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A small interferometer—A double cat's eye— For rapid-scanning FTS
Infrared Phys. Vol. 24, No. 213, pp. 329-331, Printed in Great Britain. All rights reserved
A SMALL
Copyright
INTERFEROMETER-A DOUBLE FOR RAPID-SCANNING FTS T. B.
Institute
OOZO-0891184 $3.00 + 0.00 1984 Pergamon Press Ltd
1984 c
CAT’S
EYE-
EGEVSKAYA
of Semiconductor Physics, Siberian Branch of the Academy of Sciences Novosibirsk 630090, Prospect ak. Lavrentjeva 13, U.S.S.R. (Received 26 September
of the U.S.S.R.
1983)
Abstract-The operation and construction of an interferometer with a movable beam-splitter are described. The construction of the interferometer a “Double Cat’s Eye” is the result of the combination of two “Cat’s Eye” reflectors into one. The interferometer has permissible large-angle adjustments of the optical elements, twice as much resolution on the unit of motion and non-coincidence of the interfering beams on the beam-splitter. The interferometer may be used in rapid-scanning FTS for the analysis of electromagnetic radiation in the range of 500-15,000 cm-’ with a limiting resolution of 0.8 cm-‘.
OPTICAL
DEVICE
Beam passage through the optical system of the interferometer is shown in Fig. 1. The optical system consists of two identical spherical or parabolic, concave, coaxial mirrors M, and M, (of radius R),one is at the focus of the other, and a beam-splitter BS, initially placed centrally between the mirrors. Continuous motion of the beam-splitter by a value of ItA from the initial position changes the optical path difference in the interferometer by 4 4A. The entrance radiation direction makes an angle y with the mirrors’ axis, and the entrance radiation focus F is shifted from this axis by 6 -=+R,sothat the beams, split at the beam-splitter, may not leave the interferometer before interference. Radiation goes into and out of the interferometer through the holes in the mirrors. The entrance radiation, having the divergence angle Ay is collimated by the mirror M,, then it passes to the beam-splitter, where it is split into two coherent beams A and B. After splitting both beams undergo three reflections at the mirrors, similar to those at a “Cat’s Eye” retroreflector.“’ In this case the property of retroreflection to rotate the beam by 180” to within an accuracy of several angular seconds is preserved, and the beams A’ and B’, interfering at the beam-splitter, are parallel to the beams A and B, respectively. The combined beams are focused onto the detector P by the mirror M,.
ADVANTAGE In a Double Cat’s Eye (DCE) interferometer the requirement for specifications of the small/large mirror distance in CE reflectors is eliminated. A DCE permits both high angular and linear misalignments of optical elements. Therefore a DCE does not need alignment either during assembly or operation of the interferometer. The interferometer has twice as much resolution per unit of motion, spatial non-coincidence of separated and interfering beams at the beam-splitter and the possibility to introduce additional light channels without any complication of the structure.
APPLICATION Analysis of the parameters of the interferometer with a movable beam-splitter showed that the most promising application of the interferometer lies in its application to small-field Fourier spectrometers, because of its insensitivity to misalignment, its low-to-medium resolution and its small angle of view. The interferometer can also be used in laboratory Fourier spectrometers in measurements requiring a small angle of view or using laser sources in, for example, local 329
330
T. B. ECEVSKAYA
M, Fig. 1. Beam passage
-ZA through the optical
M, system of the interferometer.
measurements of film thicknesses and in revealing the known impurities in a liquid or gaseous sample. Since a DCE interferometer operates in a rapid-scanning regime, it can be used to register processes. For example, to draw up successive pictures of a chemical reaction, proceeding with 1 set intervals. The working wavelength range is determined by the beam-splitter and the type of photodetector, but particular advantages should result from the use of the interferometer in the visible and NIR region where the requirements for permissible misalignments and optical component quality are the most stringent. CONSTRUCTION Figure 2 gives an example of the construction of a DCE interferometer with a movable splitter. The interferometer weight is 0.8 kg and has dimensions of 150 x 110 x 100 mm. The type of drive 9
with a movable splitter: 1, spring; 2, fixture; Fig. 2. A DCE interferometer mirrors; 7, beam-splitter; 8 and 9. movable and unmovable
3, coil; 4. magnet; body parts.
5 and 6.
A “Double
Cat’s Eye” interferometer
331
can be chosen at the maker’s discretion (Fig. 2 shows an electrodynamic drive with a spring parallelogram). The admissible misalignments of the optical elements are as follows: angular, _+20’; linear, + I mm; the utmost beam-splitter shift, _+3 mm (resolution limit is 0.8 cm-‘). The interferometer is intended for the 1600-5000 cm-’ range (for GeAu or InSb cooled detectors). In principle the interferometer construction may be applicable within the 500- 15,000 cm-’ range. REFERENCE 1. Beer R. and Marjaniemi
D., Appl. Opf. 5, 1191 (1966).
A SMALL
Copyright
INTERFEROMETER-A DOUBLE FOR RAPID-SCANNING FTS T. B.
Institute
OOZO-0891184 $3.00 + 0.00 1984 Pergamon Press Ltd
1984 c
CAT’S
EYE-
EGEVSKAYA
of Semiconductor Physics, Siberian Branch of the Academy of Sciences Novosibirsk 630090, Prospect ak. Lavrentjeva 13, U.S.S.R. (Received 26 September
of the U.S.S.R.
1983)
Abstract-The operation and construction of an interferometer with a movable beam-splitter are described. The construction of the interferometer a “Double Cat’s Eye” is the result of the combination of two “Cat’s Eye” reflectors into one. The interferometer has permissible large-angle adjustments of the optical elements, twice as much resolution on the unit of motion and non-coincidence of the interfering beams on the beam-splitter. The interferometer may be used in rapid-scanning FTS for the analysis of electromagnetic radiation in the range of 500-15,000 cm-’ with a limiting resolution of 0.8 cm-‘.
OPTICAL
DEVICE
Beam passage through the optical system of the interferometer is shown in Fig. 1. The optical system consists of two identical spherical or parabolic, concave, coaxial mirrors M, and M, (of radius R),one is at the focus of the other, and a beam-splitter BS, initially placed centrally between the mirrors. Continuous motion of the beam-splitter by a value of ItA from the initial position changes the optical path difference in the interferometer by 4 4A. The entrance radiation direction makes an angle y with the mirrors’ axis, and the entrance radiation focus F is shifted from this axis by 6 -=+R,sothat the beams, split at the beam-splitter, may not leave the interferometer before interference. Radiation goes into and out of the interferometer through the holes in the mirrors. The entrance radiation, having the divergence angle Ay is collimated by the mirror M,, then it passes to the beam-splitter, where it is split into two coherent beams A and B. After splitting both beams undergo three reflections at the mirrors, similar to those at a “Cat’s Eye” retroreflector.“’ In this case the property of retroreflection to rotate the beam by 180” to within an accuracy of several angular seconds is preserved, and the beams A’ and B’, interfering at the beam-splitter, are parallel to the beams A and B, respectively. The combined beams are focused onto the detector P by the mirror M,.
ADVANTAGE In a Double Cat’s Eye (DCE) interferometer the requirement for specifications of the small/large mirror distance in CE reflectors is eliminated. A DCE permits both high angular and linear misalignments of optical elements. Therefore a DCE does not need alignment either during assembly or operation of the interferometer. The interferometer has twice as much resolution per unit of motion, spatial non-coincidence of separated and interfering beams at the beam-splitter and the possibility to introduce additional light channels without any complication of the structure.
APPLICATION Analysis of the parameters of the interferometer with a movable beam-splitter showed that the most promising application of the interferometer lies in its application to small-field Fourier spectrometers, because of its insensitivity to misalignment, its low-to-medium resolution and its small angle of view. The interferometer can also be used in laboratory Fourier spectrometers in measurements requiring a small angle of view or using laser sources in, for example, local 329
330
T. B. ECEVSKAYA
M, Fig. 1. Beam passage
-ZA through the optical
M, system of the interferometer.
measurements of film thicknesses and in revealing the known impurities in a liquid or gaseous sample. Since a DCE interferometer operates in a rapid-scanning regime, it can be used to register processes. For example, to draw up successive pictures of a chemical reaction, proceeding with 1 set intervals. The working wavelength range is determined by the beam-splitter and the type of photodetector, but particular advantages should result from the use of the interferometer in the visible and NIR region where the requirements for permissible misalignments and optical component quality are the most stringent. CONSTRUCTION Figure 2 gives an example of the construction of a DCE interferometer with a movable splitter. The interferometer weight is 0.8 kg and has dimensions of 150 x 110 x 100 mm. The type of drive 9
with a movable splitter: 1, spring; 2, fixture; Fig. 2. A DCE interferometer mirrors; 7, beam-splitter; 8 and 9. movable and unmovable
3, coil; 4. magnet; body parts.
5 and 6.
A “Double
Cat’s Eye” interferometer
331
can be chosen at the maker’s discretion (Fig. 2 shows an electrodynamic drive with a spring parallelogram). The admissible misalignments of the optical elements are as follows: angular, _+20’; linear, + I mm; the utmost beam-splitter shift, _+3 mm (resolution limit is 0.8 cm-‘). The interferometer is intended for the 1600-5000 cm-’ range (for GeAu or InSb cooled detectors). In principle the interferometer construction may be applicable within the 500- 15,000 cm-’ range. REFERENCE 1. Beer R. and Marjaniemi
D., Appl. Opf. 5, 1191 (1966).