An RF-Photonic phased array antenna beamformer was previously demonstrated using cascaded fiber Bragg gratings with 1 x 2 couplers for true-time-delay beamforming. This work's focus is to design, build, and test an integrated Si-photonic beamforming circuit to replace the fiber-optics system, allowing for chip-scale beamformers with low size, weight, power, and cost. Several metastructure waveguides were designed to provide a strong slowlight effect near their transmission band edge. By tuning the wavelength near the band edge, tunable optical truetime delay is achieved. We report the design, simulation, fabrication and test of these high-contrast metastructure waveguides to provide group velocity variation against wavelength near the band-edge. Wavelength-tunable delay was verified using both an interferometric approach using an integrated Mach-Zehnder interferometer, and using a direct measurement of the true-time delay of an RF signal modulated onto a C-band optical carrier. We have also designed an integrated photonic beamforming circuit for a small array, including photodetectors, fabricated by AIM Photonics. Experimental test results for those integrated photonic circuits will be discussed. We will continue to improve our integrated photonic circuit to pursue larger array implementation. The goal is to further integrate this photonic circuit with an RF phase array antenna and demonstrate the scan of an RF beam by optical control.
The ability to tune the delay of an optical signal is a key component in photonics-based RF phased-array beamforming applications. Recent work has shown that high-contrast metastructure waveguides can be designed for a wide range of delay tuned by carrier injection or signal wavelength, enabling two-dimensional beam steering. In this work, we further explore the parameter space of these structures to maximize the delay change over optical wavelength while maintaining low insertion loss, with the goal of implementing phased-array beamforming in integrated photonic devices.
In this work, we have designed a novel Si based 1-dimensional high contrast meta-structure waveguide that has slow light effect as well as phase tunability using p-n junction. The goal is to use such waveguide to design active optical devices such as high frequency modulators and tunable filters for analog RF-photonics or data communication applications. The Si ridge waveguide has a pair of high contrast grating wings adhered to the waveguide core in the center. Grating bars at two sides of the waveguide are doped P and N-type respectively, while a p-n junction region is formed in the middle of the waveguide core. By applying a voltage to bias the p-n junction, one can sweep the free carriers to change the effective index of the waveguide as well as the dispersion property of the grating. This metastructure Si waveguide is ideal in the design of high frequency optical modulators since the slow light effect can reduce the modulator waveguide length, increase the modulation efficiency as well as compensate other nonlinearity factors of the modulator for analog applications.