Unified Optical Scanning Technology

Chapter 4.4 - Scanner Devices and Techniques: Holographic Scanners

4.4 HOLOGRAPHIC SCANNERS

Holographic scanners employ many of the disciplines utilized by rotating polygons [Bei1]. Almost all comprise a substrate that is rotated about an axis. An array of diffractive elements (often mastered interferometrically) is disposed about the periphery of the substrate, to serve as transmissive or reflective facets. As with polygons, the number of holograms n is determined, per Equation 4-5, by the desired optical scan angle and duty cycle. And, as expressed for all angular scanners, the resolution N is determined primarily by the scan angle and beam size, per Equations 3-5, 3-11, and 3-13. The similarities relate even more closely in radially symmetric systems (Section 3.4.1), in which the geometric scan functions can be identical to those of the polygon. This is illustrated below. With such similarities, what are the distinctions, advantages, and limitations of hologaphic scanning? The distinctions may be listed as follows:

  1. The substrate surface appears smooth and uniform, exhibiting no facet discontinuities.

  2. The holographic (diffractive) facets are microscopically thin grating elements, formed on the otherwise smooth substrate surface.

  3. The substrate exhibits ideal rotational symmetry, typically as a flat disk (although not so limited) mounted on a rotating shaft.

  4. The scanner may be designed to operate in optical transmission or reflection.

  5. The diffractive properties of the holographic facets exhibit spectral and angular selection characteristics.

  6. The facet grating may be 'linear' (causing only angular displacement) or lenticular (providing added optical power).

  7. The orientations of the input and output beams (with respect to the scanner) are design variables.

Consequential advantages may be listed as follows:

  1. Replacement of conventional facets with holograms eliminates surface discontinuities. This reduces significantly the aerodynamic loading and windage, allowing more efficient high-speed rotation [Shep, Lenn].

  2. Elimination of radial variations also reduces the inertial deformation of the substrate and its facet elements under high-speed rotation [Bei2].

  3. When the facets are operated in the Bragg regime (Fig. 3.8), beam misplacement due to shaft wobble is reduced significantly [Kra1,Bei1].

  4. Optically exposed facets require no physical contact during exposure, allowing precise shaft and facet indexing and positioning.

  5. Accurate replication of the diffractive elements on the substrate (from an optimized master) can reduce the production cost significantly.

  6. Beam filtering in retrocollection allows spatial and spectral selection.

  7. Facets may be varied (during exposure) in focus, size, and orientation.

Complementing the above advantages are the consequential limitations:

  1. Design, fabrication, and test of holographic scanners entails unique technical discipline, along with special instrumentation, metrology, and processing controls. It requires astute orientation in diffractive optics and a significant investment in R&D manpower and facilities.

  2. The spectral and angular beam sensitivity of diffractors (distinction nos. 5, 6, and 7) can impose beam shift complications when reconstructing at a wavelength that differs from that of exposure. Also, small wavelength shifts from laser diode drift and/or mode-hopping may appear, potentially perturbing the diffracted output beam angle.

  3. Systems that retain radial symmetry for circular scan uniformity may be limited in Bragg angle wobble reduction. They can require auxiliary wobble compensation (Chapter 5), such as anamorphic error correction. Also, for flat-field scan, they must be augmented with a flat-field lens.

  4. Systems that do depart from radial symmetry and operate in the Bragg regime can require interrelated balancing of linearity, scan angle range, wobble correction, scan bow correction, radiometric uniformity, and insensitivity to beam polarization [Kra3].

Fig. 4.10 Analogous nonholographic scanners (a) & (b) and holographic scanner (c) form identical radially symmetric outputs through angle α at focal point P. Cylinder S is concentric with the axis, or flat surface S' is perpendicular to the axis. In (a) mirror M reflects converging input beam I from lens L to point P. In (b) lens segment LS refracts focusing beam to point P. In (c) holographic segment HS diffracts focusing beam to point P. From Holographic Scanning, L. Beiser, ©1988 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

 

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