Abstract
M.Ing.
Bragg gratings exist due to permanent changes in the refractive index introduced in
the fibre through exposure to ultraviolet radiation. Standard telecommunications fibre
(SMF-28) is only slightly photosensitive at 248 nm irradiation. A comparative study
of different photosensitization techniques is undertaken on the basis of the desired
photosensitivity characteristics and efficiency of the technique used.
The analytical method is used to determine valuable properties of fibre Bragg
gratings. The bandwidth and maximum reflectivity of the fibre Bragg grating is
dependant on the change of refractive index and is determined analytically. Obtaining
very high reflectivity and bandwidth (~lnm) can only be achieved with highly
photosensitive optical fibre where refractive index changes of~ 1 o-3 are possible.
The change of the refractive index in the optical fibre is related to the phase change
by:
A new interferometric measurement technique based on this assumption allows the
direct measurement of photosensitivity. This technique is modelled with a modified
Layer Peeling algorithm.
Standard telecommunications fibre was exposed to high-pressure (26B - 160B)
hydrogen for several days. This increased the photosensitivity of the optical fibre
significantly. The photosensitivity of the fibre is directly dependent on the hydrogen
concentration inside the fibre. Refractive index changes, M ~ 1.3x10-3 were achieved
in germanium doped fibre and M ~ 5x10-3 in germanium/boron codoped fibre. The
knowledge of the hydrogen concentration inside the fibre is important in studying
photosensitivity, transmission losses and the wavelength drift after Bragg grating
manufacturing. The diffusion proceeds interstitially with no significant chemical
interaction. A hydrogen diffusion model was developed based on the transfer of heat
between two objects.
Although the photosensitivity phenomenon was discovered in optical fibre more than
20 years ago, no complete physical explanation exist for it at present. We agree that
stress relaxation and/or compaction are the main reasons for photosensitivity in
optical fibre but also that it is still a complex and multifaceted phenomenon. A study
of the thermal decay of fibre Bragg gratings suggests that fibre Bragg gratings written
in hydrogen loaded fibre is less thermally stable than gratings written in germanium
doped fibre. The analysis of accelerated ageing will predict the thermal stability of the
Bragg grating over time.