Elevated alumina content optical fibres for the mid-infrared transmission

Blaser, Dunia Beatriz (2023). Elevated alumina content optical fibres for the mid-infrared transmission. (Dissertation, University of Bern, Faculty of Science)

Text (PhD Thesis) Dunia_Blaser_Lopez_PhD_Thesis_2023_Boris_UniBe.pdf - Submitted Version Restricted to registered users only Available under License BORIS Standard License. Download (19MB) | Request a copy

Optical fibres have brought innumerable improvements in technology, not only in data
transmission, but also in the realisation and continuous improvement of devices such as lasers,
amplifiers, resonators and other applications. These devices are widely used in research,
industrial sensors, and medical imaging to name just a few. In this context, optical fibres specialising
in the transmission of light in the mid-infrared (range), capable of withstanding temperatures
above 1000°C and still maintaining their optical and mechanical properties, requires
a proper material selection.
Glass science has been extensively studied, with the introduction of rare-earth elements
(i.e., ytterbium, erbium) and passive materials (i.e., alumina) has led to new applications
in laser systems and new research opportunities within the mid-infrared transmission
range. For example, the investigation of innovative and better realisation techniques of optical
fibres with high doping percentages of elements. However, the majority of existing production
methods, which focus on the realisation of alumina doped fibres, have produced optical fibres
with limited concentrations of alumina content in both the core and cladding regions, or they
have a narrow control over the core/cladding ratio and structure, fibre dimensions, homogeneity,
and flexibility to tailor the index step between regions.
Key issues that have not been sufficiently discussed in previous studies by other authors
are: i) the diffusion phenomenon that takes place during fibre drawing, and ii) the challenging
task of making (drawing) optical fibres with high alumina content. Due to the different
melting points of silica (host glass) and alumina, these materials behave differently when
exposed at temperatures above 2000°C.
But why is it necessary to produce optical fibres with the highest possible alumina
content? The addition or high concentration of alumina, which results in aluminosilicate glass,
can improve the optical properties of the optical fibres by extending the transmission range
up to 5μm wavelength (mid-infrared). This extended range minimises signal loss (attenuation),
increasing the refractive index of the doped region (supporting more modes of light)
[1], increasing the solubility of rare-earth elements in a silica matrix, and retarding the photodarkening
effect [2]. Mechanically, alumina increases the tensile strength of the optical fibre,
allowing it to withstand higher levels of stress and strain, improving its resistance to bending
and durability. Also with its high melting point, the optical fibres are able to withstand elevated
temperatures without mechanical or optical degradation.
In light of the existing limitations in the production methods and material properties
of alumina-doped optical fibres, this research seeks to address the following fundamental
research question: How can the production process and material composition be optimised
to achieve optical fibres with significantly higher alumina concentrations, while maintaining
their optical and mechanical properties?
Within the present cumulative thesis, a number of improvements designed to enhance
the optical performance and durability of high alumina content optical fibres will be explored.
Focusing on expanding their potential applications by improving the transmission in the
mid-infrared region, or in high power lasers, amplifiers, implementation in thermal environments
(temperatures above 1000°C), and by tailoring the Refractive Index (higher index in
core) reduce the Numerical Aperture of the fibre.
One of the main objectives of this research is to develop a technique called Green Compacts.
This method enables the production of optical fibres with increased concentrations of
aluminium oxide (Al2O3) and a tailored refractive index in both the core and cladding regions,
which consequentially allows control over the Numerical Aperture of the optical fibres, resulting
in an improved spectral range and mechanical properties. An in-depth evaluation of
the impact, reproducibility and implementation as a process for rapid prototyping of optical
fibres will be demonstrated for widespread production and realisation of novel fibre designs,
confirming the significance and scope of the current research.
Another important aspect of the current study is to test different drawing parameters
and define their suitability for each preform assembly technique. These have been used in the
fabrication of the optical fibres presented in this research. An optimization of the production
process and preform assembly is to be implemented through experimentation. Leading to the
successful realisation of optical fibres with elevated alumina content and enhanced performance
and transmission characteristics.
Additionally, this research aims to investigate the maximum alumina content achievable
in the core and cladding regions of optical fibres. By reviewing and interpreting the fibre
characterisation data, production challenges that hinder the realisation of fibres with a continuous
core/cladding structure are to be identified and addressed. The impact of the diffusion
phenomena on the final properties such as refractive index of the produced optical fibres will
be studied. With proposed alternative steps to minimize the impact of such effect and ensure
the optimal performance of the fibres.
By measuring and comparing the transmission of the aluminosilicate core optical
fibres optimized for the mid-infrared, a comprehensive understanding of their capabilities
within the specific spectral range has been achieved. By using different types of commercially
available silica tubes, the transmission between 2µm and 3µm wavelength can be significantly
increased, with the aim of widening the range of applications and opportunities for optical
fibres in medical research.
In this thesis will detail a general context of optical fibres, including history and material
selection. Contained within the supportive Chapters of the current thesis, an insight
into the achievements of fibre production, tailoring of refractive index, transmission measurements,
as well as material structure is to be depicted. Supportive material and information is
also included in the Appendix section.

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