Date of Award
Doctor of Philosophy (PhD)
Eric G. Johnson
Richard J. Watkins
In this work, the dynamic generation of structured light modes was demonstrated using coherent, co-aligned beams carrying orbital angular momentum (CCOAM). These modes are created using sources with blue/green wavelengths to study the effects of propagation and applications underwater maritime environments. Three techniques are discussed and are compared to simulation using a Rayleigh-Sommerfeld propagation kernel: concentric phase plates, Mach-Zehnder Interferometry, and the HOBBIT (Higher Order Bessel Beams Integrated in Time). These three systems are used to examine the modal integrity, controllability, and unique applications.
Structured CCOAM modes were first demonstrated using a 450 nm source and concentric phase plates and were propagated through 3 meters of turbid underwater environments. Beam coherence was measured using image registration, and the wavefronts were found to maintain their structure despite propagation through extreme turbidity. In addition, the source was amplitude modulated to verify that the mode structure can carry an amplitude modulation signal.
Next, an interferometry approach is used so that the two interfering modes can be controlled separately. The relative phase is controlled between the two interfering modes by manipulating the optical path length that each mode travels using an electro-optic phase modulator. Phase modulation allows for precise yet limited control of the wavefront and structure. Two setups were examined, a fiber-to-free-space Mach-Zehnder interferometer, and a HOBBIT system with two inputs. Phase only control was demonstrated using sinusoidal modulation and an orthogonal frequency division multiplexing (OFDM) signal applied to the phase modulator. The modulated signals were successfully transmitted 3 and 6 meters through turbid water. Phase only modulation allowed for the transmission of a constant-amplitude signal, which provides nonlinear manipulation of the signal, such as amplification and harmonic generation, which are both crucial in creating high-power signals in the visible regime. The interferometry setups are very sensitive and a phase drift was found to occur due to temperature fluctuations and small movements of optical fiber in the setup, so a preliminary phase-lock loop was designed and tested to eliminate the phase drift. Without applied modulation, a RMS phase error of less than λ/30 was measured.
Lastly an acousto-optic deflector (AOD) was added to the HOBBIT setup, which adds mode tunability in addition to amplitude and phase control. The traveling acoustic wave also induces a frequency shift in the optical signal producing a continuous modulation of the output CCOAM mode. This is demonstrated by using a pulsed 450 nm diode to strobe the signal. Operation in pulsed mode enables the system to perform a self-referencing wavefront recovery from which the total OAM was extracted.
Morgan, Kaitlyn S., "A Study on the Dynamic Manipulation of Structured Light Using Orbital Angular Momentum for Wireless Underwater Links" (2020). All Dissertations. 2693.