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Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 7 — Apr. 4, 2005
  • pp: 2276–2281

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Thermal hypersensitisation and grating evolution in Ge-doped optical fibre

H. Sørensen, J. Canning, and M. Kristensen  »View Author Affiliations


Optics Express, Vol. 13, Issue 7, pp. 2276-2281 (2005)
http://dx.doi.org/10.1364/OPEX.13.002276


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Abstract

Low temperature (sub 1000°C) thermal hypersensitisation is reported in germanosilicate optical waveguides. Gratings are written using a CW 266nm laser source. In contrast to laser hypersensitisation, thermal excitation is generally dispersive involving a range of specific glass sites. More complex grating profiles presenting evidence of solid-state autocatalysis and bistability at increasingly high sensitisation temperatures are observed. More specifically, at 500°C, a behaviour resembling type IIA grating response is observed.

© 2005 Optical Society of America

1. Introduction

For many applications of UV-induced index changes in silica-glass, the photosensitivity of the pristine glass is so low that it is of limited practical use. Hydrogen loading is therefore employed in most applications [1

P. J. Lemaire, R, M. Atkins, V. Mizrahi, and W.A Reed, “High pressure H2 loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO2 doped optical fibres,” Electon. Lett. 29, 1191–1193 (2004). [CrossRef]

]. However, the out diffusion of hydrogen from the loaded optical fibre before and during UV-radiation, along with unnecessary complications associated with excess hydrogen is often a drawback for complex grating fabrication where long exposure times are necessary. Similar limitations are more pronounced in UV-written gratings and devices in planar waveguides where diffusion through a thinner cladding layer is a major problem. Methods for obtaining a permanent high UV photosensitive fibre have therefore been investigated in detail. Photo-hypersensitisation of optical fibres loaded with hydrogen using UV-light was first demonstrated using germanium doped optical fibre and a pulsed 193nm ArF laser source [2

J. Canning, R. Pasman, M. G. Sceats, and P. A. Krug, “Photosensitisation of phosphosilicate fibre Bragg gratings,” Proc. Conference on photosensitivity and quadratic non-linearity, OSA, Portland, OR , 86–89 (1995).

]. A two-step underpinning mechanism involving hydrogen catalysis was proposed. Similar hypersensitisation has since been observed at most UV wavelengths, including longer UV wavelengths at 266nm [3

H. R. Sørensen, J. Canning, and M. Kristensen, “Laser hypersensitisation using 266nm light,” Laser Phys. Lett. 2, 194–197 (2004). [CrossRef]

] and 355nm [4

A. Canagasabey and J. Canning, “UV lamp hypersensitisation of hydrogen-loaded optical fibres,” Opt. Express 11, 1585–1589 (2003). [CrossRef] [PubMed]

]. Subsequently, thermal hypersensitisation was demonstrated in phosphosilicate fibres loaded with hydrogen [5

J. Canning and P-F. Hu, “Low-temperature hypersensitisation of phosphosilicate waveguides in hydrogen,” Opt. Lett. 26, 6, 1230–1232 (2001). [CrossRef]

]. This process operates at low temperatures and is not the same as high temperature flash heating [6

M. Fokine and W. Margulis, “Large increase in photosensitivity through massive hydroxyl formation,” Opt. Lett. 25, 302–304 (2000). [CrossRef]

], another approach to increase photosensitivity by introducing a permanent OH species. It was subsequently predicted that thermal hypersensitisation is possible in pure silica and other doped fibres at temperatures which correlate with the chemical solubility of hydrogen in the particular glass system [9

J. Canning, “Hydrogen and photosensitivity,” POWAG 2002 Summer school, St. Petersburg, Russia (2002).

]. For germanosilicate and pure silica fibres this was predicted to be above 300°C and 500°C respectively. Thermal hypersensitisation is of interest because it has numerous potential advantages over laser hypersensitisation, including its implementation with existing loading or out-diffusion methods and the ability to bulk hypersensitise large quantities of optical fibre and planar waveguides. From an industrial perspective, thermal hypersensitisation is a low cost method that can be incorporated into existing grating writing processes. In this paper, thermal hypersensitisation of germanosilicate optical fibres is demonstrated for the first time at various temperatures. Complex grating evolution and hydroxyl formation as function of increasing sensitisation temperature is observed.

2. Experimental

The fibre used in these experiments contains 22mol% germanium in the fibre core. All of the gratings presented in this paper are written using a CW solid-state 266nm quadrupled Nd/YAG laser. The polarisation axis is chosen to be parallel along the fibre axis and is referred to as p-polarised. A Gaussian intensity distribution (FWHM=670µm along the fibre; FWHM=320µm transverse to the fibre) characterises the beam profile. The power of the UV-light is 120mW providing an irradiance of 56mW/cm2, which is estimated to be accurate to within 5%. A grating written into “pristine” fibre with no hydrogen loading is used as a reference for the hypersensitised fibres. Data on the grating growth process and OH formation are extracted from transmission and absorption spectra using a synchronised wavelength swept tuneable laser source (TLS) and optical spectrum analyzer (OSA). It is noted when specifically analysing absorption spectra that the role of the field distribution across the fibre may be an important factor in affecting some of the results if a parameter has any spatial dependence, such as diffusion. A schematic of the setup is shown in Fig. 1. Using the obtained grating spectra, the effective index change and index modulation are derived from

Δ neff= λB− λ B0 ΛMask
(1)
Δ nmod= λB 2πη LGratln ( 1+ R 1− R)
(2)

where λB is the Bragg wavelength of the measured grating, λB0 the Bragg wavelength of the first measured grating, ΛMask the pitch of the phasemask used, η the confinement factor of the fibre, and LGrat the length of the grating, defined as 670µm, which is the FWHM of the UV-beam along the fibre, and R representing the linear reflection coefficient of the grating. The resulting characteristic curves can be seen in Fig. 2. Typical type I grating evolution is observed in the pristine fibre, where there is a maximum index modulation, Δnmod =4×10-4 and an effective index change, Δneff =2.5×10-4 at a fluence of 435kJ/cm2.

3. Thermal hypersensitisation

Thermal hypersensitisation is performed by inserting fibres, hydrogenated at 400Bar over 14 days at room temperature, into an oven for half an hour. Oven temperatures used are 300, 360, 400 and 500°C. A practical advantage of high-temperature sensitisation is a reduced time interval for processing, since hydrogen out-diffusion occurs while heating. For example, at 300°C, a heating time of 6.5 minutes is required for 95% of the hydrogen to out-diffuse, assuming no chemical interactions and only mechanical out-diffusion. After sensitisation, gratings are written into the fibre in the manner described above. Generally, if only one relaxation process is involved, monotonic characteristic plots (index change versus ln (fluence)) are expected in Fig. 2 [8

J. Canning, “The characteristic curve and site-selective laser excitation of local relaxation in glass,” J. Chem. Phys. 120, 9715–9719 (2004). [CrossRef] [PubMed]

, 9

J. Canning, “Hydrogen and photosensitivity,” POWAG 2002 Summer school, St. Petersburg, Russia (2002).

].

Fig. 1. Experimental setup for grating inscription and measurement.
Fig. 2. Grating growth-curves in pristine and thermally hypersensitised fibres. Open symbols - index modulation, Δnmod. Closed symbols - effective index, Δnav.

Indeed, hypersensitising at 300, 360 and 400°C yields monotonically growing curves, indicating a process in which there is one dominant reaction in the glass. This also suggests that thermal hypersensitisation can access a single net process, even if multiple excitation pathways are triggered. Of the three sets of curves, 400°C yields the highest index modulation of 8.4×10-4. However, there is also an increase in the effective index not found in sensitising at 300 and 360°C, which reduces the fringe contrast. This is illustrated in Fig. 3 showing the fringe contrast of the gratings normalized to the grating’s initial fringe contrast.

Fig. 3. Fringe contrasts normalized to the initial fringe contrast of the gratings written in 300, 360, 400, 500°C hypersensitized fibre and in unloaded fibre.

The observed reduction in fringe contrast for 400 and 500°C sensitisation is an indicator of an additional net bulk index change on a larger spatial scale than the grating period. Hypersensitising at 300 and 360°C, results in an increase in the index modulation only and corresponding fringe contrasts are similar to those observed in the unloaded fibre. In fact at 360°C the fringe contrast remains close to 1 at higher fluencies indicating much more efficient use than even the pristine fibre. This suggests that the optimal temperature for a single process is around these values, close to that predicted based on the chemical solubility of hydrogen in germanate glass [9

J. Canning, “Hydrogen and photosensitivity,” POWAG 2002 Summer school, St. Petersburg, Russia (2002).

]. The prediction is based on measurements of the solubility of hydrogen in pure silica glass, the measured solubility of hydrogen in phosphorous-doped glass and enthalpy considerations. Sensitising the fibre at 400°C initiates an additional index change contribution observed by the high increase in both index modulation and effective index change where the fringe contrast is reduced significantly. We conjectured that this might be attributed to an increase in hydroxyl formation or to the formation of defects with a negative index contribution [10

M. Kristensen, “Ultraviolet-light-induced processes in germanium-doped silica,” Phys. Rev. B 64, 4201–4212 (2001). [CrossRef]

]. Recent OH-diffusion studies of silica preforms in hydrogenated atmospheres at elevated temperatures underpins an OH-related mechanism by concluding that OH-formation transforms from one to two reaction processes at a temperature around 500°C [11

P. Tandon, “Chemical annealing of oxygen hole centers in bulk glasses,” J. Non-Cryst. Sol. 336, 212–217 (2004). [CrossRef]

]. In order to investigate this, the OH-absorption peak of the hypersensitised fibre is measured. As shown in Fig. 6, no significant OH-absorption can be measured at 300°C, but at 360 and 400°C an increase in the OH-absorption peak is observed. We therefore suspect, as suggested in [7

J. Canning, “Photosensitization and Photostabilization of Laser-Induced Index Changes in Optical Fibers,” Optical Fiber Tech. 6, 275–289 (2000). [CrossRef]

], that the reduced fringe contrast involves the onset of significant OH formation in the fibre, adding additional contributions to index change. In particular, these arise from a photorefractive positive index change from OH and a reduction in the stress-related negative index change both present and generated at the core/cladding interface through a small amount of dilation associated with OH formation [12

C.M. Smith, N.F. Borelli, J.J. Price, and D.C. Allen, “Excimer laser-induced expansion in hydrogen-loaded silica,” Appl. Phys. Lett. 78, 2452–2454 (2001). [CrossRef]

]. What is interesting to note is that we are operating well below the temperature used by flash heating method, which is based on OH formation under a very short timescale [6

M. Fokine and W. Margulis, “Large increase in photosensitivity through massive hydroxyl formation,” Opt. Lett. 25, 302–304 (2000). [CrossRef]

]. By hypersensitizing the fibre at 500°C initially a fringe contrast evolution resembling the 400°C behaviour is observed, but at a fluence of 80kJ/cm2 the grating begins to decay in a manner resembling type IIA behaviour [13

I. Riant and F. Haller, “Study of the photosensitivity at 193 nm and comparison with photosensitivity at 240 nm influence on fiber tension: type IIa aging,” J. Lightwave Technol. 15, 1466–1469 (1997). [CrossRef]

] in non-hydrogenated fibres where there is a rapid decay of the fringe contrast. However, at low fluences the characteristic curve is linear indicating that hypersensitisation is adequate until an onset of additional processes begins after a threshold fluence (not dissimilar to type IIA behaviour [13

I. Riant and F. Haller, “Study of the photosensitivity at 193 nm and comparison with photosensitivity at 240 nm influence on fiber tension: type IIa aging,” J. Lightwave Technol. 15, 1466–1469 (1997). [CrossRef]

]). By spectral analysis using Lorentzian fits, we were able to distinguish the GeOH and SiOH absorption peaks at 1.42µm and 1.38µm respectively. Examples of the Lorentzian fits are shown in Fig. 4 and 5. The fitted GeOH and SiOH absorption peaks consists Gaussian shaped absorption bands centered around a central main band. These internal bands do not change shape throughout the experiment due to their weak mechanical interactions with other OH-sites in the glass as they are spatially far apart. Secondly the Lorentzian fit is centered on the dominating absorption band for both the GeOH and SiOH absorption. The sum of the dominating band and its smaller neighbouring bands is fitted well by a Lorentzian fit as is shown in Figs. 4 and Figs. 5. Normalisation and integration of the fitted areas provided the total absorption, which is a direct measure of the relative concentrations of both species. We observed that by hypersensitising at increasing temperatures, the ratio of induced Ge-OH absorption (αGeOH at 1.42µm) to Si-OH absorption (αSiOH peak at 1.38µm) is reduced and at a hypersensitisation temperature of 500°C the ratio for the first time changes in favour of silica-bonded hydroxyl, Si-OH. The results are summarised in Fig. 6 and 7.

Fig. 4. Example of Lorentzian fits made to the measured absorption spectrum for a heating time of 2½ minutes at 500°C in a 400Bar H2 loaded fibre.
Fig. 5. Example of Lorentzian fits made to the measured absorption spectrum for a heating time of 38 minutes at 500°C in a 400Bar H2 loaded fibre.
Fig. 6. Evolution of αGe-OH and αSi-OH in 400Bar H2 loaded fibre sensitised at 300, 360, 400 and 500°C
Fig. 7. Evolution of the ratio of αGe-OH to αSi-OH in 400Bar H2 loaded fibre sensitised at 300, 360, 400 and 500°C
Fig. 8. Evolution of the integrated and normalized area of the total absorption, Ge-OH and Si-OH absorption in 400Bar H2 loaded fibre hypersensitised at 500°C.

Explanation of this behaviour includes a lower bonding energy of Ge-OH, compared to Si-OH, and/or saturation effects of hydroxyl formation on Ge-sites. However, also possible is hydrogen hopping from already formed Ge-OH sites to the more stable Si-OH sites requiring either H hopping, or water formation, and diffusion. By investigating the evolution of the areas of the measured absorption peaks as shown in Fig. 8, it is observed that while some hydrogen leaves the Ge-OH and Si-OH system, as indicated by the exponentially decaying curve of the total normalized absorption area, there is also an additional interaction between Ge-OH and Si-OH sites. This can be observed as oscillations exactly out of phase in the area-plots of the Si-OH and Ge-OH sites caused by the above-mentioned H hopping, or water hopping and terminated eventually by diffusion. Hence there is evidence of a strong dynamic environment where hydrogen may be utilised in complex autocatalytic pathways primarily determined by the bi-stable steady state solution between the various OH that form away from ideal equilibrium. An analytic rate expression will therefore be complicated by the convolution of the diffusion profile together with the spatial mode field distribution in the core and cladding regions. For the purposes of this study we focus on the qualitative impact of these processes. Studying the time evolution of the Ge-OH to Si-OH absorption ratios shown in Fig. 7, it is obvious that the heating time is critical. This is also known from the dependence of OH formation in flash heating at much higher temperatures [6

M. Fokine and W. Margulis, “Large increase in photosensitivity through massive hydroxyl formation,” Opt. Lett. 25, 302–304 (2000). [CrossRef]

]. Heating at 500°C for up to 5 minutes results in a higher ratio of Ge-OH compared to Si-OH, opposite to that observed at longer heating times. Studying the slope of the ratios in Fig. 7 indicates, after an initial rapid oscillation, a negatively sloped evolution indicating the possibility of obtaining ratios below 1 in all cases. This negatively sloped time dependence leads us to believe that the duration time of heat-exposure is critical and that it might be possible to achieve “flash” heating sensitisation [6] at much lower temperatures (as low as 360°C) than has been reported provided thermal treatment is optimised and long enough in duration. Thus, overall our results are consistent with diffusion. Nevertheless, the complicated behaviour points to low temperature flash heating involving more than one process with varying degrees of stability to be expected. We are currently studying this complex grating behaviour in further detail [15

H. R. Sørensen, J. Canning, and M. Kristensen, results to be published.

].

4. Conclusion

Thermal hypersensitisation in germanosilicate optical fibres has been demonstrated. The optimal value, consistent with a linear characteristic curve corresponding to one primary relaxation in the glass, lies within the range of 300–360°C. This value agrees well with the temperatures predicted qualitatively by the chemical solubility of hydrogen in bulk stress-free Ge-doped glass. When hypersensitising at higher temperatures (e.g. 400°C) we observe an increase in the OH-formation in the fibre suspected to be one factor responsible for a reduced fringe contrast. Hypersensitising at even higher temperatures, (e.g. 500°C) results in a higher Si-OH than Ge-OH absorption, and we observe new grating behaviour resembling type IIA grating evolution of unhydrogenated fibres. Evidence is presented of hydrogen hopping and diffusion leading to conversion with continued exposure of GeOH to SiOH as being the primary mechanism for the complexity at higher temperatures. Spatial displacement of hydrogen is convoluted experimentally with the optical field distribution of the probing mode so analytic expressions describing potential chemical interactions need to be done with care. In addition the complexity of a binary material system where stresses exist and are altered, also likely affect the observations. We have demonstrated that the combination of both temperature and thermal exposure duration allows flexibility in tailoring the OH distribution in the system as well as the photosensitivity.

Acknowledgments

The authors acknowledge OFS Denmark for supplying the optical fibre and J. Canning acknowledges financial support from the Otto Mønsted Fond, Denmark.

References and links

1.

P. J. Lemaire, R, M. Atkins, V. Mizrahi, and W.A Reed, “High pressure H2 loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO2 doped optical fibres,” Electon. Lett. 29, 1191–1193 (2004). [CrossRef]

2.

J. Canning, R. Pasman, M. G. Sceats, and P. A. Krug, “Photosensitisation of phosphosilicate fibre Bragg gratings,” Proc. Conference on photosensitivity and quadratic non-linearity, OSA, Portland, OR , 86–89 (1995).

3.

H. R. Sørensen, J. Canning, and M. Kristensen, “Laser hypersensitisation using 266nm light,” Laser Phys. Lett. 2, 194–197 (2004). [CrossRef]

4.

A. Canagasabey and J. Canning, “UV lamp hypersensitisation of hydrogen-loaded optical fibres,” Opt. Express 11, 1585–1589 (2003). [CrossRef] [PubMed]

5.

J. Canning and P-F. Hu, “Low-temperature hypersensitisation of phosphosilicate waveguides in hydrogen,” Opt. Lett. 26, 6, 1230–1232 (2001). [CrossRef]

6.

M. Fokine and W. Margulis, “Large increase in photosensitivity through massive hydroxyl formation,” Opt. Lett. 25, 302–304 (2000). [CrossRef]

7.

J. Canning, “Photosensitization and Photostabilization of Laser-Induced Index Changes in Optical Fibers,” Optical Fiber Tech. 6, 275–289 (2000). [CrossRef]

8.

J. Canning, “The characteristic curve and site-selective laser excitation of local relaxation in glass,” J. Chem. Phys. 120, 9715–9719 (2004). [CrossRef] [PubMed]

9.

J. Canning, “Hydrogen and photosensitivity,” POWAG 2002 Summer school, St. Petersburg, Russia (2002).

10.

M. Kristensen, “Ultraviolet-light-induced processes in germanium-doped silica,” Phys. Rev. B 64, 4201–4212 (2001). [CrossRef]

11.

P. Tandon, “Chemical annealing of oxygen hole centers in bulk glasses,” J. Non-Cryst. Sol. 336, 212–217 (2004). [CrossRef]

12.

C.M. Smith, N.F. Borelli, J.J. Price, and D.C. Allen, “Excimer laser-induced expansion in hydrogen-loaded silica,” Appl. Phys. Lett. 78, 2452–2454 (2001). [CrossRef]

13.

I. Riant and F. Haller, “Study of the photosensitivity at 193 nm and comparison with photosensitivity at 240 nm influence on fiber tension: type IIa aging,” J. Lightwave Technol. 15, 1466–1469 (1997). [CrossRef]

14.

P. J. Lemarie, “Reliability of optical fibres exposed to hydrogen: prediction of long term loss increases,” Opt. Eng. 30, 6, 780–789 (1991). [CrossRef]

15.

H. R. Sørensen, J. Canning, and M. Kristensen, results to be published.

OCIS Codes
(050.2770) Diffraction and gratings : Gratings
(060.2290) Fiber optics and optical communications : Fiber materials
(160.5320) Materials : Photorefractive materials

ToC Category:
Research Papers

History
Original Manuscript: January 12, 2005
Revised Manuscript: March 4, 2005
Published: April 4, 2005

Citation
H. Sørensen, J. Canning, and M. Kristensen, "Thermal hypersensitisation and grating evolution in Ge-doped optical fibre," Opt. Express 13, 2276-2281 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-7-2276


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References

  1. P. J. Lemaire, R, M. Atkins, V. Mizrahi, W.A Reed, �??High pressure H2 loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO2 doped optical fibres,�?? Electon. Lett. 29, 1191-1193 (2004). [CrossRef]
  2. J. Canning, R. Pasman, M. G. Sceats, P. A. Krug, �??Photosensitisation of phosphosilicate fibre Bragg gratings,�?? Proc. Conference on photosensitivity and quadratic non-linearity, OSA, Portland, OR, 86-89 (1995).
  3. H. R. Sørensen, J. Canning, M. Kristensen, �??Laser hypersensitisation using 266nm light,�?? Laser Phys. Lett. 2, 194-197 (2004). [CrossRef]
  4. A. Canagasabey and J. Canning, �??UV lamp hypersensitisation of hydrogen-loaded optical fibres," Opt. Express 11, 1585�??1589 (2003). [CrossRef] [PubMed]
  5. J. Canning, P-F. Hu, �??Low-temperature hypersensitisation of phosphosilicate waveguides in hydrogen,�?? Opt. Lett. 26, 6, 1230-1232 (2001). [CrossRef]
  6. Fokine M., Margulis W., �??Large increase in photosensitivity through massive hydroxyl formation,�?? Opt. Lett. 25, 302-304 (2000). [CrossRef]
  7. J. Canning, �??Photosensitization and Photostabilization of Laser-Induced Index Changes in Optical Fibers,�?? Optical Fiber Tech. 6, 275-289 (2000). [CrossRef]
  8. J. Canning, �??The characteristic curve and site-selective laser excitation of local relaxation in glass�??, J. Chem. Phys. 120, 9715-9719 (2004). [CrossRef] [PubMed]
  9. J. Canning, �??Hydrogen and photosensitivity,�?? POWAG 2002 Summer school, St. Petersburg, Russia (2002).
  10. M. Kristensen, �??Ultraviolet-light-induced processes in germanium-doped silica,�?? Phys. Rev. B 64, 4201-4212 (2001). [CrossRef]
  11. P. Tandon, �??Chemical annealing of oxygen hole centers in bulk glasses,�?? J. Non-Cryst. Sol. 336, 212-217 (2004). [CrossRef]
  12. C.M. Smith, N.F. Borelli, J.J. Price, D.C. Allen, �??Excimer laser-induced expansion in hydrogen-loaded silica,�?? Appl. Phys. Lett. 78, 2452-2454 (2001). [CrossRef]
  13. Riant I., Haller F., �??Study of the photosensitivity at 193 nm and comparison with photosensitivity at 240 nm influence on fiber tension: type IIa aging,�?? J. Lightwave Technol. 15, 1466-1469 (1997). [CrossRef]
  14. P. J. Lemarie, �??Reliability of optical fibres exposed to hydrogen: prediction of long term loss increases,�?? Opt. Eng. 30, 6, 780-789 (1991). [CrossRef]
  15. H. R. Sørensen, J. Canning, M. Kristensen, results to be published.

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