Unified Optical Scanning Technology

Chapter 4.8.1 - Scanner Devices and Techniques: Operating Principles

4.8.1 Operating Principles

An acoustooptic deflector showing typical beam propagation is illustrated in Figure 4.22. Although several interactions of light and sound have been investigated intensively [Adl,D&Se,Kor,Gor,Bed], these devices operate in the Bragg diffraction domain, in a manner described

Fig. 4.22 Acoustooptic deflector. Angles exaggerated for illustration. Electrical drive upon acoustic transducer generates traveling acoustic wave in elastic medium. Resulting periodic index change simulates a thick optical grating. Input beam and output beam (at position b) form nominal Bragg angles. Relationship between the output beam positions and the acoustic frequency is tabulated. From [Bei8].

initially in Section 4.4.1 for some disktype holographic scanners. Although there is a distinction in the type and thickness of grating (a surface relief grating on the holographic disk and a volume grating of substantive thickness in the AO device), the diffraction properties are similar.

As rendered in Figure 4.22, a diffraction grating of spacing A is synthesized by the wavefront spacings of an acoustic wave traveling through a block of elastic material (which is transparent to the optical wavelength of interest). At the base of the block is fastened an acoustic (piezoelectric) transducer that converts an electrical drive signal to a pressure wave that traverses the medium at its acoustic velocity vs. At the far end, the energy in the traveling wave is consumed by the acoustic absorber to suppress standing waves. The pressure wave in the medium forms a corresponding photoelastic variation in its refractive index, forming the synthetic optical grating. An incident light beam of width D is introduced at the Bragg angle ΘB (shown exaggerated). Although the above-developed index variation is extremely small, the cumulative effect of the beam propagating through the long path within the medium (thick grating of length L > Λ2/λ) provides for a substantive diffraction efficiency. In the perfect Bragg condition, the energy in all pairs of diffraction orders is transferred to one first order at the output Bragg angle ΘB, attaining a theoretical diffraction efficiency of 100%. Operationally, depending on compliance of Bragg, beam shape and polarization, and optical wavefront purity, drive signal characteristics, and material and coating losses, the diffraction efficiency can range between 50% and 90%.

In Figure 4.22, the Input Beam is incident at the Bragg angle, ΘB = ½λ/Λ.The undeflected output beam at (a) is the undiffracted zero order. The output beam at (b) is the diffracted first order, at the Bragg angle ΘB when the drive signal is at center frequency fo. The dashed-line output beam at (c) is the deflected beam displaced "positively" from position (b) when fo is increased to fo + Δf, thereby decreasing the grating spacing. The 'negatively' deflected beam (not shown) appears on the other side of (b) when fo is decreased to fo - Δf, increasing the grating spacing. [Note: Small delta (Δ) modifying (f) signifies the change in frequency; large delta (Δf) represents the full frequency bandwidth.] The full Δf is typically limited to one octave to avoid formation of 'ghost' diffraction spots that can be generated by harmonics of the acoustic frequency.

If the Bragg angle is set symmetrically at center frequency fo, then when Δf is instituted the output beam is deflected, altering and disrupting the perfect Bragg condition. Unless rectified as indicated below, this depletes the transfer efficiency, which can be reduced by about 1/3 of its initial values as the Δf is shifted to its typical limits of about ±¼ of its center frequency. Although seldom applied, a direct attack on the source of this problem is altering the direction of the acoustic wave to track and maintain bisecting the incident and diffracted beams during scan [Kor,CG&A,Got]. A principal method utilizes a phased array of acoustic transducers, in a manner discussed for optical scanning in Section 4.10.1. Acoustic beam steering for Bragg angle sustenance was implemented successfully as early as 1966 by A. Korpel and his associates [Kor] at Zenith Radio Corporation (and witnessed in their laboratory by this author), in their pioneering develpment of an acous-tooptically deflected (15,750 scans/s) projection laser display system. The acoustic medium was water.


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