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Differential Interference Contrast measurements

(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:

ˆ Dr. Paul Stoddart for his supervision and advice on the project

ˆ Prof. Dr. Alfred Leitenstorfer and Prof. Dr. Paul Leiderer for their supervi-sion and assessing my thesis

ˆ Dr. Scott A. Wade for his help at various stages and his suggestions on how to improve my thesis

ˆ Julie Heller for her love, company and her effort to suport me

ˆ 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)

ˆ 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

ˆ The Australian Synchrotron Research Program (ASRP), the Defence Science and Technology Organisation (DSTO) and the Victorian Government for fund-ing

ˆ My family and friends for their support for the whole time of my studies.

Thank you all!

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