UNIVERSITY OF SOUTHAMPTON Scanning Near-Field Optical Microscope Characterisation of Microstructured Optical Fibre Devices. Christopher Wyndham John Hillman Submitted for the degree of Doctor of Philosophy Faculty of Science Department of Physics September 2002
UNIVERSITY OF SOUTHAMPTON ABSTRACT Faculty of Science Department of Physics Doctor of Philosophy Scanning Near-Field Optical Microscope Characterisation of Microstructured Optical Fibre Devices. By Christopher Wyndham John Hillman This thesis details work relating to the characterisation of microstructured fibres using SPM techniques. More specifically the optical properties of the fibres have been investigated by the use of a scanning near-field optical microscope and atomic force microscopy. The SNOM was constructed and fully characterised as part of this work. The current state of research into microstructured fibre fabrication, theory and applications is currently benefitting from a great deal of interest from academia and commercial investors alike. New fibre structures are being produced at a rate previously impossible. With this increase comes a need to be able to characterise more effectively the fibres that are produced. SNOM provides a number of significant features that address this issue. In this work four recently fabricated microstructured fibres have been investigated at a number of wavelengths. In each case accurate mode profiles have been measured and compared with resolution that would be extremely difficult to obtain with traditional mode profiling techniques. A theoretical model has also been used to predict the mode profiles. Measurements of the mode profiles after propagation in free space are presented and are compared to a theoretical beam propagation technique. An interferometric technique at 1550nm was used to image electric field amplitude and phase of the fibre modes, including results on the phase evolution of the mode as it propagates in free space. i
Acknowledgements My time at the ORC has, almost without exception, been a happy one. That doesn t mean to say of course that it has been easy. There are many, many people that have helped me pull through to this stage and without them I suspect I would still be stumbling around in the proverbial dark. 1 Unfortunately there is very literally not enough time to mention them properly. Guys, you know who you are and the fact that you were generous enough to help means a lot to me. Thanks. However fun this work is I would have thought twice about doing it for free. On this count the support of both the EPSRC and the ORC is very greatly appreciated. Special mention has to go to the few hardy souls that I can t afford to buy enough pints for. To Gates, a Dremel incarnate. To Bill, can that much patience be a virtue? To Steve, for tech. support above and beyond. To Tom and Simon, who rarely said No, and most importantly to Joyce, who kept on kicking me when others stopped and stopped when others carried on. My most sincere thank you to you all...i think I ll go to sleep now. 1 Proverbial Dark - Similar to the dark observed in labs just after the lights go out, but with even more stools to trip over. ii
Contents Abstract Acknowledgements i ii 1 Introduction 1 1.1 Chapter introduction....................... 1 1.2 The conception of SNOM.................... 2 1.3 The application of SNOM to optical waveguides....... 3 1.4 SNOM of Microstructured Fibres................ 4 1.5 Outline of thesis.......................... 5 1.5.1 Chapter 1......................... 5 1.5.2 Chapter 2......................... 5 1.5.3 Chapter 3......................... 6 1.5.4 Chapter 4......................... 6 1.5.5 Chapter 5......................... 6 1.5.6 Chapter 6......................... 7 1.5.7 Appendix A........................ 7 1.6 References............................. 7 2 Theory of Scanning Near-field Optical Microscopy 8 2.1 Chapter introduction....................... 8 2.2 The diffraction limit and sub-wavelength information.... 9 2.2.1 Plane Wave Decomposition............... 11 2.2.2 Sub-wavelength resolution............... 15 2.3 Height regulation and the shear force............. 17 iii
2.3.1 Detection of the shear force............... 18 2.3.2 Origins of the shear force................ 19 2.3.3 Bandwidth limit with shear force detection...... 20 2.4 Introduction to microstructured fibres............. 20 2.5 MSF Guidance Methods..................... 21 2.5.1 Holey Fibres........................ 23 2.5.1.1 HF Theory................... 24 2.5.2 Photonic Crystal Fibres................. 27 2.5.2.1 B-PBG...................... 27 2.6 MSF Fabrication.......................... 29 2.7 Detailed modal modelling..................... 29 2.7.1 Modelling Method.................... 30 2.7.2 Modelling Requirements................. 31 2.8 MSF Characterisation....................... 32 2.8.1 Current HF Characterisation techniques........ 32 2.8.1.1 Physical Structure............... 32 2.8.1.2 Optical Mode structure............ 35 2.8.1.3 Aside: Definition of Near Field...... 36 2.8.2 Application of scanning probe microscope techniques 37 2.8.2.1 AFM Holey fibre AFM results......... 38 2.8.2.2 SNOM...................... 40 2.9 Chapter Summary......................... 40 2.10 References............................. 41 3 Construction and Characterisation of a SNOM 47 3.1 Chapter introduction....................... 47 3.2 Tip fabrication and characterisation............... 48 3.2.1 Tip Pulling......................... 49 3.2.2 Aluminium Coating................... 53 3.2.2.1 Coating characterisation........... 55 3.2.3 Tip characterisation.................... 60 3.2.3.1 SEM analysis.................. 61 iv
3.3 Height regulation......................... 61 3.3.1 Tip dithering and resonance detection system.... 64 3.3.2 Surface interaction.................... 66 3.3.3 Feedback control..................... 68 3.4 Scanning control.......................... 69 3.5 Instrument optimisation and characterisation......... 70 3.5.1 Environmental Isolation................. 70 3.5.1.1 Vibrational Isolation.............. 70 3.5.1.2 Thermal Isolation............... 71 3.5.2 Data repeatability..................... 71 3.5.3 Resolution......................... 72 3.5.3.1 Lateral Topography.............. 72 3.5.3.2 Vertical Topography.............. 73 3.5.3.3 Lateral Optical................. 74 3.5.4 Shear-force artifacts................... 75 3.6 Chapter Summary......................... 75 3.7 References............................. 76 4 Scanning near-field optical microscopy of MSF 79 4.1 Chapter introduction....................... 79 4.2 Experimental Detail........................ 80 4.2.1 Holey Fibre sample details............... 80 4.2.2 SNOM technique..................... 81 4.3 SNOM Results and Analysis................... 83 4.3.1 Contact Mode Imaging.................. 83 4.3.2 Data Analysis....................... 85 4.3.2.1 Inspection.................... 85 4.3.2.2 Numerical Effective area measurement... 86 4.3.2.3 Gaussian Fitting................ 87 4.4 Modelling............................. 90 4.4.1 AFM results........................ 90 4.4.2 Modelling Results.................... 91 v
4.4.3 Mode Comparison.................... 92 4.5 Imaging Artifacts......................... 97 4.5.1 Mode broadening..................... 97 4.5.2 Side wall coupling.................... 99 4.5.3 Topographical Artifacts................. 105 4.6 Chapter Summary......................... 111 4.7 References............................. 112 5 Further applications of SNOM to MSF Characterisation 113 5.1 Chapter introduction....................... 113 5.2 Large Mode Area Fibre...................... 114 5.2.1 Mode measurements at 633nm............. 118 5.2.2 Mode Measurements at 1550nm............ 129 5.2.2.1 1550nm Detection Method.......... 129 5.2.2.2 Results..................... 132 5.2.2.3 Phase Measurements............. 136 5.2.2.4 Theoretical mode comparison........ 137 5.2.2.5 Z Scans..................... 137 5.2.3 Mode comparison at 633nm and 1550nm....... 144 5.2.4 Mode propagation at 633nm.............. 146 5.3 Other Fibre samples........................ 154 5.3.1 Sample 1.......................... 155 5.3.2 Sample 2.......................... 163 5.4 Chapter Summary......................... 165 5.5 References............................. 167 6 Conclusions and Further Work 168 6.1 Chapter introduction....................... 168 6.2 SNOM and data analysis..................... 169 6.3 Fibre Mode Profiling....................... 170 6.4 Imaging Artifacts......................... 172 6.5 Mode field propagation and normal plane scans....... 173 6.6 Further Work........................... 174 vi
A Publications 176 vii