Hollow Fiber As Absorption Spectrum Sensing For The Optimization Design Of The Air Chamber

Mid-infrared (IR) spectroscopy has a long history in physics and chemistry, and is a well-established research field for optical sensor technologies in recent decades. Each chemical species is characterized by distinctive IR spectroscopic response patterns resulting from rotational and vibrational transitions excited in this frequency regime, thereby providing a rigorous platform for gas sensing applications. Meanwhile, mid-infrared spectroscopy overwhelms near-infrared one in both number and intensity of the absorption peak. Therefore, mid-infrared waveguides with wide spectrum and low loss have a good application prospects in the research area of spectroscopy sensing.Compared with traditional silica fibers, hollow optical fibers enjoy a wider transmission spectrum and comparatively lower transmission loss. In the application of gas sensing, the hollow optical fiber simultaneously acts as a wavelength selective waveguide and miniaturized gas cell, which provides plenty of advantages. The characteristics of the sensing system are dependent on the parameters of the hollow waveguide cell.In this paper, a mathematical model was proposed to optimize the waveguide cell by considering waveguide loss, effective optical path length, and signal-to-noise ratio of the system. Simulation results show that the gas absorption intensity and system sensitivity are dependent not only on the waveguide length but also on the bore-diameter, signal-to-noise ratio and the concentration of the measured gases. The results provide optimizing methods for the spectroscopic sensing system and algorithms for error compensation.In order to verify the reliability of the theoretical model, a mid-infrared gas spectroscopic sensing system was established in the lab. The Mid-IR light was focused into an Ag/AgI hollow waveguide (1m length,1mm inner diameter) from a Fourier transform infrared (FTIR) spectrometer utilizing a DTGS (deuterated triglycine sulfate detectors) detector. A national standards gas analyzer was used as a calibration gas output, connected with a physical interface to combine the optical path with the gas channel. Quantitative measurements were carried out for the carbon dioxide (CO2) with different concentrations between5ppm~70ppm, using the characteristic absorption peak of CO2at the wavelength of4.25um. The limit of detection with ppm level CO2was achieved. Preliminary experiments on concentration detection of methane gas (CH4) were also carried out and measured data show good agreement with the simulation results. It showed that increasing system SNR beyond the efficient point can no longer improve the system sensitivity and when the system works at high concentration level or with low SNR, error compensation is necessary in the measurement.