Table of contents

The paraxial propagation of a beam incident along an optic axis of a biaxial crystal slab is studied in detail. Analytical descriptions are given for the Poggendorff bright and dark rings (associated with the conical singularity of the dispersion surface), and the axial spike (associated with the toroidal ring in the dispersion surface). The rings and spike depend on distance from the crystal. In sharpest focus, the rings are close and asymmetrical, and the spike is faint. Further away, the rings separate, they develop weak diffraction oscillations, and the spike grows in intensity. Eventually the oscillations disappear and the rings become symmetrical, and the axial spike dominates. The images depend on the profile of the incident beam; explicit formulae are given for a Gaussian beam and a coherently illuminated pinhole. Geometrical optics (extended to complex rays for the Gaussian beam) can describe some aspects of the images, in particular the Poggendorf dark ring, which arises from antifocusing and for which an explicit description is given.

We present a detailed analysis of vector modulational instabilities in ultra-small core optical fibres. The existence of new instability peaks emerging due to a strong waveguide contribution to the dispersion characteristic of these fibres is reported and their properties are analysed for the cases of low and high birefringence.

The fractional Fourier transform (FRT) is applied to a partially coherent pulse. On the basis of the definition of the FRT in the temporal domain, an analytical transformation formula is derived for the correlation function of a partially coherent Gaussian–Schell model (GSM) pulse passing through a FRT optical system by means of a tensor method. The corresponding tensor ABCD law for performing a FRT of a partially coherent GSM pulse is obtained. The properties of a partially coherent GSM pulse in the FRT plane are illustrated numerically. The derived formulae provide a powerful tool for analysing and calculating the FRT of a partially coherent GSM pulse in the temporal domain.

We present a study of fabrication and characterization of methyl red doped polystyrene waveguides. Optical absorption and mode effective refractive index measurements were performed on a large number of waveguides fabricated on glass substrates prepared by a dip coating technique. It is observed that doping of azo dye increases the index of refraction of the waveguides. The losses, however, decrease initially for dye concentration up to 0.1% and thereafter increase rapidly.

A Gaussian beam emerging out of a fibre and launched into a dye doped nematic liquid crystal experiences self-waveguiding for appropriate input power in the milliwatt range. This is a consequence of the large thermal non-linearity exhibited by such a material. Two modes have been reported, occurring for two different ranges of input powers. It has been shown previously that the high power mode occurs in the isotropic phase, the light being trapped within a tube of isotropic material. In this paper, the onset of the phase transition is experimentally detailed. A simulation of the beam propagation in both modes of self-waveguiding is presented as well, in full agreement with the experimental results obtained on the distribution of the optical field in a cross section of the waveguide. Finally, it is shown how it is possible to drive a probe beam through bent self-waveguides, generated by such isotropic tubes.

We present a spectrophotometric investigation of the processes in three azobenzene polymers induced by a linearly polarized Ar beam (488 nm). The polymers differ only in the length of the spacers attaching the chromophores to the main chain. The experimental results show that this difference determines not only the polymer structure; it also substantially influences the value of the photoinduced dichroism and its time behaviour after the illumination. We explain this on the basis of the different efficiencies of the two main photoinduced processes in the azobenzenes—the selective trans–cis isomerization and the reorientation of the trans-azobenzenes. The results from a real-time measurement of the photoinduced anisotropic changes in the refractive indices of the three polymers support this understanding.

Various all-liquid fabrication strategies to construct polymer micro-lens arrays on glass substrates are presented. Wettability patterns were made on glass substrates and a monomer was deposited selectively on circular domains on the substrate by making use of the hydrophobic effect, resulting in liquid micro-lenses. The liquid micro-lenses were solidified by polymerization. Three different variations of the selective wetting process were developed.

The polymer micro-lenses were characterized extensively by optical methods: focal length, surface roughness and optical aberrations of the lenses were determined by scanning white light interferometry. Due to their fabrication from the liquid phase, the root-mean-square surface roughness of the micro-lenses is between 10 and 35 nm, i.e. better than 1/10 wavelength. These measurements allowed comparison of the quality of lenses fabricated by these techniques.

Studies of the spatial distribution of time-harmonic optical fields inside a metallized cone, tapered to a subwavelength diameter, are reported. We consider the electric-type (TM_mn)wave with the lowest-order indices m = 0 and n = 1. For these waves we obtain exact analytical results for electromagnetic fields inside a conical waveguide with a loss-free dielectric core and perfectly conducting metallic walls. The contributions of different field components to the energy density distributions are explicitly expressed as functions of the radial and angular coordinates. We outline a technique for the evaluation of the optical fields in the near-field zone beyond the exit aperture. This provides tools for the calculation of the amplitude reflection coefficient taking into account the influence of a plane interface between the truncated cone and free space. Particular attention is paid to the calculations of the energy density at the exit aperture and the near-field transmission coefficient of an optical probe with a glass core. We present a detailed analysis of the optical transmission of such a waveguide as a function of the wavelength for large taper angles and various values of the aperture diameter reaching . The results obtained yield the range of the cone parameters and wavelengths in which one can achieve a high spatial resolution capability and sufficiently high transmission efficiency.

We investigate light-induced parametric holographic scattering in photorefractive Ba_0.77Ca_0.23TiO₃ (BCT) crystals illuminated with two coherent pump beams. On a screen behind the crystal we observe different scattering patterns which we attribute to certain phase-matching conditions. This allows for the identification of the involved four-wave mixing processes. We discuss the influence of space charge field limitations and the magnitude of the involved electrooptic tensor elements on the appearance of parametric scattering processes. In this context we compare our results with observations in other crystals.

Optically induced linear electrooptics and elastooptics effects were revealed in superconducting Hg_0.85Pb_0.18Ba_0.13CuO_4+δ (Hg-1201) (T_c = 112 K) single crystalline films deposited on an MgO surface. Investigations were done by a fast-resolved set-up, which allowed performing simultaneous measurements of birefringence in different external fields with a precision up to 10⁻⁷. It was shown that below the superconducting phase transition the piezooptic and electrooptic coefficients are substantially changed and show tremendous sensitivity to the appearance of superconductivity.

We investigated all-optical nonlinear switching and optical limiting properties of double-loop fibre Sagnac interferometers with symmetric power splitting. We show that significant nonlinear switching and optical power limiting can occur in this device due to nonlinear polarization rotation in the polarization maintaining (PM) fibres forming the Sagnac loops. This device can be considered as a lossless system when used as an optical limiter, since the excess input power, instead of being absorbed, would be reflected back and could be used for other purposes. For the same reason, this device would have higher damage threshold than the absorption-based optical limiters. Numerical simulations were conducted to explore the characteristics of this device. The switching and saturation power depends on the total length of fibres in the loop and the useful bandwidth depends on the polarization conditions and the birefringence of the PM fibres in the loop. We also investigated the intensity-dependent filtering property of this device and its relationship to the device bandwidth when used as either a nonlinear optical switch or an optical limiter.

We report a preliminary experimental study on a transmitting synthesized antenna constituted by a two-lens array. We realized experimentally the shifting of the entire far field diffraction pattern over more than 100 µrad. This corresponds to a change of the global transmitting direction, by acting simultaneously on the transmitting direction of the elementary fields (tilt) and the phase difference between the elementary fields (piston). The light flux transmitted from the source is forwarded to the elementary lenses of the antenna using optical fibres. We obtain experimentally an error signal permitting servo-control of the piston of the antenna around a given functioning point.

We report the position dependent tuning of fluorescence emission from rhodamine 6G doped polymethylmethacrylate film waveguide using a side illumination technique. The transmitted fluorescence as a function of the distance from the point of illumination is measured by translating the waveguide horizontally across a monochromatic light source. This technique has been utilized to characterize the optical loss in dye doped waveguides. We observe that the optical loss coefficients for shorter and longer distances of propagation through the dye doped waveguide are different. At longer distance of propagation a decrease in optical loss coefficient is observed.

With the rapid development of modern science, the techniques of fabricating two-dimensional surface-relief gratings with a hemispherical grid for visible light by chemical methods are proving to be successful. In this paper, we use a multi-level structure to simulate this kind of grating and adopt the Lagrange multiplier method to minimize the volume error, and the rigorous coupled-wave method is employed to analyse the vector diffraction properties of this kind of grating. By computer simulation and calculation, the relations between the reflectivity and the structure parameters of the gratings are presented, and the antireflective characteristics are also studied when visible light is incident upon them. The results show that this kind of grating is capable of reducing reflections, and could achieve very low reflectivity over a wide field of view and a wide waveband by choosing appropriate parameters. The results also show that the errors can be neglected when and the results are proved to be credible.

Light beams with elliptical flat-topped profiles are proposed. Such a beam can be expressed as a finite series of elliptical Gaussian beams with different parameters. Analytical propagation formulae for elliptical flat-topped beams passing through aligned and misaligned optical systems are derived. The fractional Fourier transform of the elliptical flat-topped beam is also studied. Our method provides a convenient way to describe the elliptical flat-topped beam and treat its propagation and transformation.

In this paper, we present a new technique for 3D shape measurement by recovering the 3D numerical reconstruction image of a digital hologram. Firstly, we fabricate a Fresnel digital hologram. Then, a number of 2D light field intensity distributions, which are in the reconstructed field of the digital hologram, are computed on the different depth planes. Finally, the focus measure evaluation of the grey level variance is applied. By finding the maximum focus measure, we decide on the depth information of each small image patch. The experiment confirms that the technique can materialize the 3D rebuilding of the reconstruction image and obtain 3D profile information on the object recorded. So digital holography can be a very promising technology for non-destructive 3D shape measurement.

A novel secure data storage architecture based on phase-encoded, thin film, Fourier-type polarization holograms is reported. Efficient data encryption and verification can be obtained by phase modulation of the reference wave introduced by a phase-type spatial light modulator that is imaged onto the hologram plane. A physical model is formulated including phase-encoding, non-linear storage medium behaviour and features of the proposed optical system. A method is proposed and implemented in the form of a custom modelling tool for generating code sets optimal in terms of code number and security level. The utility of the architecture and the achievable security level are experimentally demonstrated.

This work provides a detailed numerical analysis on the optimized compression of a dual frequency beat signal to generate a spectrally-enriched short-duration pulse train at 10 and 40 GHz frequencies. The effect of pre-chirping the beat signal on its compression characteristics using a dispersion imbalanced nonlinear optical loop mirror (DI-NOLM) is studied in detail. The output pulse quality is characterized by the shape of the pulse, peak-to-pedestal ratio, spectral content of the pulse and the energy contained in the pulse. It is found that suitable pre-chirping of the beat signal results in better shaping characteristics with significant pedestal suppression. The suitability of the shaped picosecond pulses for further spectral enrichment and the possibility of multi-wavelength generation are also studied.

Electro-optic coefficients can be defined in terms of the changes, which result from an applied electric field, to components of either the impermeability or electric susceptibility tensors. In this paper exact relationships are derived between electro-optic coefficients defined in these two ways for powers of the field up to four. Previously known approximate relationships are also discussed.

It is shown that the known rhomb-type total internal reflection phase retarders could be constructed from prisms. With all the advantages of the traditional designs preserved, these prism-type devices are of smaller volumes, convenient shapes, and light traverses shorter path lengths inside the devices. Moreover, the serious reduction in the volumes of the presented devices reduces the undesired stress birefringence which affects the retardance value. Two cases will be considered, which are the 4-reflection normal-incidence AD-1 and the super achromatic 2-reflection oblique-incidence quarter-wave phase retarders.

The rapid miniaturization in technologies for signal, image processing and optical communication increases the need for diffractive optical elements (DOEs), whose scales move closer to that of the illumination wavelength. The optical behaviour of DOEs is dependent on the polarization of the incident light, and so it is necessary to use a rigorous electromagnetic theory of diffraction to accurately design and analyse its performance. An iterative optimization algorithm for designing two dimensional, finite aperture, aperiodic diffractive optical elements based on a rigorous electromagnetic computation model—the finite-difference time-domain method—has been introduced. The algorithm can present rigorous design results with reasonable computational cost. Two kinds of DOEs designed by our algorithm, one for realizing the beam deflection used for integration optical systems, etc, and the other for realizing 1-to-2 beam split and 1-to-3 beam split, used for optical interconnections, etc, are presented. The simulated results have shown that our method has advantages of good optimization potential, high convergence and is an attractive approach for the rigorous design of DOEs.

The spectral properties of a single-mode planar deep waveguide Bragg grating (DWBG) operating on the first diffraction order are investigated by a spatial harmonics theoretical approach. This analytical method allows us to evaluate the most important features of the propagating fields. In particular, the slowdown of the group electromagnetic propagation near the photonic band-edges is accurately modelled with a minimal computational effort, which makes this tool useful for preliminary analysis of periodic waveguides.

A study is presented of the application of commercially available FBGs (fibre Bragg gratings) to measure temperature gradients. The optical measurement system is based on an electronically tuned solid-state laser, S-TLD. FBG sensor calibration is carried out using a dual Peltier water-cooled system, with which linear temperature gradients are applied to the FBG, resulting in chirping along the grating. Spectral parameters are extracted from reflection intensity spectra using LabView software. A 'look-up table' approach is then used as a means of solving the inverse problem. Quantified worst-case errors associated with each estimated temperature value are presented, and in this way the accuracy of the temperature measurements are discussed. Based on our experimental results a novel proposal to use pre-chirped FBGs to optimize temperature gradient sensing is made.

The nonlinear refraction and nonlinear absorption of ZrO₂ thin films doped with CdS and ZnS nanoparticles and various metals were investigated using the Z-scan technique. The structural properties of these films (TEM, EDX) were compared with their optical and nonlinear optical characteristics. The values of nonlinear refractive index of these films were measured to be of the order of at a laser intensity of 9 × 10⁹ W cm⁻². Sign variations of γ of ZrO₂:CdS(Cr) and ZrO₂:ZnS(Mn) films due to fifth-order nonlinearities were observed and analysed using the intensity-dependent Z-scan studies of nonlinear optical parameters.

In near-infrared diffuse optical tomography (DOT), a point excitation source model and a constant boundary conditions coefficient are usually used in image reconstruction. Here we describe an improved reconstruction algorithm that considers a distributed excitation source model and periodical boundary conditions coefficient. The improved algorithm is tested using several sets of phantom experimental data. The results show that the inclusion of the distributed source and periodical boundary conditions coefficient in our reconstruction algorithm can provide a better quality of image reconstruction, especially in terms of the reconstructed scattering coefficient value relative to that using point source and constant boundary conditions coefficient.

We report on a theoretical analysis of spatial modulation instability of coherent light propagating in a nonlinear medium with a noninstantaneous Kerr response. The latter is of a relaxing Debye type and originates from diffusive molecular reorientation. We consider the examples of harmonic and pulse-like perturbations superimposed either on a monochromatic temporal background or on a quasi-monochromatic Gaussian pulse. Our analysis reveals that the finite duration of the nonlinear response is responsible for spatiotemporal dynamic features that obviously do not exist within the framework of the usual scalar nonlinear Schrödinger equation, which models spatial modulation instability in an instantaneous Kerr medium.

Theoretical and experimental results are presented of investigation of an acousto-optic generator in which the feedback signal is formed by means of the effect of optical heterodyning. The process of oscillation establishment in the generator has been analysed by a computer simulation method. It has been shown that the effect of acousto-optic mode competition can lead to spectrum degeneracy and to the appearance of the single-mode regime of generation. The dependence of the spectral line width on system parameters has been studied as well.