Efficient and wideband acousto-optic modulation on thin-film lithium niobate for microwave-to-photonic conversion

Abstract

Microwave photonics, a field that crosscuts microwave/millimeter-wave engineering with optoelectronics, has sparked great interest from research and commercial sectors. This multidisciplinary fusion can achieve ultrawide bandwidth and ultrafast speed that were considered impossible in conventional chip-scale microwave/millimeter-wave systems. Conventional microwave-to-photonic converters, based on resonant acousto-optic modulation, produce highly efficient modulation but sacrifice bandwidth and limit their applicability for most real-world microwave signal-processing applications. In this paper, we build highly efficient and wideband microwave-to-photonic modulators using the acousto-optic effect on suspended lithium niobate thin films. A wideband microwave signal is first piezoelectrically transduced using interdigitated electrodes into Lamb acoustic waves, which directly propagates across an optical waveguide and causes refractive index perturbation through the photoelastic effect. This approach is power-efficient, with phase shifts up to 0.0166 rad / √ mW over a 45 μm modulation length and with a bandwidth up to 140 MHz at a center frequency of 1.9 GHz. Compared to the state-of-the-art, a 9 × more efficient modulation has been achieved by optimizing the acoustic and optical modes and their interactions.

Thin-Film Lithium Niobate Based Acousto-Optic Modulation Working at Higher-Order TE1 Mode

“…The respective microwave working frequencies are 4.06 GHz and 4.08 GHz. In addition, since large gap width G reduces the metal absorption loss of the optical waveguide and the whole AOM device [1][2][3][4][5], large gaps are preferred in on-chip AOM under the same fabrication processes and requirements. The maximum value locates at the width of w = 2.1 μm for both of the gap width considered.…”

“…The gap width between the optical waveguide and IDT is G. Moreover, the gap width between the IDT and the left side of the acoustic resonator is chosen as L t (=1.2 µm), to match with the wavelength of the generated SAW. Considering the current fabrication abilities, the slant angle of the TFLN waveguide sidewall is set to 70 • for the proposed AOM design [3,[19][20][21][22][23][24]26]. Note that we also etch two relatively large slots on both sides of the AOM along the y-z direction as shown in Figure 1a (light gray rectangular) to make the corrosive liquid contact with the buried oxide (BOX) layer below the AOM region and remove it via the corrosion method.…”

“…Acousto-optic modulation (AOM) connecting the electric, mechanical, and optical fields, takes a vital role in the microwave, lightwave, and quantum photonic signal processing [1][2][3][4][5]. Comparing with integrated electro-optic modulations (EOMs) [6][7][8][9][10], AOM typically works with multiple physical fields and its basic working process can be described as follows.…”

“…Secondly, the generated acoustic wave would interact with the optical mode of the waveguide, leading to a change in the mode effective index of the optical waveguide. Finally, the mode effective index change is used to modulate the output spectrum via waveguide structures, such as a Mach-Zehnder interferometer (MZI) [1,3] or microring resonator (MRR) [1,4]. On-chip integrated AOM can enhance the AOM performance and reduce the device dimension, but requires an ideal material platform to ensure efficient couplings between the different physical fields.…”

“…The proposed AOM scheme would contribute to the development of on-chip low power, low loss, and compact AOM. AOM-related applications and components, such as frequency shifter [1,5], tunable filter [1,5], beamforming [3], and cavity optomechanics [33][34][35] would also be benefited, contributing to the building of on-chip photonic systems.…”