Unified Optical Scanning Technology

Chapter - Scanner Devices and Techniques: Postobjective Configurations    Postobjective Configurations    Because postobjective operation (Section yields typically an arced scan (see Fig. 1.10 vs. Fig. 1.9 and limaçon scan [Bei2]), such curved fields serve well where advantageous. A popular example is the internal drum scanner utilizing variations of the monogon scanner of Figure 1.10. The formation of short linearized scans in postobjective polygon operation is discussed at the close of this section.

A basic multifacet scanner that merits attention is the postobjective pyramidal configuration, illustrated in Figure 4.4. It represents the extension of a rotating monogon to a regular pyramidal polygon having similar illumination. Because the initial focal point intersects the rotating axis, it retains the properties of radial symmetry (Section 3.4.1), directing the scanning beams into focal point loci on a concentric cylindrical surface. For an active input device (i.e., reading stored data on the cylindrical surface), one may consider dividing the storage medium into n sectors or strips (where n corresponds to the number of facets.) If each sector is provided with localized light collection and detection, the data on each sector may be read individually-as successive beams of the 'rotating crown' sample each sector.* This configuration would render efficient utilization of illumination.

If, however, the system is intended for writing on storage media, it is challenging to consider methods of modulating the beam(s) for more than one sector. When writing only one sector, the optical efficiency is depleted on η < 1/n, that is, wasting all the illumination but that on one facet. One may configure modulating simultaneously two opposite beams, forming two diametric channels, and perhaps even four channels in quadrant sectors, depending on the number of facets. In these 'thought experiments,' it is reasonable also to consider the options of over- or underfilling of the facets, a discussed above.

* Note that this idealized configuration may only approach the condition of n sectors equaling the number n of facets, to allow mounting space for the traversing scanning system to be supported mechanically within the concentric cylindrical enclosure.

Fig. 4.4 Postobjective pyramidal scanner. Provides radial symmetry when initial focal point intersects spinner axis, forming circular scan locus. From "Laser scanning systems," by L. Beiser in Laser Applications, Volume 2, by Monte Ross, ©1974, Elsevier Science (USA), reproduced by permission of the publisher.

In the form illustrated by Figure 4.4, the condition of radial symmetry (which scans a perfectly circular arc) is achieved by converging the initial focal point on to the rotating axis and apparently also by maintaining the axis of the objective lens coaxial with the rotating axis.* However, only the first condition is both necessary and sufficient. Consequently, the axis of the illuminating optics may be tilted with respect to the rotating axis while the initial focal point intersects the rotating axis, sustaining radial symmetry. This important option is illustrated in Figures 4.5 and 4.6. [See "Appendix B:: Circular Locus Theorem" in "Fundamental architecture of optical scanning system (Beiser,1995).

* See also Section 3.4.1 for operation in radial symmetry with collimated light parallel to the axis, forming preobjective scanning of Figure 3.6.

Fig. 4.5 Tilted-axis postobjective pyramidal scanner. Initial focal point on rotating axis provides radial symmetry. On-axis objective lens minimizes its size and aberrations. Two-facet overfilling maximizes the N.A. and scan duty cycle.

Figure 4.5 illustrates the tilted axis pyramidal scanner, drawn to scale for one design. Its scanned focal point (reflected from an over-filled uniformly illuminated keystone aperture, Section traverses a 5–in. width on curved film. At 633 nm, this f/6 cone provides high MTF at 100lp/mm (5–μm spot size) at a bandwidth of 50mHz. Figure 4.6 shows its application in recording 'cupped' film (with transitions to 'flat' on film spools at both ends). The film is maintained with concentric cylindrical curvature in the scanned region by a slotted arced guide (not shown). This basic system was deployed in a family of renowned phototransmission system programs [described briefly in Appendix 1 of Holographic Scanning (Beiser, 1988) and extended for wideband data storage and retrieval with added precise tracking of the scanned data.

In addition to allowing use of an objective lens of reduced size and cost, the principal advantages of this tilted axis configuration are—

(1) The f-number of the input objective lens is relaxed (allowed to increase) significantly, by a factor related to sinπ/n. For n = 10 facets, the objective lens f-number increases by a factor of ≈3¼ times. In the challenging high-speed scanning of an f/6 cone

Fig. 4.6 Application of tilted-axis postobjective configuration of Fig. 4.5. "Cupped" film is scanned in circular arc at very high resolution and speed. From "Laser scanning systems," by L. Beiser in Laser Applications, Volume 2, by Monte Ross, ©1974, Elsevier Science (USA), reproduced by permission of the publisher.

described above, and allowing for overfilling to uniformize the aperture, this permits design of an easily realizable objective lens.
(2) The objective lens operates in its essentially stigmatic axial region, rather than from a peripheral segment that is subject to to off-axis aberrations.

Although arced scan loci are typical in postobjective operation, significant work has been conducted [Wal] to approach a linearized portion of scan during such operation for special applications. One such development was for material surface processing utilizing a 5-kW CO2 laser. This linearizing technique balances the intrinsic arced scan with a compensating function utilizing the facet 'shift' (end of Section to contribute a complementary arc to straighten the scan locus over an adequate region of operation. Significant value is reported, as applied above. Its use in more general applications is limited to those cases that (a) require relatively narrow scan angles and field widths,

Fig. 4.7 Postobjective scanning by a monogon illustrated as in Fig. 1.10, except for its rectangular aperture. Whenoverfilled uniformly, it develops a sinc2(x, y) PSF, providing identifiable angular rotation during scan. From [Bei3].

(b) tolerate a polygon that is very large compared to the scan width, and (c) where interference between the scan plane and the input beam can be avoided. This conflict was overcome in the above application by separating the input and output beam planes angularly as in Figures 1.4 and 4.3, although not benefiting from reduction of the angle by double pass. Consequently, as burdened further because of the short focal length (short level arm) of the output beam, the resulting scan bow must be considered in light of the intended application.



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