Hybrid integration of microwave acoustics and photonics for rf microsystems on thin film lithium niobate

Hassanien, Ahmed E.

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https://hdl.handle.net/2142/115505 Copy

Description

Title

Hybrid integration of microwave acoustics and photonics for rf microsystems on thin film lithium niobate

Author(s)

Hassanien, Ahmed E.

Issue Date

2022-02-02

Director of Research (if dissertation) or Advisor (if thesis)

Gong, Songbin

Doctoral Committee Chair(s)

Gong, Songbin

Committee Member(s)

• Goddard, Lynford L.
• Jin, Jianming
• Hanumolu, Pavan K.

Department of Study

Electrical & Computer Eng

Discipline

Electrical & Computer Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Photonics
• MEMS
• RF
• 5G
• Integration
• Lithium Niobate

Abstract

Lithium niobate, a synthetic crystal known for its various properties, such as the strong electro-optic, photo-elastic, and piezoelectric effects. These properties are useful for linear and nonlinear optical applications and the generation and detection of acoustic waves. Moreover, lithium niobate 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. Lithium niobate thin-film on insulator (LNOI), a revolutionary technology, became recently available through smart cut technology, giving rise to a myriad of new devices and applications with a high level of integration and performance. This dissertation will demonstrate LNOI applications in microwave acoustics, integrated photonics, and the hybrid integration between them. First, near-zero drift and high electromechanical coupling acoustic resonators have been designed for, demonstrated with, and motivated by the development of 5G and internet of things applications. The acoustic resonator is based on Lamb-acoustic waves in a bimorph composed of lithium niobate on silicon dioxide. Our approach breaks through a performance boundary in conventional Lamb-wave resonators by introducing the bimorph while operating at higher-order resonant modes. This enables the resonator to achieve frequency scalability, a low-temperature coefficient of frequency, and high electromechanical coupling altogether. The electromechanical coupling and temperature coefficient of the resonator were analytically optimized for the A3 mode by adjusting the thicknesses of different materials in the bimorph. Resonators with different dimensions and stack thickness were fabricated and measured, resulting in a temperature coefficient of frequency ranging from -17.6 to -1.1 ppm/°C, high electromechanical coupling ranging from 13.4 to 18%, and quality factors up to 800 at 3.5 GHz. The achieved specifications are adequate for 5G sub-6 GHz frequency bands n77 and n78. Second, 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 was considered impossible in conventional chip-scale microwave/mm-wave systems. Conventional microwave-to-photonic converters, based on the resonant acousto-optic modulation, produce highly efficient modulation but sacrifice bandwidth and limit their applicability for most real-world microwave signal processing applications. This dissertation builds 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 photo-elastic effect. This approach is power efficient, with phase shifts up to 0.0166 rad/√mW over a 45 µm modulation length and with 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. Finally, in this dissertation, we designed, implemented, and characterized compact Mach-Zehnder interferometer-based electro-optic modulators. The modulator utilizes spiral-shaped photonic wires on Z-cut lithium niobate and the preeminent electro-optic effect which is applied using top and bottom electrodes. Photonic wires are made of rib etched lithium niobate waveguides with bottom silicon oxide cladding, while SU8 polymer covers the top and sides of the rib waveguides. The proposed implementation resulted in low optical losses < 1.3 dB/cm. Moreover, compact modulators that fit 0.286 cm and 2 cm long photonic wires in 110 µm × 110 µm and 300 µm × 300 µm areas, respectively, were achieved. For single arm modulation, the modulators achieved a VπL of 7.4 V.cm and 6.4 V.cm and R-C time constant limited 3-dB bandwidths of 9.3 GHz and 2.05 GHz, respectively. Push-pull modulation is expected to cut these VπL in half. The proposed configuration avoids traveling wave modulation complexities and represents a key development towards miniature and highly integrated photonic circuits.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Ahmed Hassanien

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