Circularly birefringent optical fibres: new proposals

This paper deals with a family of azimuthally inhomogeneous fibres whose guided modes have been shown, in a previous paper, to have analytical expressions. It elaborates on the per-unit-length torque-induced circular birefringence as a function of the geometrical parameters and of the refractive indices. Recipes are given for enhancing the birefringence above that of an azimuthally homogeneous fibre. The coupling loss, which is unavoidable when such a fibre is fed by a circularly symmetrical fibre or beam, is shown not to be a major concern.     

Circularly birefringent optical fibres: new proposals

A new family of azimuthally inhomogeneous fibres is proposed and analysed theoretically. In contrast with previous similar structures, in these fibres one can find exact analytical expressions for the guided fields. Such modes could also be used as zeroorder approximations in analysisng other waveguides.This paper presents the basic theory and several examples illustrating how the main parameters (radii, number and width of the sectors, and refractive indices) affect the field profiles. We also present introductory, qualitative arguments on how this improved control over the field could be exploited in order to get either high circular birefringence or good matching of the guided modes to TEM₀₀ or LP₀₁ exciting fields.     

Evolution of optical solitons in nonlinear dispersive lossy fibres

A variational technique has been applied to the evolution of optical solitons in a nonlinear dispersive lossy fibre. An analytical model has been developed to describe the interaction of two co-propagating orthogonally polarized optical pulses in a lossy fibre, governed by a pair of coupled nonlinear Schroedinger equations, and the threshold amplitude at which the solitons form a bound state has been obtained.     

Propagation of chirped nonlinear pulses in a lossy highly birefringent single-mode optical fibre

A new model of the nonlinear propagation of optical solitons with initial linear frequency chirp through lossy highly birefringent fibres, allowing the improvement of the practical results concerning the distortionless signal transmission in ultrahigh-speed communications, is presented within the framework of a system of coupled nonlinear Schroedinger (CNLS) equations. This model demonstrates the existence of bound chirped solitons in a lossy single-mode highly birefringent fibre.     

Efficient non-linear optical fibres and their applications

There seem to be three possible approaches to achieve efficient non-linear optical effects in fibres: the increase of optical intensity in fibres, the control of group velocity dispersion in fibres and the use of efficient non-linear optical materials for fibres. From this viewpoint, high-numerical-aperture single-mode fibres with high optical intensity, single-mode fibres with extremely small dispersion at the operating wavelength and LiNbO₃ single-crystal fibres with efficient non-linear effects, are now being investigated. This paper describes the fabrication of these optical fibres, and their non-linear optical applications.     

A direct analysis of the dispersion equation in birefringent optical fibres

Weakly-guiding, anisotropic step-index fibres are considered in the case where the permittivity tensor is everywhere diagonalized in the axes of the fibre. A method is shown which allows one to derive a mode and its dispersion equation with arbitrary accuracy in terms of the powers of the weak-guiding parameter. The method is applied to the derivation of the leakage loss of they-polarized LP₀₁ mode when _x>_y.     

Bit-error rate evaluation for non-linear propagation in optical fibres

The influence of soliton time jitter on the bit-error rate in long-distance single-mode optical fibre transmission lines was studied. It was assumed that mutual interactions between two adjacent solitons were prevented by periodic amplification. It was found that the upper bit rate bound of practically acceptable non-linear transmission is about 50 Gbit s^–1. Above a bit rate of 50 Gbit s^–1 the advantage of non-linear (soliton) transmission, related to dispersion-free linear transmission, disappears.     

Polarization modulational instability in a birefringent optical fiber with fourth order dispersion

We obtain conditions for the occurrence of polarization modulational instability in the anomalous and normal dispersion regimes for the coupled nonlinear Schrödinger equation modelling fourth order dispersion effects when the linearly polarized pump is oriented at arbitrary angles with respect to the slow and fast axes of the birefringent fiber.     

Chirped solitary pulses in low birefringent optical fibres: a theoretical model

Non-linear coupling of polarized solitons with initial linear frequency chirp in a low birefringent optical fibre is analysed on the basis of a theoretical model of coupled non-linear Schrödinger equations. A threshold condition is obtained for the bound state of two polarizations corresponding to a phase mismatch in solition positions.     

Polarization effects in non-linear scattering

The problem of non-linear scattering of unpolarized light is discussed. New relationships between the scattering intensities for unpolarized and polarized incident light are given. It is shown that the usual equation connecting the depolarization ratios for plane and unpolarized light does not hold for non-linear scattering processes such as hyper-Rayleigh and hyper-Raman scattering. A new relation between the two is given in terms of two additional intensity ratios. An expression for the depolarization ratio for unpolarized light is also given in terms of the hyperpolarizability tensor.     

Polarization bistability in a nonlinear birefringent Fabry-Perot cavity

Polarization bistability and switching are described theoretically and demonstrated experimentally for a birefringent nonlinear Fabry-Perot cavity.     

Design of temperature-compensated elliptical core birefringent fibres

A new type of temperature compensated birefringent optical fibre is reported. The proposed fibre is designed aiming to minimising the stress birefringence by matching the thermal expansion coefficients between the core and the cladding and maximising the geometric birefringence by increasing the refractive index difference between the core and the cladding and increasing the core ellipticity. The optimum dopant concentrations satisfying the requirements is determined by solving a set of nonlinear algebraic equations. The fibre aimed at satisfying the requirements is fabricated and characterised. The experimental results show that the temperature dependence of the fibre birefringence is 6.97 × 10⁻⁴/°C. Methods of further improvement of the compensation are discussed and an alternative dopant material, titanium, for the improvement is suggested.     

Polarization Fizeau interferometer based on birefringent thin film

A polarization Fizeau interferometer based on birefringent thin film is presented. The interferometer adopts a birefringent thin film to obtain orthogonally polarized and strictly common-path reference and test beams. Advantages include ease of implementation on large-aperture interferometer, measuring test optics from long distance, and achieving high fringe visibility. The phase shift is obtained by combining a quarterwave plate and an analyzer. The concepts illustrated are verified experimentally.     

Eigenmodes expansions in lossy open waveguides (fibres)

The properties of the radiation eigenmodes propagating in lossy open waveguides (fibres) are examined by the-operator method. The radiation eigenmode fields are demonstrated to be transversely unbounded at infinity. For these modes the orthogonal relationships are derived by regularization of divergent integrals.     

Modulation instability in lossy silicon optical waveguide

Taking into account the small linear loss of silicon-on-insulator(SOI) waveguide, conditions and gain spectra of modulation instability(MI) induced by combined effects of self-phase modulation and waveguide dispersions are investigated. The impacts of various parameters to gain spectra of MI are analyzed theoretically. Results show that strong MI takes place even in the existence of low light power. The MI peak gain is 2 to 3 orders of magnitude larger than that achieved in optical fibers with the same light power. The linear loss of waveguide impacts gain spectra of MI, even within ultra-short propagation distance. The peak gain, peak gain frequency and bandwidth of gain spectra decrease to 66.828%, 41.683% and 41.6879% of their maxima at propagation distance z = 5mm, respectively.     

Electromagnetic enhancement in lossy optical transition metamaterials

We investigate the effect of anomalous field enhancement in metamaterials (MMs) where the effective refractive index gradually changes from positive to negative values, i.e., transition MMs. We demonstrate that considerable field enhancement can be achieved in lossy optical transition MMs that have electromagnetic material properties obtained from experimental data. The field enhancement factor is found to be polarization dependent and largely determined by the material parameters and the width of the transition layer.     

Second harmonic generation in birefringent optical fibers

Second harmonic generation in optical fibers is investigated. A scheme to combine material, modal and geometrical dispersion to achieve phase matching is proposed. Conversion efficiency, as a function of the fiber's parameters and the incoming electric field strength, is computed.     

Elliptically birefringent optical fiber transmission characteristics

Abstract

This paper presents a rigorous analysis of spun hi-bi (highly birefringent) fibers, loosely categorized as elliptically bi fibers, via the initial value problem approach. Two kinds of transmission problem are treated. In the single-eigenmode transmission regime, it is found that the major technological difficulty inherent to spun hi-bi fibers concerns excitation of the eigenmode, which strictly requires that the launched light be exactly oriented in conformity with one or the other local principal axes of the fiber and, meanwhile, that the ellipticity of the launched light be exactly equal to the eigen-ellipticity of the same fiber. In the second kind of problem that involves the interaction of two eigenmodes, a basic result is derived to show that an arbitrary polarization mode of excitation will reproduce itself in integer multiples of the local beat length. Such a kind of transmission regime is relieved of the excitation difficulty but, because of being severely length-sensitive, is inherently impractical from the application viewpoint.