Simon A. Boothroyd

Interboard optical data distribution by Bessel beam shadowing

We describe the efficient conversion of light from a laser diode into a Bessel beam whose axial intensity varies almost uniformly with distance using only a holographic optical element. An interesting shadowing property of the Bessel beam is demonstrated where on blocking the intense central spot the propagating ring pattern acts to reform the central spot a short distance following the obstruction. This behaviour together with the long propagation range for the Bessel beams central spot are considered for multiboard optical interconnects.

Holographic formation of a diode laser nondiffracting beam

We report for the first time, to our knowledge, the characteristics of a so-called nondiffracting beam produced by illumination of a binary-phase reflective holographic optical element with light from a diode laser. The Bessel beam has an intensity profile whose pattern changes little over distances of order 1 m and has a 1/e amplitude radius for the central lobe of ∼100 µm. This beam may have application for alignment of integrated optic elements in which unguided diffraction-free beams are used to align glass-slab elements containing interconnection holograms to a computer board. The aligning Bessel beam would be produced on reflection from a hologram on the glass-substrate interconnecting element. A single hologram may be used for different substrates having different lengths and functionality because of the large depth of field of the Bessel beam.

Upconversion dynamics of Er3+:YAlO3

The dynamics of the green (2H11/12, 4S3/2) and red (4F9/2) upconversion emission excited by 15 ns pulses from an Er:fibre laser at 1500 nm are presented and analyzed. The upconversion process is found to fit a kinetic model, which includes both cross relaxation and energy transfer.

Determination of the phase of the complex nonlinear refractive index by transient two-wave mixing in saturable absorbers.

The ratio of the imaginary part to the real part of the complex nonlinear refractive index has been measured by transient two-wave mixing in acridine-orange-doped fluorophosphate glass, fluorescein-doped boric-acid glass, and ruby at argon-ion wavelengths. The method enables the individual signs of the components of the nonlinear index to be obtained. A simple theory based on a single homogeneously broadened absorption from the ground state agrees with measurements of the phase of the nonlinear index in acridine but not in fluorescein, where our data suggest that excited-state absorption plays an important role.

Novelty filter that uses a bacteriorhodopsin film

We propose a new novelty optical filter that uses a bacteriorhodopsin film. This filter is based on the time-dependent nonlinear diffraction efficiency of real-time holograms recorded in the film. As soon as the signal beam carrying a pattern is diffracted by the polarization hologram recorded in the bacteriorhodopsin film, it begins to erase the hologram and suppresses the diffraction of the beam at the position of the stationary part of the pattern. This filter enhances only leading edges of moving patterns. In this system undesired scattered light, which is orthogonally polarized to the diffracted beam, is discriminated by a polarizer.

Partial rotation-invariant pattern matching and face recognition with a joint transform correlator

We describe the circular-harmonic (CH) image CH(mr), which is based on CH components for rotationally invariant pattern recognition. CH components of the order m, derived from an image in polar coordinates, are used to form a two-dimensional space together with the radial variable r. Filtering the CH(mr) image leads to a reference image with some rotational invariance. For a narrow-pass filter we have a single CH component with full rotation invariance; for an all-pass filter we have the original image with no rotational invariance; for a low-pass filter we form a reference image containing multiple circular harmonics with partial rotation invariance. Results of numerical simulations and optical experiments with a joint transform correlator are given that illustrate partial-rotation-invariant recognition for human face images.

Nondegenerate two-wave mixing in Cr 3+ :Er 3+ :YAlO 3

We present the results of nondegenerate two-wave mixing measurements in Cr−3:YAlO3 and in Cr−3: Er−3:YAlO3 in which the induced index change arises from the population of the 2E excited state of the chromium. The presence of erbium ions alters the 2E-level lifetime and quenches the chromium nonlinearity. We interpret our results in terms of a model in which different lifetimes are assigned to the chromium ions. Chromium ions close enough to an erbium ion to experience quenching by nonradiative energy transfer have lifetimes that are shorter than chromium ions that are not affected by energy transfer.

Optical image subtraction in fluorescein-doped boric acid glass.

The interference of two coherent images with a controlled phase difference between them is shown by using four-wave mixing in fluorescein-dye-doped boric acid glass and a liquid-crystal television spatial light modulator. We present results showing the digital optical exclusive OR operation with milliwatt optical power and an output that is compatible with CCD camera sensitivities.

Rotation-invariant pattern recognition using a joint transform correlator

We present numerical and experimental results for a rotation-invariant pattern recognition method using a joint transform correlator. The reference image input to the correlator along with the input scene is a circular harmonic function. This positive real function is derived from a phase only circular harmonic matched filter for the reference image and is shown to give good discrimination for translation and rotation-invariant pattern recognition.

Photonic Band Structure Calculations Using a Two-dimensional Electromagnetic Simulator

Abstract The use of an electromagnetic simulator is demonstrated as a tool for exploring photonic band structure in two-dimensional dielectric arrays. The photon dispersion relation as derived from the simulation is shown to agree well with that calculated by the plane wave expansion technique. The simulation is also used to explore the transmission properties and field distributions of localized electromagnetic modes associated with defects in the regular array.