Absorption spectroscopy of xylene with a PDMS gas concentrating layer on SOI waveguides

Abstract

The presence of chemical substances on the environment, such as xylene, has harmful effects on human health. Exposure to xylene to humans causes symptoms in a range from dizziness to death, being crucial to monitor it. Silicon-on-insulator (SOI) waveguides and absorption spectroscopic technique for gas sensing presents an interesting alternative due to its high potential of provide miniaturized, compact and high sensitivity devices. Moreover, the use of gas pre-concentrating chemical films is presented as an alternative approach for enhanced gas detection using optical waveguides. In this work, we investigate gaseous xylene absorption in the near-infra-red region (NIR), exploiting evanescent field absorption (EFA) on SOI waveguides covered by a preconcentrating polydimethylsiloxane (PDMS) film. Estimations of absorption cross section and the PDMS enhancement factor have been made. It has been experimentally observed that 5000 times gas density enhancement is achievable with the PDMS films. The potential benefit of gluing fibers on the grating couplers has been studied and 5 times improvement in the measurement stability with glued fibers has been observed. However, in order to maintain a better overlap between the grating transmission spectrum and the absorption band of xylene, most experiments were run without gluing the fibers. By using a 350nm PDMS film on a 1.2 cm long SOI waveguide, an expected absorption behavior has been observed at a concentration of ∼ 4000ppm gaseous xylene; which is in a close agreement with an estimate taken with approximately 5000 times PDMS enhancement factor. A rising absorption trend has been observed in the region beyond 1665nm agreeing with the theoretical xylene absorption band. An absorption measurement for a drop of liquid xylene on a PDMS coated waveguide has been also carried out which gave a further confirmation on the absorption trend observed at ∼4000ppm xylene vapor. It is, however, observed that ensuring measurement and environmental stability can be critical in absorption experiments. For instance, in one experiment, the measured absorption appeared to extend outside the expected region likely due various factors such as mechanical stability and temperature contributing to the system noise. Future studies may focus on minimizing the measurement noise and lowering the gas detection limits.