Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1991-09-23
1994-04-05
Turner, Samuel A.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356345, G01D 902
Patent
active
053010108
DESCRIPTION:
BRIEF SUMMARY
This invention relates to interferometry and is particularly concerned with methods and apparatus for measuring or tracking variations in a parameter utilising interferometric techniques.
In conventional interferometers, light from a monochromatic coherence source is passed through a beam splitter that divides the light into two beams which, after reflection, are both returned to the beam splitter and recombined. Upon recombination, destructive or constructive interference arises dependent upon any optical path length differences in the two arms. Thus, if each arm includes a mirror for reflecting the split beam back to the beam splitter, movement of one of the mirrors towards (or away) from the beam splitter will cause destructive and constructive interference alternately to arise, so that an observer of the recombined beam would alternately see bright and dark fringes as the mirror is moved. In a conventional interferometer, counting of such fringes enables the distance through which the mirror has been moved to be measured and thus, causing the mirror to move as a function of the parameter to be measured, enables changes in that parameter to be measured.
If the range of movement of the movable mirror represents the range of values of the parameter which can be measured, it is necessary, if an absolute value for the parameter is required, to ensure that the mirror is first moved to one of its end positions and then to count fringes from that position, when setting up the apparatus. This is a disadvantage of conventional interferometers.
In a conventional interferometer as described above, the intensities of the bright and dark fringes, and therefore the contrast between those fringes, remains substantially uniform as the movable mirror moves. If a light source having a certain but limited bandwidth were used instead of the monochromatic light source, the brightness of both the bright and dark fringes and the contrast between them would all vary as the movable mirror moves. The contrast would be a maximum when the optical path length difference in the two arms of the interferometer is zero and would decrease with increasing optical path length difference (or phase difference) in the two arms. Also, the brightness of the bright fringes and the darkness of the dark fringes would be at a maximum when the optical path length difference in the two arms is at or near to zero. After the optical path length difference has increased beyond a certain point dependent upon the bandwidth of the light source, the contrast between the bright and dark fringes would have decreased to an extent that the fringes become substantially invisible.
The above described phenomenon is known and light sources of limited bandwidth whereby this phenomenon may be produced are said to have a short coherence length. Proposals have been made in the prior art, for example in U.S. Pat. Nos. 4,596,466 and 4,697,926 to provide an interferometer utilising a light source of short coherence length, instead of a monochromatic source. In the prior art proposals, the interferometer operates by causing the movable mirror to be continuously moved between two end positions representing the range of the parameter which can be measured, detecting the fringes as the mirror moves and determining the mirror position at which the contrast of the fringes is maximum. These proposals suffer from the disadvantage that, for each measurement, the movable mirror has to be moved throughout its range of movement, thus limiting the bandwidth or frequency of measurement which can be taken. Further, measurements are made relative to a mechanical or piezoelectric scan motion whose characteristics have to be calibrated in order to provide an accurate output measurement.
One object of the invention is to overcome these problems.
In one aspect, the invention provides an interferometer and a method of interferometry in which a light source of relatively short coherence length is utilised to produce interference fringes having a parameter whose magnitude varies as a function
REFERENCES:
patent: 4596466 (1986-06-01), Ulrich
patent: 4636076 (1987-01-01), Pettigrew
patent: 5037206 (1991-08-01), Etzkorn et al.
Conference Proceedings OFS 1984, 2nd International Conference on Optical Fiber Sensors, Sep. 5-7, 1984, Stuttgart, VDE-Verglag GmbH, "High-Accuracy Position-Sensing with Fibert-Coupled White-Light Interferometers".
Hazell Michael S.
Jones Robert
Welford Robert J.
Cambridge Consultants Limited
Turner Samuel A.
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