A Bragg modulator is formed in a LiNbO3 planar waveguide which is capable of propagating only the lowest order mode for light of 6328 A (vacuum) ˚
wavelength. This mode has β = 2.085 × 105 cm−1. The bulk index of refrac-
tion for LiNbO3 at this wavelength is 2.295. The acoustic wavelength in the
waveguide is 2.5 μm, the optical beam width 4.0 mm, and the interaction
length 2.0 mm. Obtain the following:
(a) The angle (in degrees, with respect to the acoustic wave propagation
direction) at which the optical beam must be introduced so as to obtain
maximum diffraction efficiency.
(b) If the input optical beam is a uniform plane wave, what is the angular
divergence (in degrees) between the half-power points of the diffracted
beam, and at what angle does it leave the modulator?
(c) Make a sketch of the modulator, labeling the angles found in a) and b).
(Do not attempt to draw the angles to scale.)