Mid-infrared gas sensors of liquid crystal type

摘要

One of the most important tasks in the environmental monitoring field is detection and concentration measurements of different ingredients in natural and artificial mixtures of substances. According to the measuring of value of IR absorption of different gases one can determine their concentration in the environment. That is why there is a great practical and on principle fundamental interest to investigate an opportunity of making the devices which could be used for measuring the absorption of some gases in IR optical range and also would not have such disadvantages as construction complexity, cumbersome embodiments, difficulty in controlling and obligatory presence of the moving parts (mirrors, prisms or gratings). The main element in a sensor of the type proposed is a liquid-crystalline cell, possessing marked dichroism of optical absorption in some spectral range (IR region). By fitting the absorbance and alignment parameters of the LC cell, it is possible to create the effect of the absorption edge shift when varying the magnitude of electric field applied. This leads to some kind of wavelength modulation of an external optical radiation in the pre-determined spectral range. Adding an appropriate light detector to the system described, one can obtain a miniature scanning spectral instrument (sensor) of a liquid crystal type. LC cells with absorption bands in spectral range near 3 urn where some atmospheric hydrocarbons absorb were investigated. The technique for reducing true external spectrum from the signal detected after the modulation process was developed.