Three-dimensional electromagnetic modeling of fiber-core effects onthe coupling characteristics of weakly fused tapered fiber-opticcouplers

Based on the full-wave formulations, which combine the finite element method add the boundary element technique, and the step-like approximation method, a three-dimensional (3-D) electromagnetic modeling approach is proposed in this work to investigate the fiber-core effects on the coupling characteristics of the weakly fused tapered couplers. We find the fiber cores have significant effects on the mode field patterns, coupling coefficients, and birefringence for the coupler with large normalized frequencies. The influence of the fiber cores on the coupling behavior for the real couplers, such as 3 dB power dividers, polarization beam splitters, and wavelength demultiplexing/multiplexing couplers, are also studied in 3-D view. It is found that the effects of fiber-cores in the weakly fused tapered couplers have to be considered for accurately modelling their coupling behavior, although the model with no core assumption can predict the trend of the coupling characteristics for the real couplers     

单模光纤组合波导的模型及熔锥工艺

本文首先用理论分析的方法证明了将单模光纤组合波导腰部区域的物理模型简化成等效平行波导的近似模型，必须作弱导近似和弱耦合近似．据此理论在弱熔工艺条件下研制成了高性能的，在１．２４～１．６０ μｍ波长范围内频谱响应平坦的２×２定向耦合器件，其具有哑铃形结构的腰部横截面，满足弱导近似和弱耦合近似的条件．     

Influence of noncircular core on the polarisation performance of single mode fibres

Measurements on borosilicate fibres with noncircular cores indicate that the noncircular geometry and the associated stress-induced birefringence alone are not sufficient to maintain polarisation in single-mode fibres. To achieve single polarisation fibres, it is necessary to deliberately enhance the anisotropic birefringence.     

Wavelength-insensitive fused polished couplers

Wavelength-insensitive fused polished couplers are fabricated from different fibres and from identical fibres where in the latter case one of the fibres is polished to such an extent that a small portion of its core is removed.<>     

Novel polarisation phenomena on anisotropic multimoded fibres

The lowest-order modes of multimoded fibres composed of birefringent material can differ radically from the familiar modes of multimoded isotropic fibres. In general, the modal fields are neither uniformly polarised nor circularly symmetric as they would be on circularly symmetric isotropic fibres that are weakly guiding. Thus multimoded anisotropic fibres can exhibit dramatic polarisation effects which may have important practical applications.     

Asymmetric robust evanescent coupler on singlemode fibre

A new method for flattening the spectral splitting ratio response of the robust evanescent fused tapered coupler is described; through side-polishing the amalgamate of two fibres that have been circularly pre-fused together, the required asymmetry between the fibres is introduced.     

Fusion mass-splicing for optical fibres by discharge heating

A mass-splicing technique for tape-unit silica fibres based on discharge fusion is demonstrated. Five pairs of fibres are simultaneously fused and spliced together by the heat from the electric discharge between two pairs of electrodes. The average splice loss for 50 ?m-core graded-index fibres is 0.22 dB.     

Fabrication of wavelength-flattened tapered couplers usingpolishing for cladding removal

Wavelength-flattened fused tapered couplers have been fabricated with low loss using a new technique. The asymmetry between the constituent fibres is induced by the rapid removal of a small amount of the cladding of one of the fibres with a polishing wheel     

Star couplers using fused biconically tapered multimode fibres

We report the fabrication and testing of 4-, 7-and 19-fibre fused biconical taper transmissive star couplers for use with single multimode fibres. Overall insertion losses on six couplers range from ?1.3 dB to ?2.6 dB. Losses are nearly independent of N, the number of fibres, but dispersion of the powers in the output fibres increases significantly with N.     

Leaky rays cause failure of geometric optics on optical fibres

Many of the rays on circular optical fibres that are predicted from geometric optics to be trapped by total internal reflection are, in fact, leaky. Thus geometric optics fails for long fibres. The letter presents a concise analytic expression for the loss of all weakly attenuating rays. The method is simple, and applicable to a large class of problems. The results provide a unifying theory of light transmission within fibres, representing a generalisation of Fresnel's and Snell's laws.     

Wavelength-flattened response in bent fibre couplers

A symmetric fused tapered coupler in singlemode fibre can be given a wavelength-flattened response by bending it in the plane of its constituent fibres. The maximum splitting ratio can be freely adjusted after coupler elongation, in contrast with methods requiring the pretreatment of the fibres.<>     

Polarisation analysis of strongly fused and weakly fused tapered couplers

The polarisation properties of both strongly fused and weakly fused single-mode tapered fibre couplers have been analysed. A comparison with experimental results shows excellent agreement with our analysis.     

Fusion junctions for glass-fibre waveguides

Various ways of making fused joints in a monomode glass-fibre waveguide have been tried. The highest efficiencies have been about 80%, with a joint in which the end faces are lightly fused together after the cores have been aligned for optimum transmission.     

Rigorous analysis of form birefringence of weakly fused fiber-opticcouplers

Based on a rigorous vectorial formulation, the surface integral equation method is used to solve the vectorial mode patterns and the coupling characteristics of weakly fused fiber-optic couplers. Form birefringence versus aspect ratio of the coupler cross-section for different normalized frequencies V are calculated. A dramatic feature-that there is a local maximum of form birefringence for the weakly fused couplers with large V-values-is found. The dependence of the aspect ratio at which the coupler is isotropic on the V-value is also presented. The fused coupler with large V-value is isotropic as the aspect ratio is close to 1.8, but its polarization effect is still significant when the aspect ratio is 2.0     

Perturbation analysis of fused tapered single-mode fibre couplers

By treating a fused tapered single-mode fibre coupler as a perturbation of the same coupler with the fibre cores ignored, simple analytical expressions are derived for the coupling coefficients of the coupler. These new expressions highlight the effects of the fibre cores on the characteristics of the coupler.     

Fusion splicing machine for optical fibres

A fusion splicing machine has been developed with an integrated fibre cutter and support for the splice case inset with splice magazine. The optical fibres and the electrodes can be manipulated, with the result that the splice losses are minimal and optical fibres with different melting points can be fused. Since the splices are not mechanically strong, tension relief facilities for the splice play an important part.     

Modelling fused single-mode-fibre couplers

The fused single-mode tapered coupler is successfully modelled by assuming that the neck section of the coupler can be represented by a rectangular dielectric waveguide. Guidance is provided at the glass-to-air (or external medium) boundary and the cores are neglected. The mechanism of field detachment from the cores is outlined and the coupler characteristics are derived from the interference between the two lowest-order modes of the large waveguide.     

Polarisation-maintaining fibre couplers with misalignedbirefringent axes

A recently developed formulation of the coupling between polarisation maintaining fibres is applied to the study of the dependence of the properties of tunable polarisation maintaining couplers on the angular misalignment between the transversal principal axes of the two individual fibres. When the cores are close enough it is shown that while the same coupling ratio can be obtained at several transversal settings of the coupler, quite different behaviour is exhibited by the extinction ratio. In particular, for a transversely tunable coupler made of 3 mm beat-length fibres with a 5° misalignment, two 3 dB settings result in polarisation isolations of 18 dB and 40 dB     

Recording of efficient high-order Bragg reflectors in opticalfibres by mask image projection and single pulse exposure with anexcimer laser

A technique using high resolution mask projection for creating efficient Bragg grating reflectors in optical fibres by single excimer laser pulse exposure is described. Using a deep UV projection system with a large image field size and micrometre resolution, gratings with periods between 1 and 6 μm have been recorded in Ge-doped fused silica fibres that produce high order Bragg reflections in the wavelength range 1.43-1.65 μm     

Generalised coupled-mode equations and their applications to fibre couplers

Generalised coupled-mode equations for two or more coupled weakly guiding fibres are presented, when the fibres are not well separated. These equations are applied to studying fibre couplers. Compared with the results derived from previous coupled-mode equations, our results are in better agreement with those calculated rigorously by a numerical method.