Solar Adaptive Optics System Based on Software Control

An adaptive optics (AO) system for solar observations at the Hida Observatory is described, which is composed of commercially available low-cost devices and fully controlled with software in standard personal computers. This AO system operates at a frame rate of more than 250 Hz by virtue of the image processing procedures used. From laboratory experiments, it was confirmed that this AO system was useful for the temporal wavefront-variation of less than 64 Hz. During solar observations at the Hida Observatory the AO system worked well, except for a few cases.     

Active control of the volume acquisition noise in functional magnetic resonance imaging: method and psychoacoustical evaluation.

Functional magnetic resonance imaging (fMRI) provides a noninvasive tool for observing correlates of neural activity in the brain while a subject listens to sound. However, intense acoustic noise is generated in the process of capturing MR images. This noise stimulates the auditory nervous system, limiting the dynamic range available for displaying stimulus-driven activity. The noise is potentially damaging to hearing and is distracting for the subject. In an active noise control (ANC) system, a reference sample of a noise is processed to form a sound which adds destructively with the noise at the listener's ear. We describe an implementation of ANC in the electromagnetically hostile and physically compact MRI scanning environment. First, a prototype system was evaluated psychoacoustically in the laboratory, using the electrical drive to a noise-generating loudspeaker as the reference. This system produced 10-20 dB of subjective noise-reduction between 250 Hz and 1 kHz, and smaller amounts at higher frequencies. The system was modified to operate in a real MR scanner where the reference was obtained by recording the acoustic scanner noise. Objective reduction by 30-40 dB of the most intense component in scanner noises was realized between 500 Hz and 3500 Hz, and subjective reduction of 12 dB and 5 dB in tests at frequencies of 600 Hz and at 1.9 kHz, respectively. Although the benefit of ANC is limited by transmission paths to the cochlea other than air-conduction routes from the auditory meatus, ANC achieves worthwhile attenuation even in the frequency range of maximum bone conduction (1.5-2 kHz). ANC should, therefore, be generally useful during auditory fMRI.     

Age effects in the human middle ear: wideband acoustical measures

Studies that have examined age effects in the human middle ear using either admittance measures at 220 or 660 Hz or multifrequency tympanometry from 200 to 2000 Hz have had conflicting results. Several studies have suggested an increase in admittance with age, while several others have suggested a decrease in admittance with age. A third group of studies found no significant age effect. This study examined 226 Hz tympanometry and wideband energy reflectance and impedance at ambient pressure in a group of 40 young adults and a group of 30 adults with age > or = 60 years. The groups did not differ in admittance measures of the middle ear at 226 Hz. However, significant age effects were found in wideband energy reflectance and impedance. In particular, in older adults there was a comparative decrease in reflectance from 800 to 2000 Hz but an increase near 4000 Hz. The results suggest a decrease in middle-ear stiffness with age. The findings of this study hold relevance for understanding the aging process in the auditory system, for the establishment of normative data for wideband energy reflectance, for the possibility of a conductive component to presbycusis, and for the interpretation of otoacoustic emission measurements.     

Optical-fiber pulse rate multiplier for ultralow phase-noise signal generation

In this Letter we report on an all optical-fiber approach to the synthesis of ultralow-noise microwave signals by photodetection of femtosecond laser pulses. We use a cascade of Mach-Zehnder fiber interferometers to realize stable and efficient repetition rate multiplication. This technique increases the signal level of the photodetected microwave signal by close to 18 dB. That in turn allows us to demonstrate a residual phase-noise level of -118 dBc/Hz at 1 Hz and -160 dBc/Hz at 10 MHz from a 12 GHz signal. The residual noise floor of the fiber multiplier and photodetection system alone is around -164 dBc/Hz at the same offset frequency, which is very close to the fundamental shot-noise floor.     

TW级钛宝石飞秒激光放大装置中的同步控制

讨论了TW级飞秒激光放大装置中的同步时序系统原理,介绍了自行设计并建造的3套频率成分各异的TW级钛宝石飞秒激光放大装置,其中"极光Ⅱ"和"Titan"为两级放大系统,前者预放大与主放大重复频率均为10 Hz,后者采用1 kHz的预放大和10 Hz的主放大,"极光Ⅲ"为多级放大系统,预放大重复频率为10 Hz,主放大分别为1 Hz和15 min输出一发。针对各个放大装置的不同特点和对同步精度的具体要求,自行设计并建造了不同的同步时序控制系统,实现了各个激光放大系统的有效运转,同步精度达到200 ps。     

Seven-terawatt Ti:sapphire laser system operating at 50 Hz with high beam quality for laser Compton femtosecond X-ray generation

We present a 7-TW Ti:sapphire laser system operating at 50 Hz for laser Compton femtosecond X-ray generation. This laser system delivers 8.4 W of average output power at a repetition rate of 50 Hz with a pulse width of 24 fs. It demonstrates successful management using a dynamically stable resonator in the regenerative amplifier and compensation for thermal lensing by a convex mirror in a ring-type four-pass power amplifier. We also present the results of closed-loop corrections for distorted wavefronts of amplified and compressed laser pulses, using an adaptive optical system consisting of a Shack–Hartmann-type wavefront sensor and a deformable mirror. This closed-loop correction results in dramatic improvements, reducing wavefront distortions below 0.05 λ rms. Received: 31 October 2002 / Revised version: 3 March 2003 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/61-3349, E-mail: ito@festa.or.jp     

Evidence for different types of mechanoreceptors from measurements of the psychophysical threshold for vibrations under different stimulation conditions

The shape of the psychophysical frequency threshold curve for vibrations presented to the skin in the frequency region 5-1000 Hz is strongly dependent on the static force that the vibrator exerts on the skin and on whether there is a rigid surround around the vibrating contactor (presence of contrast). Where there is no rigid surround, an increase in static force reduces the threshold in the high-frequency region and increases it at low frequencies. When the static forces are sufficiently large, the thresholds reach a minimum value above 30 Hz and a maximum one below 30 Hz, this being the crossover frequency. Under these conditions in the frequency region around 200 Hz, where the threshold is determined by the Pacini receptor system, the vibration sensitivities of finger pad and thenar eminence (glabrous skin) are equal, while the value for the inner side of the forearm (hairy skin) is 12 dB higher. However, when a rigid surround is used, the threshold increases above 30 Hz and decreases below 30 Hz. The latter increase in sensitivity, which is introduced by the presence of contrast cues, amounts to about 20 dB and is sharply tuned at 18 Hz for the glabrous skin of the finger. It is argued that in this case the threshold is determined by the Meissner receptor system. This increase in sensitivity is less pronounced (about 10 dB) and less sharply tuned for the other sites. Finally, when the contact of the vibrating surface to the skin is at a minimum, the vibration threshold has the same displacement value (about 3 microns) over the whole frequency region independent of the site of stimulation and whether or not a rigid surround is present.     

Subfrequency noise signal extraction in fiber-optic strain sensors using postprocessing

Laser frequency fluctuations typically limit the performance of high-resolution interferometric fiber strain sensors. Using time delay interferometry, we demonstrate a frequency noise immune fiber sensing system, where strain signals were extracted well below the noise floor normally imposed by the frequency fluctuations of the laser. Initial measurements show a reduction in the noise floor by a factor of 30, with strain sensitivities of a nanostrain/Hz at 100 mHz and reaching 100 ps/Hz at 1 Hz. Further characterization of the system indicates the potential for at least 4.5 orders of magnitude frequency fluctuation rejection.     

Thermal-noise limit in the frequency stabilization of lasers with rigid cavities

We evaluate thermal noise (Brownian motion) in a rigid reference cavity used for frequency stabilization of lasers, based on the mechanical loss of cavity materials and the numerical analysis of the mirror-spacer mechanics with the direct application of the fluctuation dissipation theorem. This noise sets a fundamental limit for the frequency stability achieved with a rigid frequency-reference cavity of order 1 Hz/ square root Hz (0.01 Hz/ square root Hz) at 10 mHz (100 Hz) at room temperature. This level coincides with the world-highest level stabilization results.     

100Hz级脉冲管制冷机整机特性研究

脉冲管制冷机在更高频率下操作有利于系统体积的减小,同时也提高了能量的密度。本文首先介绍了直线压缩机驱动直线型脉冲管制冷机的整机计算模型,对于惯性管进行了湍流修正后的阻抗相位分析,结果表明频率越高,湍流影响越小。在100Hz下,整机取得了12．4W的制冷量,冷头温度到达31．8K,整机相对卡诺效率18．4％,已经十分接近...     

Development of 1.3GHz high-T-c rf SQUID

A new high-T_c (HT_c) rf SQUID working at around 1.3GHz has been developed to avoid electromagnetic interference such as growing mobile communication jamming. This new system works in a frequency range from 1.23 to 1.42GHz (centred at 1.3GHz), which is not occupied by commercial communication. The sensor used in the 1.3GHz rf SQUID is made of a HT_c coplanar superconducting resonator and a large-area HT_c superconducting film concentrator. We have achieved in the 1.3GHz HT_c rf SQUID system a minimal flux noise of 2.5×10^{-5}Φ_0/\sqrt{Hz} and a magnetic field sensitivity of 38fT/\sqrt{Hz} in white noise range, respectively. The effective area of the concentrator fabricated on a 15×15mm^2 substrate is 1.35mm^2. It is shown that the 1.3GHz rf SQUID system has a high field sensitivity. Design and implementation of 1.3GHz HT_c rf SQUID offers a promising direction of rf SQUID development for higher working frequency ranges.     

An oscillator-amplifier distributed feedback dye laser system,pumped by a single TEA N2 laser

In this paper we describe a simple and inexpensive Distributed Feedback Dye Laser (DFDL) system for picosecond pulse generation. This system consists of an oscillator-amplifier DFDL, either in the grating or in the prism configuration: both stages are pumped by a single TEA nitrogen laser with improved beam quality. Single pulses having duration of 26 ps are obtained at a frequency up to 100Hz; the overall pulse-to-pulse stability is, at the maximum frequency, of about 25%. This system is particularly well-suited for time-resolved fluorescence microscopy studies where short tunable laser pulses of low energy are required.     

Monolithic CEO-stabilization scheme-based frequency comb from an octave-spanning laser

We demonstrate a carrier-envelope phase-stabilized octave-spanning oscillator based on the monolithic scheme. A wide output spectrum extending from 480 nm to 1050 nm was generated directly from an all-chirped mirror Ti:sapphire laser. After several improvements, the carrier-envelope offset(CEO) beat frequency accessed nearly 60 d B under a resolution of 100 k Hz. Using a feedback system with 50-k Hz bandwidth, we compressed the residual phase noise to 55 mrad(integrated from 1 Hz to 1 MHz) for the stabilized CEO, corresponding to 23-as timing jitter at the central wavelength of790 nm. This is, to the best of our knowledge, the smallest timing jitter achieved among the existing octave-spanning laser based frequency combs.     

Ultrahigh sensitivity electric field detection with a liquid electro-optical film

In this Letter, an electro-optical probe configuration with polar molecule liquids as the sensing film is proposed to improve the voltage sensitivity. This method exhibited increases in intrinsic sensitivities better than 0.1 mV/√Hz, 2 orders of magnitude larger than the normal method using a GaAs probe in the same measurement system. Based on the mechanism of orientation polarization, the electro-optic coefficient was measured to be 250 pm/V by the Teng-Man method at a modulation field of 100 Hz. This technology will be promising in applications of low-frequency field detection.     

Measurements of vocal fold tissue viscoelasticity: approaching the male phonatory frequency range

Viscoelastic shear properties of human vocal fold tissues have been reported previously. However, data have only been obtained at very low frequencies (< or = 15 Hz). This necessitates data extrapolation to the frequency range of phonation based on constitutive modeling and time-temperature superposition. This study attempted to obtain empirical measurements at higher frequencies with the use of a controlled strain torsional rheometer, with a design of directly controlling input strain that introduced significantly smaller system inertial errors compared to controlled stress rheometry. Linear viscoelastic shear properties of the vocal fold mucosa (cover) from 17 canine larynges were quantified at frequencies of up to 50 Hz. Consistent with previous data, results showed that the elastic shear modulus (G'), viscous shear modulus (G"), and damping ratio (zeta) of the vocal fold mucosa were relatively constant across 0.016-50 Hz, whereas the dynamic viscosity (eta') decreased monotonically with frequency. Constitutive characterization of the empirical data by a quasilinear viscoelastic model and a statistical network model demonstrated trends of viscoelastic behavior at higher frequencies generally following those observed at lower frequencies. These findings supported the use of controlled strain rheometry for future investigations of the viscoelasticity of vocal fold tissues and phonosurgical biomaterials at phonatory frequencies.     

Interrogation of a long-period grating using a mechanically scannable arrayed waveguide grating and a sampled chirped fiber Bragg grating

A novel technique to interrogate a long-period grating (LPG) using a mechanically scannable arrayed waveguide grating (AWG) is proposed. This technique is implemented based on space-to-wavelength mapping by mechanically scanning the input light beam along the input coupler facet of an AWG. By employing a sampled chirped fiber Bragg grating with multiple peaks as a reference, the central wavelength of the LPG is measured. An interrogation system with a resolution of 10 pm at a speed of 10 Hz is demonstrated. Furthermore, the technique proposed can potentially offer subpicometer resolution at a speed of 500 Hz.     

Phase-locked, low-noise, frequency agile titanium:sapphire lasers for simultaneous atom interferometers

We demonstrate a laser system consisting of a >1.6 W titanium:sapphire laser that is phase locked to another free-running titanium:sapphire laser at a wavelength of 852 nm with a phase noise of -138 dBc/Hz at 1 MHz from the carrier, using an intracavity electro-optic phase modulator. The residual phase variance is 2.5 x 10(-8) rad2 integrated from 1 Hz to 10 kHz. This system can phase-continuously change the offset frequency within 200 ns with frequency steps up to 4 MHz. Simultaneous atom interferometers can make full use of this ultralow phase noise in differential measurements, where influences from the vibration of optics are greatly suppressed in common mode.     

Psychophysical evidence for auditory compression at low characteristic frequencies

Psychophysical estimates of compression often assume that the basilar-membrane response to frequencies well below characteristic frequency (CF) is linear. Two techniques for estimating compression are described here that do not depend on this assumption at low CFs. In experiment 1, growth of forward masking was measured for both on- and off-frequency pure-tone maskers for pure-tone signals at 250, 500, and 4000 Hz. The on- and off-frequency masking functions at 250 and 500 Hz were just as shallow as the on-frequency masking function at 4000 Hz. In experiment 2, the forward masker level required to mask a fixed low-level signal was measured as a function of the masker-signal interval. The slopes of these functions did not differ between signal frequencies of 250 and 4000 Hz for the on-frequency maskers. At 250 Hz, the slope for the 150-Hz masker was almost as steep as that for the on-frequency masker, whereas at 4000 Hz the slope for the 2400-Hz masker was much shallower than that for the on-frequency masker. The results suggest that there is substantial compression, of around 0.2-0.3 dB/dB, at low CFs in the human auditory system. Furthermore, the results suggest that at low CFs compression does not vary greatly with stimulation frequency relative to CF.     

Automatic alignment of a rigid spacer cavity

A bench-top auto-alignment system employing an invariant Gouy phase telescope is described in this paper. Our system uses external galvanometer-actuated mirrors for the tilt and offset control of a laser beam onto a rigid spacer cavity. Wavefront sensing diagnostics which accurately decouple tilt and offset er-rors are employed, and corresponding orthogonal corrections are enabled with the aid of electronic nulling. The closed loop system achieves an alignment fluctua-tion suppression of 50.4 dB for angular tilt and 58.3 dB for lateral offset at 1 Hz, with a unity gain bandwidth in excess of 115 Hz.     

LES-FEM coupled analysis and experimental research on aerodynamic noise of the vehicle intake system

To thoroughly explore the aerodynamic noise in order to achieve a more efficient engineering application for a vehicle intake system, the large eddy simulation and the finite element method were employed in numerical simulations, and the aeroacoustic characteristics were validated through the experimental data. In this research, the k-ε model was adopted to simulate the steady state fluid dynamic, and the static pressure loss was consistent with the bench test data, indicating the computational fluid dynamics model was valid. After acquiring the data from the steady state simulation, the fluctuating pressure of the inner wall was calculated based on the transient state calculation results from the large eddy simulation. Thereafter, the finite element method was used to determine the acoustic performance of the intake system. By comparing the experiment data, the noise reduction indicated that the intake system performed well at various frequencies, e.g. 320 Hz, 520 Hz and 770 Hz, but poorly at 140 Hz, 210 Hz, 420 Hz and 600 Hz. Finally, the far-field aerodynamic noise was calculated based on FW-H equation, and the output showed that the noise of each measuring point agreed well with the test results in trend. In particular, the inlet sound pressure spectrum almost fit the test data with the airflow of 300 m³/h, and several amplitude peaks appeared at 210 Hz, 420 Hz and 600 Hz, corresponding to the low-attenuation region of the noise reduction curve. Moreover, the specific frequencies were not shifted with the airflow changing. In conclusion, the numerical simulation method proves to be effective in calculating the aerodynamic noise accurately.