Acousto-optical modulator based on elastic energy trapping in high aspect ratio interdigital transducers
Résumé
Background, Motivation and Objective Acousto-optics (AO) is a well established concept, widely used in applied photonics for the control of the propagation of light beams. Surface acoustic waves have been for long considered as the privileged way of modulating light in monolithic integrated optical devices due to their intrinsic confinement at the substrate surface. The increasing demand for compact, low-power photonic devices however calls for the development of more efficient AO modulators. A very promising direction lies in a better use of the available acoustic energy. This can be obtained through a tighter confinement of the elastic strain at the vicinity of the optical guided modes. Statement of Contribution/Methods We propose an AO intensity modulator exploiting the elastic energy trapping capabilities of an interdigital transducer with high aspect ratio electrodes (HAR IDTs). The device relies on an integrated Mach-Zehnder interferometer (MZI) consisting of optical waveguides obtained by annealed proton-exchange of an X-cut lithium niobate substrate. The acoustic modulation is applied through an 18-μm pitch HAR IDT on a single arm of the interferometer in a transverse configuration. The device was optically characterized in a butt-coupling configuration and the output signal monitored in both the time and frequency domains. The influence of the acoustic modulation on the channeled spectrum of the MZI was also investigated. Results/Discussion A strong slowing down of the acoustic wave velocity along with a tight confinement of the elastic displacement field in the electrodes is observed for the modes lying well below the sound cone, resulting in a wave velocity of 1200 m/s for the slowest mode. A leaky surface wave is also excited at about 145 MHz. A modulation of the output optical signal is observed for each of the IDT resonance frequencies. The measured acousto-optical modulation amplitude is currently limited (15% for a confined mode, 5% for the leaky SAW) but can be increased by adjusting the MZI operating point which is not optimal in the present configuration. More unexpectedly, measurements performed at zero-frequency with an optical spectrum analyzer revealed a wavelength shift of the interferometer channeled spectrum reflecting a static modulation of the optical signal. This effect is directly linked to the electro-acoustic response of the HAR IDT and results in a shift as high as 20 nm at 145 MHz.