High Power Optical Pulses through Linear, Nonlinear, and Time-Varying Media and their Applications
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Date
2023-04
Researcher
Biswas, Piyali
Supervisor
Ghosh, Somnath
Journal Title
Journal ISSN
Volume Title
Publisher
Indian Institute of Technology Jodhpur
Abstract
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.
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Citation
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.