Observation of mode coupling in bitapered air-core photonic bandgap fibers

We report the fabrication and characterization of the first bitapers in air-core photonic bandgap fibers. Like conventional fiber bitapers, these devices are found to exhibit quasi-sinusoidally modulated transmission spectra, the signature of resonant coupling to a dominant cladding mode. However, unlike in conventional tapers of comparable length, the modulation period is very short (0.5–3.3 nm), which suggests that coupling takes place to a very high order cladding mode. Such coupling is observed even in comparatively weak bitapers (diameter reduction as low as ∼2%), indicating that air-core fibers are highly sensitive to tapering. The loss of weak tapers is observed to be small (<0.3 dB), which suggests possible applications of this simple technology as in-line filters of high resolution.