Design Of Carbon Dioxide Gas Sensor Based On Infrared Absorption

The detection of carbon dioxide gas concentration is one of the important parameters in the medical clinical monitoring of human blood circulation and respiratory system.Continuous end-expiratory carbon dioxide（PETCO₂）monitoring can assess ventilation function,identify airway obstruction,reduce the incidence of hypoxemia（Oxygenation less than 90%）,and improve monitoring safety.Up to now,all the products on sale in the market are tested by non-main flow method,that is,the gas exhaled by the trachea entering the pharynx is detected by the sensing device.Therefore,during the perioperative period of gastroenteroscopy,many domestic large-scale third-class hospitals are unable to achieve continuous monitoring of PETCO₂.For the monitoring of carbon dioxide of exhalation,in this paper,the infrared absorption carbon dioxide gas sensor is studied,and the related sensing schemes are designed and tested.Based on the principle of infrared absorption of light,we can get the spectral absorption intensity of carbon dioxide from the HITRAN database by Python,the Voigt linear model is used to calculate the absorption coefficient of gas molecules.When the operating wavelength is 4.23μm,the absorption coefficient of carbon dioxide is the highest.The design of the carbon dioxide gas sensor mainly includes the following three parts.The first part is about the design of a kind of gas sensor for a mid-infrared Silicon photonic integrated chip.The substrate of the sensor is SOS（Silicon on sapphire）,and the transmission of light in different Silicon waveguide structures is compared,such as strip waveguide,SWG（Subwavelength Grating）waveguide,slot waveguide,SWGS（Subwavelength Grating Slot）waveguide.The slot waveguide and the SWGS waveguide have a higher optical power limiting factor.The parameters of the two structures,such as waveguide width,slot width,and transmission loss,are scanned.In addition,taking the slot waveguide as an example,the spiral structure,the bending structure,and the combined structure of the ring resonator and the slot waveguide are proposed,which directly or indirectly increase the gas absorption length,to integrate the sensing region into a region of several hundred microns.The second part is an experimental test scheme for monitoring end-expiratory carbon dioxide based on optical microfibers,in which the fiber acts both as a transmission light and as a sensing region to interact with the gas to be measured.By preparing optical microfibers with different diameters and monitoring exhaled carbon dioxide gas,the periodical variation of light power represents the periodic change of respiration,and the frequency of periodical variation of light power characterizes the respiratory frequency.For sensing carbon dioxide gas near the nasal cavity,two aspects of the research have been carried out in this paper:the theoretical design of the silicon waveguide chip gas sensor and the experimental test of the high evanescent field absorption sensing scheme of optical micro/nano-fibers.