Unified Optical Scanning Technology

Chapter 4.8.3.2 - Scanner Devices and Techniques: The Traveling Lens and Chirp Deflectors

4.8.3.2    The Traveling Lens and Chirp Deflectors    Both the traveling lens and the chirp deflectors utilize acoustooptics in a function-ally similar manner that differs significantly from the conventional AO deflector discussed in Sections 4.8.1 and 4.8.2. Instead of diffracting an incident beam through an angle that is determined by the drive frequency, a pressure wave, developed in the medium by a fixed frequency pattern, shapes the index variation in the medium to appear as a small lens that travels through the medium at the acoustic velocity. Illumination of this traveling simulated lens with a narrower beam forms a smaller focal point that travels across the full aperture length, forming a long linear scan at the high acoustic velocity and providing notably high resolution. A conventional AO deflector prescans the beam that illuminates and tracks the traveling synthetic lens in the linear scanner. Allowing significantly higher resolutions, these systems still serve as flying spot scanners.

The traveling lens concept was described in 1970 [FC&C] by Zenith Radio Corporation, followed by intensive investigation by the Harris Corporation [J&M]. The basic process is illustrated in Figure 4.24,

Fig. 4.24 Basic traveling lens process. As in Fig. 4.22, a traveling wave propagates "upward" at velocity V while an input beam of height D is scanned at the same velocity to track a crest in the harmonic pressure wave. The resulting focused spot of size δ « D scans also at velocity V‚ multiplying resolution by the factor D/δ. After R.J. Johnson and R.M. Montgomery, "Optical beam deflection using acoustic-traveling-wave technology," in Acousto-Optics/Instrumentation/Applications, Proc. SPIE, Vοl. 90 (1976). Reproduced by permission of the publisher.

where a cell of acoustooptic material of thickness L exhibits a traveling sinusoidal pressure wave propagating at velocity V along the X-direction (generated by a piezoelectric transducer driven by a sinusoidal r-f frequency that establishes wavelength Λ within the material.) An input beam of width D enters the cell at a location coincident with the crest of one harmonic pressure wave and tracks this crest at velocity V as described above. When the width of the beam satisfies D Λ/4, the beam encounters a near-parabolic index change, acting as a refractive cylindrical lens of length L. This provides for acceptably low-aberration focusing of the beam to the line width δ such that δ << D. Thus the resolution developed originally at the AO deflector is multiplied by the factor D/δ in the focused X–direction. Additional cylindrical focusing in the quadrature y-direction is utilized to reshape the 'line' spot to round or elliptical, as required. Resolution gains of 40 have been achieved over a scan length of up to 10 in. [J&M].

Figure 4.25 illustrates the scanning and optical transfer arrangement of a traveling lens system. A broad illuminating beam enters a conventional AO beam deflector at the left. In the upper illustration, the angular change at the output corresponds to ±Θ in Figure 4.22, with the AO aperture fully illuminated. Bragg angles at the input and output are omitted for clarity. Scan direction is such that the solid-line beam moves toward the dashed-line beam. Lens L1 is oriented approximately telecentrically (Z f1) between the AO deflector and its traveling focal point, transforming the angular scan to a linear scan. This scanning focal point in plane P1 is then relayed by lenses L2 and L3 into the traveling lens cell, plane P3. There, the synthetic lens (shown as a real lens) starts at the bottom of the cell illuminated with the beam height D as in Figure 4.24, which tracks the lens as it travels upward at velocity v^ over the magnified scan distance Ls. Lenses L1 and L2 may be regarded as a beam compressor, acting in a manner very similar to that in Figure 1.7, thereby magnifying the small scan angle of the AO deflector (shown in Fig. 1.7 as Θ/2 enlarged to the output angle Θ'/2, as noted in the caption). At Plane 3, the traveling lens focuses the incoming collimated beam to the scanning focal point δ outside the cell, as represented in Figure 4.24. Inside the cell, the traveling wave extends over the full thickness of the cell (L in Fig. 4.24) focusing in a continuum, as by a graded-index cylinder of length L. Because the traveling lens cell provides optical power anamorphically in the plane of the paper, quadrature optical power is provided in and out of the paper by cylindrical lens L4 to form a corrected two-dimensional spot. The bottom illustration shows the optical action in that plane.

Fig. 4.25 Traveling lens system. Top View illustrates an acoustooptic beam deflector (AOBD) as in Fig. 4.22 with optical scan angle magnification as in Fig. 1.7. They provide the prescanned beam of height D that tracks the traveling lens in plane P3 to focus (per Fig. 4.24) in the plane of the paper. Side View shows the contribution of the cylindrical lens L4, which focuses the beam in the quadrature direction. After R.J. Johnson and R.M. Montgomery, "Optical beam deflection using acoustic-travel-ing-wave technology," in Acousto-Optics/Instrumentation/Applications, Proc. SPIE,Vol. 90 (1976). Reproduced by permission of the publisher.

In 1979, Workers at Rockwell International and Harris Corporation [YW&M] reported an advance that significantly reduces the substantive electrical power consumed by the traveling lens. This is accomplished by replacing the continuous r-f signal (forming the continuous sinusoid in Fig. 4.24), with a narrow r-f pulse, utilizing its central lobe as an isolated lens traveling at velocity vs. This conserves the energy otherwise dissipated in the unused portion of the full field of Figure 4.24. Power reductions by a factor of 25 have been observed. Some demanding special requirements must be accommodated, as discussed in the referenced work.

The chirp deflector [Bade] utilized a different r-f drive technique in the traveling lens system, forming the lens diffractively rather than refractively. By driving the scanner with bursts of r-f signal that shape the synthetic lens, it not only conserves power as described above but allows for adjusting the focal length of the lens while maintaining a large acoustooptic index change.

The basis of forming the diffractive lens is gleaned from the grating equation (Eq. 4-25) for small angles, with Θ1 = 0 and n = 1,

indicating that Θ is inversely proportional to the acoustic wavelength Λ or directly proportional to the drive frequency f. When the frequency is altered rapidly such that portions of the incident beam are exposed to progressively varying wavelengths (grating spacings), the beam will converge or diverge, depending on the direction of the frequency change. The lensing is converging for increasing frequency (df/dt > 1), and conversely for decreasing frequency. With a linear frequency change, identified as a 'chirp,' the focal length of the beam is given by [G&M]

The chirp deflector is illustrated in Figure 4.26a, in which the scanning mode is directly analogous to the operation of the traveling lens cell at the far right of Figure 4.25, having the refractive lens replaced by a diffractive one. With the chirp pulse traveling 'upward' at velocity vs, it is comprised of a decreasing acoustic wavelength, that is, driven by an increasing signal frequency to form a positive lens. The input laser beam scans upward at the same velocity to track the chirp diffractor in its

Fig. 4.26 The chirp deflector. Utilizes a diffractive traveling lens. The drive signal to the acoustic transducer (Fig. 4.22) forms a positive "chirp"—a rapid increase in frequency—that synthesizes a lens of positive focal length traveling "upward" at velocity Vs. (a) Scanning Mode: Prescanned beam tracks the chirp at the same velocity. (b) Flooded Mode: Wide beam fills the full aperture, eliminating need for prescan, but sacrificing beam energy while illuminating the nonlens region. After L. Bademian, "Acousto-optic laser scanning," in Optical Engineering, Vol. 20 (1981). Reproduced by permission of the publisher.

formation of the scanning focal point. Figure 4.26b illustrates a flooded mode of operation, scanning the same distance Ls. By filling the acoustic aperture with the input beam, as in Figure 4.23, the need for prescanning is eliminated and flyback time may be reduced effectively to zero. As in most overfilled systems except, for example, in the Scophony, the main trade-off is the waste of input beam energy when not illuminating an active lens region.

 

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