A Study on the Dynamic Manipulation of Structured Light Using Orbital Angular Momentum for Wireless Underwater Links

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

Optical Communication

Optical communication is very much useful in telecommunication systems, data processing and networking. It consists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to its desired destination, and a receiver that reproduces the message from the received optical signal. It presents up to date results on communication systems, along with the explanations of their relevance, from leading researchers in this field. The chapters cover general concepts of optical communication, components, systems, networks, signal processing and MIMO systems. In recent years, optical components and other enhanced signal processing functions are also considered in depth for optical communications systems. The researcher has also concentrated on optical devices, networking, signal processing, and MIMO systems and other enhanced functions for optical communication. This book is targeted at research, development and design engineers from the teams in manufacturing industry, academia and telecommunication industries

Overcoming Atmospheric Effects in Quantum Cryptography

Quantum Computers will have the potential to greatly assist us in problems such as searching, optimization and even drug discovery. Unfortunately, among these newfound capabilities is one which allows one to break RSA encryption in orders of magnitude less time. One promising countermeasure to secure our communication today and in the future is the one time pad, although it is very difficult to generate and distribute. Quantum Key Distribution offers a practical method for two authenticated parties to generate a key. Whereby the parties, Alice and Bob, share quantum states and use physical laws to place an upper bound on the information an eavesdropper could possibly have. QKD has matured tremendously since it was proposed in 1984, but gaps still remain between the device models used in security proofs and what can be achieved in deployments. In particular, the atmosphere presents a challenging environment for QKD, and due to the wireless nature of communications today, we cannot avoid it. QKD must be robust to atmospheric loss as well as transmittance fluctuation due to turbulence. In this dissertation, I perform an experiment to make the BB84 protocol more resilient by monitoring the transmittance of a channel and finding time periods where the transmittance is low, hence error rate is high and discard data collected during such timeframes. We conduct an experiment to test Hong Ou Mandel visibility, which is at the heart of Measurement Device Independent QKD, by varying the detector parameters and the photons\u27 level of identity. I use an observation where we find that the interference effect is highly dependent on the similarity of photon numbers at the measurer\u27s beamsplitter to develop a scheme in MDI QKD where the key rate is kept high through tethering the transmittance fluctuations of Alice and Bob\u27s channels with dynamic attenuation. The results show an improved robustness when in a turbulent atmosphere, even when accounting for nonzero minimum loss

Practical free-space quantum key distribution

Within the last two decades, the world has seen an exponential increase in the quantity of data traffic exchanged electronically. Currently, the widespread use of classical encryption technology provides tolerable levels of security for data in day to day life. However, with one somewhat impractical exception these technologies are based on mathematical complexity and have never been proven to be secure. Significant advances in mathematics or new computer architectures could render these technologies obsolete in a very short timescale. By contrast, Quantum Key Distribution (or Quantum Cryptography as it is sometimes called) offers a theoretically secure method of cryptographic key generation and exchange which is guaranteed by physical laws. Moreover, the technique is capable of eavesdropper detection during the key exchange process. Much research and development work has been undertaken but most of this work has concentrated on the use of optical fibres as the transmission medium for the quantum channel. This thesis discusses the requirements, theoretical basis and practical development of a compact, free-space transmission quantum key distribution system from inception to system tests. Experiments conducted over several distances are outlined which verify the feasibility of quantum key distribution operating continuously over ranges from metres to intercity distances and finally to global reach via the use of satellites

Modern Applications in Optics and Photonics: From Sensing and Analytics to Communication

Optics and photonics are among the key technologies of the 21st century, and offer potential for novel applications in areas such as sensing and spectroscopy, analytics, monitoring, biomedical imaging/diagnostics, and optical communication technology. The high degree of control over light fields, together with the capabilities of modern processing and integration technology, enables new optical measurement systems with enhanced functionality and sensitivity. They are attractive for a range of applications that were previously inaccessible. This Special Issue aims to provide an overview of some of the most advanced application areas in optics and photonics and indicate the broad potential for the future

Advanced DSP Techniques for High-Capacity and Energy-Efficient Optical Fiber Communications

The rapid proliferation of the Internet has been driving communication networks closer and closer to their limits, while available bandwidth is disappearing due to an ever-increasing network load. Over the past decade, optical fiber communication technology has increased per fiber data rate from 10 Tb/s to exceeding 10 Pb/s. The major explosion came after the maturity of coherent detection and advanced digital signal processing (DSP). DSP has played a critical role in accommodating channel impairments mitigation, enabling advanced modulation formats for spectral efficiency transmission and realizing flexible bandwidth. This book aims to explore novel, advanced DSP techniques to enable multi-Tb/s/channel optical transmission to address pressing bandwidth and power-efficiency demands. It provides state-of-the-art advances and future perspectives of DSP as well

Novel Insights into Orbital Angular Momentum Beams: From Fundamentals, Devices to Applications

It is well-known by now that the angular momentum carried by elementary particles can be categorized as spin angular momentum (SAM) and orbital angular momentum (OAM). In the early 1900s, Poynting recognized that a particle, such as a photon, can carry SAM, which has only two possible states, i.e., clockwise and anticlockwise circular polarization states. However, only fairly recently, in 1992, Allen et al. discovered that photons with helical phase fronts can carry OAM, which has infinite orthogonal states. In the past two decades, the OAM-carrying beam, due to its unique features, has gained increasing interest from many different research communities, including physics, chemistry, and engineering. Its twisted phase front and intensity distribution have enabled a variety of applications, such as micromanipulation, laser beam machining, nonlinear matter interactions, imaging, sensing, quantum cryptography and classical communications. This book aims to explore novel insights of OAM beams. It focuses on state-of-the-art advances in fundamental theories, devices and applications, as well as future perspectives of OAM beams