Interferometric Approach Research Articles

Coherence-imaging approach to time-resolved charge-exchange recombination spectroscopy in high-temperature plasma

A coherence-based, or interferometric approach to spectral analysis of charge-exchange recombination (CXR) emission radiation from high-temperature plasma probed or heated using energetic neutral beams, offers a number of advantages over wavelength-domain instruments. The spectral-line shift and broadening are obtained from measurements of the spectral coherence at a given fixed time delay. The coherence is monitored by first approximately isolating the spectral line of interest using an interference filter and subsequently imaging the spectral scene using a field-widened electro-optic path-delay-modulated polarization interferometer. Interferometers have the advantage of high-light throughput (no slit aperture). Moreover, because the spectral information is encoded at harmonics of the electro-optic modulation frequency, a single detector suffices to capture the spectral information, thereby opening the possibility for time-resolved two-dimensional spectral imaging. When unwanted spectral features are passed by the interference filter, the interpretation of the coherence phase and amplitude images can become ambiguous. By modulating the particle beam source, however, we show that coherence imaging using a single-delay modulatable interferometer can distinguish and characterize the Doppler-broadened CXR emission component against a significant background of continuum and intrinsic radiation, or pollution from nearby spectral features.

Multiaxial beam combiner for optical coherence tomography

We propose a new interferometric approach for optical coherence tomography applications. The device uses the Young's-hole principle to produce the fringes. The low-cost and versatile ion-exchange technology has been chosen to fabricate an integrated optics chip. This new approach has allowed us to eliminate optical delay lines presenting mechanical instabilities. We present fringes using both laser and white light.

Waveform restoration in semiconductor optical amplifier using fiber loop mirror

In this letter, we present a simple and effective interferometric approach for waveform restoration in a semiconductor optical amplifier (SOA) using a fiber loop mirror. Pattern-dependent distortion caused by gain dynamics of the SOA can be compensated. With our scheme, an enhanced input power dynamic range of 7 dB was achieved. For data sequences with different pattern lengths, both significant waveform and extinction ratio improvement were obtained.

Performance characterisation of hybrid STAP architecture incorporating elevation interferometry

Radar space–time adaptive processing (STAP) techniques have classically focused on azimuth–Doppler adaptivity while placing minimal emphasis on elevation. Elevation beamforming offers significant clutter suppression improvement, allowing further suppression of interference sources having identical Doppler and azimuth. This work incorporates elevation adaptivity through an interferometric approach, greatly improving clutter suppression while providing an often overlooked target height discrimination capability. A mathematical construct encapsulating the multistage processing framework is developed for the proposed technique. This framework allows extension of the traditional factored time–space (FTS) technique into the azimuth–Doppler–elevation hypercube and represents a subclass of more generalised hybrid methods. The proposed concept is validated through results based on simulated airborne radar data. Target detection improvement of the order of 25 dB, when compared to standard two-dimensional FTS processing, is demonstrated for an 8×8 nonuniform rectangular array. Elevation pattern data are provided to illustrate achievable null width/depth capabilities. These data also indicate target height discrimination is inherently provided and further development is warranted.

Ice-sheet velocity mapping: a combined interferometric and speckle-tracking approach

AbstractThe first and second RADARSAT Antarctic Mapping Missions (AMM-1 and -2) have now acquired interferometric synthetic aperture radar (SAR) overmuch of the ice sheet. The RADARSAT 24 day repeat cycle is nearly ideal for measuring slow ice motion (e.g. <100ma–1), but application of SAR interferometry is limited in faster-moving areas. With a 1day repeat period, ERS-1/-2 tandem SAR data are much better matched to fast motion, but are not always available. Fortunately, several authors have demonstrated the ability to measure velocity in fast-moving areas by tracking SAR speckle from image to image, which works well even in the absence of visible features. While these estimates have intrinsically lower resolution and poorer accuracy than direct phase measurements, they serve well in areas where there are no data suitable for conventional interferometry. This paper describes algorithms I have developed for merging interferometric and speckle-tracking data from multiple swaths to form a single seamless mosaic of velocity. At each point in the mosaic, all the available data are combined to produce estimates of the velocity and the associated error. This technique is demonstrated using RADARSAT data collected over Lambert Glacier, Antarctica, during AMM-1 and -2.

Interferometric radio-frequency emitter location

A generalised correlation technique is developed as the basis for radio-frequency emitter location employing two or more intercept platforms. The technique extends the applicability of the interferometric correlation approach, as exemplified by the use of the ambiguity function in satellite interferer location, to a broader range of emitter location problems. In particular, the proposed approach allows accurate localisation of RF sources in situations where the motion of the intercept platforms would seriously degrade the performance of the more conventional approach. A generalised correlation function is derived which utilises differential time delay and differential phase which are, respectively, related to conventional time difference of arrival, and frequency difference of arrival and higher-order time derivatives. Criteria expressing the significance of the higher-order terms are also derived. Case studies are presented which illustrate the effectiveness of the proposed approach and emphasise important aspects of the theory. It is envisaged that the proposed approach will be highly effective in most civil and military emitter localisation applications.

An interferometric technique for measuring gas flow velocity at large peclet numbers

A technique for determining the gas flow velocity at large Peclet numbers using an interferometric approach has been developed. It is shown that if a heat source is introduced in a uniform gas flow, areas under curves describing shifts of interference maxima are approximately equal to each other and tend to a certain limiting value. The limiting area is inversely proportional to the gas flow velocity and does not depend on the heat conduction and the type of flow. A theoretical expression has been derived that gives the relative error of velocity determinations by the method developed.

Bistatic synthetic aperture target detection and imaging with an AUV

The acoustic detection and classification of completely and partially buried objects in the multipath environment of the coastal ocean presents a major challenge to the mine countermeasures (MCM) community. However, the rapidly emerging autonomous underwater vehicle (AUV) technology provides the opportunity of exploring entirely new sonar concepts based on mono-, bi- or multi-static configurations. For example, the medium frequency regime (1-10 kHz) with its bottom penetration advantage may be explored using large synthetic apertures, where acoustic information is accumulated over a series of sonar pings. The performance of such approaches is highly dependent on accurate platform navigation and timing, which poses a significant challenge to AUV developers, particularly because the navigation procedures are themselves dependent on the complicated multipath acoustic environment. Data from the GOATS'98 experiment have been analyzed to investigate the feasibility of combining seabed scattering data from consecutive pings of a fixed parametric source to form a bistatic synthetic aperture for target localization and imaging with an AUV based receiving platform. The paper describes different levels of bistatic processing including both incoherent and coherent beamforming and very large aperture interferometric approaches, and the associated performance tradeoffs are discussed.

Bistatic target detection, interferometry, and imaging with an autonomous underwater vehicle platform

The acoustic detection and classification of completely and partially buried objects in the multipath environment of the coastal ocean presents a major challenge to the underwater acoustics community. However, the rapidly emerging autonomous underwater vehicle (AUV) technology provides the opportunity of exploring entirely new sonar concepts based on mono-, bi-, or multistatic configurations. For example, the medium frequency regime (1–10 kHz) with its bottom penetration advantage may be explored using large synthetic apertures, where acoustic information is accumulated over a series of sonar pings. The performance of such approaches is highly dependent on accurate platform navigation and timing, which poses a significant challenge to AUV developers, particularly because the navigation procedures are themselves dependent on the complicated multipath acoustic environment. Using experimental data from the GOATS’98 SACLANTCEN/MIT experiment, this paper describes an investigation into the feasibility of combining seabed scattering data from consecutive pings of a fixed parametric source to form a bistatic synthetic aperture for target localization and imaging with an AUV-based receiving platform. The paper describes different levels of bistatic processing including both incoherent and coherent beamforming and very large aperture interferometric approaches, and the associated performance trade-offs are discussed. [Work supported by ONR and SACLANT.]

Practical common-path interferometry for real-time thermal/fluid flow measurements

Interferometric technique can be a useful gross-field diagnostic tool for quantitative and qualitative evaluation of thermal/fluid flow phenomena. Here, we present a practical interferometric approach based on a common-path configuration of the interfering beams with some results for demonstration. It incorporates advances in the modern optical technology to fully exploit the potential of interferometric techniques for on-site applications. The results of the initial testing are encouraging. The approach can allow one to build a simple and compact system insensitive to external disturbances for real-time monitoring with good measurement accuracy. The system errors contributed by low-quality optics can be eliminated by employing a moire technique. The approach based on the numerical moire, which utilizes the images directly captured by a solid-state camera to avoid wet chemical processing, appears to be promising.

Reflection near-field scanning optical microscopy: an interferometric approach

An all-fiber near-field scanning optical microscope (NSOM) is demonstrated which allows significant signal enhancement through interferometric mixing and permits phase contrast. An uncoated fiber tip is used both as source and detector in a reflection mode configuration. Pseudoheterodyne detection in a Mach-Zehnder interferometer used in conjunction with the reflection mode NSOM allows the imaging of both opaque and transparent samples with a large signal enhancement. A phase contrast can be observed that may prove useful in the imaging of certain samples such as biological specimens. Images of a diffraction grating and red blood cells are presented in three different operating modes: conventional reflection, reflection with enhanced amplitude contrast, and reflection with phase contrast.

First results with the airborne single-pass DO-SAR interferometer

The first European airborne single-pass interferometric SAR experiment for topographic applications, which was performed during May 1994, is presented. The study goal is the validation of the DO-SAR system as a standard tool to derive high-precision digital elevation models using interferometric techniques. The DO-SAR sensor as well as the processing capabilities are presented in detail. After description of the test area and the flight campaign, the interferometric SAR processing approach is discussed. The first results are very promising and confirm the expected absolute height accuracy of approximately 2-5 m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical instrumentation for eye length measurement using a short coherence length laser-based interferometer approach

An optical interferometric approach is described for the measurement of axial eye length. The technique uses the short coherence length characteristics of the light source to locate, interferometrically, the position of the front surface of the eye and the retina. Results are presented from studies on an idealized model eye and the precision of the measurement is discussed. The advantages of the technique and a comparison with previously used short coherence measurements are also reviewed.

An image of the Sunyaev–Zel'dovich effect

THE Sunyaev–Zel'dovich effect1 is a distortion imposed on the 2.7-K cosmic microwave background radiation when the microwave photons are scattered by the hot gas in galaxy clusters. At radio wavelengths it appears as a decrement (of about 0.5 mK) in the brightness temperature of the background radiation. Measurements of this effect can provide information about the physics of galaxy clusters, and can also potentially be used to determine the Hubble constant, by constraining the size of the cluster along the line of sight. Successful detections have been made with singledish radio telescopes2,3, from which one-dimensional profiles of the temperature decrement are constructed. But these observations are susceptible to several systematic errors, such as corruption of the measured signal by radio sources located close to the line of sight. To circumvent these problems, we have obtained a two-dimensional image, using an interferometric (rather than single-dish) approach, of structure in the microwave background in the vicinity of the galaxy cluster Abell 2218. Our measurements, combined with X-ray observations of the scattering gas in the same cluster, support a low value of the Hubble constant.

Fiber-optic Bragg-grating differential-temperature sensor

A differential temperature sensor based on fiber-optic Bragg-grating elements is described. A high sensitivity to thermally induced Bragg wavelength shifts is obtained using an interferometric detection approach. Results presented show a temperature resolution of <0.05 degrees C, corresponding to a Bragg wavelength shift resolution of <6*10/sup -4/ nm.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-resolution distance measurement by dynamical spectral filtering of a superluminescent source

A nonincremental interferometer is shown in which the dynamical spectral filtering of a superluminescent source induces a broadband sinusoidal frequency modulation. The method reaches very high resolution limits in distance measurements (a theoretical expectation of 10(-12) m/ radicalHz is obtained) while maintaining all the advantages of heterodyne phase-recovery processing. Comparison with other nonincremental interferometric approaches is given.

Interferometry On Wolter X-Ray Optics: A Possible Approach

A novel interferometric approach for obtaining surface interferograms on Wolter-type optics is presented. A subaperture simulation experiment shows the data analysis steps required. A second, independent test basically confirms the simulation results. Splicing of the subaperture interferograms around the circumference is discussed.

Amplitude And Phase Characterisation Of Polished Directional Half-Couplers With Variable Refractive Index Overlays

We report a characterisation technique for both amplitude and phase variations of polished fibre half-couplers in the proximity of variable index bulk overlays. Experimental results are compared with simple theoretical models. In addition, simple derivative fibre-based devices are reported. Amplitude measurements were performed using a single fibre embedded in a block with a 25 cm radius groove, polished to within 1um of the core. A linear (in decibels) change in attenuation resulted for overlay indices greater than the effective mode index. An attenuation change of 40 dB was observed for a 0.0026 change in index. To observe phase changes, an interferometric approach was taken. Two fibres were embedded, one above the other, in a block identical to that above. The core of the top fibre only was exposed by polishing. Low loss 50:50 fused taper couplers were spliced to both sides of the block, forming a fibre Mach-Zehnder interferometer. For overlay indices less than the effective mode index, a phase change of rad was observed for an index change of 0.0045, with an interaction length of 700um.

Optical implementations of the perfect shuffle interconnection

The concept of a perfect shuffle is reviewed. Holographic approaches based on implementing a point spread function equivalent to the sum of two shifted delta functions are suggested. Interferometric approaches based on splitting an image and then combining them in a shifted manner are also suggested.

Interferometric approach in the NNSS data processing

Simultaneously received transmission from NNSS satellites by two or more Doppler receivers can be processed as interferometric observables. The receivers are physically unconnected and the coherence of the phase information recordings (Doppler counts) is ensured by highly stable independent local oscillators similarly as in VLBI. The purpose of the interferometric processing is to obtain high precision geodetic distance determinations using NNSS Doppler data.