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Prism Window for Optical Alignment

Prism windows could be generally useful in manufacture of optical instruments.

A prism window has been devised for use, with an autocollimator, in aligning optical components that are (1) required to be oriented parallel to each other and/or at a specified angle of incidence with respect to a common optical path and (2) mounted at different positions along the common optical path. The prism window can also be used to align a single optical component at a specified angle of incidence. Prism windows could be generally useful for orienting optical components in manufacture of optical instruments.

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Figure 1. A Prism Window comprising two beam-splitter windows bonded together at a predetermined angle equal to twice a specified angle of incidence is used in conjunction with an autocollimator to align an optical component at the specified angle of incidence.
Heretofore, for aligning multiple optical components in a large optical assembly for which there is a requirement that no such component completely obstruct the alignment optical path to any other such component, it has been common practice to use a single large-aperture autocollimator or interferometer. How ever, the sizes of optical assemblies amenable to this alignment practice are limited by the sizes of apertures of commercially available autocollimators and interferometers. Moreover, in some cases, it may be necessary to remove some optical components to prevent obscuration of other optical components or to make room for the autocollimator or interferometer. In contrast, the prism window makes it possible to use an autocollimator or other suitable instrument of narrow aperture to align multiple optical components in a possibly large optical assembly, without need to remove one or more optical components to prevent obscuration of other optical components or to make room for the alignment instrumentation.

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Figure 2. Multiple Optical Components are aligned in the manner of Figure 1 by repetition of the basic alignment procedure with the prism window suitably positioned in relation to each component to be aligned.
“Prism window” as used here should not be confused with “prism window” used in U.S. Patent 4,772,094 to denote an assembly of prisms configured as a stereoscopic viewing device. Instead, as used here, “prism window” denotes an application- specific unit comprising two beam-splitter windows that are bonded together at an angle chosen to obtain the specified angle of incidence.

Figure 1 illustrates a simple example of the use of a prism window and an autocollimator to align one optical component in a horizontal plane of incidence. In this example, the autocollimator is nominally aimed horizontally and the prism window is mounted on a flat, smooth, nominally horizontal platform that can be adjusted slightly in rotation about any or all of three axes to bring the prism window into alignment with the autocollimator.

First, the surface S1 of the prism window is aligned with the autocollimator by performing such adjustments while using the autocollimator in the conventional manner to center light reflected from surface S1. Next, surface S2 is brought into alignment by rotating the platform about the axis parallel to the optical axis of the collimator until the table is as nearly level as possible, as indicated by a commercial level meter or any other suitable means. Finally, the optical component to be aligned is placed at or near the desired position and adjusted in tilt and tip. Alignment of this component is deemed to be achieved when, as observed via the autocollimator, light reflected from surfaces S1 and S2 is centered.

Figure 2 illustrates an example of the use of a prism window in conjunction with an autocollimator to align multiple optical components with respect to a multi-leg common optical path. In this case, the procedure described above for the single-component case must be repeated, with appropriate positioning of the prism window with respect to each component to be aligned.

This work was done by Hong Tang of Caltech for NASA’s Jet Propulsion Laboratory.
NPO-45546

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