[论文解读] Fast Switching Dual Fabry-Perot-Cavity-based Optical Refractometry for Assessment of Gas Refractivity and Density - Estimates of Its Precision, Accuracy, and Temperature Dependence

Martin Zelán, Isak Silander+2 more|arXiv (Cornell University)|Apr 3, 2017

Spectroscopy and Laser Applications被引用 3

一句话总结

本文提出了一种快速切换双法布里-珀罗腔（FS-DFCB-OR）光学折射率计，可实现高精度、无漂移的气体折射率与密度测量，精度达亚ppm级别。其在标准温度和压力下精度可达10⁻⁹，温度依赖性极低（约10⁻⁸/°C），并表明由于内部参考稳定，内部精度可比绝对精度高几个数量级。

ABSTRACT

Dual Fabry-Perot-Cavity-based Optical Refractometry (DFCB-OR) have been shown to have excellent potential for characterization of gases, in particular their refractivity and density. However, its performance has in practice been found to be limited by drifts. To remedy this, drift-free DFPC-OR (DF-DFCB-OR) has recently been proposed. Suggested methodologies for realization of a specific type of DF-DFCB-OR, termed Fast Switching DFCB-OR (FS-DFCB-OR), have been presented in an accompanying work. This paper scrutinizes the performance and the limitations of both DF- and FS-DFCB-OR for assessments of refractivity and gas density, in particular their precision, accuracy, and temperature dependence. It is shown that both refractivity and gas density can be assessed by FS-DFCB-OR with a precision in the 10$^{-9}$ range under STP conditions. It is demonstrated that the absolute accuracy is mainly limited by the accuracy by which the instantaneous deformation of the cavity or the higher order virial coefficients can be assessed. It is also shown that the internal accuracy, i.e. the accuracy by which the system can be characterized with respect to an internal standard, can be several orders of magnitude better than the absolute. It is concluded that the temperature dependence of FS-DFCB-OR is exceptionally small, typically in the 10$^{-8}$ to 10$^{-7}$/C range, and primarily caused by thermal expansion of the FPC-spacer material. Finally, this paper discusses means on how to design a FS-DFCB-or system for optimal performance and epitomizes the conclusions of this and our accompanying works regarding both DF- and FS-DFCB-OR in terms of performance and provides an outlook for both techniques. Our works can serve as a basis for future realizations of instrumentation for assessments of gas refractivity and density that can fully benefit from the extraordinary potential of FPC-OR.

研究动机与目标

• 开发一种无漂移、高精度的光学折射率测量系统，以实现对气体折射率和密度的精确评估。
• 解决传统双法布里-珀罗腔（DFCB-OR）系统因热和机械漂移导致的性能限制。
• 评估快速切换DFCB-OR（FS-DFCB-OR）配置在折射率和密度测量中的精度、准确度及温度依赖性。
• 为优化FS-DFCB-OR系统在气体折射率与密度测量中的最大性能提供设计指导。

提出的方法

• 该系统采用两个在快速切换模式下运行的法布里-珀罗腔，以实现参考气体与样品气体之间的差分测量。
• 采用双腔结构以消除共模漂移，特别是热和机械不稳定性。
• 该方法依赖于对腔模相位或频率的跟踪，通过光程长度变化提取折射率和密度。
• 应用精确的第二 virial 系数表达式及腔体形变模型，以校正非理想气体行为和机械畸变。
• 通过内部参考进行校准，使系统具备独立于绝对校准不确定性的高内部精度。
• 对腔体间隔材料的热膨胀进行建模，以量化并最小化温度引起的误差。

实验结果

研究问题

• RQ1 在标准条件下，FS-DFCB-OR在测量气体折射率和密度方面可实现的精度是多少？
• RQ2 FS-DFCB-OR的绝对准确度在多大程度上取决于腔体形变和高阶virial系数测量的精度？
• RQ3 FS-DFCB-OR系统的温度依赖性如何？其主导物理因素是什么？
• RQ4 FS-DFCB-OR的内部准确度与绝对准确度相比如何？这种差异由什么因素实现？
• RQ5 哪些设计原则可使FS-DFCB-OR在高精度气体计量中实现最优性能？

主要发现

• 在标准温度和压力（STP）条件下，FS-DFCB-OR在折射率和气体密度测量中可实现10⁻⁹的精度。
• 绝对准确度主要受限于腔体形变测量和高阶virial系数测量的不确定性。
• 内部准确度——即系统相对于内部标准分辨差异的能力——可比绝对准确度高几个数量级。
• 系统的温度依赖性极低，通常在10⁻⁸至10⁻⁷ /°C范围内，其中腔体间隔材料的热膨胀是主要来源。
• 系统对漂移具有强鲁棒性，适用于长期、高精度的气体折射率测量。
• 研究结果为实现下一代基于法布里-珀罗腔的光学折射计奠定了基础，可充分释放该技术在折射率测量中的潜力。