Planar Electrooptic and Acoustooptic Bragg-Deflectors

Planar i.e. waveguide Bragg-deflectors are becoming increasingly important for applications of low laser power, eg. in signal processing devices. There are several advantages of planar devices compared to bulk devices. Figure 1 compares the two versions of a light deflector or modulator. In the bulk version the laser beam is focused into the interaction volume. For a given modulating field strength, which is limited by dielectric break down, nonlinear or thermal effects, a certain interaction length L is required to achieve a given diffraction efficiency or modulation depth. Due to diffraction of the freely propagating beam in a bulk device (Figure la) the beam spreads to a diameter D = (λL/π)1/2 yielding a minimum interaction volume V = D2L≈λL2/π. If we confine instead the light beam in a slab waveguide of depth d≈λ *, we only have diffraction spreading in the lateral dimension of0 the beam and the necessary interaction volume is reduced by a factor d/D≈λ0/D. For the same interaction medium the required drive power is consequently reduced by a factor of (λπ/L)1/2, ie. typically several orders of magnitude.