Acousto-optical modulation of thin film lithium niobate waveguide devices
Acousto-optic (AO) effect, which describes the interaction between light and sound waves, is extensively used to build up frequency shifter, tunable filter and light beam deflectors, leading to applications including laser Q-switching/mode-locking, signal processing and manipulation of light beams. Among all the acousto-optic active materials, lithium niobate (LN) is one of the most attractive materials because of its high piezoelectric coefficients enabling efficient generation of surface acoustic waves (SAW) through interdigital transducer (IDT) technology. Compared with the state-of-art AO modulator fabricated on other piezoelectric materials such as GaAs, LN exhibits a higher electromechanical coupling coefficient, which dramatically simplifies the design of SAW devices and facilitates matching to radio frequency (RF) driving sources in a much smaller form factor. In addition, the strong photo-elastic effect enables efficient photon-phonon interactions in such material. These properties have made LN one of the most promising materials for realizing integrated AO devices for inertial sensing, holographic storage/imaging and beam steering applications. However, conventional AO devices built on bulk LN wafers suffer from the limited AO interaction due to the weak light confinement in waveguide structures created by titanium-indiffusion and proton exchange methods. Typical waveguide mode size is in the dozens of square micrometers (μm2), and the bend radius on the order of centimeters, resulting in relatively large device footprint and hindering high density integration.
