Early methods of optical fiber communication systems haven't been much promising in terms of efficiency. The presence of various impairments in the fiber channel has forced researchers to uncover solutions in order to minimize those effects. With the advancement of technology, optical solutions were finally easier to implement in the system. To this day, optical compensation methods are still found to be as the best way to minimize fiber impairments. However, such technique does introduce enormous complexity to the system, in addition to a large cost. For that reason, the main focus had to shift to an alternative method. Electrical compensation techniques have provided the factor of simplicity to the optical communication system, not to mention that they are relatively cheaper than optical compensators. Furthermore, electrical schemes were found to handle fiber impairments in a relatively efficient manner. In this thesis, an optical fiber communication scheme using the direct-detection method is simulated. A frequency shifter in the optical domain will be used for the system to have a coherent like detection. At the receiver's side, a linear equalizer is realized to offset the linear effects caused by the fiber. To our knowledge, this will mark the first direct detection transmission system to pass the one thousand kilometre mark in fiber length. Furthermore, we simulate another optical fiber communication design using the coherent detection. A nonlinear compensator adapting the Volterra approach will be used to offset nonlinear impairments. Such performance will be compared to that of a linear compensator. Design trade-offs will be analyzed, and the nonlinear compensator is found to a improve performance when a dispersion compensation fiber (DCF) is introduced in the optical domain. / Thesis / Master of Applied Science (MASc)

Telecommunication networks based on optical fiber technology have become a major information-transmission system, satisfying the growing demand for bandwidth due to increased internet traffic and other applications such as video on demand, etc. Fiber Bragg gratings (FBGs), in recent years, have emerged as critical components for enabling high-capacity transmission since their response can be tailored to meet the needs of specific applications. FBGs are currently the focus of intense research interest in both the fiber communications and sensing fields. Optical fiber Bragg grating structures in single-mode fiber (SMFBGs) have been studied extensively since the discovery of photosensitivity in germanium-doped silica fiber. They have been used in numerous applications ranging from wavelength-selective filtering in wavelength-division-multiple-access (WDMA) systems to temperature and strain sensing. To a lesser extent, Bragg gratings in multimode fibers have also received attention because of easy coupling with light sources. Most of the MMFBGs related research work has demonstrated the formation of a Bragg grating in a graded-index MMF and briefly reported the measured transmission spectrum. So far, there are few theoretical studies on Bragg gratings in multimode fibers. In this thesis, we investigate Bragg gratings in multimode optical fibers both theoretically and experimentally. A comprehensive numerical model for MMFBGs has been established and the corresponding computer simulation software (MMFBG simulator combined with mode solver) developed. The optical properties of MMFBGs were systematically studied for the first time using our own MMFBG numerical software package. It effectively assists the design modeling for MMFBG-based optical devices. Bragg gratings in multimode fiber were also investigated experimentally. Our theoretical simulation results show good agreement with experiments and offer the insightful explanations for the underlying physics of the device. First, the guided modes were modeled and simulated for step index multimode fibers and graded index multimode fibers with emphasis on parabolic fiber structure. These are popular, standard and commercially available MM fibers, and employed throughout our experiments. This allows us for the simulation of fiber characteristics such as cut-off wavelength, mode effective index, propagation constants and optical field distribution. It also allows for calculation of mode coupling coefficients by overlap integral between any chosen guided modes. Therefore, it serves as a powerful model for the design and analysis of optical fibers. Second, the generalized MMFBG coupled mode theory formalism is derived. The physical mechanism of the behavior of MMFBGs is studied and discussed. The general solution to the MMF Bragg grating problem is achieved by Runge-Kutta, Newton-Raphson and shooting numerical methods. Our theoretical treatment, in particular, offers the advantages which can deal with not only self-coupling but also more complicated cross-coupling interactions and can solve arbitrary large number of mode coupling problems throughout the entire spectra simultaneously for multimode FBGs, thus allowing for a precise and quantitative study of MMFBGs. Such an intensive multimode fiber Bragg grating physical modeling and simulations have not been reported previously. It provides an effective means for the design and analysis of optical fiber devices based on Bragg gratings. Third, the optical properties of multimode FBGs were studies experimentally. Numerical predications of the grating spectral characteristics under fabrication and experimental condition are calculated. The results of the numerical calculations are compared with experimentally measured spectra of multimode gratings written by ultraviolet irradiation of deuterium-sensitized fiber with grating reflectivities ranging from 78% to 99.39%. Good agreement is obtained between the theoretical simulations and the experimental results. Thus, we provide quantitative explanations for the observed experimental phenomena. These explanations give both physical insight and a more complete understanding of the nature of the interaction between the wave propagation and multimode fiber gratings. Furthermore, the spectral simulation of the actual experiments prepares a theoretical guidance for the advanced experimental investigation and also presents a step toward MMFBG device design. Finally, the optical properties of MMFBGs were also studied theoretically. To our knowledge, this is the first detailed analysis and thorough investigation on grating characteristics in MMF. It is demonstrated that the transmission and reflection spectra of fiber Bragg gratings in multimode optical fibers strongly depend on the length of grating, index modulation, period of grating, mode excitation condition and physical structure of MMF. The simulation results allow us to deeply comprehend and visualize the more sophisticated behavior within a multimode fiber grating, and will also allow us to confidently predict and evaluate the performance of more complex structure MMFBGs. It provides the fundamental principles for designing the targeted spectrum performance and settles the theoretical rationale for realizing the practical applications. Overall, the comprehensive numerical model and MMFBG solver package developed in this thesis opens a clear and broad window for understanding MMFBG mechanisms from the physical point of view. Various simulation results and spectral characteristics have been researched and discussed under both ideal and experimental conditions for the purpose of experimental analysis and device design. The results of our study indicate that a new class of potential applications based on MMFBGs can be expected in optical fiber sensors and advanced communication systems. / Thesis / Master of Applied Science (MASc)

Although fiber optic communication systems have enjoyed tremendous developments since their inception roughly fifty years ago, there are still improvements to be made. One such area for growth, relating more to long-haul applications of optical fibers, is the mitigation of nonlinear penalties. These nonlinear penalties appear due to the transmitted signal affecting the material properties of the fiber itself, and are more prominent when said signal has more energy in it—a conflicting effect from the traditional stance where more energy is considered beneficial. These penalties come in different forms, although they all have the same degrading effects for the system in question. Therefore, it is imperative to find new techniques that can mitigate these nonlinear penalties to ensure the optimal performance for our systems. One such technique, that we call multi-subcarrier processing, is the interest of the thesis here. The first section proposes a new method to compensate for the receiver side digital signal processing. More specifically, it introduces an algorithm that can remove the intermediate frequency offset that the signal acquires after passing through the system, which is necessary for coherent systems employed today. As some traditional algorithms used in the signal processing no longer work for multi-subcarrier systems, this new algorithm is necessary to compensate effectively for said offset. The second section introduces a new signal processing technique for the multiplexing and demultiplexing of multi-subcarrier signals. By making use of the well-known Fast Fourier Transform algorithm, the computational cost for generating said signals can be drastically reduced as the number of subcarriers grow larger. As systems continue the trend of operating at greater baud rates, the savings introduced here should be of even greater significance for commercial systems of the future. / Thesis / Master of Applied Science (MASc)

To fulfill the objective of providing robust and reliable fiber optic pressure sensors capable of operating in harsh environments, this dissertation presents the detailed research work on the design, modeling, implementation, analysis, and performance evaluation of the novel fiber optic self-calibrated interferometric/intensity-based (SCIIB) pressure sensor system. By self-referencing its two channels outputs, for the first time to our knowledge, the developed SCIIB technology can fully compensate for the fluctuation of source power and the variations of fiber losses. Based on the SCIIB principle, both multimode and single-mode fiber-based SCIIB sensor systems were designed and successfully implemented. To achieve all the potential advantages of the SCIIB technology, the novel controlled thermal bonding method was proposed, designed, and developed to fabricate high performance fiber optic Fabry-Perot sensor probes with excellent mechanical strength and temperature stability. Mathematical models of the sensor in response to the pressure and temperature are studied to provide a guideline for optimal design of the sensor probe. The solid and detailed noise analysis is also presented to provide a better understanding of the performance limitation of the SCIIB system. Based on the system noise analysis results, optimization measures are proposed to improve the system performance. Extensive experiments have also been conducted to systematically evaluate the performance of the instrumentation systems and the sensor probes. The major test results give us the confidence to believe that the development of the fiber optic SCIIB pressure sensor system provides a reliable pressure measurement tool capable of operating in high pressure, high temperature harsh environments. / Ph. D.

This dissertation proposes a miniaturized optical fiber based sensor system for the measurement of 3-dimensional vector magnetic fields. The operation of the sensor system is based on the detection of magnetostrictive dimensional changes in the sensor gage using a modified extrinsic Fabry-Perot Interferometer configuration. Because of the magnetostrictive reflector the gap length depends on the magnetic fields applied to the sensor. Since the diameter of the magnetostrictive sensor gage is 125 micrometer which is the same as that of the input/output fiber, the sensor is simply constructed by inserting the sensor gage and the input/output fiber into a small glass tube. The glass tube serves as both an aligner for the sensor gage and input/out fiber, and a passive temperature compensator. In addition, it also enhances the mechanical strength and compactness of the sensor. This sensor design shows 98 percent suppression of the thermally induced sensor output changes. The linear output of the sensor system is enhanced by transverse field annealing which increases magnetostrictive induction in the ferromagnetic sensor gage material and controls the sensor gage geometry. A 5-times increase in sensor sensitivity is obtained with the transverse field annealing and the use of a new magnetostrictive material. A modified sensor gage endface demonstrates 92 percent of fringe visibility, which further improves the performance of the interferometer. The signal fading in the interferometric sensors at the peak or bottom of a fringe is reduced by using a quadrature signal demodulation method. The system has been shown to have a resolution better than 100 nT over a measurement range from 100 to 40,000 nT. This research is supported financially by the Phillips Laboratory of the U.S. Air Force. / Ph. D.

For the purpose of developing a new highly-sensitive and reliable fiber optical acoustic sensor capable of real-time on-line detection of acoustic emissions in power transformers, this dissertation presents the comprehensive research work on the theory, modeling, design, instrumentation, noise analysis, and performance evaluation of a diaphragm-based optical fiber acoustic (DOFIA) sensor system. The optical interference theory and the diaphragm dynamic vibration analysis form the two foundation stones of the diaphragm-based optical fiber interferomtric acoustic (DOFIA) sensor. Combining these two principles, the pressure sensitivity and frequency response of the acoustic sensor system is analyzed quantitatively, which provides guidance for the practical design for the DOFIA sensor probe and system. To meet all the technical requirements for partial discharge detection, semiconductor process technologies are applied, for the first time to our knowledge, in fabricating the micro-caved diaphragm (MCD) used for the DOFIA sensor probe. The novel controlled thermal bonding method was proposed, designed, and developed to fabricate high performance DOFIA sensor probes with excellent mechanical strength and temperature stability. In addition, the signal processing unit is designed and implemented with high gain, wide band response, and ultra low noise. A systematic noise analysis is also presented to provide a better understanding of the performance limitations of the DOFIA sensor system. Based on the system noise analysis results, optimization measures are proposed to improve the system performance. Extensive experiments, including the field testing in a power transformer, have also been conducted to systematically evaluate the performance of the instrumentation systems and the sensor probes. These results clearly demonstrated the feasibility of the developed DOFIA sensor for the detection of partial discharges inside electrical power transformers, with unique advantages of non-electrically conducting, high sensitivity, high frequency response, and immunity to the electro-magnetic interference (EMI). / Ph. D.

Interferometric fiber optic sensors have been extensively used to measure a large variety of physical, chemical and biomedical parameters due to their superior performance. At the Center for Photonics Technology of Virginia Tech, a variety of interferometric fiber optic sensors have been developed in recent years, for efficient oil recovery, partial discharge detection in high voltage transformers, pressure sensing in gas turbine engines, and temperature measurements in gasifiers and boilers. However, interrogating an interferometric sensor involves accurate recovery of a measurand from the phase-modulated lightwaves, and has been a challenge for high performance, high speed, and low-cost, to current white-light interferometry (WLI) techniques, such as the widely used scanning WLI (S-WLI) and spectral-domain WLI (SD-WLI). The performance of a white-light interferometric sensing system depends not only on the design of the probes, but also, to a great extent, on the interrogation strategy to be used. In this Ph.D. research, a tunable optical filter based WLI (TOF-WLI) is proposed and validated as a low cost, yet high performance, solution to the interrogation of various types of interferometric sensors. In addition to the capability of linear/quadrature demodulation, TOF-WLI retains all the features of WLI, is compatible with the SD-WLI, and can be tailored for both static and wideband signals. It also has great potential in surface metrology and biomedical imaging as well as optical spectroscopy. The key, to the success of this new approach in competition with the other available WLI techniques, is that the tunable optical filter (TOF) must be specially designed for sensing and extremely low cost. Therefore, two novel TOFs, a diffraction grating tunable filter (DG-TOF) and an extrinsic Fabry-Perot tunable filter (EFP-TF), are proposed and demonstrated. Laboratory and field test results on using the DG-TOF WLI for partial discharge and thermal fault detection in high voltage power transformers, and the EFP-TF WLI in temperature sensor systems and a turbine engine monitoring system will also be presented to demonstrate the feasibility for efficient sensor interrogation. / Ph. D.

In this dissertation the performance of a novel variant of a return-to-zero (RZ) modulation format, based on square-wave phase modulation and filtering of a continuous-wave (CW) signal, is investigated and compared with various modulation formats considered in the literature. We call this modulation format continuous-wave square-wave (CWSW). With CWSW an RZ pulse train is generated by phase modulating the CW signal by a periodic square-wave phase function having an amplitude of and frequency of half the bit rate, and then filtering the signal. The filter performs phase-to-amplitude conversion, resulting in an alternate-sign RZ pulse train, which is shown to be resistant to fiber dispersion. The alternate-sign RZ pulse train is then amplitude modulated with the data before the transmission. Alternate signs between adjacent pulses makes this signal format robust to impairments caused by the optical fiber, similar to a conventional alternate-sign RZ signal format. However, the unique property of the CWSW signal format is that individual pulses can induce peak intensity enhancement (PIE), a phenomenon by which the peak of a pulse increases during the initial propagation in the presence of dispersion. The PIE in effect delays the decrease in the pulse peak, which represents the signal level for bit 1. Thus, the eye opening at the receiver is improved. An analytically tractable model is developed to explain the occurrence of the PIE, which cannot be achieved with a conventional pulse shape. The sources of performance degradations for different modulation formats in single-channel 40 Gb/s systems are also discussed in this dissertation. Various transmission system configurations of practical interest are considered and the performance of CWSW is compared with alternative modulation formats. It is found that the CWSW signal format performs significantly better than the other considered modulation formats in systems not employing dispersion compensation and is comparable to the others in dispersion-managed systems. Furthermore, the transmitter configuration of the CWSW signal format is simpler than the other approaches. / Ph. D.

Immune checkpoint blockade antibodies have promising clinical applications but suffer from disadvantages such as severe toxicities and moderate patient-response rates. None of the current delivery strategies, including local administration aiming to avoid systemic toxicities, can sustainably supply drugs over the course of weeks; adjustment of drug dose, either to lower systemic toxicities or to augment therapeutic response, is not possible. Herein, an implantable miniaturized device has been developed using electrode-embedded optical fibers with both local delivery and measurement capabilities over the course of a few weeks. The combination of local immune checkpoint blockade antibodies delivery via this device with photodynamic therapy elicits a sustained anti-tumor immunity in multiple tumor models. Named Implantable Miniature Optical Fiber Device (IMOD), this device uses tumor impedance measurement for timely presentation of treatment outcomes, and allows modifications to the delivered drugs and their concentrations, rendering IMOD as outstandingly valuable for on-demand delivery of potent immunotherapeutics without exacerbating toxicities. Rigorous studies performed using IMOD are presented and discussed in the follow chapters, followed by exploration of proposed work to expand the breadth of functions offered by this implantable biomedical platform. / Doctor of Philosophy / Aside from efficient energy and data transfer, optical fibers today are used in varying fields including optogenetics and neuroscience. However, merging fiber optics with therapeutics against cancer has rarely been reported. We establish a versatile polymer/drug integrated optical fiber for both diagnosis and treatment of cancers, with minimum mechanical invasiveness. Release profiles of polymer/drug nanoparticles loaded onto our fibers, regardless of their hydrophilicity, can be adjusted to accommodate both short-term and long-term delivery specifications. This enhances intratumoral drug accumulation with minimal systemic toxicity, thus overcoming the dosing obstacle. The optical fibers are also ideal to be utilized during photodynamic therapy (PDT), since photosensitizers can be easily incorporated and activated by near-infrared light traveling through the fibers. Hollow channel within the optical fiber allows for repetitive on-demand delivery of immune checkpoint inhibitors to surrounding tumor tissue, thus stimulating and reactivating cytotoxic and helper T cells. The synergistic combination of PDT and immunotherapy can potentially boost the tumor-targeted treatment outcome by numerous folds. Lastly, our optical fibers are adaptable to integrate biosensing functionality. Devices are built upon the optical fibers to monitor treatment outcome along tumor regression. Our data establishes a correlation between tumor impedance and tumor volumes, thus allowing us to track tumor progression and treatment response towards administered treatments.

Fiber-optic Bragg grating (FBG) sensors are a very attractive technology for the measurement of strain and temperature. They have many advantages over conventional sensors in sensing applications such as sensitivity, immunity to electromagnetic interferences,large bandwidths,capability of remote operation and the potential power to sense micro strain at high temperature. They can be directly embedded into many structures such as concrete to evaluate the material deformation. FBGs are fabricated by photo-inscribing through a phase mask technology on a photosensitive fiber. A periodic refractive index is formed in the fiber core, introducing a reflection at the Bragg wavelength. Since the FBG is characterized by a low insertion loss and controllable reflectance, it has the potential to be multiplexed in very large numbers. The major purpose of this dissertation research is to develop an innovative, high- resolution fiber Bragg grating sensing system using photon-counting optical time domain reflectometry (pc-OTDR) based multiplexing technology. The system uses a Fresnel reflection OTDR with a zero deadzone to detect FBG sensors, which improves both the system detection ability and spatial resolution. A low reflectance FBG with broad bandwidth has been developed that is appropriate for the pc-OTDR measurement. Hundred of multiplexed sensors have been implemented in this system. Two theoretical analyses and preliminary results are presented. The greatest advantage of the system is to increase the maximum multiplexing sensor number to one thousand within a short fiber range. Self-referencing demodulation is necessary to eliminate multiplexed system noise caused by the source power fluctuation and fiber bending effects. A referencing FBG with a different wavelength from the sensing FBG has to be introduced to achieve compensation of disturbances in the measurement. The spectral properties of the FBGs and the combination of WDM/TDM are also discussed to evaluate multiplexing sensor performance. The sensor crosstalk and other noise performances are assessed to evaluate the possibility of large scale multiplexing. / Ph. D.