Showing papers on "Astronomical interferometer published in 1999"

Measurement of gravitational acceleration by dropping atoms

Abstract: Laser-cooling of atoms and atom-trapping are finding increasing application in many areas of science1 One important use of laser-cooled atoms is in atom interferometers2 In these devices, an atom is placed into a superposition of two or more spatially separated atomic states; these states are each described by a quantum-mechanical phase term, which will interfere with one another if they are brought back together at a later time Atom interferometers have been shown to be very precise inertial sensors for acceleration3,4, rotation5 and for the measurement of the fine structure constant6 Here we use an atom interferometer based on a fountain of laser-cooled atoms to measure g, the acceleration of gravity Through detailed investigation and elimination of systematic effects that may affect the accuracy ofthe measurement, we achieve an absolute uncertainty of Δg/g ≈ 3 × 10−9, representing a million-fold increase in absoluteaccuracy compared with previous atom-interferometer experiments7 We also compare our measurement with the value of g obtained at the same laboratory site using a Michelson interferometer gravimeter (a modern equivalent of Galileo's ‘leaning tower’ experiment in Pisa) We show that the macroscopic glass object used in this instrument falls with the same acceleration, to within 7 parts in 109, as a quantum-mechanical caesium atom

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.

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.

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.

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.

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.

Heterodyne interferometer with subatomic periodic nonlinearity.

Abstract: A new, to our knowledge, heterodyne interferometer for differential displacement measurements is presented. It is, in principle, free of periodic nonlinearity. A pair of spatially separated light beams with different frequencies is produced by two acousto-optic modulators, avoiding the main source of periodic nonlinearity in traditional heterodyne interferometers that are based on a Zeeman split laser. In addition, laser beams of the same frequency are used in the measurement and the reference arms, giving the interferometer theoretically perfect immunity from common-mode displacement. We experimentally demonstrated a residual level of periodic nonlinearity of less than 20 pm in amplitude. The remaining periodic error is attributed to unbalanced ghost reflections that drift slowly with time.

Extreme-ultraviolet phase-shifting point-diffraction interferometer: a wave-front metrology tool with subangstrom reference-wave accuracy

Abstract: The phase-shifting point-diffraction interferometer (PS/PDI) was recently developed and implemented at Lawrence Berkeley National Laboratory to characterize extreme-ultraviolet (EUV) projection optical systems for lithography. Here we quantitatively characterize the accuracy and precision of the PS/PDI. Experimental measurements are compared with theoretical results. Two major classes of errors affect the accuracy of the interferometer: systematic effects arising from measurement geometry and systematic and random errors due to an imperfect reference wave. To characterize these effects, and hence to calibrate the interferometer, a null test is used. This null test also serves as a measure of the accuracy of the interferometer. We show the EUV PS/PDI, as currently implemented, to have a systematic error-limited reference-wave accuracy of 0.0028 waves (lambda/357 or 0.038 nm at lambda = 13.5 nm) within a numerical aperture of 0.082.

Real-time, interferometrically measuring atomic force microscope for direct calibration of standards

Abstract: An atomic force microscope with a high-resolution three-axis laser interferometer for real-time correction of distorted topographic images has been constructed and investigated. With this apparatus, standard samples for a scanning probe microscope can be directly calibrated on the basis of stabilized wavelength of He–Ne lasers. The scanner includes a three-sided mirror block as a mobile target mirror for the interferometer, which realizes a rectangular coordinate system. The position coordinates of the sample is independently and simultaneously acquired with high-resolution (0.04 nm) X/Y/Z interferometer units and fed back for XY servo scanning and height image construction. The probe is placed on the sample surface at the intersection of the three optical axes of the interferometer with good reproducibility, so that the Abbe error caused by the rotation of the scanner is minimized. Image distortion in the XY plane and vertical overshoot/undershoot due to nonlinear motion of piezo devices, hysteresis, and creep are eliminated. The optical properties of the interferometers, mechanical characteristics of the scanner, and system performances in dimensional measurements for calibration standards are demonstrated.

Fiber laser hydrophone array

Abstract: In recent years growing interest has surrounded the development of fiber laser sensors (FLS). This is due to their ultra high sensitivity to temperature and strain as well as their ability to be multiplexed along a single fiber using WDM techniques. It is their extreme sensitivity that has led to them being considered as acoustic pressure sensors rather than standard fiber Bragg gratings. The work presented here describes the development of an array of FLS configured as hydrophones. We discuss the design of the single mode fiber laser used throughout our system; comparing examples based upon distributed Bragg reflectors (DBR) and distributed feedback (DFB). In addition we discuss both the theoretical and experimental acoustic sensitivity enhancements obtained by the application of an elasto-plastic coating to the FLS. The array configuration is described, as is the heterodyne interrogation scheme using an unbalanced Mach-Zehnder interferometer with WDM channel selection. Results from the measurement of the minimal detectable acoustic signal of a bare fiber laser are shown to be -69 dB re.Pa/(root)Hz at 1 kHz when using a 200 m path imbalanced readout interferometer. Further gains in the sensitivity due to the application of various coatings are reported, as is a full characterization of an array of fiber laser hydrophones. Finally we discuss the future research of the FLS, and the areas in which the technology is particularly applicable.

Integrated optics for astronomical interferometry. I. Concept and astronomical applications

Abstract: We propose a new instrumental concept for long-baseline optical single-mode interferometry using integrated optics which were developed for telecommunication. Visible and infrared multi-aperture interferometry requires many optical functions (spatial filtering, beam combination, photometric calibration, polarization control) to detect astronomical signals at very high angular resolution. Since the 80's, integrated optics on planar substrate have become available for telecommunication applications with multiple optical functions like power dividing, coupling, multiplexing, etc. We present the concept of an optical / infrared interferometric instrument based on this new technology. The main advantage is to provide an interferometric combination unit on a single optical chip. Integrated optics are compact, provide stability, low sensitivity to external constrains like temperature, pressure or mechanical stresses, no optical alignment except for coupling, simplicity and intrinsic polarization control. The integrated optics devices are inexpensive compared to devices that have the same functionalities in bulk optics. We think integrated optics will fundamentally change single-mode interferometry. Integrated optics devices are in particular well-suited for interferometric combination of numerous beams to achieve aperture synthesis imaging or for space-based interferometers where stability and a minimum of optical alignments are wished.

Tropospheric phase calibration in millimeter interferometry

Abstract: We review millimeter interferometric phase variations caused by variations in the precipitable water vapor content of the troposphere, and we discuss techniques proposed to correct for these variations. We present observations with the Very Large Array (VLA) at 22 and 43 GHz designed to test these techniques. We find that both the fast switching and paired array calibration techniques are effective at reducing tropospheric phase noise for radio interferometers. In both cases, the residual rms phase fluctuations after correction are independent of baseline length for b > beff. These techniques allow for diffraction-limited imaging of faint sources on arbitrarily long baselines at millimeter wavelengths. We consider the technique of tropospheric phase correction using a measurement of the precipitable water vapor content of the troposphere via a radiometric measurement of the brightness temperature of the atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz for the millimeter array (MMA) and to 120 mK for the VLA at 22 GHz. The minimum gain stability requirement is 200 at 185 GHz at the MMA, assuming that the astronomical receivers are used for radiometry. This increases to 2000 for an uncooled system. The stability requirement is 450 for the cooled system at the VLA at 22 GHz. To perform absolute radiometric phase corrections also requires knowledge of the tropospheric parameters and models to an accuracy of a few percent. It may be possible to perform an “empirically calibrated” radiometric phase correction, in which the relationship between fluctuations in brightness temperature differences and fluctuations in interferometric phases is calibrated by observing a strong celestial calibrator at regular intervals. A number of questions remain concerning this technique, including the following: (1) Over what timescale and distance will this technique allow for radiometric phase corrections when switching between the source and the calibrator? (2) How often will calibration of the TrmsB – Orms relationship be required?

Progress in multipass tandem Fabry–Perot interferometry: I. A fully automated, easy to use, self-aligning spectrometer with increased stability and flexibility

Abstract: A computer control for a Sandercock-type multipath tandem Fabry–Perot interferometer is described, which offers many advantages over conventionally used analog control: The range of stability is increased due to active control of the laser light intensity and the mirror dither amplitude. The alignment is fully automated enabling start of a measurement within a minute after start of align, including optionally finding the optimum focus on the sample. The software control enables a programmable series of measurements with control of, e.g., the position and rotation of the sample, the angle of light incidence, the sample temperature, or the strength and direction of an applied magnetic field. Built-in fitting routines allow for a precise determination of frequency positions of excitation peaks combined with increased frequency accuracy due to a correction of a residual nonlinearity of the mirror stage drive.

Analysis of generalized Mach-Zehnder interferometers for variable-ratio power splitting and optimized switching

Abstract: The nonideal integrated optical N/spl times/N generalized Mach-Zehnder interferometer (GMZI) employing multimode interference (MMI) couplers is analyzed using transfer matrix techniques. Deviations in the phase relations and the power splitting ratio of the MMI couplers are included in the theory, along with the effects of phase errors in the interferometer arms. The predictions of the theory are compared to the response of a 4/spl times/4 GMZI which has been fabricated. The device is operated as both a variable-ratio power splitter and a switch by compensating for the phase errors in the interferometer arms, but the performance is ultimately limited by the nonideal imaging in the MMI couplers. The practicality of these applications is investigated by performing a tolerance analysis for the operation of 1/spl times/N power splitters and switches for N up to 10.

Integrated optics for astronomical interferometry

Abstract: We propose a new instrumental concept for long-baseline optical single-mode interferometry using in- tegrated optics which were developed for telecommunica- tion Visible and infrared multi-aperture interferometry requires many optical functions (spatial ltering, beam combination, photometric calibration, polarization con- trol) to detect astronomical signals at very high angular resolution Since the 80's, integrated optics on planar sub- strate have become available for telecommunication appli- cations with multiple optical functions like power divid- ing, coupling, multiplexing, etc We present the concept of an optical/infrared interferometric instrument based on this new technology The main advantage is to provide an interferometric combination unit on a single optical chip Integrated optics are compact, provide stability, low sensitivity to external constrains like temperature, pres- sure or mechanical stresses, no optical alignment except for coupling, simplicity and intrinsic polarization control The integrated optics devices are inexpensive compared to devices that have the same functionalities in bulk op- tics We think integrated optics will fundamentally change single-mode interferometry Integrated optics devices are in particular well-suited for interferometric combination of numerous beams to achieve aperture synthesis imaging or for space-based interferometers where stability and a minimum of optical alignments are wished

Optical roughness measurements using extended white-light interferometry

Abstract: Roughness measurements of engineering surfaces are an important and interesting application for optical sensors The increased speed, the ability to measure 3-D profiles, and the noncontact mode of optical sensors are of great interest for practical uses A most challeng- ing task is achieving comparability to the established methods, such as tactile stylus sensors, which are the most commonly used instruments in industry We show that white-light interferometry is a useful method to measure in a microscopic range as well as with an extended field of view The determination of the fringe contrast as a function of the focus position enables highly resolved data acquisition on engineering sur- faces with a mean roughness of the order of a few nanometers to several micrometers The measurements show that amplitude roughness pa- rameters of engineering surfaces are not strongly affected by the spatial resolution A good agreement between the results of a stylus sensor and the results of our optical method are demonstrated on different types of surfaces © 1999 Society of Photo-Optical Instrumentation Engineers (S0091-3286(99)00906-X)

Integrated optics for astronomical interferometry - I. Concept and astronomical applications

Abstract: We propose a new instrumental concept for long-baseline optical single-mode interferometry using integrated optics which were developed for telecommunication. Visible and infrared multi-aperture interferometry requires many optical functions (spatial filtering, beam combination, photometric calibration, polarization control) to detect astronomical signals at very high angular resolution. Since the 80's, integrated optics on planar substrate have become available for telecommunication applications with multiple optical functions like power dividing, coupling, multiplexing, etc. We present the concept of an optical/infrared interferometric instrument based on this new technology. The main advantage is to provide an interferometric combination unit on a single optical chip. Integrated optics are compact, provide stability, low sensitivity to external constrains like temperature, pressure or mechanical stresses, no optical alignment except for coupling, simplicity and intrinsic polarization control. The integrated optics devices are inexpensive compared to devices that have the same functionalities in bulk optics. We think integrated optics will fundamentally change single-mode interferometry. Integrated optics devices are in particular well-suited for interferometric combination of numerous beams to achieve aperture synthesis imaging or for space-based interferometers where stability and a minimum of optical alignments are wished.

TIMED Doppler Interferometer (TIDI)

Abstract: The TIMED Doppler Interferometer (TIDI) is a Fabry-Perot interferometer designed to measure winds, temperatures, and constituents in the mesosphere and thermosphere (60 - 300 km) region of the atmosphere as part of the TIMED mission. TIDI is a limb viewer and observes emissions from OI 557.7 nm, OI 630.0 nm, OII 732.0 nm, O 2 (0-0), O 2 (0-1), Na D, OI 844.6 nm, and OH in the spectral region 550 - 900 nm. Wind measurement accuracies will approach 3 ms -1 in the mesosphere and 15 ms -1 in the thermosphere. The TIDI instrument has several novel features that allow high measurement accuracies in a modest-sized instrument. These include: an optical system that simultaneously feeds the views from four scanning telescopes which are pointed at plus or minus 45 degrees and plus or minus 135 degrees to the spacecraft velocity vector into a high-resolution interferometer, the first spaceflight application of the circle-to-line imaging optic (CLIO), and a high quantum efficiency, low noise CCD.

Soft-x-ray laser interferometry of a plasma with a tabletop laser and a Lloyd's mirror.

Abstract: Interferometry using optical lasers is a powerful technique for the diagnostics of plasmas that provides time-resolved two-dimensional maps of the electron density. However, free‐free absorption and refraction of the probe beam limit the maximum electron density, plasma size, and plasma density gradients that can be probed with optical wavelengths. The development of soft-x-ray lasers has opened the possibility of extending plasma interferometry to shorter wavelengths, significantly expanding the range of plasma parameters that can be probed. Recently, a 15.5-nm Ne-like Y laser pumped by the Nova laser at Lawrence Livermore National Laboratories was used in combination with a skewed Mach‐Zehnder interferometer to probe very high-density laser-created plasmas. 1,2 The amplitude-division interferometer used in those pioneering experiments was implemented by use of thin multilayer beam splitters and multilayer-coated mirrors developed for that wavelength. The advent of gain-saturated discharge-pumped tabletop soft-x-ray lasers 3,4 and the development of several saturated 5 or nearly saturated 6,7 soft-x-ray lasers pumped by relatively compact optical lasers will allow the probing of a great variety of dense plasmas. However, multilayer beam splitters cannot be developed for the wavelengths corresponding to some of these lasers at present, owing to the lack of materials with adequate optical constants. Alternatively, amplitude-division interferometers based on diffraction gratings 8 and wavefront-division interferometers 9,10 have been discussed.

Thermal tuning of a fiber-optic interferometer for maximum sensitivity

Abstract: We describe a fiber-optic interferometer that employs wavelength changes to achieve maximum sensitivity. Wavelength changes are induced by adjusting the operating temperature of the laser, eliminating the need for an actuator to vary the spacing between the sensing fiber and the object to be monitored. The instrument and techniques described are suitable for cryogenic, high vacuum applications such as magnetic resonance force microscopy, where space is limited and micromanipulation can be challenging. The noise floor of 1.6×10−3 nm/Hz is adequate for monitoring subangstrom displacement of force microscope cantilevers.

Spatial phase-stepped interferometry using a holographic optical element

Abstract: We describe two new techniques for spatial phase stepping using a computer-generated holographic optical element (HOE). The techniques are implemented with electronic speckle pattern interferom- etry, although they could be applied to other optical metrology methods. The first technique uses an HOE that introduces a known phase step between the 61 diffracted orders, without being translated. The interfer- ence phase corresponding to object deformation can be calculated from a single TV frame, and the technique is therefore suitable for the mea- surement of transient deformations. We apply the technique to measure the phase changes caused by refractive index variations in an evolving thermal plume. The second technique requires the HOE to be used in conjunction with a phase grating. In this technique, the HOE and the phase grating are used sequentially for the two exposures required in speckle interferometry (made before and after object deformation) and is therefore suitable for making dynamic deformation measurements with a double-pulsed interferometer. A root mean square (rms) precision of 2p/20 rad for the phase measurements is obtained. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00712-6)

Thermally actuated interferometric sensors based on the thermal expansion of transparent elastomeric media

Abstract: In this article the fabrication and characterization of two thermally actuated optical devices for the measurement of temperature and power are described. A transparent polymer having a high coefficient of thermal expansion—poly(dimethylsiloxane) (PDMS)—was used as the temperature-sensitive medium. Changes in the dimensions of the polymer on heating caused the observed optical responses of both devices. The temperature sensor based on the Fabry–Perot cavity measures temperature differences to a precision of 0.005 °C within the linear working ranges of the device. The power sensor uses the architecture of a Mach–Zender interferometer; it is suitable for measurements of powers in the mW/cm2 range, delivered optically to the surface of the device in the visible wavelength region. The devices are inexpensive, easy to fabricate, and mechanically rugged. They offer alternatives to other sensors for measuring temperature and power.

Search techniques for gravitational waves from black-hole ringdowns

Abstract: Of all the astronomical sources of gravitational radiation, the ringdown waveform arising from a small perturbation of a spinning black hole is perhaps the best understood: for the late stages of such a perturbation, the waveform is simply an exponentially damped sinusoid. Searching interferometric gravitational wave antenna data for these should be relatively easy. In this paper, I present the results of a single-filter search for ringdown waveforms arising from a 50 solar mass black hole with $98%$ of its maximum spin angular momentum using data from the Caltech 40-m prototype interferometer. This search illustrates the techniques that may be used in analyzing data from future kilometer-scale interferometers and describes some of the difficulties present in the analysis of interferometer data. Most importantly, it illustrates the use of coincident events in the output of two independent interferometers (here simulated by 40-m data at two different times) to substantially reduce the spurious event rate. Such coincidences will be essential tools in future gravitational wave searches in kilometer-scale interferometers.

Standing wave detection and interferometer application using a photodiode thinner than optical wavelength

Abstract: A thin film photodiode whose active layer is thinner than the wavelength of the incident light is described. A part of the incident photon is detected and the rest transmits through the thin film photodiode without absorption. Being inserted in the optical field, this sensor is applied to construct the new interferometer detecting the intensity profile of the standing wave of the thinnest interference fringe.

Antenna and method for two-dimensional angle-of-arrival determination

Abstract: An antenna and system for determining two-dimensional angle-of-arrival includes circular array interferometers for azimuth, linear interferometers for elevation, and a mast for positioning the interferometers in a vertically stacked orientation. The circular array interferometers exhibit azimuth constant-phase-difference contours orthogonal to elevation constant-phase-difference contours exhibited by the linear interferometers. An antenna and system for determining azimuth of input signals includes ambiguous and non-ambiguous circular array interferometers. The non-ambiguous circular array interferometer resolves ambiguities introduced by the ambiguous circular array interferometer, the at least one ambiguous circular array interferometer operatively coupled to a low-order Butler matrix and power divider/combiner such that a high order Butler matrix is not required to generate omnidirectional phase modes from the ambiguous circular array interferometer. A method for determining phase differences indicative of two-dimensional angle-of-arrival includes determining a first phase difference indicating azimuth using circular array interferometers, determining a second phase difference indicating elevation using linear interferometers, and orienting the interferometers in a vertically stacked orientation such that the azimuth constant-phase-difference contours are orthogonal to the elevation constant-phase-difference contours. A method for determining phase difference representative of azimuth includes determining a phase difference indicative of azimuth using ambiguous circular array interferometer and non-ambiguous circular array interferometers, and resolving ambiguities in azimuth introduced by the ambiguous circular array interferometer using the non-ambiguous circular array interferometer, the at least one ambiguous circular array interferometer operatively coupled to a low-order Butler matrix and power divider/combiner such that a high order Butler matrix is not required to generate omnidirectional phase modes from the ambiguous circular array interferometer. A method for determination of polarization of input signals with only one added element.

A linear/angular interferometer capable of measuring large angular motion

Abstract: A novel linear and angular interferometric heterodyne system, capable of measuring large angular motion, is proposed. The interferometric system has a high linear-displacement resolution of 1.24 nm while the angular-displacement resolution can reach 0.0025 arcsec. By combining the advantages of the interferometer with a retroreflector and the interferometer with a plane-mirror reflector, the proposed interferometric measurement system allows a desirable transverse motion and rotational motion along any of the three orthogonal directions. Using a retroreflector with a diameter of 38.1 mm, the translational range in the transverse directions is no less than 20 mm × 10 mm while the angular range along any axis is no less than 10°. The angular readings can be used to compensate the linear outputs in real time when the Abbe error occurs. This ability has been demonstrated by the experiments.