Variational Method in the Study of Optical Fibres

Abstract

The dispersion characteristics are needed to evaluate the performance of optical fibres. In general, these will have to be obtained numerically. For this, a computationally fast method is required which will give accurate values of the propagation constants (eigenvalues). In this work, the variation method is used with the propagation equation imposed as a constraint. This method is computationally fast and gives accurate results. In an earlier work, the LP₀₁ mode had been dealt with for the parabolic-profile fibre and it was speculated that the same method could be extended to the higher-order modes. In the present work, the LP₁₁ and LP₁₂ modes have been dealt with by the same method for the parabolic-profile fibre, thus confirming the speculation.     

Strain-guided Optical Fibres

Abstract

An analysis is made of optical guiding in the strained cladding of a glass fibre the core of which exhibits a large thermal expansivity. The behaviour of specimens of fibre designed to conform to the assumptions built into the analysis is described and the significance of strained-layer structures for the realisation of ultra-weak guiding is discussed.     

Thermo-optical Interactions in Optical Fibres Caused by Axial Heat-flux Propagation

Abstract

Mode distribution changes were observed when fibre cladding was locally heated by a focused c.w. laser. The time-evolution of mode-coupling was experimentally obtained with the help of a linear photodiode array. A simplified theoretical model of fibre axial temperature flow gave a good agreement with experiment.     

Measuring Strain Using Optical Fibres

Abstract: Optical fibres have emerged as versatile strain transducers. They can function as direct but topologically simple replacements for a range of strain gauges or can implement new measurements in distributed and integrating formats. Like all strain measurement techniques the properties of the transducer dictate its suitability for specific applications. In the case of optical fibres some of these applications are entirely new and in particular facilitate the techniques of "smart structures" fabricated from new advanced materials. This paper presents an overview of the principal approaches to optical fibre strain measurement and endeavours to set these into an applications context. In particular we shall highlight some of the more novel applications which are enabled through the use of fibre optic strain transducers.     

Reversible Image Coding by the Use of Optical Fibres

We examine the possibility of using optical fibres to reversibly modify a picture in order to encode and to decode it. The decoder is to receive the coded picture in the form of a photographic transparency and the decoding process requires possession of the proper apparatus in addition to the code key. The new feature of the proposed devices lies in the greatly enhanced degree of coding security. We discuss two different methods of constructing the necessary device, and we estimate theoretically the expected degree of security for each.     

Spatial Coherence and Index-profiling in Optical Fibres

A simple relation is proved to exist between the coherence properties of the light emerging from the end cross-section of multi-mode optical fibres, excited by means of spatially incoherent sources, and their refractive index distributions. Experimental results showing the possibility of visualizing the index profile are reported.     

Optical Fourier Transforms of Birefringent Fibres

Abstract

A cylindrical birefringent fibre orientated diagonally between two crossed polarizers can be regarded as a single-, double- or multiple-slit source of light, depending on the refractive index of the medium which surrounds the fibre. When normally trans-illuminated by monochromatic light and observed through a polarizing microscope fitted with a condenser-slit diaphragm, the fibre produces specific interference patterns in the exit pupil of the microscope objective. These patterns manifest themselves as optical Fourier transforms. Their properties and possible applications are discussed. The experiments were performed using polymeric textile fibres.     

Amplification of Picosecond Pulses in Neodymium-doped Single-mode Optical Fibres

Abstract

The picosecond pulses from the output of a fibre-grating pair compressed mode-locked c.w. pumped Nd : YAG laser have been amplified in a neodymium-doped single mode optical fibre using a counter propagating c.w. pump geometry. Gains of up to times ten were measured in a 2·5 m fibre length for input signal energies of up to 0·2 pJ per pulse.     

A Modal Approach to Propagation in Graded Optical Fibres

By means of a new and simple formalism it is shown that the field equations usually used in the literature can be deduced under the only assumption of weakly guiding fibres. The formalism provides simple expressions of characteristic equation, power flow and group velocity, susceptible to an efficient numerical approach. Some preliminary results obtained in actual cases are given and a comparison between two asymptotic solutions is performed.     

Calculation of Nonlinear Waves Guided by Optical Fibres Using the Resonance Technique

Abstract

A recently developed, simple, and easily programmable method based on a resonance technique for the analysis of nonlinear guided modes for planar geometry has been extended to include fibre configurations. We consider here nonlinear media of the general type (non-Kerr media) including the more complicated but realistic case of saturable media. In the present paper, as an example of the proposed methodology, the following configuration is treated: a circular optical fibre consisting of a linear core bounded by a nonlinear cladding medium. The numerical results obtained by the method, correspond to the ‘weak guidance’ approximation, including both field distribution and power calculations.     

Subtraction Interferometry in the Study of Radiation Damage in Optical Fibres

Two interferometric methods in subtraction are described which are useful in the study of small changes in dimension and refractive indices of optical fibres. Examples of application are shown in the case of γ -irradiated fibres.     

Spatial Chaotic Power Cross-talk in Nonlinear Multibeam Co-propagation in Optical Fibres

Abstract

We investigate spatial power coupling and chaotic cross-talk when beams co-propagate in multimode optical fibres, specifically among four beams that belong to two weakly degenerate mode families. The nonlinear mechanism responsible for the power and phase coupling is the optical Kerr effect in fibres. The power of each of the modes is theoretically demonstrated to be spatially unstable and chaotically dependent on launch conditions. It is shown that the spatial instabilities and irregular energy exchange occur over broad operating conditions as long as the system deviates from its spatial steady states.     

On the Phenomenology of Self-organized Quasi-phase-matched Second-harmonic Generation in Optical Fibres

Abstract

A simple phenomenological theory of second-harmonic generation in centrosymmetric media is presented in this analytical paper. It is shown that in the medium without memory the energy transfer between interacting waves is negligibly small. For the medium with an optical memory (like phosphorus-germanium-doped glass fibres), the process of formation of χ ⁽²⁾ grating, owing to the third-order optical rectification and the electric-field-induced deprivation of centrosymmetric structure, is described from the phenomenological point of view. Second harmonic generation at periodical χ ⁽²⁾ grating is studied. In the case of resonant interaction, second-harmonic radiation can be systematically amplified. For the non-monochromatic and pulse-form radiations the effective length of the medium is limited largely due to the Kerr nonlinearities. The estimated effective fibre length is in good agreement with previous experiments.     

光谱拟合法确定一种钙钛矿结构薄膜材料的复数光学常数

为了获得一种钙钛矿结构镧锶锰氧La0.8Sr0.2MnO3薄膜材料的复数光学常数,利用光学薄膜原理和数学优化方法,并基于钙钛矿结构材料的色散模型,对磁控溅射技术制备的不同厚度的镧锶锰氧薄膜在波数400～1250cm-1范围内的反射光谱进行了全谱拟合,并由拟合参数确定了薄膜的复数光学常数.拟合结果显示,测试光谱和拟合光谱较为一致,说明钙钛矿结构材料的色散模型适用于描述La0.8Sr0.2MnO3薄膜的光学特性,利用该色散模型并通过光谱拟合法获得La0.8Sr0.2MnO3薄膜的复数光学常数是获得此材料光学特性的一种有效的方法.     

Analysis of Modulational Instability in Monomode Optical Fibres with High-order Nonlinearity

Abstract

The modulational instability (MI) in monomode optical fibres with fifth-order nonlinearity, fibre loss, higher-order dispersion, and the temporal variation of third-order nonlinearity is studied theoretically. The conditions for the existence of the MI and the maximal modulational growth are given and discussed in detail. The results obtained show that the key factor dominating the producing condition of the MI is the power P of the continuous wave initially launched into the optical fibres. If P falls into 3/10<P/P ₀ <1/2 where P ₀ is defined as characteristic power, the MI can be produced in the range of not only anomalous group velocity dispersion but also the normal in which the final evolution state of the modulated wave is dark soliton.