Effect of acoustic birefringence on acoustooptic interaction in birefringent two-mode optical fibers

The acoustic birefringence in a highly birefringent (Hi-Bi) two-mode optical fiber is measured and shown to be about 0.8%. The birefringence is primarily caused by a cladding ellipticity in excess of 1%. The effect of the softer stress-inducing elements is small in comparison. The effects of misalignment of the acoustic and optical axes is studied in detail using both continuous-wave and pulsed acoustooptic interaction. In the Hi-Bi fiber, the misalignment is approximately 20/spl deg/. The acoustic birefringence and misalignment have a profound effect on the spectral characteristic of tunable acoustooptic notch- and passband filters, giving rise to double spectral peaks. The birefringence is not specific to Hi-Bi fibers. The same effect is demonstrated in a birefringent elliptic core fiber, and it should be expected also in standard telecom single-mode fibers. In tunable acoustooptic notch filters based on coupling to cladding modes in standard telecom fibers with ellipticity of the order of 1% the acoustic birefringence may cause double absorption peaks separated by more than 4 nm. It is also shown that acoustic birefringence provides guiding of the spatial orientation of the acoustic flexural waves, even if the fiber is twisted. This guiding effect eases the construction of acoustooptic components with long interaction length.     

隧道中低频导行电磁波的传播特性

在低频段上，导行电磁波的衰减率很小，随频率的升高而缓慢增加，不论单线波模还是双线波模其近壁效应都很小。用数值分析方法术解了单线波模方程和双线波模方程，得出的主要结论为它们的衰减率与隧道壁电导率和隧道半径有撤弱依赖关系。理论分析含单导体的任意横截面隧道可知低频段上导行电磁波的衰减率与隧道横截面积大小有微弱依赖关系，而与横截面形状无关。     

High resolution spatial interferometry of guided optical waves

The spatial variation of the phase difference Φ(z) between collinear guided waves is measured interferometrically by scanning a localised perturbation along the guide. Milliradian phase resolution is achieved by the use of a quadrature signal. The method is demonstrated with TE⇔TM coupling in Ti:LiNbO₃. It yields the absolute value and the uniformity of the birefringence Δβ(z)=dΦ/dz     

1-D acoustic cavity in optical fibers using two acoustic Bragggratings

We report the fabrication and characterization of a one-dimensional acoustic cavity made on a tapered single-mode optical fiber with two acoustic mirrors. The mirrors are acoustic Bragg gratings formed by modulating the fiber diameter periodically. The cavity exhibits resonances with quality factors of around 300. Higher Q factors are achievable by increasing the number of periods in the gratings. A simple transfer matrix model successfully simulates the acoustic properties of the cavity. This type of acoustic cavity is of interest for its potential applications in acoustooptic components and the development of devices based on stimulated Brillouin scattering     

Nonlinear interaction of acoustic surface waves with carriers

We calculate the 2nd-order signal generation due to the space-charge nonlinearity induced by two interacting surface waves in a piezosemiconducting material. Using the Bleustein?Gulyaev mode as a typical example, it is shown that this 2nd-order term is not zero for two oppositely travelling surface waves of the same frequency ? in the absence of electron drift. This calculation clearly shows that this nonzero signal arises from the decrease in the ultrasonic electric field toward the interior of the sample. The general result can be extended easily to other types of elastic surface waves. Complementary experiments are also reported on the interaction of surface waves propagating in the same direction, and it is shown that there is a satisfactory agreement between the experimental data and the results of the analysis.     

Acousto-optic diffraction of optical guided waves in crystal quartz

A waveguide supporting a single propagating mode has been fabricated by ion beam modification of X-cut crystal quartz. The gas ion implantation reduced the index of a subsurface layer, and guiding occurred in the surface region. The acoustic properties of the quartz are little affected. Bragg diffraction of guided light by surface acoustic waves was observed with efficiencies of approximately 0.25% per milliwatt acoustic power.     

在损耗光纤中的暗孤子传输特性

本文采用数值分析和扰动法研究了光纤损耗的存在对正常色散区暗孤子的影响，结果表明：暗孤子的脉宽在相同损耗光纤中总比亮孤子扩展得更慢；对于零色散波长大于１．５５μｍ的色散移位光纤，暗孤子的放大周期比亮孤子的放大周期大一倍。这对于未来的海底光纤通信将具有很大的吸引力。关于暗孤子的二阶理论，现有的文章很少，仅文献［１］也只做了Ｈ１的理论推导工作，且误差还很大。本文首次推导出Ｈ１的精确解、并得出了关于暗孤子准确、完善的二阶理论结果。     

Propagation characteristics of femtosecond pulses in erbium-dopedmonomode optical fibers

The spectral and temporal characteristics of femtosecond optical pulses (durations of ~120 fs, wavelength of 1.5 μm) propagating in erbium-doped monomode fibers have been experimentally studied in detail. Distinct spectral broadening and shifting processes have been observed for three types of fibers (two different dopant concentrations and one erbium-free sample for comparison). Whereas only a small amount of spectral broadening occurs in the Er-free fiber, there is a major spectral extension (1.3-1.7 μm) for the low-doped erbium fiber, in which a self-phase-modulation mediated four-wave-mixing process has been observed. Autocorrelation data for the exiting pulses indicate the existence of strong pulse shaping and breakup effects in the Er-doped fibers     

Coupled waves on optical fibers by power conservation

Using only power conservation, the authors derive the coupling coefficients of coupled-mode theory and a new power conservation law. The derivation provides physical insight into the mechanism of wave coupling on optical fibers. It shows explicitly that the coupled-mode approximation is not generally applicable to Maxwell's equations, since the polarizability of perturbations is omitted. Examples include nonlinear fiber couplers and fiber nonuniformities, with emphasis on their polarization characteristics. An important class of unsolved problems is revealed     

Guided optical waves in fibers with negative dielectric constant

We classify and discuss all optical guided modes able to propagate in ideal metallic-dielectric structures having the symmetry of a step index fiber. The cases of a metallic core with a dielectric cladding and a dielectric core with a metallic cladding are considered. In each case, the intensity profile of the allowed guided modes is shown. The geometric and frequency dispersion of the effective index of the modes is discussed as a function of the optogeometrical parameters of the fibers     

Variable delay for acoustic surface waves by electro-acousto-optic interaction

Continuosly variable analogue delay is demonstrated by using reflections from a laser-light-induced impedence discontinuity on a photoconductive layer evaporated on an LiNbO3 delay line at 75 MHz. ?30dB reflection loss is obtained with a delay range of 20 ?s.     

Nonlinear interaction of acoustic surface waves in epitaxial gallium arsenide

A technique is described for obtaining the nonlinear interaction between two surface waves propagating in a piezoelectric semiconductor. The coupling between the waves is associated with the transverse electric fields of the surface wave and the carriers in the semiconductor. The effective carrier concentration for the interaction has been altered by forming a surface-depletion region beneath a metal?semiconductor contact. This scheme has been used to produce the convolution of two oppositely travelling acoustic waves in epitaxial gallium arsenide, and it has been found that a strong nonlinearity can be obtained at small acoustic-power levels.     

Propagation of waves in optical waveguides with various dielectric and metallic claddings

A new graphic method of obtaining easily the phase constant for a propagating wave is described and illustrated by a number of solutions for an optical waveguide covered with various dielectric and metallic claddings. It contains, implicitly, solutions for surface waves.     

Propagation characteristics of single-mode optical fibers with arbitrary complex index profiles

A rapidly converging numerical technique for the evaluation mode characteristics of circularly symmetric optical fibers with an arbitrary complex refractive index profile is presented. This method is based on transmission-line principles. From Maxwell's equations, we derive a transmission-line equivalent circuit for the optical fiber refractive index profile and we demonstrate how it can be used to determine the mode effective index (normalized propagation constant) of cylindrical dielectric waveguides. To illustrate the effectiveness of the procedure, we have applied it to circularly symmetric fibers with complex step, parabolic, and segmented optical refractive index profiles. We have used this method to evaluate and manipulate the gain in a typical 980-nm pumped erbium-doped fiber as well as for calculating attenuation of optical fibers when radial loss factors are present.     

Propagation constant of weakly guiding optical fibers: a neweigenvalue condition

After Laplace transforming the reduced radial Helmholtz equation for a weakly guiding optical fiber, we derive an eigenvalue condition for its propagation constant. It is expressed in terms of the ratio of two Laplace-transformation integrals. We show the usefulness of this new condition in connection with the numerical solution of the scalar Helmholtz equation     

End-fire array of virtual acoustic sources produced by the interaction of cylindrically spreading acoustic waves

An experimental study of the interaction between cylindrically spreading acoustic waves in water is described, and experimental results are given. The experimental results are compared with theoretical work separately published by one of the authors.     

Demonstration of birefringent optical fibre frequency shifter employing torsional acoustic waves

A fibre-optic frequency shifter utilising torsional acoustic waves to produce a travelling perturbation which couples the polarisation modes of a linearly birefringent optical fibre is demonstrated. The shift in optical frequency was 3.195 MHz and an optical coupling efficiency of 6% was obtained with 780 mW of electrical power.<>     

Propagation characteristics of the pseudo-Rayleigh mode of the ridge guide for acoustic surface waves

An approximate but simple dispersion relation is derived for the pseudo-Rayleigh mode of the ridge waveguide. The wave is essentially nondispersive and its phase velocity is slightly lower than that of the Rayleigh wave.     

The effects of surface acoustic waves on the optical properties ofAlGaAs-GaAs quantum-well structures

The effects of a propagating surface acoustic wave (SAW) on the transitions in single- and multiple-quantum-well structures and the change in their absorption and refractive index are presented here. The fundamental difference between the effects of an applied electric field and a propagating SAW are those due to a linear and a nonlinear induced field, respectively. The electron and hole energy eigenvalues and envelope functions of the unperturbed structure are determined. The effect of the SAW-induced strain and electric fields on the confining potential of the quantum-well (QW) structure is then calculated and the energy eigenvalues and envelope functions for this perturbed case are determined. The complex refractive index of the structure is then determined for both the perturbed and unperturbed cases, to give the change in refractive index and the absorption coefficient as a function of SAW wavelength, amplitude, barrier composition, well width, and number of wells. The wavelength (λ_ac) and power of the SAW were found to be important in setting the induced changes in the refractive index