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A prototype heterodyne stellar interferometer has been built in order to demonstrate the feasibility of heterodyne techniques in measuring angular diameters of bright infrared stars. The first system tests were performed in December 1972. Attention is given to investigations concerning the possibility that optical air turbulence within the structure of the solar telescope employed can possibly destroy the phase coherence of the fringe signals.
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A compact, high-resolution angle measurement instrument has been developed that is based on a heterodyne interferometer. The common-path heterodyne interferometer metrology is used to measure displacements of a reflective target surface. In the interferometer setup, an optical mask is used to sample the measurement laser beam reflecting back from a target surface. Angular rotations, around two orthogonal axes in a plane perpendicular to the measurement- beam propagation direction, are determined simultaneously from the relative displacement measurement of the target surface. The device is used in a tracking telescope system where pitch and yaw measurements of a flat mirror were simultaneously performed with a sensitivity of 0.1 nrad, per second, and a measuring range of 0.15 mrad at a working distance of an order of a meter. The nonlinearity of the device is also measured less than one percent over the measurement range.
An improved method and apparatus have been devised for measuring cyclic errors in the readouts of laser heterodyne interferometers that are configured and operated as displacement gauges. The cyclic errors arise as a consequence of mixing of spurious optical and electrical signals in beam launchers that are subsystems of such interferometers. The conventional approach to measurement of cyclic error involves phase measurements and yields values precise to within about 10 pm over air optical paths at laser wavelengths in the visible and near infrared. The present approach, which involves amplitude measurements instead of phase measurements, yields values precise to about .0.1 microns . about 100 times the precision of the conventional approach. In a displacement gauge of the type of interest here, the laser heterodyne interferometer is used to measure any change in distance along an optical axis between two corner-cube retroreflectors. One of the corner-cube retroreflectors is mounted on a piezoelectric transducer (see figure), which is used to introduce a low-frequency periodic displacement that can be measured by the gauges. The transducer is excited at a frequency of 9 Hz by a triangular waveform to generate a 9-Hz triangular-wave displacement having an amplitude of 25 microns. The displacement gives rise to both amplitude and phase modulation of the heterodyne signals in the gauges. The modulation includes cyclic error components, and the magnitude of the cyclic-error component of the phase modulation is what one needs to measure in order to determine the magnitude of the cyclic displacement error. The precision attainable in the conventional (phase measurement) approach to measuring cyclic error is limited because the phase measurements are af-
A need has arisen for efficient, blazed, symmetric gratings for use as beam splitters in far and extreme ultraviolet interferometers. In particular, the development of an all-reflection, far ultraviolet spatial heterodyne interferometer can benefit tremendously from such a grating. To fulfill this need, we have manufactured a mechanically ruled grating with a V-groove profile blazed for H Lyman-alpha at 1216 A. We present the grating performance at Lyman-alpha in the context of its application to the spatial heterodyne interferometer.
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Electron and metastable density measurements in argon discharge tube using laser heterodyne interferometer
The present invention relates to a heterodyne interferometer system with a pre-processing of the target signal to isolate and remove self-interference signals using a known phase modulation of the carrier signal's frequency. Where self- interference signals do not include a time delay inherent in the target signal that travels to the target reflector, by selecting a modulation frequency tuned to the time delay and then filtering the resultant signal the target beam can be isolated and the self-interference signal can be effectively removed The system includes a modulation unit to apply a phase modulation to the carrier signal, and a mixing unit that demodulates the target signal at the modulation frequency to isolate the target beam.
The Digital Phase Meter is based on a modified phase-locked loop. When phase alignment between the reference input and the phase-shifted metrological input is achieved, the loop locks and the phase shift of the digital phase shifter equals the phase difference that one seeks to measure. This digital phase meter is being developed for incorporation into a laser heterodyne interferometer in a metrological apparatus, but could also be adapted to other uses. Relative to prior phase meters of similar capability, including digital ones, this digital phase meter is smaller, less complex, and less expensive. The phase meter has been constructed and tested in the form of a field-programmable gate array (FPGA).
Requirements for spaceflight optical instruments usually dictate that for the structures be rigid, lightweight, and thermally stable. In addition, for interferometric far ultraviolet (FUV) spectrometers, the requirements for torsional deflection are more severe than with conventional spectrometers. To meet the challenge for rigid and lightweight optical instruments, this paper explores the design of a high-stiffness structure for the support of an FUV spatial heterodyne interferometer where the torsional deflection of the instrument is on the order of 10 arc seconds. The structure is based on use of a thin, hollow section beam with weight-relieving between optical elements. The design also uses a modular and self-contained positioning mechanism that is removed after final optical alignment. Several specific material properties are presented as criteria for material selection. The parameters which affect the particular design requirements are identified with respect to the desired material properties and physical design features. Although large thin sections are susceptible to thermal gradients, this could be minimized by a trade-off for weight, where adequate margin exists. This paper describes the preliminary design for the structure and presents an analysis to verify compliance with the requirements.
Spaceflight optical instruments have two conflicting requirements. They need to be both rigid and lightweight. In addition, for interferometric far ultraviolet spectrometers, the requirements for precision positioning are more severe than for conventional spectrometers. To meet the challenge of lightweight optical instruments, a modular adjustment mechanism was developed to position two orthogonal axes of a universal three-axis gimbal support system with a positioning accuracy on the order of 10 arc seconds. The mechanism was designed as a self-contained assembly which can be removed after final alignment of a spacebound optical instrument to reduce its in-flight mass. To demonstrate the concept, a number of these assemblies were made and mounted on two of the positioning axes of a far ultraviolet spatial heterodyne interferometer. A shaft clamp was used on each positioning axis to retain the adjusted position. This paper describes the design of the mechanism and presents optical test results.
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A document discusses a component of a laser metrology system designed to measure displacements along the line of sight with precision on the order of a tenth the diameter of an atom. This component, the phasemeter, measures the relative phase of two electrical signals and transfers that information to a computer. Because the metrology system measures the differences between two optical paths, the phasemeter has two inputs, called measure and reference. The reference signal is nominally a perfect square wave with a 50- percent duty cycle (though only rising edges are used). As the metrology system detects motion, the difference between the reference and measure signal phases is proportional to the displacement of the motion. The phasemeter, therefore, counts the elapsed time between rising edges in the two signals, and converts the time into an estimate of phase delay. The hardware consists of a circuit board that plugs into a COTS (commercial, off-the- shelf) Spartan-III FPGA (field-programmable gate array) evaluation board. It has two BNC inputs, (reference and measure), a CMOS logic chip to buffer the inputs, and an Ethernet jack for transmitting reduced-data to a PC. Two extra BNC connectors can be attached for future expandability, such as external synchronization. Each phasemeter handles one metrology channel. A bank of six phasemeters (and two zero-crossing detector cards) with an Ethernet switch can monitor the rigid body motion of an object. This device is smaller and cheaper than existing zero-crossing phasemeters. Also, because it uses Ethernet for communication with a computer, instead of a VME bridge, it is much easier to use. The phasemeter is a key part of the Precision Deployable Apertures and Structures strategic R&D effort to design large, deployable, segmented space telescopes.
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