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Optics Express

Optics Express

  • Editor: J. H. Eberly
  • Vol. 9, Iss. 10 — Nov. 5, 2001
  • pp: 476–482
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Stability of thermally hypersensitised phosphosilicate waveguides and the characteristic growth curve

J. Canning and P-F. Hu  »View Author Affiliations


Optics Express, Vol. 9, Issue 10, pp. 476-482 (2001)
http://dx.doi.org/10.1364/OE.9.000476


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Abstract

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.

© 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

1. J. Canning, “Photosensitisation and photostabilisation of laser induced index changes in optical fibres,” Opt. Fibre. Tech. , 6275–289, (2000) [CrossRef]

,2

2. J. Canning, “Contemporary Thoughts on Glass Photosensitivity and their Practical Application,” Materials Forum , 25101–128, (2001)

]. 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

3. J. Canning, M. Åslund, and P-F. Hu, “UV-induced absorption losses in hydrogen-loaded optical fibres and in pre-sensitised optical fibres,” Opt. Lett. , 251621–1623, (2000) [CrossRef]

5

5. M. Åslund and J. Canning, “Annealing properties of gratings written into UV-presensitised hydrogen out-diffused optical fibre,” Opt. Lett. , 25692–694, (2000) [CrossRef]

]. 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

1. J. Canning, “Photosensitisation and photostabilisation of laser induced index changes in optical fibres,” Opt. Fibre. Tech. , 6275–289, (2000) [CrossRef]

10

10. J. Canning and K. Sommer, “Hypersensitisation of Rare-Earth Doped Waveguides for DFB Amplifier and Laser Applications,” Accepted to Opt Lett. (2001)

]. 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

11. J. Canning, K. Sommer, M. Englund, and S. Huntington, “Direct evidence of two types of UV-induced glass changes in silicate-based optical fibres,” Adv. Mater. , 13970–973, (2001) [CrossRef]

].

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

12. J. Canning and P-F. Hu, “Low temperature hypersensitisation of phosphosilicate waveguides in hydrogen,” Opt. Lett. , 261230–1232, (2001) [CrossRef]

]. 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

12. J. Canning and P-F. Hu, “Low temperature hypersensitisation of phosphosilicate waveguides in hydrogen,” Opt. Lett. , 261230–1232, (2001) [CrossRef]

14

14. P. Hu, J. Canning, K. Sommer, and M. Englund, “Phosphosilicate optical fibres: a grating host for all windows?” Proceedings of Optoelectronics and Optical Communications Conference (OECC/IOOC 2001), Sydney, Australia, pp.24–25, (2001)

] 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

15. J. Canning, M.G. Sceats, H.G. Inglis, and P. Hill, “Transient and permanent gratings in phosphosilicate optical fibres produced by the flash condensation technique,” Opt. Lett. , 202189–2191, (1995) [CrossRef] [PubMed]

]. 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

16. A.L.G. Carter, S.B. Poole, and M.G. Sceats, “Flash-condensation technique for the fabrication of high phosphorous-content rare-earth doped fibre,” Electron. Lett. , 282009–2011, (1992) [CrossRef]

]. 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.

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

17. H.I. Bjelkhagen, Silver-halide Recording Materials, Springer Series in Optical Science, Vol. 66, Springer-Verlag, Berlin, (1995)

]. 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

15. J. Canning, M.G. Sceats, H.G. Inglis, and P. Hill, “Transient and permanent gratings in phosphosilicate optical fibres produced by the flash condensation technique,” Opt. Lett. , 202189–2191, (1995) [CrossRef] [PubMed]

]. 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].

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

15. J. Canning, M.G. Sceats, H.G. Inglis, and P. Hill, “Transient and permanent gratings in phosphosilicate optical fibres produced by the flash condensation technique,” Opt. Lett. , 202189–2191, (1995) [CrossRef] [PubMed]

], derived from classical diffusion solutions for a cylinder [23

23. J. Crank, Mathematics of Diffusion, Oxford U. Press, London, (1975)

]:

CtC=2n=1exp(jn2Dtmb2)jnJ1(jn)
(1)

where

tm=t+w28D
(2)

and [24

24. P.J. Lemaire, “Reliability of optical fibres exposed to hydrogen: prediction of long-term loss increases,” Opt. Eng. 30780 (1991) [CrossRef]

]

D=(2.83x104)exp(40.19kJ/molRT)cm2/s
(3)

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.

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

25. H.I. Inglis, “Photosensitivity in germanosilicate optical fibres,” PhD. Dissertation, Physical and Theoretical Chemistry Department, University of Sydney, (1997)

] 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

1.

J. Canning, “Photosensitisation and photostabilisation of laser induced index changes in optical fibres,” Opt. Fibre. Tech. , 6275–289, (2000) [CrossRef]

2.

J. Canning, “Contemporary Thoughts on Glass Photosensitivity and their Practical Application,” Materials Forum , 25101–128, (2001)

3.

J. Canning, M. Åslund, and P-F. Hu, “UV-induced absorption losses in hydrogen-loaded optical fibres and in pre-sensitised optical fibres,” Opt. Lett. , 251621–1623, (2000) [CrossRef]

4.

M. Åslund, J. Canning, and G. Yoffe, “Locking in photosensitivity in optical fibres and waveguides,” Opt. Lett. , 241826–1828, (1999) [CrossRef]

5.

M. Åslund and J. Canning, “Annealing properties of gratings written into UV-presensitised hydrogen out-diffused optical fibre,” Opt. Lett. , 25692–694, (2000) [CrossRef]

6.

J. Canning, “Improving the manufacture of fibre Bragg gratings,” SPIE Vol. 3896769–778, (1999)

7.

J. Canning and P-F. Hu, “Eliminating UV-induced losses during UV-exposure of photo-hypersensitised optical fibres,” Proceedings of Bragg Gratings, Photosensitivity, and Poling In Glass Waveguides, Stresa, Italy, paper BthA6-1, (2001)

8.

K.P. Chen, P.R. Hermann, and R. Tam, “157nm F2 laser photosensitivity and photosensitisation in optical fibres,” Proceedings of Bragg Gratings, Photosensitivity, and Poling In Glass Waveguides, Stresa, Italy, paper BthA5-1, (2001)

9.

K.P. Chen, P.R. Hermann, and R. Tam, “Trimming phase and birefringence errors in photosensitivity-locked planar optical circuits,” Accepted for IEEE Phot. Tech. Lett., (2001)

10.

J. Canning and K. Sommer, “Hypersensitisation of Rare-Earth Doped Waveguides for DFB Amplifier and Laser Applications,” Accepted to Opt Lett. (2001)

11.

J. Canning, K. Sommer, M. Englund, and S. Huntington, “Direct evidence of two types of UV-induced glass changes in silicate-based optical fibres,” Adv. Mater. , 13970–973, (2001) [CrossRef]

12.

J. Canning and P-F. Hu, “Low temperature hypersensitisation of phosphosilicate waveguides in hydrogen,” Opt. Lett. , 261230–1232, (2001) [CrossRef]

13.

J. Canning, K. Sommer, and M. Englund, “Fibre gratings for high temperature sensor applications,” Meas. Sci. Technol. , 12824–828, (2001) [CrossRef]

14.

P. Hu, J. Canning, K. Sommer, and M. Englund, “Phosphosilicate optical fibres: a grating host for all windows?” Proceedings of Optoelectronics and Optical Communications Conference (OECC/IOOC 2001), Sydney, Australia, pp.24–25, (2001)

15.

J. Canning, M.G. Sceats, H.G. Inglis, and P. Hill, “Transient and permanent gratings in phosphosilicate optical fibres produced by the flash condensation technique,” Opt. Lett. , 202189–2191, (1995) [CrossRef] [PubMed]

16.

A.L.G. Carter, S.B. Poole, and M.G. Sceats, “Flash-condensation technique for the fabrication of high phosphorous-content rare-earth doped fibre,” Electron. Lett. , 282009–2011, (1992) [CrossRef]

17.

H.I. Bjelkhagen, Silver-halide Recording Materials, Springer Series in Optical Science, Vol. 66, Springer-Verlag, Berlin, (1995)

18.

L. Dong, J. L. Archambault, L. Reekie, P. St. J. Russell, and D. N. Payne, “Photoinduced absorption change in germanosilicate preforms: evidence for the color-center model of photosensitivity,” Appl. Opt. 343436–3440, (1995) [CrossRef] [PubMed]

19.

K.W. Raine, R. Feced, S.E. Kanellopoulos, and V.A. Handerek, “Measurement of stress at high spatial resolution in UV exposed fibres,” 4th Optical Fibre Measurements Conference (OFMC’97), National Physical Laboratory, Teddington, UK, pp..200–204, (1997)

20.

V. Grubsky, D.S. Starobudov, and J. Feinberg, “Mechanisms of index change induced by near-UV light in hydrogen loaded fibres,” Proceedings of Conference on Photosensitivity and Quadratic Non-Linearity, Optical Society of America, Williamsburg, Virginia, USA, p98, (1997)

21.

M. Fokine and W. Margulis, “Large increase in photosensitivity through massive hydroxyl formation,” Opt. Lett. , 25302–304, (2000) [CrossRef]

22.

A. Wootten, B. Thomas, and P. Harrowell, “Radiation-induced densification in amorphous silica: a computer simulation study,” J. Chem. Phys. , 1153336–3341, (2001) [CrossRef]

23.

J. Crank, Mathematics of Diffusion, Oxford U. Press, London, (1975)

24.

P.J. Lemaire, “Reliability of optical fibres exposed to hydrogen: prediction of long-term loss increases,” Opt. Eng. 30780 (1991) [CrossRef]

25.

H.I. Inglis, “Photosensitivity in germanosilicate optical fibres,” PhD. Dissertation, Physical and Theoretical Chemistry Department, University of Sydney, (1997)

OCIS Codes
(050.2770) Diffraction and gratings : Gratings
(060.2290) Fiber optics and optical communications : Fiber materials
(160.4670) Materials : Optical materials
(230.1150) Optical devices : All-optical devices
(260.5130) Physical optics : Photochemistry

ToC Category:
Research Papers

History
Original Manuscript: October 2, 2001
Published: November 5, 2001

Citation
John Canning and P.-F Hu, "Stability of thermally hypersensitised phosphosilicate waveguides and the characteristic growth curve," Opt. Express 9, 476-482 (2001)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-9-10-476


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References

  1. J. Canning, "Photosensitisation and photostabilisation of laser induced index changes in optical fibres," Opt. Fibre Tech. 6 275-289, (2000) [CrossRef]
  2. J. Canning, "Contemporary Thoughts on Glass Photosensitivity and their Practical Application," Materials Forum 25 101-128, (2001)
  3. J. Canning, M. Åslund and P-F. Hu, "UV-induced absorption losses in hydrogen-loaded optical fibres and in pre-sensitised optical fibres," Opt. Lett. 25 1621-1623 (2000) [CrossRef]
  4. M. Åslund, J. Canning and G. Yoffe, "Locking in photosensitivity in optical fibres and waveguides," Opt. Lett. 24 1826-1828 (1999) [CrossRef]
  5. M. Åslund and J. Canning, "Annealing properties of gratings written into UV-presensitised hydrogen out-diffused optical fibre," Opt. Lett. 25 692-694 (2000) [CrossRef]
  6. J. Canning, "Improving the manufacture of fibre Bragg gratings," SPIE Vol. 3896 769-778, (1999)
  7. J. Canning and P-F. Hu, "Eliminating UV-induced losses during UV-exposure of photo-hypersensitised optical fibres," Proceedings of Bragg Gratings, Photosensitivity, and Poling In Glass Waveguides, Stresa, Italy, paper BthA6-1, (2001)
  8. K.P. Chen, P.R. Hermann and R. Tam, "157nm F2 laser photosensitivity and photosensitisation in optical fibres," Proceedings of Bragg Gratings, Photosensitivity, and Poling In Glass Waveguides, Stresa, Italy, paper BthA5-1, (2001)
  9. K.P. Chen, P.R. Hermann and R. Tam, "Trimming phase and birefringence errors in photosensitivity-locked planar optical circuits," Accepted for IEEE Phot. Tech. Lett. (2001)
  10. J. Canning and K. Sommer, "Hypersensitisation of Rare-Earth Doped Waveguides for DFB Amplifier and Laser Applications," Accepted to Opt Lett. (2001)
  11. J. Canning, K. Sommer, M. Englund and S. Huntington, "Direct evidence of two types of UV-induced glass changes in silicate-based optical fibres," Adv. Mater. 13 970-973 (2001) [CrossRef]
  12. J. Canning and P-F. Hu, "Low temperature hypersensitisation of phosphosilicate waveguides in hydrogen," Opt. Lett. 26, 1230-1232 (2001) [CrossRef]
  13. J. Canning, K. Sommer and M. Englund, "Fibre gratings for high temperature sensor applications," Meas. Sci. Technol. 12, 824-828 (2001) [CrossRef]
  14. P. Hu, J. Canning, K. Sommer and M. Englund, "Phosphosilicate optical fibres: a grating host for all windows?" Proceedings of Optoelectronics and Optical Communications Conference (OECC/IOOC 2001), Sydney, Australia, pp.24-25 (2001)
  15. J. Canning, M.G. Sceats, H.G. Inglis and P.Hill, "Transient and permanent gratings in phosphosilicate optical fibres produced by the flash condensation technique," Opt. Lett. 20, 2189-2191 (1995) [CrossRef] [PubMed]
  16. A.L.G. Carter, S.B. Poole and M.G. Sceats, "Flash-condensation technique for the fabrication of high phosphorous-content rare-earth doped fibre," Electron. Lett. 28, 2009-2011 (1992) [CrossRef]
  17. H.I. Bjelkhagen, Silver-halide Recording Materials, Springer Series in Optical Science, (Springer-Verlag, Berlin, 1995) Vol. 66.
  18. L. Dong, J. L. Archambault, L. Reekie, P. St. J. Russell and D. N. Payne, "Photoinduced absorption change in germanosilicate preforms: evidence for the color-center model of photosensitivity," Appl. Opt. 34, 3436-3440 (1995) [CrossRef] [PubMed]
  19. K.W. Raine, R. Feced, S.E. Kanellopoulos and V.A. Handerek, "Measurement of stress at high spatial resolution in UV exposed fibres," 4 th Optical Fibre Measurements Conference (OFMC'97), National Physical Laboratory, Teddington, UK, pp. 200-204 (1997)
  20. V. Grubsky, D.S. Starobudov and J. Feinberg, "Mechanisms of index change induced by near-UV light in hydrogen loaded fibres," Proceedings of Conference on Photosensitivity and Quadratic Non-Linearity, (Optical Society of America, Washington, D.C., 1997) p.98.
  21. M. Fokine and W. Margulis, "Large increase in photosensitivity through massive hydroxyl formation," Opt. Lett. 25, 302-304 (2000) [CrossRef]
  22. A. Wootten, B. Thomas and P. Harrowell, "Radiation-induced densification in amorphous silica: a computer simulation study," J. Chem. Phys. 115, 3336-3341 (2001) [CrossRef]
  23. J. Crank, Mathematics of Diffusion, (Oxford Universty Press, London, 1975)
  24. P.J. Lemaire, "Reliability of optical fibres exposed to hydrogen: prediction of long-term loss increases," Opt. Eng. 30, 780 (1991) [CrossRef]
  25. H.I. Inglis, "Photosensitivity in germanosilicate optical fibres," PhD. Dissertation, Physical and Theoretical Chemistry Department, University of Sydney, (1997).

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