塑料光纤中模式耦合的研究

利用显式有限差分法数值解阶跃多模光纤的功率流方程，研究阶跃塑料光纤中不同入射光束的模式耦合模式及模式耦合引起的衰减，得出塑料光纤达到模均衡分布的模式耦合长度是20m左右。     

Investigation of mode coupling in step index plastic optical fibers using the power flow equation

Using the power flow equation, we have examined the mode coupling caused by intrinsic perturbation effects of the step index plastic optical fiber. A numerical solution has been obtained by the explicit finite difference method. Results show the state of mode coupling along the fiber. They indicate that the equilibrium mode distribution is achieved at much shorter lengths compared to the case with glass fibers.     

Frozen-in viscoelasticity for novel beam expanders and high-power connectors

Frozen-in viscoelasticity has a significant, yet previously overlooked, impact on the refractive index profile of optical fibers and can serve as the basis for novel fiber devices. Viscoelastic strains can be frozen into an optical fiber during draw, or by cooling down a drawn fiber from high temperature while under tension. The resulting refractive index perturbation can exceed 0.003, exhibits minimal birefringence, and is concentrated in the highest viscosity region of the fiber. We have developed a quantitative theory for the refractive index perturbation, which is in excellent agreement with measured data. We have harnessed frozen-in viscoelasticity to create novel high-performance all-fiber beam expanders achieving mode field diameters in excess of 40 /spl mu/m. These beam expanders are suitable for low-cost high-volume manufacturing and can serve as the basis for high-power single-mode fiber (SMF) connectors, since their lower power density improves reliability during high-power operation. Because of the significant reduction in free-space diffraction associated with the enlarged mode field diameter, these beam expanders may be suitable for MEMS applications, or for coupling between optical fiber terminations and optical fiber sources or detectors.     

An integrated optic frequency translator for microwave lightwavesystems

A novel integrated optic component that performs a microwave frequency translation of an optical wave is presented. Its principle of operation and characteristics are derived from a perturbation analysis of the supermode propagation in a Y-fed directional coupler subjected to an electrooptic effect associated with a propagating microwave field. It is found that under appropriate values of the microwave field amplitude and interaction length, a very efficient conversion of optical power can be achieved. The optimum frequency shift of this transfer is controlled through the coupling length of the directional coupler. An approximate analytical solution of the coupled mode equations, which considers only the coupling between the two spectral components of the supermodes that are phase matched, has been performed     

Modes and Pseudomodes in Dielectric Waveguides (Correspondence)

It is shown that a medium whose dielectric constant has a square-law distribution with complex coefficients possesses guided mode solutions even under the extreme condition that both the real and the imaginary part of the index of refraction have their lowest value at the axis of the waveguide. However, the resulting modes are unstable (pseudomodes). Stable guided modes exist if the imaginary part of the refractive index has its highest value on axis.     

Quality factors and effective-average modal gain or loss ininhomogeneous spherical resonators: application to two-photon absorption

A method is presented for calculating the quality factors (Q's) and the effective-average gain and loss coefficients for the modes of a dielectric sphere which has a spatial perturbation of the imaginary part of the refractive index. The method is applicable to gain calculations in spherical lasers, to spatial hole burning and to stimulated Raman scattering in spheres. A time-independent perturbation method is also used to compute the Q's of the resonances. Results computed using both methods are compared with analytical results for layered spheres. The methods are used to compute the near-resonance backscattering by a sphere having an intensity-dependent (two-photon absorption) loss     

Spot-size conversion using uniform waveguide sections for efficientlaser-fiber coupling

Using numerically simulated results, it is shown that an efficient laser-to-fiber coupling is possible by incorporating a uniform spot-size converter (SSC) with two nonidentical but phase matched optical waveguides. Using a “diluted” waveguide with a low index contrast, the power coupling loss can be significantly reduced from 10.4 dB to only 1.15 dB and at the same time 1.0 dB alignment tolerances can also be improved to achieve ±1.8 μm. Besides the improved coupling, the beam divergence of the far-field is also reduced considerably to 9°, accompanied by a significant lowering of the reflected power from the fiber interface     

Non-evanescent adiabatic directional coupler

A directional coupling mechanism based on an adiabatic coupling between three optical modes is suggested. The optical power transfer between two waveguides which are far apart is mediated by adiabatic coupling between zero-order optical modes of the individual waveguides and a high-order intermediate mode. The analytical model for an adiabatic three-mode coupling based on a scalar wave equation is presented. The directional coupling via the adiabatic mode coupling between copropagating modes is described and compared with a nonadiabatic directional coupling assisted by periodic perturbation. It is shown that adiabatic directional coupling has much less sensitivity to the mode parameters and to the wavelength     

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     

Intermodal dispersion and mode coupling in perfluorinatedgraded-index plastic optical fiber

Graded-index multimode perfluorinated plastic optical fibers typically exhibit bandwidths much greater than would be expected from their index profiles. To resolve this discrepancy, we have conducted the first measurements of differential mode delay in such fibers. These measurements show intermodal dispersion that increases as the square root of fiber length, implying strong mode coupling in these fibers. Significant power transfer between modes occurs at lengths less than 20 m, so that mode coupling results in improved bandwidth on length scales relevant for local area networks. The observed coupling arises from extrinsic nonuniformities of the waveguide     

Analysis of the degree of polarization of optical waves in a single-mode optical fiber with random coupling

A single-mode optical fiber model is proposed with discrete mode-coupling centers having random coupling coefficients at regular intervals. The degree of polarization in a single-mode fiber with random-mode coupling does not approach zero with increasing fiber length. Numerical results show that the ensemble average of the degree of polarization approaches a nonzero value the magnitude of which depends on the coupling intensity, the light source spectrum, the fiber birefringence, and the spatial frequency of coupling centers, The theoretical and numerical results clarify the limitation of the perturbation iteration method for long optical fibers.     

Bandpass filter design using a microstrip triangular loop resonator with dual-mode operation

A bandpass filter (BPF) design using a dual-mode microstrip triangular loop resonator is presented for the first time. The circuit produces frequency responses with one real finite frequency transmission zero and one imaginary finite frequency zero on either side of the passband. Depending on the perturbation arrangement, the nature of the coupling between degenerate modes causes the zeros to exchange their axis locations from real to imaginary and from imaginary to real while keeping their magnitude. This behavior results in frequency responses that are suitable for applications with asymmetrical requirements. Results show 8% bandwidth filters with insertion loss ranging from 0.82 dB to 1.4dB at 10 GHz. Advantages in size reduction and design flexibility are demonstrated when the triangular loop is compared to other dual-mode resonators.     

Guided-mode and leaky-mode analysis by imaginary distance beampropagation method based on finite element scheme

As a simple analysis method to solve eigenmodes of optical waveguides, we present an imaginary distance beam propagation method (BPM) based on finite element scheme. The matrices used in the beam propagation analysis are essentially complex, so lossy optical waveguides can be easily treated. Moreover, employing the transparent boundary condition or perfectly matched layer boundary condition, the validity of a which has been already confirmed in the real distance BPM, we can easily treat not only guided modes but leaky ones. To show the validity and usefulness of this approach, eigenmodes of twoand three-dimensional leaky waveguides, and optical fibers are calculated     

Réflectomètre à haute résolution

Measurement capabilities of a single mode optical time domain reflectometer (Otdr) are given. Fault location accuracy is 1 m (when the group index of refraction is known) and spatial resolution is 3 m.     

聚合物光纤中的模式耦合及其对带宽的提高

光在聚合物光纤(POF)中传输时存在较强的模式耦合,由于模式耦合的影响,聚合物光纤的传输带宽得以提高。根据能流方程对阶跃型聚合物光纤光传输中的模式耦合进行了研究,从实验上测量了聚合物光纤的模式耦合系数,测量得到的模式耦合系数为7.61×10-4rad2/m。用测得的模式耦合系数对聚合物光纤中的模式耦合进行数值模拟,得到聚合物光纤的模式耦合长度约为20 m。由模式耦合长度可以得到聚合物光纤的真正带宽,其在150 m传输距离的带宽约为130 Mbit/s。为了验证这一结论,进行了125 Mbit/s.150 m的局域网(LAN)通信实验,通过对发射、接收波形及通信眼图的分析表明,由于模式耦合提高了聚合物光纤的传输带宽,使其可以在150 m的距离上进行百兆速率的通信传输。     

Low-Chirp 85-Gb/s Duobinary Modulator in InP Using Electroabsorption Modulators

In 2005, Griffin showed that an InP phase-shift-keying/duobinary Mach–Zehnder interferometer (MZI) modulator could achieve good transmission performance at 10 Gb/s, despite modest residual amplitude modulation in the phase modulators in the MZI (ratio of real to imaginary part of index change was $sim$0.1). Here we show that even strong amplitude modulation in the “phase” modulators (ratio of real to imaginary part of index change is $sim$ 1.0) gives good transmission performance. Allowing for strong amplitude modulation allows a significant increase in modulator bandwidth. We demonstrate low-chirp 85.4-Gb/s optical duobinary generation in a fully packaged InP photonic integrated circuit.      

倾斜长周期光纤光栅薄膜传感器特性研究

为设计高灵敏光纤光栅薄膜传感器,基于耦合模理论研究了表面镀制高折射率敏感薄膜的倾斜长周期光纤光栅（TLPFG）耦合特性和薄膜折射率传感特性。TLPFG能激发高阶（角向序数l1）包层模耦合,高阶包层模耦合系数随倾角增大而增大,80时,高阶包层模耦合系数远小于1阶包层模耦合系数。研究表明,光栅倾角变化不影响包层模耦合谐振波长,但影响光谱透过率。进一步研究了TLPFG在80时1阶5次包层模耦合的光谱透过率对薄膜折射率变化的响应,分析了光栅结构参数（L、、）和薄膜参数（h3,n3）对薄膜折射率响应灵敏度的影响,结果表明,响应灵敏度相比于非倾斜LPFG可提高一个数量级,对薄膜折射率的分辨率可达10-9。     

Optical properties of nanocrystal layers embedded in a carrier medium

A matrix ordered nanocomposite formed from a monolayer of spherical particles that is embedded in a carrier medium is considered. An analytic theory describing the optical properties of such a nanocomposite is proposed. It is shown that the monolayer is an imaginary interface with non-Fresnel reflection and transmission coefficients and that the Airy relationships for a thin film deposited on a substrate can be applied to the aforementioned structure. Conditions for constructive and destructive interferences of the fields reflected by the real and imaginary interfaces are determined. With the use of these conditions, it is possible to vary the total reflectance and transmittance of the structure. Expressions for the effective refractive index of the matrix medium are derived, and it is shown that the presence of a nanoparticle monolayer substantially changes this index. The results are verified through comparison with those obtained from the exact numerical analysis.     

Optical properties of an InGaAs-InP interdiffused quantum well

A comprehensive model is developed for the calculation of polarization-dependent absorption coefficients and refractive index of the InGaAs-InP interdiffused multiple-quantum-well at room temperature for wavelengths ranging from 1.1 to 2.4 μm. Groups III and V types of interdiffusion are considered separately. The as-grown structure is a latticed-matched In_0.53Ga_0.47As-InP structure with a well width of 60 Å. The optical transitions consist of a full quantum-well calculation together with Γ,X, and L valleys contributions and through the Kramers-Kronig transformation to link the real and imaginary parts of the dielectric functions. The results show that group-III-only interdiffusion produces compressive strain and results in a band-edge red shift and refractive index enlargement, while the tensile strain induced by group-V-only interdiffusion results in a vice verse effect. This provides a left and right tunable band edge and positive and negative index steps dependent on the interdiffusion process. A small and constant birefringence of 0.005 at around 1.55 μm can also be obtained over a 50-nm wavelength range by using group-V-only interdiffusion. These properties have strong implications in realizing a tunable and high-performance device as well as for photonic integrations