The development of photonic integrated circuit (PIC) applications prompted by optical transceivers for data centers, microwave photonic-based signal processing, quantum computing, spectroscopy, and holography, demands more efficient means to control lightwave propagation. One efficient method to achieve this objective is to utilize light-matter interactions through acousto-optic (AO) devices.
Fundamentally, AO devices enable the interaction via perturbing the refractive index in an optical medium by acoustic waves. The perturbation is made possible by the photo-elastic effect in the medium where acoustic and optical waves can be launched and guided independently. Several practical bulk-wave AO devices have been realized, including optical modulators, frequency shifters, switches, tunable filters, isolators, spectrum analyzers, scanners, and correlators.
Piezoelectric thin films such as gallium arsenide (GaAs) and lithium niobate (LiNbO3, or LN) are promising candidates for AO devices. These films have high refractive index contrast to their surroundings for lightwave confinement and are compatible with generating acoustic waves using simple interdigitated transducers. AO devices on different substrates have recently been used for many applications, including modulators, frequency shifters, and tunable filters and applications spanning phase-sensitive imaging, and 3D holography.
The advances of microwave photonics have recently been accelerated by unprecedented microwave-to-photonic conversion demonstrated in thin-film lithium niobate on insulator. Lithium niobate is a synthetic crystal known for its various properties, such as its strong electro-optic, photo-elastic, and piezoelectric effects. These properties are helpful for linear and non-linear optical applications and the generation and detection of acoustic waves.
Moreover, LN has a negative uniaxial birefringence with a high refractive index (~2.13 at 1550 nm) and a high index contrast to many dielectrics, permitting strong confinement of optical modes and PIC miniaturization.
In previous research efforts, optical waveguides are inserted into resonant acoustic cavities, producing efficient AO modulators but sacrificing bandwidth (<0.1%) and limiting their applicability for most real-world microwave signal processing applications.
The research group, led by Prof. Songbin Gong in collaboration with Prof. Lynford L. Goddard from University of Illinois at Urbana-Champaign, developed highly efficient and wideband microwave-to-photonic modulators using the acousto-optic effect in suspended lithium niobate thin films. The research results are published in Photonics Research, Volume 9, No. 7, 2021(Ahmed E. Hassanien, Steffen Link, Yansong Yang, Edmond Chow, Lynford L. Goddard, Songbin Gong. Efficient and wideband acousto-optic modulation on thin-film lithium niobate for microwave-to-photonic conversion[J]. Photonics Research, 2021, 9(7): 07001182).
In this article, we employ traveling acoustic waves to pass through the optical waveguide, eliminating the resonant nature in prior approaches, resulting in highly desirable wideband modulators. This approach provides filtration to the input microwave signal without any additional circuitry due to the bandpass spectral response of the microwave transducers, which makes it a promising candidate for 5G and IoT applications where an optical signal is used for direct communication between 5G base stations and data centers. Other applications, such as frequency comb generation, can also benefit from wideband and efficient AO modulators.
They present the design, implementation, and measurements of an efficient AO modulator using an unbalanced Mach Zehnder interferometer (MZI) on thin-film LN. The thin film is fully suspended, enabling the generation of Lamb acoustic waves (plate waves) that possess higher electromechanical coupling than surface acoustic waves, resulting in the significantly more efficient microwave to acoustic conversion.
Microscope Image of The Fabricated Acousto-optical Modulator.
Acoustic modes are confined within the suspended film by the velocity mismatch boundary condition at the LN/air interface. On the other hand, optical modes are confined to the plane by the index contrast at the LN/air interface and guided laterally by a photonic crystal waveguide made of a square lattice of air holes inside the LN suspended film.
The confinement of waves within the thin film features a unity overlap between the acoustic and optical modes resulting in the efficient microwave-to-photonic conversion. AO modulators with a phase shift up to 0.0166 rad/√mW, a center frequency of 1.9 GHz, and a bandwidth up to 140 MHz have been demonstrated.
Moreover, a narrowband AO modulator with an optical waveguide inserted inside an acoustic cavity is reported in this article to be compared with the state-of-the-art AO modulators where a 9× more efficient modulation has been achieved by optimizing the acoustic and optical modes and their interactions.
