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Integrated True Time Delays and Optical Beam Forming Networks for Wideband Wireless Communications

  • Author(s): Liu, Yuan
  • Advisor(s): Klamkin, Jonathan
  • et al.

Millimeter waves (mmW) ranging in frequency from 30 to 300 GHz provide a tremendous amount of spectrum to meet the rapid growth of wireless data traffic demand. The W-band (75 - 100 GHz) mmW frequencies are particularly attractive as they can support links faster than 100 Gbps. Microwave photonics (MWP) is promising for mmW signal generation due to the frequency-independent low optical loss. For mmW phased array

antennas (PAAs), wide bandwidth and low loss optical true time delays (TTDs) mitigate the beam squint issue and reduce power consumption in comparison to RF phase shifters that require driver amplifiers. In combination with integrated photonics where large scale and compact TTDs can be realized, integrated MWP is particularly suitable for mmW PAA beamforming ultra-high speed communications.

Among the various tunable integrated TTD implementations, switched delay lines (SDLs) and optical ring resonators (ORRs) are particularly promising for large scale optical beamforming networks (OBFNs) for PAAs. In this thesis, both SDL and ORR based TTD devices have been investigated, with emphasis on the reduction of ripple in the delay spectrum. A ripple-free SDL architecture was developed based on Mach-Zehnder interferometer (MZI) optical switches, and a 1x4 OBFN using 5-stage SDLs was realized for linear PAAs. Ultra-low loss silicon nitride was used as an integrated photonics technology platform. Six delay configurations with 0, 1.5, 3, 4.5, 6 and 7.5 ps path delay incremental steps were demonstrated for the SDL-OBFN. The ORR based delay lines were optimized to minimize the ripple in the delay response by applying a genetic algorithm, which revealed a tradeoff between the ripple level, delay value, bandwidth and total number of rings used. A 1x4 3-ORR (three rings in each path) based OBFN was optimized, An ORR-OBFN architecture where one ORR is shared by two adjacent paths was determined to reasonably balance the ripple level and system complexity. Two 3-ORR based 1×4 OBFNs with the architecture of sharing one and two ORRs were realized on the silicon nitride platform as well. With optimized tuning, a single 3-ORR delay line generated continuous tuning ranges of 209 ps and 172 ps for bandwidths of 6.3 GHz and 8.6 GHz, respectively. Delay responses with delay increment of 4.6 ps for linear PAAs were generated by the two ORR-OBFNs, both of which demonstrated that the OBFN with one ORR shared exhibits much more flattened delay spectrum as expected. Based on the results, both the SDL-OBFN and ORR-OBFN are considered extremely promising for directional beamforming with PAAs.

Integrated OBFN chips were packaged and used in experiments for mmW signal generation and beamsteering. The ORR-OBFN was utilized for a 41 GHz mmW signal generation experiment. The SDL-OBFN was utilized for a W-band signal beamsteering experiment. Six beam angles from -51° to 31°, with at least 15 GHz TTD bandwidth from 85 to 100 GHz, and an SNR of 35 dB were demonstrated. Also, a 94 GHz W-band signal with 3 Gbps amplitude shift keying (ASK) data was generated and the spectrum was measured. To the author’s best knowledge, this is the first report of W-band beamsteering and data transmission using an integrated OBFN.

To further improve the scalability and reduce the power consumption and complexity of the current PAA system, the ongoing work on OBFN-photodiode-antenna integration has been considered. Also, a novel recirculating delay loop (RDL) based OBFN architecture was proposed. This re-uses a single TTD element for the signal delays in all channels. Some of the required components for this RDL-OBFN have been realized.

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