Optical and mechanical design of an InP-based tunable detector for gas-sensing applications

Abstract

Ithas been shown that it is possible to produce highlyselective and continuously tunable filters based on InP material usingsurface micro-machining. One interesting issue for this kind of deviceis NIR absorption spectroscopy for gas analysis. In this work,we present the design of a Resonant Cavity Enhanced tunablephotodiode for operation around 1.6 micrometer near the C-H stretchingfrequency for organic molecules such as benzene. For this typeof application, the required performances are a large tunability, ahigh selectivity, a weak temperature dependence and a constant absorptionlevel over the tuning range. To meet these requirements themicro-system must be optimized from the optical and mechanical pointof view. The RCE photodiode structure is composed of anair/InP bottom Bragg mirror and a dielectric top Bragg mirror.The cavity includes an air-gap and the InP layer containinga p.i.n. photodiode with absorption in a few strained InGaAsQuantum Wells (QWs). Tuning is obtained by actuating electrostatically theair micro-cavity thickness. A prospective device meeting the optical requirementshas been designed. It is based on an absorption regioncomposed of three InGaAS QWs conveniently located in the cavitystanding wave pattern in order to optimize the resonant absorptionover the tuning range. Optical simulation shows that an absorptionlevel greater than 50% can be achieved. The temperature dependenceof the resonance wavelength can be kept below 0.08 nm/(Delta)T(C degrees) at room temperature. The mechanical properties of themicromachined structure has been investigated using finite element analysis. ©2003 COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.