Controlling phonons and photons at the wavelength-scale: silicon photonics meets silicon phononics

Radio-frequency communication systems have long used bulk- and surface-acoustic-wave devices supporting ultrasonic mechanical waves to manipulate and sense signals. These devices have greatly improved our ability to process microwaves by interfacing them to orders-of-magnitude slower and lower loss mechanical fields. In parallel, long-distance communications have been dominated by low-loss infrared optical photons. As electrical signal processing and transmission approaches physical limits imposed by energy dissipation, optical links are now being actively considered for mobile and cloud technologies. Thus there is a strong driver for wavelength-scale mechanical wave or "phononic" circuitry fabricated by scalable semiconductor processes. With the advent of these circuits, new micro- and nanostructures that combine electrical, optical and mechanical elements have emerged. In these devices, such as optomechanical waveguides and resonators, optical photons and gigahertz phonons are ideally matched to one another as both have wavelengths on the order of micrometers. The development of phononic circuits has thus emerged as a vibrant field of research pursued for optical signal processing and sensing applications as well as emerging quantum technologies. In this review, we discuss the key physics and figures of merit underpinning this field. We also summarize the state of the art in nanoscale electro- and optomechanical systems with a focus on scalable platforms such as silicon. Finally, we give perspectives on what these new systems may bring and what challenges they face in the coming years. In particular, we believe hybrid electro- and optomechanical devices incorporating highly coherent and compact mechanical elements on a chip have significant untapped potential for electro-optic modulation, quantum microwave-to-optical photon conversion, sensing and microwave signal processing.Comment: 26 pages, 5 figure

A novel interplanetary communications relay

A case study of a potential Earth-Mars interplanetary communications relay, designed to ensure continuous communications, is detailed. The relay makes use of orbits based on artificial equilibrium points via the application of continuous low thrust, which allows a spacecraft to hover above the orbital plane of Mars and thus ensure communications when the planet is occulted with respect to the Earth. The artificial equilibria of two different low-thrust propulsion technologies are considered: solar electric propulsion, and a solar sail/solar electric propulsion hybrid. In the latter case it is shown that the combination of sail and solar electric propulsion may prove advantageous, but only under specific circumstances of the relay architecture suggested. The study takes into account factors such as the spacecraft's power requirements and communications band utilized to determine the mission and system architecture. A detailed contingency analysis is considered for recovering the relay after increasing periods of spacecraft motor failure, and combined with a consideration for how best to deploy the relay spacecraft to maximise propellant reserves and mission duration

Experiments on synthetic dimensions in photonics

The first and introductory section of the dissertation presents the working principle of a one- and two-dimensional photonic mesh lattice based on the time-multiplexing technique. The basis of a random walk interrelated to the corresponding light and quantum walk is comprehensively discussed as well. The second part of the dissertation consists of three experiments on a one-dimensional photonic mesh lattice. Firstly, the Kapitza-based guiding light project models the Kapitza potential as a continuous Pauli-Schrödinger-like equation and presents an experimental observation of light localization when the transverse modulation is bell-shaped but with a vanishing average along the propagation direction. Secondly, the optical thermodynamics project experimentally demonstrates for the first time that any given initial modal occupancy reaches thermal equilibrium by following a Rayleigh-Jeans distribution when propagates through a multimodal photonic mesh lattice with weak nonlinearity. Remarkably, the final modal occupancy possesses a unique temperature and chemical potential that have nothing to do with the actual thermal environment. Finally, the quantum interference project discusses an experimental all-optical architecture based on a coupled-fiber loop for generating and processing time-bin entangled single-photon pairs. Besides, it shows coincidence-to-accidental ratio and quantum interference measurements relying on the phase modulation of those time bins. The third part of the dissertation comprises two experiments on a two-dimensional photonic mesh lattice. The first project discusses the experimental realization of a two-dimensional mesh lattice employing short- and long-range interaction. To some extent, the second project presents a nonconservative system based on a two-dimensional photonic mesh lattice exploiting parity-time (PT) symmetry

A LINEARIZATION METHOD FOR A UWB VCO-BASED CHIRP GENERATOR USING DUAL COMPENSATION

Ultra-Wideband (UWB) chirp generators are used on Frequency Modulated Continuous Wave (FMCW) radar systems for high-resolution and high-accuracy range measurements. At the Center for Remote Sensing of Ice Sheets (CReSIS), we have developed two UWB radar sensors for high resolution measurements of surface elevation and snow cover over Greenland and Antarctica. These radar systems are routinely operated from both surface and airborne platforms. Low cost implementations of UWB chirp generators are possible using an UWB Voltage Controlled Oscillator (VCO). VCOs possess several advantages over other competing technologies, but their frequency-voltage tuning characteristics are inherently non-linear. This nonlinear relationship between the tuning voltage and the output frequency should be corrected with a linearization system to implement a linear frequency modulated (LFM) waveform, also known as a chirp. If the waveform is not properly linearized, undesired additional frequency modulation is found in the waveform. This additional frequency modulation results in undesired sidebands at the frequency spectrum of the Intermediate Frequency (IF) stage of the FMCW radar. Since the spectrum of the filtered IF stage represents the measured range, the uncorrected nonlinear behavior of the VCO will cause a degradation of the range sensing performance of a FMCW radar. This issue is intensified as the chirp rate and nominal range of the target increase. A linearization method has been developed to linearize the output of a VCO-based chirp generator with 6 GHz of bandwidth. The linearization system is composed of a Phase Lock Loop (PLL) and an external compensation added to the loop. The nonlinear behavior of the VCO was treated as added disturbances to the loop, and a wide loop bandwidth PLL was designed for wideband compensation of these disturbances. Moreover, the PLL requires a loop filter able to attenuate the reference spurs. The PLL has been designed with a loop bandwidth as wide as possible while maintaining the reference spur level below 35 dBc. Several design considerations were made for the large loop bandwidth design. Furthermore, the large variations in the tuning sensitivity of the oscillator forced a design with a large phase margin at the average tuning sensitivity. This design constraint degraded the tracking performance of the PLL. A second compensation signal, externally generated, was added to the compensation signal of the PLL. By adding a compensation signal, which was not affected by the frequency response effects of the loop compensation, the loop tracking error is reduced. This technique enabled us to produce an output chirp signal that is a much closer replica of the scaled version of the reference signal. Furthermore, a type 1 PLL was chosen for improved transient response, compared to that of the type 2 PLL. This type of PLL requires an external compensation to obtain a finite steady state error when applying a frequency ramp to the input. The external compensation signal required to solve this issue was included in the second compensation signal mentioned above. Measurements for the PLL performance and the chirp generator performance were performed in the laboratory using a radar demonstrator. The experimental results show that the designed loop bandwidth was successfully achieved without significantly increasing the spurious signal level. The chirp generator measurements show a direct relationship between the bandwidth of the external compensation and the range resolution performance

Benchmarking five numerical simulation techniques for computing resonance wavelengths and quality factors in photonic crystal membrane line defect cavities

We present numerical studies of two photonic crystal membrane microcavities, a short line-defect cavity with relatively low quality ($Q$) factor and a longer cavity with high $Q$. We use five state-of-the-art numerical simulation techniques to compute the cavity $Q$ factor and the resonance wavelength $\lambda$ for the fundamental cavity mode in both structures. For each method, the relevant computational parameters are systematically varied to estimate the computational uncertainty. We show that some methods are more suitable than others for treating these challenging geometries.Comment: Revised and final version for publication. 28 pages, 10 figures, 7 table

Stabilized high-power laser system for the gravitational wave detector advanced LIGO

An ultra-stable, high-power cw Nd:YAG laser system, developed for the ground-based gravitational wave detector Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory), was comprehensively characterized. Laser power, frequency, beam pointing and beam quality were simultaneously stabilized using different active and passive schemes. The output beam, the performance of the stabilization, and the cross-coupling between different stabilization feedback control loops were characterized and found to fulfill most design requirements. The employed stabilization schemes and the achieved performance are of relevance to many high-precision optical experiments