A low-concentration gas sensor based on the photonic bandgap fiber cell

Abstract

Microstructured optical fibers (MOFs) using photonic crystals described in the pioneering research of Yablonovitch1 and John2 were proposed in the 1990s. MOFs are a group of pure silica light-guiding fibers in which air holes running along their lengths (i.e., in the axial direction) are introduced to the fibers. Photonic bandgap fibers (PBGFs) are MOFs composed of a honeycombed arrangement of air holes in the microstructured cladding surrounding a hollow core. In PBGFs, light is propagated along the core due to the so-called photonic bandgap phenomenon, resulting in low bending losses. PBGFs are already commercially produced. Our research investigates the microcapillary gas flow phenomenon inside these fibers and examines various properties of PBGFs in general. We are interested in their optimal design, effective cutting and splicing techniques, and applications in lowconcentration gas-measurement systems for the semiconductor manufacturing industry.3–8 The idea of using MOFs for gas sensing is not new. Data has been obtained on the biosensing and detection of acetylene,9 hydrogen,10 and methane11 using photonic crystal fibers. PBGFs were used for acetylene detection by Ritari et al.12 in 2004 and Petrovich et al.13 in 2005. There are still relatively few reports using PBGFs, however, and most of the measurements listed here have been carried out in uncontrolled flow conditions. In the PBGFs used in this study, up to 65% of the fiber crosssection consisted of solid undoped silica, but less than 5% of light propagated in the glass. More than 90% of optical power was located in the hollow core or in the holes of the cladding. We tested our design with ammonia (NH3) and carbon dioxide (CO2) gasses. NH3 and CO2 absorption spectra show several Figure 1. Schematic diagram of the gas flow system. (1) PBGF, (2) gas cell, (3) gas tank, (4) gas-mixing device, (5) rotary pump, (6) pressure control.