"Hydroxyl ions are a common contaminant in optical fibers, and are responsible for strong absorption centered at 1380 nm that becomes significant over long optical path lengths. Recently, however, special fabrication methods have been developed that minimize the hydroxyl ion contamination, permitting use of the entire 1300-1700 nm spectral region for telecommunications. There is therefore interest in examining the Nd 4F3/2 to 4I13/2 transition for a potential optical amplifier at 1400 nm. In this thesis, the excited state absorption cross section and the overall gain/loss spectrum of neodymium in a silica glass fiber were determined for the 1200-1500 nm wavelength region using the pump-probe method. The ground state absorption cross section was also determined from transmission measurements, and the stimulated emission cross section was calculated using the fluorescence spectrum and the McCumber relation. Oscillator strengths for absorption and emission transitions were calculated in the 800-1600 nm wavelength range using the Judd-Ofelt method. The above procedures were followed for both the Nd-doped fiber, as well as an erbium-doped silica fiber. The shape of the Nd emission spectrum is also noteworthy, since the characteristic Nd peak at 1064 nm is not observed, although there is strong emission at 1092 nm. The pump-probe measurements revealed significant excited state absorption loss between 1200 and 1350 nm, due to excitation from the 4F3/2 to the higher 4G9/2 and 4G7/2 states. Between 1350 and 1475 nm, there was no net gain or loss that could be observed beyond the level of the noise. For the glass fibers studied, it appears that in the spectral region of interest for an optical amplifier, the stimulated emission and excited state absorption cancel one another out."

Liang Tak-keung. / "June 2004." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

"January 2004." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

Thesis (M. Phil.)--University of Hong Kong, 2009. / Includes bibliographical references. Also available in print.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

This thesis is dedicated to the study of gain transients of discrete fiber Raman amplifiers and to the all-optical gain-clamping technique which is used to mitigate those transients. / First, we study the standing-wave and the traveling-wave gain-clamping techniques when applied to a single discrete fiber Raman amplifier in the context of WDM channel add and drop. We take into account the operational regime of the amplifier and the location of the surviving channel in the amplification band. We demonstrate that the gain-clamped amplifier has to be operated in a regime below the critical regime to ensure that gain-clamping will be in effect. The efficiency of gain-clamping also depends on the feedback level of the lasing signal and on the implementation. / Next, we investigate the dynamic behaviour of a single discrete fiber Raman amplifier fed by multi-channel packet traffic. Our study shows that the efficiency of the gain-clamping technique to reduce the gain transients is dependent upon the operational regime of the amplifier and the packet duration. However, we also demonstrate that gain-clamping is not required to control the gain transients as the gain variations of the unclamped amplifier are small enough to be neglected. / We then theoretically analyse the dynamic response of cascades of discrete fiber Raman amplifiers subject to WDM channel add and drop. We consider cascades of mixed unclamped and gain-clamped amplifiers, varying the number and the position of the gain-clamped amplifiers in the cascade and taking into account the location of the surviving channel and the operational regime of the amplifiers. Our results show that the location of the gain-clamped amplifiers in a mixed cascade affects the transient characteristics and that it is possible to control the transients within tolerable limits. / Finally, we investigate the gain transients that occur in hybrid amplifiers in the presence of channel add and drop. We demonstrate that the gain-clamping technique can be used to mitigate the gain transients in hybrid amplifiers and that the surviving channel location does not influence the transient characteristics, contrary to the case of single and cascaded fiber Raman amplifiers.

This thesis reports original work on suppression of transient gain excursions in an erbium-doped fibre amplifier (EDFA). The work presented in this thesis is a detailed investigation of four closed-loop systems that control the EDFA gain dynamically. The performance of the four closed-loop systems is evaluated by analytical work, supplemented by computer simulations and insystem measurements performed on a hardware EDFA. In addition, a stability analysis of the four closed-loop systems is presented. In the stability analysis presented in this thesis, nonlinear nature of the four closed-loop systems is taken into consideration. In the stability analysis, in addition to proving that the four closed-loop systems considered are stable, it is proven that for any practical values of the EDFA gain at the initial time of observation, the EDFA gain is restored to the desired value in steady state. These outcomes of the stability analysis are supported by simulation results and experimental results. Errors in system modelling, change in the operating point of the nonlinear closed-loop system, and measurement noise are important aspects of practical implementations of systems that control the EDFA gain dynamically. A detailed analysis of the effects these practical aspects have on the performance of the four closed-loop systems considered is presented. The analysis is validated using computer simulations and experimental measurements. In most of the work reported in the literature on controlling the EDFA gain, controllers that include feedforward and/or feedback components are employed. In the traditional approaches to combining the feedforward and the feedback components, large transient excursions of the EDFA gain can still occur due to errors in the control provided by the feedforward component. In this thesis, a novel approach to combining the feedforward and the feedback components of the controller is presented. Based on the analytical work, the computer simulations and the experimental work presented in this thesis, the novel approach provides a significant reduction in the excursions of the EDFA gain in the transient period.

Thesis (Ph.D.)--McMaster University, 2005. / Supervisor: W.-P. Huang Includes bibliographical references (leaves 124-136).

Thesis (Ph.D.)--University of Western Australia, 2007.

Thesis (M. Phil.)--University of Hong Kong, 2008. / Includes bibliographical references (leaf 100-105) Also available in print.