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 λ/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
