Bragg Cell Research Articles

Space-integrating acousto-optic processor for rapid Global Positioning System signal acquisition

A space-integrating acousto-optic processor conceived for Global Positioning System (GPS) signal acquisition is presented. The main advantage of the proposed system, as compared with traditional ones, is that only one parameter estimation, Doppler shift, is needed, instead of two. The GPS clear and acquisition (C/A) codes are matched to a paratellurite Bragg cell in time-compression hardware with a compression factor of 20. This technique is further exploited for accelerating Doppler estimation. Experimental results show an acquisition time of at most 1 ms.

Acousto-optic liquid-crystal analog beam former for phased-array antennas

A compact phased-array antenna acousto-optic beam former with element-level analog phase (0-2π) and amplitude control using nematic-liquid-crystal display-type technology is experimentally demonstrated. Measurements indicate > 6-bit phase control and 52.6 dB of amplitude-attenuation control. High-quality error calibration and antenna sidelobe-level control is possible with this low-control-power analog beam former. Optical system options using rf Bragg cells or wideband Bragg cells are discussed, with the rf design being the current preferred approach. Transmit-receive beam forming based on frequency upconversion-downconversion by electronic mixing is introduced for the rf Bragg-cell beam former, and comparisons with digital beam forming are highlighted. A millimeter-wave signal generation and control optical architecture is described.

In-line interferometric time-integrating acousto-optic correlator

A novel, efficient, stable, in-line interferometric, two Bragg cell, acousto-optic architecture is introduced for signal correlation and spectrum analysis. The processor employs an image-inversion technique to produce a correlation output that does not have to be generated on a fast spatial carrier, thus reducing the burden on the required space-bandwidth product on the output time-integrating detector array. The system is particularly suited for wide-instantaneous-bandwidth signal processing and is demonstrated as an autocorrelator for dc-to-10-MHz AM signals on a 100-MHz carrier. A fixed-spatial-carrier outputbased correlator and spectrum-analyzer architecture that is particularly limited in the wideband mode is demonstrated for comparison purposes with the proposed flexible-spatial-carrier-design-based in-line acousto-optic correlator.

Growth and characterization of mercurous halide crystals: mercurous bromide system

Mercurous bromide single crystals were grown by the physical vapor transport method in a sealed quartz tube. Growth rates were measured for crystals growing in different orientations. Experimental data followed the equation l t = l ∞− A exp(− t/ τ), where l t is the length of the crystal at any time t, l ∞ is the final statio nary length, and A and τ are constants. Crystals growing in 〈001〉 orientation had much higher velocity than crystals growing in 〈110〉 orientation. Large crystals were grown in 〈110〉 orientation by using preoriented seeds, an orientation needed for acousto-optic devices. The optical quality of mercurous bromide crystals was evaluated by fabricating a 5 cm long crystals. The acoustic velocity measurement data showed that the value for the slow shear velocity propagating along 〈110〉 is 2.73 × 10 4 cm/s, consistent with the values reported earlier. This translates with a high value of photelastic coefficient, into a high acousto-optic diffraction efficiency; 2600 with respect to quartz. These results showed that mercurous bromide has potential as an efficient Bragg cell with a delay time greater than 200 μs in a compact package.

Flame-induced laser Doppler velocimetry velocity bias

Laser Doppler velocimetry (LDV) measurements are often made in the presence of transient index of refraction variations which bias the measured velocity. Transient temperature variation (e.g. as found in turbulent combustion or turbulent convective heat transfer situations) is the most common mechanism driving index of refraction changes in systems probed with LDV. The physical phenomena inducing this bias, referred to here as flame-induced velocity bias (FIVB), are outlined. Combustion system velocity RMS biases of 10% are possible. The literature currently lacks a model to predict, and a list of criteria to minimize, FIVB. To fill this need a model is developed to predict the magnitude of FIVB. Predictions are compared to experimental measurements of FIVB in a pool fire to confirm the validity of the model. Specific criteria identified for minimizing the FIVB include maximizing the beam crossing angle magnitude; using a Bragg cell frequency shift to reduce sampling times to less than 1 mu s; using the longest laser wavelength available; orienting beams parallel to index of refraction gradients to minimize the beam incidence angles; and when measuring velocities perpendicular to the main flow direction, orienting the beams such that the main flow is along the bisector of the beam crossing angle. Contrary to intuition, minimizing beam steering displacements by minimizing flame front to probe volume distance may increase the FIVB.

Compact acousto-optic processor for synthetic aperture radar image formation

A compact implementation of a real-time acousto-optic synthetic aperture radar (SAR) imager is described. The architecture generates SAR imagery by decomposing the required 2D integration into a cascade of two 1D integrations, in a manner similar to that used in computed tomography for medical imaging applications. To achieve low power consumption, the required integrations are performed in the analog optical domain, with a crossed Bragg cell configuration, using a combination of spatial and temporal integration of optical signals. The time integrating and space integrating modules of the architecture are coupled via a common path interferometer. Subimages are formed on a 2D CCD detector array that is rotated during image formation to avoid a computationally difficult digital interpolation operation. The complex-valued subimages are combined in a digital frame buffer for dynamic range enhancement before being displayed. To accommodate needed flexibility, the required filter functions are calculated in a digital controller and downloaded through the Bragg cells. Results of a laboratory demonstration are presented. Performance projections suggest that the architecture may offer an advantage over an all-electronic approach for high-resolution applications which are severely constrained in power consumption. >

Performance of mercurous chloride acousto-optic devices

Abstract The suitability of mercurous chloride for commercial applications was examined by fabricating Bragg cells and acousto-optic tunable filters (AOTF). The acoustic attenuation measurements showed that for operation at a center frequency of 30 MHz, it should be possible to build a Bragg cell with over 8000 resolution elements. For a frequency of 50 MHz, 6000 resolution elements can be achieved with an aperture smaller than 10 cm which is a significant advancement in the state of the art, and is superior to that tellurium dioxide. The AOTF was tested by measuring the transmission at 3.39 μm. The peak response was observed at 18.9 MHz and the fwhm was about 0.58 MHz, corresponding to a 3% of center wavelength resolution, which were excellent agreement with the theory.

Simultaneous measurements of in-plane and out-of-plane surface displacements of shell vibrations at several points by an array of miniature laser probe heads

Experiments are being conducted to measure directly by laser Doppler interferometry the in-plane and out-of-plane components of the surface motion of a vibrating shell simultaneously at several surface points. The proposed design requires three Bragg cells to generate three coherent optical beams with different carrier frequencies. Three 1×4 fiber optic connectors are used to transmit each beam to four surface points. At each point, both the out-of-plane component and one of the in-plane components of the displacement vector are measured. The light scattered by the vibrating surface is Doppler shifted and collected into a large core multimode fiber situated close to the surface and terminated by a photodiode. The signals are acquired and digitized for direct phase demodulation on a computer by means of Hilbert transforms. [Work supported by ONR.]

A novel integrated-optic WDM cross-connect for wide area all optical networks

The authors propose a novel design for an integrated-optic wavelength-division-multiplexing (WDM) cross-connect that can be used in routing network applications at various wavelengths. The device is based on integrating a two-dimensional array of Bragg diffraction cells at cross points of input/output (I/O) coplanar waveguides. The wavelength tuning of the individual Bragg cells can be chosen to produce a WDM cross-connect with any number of required I/O interconnections. These cross-connects have proved extremely useful in the design and deployment of wide-area/nationwide all-optical networks. >

Acousto-Optical Spectrometers for broadband millimeter radioastronomy at I.R.A.M.

The Observatoire de Paris, in collaboration with the Institute of Millimeter Radio Astronomy (I.R.A.M.), has designed and built a 500 MHz bandwidth, 1 MHz resolution Acousto-Optical Spectrometer for the 30 m radiotelescope in Granada (Spain). The main characteristics of the device are shortly described. A dedicated Bragg cell was designed and built in order to achieve high efficiency and wide band. The performances obtained as well in lab as on site, and extensively measured by comparison with those of reference filterbanks, show that the AOS is fully qualified for high sensitivity, broadband millimeter radioastronomy. Albeit its limitation in frequency resolution, for which other technics as digital autocorrelation are more appropriate, the main advantages of such an AOS are wide band, compactness, and low cost. Its high reliability is evidenced by two years of uninterrupted operations on site.

Diode-pumped Q-switched fiber laser

A Q-switched fiber laser is developed for eyesafe ranging applications. Using a highly doped erbium fiber pumped with a 30-mW laser diode at 980 nm, pulses of 290-W peak power and 20-ns duration are obtained. The laser consists of all-fiber elements, with the exception of a high-efficiency bulk acousto-optic Bragg cell aligned to the laser on the diffracted beam to provide <i>Q</i>-switching with high hold-off. Three emission bands of erbium are made to lase in continuous wave (cw) and <i>Q</i>-switched operation. It is established that optimal cw lasing is achieved at 1560 nm, and maximum intensity Q-switch pulses are produced at 1530 nm. The laser was packaged and tested in the field, demonstrating the readiness of this technology for ranging applications.

Feynman diagram analysis of intermodulation products in Bragg cells

The elastic process of intermodulation generation in multifrequency acousto-optic diffraction was analyzed in detail by using the Feynman diagram method. The complete perturbation solution, which can be further simplified into an analytical form, was obtained for the strains of various acoustic waves in an acousto-optic medium. This makes it possible to predict accurately the intermodulation products in multifrequency acousto-optic diffraction and to give the exact value of the critical acousto-optic interaction length above which the dynamic process is dominant.

Crossed Bragg cell implementation of a Fourier-plane filter for optical image correlators

A two-dimensional optical image processor is described and demonstrated in which the point-spread function is generated by crossed acousto-opticells in the Fourier plane of a coherent optical correlator. Coherence effects and generation of nonseparable point-spread functions are considered.

Acousto-optic wavelength division multiplexing system

A novel wavelength division multiplexing/demultiplexing (WDM) system using acousto-optic Bragg cell is presented. The system is adaptable for any wavelength deviation of light sources caused by temperature changes and allows changing the operating wavelengths over a wide range. High diffraction efficiency, high resolution, and a large number of channels are obtained. Computer simulation for a five-channel system in the 0.8-μm region is presented. © 1992 John Wiley & Sons, Inc.

Scanning differential interference contrast microscope

An optical microscope has been developed based on the differential interference contrast method to evaluate the roughness of supersmooth surfaces. The instrument uses a Bragg cell and a translation stage driven by a DC motor to produce an image of an area of a sample. Its lateral resolution is 2 μm and its vertical resolution is subangstrom. It takes only 7 seconds to scan an area of 1 mm 2. Three different curve fits can be used to remove the tilt, the curvature, and the low spatial frequency features of the sample. A figure for surface roughness is produced that is repeatable to 0.01 Å. The instrument is described and the noise sources and repeatability are discussed. The results of measurements of ring laser gyroscope mirror substrates are shown.

Influence of a Shroud on Swirler Flow Fields

The flow fields for swirlers with and without a shroud were measured using a twocomponent laser-Doppler velocimeter (LDV) system. The primary goal of this study is to investigate the effect of shrouds on swirler flow fields, in order to provide useful design information for the manufacture of gas turbine fuel nozzles, and to supply benchmark data for comparison with numerical predictions. As a result of the measurements, the radial distributions of three mean velocity components, turbulence intensity, and shear stresses were obtained at five locations (x/d = 0.1, 1, 2, 4, and 8) along the axis of the swirlers. The LDV system was operated in the 20 deg off-axis forward scatter mode with beam expanders and Bragg cell frequency shifting on both components. The flow was seeded by 1 μm mean diameter atomized particles of glycerol and water (50/50) mixture. Comparison of flow with and without the shroud showed that the jet diameter was much smaller, and the flow deceleration in the downstream direction was faster, due to the influence of the shroud, at the same supply pressure (750 mm H2O). As a result of the significant reduction in the swirl number due to the addition of the shroud, the recirculation zone disappeared. In addition to its influence on recirculation, the shroud caused a radially inward shift of the maximum mean and turbulence parameters at all axial locations. The anisotropy of turbulence increased as compared to the values for the swirler without the shroud.

Piezoelectric transducer to generate bulk waves

The transducer is chiefly designed to generate acoustic waves in the microwave range in a crystalline support of a bulk wave delay line or a Bragg cell. The transducer has a plane contact region of a ground electrode partially covering a face of the substrate, a piezoelectric plate on the contact region and a second electrode defining the piezoelectrically active region of the transducer. So as not to make a direct solder on the electrode and thus, in particular, reduce the admittance of the transducer as compared with known transducers, the second electrode is connected to a large connection region, placed on the support face, by a link bridge. The connection region supports the connection by soldering. The admittance is thus independent of the thickness of the solder.

Temperature sensitivity of acoustic velocity and frequency registration in acousto-optic spectrum analyzers

An upper bound DeltaT is derived for the temperature variation over which frequency registration is maintained. DeltaT is determined for a TeO(2) slow-shear-mode Bragg cell.

Permanent magnet-based guided-wave magnetooptic Bragg cell modules

Compact magnetostatic forward volume wave-based guided-wave magnetooptic (MO) Bragg cell modules have been realized by utilizing a pair of small samarium-cobalt permanent magnets together with a pair of current-carrying coils. A highly uniform DC magnetic field, has been obtained in the air gap where yttrium iron garnet-gadolinium gallium garnet (YIG-GdGG) waveguide samples are inserted. A tunable DC magnetic field as large as 2446 Oe corresponding to a tunable carrier frequency band of 6.85 GHz has been achieved. The resulting MO Bragg cell modules, at the optical wavelength of 1.303 mu m, with carrier frequencies ranging from 2.0 to 12.0 GHz have provided performance characteristics comparable to those obtained by using a bulk electromagnet. Compact MO Bragg cell modules have potential applications, such as scanning, switching of light beams, and real-time processing of wide-band microwave signals without requiring frequency down-conversion.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Gaussian Schell-model beams generated with synthetic acousto-optic holograms

The recently introduced concept of a synthetic acousto-optic hologram [ J. Appl. Phys.67, 49 ( 1990)] is applied to convert a Gaussian laser beam into a partially coherent anisotropic Gaussian Schell-model (AGSM) beam. Real-time reconfigurability of the coherence properties is achieved by this technique, which features scattering of the laser beam by an electronically synthesized, digitally phase-modulated volume grating that propagates in an acousto-optic Bragg cell. The coherence and intensity distributions of the fields obtained by different types of phase modulation are investigated theoretically. We demonstrate some particularly interesting AGSM sources and fields: a secondary elliptical AGSM source with a circularly symmetric far-field intensity distribution and an AGSM field that retains the eccentricity of its intensity profile in the propagation through any centrosymmetric (paraxial) optical system.