Chip-to-chip optical wireless link feasibility using optical phased arrays on silicon-on-insulator

One- and two-dimensional integrated optical phased arrays (OPAs) on silicon-on-insulator have been fabricated and measured having directivities of more than 40dBi and steering ranges up to 10 degrees . These OPAs would allow data rates of 100Mbps at distances up to 0.5m

Scaling Optical Phased Arrays

Optical phased array (OPA) technology can improve the useful capabilities of lasers by controlling the relative optical phase of an array of emitting apertures. Combining lasers in this way can produce a beam with increased optical intensity, rapid beam pointing and the potential to perform adaptive optics to correct for atmospheric turbulence. Future applications of optical phased arrays, particularly ground-to-space laser transmission, require both the ability to combine individual high power optical sources and emitter counts greater than in existing implementations. Breakthrough Starshot proposes the ambitious goal of accelerating a light sail to 20% of the speed of light to reach the nearest solar system within a human lifespan, which may require upwards of 100 GW total power from 100 million emitters in a ground-to-space array. At a reduced scale, improved tracking and eventual manoeuvring of orbital space debris could be also improved through the use of a ground-to-space OPA. The research in this thesis presents improvements and techniques for internally-sensed optical phased array designs to allow the scaling to greater numbers of combined optical sources. While primarily motivated by enabling large scale ground-to-space optical phased arrays, consideration is given to more conventional systems and benefits to non-optical phased array applications. A thorough investigation of the limits of digitally enhanced heterodyne interferometry (DEHI), a technique which allows the simultaneous measure of multiple optical phase signals, forms the first major component of this work. In addition to optical phased arrays, this technique has potential applications in acoustic sensing, wavefront sensing, satellite interferometry and fibre frequency references. A combination of analytical, simulation and experimental work was performed to better predict crosstalk between optical phase measurements and establish a set of robust parameters to improve phase measurement performance. Recommendations for requirement dependent parameter choices when using this technique are presented. The optical phased array underpinning this work is internally sensed, implying the ability to measure differential emitter paths without the need for external beam sampling. However, a lingering challenge in previous implementations arose from the double-pass of internal optical pathlengths, resulting in a pi-phase ambiguity in the sensing. The second key challenge addressed in this thesis is a proof-of-concept experimental solution to resolve this ambiguity, demonstrated with a novel waveguide optical head created using three-dimensional laser inscription. In culmination, this research presents a conceptual design for a ground-to-space optical phased array to act as the "photon engine" component of the Breakthrough Starshot program. This design involves a system to interferometrically link multiple sub-arrays in a scalable hierarchy. The active control of differential pathlengths in the hierarchy are enabled using a combination of wavelength division multiplexing and DEHI. Internal array measurements in this design are partnered with measurements of a satellite mounted laser beacon for atmospheric phase sensing and to account for unevenness of the array surface across a kilometre scale. The satellite laser beacon is designed to operate at a different wavelength from the arrays outgoing beam, allowing weak beacon light to be distinguished from high powered emitter scattering. An associated technique established and modelled in this work is how phase measurements can be accurately mapped between wavelengths. Multiple variations of the complete array are modelled to assess fundamental performance limits of the sensing system with realistic system parameters for the combination of 100 million emitters

PHASAR-based PICs for WDM-applications

Wavelength multiplexers, demultiplexers and routers based on optical phased arrays play a key role in multi-wavelength telecommunication links and networks. Photonic integration of PHASARS with active components will provide the functionality required in tomorrows multi-wavelength network

Circular Optical Phased Array with Large Steering Range and High Resolution

Light detection and ranging systems based on optical phased arrays and integrated silicon photonics have sparked a surge of applications over the recent years. This includes applications in sensing, free-space communications, or autonomous vehicles, to name a few. Herein, we report a design of two-dimensional optical phased arrays, which are arranged in a grid of concentric rings. We numerically investigate two designs composed of 110 and 820 elements, respectively. Both single-wavelength (1550 nm) and broadband multi-wavelength (1535 nm to 1565 nm) operations are studied. The proposed phased arrays enable free-space beam steering, offering improved performance with narrow beam divergences of only 0.5° and 0.22° for the 110-element and 820-element arrays, respectively, with a main-to-sidelobe suppression ratio higher than 10 dB. The circular array topology also allows large element spacing far beyond the sub-wavelength-scaled limits that are present in one-dimensional linear or two-dimensional rectangular arrays. Under a single-wavelength operation, a solid-angle steering between 0.21π sr and 0.51π sr is obtained for 110- and 820-element arrays, respectively, while the beam steering spans the range of 0.24π sr and 0.57π sr for a multi-wavelength operation. This work opens new opportunities for future optical phased arrays in on-chip photonic applications, in which fast, high-resolution, and broadband beam steering is necessary.This work was supported by the Natural Sciences and Engineering Research Council of Canada’s Collaborative R&D Grant Program by collaborating with Optiwave Systems, Inc., Slovak Grant Agency VEGA 1/0113/22, and Slovak Research and Development Agency under the project APVV-21-0217. Partial funding for open access charge: Universidad de Málag

Internally Sensed Optical Phased Arrays

The performance of existing ground-based space debris laser ranging systems can be improved by directing more light onto space debris by coherently combining multiple lasers using an optical phased array (OPA). If the power delivered to target is sufficiently high then these systems may also provide the capability to remotely manoeuvre space debris via photon radiation pressure and/or ablation. By stabilising the relative output phase of multiple lasers, OPAs form a coherent optical wave-front in the far field. Since the phase of each laser can be controlled independently, they also have the ability to dynamically manipulate the distribution of optical power in the far field, potentially enabling them to compensate for atmospheric turbulence. This beam-forming functionality, combined with their inherent scalability and high power handling capabilities make OPAs a promising technology for future space debris laser ranging and manoeuvring systems. In this thesis, we describe the iterative development of a high-power compatible internally sensed OPA, which---in contrast to externally sensed OPAs that sense the output phase of each laser externally using free-space optics---relies on the small fraction of light that is reflected back into the fibre at the output of the OPA to stabilise its relative output phase. This allows internally sensed OPAs to be implemented entirely within fibre without any dependence on free-space optics at the output, offering potential advantages over externally sensed techniques when operating in the presence of shock and vibration. A proof-of-concept experiment demonstrated the viability of internal sensing, but also highlighted a number of weaknesses that would affect its utility, specifically in supporting high optical powers greater than 100s of mW. An improved high-power compatible internally sensed OPA was designed to overcome these restrictions by isolating sensitive optical components from high optical powers using asymmetric fibre couplers. This concept was initially demonstrated experimentally using slave lasers offset phase-locked to a single master laser, and then again using fibre amplifiers in a master oscillator power amplifier configuration. The experimental demonstration of the fibre amplifier compatible OPA stabilised the relative output phase of three commercial 15 W fibre amplifiers, demonstrating a root-mean-squared output phase stability of $\lambda/194$, and the ability to steer the beam at up to 10 kHz. The internally sensed OPA presented here requires the simultaneous measurement, and control of the phase of each emitter in the OPA. This is accomplished using digitally enhanced heterodyne interferometry and digitally implemented phasemeters, both of which rely heavily on high-speed digital signal processing resources provided by field-programmable gate-arrays