The propagation loss of a fiber can be
increased by coupling core mode and surface mode
which will deteriorate the performance of photonic
bandgap fiber (PBGF). In this paper, we presented an air-
core PBGF for gas sensing applications. By designing Λ =
2.63 μm, d = 0.95 Λ, and R core = 1.13 Λ, where Λ is the
distance between the adjacent air holes, the fiber was
single-mode, no surface mode was supported with fiber,
and more than 90% of the optical power was confined in
the core. Furthermore, with optimizing the fiber structural
parameters, at wavelength of l = 1.55 μm that is in
acetylene gas absorption line, significant relative sensitiv-
ity of 92.5%, and acceptable confinement loss of
0.09 dB/m, were simultaneously achieved.

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• Improved gas sensor with air-core photonic bandgap fiber