Showing papers on "Interferometry published in 1999"

Coherence estimation for SAR imagery

Abstract: In dual- or multiple-channel synthetic aperture radar (SAR) imaging modes, cross-channel correlation is a potential source of information. The sample coherence magnitude is calculated over a moving window to generate a coherence magnitude map. High-resolution coherence maps may be useful to discriminate fine structures. Coarser resolution is needed for a more accurate estimation of the coherence magnitude. In this study, the accuracy of coherence estimation is investigated as a function of the coherence map resolution. It is shown that the space-averaged coherence magnitude is biased toward higher values. The accuracy of the coherence magnitude estimate obtained is a function of the number of pixels averaged and the number of independent samples per pixel (i.e., the coherence map resolution). A method is proposed to remove the bias from the space-averaged sample coherence magnitude. Coherence magnitude estimation from complex (magnitude and phase) coherence maps is also considered. It is established that the magnitude of the averaged sample coherence estimate is slightly biased for high-resolution coherence maps and that the bias reduces with coarser resolution. Finally, coherence estimation for nonstationary targets is discussed. It is shown that the averaged sample coherence obtained from complex coherence maps or coherence magnitude maps is suitable for estimation of nonstationary coherence. The averaged sample (complex) coherence permits the calculation of an unbiased coherence estimate, provided that the original signals can be assumed to be locally stationary over a sufficiently coarse resolution cell.

Self-referencing spectral interferometry for measuring ultrashort optical pulses

Abstract: This paper describes a novel self-referencing interferometric method for measuring the time-dependent intensity and phase of ultrashort optical pulses. The technique, spectral phase interferometry for direct electric-field reconstruction (SPIDER), measures the interference between a pair of spectrally sheared replicas of the input pulse. Direct (noniterative) inversion of the interferogram yields the electric field of the input pulse without ambiguity. The interferogram, which is solely a function of frequency, is resolved with a spectrometer and recorded with a slow detector. Moreover, the geometry is entirely collinear and requires no moving components. This paper describes in detail the principle of operation, apparatus, and calibration of SPIDER and gives experimental examples of reconstructed pulses.

Improved three-dimensional imaging with a digital holography microscope with a source of partial spatial coherence.

Abstract: A digital holographic technique is implemented in a microscope for three-dimensional imaging reconstruction. The setup is a Mach–Zehnder interferometer that uses an incoherent light source to remove the coherent noise that is inherent in the laser sources. A phase-stepping technique determines the optical phase in the image plane of the microscope. Out-of-focus planes are refocused by digital holographic computations, thus considerably enlarging the depth of investigation without the need to change the optical focus mechanically. The technique can be implemented in transmission for various magnification ratios and can cover a wide range of applications. Performances and limitations of the microscope are theoretically evaluated. Experimental results for a test target are given, and examples of two applications in particle localization and investigation of biological sample are provided.

Remote opto-chemical sensing with extreme sensitivity: design, fabrication and performance of a pigtailed integrated optical phase-modulated Mach-Zehnder interferometer system

Abstract: This paper describes the design, fabrication and testing of a pigtailed integrated optical (IO) phase-modulated Mach–Zehnder interferometer (MZI) including both the optical chip and the electronics. The optical chip is realised in SiON technology. The IO components (the sensing function, the straight waveguiding channels, the phase modulator, the polariser, the splitter, the combiner and the fibre-to-chip connection unit) are individually optimised and interconnected by using transversal adiabatic tapers. To obtain a high waveguide evanescent field sensitivity, the sensor is designed for — but not limited to — a wavelength of 632.8 nm. The integrated MZI is actively phase-modulated by virtue of the electro-optic effect of the incorporated material zinc oxide (ZnO). The electro-optical voltage–length product Vπ is 16 V cm at frequencies above 10 Hz. The polariser is a distributed function, that effectively filters TM-polarised light (TE/TM polarising ratio >30 dB). The fibre pigtail, affording remote optical sensing, is based on a cheap, easy-to-use fibre-to-chip connection with a typical coupling efficiency of 50%, while the device throughput (“insertion loss”) is −20 dB. The drive- and demodulation electronics enable a phase resolution 5×10−5×2π, corresponding to a refractive index resolution of 2×10−8. The sensing system as has been realised up to now shows a phase resolution of 1×10−4×2π, its long-term stability (hours) being ≤3×10−4×2π. This corresponds to a refractive index resolution of 5×10−8, and a long-term stability of 10−7.

Optimal interferometer designs for optical coherence tomography

Abstract: We introduce a family of power-conserving fiber-optic interferometer designs for low-coherence reflectometry that use optical circulators, unbalanced couplers, and (or) balanced heterodyne detection. Simple design equations for optimization of the signal-to-noise ratio of the interferometers are expressed in terms of relevant signal and noise sources and measurable system parameters. We use the equations to evaluate the expected performance of the new configurations compared with that of the standard Michelson interferometer that is commonly used in optical coherence tomography (OCT) systems. The analysis indicates that improved sensitivity is expected for all the new interferometer designs, compared with the sensitivity of the standard OCT interferometer, under high-speed imaging conditions.

The Palomar Testbed Interferometer

Abstract: The Palomar Testbed Interferometer (PTI) is a long-baseline infrared interferometer located at Palomar Observatory, California. It was built as a testbed for interferometric techniques applicable to the Keck Interferometer. First fringes were obtained in 1995 July. PTI implements a dual-star architecture, tracking two stars simultaneously for phase referencing and narrow-angle astrometry. The three fixed 40 cm apertures can be combined pairwise to provide baselines to 110 m. The interferometer actively tracks the white-light fringe using an array detector at 2.2 microns and active delay lines with a range of +/-38 m. Laser metrology of the delay lines allows for servo control, and laser metrology of the complete optical path enables narrow-angle astrometric measurements. The instrument is highly automated, using a multiprocessing computer system for instrument control and sequencing.

Thickness-profile measurement of transparent thin-film layers by white-light scanning interferometry

Abstract: White-light scanning interferometry is increasingly used for precision profile metrology of engineering surfaces, but its current applications are limited primarily to opaque surfaces with relatively simple optical reflection behavior. A new attempt is made to extend the interferometric method to the thickness-profile measurement of transparent thin-film layers. An extensive frequency-domain analysis of multiple reflection is performed to allow both the top and the bottom interfaces of a thin-film layer to be measured independently at the same time by the nonlinear least-squares technique. This rigorous approach provides not only point-by-point thickness probing but also complete volumetric film profiles digitized in three dimensions.

Power-efficient nonreciprocal interferometer and linear-scanning fiber-optic catheter for optical coherence tomography

Abstract: A nonreciprocal fiber-optic interferometer is demonstrated in an optical coherence tomography (OCT) system. The increased power efficiency of this system provides a 4.1-dB advantage over standard Michelson implementations. In addition, a new linear-scanning fiber-optic catheter is demonstrated that avoids the rotary optical junction that is required in circumferential scanning systems. These advancements have permitted the clinical implementation of OCT imaging in the human gastrointestinal tract.

Gravity-wave interferometers as quantum-gravity detectors

Abstract: Nearly all theoretical approaches to the unification of quantum mechanics and gravity predict 1, 2, 3, 4 that, at very short distance scales, the classical picture of space-time breaks down, with space-time becoming somewhat ‘fuzzy’ (or ‘foamy’). The properties of this fuzziness and the length scale that characterizes itsonset are potentially a means for determining which (if any) of the existing models of quantum gravity is correct. But it is generally believed 5 that these quantum space-time effects are too small to be probed by technologies currently available. Here Iargue that modern gravity-wave interferometers are sensitive enough to test certain space-time fuzziness models, because quantum space-time effects should provide an additional source of noise in the interferometers that can be tightly constrained experimentally. The noise levels recently achieved in one interferometer 6 are sufficient to rule out values of the length scale that characterizes one of the space-time fuzziness models down to the Planck length (∼10 −35 m) and beyond, while the sensitivity required to test another model should be achievable with interferometers now under construction.

Time-delay interferometry for space-based gravitational wave searches

Abstract: Ground-based, equal-arm-length laser interferometers are being built to measure high-frequency astrophysical graviatational waves. Because of the arm-length equality, laser light experiences the same delay in each arm and thus phase or frequency noise from the laser itself precisely cancels at the photodetector.

Determination of interferometer phase distributions by use of wavelets.

Abstract: A new technique for directly extracting phase gradients from two-dimensional (2-D) interferometer fringe data is presented. One finds the gradients by tracking the maximum modulus of the continuous wavelet transform of the fringe data and the phase distribution that is obtained, with a small error, by integration. Problems associated with phase unwrapping are thereby avoided. The technique is compared with standard methods, and excellent agreement is found. In common with Fourier-transform methods, the technique is capable of extracting the full 2-D phase distribution from a single image.

Spatially resolved spectral interferometry for determination of subsurface structure

Abstract: We describe an instrument capable of obtaining two-dimensional images of subsurface structure in real time with no moving parts. The technique is based on spectral interferometry and uses an imaging spectrograph to obtain spatially resolved spectra. A test sample consisting of microscope coverslips and a Ronchi grating was measured, illustrating the system’s depth resolution of 38 ?m and transverse resolution of at least 12.7 ?m. The technique is readily adaptable to endoscopic delivery as well as three-dimensional real-time image acquisition.

Dispersion effects in partial coherence interferometry: implications for intraocular ranging

Abstract: In nondispersive media, the minimum distance that can be resolved by partial coherence interferometry (PCI) and optical coherence tomography (OCT) is inversely proportional to the source spectral bandwidth. Dispersion tends to increase the signal width and to degrade the resolution. We analyze the situation for PCI ranging and OCT imaging of ocular structures. It can be shown that for each ocular segment an optimum source bandwidth yielding optimum resolution exists. If the resolution is to be improved beyond this point, the group dispersion of the ocular media has to be compensated. With the use of a dispersion compensating element, and employing a broadband superluminescent diode, we demonstrate a resolution of 5 μm in the retina of both a model eye and a human eye in vivo. This is an improvement by a factor of 2-3 as compared to currently used instruments. © 1999 Society of Photo-Optical Instrumentation Engineers.

Arrangement for spectral interferometric optical tomography and surface profile measurement

Abstract: An arrangement is provided for optical surface profile measurement and for obtaining optical sectional images of transparent, partially transparent and opaque objects by the spectral interferometric OCT method. In the spectral interferometric OCT method, the depth position of the object locations from which light is diffusely reflected is given by the light diffusely reflected by the object through a Fourier transform. Because of the path difference between the object light and reference light which is required for this purpose, large spatial frequencies occur in the spectrum which impair the resolution capacity of this method. According to the invention, the reference light is used to measure the phase of the wavelength spectrum by use of discrete phase displacements from the measured spectral intensities. This is also possible when the path difference between the object light and reference light is zero and a worsening of resolution therefore does not occur in this case.

Dynamic coherent focus OCT with depth-independent transversal resolution

Abstract: We present a new OCT technique which renders the transversal resolution depth independent. This is achieved by an optical setup which shifts the focus of the beam illuminating the object through the object depth without changing the path length in the corresponding interferometer arm. Therefore, the coherence gate remains at the beam focus without any readjustment of the reference arm. Depth resolution was tested with the help of microscopy cover-plates and transversal resolution was tested with the help of Ronchi rulings. Resolution was 100 lines mm−1 over an object depth of 430 μm. For a first demonstration of the properties of this dynamic coherent focus scheme in a biologic system a section of a human cornea was used. We expect that this technique can further be improved to obtain transversal resolution down to the 1-μm range

Characterization of sub-6-fs optical pulses with spectral phase interferometry for direct electric-field reconstruction.

Abstract: We demonstrate spectral phase interferometry for direct electric-field reconstruction (SPIDER) as a novel method to characterize sub-6-fs pulses with nanojoule pulse energy. SPIDER reconstructs pulse phase and amplitude from a measurement of only two optical spectra by use of a fast noniterative algorithm. SPIDER is well suited to the measurement of ultrabroadband pulses because it is quite insensitive to crystal phase-matching bandwidth and to unknown detector spectral responsivity. Moreover, it combines highly accurate pulse-shape measurement with the potential for online laser system diagnostics at video refresh rates.

Non-invasive method and low-coherence apparatus system analysis and process control

Abstract: The disclosure relates to measuring devices that are particularly suited for the purpose of in-situ characterization of particles present in fluid substances or in air using a low-coherence interferometer. Specifically, the characterization includes average size, size distribution, volumetric density, and composition. The low-coherence interferometer utilizes a split band of radiation to illuminate a sample probe and a reference probe then combines the reflected radiation from both probes to determine the photon pathlength distribution of the tested particulate or colloidal containing stream and from this information determine the size characteristics of said stream. The methodology is relevant to possible spatially distributed control of chemical processes such as emulsion polymerization to produce paints, coatings, synthetic rubbers, or crystallization processes in pharmaceuticals, food, and bulk chemicals industries. Another application relates to on-line control of particle size and volumetric density is in combustion for diagnostics. The invention can be used for the characterization of coal particles, dense sprays and solid propellants or any other system, which is too dense for conventional optical measurement techniques. Beside the intrinsic particulate nature of these systems, random index of refraction variations are also created due to turbulence/temperature interactions. The remote optical characterization of systems with high-concentration of suspended solids is also important for water quality control and pollution monitoring.

Generation of optical macroscopic quantum superposition states via state reduction with a mach-zehnder interferometer containing a kerr medium

Abstract: A method for producing macroscopic quantum superposition states (generally known as Schr\"odinger cat) states for optical fields is presented. The proposed method involves two modes of the field interacting dispersively in a Kerr medium where one of the modes is an arm of a Mach-Zehnder interferometer and the other mode is external to it. If the external mode initially contains a macroscopic quantum state, such as a coherent state, and the vacuum and a single photon state are the inputs to the interferometer, the external field state becomes entangled with the number states associated with the two paths of the interferometer. Selective measurement at the output ports of the interferometer project the external mode into the desired cat states. It is pointed out that the method can also be used to generate cat states out of multimode states initially containing correlations.

Fringe Visibility Estimators for the Palomar Testbed Interferometer

Abstract: The Palomar Testbed Interferometer (PTI) is a long-basline infrared interferometer located at Palomar Observatory, California.

Guiding of laser pulses in plasma channels created by the ignitor-heater technique

Abstract: Experimental and theoretical investigations of laser guiding in plasma channels are reported. Intense (<5×1017 W/cm2), short (75 fs) laser pulses have been injected and guided in channels produced using a novel ignitor-heater technique, which uses two laser pulses. The ignitor, an ultrashort (<100 fs) laser pulse, is brought to a line focus to ionize the gas jet. The heater pulse (160 ps long) is subsequently used to heat the existing spark via inverse Bremsstrahlung. The hydrodynamic shock expansion creates a channel. This technique allows the creation of slab or cylindrical channels in low atomic number gases, e.g., hydrogen. The channel profile was diagnosed with time resolved longitudinal interferometry. The effects of laser beam size and divergence mismatch at the channel entrance and leakage of the laser energy out of the channel are studied theoretically and experimentally in one and two transverse dimensions. An all-optical channel wake diagnostic based on Fourier domain interferometry is discusse...

Dynamic range of optical reflectometry with spectral interferometry

Abstract: I present an optical reflectometry (OR) technique with spectral interferometry (SIOR) that realizes high dynamic range compared with a conventional OR system using the delayed heterodyne technique (DHOR), and report on the application of this system to noninvasive in vivo measurements of the structure of the skin and the nail of an index finger. The theoretically derived dynamic range of SIOR is m/4-times superior to that of DHOR, where m is the number of independent image pixels. A dynamic range of 105 db was experimentally realized, which is comparable to the theoretically expected dynamic range of 112 db.

Differential phase contrast in optical coherence tomography.

Abstract: We report on a modification of optical coherence tomography (OCT) that allows one to measure small phase differences between beams traversing adjacent areas of a specimen. The sample beam of a polarization-sensitive low-coherence interferometer is split by a Wollaston prism into two components that traverse the object along closely spaced paths. After reflection at the various sample surfaces, the beams are recombined at the Wollaston prism. Any phase difference encountered between the two beams is converted into a change of polarization state of the recombined beam. This change is measured, and the resulting signals are converted to differential phase-contrast OCT images. The first images obtained from simple test objects allowed us to determine path-difference gradients with a resolution of the order of 5×10-5.

Method and system for profiling objects having multiple reflective surfaces using wavelength-tuning phase-shifting interferometry

Abstract: The invention features methods and systems for interferometrically profiling a measurement object having multiple reflective surfaces, e.g., to profile a selected one of the multiple reflective surfaces. The methods and systems involve: positioning the measurement object within an unequal path length interferometer (e.g., a Fizeau interferometer) employing a tunable coherent light source; recording an optical interference image for each of multiple wavelengths of the light source, each image including a superposition of multiple interference patterns produced by pairs of wavefronts reflected from the multiple surfaces of the measurement object and a reference surface; and extracting phases of a selected one of the interference patterns from the recorded images by using a phase-shifting algorithm that is more sensitive (e.g., at least ten times more sensitive) to a wavelength-dependent variation in the recorded images caused by the selected interference pattern than to wavelength-dependent variations in the recorded images caused by the other interference patterns.

Surface plasmon resonance interferometry for biological and chemical sensing

Abstract: Surface plasmon resonance (SPR) interferometry is reported as a novel technique for biological and chemical sensing, which employs not only the amplitude of a resonantly reflected light wave, but its phase as well. In this connection, the phase behavior under SPR has been comprehensively described by theoretical analysis, numerical simulations, and a number of experiments. Near optimum SPR conditions, a resonant phase dependence is step-like, the ‘step’ being at the reflectivity minimum. For SPR-based sensors, the slope of the ‘step’ can always be made by several orders steeper than that of the resonant reflectivity contour. The ‘step’ has been imaged by the fringe of a 2-dimensional interference pattern where one coordinate was the incidence angle, and the other was the phase. The inversion of the ‘step’ has been observed for the first time during antigen–antibody binding, when the system passes through the optimum SPR conditions. Monitoring the inversion provides for ultra-high sensitivity to an analyte while recording angular position of the ‘step’, does for dynamic range as wide as that of traditional SPR sensors. The SPR interferometry technique has confirmed theoretical findings and opened up new possibilities for (bio)chemical sensing.

The structure of oriented vegetation from polarimetric interferometry

Abstract: Polarimetric radar interferometry is much more sensitive to the distribution of oriented objects in a vegetated land surface than either polarimetry or interferometry alone. This paper shows that single-baseline polarimetric interferometry can be used to estimate the heights of oriented-vegetation volumes and underlying topography, while at least two baselines are needed for randomly oriented volumes. Single-baseline, calculated vegetation-height accuracies are in the range of 2-8 m for reasonable levels of vegetation orientation in forest canopies.

Phase-coherent amplification of matter waves

Abstract: Phase-coherent matter-wave amplification was demonstrated using Bose- Einstein–condensed rubidium-87 atoms. A small seed matter wave was created with coherent optical Bragg diffraction. Amplification of this seed matter wave was achieved by using the initial condensate as a gain medium through the superradiance effect. The coherence properties of the amplified matter wave, studied with a matter-wave interferometer, were shown to be locked to those of the initial seed wave. The active matter-wave device demonstrated here has great potential in the fields of atom optics, atom lithography, and precision measurements.

Experimental distinction between phase shifts and time delays: Implications for femtosecond spectroscopy and coherent control of chemical reactions

Abstract: Two different definitions of phase shifts and time delays are contrasted and shown to match different experimental methods of generating delayed pulses. Phase shifts and time delays are usually defined in terms of a carrier wave in magnetic resonance, but definitions based on the envelope of a single pulse are useful in optics. It is demonstrated experimentally that a frequency domain measurement using spectral interferometry can simultaneously measure phase shifts with an accuracy of 0.1 rad (2σ) and time delays with a precision of 40 attoseconds (2σ) for 25 femtosecond optical pulses. Envelope time delays are generated by pathlength differences in an interferometer. Constant spectral phase shifts are demonstrated by diffracting pulses from a variable phase volume diffraction grating. Experimental requirements for phase-resolved spectroscopy are outlined. The theory of phase-locked pulse pair techniques is reexamined, and it is concluded that linear experiments with phase-locked pulse pairs are completely equivalent to Fourier transform absorption spectroscopy and do not measure the refractive index or real part of the susceptibility. It is shown that Fourier sine and cosine transformations of truncated time domain signals which do not match the symmetry of the complete signal can produce a false dispersive susceptibility because they are equivalent to Kramers–Kronig inversion of finite bandwidth absorption data. A procedure for shifting π/2 phase-locked transients by a quarter cycle of delay to generate a transient with a π/2 spectral phase shift is given. Equations used to calculate femtosecond nonlinear optical signals have assumed carrier wave delays. Modifications to these equations are required when envelope delays are generated by interferometer pathlength differences and modified equations are given. The modified equations yield significantly different results for phase-resolved or interferometric experiments. In particular, the modified equations are needed to calculate indirectly (interferometrically) detected frequencies and the real and imaginary parts of two-dimensional Fourier transform spectra. The role of the refractive index and real part of the frequency domain susceptibility in nonlinear experiments with phase-locked pulse pairs is explored. It is concluded that experiments such as the heterodyne detected stimulated photon echo are insensitive to nonlinear refractive index changes under some circumstances. Finally, modifications of some equations used in the theory of coherent control are needed to match theory with experimental practice.

Interferometric observation of cosmic microwave background anisotropies

Abstract: We present a formalism for analyzing interferometric observations of the cosmic microwave background anisotropy and polarization. The formalism is based on the l-space expansion of the angular power spectrum favored in recent years. Explicit discussions of maximum likelihood analysis, power spectrum reconstruction, parameter estimation, imaging, and polarization are given. As an example, several calculations for the Degree Angular Scale Interferometer and Cosmic Background Interferometer experiments are presented.

Cancellation of laser noise in an unequal-arm interferometer detector of gravitational radiation

Abstract: In this paper we present a method for exactly cancelling the laser noise in a one-bounce unequal-arm Michelson interferometer. The method requries separate measurements of the phase difference in each arm, made by interfering the returning laser light in each arm with the outgoing light.

Topography of the lunar poles from radar interferometry : A survey of cold trap locations

Abstract: Detailed topographic maps of the lunar poles have been obtained by Earth-based radar interferometry with the 3.5-centimeter wavelength Goldstone Solar System Radar. The interferometer provided maps 300 kilometers by 1000 kilometers of both polar regions at 150-meter spatial resolution and 50-meter height resolution. Using ray tracing, these digital elevation models were used to locate regions that are in permanent shadow from solar illumination and may harbor ice deposits. Estimates of the total extent of shadowed areas poleward of 87.5 degrees latitude are 1030 and 2550 square kilometers for the north and south poles, respectively.