June 1966 L E T T E R S TO T H E E D I T O R 831

Stability of Fabry-Perot Étalons F. M. PHELPS, III

Infrared Physics Laboratory, Institute of Science & Technology, The University of Michigan, Ann Arbor, Michigan 48107

AND

K. B. NEWBOUND

Physics Department, The University of Alberta, Edmonton, Alberta, Canada (Received 3 February 1966)

INDEX HEADINGS : Interferometer, Spectroscopy.

IN 1965 Phelps1 described a new method of making Fabry-Perot étalons which proved to be simple, effective, and in­

expensive. The purpose of this letter is to discuss the stability of the 5-cm étalon which was one of the original set of 6 étalons made at the University of Alberta during 1960-1961; and incidentally to point out an error in Ref. 1.

In Fig. 2 of Ref. 1, the pressure rod which holds the optical flat in contact with the boss is shown as hinged at the upper end where it is connected to the horizontal member of the adjusting assembly. Actually, however, the figure should have shown the upper end of the pressure rod to be of smaller diameter and threaded at the tip so that it can pass through a matching hole in the horizontal member and be held in place snugly with a small nut. One of the authors (F.M.P.) is indebted to N. Riebeek, Chief Instrument Maker of the University of Alberta Physics Department Shop, for discovering this error when he began work on a new set of étalons for the Infrared Physics Laboratory of The University of Michigan.

As mentioned in Ref. 1, étalons built according to Phelps' recipe remain in adjustment for long periods. This claim received additional support from the following observation. The étalon vacuum chamber was demounted from the spectrograph and left at atmospheric pressure for almost two years with the 5-cm étalon inside it. We opened the chamber on 3 December 1965, carefully extracted the étalon, and tested its adjustment with a 198Hg elec-trodeless lamp. Over the central region (the region which would produce the interference patterns the spectrograph would record) the variation of étalon thickness was no more than 1/20 fringe. Only a very slight change in spring tension was required to return the étalon to optimum adjustment. On checking our record book we found that the étalon was assembled, adjusted, and placed in the vacuum chamber before 11 December 1963. (On that date we exposed a plate to the spectrum of 86Kr, as previously reported in this journal by Phelps,2 for the specific purpose of checking on the possibility of a double line at 6013 Å.)

The important point, of course, is that the adjustment of this étalon did not change appreciably in two years! We attribute this remarkable stability to the large boss areas employed, 25 mm2

each. I t may be interesting to note that we demand neither highly polished bosses nor optical contact between a boss and the optical flat. However, we do insist that the separations of opposite pairs of bosses be equal to within at least ¼ wavelength of light (½ fringe) before the final adjustments are made with the pressure rods and springs. This is in sharp contrast to the experience of other workers who, at times, have reported that their étalons require adjustment as often as once a day, or at least every week.

1 F. M. Phelps, I I I . J. Opt. Soc. Am. 55, 3, 293 (1965). 2 F. M. Phelps, III . J. Opt. Soc. Am. 54, 864 (1964).

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