However there is a way to get around this problem…
The emergence of hollow core photonic bandgap fibers (HC-PBGF) towards the end of the 20th century greatly enriched our understanding of optical waveguides and expanded the horizon of specialty optical fibers. While the basic material is still silica glass, more than 99% of the light can be efficiently guided in an air core. Wheeler and colleagues have now reported significant progress in the fabrication of low loss HC PBGF for long wavelength transmission in the mid-infrared (MIR). They have successfully reduced the lowest recorded attenuation level of HC-PBGF from 1 dB/m (reported by Urich et al. in 2012) down to 0.05 dB/m in the MIR (beyond 3 μm), by adopting a larger core size of 50 µm (a 19-cell HC-PBGF is used). This fiber exhibited a transmission window from 3.1 μm to 3.8 μm and the reported minimum loss was obtained after a purging process was used to remove HCl molecules (generated in the course of fibre fabrication) from the hollow core.
Compared with the hollow core negative curvature fibers (HC NCF) reported by Yu et al. in 2012, which have already achieved even lower loss and broader transmission window in the MIR, a key advantage of this HC-PBGF reported by Wheeler and colleagues lies in its resistance to bending losses. The authors report that, even when HC PBGF was bent in a coil of 5 cm in diameter, the bend losses of the fabricated HC-PBGF were less than 0.5dB/turn over the entire transmission spectral region and less than 0.25dB/turn over a 300 nm bandwidth. Such low bend losses allow increased flexibility and accessibility in applications such as high power laser delivery, potentially an important application area.
The authors also expect that, by enlarging the core to a 37 cell defect, the losses can be further reduced without increasing the bend sensitivity, because the reported 0.1% overlap between the guided mode and the silica cladding would decrease even further.
The combination of a relatively low attenuation and bending losses will add more flexibility and accessibility in practical applications of these silica based hollow core optical fibres in unconventional long-wavelength regimes, breaking those boundaries that have for too long limited the development of optical fiber devices and systems in the mid-infrared. This new world that is being explored may lead to unpredictable discoveries and novel applications in the field of optical sensors, optical beam delivery and optical communications.
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