Unified Optical Scanning Technology

Chapter 4.3.5.3 - Scanner Devices and Techniques: Overfilling, Double-Pass, and Facet Tracking

4.3.5.3    Overfilling, Double-Pass, and Facet Tracking     Although discussion in the preceding sections addresses the most popular form of polygon operation, other techniques may be utilized advantageously. An important variation to the above underfilled prismatic polygon is the overfilled polygon (Section 4.3.3), a polygonal adaptation of Figure 1.4, represented in Figure 4.3 in double-pass form. Such operation could be accomplished by illuminating the polygon asymmetrically as in Figure 4.2. However, the symmetry of illumination on the facets (in the scan direction) is advantageous for Figure 4.3; it reduces the angle α (Fig. 4.2) to zero. This option is extremely conservant in polygon diameter, for the full facet serves as the beam aperture. When overilluminated (overfilled), the duty cycle can approach 100% by illuminating two adjacent facets. In contrast, when underfilled, obtaining a high duty cycle requires that the facet be much wider than the beam, incurring a substantial increase in the polygon diameter. Thus Figure 4.3 represents a class of polygon scanners capable of providing the combination of very high resolution and speed simultaneously [Bei2]. Its principal trade-off is the loss of illuminating flux when an input Gaussian beam is expanded sufficiently to overfill two facets while approaching uniform flux density on each active facet. Another precaution is the need for minimizing the angular separation (β; Fig. 1.3) between the

Fig. 4.3 Prismatic Polygon in double-pass configuration. Input beam, expanding beyond focus, is directed by folding mirror through flat-field lens, filling polygon facets with collimated light. Scanned reflected beam is refocused by flat-field lens, forming scan line. Input and output beams are skewed slightly (vertically) for separation by folding mirror. From [Bei3].

input and output beams, to allow formation of a usefully straight (unbowed) scanned locus.

The illumination of two adjacent facets by one expanded beam is the most accessible form of filling the facet while attaining a high duty cycle. However, a quest for this advantage without serious loss of light flux is represented by the technique of 'facet tracking." This 'simple' principle of scanning the illuminating beam synchronously to track the center of the moving facet has, in practice, required novel beam manipulation technique that impose added complexity. Three methods of facet tracking are identified in Section 4.3.4.

Because overfilling and facet tracking can be applied to pyramidal as well as prismatic polygons, it is appropriate to review the characteristics of the pyramidal polygon, particularly when operating in radial symmetry. [Although it is possible to configure the prismatic polygon in radial symmetry (per Fig. 4.10), it is rarely operated in that mode.]

Referring to Section 3.4.1 and its Figure 3.6, note that in radial symmetry, scan magnification m = 1. Also notable is the relative freedom of separating the input and output beams of the pyramidal polygon (see Section 4.3.5), allowing for a minimal pupil relief distance and a correspondingly reduced size of the flat-field lens (in preobjective scan) achieving otherwise similar performance. Also provided is the nominally perfect straight scan line, suffering no bow in propagating axially, through the flat-field optics (in contrast to the possible bow in the above-described double-pass mode of Fig. 4.3.)

 

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