Showing papers on "Astronomical interferometer published in 1986"

Tools of Radio Astronomy

Abstract: Radio Astronomical Fundamentals.- Electromagnetic Wave Propagation Fundamentals.- Wave Polarization.- Signal Processing and Receivers.- Fundamentals of Antenna Theory.- Filled Aperture Antennas.- Observational Methods.- Interferometers and Aperture Synthesis.- Emission Mechanisms of Continuous Radiation.- Some Examples of Thermal and Nonthermal Radio Sources.- Spectral Line Fundamentals.- Line Radiation of Neutral Hydrogen.- Recombination Lines.- Molecules in Interstellar Space.

Distance measurement by the wavelength shift of laser diode light

Abstract: This paper proposes a method for measuring distance larger than the wavelength of light with an interferometer using a laser diode. This method uses the fact that the wavelength of the emitted light of a laser diode varies in proportion to the diode’s injection current. The phase difference between the two interfering beams varies due to the sinusoidal variation of wavelength. The variation of the phase difference is detected by the optical heterodyne method. The magnitude of the variation is proportional to the measuring distance and the light wavelength shift. If the wavelength shift is known, a distance larger than the wavelength can be obtained from measurement of the phase variation. This method is a kind of multiwavelength interferometry using a single light source. We have done some fundamental experiments with this method and have confirmed its applicability to practical applications.

Laser interferometer measurements of refractive index in shock-compressed materials

Abstract: Laser interferometer systems provide a means for probing the refractive index of transparent specimens subjected to dynamic compression. Previous interferometer measurements of optical properties under shock loading are reviewed for polymethyl methacrylate, fused silica, sapphire, nitromethane, and an aqueous solution of zinc chloride; various degrees of departure from Gladstone-Dale behavior are noted for these materials. In addition, a detailed summary of recent optical studies of lithium fluoride (LiF) is provided. Interferometer data from plate-impact experiments verify sustained LiF transparency for Hugoniot stresses to at least 160 GPa, and establish the variation of LiF refractive index for shock amplitudes ranging from 1.58 to 115 GPa. The refractive-index data for LiF agree with earlier static and shock-wave data, and exhibit a pronounced deviation from predictions based on the Gladstone-Dale, Lorentz-Lorenz, and Drude relations. A modified form of the Gladstone-Dale relation is presented which correctly models the latest LiF measurements. Potential applications of LiF and other window materials to dynamic high-pressure experimentation are discussed.

Parallel image subtraction using a phase-conjugate Michelson interferometer.

Abstract: A phase-conjugate Michelson interferometer using an internally self-pumped barium titanate crystal as reflectors has been constructed to perform parallel image subtraction, intensity inversion, and exclusive or logic operation. These operations are independent of the optical path differences and phase aberration.

Compression of femtosecond optical pulses with dielectric multilayer interferometers

Abstract: The group velocity dispersion of a multilayer thin film Gires-Tournois interferometer used for reflection of ultrashort optical pulses can be continuously tuned from positive to negative values at an extremely low loss in pulse energy. Thus, this device can be applied for compression of femtosecond pulses independent of the sign of the frequency chirp by simple angle tuning of the interferometer. This has been demonstrated with up-chirped 210 fs pulses which have been compressed to an almost transform-limited duration of 115 fs.

Optical ranging by wavelength multiplexed interferometry

Abstract: A new optical technique is described for measurement of absolute distance. The approach is based upon a wavelength multiplexed heterodyne interferometer with FM demodulation. By temporally multiplexing discrete wavelengths in a heterodyne interferometer, a complete elimination of interferometric range ambiguity can be achieved while maintaining the high range sensitivity and resolution of interferometry. The basic theory is presented and an algorithm is described for measurement of range over meter distances with submicrometer resolution. The experimental implementation of the wavelength multiplexed interferometer is described and ranging results with 2 μm resolution from 20 cm are presented. A scanned three‐dimensional map of a surface contour with 3‐mm topography is also presented.

Quadrature outputs from fiber interferometer with 4 × 4 coupler

Abstract: A broad stripe waveguide of width W and length L acts as a self-imaging 4 x 4 directional coupler if L = NW(2)/lambda, where N is the effective index. When the arms of a fiber-optic Mach-Zehnder interferometer are connected to two input ports of this coupler, the powers at the four output ports vary in phase quadrature.

Internally coupled Fabry-Perot interferometer for high precision wavelength control of tunable diode lasers.

Abstract: An internally coupled confocal Fabry-Perot interferometer (FPI) has been developed for both high precision wavelength calibration and stabilization of tunable diode lasers (TDL). Our FPI is tunable and thermally stable and works over a large wavelength range (0.6–30 μm)—characteristics that cannot be simultaneously realized with conventional etalons. As part of a versatile wavelength control system the instrument has already considerably improved the quality of our diode laser spectra and will facilitate the use of TDLs in sub-Doppler spectroscopy and as local oscillators in heterodyne radiometers.

Electro-optical beamforming network for phased array antennas

Abstract: A fiber optic device 50 designed to steer the radiation beam of a phased-array antenna 10 has been demonstrated. A radio frequency (RF) signal is generated via photomixing at the output of a single-mode fiber optic interferometer. The phase of the electrical signal is shifted over several cycles in direct proportion to a voltage applied to an optical modulator 34, 60. The modulator consists of a Pockels-type optical phase modulator located in one arm of the heterodyne interferometer. Rapid changes in RF phase are feasible. A miniature low-voltage version of the device 50, 72, based upon integrated optics, has been devised.

Distortion free interferograms in Fourier transform spectroscopy with nonlinear detectors.

Abstract: A new method for removing the most important distortions from Fourier transform spectra recorded with nonlinear detectors is reported and illustrated with practical examples. The removal of these distortions is obtained with a two-output interferometer configuration. This essentially works because the resulting interferogram is made up of the difference between the signals of the two detectors and because these signals have the same amplitude and opposite phases. The proposed method leaves the detectors unchanged and makes use of their distorted signals. The correction is not restricted to the nonlinear effects of the only detectors but also of the entire electronic system amplifying each detector signal. It easily applies to absorption, emission, low and high resolution spectra, as well as spectra of short and long lifetime phenomena. The benefit of the method is not restricted to Fourier transform spectroscopy. It is even not restricted to techniques making use of detectors. Its advantages are available for any data treatment where the important information is made up of the difference between nonlinear functions (or signals) having similar amplitudes and opposite phases, whatever the number of pairs of such distorted functions (or signals) may be.

Precise measurements of optical dispersion using a new interferometric technique.

Abstract: Measurements of the dispersion of the refractive index using a nonlinear interferometer are described. A sample of the optical material to be measured is interposed between two optically nonlinear crystals, and a moderately intense laser is passed through the combination. By observing the interference between the second harmonics produced in the two nonlinear crystals, the difference between the refractive indices of the sample at the laser frequency and its second harmonic frequency can be determined very precisely. We have used the interferometer to measure the dispersion in several gases between 1064 and 532 nm. We have also used it to determine the dispersion of two transparent solids. The method can measure the index difference to better than 0.1%, which for gases such as helium represents an absolute accuracy of ∼10−9.

Coherent electromagnetic energy emitter locator

Abstract: An emitter location system includes a single carrier based first and second short baseline interferometers, and a long baseline interferometer operatively connected to a computing means. The computing means includes a three level processor. The short baseline interferometers provide phase measurements for level one combination into total phase measurements and estimates of the angle of incidence of the incoming electromagnetic energy. After test for acceptance, the estimated angle of incidence is passed for level two processing which includes the simultaneous processing of the estimated incident angle with the phase measurement of the long baseline interferometer for error correction to provide an improved estimated incident angle. After test for acceptance, the improved angle of incidence measurement is passed for determining the angle of the incident wave for level three processing. Level three processing utilizes the bearing estimates and the carrier position and heading information for an inertial navigation system to relate the information to a fixed coordinate system. Then the location of the emitter is determined using least square method.

Field Of View Considerations For Telescope Arrays

Abstract: Often it is desirable to have a large field of view when using telescope arrays for interferometric imaging. I examine for different types of arrays what the limitation of the field of view is and what can he done to optimize it.

Synthetic interferometric distance-measuring system using a CO2 laser.

Abstract: A new synthetic interferometric distance-measuring (SIDM) system is described using two-wavelength simultaneous (synthetic) interferometry with a CO2 laser. The measuring procedure of the SIDM system is based on phase measurement such as with conventional light-modulation distance-measuring instruments. The SIDM system is excellent in length resolution since it uses the synthetic wavelengths of 5.5 mm and 300 μm and a regular single wavelength, interferometrically. It is applicable to the measurement of distances up to 100 m with an accuracy of 1 × 10−7. Its performance is confirmed by comparing it with an HP laser interferometer using a 50-m movable carriage in the NRLM 300-m optical tunnel.

Semiconductor wafer and technical flat planeness testing interferometer

Abstract: An interferometer for planeness testing of technical surfaces is described and real-time interferometric evaluations of 10-cm (4-in.) diam silicon wafers were carried out. The interferometer, using the principles of phase sampling interferometry, is a combination of an oblique incidence Fizeau interferometer with a moire interferometer. The interference pattern is a pure two-beam pattern. To get low frequency moire fringes the live interference fringes must have about the same spatial frequency as the moving grating used for reference phase stepping. Assuming that an automatic alignment device is included in the design, up to three wafers per minute could be measured and classified.

Reduction of radiation-pressure-induced fluctuations in interferometric gravity-wave detectors

Abstract: The performance of an interferometric gravity-wave detector is analyzed, treating both the motion of the mirrors and the optical fields quantum mechanically, in continuous time. Kerr cells in each arm of the interferometer are used to counter the effects of radiation-pressure fluctuations, a feedback loop is used to keep the interferometer operating at the proper null, and a large mean-field assumption is used to linearize the analysis. When narrow-band disturbances are being measured, the performance of this interferometer can greatly surpass the standard quantum limit. When the Kerr cells are removed, the interferometer operates at the standard quantum limit, and this analysis agrees with earlier studies.

Automatic Gauge Block Measurement Using Multiple Wavelength Interferometry

Abstract: A new multiple wavelength interferometer makes use of stabilised helium-neon and argon ion lasers to measure the length of engineer's gauge blocks. The combination of television and computing techniques to analyse the resulting interferogram makes the instrument less dependent on operator skills and is able to provide data about the gauge surface.

Method and apparatus for processing analog optical wave signals

Abstract: Discrete segments of an analog optical wave signal are collected in individual waveguides of a waveguide array. In the waveguide array, the optical signal is processed to provide an analysis of the signal or to control the signal. The signal can be analyzed by Mach-Zehnder interferometers which can provide an indication of the distance of a subject from the array. By controlling the dielectric constants of the waveguides electronically, the array can serve as an aberration free focusing lens or as an aberration compensation lens. Rapid electronic control of the focal length of the lens makes the lens particularly applicable to automatic focusing systems, systems which view only objects within a selected range of distances from the array, and the compiling of in focus segments to form a wide depth of field image.

A dual interferometer implemented in parallel on a single birefringent monomode optical fibre

Abstract: An interferometric optical fibre sensor has been constructed employing a highly birefringent monomode optical fibre sensing element. The sensing element is configured such that two separate division-of-amplitude interferometers are effectively realised within a single fibre, each corresponding to one of the orthogonal polarisation eigenmodes. Both the optical phase change of each interferometer and the differential optical phase change induced by a physical change within the fibre may be separately recovered. The high resolution of the conventional interferometric sensor is therefore maintained with a greatly increased unambiguous dynamic range corresponding to the period of the differential phase change. The system has been demonstrated as a temperature sensor.

Optically Multiplexed Interferometric Fiber Optic Sensor System

Abstract: A sensor system where a number of Fabry-Perot interferometer sensors are placed in series on a single-mode fiber is discussed. Each Fabry-Perot sensor is resonant within a unique bandpass of optical wavelengths and transmits with small attenuation at all other wavelengths. Fiber Fabry-Perot interferometers with narrow-band resonances are formed by placing a matched pair of bandpass reflectors on the fiber. The sensor system has three major subsystems: a swept frequency laser, frequency selective Fabry-Perot sensors, and demodulation hardware. A grating formed by holographic process is placed in the evanescent field to produce a frequency selective reflector. Individual sensors are polled by a frequency swept laser. As the laser wavelength is scanned over the resonance bandpass of the first sensor, Fabry-Perot fringes from the first sensor are read out. As the laser continues to sweep its optical frequency, the first sensor's gratings no longer act as reflectors; the sensor becomes transparent to the optical probe. After a sufficient optical wavelength guard band, the resonance band of the second sensor is reached and fringes from the second sensor are read out. Fabry-Perot phase and fringe order information are recovered by a combination of optical and electronic signal processing. The optical processor is a second Fabry-Perot interferometer which is used as an analyzer to aid recovery of the phase information.

Monopulse Resolution of Interferometric Ambiguities

Abstract: Ambiguities in interferometers with high angular accuracy must be resolved to achieve a practical system design. A new technique for ambiguity resolution is described and is based on monopulse circuitry used with the interferometric elements. The overall angular accuracy of the system is achieved by the interferometer; the angular accuracy of the monopulse subsystem is used to resolve interferometric ambiguities. An expression for the probability of correct ambiguity resolution is derived as a function of element size and monopulse accuracy which indicates that high probability of ambiguity resolution results when the size of the interferometric elements are a fraction of the interferometric baseline. Finally, a comparison between conventional monopulse and interferometric system designs is made for the three principal parameters, signal sensitivity, angular accuracy, and field of view, that dictate the appropriate choice for a particular application. Interferometric systems are more appropriate than monopulse systems for those applications in which angular accuracy and field of view are more important than signal sensitivity.

Exact evaluation of the transmitted amplitude for a Fabry-Perot interferometer with surface defects

Abstract: An exact easy-to-compute formula has been derived for the transmitted amplitude of a Fabry-Perot interferometer for various surface irregularities. Some results for various surface defects obtained with this have been compared with the results of a Monte Carlo calculation which simply calculates the transmitted intensity as an incoherent sum from a distribution of elemental interferometers. Out of three possible defect distributions, namely, a random distribution of surface defects, a small curvature, and a small inclination, the random distribution is found to affect the line shape in a more severe way.

Fabry-Perot interferometers with three mirrors.

Abstract: The properties of two-mirror Fabry-Perot interferometers can be improved by adding one or more partially reflecting mirrors. In this paper we show how three-mirror Fabry-Perot interferometers provide better control on the shape of the transmission band, the bandwidth, and the suppression of sidebands. We present measurements on three-mirror Fabry-Perot interferometers in the submillimeter wavelength region for λ = 0.64 mm.

Multichannel Grating Phase-Shift Interferometers

Abstract: When a beam is focused into grating elements, it is diffracted and phase-shifted. These fundamental characteristics of gratings have been found very useful for converting conventional interferometers into real-time wavefront sensors for digital computer control and data analysis. The principle of phase-shifting interferometry has been combined with above characteristics of the gratings to produce instantaneous multichannel interferograms. Attempts have been made successfully to apply these principles to three well-known interferometers, (1) the point-diffraction interferometer, (2) the radial-shear interferometer, and (3) the double-frequency crossed-grating lateral-shear interferometer. The advantages of these new approaches are simplicity and versatility, in particular, these wavefront sensors can measure wavefronts from both continuous and pulsed sources. Optical principles and the laboratory results will be shown.

Test And Evaluation Of The Hubble Space Telescope 2.4-meter Primary Mirror

Abstract: Manufacture of the 2.4-meter diffraction-limited hyperboloidal primary mirror for the Hubble Space Telescope encompassed several state-of-the-art processes. These included computer-controlled polishing and interferometric test and evaluation. To advance the manufacturing processes to the 0.008μm rms surface figure level achieved on the primary mirror, full-aperture and sub-aperture interferometric test configurations were developed in conjunction with a precision interferogram analysis facility. Full-aperture interferometric testing of the uncoated mirror took place in air with the test chamber providing the necessary thermal and vibrational stability. A remotely-controlled, six-degree-of-freedom table was used to orient and position the mirror with respect to the fixed geometry metrology unit consisting of a Co-axial Reference Interferometer (CORI) and a reflective null corrector. The CORI generated a high quality spherical wavefront which, in turn, was converted to a hyperboloidal wavefront characteristic of the desired mirror surface by the reflective null corrector. Interferograms in the form of photographic negatives recorded the departures from the highly corrected null condition to provide surface contour and aberration data. The high spatial frequency surface defects, so important to the ultraviolet (UV) performance of the Space Telescope, were measured via sub-aperture interferometry. A laser interferoscope and precision test plates with reference surface radii appropriate to several zones on the primary mirror were employed to produce high fidelity measurements of "mid-frequency" defects at the 0.002μm rms level.

Integrated optic temperature and humidity sensors

Abstract: Temperature sensor of the integrated optic structure was constructed using a Michelson interferometer configuration with an asymmetric X junction and a grooved reflector on a LiNbO 3 substrate. With this structure, larger difference in the optical paths lengths for two beams is provided to realize higher sensitivity. Also a humidity sensor of the optical guided-wave structure is proposed and its first known demonstration is reported.

The Long Wavelength Spectrometer (LWS) for ISO

Abstract: A spectroscopic instrument for the ESA Infrared Space Observatory is described, covering the wavelength range 45 to 180 microns and with operating modes offering a choice of two resolving powers. The lower resolving power of around 230 is given by a diffraction grating system, with the spectrum recorded by a line of 10 detectors. The high resolving power of around 1.5E4 is achieved by switching a Fabry-Perot (F-P) interferometer into the parallel part of the instrument beam. Either of two F-Ps can be selected, one optimised for the 45 to 90 micron wavelength range and the other for 90 to 180 microns. The operating modes offered include differential measurement, using wavelength switching and synchronous detection, and absolute measurement with an internal chopper/reference. Development aspects of the instrument are also discussed.

Radio-astronomy precedent for optical interferometer imaging.

Abstract: Radio-astronomical correlator arrays have demonstrated the ability of producing images of cosmic radio sources with extremely high resolution, dynamic range, and precision. A large body of experience and theory has been developed in connection with this technology that may form useful precedents for the development of similar technology in the optical and infrared regions of the electromagnetic spectrum. The production of images of sources from measurements of their mutual coherence functions is demonstrated, and the capabilities and limitations of the technique are discussed. A prototype optical instrument is proposed, and its advantages and liabilities are identified.

Lens collimation and testing using a Twyman-Green interferometer with a self-pumped phase-conjugating mirror

Abstract: Ordinarily Twyman-Green interferometers are employed testing optical elements. In a modification of the basic configuration, the ordinary mirror in the test arm is replaced with a self-pumped phase-conjugating mirror using a barium titanate crystal. It is shown that, with a redefinition of components, the new configuration permits retention and improvement of the optical element testing function while simultaneously serving as a sensitive test for collimation. The optical path difference resulting from the double pass in the original Twyman-Green interferometer approximately equals that of the single pass and phase conjugation in the modification.