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6.22 LiNbO3 SAW AO modulator A SAW AO modulator is shown in Figure 6.36 and operates at a wavelength of 1.5 µm. The interdigital electrodes are driven from a 500 MHz RF source. What should be the periodicity of the interdigital electrodes? What is the deflection angle? If the length L of the transducer is 5 mm, and the power in the acoustic wave is 200 mW, estimate the diffraction efficiency.

FIGURE 6.36 A simplified and schematic illustration of a surface acoustic wave (SAW)-based waveguide AO modulator. The polarity of the electrodes shown is at one instant, since the applied voltage is from an AC (RF) source. Incident light Acoustic wavefronts Acoustic absorber Acoustic absorber Modulating RF voltage Interdigital electrodes Thin film waveguide Diffracted light Doped region with higher n Piezoelectric crystal (LiNbO3) substrate