Validating one single model which describes photosensitivity in germanosilicate fibres leads to difficulties as there is usually more than one mechanism involved. Moreover, the fibre (type, doping, temperature), radiation (wavelength, power, fluence) or
16 3. Theory
Figure 3.2: Plot of the maximum (saturated) photoinduced index change observed in Corning SMF-28 fibre as a function of processing time under the flame brush.
Reproduced from Ref. [15].
preprocessing (as described above) can all play a role in causing the changes that occur in any given sample.
The current consensus explains photosensitivity as being initiated through the forma-tion of colour-centres [24] and compacforma-tion of the irradiated glass [22]. However, both of colour-centres and compaction are themselves not fully understood yet. Moreover, whether colour-centres or compaction serves as the dominant effect is determined by reference to the range of experimental factors considered above, which obviously vary from case to case.
The theories underpinning these two models are described below. For a wider overview over different photosensitivity models and these two in particular refer to [35].
3.1.2.1 The Colour-centre Model
A colour-centre or F-centre (derived from the German wordFarbzentrum for colour-centre) refers to where there is a missing anion causing a vacancy that is filled with one or several electrons. The resulting oxygen deficiency in the silicate glass matrix is called Ge-Si or Ge-Ge “wrong bond”. Color-centres cause the atom to absorb photons of a different frequency (colour) after irradiation. Such defects are important where optical fibres are concerned because their absorption bands cause deleterious transmission losses. Colour-centres are also responsible for non-linear optical fibre transmission, where the transmission changes over time and with light intensity.
Consequently, a great deal of research has been directed towards minimising the
3.1. Photosensitivity 17 formation of colour-centre defects in glass. With their link to fibre Bragg gratings, however, they are playing a big role within fibre optics and the colour-centre model has received a great deal of attention.
According to the colour-centre model of photosensitivity, the colour-centre defects that are formed during the fabrication process of germanosilicate fibres are responsi-ble for their intrinsic photosensitivity by absorbing UV radiation in the wavelength range between 240-250 nm (∼5 eV), which leads to bleaching of the colour-centres and a growth of absorption features at shorter wavelengths (see Figure 3.3).
200 220 240 260 280 300 320 340 360
0
Figure 3.3: Transmission loss spectrum of a fibre preform before and after irradi-ation with 244nm UV light. Reproduced from Ref. [34].
The colour-centre model was first proposed by Hand and Russell in [24]. They dis-covered that photoinduced changes in the material properties of the glass introduce new localised electronic excitations and transitions of defects. Due to their strong op-tical absorption, it is contended that it is precisely these colour-centre defects which give rise to the change in the refractive index associated with photosensitivity. The bleachable wrong bond defects, that initially absorb the light, are transformed into defects that are more polarisable by virtue their electronic transitions take place at longer wavelengths or that they have stronger transitions.
Any change in the refractive index is associated with the photoinduced change in absorption trough the Kramers-Kronig relation, given as
∆nef f(λ) = 1 whereP is the principal part of the integral, λis the wavelength, and αef f(λ) is the effective change in the absorption coefficient of the defect which is given by
∆αef f(λ) = (1/L)
Z L
0 ∆α(λ, z) dz (3.2)
18 3. Theory whereLis the sample thickness. In this manner, it is factored in that the bleaching beam is strongly attenuated as it passes through the sample, and therefore bleaching does not occur uniformly with increasing depth. A Gaussian distribution provides an appropriate model for ∆αef f(λ) and the Kramers-Kronig relation in Equation 3.1 on the previous page may be used to calculate the change in the refractive index induced by bleaching of the absorption bands.
Many studies have indeed reported index changes that are consistent with estima-tions made by means of the colour-centre model, thus supporting its application as a model suitable for assessing photosensitivity [18].
Also supporting the colour-centre theory are findings that the bleaching of the 240 nm band can be reversed by heating the fibre. It is known that thermally acti-vated processes serve to anneal colour-centres by exciting the trapped electrons out of the anion vacancy sites. A few groups report to have written and then completely erased gratings by annealing at temperatures above 800 [13, 18].
3.1.2.2 The Compaction Model
Albert et al. have reported in [11] a growth rate of fibre Bragg gratings that was linearly proportional to the 193 nm laser pulse energy density in fibres highly doped with germanium, but proportional to the square of the pulse energy density for standard telecommunication fibres with low germanium concentration. Furthermore, the achieved index changes in the latter case were an order of magnitude greater than previously achieved in fibres without sensitisation treatment. Further experiments with fibres treated to entirely remove germanium oxygen deficiency centres ruled out both E’ colour-centres and oxygen deficiency centres as a cause for the index change.
A model that explains the change in refractive index considering the mentioned observations is the model of densification or compaction of the material when being irradiated with high energy light.
According to the compaction model, exposure of SiO2 glass to radiation can cause a local increase of density i.e. volume compaction. Since the refractive index is known to be linked to the material density (see Figure 3.4 on the facing page), a local increase in density will, in general, lead to a local increase in refractive index.
This increase can be estimated by means of the Lorentz-Lorentz relation
∆n
n = ∆ρ
ρ (1 + Ω)(n2−1) (n2+ 2)
6n2 . (3.3)
For densified silica glasses the relationship between refractive index and density has been found to be linear as shown in Figure 3.4 on the next page
From [42] we obtain the functional relation for n(ρ) to be
n(ρ) = 1.037 + 0.195·ρ. (3.4)
3.1. Photosensitivity 19
2,15 2,20 2,25 2,30 2,35 2,40 2,45 2,50 2,55 2,60
1,46 1,48 1,50 1,52 1,54
Refractive Index
Density (g/cm3)
Figure 3.4: The relationship between density and refractive index in silica glass.
Reproduced from Ref. [42].
Primak established a mathematical description of the compaction κ depending on the deposited dose D in the material [40]. Compaction is hereby defined as the negative change in volume
−κ= V0−V
V0 = ∆V
V0 (3.5)
where ∆V is the difference between the original volume V0 and the present volume V after the process of densification. According to Primak’s studies, the compaction follows a 2/3rds power relation of the deposited dose:
κ=C·D2/3 (3.6)
where C denotes a constant factor and D is the dose profile. The compaction occurring in fibre Bragg gratings in germanium-boron codoped fibres and hydro-genised fibres was observed by means of AFM measurements in research completed in 2002 [43].
The exact mechanism by which compaction occurs is not fully understood, however it can be taken for granted that radiation-induced volume compaction is achieved by a two-photon effect and is a result of reorganisation of existing bond structures rather than bond rupturing or the formation of defect centres. Research by different groups [16, 33] indicate that a relaxation process may provide a means of crossing an energy barrier and thus allowing the system to be in a lower free-energy state.
The processes leading to compaction appear to be triggered by ionisation events that occur when samples are exposed to radiation with an energy high enough to bridge the SiO2 band gap of ∼8 eV. This provides compelling arguments for writing re-fractive index structures at shorter wavelengths via a compaction mechanism rather
20 3. Theory than the common method using the 240 nm (∼5 eV) absorption band. Whereas this requires pretreatments of the fibres to increase their photosensitivity as well as post-annealing in order to prevent Bragg wavelength shifts, index changes can be induced by compaction in untreated fibres and tend to be more stable.