Beam delivery characteristics of optical fibres for carbon monoxide lasers

An experimental investigation has been conducted of the propagation characteristics of several types of optical fibres which are candidates for use in optical beam delivery systems for carbon monoxide lasers. Both solid core (chalcogenide) fibres and hollow core (with dielectric or dielectric coated metal) waveguides have been investigated. Such experiments have included an assessment of both the power transmission characteristics, and the effects of waveguide transmission on the optical quality of the beam at the exit of the fibre. The experiments indicated lowest loss for the chalcogenide solid core fibre with a value of 0.4 dB m⁻¹.     

Perturbation theory of optical fibres with power-law core profile

The characteristics of graded-index optical fibres with power-law core profile and homogeneous cladding have been studied using perturbation theory. Analytical expressions are derived for the cut-off frequency, propagation constant and, in the case of leaky modes, the attenuation coefficient of an arbitrary mode on such a fibre. For the case of monomode fibres, excellent agreement is found with existing theoretical results for the propagation constant of the HE₁₁ mode and cut-off frequency of the TE₀₁ mode.On leave from Department of Physics, IIT, New Delhi, India.     

Modes in weakly guiding elliptical optical fibres

Approximate and much simplified dispersion relations are obtained for the problem of optical wave propagation within weakly guiding elliptical fibres. The refractive index difference between the core and its cladding of weakly guiding optical fibres that are contenders for use as practical optical communication lines is very small; i.e., (n ₁/n₀–1) 1 wheren ₁ is the core index andn ₀ is the cladding index. These greatly simplified dispersion relations are then used to calculate the propagation constants for several higher order modes on an elliptical optical fibre.Supported partly by NELC, San Diego; this paper was presented at 1974 URSI Electromagnetic Waves Conference, London.     

Polarization properties of elliptical core non-hexagonal symmetry polymer photonic crystal fibre

In this paper, polarization properties and propagation characteristics of polymer photonic crystal fibres with elliptical core and non-hexagonal symmetry structure are investigated by using the full vectorial plane wave method. The results show that the birefringence of the fibre is induced by asymmetries of both the cladding and the core. Moreover, by adjusting the non-symmetrical ratio factor of cladding $\eta$ from 0.4 to 1 in step 0.1, we find the optimized design parameters of the fibre with high birefringence and limited polarization mode dispersion, operating in a single mode regime at an appropriate wavelength range. The range of wavelength approaches the visible and near-infrared which is consistent with the communication windows of polymer optical fibres.     

Semiconductor optical fibres: progress and opportunities

This paper reviews recent progress in the nascent field of semiconductor optical fibres, from the fundamentals through to device demonstration. The incorporation of semiconductor materials into both the step‐index and microstructured fibre geometries provides a route to introducing new optoelectronic functionality into existing glass fibre technologies. Herein, the various fabrication methods that have been developed as of to date are described, and their compatibility with the different semiconductor materials and fibre designs discussed. Results will be presented on the optical transmission properties of several fibre types, with particular attention being paid to the observation of nonlinear propagation in silicon core fibres. Finally, some speculation regarding the future prospects and applications of this new class of fibre will be provided.     

Wave propagation in a single-mode fibre with dip in the refractive index

A theoretical study has been carried out on propagation in single-mode fibres having a dip in the refractive index at the centre of the core. The energy of the mode gradually spreads from the centre of the core with increase of the degree of the dip. Nevertheless the near and far-field distributions of such single-mode fibres are very nearly Gaussian in shape. A simple method of estimating the limit of the single-mode region for any kind of index profile is presented. For a particular class of profiles, dip widths up to 30% have a negligible effect on the propagation characteristics.     

室温条件下掺铥光纤放大器中光波群速减慢的研究

从掺铥离子光纤的速率方程和传输方程出发,建立了掺铥离子光纤放大器中光速减慢的理论模型,分析并讨论了介质的增益与泵浦光功率之间的关系。当掺铥离子光纤处于吸收区域时,粒子布居振荡导致光脉冲经历了饱和吸收过程,此时光脉冲传输延迟;当掺铥离子光纤处于增益区域时,粒子布居振荡导致光脉冲经历了增益饱和过程,此时脉冲传输超前。依据该理论模型进行了理论仿真计算,同时进行了室温条件下掺铥离子光纤中光波群速减慢传输的研究。     

Modelisation de la propagation dans les fibres optiques

This paper presents a new method for investigating the effects of mode conversion due to microbends on the transmission characteristics (attenuation, frequency response ...) of multimode optical fibres. The method applies to both step-index and graded-index fibres and takes into account any dependence of the mode coupling coefficient upon propagation angle. The general theory of propagation with mode coupling is developed and we introduce the concept of modes. The propagation problem is then simplified by using hypotheses based on exerpimental results and the solution is obtained with the help of a computer. Practical results concerning the increase of attenuation due to microbends are presented, from which the precautions to be taken when cabling fibres are deduced. It is also shown that in every practical case encountered, the prediction of the total attenuation and of the frequency response of a given fibre, when the launching conditions are specified is obtained from analytical expressions which approximate well with the exact results obtained from the computer. All these results make the method powerful and very useful for system designers when evaluating the effect of introducing an optical fibre into a transmission system.     

Measurement of net group and reshaping delays for optical pulses in dispersive media

We experimentally demonstrate the direct measurement of net group and reshaping delays for arbitrary optical pulses in dispersive media, verifying the earlier prediction of Peatross et al. [Phys. Rev. Lett. 84, 2370 (2000)]. Incoherent pulse propagation in an absorptionless system is well described by net group delay; even the medium causes a great deal of deformation in the transmitted pulse. However, in the case of phase modulated chirping pulses in a resonant absorber, the so-called superluminal or subluminal propagation velocity is strongly influenced by the reshaping delay.     

Partially coherent optical waves in random gradient fibres

The excitation characteristics and spatial coherence of partially coherent optical waves in gradient fibres are demonstrated for incident light waves radiated from semiconductor lasers and light emitting diodes. Lower modes are efficiently excited in the case of coherent laser beams, while incident waves of low coherence such as lightwaves of LEDs excite higher modes. Pulse propagation of partially coherent optical waves in dispersive gradient fibres is also discussed for random index fluctuations. Mode coupled equations for temporal correlation functions of the electric field that are generalizations of coupled power equations are found. Mode filtering and pulse improvement with a lossy inhomogeneous cladding are described.     

A numerical technique for solving the scalar wave equation for Gaussian and smoothed-out profiles

A numerical technique, based on transforming the scalar wave equation to a finite interval before discretization is applied, is proposed for the solution of the scalar wave equation defining the modal fields and propagation constants of optical fibres. It is used to compute the propagation constants and modal fields of weakly guiding fibres of circular cross-section when the refractive index profile has a Gaussian or smoothed-out form. The smoothed-out profiles are studied because they vary continuously from the Gaussian to the step profile. Results are compared with and shown to be accurately approximated by simply explicit functions, dependent only on the fibre parameter.     

Study of Exact Solutions to Cubic-Quintic Nonlinear Schrödinger Equation in Optical Soliton Communication

A systematic method which is based on the classical Lie group reduction is used to find the novel exact solution of the cubic-quintic nonlinear Schrödinger equation (CQNLS) with varying dispersion, nonlinearity, and gain or absorption. Algebraic solitary-wave as well as kink-type solutions in three kinds of optical fibers represented by coefficient varying CQNLS equations are studied in detail. Some new exact solutions of optical solitary wave with a simple analytic form in these models are presented. Appropriate solitary wave solutions are applied to discuss soliton propagation in optical fibres, and the amplification and compression of pulses in optical fibre amplifiers.     

10 Gbits−1 optically amplified single-channel systems operating in links encompassing conventional step-index fibres

In this work a numerical investigation is reported on the propagation regimes that allow single-channel systems to obtain the maximum capacity in links with optical amplification encompassing conventional step-index fibres. Solitons and NRZ signals are considered. We show how the periodical introduction of pieces of dispersion compensating fibres can greatly improve the system performance. The polarization properties of the signals are investigated to increase the system performance and     

Real-time non-linear spatial filtering with a leaky OASLM

In this paper we demonstrate image processing techniques such as edge enhancement and phase contrast by using an optically addressed liquid crystal spatial light modulator (OASLM) in the frequency plane of an optical 4f processor. The transfer function of the device is derived on the basis of the Jones formalism. Faced with a lack of a general theory for such non-linear optical processors, we show that an analogy to the propagation of optical pulses in fibres is helpful for the understanding of the image processing operation.     

Effects of joints on mode power distribution and losses in multimode optical fibres

The transformation of mode power distribution at a joint between two different (even elliptical) optical fibres is studied. Simple analytical expressions are obtained also in the most general coupling and mismatch configuration, provided that coupling errors and mismatch parameters are small. Detailed calculations of joint losses are performed in each case, including propagation effects in both fibres. Original results are obtained particularly in the cases of misalignment, end separation between the fibres, mismatches in index profiles and coupling between two elliptical fibres. A strong dependence on the launching conditions and the amount of mode coupling is evidenced in all cases.     

Collisions and turbulence in optical rogue wave formation

We discuss optical rogue wave generation in terms of collisions and turbulence processes. Simulations of picosecond pulse propagation in optical fibres show rogue soliton generation from either third-order dispersion or Raman scattering independently. Simulations of rogue soliton emergence with dispersive perturbation in the long-distance limit are also presented.     

Determination of the optical and geometrical properties of fibres having skin–core structure

A method is suggested to determine both the refractive index and the transverse sectional shape and area of fibres, having skin–core structure, at the same time for the same region of the fibre. The method depends on using a fibre rotator device attached with Pluta polarizing interference microscope, to record the variation of the fibre thickness at each angle of rotation. Nylon 6 fibres having skin–core structure were used in this study. Beck-line method was used to determine the refractive index of the skin for light vibrating parallel and perpendicular to the fibre axis. To confirm the results of the suggested method, the optical microscope was used to determine the transverse sectional shapes of bundles of nylon 6 fibres. The mean refractive indices of the skin and core of nylon 6 fibres were determined. Microinterferograms are given for illustration.     

Light propagation in multi‐step index optical fibres

This paper reviews the theoretical analysis of light propagation we have carried out on multimode multi‐step index (MSI) optical fibres. Starting from the Eikonal equation, we derive the analytical expressions that allow calculating the ray trajectories inside these fibres. We also analyse the effects of leaky rays on the transmission properties of MSI fibres. For this purpose, a single analytical expression for the evaluation of the ray power transmission coefficient is calculated. Afterwards, we investigate the effects of extrinsic and intrinsic coupling losses on the performance of MSI fibres, providing analytical expressions to calculate the coupling loss and, also, determining the most critical parameters. Finally, we carry out a comprehensive numerical analysis of the fibre bandwidth under different source configurations.