Internally Sensed Optical Phased Arrays
Creators: name: Roberts, Lyle Edward
Publisher: name: The Australian National University
Year: 2016
Subjects: Optical Phased Arrays
Control Systems
Lasers
Optics
Stabilisation
Stabilization
Coherent Beam Combining
Space Debris
Orbital Debris
Interferometry
Digitally Enhanced Heterodyne Interferometry
Phasemeters
Digitally Implemented Phasemeters
Field Programmable Gate Array
FPGA
OPA
CBC
Internal sensing
Internally sensed

description: 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.