Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1994-05-06
1996-08-13
Turner, Samuel A.
Optics: measuring and testing
By particle light scattering
With photocell detection
356346, G01B 902
Patent
active
055461840
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to optical measuring instruments and, in particular to a bidirectional fringe-counting two-beam interferometer system.
Stimulated by the development of the 633 nm helium-neon laser in the early 1960's with its intense collimated beam and narrow bandwidth, interferometric length measurement has become one of the most widely implemented techniques of precision measurement. It is used for measurements of length ranging from less than a micrometer, to distances of tens of meters. For the highest accuracy (approaching 1 part in 10.sup.9, the laser source must be stabilized in frequency and calibrated by comparison with a reference laser and, where the measurement application is in the free atmosphere, it is essential to continuously to correct the wavelength of the radiation for the refractive index of the air. Over shorter distances where a high measurement precision is required, it is necessary to resolve fractions of an optical fringe. In these applications unwanted reflections and polarization effects become fundamental limitations as they cause fringe distortion and limit the fidelity of the electronic signals.
BACKGROUND OF THE INVENTION
Bidirectional counting techniques are used to correct automatically for vibration or retraced motion ensuring that the fringe count represents the displacement of the moving reflector. For optimum performance bi-directional counters require two signals with constant average dc levels and sinusoidal components, related to the optical path difference in the interferometer, that are in phase quadrature. The counter logic is set to respond each time one of the signals passes through its average value. Unfortunately, in practice this dc level is subject to variations. For example, its value is dependent on the intensity of the light source. Removal of the dc component by capacitance coupling, so that the average signal level is always zero, is not completely effective because the frequency of the sinusoidal component may be very low and, indeed, zero if the corner cube retro-reflector attached to the workpiece is stationary.
It is common practice either to employ some form of modulation of the interferometer signal or to maintain the average signal level at zero volts by an electronic subtraction process which removes variations in the average signal level from the photodetector signals and avoids the need for modulation. However, instruments using these methods involve the use of polarization techniques, introducing a 90.degree. phase difference between the signals obtained from two orthogonally polarized components by means of a phase retardation plate. This results in the imposition of alignment requirements on the polarization azimuths of the optical components in the interferometer system and the radiation source and in addition increases the overall cost.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a bidirectional fringe-counting two-beam interferometer system that can be used for precision length measurement with any laser source that is adequately coherent for the application and with any orientation of polarization of the input laser beam.
According to the invention a single frequency bidirectional fringe-counting interferometer system comprises measuring beam, the beams having a phase difference of substantially 90.degree.; interfering beams having a path length difference related to a displacement to be measured; to provide related electrical signals each having an ac and a dc component, the signals having a difference in phase equal to the phase difference introduced by the beamsplitter means: modify it to equal 90.degree.; difference and to compare it with a preset rate of change and when the sensed rate exceeds the preset rate to subtract the respective dc components from the two signals; signals and when they are not equal to normalize the ac components; and fringe fraction.
Preferably, before each measurement of path length difference, the system is initially calibrated by sensing the
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British Technology Group Ltd.
Turner Samuel A.
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