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Low temperature (80°C) hypersensitised hydrogen-loaded phosphosilicate optical fibre is found to be unstable, decaying progressively at room temperature. However, the hypersensitisation process linearises the grating growth characteristic curve. Further, a negative index contribution is inferred at low fluence in the presence of hydrogen.
©2001 Optical Society of America
1. Introduction
Hypersensitisation is a process recently developed where the intrinsic glass photosensitivity of waveguides is permanently enhanced with hydrogen and a preliminary pre-exposure [1,2]. Subsequent out-diffusion of the hydrogen leaves behind an optical waveguide with an enhanced photosensitive response more suited for component fabrication for a number of reasons. Issues such as unwanted absorption bands, hydrogen out-diffusion and an unstable index change contribution are removed by this technique [3–5]. Most of the means of hypersensitisation have involved photolytic irradiation with UV light from various laser sources, including pulsed exciplex (ArF - 193nm, KrF - 248nm), pulsed excimer (F2 - 157nm), and CW frequency doubled Ar+ lasers (244nm) [1–10]. The waveguide material analysed to date and found to be responsive to the treatment includes pure silica, germanosilicate with and without boron and phosphosilicate, emphasising the generality of the hypersensitisation process. The underlying benefit of this process is to remove the undesirable component to index change that is present under conventional exposure of UV irradiation of optical waveguides loaded with hydrogen. This additional index change arises from stresses at the core/cladding interface that increase as densification takes place at the core [11].
More recently, low temperature hypersensitisation of phosphosilicate waveguides has been demonstrated and shown to offer similar advantages to photo-hypersensitisation in most of these areas [12]. In this process, the initial sensitisation step ordinarily carried out with irradiation, is instead, carried out by heating during the hydrogen loading phase at moderately low temperatures, typically 80°C. However, low temperature thermal hypersensitisation is a very low energy process and there are questions regarding its intrinsic stability compared to photolytic treatment where bonds are broken as well as local heating arising. In this paper we show that in phosphosilicate optical fibres low temperature hypersensitisation is not stable over time and decays gradually, increasing the total fluence required to write similar strength Bragg gratings. Nevertheless, the advantages of low loss and stability reported earlier for photo-hypersensitised phosphosilicate optical fibres [12–14] remain. Further, another important reason for still using thermal hypersensitisation is the linearised grating growth characteristic curve, which simplifies the reproducibility of UV writing or processing of optical components. In addition, it is found that over short exposure fluence, the grating strength achievable is found to be larger for the hypersensitised fibre than for fully-hydrogen loaded fibre. This indicates that the subsequent contribution of hydrogen in conventional photosensitivity work through OH formation has a negative index contribution.
2. Hypersensitisation and Grating Writing
Thermal sensitisation of a fibre during hydrogen loading is predicated on the basis that the grating strength in phosphosilicate fibre did not correlate with the in-diffusion rate of molecular hydrogen [15]. In order to investigate a similar correlation between out-diffusion and grating strength, we measured the grating growth curve for hydrogen loaded phosphosilicate optical fibre at various stages of out-diffusion, including time well outside complete out-diffusion: i.e. in the hypersensitisation domain. In this way characteristic growth curves were obtained for fully hydrogen loaded fibre and for fibre at different stages after out-diffusion.
The phosphosilicate optical fibre is fabricated by modified MCVD where the phosphate is introduced by flash condensation using phosphoric acid [16]. Up to 17mol% of P2O5 is readily incorporated into the silica core. The V-parameter is matched to standard telecommunications grade fibre.
Figure 1. Typical transmission spectrum of a grating written into hypersensitised phosphosilicate optical fibre. The resolution is 0.1nm.
Hydrogen loading of the phosphosilicate fibres was carried out at 200atm pressure and 80°C for 14 days, well beyond the diffusion saturation time of less than a day at the conditions employed. This ensured that low energy thermal hypersensitisation has occurred. As soon as the loading was complete, gratings were written into various samples of the fibre at different times using 193nm direct grating writing through an optical phase mask. 193nm from an ArF laser is chosen since longer wavelengths do not produce strong gratings in reasonable time within phosphorus doped fibres [15]. Figure 1 shows a typical transmission spectrum of such a grating written into a fibre 14 days after hydrogen begins out-diffusing. The spectrum is resolution limited by the resolution of the optical spectrum analyser used, ~0.1nm. In previous work, it was shown that once these gratings are written they have excellent thermal stability making these gratings suitable for operation within high temperature (upwards of 700°C) environments [13]. This compares dramatically when no hypersensitisation is used and the gratings decay within minutes [15]. It is therefore of interest to see whether the thermal hypersensitisation process itself is of similar stability.
3. Characteristic Curves
Figure 2a and b is a plot of the average induced index change with UV light during writing of several gratings, which is begun at increasing time away from the loading phase. The average index is determined from the shift in Bragg wavelength. This index profile describes the characteristic photosensitive response of the waveguide and the plot is therefore defined to be the characteristic curve of the material, consistent with the terminology used for photochromic materials generally [17]. Figure 2b is an expanded version of figure 2a so that the characteristic response at low fluence can be analysed. What is immediately noticeable is that whilst hydrogen remains present during irradiation, the characteristic growth curve is not linear. However, the curvature decreases until close to 14 days, whereupon most of the hydrogen has out-diffused, and the curve is linearised. This in itself is of interest since it implies that some slow thermally driven, chemical sensitisation has indeed taken place during loading as previously proposed [15]. Further, it can be observed generally that at low fluence the index change grows as hydrogen out-diffuses (shown clearly in figure 2b) whilst at larger fluence the reverse is true.
4. Discussion
Figure 2. a - top) Photosensitive response curve of phosphosilicate optical fibre at various times after hydrogen loading (Time is indicated on days on the right).
Figure 3. Plot of recovered fraction of normalised reflectivity after 3mins cooling inbetween temperatures during isochronal annealing for a grating written into fully-hydrogen loaded (open squares) and hypersensitised (filled squares) phosphosilicate optical fibres. Details of the isochronal annealing experiments can be found in [13].
The observation of lower index change at low fluence within the hydrogenated samples is consistent with a negative index contribution arising when hydrogen is present, indicating that there is an initial growth in a negative index change as a result of continued formation of hydrogen species. The timescales can only be explained if these species are forming as a result of the initial index change at the core. Hence they must be related to the growing tensile stress at the core/cladding interface, which is entirely consistent with previous work showing that there is a spatially distinct index contribution arising at the core/cladding interface [11]. Since this negative index contribution eventually is overridden by further positive index growth within the fully hydrogen loaded sample, its magnitude can be approximately determined by taking the difference at low fluence between the fully hydrogen loaded case (time zero after loading) and that where the characteristic curve with the largest index modulation value at the same fluence. From figure 2b this is found to be ~10-4 - the magnitude is consistent with that expected from polarisability changes when hydrogen species are formed [18]. It cannot arise from a stress contribution since this contribution is passivated by the formation of such species.
From previous isochronal annealing data [13], hypersensitised fibre had a significantly enhanced elastic relaxation at higher temperatures than fibre irradiated fully when hydrogen is present. Figure 3 summarises this data for comparison. It can be observed that the grating written into a fully hydrogen-loaded phosphosilicate optical fibre has significantly reduced recovery of reflectivity after heating, indicating that the elastic component to the index change generated by core/cladding stresses whilst heating has been passivated. It also indicates that a stress grating must be present since the reflectivity monitors an index modulation with a period of ~0.5µm, consistent with that directly observed for gratings written into pristine germanosilicate optical fibre [19]. It can therefore be concluded that such a stress grating is negligible in the fully hydrogen loaded case and has been passivated by hydrogen reactions at the core/cladding interface [11]. Consequently, the stress contribution will be higher in the hypersensitised sample, supporting a previous argument that the role of hydrogen in the hypersensitised fibre is to drive the core index change that occurs in irradiated pristine fibre further [11]. The predominant species involved with UV irradiation of hydrogen loaded optical fibres are the hydroxyl groups [4,20,21] and it can be concluded that these are the source of negative index change. However, it has been recently proposed that densification of silica with 193nm arises because there is rapid local cooling analogous to fast quenches used to form high density forms of silica [22]. If this cooling is slow then the relaxation results in dilation of the network - conceivably, the presence of both stress and passivating OH at the core/cladding interface can lead to a slowdown in thermal quenching at the interface. In any case, the final positive index change is likely to arise from densification at the core.
Figure 4 illustrates the behaviour in more detail. The normalised index change obtained as a function of post-hydrogen loading time is shown at both low and high fixed fluence. For comparison, the expected out-diffusion normalised to the initial hydrogen concentration, is also displayed. The out-diffusion calculation follows that used in [15], derived from classical diffusion solutions for a cylinder [23]:
where
and [24]
Neither case follows the diffusion profile indicating that thermal sensitisation as predicted in [15] is present. Further, at low fluence the contribution from a sizeable negative component is maximum when the hydrogen concentration is maximum, again, indicating that hydrogen interactions are the source of component. In line with previous work on hypersensitisation where the index change arising from hydrogen contributions appears localised to the diffuse core/cladding boundary as a result of tensile stress increase [11], it may be concluded that this negative contribution arises around the core/cladding interface.
Figure 4. Normalised index change at fixed low and high fluence both as a function of out-diffusion time are shown. The normalised out-diffusion profile is also superimposed to illustrate the deviation from a simple linear proportionality between index change and hydrogen.
Hence, the overall positive index change is less than expected until at greater fluence the positive index change due to densification at the core dominates strongly. In the hypersensitised case, where hydrogen is removed before significant reactions take place, the absence of a negative contribution to index change results in a larger positive index change at lower fluence despite the overall positive index change due to changes in the core being less. This is why as the hydrogen out-diffuses from the core there is a noticeable increase in positive index change at low fluence compared to the decrease observed at large fluence. As a result there is a linearisation of the characteristic curve. Combined with the results presented in figure 2, it can be concluded that the negative index contribution does not arise directly from tensile stress growth at the interface since this remains present and is unpassivated in the hypersensitised fibre. After the hydrogen has sufficiently out-diffused the decay profiles are the same, within experimental error, for both low and high fluence, indicating no additional phenomena in the hypersensitised fibre.
As mentioned, the elimination of hydrogen in the hypersensitised fibre results in a linearised characteristic curve as shown in figure 2. This indicates the contribution from a negative index change plays a role in generating the non-linear response in the characteristic curve of fully hydrogen-loaded fibre. Hence, the second mechanism describing hydrogen interactions which leads to increased positive index change with continued irradiation [1,11], is convoluted with the presence of the negative index change. One can make preliminary conclusions that the presence of hydrogen increases the total densification possible at the core. This is consistent with the previous observations of hydrogen hopping in irradiated hypersensitised fibre where the OH peak characterising Ge or P sites shifts towards the shorter wavelength associated with SiOH [3,7,14]. It too indicates that the hydrogen plays a catalytic-like role in aiding structural change such as densification. However, this positive index change itself has two components in the fully H2-loaded case since annealing removes one over the other [25]. The reduced positive index change at larger fluence adds further support that the second positive component is also related to hydrogen reactions, possibly as a result of the additional contribution from passivation of the tensile stress [1,2,11].
Despite the instability of the low temperature thermal hypersensitisation process, it is clearly of benefit to allow hydrogen to out-diffuse from the fibre prior to grating writing. A linearised photosensitive response curve has advantages in a fabrication environment since it allows improved reliability and reproducibility in component manufacture and removes the high tolerance demanded on predicting grating device performance. Further, the variation in annealing decay found in fully hydrogen-loaded samples where two index contributions are present is removed. This means the uncertainty associated with the different decay rates between localised regions of high and low index change [25] is removed. The ability to extract a higher positive refractive index at lower fluence can also lead to more efficient processing times when the index change required is not large.
7. Conclusion
In conclusion, low temperature hypersensitisation of phosphosilicate optical fibres is found to be relatively unstable. However, hypersensitisation leads to a linearised photosensitive response curve and higher refractive index change at lower fluence than a fully hydrogen loaded optical fibre. There is an optimal value of sensitisation fluence to enjoy the benefits of a linearised grating growth curve balanced against the increase in writing fluence the further away from this optimal value. Together with the improved thermal stability of gratings subsequently written into such fibre and reduced OH formation, these are important advantages that can allow improved production efficiency in a manufacturing environment.
Acknowledgments:
J. Canning acknowledges an Australian Research Council Large Grant and a QEII Fellowship.
References and links
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