Unified Optical Scanning Technology

Chapter 4.8.3 - Scanner Devices and Techniques: Alternate Acoustooptic Deflection Techniques

4.8.3 Alternate Acoustooptic Deflection Techniques    The Scophony Scanner    Although AO deflection is dominated by the implementations discussed above, several variations are significant. The earliest investigated, only five years after the theoretical exposition of acoustooptics in 1932 [D&Se], was the technique applied to the Scophony TV receiver [Oko]. This more expansive concept, which remains known as the Scophony process, was analyzed more completely [Joh,JG&S] and continues to be encountered in varying amounts of application of its principles.

All active scanners considered here-other than the Scophony-are known as flying spot scanners, as introduced in Section 1.2. That is, they all cause the motion of a single spot or image element, typically 'flying' at a high velocity. This descriptor has been used since the early years of cathode-ray tube (CRT) scanning [B&Y]. In the Scophony scanner, however, the acoustic traveling wave usually encompasses more than one image element. Although the earliest Scophony scanner was illuminated with incoherent light, and the imaged multiple spots remained incoherently related, one must now consider the consequence of illuminating the AO device with a laser, spatially coherent.

When the laser-illuminated AO device scans a single spot, although the beam is spatially coherent, its motion in the image plane destroys the relative coherence of adjacent spots, resulting in spot resolutions typified by an incoherent source. The resulting modulation transfer functions (MTFs) also remain as of an incoherent source, and are also so represented in Chapter 3. However, the virtue of the Scophony scanner depends on the acoustic wave operating on a broad illuminating beam subtending several information elements. This is accomplished as the acoustic wave, now modulated by an information stream, propagates through this broad beam, affecting the medium correspondingly in elements of grating intensity. The modulating acoustic drive signal is now characterized as AM, not FM.The Scophony process is a modulation, which causes a stream of diffractive elements to travel at the acoustic velocity. When this pattern is reimaged, it will travel at this velocity magnified by any optical magnification. Thus the Scophony modulator requires a subsequent deflector to cancel the motion of the rapidly moving array of spots to form a stationary image. Consider Figure 4.23.

A laser and beam expander illuminate the Scophony modulator with, as indicated above, a broad beam. Otherwise, the configuration is very similar to that of the AO deflector, while the acoustic drive center

Fig. 4.23 Scophony scanner optics. Expanded laser illumination enters modulator as in Fig. 4-22. Two bursts of acoustic wave, traveling "upward," cause diffraction of two beams that are then focused by lens of focal length F at point P on def1ector plane. Underviated zero order is also focused at deflector plane, where it is absorbed by stop S. Diffracted beams continue through 2nd focal distance MF (M = magnification) twice, arriving at image plane traveling "downward." Deflector (not shown) in vicinity of point P scans beams in reverse direction to immobilize them on the image plane. Sub-sequent scans lay down subsequent beam positions in a raster, forming a correct image. From [Joh].

frequency (fo in Fig. 4-22) remains constant. When not driven (fo off), the zero order propagates through directly. When driven, the diffracted components exit, satisfying the nominal Bragg condition for high efficiency. The illustration represents a binary on-off acoustic drive, simulating 100% AM modulation. Two bursts of drive, forming two diffraction gratings, are shown traveling upward through the medium at the acoustic velocity. The exiting diffracted beams and the zero-order beam encounter a first lens of focal length F such that they focus at the surface marked "deflector plane.' Because both diffracted beams are collimated and are incident at the lens at the same angle, they form a single focal point P. The zero order is blocked by a "stop." The two diffracted beams continue toward a second lens of focal length MF (M = optical magnification) to image two related moving spots on the image plane. A 'conventional' deflector is now added in the vicinity of point P to scan the incident beams in a reverse direction at such speed as to immobilize the output image. Thus a stationary object that is represented by the previously scanned moving data points in the Scophony modulator will be portrayed properly as a stationary image. The inverse velocity must be exact; otherwise, cancellation will be imperfect, and the image will drift.

This imaging system is a simple form of schlieren optics, in which the scattered (diffracted) information components are rendered clear optical passage. The focal region near P is a good location for a deflector because of its small size (exhibiting the Fourier transform of the grating transmission). Derived from a large diffraction region, it con-verges and expands rapidly around focus (depending on f-number), motivating use of a mirrored deflector such as a polygon scanner, rather than one having substantive thickness, such as an AO scanner. The relative coherence of adjacent spots, identified above, now merits attention. Fully coherent imaging of high-contrast objects (such as bar targets) yields significant edge ringing, degrading resolution [P&T,Joh]. This may be alleviated to the degree that coherence is reduced. Some Scophony systems reduce the number of data elements appearing within the diffracting aperture, thereby approaching the resolution characteristics of coherent single spot scanning noted above. An objective of the Scophony system is to maintain a wide illuminated aperture, to maximize acoustooptic modulation efficiency.



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