Showing papers on "Astronomical interferometer published in 2004"

Exploring The Saturn System In The Thermal Infrared: The Composite Infrared Spectrometer

Abstract: The Composite Infrared Spectrometer (CIRS) is a remote-sensing Fourier Transform Spectrometer (FTS) on the Cassini orbiter that measures thermal radiation over two decades in wavenumber, from 10 to 1400 cm− 1 (1 mm to 7μ m), with a spectral resolution that can be set from 0.5 to 15.5 cm− 1. The far infrared portion of the spectrum (10–600 cm− 1) is measured with a polarizing interferometer having thermopile detectors with a common 4-mrad field of view (FOV). The middle infrared portion is measured with a traditional Michelson interferometer having two focal planes (600–1100 cm− 1, 1100–1400 cm− 1). Each focal plane is composed of a 1× 10 array of HgCdTe detectors, each detector having a 0.3-mrad FOV. CIRS observations will provide three-dimensional maps of temperature, gas composition, and aerosols/condensates of the atmospheres of Titan and Saturn with good vertical and horizontal resolution, from deep in their tropospheres to high in their mesospheres. CIRS’s ability to observe atmospheres in the limb-viewing mode (in addition to nadir) offers the opportunity to provide accurate and highly resolved vertical profiles of these atmospheric variables. The ability to observe with high-spectral resolution should facilitate the identification of new constituents. CIRS will also map the thermal and compositional properties of the surfaces of Saturn’s icy satellites. It will similarly map Saturn’s rings, characterizing their dynamical and spatial structure and constraining theories of their formation and evolution. The combination of broad spectral range, programmable spectral resolution, the small detector fields of view, and an orbiting spacecraft platform will allow CIRS to observe the Saturnian system in the thermal infrared at a level of detail not previously achieved.

Pixelated Phase-Mask Dynamic Interferometer

Abstract: We have demonstrated a new type of dynamic measurement system that is comprised of a micropolarizer array and can work with any type polarization interferometer to measure a variety of physical properties. The unique configuration overcomes many of the limitations of previous single frame, phase-shift interferometer techniques. In particular it has a true common path arrangement, is extremely compact, and is achromatic over a very wide range. We demonstrated high quality measurement with both a Twyman-Green and Fizeau type interferometer. The technique is useful for many applications where vibration or motion is intrinsic to the process.

Electro-Optical Instrumentation: Sensing and Measuring with Lasers

Abstract: Preface. 1. Introduction. Looking Back to Milestones. References. 2. Alignment, Pointing, and Sizing Instruments. Alignment. Pointing and Tracking. Laser Level. Wire Diameter Sensor. Particle Sizing. References. 3. Laser Telemeters. Triangulation. Time-of-Flight Telemeters. Instrumental Developments of Telemeters. Imaging Telemeters. The LIDAR. References. 4. Laser Interferometry. Overview of Interferometry Applications. The Basic Laser Interferometers. Performance Parameters. Ultimate Limits of Performance. Read-Out Configurations of Interferometry. Laser Vibrometry. Other Applications of Injection Interferometry. White Light Interferometry. References. 5. Speckle-Pattern Instruments. Speckle Properties. Speckle in Single-Point Interferometers. Electronic Speckle Pattern Interferometry. References. 6. Laser Doppler Velocimetry. Principle of Operation. Performance Parameters. Electronic Processing of the Doppler Signal. Optical Configurations. References. 7. Gyroscopes. Overview. The Sagnac Effect. Basic Gyro Configurations. Development of the RLG. Development of the Fiber Optics Gyro. The Resonant FOG and Other Configurations. The 3x3 FOG for the Automotive. The MEMS Gyro and Other Approaches. References. 8. Optical Fiber Sensors. Introduction. The Optical Strain Gage: A Case Study. Readout Configuration. Multiplexed and Distributed OFS. References. Appendix A0: Nomenclature. Appendix A1: Lasers for Instrumentation. Laser Basics. Frequency Stabilization of the He-Ne Laser. Semiconductor Narrow-Line and Frequency Stabilized Lasers. Diode-Pumped Solid-State Lasers. Laser Safety Issues. References. Appendix A2: Basic Optical Interferometers. Configurations and Performances. Choice of Optical Components. References. Appendix A3: Propagation through the Atmosphere. Turbidity. Turbulence. References. Appendix A4: Optimum Filter for Timing. Appendix A5: Propagation and Diffraction. Propagation. The Fresnel Approximation. Examples. References. Appendix A6: Source of Information on Electro-Optical Instrumentation. Index.

Time-Delay Interferometry

Abstract: Equal-arm interferometric detectors of gravitational radiation allow phase measurements many orders of magnitude below the intrinsic phase stability of the laser injecting light into their arms. This is because the noise in the laser light is common to both arms, experiencing exactly the same delay, and thus cancels when it is differenced at the photo detector. In this situation, much lower level secondary noises then set overall performance. If, however, the two arms have different lengths (as will necessarily be the case with space-borne interferometers), the laser noise experiences different delays in the two arms and will hence not directly cancel at the detector. In order to solve this problem, a technique involving heterodyne interferometry with unequal arm lengths and independent phase-difference readouts has been proposed. It relies on properly time-shifting and linearly combining independent Doppler measurements, and for this reason it has been called Time-Delay Interferometry (or TDI). This article provides an overview of the theory and mathematical foundations of TDI as it will be implemented by the forthcoming space-based interferometers such as the Laser Interferometer Space Antenna (LISA) mission. We have purposely left out from this first version of our ``Living Review'' article on TDI all the results of more practical and experimental nature, as well as all the aspects of TDI that the data analysts will need to account for when analyzing the LISA TDI data combinations. Our forthcoming ``second edition'' of this review paper will include these topics.

Imaging extrasolar planets by stellar halo suppression in separately corrected color bands

Abstract: Extrasolar planets have not been imaged directly with existing ground or space telescopes because they are too faint to be seen against the halo of the nearby bright star. Most techniques being explored to suppress the halo are achromatic, with separate correction of diffraction and wave-front errors. Residual speckle structure may be subtracted by differencing images taken through narrowband filters, but photon noise remains and ultimately limits sensitivity. Here we describe two ways to take advantage of narrow bands to reduce speckle photon flux and to obtain better control of systematic errors. Multiple images are formed in separate color bands of 5%-10% bandwidth and recorded by coronagraphic interferometers equipped with active control of wave-front phase and/or amplitude. In one method, a single deformable pupil mirror is used to actively correct both diffraction and wave-front components of the halo. This yields good diffraction suppression for complex pupil obscuration, with high throughput over half the focal plane. In a second method, the coronagraphic interferometer is used as a second stage after conventional apodization. The halo from uncontrollable residual errors in the pupil mask or wave front is removed by destructive interference made directly at the detector focal plane with an "antihalo," synthesized by spatial light modulators in the reference arm of the interferometer. In this way very deep suppression may be achieved by control elements with greatly relaxed, and thus achievable, tolerances. In both examples, systematic errors are minimized because the planet imaging cameras themselves also provide the error-sensing data.

A catalog of bright calibrator stars for 200-meter baseline near-infrared stellar interferometry

Abstract: We present in this paper a catalog of reference stars suitable for calibrating infrared interferometric observations. In the K band, visibilities can be calibrated with a precision of 1% on baselines up to 200 meters for the whole sky, and up to 300 meters for some part of the sky. This work, extending to longer baselines a previous catalog compiled by Borde et al. (2002), is particularly well adapted to hectometric-class interferometers such as the Very Large Telescope Interferometer (VLTI, Glindemann et al. 2003) or the CHARA array (ten Brummelaar et al. 2003) when observing well resolved, high surface brightness objects (K<8). We use the absolute spectro-photometric calibration method introduced by Cohen et al. (1999) to derive the angular diameters of our new set of 948 G8--M0 calibrator stars extracted from IRAS, 2MASS and MSX catalogs. Angular stellar diameters range from 0.6 mas to 1.8 mas (median is 1.1 mas) with a median precision of 1.35%. For both the northern and southern hemispheres, the closest calibrator star is always less than 10 degree away.

Birefringent Fourier-transform imaging spectrometer.

Abstract: Fourier-transform imaging spectrometers offer important advantages over other spectral imaging modalities, such as, a wider free spectral range, higher spectral resolutions and, in low-photon-flux conditions, higher signal-to-noise ratios can be achieved. Unfortunately, for application in harsh environments, deployment of Fourier-transform instruments based on traditional moving-mirror interferometers is problematic due to their inherent sensitivity to vibration. We describe a new Fourier-transform imaging spectrometer, based on a scanning birefringent interferometer. This system retains the advantages of traditional Fourier transform instruments, but is inherently compact and insensitive to vibration. Furthermore, the precision requirements of the movement can be relaxed by typically two orders of magnitude in comparison to a traditional two-beam interferometer. The instrument promises to enable application of Fourier-transform imaging spectrometry to applications, such as airborne reconnaissance and industrial inspection, for the first time. Example spectral images are presented.

The Michigan Infrared Combiner (MIRC): IR imaging with the CHARA Array

Abstract: We present the design of the Michigan Infra-Red Combiner (MIRC). MIRC is planned for deployment at the Georgia State University CHARA array to simultaneously combine all six telescope beams in an image-plane combiner. The novel design incorporates spatial-filtering with single-mode fiber optics, a synthetic (densified) pupil, and a low-resolution spectrometer to allow good calibration and efficient aperture synthesis imaging in the near-infrared. In addition, the focalization and spectrometer optics can accommodate an integrated optics component with minimal re-alignment. The MIRC concept can be scaled-up for interferometer arrays with more telescopes.

Generation of a pulsed polarization entangled photon pair using a Sagnac interferometer

Abstract: In this paper, we report on a scheme to generate a pulsed polarization entangled photon pair through the use of a Sagnac interferometer. To demonstrate its workability, we experimentally obtained two-photon quantum interference for the polarization variable. The main advantage of this scheme is its exceptional stability, compared to other schemes based on the interferometric technique. It does not need any active or passive techniques to stabilize the interferometer, even the instrument is exposed to a relatively turbulent environment.

Frequency scanning interferometry in ATLAS: remote, multiple, simultaneous and precise distance measurements in a hostile environment

Abstract: ATLAS is the largest particle detector under construction at CERN Geneva. Frequency scanning interferometry (FSI), also known as absolute distance interferometry, will be used to monitor shape changes of the SCT (semiconductor tracker), a particle tracker in the inaccessible, high radiation environment at the centre of ATLAS. Geodetic grids with several hundred fibre-coupled interferometers (30 mm to 1.5 m long) will be measured simultaneously. These lengths will be measured by tuning two lasers and comparing the resulting phase shifts in grid line interferometers, (GLIs) with phase shifts in a reference interferometer. The novel inexpensive GLI design uses diverging beams to reduce sensitivity to misalignment, albeit with weaker signals. One micrometre precision length measurements of grid lines will allow 10 μm precision tracker shape corrections to be fed into ATLAS particle tracking analysis. The technique was demonstrated by measuring a 400 mm interferometer to better than 400 nm and a 1195 mm interferometer to better than 250 nm. Precise measurements were possible, even with poor quality signals, using numerical analysis of thousands of intensity samples. Errors due to drifts in interferometer length were substantially reduced using two lasers tuned in opposite directions and the precision was further improved by linking measurements made at widely separated laser frequencies. © 2004 IOP Publishing Ltd.

Frequency domain interferometer simulation with higher-order spatial modes

Abstract: FINESSE is a software simulation allowing one to compute the optical properties of laser interferometers used by interferometric gravitational-wave detectors today. This fast and versatile tool has already proven to be useful in the design and commissioning of gravitational-wave detectors. The basic algorithm of FINESSE numerically computes the light amplitudes inside an interferometer using Hermite–Gauss modes in the frequency domain. In addition, FINESSE provides a number of commands for easily generating and plotting the most common signals including power enhancement, error and control signals, transfer functions and shot-noise-limited sensitivities. Among the various simulation tools available to the gravitational wave community today, FINESSE provides an advanced and versatile optical simulation based on a general analysis of user-defined optical setups and is quick to install and easy to use.

Computer-generated holograms in interferometric testing

Abstract: With surface-relief structures, optical functions that are required for radiation power management such as antireflection, light trapping, or light distribution and redirection can be obtained for new applications in solar energy systems and in displays. There, structures with submicrometer features must be distributed over large areas homogeneously. We address the design and the whole experimental process chain from the microstructure origination on large areas to the replication and the system integration in the specific application. Topics are antireflective surfaces for solar systems and displays, light trapping in polymer solar cells, sun protection systems for facades, and diffusers for projection displays and in glazing. For the microstructure origination we investigate the suitability of holographic recording in photoresist using a large-scale interferometer. We use an argon ion laser as a coherent light source at a wavelength of 364 nm. Periodic and stochastic interference patterns are recorded in positive photoresist with the interferometer setup. In the case of periodic structures, grating periods between 200 nm and 20 µm are realized. By carefully modeling the resulting resist profiles it is possible to originate even prismatic surface-relief profiles. Structures with good homogeneity are originated on areas of up to 4800 cm2 by optimizing the interferometer setup and the photoresist processing.

Systematic errors in nulling interferometers.

Abstract: Nulling interferometers combine on-axis suppression with high angular resolution, making them ideal instruments for the direct detection of faint planets close to their parent star Analysis is developed to show that it is systematic errors, resulting from fluctuations in the null depth, that drive the instrument performance A second-order combination of amplitude and phase errors is the dominant contributor In the calculated example, the detection of an Earthlike planet around a Sunlike star at 15 pc requires that the arms of the interferometer must be phased to within ∼15 nm and have their amplitudes matched to ∼01%

High stability multiplexed fibre interferometer and its application on absolute displacement measurement and on-line surface metrology

Abstract: We propose a self-reference multiplexed fibre interferometer (MFI) by using a tunable laser and fibre Bragg grating (FBG). The optical measurement system multiplexes two Michelson fibre interferometers with shared optical path in the main part of optical system. One fibre optic interferometer is used as a reference interferometer to monitor and control the high accuracy of the measurement system under environmental perturbations. The other is used as a measurement interferometer to obtain information from the target. An active phase tracking homodyne (APTH) technique is applied for signal processing to achieve high resolution. MFI can be utilised for high precision absolute displacement measurement with different combination of wavelengths from the tuneable laser. By means of Wavelength-Division-Multiplexing (WDM) technique, MFI is also capable of realising on-line surface measurement, in which traditional stylus scanning is replaced by spatial light-wave scanning so as to greatly improve the measurement speed and robustness. © 2004 Optical Society of America.

Exact analysis of low-finesse multimode fiber extrinsic Fabry-Perot interferometers.

Abstract: A straightforward theory is presented to accurately model the light inferences in a low-finesse multimode fiber extrinsic Fabry-Perot (FP) interferometer. The effect on the fringe visibility of the gap length, sensor structure imperfections, and modal power distributions is explored. The analysis is particularly useful in the design and optimization of sensors that use an extrinsic FP cavity as the sensing element.

On the origin of H2CO abundance enhancements in low-mass protostars

Abstract: High angular resolution H2CO 218 GHz line observations have been carried out toward the low-mass protostars IRAS 16293-2422 and L1448-C using the Owens Valley Millimeter Array at ~2" resolution. Simultaneous 1.37 mm continuum data reveal extended emission which is compared with that predicted by model envelopes constrained from single-dish data. For L1448-C the model density structure works well down to the 400 AU scale to which the interferometer is sensitive. For IRAS 16293-2422, a known proto-binary object, the interferometer observations indicate that the binary has cleared much of the material in the inner part of the envelope, out to the binary separation of ~800 AU. The morphology and velocity structure of the H2CO array data have been used to investigate whether the abundance enhancements inferred from single-dish modelling are due to thermal evaporation of ices or due to liberation of the ice mantles by shocks in the inner envelope. A scenario in which the H2CO abundance drops in the cold dense part of the envelope where CO is frozen out but is undepleted in the outermost region provides good fits to the single-dish and interferometer data on short baselines for both sources. Emission on the longer baselines is best reproduced if the H2CO abundance is increased by about an order of magnitude from ~1E-10 to ~1E-9 in the inner parts of the envelope due to thermal evaporation when the temperature exceeds ~50K. Other scenarios, including weak outflow-envelope interactions and photon heating of the envelope, are discussed and predictions for future generation interferometers are presented, illustrating their potential in distinguishing these competing scenarios.

Review: Quantum noise in gravitational-wave interferometers

Abstract: We present an overview of quantum noise in gravitational-wave interferometers. Current gravitational-wave detectors are modified variants of a Michelson interferometer and the quantum noise limits are strongly influenced by the optical configuration of the interferometer. We describe recent developments in the treatment of quantum noise in the complex interferometers of present-day and future gravitational-wave detectors and explore prospects for beating the standard quantum limit by use of both injected and ponderomotive squeezing in future interferometers.

An interferometry imaging beauty contest

Abstract: We present a formal comparison of the performance of algorithms used for synthesis imaging with optical/infrared long-baseline interferometers. Six different algorithms are evaluated based on their performance with simulated test data. Each set of test data is formated in the interferometry Data Exchange Standard and is designed to simulate a specific problem relevant to long-baseline imaging. The data are calibrated power spectra and bispectra measured with a ctitious array, intended to be typical of existing imaging interferometers. The strengths and limitations of each algorithm are discussed.

Hard x-ray phase imaging and tomography with a grating interferometer

Abstract: We have developed a two-grating interferometer for hard X rays that can be used for phase imaging and tomography. A silicon phase grating positioned just downstream of the object under study splits the distorted wavefront into essentially a positive and a negative first-order beam. At a given distance from this beam-splitter grating, where the two beams still mostly overlap, they form a pattern of interference fringes that is distorted according to the wavefront distortions. The fringes may be finer than the resolution of an area detector used to record the signal, but an absorption grating with suitable pitch, put in front of the detection plane, allows the detection of intensity variations that correspond to the derivative of the wavefront phase taken along the direction perpendicular to the grating lines. A combination of this technique with the phase-stepping method, in which several exposures are made which differ in the phase of the fringe pattern, allows to eliminate effects of non-uniform intensity due to inhomogeneous illumination and edge-enhancing inline phase contrast. Several examples of tomograms taken under different experimental conditions are shown, including a polychromatic "pink-beam" setup.

Resolving quadrature fringes in real time.

Abstract: In many interferometers, two fringe signals can be generated in quadrature. The relative phase of the two fringe signals depends on whether the optical path length is increasing or decreasing. A system is developed in which two quadrature fringe signals are digitized and analyzed in real time with a digital signal processor to yield a linear, high-resolution, wide-dynamic-range displacement transducer. The resolution in a simple Michelson interferometer with inexpensive components is 5 × 10-13 m Hz-1/2 at 2 Hz.

Three-dimensional deformation measurement from the combination of in-plane and out-of-plane electronic speckle pattern interferometers

Abstract: An optical setup that can be switched to produce in-plane and out-of-plane sensitivity interferometers was designed for three-dimensional deformation measuring by electronic speckle pattern interferometry. Divergent illumination is considered in the evaluation of sensitivity vectors to measure both in-plane and out-of-plane displacement components. The combination of these interferometers presents the advantage of greater sensitivity in directions u, v, and w than a typical interferometer with three illumination beams provides. The system and its basic operation are described, and results with an elastic target that is exposed to a mechanical load are reported.

Apparatus and method for correcting errors generated by a laser with non-ideal tuning characteristics

Abstract: The present invention is an apparatus and method for correcting errors generated by a laser with non-ideal tuning characteristics, the apparatus comprises a laser having non-ideal tuning characteristics. At least three interferometers are positioned in an operable relationship to the laser, wherein a sampling interferometer and at least one auxiliary interferometer correct for residual errors resulting from the laser; and wherein a measurement interferometer makes a measurement. A signal acquisition system is positioned in an operable relationship to each interferometer and a processor is positioned in an operable relationship to the signal acquisition system.

Removing nonlinearity of a homodyne interferometer by adjusting the gains of its quadrature detector systems.

Abstract: Most homodyne interferometers have a quadrature detector system that includes two polarizing beam splitters that cause nonlinearity of the order of a few nanometers by phase mixing Detectors should have the same gains to reduce nonlinearity under the assumption that there is no loss in optical components However, optical components exhibit some loss We show that nonlinearity can be reduced to an order of 001 nm when the detector gains are adjusted by simulation to include the optical characteristics The compensated nonlinearity is 18 times smaller than that when the four detector gains are set to be equal

Tunable photonic-crystal waveguide Mach-Zehnder interferometer achieved by nematic liquid-crystal phase modulation.

Abstract: Photonic crystals (PCs) have many potential applications because of their ability to control light-wave propagation and because PC-based waveguides may be integrated into optical interferometers. We propose a novel tunable PC waveguide Mach–Zehnder interferometer based on nematic liquid crystals and investigate its interference properties numerically by using the finite-difference time-domain method. We can change the refractive indices of liquid crystals by rotating the directors of the liquid crystals. Then we can control the phase of light propagation in a PC waveguide Mach-Zehnder interferometer. The interference mechanism is a change in the refractive indices of liquid-crystal waveguides. The novel interferometer can be used either as an optically controlled on–off switch or as an amplitude modulator in optical circuits.

Tests with a Carlina-type hypertelescope prototype I. Demonstration of star tracking and fringe acquisition with a balloon-suspended focal camera

Abstract: Labeyrie (1996, A&A, 118, 517) established the feasibility of snapshot images with a multi-aperture interferometer having a densified exit pupil. The numerous widely spaced mirrors in these instruments, called hypertelescopes, do not alleviate the usual difficulty of adjusting and phasing interferometers. A simplification is however possible, in the form of the optical and mechanical architecture called Carlina (Labeyrie et al. 2002, Proc. SPIE, 4838). It is configured like a diluted version of the Arecibo radio-telescope. Above the diluted primary mirror, made of fixed co-spherical segments, a helium balloon carries a gondola containing the focal optics and detector. We describe in more detail the Carlina concept, including versions equipped with an equatorial drive and a coude train. The optical design with a clam-shell corrector of spherical aberration is optimized with a ray-tracing code. A two-element prototype of a sparse aperture, multi-element, optical dish has been built using a steerable balloon-suspended secondary optical structure. Following imaging and tracking tests with a single mirror, which give encouraging results, fringes have been obtained on Vega with a pair of closely spaced mirrors. We developed adjustment techniques for co-spherizing the mirrors within one or a few microns, using a light source at the curvature center. The absence of delay lines is a major simplification with respect to conventional interferometers, paving the way towards using hundreds or thousands of sub-apertures for producing direct images with rich information content. These results demonstrate the short-term feasibility of large Carlina hypertelescopes, with effective aperture size possibly reaching 1500 m at suitable terrestrial sites. Such interferometers will provide snapshot images of star surfaces, and of exo-planets if equipped with an adaptive coronagraph. Collecting areas comparable to those of ELTs appear feasible at a lower cost, while providing a higher resolution and similar limiting magnitude.

Novel folded Mach-Zehnder interferometers and optical sensor arrays

Abstract: The invention provides novel “folded” Mach-Zehnder interferometers (“folded” MZI's), methods for making folded MZI's, and systems and devices incorporating them. The novel folded MZI's are elaborated from conventional MZI structures by cutting across the interferometer arms of a conventional MZI structure and creating reflectors on the exposed ends of the interferometer arms to form two “folded” MZI's from a single conventional Mach-Zehnder interferometer structure. The novel folded MZI's show promise as sensors having a reduced size and enhanced sensitivity relative to sensors incorporating conventional Mach-Zehnder Interferometers.

Full-field optical coherence microscopy

Abstract: We present the new advances in full field optical coherence microscopy, an alternative method to conventional optical coherence tomography (OCT). The experimental setup is based on Linnik interferometer illuminated with a tungsten halogen lamp. En face tomographic images are obtained in real-time without scanning by computing the difference of two phase-opposed interferometric images recorded by a high-resolution CCD camera. The short coherence length of the source and the compensation of dispersion mismatch in the interferometer arms yield an optical sectioning ability with 0.8 μm resolution in water. Transverse resolution of 1.0 μm is achieved by using microscope objectives with a numerical aperture of 0.5. A shot-noise limited detection sensitivity of 86 dB can be reached with 2 s acquisition time. High-resolution images of the anterior segment of the rat eye are shown.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Long-term study of the seismic environment at LIGO

Abstract: The LIGO experiment aims to detect and study gravitational waves using ground-based laser interferometry. A critical factor to the performance of the interferometers, and a major consideration in the design of possible future upgrades, is isolation of the interferometer optics from seismic noise. We present the results of a detailed programme of measurements of the seismic environment surrounding the LIGO interferometers. We describe the experimental configuration used to collect the data, which were acquired over a 613 day period. The measurements focused on the frequency range 0.1–10 Hz, in which the secondary microseismic peak and noise due to human activity in the vicinity of the detectors was found to be particularly critical to the interferometer performance. We compare the statistical distribution of the data sets from the two interferometer sites, construct amplitude spectral densities of seismic noise amplitude fluctuations with periods of up to 3 months and analyse the data for any long-term trends in the amplitude of seismic noise in this critical frequency range.

Roll angular displacement measurement system with microradian accuracy

Abstract: A novel compact roll angular displacement measurement system is presented. It consists of a linearly orthogonally polarized, heterodyne laser source as the detection probe beam, a wave-retardation plate or a set of a half-wave plate plus a wave-retardation plate as the angle-sensing element to form an interferometric system. The roll angular displacement of a moving device can be detected by measuring the phase difference between the reference and measurement beams. The experimental results fit well with the theoretical analysis and a measurement resolution of sub-microradian over a half-degree is achieved experimentally.

Quantum microscopy using photon correlations

Abstract: Higher-order correlations of the radiation field improve resolution in stellar interferometers, as in the Hanbury-Brown–Twiss effect. It is also possible to improve microscopic resolution beyond the Rayleigh limit by using quantum light fields composed of entangled photons. Focusing on two photons, we distinguish two types of entanglement: frequency entanglement, where the photons in different paths are correlated in frequency, and path entanglement, where the correlation between paths is in photon number. Two paradigms of quantum microscopy are discussed: spectral microscopy, where path- and frequency-entangled photons produced in cascade decay of two atoms make possible sub-natural linewidth resolution of atomic levels, and spatial microscopy, where path-entangled photons emitted by an atomic array produce sub-wavelength diffraction resolution as compared to an equivalent classical grating. These scenarios require two-photon correlation or coincidence measurements. The connection between the two paradigms, and the two types of entanglement, highlights the link between the temporal and spatial aspects of quantum interferometry.