Adiabatic fused tapered couplers

Pre-fattening of one fibre is shown to be a vehicle for achieving wavelength independent adiabatic fused tapered couplers. Compared with the conventional pre-tapered fused tapered coupler, this adiabatic coupler has a significant improvement in its ease of manufacture     

熔锥型宽带光纤耦合器的研究

为了探究非对称耦合器的特性, 利用具有折射率差异的两根光纤制作了宽带耦合器。采用数值计算模拟了不同折射率差光纤耦合器的光场分布以及输出光功率随拉伸长度的变化曲线, 并分析了两种宽带耦合器的带宽差异以及熔融度对功率转换比的影响; 采用光束传播法, 通过仿真模拟得到了理论带宽。结果表明, 耦合的功率转换比随两根光纤的不对称情况而变化, 功率转换比调节到耦合器分束比的大小时, 耦合器带宽最宽; 熔融度对宽带耦合器分束比有一定的调节作用; 3dB光纤耦合器在C+L波段波长响应平缓, 带宽范围达到150nm; 分束比3:7和1:9的耦合器带宽范围分别是210nm和330nm; 分束比1:99的耦合器带宽范围是420nm。此研究结果对制作非对称宽带耦合器提供了参考依据。     

Variational analysis of fused tapered couplers

A variational technique is used to evaluate the coupling ratio of a fused coupler. The technique can be used for both strongly and lightly fused couplers. Simple formulas are presented for use in regions of high V-value and are shown to be in agreement with the more lengthy beam propagation calculations. The importance of the fibre core in determining the field shape is highlighted. A realistic coupler model containing a gradual taper is presented and the strong wavelength response of the coupling ratio is confirmed by numerical means.     

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.     

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.     

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.     

熔融拉锥型单模光纤光功率比耦合器的研制

本文由耦合波方程出发推导了熔融拉锥单模光纤耦合器的功率耦合比与拉伸长度的关系,并数值计算了光纤耦合器的光功率分配。实验上利用实验室现有的设备制作出2×2型、单窗口1×2型1550nm宽带耦合器,并完成了对制备出的光纤耦合器的光功率检测。通过理论与实验的比较得出熔融拉锥机制备的单模光纤光功率比耦合器的耦合曲线与理论上的耦合曲线基本一致,制备出的耦合器符合耦合器检验标准。     

Completely fused tapered couplers: comparison of theoretical and experimental results

Completely fused single-mode fibre couplers, characterised by an almost circular cross-section at the waist, have been fabricated. An analytic model which takes into consideration the taper and variable aspect ratio of the coupler has been developed. Good agreement is obtained between the theoretical and experimental values.     

Single-mode fused biconical couplers for wavelength divisionmultiplexing with channel spacing between 100 nd 300 nm

The authors report on single-mode fused biconical couplers tailored to the requirements of wavelength-division-multiplexing systems. The wavelengths of the two channels can be chosen arbitrarily for wavelength separations between 100 and 30 nm. For separations between 150 and 300 nm, the couplers can be manufactured with an accuracy of ±5 nm at the two operating wavelengths. For decreasing wavelength separation of the channels, the influence of the state of polarization results in an increased inaccuracy of the two wavelengths, increased far-end crosstalk, and power fluctuations after the coupler. This sets a lower limit for channel separation. Good agreement between theory and experiment is achieved     

Wavelength-flattened fused couplers

A technique is reported for the fabrication of fused couplers, which yields devices having splitting ratios substantially independent of wavelength. A low-loss, stable, single-mode 2×2 port coupler has been fabricated using this technique. The coupler has 50/50% coupling at both 1.3 and 1.52 ?m and 50% ± 9% coupling over the wavelength range 1.23 to 1.57 ?m.     

Performance tuning of 1*7 wavelength-flattened fused fibre couplers

In order to successfully fabricate monolithic 1*7 wavelength-flattened couplers, careful control over the degree of fusion of the structure is required. The variation of coupling characteristics with the degree of fusion is discussed. Two degrees of fusion are identified where an equal power split wavelength-flattened response may be obtained.<>     

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.     

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.<>     

All-silica package for tapered couplers

The primary packaging of singlemode tapered couplers, by fusing the fibre leads with a negligible loss to a silica substrate, is reported. This procedure eliminates adhesive, and ensures matched coefficients of thermal expansion throughout.<>     

Wavelength dependence of fused couplers

The transmission characteristics of fused couplers are evaluated as a function of the cross-sectional radius and the degree of fusion. The wavelength dependence of the coupling ratio is then examined with a view to minimising the wavelength dependence in wavelength-multiplexed optical networks. The effect of changing the external refractive index is also considered.     

Optical wavelength filter with tapered couplers

Optical wavelength filters based on tapered directional couplers are analyzed by nonorthogonal coupled-mode theory. The nonorthogonal coupled-mode formulation in general yields more accurate results than the conventional orthogonal coupled-mode theory. The focus is on the novel features of the tapered couplers used as wavelength filters. It is demonstrated that, in addition to reducing the crosstalk and suppressing the sidelobes, the tapered structure can also make the filtering response independent of the device length beyond a critical coupling length,.<>     

Back-reflexion from fused biconic couplers

Several theories are investigated to account for the origin of back-reflexions from fused biconic couplers. Amongst the effects causing such variation are those considered in this paper, namely, tapering, surface roughness on the cladding-air interface, fiber bends and twists and, finally, stress arising from bending and packaging. The basic investigative tool is the coupled local mode equations solution for the coupling between the forward- and backward-propagating modes arising in response to these effects. Theoretical analysis shows the stress optic effect to be the most likely reflexion origin and experimental results support this assertion, as well as ruling out tapering and surface roughness as likely causes     

Single-mode fused biconical coupler optimized for polarizationbeamsplitting

A single-mode fused biconical coupler for polarization beamsplitting is described. In the coupling region, standard communication fibers are used. The output fibers are polarization-maintaining. To stop the coupler drawing process accurately, the degree of polarization is measured during the drawing and is used as a criterion. Losses of 0.2 dB are typical, and a minimum extinction ratio of -27 dB is achieved. A usable spectral window as broad as 17 nm with an extinction ratio smaller than -15 dB for both output fibers is obtained. Any specified wavelength for the minimum of the extinction ratio is reached within ±3 nm     

Power exchange in tapered optical couplers

The power exchange between nonparallel optical waveguides or tapered couplers is analyzed by the coupled-mode theory based on local modes of the individual waveguides (local waveguide modes). A self-consistent nonorthogonal coupled-mode formulation is presented and transformed into an orthogonal form which is equivalent to the coupled-mode equations for the local modes of the coupled waveguides (local array modes). The coupled-mode equations are then solved numerically and the effect of the taper on the power exchange between the two guides is studied     

Finite-element analysis for fused couplers

By using the finite-element method we show that the polarisation beam splitting property of realistic fused optical couplers is well modelled by an elliptical cross-section, but differs substantially from a rectangular cross-section.