Different kinds of optical ammonia concentration sensors, based on monitoring the absorption or fluorescence characteristics of sensing films deposited onto an optical fiber, have been reported [8-9]. An integrated optical ammonia sensor based on a Y-junction has been reported in . This sensor employs a deposited sensing film whose absorbance (dependent on ammonia concentration in the surrounding ambient) is continuously measured and exhibits a detection limit around 1ppm. Ammonia concentration estimation by this sensor implies an optical power differential measure at the output of the two Y-junction arms.In recent years, integrated optical sensors have attracted considerable attention because of their immunity to electromagnetic interference, high sensitivity, good compactness and robustness and high compatibility with fiber networks .
A great variety of guided-wave optical sensors has been proposed, such as those based on directional couplers , Mach�CZehnder interferometers , grating-assisted couplers , and optical microcavities .In particular, optical microring resonators, widely used in add�Cdrop filters, optical switches, ring lasers and WDM multiplexers, are showing very attractive features for sensing applications, permitting to realize highly sensitive immunochemical optical biosensors [16-18]. In this paper we design, optimize and 3D simulate an integrated optical microring resonator-based ammonia sensor. Device sensitivity dependence on waveguide optical and geometrical parameters is investigated. Sensor detection limit is also analyzed.
2.?Sensing PrincipleThe architecture of a very compact microring resonator-based sensor in Silicon on Insulator (SOI) technology is sketched in Figure 1(a). A ridge structure has been adopted as waveguide (this kind of sub-micrometer guiding structure is also indicated as silicon photonic wire), as in Figure Brefeldin_A 1(b), and Polymethylmethacrylate (PMMA) doped with Bromocresol Purple (BCP) has been used as cladding layer.Figure 1.(a) Ammonia sensor architecture. (b) Ridge guiding structure adopted in sensor design.Optical absorption changes of PMMA-BCP system, due to interaction with ammonia, has been proved  either when PMMA-BCP is exposed to dry ammonia (in this case PMMA-BCP sample is put in a chamber filled by ammonia diluted with pure nitrogen to a molar concentration of 5%) or when PMMA-BCP is exposed to a vapor of conventional medical ammonia spirit (65% alcohol).