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Dual beam interferometer with autoequalization of path length David C. Auth University of Washington, Department of Electrical En- gineering, Seattle, Washington 98195. Received 19 June 1976. In the operation of dual-path interferometers with optical sources of short temporal coherence, the necessity for equal- ization of path length requires careful and usually tedious adjustments. The use of compensating plates is well known in such applications. 1 For continuous wave visible light sources the alignment is much easier to accomplish since the human operator can make fine adjustments while maintaining visual surveillance of fringe visibility. Since many unique interference experiments can be performed utilizing short, sometimes invisible, pulses of laser radiation, a means of predetermining path length equality is highly desirable. In some cases a cw visible laser may be employed for this deter- mination. This technique may not always be feasible, such as, for example, when the optics will not transmit or reflect. visible radiation or when the fringes are too small or too weak to enable easy visualization. The mode-locked Nd:glass laser can have pulse lengths 2 of ~ 10 -2 cm and thus require ex- tremely precise path length equalization to enable production of high contrast optical fringes. A novel interferometer has been devised that greatly simplifies alignment and path equalization. This device is depicted in Fig. 1. The pellicle beam splitter consists of a plastic membrane 2-8 μm thick. This membrane is optically flat with parallel sides; and, because of its thin dimension, contributes little to the path length in one leg of the inter- ferometer. Of particular interest is the fact that when the two front surface mirrors are parallel to the beam splitter, the optical path length of the two legs must be equal. This as- sumes negligible thickness for the beam splitter. Since mu- tual parallelism can easily be acquired by adjusting the three planes of the two mirrors and the beam splitter so each is perpendicular to the same line, a rapid and simple means exists for assuring path length equality. As shown in Fig. 1, a He-Ne alignment laser provides the common line for per- pendicularity of each of the three planes. By observing partial 2302 APPLIED OPTICS / Vol. 15, No. 10 / October 1976 (even minute) back reflections from each plane surface, ex- tremely rapid adjustment is possible. Large variations in reflectivity of the three surfaces can be accommodated by observing alternately forward and backward reflections and blocking individual beams. Other methods may be utilized to ascertain parallelism of the three surfaces. If the mirrors are totally opaque, partial reflection from an edge or hole may be appropriate. If the beam splitter and mirrors are not overlapping, the alignment beam may be moved transversely on a horizontal translation stage and each reflector adjusted separately. As can be seen in Fig. 1, the optical rays always define a parallelogram regardless of the angular values of θ and φ. Furthermore, the position of the beam splitter need not be at the midpoint between the two mirrors to assure path length equality. If the incoming beam assumes arbitrary values of θ and φ as well as the location of intercept on the beam splitter, path length equality is nonetheless preserved. If is, of course, necessary that none of the beams miss the appropriate re- flectors. A device of this type has been constructed and tested. A low power He-Ne laser served as the alignment beam, and a high power mode-locked Nd:glass laser provided the short coherence length incoming pulsed beam. Two separate ex- periments were performed. The first consisted of laser evaporative etching of diffraction gratings in metallic films of bismuth and aluminum. In this experiment gratings of approximately 2-μm spacing were formed by evaporation of metal at the antinodes of the interference of the two optical beams at a plane located at the point of intersection. Gratings with high modulation index were obtained using this system. In another experiment holographic recording was accom- plished at the plane of recombination of the two beams. An object was placed in one of the optical legs between the front surface mirror and the plane of recombination. A bismuth film 3 recorded a hologram of transmission of the object. A beam splitter may be placed at the plane of recombination to superimpose the two beams in a manner similar to the Mach-Zehnder interferometer. The pellicle beam splitter behaves as a Fabry-Perot interferometer of low finesse and large free spectral range. As a result, its reflection and transmission may be modulated quite dramatically by changing the polarization of the incoming beam with respect to the plane of incidence and by changing the value of φ. Thus a wide range of reflection/transmission ratios can be obtained simply by varying the angle φ. This of course varies the ratio of optical intensity in the two legs of the interfer- ometer. Because of the differential reflection of s and ρ waves 4 at the pellicle, it may be desirable to use pure s or pure Fig. 1. Schematic diagram of interferometer with autoequalization of path length.
