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

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 low-concentration gas-measurement systems for the semiconductor manufacturing industry.^8–13

The idea of using MOFs for gas sensing is not new. Data has been obtained on the biosensing and detection of acetylene,³ hydrogen,⁴ and methane⁵ using photonic crystal fibers. PBGFs were used for acetylene detection by Ritari et al.⁶ in 2004 and Petrovich et al.⁷ 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 cross-section 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 peaks around the 1500nm wavelength. Accordingly, a 1550nm emission wavelength tunable laser was paired with a PBGF of a 1550nm center operating wavelength and 10.9μm core. Laser light was introduced to the core of the fiber where the test gas flowed continuously under known parameters. Figure 1 shows a gas flow diagram of the sensing device.

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.

The light was partly absorbed by the gas within the optical fiber. This absorption was directly proportional to the concentration of NH₃ and CO₂ in the carrying gas (high-purity nitrogen). By adjusting the flow rate in the gas cell, we were able to produce clear absorption peaks that allowed accurate quantification of the investigated molecules. The optical system is detailed in Figure 2.

Figure 2. Photograph of the optical system showing laser light and sample inputs, optical fiber, and signal sensing.

Results of the test using 10ppm NH₃ and a 1m length of PBGF with a 10.9μm core are presented in Figure 3. The maximum theoretical sensitivity using 1m fiber ranged to 0.18ppb. The sensitivity tends to grow with increasing fiber length.

Figure 3. Absorbtion peak of 10ppm ammonia in a 1m PBGF with core diameter 10.9μm. a.u.: Arbitrary units.

For continuous monitoring at parts-per-billion levels, the development of larger core diameter PBGFs to improve the gas flow conditions seems to be crucial. Proper cutting of the PBGF was also important for accurate results. After removal of the coating, the length of the fiber was adjusted using several methods. The best results were obtained using argon ion beam cutting (see Figure 4). The argon ion beam method may be especially advantageous for precise input of the light into the fiber core and for preventing surface reflections, which can influence the measurement spectra.

Figure 4. Comparison of PBGF cutting methods. (A) Fiber cleaver. (B) Argon ion beam. Laser microscope, 1000(×).

The PBGF-based sensing device described above represents a cheaper and more compact alternative to current methods (such as those based on Fourier transform infrared spectroscopy). Despite several technical problems, including fabrication of the larger core fiber, PBGFs have a huge potential in gas sensing. For instance, in addition to the detection of trace levels of pollutants, fibers could be employed in biosensing substances present in breath and body odors, or in noninvasive diagnosis of cancer and other diseases.

This research was supported by a Grant-in-Aid of the Japanese Ministry of Ecology and Natural Resources. The authors would like to thank Lev Zimin and Tatsuhiko Saitoh for fruitful advice and help with experiments.