Single-mode fiber transmission can no longer satisfy exponentially growing capacity demand. Space-division multiplexing (SDM) appears to be the only way able to dramatically improve the transmission capacity, for which, novel optical fiber is one of the key technologies. Such fibers must possess the following characteristics: 1) high mode density per cross-sectional area and 2) low crosstalk or low modal differential group delay (DMGD) to reduce complexity of digital signal processing. In this dissertation, we explore the design and characterization of three kinds of fibers for SDM: few-mode fiber (FMF), few-mode multi-core fiber (FM-MCF) and coupled multi-core fiber (CMCF) as well as their applications in transmission and networking. For the ultra-high density need of SDM, we have proposed the FMMCF. It combines advantages of both the FMF and MCF. The challenge is the inter-core crosstalk of the high-order modes. By applying a hole-assisted structure and careful fiber design, the LP11 crosstalk has been suppressed down to -40dB per km. This allows separate transmission on LP01 and LP11 modes without penalty. In fact, a robust SDM transmission up to 200Tb/s has been achieved using this fiber. To overcome distributed modal crosstalk in conjunction with DMGD, supermodes in CMCFs have been proposed. The properties of supermodes were investigated using the coupled-mode theory. The immediate benefits include high mode density and large effective area. In supermode structures, core-to-core coupling is exploited to reduce modal crosstalk or minimize DMGD. In addition, higher-order supermodes have been discovered in CMCFs with few-mode cores. We show that higher-order supermodes in different waveguide array configurations can be strongly affected by angle-dependent couplings, leading to different modal fields. Analytical solutions are provided for linear, rectangular and ring arrays. Higher-order modes have been observed for the first time using S2 imaging method. Finally, we introduce FMF to gigabit-capable passive optical networks (GPON). By replacing the conventional splitter with a photonic lantern, upstream combining loss can be eliminated. Low crosstalk has been achieved by a customized mode-selective photonic lantern carefully coupled to the FMF. We have demonstrated the first few-mode GPON system with error-free performance over 20-km 3-mode transmission using a commercial GPON system carrying live Ethernet traffic. We then scale the 3-mode GPON system to 5-mode, which resulted in a 4dB net gain in power budget in comparison with current commercial single-mode GPON systems.

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Aiming to reduce research and development times in the field of silicon photonics, this paper presents a method for automatized device testing. Focus lies on automatic optical coupling between the grating couplers on a chip and optical fibers and efficient switching between devices when performing laboratory tests on silicon photonic chips. A lab setup with high precision motorized stages has been built and an algorithm for finding the best optical coupling between fiber and chip, based on the light distribution properties of the fiber, has been implemented. The project results shows that, while these methods have the potential of considerable time savings, further testing is needed.

A long-period grating is obtained by introducing a periodic refractive index modulation in the core of a hydrogen-sensitized germanosilicate fiber. The phase-matching condition causes light from the fundamental guided mode to couple to discrete, forward-propagating cladding modes. These cladding modes attenuate rapidly on propagation and result in loss bands at distinct wavelengths in the grating transmission spectrum. We present a comprehensive analysis of the spectral modulation provided by long-period gratings. An analytical model is developed to predict the location of the resonance bands as functions of the grating period and the parameters of the host fiber. These gratings with small insertion loss and negligible back-ret1ection are shown to possess two different regions of operation, namely, normal and anomalous. The fabrication and high temperature annealing of these devices is detailed, and a novel method to obtain these gratings without employing ultra-violet radiation is presented. Long-period gratings are proposed as simple yet versatile optical fiber sensors. It is demonstrated that external temperature and axial strain introduce large spectral shifts in the resonance bands. A theoretical evaluation of the sensitivity reveals a strong dependence on the properties of the optical fiber, the grating periodicity, the order of the cladding mode, the writing and annealing conditions, and the index of refraction of the surrounding medium. Temperature-insensitive and strain-insensitive long-period gratings written in standard optical fibers are studied for their sensing characteristics. Long period grating-based refractive index sensors are obtained without etching the cladding of the fiber. It is demonstrated that long-period grating sensors can be implemented with simple demodulation schemes. Applications of these devices to structural health monitoring and biochemical sensing are presented. Finally, long-period gratings are demonstrated as effective sensors that can be used to separate temperature and axial strain acting simultaneously on the fiber. Strain-insensitive gratings are used to extend the dynamic range of the system in the presence of non-linearities and cross-sensitivities. / Ph. D.

A sensor is described which utilizes the phenomenon of surface plasmon resonance to detect changes in refractive index of chemical or biochemical samples applied to a surface modified optical fiber. The sensor is constructed by polishing a short section of the lateral surface of an optical fiber to its evanescent field surrounding the fiber core. One or more thin films are applied to the polished section of the fiber to produce the sensing element. One of the films is the metal silver, which acts as the support for the surface plasmon. Under the proper conditions, TM polarized energy propagating in the fiber can be coupled to a surface plasmon electromagnetic mode on the metal film. This coupling depends on the wavelength, the nature of the fiber, the refractive index and thickness of the thin films applied to the fiber, and the refractive index of a chemical sample in contact with the modified surface. The fiber to plasmon coupling is seen as a large attenuation of the light reaching the distal terminus of the fiber. / Ph. D.

Present analog optical fiber multi-channel video transmission systems are very sensitive to laser nonlinearities and are consequently limited in the optical modulation depth (OMD) that may be used. This, in turn limits the power budget achievable, signal-to-noise ratio, and the channel capacity. In this dissertation a new analog transmission technique for multi-channel TV transmission on fiber USIng frequency modulation/pulse amplitude modulation/time division multiplexing (FM/TDM) is described and compared with present digital and analog systems. Parameters for the proposed system are selected and the relationship between the performance and parameter values is discussed. Analysis and simulations indicate that the proposed system has a very low sensitivity to nonlinearities and is similar to that of digital systems, and much better than current Frequency Modulated/Frequency Division Multiplexed (FM/FDM) systems. This permits the use of higher OMD (as high as in digital systems), which results in achieving a high signal-to-noise ratio and a large power budget. Analysis of the number of channels as a function of adjacent channel intersymbol interference indicates that the proposed system has a better spectral efficiency than present analog systems. Simulations are also used to predict the performance of the proposed system with laser diodes poorer than the ones presently used for multi-channel analog systems. Considerably poorer lasers may be used while achieving acceptable transmission quality. Finally, carrier-to-noise penalty caused by timing errors and jitter effects are analyzed. / Ph. D.

The objective of this dissertation is to develop, analyze, and implement optical fiber-based sensors for the nondestructive quantitative evaluation of advanced civil structures. Based on a comparative evaluation of optical fiber sensors that may be used to obtain quantitative information related to physical perturbations in the civil structure, the extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensor is selected as the most attractive sensor. The operation of the EFPI sensor is explained using the Kirchhoff diffraction approach. As is shown in this dissertation, this approach better predicts the signal-to-noise ratio as a function of gap length than methods employed previously. The performance of the optical fiber sensor is demonstrated in three different implementations. In the first implementation, performed with researchers in the Civil Engineering Department at the University of Southern California in Los Angeles, optical fiber sensors were used to obtain quantitative strain information from reinforced concrete interior and exterior column-to-beam connections. The second implementation, performed in cooperation with researchers at the United States Bureau of Mines in Spokane, Washington, used optical fiber sensors to monitor the performance of roof bolts used in mines. The last implementation, performed in cooperation with researchers at the Turner- Fairbanks Federal Highway Administration Research Center in McLean, Virginia, used optical fiber sensors, attached to composite prestressing strands used for reinforcing concrete, to obtain absolute strain information. Multiplexing techniques including time, frequency and wavelength division multiplexing are briefly discussed, whereas the principles of operation of spread spectrum and optical time domain reflectometry (OTDR) are discussed in greater detail. Results demonstrating that spread spectrum and OTDR techniques can be used to multiplex optical fiber sensors are presented. Finally, practical considerations that have to be taken into account when implementing optical fiber sensors into a civil structure environment are discussed, and possible solutions to some of these problems are proposed. / Ph. D.

A kind of intensity modulated fiber sensor utilizing axial offset parameter is proposed. The theoretical analysis and experimental characteristics of this sensor are described. All the theoretical results derived in this thesis are based on assuming a uniform power distribution in the fibers. An expression of coupling efficiency of central dipped parabolic graded index fibers due to axial offset is derived. The results show less sensitivity to axial offset for the central dipped fibers than for the parabolic profile fibers without a dip. Expressions of coupling efficiency of graded index fibers due to axial offset for several different values of a are also derived. The results show that sensitivity increases as the value of a decreases. A general expression of coupling efficiency which is valid for small values of axial offset is derived. This expression exhibits a linear relationship between coupling efficiency and small axial offset. Coupling efficiencies versus fiber end separation and axial offset of step index fibers have been measured. The measurements show that coupling efficiency is much more sensitive to axial offset than end separation. A simple construction of the axial offset fiber sensor is described. An approximate linear relationship between the output power and the mechanical loading has been obtained for this sensor. Several ways of increasing the sensitivity of this sensor are discussed. / M.S.

All-fiber optical sensors have established themselves in the market for a decade. Among different categories of these sensors is the intensity-based type whose advantages make it suitable for many practical applications. Frustrated Total Internal Reflection (FTIR) is a phenomenon that shows a great potential to be used as a transduction mechanism in this type of sensors. Although physical interpretation of the phenomenon itself is rather complicated, it can be mathematically formulated in a non-extensive manner. The first attempt is to build a physical understanding of the phenomenon as well as a simple mathematical model based on planar waves. Then, it will be expanded it to waves inside a waveguide structure like optical fibers in a concrete way based on computer simulations and actual experiments, a task that has never been done before. This would make it possible to address the practical issues involved afterwards. Again, the obtained experimental results as well as simulations help analyze advantages and disadvantages of our basic structure and move on to design a practically viable all-fiber FTIR sensor. A prototype vacuum pressure sensor has been designed and tested to show the potential in this category of optical sensors. Some other applications are also briefly discussed to give us a good feeling of the restrictive problems and future prospective of the ongoing related research. / Master of Science

Several techniques for measuring high frequency vibrations are presented. The goal of the study is to develop a sensor for detecting acoustic emissions (AE) inside composite structures. The basics of wave propagation inside of materials has been presented along with an overview of typical acoustic emission testing. Surface acoustic waves (SAWs) were studied first and a novel, noncontact optical interferometric technique for measuring absolute amplitudes is presented. This technique has the added advantages in that it does not require that the interferometer be stabilized or phase biased. It is insensitive to laser fluctuations, random phase drifts, polarization changes and changes in mixing efficiency of the interferometer. SAW amplitudes between 7 and 2.5 angstroms were measured with the described technique. An intrinsic Fabry-Perot type interferometer was demonstrated for detecting SAW's and was then embedded into carbon fiber composite panels which were then put through tensile tests. AE's were captured, centered around 300 KHz, as is expected of a composite. These tests were repeatable and indicate that qualitative measurements of AE can be made. This sensor configuration was also used for detecting a variety of taps on the composite panel as well as pencil lead breaks, a standard calibration procedure for AE testing. / Master of Science