Photonic crystal fibers and optical true time delay engines for wideband arrays.

摘要

This dissertation addresses two important research topics related to wideband beam-steering arrays with optical true time delay (TTD). The first topic presents the feasibility of using large-mode-area (LMA) photonic crystal fibers (PCF) as delay elements in White cell (WC) TTD to produce long-delays (0.2-20ns). The measured coupling loss from free-space into LMA-PCF is -2.15dB. However, it is shown in simulation that the loss decreases to -0.5dB by using additional field lenses. The measured and simulated data also show that LMA-PCF is not that sensitive to misalignments. Therefore, LMA-PCF is the most efficient delay element for a WC-TTD engine in terms of loss and volume minimization.;The second topic is the demonstration of beam-steering in a wideband array with WC produced delays. A Quadratic WC was designed and simulated to produce four simultaneous delays (with Deltat=25ps) for a four-sub-array system (for frequencies 2-18GHz) where each sub-array consists of two Vivaldi antennas. Simulation results show that aberrations in the long-delays generated with a lens-train suffer from a 3.2dB higher loss than that produced in free-space. Therefore, a Quartic cell is proposed with commercially available optics which does not use any glass blocks (for short-delays) or lens-trains (for long-delays) as delay elements. A proof-of-concept MEMS-based Quadratic cell was designed and aligned with optics available at ESL for Deltat=500ps. The measured aberrated beams incident on the MEMS were 37.5mum, which is larger than the expected beamsize of 25im. This caused a -10.6dB cross-talk between the null and the short-delay arms.;A parametric study conducted of an ideal four-sub-array system formed by isotropic radiators showed that it can scan +/-50° with Deltat=25ps and 4° error in the beam-direction. Next, a four-sub-arrayed Vivaldi-array was built, the measured scattering matrix of it shows severe mutual coupling at low frequencies (2-5GHz). However, the array performs as expected in the range of 6-11GHz. The effect of lens-train optical loss is not negligible (-11dB) for the beam-steering. Although the delays are independent of frequency, the mutual coupling is not. Therefore, truly frequency-independent beam steering arrays will require algorithms for the delays to compensate for the frequency-dependent mutual coupling.