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Near-field scanning optical microscopy (NSOM)has become a
widespread tool in biological and physical imaging. Aperture based NSOM
relies on the use of a tapered optical fibre probe. These probes are usually
coated with metal leaving an aperture with dimensions as small as 50nm at the
tip. This tip is then scanned very close to the structure of interest and the
object is either illuminated with light radiated from the end of the probe or
light transmitted through or reflected from the structure is collected
through the probe. In this way an image of the object with a resolution
comparable to the size of the aperture is obtained.
For over a decade now, we have been interested in extending these
techniques to mapping both the intensity and phase of optical wavefields. In
particular, we have produced precision maps of the intensity distribution
near the foci of both high and low numercial aperture lenses. Recently, this
work has been extended to studying phase singularities and we have observed both
edge dislocations vortex-like structures at the focus of a lens. Another
important application of this research is to the precision measurement of the
mode structures within optical fibres and waveguides. From these measurements
information about the fibre or waveguide, including losses and the refractive
index profile, can be inferred. This work is currently being done in
collaboration with researchers at Victoria University and together we are
exploring a range of techniques, including NSOM, confocal microscopy and
Differential Interference Contrast (DIC) Microscopy to the characterisation
of a wide range of photonic devices central to developments in optical
telecommunciations. In addition, these methods have permitted us to study devices such as waveguides and gratings produced in bulk silica and optical fibre using high energy focussed ion beams. The fabrication of these devices is undertaken in collaboration with members of the School of Physics' Microanalytic Research Centre.
In collaboration with researchers in the University of
Melbourne Conservation Service and the Ian Potter Museum of Art, we have
commenced studies into the application of a range of novel imaging techniques
to issues of importance in artwork conservation. These include the use of
intensity-based phase imaging techniques pioneered within the Optics Group.
It is well-known that partially coherent wavefields have the potential to carry more information than a coherent field. The nature of partially coherent fields is generally fully described via either the mutual optical intensity or the Wigner Distribution Function. With my colleagues within the optics group I am involved in developing new, primarily non-interferometric, methods for the determination of the coherence properties of classical fields. Such techniques will be invaluable in characterising the scattering properties of materials, synchrotron sources and atomic beams as well as potentially underpinning new biomedical imaging methods.
Selected Publications:
- N.M. Dragomir, C. Rollinson S. 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, Optics Letters 28, 789-791, 2003.
- A. Roberts, E. Ampem-Lassen, A. Barty, K.A. Nugent, G. Baxter, N. Dragomir and S.T. Huntington, Refractive Index Profiling of optical fibers with axial symmetry using quantitative phase microscopy, Optics Letters 27, 2061-2063, 2002.
- A. Roberts, K. Thorn, M.L. Michna, N. Dragomir, P. Farrell and G. Baxter, Determination of bending induced stress in optical fibers using quantitative phase imaging, Optics Letters 27, 86-88, 2002.
- J.N. Walford, K.A. Nugent, A. Roberts and R.E. Scholten, High resolution phase imaging of phase singularities in the focal region of a lens, Optics Letters 27, 345-347, 2002.
- S.K. Rhodes, K.A. Nugent and A. Roberts, Precision measurement of the electromagnetic fields in the focal region of a high-numerical aperture lens using a tapered fibre probe, Journal of the Optical Society of America A 19, 1689 -1693, 2002.
- M.L. von Bibra, A. Roberts and J. Canning, Fabrication of long-period fiber gratings by use of focused ion-beam irradiation, Optics Letters, 26, 765-767, 2001.
- A. Barty, K.A. Nugent, D. Paganin, and A. Roberts, Quantitative optical
phase microscopy, Optics Letters 23, 817-819, 1998.
- A. Roberts and M.L. von Bibra, Fabrication of buried channel waveguides in
fused silica using focussed MeV proton beam irradiation, Journal of Lightwave
Technology, JL-14, pp. 2554-2557, 1996.
- D.J. Butler, K.A. Nugent and A. Roberts, Characterisation of optical
fibres using scanning near-field optical microscopy, Journal of Applied
Physics, 75, pp. 2753-2756, 1994.
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