Fibre imaging bundles for full-field optical coherence tomography

An imaging fibre bundle is incorporated into a full-field imaging OCT system, with the aim of eliminating the mechanical scanning currently required at the probe tip in endoscopic systems. Each fibre within the imaging bundle addresses a Fizeau interferometer formed between the bundle end and the sample, a configuration which ensures down lead insensitivity of the probe fibres, preventing variations in sensitivity due to polarization changes in the many thousand constituent fibres. The technique allows acquisition of information across a planar region with single-shot measurement, in the form of a 2D image detected using a digital CCD camera. Depth scanning components are now confined within a processing interferometer external to the completely passive endoscope probe. The technique has been evaluated in our laboratory for test samples, and images acquired using the bundle-based system are presented. Data are displayed either as en-face scans, parallel to the sample surface, or as slices through the depth of the sample, with a spatial resolution of about 30 ï ­m. The minimum detectable reflectivity at present is estimated to be about 10-3, which is satisfactory for many inorganic samples. Methods of improving the signal-to- noise ratio for imaging of lower reflectivity samples are discuss

Optical coherence tomography with a Fizeau interferometer configuration

We report the investigation of a Fizeau interferometer-based OCT system. A secondary processing interferometer is necessary in this configuration, to compensate the optical path difference formed in the Fizeau interferometer between the end of the fibre and the sample. The Fizeau configuration has the advantage of 'downlead insensitivity', which eliminates polarisation fading. An optical circulator is used in our system to route light efficiently from the source to the sample, and backscattered light from the sample and the fibre end through to the Mach-Zehnder processing interferometer. The choice of a Mach- Zehnder processing interferometer, from which both antiphase outputs are available, facilitates the incorporation of balanced detection, which often results in a large improvement in the Signal-to-Noise ratio (SNR) compared with the use of a single detector. Balanced detection comprises subtraction of the two antiphase interferometer outputs, implying that the signal amplitude is doubled and the noise is well reduced. It has been discerned that the SNR drops when the refractive index variation at a boundary is small. Several OCT images of samples (resin, resin + crystals, fibre composite) are presented

Single camera 3D planar Doppler velocity measurements using imaging fibre bundles

Two frequency planar Doppler Velocimetry (2ν-PDV) is a modification of the Planar Doppler Velocimetry (PDV) method that allows velocity measurements to be made, quickly and non intrusively, across a plane defined by a laser light sheet. In 2ν-PDV the flow is illuminated sequentially with two optical frequencies, separated by about 700MHz. A single CCD viewing through an iodine absorption cell is used to capture images under each illumination. The two images are used to find the normalised transmission through the cell, and the velocity information is encoded as a variation in the transmission Use of a single camera ensures registration of the reference and signal images and removes issues associated with the polarization sensitivity of the beam splitter, which are major problems in the conventional approach. A 2ν-PDV system has been constructed using a continuous-wave Argon ion laser combined with multiple imaging fibre bundles, to port multiple views of the measurement plane to a CCD camera, allowing the measurement of three velocity components.EPSR

Single camera 3D planar Doppler velocity measurements, using two frequency planar Doppler velocimetry (2v-PDV) and imaging fibre bundles

A modified Planar Doppler Velocimetry (PDV) technique, two frequency PDV (2v-PDV), is described that allows measurements of the velocity field over a plane defined by a laser light sheet using sequential illumination of the flow with two closely separated (&IGHz) frequencies of laser light. This allows a common-path imaging head to be used containing a single CCD camera instead of the usual camera pair. The problem of image misalignment is now avoided and the polarisation sensitivity of the beam splitter used in two camera imaging heads is also removed. Cost efficiency is improved by the simplification of the system. This paper describes the development of a 2v-PDV system using a continuous wave argon ion laser capable of making time-averaged velocity measurements. Initially a single velocity component system was constructed using acousto-optic modulators to produce the two illumination frequencies required. The system was then expanded to make 3D velocity measurements using a single CCD camera and multiple coherent imaging fibre bundles. Measurements were made on the rotating disc, in order to assess error level in the measurements, and on a seeded axisymmetric air jet. A method of improving the sensitivity of the 2v-PDV system is demonstrated by using both the rising and falling slopes of the iodine absorption line. Reductions in the error levels of velocity measurements of approximately 40% can be achieved using this increased sensitivity method

Hyper-Rayleigh scattering in centrosymmetric systems

Hyper-Rayleigh scattering (HRS) is an incoherent mechanism for optical second harmonic generation. The frequency-doubled light that emerges from this mechanism is not emitted in a laser-like manner, in the forward direction; it is scattered in all directions. The underlying theory for this effect involves terms that are quadratic in the incident field and involves an even-order optical susceptibility (for a molecule, its associated hyperpolarizability). In consequence, HRS is often regarded as formally forbidden in centrosymmetric media. However, for the fundamental three-photon interaction, theory based on the standard electric dipole approximation, representable as E13, does not account for all experimental observations. The relevant results emerge upon extending the theory to include E12M1 and E12E2 contributions, incorporating one magnetic dipolar or electric quadrupolar interaction, respectively, to a consistent level of multipolar expansion. Both additional interactions require the deployment of higher orders in the multipole expansion, with the E12E2 interaction analogous in rank and parity to a four-wave susceptibility. To elicit the correct form of response from fluid or disordered media invites a tensor representation which does not oversimplify the molecular components, yet which can produce results to facilitate the interpretation of experimental observations. The detailed derivation in this work leads to results which are summarized for the following: perpendicular detection of polarization components both parallel and perpendicular to the pump radiation, leading to distinct polarization ratio results, as well as a reversal ratio for forward scattered circular polarizations. The results provide a route to handling data with direct physical interpretation, to enable the more sophisticated design of molecules with sought nonlinear optical properties

Gravitons and Lightcone Fluctuations II: Correlation Functions

A model of a fluctuating lightcone due to a bath of gravitons is further investigated. The flight times of photons between a source and a detector may be either longer or shorter than the light propagation time in the background classical spacetime, and will form a Gaussian distribution centered around the classical flight time. However, a pair of photons emitted in rapid succession will tend to have correlated flight times. We derive and discuss a correlation function which describes this effect. This enables us to understand more fully the operational significance of a fluctuating lightcone. Our results may be combined with observational data on pulsar timing to place some constraints on the quantum state of cosmological gravitons.Comment: 16 pages and two figures, uses eps

Inversely Unstable Solutions of Two-Dimensional Systems on Genus-p Surfaces and the Topology of Knotted Attractors

In this paper, we will show that a periodic nonlinear, time-varying dissipative system that is defined on a genus-p surface contains one or more invariant sets which act as attractors. Moreover, we shall generalize a result in [Martins, 2004] and give conditions under which these invariant sets are not homeomorphic to a circle individually, which implies the existence of chaotic behaviour. This is achieved by studying the appearance of inversely unstable solutions within each invariant set.Comment: 19 pages with 20 figures, AMS La-TeX, to be published in International Journal of Bifurcation and Chao

Reply to Comment on "Completely positive quantum dissipation"

This is the reply to a Comment by R. F. O'Connell (Phys. Rev. Lett. 87 (2001) 028901) on a paper written by the author (B. Vacchini, ``Completely positive quantum dissipation'', Phys.Rev.Lett. 84 (2000) 1374, arXiv:quant-ph/0002094).Comment: 2 pages, revtex, no figure

The Health of Aging Lesbian, Gay and Bisexual Adults in California

Examines rates of diabetes, hypertension, poor mental health, physical disability, and fair or poor self-reported health status among homosexual adults ages 50 to 70 compared with their heterosexual peers. Recommends enhancing policies and practices