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Call number: LD2668 .T4 1984 S33 / Master of Science

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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Doctor of Philosophy / Department of Physics / Brian R. Washburn / Mode-locked lasers that produce short optical pulses in the mid-infrared wavelength region have been sought out for a wide range of applications such as free space communication, molecular spectroscopy, medical diagnostics, and remote sensing. Here, a thulium and holmium (Tm/Ho) co-doped fiber laser that mode-locks in both the solitonic and stretched-pulse regimes is used to produce ultra-short pulses in the 2 [mu]m region. Nonlinear polarization rotation technique is used where fiber nonlinearity is responsible to mode-lock the laser. The anomalous group velocity dispersion of both the single mode and gain fibers used limit the laser operation in the solitonic regime where spectral bandwidth is 10 nm and hence the pulse duration is limited to 996 fs. In order to increase the spectral bandwidth and hence get the shorter pulses the anomalous dispersion of these fibers has to compensate using normal group velocity dispersion fiber in the laser cavity. High numerical aperture fibers, which have normal group velocity dispersion around 2 [mu]m due to its large and positive waveguide dispersion, can be used to compensate the anomalous dispersion of the gain and single mode fibers. We used a high numerical aperture fiber called UHNA4 in the laser cavity in order to compensate the anomalous dispersion of other fibers and mode-locked the laser in stretched pulse regime. The spectral bandwidth of the laser increased to 31 nm with corresponding pulse duration of 450 fs measured from the interferometric autocorrelation. The laser dynamics of the Tm/Ho co-doped fiber laser is also studied while going from the stretched-pulse to solitonic regime by fiber cut-back measurements of normal dispersion fiber. It was clearly observed that both the spectral bandwidth and the pulse duration changed significantly going from one region to the other.

The computation of a piecewise smooth function that approximates a finite set of data points is decomposed into two decoupled tasks: first, the computation of the locally smooth models, and hence, the segmentation of the data into classes that consist on the sets of points best approximated by each model, and second, the computation of the normalized discriminant functions for each induced class. The approximating function is then computed as the optimal estimator with respect to this measure field. Applications to image processing and time series prediction are presented as well.

Control algorithms which exploit the unique properties of chaos can vastly improve the design and performance of many practical and useful systems. The program Perfect Moment is built around such an algorithm. Given two points in the system's state space, it autonomously maps the space, chooses a set of trajectory segments from the maps, uses them to construct a composite path between the points, then causes the system to follow that path. This program is illustrated with two practical examples: the driven single pendulum and its electronic analog, the phase-locked loop. Strange attractor bridges, which alter the reachability of different state space points, can be used to increase the capture range of the circuit.

Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 73-75). Also available on the World Wide Web.

The graphene-polymer SA thin film using solution blending method and atomic layer graphene as saturable absorber (SA) used to generate femtosecond laser pulse were measured. Stable soliton-like pulses with the pulsewidth of 403 fs and 432 fs, the spectral linewidth of 6.32 nm and 6.16 nm, and the time-bandwidth product of 0.315 and 0.329 using graphene-PVA film and atomic layer graphene as SA were achieved, respectively, in mode-locked Er-doped fiber ring laser. The graphene-PVA SA suffered from larger loss caused by graphene flake aggregating, while the atomic layer graphene had smaller nonsaturable loss which exhibited lower mode locking threshold power. Atomic layer graphene also had stable fabricated process and controllable modulation depth depended on its layer numbers. To compare the mode locking performance of single wall carbon nanotubes (SWCNTs) and graphene SA, the same solution blending fabricated sample was used. Under similar nonsaturable loss and modulation depth, the SWCNTs SA with optimized concentration of 0.5wt% and thickness of 188£gm had shortest pulsewidth of 440 fs and 3-dB spectral linewidth of 6 nm. The shortest pulsewidth of 403 fs and broad spectral linewidth of 6.32 nm was obtained using graphene SA with concentration of 6.25wt% and thickness of 18£gm. Graphene has broad band absorbance and larger modulation depth, the experimental result indicates that graphene SA can generate shorter pulse and has chance to become the potential candidate of SA.

A mode locked fiber-ring laser utilizing an optical spot-size converter (OSSC) integrated electroabsorption modulator (EAM) and Erbium-doped fiber amplifiers (EDFA) is demonstrated in this work. By taking advantage of OSSC, the EAMs not only have high-speed performance, but also have high tolerance alignment stability and high power handling capability due to the distributive effect. Thus, by the saturation absorbing and highly nonlinear transmission properties of such EAMs, it can be realized that short optical switching widow operations with high pulse energy excitation driven by EDFA can be used to hybrid mode locked ring laser application. The repetition rate of 10GHz in the mode locked fiber-ring laser is set by a RF synthesizer to driven EAM, creating short time gating widows. EDFA with an optical filter of bandwidth 1.2nm is used for optical gain. By adjusting the operation point in EAM to a bias of 1.72V and a RF-power of Vpp=2.2V, a nearly transform-limited autocorrelated Gauss pulse of 11.72ps (FWHM, the extracted pulsewidth is 8.28 ps) with average power level of 1.2mW is obtained. The time-bandwidth product is 0.478. The optical pulsewidths are mainly limited by the optical filter. Without any feedback circuit, pulse jitter of 993fs measured from a high-speed sampling scope (Agilent 86100A, bandwidth =30GHz) is found from the mode locked pulses, indicating the hybrid mode locked operation can be achieved by the EAM saturation absorbing properties. By measuring the photocurrent of EAM, the highest photocurrent occurs in the conditions of the highest optical transmission and also the shortest optical pulses, suggesting the saturation absorbing of EAM is the main mechanism dominating mode-lock operation. The mode locked operation with repetition rates of 10GHz to 20GHz are also obtained, indicating high-speed operation can be boosted from OSSC integrated EAM to hybrid-type mode locked fiber-ring laser.