Unified Optical Scanning Technology

Chapter 4.9.1 - Scanner Devices and Techniques: Implementation Methods

4.9.1 Implementation Methods

A time-dependent index gradient was proposed [G&B] utilizing resonant acoustic pressure variations in a cell of a transparent material. Although this appears similar to acoustooptic deflection (Section 4.8), it differs fundamentally in that the cell is terminated reflectively to support a standing wave rather than absorptively to maintain the traveling wave. The acoustic wavelength is much longer than the beamwidth to impart the refractive effect rather than much shorter for diffraction. An approach to a linearly varying index gradient utilizes a quadrapolar array of electrodes bounding an electrooptic material [Fow‚Los].

Fig. 4.28 Iterated electrooptic prism deflector. Alternating crystallographic z-axes provide alternating index changes that are varied in magnitude and direction by the applied signal voltage to form the tandem deflecting transitions. A uniform electric field is established in the z-direction by the parallel plate electrodes. [Bei2]

A continuous index gradient can be simulated with the use of alternating electrooptic prisms. A single-stage biprism is illustrated in Figure 4.27b and an iterated array increasing length and effect appears in Figure 4.28. Each interface imparts cumulative retardation across the beam. The direction and rate of retardation are controlled by the electrooptic index changes in the material. Because most electrooptic coefficients are extremely low, high drive voltages are required to achieve even moderate resolutions (to N 100). These devices can, however, scan at very high speeds (to 105 per s) while imposing very low time delays (<0.1μ, in contrast to the significant τ of AO devices), allowing broadband negative feedback with stable angular control of the beam. Significant review, experiment, and test are reported for the electrooptic deflector [L&Z, Bei2, I&L].

Whereas Figure 4.27 illustrates the passage of a generally collimated beam through the EO material, it is often desired to propagate a convergent beam, to form the focal point just beyond the deflector. There is also motivation, when the beam is proximate to the cell walls and executing significant deflection, to converge the beam inside a long cell to avoid encountering the walls during scan. Anticipating aberration resulting from gradient deflection of a convergent beam, this condition was analyzed and reported [Bei6] to exhibit a direct relationship to the scan angle magnitude and the f-number of the focusing bundle. The results of this work are summarized and analyzed further [Bei2] to include subsequent related consideration of beam convergence in EO deflectors [F&S, L&Z].

 

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