Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-08-10
2002-11-19
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S493000, C356S498000
Reexamination Certificate
active
06483593
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to methods and apparatus for determining the displacement or location of a target and, more particularly, to heterodyne interferometers and associated interferometric methods.
BACKGROUND OF THE INVENTION
Heterodyne interferometers are utilized in a variety of commercial and noncommercial applications. For example, optical heterodyne interferometers commonly measure displacement or are used in sensors to measure force, pressure or other physical quantities that create a measurable displacement in a respective transducer.
Heterodyne interferometers include a stable optical source, such as a laser, for providing coherent optical signals. The optical source can provide signals directly to the system or the optical source can be remotely located and connected to the system via optical fibers. Regardless of the location of the optical source, conventional heterodyne interferometers require coherent signals having two different frequencies. In addition, most conventional heterodyne interferometers require that the signals having the first frequency be orthogonally polarized relative to the signals having the second frequency. These heterodyne interferometers are therefore classified as polarizing interferometers. For example, the Hewlett Packard 10715 differential interferometer is a type of polarizing heterodyne interferometer as described in an article by C. Steinmetz, et al., entitled Accuracy Analysis and Improvements to the Hewlett-Packard Laser Interferometer System, SPIE 816, Interferometric Metrology, p. 79 (1987). Similarly, the Hewlett Packard 5527 Laser Position Transducer System is another type of polarizing heterodyne interferometer as described in HP product brochure No. 5964-6190 E entitled Optics and Laser Heads for Laser-Interferometer Positioning Systems (1995).
More specifically, conventional polarizing interferometers include a first laser source for providing a first beam having a first frequency and a first linear polarization and a second laser source having a second frequency and a second linear polarization that is orthogonal to the first polarization state. A polarizing interferometer also includes reference and measurement arms as well as a polarizing beamsplitter for separating the first and second beams based upon their polarization such that one of the beams is directed to the measurement arm of the interferometer, while the other beam is directed to the reference arm of the interferometer. Upon returning from the measurement and reference arms, the first and second beams are mixed by a polarization analyzer or other polarization manipulating optical elements so as to create an interference pattern. While the reference arm typically has a fixed or predetermined length, the measurement arm has a length that is defined by the position of a target. As such, as the target is displaced, the optical length of the measurement arm is accordingly altered. By measuring the phase of the resulting fringes created by the interference of the first and second beams, however, the heterodyne interferometer permits the displacement of the target to be determined.
Unfortunately, the first and second beams that are produced by the laser sources generally do not have pure linear polarization. Thus, the first beam that is substantially polarized with the electric field parallel to the plane of incidence of the interferometer beamsplitter, P-polarized, also generally has some component of electric field perpendicular to the plane of incidence, S-polarized. Likewise, the second beam that is primarily S-polarized, also generally has some P-polarization. In addition, polarizing beamsplitters are not perfect and therefore do not completely separate signals that are orthogonally polarized. See N. Bobroff, “Recent Advances in Displacement Measuring Interferometry”, Meas. Sci. Tech. Vol. 4, pp. 907-26 (1993). As such, while a polarizing beamsplitter generally separates the beams according to their polarization state such that S-polarized signals are directed along one path and P-polarized signals are directed along another path, imperfections in conventional polarizing beamsplitters allow at least some P-polarized signals to mix with the generally S-polarized beam and, correspondingly, permit at least some S-polarized signals to mix with the generally P-polarized beam.
The mixture of the polarization states downstream of the polarizing beamsplitter is commonly termed “polarization crosstalk”. As a result of polarization crosstalk, conventional polarizing interferometers having a nonlinear error in the final phase measurement. The error is periodic with the period of one wavelength of the optical path change. Since the phases of the beams that give rise to this nonlinear error can drift, the magnitude of the error can also vary over time. In order to minimize the variations in the magnitude of the nonlinear error, some polarizing interferometers rapidly dither the reference mirror that defines the optical length of the reference arm so as to average out the periodic error. While generally effective, this technique requires the interferometer to be substantially more complex.
In addition, U.S. Pat. No. 4,693,605 to Gary E. Sommargren describes an interferometer in which mixing of the different polarization states is minimized by reducing the number of reflections in the interferometric system. While somewhat helpful, this technique appears only to reduce, but not totally solve the problems created by polarization crosstalk. In addition, an article by W. Hou, et al. entitled “Investigation and Compensation of the Nonlinearity of Heterodyne Interferometers”, Precision Engineering, Vol. 12, p. 91 (1992), describes a technique for compensating for some of the nonlinear errors in the final phase measurement. While also somewhat beneficial for reducing the nonlinear errors, this technique does not compensate for all nonlinear errors and is more complex by requiring twice the usual number of photodetectors and phase-measurement channels. As such, while several techniques have been developed for reducing the polarization crosstalk of heterodyne interferometers, these techniques do not eliminate the polarization crosstalk and typically increase the complexity of the interferometric system.
Nonpolarizing heterodyne interferometers have also been developed. By avoiding the mixing of beams of different polarization states, nonpolarizing interferometers reduce or eliminate the nonlinear errors in the final phase measurement that otherwise arise as a result of polarization crosstalk. See, for example, M. Tanaka, et al. “Linear Interpolation of Periodic Error in a Heterodyne Laser Interferometer at Subnanometer Levels”, IEEE Trans. Instrum. Meas., Vol. 38, No. 2, pp. 552-54 (April 1989); Jack A. Stone, et al., “Wavelength Shift Interferometry: Using a Dither to Improve Accuracy”, Proc. of the Eleventh Annual Meeting of the American Society for Precision Engineering,” pp. 357-62 (Nov. 9-14, 1996); and Chien-ming Wu, et al., “Heterodyne Interferometer with Subatomic Periodic Nonlinearity,” Applied Optics, Vol. 38, pp. 4089-94 (1999). Unfortunately, conventional nonpolarizing interferometers suffer from several disadvantages including increased complexity created by additional optical components that must remain accurately aligned.
In addition to problems related to polarization crosstalk, conventional heterodyne interferometers are typically limited by the requirement that the target move only in a predetermined direction of interest that the interferometer is designed to measure. As such, motion of the target in a plane orthogonal to the direction of interest typically interferes with the measurement and should be avoided. In addition, tilting of the target or the stage on which the target is mounted can adversely influence the measurement. Accordingly, conventional heterodyne interferometers generally limit the target to movement in the direction of interest and do not permit movement in a plane orthogonal to the direction of interest or
Bell John A.
Capron Barbara A.
Leep David A.
Alston & Bird LLP
Font Frank G.
Lee Andrew H
The Boeing Company
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