Linear encoders with a zero index are first types of encoders being built with SPPE technology. The possible measurement ranges for linear encoders are limited only by the available lengths for grating scales. Since SPPEs can use larger pitch holographic gratings, highly accurate scales with lengths up to 500 mm or more are available at reasonable costs.

Rotary encoders, with very fine angular resolutions (e.g., on the order of nano-radians) and support for high angular velocities and zero index, in a compact form factor are also possible using SPPE technology. Absolute rotary encoders are also possible.

The scanned laser light used in an SPPE may be focused onto the grating surface creating one or more spots each smaller than the pitch of the grating, or a much broader diffracted beam may be used. There are distinct advantages to each approach. For instance a focused spot may be more desirable in a compact, sealed disk-type rotary encoder, while a broad diffracted beam may be more desirable for an exposed linear encoder to give more tolerance against dust and scratches.

While it is typical in the industry to use a one dimensional grating scale with each linear position encoder, with SPPE technology, a two dimensional holographic grating scale may also be used with the linear encoders. This has some advantages for certain applications. For instance the motion of an X-Y stage can be measured in both directions by using a two-dimensional planar grating mounted on the bottom (or the top) surface of the stage, rather than mounting scales on the sides of the stage.

SPPEs can also be built to measure both the transverse motion relative to the grating as well as changes in the gap distance (commonly called the Z-axis) between the encoder head and grating scale. While these gap distance measurements have less resolution (typically about 1 nanometer), that is often adequate for Z-axis control in many applications.

Employing these SPPE capabilities, complete six degree-of-freedom (6-DoF) measurement and control is possible using just three encoders (each measuring gap distance) while using two-dimensional holographic grating, as shown below.

Of course 6-DoF configurations may also be formed in more traditional fashions using larger numbers of encoders each with a separate grating scale.

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