Torsional Acoustic Wave Research Articles

Time–Frequency-Based Quench Detection for HTS VIPER Cable Using Torsional Acoustic Wave

A new quench detection and diagnostic system is required for the reliable operation of high-field superconducting magnets for fusion which is fabricated with high-temperature superconductor (HTS) cables. The conventional voltage-based diagnostic methods have a limitation in detecting local temperature changes in large-scale HTS magnets. To address the limitation, an acoustic-based diagnostic method using torsional waves applied by piezoelectric transducers is proposed. In addition, from the sensor data collected by the piezoelectric transducers, a new monitoring index that can measure the temperature change is extracted via chirplet transform (CT), which is one of the time–frequency analyses. The proposed quench detection method using torsional wave guiding is implemented in a vacuum pressure-impregnated, insulated, partial dislocation, extruded, and roll-formed (VIPER) cable, and the performance of the new index, time–frequency-based phase delay, is validated. This work proves that the propagation characteristics of torsional acoustic waves depend on the strength and location of the heat and that the change can be monitored via the proposed time–frequency-based phase delay. The preliminary development and results of a new quench detection method are discussed to validate future use in complex environments such as fusion power plants.

Characterization of structural irregularities in highly birefringent photonic crystal fiber using torsional acoustic polarization coupling

We demonstrate a novel method to characterize structural irregularities in a highly birefringent photonic crystal fiber (HB PCF) with high accuracy based on polarization coupling by torsional acoustic wave. Birefringence variation induced by the irregularities of air-hole structure in the fiber cross-section is analyzed via the transmission spectra of the acousto–optic coupling between two orthogonal polarization modes propagating along the fiber. The estimated maximum birefringence variation of two sections in the same batch of the HB PCF is 3 × 10 − 6 and 10 − 5 , respectively, from nominal birefringence value of 4.86 × 10 − 4 . The comparison between the experimental results and the numerical simulation resulted in the estimation of few tens of nm variation in the air-hole structure in the PCF.

Sidelobe suppression in all-fiber acousto-optic tunable filter using torsional acoustic wave

We propose two techniques to suppress intrinsic sidelobe spectra in all-fiber acousto-optic tunable filter using torsional acoustic wave. The techniques are based on either double-pass filter configuration or axial tailoring of mode coupling strength along an acousto-optic interaction region in a highly birefringent optical fiber. The sidelobe peak in the filter spectrum is experimentally suppressed from -8.3 dB to -16.4 dB by employing double-pass configuration. Axial modulation of acousto-optic coupling strength is proposed using axial variation of the fiber diameter, and the simulation results show that the maximum side peak of -9.3 dB can be reduced to -22.2dB. We also discuss the possibility of further spectral shaping of the filter based on the axial tailoring of acousto-optic coupling strength.

Polarization-Independent All-Fiber Acousto-Optic Tunable Filter Using Torsional Acoustic Wave

The intrinsic polarization dependence of an all-fiber tunable wavelength filter based on polarization coupling by torsional acoustic wave is eliminated by employing a fiber loop configuration. In the fiber loop, two orthogonal polarizations split from input light propagate the single acousto-optic interaction region in opposite directions before being recombined to produce a polarization-independent filter spectrum. The polarization-dependent loss was estimated to be less than 0.1 and 0.3 dB within passband in bandpass and notch type filters, respectively.

Analyses of unintentional intensity modulation in all-fiber acousto-optic tunable filters

We theoretically and experimentally analyze unintentional intensity modulation phenomena in two types of all-fiber acousto-optic tunable filters utilizing flexural and torsional acoustic waves. Output filter signal at a resonant wavelength shows time-varying oscillations with even- and odd-order harmonics of applied acoustic frequency, which are explained by two factors of static mode coupling and acoustic back reflection. The magnitudes of static coupling and acoustic reflection in our devices are estimated from the measured first and second harmonic modulation powers.

Polarization-coupling all-fiber acousto-optic tunable filter insensitive to fiber bend and physical contact

We show that an all-fiber acousto-optic tunable filter based on polarization mode coupling using torsional acoustic wave is immune to the fiber bend and physical contact in the acousto-optic interaction region. We also propose and demonstrate a novel strain-free and size-reduced tunable filter with a 4-m-long fiber acousto-optic interaction region looped into a 5- cm-diameter coil.

Axial strain dependence of all-fiber acousto-optic tunable filters

We report the axial strain dependence of two types of all-fiber acousto-optic tunable filters based on flexural and torsional acoustic waves. Experimental observation of the resonant wavelength shift under applied axial strain could be explained by theoretical consideration of the combination of acoustic and optical effects. We discuss the possibility of suppressing the strain effect in the filters, or conversely, the possibility of using the strain dependence for wavelength tuning or strain sensors.

Polarization coupling in a highly birefringent photonic crystal fiber by torsional acoustic wave

We demonstrate and analyze the acousto-optic coupling between two optical polarization modes of the LP(01) mode propagating in a highly birefringent photonic crystal fiber. The coupling is realized based on wavelength selective acousto-optic coupling by traveling torsional acoustic wave in an all-fiber tunable polarization filter configuration. The dispersion properties of the torsional acoustic wave in the photonic crystal fiber and the influence of axial non-uniformity in the modal birefringence on the filter transmission are discussed in detail.

Highly efficient all-fiber tunable polarization filter using torsional acoustic wave

We demonstrate an all-fiber tunable polarization filter with high coupling efficiency based on acousto-optic coupling between two optical polarization modes of the LP(01) mode propagating in a highly birefringent single mode optical fiber. An over-coupling between the two polarization modes is realized over the wavelength range from 1530 nm to 1610 nm using traveling torsional acoustic wave. The measured 3-dB optical bandwidth of the filter was 4.8 nm at the wavelength around 1550 nm. The details of the filter transmission and the coupling characteristics are discussed.

Coaxial optical-fiber frequency shifter

A fiber-optic frequency shifter utilizing a traveling torsional acoustic wave to couple the polarization eigen-modes of a highly linearly birefringent optical fiber is demonstrated. The frequency-shifter configuration is such that the acoustic-wave generator and optical fiber are coaxial. The device is thus more rugged than previous free-fiber frequency shifters, which have incorporated a T junction between the acoustic generator and optical fiber. The optical frequency shift was 3.204 MHz, and an optical coupling efficiency of 12% was obtained with 4 W of electrical drive power applied to the acoustic transducer.

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 bircfringent 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.

Torsional wave generator

First Page

High temperature elastic properties of polycrystalline niobium, tantalum, and vanadium

A pulse-echo technique has been used to measure the temperature variation of the velocity of extensional and torsional acoustic waves in polycrystalline Nb(Cb), Ta, and V up to ∼2000 K. The concomitant temperature variation of the elastic moduli do not show any anomalies which would be consistent with a “lattice softening” theory for the departure from linearity of the Arrhenius plots for the diffusivity of interstitial atoms in the Group V metals.