Single-mode fiber directional couplers fabricated by twist-etching techniques (stabilization)

Rugged single-mode fiber couplers have been fabricated by two different techniques: 1) twist-etch RTV potting, and 2) twist-etch gel glass potting. The performances, in terms of optical stability, mechanical ruggedness, and lifetime, are compared between the couplers made by these two techniques, and also with those of the original twist-etched (bottle) coupler reported earlier.     

Modulation instability of dispersion-shifted fiber couplers

The nonlinear coupled-mode equations are rewritten by even and odd modes.We study modulation instability(MI) of dispersion-shifted fiber couplers when either even or odd mode is launched alone by using zero-dispersion wavelength relatively long(quasi-cw) pulses.The result shows that there are new types of MI in both the normal-dispersion and the anomalous-dispersion regimes.MI is concerned with forth-order dispersion and has no relation with third-order dispersion.Quasi-cw can be changed into pulses array under certain conditions.We can extract super short pulse from this.Furthermore,the bandwidth of gain spectra widens and its strength accretes as the input power increases.     

Modulation instability of dispersion-shifted fiber couplers

The nonlinear coupled-mode equations are rewritten by even and odd modes. We study modulation instability (MI) of dispersion-shifted fiber couplers when either even or odd mode is launched alone by using zero-dispersion waveleng threlatively long (quasi-cw) pulses. The result shows that there are new types of MI in both the normal-dispersion and the anomalous-dispersion regimes. MI is concerned with forth-order dispersion and has no relation with third-order dispersion.Quasi-cw can be changed into pulses array under certain conditions. We can extract super short pulse from this. Furthermore,the bandwidth of gain spectra widens and its strength accretes as the input power increases.     

Dielectric mirror embedded optical fiber couplers

Dielectric mirrors in multimode and single-mode silica fibers have been produced by a fusion-splicing technique. Mirrors oriented normally and at a 45° angle to the axis have been studied. Fibers with angled internal mirrors serve as compact directional couplers with low excess optical loss (≈0.2 dB for multimode and 0.5 dB for single mode at 1.3 μm) and excellent mechanical properties. The reflectance is found to be wavelength dependent and strongly polarization dependent. Far-field scans of the reflected output power measured with a white-light source show a pattern that is almost circularly symmetric. The splitting ratio in a multimode coupler measured with a laser source is much less dependent on input coupling conditions than in conventional fused biconical-taper couplers. Spectral properties of multilayer mirrors oriented normally to the fiber axis have been investigated experimentally, and a matrix analysis has been used to explain the results     

Star couplers with gain using fiber amplifiers

The authors propose a novel technique for constructing large passive star couplers with gain using high-power erbium-doped fiber (EDF) amplifiers. In contrast to other proposed techniques where each output port of the coupler requires its own EDF amplifier, the proposed alternative requires few EDF amplifiers with sufficient output saturation power levels. When used in an FDM/WDM star network, the size of the coupler can be increased by a factor of m using m narrowband EDF amplifiers. Moreover, this alternative avoids coupling the pump-laser light through the star coupler itself, alleviating the requirement on designing large bandwidth couplers to accommodate the signal and the pump wavelengths     

Stochastic instability in nonlinear anisotropic fiber couplers

Transmission characteristics of a nonlinear coherent coupler composed of anisotropic fibers are investigated. The system mode approach and the channel normal mode approach are compared and shown to produce identical results so long as weakly guiding and weakly coupling structures are involved, as in the linear case. It is found that the critical state at which the power is equally divided into two channels at any long coupler lengths, as predicted in the early literature, does not exist in a conventional nonlinear coherent coupler unless the initial excitation beam is oriented along the coupler geometric axes. The existence of stochastic instability in a nonlinear anisotropic fiber coupler is shown even when the modes of the individual cores are excited. This result is different from the conclusion reported in the early literature with the assumption of negligible intensity-induced power exchange between the axes. The reason for such instability and the way to avoid it are also discussed     

Anisotropic fiber couplers with nonaligned optical axes

Evanescent-type couplers composed of identical anisotropic cores with nonaligned stress axes reveal interesting propagation characteristics, such as power transfer occurring on two different length scales and reduced polarization holding capacity. The four normal modes have different propagation constants and electric field vectors which are skew to each other     

塑料光纤耦合器的分析和研制

研究了一种新型的塑料光纤耦合器件.采用超声波熔接法制作塑料光纤耦合器,制作过程方便可行.分析了这种耦合器的长度和压缩厚度的变化对耦合比的影响,实验测试结果和计算机模拟分析基本一致.提出了降低工艺过程中的附加损耗的措施,实际制得的塑料光纤耦合器的耦合比和附加损耗能满足短距离通信上的使用要求.     

Modal cutoffs in single- and few-mode fiber couplers

Optical coupling, or crosstalk, between two parallel, identical single- or few-mode fibers is describable as a superposition of the modes of the composite waveguide. Like the modes of a single-fiber waveguide, the modes of the two-fiber waveguide are cut off. Exact and simple accurate asymptotic formulae are derived giving the cutoff value as a function of separation. For single-mode couplers, perturbation theory gives the fundamental and second modesPsi+andPsi-as the sum and difference of the fundamental modesPsi_{1}andPsi_{2}of each fiber. In realityPsi+is the fundamental mode of the composite waveguide, whereasPsi-is the second mode and has a finite cutoff. This limits the smallestV-value for which low-loss coupling is possible. The limitation is least severe for step-profile fibers.     

Optical fiber couplers: scalar supermodes and polarizationcorrections

Scalar field corrections of first order in weak coupling are constructed for the supermodes of couplers composed of two identical step-index optical fiber cores by a novel perturbation technique. The perturbations of the propagation constants lead to the well-known result for the scalar coupling coefficient. However, from the corrected fields the direction-dependent polarization corrections are accurately calculated for the first time, yielding accurate results for both the form birefringence, and the polarization beam splitting     

Polarization properties of fused fiber couplers and polarizingbeamsplitters

The polarization properties of fused taped fiber couplers were investigated experimentally by changing cross-sectional shapes at the coupler waist. The coupling difference between x and y polarized light was studied for many test fused fiber couplers with various cross-sectional shapes. On the basis of the experimental results, two polarizing beamsplitters with the aspect ratio of 1.1 and 1.8 were produced, and their chromatic characteristics were measured. It was evident that the aspect ratio should be more than 1.7 to manufacture polarizing beamsplitters with few power transfer cycles, i.e. long spectral period. Splitting ratios of polarizing beamsplitters with the aspect ratio of 1.1 and 1.8 were 17.6 and -12.7 dB for x polarized light and -16.1 and 11.7 dB for γ polarized light, respectively. Their losses were less than 0.6 and 0.3 dB     

Accurate analysis of single-mode graded-index fiber directional couplers

We present an accurate coupled-mode analysis to evaluate the coupling characteristics of identical single-mode graded-index fiber directional couplers. We also give an empirical relation for calculating the coupling length of a fiber coupler formed by fibers having profile parameterq, normalized frequencyV, and normalized separationd/a;dis the distance between the fiber centers anda, the radius of the fiber core.     

Reproducible fabrication method for polarization preservingsingle-mode fiber couplers

The adaptation of the biconically-tapered-fused (BTF) fiber coupler fabrication approach to polarization preserving fiber couplers is reported. Two practical methods for identifying and aligning the birefringent axes of the fibers forming the coupler are described. The fabrication approaches are highly reproducible and the 3-dB couplers fabricated exhibit less than 1.0-dB insertion loss, controllable splitting ratio, and an average polarization extinction ratio of 15 to 20 dB     

Wavelength selective optical fiber couplers based on longitudinalFabry-Perot structures

A new approach for the development of wavelength selective optical fiber couplers is proposed. It is based on a standard polished fiber coupler in which an intermediate planar multilayer structure, which acts as a transversal optical filter, is included between the two fibers. The simplest multilayer structure, with interesting potential applications, is constituted by high refractive index single-layer Fabry-Perot filters. Experimental results for several single-layer Fabry-Perot filters, as well as a phenomenological model that provides accurate predictions about the wavelength selective coupling ratio, are presented     

The acousto-optic effect in single-mode fiber tapers and couplers

All-fiber acousto-optic devices based on the null fused taper coupler have been successfully demonstrated as frequency shifters, variable splitters, switches and tunable filters. In this paper, the interaction upon which these devices are based has been extensively analyzed under a set of approximations that are valid in most cases. Simple analytical expressions for the important properties are derived, which provide a set of design rules for such devices     

熔融拉制微纳光纤耦合器的仿真模拟

为了对微纳光纤耦合器进行研究,采用光束传播法,在不同熔融区长度和不同波长输入光情况下对微纳光纤耦合器的熔融拉制过程进行数值模拟,取得了输出光功率随拉伸长度变化的曲线和光场分布,并分析了耦合器的3个阶段的模场变化和光场特点。结果表明,当拉伸到微纳光纤耦合器失去有效耦合阶段时,两光纤的输出光功率趋于相等且不再随拉伸长度的变化而变化;熔融拉锥耦合器在各个阶段的光场分布特点不同;熔锥型的微纳光纤耦合器失去有效耦合与熔融区的光纤直径直接关联,且此光纤直径与输入光的波长有关,波长越小,熔融区需经拉伸达到的光纤直径越小。这一结果对研究微纳光纤耦合器失去有效耦合的成立条件是有帮助的。     

Analysis of lapped fiber couplers with an intermediate layer

The effect of a matching or sealing layer inserted in the coupling region of a lapped fiber coupler is analyzed. It is found that the first-order correction factor to the coupling coefficient is proportional to the optical path increment but is independent from bending radius and fiber separation. Specific data with reference to wavelength division multiplexing couplers are presented     

All-optical pulse switching in twin-core fiber couplers withintermodal dispersion

Due to the dispersion mismatch between the two supermodes of a two-channel coupler, pulse switching cannot be analyzed without taking into account the effect of intermodal dispersion. Using full numerical simulations, we show the influence of intermodal dispersion on all-optical switching with ultrashort pulses in twin-core fiber couplers. For the nonlinear regime, however, only subpicosecond and femtosecond pulses are affected in the cross state since the parallel state remains almost unaffected     

Fused taper infrared optical fiber couplers in chalcogenide glass

We have fabricated low loss, high coupling ratio bidirectional optical couplers for infrared light using multimode step-index chalcogenide fiber. Using a fusion technique similar to that commonly employed with silica fibers, we have found a temperature regime where fusion can occur while interdiffusion of the core and cladding materials will not. The resultant device has a roughly 3:1 coupling ratio, less than 0.3 dB of excess insertion loss, and preserves the guided modes of the fiber intact