Flexural Acoustic Waves Research Articles

Characterization of flexural acoustic waves in optical fibers using a fiber-tip interferometer

An experimental study using a fiber-tip interferometer (FTI) to characterize traveling flexural acoustic waves (TFAWs) along an optical fiber is reported. The measurements carried out with the FTI are performed following two different procedures: one of them relies on adjusting the interferometer at the quadrature condition and the other extracts the information from the amplitudes of the fundamental and second harmonic of the interference signal. From our measurements, the detection limit of the FTI is 0.5 nm and the upper limit of the linear regime is 30 nm. These results validate the use of the FTI to detect nanometric displacements associated to the amplitude of acoustic waves along an optical fiber. Parameters such as the acoustic wavelength, attenuation coefficient, and phase velocity are measured for a range of acoustic frequencies around the 2 MHz region. Furthermore, and supported by the in-fiber acousto-optic (AO) effect produced by TFAWs, parameters associated to an acoustic wave packet such as the group velocity and the group attenuation coefficient are also determined. This set of experimental results provide useful information of the mechanisms underlying the propagation of TFAWs along an optical fiber and its corresponding AO effect.

Optical Reflection Induced by Flexural Acoustic Wave on Fiber Bragg Grating

Flexural acoustic waves can induce optical reflection peaks on one side of the Bragg wavelength in the reflection spectrum of a fiber Bragg grating, thus laying a cornerstone of active acoustic-optic fiber devices. The acoustically-induced refractive index distribution of FBG is deduced. The mechanism of forming acoustically-induced reflection peaks is investigated using mode coupling theory in vector mode presentation. A precise picture is depicted with the selection rules for mode coupling and observation of phase matching, such that the two-step and the three-step processes for acoustically-induced reflection peaks are identified with experimental confirmation. While the three-step coupling theory is discovered, our theoretical framework also refines the former scalar-mode two-step coupling theory. Empowering the theoretical prediction to perfectly match experiment, this work will facilitate the design and application of novel active fiber-optic devices based on acoustic-optic interactions.

Experimental demonstration of acoustically induced polarization-dependent fiber optical vortex inversion.

A recently proposed theoretical model of acousto-optic interaction in optical fibers with a traveling flexural acoustic wave of the fundamental order [M.A. Yavorsky, et al., Opt. Lett.44, 598 (2019)10.1364/OL.44.000598] is experimentally examined. We show the effect of inversion of topological charge of optical vortices, which is governed by the direction of incident linear polarization. This vector effect of a coupling of polarization and orbital degrees of freedom proves the inconsistency of the conventional microbending-based model and confirms the recently suggested approach of the description of acousto-optic interaction that is based on the actual displacement vector. In addition, the obtained results demonstrate the realization of a controlled-NOT gate for orbital angular momentum (OAM) states.

Laser2: A two-domain photon-phonon laser.

The laser is one of the greatest inventions in history. Because of its ubiquitous applications and profound societal impact, the concept of the laser has been extended to other physical domains including phonon lasers and atom lasers. Quite often, a laser in one physical domain is pumped by energy in another. However, all lasers demonstrated so far have only lased in one physical domain. We have experimentally demonstrated simultaneous photon and phonon lasing in a two-mode silica fiber ring cavity via forward intermodal stimulated Brillouin scattering (SBS) mediated by long-lived flexural acoustic waves. This two-domain laser may find potential applications in optical/acoustic tweezers, optomechanical sensing, microwave generation, and quantum information processing. Furthermore, we believe that this demonstration will usher in other multidomain lasers and related applications.

Soft flexural waves and self-localization of wrinkling modes of single- and few-layer graphene under compression in a compliant matrix

This Research Letter studies the evolution under external compression of the wrinkling modes of single- and few-layer graphene or another two-dimensional atomic crystal embedded in or placed on a compliant matrix, or on a dense fluid in a gravitational field. An analytical model based on the nonlinear bending elasticity of the graphene is developed, which shows that the compressive surface stress causes spatial localization of the extended sinusoidal wrinkling mode with a solitonlike envelope, with the localization length decreasing with overcritical external strain. The parameters of the extended sinusoidal wrinkling mode are determined from the conditions of the anomalous softening of the flexural surface acoustic wave with predominant out-of-plane and suppressed in-plane surface displacements, propagating along the graphene in a compliant anisotropic matrix. Self-localization of the wrinkling modes finally results in the formation of strongly localized modes with approximately one-period sinusoidal profiles and external-strain-independent wavelengths. Self-localization of the wrinkling modes is governed by the derived Ginzburg-Landau-type nonlinear envelope-function equation with a negative dispersive term, which we relate with the graphene-nonlinearity-induced repulsion between soft flexural surface acoustic waves with negative effective mass. One- and two-dimensional wrinkling patterns and two types of strongly localized graphene wrinkling modes with different symmetry are described.

An All-Fiber Mode-Locked Pulse Laser by Fiber Bragg Grating-Based Acousto-Optic Frequency Shifter

An all-fiber frequency shifted feedback laser is developed to export mode-locked pulses with a narrow spectral bandwidth (238 MHz). A simple cavity structure is established by employing a fiber Bragg grating-based acousto-optic frequency shifter. An exotic reflection peak in a large spectral separation from the Bragg reflection peak is generated by applying flexural acoustic waves, and is utilized for narrow-band filtering and frequency shifting in the laser cavity. Stable mode-locked pulse output can be obtained with the cavity length as short as 16.5 m, whereas increasing the cavity length can reduce the pump power to 37 mW for mode-locked pulse output. A maximum power conversion efficiency of 17% with an average output power of 103 mW were obtained experimentally at a pump power of 600 mW, with corresponding pulse width, pulse energy and peak power of 2 ns, 88 nJ and 4.5 W, respectively. A time-bandwidth product of mode locked pulses of 0.476 denotes nearly Fourier-transform-limited performance. This work provides a new technique to implement efficient, compact and low-cost all-fiber FSFLs for narrow spectral bandwidth mode-locked pulse output.

Investigation of layered composite plates under acoustic emission using an appropriate finite element model

Structural damages generate acoustic emission (AE) in the media and cause extensional and flexural acoustic waves. Often in structures like plates, flexural mode is predominant. In this study, the flexural mode AE waveforms due to simulated damage are studied for multi-layered composite plates. A generalized refined 2D plate theory, which satisfies the transverse shear stress continuity at the layer interfaces, is proposed here for modelling the plates. This formulation is implemented through finite element method (FEM) where a four-node rectangular element, that satisfies C1 continuity, is used. Plates, having different thickness ratios, are studied through numerical examples using the model. Results are validated wherever applicable and some new results are obtained. The results indicate that the proposed model can simulate the flexural waveforms realistically for ‘very thin’ to ‘moderately thick’ plates. It is also found that the present model is as accurate as 3D FEM but it possesses much better computational efficiency.

Multi-Orthogonal High-Order Mode Converter Based on Acoustically Induced Fiber Gratings

We experimentally investigate the generation of high-order modes (HOMs) with flexible switchable output. Cascaded acoustically induced fiber gratings (AIFGs) act as mode converters. A periodic micro-bending in the AIFG is produced by travelling flexural acoustic wave, and two modes with adjacent azimuthal numbers can be coupled to each other. Different HOMs (LP21a/b and LP11a/b) could be obtained just by tuning the frequency of the radio frequency (RF) driving signal. The dependence of RF frequencies on the two acousto-optic (AO) coupling stages is theoretically analyzed and experimentally verified. Based on cascaded AIFGs mode converter, a multiple-level intensity modulator and an all-fiber Yb-doped laser with flexible mode switching output of HOMs (LP21a/b and LP11a/b) are demonstrated. Moreover, by optimizing the outer diameter of the second part of the AO coupling regions, the mode conversion between LP01 and LP21 can be realized by applying a single frequency on the piezoelectric transducer (PZT).

Experimental study of an in-fiber acousto-optic tunable bandpass filter for single- and dual-wavelength operation in a thulium-doped fiber laser.

A tunable single- and dual-wavelength thulium-doped all-fiber laser is demonstrated based on the implementation of an in-fiber acousto-optic tunable bandpass filter (AOTBF). The AOTBF is fabricated to be operated in the 1.9 µm region, and takes advantage of the intermodal coupling effect produced by traveling flexural acoustic waves in an optical fiber. It exhibits a 3-dB bandwidth of 2.04 nm with an insertion loss of 4.75 dB. The tuning properties of the AO device allows a continuous-wave operation with characteristics of wide tuning range (211.5 nm), narrow linewidth (50 pm) and high signal-to-noise ratio (60 dB). In the dual-wavelength regime, the laser is capable of independent tuning of each of the laser lines, achieving a tunable dual-wavelength emission that extends from 1802.67 to 1932.75 nm. A controllable wavelength spacing with minimum and maximum separations of 1.04 and 130.08 nm is obtained.

Dual-wavelength narrowband all-fiber acousto-optic tunable bandpass filter based on dispersion-compensating fiber

A dual-wavelength narrowband all-fiber acousto-optic tunable bandpass filter was proposed and fabricated based on vector mode coupling induced by the acoustic flexural wave in dispersion-compensating fiber with a ring core. In the experiment, the resonant dual wavelengths could be electrically tuned with a range of 1526 nm to 1565 nm and the 3 dB bandwidth was less than 1 nm, which coincides with the calculation results. The structure shows the advantages of fast tuning speed, easy fabrication, and zero frequency shift, which can be applied in fast-tunable dual-wavelength lasers, WDM networks, and microwave and terahertz-wave generation.

Polarization-dependent orbital angular momentum flipping in fibers with acousto-optic interaction

In this paper we consider the evolution of linearly polarized optical vortices in circular optical fibers with a propagating fundamental flexural acoustic wave. A new polarization-dependent mode conversion is found out, in which the sign of the topological charge (and orbital angular momentum) of the outgoing vortex beam is governed by the direction of the incident linear polarization. This effect can be used for implementing polarization-controlled orbital angular momentum flipping. This paves the way to implementation of the all-fibre stable controlled-NOT gate, in which the linear polarization carries the control qubit and the topological charge carries the target. Such a gate is able to produce optical beams with entanglement between polarization and orbital degrees of freedom in regime of linear optics. Yet, such orbital angular momentum controlling should be useful in micromechanics, classical and quantum information encoding, and classical simulation of quantum algorithms.

Broadband tuning of a long-cavity all-fiber mode-locked thulium-doped fiber laser using an acousto-optic bandpass filter

A long-cavity passively mode-locked thulium-doped all-fiber laser is reported incorporating a tapered acousto-optic tunable bandpass filter (AOTBF). The operation of the AOTBF relies on the intermodal coupling between core and cladding modes when a flexural acoustic wave propagates along an 80μm tapered fiber. The filter works in transmission and exhibits a 3dB bandwidth of 9.02nm with an insertion loss of 3.4dB. The laser supports ultrashort pulse generation at a low repetition rate of 784.93kHz. Optical pulses with 2.43nm of optical bandwidth and 2.1ps pulse duration were obtained in a broad tuning range from 1824.77 to 1905.16nm.

Manipulation of a Micro Droplet in a Capillary Tube by Using a Flexural Acoustic Wave

Manipulation of a Micro Droplet in a Capillary Tube by Using a Flexural Acoustic Wave

All-fiber frequency shifter consisting of a fiber Bragg grating modulated via an acoustic flexural wave for optical heterodyne measurement.

We present an all-fiber frequency shifter (AFFS) consisting of a fiber Bragg grating (FBG) modulated via an acoustic flexural wave for optical heterodyne measurement. The AFFS can efficiently generate the frequency-shifted signal due to the resonance peak with a high-reflection efficiency and being completely separated from the reflection spectrum of the original FBG, simultaneously. The experimental result shows that the minimal measurable vibration amplitude and the resolution of the all-fiber optical heterodyne measurement setup constructed with the AFFS are 0.06nm and 30pm in the region of tens to hundreds of kilohertz, respectively.

Wavelength Tunable Filter Based on Acousto-Optic Modulation of a Double-Core Fiber

A tunable notch filter is acoustically modulated in the transmission spectrum of a double-core optical fiber. Flexural acoustic waves dynamically change the coupling power and the wavelength of the propagating modes, allowing to switch and tune wavelengths between the two cores. A maximum modulation depth of 5.4 dB and a tuning wavelength range of 8.4 nm are achieved with the present configuration. The results indicate new possibilities for application in space division multiplexing systems, dynamic couplers, gain equalizers, and fibers lasers.

Revised model of acousto-optic interaction in optical fibers endowed with a flexural wave.

We propose a novel model of the acousto-optic interaction in circular fibers endowed with the lowest-order flexural acoustic wave that is based on the actual distribution of the acoustically induced displacement vector. The corresponding expression for the fiber's permittivity is derived and compared with the commonly used one. The resonance optical fiber modes and the propagation constants are found. It is predicted that the lengths of the well-known acoustically driven mode conversion LP0,n→LP1,n' should be slightly different for the x- and y-polarized incident fundamental modes. Moreover, we unveil a new polarization-dependent mode conversion in which the azimuthal mode number ℓ, as well as the optical frequency of the generated standard fiber mode LPℓ,n' , is governed by the linear polarization direction of the incident zero-order beam LP0,n.

Q-switching and mode locking pulse generation from an all-fiber ring laser by intermodal acousto-optic bandpass modulation

Q-switched and mode-locked (QML) pulse generation from an all-fiber ring laser based on intermodal acousto-optic bandpass modulation is reported. The modulator relies on full-acousto-optic mode re-coupling cycle induced by a standing flexural acoustic wave, with a transmission response that is controlled by amplitude modulation of the acoustic wave signal. The Q factor of the cavity is controlled by a rectangular pulse wave with variable frequency and duty cycle, whereas mode locking is achieved by amplitude modulation derived from a standing flexural acoustic wave. The best QML pulses were obtained at 0.5 kHz repetition rate, with a pump power of 549.2 mW, at the optical wavelength of 1568.2 nm. A maximum overall energy of 2.14 µJ at an average output power of 1.07 mW was achieved, corresponding to a burst of mode-locked sub-pulses of 100 ps pulse duration within a QML envelope of 3.5 µs.

Actively mode-locked all-fiber laser by 5 MHz transmittance modulation of an acousto-optic tunable bandpass filter

Active mode-locking of an all-fiber ring laser by transmittance modulation of an in-fiber acousto-optic tunable bandpass filter (AOTBF) is reported. Cavity loss modulation is achieved by full acousto-optic mode re-coupling cycle induced by standing flexural acoustic waves. The modulator permits the implementation of 28 dB of nonresonant light suppression, 1.4 nm of modulation bandwidth, 74% of modulation depth and 4.11 dB of optical loss in a 72.5 cm-long all-fiber configuration. The effects of the modulated AOTBF on the laser performance are investigated. Transform-limited optical pulses of 8.8 ps temporal width and 6.0 W peak power were obtained at 4.87 MHz repetition rate.

All-fiber cylindrical vector beams laser based on an acoustically-induced fiber grating

We presented a method to construct an all-fiber laser by using an acoustically-induced fiber grating (AIFG) in two-mode fiber (TMF) within the laser cavity to generate cylindrical vector beams (CVBs) in communication band. Based on the polarization dependent vector mode coupling characteristic of an AIFG, the CVBs were generated by respectively coupling and modes to TM01 and TE01 modes, which have radial and azimuthal polarizations characteristics, respectively. The output of the all-fiber laser could be adjusted from a linearly-polarized Gaussian mode to a radially or an azimuthally polarized vector mode through a polarization controller to adjust the linear polarization direction of the input mode of and an acoustic flexural wave applied on TMF. This work provides a method for generating CVBs with convenient mode conversion characteristics.

Bending instability of few-layer graphene embedded in strained polymer matrix

In this Letter we describe analytically and simulate numerically the softening of flexural surface acoustic waves, localized in the plane of few-layer graphene embedded in soft matrix of low-density polyethylene. The softening of surface acoustic wave is triggered by the compressive strain in the matrix, which results in compressive surface stress in the few-layer graphene. Softening of the flexural surface acoustic wave leads to spatially periodic static bending deformation (modulation) of the embedded nanolayer with the definite wave number. Few-layer graphene with different numbers of graphene monolayers is considered. We describe the different models of interlayer bonding of graphene monolayers in a few-layer graphene, which correspond to the weak and strong interlayer bonding. The considered models give substantially different scaling of the wave number of periodic bending deformation and of the threshold compressive strain in the matrix as functions of the number of graphene monolayers in the few-layer graphene. Both the wave number of periodic bending deformation and the values of the threshold compressive surface stress in the few-layer graphene and of the compressive strain in the matrix are very well confirmed by the numerical simulations. Bending instability of few-layer graphene can be used for the study of bending stiffness and two-dimensional Young modulus of the graphene nanolayers, embedded in a soft matrix.