COHERENCE PROPERTIES OF SUPERCONTINUUM GENERATED IN HIGHLY NONLINEAR PHOTONIC CRYSTAL FIBERS.
Abstract: In this dissertation, experimentally measured spectral and coherence
evolution of supercontinuum (SC) is presented. Highly nonlinear soft-glass photonic crystal
fibers (PCF) were used for SC generation, including lead-silicate (Schott SF6) PCFs of a
few different lengths: 10.5 cm, 4.7 mm, and 3.9 mm, and a tellurite PCF of 2.7 cm. The pump
is an optical parametric oscillator (OPO) at ... read more1550 nm with pulse energy in the order of
nanojoule (nJ) and pulse duration of 105 femtosecond (fs). The coherence of SC was measured
using the delayed-pulse method, where the interferometric signal was sent into an optical
spectrum analyzer (OSA) and spectral fringes were recorded. By tuning the pump power,
power-dependent evolution of spectrum and coherence was obtained. Numerical simulations
based on the generalized nonlinear Schrödinger equation (GNLSE) were performed. To match
the measured data, the simulated spectral evolution was optimized by iteratively tuning
parameters and comparing features. To further match the simulated coherence evolution with
the measurement, shot noise and pulse-to-pulse power fluctuation were added in the pump,
and the standard deviation of the fluctuation was tuned. Good agreement was obtained
between the simulated and the measured spectral evolution, in spite of the unavailability
of some physical parameters for simulation. It is demonstrated in principle that, given a
measured spectral evolution, the fiber length, and the average power of SC, all other
parameters can be determined unambiguously, and the spectral evolution can be reproduced in
the simulations. Most importantly, the soliton fission length can be simulated accurately.
The spectral evolution using the 4.7- and the 3.9-mm SF6 PCFs shows a pattern dominated by
self phase modulation (SPM). This indicates that, these fiber lengths are close to the
soliton fission length at the maximum power. The spectral evolution using the 10.5-cm SF6
PCF and the 2.7-cm tellurite PCF shows a soliton-fission-dominated pattern, indicating
these lengths are much longer than the soliton fission length at the maximum power. For the
coherence evolution using the SF6 PCFs, the simulations and the measurements show
qualitative agreement, confirming the association between coherence degradation and soliton
fission. For the case of the tellurite PCF, nearly quantitative agreement is shown, and it
is shown that the solitonic coherence degrades slower than the overall coherence.
Fluctuation of coherence occurs at the regime where the coherence starts to degrade, in the
measurement and the simulations of the SF6-PCF case. It is shown that the cause is the
pulse-to-pulse power fluctuation in the pump. The pulse-to-pulse stability of spectral
intensity is another characterization of SC stability, other than the coherence. It is
shown by simulations that these two exhibit different dynamics, and have low
Thesis (Ph.D.)--Tufts University, 2015.
Submitted to the Dept. of Physics.
Advisor: Fiorenzo Omenetto.
Committee: Peggy Cebe, Daniel Kane, Anthony Mann, Austin Napier, Krzysztof Sliwa, and Roger Tobin.
Keywords: Physics, and Optics.read less