Low-loss chip-scale programmable silicon photonic processor

Chip-scale programmable optical signal processors are often used to flexibly manipulate the optical signals for satisfying the demands in various applications,such as lidar,radar,and artificial intelligence.Silicon photonics has unique advantages of ultra-high integration density as well as CMOS compatibility,and thus makes it possible to develop large-scale programmable optical signal processors.The challenge is the high silicon waveguides propagation losses and the high calibration complexity for all tuning elements due to the random phase errors.In this paper,we propose and demonstrate a programmable silicon photonic processor for the first time by introducing low-loss multimode photonic waveguide spirals and low-random-phase-error Mach-Zehnder switches.The present chip-scale programmable silicon photonic processor comprises a 1×4 variable power splitter based on cascaded Mach-Zehnder couplers(MZCs),four Ge/Si photodetectors,four channels of thermally-tunable optical delaylines.Each channel consists of a continuously-tuning phase shifter based on a waveguide spiral with a micro-heater and a digitally-tuning delayline realized with cascaded waveguide-spiral delaylines and MZSs for 5.68 ps time-delay step.Particularly,these waveguide spirals used here are designed to be as wide as 2μm,enabling an ultralow propagation loss of 0.28 dB/cm.Meanwhile,these MZCs and MZSs are designed with 2-μm-wide arm waveguides,and thus the random phase errors in the MZC/MZS arms are negligible,in which case the calibration for these MZSs/MZCs becomes easy and furthermore the power consumption for compensating the phase errors can be reduced greatly.Finally,this programmable silicon photonic processor is demonstrated successfully to verify a number of distinctively different functionalities,including tunable time-delay,microwave photonic beamforming,arbitrary optical signal filtering,and arbitrary waveform generation.

Hybrid material integration in silicon photonic integrated circuits

Hybrid integration ofⅢ-Ⅴand ferroelectric materials is being broadly adopted to enhance functionalities in silicon photonic integrated circuits(PICs).Bonding and transfer printing have been the popular approaches for integration of III–V gain media with silicon PICs.Similar approaches are also being considered for ferroelectrics to enable larger RF modulation bandwidths,higher linearity,lower optical loss integrated optical modulators on chip.In this paper,we review existing integration strategies ofⅢ-Ⅴmaterials and present a route towards hybrid integration of bothⅢ-Ⅴand ferroelectrics on the same chip.We show that adiabatic transformation of the optical mode between hybrid ferroelectric and silicon sections enables efficient transfer of optical modal energies for maximum overlap of the optical mode with the ferroelectric media,similar to approaches adopted to maximize optical overlap with the gain section,thereby reducing lasing thresholds for hybridⅢ-Ⅴintegration with silicon PICs.Preliminary designs are presented to enable a foundry compatible hybrid integration route of diverse functionalities on silicon PICs.

Ultralow-power polymer electro–optic integrated modulators

On-chip integration of electronics and photonics have attracted substantial amount of interest in recent decades. Major obstacles to the realization of this integration are size mismatch between electronic and photonic circuits, as well as issues with ever-increasing requirements for energy efficiency, bandwidth, optical loss, and drive voltage. Another important issue is the absence of photonic materials that make such integration commercially possible in foundry-compatible processes. Future integration involves combination of various materials and platforms. During the last decade there has been an increasing interest in exploiting various photonic platforms to overcome these obstacles. Integration of silicon photonics[1–3] with technologies such as plasmonics[4–6], photonic crystal architectures[7], and hybrid materials[8] have been widely pursued for photonic integration.

Changes in interstitial fluid flow,mass transport and the bone cell response in microgravity and normogravity

In recent years,our scientific interest in spaceflight has grown exponentially and resulted in a thriving area of research,with hundreds of astronauts spending months of their time in space.A recent shift toward pursuing territories farther afield,aiming at near-Earth asteroids,the Moon,and Mars combined with the anticipated availability of commercial flights to space in the near future,warrants continued understanding of the human physiological processes and response mechanisms when in this extreme environment.Acute skeletal loss,more severe than any bone loss seen on Earth,has significant implications for deep space exploration,and it remains elusive as to why there is such a magnitude of difference between bone loss on Earth and loss in microgravity.The removal of gravity eliminates a critical primary mechano-stimulus,and when combined with exposure to both galactic and solar cosmic radiation,healthy human tissue function can be negatively affected.An additional effect found in microgravity,and one with limited insight,involves changes in dynamic fluid flow.Fluids provide the most fundamental way to transport chemical and biochemical elements within our bodies and apply an essential mechano-stimulus to cells.Furthermore,the cell cytoplasm is not a simple liquid,and fluid transport phenomena together with viscoelastic deformation of the cytoskeleton play key roles in cell function.In microgravity,flow behavior changes drastically,and the impact on cells within the porous system of bone and the influence of an expanding level of adiposity are not well understood.This review explores the role of interstitial fluid motion and solute transport in porous bone under two different conditions:normogravity and microgravity.

Silicon nonlinear switch as a conditional circulator for monostatic LiDAR systems

All-optical silicon-photonics-based LiDAR systems allow for desirable features in scanning resolution and speed,as well as leverage other advantages such as size, weight, and cost. Implementing optical circulators in silicon photonics enables bidirectional use of the light path for both transmitters and receivers, which simplifies the system configuration and thereby promises low system cost. In this work, to the best of our knowledge, we present the first experimental verification of all-passive silicon photonics conditional circulators for monostatic LiDAR systems using a nonlinear switch. The proposed silicon nonlinear interferometer is realized by controlling signal power distribution with power-splitting circuits, allowing the LiDAR transmitter and receiver to share the same optical path. Unlike the traditional concept requiring a permanent magnet, the present device is implemented by using common silicon photonic waveguides and a standard foundry-compatible fabrication process. With several additional phase shifters, the demonstrated device exhibits considerable flexibility using a single chip, which can be more attractive for integration with photodetector arrays in LiDAR systems.