4.4.2 Implementation of Holographic Scanners
It is informative to consider some earlier holographic scanners and their relationship to to contemporary designs. An example of one of the first systems is illustrated in Figure 4.12, representing the family of Holofacet scanners [Bei1]. The most prominent of that group (the first patented holographic scanner) was tested in the early 1970s to the highest resolution and speed: N = 20,000 elements/scan at 200 Mpixels/s. Designed for photoreconnaissance imaging, that apparatus
Fig. 4.12 Reflective Holofacet scanner, underfilled. Input lens converges beam on rotating axis. Output scanned collimated beam is focused by flat-field lens upon flat image surface. Applied as microimage recorder (100lp/mm over 11 -mm format). From Holographic Scanning, L. Beiser, ©1988 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.
is now in the permanent collection of the Smithsonian Institution. The Figure 4.12 version also emulates a reflective pyramidal polygon with the input beam converging to focus on the axis (see Fig. 4.5). It allows, however, formation of a flat field. This option of reconstructing a collimated beam for entry into a flat-field lens is available uniquely with holographic facets when the holograms are exposed with a collimated 'object' beam (and a required 'reference' beam.) No other scanning technique (reflective, acoustooptic, electrooptic, phased array, etc.) provides for arbitrary formation of the output beam configuration. For reference, flat-field operation of a pyramidal polygon is represented in Figure 3.6, in which the polygon is illuminated with a collimated beam parallel to the axis. This illumination option for operation in radial symmetry is also available in the holographic systems described below.
The Figure 4.12 configuration was designed for microimage record-in at 100lp/mm over an 11-mm flat image surface at high speed. This corresponds to 2200 spots of 5-μm width over the 11-mm format. If imaged with a lens of increased focal length, as for reprographics application, this forms 260 DPI (dots per inch) over an 8½ in. format. As is typical where laser power is to be conserved, the facets are underfilled
Fig. 4.13 Transmissive Holofacet scanner utilizing solid glass substrate. Provides large-aperture scanned beam for high resolution. Solid substrate maintains stability and surface integrity at higher speeds than does the thin-walled design of Fig. 4.14. From Holographic Scanning, L. Beiser, ©1988 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.
and sufficiently wide to provide a high duty cycle (see Section 4.3.2). The very high-resolution and high-speed reconnaissance configurations utilized the important option of overfilling the facets (see Section 4.3.3). This is most notable in the system offering the highest single-channel performance, 50,000 elements/scan at 500 Mpixels/s; the Ultraimage Cylindrical Holofacet Scanner [Bei1]. In contrast to a laser printer forming 3000 elements/scan at 10 }Mpixels/s, it provides effectively montaged images of 50 such printers.
Although the above-described scanners were designed to diffract in reflection, as formed on a solid beryllium substrate (to maximize inertial stability at high speed), they may be formed on transparent substrates to operate in transmission at appropriate lower speeds. Two such
Fig. 4.14 Transmissive Holofacet scanner for business graphics applications. Under-filled facets direct collimated output beam through flat-field lens, forming straight line scan locus P. From Holographic Scanning, L. Beiser, ©1988 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.
arrangements appear in Figures 4.13 and 4.14 [Bei1]. Both are radially symmetric, with the input beam focused on the rotating axis. Figure 4.13 shows the use of a thick glass substrate to provide internal elastic constraint during higher-speed rotation. In Figure 4.14, the holograms are formed on a thin glass substrate that tolerates the lower-speed operation. The Figure 4.13 design forms a circular scan locus per Figures 4.5 and 4.6, whereas that of Figure 4.14 develops a flat field for reprographics, typical of business graphics scanning and recording.
A transmissive holographic scanner for reprographics was formed on a cylindrical glass substrate, as described in 1975 5P&W] and illustrated in Figure 4.15. Serving as a document scanner, it collects a portion of the scanned radiation that is backscattered from the document. This, in turn, is rediffracted by the 'Hololens" and effectively 'descanned' to converge toward point O on the rotating axis. The now fixed converging beam is intercepted by the mirror and reflected toward a small detector that is fixed at the reflected focal point O' to develop the scanned signal. This system utilizes reciprocal diffractive properties, whereby reillumination of the hologram by the negative of the 'object' wave reconstructs the negative of the 'reference' wave. Because this is true for any hologram, it is an option for any holographic scanner configuration. It is noteworthy that retroreflected signal detection (retrocollection) is not unique to holographic scanning (Section 188.8.131.52). Such reciprocity properties may serve almost any optical scanner, so long as the acceptance aperture subtends sufficient return signal to override systematic noise. Diffractive elements can also provide spectral filtering.
Early in Section 4.4.1 there is a discussion of one of the most significant advances in holographic scanning, that is, the utilization of a rotating transparent disk having an array of linear plane grating 'facets' disposed along its peripheral radius and operating in the Bragg regime. The principal beam configuration is illustrated in Figures 3.8 and 4.11a. This arrangement, which became a model for contemporary design, was initiated in the early 1980s 5Kra1,Kra2,Bei1]. Although holographic scanning allows the options of many variational configurations, utilizing transmissive or reflective substrates, having linear or lenticular gratings, maintaining rotational symmetry or asymmetry, or formed interferometrically or by computer pattern generation, the most popular format is the above transmissive disk disposed with a peripheral array of linear grating 'facets' operating in the Bragg regime. Although the output beam is uniquely not normal to the rotating axis, the scanned beam is constrained within a plane with reasonable integrity over a useful scan angle. Thus, with typical collimated input
Fig. 4.15 Transmissive cylindrical holographic scanner. Input beam, focused on the rotating axis at O, expands and underfills a hololens that diffracts the beam to focus on P along the arced image surface. Returning dashed lines designate optional collection of backscattered radiation for deriving a document scanning signal at the detector at O'. From Holographic Scanning, L. Beiser, © 1988 John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.
and output beams, a flat-field lens serves to focus the scanned beam along a straight line.
The first operational form utilized the analytically optimal Bragg angle of 45° (βi = βo 45° in Fig. 3.8) to yield a sufficiently straight line over a large enough scan angle to be useful. This Bragg orientation, per grating Equation 3-14 yields scan magnification m = λ/d = √2, forming the output scan angle, which is √2 larger than the disk rotation angle. However, two restrictions are imposed:
- High diffraction efficiency from relief gratings (e.g., photoresist) requires high depth-to-spacing ratios, whereas the spacing d = λ/√2 must be extremely narrow. This is not only difficult to achieve holographically, but also difficult to replicate.
- Such gratings exhibit a high polarization selectivity, imposing a variation in diffraction efficiency during scan when illuminated with a polarized input beam.
Fig. 4.16 Plane linear grating (Hologon) holographic disc scanner. 30° Bragg angle provides more fabricatable and polarization-insensitive grating structure. Requires, however, bow compensation prism to maintain scan beam in plane perpendicular to paper. From [Bei8].
These limitations are accommodated by reducing the Bragg angle and by introducing (in one method) a bow compensation prism to straighten the scan line. Figure 4.16 illustrates such a system: a high-performance 'Hologon" scanner designed for application in the graphic arts. With the Bragg angle reduced to 30°, the magnification (Equation 3-14) is reduced to m = λ/d = 1 (as in radially symmetric systems). This increases d to equal λ for a more realizable grating depth to render a higher diffraction efficiency. It also reduces significantly the angular sensitivity of the grating to input beam polarization.
In less demanding tasks than the above graphic arts 'typesetting' function, (e.g., laser printing), the elegance of the original 45° Bragg configuration has been adapted to achieve self-focusing, as illustrated in Figure 4.17. An added holographic lens complements the Wavelength
Fig. 4.17 Holographic disk scanner with corrective holographic lens that balances the cross-scan error due to wavelength shift and adds optical power to focus the scanned beam. 45° Bragg angle utilizes no bow compensation. Focusing holographic facets imposes precise centering of scanner disk. From L. Beiser, Laser Scanning Notebook, SPIE(1992). Reproduced by permission of the publisher.
shift of the diode laser and shapes the laser output for illumination of the scanner [Kay,Ike,Yam]. However, the control of such multifunction systems is compounded by the balancing of characteristics required to achieve their objectives. Notable is the critical orientation of the lenticular holograms on the disk and of the centration of the rotating disk [Bei1] to approach repeatedly precise positioning of the scan lines.
A system requiring significant analytic evaluation to optimize performance of several interrelated elements is represented in Figure 4.18, the main diagram of the 17 figures in its patent [C&R]. Although this illustration appears to show the path of a typical beam of finite width, it actually represents the principal rays of three beams of slightly different wavelength, as derived from a (drifting or mode-shifting) diode laser. Lines P1, P2, and P3 trace different positions (displacement highly exaggerated) of three such instances, where λ3 > λ2 > λ1 (Typ. ±1nm). Corrector hologram 108 diffracts a beam of longer wavelength to a shorter radius at scanner disk 103, such that the scanned angle and resulting line width remain constant; the opposite correction is for a shorter wavelength. However, because hologram 108 is oriented differently than in Figure 4.17, nonparallel to the scanner disk [Kay], it increases the cross-scan error. This, in turn, is compensated by cross-scan corrector hologram 101. Cylindrical element 105 is an alternate component to correct the astigmatism introduced by hologram 101. Along with cylindrically curved mirror 107, all these components inter-
Fig. 4.18 Integrated holographic scanner compensates for diode laser wavelength shifts for both along-scan and cross-scan directions. Rays P1, P2, and P3 at three different wavelengths are restored by hologram 108 to provide uniform scan line lengths and by hologram 101 to form a common focal line. Utilizing no refractive fIat-field lens, curved mirror 107 forms telecentric output and corrects scan bow. Interactions complicate analysis and design. Achieves use of replicated holograpic scanner disks. From [C&R].
relate and cooperate to form a telecentric focused line over its 9-in. scanned width.
This holographic disk operates with a Bragg angle of approximately 30°. Recalling the consequences of the Figure 4.16 Hologon system, which benefits from the favorable characteristics of this Bragg angle (unity magnification and more replicable grating contour), unfortunately, scan bow is introduced. Whereas that scanner utilizes an added prism to complement the bow, this system employs a tilted-axis orientation of the curved mirror to generate a complementary bow, furthering the interrelation of components. With production economy in mind, the developer of this system devoted extensive research in injection and cold-form molding and fabrication techniques [Row]. Utilizing the above-discussed grating replicability feature on the master holograms, they succeeded in the quantity reproduction of a variety of precision holographic elements and scanner disks.
Many additional holographic scanners have been implemented. Notable are two innovative adaptations [Kra3] of the more familiar pyramidal polygons. Because they retain the radial symmetry of the polygon types, the scan beams can propagate perpendicular to the rotating axis and develop perfectly straight scan lines. The first is effectively a transformation of Figure 1.9, in which the 45° mirror is replaced with a complementary-angled 45° plane linear grating hologram. Thus, with typical collimated input illumination along the rotating axis, the diffracted beam scans through a flat-field lens, as in Figure 1.9. Because the Bragg angle is 45°, this scanner exhibits the optimal effects of reduced wobble coupled, however, with the two limitations identified above of challenging grating dimensions and polarization sensitivity. The second configuration is a multifacet extension of the first, essentially an analog of Figure 3.6, underfilled. It is comprised of four individual (plane linear grating) holographic facets mounted at 45° in a cylindrical carrier, with the pyramidal apex facing away from (rather than toward) the input beam. The facets form a true four-sided pyramid. As such, they exhibit the radial discontinuities of a polygon scanner (e.g., differential inertial stress). And, assembled individually as completed holograms, they require orientation and tilt adjustment to attain accurate scan line repeatability. Alternatively, the sensitized plates may be mounted for exposure in an armature having identical bearing support. Indexed angularly, exposed and processed in situ, the armature is then assembled into the scanner, holograms well oriented. A similar self-aligning technique was employed more easily on the continuous substrates, for example, spherical per Figure 4.12, and disk of Figures 4.16 and 4.18. Holographic Scanning (Beiser, 1988) provides a comprehensive analytic and historic review of much additional holographic scanner development.