Scaling Optical Phased Arrays

Abstract

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. Show more