Showing papers on "Astronomical interferometer published in 1985"

Monomode optical fibre interferometers for precision measurement

Abstract: The evolution of optical interferometers into extreme resolution large dynamic range sensors implemented in a guided wave form is discussed. The design and performance of fibre optic interferometric sensors optimised for specific periodic and quasi-steady-state measurands such as rotation, magnetic field, temperature, etc., are described.

Multimirror Fabry–Perot interferometers

Abstract: The addition of one or more partially reflecting mirrors to a traditional two-mirror Fabry–Perot interferometer results in a multimirror Fabry–Perot interferometer. A superposition of all possible multiple reflected beams is described with a general theory for multimirror interferometers, featuring matrices analogous to the theory of multilayer thin films. However, the parameters in the matrix elements are mirror reflection coefficients and spacings instead of the usual refractive indices and layer thicknesses of thin films. The transmission characteristics of two-, three-, and four-mirror Fabry–Perot optical filters are described. It is shown that a suitable choice of reflection coefficients results in transmission properties that can be described approximately with Butterworth profiles, which are known from network analysis of electrical circuits.

WAMDII: wide-angle Michelson Doppler imaging interferometer for Spacelab

Abstract: A wide-angle Michelson Doppler imaging interferometer (WAMDII) is described that is intended to measure upper atmospheric winds and temperatures from naturally occurring visible region emissions, using Spacelab as a platform. It is an achromatic field-widened instrument, with good thermal stability, that employs four quarterwave phase-stepped images to generate full images of velocity, temperature, and emission rate. For an apparent emission rate of 5 kR and binning into 85 × 105 pixels, the required exposure time is 1 sec. The concept and underlying principles are described, along with some fabrication details for the prototype instrument. The results of laboratory tests and field measurements using auroral emissions are described and discussed.

High-accuracy displacement interferometry in air.

Abstract: The absolute accuracy of laser interferometers employed in displacement metrology is limited by two dominant factors: uncertainties in the source vacuum wavelength and the refractive index of the ambient air. In this paper we describe an interferometer system designed to minimize these uncertainties. Based on a commercial interferometer, the new system features direct measurement of the vacuum wavelength by frequency comparison with a portable iodine-stabilized He–Ne laser. The refractive index of air is computed from accurately measured values of pressure, temperature, and relative humidity. Combined with a desktop computer, the interferometer system permits the automated field measurement of displacement errors (such as those associated with precision machine tools) with an absolute accuracy of 8.5 parts in 108. Performance of the interferometer in field metrology is illustrated by the results of recent validation testing of the large optics diamond turning machine (LODTM) at Lawrence Livermore National Laboratory. These results highlight the need for new measurements of the absolute refractive index of standard air in order to reduce a limiting uncertainty on such measurements of ±5 parts in 108.

Rough surface interferometry with a two-wavelength heterodyne speckle interferometer.

Abstract: If a rough surface is illuminated by a coherent lightwave of wavelength λ1, it is not possible to determine the surface profile from the phases of the speckle field formed by the scattered light. If the rough surface is illuminated, however, by an additional coherent wave of wavelength λ1, the phase differences between the two speckle fields do contain information about the macroscopic surface profile even if subject to a statistical error. It is shown that (1) the macroscopic surface profile may be determined from the phase differences if the effective wavelength Λ = λ1λ2/|λ1−λ2| is sufficiently larger than the standard deviation of the microscopic profile of the illuminated surface, and (2) the statistical error is reasonably small if the phase measurements are obtained from speckles of sufficient intensity. Using a heterodyne interferometer we demonstrate the feasibility of this technique. In the first experiment we determine the radius of curvature of a rough spherical surface. In the second experiment the macroscopic surface contour on two ophthalmic lenses of the same power variation, one with a grounded surface and the other with a polished surface, was determined.

Shearing interferometer for phase shifting interferometry with polarization phase shifter.

Abstract: A cyclic interferometer can be used for both lateral and radial shearing. Being common-path this interferometer is quite stable. The use of a cyclic shear interferometer for phase shifting interferometry is discussed. The phase shifting is provided here by polarization techniques. Some useful cyclic phase shifting interferometer arrangements are discussed, and the working of a polarization phase shifting cyclic shear interferometer is demonstrated.

Interferometer system for controlling non-rectilinear movement of an object

Abstract: A system for accurately controlling the movement of an object through a predetermined non-rectilinear path in a predetermined volume having X, Y and Z axes. The system uses three tracking laser interferometers rigidly positioned relative to the volume and a single retroreflector rigidly coupled to the object to control movement of the object non-rectilinearly through the volume along a predetermined path. Using two more tracking laser interferometers and an additional retroreflector rigidly coupled to the object, the system controls angular orientation of the object about one of the axes. Adding a sixth tracking laser interferometer and a third retroreflector to the object, the system can also control angular orientation about the other two axes. Each tracking laser interferometer comprises an interferometer and a tracking mirror which is pivoted to maintain the interferometer output beam on its associated retroreflector.

Photodiode array Fourier transform spectrometer with improved dynamic range.

Abstract: An unconventional Fourier transform spectrometer is described which does not require mechanical scanning. The instrument uses a photodiode array as detector and incorporates features whereby the systematic background noise associated with this type of device may be compensated for to give good quality interferograms. Sample spectra are presented to demonstrate the performance of the instrument.

Image transmission and interferometry with multimode fibers using self-pumped phase conjugation

Abstract: A double‐pass image transmission through a single multimode fiber is demonstrated, using a passive phase conjugate mirror. An application to interferometry based on phase sensing is demonstrated, by implementing the multimode fiber and the passive phase conjugate mirror as one arm of a Michelson interferometer. Due to the unique properties of the self‐pumped conjugator, nonuniform distortions caused by modal dispersion in the fiber and other aberrations are cancelled out, while uniform phase changes are detected.

Multiple mirror telescope as a phased array telescope.

Abstract: By adjusting the optical path lengths of its individual beams, it is possible to make the multiple mirror telescope (MMT) into a phased array with a 6.86-m base line. A coherent phased focus can be achieved with tilted focal planes if the tilt angle is chosen so that the internal phase differences exactly compensate the external phase differences. This amounts to a slight change in configuration so that the beams are brought together at f/8.39 rather than the originally designed f/9. We summarize experiments which have used the MMT subapertures as a phased array and as a coherent phased telescope and present a simple analysis of the titled focal plane geometry for coherent observation. The phased operation of the MMT is important not only for obtaining high angular resolution but also for obtaining the higher detection sensitivity which results from the better discrimination against the sky emission background for IR diffraction-limited images. Full-aperture (six-beam) diffraction-limited results for the unresolved source Gama Orionis, the well-known close binary Capella, and the resolved red supergiant Betelgeuse (including a diffraction-limited differential speckle image of the latter) are presented as preliminary demonstration of the potential capabilities of this configuration.

Improved Version of a Polarized Beam Heterodyne Interferometer

Abstract: The heterodyne interferometer is a very sensitive instrument with a wide dynamic range, suitable for measuring surface displacements of amplitude down to mented present some disadvantages. T heir optical mounting is space demanding and alignments are critical. In addition, some inherent instabilities have also been identified. We have constructed a new version of the instrument which considerably reduces these disadvantages. The combination of a colinear beam Bragg cell and of a polarizing beam splitter makes the optical arrangement compact (less than lOcm long) and simplifies the alignment p rocedure. Ekper*pental results show that the sensitivity, lo+ A/ &z,reaches the t heoretical value and that the instabilities a re reduced by more than one order of magnitude, down to 0.3% of the output signal. 1. However, the interferometers p resently imple

Fully compensated Michelson interferometer of fixed-path difference.

Abstract: A Michelson interferometer of fixed-path difference, capable of scanning over only one fringe, has been shown to be useful for the measurement of Doppler shifts from isolated emission lines. It is straightforward to field widen this type of configuration, but if different wavelengths are to be selected, the field widening must be achromatic. Further, for Doppler shift measurement, one requires a path difference that is highly stable with respect to temperature. The generalized requirements for meeting all these conditions are set out and a configuration that satisfies them is described. Sample characteristics are given for a number of different systems.

Gas bearing X-Y-θ stage assembly

Abstract: An improved gas bearing X-Y-θ stage assembly is disclosed which has a base, a stage element with three retroreflectors fixedly mounted thereto, and a control system for controlling the movement of the stage element. The control system is made up of three interferometer measuring systems for measuring parameters associated with changes in position of the three retroreflectors as the stage element moves. Also, the control system includes a transport system for moving the three interferometers to maintain optical coupling between the interferometers and retroreflectors. In addition, the control system includes control circuitry for causing the stage element to move to a preselected X-Y-θ position in response to signals from the interferometer measuring systems. In one embodiment, a first interferometer of a first one of the three interferometer measuring systems is moved linearly by the transport system on a gas bearing slide in a direction parallel to the Y-axis of the system, while second and third interferometers of the other two interferometer measuring systems are moved together on a second gas bearing slide parallel to the X-axis of the system.

Time reversal by an interferometer with coupled phase-conjugate reflectors.

Abstract: A new type of interferometer has been constructed that uses a beam splitter and two self-pumped BaTiO3 crystals as phase-conjugate reflectors in place of the usual interferometer mirrors. Counterpropagating beams of light are spontaneously generated between the two crystals, coupling the pair of phase conjugators. This optical oscillation locks the relative phase of the two phase-conjugate reflections such that they recombine at the beam splitter to form only a single beam as though they were truly time-reversed waves.

The imaging Doppler interferometer: Data analysis

Abstract: Imaging Doppler interferometry has been developed as a pulsed-radar technique for obtaining three-dimensional images of mesospheric scattering. Here we discuss the analysis procedure and algorithms and show the results of processing synthetic data records for a variety of situations.

Multichannel far-infrared phase imaging for fusion plasmas

Abstract: A 20-channel far-infrared imaging interferometer system has been used to obtain single-shot density profiles in the UCLA Microtor tokamak. This system differs from conventional multichannel interferometers in that the phase distribution produced by the plasma is imaged onto a single, monolithic, integrated microbolometer linear detector array and provides significantly more channels than previous far-infrared interferometers. The system has been demonstrated to provide diffraction-limited phase images of dielectric targets.

Monomode fiber-optic strain gauge with simultaneous phase- and polarization-state detection

Abstract: A fiber-optic sensor for the measurement of strain is described, in which the measurand-induced changes in both phase and polarization state are simultaneously recovered. A simple optical arrangement based on the fiber Fabry–Perot interferometer is used to realize a high-resolution instrument with enhanced measurement range.

Proposal for a new all-optical waveguide functional device.

Abstract: A new type of all-optical logic gate is described that employs a single-mode waveguide, unequal-arm-length Mach–Zehnder interferometer fabricated using materials that exhibit an optical Kerr effect. Because of the unique properties of the single-mode Y junction, there is no reflected light output from the element. This interferometer is therefore suitable for digital signal processing because an optical isolator is not needed for connection as a serial array to achieve sharp on–off characteristics.

TFTR Michelson interferometer electron cyclotron emission diagnostic

Abstract: In July 1984, a Fourier transform spectrometer employing a fast‐scanning Michelson interferometer began operating on TFTR. This diagnostic system can measure the electron cyclotron emission spectrum 72 times per s with a time resolution of 11 ms and a spectral resolution of 3.6 GHz. The initial operating spectral range is 75–540 GHz, which is adequate for measuring the first three cyclotron harmonics at present TFTR magnetic field levels. The range can be extended easily to 75–1080 GHz in order to accommodate increases in toroidal magnetic field or to study superthermal ECE. The measured spectra are absolutely calibrated using a liquid nitrogen cooled blackbody reference source. The second harmonic feature of each spectrum is used to calculate the absolute electron temperature profile.

Optimization of a field-widened Michelson interferometer.

Abstract: This paper considers the optical design of a wide-angle fixed-path Michelson interferometer consisting of two arm glasses and an air gap. It is shown that this configuration can be optimized to give (a) extra large fringes (over 50 degrees in diameter) over a range of wavelength, (b) a path difference nearly independent of wavelength, or (c) a path difference specified differently at two different wavelengths for observing a pair of doublets. Specific examples refer to the airglow wavelengths of 557.7, 630.0, 732.0 nm and others, and to a path difference of 4.5 cm. The properties of different glass combinations are discussed.

Rainbow schlieren vs Mach-Zehnder interferometer - A comparison

Abstract: The rainbow schlieren apparatus is simpler, cheaper, and more easily built to large scale than the interferometer. The accuracies of the two instruments are similar but only if refraction is properly accounted for in interferometry. The measurement thresholds of both instruments are similar. The rainbow schlieren device provides more detailed information because the detection threshold of the rainbow schlieren is an order of magnitude better than that of the interferometer.

Multiple aperture exit plate for field-widening a Fabry-Perot interferometer.

Abstract: Replacement of the central exit aperture of a conventional Fabry-Perot (F.P.) photoelectric interferometer with a multiple annulus exit plate provides a substantial gain in luminosity while maintaining resolving power in the study of extended sources of radiation. The design, fabrication, assembly, and testing of multiple aperture exit plates (MAEPs) which pass 6 interference orders via a central aperture and five concentric annular openings are described. The measured luminosity gains and inferred aperture finesses of MAEPs used with 150- and 100-mm aperture F.P.s are quite close to the design values of 6 and 14.5, respectively.

Conversion of optical path length to frequency by an interferometer using photorefractive oscillation

Abstract: Frequency detuning effects in photorefractive oscillators are used in a new type of (passive) interferometry which converts optical path length changes to frequency shifts. Such an interferometer is potentially more accurate than conventional interferometers which convert optical path length changes to phase or intensity changes.

1‐millimeter wave interferometer for the measurement of line integral electron density on TFTR

Abstract: A two‐pass interferometer at 285 GHz has been developed to measure the line‐integrated electron density on the horizontal midplane of the toroidal fusion test reactor (TFTR). Presently, the interferometer employs a 2‐mW solid‐state source to supply the launch wave and a 2‐mm klystron oscillator and, a harmonic mixer to provide a superheterodyne front end. The transmission system consists of 25 m of C‐band rectangular waveguide, adjustable miter bends, and a spherical mirror in the vacuum vessel with a total round trip transmission loss of 21 dB. The interferometer signal to noise is ≳50 dB. Utilization of a feed forward tracking system provides long‐term stable operation. The interferometer routinely provides real‐time feedback control for the gas injection system and a permissive for neutral beam operation.

A high-frequency GaAs optical guided-wave electrooptic interferometric modulator

Abstract: The characteristics and frequency response of a GaAs monolithic guided-wave interferometric modulator operating at 1.3 μm are presented. The interfetometer consists of three-guide coupler input and output sections and single-mode p+-n--n+ slab-coupled rib-waveguide active arms. The measured electrical bandwidth of an interferometer with 2 mm long active arms was 2.2 GHz and limited by parasitics. This corresponds to a small signal optical bandwidth of ≈ 3.0 GHz when the interferometer is biased in a linear portion of the optical output versus voltage characteristics. Reduction of parasitics should result in a substantial increase in the bandwidth of these devices.

Calibration Of Surface Roughness Transducers At Angstrom Levels Using X-Ray Interferometry

Abstract: An x-ray interferometer for calibration of microdisplacement transducers at sub nm levels is under development. Using the lattice spacing of high perfection semiconductor industry grade silicon as a reference, the objective is to produce a portable, absolute length standard with a precision of 1 part in 108 traceable to primary standards. The interferometer is of the Hart design and fabricated from a monolith of single crystal silicon. The triple-blade construction utilises Lauecase diffraction in beam splitter, mirror and analyser and is capable of an ultimate resolution of about 10 pm. The system is now operational, and the first trials, reported in this paper, have been entirely successful.

Microwave interferometer for shock wave, detonation, and material motion measurements

Abstract: A microwave interferometer system which provides a continuous measurement of the position of interfaces, such as shock fronts, detonation fronts, or material surfaces, has been developed. The use of low‐mass microcoaxial cable, some of which is commercially available, and stripline for conducting the microwave signal into the interior of an experiment makes the perturbation of the measuring system on the experiment small. Other microwave systems have measured the time of peak amplitude of a reflected signal, but the present system with proper matching produces a continuous record of position as a function of time. It is believed that this technique can be used to replace most uses of electrical contact or fiber‐optic pins in high‐explosive experiments. The amount of data far exceeds that which it is possible to obtain from pins or optical fibers. The matching of the microcoax to the microwave generator has an important effect on the accuracy of the technique. The effect of matching on accuracy will be des...

Single-longitudinal-mode operation of a TEA CO 2 laser with a modified Fabry-Perot interferometer

Abstract: An interferometric device consisting of two metallic mirrors and a dielectric beam splitter of low reflectivity is used to reduce the spectral emission of a CO2 TEA laser to a single longitudinal mode (SLM). The finesse of the interferometer can be varied continuously by turning the polarization of the laser beam or in steps by using different beam splitters. The resonant reflectivity of the device was found to vary from 20 to 90% depending on transverse mode matching and alignment. With a low reflectivity beam splitter, the reliability of producing SLM pulses reached 100%. As the beam splitter reflectivity was increased, the interferometer finesse decreased resulting in a low reliability of producing SLM pulses. The maximum free spectral range which gave SLM reliably was 5 GHz. We also replaced one of the mirrors of the interferometer by a grating, and we observed SLM output on forty lines of the CO2 spectrum with a reproducibility of 80% or better. We discuss how the device could be used for intracavity pulse selection.

Fiber-optic rotation rate sensor having dual interferometer loops

Abstract: A fiber optic rotation rate sensor employs two fiber loop interferometers. Two Sagnac fiber optic interferometers are interleaved such that both fibers are contained in each each interferometer. A coherent light beam at one frequency is proprogated in only one of the fibers, and an opticl beam at another frequency is proprogated in the other fiber. The beam emerging from one of the fibers is combined with the beam emerging from the other of the fibers for each of the two ends of each fiber to produce a pair of interference signals each having a frequency equal to the difference between the two light beam frequencies. The relative phase of these two signals is then determined to provide an indication of the rate of angular rotation of the fiber optic loops, and this determination is independent of any temperature-related characteristics that produce offset phase errors.

Beam splitter for a wide-angle Michelson Doppler imaging interferometer.

Abstract: The design and construction of a hexagonal interferometer beam splitting prism is described in which the light falls onto the all-dielectric partially reflecting coating at an angle of 30° with respect to the normal. For a beam that undergoes one transmission and one reflection by the device, the average effective transmittance ½(T‖R‖ +T⊥R⊥) in the 0.55–0.75-μm spectral region is ∼0.23. This quantity is not strongly dependent on the plane of polarization of the incident light, nor on small changes of the angle of incidence from the design angle of 30°. Ways to improve further the performance of the beam splitter are indicated. Coating designs for other angles of incidence are also given.