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Optical True Time Delay Device for mm-Wave Antenna Array Beamforming

Almhmadi, Raed Ali M
http://rave.ohiolink.edu/etdc/view?acc_num=osu1566073503380698

Abstract Details

2019, Doctor of Philosophy, Ohio State University, Electrical and Computer Engineering.
The demand for higher-data-rate, energy efficient, and tunable devices continues to grow as more and more devices are connected together in the context of internet of things. To sustain the exponential growth in wireless connectivity, ultra-wideband communication scenarios are targeted for 5th generation (5G) wireless systems, where much higher data rates can be obtained due to the utilization of millimeter-Wave (mmW) bands, beyond 26GHz. Due to much increased path losses at such high-frequencies, high-bandwidth mobile network nodes typically utilize phased arrays to compensate. Real-time steering of the mmW phased array enables direct, node-to-node communication links with minimum energy consumption. Array beam steering can be realized through two approaches, either by adding a progressive phase shift to each signal, or by delaying each signal through true time delay device. Phase shifters are frequency dependent devices, thus, they are bandwidth limited, thus not suitable in modern 5G communication systems. In this dissertation, a tunable optical true time delay device is designed based on the control of group delay in three coupled periodic silicon ridge waveguides, which allows ultra-wideband antenna array beamforming. The tunable delay is achieved by harnessing the wave slow-down exhibited in the device as a consequence of its unique dispersion characteristics, exhibiting a stationary inflection point (SIP) with vanishing group velocity. Slowing-down electromagnetic waves through dispersion engineering has been instrumental for many applications in optical and microwave regimes. Control of wave speed can be utilized to design phase shifters, improve light-matter interaction for linear and nonlinear optics and lasing, and provide true time delay signals. Optically, slowing down light to a stand-still can be realized through two approaches, namely strongly-dispersive media, and periodically inhomogeneous media. In this work, a conventional Silicon ridge optical waveguide topology is used with periodic circular holes etched into the ridge to induce bandgaps in its propagation characteristics. Subsequently, 3 such guides are brought together to allow for mode coupling, leading to unprecedented mode diversity for the first time. The structure is then tuned to exhibit the SIP within its propagation band. As such, these simple implementations exhibit many of the proprieties that can only be otherwise realized in multi-dimensional structures, but with reduced footprint and fabrication complexity. To realize the frozen mode at the SIP, a finite length structure made of 3-way coupled periodic silicon ridge waveguides is designed. The number of unit cells used in the structure is tuned to enable effective excitation of the slow mode using a single fiber coupled ridge waveguide port. Likewise, the output signal is designed to be strong at a single output port, where the delayed light is coupled out for subsequent manipulation of processing. Using this design, the velocity of the transmitted signal was 560 times slower than speed of light in free space with 80% transmission at the SIP frequency. Tuning and controlling of the group velocity dispersion in the structure are also required to achieve antenna array beamforming. To do so, anisotropic liquid crystal cladding layer is incorporated into the ridge waveguide geometry to enable voltage tuning of its dispersion characteristics. Since the permittivity of the liquid crystal can be controlled via an external applied electric field, this approach results in a simple, voltage-controlled true time delay device that is ideal for mmW beamforming applications. We show that the change of the liquid crystal’s permittivity from 2.4 to 3, under varying applied electric field, can tune the SIP over an extremely-broad frequency range of 4.6 THz. Moreover, incorporation of liquid crystal cladding layer allows to control the group delay of the propagating modes at the SIP from 6 ps to 27 ps. As an example to demonstrate the potential benefits of this novel phenomenon, a 11 um long device consisting of 30 unit cells is introduced that provides 730-times slower group velocity as compared to free space. This true time delay device can be used for UWB beamforming at 60GHz by incorporating a tunable delay device with each antenna element. This topology provides ±60o beam scanning over extremely large bandwidths while maintaining small footprint and continuous beam scanning provided by voltage tuning of the delay makes it ideal for 5G and automotive radar applications.
Kubilay Sertel, Prof. (Advisor)
87 p.
Electrical Engineering; Electromagnetics
Periodic ridge waveguide; slow light; stationary inflection point; mm-Wave beamforming;

Recommended Citations

Citations

  • Almhmadi, R. A. M. (2019). Optical True Time Delay Device for mm-Wave Antenna Array Beamforming [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1566073503380698

    APA Style (7th edition)

  • Almhmadi, Raed. Optical True Time Delay Device for mm-Wave Antenna Array Beamforming. 2019. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1566073503380698.

    MLA Style (8th edition)

  • Almhmadi, Raed. "Optical True Time Delay Device for mm-Wave Antenna Array Beamforming." Doctoral dissertation, Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1566073503380698

    Chicago Manual of Style (17th edition)

osu1566073503380698
378
© 2019, some rights reserved.Creative Commons License Optical True Time Delay Device for mm-Wave Antenna Array Beamforming by Raed Ali M Almhmadi is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by The Ohio State University and OhioLINK.