈的背景光噪声, 以及对Mie-Rayleigh散射提供70 dB以上的带外抑制率. 本文提出了以可见光波段取样光纤布拉格光栅为核心的多级级联的特征光谱提取光路, 构建高抑制率的全光纤拉曼测温分光系统, 以实现大气温度的全天时和高精度探测. 根据分光系统光路的传输特性, 采用传输矩阵模型, 优化设计了影响取样光纤布拉格光栅带外抑制率的主要因素(折射率调制深度、栅区总长度、取样周期和占空比), 得到了优化的光谱分光系统参数. 利用该分光系统可实现太阳背景光强度和Mie-Rayleigh散射信号强度分别比转动拉曼散射信号强度弱40 dB和50 dB, 信噪比高于100时, 白天探测高度可达1.6 km. 该全光纤分光系统具有小型化、抗干扰和稳定性高的优点, 可为陆基及星载拉曼测温激光雷达提供一种全新的解决方案.

Design and optimization of all-fiber rotational Raman spectroscope for temperature measurement based on sampled fiber Bragg grating

Atmospheric temperature is a key parameter to characterize the state of the atmosphere. Owing to the independence of the aerosol effect for profiling the temperture, the pure rotational Raman lidar has become one of valid tools. To achieve all-time and high-precision active remote sensing, strong background noise needs to be filtered out, and the inhibition rate outside the band of more than 70 dB is needed for Mie-Rayleigh scattering in a rotational Raman temperature measurement lidar. In this paper, a multiple cascaded light path based on sampled fiber Bragg grating (SFBG) and fiber Bragg grating (FBG) in visible spectrum is presented to obtain characteristic spectrum. All-fiber spectroscopic system with high inhibition rate for Raman thermometry is set up based on the above light path. The core device consists of single mode fibers (460-HP) to ensure the compatibility with optical fiber. The main factors affecting the inhibition rate outside the band of sampled fiber Bragg grating, including refractive index modulation depth, total length of grating, sampling period and duty, are optimally designed by using mode coupling theory and tranmission matrix model. Then the optimized parameters of spectroscope are obtained. The results show that the inhibition rate outside the band is proportional to the refractive index modulation depth and duty, when the total length of grating is a constant. However, a larger sidelobe jamming will be caused by overlarge refractive index modulation depth. The less amount and widened full width half maximun of reflectivity peak appear following overlarge duty. In the Raman spectroscopic system of this paper, the inhibition rates outside the bands of SFBG and FBG are 30 dB and 20 dB, respectively. The inhibition rate of more than 70 dB is realized for Mie-Rayleigh scattering, after passing through two FBGs and one SFBG. The simulated optimum parameters of SFBGs are the effective index of the guide mode of 1.465, the saturation index variation of 0.00005, the SFBG length of 20 mm, the sampled period of 0.4 mm, and the Bragg wavelengths of 528.51 nm and 530.76 nm. By using the American standard model and atmospheric scattering signal model, the all-time signal-to-noise ratio (SNR) and inhibition rate of Mie-Rayleigh scattering and solar background light are simulated and analyzed. The results show that the intensities of solar background light and Mie-Rayleigh scattering signal are weaker than Raman scattering signals at 40 dB and 50 dB, respectively. The detection height in daytime and night can reach up to 1.6 km and 2.6 km under the condition of SNR of more than 100, respectively. Owing to these advantages such as miniaturization, anti-interference and high stability, this spectroscope provides a viable solution for filter systems of ground-based and spaceborne lidars.