Scanning Fabry-Perot Interferometer for Field Use

P. J. Brannon Sandia Laboratories, Albuquerque, New Mexico 87115. Received 16 February 1971.

A scanning Fabry-Perot interferometer for field use should be resistant to rough handling and require only minor alignment adjustments on arriving from the laboratory to the field. Also it would be convenient if these adjustments could be made re­motely and easily.

The instrument reported here meets these requirements. The minor field adjustments were made with three piezoelectric fine tuners similar to those reported by Jones and Greig.1 The gross alignment was made at the laboratory. This was accomplished by shimming with shim stock and compressing 0.1-mm lead wash­ers on each of the three stainless steel spacing rods (see Fig. 1).

August 1971 / Vol. 10, No. 8 / APPLIED OPTICS 1977

Fig. 1. Interferometer.

Fig. 2. Interferometer assembly.

The length of these rods determine the plate spacing. Waller et al.2 have used a similar method for both gross and fine adjustment. They compress or stretch three thinwall Invar tubes. That system, however, is expensive to machine as compared to three lead washers. Slater et al.3 have also used tension bolts through the centers of tubes to make fine adjustments.

The assembled instrument is shown in Fig. 1. The barium titanate piezoelectric translator is cemented to the end plate and Invar ring with epoxy. Others1 ,4 ,5 have used similar trans­lators. The piezoelectric tuning stacks consist of four barium titanate disks sandwiched with thin brass electrical contact tabs. They were cemented together with conducting epoxy. The assembled stack was also cemented to the end plate and Invar-ring with epoxy. Less than 1000 V were necessary for fine tuning and driving the translator. The interferometer plates were cemented to the Invar mounting ring.

The interferometer was field tested both on the ground and in a C135 aircraft. Since the environment of the aircraft was the most severe, only the results for it are given. The vibrations of the aircraft required special shock mounting of the interferome­ter. Figure 2 shows the interferometer mounted in its outer housing. The GE SC5601 silicon rubber caps on both ends of

the interferometer acted as shock mounts. Thin sheet metal cover plates were bolted on three sides of the rectangular outer housing end plates. The third side was bolted to a rail, and the whole assembly was bolted to the airframe. The interferometer was mounted vertically in the aircraft. Also, to improve the mechanical stability, the three spacer rods in Fig. 1 were replaced by a 9.5-mm wall cylinder of similar shape as the outer housing cylinder shown in Fig. 2.

The finesse measured in the laboratory was much less than the theoretical value of 19. This value was obtained from an ap­proximate formula given by Cooper and Grieg.6 The theoretical flatness and reflectivity finesses were 25 and 30, respectively. The scanning aperture finesse was negligible in this case. Aper-turing the 5.08-cm interferometer plates down (which in effect corresponds to increasing the flatness finesse) yielded a finesse-of 26. This indicated that the plates were warped. The warpage probably occurred during the cementing process. The finesse did not change for free spectral range scanning times of 1 sec, and 30 msec.

The finesse was measured periodically during a 5-h flight aboard the C135 aircraft. I t was also measured before and after the-flight with aircraft engines off. The measured finesse was 11 with a maximum variation of 8% during the flight. No changes due to vibrations were evident. The free spectral range of 4 Å was scanned in 60 sec. The large variation in finesse was a t ­tributed to thermal scanning; that is, the spacing between the plates changed because of temperature variations. Random tem­perature variations were estimated to be as high as 11.1°C. Evi­dence that thermal scanning was responsible for the change in finesse was the direct correlation between the change in spacing-of two successive orders used to measure the finesse and the change in finesse. When the finesse decreased or increased by a certain percentage, the spacing had a corresponding decrease or increase.

I t was concluded from the above measurements that the inter­ferometer was mechanically stable and was a useful field instru­ment. The thermal stability probably could be improved by adding a temperature controlled heating pad. This is presently being done. Another way of eliminating the thermal scanning-problem is to scan the free spectral range at a much faster rate. Thermal scanning would then be too slow to change the measured finesse significantly.

The author wishes to thank J. M. Hoffman for his encourage­ment and 0 . E. Smith for help in performing tests on the inter­ferometer.

This work was supported by the United States Atomic Energy Commission.

References 1. W. W. Jones and J. R. Greig, Technical Rept. 943, University

of Maryland, Dept. of Physics and Astronomy. 2. Private communication expressed through J. M. Hoffman

from W. Waller and others of AWRE, Culhem, U. K. (1969). 3. P. N. Slater, H. T Betz, and G. Henderson, Jap. J. Appl.

Phys., 4 , Suppl. I, 440 (1965). 4. K. D. Mielenz, R. B. Stephens, and K. F . Nefflen, J. Res.

Nat . Bur. Stand. 68C, 1 (1964). 5. V. G. Loloshinkov, M. A. Mazing, S. L. Mendelstrum, and

Yu P. Marasanov, Opt. Spectrosc. 11, 302 (1961). 6. J. Cooper and J. R. Grieg, J. Sci. Instrum. 40, 433 (1963).

1978 APPLIED OPTICS / Vol. 10, No. 8 / August 1971