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Journal of the Optical Society of America

Journal of the Optical Society of America

  • Vol. 54, Iss. 12 — Dec. 1, 1964
  • pp: 1474–1484

High-Resolution Fourier Transform Spectroscopy in the Far-Infrared

P. L. RICHARDS  »View Author Affiliations

JOSA, Vol. 54, Issue 12, pp. 1474-1484 (1964)

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The construction and performance of three far-infrared spectrometers are described. These are a large-aperture diffraction-grating monochromator using six 28×35-cm interchangeable echelette gratings to span the frequency range from 10 to 150 cm-l, a set of 30-cm-square lamellar grating plates which can be inserted into the grating monochromator to convert it into a lamellar grating interferometer, and an 18-cm Michelson interferometer. The two interferometers are used with an automatic digital data recording system which records the interferograms on punched cards so that the spectra can be obtained by numerical transformation on a digital computer. All three instruments have been operated with the same detector and the same source, thus providing, for the first time, a controlled test of the relative merits of these three types of spectrometers. As expected theoretically, the two interferometers performed similarly except for differences due to beamsplitter efficiency and mechanical accuracy. Owing to their ability to look at all parts of the spectrum simultaneously and to achieve high resolution with large aperture, however, both interferometers proved far superior to the grating spectrometer, giving some of the highest resolution spectra yet obtained in the far-infrared. Examples are presented demonstrating resolution of ~0.1 cm-1 over the frequency range from 3 to 80 cm-l.

P. L. RICHARDS, "High-Resolution Fourier Transform Spectroscopy in the Far-Infrared," J. Opt. Soc. Am. 54, 1474-1484 (1964)

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  1. See, for example, R. S. Ohl, P. P. Budenstein, and C. A. Burrus, Rev. Sci. Instr. 30, 765 (1959).
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  20. The optical system used here can transmit false energy due to double diffraction. Energy at frequencies close to those which pass through the exit slit is roughly focused on the slit jaws or the face of the grating. Part of this energy finds its way back along the optical path, is diffracted by the grating, and irradiates a region around the entrance slit. Since the grating is not, in general, in the focal plane of the collimating mirror, this region is large enough for some of the doubly diffracted energy to spill out the exit slit as false energy. To avoid this we mask a horizontal strip of the grating somewhat wider than the slit height when maximum radiation purity is required. This difficulty could be avoided by using an optical system2 in which the entrance and exit slits are widely separated, at the cost of an increase in the over-all size of the instrument, and some impairment of its performance as a lamellar grating interferometer.
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