Table of contents

The spatiotemporal transformation of a focused femtosecond laser pulse during nonlinear propagation in fused silica was investigated numerically. We found that the on-axis intensity of the pulse experiences a shortening of pulse duration, formation of a double peak and then an evolution to one peak as a result of the interplay of plasma-induced defocusing, self-focusing and diffraction. By contrast, the temporal profile of the space integrated power of the pulse retains the initial shape but with a slight depletion due to the nonlinear loss. As a result, although it is possible to obtain a shortened pulse by selecting the central portion of the pulse at appropriate locations, the percentage of available energy cannot exceed the percentage of pulse shortening.

A mathematical model describing the absorption and oscillation processes of an intracavity Cr⁴⁺:YAG crystal pumped by a Nd-glass laser has been developed, in order to describe the temporal behaviour of a laser-absorber system. The model assumes that the Cr⁴⁺ ions are excited to a higher level by excited state absorption, followed by relaxation directly to the upper laser level through a fast channel, and indirectly through a slow proposed intermediate channel at different lifetimes.

The model offers simple kinetic mechanisms for pulsed solid state lasers and also explains the influence of the variations of the laser input parameters (pumping rate, maximum amplification coefficient and loss coefficient) on the output pulse characteristics of passive Q-switched Nd-glass and pulsed Cr⁴⁺:YAG lasers. The model estimates the temporal behaviour of the population densities of different levels and laser beam densities, and predicts the nanosecond output laser pulses of passive Q-switched Nd-glass lasers and pulsed Cr⁴⁺:YAG lasers. The calculated results are in good agreement with the available experimental and theoretical data in the literature.

The control of the exposure patterns, which maximizes the capacity of a holographic recording medium, is of critical importance in any holographic data storage system. In this paper, we develop a method to theoretically explore linear photopolymer storage media in which a polymerization driven monomer diffusion process takes place. Using this method we examine a technique, involving randomly shifting the exposure pattern between exposures, to increase material capacity. The randomization acts to reduce the effects of past exposures by reducing the monomer concentration spatial distribution which arises due to those exposures. Following a detailed description of a series of assumptions and approximations, we derive an analytic formula, which allows us to explore the results of applying the technique. It is shown that, in the particular situation discussed, randomization can provide a useful tool in reducing constraints on the relaxation times necessary between exposures.

Seeing monitoring in astronomy is widely based on the statistical analysis of angle-of-arrival (AA) fluctuations, which are usually modelled in the framework of the near-field approximation where diffraction through turbulence is ignored. They are consequently believed to be completely independent of wavelength. We discuss in this paper the influence of Fresnel diffraction from distant turbulence layers on multi-wavelength (polychromatic) AA fluctuations. For this purpose we propose a model for polychromatic AA fluctuations in weak turbulence conditions and derive an analytical model for their variance in the case where scintillation is ignored. We also present a numerical simulation that includes scintillation and justifies that this latter may be neglected in the analytical model.

The immersion factors I_f of sample RAMSES radiance and irradiance hyper-spectral radiometers were experimentally determined in the 400–700 nm spectral range using state of the art protocols, with the aim of supporting the use of these sensors in accurate radiometric measurements for ocean colour applications. Values of I_f obtained from the characterization of RAMSES-ACC-VIS irradiance sensors showed a dispersion of 2% in the blue increasing to 4% in the red spectral region. Results obtained from the characterization of RAMSES-MRC-VIS radiance sensors having a 20° in-air full-angle field of view highlighted an average spectral bias of −2.4% in the theoretical I_f values determined just using the refractive indices of the intervening medium and of the optical window. This average spectral bias exhibited values of −0.8% for the RAMSES-ARC-VIS radiance sensors having a 7° in-air full-angle field of view. Using these results, reference I_f values have been proposed for the RAMSES series of hyper-spectral radiometers.

The structural modes of a 2D periodic array of metallic cylinders near a dielectric interface are studied, for large and small wavelength-to-period ratios. We show that there are two clearly distinguished kinds of resonances: waveguide modes of the gap between the array and the interface, and eigenmode excitations (plasmons). While waveguide modes are present for both polarizations, surface plasmon polaritons (SPPs) only occur for p polarization, and, under particular conditions, this mechanism can produce an efficient coupling (about 60%) between the incident evanescent wave and the eigenmodes supported by the structure, which produces an enhanced transmitted propagating order. The response of the structure is analysed in detail by varying its relevant parameters, paying particular attention to the interplay between SPPs and Rayleigh anomalies, and its effect on the grating response.

We realized a planar multireflection glass system to investigate the localized surface plasmon resonance (LSPR) of gold nanoparticles. These nanospheres were realized in situ using an original and simple chemical growth method that is described. We were able to observe resonance phenomena as reflected by variations in the spectrum and as enhancements in the refractive index sensing ability. The system was able to clearly discern 2% sucrose solution and demonstrated outstanding linearity and reproducibility. We believe this study could be useful for elucidating the fundamental processes in nanoparticle LSPR and contribute to the realization of new and more efficient sensors.

This paper describes a laboratory microsystem (Microlab) used to measure the concentration of biomolecules in biological fluids. Rather than just one measurement channel, it comprises 16 optical channels that enable the measurement of the concentration of 16 different biomolecules with the same device. An array of 16 optical filters based on Fabry–Perot thin-film optical resonators has been designed. Each optical channel is sensitive in a single wavelength with a FWHM less than 6 nm and with a peak intensity higher than 86%. The 16 optical channel array fabrication requires only four masks, used with different deposition time. The Microlab can easily be tuned during fabrication to analyse different biomolecules only by adjusting the deposition times without affecting the device layout. A commercially available passband optical filter with a passband wavelength in the range 450–650 nm is used. The Microlab requires only a white light source for illumination due to the use of selective optical filters. The quantitative measurement of uric acid in urine is demonstrated. Such a device is extremely suitable for clinical diagnosis application in clinical laboratories and at a patient's home because of its small size, low cost and portability.

The problem of phase reconstruction from intensity is considered in the paraxial geometrical optics regime. The data consist of intensity measurements on two planes orthogonal to the direction of propagation. A new variational principle is used to find the ray mapping between the planes. The mapping is found by minimizing an appropriate function. The minimization process involves a nonlinear partial differential equation. A numerical algorithm for solving this equation is described. The phase is then integrated from the ray mapping. Finally, the method and code are examined by applying them to simulated data.

An improved method for testing the collimation of an incoherent optical beam using Lau interferometry is presented. The experimental setup consists of a white light source and a set of three identical gratings. A source grating G₁ is placed in front of a collimating lens. Beyond the collimating lens gratings G₂ and G₃ are so aligned that the lines of the gratings make a small but equal and opposite angle with the vertical. The self-image of grating G₂ is superimposed on the grating G₃, resulting in the generation of moiré fringes. Decollimation of the optical beam results in a change in the dimensions of the self-image. This change is detected using the magnification effect of the moiré method. The horizontal moiré fringes are indicative of collimation of the optical beam. Deviation from the horizontal inclination of the moiré fringes indicates setting in of decollimation. Theoretical and experimental investigations to determine the sensitivity achievable in this collimation testing technique are undertaken and results of the investigations are reported. Experimental results are in close agreement with the theoretical predictions.

An improved description of Jones vectors of the electric fields of incident and refracted rays in a tilted birefringent plate were derived. The incident and refracted electric fields were both expressed as linear combinations of ordinary (O) and extraordinary (E) components. For the O polarization inside the plate, its electric field was directly obtained according to the ellipsoid of wave normals. The 'O component' of the incident electric field was educed by using Fresnel's equations. For the E polarization inside the plate, its displacement vector was decided by the ellipsoid of wave normals, but the electric field vector was educed with the aid of material equations. Moreover, in order to get the 'E component' of the incident electric field, Fresnel's equations were modified to satisfy the boundary conditions. Numerical examples were given to compare the difference between conventional and modified Fresnel's equations. Finally, Jones vectors for the electric fields of the incident and refracted rays were obtained.

A four-frame phase shift method and an associated algorithm using unequal phase steps are presented. The unique advantage of this method is that it becomes insensitive to phase shifter nonlinearity because of the performance of a special procedure, in which the phase shifts are shared out between the reference beam and the object beam. By this means, any phase shifter can work as long as one phase shift is accurately known. On the basis of the technique, a simple calibration method for the linear phase shifter is suggested. The influences of phase shifter miscalibration, detector nonlinearity and random noise on the algorithm are investigated, and the optimal phase shifts are given.

If the diffraction pattern of the hyperbolic umbilic diffraction catastrophe is produced by an optical system of increasing aperture, it passes continuously from the two-dimensional system of Airy rings in the focal plane, made by a very small aperture, to the full three-dimensional pattern corresponding to infinite aperture. The paper studies this transition by examining the truncated diffraction integral and following the evolution of the wave dislocation lines (phase singularities) on which the pattern is based. The seed of the evolution from a two-dimensional to a three-dimensional pattern turns out to be already present asymptotically even for the smallest aperture: namely, a column of small dislocation rings very close to the axis that stream in procession towards the focal plane, and become dislocations lying in the Airy fringe surfaces that run parallel to the main fold caustic, only to disappear ultimately by retreat to infinity. The evolution into the final dislocation pattern takes place by a sequence of primitive local topological events, such as reconnection (hyperbolic interchange) and ring creation.

A new method to measure the modulator chirp parameter is proposed. This method will be performed in two steps. In the first step, the frequency separation between two optical signals passing through a phase conjugator is changed. This produces a resonance reference frequency as a result of the accumulated fibre chromatic dispersion (CD). In the second step, a RF modulated signal passes though the same length of fibre as in the first step. A second resonance frequency is produced as a result of fibre CD and modulator chirp. The difference between the two resonance frequencies is used to measure the modulator chirp. The new method successfully achieves a wide measurement range with low modulating frequencies.

We investigate triplet bound states with a new symmetry, called 'cis', using the cubic–quintic CGL equation. We show that the leading term of the functional J[ψ], which governs the evolution of the momentum of the solution to the CGL equation, vanishes for the cis-symmetry. Numerical investigations show that stable cis triplet bound states are solutions of the CGL equation. Quasi-stable cis-states are also found, and also a stable quasi-stationary asymmetrical triplet state. Then we show that it is possible to experimentally distinguish between the trans and cis triplet states, using either the optical spectrum or the collinear autocorrelation trace.

This paper presents approaches to achieve high reflective metal mirrors from 1064 nm—the near infrared (NIR)—down to 150 nm—the vacuum ultraviolet (VUV) spectral region. Metal (aluminium or silver) sublayers with dielectric (oxide and fluoride) protected and enhanced optical coatings are sought as solutions for high reflectance with low thickness and stress. Aluminium and silver with oxide enhanced optical coatings can provide reflectance above 99% for wavelengths at 248, 308, 633 and 1064 nm, with total thickness varying from 400 to 950 nm. As for the VUV spectral region, high reflectance around 90% has been realized based on an aluminium layer with fluoride capping layers. Single fluoride and oxide materials have achieved reflectance above 90% at 193 nm. Due to the short total thickness and specially optimized deposition process, these highly reflecting coatings possess stresses lower than ± 60 MPa, which is very promising for stress-sensitive substrates, such as micro-structured mirrors.

Resonant grating waveguide structures are discussed as extreme narrow-band reflection filters. Their spectral response may shift due to environmental impact, e.g. from temperature, pressure or humidity, making it interesting for sensor applications. The present paper derives approximate analytical expressions for spectral rejection band shifts caused by changes in the refractive index of the waveguiding layer and compares these expressions with corresponding shifts of the reflection band of high–low multilayer stacks due to environmental changes. It is shown that a minimum shift is expected for the highest grating period that fulfils the coupling condition.

A theoretical approach to determine the optimal form of the near-field optical microscope probe is proposed. An analytical expression of the optimal probe form with subwavelength aperture has been obtained. The advantages of the probe with the optimal form are illustrated using numerical calculations. The conducted calculations show ten times greater light throughput compared to the conical probe form and the reception possibility of the more compactly localized light at the output probe aperture, which could indicate better spatial resolution of the optical images in the near-field optical technique using an optimal probe.

We present a design for a high-efficiency polarization beam splitter based on a two-dimensional hexagonal polymer photonic crystal, which can be fabricated directly in SU-8 photoresist using interference lithography. Computer simulations show that more than 99.9% of TM-polarized light is reflected by the polarization beam splitter, whereas 98.9% of TE-polarized light propagates through the polarization beam splitter over the wavelength range 1.53–1.62 µm (C and L bands for optical communication) with good angular insensitivity of about 10°. The present polarization beam splitter has a reasonably good tolerance of fabrication errors.

This paper proposes a modified Monte Carlo method, namely the multilayer Monte Carlo simulation method, for simulating the return signal of light scattered by bubbles underwater. A curve representing the backscattering signal is obtained. The results predict a second backscattering peak in the total backscattering signal curve caused by the bubble layer underwater. The return time of the second peak is used to estimate the position of the bubble layer. Both the bubble layer's thickness and the distance between the emitter and the bubble layer will obviously affect the backscattering signal. A new theoretical method for detecting the bubble layer underwater and estimating its properties is presented.

We investigated electromagnetic wave propagation through one-dimensional stacks of alternating positive and negative refractive index layers arranged as truncated quasi-periodic (fractal) Cantor multilayers. We considered the occurrence of phase compensation in these structures. In our calculation we utilized the transfer matrix technique and applied the Kirchhoff's second law to calculate emittance and absorptance modification by negative index metamaterials. We took into account dispersion and absorptive losses and analysed both on-axis and off-axis radiation. We showed that Cantor structures formed by inserting negative refractive index layers as a substitution part in the multilayer lattices enable tailoring of both spectral and angular dependences of emittance/absorptance. We conclude that phase compensation could significantly extend the applicability of Cantor-type multilayers containing negative refractive index materials.