Tunable hybridization at midzone and anomalous Bloch-Zener oscillations in optical waveguide ladders

We have studied the optical oscillation and tunneling of light waves in optical waveguide ladders (OWLs) formed by two coupled planar optical waveguide arrays. For the band structure, a midzone gap is formed owing to band hybridization, and its wavenumber position can be tuned throughout the whole Brillouin zone, which is different from the Bragg gap. By imposing a gradient in the propagation constant in each array, Bloch-Zener oscillation (BZO) is realized with Zener tunneling between the bands occurring at the midzone, which is contrary to the common BZO with tunneling at the center or edge of the Brillouin zone. The occurrence of BZO is demonstrated by using the field-evolution analysis. The tunable hybridization at the midzone enhances the tunability of BZO in the OWLs. This Letter may offer new insights into the coherent phenomena in optical lattices.     

Bright screening solitons in a photorefractive waveguide

Characteristics of propagation of bright screening solitons are studied in a photorefractive waveguide. The investigation is performed under paraxial wave approximation and Wentzel–Kramers–Brillouin–Jefferys approximation. A Gaussian ansatz for the soliton shape is used instead of the numerical solutions. The planar waveguide structure intensifies the self-focussing while decreasing the minimum or threshold power required for self trapping. The waveguide structure embedded in the crystal can self trap a soliton of power lower than the threshold power. As the waveguide parameter increases, minimum required power to self trap the beam decreases. The existence of bistable states is also predicted. Four regimes of power are identified in which the solitons behaviour is studied. Propagation of screening solitons in a photorefractive waveguide is studied in the absence of the photovoltaic and pyroelectric effect for the first time.     

Periodic spatial rearrangement of the interference structure and the diffraction focusing of acoustic fields in oceanic waveguides

It is demonstrated that the periodic (along the path) rearrangement of the spatial (in depth and horizontal distance) interference structure of the acoustic field generated in an oceanic waveguide by a CW point source is accompanied by a diffraction focusing of the field with the corresponding spatial period. It is found that, in an oceanic waveguide with a monotone angular dependence of the Brillouin ray cycle, the spatial period of both the rearrangement of the interference structure and the diffraction focusing of the field is directly proportional to the square of the characteristic vertical scale of the waveguide and inversely proportional to the wavelength; in a waveguide with a nonmonotone dependence, the corresponding spatial period is directly proportional to the cube of the aforementioned waveguide scale and inversely proportional to the square of the wavelength.     

Coupling characteristics of a three channel waveguide directional coupler

We develop an accurate analysis for a three channel waveguide directional coupler which takes the effect of corner regions into account through the first order perturbation theory. The applicability of the earlier approaches namely (i) effective-index method (ii) planar waveguide approximation for such couplers has also been discussed. It is found that while the effective-index approach used by earlier authors underestimates the coupling length, the planar waveguide approximation overestimates the same.     

Features of the Invariant of the Spatial–Frequency Interference Structure of Acoustic Fields in Oceanic Waveguides

The paper considers the regular patterns that can be manifested in the behavior of the invariant of the spatial–frequency interference structure of an acoustic field in oceanic waveguides that are homogeneous and inhomogeneous along a track. Using the WKB and adiabatic approximations, an analytic expression is obtained for the invariant that, when certain conditions are fulfilled, reduces to the well-known classical expression independent of the mode numbers, their parity, or the emission frequency. It has been established that the approximate classical expression for the invariant correctly describes the slope of interference lines only in ranges of variation in the grazing angles of modes where the dependence of the cycle length of their corresponding Brillouin waves on the ray parameter is quite smooth and monotonic. The paper studies the formation of the spatial–frequency interference structure of an acoustic field propagating from a shallowwater isovelocity waveguide to a relatively deep-water waveguide with a near-surface sound channel.     

Acoustic longitudinal mode coupling in w-shaped Al/Ge Co-doped fibre

This paper proposes a novel fibre structure aiming at distributed temperature and strain sensing. Utilizing Al₂O₃ and GeO₂ as dopants to form a w-shaped acoustic waveguide, it realizes modal coupling between longitudinal acoustic modes of its inner and outer core layers, leading to a dual-peak or multi-peak Brillouin gain spectrum. The relationship between the acoustic mode coupling properties and the fibre materials, doping concentrations and structural parameters are investigated, showing that the positions of mode coupling points in acoustic dispersion curves and the coupling intensities can be designed flexibly. A specific fibre design for the discriminative sensing of temperature and strain under a pump wavelength of 1.55 μm is given. The responses of its Brillouin gain properties on temperature and strain are analysed theoretically, demonstrating its potential for distributed fibre Brillouin sensing.     

Observation of inhomogeneous spectral broadening of stimulated brillouin scattering in an optical fiber

We report experimental results which show that the stimulated Brillouin scattering (SBS) spectrum in an optical fiber is inhomogeneous, exhibiting spectral broadening and hole burning under cw monochromatic laser excitation. This phenomenon arises from the waveguide interaction of the pump and Stokes signals and is a fundamental property of SBS in waveguiding systems due to their ability to confine a fan of radiation wave vector directions.     

High reflectivity phase conjugation in magnetized diffusion driven semiconductors

Optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in magnetized diffusion driven semiconductors under the off-resonant transition regime has been investigated theoretically. The model is based upon the coupled-mode approach and incorporates the effect of pump absorption through the first-order induced polarization. The linear dispersion is found not to affect the reflectivity of the phase conjugate Stokes shifted Brillouin mode. The reflectivity of the image radiation is dependent upon the Brillouin susceptibility and can be significantly enhanced through n-type doping of the crystal and the simultaneous application of magnetic field. Moreover, the threshold of the pump intensity required for the occurrence of SBS in the crystal with finite optical attenuation can be considerably diminished through a suitable choice of the excess carrier concentration and the magnetic field. Consequently, OPC-SBS becomes a possible tool in phase-conjugate optics even under not-too-high power laser excitation by using moderately doped n-type semiconductors kept under the influence of magnetic field. Numerical estimates made for n-InSb crystal at 77 K duly irradiated by nanosecond pulsed 10.6 μm CO₂ laser show that high OPC-SBS reflectivity (70%) can be achieved at pump intensities below the optical damage threshold if the crystal is used as an optical waveguide with relatively large interaction length (L ∼5 mm) which proves its potential in practical applications such as fabrication of phase conjugate mirrors.     

Direct observation of Bloch harmonics and negative phase velocity in photonic crystal waveguides

The eigenfield distribution and the band structure of a photonic crystal waveguide have been measured with a phase-sensitive near-field scanning optical microscope. Bloch modes, which consist of more than one spatial frequency, are visualized in the waveguide. In the band structure, multiple Brillouin zones due to zone folding are observed, in which positive and negative dispersion is seen. The negative slopes are shown to correspond to a negative phase velocity but a positive group velocity. The lateral mode profile for modes separated by one reciprocal lattice vector is found to be different.     

Negative Refraction in One-Dimensional Photonic Crystals with Tilted Interface

We investigate the wave propagation through the tilted interface of one-dimensional photonic crystals. Negative refraction can be realized by excitation of the Bloch states in the extended Brillouin zone with suppressed reflection. Equi-frequency surface analysis shows that the positive refraction, negative refraction or birefringence in this configuration can be achieved under a proper incident angle, which is confirmed by finite-difference timedomain simulations. The results may be useful in applications in the new devices based on one-dimensional photonic or optical waveguide arrays.     

Plasmonic Zener tunneling in metal-dielectric waveguide arrays

We elucidate in this Letter plasmonic Zener tunneling (PZT) in metal-dielectric waveguide arrays (MDWAs) by using numerical simulations and theoretical analyses. PZT in MDWAs occurs at the waveguide entrance and wherever the beam completes Bloch oscillations, because the bandgap between the first and second bands is minimal at the center of the first Brillouin zone. This feature significantly differs from that of optical Zener tunneling in dielectric waveguide arrays. The dependence of the simulated tunneling rate on the gradient of the relative permittivity of the dielectric layers correlates with the tunneling theory, thus confirming the occurrence of PZT in MDWAs.     

Cavity enhanced stimulated Brillouin scattering in an optical chip for multiorder Stokes generation

We report the first demonstration of on-chip cascaded stimulated Brillouin scattering (SBS). Cascaded SBS is characterized in a 4 cm long chalcogenide (As?S?) rib waveguide where the end facet reflections provide a monolithic Fabry-Perot (FP) resonator. The presence of the FP cavity reduces the Brillouin gain threshold, which enables observation of cascaded SBS at reduced pump powers. We observe up to three orders of Stokes waves in the backscattered signal at a coupled peak power of 1.34 W. Anti-Stokes waves due to four-wave mixing between the pump and the Stokes wave were observed in the forward spectrum.     

Ultrasmall demultiplexer by use of one-dimensional photonic crystal

Silica-based one-dimensional photonic crystal (1D-PC) is fabricated by use of a high-spatial-frequency grating with input and output surfaces tilted with respect to its periodic direction. An incident beam is coupled with the first photonic band in the second Brillouin zone of the 1D-PC. The output beam angle changes 3 degrees with a wavelength change of 1%. A prototype of an ultrasmall demultiplexer is demonstrated by use of a silica slab waveguide with 1D-PC.     

单轴各向异性异向介质平板波导中的导模特性

 推导了介电常数张量和磁导率张量中各分量带有不同符号的单轴各向异性异向介质平板波导的导行条件。根据分量符号的正负组合,分情况讨论了导行TE波的偶模特性,画出了平板波导中为实横波数时的布里渊图。结果表明,在特定条件下此平板波导中有无穷多导模,这与传统介质波导及各向同性异向介质平板中只有有限数量模式存在有很大不同。此外,还发现当分量参数满足一定条件时,出现了多个虚横波数的传输模式解,并且该模式存在高频截止。     

Artificial cochlea and acoustic black hole travelling waves observation: Model and experimental results

An inhomogeneous fluid structure waveguide reproducing passive behaviour of the inner ear is modelled with the help of the Wentzel–Kramers–Brillouin method. A physical setup is designed and built. Experimental results are compared with a good correlation to theoretical ones. The experimental setup is a varying width plate immersed in fluid and terminated with an acoustic black hole. The varying width plate provides a spatial repartition of the vibration depending on the excitation frequency. The acoustic black hole is made by decreasing the plate?s thickness with a quadratic profile and by covering this region with a thin film of viscoelastic material. Such a termination attenuates the flexural wave reflection at the end of the waveguide, turning standing waves into travelling waves.     

Time delay enhancement in stimulated-Brillouin-scattering-based slow-light systems

We show a simple method of time delay enhancement in slow-light systems based on the effect of stimulated Brillouin scattering. The method is based on the reduction of the absolute Brillouin gain by a loss produced by an additional pump laser. With this method we achieved pulse delays of nearly 100 ns in a standard single-mode fiber. In the presented approach the delay or acceleration of optical signals is decoupled from their amplification or attenuation, which allows the adaptation of the pulse amplitudes to the given application.     

Spatiotemporal dynamics of magnetostatic and spin waves in a transversely confined ferrite waveguide

Spatiotemporal dynamics of the multimodal propagation of magnetostatic waves in a transversally confined ferrite waveguide and the parametric excitation of spin wave under conditions of three-magnon decay processes are experimentally investigated by the Brillouin spectroscopy.     

Modelling of the surface plasmon resonance waveguide sensor with Bragg grating

The operation of a novel device – a waveguide surface plasmon resonance sensor with a UV-written Bragg grating – is theoretically analysed using two methods. In the simple perturbation approach, the metal/dielectric layer system supporting the resonance excitation of the surface plasma wave is considered to be a perturbation of the original dielectric waveguide with Bragg grating that is analysed using a coupled-mode theory. The second approach consists of the rigorous method of bi-directional mode expansion and propagation using the Floquet mode formalism developed recently for the analysis of waveguide grating structures. The results of both approaches are mutually compared, and the operation characteristics of this novel sensing device are briefly described.     

Resonance and Antiresonance in Electronic Transport Process Through a T-Shaped Quantum Waveguide

By means of the transfer matrix approach, the linear conductance spectrum for electronic transport through a T-shaped quantum waveguide is calculated. The resonant peaks and the antiresonant dips in the conductance spectrum are mainly focused. The previous prediction about their positions by other theoretical approaches is checked. In addition, a function of spin filtering is suggested based on the interplay of the resonance and antiresonance in this T-shaped quantum waveguide.     

基于波导不变量的空中和水下声源分辨方法

波导不变量反映了水下声场多途传播的干涉结构特征。将波导不变量的提取分析应用于空气中声源产生的水下声场,通过数值仿真分析了波导不变量分布随深度的变化,提出了利用小深度接收声场提取的波导不变量分布来分辨空中和水下声源的方法。分析了空气中声速和风速剖面对该方法的影响,指出在实际海上环境条件下,当出现逆温现象或者当接收器在声源下风方向时,该方法是有效的。