Nonlinear Phenomena In Higher Order Mode Fiber For Development Of Light Sources For Biomedical Imaging
Higher order mode (HOM) fibers present a breakthrough in fiber design, in its ability to provide anomalous dispersion in solid, silica-based fiber below 1300 nm. Anomalous dispersion is especially interesting for purposes of short-pulse propagation, since soliton propagation requires this sign of dispersion to balance the nonlinear phase induced by self-phase modulation. Since silica has normal material dispersion below 1300 nm, and a fundamental mode propagating in single-mode fiber has negative waveguide dispersion, access to anomalous dispersion below 1300 nm has been limited to special microstructured fibers. These microstructured fibers, however, are limited to regimes of power too extreme for biomedical applications. Hollow-core photonic bandgap fibers (PBGF), with their low nonlinearity, require pulse energies of many hundreds of nanojoules, while solid-core photonic crystal fibers (PCF), with their tight confinement, support only fractions of nanojoule-pulses. HOM fibers, are all-solid silica but can attain anomalous waveguide dispersion (and thus anomalous dispersion below 1300 nm) by propagating light solely in a higherorder mode (LP02) of the fiber. With moderate nonlinearity (compared to PBGF) and relatively large mode area (compared to PCF), HOM fibers aim to provide 1-10 nJ pulses at wavelengths suitable for biomedical imaging applications. We demonstrate the nonlinear phenomena of soliton self-frequency shift and Cerenkov radiation generation in HOM fibers for providing short energetic pulses. By pumping an HOM fiber with a free-space femtosecond fiber laser, Raman-shifted solitons and a compressible radiation band (Cerenkov radiation) on the long-wavelength side of the zero-dispersion wavelength were observed, spanning 1064 nm to 1450 nm. In addition, with longer fiber lengths and a picosecond fiber laser source, we were able to demonstrate an all-fiber system for generating shifted solitons and Cerenkov radiation. This all-fiber system was successfully used as a light source for two-photon fluorescence microscopy, and the focusing properties of the LP02 mode were carefully characterized.
nonlinear fiber optics; specialty fibers; nonlinear microscopy
Wise, Frank William; Pollock, Clifford Raymond
Ph.D. of Applied Physics
Doctor of Philosophy
dissertation or thesis