Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1994-07-18
1995-11-28
Turner, Samuel A.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356354, 356360, 356345, G01B 902
Patent
active
054713021
DESCRIPTION:
BRIEF SUMMARY
INTRODUCTION
The present invention relates to an interferometric probe for measuring the distance to a surface of an object.
The invention is an improvement in the interferometric probe described in European Patent Application No. 480028 Al in that it provides an improvement in the signal strength generated by the probe.
European Patent Application No. 480028 Al describes a probe in which a measuring beam of radiation generated within the probe is focused onto the surface of an object. Radiation reflected back from the surface of the object is passed to an detector system which generates interference between the reflected radiation and a reference beam of radiation generated within the probe to create an interference pattern on a detector system. A reflector in the measuring beam path is mounted on an oscillating membrane to vary the length of the path of the measuring beam.
By determining the position of the oscillating reflector at the instant of maximum contrast in the interference pattern detected by the detector, the point at which the path lengths of the measuring beam and the reference beam are equal can be determined. This provides an accurate measure of the position of the surface of the object.
The object of the present invention is to improve the level of the signal generated by the detector system from the interference pattern generated by the interference of the measuring and reference beams.
This object is achieved in accordance with the present invention by focusing the reference beam onto a diffraction grating and creating interference between the reflected measuring beam from the object and a plurality of orders of diffraction of the reference beam emanating from the diffraction grating.
By this means a greater part of the radiation reflected from the object can be used in signal generation than in the known probe, resulting in a greater signal to noise ratio and greater accuracy in the measurements made by the probe.
According to the present invention an interferometric probe comprises:
a radiation source for providing a beam of coherent radiation,
a beam splitter for generating from said radiation beam, a measuring beam directed towards the surface of the object the distance to which is to be measured, and a reference beam directed towards a reflector, and for recombining portions of the measuring and reference beams which are reflected respectively from the object and the reflector, a detector system for providing a signal indicative of interference in the re-combined beam,
characterised in that the reflector is a diffracting reflector which produces a plurality of diffracted beams, and the detector system produces signals indicative of the interference between the reflected radiation from the surface of the object and at least two of said diffracted beams from the reflector.
In a preferred embodiment of the invention the source is an optical source and the wavelength and coherence of the radiation from the optical source is chosen to produce a speckle pattern in the beam reflected from the object, and the reflected beam is made to interfere with the diffracted beams from the diffraction grating. The optical source may be a super-luminescent diode or a laser.
The detector system may take the form of an array of photodetectors each detector of the array receiving radiation from one of the diffracted orders.
In order to get optimum signal strength in such a system the aperture of the detector should be substantially equal to size of the speckles in the reflected measuring beam. Thus in the preferred embodiment a focusing lens is included at least in the measuring beam path to focus the measuring beam onto the surface of an object.
The radiation beam from the source may also be focused onto the diffracting reflector whereby more radiation is available in the various orders diffracted therefrom and the reflected orders emerge at an increased angle. This is preferably achieved by placing the focusing lens between the radiation source and the beam splitter whereby the same lens focuses both
REFERENCES:
W. Jaerisch et al.; "Interferometric surface mapping with variable sensitivity;" Applied Optics/vol. 17, No. 5/1 Mar. 1978.
Kim Robert
Renishaw plc
Turner Samuel A.
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