Whayne Padden

Recent progress in microstructured polymer optical fibre fabrication and characterisation

Recent progress in microstructured polymer optical fibre fabrication and characterisation will be presented. A wide range of different optical functionalities can now be obtained by modifications of the microstructure, as is demonstrated by the fibres presented here. Microstructured fibres that are single-mode, highly birefringent or show twin-core coupling are described, in addition to graded-index microstructured fibres and hollow core fibres, the latter case being where light is guided in an air core. Microstructured polymer optical fibres are an exciting new development, offering opportunities to develop fibres for a wide range of applications in telecommunications and optical sensing.

Coupling in a twin-core microstructured polymer optical fiber

Theoretical calculations and experimental results are reported for a microstructured polymer optical fiber twin-core coupler. Beat lengths are calculated using a fully vectorial, Fourier decomposition method, which show that the beat length is extremely sensitive to any core asymmetry. Reasonably good agreement between theory and experiment is obtained.

Manipulating and controlling the evanescent field within optical waveguides using high index nanolayers

We propose and demonstrate, through simulation and experiment, how the interaction of an optical field within a waveguide designed for chemical sensing and, more generally, evanescent field spectroscopy can be enhanced substantially by strategic deposition of high index surface layers. These layers draw out the optical field in the vicinity of probing and take advantage of field localisation through optical impedance matching. Localisation of the evanescent field to the inner layer in turn is accompanied by whispering gallery modes within the channels of a structured cylindrical waveguide, further enhancing sensitivity. A novel demonstration based on self-assembled layers made up of TiO2 within a structured optical fibre is demonstrated, using a simple porphyrin as the spectroscopic probe. This technique offers optimisation of the limitations imposed on practical waveguide sensors that are highly sensitive but nearly always at the expense of low loss. The principles have potential ramifications for nanophotonics more generally and these are discussed.Controlling the evanescent field within platform waveguide technologies underpins waveguide nanophotonics and is critical to optimising the interaction with integrated specialised materials or devices under test. Unfortunately, this interaction is often small since the evanescent field is a fraction of the total optical field. Here we propose and demonstrate, through simulation and experiment, how the waveguide evanescent field can be enhanced substantially by using high index interface layers, which draw out the optical field in the probe vicinity taking advantage of field localisation. This can be further enhanced by extended resonant and gallery modes within the channels of a structured cylindrical waveguide. Several orders of magnitude increased sensitivity with minimal added insertion loss is obtained using self-assembled layers of TiO2 (B) nanoparticles and porphyrin within a silica structured optical fibre. The combination of novel photonics with specialty material integration highlights the potential scope for physics, chemistry, sensing and materials research.

Characterisation of phase-shifts in gratings fabricated by over-dithering and simple displacement

Abstract Several different gratings utilizing phase-shifts in their designs are fabricated and characterised: a uniform grating with a single phase shift in the centre, a sinc-profile grating (with intrinsic phase shifts at the zeros), and a stretched cosine apodised grating with multiple and arbitrary phase shifts. The gratings are fabricated by either over-dithering the phase mask or a simple displacement of the phase-mask. The gratings are then characterised by measuring their spectral and group delay characteristics and using a grating reconstruction algorithm to determine the apodisation and phase profiles of the grating. The relative susceptibility of the different fabrication methods to errors is discussed along with the role of grating reconstruction in characterisation of gratings.

Light acceptance properties of multimode microstructured optical fibers: Impact of multiple layers

We present a study of the large numerical aperture and high capture efficiency in a class of microstructured optical fibers, also called ???air-clad??? fibers. We employ a recently developed method where the leaky modes supported by a waveguide are used to determine the far-field angular intensity distributions. These distributions are subsequently used to calculate the capture efficiency and numerical aperture. Their dependence on length, wavelength, bridge thickness and number of layers is presented. Based on the physical insights provided by the analysis, two simplified heuristic models are presented which are valid for either single layer or multiple layer fibers. They show good agreement with the full numerical calculations.

Interactive design and fabrication of complex FBGs

Gratings with a single phase shift or several phase shifts are fabricated by over-dithering the phase mask. The gratings are then characterised by measuring their spectral and group delay characteristics and using a reconstruction algorithm.

Imaginary refractive index waveguides and applications

A new form of optical waveguide, the gain/loss waveguide (GLW), is proposed and demonstrated through numerical simulation. Both step- and photonic crystal gain/loss fibres with no variation in the real part of the refractive index are shown to be feasible. The implications in designing novel waveguides and devices are discussed.

The acoustic field in biomedical tissue with midscale inhomogeneities

Biomedical ultrasound is often used for investigations within and close to tissue inhomogeneities, such as lesions and plaques, that are midsized compared with the ultrasound wavelength. The scaled wavenumber is typically in the range 1 to 100. Even with small (less than 10%) sound speed variations, such objects are associated with very complicated diffractive field magnitude modulations. The corresponding phase modulations are much more regular, and this observation is the basis for the method described in this paper. The acoustic field can be expressed in terms of a scattering integral. For biomedical parameters, calculations with the widely used Born approximation give accurate results in only very limited circumstances. In this paper we demonstrate the importance of the initial phase estimate, and introduce the phase corrected scattering integral (PCSI) method. We show that remarkably accurate results for the acoustic field can be obtained from a single evaluation of the scattering integral if this incorporates an initial estimate of the phase modulation imposed by the inhomogeneity. A simple ray model can be used to find the phase correction. The PCSI method deals very effectively with scattering due to small changes in sound speed and irregular geometry, both characteristic of biomedical problems.

P1A-12 Using the Phase Modulation Imposed by Tissue Inhomogeneity to Determine the Full Acoustic Near Field

Ultrasound is used in many different clinical contexts, and often in tissue with inclusions such as cavities, vessels or lesions. If the acoustic impedance of the medium within the inclusion is different to that of the surroundings, the propagating ultrasonic field will be altered, especially near the objects boundaries and in the distal shadow zone. The diffractive field magnitude modulations are complicated. The corresponding phase modulations are, however, quite regular, and this observation is the basis for the method presented in this paper. The tissue objects we model are small to mid-sized compared to the ultrasound wavelength, with scaled wave number ka ~ 1-60, and the sound speed variations are lsim 10%. The acoustic field at any point can be expressed, in integral form, using the Greens function as the kernel. The integrand also involves the unknown field inside the tissue inhomogeneity, which, in the widely used Born approximation, is replaced by the incident wave. Our results for biomedical parameters show that the field magnitude inside the object is greatly overestimated, with errors much greater than 100% for sound speed differences of more than a few percent. In this study we present a different treatment of the integrand that produces far more accurate results. We show that seeding the integrand with a better estimate of the phase modulation imposed by the tissue gives much more accurate and reliable results for the full field, both within and near the object. Only one evaluation of the scattering integral is required. We call this approach the Phase Corrected Scattering Integral (PCSI). Results are surprisingly accurate even when the tissue phase is estimated using a simple ray model. The range and accuracy of the PCSI approach is demonstrated in two dimensions using cylindrical geometry, and in three dimensions using spherical geometry. The method can be readily adapted to more complex scattering objects, such as non-circular cross sections, shells, nonuniform sound speed distributions and even multiple scattering objects.

Temperature independent polarisation maintaining fibre for sensing and interferometry

Polarisation maintaining fibres used for sensing and interferometry typically have high birefringence [1-3] and are known as HiBi fibres. Since photonic crystal fibre (PCF) was first reported [4,5], HiBi PCFs with birefringence comparable to and greater than conventional highly stressed bow-tie and PANDA fibre have been demonstrated [6-10]. Very high levels of form birefringence in PCFs have been possible due to the flexibility in geometry and the high refractive index contrast offered by making a fibre with an air silica structure (ASS). In this paper we present experimental results that show effective temperature independent, or athermal, birefringence in a HiBi-PCF [11-14]. This is expected to be beneficial for a number of sensing and interferometric applications. For example, fibre optic gyroscopes (FOG) generally use very long lengths of coiled HiBi fibre in a Sagnac configuration to attain suitable sensitivity. FOG cost is, however, a significant driving factor in limiting the expansion of FOGs into new lower cost applications. FOG performance has been primarily limited by environmental temperature sensitivity [15,16] and stabilisation routes, using temperature-stabilised packaging, add too much to their cost. The use of a passive, temperature insensitive HiBi-PCF is a much lower cost alternative that does not require active stabilisation, thereby potentially overcoming these limitations and potentially opening up a new low cost market for FOG technology whilst retaining high performance.