Table of contents

We investigate the upper limits on the nonlinear coefficients obtainable in various silica-based photonic crystal fibre designs. For a fixed wavelength, the highest nonlinearities are found to occur when a substantial part of the field energy resides in air. The case of a silica strand in air is found to constitute an upper bound on the nonlinear coefficients obtained; however, all the designs investigated allow nonlinear coefficients larger than 50% of this limiting value. On the other hand, the choice of fibre design is found to have a significant influence on the dispersion properties.

We characterize a twisted nematic liquid crystal light modulator intended to modulate a laser beam in the mid-infrared spectral region. In particular, to test the device, we employ a CO₂ laser operating at 10.6µm. We analyse the optical characteristics and the response times of the modulator and consider the possibility of using it as a single element of a multiple-layered liquid crystal device.

This paper describes an advanced rangefinder equipped with a new image sensor capable of detecting the incident angle of a light stripe. The triangulation-based rangefinding approach is the one that is most widely used. However, ordinary triangulation-based rangefinders have hitherto been limited to measuring Lambertian objects because triangulation-based approaches are particularly susceptible to the object's surface reflectivity. We previously proposed a new type of rangefinder that relaxed the above-mentioned restrictions on ordinary rangefinders. However, the previous rangefinder had two unsolved problems: the image sensor used for the rangefinder was one-dimensional, and the experimental verification of the applied object was limited to specular objects and Lambertian objects. We here propose a rangefinder with an area image sensor, extending the capabilities of our previous sensor, which is capable of detecting the two-dimensional incident position and angle of incident light. We have made a prototype rangefinder, and experimentally compared it to our previous rangefinder and to a conventional rangefinder. The experimental results of the proposed rangefinder demonstrate that it measures three-dimensional (3D) shapes objects faster than the previous rangefinder and produces the same high level of accuracy in measuring 3D shapes regardless of the surface reflectance of the given objects.

A time-resolved diagnostic technique was used to investigate the emission spectra from the plasmas produced by 1.06 µm, 10 ns pulsed-laser ablation of metal Cu. The spectral line broadening was analysed according to the obtained spectra of the excited atoms, several broadening factors were estimated according to our experimental conditions and the results indicate that Stark broadening is the main broadening mechanism. Under the assumption of local thermodynamic equilibrium, electron number density was deduced from the Stark-broadening measurements. The time evolution and space profile of electron density were obtained. The combined results indicate that impact excitation and ionization occurring after the end of the laser pulse is more important than photon-induced processes produced by the laser pulse.

We have investigated optical transmission due to resonant tunnelling through metal/dielectric multilayers using the tight binding model and the invariant embedding method. The tight binding model for the resonant modes in metal/dielectric multilayers describes well the splitting of resonant modes and can predict the frequencies of split resonant modes in the transmission spectra of the metal/dielectric multilayers. We also show that the transmittance of a metal/dielectric multilayer increases as the thickness of the outer metal layers decreases with the total metal thickness kept constant. This is because the transmission of resonant modes increases as the thickness of the outer metal layers decreases. The thickness of the outer metal layers is found to change not only the transmittance but also the resonant transmission peak positions. When the thickness of each dielectric layer is fixed, the bandwidth of the transmission spectrum is mainly affected by the thickness of the metal layers inserted between the dielectric layers since this thickness determines the coupling strength between the resonant modes.

Planar structures containing oriented and ordered metallic nanoparticles with shapes lacking an inversion centre can act as a nonlinear medium for generation of second harmonic optical radiation by a process whose directional features resemble those of phase matched second harmonic generation (SHG) in bulk media. The nonlinearity of the metallic patterns stems from the asymmetric modulation of the local field inside nanoparticle by electron oscillations and is deeply rooted in the nanostructured nature of the system. The SHG efficiency is inversely proportional to the second power of the nanoparticle size.

In this paper we experimentally investigate the effect of non-scattering and low scattering regions in turbid media on diffusion-approximation-based image reconstruction in diffuse optical tomography. Using tomographic dc measurements, we demonstrate that both absorption and scattering images of non-scattering and low scattering heterogeneities embedded in a highly scattering background can be simultaneously reconstructed with our diffusion-approximation-based algorithm. The results show that the reconstructions of non-scattering and low scattering targets are quantitatively accurate in terms of the target size, location, and scattering coefficient value, while the absorption coefficient value of the recovered targets is unphysically underestimated under the conditions we examined.

We present a novel Bayesian method for pattern recognition in images affected by unknown optical degradations and additive noise. The method is based on a multiscale/multiorientation subband decomposition of both the matched filter (original object) and the degraded images. Using this image representation within the Bayesian framework, it is possible to make a coarse estimation of the unknown optical transfer function, which strongly simplifies the Bayesian estimation of the original pattern that most probably generated the observed image. The method has been implemented and compared to other previous methods through a realistic simulation. The images are degraded by different levels of both random (atmospheric turbulence) and deterministic (defocus) optical aberrations, as well as additive white Gaussian noise. The Bayesian method proved to be highly robust to both optical blur and noise, providing rates of correct responses significantly better than previous methods.

The Fourier modal method for modelling crossed gratings is revisited in the case of symmetrical grating profiles. It is shown that when a plane wave is symmetrically incident on a grating, whose surface profile is mirror-reflection symmetric with respect to the two planes that are perpendicular to the grating plane and contain the two orthogonal periodic directions of the grating, the computational effort can be substantially reduced. For example, at normal incidence with polarization parallel to one of the periodic directions, the special formulation takes only about 1/64 of the computation time that the general formulation takes. Because the use of the symmetry conserves memory and speeds up computation, more accurate results can be obtained and more difficult problems can be treated.

In magnetophotoelasticity, photoelastic models are investigated in transmitted light with the wave normal parallel to the magnetic field. Transformation of polarization in the model is caused both by stress birefringence and by the Faraday effect. Due to the latter, the integral Wertheim law is not valid and it is possible to measure stress distributions that are in equilibrium through the model like bending stresses in plates and parabolic residual stresses in glass plates. One of the drawbacks of magnetophotoelasticity is the need for very precise optical measurements since the angle of rotation of the plane of polarization is small. In this paper we investigate the case when multiple reflection of the light is used to increase the rotation of the plane of polarization. It is shown that 2n multiple reflections correspond to a (2n+1)^1/2-fold increase of the magnetic field.

A simple and efficient analytical method is presented for studying leaky modes in a buried rectangular waveguide on a high-refractive-index substrate. The cross-sectional profile of the refractive index for the buried rectangular waveguide is decomposed into three parts. The two main parts correspond to two independent multilayered slab waveguide structures and the remaining part is treated as a small perturbation term. The contributions from all three parts to the leakage loss are given analytically. Numerical results are presented and compared with those calculated with the semi-vectorial finite difference method. Fast speed, satisfactory accuracy and simplicity are the main advantages of the present method.

In this paper, we study the generation of high-quality ultrashort soliton pulses in dispersion-decreasing fibres with the inclusion of fibre loss in the integrable case. Five closely shaped profiles, namely linear, hyperbolic, exponential, logarithmic, and Gaussian, have been considered. The effects of these dispersion profiles and fibre loss on the performance of pulse compression in dispersion-decreasing fibres have been analysed. The pulse is compressed adiabatically by keeping the soliton property of conservation of pulse area provided the fibre loss is less than the critical value. Both analytical and graphical results show that, out of the five different profiles considered, the Gaussian profile is the optimum profile for achieving maximum pulse compression.

The use of bleaching techniques is perhaps one of the most popular methods of creating phase holograms from holographic emulsions. In this paper we will study the influence of the different experimental conditions on the refractive index modulation created inside the hologram when bleaching techniques are used. A comparison will be made between three different kinds of bleach: a reversal bleach, a conventional bleach and a fixation-free rehalogenating bleach. It will be demonstrated that the necessary refractive index modulation can be achieved with conventional and fixation-free rehalogenating bleaches. The values of the diffraction efficiency are only limited by the absorption and scattering, which is very high in the case of conventional bleaches. It will also be found that the lowest values of absorption and scattering are obtained when the reversal bleach is used to obtain the volume holograms. In this case, the refractive index modulation stored in the hologram is not high and so the final diffraction efficiency is limited.

Based on the fact that a hard aperture function can be expanded by an approximate sum of complex Gaussian functions with finite numbers, the approximate analytical expressions for the off-axial Hermite–cosh-Gaussian beams and off-axial cosh-Gaussian beams passing through an apertured paraxial ABCD optical system have been derived. By doing some numerical simulations the results obtained by the approximate analytical expression are compared with those obtained by the diffraction integral formula directly. It is shown that the former has good consistency with the latter and that our method can significantly improve the numerical calculation efficiency. The approximate analytical expressions for the kurtosis parameter and beam radius of an off-axial Hermite–cosh-Gaussian beam through an apertured paraxial ABCD optical system are also derived.

The analysis of the resonant mechanism for silicon micro-resonators under optical excitation based on the one-dimensional heat-flow theory and the thermal theory of elasticity is presented. Three effects, including the optical pressure effect, the longitudinal thermal strain effect and the Bi-coating effect, were investigated. After comparison with the other effects, the Bi-coating effect was found to be the most important effect in the optical excitation. A new amplitude model of a bi-layered silicon micro-resonator was deduced which was verified by experimental results.

A new method based on optical tweezers technology is reported for the isolation of a single chromosome. A rice cell suspended in liquid was first fragmented by laser pulses (optical scalpel). Then a single released chromosome from the cell was manipulated and pulled away from other cells and oddments by optical tweezers without any direct mechanical contact. Finally the isolated single chromosome was extracted individually into a glass capillary nearby. After molecular cloning of the isolated chromosome, we obtained some specific DNA segments from the single chromosome. All these segments can be used for rice genomic sequencing. Different methods of extracting a single chromosome are compared. The advantages of optical micromanipulation method are summarized.

This paper presents a kind of inorganic–organic hybrid SiO₂/ZrO₂ sol–gel material successfully developed for use in a single-step fabrication of a refractive spherical microlens array (MLA). The single-step fabrication method employs direct laser writing technique to generate a designed spherical MLA on a layer of the sol–gel film. The method offers unique advantages over the conventional photoresist-based fabrication technology by eliminating the need for using a complex ion beam etching process and costly high-energy beam-sensitive grey scale mask. The measured results show that the microlenses have uniform dimensions, smooth surfaces, and strong focusing function. Furthermore, the fabricated MLA, as a coupler between a laser diode and a single mode fibre, has greatly improved the coupling efficiency between them.

In this paper we present a number of measurements of the magneto-optic (MO) polarization rotation effect in MO glasses. We then show three design concepts of MO based devices that are of interest for optical communications applications. The first device is a tunable optical polarization controller, the second is a polarization filter that passes through only a specific point on the Poincaré sphere and the third is a MO Fabry–Perot resonator that enhances the MO effect and can be used inside an all-optical switch or as part of a polarization coding module.

This paper introduces the principle and methods of using a cross diffraction grating as a 2D displacement sensor. According to the characteristic that second-order diffraction beams will produce a phase shift with the grating's motion and grating diffraction beams' tracing, the arrangement of the light routine is designed to form interference strips by congregating them. Based on arrangement of the light routine and some knowledge including the properties of light travel and reflection, the principle of vertex coordinate transformation and the solution for the intersection point of a line and plane, the direction and allocation of all diffraction beams are deduced. As a result, the relationship model between the diffraction beam (direction and position of the projective point on a detector) and the grating deflection around the X,Y, and Z axes and translation along the Z axis is built. The projective point and interference stripes' shape are simulated by Matlab and the sources of influencing the stripes' quality are analysed: for interference strips with relative X motion, the direction undergoes a great change when rotating around the X axis, the stripes' space is changed dramatically for deflection around the Z axis, direction and the space is hardly changed with movement along the Z axis and rotation around the Y axis. At the end, this paper discusses the qualitative relation between measurement error and abnormal translation or deflection of the grating.

We describe a simple experiment which allows unequivocal determination of optical phase change upon reflection of light at the mica–silver interface. While the physical origin of such a phase change at the dielectric–metal interface is well understood to lie in absorption of electromagnetic energy by the metal, inconsistency and ambiguity has persisted as to what its sign and magnitude should be in the field of thin film optics. Most commonly, it has been assigned to be negative for mathematical convenience or just arbitrarily. Our finding shows that with the convention exp(−i ωt) for time dependence of the electromagnetic wave, the phase change at the interface between mica and the thin silver film is necessarily positive and its magnitude falls between π and 3π/2 for silver thicknesses down to nanometres. This gives a physically reasonable correspondence to an increased equivalent thickness of the dielectric material, and it clarifies the assignment of interference orders in the harmonic series in a spectrum.

The optimum choice of k-point for supercell calculations of defect states in a three-dimensional photonic crystal is investigated for the case of a supercell with a simple cubic (SC) structure. By using the k-point (1/4,1/4,1/4) it is possible to eliminate the symmetric part of the repeated-image couplings for the first three neighbour shells in the SC lattice. This result is shown to hold also for the case of non-equivalent axes (e.g. a distorted lattice, or an asymmetric defect structure). A specific example of a donor defect in a woodpile structure demonstrates that use of this k-point can lead to order-of-magnitude gains in computational efficiency.

Based on the datum-line of the non-diffracting beam produced by an axicon, which is a thin lens with a weak conical surface on one face, a new measurement method of spatial straightness error has been developed in this paper, in combination with moiré-fringe technology. When a ring grating is illuminated by a non-diffracting beam, a moiré-fringe image is formed, from which the deviation of the centre of the ring grating to the centre of the Bessel fringe ring on the section of the non-diffracting beam, the offset, can be computed. The centre of the Bessel fringe ring embodies the non-diffracting beam datum-line, so the offset is also to the non-diffracting beam datum-line. Then, the spatial straightness error can be obtained from the set of the offsets corresponding to the section along the non-diffracting beam. In order to verify the performance of the new method, theoretical and experimental analyses were conducted. The results show that the presented method is effective and possesses a high system resolution in measuring the spatial straightness error. The measurement uncertainty is close to 0.28 µm in the measuring distance range 0.4–1.8 m behind the axicon.

Periodic layered media can reflect strongly for all incident angles and polarizations in a given frequency range. Quarter-wave stacks at normal incidence are commonplace in the design of such omnidirectional reflectors. We discuss alternative design criteria to optimize these systems.

In this paper a new technique is presented for adjusting the playback wavelength of reflection holograms recorded in methylene-blue sensitized dichromated gelatin (MBDCG). During the fabrication of the MBDCG medium, a water-soluble organic reagent is introduced homogeneously into the photosensitive layer as a preswelling reagent and wavelength adjuster. This method has a wide wavelength adjustment range (up to 200 nm, almost covering the entire visible spectral region), high diffraction efficiency and signal-to-noise ratio, and can be applied to quantitatively adjust the playback wavelength of reflection hologram by changing the concentration of the preswelling reagent. Its possible applications include colour image display, holographic optical elements, optical communication and optical anti-counterfeiting.

In this work we study the two-dimensional (2D) multimode interference and self-imaging in a dielectric optical waveguide. The studied waveguide is assumed to be multimode in both the transverse directions x and y. The conditions required for single and multiple image formations are developed. New types of image are also found. The obtained results are verified using a three-dimensional FDBPM technique. Applications of the 2D interference in waveguides for 2D optical power splitting are also investigated.

We show that free-space pulse solutions of Maxwell's equations which are localized in space and time have energy greater than c times their momentum. Thus a Lorentz transformation to a zero-momentum frame is always possible, in contradistinction to Einstein's light quantum, for which a zero-momentum frame does not exist. However, free-space pulse solutions of Maxwell's equations are not subluminal: their momentum is less than their energy divided by c due to necessary spreading or convergence.

Based on the fact that a hard-edge aperture function can be expanded into a finite sum of complex Gaussian functions, the approximate analytical expressions for the output field distribution of a flattened Gaussian beam passing through the apertured fractional Fourier transforming systems are derived. By using the approximate analytical formulae and diffraction integral formulae, some numerical simulation comparisons are done, and it is shown that our method can significantly improve the numerical calculation efficiency.