Modeling of Performance of Mid-Infrared Gas Sensors Based on Immersion Lens Diode Optopairs G.Yu.Sotnikova, S.E.Aleksandrov, G.A.Gavrilov, A.A.Kapralov, B.M.Matveev, M.A.Remennyy Ioffe Physico-Technical Institute 26 Polytekhnicheskaya St.Petersburg, 194021, Russian Federation
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[email protected] Abstract-We present model for determination of performance of optical sensor that takes into account fine structure of gas absorption band and specific features of immersion lens LEDs and photodiodes operating in the mid-IR spectral range (3-5 μm). The model incorporates an instrument and transfer functions that enables evaluation of accuracy, sensitivity and limit of detection (LOD) of various optical sensors. The predictions of the model were compared with the experimental values obtained with the CO2 analyzer based on immersion lens LEDs and photodiodes operating near 4.2 μm spectral band. I.
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SENSOR INSTRUMENT FUNCTION
Optical gas sensors belong to a class of nondispersive spectrum devices so their instrument function is determined by spectral characteristics of the elements, primarily radiation sources and detectors. In the present study we considered diodes fabricated through the use of flip-chip technology as they exhibits best performance compared to conventional point contact devices. Low thermal resistance, uniformity of the spatial emission distribution, low serial and contact resistance at high contact reflectivity are the key properties that make them brighter than any other LED types in the mid-IR region [1]. In addition to this the existence of a contact free surface area of the chip furnishes an additional important advantage, specifically, the possibility of immersion coupling of a LED or/and photodiode to optical lenses. The former suppress total internal reflections with the result of 3-5 fold efficiency increase if high refractive index glue is used [2]. In the latter case, it is possible to obtain PDs with a large effective sensitive area, in which the detectivity increases by a factor (Aopt /Аp-n)1/2 , where Aopt – is the photosensitive area and , Аp-n – is the area of the p–n junction (or mesa). One of the important consequence of the flip-chip bonding (or in other words, creation of backside illuminated devices) is narrow spectral response associated with the radiation absorption in intermediate layers and thick substrate as well as in the lens. Sharp sensitivity drop at the longwave side at hv
