Audio Frequency Region Research Articles

Stable continuous-wave single-frequency intracavity frequency-doubled laser with intensity noise suppressed in audio frequency region**Project supported by the National Key R&D Program of China (Grant No. 2016YFA0301401).

We demonstrated a continuous wave (cw) single-frequency intracavity frequency-doubled Nd:YVO4/LBO laser with 532 nm output of 7.5 W and output of 3.1 W, and low intensity noise in audio frequency region. To suppress the intensity noise of the high power 532 nm laser, a laser frequency locking system and a feedback loop based on a Mach–Zehnder interferometer were designed and used. The influences of the frequency stabilization and the crucial parameters of the MZI, such as the power splitting ratio of the beam splitters and the locking state of the MZI, on the intensity noise of the 532 nm laser were investigated in detail. After the experimental optimizations, the laser intensity noise in the frequency region from 0.4 kHz to 10 kHz was significantly suppressed.

Generation of low-frequency squeezed states

Squeezed state of light is an important resource of optical measuerments below the shot noise limit and has been used to improve measurement sensitivity in many areas such as gravitational wave detection, especially in audio frequency region. Compared with the high-frequency squeezed states, the generation of the low-frequency squeezed states is more difficult, because it is limited by several technical noise sources. In this paper we report the observation of more than 2 dB of vacuum squeezing at 1064 nm in the gravitational-wave detection band down to 3 kHz with a double-resonant optical parametric oscillator (OPO). The OPO has a configuration of linear cavity consisting of an input coupling mirror with a transmission of 11% at 532 nm and an output coupling mirror with the transmission of 12% at 1064 nm. The nonlinear materials in the OPO is type-I periodically poled potassium titanyl phosphate (PPKTP) crystal which is chosen for this experiment due to its higher nonlinearity, broader phase matching temperature, and smaller photo-thermal effect. The OPO is pumped by the light of 532 nm from Nd: YVO4/KTP solid-state laser of maximum optical power 3 W. To avoid various noise coupled from the seed beam, the OPO is seeded by vacuum fluctuations instead of coherent field at the fundamental wavelength (1064 nm). A Pound-Drever-Hall (PDH) locking scheme is used to lock the OPO cavity length with the signal derived from the reflected pump beam, so as to lock the pump field and also lock the fundamental field. To make both the pump and seed beams resonant simultaneously, the temperature of the PPKTP is carefully adjusted. The squeezed state can be detected on a homodyne detection by interfering it with the local oscillator (LO) and detected by a balanced detector with two photodiodes (EXT500 T) but having the same quantum efficiency of 86% at 1064 nm. The subsequent electronic noise is analyzed with a low-frequency spectrum analyzer, which shows that the audio noise sources from lab enviroment, locking quality, escape efficiency, propagation loss, homodyne efficiency and detection efficiency have effect on the squeezing pruced by an OPO.

Structural and dielectric properties of thiazolium chlorobismuthate(III) and chloroantimonate(III)

The crystal structures of [C 3H 4NS] 6[ M 4Cl 18]·2H 2O ( M = Sb, Bi) consist of three nonequivalent thiazolium cations and discrete centrosymmetric anions [ M 4Cl 18] 6−. The title compounds appear to be isomorphous and crystallize in the triclinic space group, P 1 ¯ (at 100 K). One of three thiazolium cations was found to be disordered (two-site model). [C 3H 4NS] + cations are hydrogen bonded to [ M 4Cl 18] 6− units and water molecules. The water molecules (O w) via the O–H…Cl hydrogen bonds are involved in the one-dimensional polymeric chains, which extend along the c-axis. The dielectric dispersion studies on two thiazolium analogs in the audio-frequency region disclosed complex relaxation processes. The dielectric response indicates the presence of two independent relaxators, which are characterized by polydispersive nature. The relaxation processes taking place in the both studied crystals are due to the motion of dipolar thiazolium cations.

Dielectric critical slowing down in ferroelectric(C3N2H5)5Sb2Br11

Dielectric dispersion studies have been carried out on the pentakis(imidazolium) undecabromodiantimonate(III),(C3N2H5)5Sb2Br11, single crystal in the audio-frequency region. It is found thatlow-frequency dielectric dispersion, observed for the field along thea-axis of the monoclinicsymmetry (P 21/n), reveals a polydispersive nature near the paraelectric–ferroelectric phase transition temperature(Tc = 145 K). The dielectric dispersion results can be fitted with the Cole–Cole formula.Unexpectedly, the mean relaxation frequency obeys the Vogel–Fulcher law with freezingtemperature of 143 K. Near the ferroelectric phase transition temperature, the process ofdielectric critical slowing down was observed. Such a dynamic dielectric response ischaracteristic of ferroelectrics with an ‘order–disorder’ transition mechanism.

Highly Efficient Piezoelectric Actuators for Active Vibration Control

ABSTRACTAn high performance / inexpensive diskbender actuator was produced by combining efficient design and fabrication methods and a new technique to operate these actuators was developed and tested, which can enhance the displacement and force capabilities by almost a factor of 2 by using the same maximal driving voltage.The properties of these actuators are intermediate between those of standard bimorphs, used for very large displacements but providing rather small forces, and those of low voltage stack multilayers, which provide quite large forces but are generally heavier, larger and expensive for equivalent displacements. The absence of any external mechanical amplification mechanism and their geometry make these actuators particularly suitable for active vibration damping applications within buildings affected by perturbations of hundreds of μm or for noise control by emission of controlled sound in antiphase. The class of displacement/force, which can be obtained with suitably dimensioned actuators, provides sufficient high motion even for the lower audio frequency region (400–1500 Hz).In order to lower the driving voltages, multilayer diskbenders were also fabricated with the same technique. The number of layers does not influence the actuator displacement and force properties but the increased capacity of the actuator may require sophisticated driving amplifiers.

Structure, phase transitions and molecular motions in 4-aminopyridiniumperchlorate

The crystal structure of the 4-aminopyridinium perchlorate (4-apyH)ClO4 has beendetermined at 100 K by means of x-ray diffraction as monoclinic, with space groupP 21,with Z = 8.The crystal undergoes two structural phase transitions: one of first-order type,reversible, at 241/243 K (on cooling/heating respectively) and one ofweakly first-order type, irreversible, at 277 K (on heating). The crystaldynamics is discussed on the basis of the temperature dependence of the 1 Hnuclear magnetic resonance second moment(M2) and spin–latticerelaxation time T1.Both phase transitions are interpreted in terms of thechanges in the motional state of (4-apyH)+ cations and ClO4−anions. The dielectric dispersion studies disclose a relaxation process over thehigh-temperature phase (above 241 K) in the audio-frequency region. Thedielectric results are described by a Cole–Cole equation. The title crystal revealspyroelectric properties below 241 K. The ferroelastic domain structure of(4-apyH)ClO4 is observed over the whole temperature range studied.

Structure, phase transition and molecular motions in (C5H5NH)BiCl4

The crystal structure at 293 K of the new pyridinium compound, (C5H5NH)BiCl4, has been determined by X-ray diffraction as monoclinic, space group Cc, Z = 4. The crystal is built up of one-dimensional (BiCl4−)n chains and pyridinium C5H5NH+ cations. A structural phase transition of first-order type is detected by differential scanning calorimetry (DSC) and dilatometric measurements at 114/110 K (on heating–cooling, respectively). Proton NMR second moment and spin–lattice relaxation time studies confirmed the order–disorder mechanism of the phase transition at 110 K. It was connected with the reorientational motion of the pyridinium cation. Dielectric investigations reveal absorption and dispersion in the audio-frequency region in both the high and low temperature phases. The experimental results were analysed in the high temperature phase in terms of the Havriliak–Negami formula. In the low temperature phase the Cole–Cole relation for a single relaxator was applied. Above the phase transition point the macroscopic relaxation time exhibits non-Arrhenius behaviour, whereas below Tc it is perfectly described by a pure Arrhenius relation.

Dielectric properties of plasma polymerized furan thin film capacitors

The dielectric properties of radio frequency plasma polymerized Furan film capacitors were determined in the frequency range 1 kHz–1 MHz at various temperatures in the range 303–423 K. The dielectric constant was found to increase with the increase in the film thickness. The large increase in capacitance towards the low frequency region indicates the possibility of an interfacial polarization mechanism prevailing in that region. From the experimental results, the dielectric constant and its variation with frequency and temperature, the low dielectric losses and the stability of the films, shows that, plasma polymerized Furan film is a very good dielectric material for the fabrication of high voltage capacitors in the audio frequency region.

A novel dual-band equalizer design for loudspeaker systems

Many equalization methods based on DSP technology have been proposed during the past decade. However, nearly all the methods demand low-frequency components to be processed separately because of deficient resolution. A dual-band equalization method for loudspeaker systems is presented in this paper. In this method, the conventional equalization based on deconvolution of the system response and a recently introduced method based on warped filter design are combined, to achieve a refined performance in the whole audio frequency region with lower order filter. Computer simulations are given to demonstrate the effectiveness.

Dielectric Properties of Electrorheological Fluids Containing Zeolite Particles

Relationships between electrorheological (ER) effects and dielectric properties of suspensions are investigated from measurements of ER and dielectric properties of suspensions containing particles of type A zeolite such as 3A, 4A and 5A. In each suspension containing zeolite 3A or 4A particles, the ER effect is recognized, i.e., an increase in the apparent viscosity is observed when an electric field is applied, and the dielectric property is characterized by a dielectric dispersion in audio frequency region. On the other hand, the suspension containing zeolite 5A particles does not show a distinct ER effect and has a low dielectric constant without dielectric dispersion above 1 Hz. These results indicate that relaxation time of the induced polarization is associated with the ER effect; for the appearance of the ER effect the relaxation time should be shorter than a characteristic time. It is suggested that the characteristic time is associated with an angular velocity of rotating particles under a steady shear flow and also that the relaxation time of the polarization is mainly determined by the electric conductivity of the particle

Frequency Dependence of Resistivity of High-Purity Copper at Low Temperatures

The frequency dependence of the surface impedance of different kinds of high-purity copper was measured in the audio frequency region and at low temperatures. The impedance of a coil with and without a rod-shaped sample was measured as a function of frequency. From these values, the surface impedance for copper of different purities was calculated. It was found that the real part of the difference in the coil impedance is proportional to ωn, where ω is the angular frequency and n ranges from 1/2 (normal skin effect) to 2/3 (anomalous skin effect). This finding suggests that the surface impedance of high-purity copper at low temperatures can be better explained in terms of the anomalous skin effect than the normal skin effect.

Compliances of wood for violin top plates

In the orthotropic approximation, nine independent elastic compliance elements are needed to describe the elastic behavior of wood. Many are difficult to determine experimentally, particularly in the audio frequency region, where their dynamic consequences are of interest to violin makers. This study computes the normal mode frequencies of a representative arched shell, using a commercial finite element package, in order to compute a ‘‘dependence matrix,’’ Daij =(sij/f a)(df a/dsij), expressing the relative fractional change of the ath normal mode frequency f a to the fractional change of the sij compliance element. Since the sum of Daij over all nine compliances is − 1/2 for each normal mode a, the relative importance of each compliance element can be read directly from Daij for a given set of compliances and a given plate geometry. This study concludes that only three compliance elements are of critical importance for violin top plates. One of the consequences is that the substitution of manmade composite materials with higher symmetry properties (and, hence, fewer compliance elements) for Norway spruce is feasible.

An effective utilization of spectral spread in holographic passive imaging systems

Holographic passive imaging systems are examined from the viewpoint of an effective utilization of thsep ectral spread of an object emitting random waves, and two effective methodsf or improving images are proposed. One is the smoothing of sample spectra by a spectral window at the stage of hologram signal estimation by taking into account thep rocess of wave propagation in order to obtain precise hologram signals with reduced noise. The other is the synthesis of complex valued imageso ver a wide frequency range to get a highly resolved image in the range direction. These processings are discussed theoretically and examined experimentally by using a bispectral passive imaging system which had been designed and constructed for machine system diagnosis in the audio frequency region. The results show thek effectiveness.

A. C. behaviour and dielectric relaxation in indium oxide films

Vacuum deposited blackish indium oxide films (In-O) as well as the oxidised films (In2O3) were studied for their a.c. behaviour at different temperatures and at various film thicknesses in the audio frequency region. e of In-O films was thickness dependent and also showed dielectric relaxation at lower frequencies due to the dipolar orientation arising from their non-stoichiometric nature. However at liquid nitrogen temperature region e was thickness independent similar to the oxidised films which neither showed any relaxation effect nor any thickness dependent e. The results have been discussed from the classical theory of dielectric polarisation.

Dielectric relaxation in siloxane polymers

The dielectric α relaxation arising from segmental motion in polydimethylsiloxane and polyphenylmethylsiloxane has been studied in the audio frequency region and at lower frequencies using d.c. step response measurements.The macroscopic decay function for polyphenylmethylsiloxane conforms with the empirical relationship ϕ(t)= exp –(t/τ0)β, 0 < β⩽ 1.

Interfacial forces and the brillouin spectrum of the interfacial light-scattering power

Light scattered from a fluid surface has a spectrum that involves lines slightly displaced from the incident frequency. The detection of these lines requires resolution far beyond that of normal spectrometers. Laser heterodyne beat frequency techniques developed recently allow resolution of Brillouin lines down to the low audio-frequency region. We have developed a simple spectrometer that detects the Brillouin spectra that are associated with the interaction of light and random surface ripples. The theory and experimental techniques involved are discussed. The capillary ripple dispersion equations have been solved numerically in order to provide theoretical estimates of the frequency of the splitting and estimates of the line shape. A doublet in the photocurrent power spectrum was predicted and was observed with increasing wave number for the air-water interface. The Brillouin shift observed for the air-water interface obeyed the Kelvin dispersion relation. Applications of this new technique to the study of interfacial forces are discussed.

Determination of Dielectric Behavior at Low Frequencies from Measurements of Anomalous Charging and Discharging Currents

The relation between anomalous charging or discharging currents and ac dielectric behavior of a material is discussed together with the use of the Hamon approximation for calculating the loss factor from these currents.The validity of the assumption used in deriving the Hamon approximation is discussed with reference to the behavior of a material with a Cole-Cole distribution of relaxation times. An analytical derivation of the Hamon approximation which has appeared in the literature is described together with a discussion of the limits of its validity. A description of the experimental methods which have been used to measure these anomalous currents (dc transient technique) is given, and experimental data in the literature showing the continuity of the loci of relaxation processes and values of loss factor from the audio frequency region into the very low-frequency region (e.g., ${10}^{\ensuremath{-}4}$ to ${10}^{\ensuremath{-}2}$ cps) is discussed.Solids often have long relaxation times at ordinary temperatures and are then suitable for examination with the dc transient technique. Moreover, the components of loss due to relaxation and conduction may be separately determined using this technique, and this is an essential prerequisite to an understanding of the different mechanisms involved. When peaks in loss factor are obtained, all the usual dielectric information may be obtained---including distribution of relaxation times, activation energy and entropy for dipole orientation, and dipole-moment values for the structural units involved.

Sound Attenuation in Liquid-Solid Mixtures

The sound attenuation in liquid-solid mixtures in the audio-frequency region seems to be caused by the viscous drag loss, due to the difference in velocity between solid and liquid. Attenuation caused by dilute concentrations of particulate matter in the oceans agrees with published experimental results at audio frequencies. General agreement is conceived with published experiments on marine sediments.

Hall effect wattmeters

A transmission-type wattmeter can be formed utilizing the Hall effect to give wide-range multiplication over a frequency range extending from the low audio-frequency region to the SHF band. At the low frequencies, the Hall device can take the form of a thin wafer of indium antimonide mounted in series with the center leg of a cup core magnetic circuit. An audio-frequency wattmeter employing such a structure has been analyzed, constructed and tested. The wattmeter has a range of zero to 400 milliwatts when used in a 600-ohm circuit, and displays a frequency error of less than plus and minus 3.5 per cent over a frequency range of 100 to 6000 cps for a unity power factor load. In the SHF range, a cavity and electric probe are arranged to provide excitation for the indium antimonide wafer in the manner first employed by Barlow. The cavity, about one wavelength long, is separated from the waveguide in which power flow is to be measured by a thin brass wall, and is coupled to the waveguide by a rectangular slot in the top wall of the waveguide. The semiconductor wafer is placed in the center of the cavity, at a position having a maximum in magnetic field and a minimum in electric field for a given cavity excitation value. The electric probe, which extends from the cavity to the waveguide, provides excitation current for the semiconductor element. Three tuning adjustments are required, two for the cavity and one for the probe. A wattmeter embodying this technique has been found to provide a sensitivity of one dc microvolt per milliwatt of transmitted power at a frequency of 9.40 Gc.

Segmental Motion in Polypropylene

Molecular motion in polypropylene has been studied both by measurements of dynamic mechanical properties and by observations of nuclear magnetic resonance line shapes. The mechanical measurements were made in the audio-frequency region over the temperature range from 80°K to the melting point. Both completely amorphous and highly crystalline samples of polypropylene were investigated. In the amorphous sample, two transitions were observed, a broad but weak secondary one near 235°K and a relatively sharp primary one occurring near 270°K. For the highly crystalline polypropylene, two transitions centered at 250°K and 300°K, respectively, are associated with increased mobility in the amorphous regions and a third higher temperature transition is associated with crystalline melting. These transitions have also been investigated by observing the changes occurring in NMR line shapes as the temperature is varied.