UV-transparent optical fiber coatings

Abstract

This thesis describes the design and synthesis of a thin-film optical fiber coating that is up to 50 % transparent to ultraviolet (UV) light at the wavelength emitted by KrF excimer lasers (248 nm), for a coating thickness of 80 um. This allows fiber Bragg gratings (FBGs) to be written directly through the coating into optical fibers, causing a permanent periodic refractive index modulation in the fiber core.

Initial efforts were directed towards analysis of existing commercial coating formulations to determine the typical composition of optical fiber coatings. Aliphatic poly(urethane acrylate) oligomers (PUAs) were synthesised and used in the preparation of UV-curable formulations. However, the resulting polymer films were opaque at wavelengths below 300 nm, owing to the presence of highly absorbing photoinitiators. Optimisation of the coating formulation involved changes to the concentration of photoinitiator, which was optimised from 2.00 wt. % to below 0.1 wt. %. The resulting polymer coating exhibited up to 18 % transparency at 248 nm. A long-chain aliphatic PUA analogue was successfully synthesised with a lower degree of urethane-containing hard domains, leading to the preparation of a polymer coating that exhibited up to 40 % transparency at 248 nm.

Changes in the class of photoinitiator used during photopolymerisation highlighted the significance of its absorptions on the UV transmission spectra of polymer films, as spectral bands corresponding to photoinitiator transitions could be seen in coating spectra. Thus, efforts were directed towards the design of novel acylphosphine oxide (APO) photoinitiators, which have their electronic transitions shifted away from the wavelength emitted by KrF excimer lasers (248 nm). A computational study found that para- electron withdrawing groups shifted the wavelength of π→π* transitions away from 248 nm in benzaldehyde photoproducts. The most promising candidates for low absorbing APOs were synthesised and used to initiate the photopolymerisation of acrylates. The benzoyl function was the focal point for structural modifications because it produces the least reactive and most strongly absorbing radical upon photolysis. However, a significant increase in transparency was observed upon functionalisation of the phosphine oxide function of an APO, to introduce aliphatic groups.

The UV-transparent optical fiber coating was produced on a 100 g scale and used to coat a single-mode germanium doped silica optical preform, which was successful, and the coating exhibited a high degree of cure (90 %). An array of FBGs were successfully inscribed to the coated fiber at 248 nm using a phase mask technique, forming gratings with 30 % reflectivity. It was reported that the fiber handled well in terms of its physical characteristics and was comparable to current commercial optical fiber coatings, although slightly rubberier in nature. The mechanical properties of PUA coatings were measured and optimisation of the glass transition temperature (Tg) of the low absorbing oligomer was achieved by further modifications to the oligomer structure. This was successful, however transparency was reduced by 10 %. Future work should be directed towards the removal of the moderately absorbing π-bonds within the coating formulation, through the synthesis of rigid, saturated organic coatings.

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Identifiers

ORCID for David Harrowven: orcid.org/0000-0001-6730-3573

ORCID for Graham Langley: orcid.org/0000-0002-8323-7235

ORCID for Sam Thompson: orcid.org/0000-0001-6267-5693

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