High Power Optical Pulses through Linear, Nonlinear, and Time-Varying Media and their Applications

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Biswas, Piyali
Ghosh, Somnath
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Indian Institute of Technology Jodhpur
Optical pulses are electromagnetic waves that exist for a particular duration of time. Such short-lived light pulses are generated from lasers and are very useful for data transmission, imaging, medical surgeries, various time-resolved measurements, optical signal processing, and many more. However, the transmission of such optical pulses through the state-of-the-art photonic devices eventually encounters certain challenges due to the unavoidable light-matter interaction processes that, in a large amount, controls the targeted outcome. Essentially, shorter optical pulses, with duration ranging from a few picoseconds to several hundreds of femtoseconds, severely suffer higher order dispersive and nonlinear effects. However, a judicious management of such linear and nonlinear processes may result in certain exclusive pulse dynamics. This thesis entirely focuses on the investigation of such exclusive and robust characteristics of ultrashort optical pulses during their propagation through specially designed photonic structures exhibiting linear dispersive, nonlinear, and time-varying optical effects for specialty applications. Two device platforms, that have immense potential to change the course of scientific and technological flow, have been chosen to study the ultrashort pulse dynamics. First, optical fibers have been chosen as the waveguide structure supporting confinement of light through spatially varying refractive index profile, and then a linear dispersive bulk medium with a temporally varying refractive index profile. The specific purpose of high power short pulse delivery demands robust pulse characteristics to withstand higher order dispersive and nonlinear effects. Further investigations in this direction has demonstrated Solitons and Similaritons (widely known as Parabolic pulses) to be such special type of pulses which are robust against any detrimental optical effect once formed. Formation of these pulses requires pulse reshaping techniques that can be efficiently realized in optical fibers owing to their design-flexible structural and physical properties to guide and manipulate light. Especially, the photonic bandgap fibers (PBF) where the special arrangement of high and low index material in the cladding surrounding the low index core provides ample possibilities of fiber parameter customization necessary for pulse reshaping. Based on such PBF geometry, a group of all-solid specialty optical fibers, have been presented, to accomplish reshaping of high power ultrashort pulses through longitudinal fiber tapering into either similaritons or solitons such that they can propagate over long distances without any temporal as well as spectral distortions. Firstly, an approach based on input pulse customization technique has been implemented to realize a stable self-similar delivery of parabolic pulses through a designed longitudinally tapered fiber with standard Bragg fiber crosssection in the near-infrared wavelength range. Formation of parabolic pulses from a backgroundguided combined input pulse, and its stable propagation with self-similar evolution through the tapered fiber has been presented over kilometer long distances providing a comparatively better outcome. Further, to achieve stable delivery of self-similar parabolic pulses in mid-infrared, a novel fiber customization approach exhibiting a rapidly varying longitudinal dispersion profile with near-zero average normal dispersion has been adapted to design and optimize the specialty fiber. A detailed study on the roles of higher order dispersion and nonlinear effects in such dispersion oscillating fiber has been presented, along with the proposal of a two-fold fiber engineering scheme to eliminate such higher order detrimental effects. Furthermore, based on the fundamental light guiding principle of PBFs, a multicore specialty bandgap fiber supporting an ultra-wide low-loss fiber bandwidth owing to the concentrically arranged effectively formed cores have been proposed to deliver stable femtosecond solitons and/or similaritons over kilometer long distances, eliminating the challenge of bandwidth limitation in fibers. Finally, to meet the requirement of robust transport of high power short optical pulses, a very special type of multilayered fiber, known as topological fiber, has been demonstrated where light guidance occurs at the interface of two topologically distinct periodic structures. Such guided interface states of light are inextricably tied to the topology of the entire system through an invariant quantity which inherently provides the immunity to backreflections, defects or dislocations. A detailed pulse propagation study through such topological interface states has also been presented which is envisaged to pave the way towards futuristic fiber optic technology. On the other hand, the behavior of ultrashort optical pulses in a time-dynamic medium has been investigated which has tremendous potential towards new generation all-optical integrated photonic devices. Light dynamics in such a linear dispersive medium with refractive index as a function of time has led to certain exquisite optical phenomena such as asymmetric pulse transmission and wavelength conversion. The effect of presence of deliberate gain and loss in the medium has been analyzed thoroughly. Furthermore, the dispersive nature of the medium has been investigated separately to study its effect on the asymmetric propagation of pulses. Moreover, the shifting of pulse central wavelength with respect to the input in such a linear time-dynamic medium has been found to assent the phenomenon of spectral nonreciprocity, which was primarily observed in presence of nonlinearity or magnetic effects in cavities or waveguides. Finally, a nonreciprocal behavior of optical pulses through such linear time-varying medium has been demonstrated with appropriate theoretical explanation, which will surely open up a new avenue for next-generation integrated photonic devices.
Biswas, Piyali. (2023).High Power Optical Pulses through Linear, Nonlinear, and Time-Varying Media and their Applications (Doctor's thesis). Indian Institute of Technology Jodhpur, Jodhpur.