Gas sensors using single layer patterned interference optical filters

Abstract

Abstract A method for fabricating filters for fiber optic sensors is presented. The interference filter's construction is laid on it's side to allow for the use of air as the low refractive index material. Bandpass filters tuned to the absorption line of a trace gas can then be used as a sensitive means of detecting gas concentration. Complex filter designs can be fabricated in a single patterned layer. A Cl O LCe 4 gas sensor is presented as a design example. Keywords: fiber optic sensors, patterned filters, interference filter design 1: Introduction Optical filtersI passive optical switches and polarizing elements are used extensively in a wide range of sensors 1IOIP and sensing technologies for medicalI environmental and threat detection. The design of a complex optical interference filter used to detect a specific chemical can involve 100 or more discrete alternating layers of high and low index materials. The cost of these filters can be quite high owing to long deposition times and limited yield. te are developing an approach which will allow these filters to be deposited as a singleI printable layer for use in fiber optic and wave guide based sensor systems. rsing emerging 3D printing techniquesI extremely low costI high volume fiber optic sensors can be fabricated. Complex visible through far infrared filters consisting of hundreds of discrete layers can be printed in a single layer deposition. The corresponding low index material can be airI or a second deposition, or both with some layers left as air while other layers are added to provide for other design considerations such as ultra-narrowI high optical density reflection notches and stable performance over a wide range of incidence angle. There are several advantages to using air as the low index layer. crom a design perspectiveI air is non-dispersive and offers the highest index contrast with the material chosen for the high index layers. By designing the filter to pass or reflect at a specific trace gas absorption bandI the transmission or reflection of the open air filter design is a sensitive function of the gas absorption. This paper presents filter construction and a study of design trade-offs for these open air printed (OAP) filters. Optical interference filters consist of multiple groups of high and low refractive index materials. By precisely controlling the thickness of each layerI reflected light from each interface can constructively or destructively interfere to produce unique spectral performance as a function of wavelength