Multipass interferometer using a filamentary spatial filter

A multiple pass interferometer using a filamentary spatial filter is developed. We can change the sensitivity of the interference fringe pattern by selecting the order of multiply reflected image without reduction of the fringe visibility.     

Measured wavenumber: frequency spectrum associated with acoustic and aerodynamic wall pressure fluctuations

Direct measurements of the wavenumber-frequency spectrum of wall pressure fluctuations beneath a turbulent plane channel flow have been performed in an anechoic wind tunnel. A rotative array has been designed that allows the measurement of a complete map, 63×63 measuring points, of cross-power spectral densities over a large area. An original post-processing has been developed to separate the acoustic and the aerodynamic exciting loadings by transforming space-frequency data into wavenumber-frequency spectra. The acoustic part has also been estimated from a simple Corcos-like model including the contribution of a diffuse sound field. The measured acoustic contribution to the surface pressure fluctuations is 5% of the measured aerodynamic surface pressure fluctuations for a velocity and boundary layer thickness relevant for automotive interior noise applications. This shows that for aerodynamically induced car interior noise, both contributions to the surface pressure fluctuations on car windows have to be taken into account.     

Low pressure gain measurements in KrF

Optical gain at 248 nm is measured in low pressure krypton/fluorine mixtures excited by an electron-beam. Measurable gain is observed down to a pressure of 50 mbar. Results are compared with the predictions of a simple kinetic model. The application of low pressure operation to ultra high power short pulse amplifiers is discussed.     

Hybrid multinary modulation using a phase modulating spatial light modulator and a low-pass spatial filter

We propose a method for performing binary intensity and continuous phase modulation of beams with a spatial light modulator (SLM) and a low-pass spatial filtering 4-f system. With our method it is possible to avoid the use of phase masks in holographic data storage systems or to enhance the phase encoding of the SLM by making it capable of binary amplitude modulation. The data storage capabilities and the limitations of the method are studied.     

Mode analysis with a spatial light modulator as a correlation filter

A procedure for the real-time analysis of laser modes using a phase-only spatial light modulator is outlined. The procedure involves encoding into digital holograms by complex amplitude modulation a set of orthonormal basis functions into which the initial field is decomposed. This approach allows any function to be encoded and refreshed in real time (60 Hz). We implement a decomposition of guided modes propagating in optical fibers and show that we can successfully reconstruct the observed field with very high fidelity.     

Adaptive feedforward control of a rectangular panel using a wave filter constructed with point sensors

This paper presents the wave filtering method for a rectangular panel, which is necessary for a feedforward wave control system, and clarifies its validity in the control system. Firstly, a wave solution of a rectangular panel is derived to describe the wave dynamics of the structure. This is followed by the derivation of the wave filtering equations that indicates the structure of the filter. It is found that the proposed wave filter consists of a modal filter, a rearrangement filter and a wave decomposition filter. Then, from the viewpoint of numerical simulations, the characteristics of the wave propagation in a rectangular panel as well as the accuracy of the wave fitter are verified. For the evaluation of the filter accuracy, the condition number is used as a performance index. Finally, an experiment on the adaptive feedforward control system using the wave filter is carried out, demonstrating that the reflected wave absorbing control enables the inactivation of vibration modes.     

All-optical analog-to-digital conversion system with a spatial coding method using designed filter

We propose and demonstrate an all-optical analog-to-digital conversion system with a spatial coding method using a designed filter. To make available codes more flexible, the design technique for a computer-generated hologram is introduced to our spatial coding scheme. By designing the spatial coding filter appropriately, this coding scheme enables us to output various digital signals as spatial patterns. By combining this coding method with the optical quantization method using soliton self-frequency shifting in a fiber, the proposed system converts analog sampled signals into arbitrary digital signals. Preliminary experimental results show that 3-bit Gray codes are successfully output as spatial patterns using the designed spatial coding filter.     

Assessment of acoustic pressure holograms from membrane velocity measurements

The aim of this paper is to demonstrate experimentally the possibility of acquiring acoustic pressure holograms using a light membrane and a scanning laser vibrometer. The velocity of a light membrane placed in an acoustic field can be measured without contact by means of a laser vibrometer. The ideal membrane must be optically reflective, acoustically transparent (having as little mass as possible), impermeable, and mounted without tension. The measured velocity is equal for continuity reasons to the normal acoustic velocity, but differs from the acoustic velocity without the membrane because the membrane is never completely transparent to acoustic waves. The effect of the mass of the membrane can be taken into account to correct this difference. Then, acoustic pressure holograms can be deduced from velocity holograms using the 2D Discrete Fourier Transform. An experimental validation is carried out; acoustic pressures derived from laser measurements are compared with microphone measurements, with a very satisfying match over a wide frequency range.     

Low temperature radio-frequency transverse susceptibility measurements using a CMOS oscillator circuit

A transverse susceptibility (TS) measurement system based on a simple inverter CMOS cell oscillator cross-coupled to a LC tank is presented. The system has been implemented to operate at a Quantum Design Physical Properties Measurement System (PPMS). We introduce several improvements with respect to similar currently operating TS measurement equipments. The electronics have been redesigned to use CMOS transistors as active devices, which simplifies the circuit design and enlarge the tuning range, thus making the proposed electronic block more feasible, predictable, and precise. Additionally, we propose a newly designed sample holder, which facilitates the procedure to change a sample and improves reproducibility of the circuit. Our design minimizes the thermal leak of the measuring probe by one order of magnitude, allowing to measure from 1.8 K in standard PPMS systems, thanks to the use of a low temperature beryllium–copper coaxial cable instead of the conventional RG402 Cu coaxial cable employed in the insert for the PPMS in similar systems. The data acquisition method is also simplified, so that the measuring sequences are implemented directly in the PPMS controller computer by programming them in the Quantum Design MultiVu software that controls the PPMS. We present the test measurements performed on the system without sample to study the background signal and stability of the circuit. Measurements on a Gd₂O₃ calibrating sample yield to the estimation of the system sensitivity, which is found to be on the order of 10⁻⁶ emu. Finally, measurements on a TmCo₂ Laves phase sample with a ferrimagnetic transition temperature around 4 K are described, demonstrating that the developed system is well suited to explore interesting magnetic phenomena at this temperature scale.     

Scanning electron microscope resolution: 2. Resolution measurements using structures with a rectangular profile

Different techniques for estimating the resolution of a scanning electron microscope are considered. A method for determining the resolution using reduced standard deviations of the distances between reference points which characterize features of the profile of the test object measured at video signals is proposed. Measurements of the resolution of scanning electron microscopes using test objects that represent slitlike grooves in silicon with a rectangular profile are exemplified.     

Suppression of acoustic noise in the measurements of wall pressure fluctuations

It is shown that, in hydrodynamic noise measurements in the presence of acoustic noise acting upon the pressure fluctuation receiver, spatial filtering methods should provide the best results. Active methods are developed for suppressing the acoustic noise that affects a miniature receiver in the course of turbulent pressure fluctuation measurements. The methods are based on complicating the structure of the measuring transducer by introducing an extra compensating sensing element whose characteristics are identical with those of the main sensing element. The spatial filtering of small-scale turbulent pressure fluctuations by a finite-size electroacoustic transducer is used as the basis for the development of noise-compensated measuring systems, as well as methods of measuring the turbulent pressure fluctuations by receivers with noise compensation. A numerical study of the wave-number filtering of acoustic noise in wall pressure measurements by a noise-compensated receiver is performed.     

Fiber-optic hydrophone using a cylindrical Helmholtz resonator as a mechanical anti-aliasing filter

Two Michelson interferometric fiber-optic hydrophones that use panda polarization-maintaining fibers and devices have been constructed and tested. The low-frequency acoustic sensitivities of both hydrophones are -159 dB re 1 rad/microPa. One of the hydrophones tested has a small cylindrical Helmholtz resonator that has a break point near 1200 Hz and a measured roll-off of approximately 50 dB/octave, and it is a hydrostatic-pressure-insensitive design. This hydrophone is a prototype device for a class of sensors that can be used to eliminate aliasing in future sonar systems. To our knowledge, this is the first time that such a fiber-optic hydrophone has been reported.     

Modeling a sensitive pressure and temperature sensor using rectangular PCF by 2-D FDTD technique

We propose a sensitive pressure and temperature sensor depends on hollow core rectangular photonic crystal fiber. The proposed modeling is carried out by implementing 2-D finite difference time domain (FDTD) method. The two parameters like pressure and temperature plays a vital role in reservoir engineering to increase the production rates of oil well and our sensor technique is depend on the transmission peak wavelength shift which is caused by temperature/pressure change, and geometrical parameter of the structure of rectangular PCF. Here we have done simulation for various work using 2-D FDTD method application to sensing. We have shown the proposed design which provides the sensitivity with linear dependence of the resonance wavelength over refractive index of PCF holes at a operating wavelength of 1.55 μm.     

Electron density measurements in a low pressure discharge using doppler-free two-photon spectroscopy

A new diagnostic technique is presented for the determination of the local electron density in a low pressure discharge. Electron densities ranging from 10¹⁸ to 10¹⁹ m^-3 are determined with a spatial resolution better than 0.5 mm in the positive column of a Na-Ne discharge. The technique is based on the measurement of the Stark broadening and Stark shift of the sodium 3S-4D transition using Doppler-free two-photon spectroscopy.     

Peering into darkness with a vortex spatial filter

I propose to use as a window the dark core of an optical vortex to examine a weak background signal hidden in the glare of a bright coherent source. Applications such as the detection of an astronomical object, forward-scattered radiation, and incoherent light are described whereby signal enhancements of at least 7 orders of magnitude may be achieved.     

Solar magnetic fields measurements with a magneto-optical filter

Summary The presence of a magnetic field at different levels inside the Sun has crucial implications for helioseismology. The solar oscillation observing program carried out since 1983 at Mt. Wilson with Cacciani magneto-optical filter (MOF) has recently been modified to acquire full-disk magnetograms with 2 arcsec spatial resolution. We present here a method for the correct determination of magnetic maps which are free of contamination by velocity signal. We show that no cross-talk exists between the Doppler and Zeeman shifts of the NaD lines, provided that instrumental polarization effects are taken into account. We also used the observed line-ofsight photospheric field to map the vector field in the inner corona, above active regions, in the ?current free? approximation.

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Riassunto La presenza di un campo magnetico a differenti livelli dell'interno solare ha importanti implicazioni nell'eliosismologia. Il programma osservativo a Mt. Wilson, in corso dal 1983 con il filtro magneto-ottico di Cacciani, è stato recentemente arricchito con l'acquisizione anche di mappe magnetiche. Si illustra la procedura di riduzione dei dati per una corretta determinazione dei campi magnetici e di velocità. Viene poi utilizzato il campo longitudinale fotosferico per ricostruire il campo magnetico vettoriale a livello coronale, al di sopra delle regioni attive, nell'approssimazione ?current free?.

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Резюме Налччие магнитного поля на различных уровнях внутри Солнца имеет существенное значение для гелиосейсмологии. Программа наблюдения солнечных осцилляций, проводимая с 1983 года на г. Вильсон с помощью магнитно-оптического фильтра, была недавно модифицирована. В этой работе мы предлагаем метод для определения магнитных карт, которые свободны от погрешностей, обусловленных сигналом скорости. Мы показываем, что при учете инструментальных эффектов поляризации, не существует связи между доплеровским и зеемановским сдвигами линий NaD. Мы также используем наблюдаемое поле фотосферы для отобразения векторного поля во внутренней короне, выше, активных областей.

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Paper presented at the 4th Cosmic Physics National Conference, Capri, September 11–14, 1988.

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To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Eigenmode analysis of phased-coupled VCSEL arrays using spatial coherence measurements

We apply the modal coherence theory to evaluate the spatial mode structure of a 2×2 phase-coupled array of vertical cavity surface emitting lasers (VCSELs). The eigenmode structure is extracted for different pump currents by measuring the degree of spatial coherence of all VCSEL pairs in the array. The results reveal the impact of optical disorder and spatial hole burning on the modal discrimination. The approach is useful more generally for the evaluation of spatial mode content of other laser array.     

Eigenfrequency of a Helmholtz resonator at the wall of a rectangular duct

The eigenfrequency of a Helmholtz resonator fixed at the wall of a rectangular duct is estimated. Special attention is given to determining the attached length of the resonator neck as viewed from the duct. The dependences of the attached neck length and the eigenfrequency on the duct configuration are analyzed. Theoretical results are compared with the data of numerical calculations performed by the finite-element method.     

Radiation impedance of a piston at the wall of a rectangular duct

The radiation impedance of a rectangular piston positioned at the wall of a rectangular duct is theoretically investigated. The dependence of the attached length of the piston on the width of the duct is determined. The dependence shows that, unlike the radiation resistance, the attached length decreases with a decrease in duct width. The results of the study are compared with the data obtained for the attached length of a square orifice in a rectangular baffle in a duct.     

Distortion of ultrashort laser pulses in a spatial filter

By means of Collins diffraction integral, the propagation expression of an ultrashort laser pulse passing through a spatial filter is derived. The effects of the magnification of spatial filter on pulse broadening and distortion of pulse-front for the ultrashort optical pulse are analytically deduced and numerically simulated. It is found that pulse broadening and propagation time difference of a laser pulse getting through spatial filter is proportional to the magnification. As a conclusion, in an ultrashort pulse laser system with a large aperture, the effect of pulse broadening and distortion of pulse-front should be considered.