True-time delay line based on 1-D fishbone structure reduces loss and footprint for on-chip applications
True-time delay line based on 1-D fishbone structure reduces loss and footprint for on-chip applications lead image
Effectively manipulating microwave signals without loss, distortions, or high cost is no easy task. Doing so with pulsed signals, especially the ultrashort pulses common today, involves a host of additional difficulties to address broadband applicability. Optical true-time delay (TTD) devices that add time delays across incident frequencies, demonstrate more desirable responses to broadband signal processes like beam steering compared with phase arrays that can be highly sensitive to incoming angle.
A novel 1-D photonic crystal waveguide (PCW) described in Applied Physics Letters, based on a fishbone architecture, modulates the optical mode using periodic sidewalls along the propagation direction, generating the structural slow light. The on-chip design addresses many common problems encountered with conventional 2-D slow light PCW-based TTD lines, including high optical loss, low fabrication tolerance and large size. “These improvements are achieved by having less etched surface area overlapped with optical modes and index guiding of the 1-D fishbone PCW, making it more promising for on-chip TTD applications,” said co-author Chi-Jui Chung.
The waveguides feature four-channel optical time-delay lines consisting of patterned silicon etched on a silicon dioxide supporting layer. Light is confined along the propagation direction via the photonic crystal defects, similar to 2-D PCW structures. A key beneficial difference to the 1-D structure is found along the in-plane direction perpendicular to the propagation. Along this axis, light is directed via index guiding as opposed to bandgap guiding, allowing for a smaller volume of the TTD module.
The device is currently tunable for delay times up to the hundreds of picoseconds per millimeter. The authors look to increase the delay by lengthening the waveguide in an effort to reach the delays needed for today’s real microwave applications. “We are trying to fabricate and test an elongated 1-D PCW while we are also developing similar low-loss architectures to achieve the one nanosecond goal,” said Chung.
Source: “On-chip optical true time delay lines featuring one-dimensional fishbone photonic crystal waveguide,” by Chi-Jui Chung, Xiaochuan Xu, Gencheng Wang, Zeyu Pan, and Ray T. Chen, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5006188 .
