Table of contents

Guest editors: Dr Peter Loosen Fraunhofer Institute for Laser Technology, Germany Dr Michael Mikulla Fraunhofer Institute for Applied Solid State Physics, Germany

A future issue of Journal of Optics A: Pure and Applied Optics will be devoted to recent advances in theoretical, experimental and technical aspects of high-power semiconductor lasers and systems.

Topics to be covered include, but are not limited to:

new designs of single-emitters and diode laser bars for increased output power and beam quality reliability and lifetime of diode laser bars packaging of high-power diode laser bars hybrid systems for increased brightness of diode lasers techniques for beam shaping and fibre coupling diode laser stacks and systems application of high-power diode lasers in materials processing, medicine and optical telecommunications high-power diode lasers as pump sources for fibre lasers and amplifiers

The topical issue is scheduled for publication in January 2003. All papers will be refereed according to the high standards of the journal and authors will receive 25 free offprints of their published paper. There are no page charges for publication. Manuscripts should be prepared following the general guidelines for authors published in the journal. Full instructions can be found at Notes for Authors. Please include a covering letter stating that the submission is intended for the High-power semiconductor lasers and systems special issue, to avoid treatment as a regular submission.

Manuscripts should be submitted to the Publisher by 1 June 2002, although authors are strongly encouraged to submit their work as soon as possible.

The thermal distortions in a continuous-wave end-pumped Nd:YVO₄ laser are experimentally investigated by use of an external TEM₀₀ probe beam (TEM standing for transverse electric magnetic). After a round-trip of this probe through the active medium, three irradiance profiles are recorded along its propagation axis. The distorted phase pattern of this probe beam is then reconstructed from these profiles by means of a Gerchberg-Saxton-like iterative algorithm. To identify the contribution of each kind of aberration in the phase profile, we have computed a Zernike polynomial expansion, and have shown that the main terms constituting the aberration are (in order) defocusing (optical path difference (OPD) 0.7 λ FWHM (full width at half-maximum)), spherical aberration (OPD 0.3 λ FWHM), and first-order astigmatism (OPD of ±0.14 λ_p FWHM). Moreover, all higher-order terms result in a significant phase peak (λ/4 FWHM over 400 µm).

We present a theoretical, numerical and experimental study of the power transfer at misaligned connections between multimode fibres, demonstrating that the coupling coefficients between incident and excited modes depend on the orientation of the incident modes with respect to the misalignment direction. Taking into account this parameter we derive simple analytical expressions for the power coupling coefficients. These results, valid for fibres of any index profile, help us to better understand mode propagation phenomena and contribute to more precise power coupling calculations, especially in the case of weakly multimode systems where modal noise is more important.

A theoretical investigation of mixed isotropic-anisotropic Bragg diffraction of light by ultrasound is presented. This case of acousto-optic interaction appears as isotropic and anisotropic scattering taking place simultaneously, and is typical only of a birefringent crystal. The problem of light diffraction is examined under the approximation of plane waves using coupled waves theory. In the case of absolute Bragg matching, an analytical solution of the diffraction problem has been obtained. The mixed scattering has also been considered in the case of weak violation of phase synchronism conditions. Results of numerical solutions with respect to the light intensities of the diffracted beams are presented and the characteristic features of the mixed interaction are described.

Through simulations and experiments we found considerable photonic bandgaps (PBGs) in two-dimensional structures with short-range periodicity (SRP) and without any spatial symmetry. A multiple-scattering method is used in our numerical investigations. Our experiments are performed in the microwave region with a microwavevector network analyser. The short-range periodic structures used in the investigations are produced through the arrangement of four certain basic units in a Fibonacci sequence in two dimensions. It is interesting that the PBG positions and widths are invariant for different translational cutouts and for different thickness of these SRP structures, showing that the PBG properties of the structures under consideration are decided by their SRP and not by their patterns. The physics behind the results is explored.

We present a simple way of adjusting the output performance of end-pumped passively Q-switched lasers. The method is based on changing the size of the pump mode within the active medium, and the approach is demonstrated both in theory and in practice. We show that it is possible to modify the repetition rate from 7 kHz to 1 MHz and the pulse length from 6 to 30 ns for the laser used in the experiments.

A new set of flattened light beams termed higher-order cosh-Gaussian beams which may be a solution of the paraxial wave equation for propagation in complex optical systems is introduced. Based on this, the beam propagation factor (M²_x-factor) is derived, schemes to generate the light beams are proposed and the propagation features in free space are investigated.

We consider the conversion processes of a `whispering gallery' light wave in a taper fibre in which a travelling acoustic wave propagates into its taper end. Both spectral and temporal approaches are used. The results are compared with the results of the classical theory of interaction of many plane waves in a nonlinear medium. It is shown that phase matching between light and acoustic waves is supported automatically due to the corresponding increase in the light frequency in the process of joint movement of the waves in the taper end of the fibre. The material dispersion of the optical medium, the variance in temperature and the precision of geometrical sizes of the fibre do not considerably affect the phase matching.

Polarization dependence of stimulated Raman scattering is a fundamental physical effect, which influences the design and application of Raman amplifiers for optical networks. We report experimental data of the study of polarization dependence of Raman gain in dispersion compensating fibres (DCF). The investigation displays a strong variation of the polarization dependence of Raman gain versus average polarization mode dispersion (PMD) values of DCF for both co-propagating and counter-propagating pump configurations. The reported data show, for the first time to our knowledge, that, for large PMD values, such a dependence may be as small as to be comparable with the accuracy of the measurements. The results of the study may be used to simplify the design of Raman amplifiers to avoid depolarization of pump sources that are currently used to decrease polarization dependence of Raman gain.

Under the condition of minimum average pump-spot sizes analytical expressions have been derived for the mode size and maximum output efficiency by including the effect of pump-beam quality, length and properties of the active medium. These expressions provide a straightforward procedure to design fibre-coupled laser diode end-pumped lasers with small active medium.

In an optical amplifier, when the gain is non-linear due to gain saturation, the intensity fluctuation statistics are significantly altered from the statistics of the negative-exponential intensity probability distribution function (IPDF) for spontaneous emission and unsaturated amplifiers. We have systematically computed bivariate moments, trivariate moments and central moments, tetravariate moments, as well as bivariate and trivariate cumulants of intensity fluctuations for non-linear coupled-mode optical systems starting from a k-variable multivariate IPDF of a system of n(k<n) coupled modes. The multivariate IPDF is a negative polynomial that rests on the assumption that the variation of intensity has a maximum-intensity distribution with the constraint that the total intensity is constant. The assumption holds for multimode optical systems including a thin, long, mirrorless, gas laser where under heavy gain saturation the output tends to become constant although the intensity of any mode displays rapid fluctuations about the average intensity. The multivariate IPDF reduces to the negative-exponential IPDF of spontaneous emission when n is infinite, as expected. The new results for the multivariate characteristics of the intensity fluctuations have shown characteristic signatures that depend on the number of interacting modes, n. The results are also useful for testing the multivariate IPDFs and identifying n for heavily saturated coupled-mode optical systems.

On the basis of the paraxial wave equation of ultrashort pulsed beams, the free-space propagation equation of the space-time Wigner distribution function has been derived. The propagation laws for the moments of ultrashort pulsed beams are given. A class of propagation-invariant moments is discovered. The second-order temporal central moment is a parabolic function of the propagation distance and the root-mean-square width of ultrashort pulses increases during the propagation.

Ganci's experiment is analysed by means of matrix optics and numerical simulation. It is shown that the boundary diffraction wave with phase jump π is a physical reality.

In this paper, we have studied the absorption and scattering of a Gaussian beam and a plane wave by an oblate particle. The scattered intensities distribution for an oblate particle with different size parameters are computed in the Gaussian beam and plane wave.

Because of the large beam diameter and special characteristics of the non-diffracting beam produced by an axicon, the non-diffracting beam can tolerate the eccentricity of the incident laser beam's axis to the axicon. The non-diffracting beam obtains the additional tolerance property to the incident light beam's deflection when adopting a kind of incident divergent sphere Gauss wave, and this is proved in this paper through theoretical analysis and practical tests. The obtained conclusion is that the axis of the non-diffracting beam produced by an axicon is very stable and can be adopted as the datum line to measure the spatial straightness error in continuous working distance, which may be short, medium or long.

The variational principle is employed to study chirped solitons that propagate through optical fibres and is governed by the dispersion-managed nonlinear Schrödinger's equation. In this paper, we have considered both kinds of fibres, namely polarization preserving and birefringent types. The study is extended to obtain the adiabatic evolution of the parameters of such solitons in the presence of perturbation terms for both types of fibres.

This paper suggests the possibility of partial compensation of spherical aberration with a polarization-masked lens aperture. The principle is based on the fact that any pre-specified phase step can be introduced between two zones of a lens aperture masked by suitably oriented linear polarizers and employing an elliptically polarized imaging beam. By comparison of the point-spread function of such a polarization-masked imaging system with that of an ideal lens, it has been shown that the effect of spherical aberration can be appreciably compensated by making an appropriate choice of the polarization parameters involved.

Fabrication of microstructures by tight-focusing intense femtosecond laser pulses into wide-bandgap materials, such as fused silica, K9 optical glass and LiNbO₃ crystal, was investigated. Micrometre or sub-micrometre voids and filaments have been observed in all three wide-bandgap materials. Another microstructure, a void string, has been observed only in fused silica. The formation of micrometre or sub-micrometre voids, filaments and void strings is discussed with the multiphoton ionization process, the avalanche ionization process and the nonlinear laser pulse self-action effect when femtosecond laser pulses propagate in wide-bandgap materials.

Here, exact analytical forms of the eigenpolarizations in the anisotropic dielectric-magnetic media discussed in our previous paper (Khorasani S 2001 J. Opt. A: Pure Appl. Opt.3 144) are reported and extended to gyrotropic media.