Bello Valentina, Simoni Alberto, Merlo Sabina
Department of Electrical, Computer and Biomedical Engineering, University of Pavia, 27100 Pavia, Italy.
Sensors (Basel). 2020 Feb 14;20(4):1043. doi: 10.3390/s20041043.
In this work, we demonstrate spectral phase-shift interferometry operating in the near-infrared wavelength range for refractive index (RI) monitoring of fluidic samples in micro-capillaries. A detailed theoretical model was developed to calculate the phase-sensitive spectral reflectivity when low-cost rectangular glass micro-capillaries, filled with samples with different refractive indices, are placed at the end of the measurment arm of a Michelson interferometer. From the phase-sensitive spectral reflectivity, we recovered the cosine-shaped interferometric signal as a function of the wavelength, as well as its dependence on the sample RI. Using the readout radiation provided by a 40-nm wideband light source with a flat emission spectrum centered at 1.55 µm and a 2 × 1 fiberoptic coupler on the common input-output optical path, experimental results were found to be in good agreement with the expected theoretical behavior. The shift of the micro-capillary optical resonances, induced by RI variations in the filling fluids (comparing saline solution with respect to distilled water, and isopropanol with respect to ethanol) were clearly detected by monitoring the positions of steep phase jumps in the cosine-shaped interferometric signal recorded as a function of the wavelength. By adding a few optical components to the instrumental configuration previously demonstrated for the spectral amplitude detection of resonances, we achieved phase-sensitive detection of the wavelength positions of the resonances as a function of the filling fluid RI. The main advantage consists of recovering RI variations by detecting the wavelength shift of "sharp peaks", with any amplitude above a threshold in the interferometric signal derivative, instead of "wide minima" in the reflected power spectra, which are more easily affected by uncertainties due to amplitude fluctuations.
在这项工作中，我们展示了在近红外波长范围内运行的光谱相移干涉测量技术，用于监测微毛细管中流体样品的折射率（RI）。我们建立了一个详细的理论模型，以计算当将填充有不同折射率样品的低成本矩形玻璃微毛细管放置在迈克尔逊干涉仪测量臂末端时的相敏光谱反射率。从相敏光谱反射率中，我们恢复了作为波长函数的余弦形干涉信号，以及它对样品RI的依赖性。使用由中心波长为1.55 µm、带宽为40 nm且发射光谱平坦的宽带光源提供的读出辐射，并在公共输入-输出光路上使用2×1光纤耦合器，实验结果与预期的理论行为吻合良好。通过监测作为波长函数记录的余弦形干涉信号中陡峭相位跳变的位置，清晰地检测到了填充流体中RI变化（将盐溶液与蒸馏水比较，异丙醇与乙醇比较）所引起的微毛细管光学共振的位移。通过在先前用于共振光谱幅度检测的仪器配置中添加一些光学元件，我们实现了作为填充流体RI函数的共振波长位置的相敏检测。主要优点在于通过检测干涉信号导数中高于阈值的任何幅度的“尖峰”的波长位移来恢复RI变化，而不是检测反射功率谱中更容易受到幅度波动引起的不确定性影响的“宽最小值”。
