Fringe Counting System Research Articles

Fringe counting method for synthetic phase with frequency-modulated laser diodes

Fringe counting method with laser diodes (LDs) for displacement measurement has been constructed. Two LDs are frequency modulated by mutually inverted sawtooth currents on an unbalanced two-beam interferometer. The mutually inverted sawtooth-current modulation of LDs produces interference fringe signals with opposite signs for respective wavelengths. The two fringe signals are fed to an electronic mixer to produce a synthetic fringe signal with a reduced sensitivity to the synthetic wavelength. Synthetic fringe pulses derived from the synthetic fringe signal make a fringe counting system possible for faster movement of the tested mirror.

FIR Laser Tangential Interferometry/Polarimetry on NSTX

A tangential far infrared interferometer/polarimeter system has been upgraded for the measurement of two-dimensional electron density and toroidal magnetic field profiles on the National Spherical Torus Experiment (NSTX). The system employs a three-wavelength configuration that simultaneously produces interferometric phase shift and Faraday rotation polarimetric data from a single detector for each channel. Three CH/sub 3/OH lasers (/spl lambda/=119 /spl mu/m) including Stark-tuned laser cavities are optically pumped by a CO/sub 2/ laser tuned to 9.7 /spl mu/m. For high time-resolution measurements, the modulation frequency in the heterodyne interferometer is operated at /spl sim/ 7 MHz via the Stark-tuned laser, serving as the local oscillator in this system. Unwanted phase changes introduced by mechanical vibrations originating from various sources were eliminated by extensive work including a specially designed vibration-free stand for the retroreflector. The system is designed to have a full remote control capacity using the LABVIEW program via fiber links and real time signal processing circuits with a tracking receiver technique. A high IF frequency and a low noise fringe counting system enabled the system to measure wide-band density fluctuations such as microturbulence and magnetohydrodynamics. In this paper, a comparison study of the measured electron density with Thomson scattering data and para/diamagnetism effects measured via polarimetry signal and compared with the calculated value using the EFIT equilibrium code are presented. A recent upgrade plan together with the technical details of the system development will also be described.

An interferometer system for ultraviolet and visible Fourier transform spectrometry

An interferometer system has been designed and built specifically for the implementation of Fourier transform spectroscopy in the ultraviolet and visible spectral regions. The moving mirror is attached to a rectangular air-bearing/permanent magnet assembly. Fixed, twin coils provide the force by which the mirror is translated. A unique two-channel zero-force servo system has been developed to maintain a constant mirror velocity on both the forward and backward strokes of the moving mirror. The travel of the moving mirror is automatically controlled to be symmetrically centered about the position of zero path difference. The drive length is precisely presettable and is controlled utilizing an absolute reference laser fringe counting system. The maximum travel is 1.04 cm on either side of the position of zero path difference. A full stroke drive is equivalent to 65,536 cycles of the 1 HeNe reference laser-fringe ugnai and one stroke takes 6.5 s. The mirror automatically cycles back and forth enabling the signal averaging of repetitively generated inlerferograms. Oe-aliased spectra can be measured using a frequency multiplied version of the reference laser fringe. A phase-locked loop-circuit provides data acquisition clocking signals with rates of four times and eight times that of the basic laser-fringe signal. All data acquisition and processing is carried out using a Macintosh II computer.

Laser interferometric system for displacement measurement with high precision

A practical method for measuring displacement with a newly developed laser interferometric system is proposed. The new system has a simplified optical layout, fewer optical elements, and a shortened pass. As a result, the thermal drift and non-linearity of the displacement measurement have been decreased in both the bi-frequency heterodyne system and the fringe-counting system. In addition, simplified electrical fringe counting logic has shortened the time needed to obtain the measured value without losing accuracy. This method achieved (i) sub-nanometer resolution (0.6 nm/LSB), (ii) high stability (2.5 nm/day), (iii) high linearity (less than 1 LSB), and (iv) high following speed (more than 1000mm s-1).

Fringe counting system for control of a Fabry-Perot spectrometer

We have developed optical measurement and control techniques that allow stable, precise adjustment and tracking of Fabry-Perot plate parallelism and spacing over a distance of many fringes with a precision of 20 nm. This extension of the range of optical position sensing has proved useful in controlling a cryogenic, electromagnetically driven, infrared spectrometer for astronomy.

Fourier emission infrared microspectrophotometer for surface analysis—I. Application to lubrication problems

A Beckman-RIIC FS-720 far-infrared interferometer was converted into an emission microspectrophotometer for surface analysis. To cover the mid-infrared as well as the far-infrared the Mylar beamsplitter was made replaceable by a germanium-coated salt plate, and the Moiré fringe counting system used to locate the moveable Michelson mirror was improved to read 0.5 μm of mirror displacement. Digital electronics and a dedicated minicomputer were installed for data collection and processing. The most critical element for the recording of weak emission spectra from small areas was, however, a reflecting microscope objective and phase-locked signal detection with simultaneous referencing to a blackbody source. An application of the technique to lubrication problems is shown.

The stress-strain behavior of the corneoscleral envelope of the eye I. Development of a system for making in vivo measurements using optical interferometry

A system is described which is capable of measuring small changes in corneoscleral dimensions of eyes under varying conditions of intraocular pressure stress in vivo. The system consists of an optical interferometer, a fringe counting system, and an automatic recording system for digital data storage. The light source for the interferometer is a helium-neon laser. Corner cube reflectors serve as mirrors in both arms of the interferometer making optical alignment virtually automatic and eliminating problems associated with small-angle ocular rotations. One corner cube is mounted on an acrylic corneal contact lens, the combined weight being less than 100 mg. This assembly is glued to the cornea of an experimental animal and serves as the spatial reference for the anterior surface of the eye. The skull of the anesthetized animal, rigidly fixed in a special holding apparatus, serves as the posterior reference. The fringe counting system is capable of tracking the position of the apex cornea to ±0·316 μm at velocities up to 600 μm/sec. Relative position of the corneal apex is recorded and stored in digital format for later computer analysis. The technique of using the shift in the position of the corneal apex to estimate changes in intraocular volume during pressure stress is discussed.

Some Aspects of Fringe Counting in Laser Interferometers

A number of arrangements are possible for deriving signals in phase quadrature from interferometers, so that bi-directional counting of the interference fringe movements can be carried out. Arrangements which avoid splitting the image field are preferred for laser light sources, and can give a better signal-to-noise ratio. In environments where the interferometer is subjected to vibration, the fringe counting system may be required to handle rapid reversals in the direction of movement. In these circumstances the rate of production of rapid counting signals of alternate direction may be considerably reduced by using an appropriate logic system between the interferometer and the counter.