Abstract

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<sup>-l</sup>, 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<sup>-1</sup> over the frequency range from 3 to 80 cm<sup>-l</sup>.

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  1. See, for example, R. S. Ohl, P. P. Budenstein, and C. A. Burrus, Rev. Sci. Instr. 30, 765 (1959).
  2. For example, see H. Yoshinaga et al., J. Opt. Soc. Am. 48, 315 (1958); D. Bloor el al., Proc. Roy. Soc. (London) A260, 510 (1961); and V. N. Murzin and A. I. Demshina, Opt. i Spektroskopiya 13, 826 (1962) [English transl.: Opt. Spectry. 13, 467 (1963)].
  3. For a discussion of the early development of Fourier transform spectroscopy, see J. Strong, J. Opt. Soc. Am. 47, 354 (1957).
  4. J. Strong and G. A. Vanasse, J. Opt. Soc. Am. 49, 844 (1959).
  5. P. Jacquinot, Rept. Progr. Phys. 23, 267 (1960).
  6. J. Connes, Rev. Opt. 40, 45, 116, 171, and 231 (1961).
  7. L. Genzel, J. Mol. Spectry. 4, 241 (1960).
  8. For an interferometer whose limiting aperture subtends a solid angle Ω=πθ2 at a collimating mirror, the axial ray through the interferometer has a path difference cosθ~1-Ω/2π times that of an extremal off-axis ray. This spread in values, δΔ/Δ, of path difference corresponds to a spread in frequency of δν/ν=δΔ/Δ=Ω/2π, thus limiting the resolving power to values less than R=ν/δν=2π/Ω. Another effect of the finite aperture is that the mean path difference for all rays entering the interferometer is 〈cosθ〉 = (1-Ω/4π) times that of an axial ray. Thus, if the axial path difference is used in Eq. (3), the frequencies of spectral elements are overestimated by the factor 1+Ω/4π. For the interferometers described in Secs. III and IV, Ω=4.5×10-4 and 1.8×10-3 sr, respectively. The limitation on resolution is negligible for frequencies below 1000 cm-1, but significant frequency corrections must be made for high-resolution spectra at much lower frequencies.
  9. P. Fellgett, J. Phys. Radium 19, 187, 237 (1958).
  10. Assuming otherwise comparable efficiency for the monochromator and interferometer.
  11. L. Genzel and R. Weber, Z. Angew. Phys. 10, 127 (1957); 10, 195 (1958).
  12. A. S. Barker, Jr., and M. Tinkham, Bull. Am. Phys. Soc. 6, 112 (1961).
  13. E. E. Bell (private communication).
  14. Block Associates, Inc., Cambridge 39, Massachusetts.
  15. H. A. Gebbie and G. A. Vanasse, Nature 178, 432 (1956).
  16. S. Goldman, Inforination Theory (Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1953), pp. 67 ff.
  17. R. C. Ohlman, P. L. Richards, and M. Tinkham, J. Opt. Soc. Am. 48, 531 (1958).
  18. P. Jacquinot, J. Opt. Soc. Am. 44, 761 (1954).
  19. This design is similar to those used by T. K. McCubbin, Jr., and W. M. Sinton, J. Opt. Soc. Am. 42, 113 (1952); and by P. L. Richards and M. Tinkham, Phys. Rev. 119, 575 (1960).
  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.
  21. P. L. Richards, Phys. Rev. Letters 7, 412 (1961).
  22. J. Strong and G. A. Vanasse, J. Phys. Radium 19, 192 (1958).
  23. J. Strong and G. A. Vanasse, J. Opt. Soc. Am. 50, 113 (1960).
  24. See, for example, H. H. Skilling, Fundamentals of Electric Waves (John Wiley & Sons, Inc., New York, 1948), p. 204.
  25. H. A. Gebbie, 1959 Symposium on Interferometry, Teddington, England (unpublished).
  26. K. D. Möller and R. V. McKnight, J. Opt. Soc. Am. 53, 760 (1963).
  27. R. F. Renk and L. Genzel, Appl. Opt. 1, 643 (1962).
  28. L. Genzel (private communication).
  29. P. F. Parshin, Opt. i Spektroskopiya 13, 740 (1962) [English transi.: Opt. Spectry. 13, 418 (1962)].
  30. G. A. Vanasse, J. Opt. Soc. Am. 52, 472 (1962).
  31. E. V. Loewenstein, Appl. Opt. 2, 491 (1963).
  32. D. W. Williamson, J. Opt. Soc. Am. 42, 712 (1952).
  33. P. L. Richards, J. Appl. Phys. 34, 1237 (1963); 35, 850 (1964).
  34. W. S. Boyle and K. P. Rodgers Jr., J. Opt. Soc. Am. 49, 66 (1959).
  35. J. E. Kunzler, T. H. Geballe, and G. W. Hull, Rev. Sci. Instr. 28, 96 (1957).
  36. F. J. Low, J. Opt. Soc. Am. 51, 1300 (1961).
  37. J. Connes6 has made some comparisons with grating spectrometers in the near-infrared on weak source experiments.
  38. N. G. Yaroslavski and A. E. Stanevich, Opt. i Spektroskopiya 5, 384 (1958); 7, 626 (1959). [English transl.: Opt. Spectry. 7, 380 (1959)].
  39. E. Archbold and H. A. Gebbie, Proc. Phys. Soc. (London) 80, 793 (1962).
  40. H. A. Gebbie, K. J. Habell, and S. P. Middleton, Proceedings of the Conference on Optical Instruments and Techniques (Chapman and Hall, Ltd., London, 1962), p. 43.
  41. P. F. Parshin, Opt. i Spektroskopiya 14, 388 (1963) [English transl.: Opt. Spectry. 14, 207 (1963)].
  42. E. H. Putley, J. Phys. Chem. Solids 22, 241 (1961).
  43. D. W. Goodwin and R. H. Jones, J. Appl. Phys. 32, 2056 (1961).
  44. M. A. C. S. Brown and M. F. Kimmitt, Brit. Commun. Electron. 11, 608 (1963).

Archbold, E.

E. Archbold and H. A. Gebbie, Proc. Phys. Soc. (London) 80, 793 (1962).

Barker, Jr., A. S.

A. S. Barker, Jr., and M. Tinkham, Bull. Am. Phys. Soc. 6, 112 (1961).

Bell, E. E.

E. E. Bell (private communication).

Boyle, W. S.

W. S. Boyle and K. P. Rodgers Jr., J. Opt. Soc. Am. 49, 66 (1959).

Brown, M. A. C. S.

M. A. C. S. Brown and M. F. Kimmitt, Brit. Commun. Electron. 11, 608 (1963).

Budenstein, P. P.

See, for example, R. S. Ohl, P. P. Budenstein, and C. A. Burrus, Rev. Sci. Instr. 30, 765 (1959).

Burrus, C. A.

See, for example, R. S. Ohl, P. P. Budenstein, and C. A. Burrus, Rev. Sci. Instr. 30, 765 (1959).

Connes, J.

J. Connes, Rev. Opt. 40, 45, 116, 171, and 231 (1961).

J. Connes6 has made some comparisons with grating spectrometers in the near-infrared on weak source experiments.

Fellgett, P.

P. Fellgett, J. Phys. Radium 19, 187, 237 (1958).

Geballe, T. H.

J. E. Kunzler, T. H. Geballe, and G. W. Hull, Rev. Sci. Instr. 28, 96 (1957).

Gebbie, H. A.

E. Archbold and H. A. Gebbie, Proc. Phys. Soc. (London) 80, 793 (1962).

H. A. Gebbie, K. J. Habell, and S. P. Middleton, Proceedings of the Conference on Optical Instruments and Techniques (Chapman and Hall, Ltd., London, 1962), p. 43.

H. A. Gebbie and G. A. Vanasse, Nature 178, 432 (1956).

H. A. Gebbie, 1959 Symposium on Interferometry, Teddington, England (unpublished).

Genzel, L.

R. F. Renk and L. Genzel, Appl. Opt. 1, 643 (1962).

L. Genzel (private communication).

L. Genzel and R. Weber, Z. Angew. Phys. 10, 127 (1957); 10, 195 (1958).

L. Genzel, J. Mol. Spectry. 4, 241 (1960).

Goldman, S.

S. Goldman, Inforination Theory (Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1953), pp. 67 ff.

Goodwin, D. W.

D. W. Goodwin and R. H. Jones, J. Appl. Phys. 32, 2056 (1961).

Habell, K. J.

H. A. Gebbie, K. J. Habell, and S. P. Middleton, Proceedings of the Conference on Optical Instruments and Techniques (Chapman and Hall, Ltd., London, 1962), p. 43.

Hull, G. W.

J. E. Kunzler, T. H. Geballe, and G. W. Hull, Rev. Sci. Instr. 28, 96 (1957).

Jacquinot, P.

P. Jacquinot, Rept. Progr. Phys. 23, 267 (1960).

P. Jacquinot, J. Opt. Soc. Am. 44, 761 (1954).

Jones, R. H.

D. W. Goodwin and R. H. Jones, J. Appl. Phys. 32, 2056 (1961).

Kimmitt, M. F.

M. A. C. S. Brown and M. F. Kimmitt, Brit. Commun. Electron. 11, 608 (1963).

Kunzler, J. E.

J. E. Kunzler, T. H. Geballe, and G. W. Hull, Rev. Sci. Instr. 28, 96 (1957).

Loewenstein, E. V.

E. V. Loewenstein, Appl. Opt. 2, 491 (1963).

Low, F. J.

F. J. Low, J. Opt. Soc. Am. 51, 1300 (1961).

McCubbin, Jr., T. K.

This design is similar to those used by T. K. McCubbin, Jr., and W. M. Sinton, J. Opt. Soc. Am. 42, 113 (1952); and by P. L. Richards and M. Tinkham, Phys. Rev. 119, 575 (1960).

McKnight, R. V.

K. D. Möller and R. V. McKnight, J. Opt. Soc. Am. 53, 760 (1963).

Middleton, S. P.

H. A. Gebbie, K. J. Habell, and S. P. Middleton, Proceedings of the Conference on Optical Instruments and Techniques (Chapman and Hall, Ltd., London, 1962), p. 43.

Möller, K. D.

K. D. Möller and R. V. McKnight, J. Opt. Soc. Am. 53, 760 (1963).

Ohl, R. S.

See, for example, R. S. Ohl, P. P. Budenstein, and C. A. Burrus, Rev. Sci. Instr. 30, 765 (1959).

Ohlman, R. C.

R. C. Ohlman, P. L. Richards, and M. Tinkham, J. Opt. Soc. Am. 48, 531 (1958).

Parshin, P. F.

P. F. Parshin, Opt. i Spektroskopiya 13, 740 (1962) [English transi.: Opt. Spectry. 13, 418 (1962)].

P. F. Parshin, Opt. i Spektroskopiya 14, 388 (1963) [English transl.: Opt. Spectry. 14, 207 (1963)].

Putley, E. H.

E. H. Putley, J. Phys. Chem. Solids 22, 241 (1961).

Renk, R. F.

R. F. Renk and L. Genzel, Appl. Opt. 1, 643 (1962).

Richards, P. L.

P. L. Richards, Phys. Rev. Letters 7, 412 (1961).

R. C. Ohlman, P. L. Richards, and M. Tinkham, J. Opt. Soc. Am. 48, 531 (1958).

P. L. Richards, J. Appl. Phys. 34, 1237 (1963); 35, 850 (1964).

Rodgers Jr., K. P.

W. S. Boyle and K. P. Rodgers Jr., J. Opt. Soc. Am. 49, 66 (1959).

Sinton, W. M.

This design is similar to those used by T. K. McCubbin, Jr., and W. M. Sinton, J. Opt. Soc. Am. 42, 113 (1952); and by P. L. Richards and M. Tinkham, Phys. Rev. 119, 575 (1960).

Skilling, H. H.

See, for example, H. H. Skilling, Fundamentals of Electric Waves (John Wiley & Sons, Inc., New York, 1948), p. 204.

Stanevich, A. E.

N. G. Yaroslavski and A. E. Stanevich, Opt. i Spektroskopiya 5, 384 (1958); 7, 626 (1959). [English transl.: Opt. Spectry. 7, 380 (1959)].

Strong, J.

J. Strong and G. A. Vanasse, J. Phys. Radium 19, 192 (1958).

J. Strong and G. A. Vanasse, J. Opt. Soc. Am. 50, 113 (1960).

For a discussion of the early development of Fourier transform spectroscopy, see J. Strong, J. Opt. Soc. Am. 47, 354 (1957).

J. Strong and G. A. Vanasse, J. Opt. Soc. Am. 49, 844 (1959).

Tinkham, M.

R. C. Ohlman, P. L. Richards, and M. Tinkham, J. Opt. Soc. Am. 48, 531 (1958).

A. S. Barker, Jr., and M. Tinkham, Bull. Am. Phys. Soc. 6, 112 (1961).

Vanasse, G. A.

J. Strong and G. A. Vanasse, J. Opt. Soc. Am. 49, 844 (1959).

J. Strong and G. A. Vanasse, J. Opt. Soc. Am. 50, 113 (1960).

J. Strong and G. A. Vanasse, J. Phys. Radium 19, 192 (1958).

H. A. Gebbie and G. A. Vanasse, Nature 178, 432 (1956).

G. A. Vanasse, J. Opt. Soc. Am. 52, 472 (1962).

Weber, R.

L. Genzel and R. Weber, Z. Angew. Phys. 10, 127 (1957); 10, 195 (1958).

Williamson, D. W.

D. W. Williamson, J. Opt. Soc. Am. 42, 712 (1952).

Yaroslavski, N. G.

N. G. Yaroslavski and A. E. Stanevich, Opt. i Spektroskopiya 5, 384 (1958); 7, 626 (1959). [English transl.: Opt. Spectry. 7, 380 (1959)].

Yoshinaga, H.

For example, see H. Yoshinaga et al., J. Opt. Soc. Am. 48, 315 (1958); D. Bloor el al., Proc. Roy. Soc. (London) A260, 510 (1961); and V. N. Murzin and A. I. Demshina, Opt. i Spektroskopiya 13, 826 (1962) [English transl.: Opt. Spectry. 13, 467 (1963)].

Other (44)

See, for example, R. S. Ohl, P. P. Budenstein, and C. A. Burrus, Rev. Sci. Instr. 30, 765 (1959).

For example, see H. Yoshinaga et al., J. Opt. Soc. Am. 48, 315 (1958); D. Bloor el al., Proc. Roy. Soc. (London) A260, 510 (1961); and V. N. Murzin and A. I. Demshina, Opt. i Spektroskopiya 13, 826 (1962) [English transl.: Opt. Spectry. 13, 467 (1963)].

For a discussion of the early development of Fourier transform spectroscopy, see J. Strong, J. Opt. Soc. Am. 47, 354 (1957).

J. Strong and G. A. Vanasse, J. Opt. Soc. Am. 49, 844 (1959).

P. Jacquinot, Rept. Progr. Phys. 23, 267 (1960).

J. Connes, Rev. Opt. 40, 45, 116, 171, and 231 (1961).

L. Genzel, J. Mol. Spectry. 4, 241 (1960).

For an interferometer whose limiting aperture subtends a solid angle Ω=πθ2 at a collimating mirror, the axial ray through the interferometer has a path difference cosθ~1-Ω/2π times that of an extremal off-axis ray. This spread in values, δΔ/Δ, of path difference corresponds to a spread in frequency of δν/ν=δΔ/Δ=Ω/2π, thus limiting the resolving power to values less than R=ν/δν=2π/Ω. Another effect of the finite aperture is that the mean path difference for all rays entering the interferometer is 〈cosθ〉 = (1-Ω/4π) times that of an axial ray. Thus, if the axial path difference is used in Eq. (3), the frequencies of spectral elements are overestimated by the factor 1+Ω/4π. For the interferometers described in Secs. III and IV, Ω=4.5×10-4 and 1.8×10-3 sr, respectively. The limitation on resolution is negligible for frequencies below 1000 cm-1, but significant frequency corrections must be made for high-resolution spectra at much lower frequencies.

P. Fellgett, J. Phys. Radium 19, 187, 237 (1958).

Assuming otherwise comparable efficiency for the monochromator and interferometer.

L. Genzel and R. Weber, Z. Angew. Phys. 10, 127 (1957); 10, 195 (1958).

A. S. Barker, Jr., and M. Tinkham, Bull. Am. Phys. Soc. 6, 112 (1961).

E. E. Bell (private communication).

Block Associates, Inc., Cambridge 39, Massachusetts.

H. A. Gebbie and G. A. Vanasse, Nature 178, 432 (1956).

S. Goldman, Inforination Theory (Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1953), pp. 67 ff.

R. C. Ohlman, P. L. Richards, and M. Tinkham, J. Opt. Soc. Am. 48, 531 (1958).

P. Jacquinot, J. Opt. Soc. Am. 44, 761 (1954).

This design is similar to those used by T. K. McCubbin, Jr., and W. M. Sinton, J. Opt. Soc. Am. 42, 113 (1952); and by P. L. Richards and M. Tinkham, Phys. Rev. 119, 575 (1960).

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.

P. L. Richards, Phys. Rev. Letters 7, 412 (1961).

J. Strong and G. A. Vanasse, J. Phys. Radium 19, 192 (1958).

J. Strong and G. A. Vanasse, J. Opt. Soc. Am. 50, 113 (1960).

See, for example, H. H. Skilling, Fundamentals of Electric Waves (John Wiley & Sons, Inc., New York, 1948), p. 204.

H. A. Gebbie, 1959 Symposium on Interferometry, Teddington, England (unpublished).

K. D. Möller and R. V. McKnight, J. Opt. Soc. Am. 53, 760 (1963).

R. F. Renk and L. Genzel, Appl. Opt. 1, 643 (1962).

L. Genzel (private communication).

P. F. Parshin, Opt. i Spektroskopiya 13, 740 (1962) [English transi.: Opt. Spectry. 13, 418 (1962)].

G. A. Vanasse, J. Opt. Soc. Am. 52, 472 (1962).

E. V. Loewenstein, Appl. Opt. 2, 491 (1963).

D. W. Williamson, J. Opt. Soc. Am. 42, 712 (1952).

P. L. Richards, J. Appl. Phys. 34, 1237 (1963); 35, 850 (1964).

W. S. Boyle and K. P. Rodgers Jr., J. Opt. Soc. Am. 49, 66 (1959).

J. E. Kunzler, T. H. Geballe, and G. W. Hull, Rev. Sci. Instr. 28, 96 (1957).

F. J. Low, J. Opt. Soc. Am. 51, 1300 (1961).

J. Connes6 has made some comparisons with grating spectrometers in the near-infrared on weak source experiments.

N. G. Yaroslavski and A. E. Stanevich, Opt. i Spektroskopiya 5, 384 (1958); 7, 626 (1959). [English transl.: Opt. Spectry. 7, 380 (1959)].

E. Archbold and H. A. Gebbie, Proc. Phys. Soc. (London) 80, 793 (1962).

H. A. Gebbie, K. J. Habell, and S. P. Middleton, Proceedings of the Conference on Optical Instruments and Techniques (Chapman and Hall, Ltd., London, 1962), p. 43.

P. F. Parshin, Opt. i Spektroskopiya 14, 388 (1963) [English transl.: Opt. Spectry. 14, 207 (1963)].

E. H. Putley, J. Phys. Chem. Solids 22, 241 (1961).

D. W. Goodwin and R. H. Jones, J. Appl. Phys. 32, 2056 (1961).

M. A. C. S. Brown and M. F. Kimmitt, Brit. Commun. Electron. 11, 608 (1963).

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