Coherent detection with subsequent digital signal processing (DSP) is developed, analyzed theoretically and numerically and experimentally demonstrated in various fiber-optic transmission scenarios. The use of DSP in conjunction with coherent detection unleashes the benefits of coherent detection which rely on the preservation of full information of the incoming field. These benefits include high receiver sensitivity, the ability to achieve high spectral-efficiency and the use of advanced modulation formats. With the immense advancements in DSP speeds, many of the problems hindering the use of coherent detection in optical transmission systems have been eliminated. Most notably, DSP alleviates the need for hardware phase-locking and polarization tracking, which can now be achieved in the digital domain. The complexity previously associated with coherent detection is hence significantly diminished and coherent detection is once again considered a feasible detection alternative. In this thesis, several aspects of coherent detection (with or without subsequent DSP) are addressed. Coherent detection is presented as a means to extend the dispersion limit of a duobinary signal using an analog decision-directed phase-lock loop. Analytical bit-error ratio estimation for quadrature phase-shift keying signals is derived. To validate the promise for high spectral efficiency, the orthogonal-wavelength-division multiplexing scheme is suggested. In this scheme the WDM channels are spaced at the symbol rate, thus achieving the spectral efficiency limit. Theory, simulation and experimental results demonstrate the feasibility of this approach. Infinite impulse response filtering is shown to be an efficient alternative to finite impulse response filtering for chromatic dispersion compensation. Theory, design considerations, simulation and experimental results relating to this topic are presented. Interaction between fiber dispersion and nonlinearity remains the last major challenge deterministic effects pose for long-haul optical data transmission. Experimental results which demonstrate the possibility to digitally mitigate both dispersion and nonlinearity are presented. Impairment compensation is achieved using backward propagation by implementing the split-step method. Efficient realizations of the dispersion compensation operator used in this implementation are considered. Infinite-impulse response and wavelet-based filtering are both investigated as a means to reduce the required computational load associated with signal backward-propagation. Possible future research directions conclude this dissertation.

京都大学 / 新制・課程博士 / 博士(情報学) / 甲第24937号 / 情博第848号 / 新制||情||142(附属図書館) / 京都大学大学院情報学研究科先端数理科学専攻 / (主査)教授 林 和則, 教授 青柳 富誌生, 准教授 寺前 順之介 / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DFAM

Time-resolved terahertz (THz) spectroscopy is an emerging optical characterization technique with the potential of becoming standard practice in fundamental research and in industry. These systems have the ability to be extremely broadband and can retrieve the phase and amplitude information of a THz pulse transmitted through a material, allowing the complex dielectric function of the material to be extracted. Although these systems are already extremely impactful, they nonetheless have their shortcomings. Namely, the data acquisition time required for a single measurement hinders their practicality in an industrial setting or retrieving interesting dynamics within a sample that are occurring on faster timescales. Moreover, broadband THz systems carry a significant financial burden as they rely on ultrafast near-infrared (NIR) sources delivering sub-100 fs pulses, limiting their accessibility. In this work, we address these issues plaguing time-resolved THz systems with the implementation of optical fibers. We begin by describing the physical processes governing ultrashort pulse propagation in fiber and the generation and detection of THz pulses via nonlinear effects in semiconductor crystals. We then design and demonstrate a THz detection scheme able to resolve each of the generated THz pulses at a repetition rate of 50 kHz. This includes using fiber to generate a chirped NIR supercontinuum, imprinting THz waveforms onto the NIR spectrum and thereby enabling single-shot THz detection; and using fiber to achieve photonic time-stretch, a technique allowing us to detect each of the THz-encoded NIR pulses with high-speed electronics at a rate determined by the repetition rate of the laser. The resulting system is then used to track carrier dynamics in a semiconductor as they are dynamically accumulating and recombining. We then combine nonlinear propagation in optical fiber with a time-resolved THz system allowing us to achieve broadband THz generation and detection. We describe two systems relying on this general scheme: The first system relies on a compact and cost-effective laser source and a standard fiber to generate and detect a THz spectrum extending up to 6 THz, alleviating the financial strain imposed by systems relying on sophisticated laser sources without sacrificing performance. The other system relies on an amplified laser source and gas-filled hollow-core photonic crystal fiber (PCF) to generate a tunable spectrum up to 20 THz. Finally, we investigate the generation of a supercontinuum spanning more than two octaves inside a highly nonlinear solid-core PCF. For the first time, we explore both the spectral intensity and polarization structure of such a broad optical spectrum approaching the mid-infrared region.

High-power fiber lasers have emerged as a cornerstone in the realm of laser technology. Characterized by their exceptional efficiency, ruggedness, and versatility, fiber lasers are experiencing widespread use in manufacturing, medical, defense, science, and in long range sensing. Unfortunately, high-power applications require strict spatial and spectral performance characteristics to be maintained, which has yet to be perfected. This dissertation discusses the power scaling of ytterbium-doped fiber laser amplifiers, presenting three significant advancements. First, a novel photonic lantern-based method is introduced for real-time monitoring of laser beam modal content and beam quality. Initial tests highlight the photonic lantern's efficiency in predicting the onset of modal instability while simultaneously measuring the laser's output beam quality, M2. Second, this work achieved 2.2 kW single-mode narrow-linewidth laser delivery through a 5-tube nested antiresonant hollow core fiber, maintaining over 95% transmission efficiency and near diffraction-limited beam quality. Lastly, this research explores active-gain fiber designs to mitigate nonlinear effects for further power scaling. One design employing confined-doping strategies, achieving a 2.4x increase in the maximum output power before the onset of stimulated Brillouin scattering. Additionally, a second experiment employing a bend-insensitive fiber design demonstrated a transverse modal instability threshold nearly 3x that of its step-index counterpart. Collectively, this work presents a novel approach to power scale, deliver, and monitor multi-kW Yb-doped fiber laser amplifiers enabling the next-generation of applications requiring the strictest spatial and spectral performance.

The effect of varying the angle of incidence of a Gaussian beam from a He-Ne laser incident upon a large radius optical fiber is theoretically and experimentally investigated. The modes in a weakly-guiding, step index fiber were determined by using an analytical approximation technique to calculate the corresponding eigenvalues. An expression was developed for the fractional power per mode as a function of the angle of incidence for such a fiber. This expression was used to calculate the fractional power per mode for the lowest order 171 modes. This allowed the calculation of the fractional power per order and total power. By comparing these theoretical results to our experiment results, it is shown that the theoretical method is accurate at normal incidence and gives qualitative but not quantitative agreement at larger angles.

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<p> In this thesis, we have developed a full time domain approach for the simulation of pulse propagation in the optical fiber. Same as split-step method in frequency domain, this approach also treats the linear and nonlinear process alternately. To avoid the back and forth transformation between time and frequency domains, a digital Infinite Impulse Response (IIR) filter is used to treat the linear propagation directly in time domain. The signal samples pass through a pre-extracted IIR digital filter where the convolution is simply replaced by a series of operations that consist of shift and multiplication only. </p> <p> Compared with frequency domain method, this approach is fully realized in a "data-flow" fashion. Compared with time domain finite impulse response (FIR) method, this approach can save more memory and computation time. </p> <p> This approach is verified by comparing with the conventional frequency domain split-step Fourier method, and it is applied to the simulation of the pulse propagation, including polarization mode dispersion (PMD) effect in the optical fiber. </p> / Thesis / Master of Applied Science (MASc)

The increasing demand on user rates in the fifth generation (5G) requires network architectures that can support high data rates with acceptable reliability. In order to increase the data rates in the presence of the current spectrum crisis, shrinking cells and reusing the spectrum is a proposed solution. Conventional implementation of dense cells requires a large number of expensive BSs to locally process and decode users’ signals. Another limiting factor that degrades the performance in a dense network is the inter-cell interference. A cloud radio access network (CRAN) is a promising solution to those cost, complexity, and interference challenges. A typical CRAN architecture consists of simplified low-cost base stations (BSs), termed radio units (RUs), that collect the radio frequency (RF) user equipments’ (UEs) signals and forward them over the fronthaul links to the central office (CO) where signal processing is done over shared resources. Besides the reduced cost and complexity of a CRAN, the joint processing at the CO enables joint interference mitigation techniques. However, the performance of CRANs depends critically on the availability of reliable fronthaul links with large bandwidth that may be expensive. Analog optical fronthaul links provide high data rates at lower cost and complexity since UEs’ signals are optically analog-modulated without digitalization, however, they suffer from other channel impairments and nonlinearities. In this thesis, analog optical fronthaul topologies are considered in which radio signals are forwarded over free-space optical (FSO) links, termed radio-over-free-space optical (RoFSO) links, and optical fiber (OF) links, termed radio-over-fiber (RoF) links. Firstly, a CRAN with mixed RF/RoFSO fronthaul is considered to investigate the performance improvement when RF fronthaul links are replaced one-by-one by RoFSO links. A novel joint optimization problem is introduced for the given architecture in which the weighted sum of UEs’ rates is maximized by jointly designing RF and RoFSO links. The optimization problem is solved over different numbers of RF and RoFSO links and under various weather conditions. Under favorable weather conditions, the replacement of 1 RF link by a RoFSO link is shown to increase the 50th percentile of UEs’ rates by 7 times. Secondly, the reliability of a CRAN with two-hop RoFSO/RoF fronthaul links is derived along with other performance metrics such as the average bit-error rate and the cumulative distribution function of UEs’ rates. For the given architecture, the Gaussian noise model of fiber nonlinearity is applied and an optimal OF average optical power is derived to minimize the outage probability. Using the optimal power, and under favorable weather conditions, the 50th percentile of user rate exceeds 1:5 Gbps. Finally, a CRAN with passive all-optical two-hop fronthaul links is considered where optical signals from the first RoFSO fronthaul hop are passively coupled into the RoF fronthaul link. The fronthaul outage probability is derived in the context of network planning to provide guidance on designing a set of system parameters. Those parameters include coverage area radius, density of RUs, RoFSO gain, RoFSO optical power and RoF length. / Thesis / Doctor of Philosophy (PhD) / The upcoming generation of wireless communications, termed fifth generation (5G), promises faster data rates and lower latency. In order to achieve this, more base stations (BSs) have to be deployed which increases the cost and complexity of the network. A solution to this challenge is to install simple BSs, i.e. radio units (RUs), that collect signals from users and forward them to a central office (CO) for joint processing which is referred to as a cloud radio access network (CRAN). The fronthaul network in a CRAN connects the RUs to the CO and it can be implemented using different kinds of links. While there are several fronthaul media (e.g., radio frequency (RF), free-space optical (FSO) links, copper lines, satellite communications, and optical fiber (OF)), optical links provide high data rates that are promising to achieve the 5G requirements. In this thesis, a novel architecture of a CRAN is considered in which analog optical links, namely FSO links and OF links, are used for fronthauling. Performance improvement in terms of rate and reliability is investigated and optimized through different design tools. In response to the challenges introduced by the proposed architecture, such as the nonlinearities of analog FSO and OF links, various design parameters are proposed in the optimization problems to tackle those challenges. Furthermore, a network planning framework is introduced to provide guidance and insights on designing the network.

This thesis contains the details of an experimental study on the local void fraction measurements in large-diameter vertical pipes using optical fiber probes. The experiments were conducted in vertical transparent acrylic pipe of a 20-cm diameter. An experimental test facility used for performing of experiments, was designed as a low-pressure air-water loop, which can operate in either a natural circulation mode or a forced circulation mode. Radial void fraction profiles were measured using an optical fiber probe. An average cross-sectional void fraction was calculated by integration of the data obtained by the optical fiber probe. The average void fraction was also calculated using two-phase pressure-drop measurements . The results were compared and the resulting good accuracy of the optical fiber probe was determined. The flow regime results were plotted in terms of superficial gas and liquid velocities using flow regime maps of several researchers. Absence of the slug flow regime in large-diameter pipes was observed during the experiments. The data were correlated using the drift-flux model. A near unity distribution parameter showed that nearly uniform radial distribution of the void fraction dominates in two-phase flow through large-diameter vertical pipes. / Thesis / Master of Engineering (ME)