(S. A. Wade, Department of Mechanical Engineering, Monash University, Mel-bourne)
Initial measurements of transmission spectra of the exposed fibres have yet to re-veal any spectra that could evidence a change in refractive index as a result of the synchrotron exposure. But scattering of visible light from the side of the fibres has been observed in the exposed region. Further investigations of the fibre by means of Differential Interference Contrast (DIC) microscopy [19, 29] were carried out on one of the samples that showed light scattering. C. Rollinson at Victoria University assisted with taking the images.
Experimental
The LPG mask that was used during the synchrotron experiments consisted of 150 µm block regions and 150 µm pass regions over a total length of 20 mm as shown in Figure A.1 on the following page. Any possible refractive index variations in the fibre would be expected to follow the same periodicity.
To take the DIC images, the GF1 fibre (sample #34A) was set up in an arrange-ment shown in Figure A.2 on the next page. The fibre was rotated in 30◦ steps to give consideration to the possibility that the potential refractive index change was depending on the orientation of the fibre during the exposure. A temperature controlled refractive index oil (Cargille Series AA 1.458) was used to obtain opti-mum images and an oil temperature of 28.5 was found to produce images most rich in contrast. The DIC set-up used in these measurements allowed for visualise
58 A. Differential Interference Contrast measurements
Optical fibre LPG phase mask
Synchrotron radiation
150 µm
Figure A.1: LPG mask used in experiments.
maximum refractive index contrast but not to measure the absolute refractive index values.
Microscope slides
SMF-28 fibre spacer
SMF-28 fibre spacer
Test fibre
Rotation device
Index matching oil Microscope objective
Figure A.2: Experimental arrangement used to obtain DIC images.
Photos were taken with a zoom of×1 for the rotated fibre, ×2 and ×7.5 at a fixed rotation. The images obtained at×1 correspond to an area of 353×353 µm, for ×2 an area of 176×176 µm and an area of 47×47µm for ×7.5. A background images without the fibre was also taken to allow non-fibre or instrumentation variations to be subtracted.
Results
Examples of DIC images of the rotated fibre, with the background subtracted are shown in Figure A.3 on the facing page. One can easily distinguish the broken end of the fibre, the outside edge of the fibre cladding, the depressed cladding region and the fibre core in the images as either a dark line (i. e. a change in the refractive index from a high region to a lower region) or as a light shade line (i. e. the refractive index changes from a low region to a higher region). A line scan of a DIC image perpendicular to the fibre direction shows the refractive index variations across the
59 fibre in Figure A.4. No attempt to enhance the contrast of the DIC images has been made.
Figure A.3: DIC images of GF1 fibre. (a) Magnification×1 (image height 353µm).
(b) Magnification×7.5 (image is 47×47 µm. Markers: (1) core region, (2) edge of depressed cladding, (3) edge of cladding.
-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70
Figure A.4: Line scan across DIC image showing refractive index variations. (a) magnification ×1). (b) Magnification ×7.5.
In order to see if any refractive index variations due to the mask could be determined, vertical line scans, i.e. parallel to the fibre direction, were obtained at various rotations. Any variations in the refractive index would come up as peaks or dips in the line scans. Figure A.5 on the following page shows the results of line scans taken of the DIC images on the left hand side in the cladding regions close to the outer edge of the fibre.
In the vertical line scan analysis some slight variations in the grey scales can be spotted which are believed to reflect possible refractive index changes caused by the synchrotron radiation. These changes are highlighted by the arrows in the 0◦
60 A. Differential Interference Contrast measurements
Figure A.5: (a) Example of DIC image indicating approximately where the line scan was taken to investigate refractive index variations along the fibre length. (b) Line scans at a rotation of 0◦ and (c) 270◦.
and 270◦ plots in Figure A.5. The approximate distance between these variations is 157µm, with the top change occurring approximately 45µm from the broken end of the fibre. The observed changes are quite small and further analysis has to be done.
However the distance between the refractive index changes does appear to coincide quite well with the ∼150 µm periodicity of the LPG mask used.
Conclusion
Differential interference contrast images were obtained of a sample of GF1 fibre which had been exposed to synchrotron radiation through a long period grating phase mask. The results appear to suggest that slight variations in refractive index, with a periodicity similar to the phase mask, have occurred as a result of the exposure.
Acknowledgments
There are many people that have been supporting me throughout this diploma thesis project. Without their help it would not have been possible for me to complete this work. Among these people I would like to thank in particular:
Dr. Paul Stoddart for his supervision and advice on the project
Prof. Dr. Alfred Leitenstorfer and Prof. Dr. Paul Leiderer for their supervi-sion and assessing my thesis
Dr. Scott A. Wade for his help at various stages and his suggestions on how to improve my thesis
Julie Heller for her love, company and her effort to suport me
James Wang for his assistance with the AFM at Swinburne University, Jack Jasieniak for assisting me with the ellipsometer experiments and Dr. Arnan Mitchell for allowing me to access the stylus profilometer at RMIT (Royal Melbourne Institute of Technology)
Umicore Electro-Optic Materials, UGQ Optics, Amorpheus Materials, Kevin Laws (University of New South Wales) and Scott A. Wade for providing the glass and fibre samples
The Australian Synchrotron Research Program (ASRP), the Defence Science and Technology Organisation (DSTO) and the Victorian Government for fund-ing
My family and friends for their support for the whole time of my studies.
Thank you all!
Bibliography
[1] Datasheet AMTIR-1.
[2] Datasheet AMTIR-6.
[3] Datasheet B270 SCHOTT Superwite.
[4] Datasheet Fused Quartz Vitreosil 077.
[5] Datasheet fused silica Spectrosil.
[6] Datasheet GASIR-1.
[7] Datasheet lead glass.
[8] Datasheet SCHOTT Borofloat.
[9] Synchrotron light source.
[10] H. Akazawa. Large changes in refractive index by synchrotron-radiation-driven compaction of hydrogenated silicon nitride films. Applied Physics Letters, 80(17):3102–3104, 2002.
[11] J. Albert, B. Malo, K. O. Hill, D. C. Johnson, F. Bilodeau, and S. Theri-ault. Comparison of one-photon and two-photon effects in the photosensitivity of germanium-doped silica optical fibres exposed to intense arf excimer laser pulses. Applied Physics Letters, 67(24), 1995.
[12] E. Ampem-Lassen, S. T. Huntington, N. M. Dragomir, K. A. Nugent, and A. Roberts. Refractive index profiling of axially symmetric optical fibers: a new technique. Optics Express, 13(9):3277–3282, 2005.
64 Bibliography [13] R. M. Atkins, V. Mizrahi, and T. Erdogan. 248 nm induced vacuum uv spectral changes in optical fiber preform cores: support for a colour center model of photosensitivity. Electronic Letters, 29(4):385–386, 1993.
[14] R. M. A. Azzam and N. M. Bashara. Ellipsometry and Polarized Light. North-Holland Books, 1977.
[15] F. Bilodeau, B. Malo, J. Albert, D. C. Johnson, K. 0. Hill, Y. Hibino, M. Abe, and M. Kawachi. Photosensitization of optical fiber and silica-on-silicon/silica waveguides. Optical Letters, 18(12):953–955, 1993.
[16] N. F. Borrelli, C. Smith, D. C. Allan, and T.P. Seward. Densification of fused silica. Journal of Applied Physics, 82(3):1065–1071, 1997.
[17] A. Canagasabey, J. Canning, and N. Groothoff. 355 nm hypersensitization of optical fibres. Optical Letters, 28(13):1108–1110, 2003.
[18] L. Dong. Photoinduced absorption change in germanosilicate preforms:
evidence for the color-center model of photosensitivity. Applied Optics, 34(18):3436, 1995.
[19] N. M. Dragomir, C. Rollinson, S. A. Wade, A. J. Stevenson, S. F. Collins, G. W. Baxter, P. M. Farrell, and A. Roberts. Nondestructive imaging of a type i optical fiber bragg grating. Optical Letters, 28:789–791, 2003.
[20] W. M. Duncan and S. A. Henck. Insitu spectral ellipsometry for real-time measurement and control. Appl. Surf. Sci., 63(9):9–16, 1993.
[21] G. Emmerson. Photosensitivity locking techniques applied to uv written planar bragg gratings. Electronic Letters, 39(6):517–518, 2003.
[22] H. G. Limberger et al. Compaction- and photoelastic-induced index changes in fiber bragg gratings. Applied Physics Letters, 68(22):3069–3071, 1996.
[23] K. O. Hill et al. Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication. Applied Physics Letters, 32(10):647–649, 1978.
[24] D. P. Hand and P. St. J. Russell. Photoinduced refractive index changes in germanosilicate fibers. Optical Letters, 15(2):102–104, 1990.
[25] S. A. Henck, W. M. Duncan, L. M. Lowenstein, and S. W. Butler. In situ spectral ellipsometry for real-time thickness measurement: Etching multilayer stacks. Vac. Sci. Technol. A, 11(4):1179–1185, 1993.
[26] A. Kameyama, A. Yokotani, and K. Kurosawa. Second order optical non-linearity and change in refractive index in silica glasses by a combination of
Bibliography 65 thermal poling and x-ray irradiation. Journal of Applied Physics, 95(8):4000–
4006, 2004.
[27] J. Kobayashi, T.Maruno, T. Ishii, T. Tamamura, and T. Horiushi. Direct fab-rication of polyiamide waveguide grating by synchrotron radiation. Applied Physics Letters, 73(23):3336–3338, 1998.
[28] M. Konstantaki, G. Tamiolakis, A. Argyris, A. Othonos, and A. Ikiades. Effects of ge concentration, boron co-doping, and hydrogenation on fiber bragg grating characteristics.Microwave and Optical Technology Letters, 44(2):148–152, 2005.
[29] B. Kouskousis, N. M. Dragomir, C. Rollinson, S. A. Wade, S. F. Collins, G. W.
Baxter, and A. Roberts. Quantitative investigation of the refractive-index mod-ulation within the core of a fiber bragg grating. Optics Express, 14(12):10332–
10338, 2006.
[30] D. K. W. Lam and B. K. Garside. Characterization of single-mode optical fiber filters. Applied Optics, 20:440–445, 1981.
[31] C. L. Liou, L. A. Wang, and M. C. Shih. Characteristics of hydrogenated fibre bragg gratings. Applied Physics A, 64:191–197, 1997.
[32] P. Meyer, J. Schulz, and L. Hahn. DoseSim: MS-Windows graphical user inter-face for using synchrotron x-ray exposure and subsequent development in the LIGA process. Review of Scienctific Instrument, 74(2):1113–1119, 2003.
[33] S. J. Mihailov, C. W. Smelser, D. Grobinc, R.B. Walker, P. Lu, H. M. Ding, and J. Unruh. Bragg gratings written in all-sio2and ge-doped core fibres with 800 nm femtosecond radiation and a phase mask. Journal of Lightwave Technology, 22(1):94–100, 2004.
[34] A. Othonos. Fiber bragg gratings. Review of Scienctific Instrument, 68(12):4309–4341, 1997.
[35] A. Othonos and K. Kalli.Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing. Artech House Books, 2000.
[36] W. Primak. Radiation damage in insulators. Phys. Rev., 92:1064, 1953.
[37] W. Primak. Radiation induced dilations in vitreous silica. Phys. Rev., 128(6):2580, 1962.
[38] W. Primak. Mechanism for the radiation compaction of vitreous silica. Journal of Applied Physics, 43(6):2745–2754, 1972.
[39] W. Primak. Threshold for radiation effects in silica. Phys. Rev. B, 6(12):4846–
4852, 1972.
66 Bibliography [40] W. Primak and R. Kampwirth. The radiation compaction of vitreous silica.
Journal of Applied Physics, 39(12):5651–5658, 1968.
[41] R.E. Schenker and W. G. Oldham. Ultraviolet-induced densification in fused silica. Journal of Applied Physics, 82(3):1065–1071, 1997.
[42] C. Z. Tan, J. Arndt, and H. S. Xie. Optical properties of densifed silica glasses.
Physica B, 252:28–33, 1998.
[43] C. Y. Wei, C. C. Ye, S. W. James, P. E. Irving, and R. P. Tatam. AFM obser-vation of surface topography of fibre bragg gratings fabricated in germanium-boron codoped fibres and hydrogen-loaded fibres.Optical Materials, 20:283–294, 2002.
[44] K. A. Zagorulko, P. G. Kryukov, Y. V. Larionov, A. A. Rybaltovsky, and E. M.
Dianov. Fabrication of fiber bragg gratings with 267 nm femtosecond radiation.
Optics Express, 12(24):5996–6001, 2004.