Distributed optical fiber sensors offer unprecedented advantages, and the most remarkable one is the ability to continuously measure physical or chemical parameters along the entire optical fiber, which is attached to the device, structure and system. As the most recently investigated distributed optical fiber sensors, phase-sensitive optical time domain reflectometry (φ-OTDR), Brillouin optical time domain analysis (BOTDA) and Brillouin dynamic grating-optical time domain reflectometry (BDG-OTDR) techniques have been given tremendous attention on the advantage of quantitative measurements ability over high sensitivity and absolute measurement with long sensing distance, respectively. However, the accompanying limitations in terms of static measurement range, acquisition rate, laser frequency drifting noise, and spatial resolution limitations in these techniques hinder their performance in practical applications. This thesis pays particular attention to the above three distributed sensing techniques to explore the fundamental limitations of the theoretical model and improve the sensing performance. Before presenting the novel sensing scheme with improved sensing performance, an introduction about distributed fiber optical sensing, including three main light scattering mechanisms in optical fiber, the recent advancements in distributed sensing and key parameters of Rayleigh scattering- and Brillouin scattering-based sensing systems. After that, a study on the theoretical analysis of large chirping rate pulse generation and the theoretical model of using chirped pulse as interrogation signal in φ-OTDR, BOTDA and BDG-OTDR systems are given. In the disruptive experimental implementations, the sensing performance has been improved in different aspects. By using a random fiber grating array as the distributed sensor, a high-precision distributed time delay measurement in a CP φ-OTDR system is proposed thanks to the enhanced in-homogeneity and reflectivity. In addition, a simple and effective method that utilizes the reference random fiber grating to monitor the laser frequency drifting noise is demonstrated. Dynamic strain measurement with a standard deviation of 66 nε over the vibration amplitude of 30 με is achieved. To solve the limited static measurement range issue, a multi-frequency database demodulation (MFDD) method is proposed to release the large strain variation induced time domain trace distortion by tuning the laser initial frequency. The maximum measurable strain variation of about 12.5 με represents a factor of 3 improvements. By using the optimized chirped pulse φ-OTDR system, a practical application of monitoring the impact load response in an I-steel beam is demonstrated, in which the static and distributed strain variation is successfully reconstructed. To obtain an enhanced static measurement range without a complicated database acquisition process, a photonic approach for generating low-frequency drifting noise, arbitrary and large frequency chirping rate (FCR) optical pulses based on the Kerr effect in the nonlinear optical fiber is theoretically analyzed and experimentally demonstrated by using both fixed-frequency pump and chirped pump. Due to the Kerr effect-induced sinusoidal phase modulation in the nonlinear fiber, high order Kerr pulse with a large chirping rate is generated. Thus the static measurement range of higher order Kerr pulse is significantly improved. Chirped pulse BOTDA based on non-uniform fiber is also analyzed, showing a high acquisition rate that is only limited by the sensor length and averaging times due to the relative Brillouin frequency shift (BFS) changes are directly extracted through the local time delays between adjacent Brillouin traces from two single-shot measurement without frequency sweep process. BFS measurement resolution of 0.42 MHz with 4.5 m spatial resolution is demonstrated over a 5 km non-uniform fiber. A hybrid simultaneous temperature/strain sensing system is also demonstrated, showing a strain uncertainty of 4.3 με and temperature uncertainty of 0.32 °C in a 5 km non-uniform fiber. Besides, the chirped pulse is also utilized as a probe signal in the Brillouin dynamic grating (BDG) detection along the PM fiber for distributed birefringence variations sensing. The strict phase-matching condition only enables part of the frequency components within the chirped probe pulse to be reflected by BDG, giving an adjustable spatial resolution without photo lifetime limitation. The spatial resolution is determined by the frequency chirping rate of the probe pulse.

The most promising strategy for reducing the cost of manufacturing polymer matrix composites while improving their reliability is the use of sensors during processing to permit control of the cure cycle based on measurements of the material's internal state. While sensors have been demonstrated that infer the material state indirectly through measurements of acoustic impedance, electrical impedance, or refractive index, sensors that directly measure parameters critical to composite manufacturing, such as resin rheology and resin hydrostatic pressure, would improve characterization of thermoset resins during cure. Here we describe the development of a multifunctional fiber optic sensor that may be embedded in a composite part during lay-up to monitor the state of the polymer matrix during processing. This sensor will output quantitative data which will indicate the viscoelasticity of the thermoset matrix resin. The same sensor will additionally function as a strain sensor following fabrication, capable of monitoring residual strains due to manufacturing or in-service internal strains. / Ph. D.

A small shear sensor utilizing an array of carbon nanotubes to support a sensor head was developed for use in steady, high speed, 2D flow. The sensor is a non-intrusive, direct measurement device with a 2 x 2 mm square sensor head surrounded by a small gap on each side (~0.004 inches). The translation of the sensing element is due to the nanotubes bending when a shear force is applied to the sensor head. Displacements are measured by an interferometric technique using fiber-optics to measure the distance the sensor head travels by viewing a polished side of the head. The fiber-optical displacement sensor is bonded to a stationary substrate so that all measurements are relative to a fixed position. Arrays of carbon nanotubes were grown on bare 2 x 2 mm square silicon chips. The nanotubes were grown to heights of 75 microns with a thin layer of amorphous carbon on top. The silicon chips were then flipped, and the amorphous layer of carbon was bonded to bare 1 x 1 cm silicon substrates, making the bottom of 2 x 2 mm silicon chip the sensor head. The sensors were calibrated at Luna Innovations using a point-load technique. Four of the six sensors could not be successfully calibrated because they were fatally damaged during the last step of the calibration process. Wind tunnel tests were conducted on the one sensor that survived the calibration. An arrangement was designed and built from aluminum to test the performance of the sensor in the Virginia Tech Supersonic Wind Tunnel. Seven test runs were conducted in this cold-flow facility at a nominal Mach number of 2.4 and stagnation pressures ranging from 50 - 90 psia. Two test runs gave skin friction values 3 - 20% lower than those values predicted by indirect measurement techniques before the sensor was damaged. While these first results are encouraging, further studies are clearly needed. Due to distinct anomalies in the displacement data during test run 3, it was concluded that the sensor was damaged during this run. Possible explanations of the failure of this sensor are offered along with suggestions for future work. / Master of Science

In digital communication systems, several types of impairments may be introduced to the signal. These impairments result in degraded system performance; for example, high bit-error-rate or power penalty. For optical communication systems, in this thesis, these impairments are categorized into four types; that is, thermal noise, shot noise, signal-dependent noise, and intersymbol interference (ISI). By using a Gaussian approximation, effects of the first three impairments are analyzed. It is shown that signal-dependent noise introduces an error floor to the system and the bit-error-rate is considerably degraded if a nonzero-extinction ratio is applied to the system. It is shown that if the decision threshold at the decision circuit is set improperly, more received power is required to keep the bit-error-rate constant. Three main components in the system (i.e., transmitter, optical fiber, and receiver) are modeled as Butterworth filters. ISI from this model is determined by computer simulation. A high ISI is from a small system bandwidth. It is shown that a minimum power penalty can be achieved if the transmitter and receiver bandwidths are matched and fixed, and the ratio of fiber bandwidth to bit rate is 0.85. Comparing ISI from this model to ISI from raised cosine- rolloff filters, it is shown that at some particular bandwidths ISI from raised cosine-rolloff filters is much lower that that from this model. However, if the transmitter and receiver bandwidths are not matched and are not equal to these bandwidths, ISI from this model is lower than ISI from raised cosine-rolloff filters. / Master of Science

This research investigates alternate methods of implementing a redundant communications ring for polled network applications. FDDI and a unique solution (the tone method) designed specifically for this application are compared. In polled networks one of the most critical parameters is the time required to interrogate all of the nodes in the network. This parameter, called the poll time, is compared for the two solutions under various operating conditions, including the instances when the ring is intact and when the ring has experienced a failure, causing it to operate in the redundant mode. As expected, the solution designed specifically for this application performed better than FDDI. The tone method, which requires very little overhead and is not limited by distance or number of nodes, is described in detail, as are the results of poll time comparisons. / Master of Science

Current analysis of the life and death cycles of in vitro cellular systems is based on visual observation methods relying upon morphological changes monitored using a microscope. Data collected from these techniques are not as precise as scientists desire them to be. The methods are discontinuous, indirect, costly, and time and labor intensive. The human element plays a significant part in error propagation as individual style of the researcher lends to skewing the data. Experimental results will differ greatly from laboratory to laboratory just because the methods of monitoring cellular activity are not standardized. The researcher uses experience to determine the best way to collect data quickly and "accurately" according to his or her definition. There is a great need not only to standardize data collection processes, but also to eliminate human error induced by lack of experience or fatigue. This research proposes a fiber optic based monitoring system as a possible solution to eliminate a number of problems with current cellular data collection methods and to increase the data collection rate tremendously since the process could be automated. / Master of Science

The purpose of this thesis is to investigate the effect of dispersion on a spectrum-sliced WDM (SS-WDM) system, specifically a system employing a single-mode optical fiber. The system performance is expressed in term of the receiver sensitivity defined as the average number of photon per bit <i>N_p</i>required for a given probability of bit error <i>P_e</i>. The receiver sensitivity is expressed in terms of two normalized parameters: the ratio of the optical bandwidth per channel and the bit rate <i>m</i>=<i>B</i>₀<i>/R_b</i>=<i>B</i>₀<i>T</i>, and the transmission distance normalized by the dispersion distance <i>z/L_D</i>. The former represents the effect of the excess beat noise caused by the signal fluctuation. The latter represents the effect of dispersion. The excess beat noise can be reduced by increasing the value of <i>m</i> (increasing the optical bandwidth<i> B</i>₀ for a given bit rate<i> R_b</i>). However, a large <i>m</i> implies that the degradation due to the dispersion is severe in a system employing a single-mode fiber. Therefore, there should be an optimum <i>m</i> resulting from the two effects. The theoretical results obtained from our analysis have confirmed this prediction. It is also shown that the optimum <i>m</i> (<i>m_opt</i>) decreases with an increase in the normalized distance. This suggests that the dispersion strongly affects the system performance. The increase in the excess beat noise is traded for the decrease in the dispersion effect. Additionally, the maximum transmission distance is relatively short, compared to that in a laser-based system. This suggests that the SS-WDM systems with single-mode fibers are suitable for short-haul systems, such as high-speed local-access network where the operating bit rate is high but the transmission distance is relatively short. / Master of Science

<p> This thesis deals with the limiting factors in the design of a long-haul fiber-optic communication system, and the techniques used to suppress their resulting impairments. These limiting factors include both linear and nonlinear effects, such as fiber chromatic dispersion and the Kerr nonlinearity, and the modulator-induced nonlinearity. </p> <p> In Chapter 3, the conditional probability density function (PDF) of the received elect rical signal given transmitted bit '1 '/'0' for a coherent fiber-optic transmission system based on binary phase shift keying (BPSK) is mathematically derived. Both amplified spontaneous emission (ASE) noise and fiber nonlinearity are taken into account . The results show that the conditional PDF of given bit '1' or '0' is asymmetric when intrachannel four-wave mixing (IFWM) is dominant, while it becomes nearly symmetric when the variance of ASE is much larger than that due to IFWM. The standard deviation of the received signal is calculated analytically. The system parameters, including optimum dispersion map and pre-compensation ratio, are optimized by analytically calculating variance of IFWM. Significant computation efforts can be saved using this approach as compared to full numerical simulations of the nonlinear Schrodinger equation, without losing much accuracy. </p> <p> In Chapter 4, an improved 4-f time-lens configuration is proposed. Fourier transform (FT) and inverse Fourier transform (IFT) can be realized using time lenses such that there is no need for time reversal at the end. A typical 4-f configuration consists of two 2-f systems and a temporal filter. The first 2-f system consisting of a time lens and two dispersive elements produces the Fourier transform (FT) of the input signal. The temporal filter modifies the spectrum. The next 2-f system produces the inverse Fourier transform (IFT). A wavelength division demultiplexer and a higher-order dispersion compensator based on 4-f configuration are numerical implemented. One of the advantages of the time-lens-based temporal filtering technique is that the transfer function of the temporal filter can be dynamically altered by changing the input voltage to the temporal filter (amplitude/phase modulator) and therefore, this technique could be used for dynamic switching and multiplexing in optical networks. </p> <p> In chapter 5, a direct-detection optical orthogonal frequency division multiplexing (DD-0-0FDM) is realized using time lenses. Typically, in OFDM systems, discrete Fourier transform (DFT) is used at the transmitter and inverse discrete Fourier transform (IDFT) is used at the receiver. In this chapter, it is proposed to use continuous Fourier transform (FT) and inverse Fourier transform (IFT) using time lenses that replace DFT and IDFT in the electrical domain. The third- and higher-order dispersive effects can be considerably reduced using the proposed DD-0-0FDM scheme. </p> <p> In Chapter 6, a coherent optical orthogonal frequency division multiplexing (OFDM) (C0-0-0FDM) scheme using time lenses is analyzed. The comparison of performance between the proposed scheme and the conventional optical OFDM scheme using fast Fourier transform (FFT) and inverse FFT in the electrical domain is made. Both the Mach-Zehnder modulator (MZM) induced and fiber induced nonlinearities are investigated. Results show that the time-lens-based C0-0-0FDM performs almost the same as the FFT-based C0-0-0FDM when the message signal launched to MZM is low so that MZM operates in the linear region. The nonlinearity of MZM degrades the performance of FFT-based C0-0-0FDM drastically when the power of message signal becomes sufficiently large, but only has negligible impact on the time-lens-based C0-0-0FDM. A periodical driving voltage has been proposed to set up the time lens such that the maximally required driving voltage level is kept low within the time frame. The advantages using the time-lens-based C0-0-0FDM are that (i) FT can be done in optical domain almost instantaneously, whereas the FFT in digital domain is slow and requires significant computational efforts, (ii) optical domain Fourier transform has a large bandwidth (~THz) and therefore, FT /IFT can be performed at a large symbol rate. </p> <p> In Chapter 7, the digital backward propagation (DBP) has been studied both in orthogonal frequency-division multiplexing ( OFDM) and single-carrier (SC) fiber-optic transmission systems. 16 quadrature amplitude modulation (QAM) is used for both systems with the bit rate of 100 Gbjs. The results show that OFDM and SC with Nyquist pulses (SC-Nyquist) have a superior performance as compared to SC with raisedcosine pulses (SC-NRZ) when the DBP is used. The impact of electrical filter bandwidth and nonlinear phase/amplitude noise has also been investigated. The performance of perfect-BP-based OFDM/SC initially improves when the electrical filter bandwidth increases at high signal-to-noise ratio (SNR). The comparison of the effects of nonlinear phase/amplitude noise among OFDM, SC-Nyquist and SC-NRZ systems is made and it is shown that SC-NRZ systems significantly suffer from the effects of nonlinear phase/amplitude noise, which explains the performance advantage of OFDM/SC-Nyquist over SC-NRZ when the DBP used. </p> / Thesis / Doctor of Philosophy (PhD)

International Telemetering Conference Proceedings / October 26-29, 1992 / Town and Country Hotel and Convention Center, San Diego, California / In this paper, we investigate high-capacity fiber-optic networks for real-time telemetry applications. The network topologies and related network components are analyzed for telemetry fiber-optic Local Area Network (LAN) and Metropolitan Area Network (MAN) as well as MAN internetworking with LANs. Two types of multiplexing techniques, namely, Wavelength Division Multiplexing and Time Division Multiplexing, are proposed to support real-time high-capacity telemetry applications, and the perspective of such networks is also considered. Finally, the optical modulation technique and the choice of optical devices are discussed, which are based on improving the reliability of fiber-optic LANs and MANs.

Parallel and distributed computing systems become more and more powerful and hence place increasingly higher demands on the networks that interconnect their processors or processing nodes. Many of the applications running on such systems, especially embedded systems applications, have real-time requirements and, with increasing application demands, high-performance networks are the hearts of these systems. Fiber-optic networks are good candidates for use in such systems in the future. This thesis contributes to the relatively unexplored area of fiber-optic networks for parallel and distributed real-time computer systems and  suggests and evaluates several fiber-optic networks and protocols. Two different technologies are used in the networks, WDM (Wavelength Division Multiplexing) and fiber-ribbon point-to-point links. WDM offers multiple channels, each with a capacity of several Gbit/s. A WDM star network in which protocols and services are efficiently integrated to support different kinds of real-time demands, especially hard ones, has been developed. The star-of-stars topology can be chosen to offer better network scalability. The WDM star architecture is attractive but its future success depends on components becoming more commercially mature. Fiber-ribbon links, offering instead an aggregated bandwidth of several Gbit/s, have already reached the market with a promising price/performance ratio. This has motivated the development and investigation of two new ring networks based on fiber-ribbon links. The networks take advantage of spatial bandwidth reuse, which can greatly enhance performance in applications with a significant amount of nearest downstream neighbor communication. One of the ring networks is control channel based and not only has support for real-time services like the WDM star network but also low level support for, e.g., group communication. The approach has been to develop network protocols with support for dynamic real-time services, out of time-deterministic static TDMA systems. The focus has been on functionality more than pure performance figures, mostly on real-time features but also on other types of functionality for parallel and distributed systems. Worst-case analyses, some simulations, and case studies are reported for the networks. The focus has been on embedded supercomputer applications, where each node itself can be a parallel computer, and it is shown that the networks are well suited for use in the radar signal processing systems studied. Other application examples in which these kinds of networks are valuable are distributed multimedia systems, satellite imaging and other image processing applications. / Technical report / School of Electrical and Computer Engineering, Chalmers University of Technology, Göteborg, Sweden, 0282-5406 ; 379