Hollow Core optical fibre for the mid infrared

  • Adam Earthey

Student thesis: Doctoral ThesisPhD


Over the past decade there has been a significant amount of work in developing hollow core optical fibres made of silica glass that have a core surround that has a negative curvature. This continued development is in order to allow silica fibres to transmit at wavelengths where the material absorption is too high to guide with traditional step index fibres. These hollow fibres do not guide by total internal reflection as the refractive index step is the wrong way around. They guide light by having a high reflectivity at the core boundary that keeps the light guided in the air core of the fibre. These hollow core designs mean the light has
to interact less with the highly absorbing glass, and has allowed their transmission to be pushed in the ultraviolet and infrared.

This work takes an alternate approach to the usual methods employed to overcome current fibre performance limits. Rather than developing a new cross sectional design that has lower fibre losses, or by fabricating current designs as perfectly as possible, or using different glasses altogether, instead we will incorporate alternative materials into the region surrounding the core by using a chemical deposition process. These multi-material fibres allows us to circumvent the attenuation limits of silica glass for guiding in the mid infrared spectral range.

I will use the theory of Bragg fibres to model a system of concentric dielectric and air rings that guide by the same high reflection mechanism as silica hollow negative curvature fibres and will show that the values of attenuation for this model can be used to compare to the measured attenuation of fabricated silica hollow core negative curvature fibres. I will investigate the attenuation behaviour of a composite material guiding wall structure comprised of silica and sapphire (Al2O3) at 5 µm wavelength, as sapphire is the material we are able to deposit with our facilities. At 5 µm the material loss of silica is extremely high, and it will be shown that it is the dominant contributor to the attenuation of the fibre, even
when the silica is only 5% of the guiding structure.

Sapphire has a higher refractive index and a lower material absorption than silica, so I will explore how the higher refractive index and lower absorption each contribute independently to the improvements of attenuation to help gain insight into the underlying balance between confinement loss and material loss.

Atomic layer deposition experiments were carried out that confirm the feasibility of depositing dielectric materials inside hollow optical fibres but find that with the current equipment available the penetration depth encounters severe diminishing returns and only 25 mm of fibre length was able to be coated.

I will also give an outline of the fabrication process used for making these silica negative curvature fibres. The main challenge is that the fibre structure scales with the guiding wavelength, so the fibres become larger for guiding in the infrared. This becomes a problem for the standard two stage stack and draw technique that draws the fibre to an intermediate size, before drawing down to fibre. I fabricated a fibre with a 100 µm core that has a guidance band from 2.7 to 4 µm wavelength using the two stage technique. To draw a fibre with a 200 µm core that guided at 4.6 µm at 1 dB/m, I needed to develop a new pressurisation technique that allowed me to draw straight to fibre, without an intermediate stage.
Date of Award14 Feb 2022
Original languageEnglish
Awarding Institution
  • University of Bath
SponsorsSpectrum Technologies
SupervisorAndrew Johnson (Supervisor) & William Wadsworth (Supervisor)

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