Robert S. Windeler

Visible continuum generation in air–silica microstructure optical fibers with anomalous dispersion at 800 nm

We demonstrate experimentally for what is to our knowledge the first time that air–silica microstructure optical fibers can exhibit anomalous dispersion at visible wavelengths. We exploit this feature to generate an optical continuum 550 THz in width, extending from the violet to the infrared, by propagating pulses of 100-fs duration and kilowatt peak powers through a microstructure fiber near the zero-dispersion wavelength.

Ultrahigh-resolution optical coherence tomography using continuum generation in an air–silica microstructure optical fiber

We demonstrate ultrahigh-resolution optical coherence tomography (OCT) using continuum generation in an air-silica microstructure fiber as a low-coherence light source. A broadband OCT system was developed and imaging was performed with a bandwidth of 370 nm at a 1.3-mu;m center wavelength. Longitudinal resolutions of 2.5 microm in air and ~2 microm in tissue were achieved. Ultrahigh-resolution imaging in biological tissue in vivo was demonstrated.

Microstructured optical fiber devices

We present several applications of microstructured optical fibers and study their modal characteristics by using Bragg gratings inscribed into photosensitive core regions designed into the air-silica microstructure. The unique characteristics revealed in these studies enable a number of functionalities including tunability and enhanced nonlinearity that provide a platform for fiber device applications. We discuss experimental and numerical tools that allow characterization of the modes of the fibers.

Optical properties of high-delta air–silica microstructure optical fibers

We analyze the waveguide properties of microstructure optical fibers consisting of a silica core surrounded by a single ring of large air holes. Although the fibers can support numerous transverse spatial modes, coupling between these modes even in the presence of large perturbations is prevented for small core dimensions, owing to a large wave-vector mismatch between the lowest-order modes. The result is an optical fiber that can appear single mode with propagation properties that can be achieved only in multimode waveguides.

Supercontinuum generation in air–silica microstructured fibers with nanosecond and femtosecond pulse pumping

We study the generation of supercontinua in air–silica microstructured fibers by both nanosecond and femtosecond pulse excitation. In the nanosecond experiments, a 300-nm broadband visible continuum was generated in a 1.8-m length of fiber pumped at 532 nm by 0.8-ns pulses from a frequency-doubled passively Q-switched Nd:YAG microchip laser. At this wavelength, the dominant mode excited under the conditions of continuum generation is the LP11 mode, and, with nanosecond pumping, self-phase modulation is negligible and the continuum generation is dominated by the interplay of Raman and parametric effects. The spectral extent of the continuum is well explained by calculations of the parametric gain curves for four-wave mixing about the zero-dispersion wavelength of the LP11 mode. In the femtosecond experiments, an 800-nm broadband visible and near-infrared continuum has been generated in a 1-m length of fiber pumped at 780 nm by 100-fs pulses from a Kerr-lens model-locked Ti:sapphire laser. At this wavelength, excitation and continuum generation occur in the LP01 mode, and the spectral width of the observed continuum is shown to be consistent with the phase-matching bandwidth for parametric processes calculated for this fiber mode. In addition, numerical simulations based on an extended nonlinear Schrodinger equation were used to model supercontinuum generation in the femtosecond regime, with the simulation results reproducing the major features of the experimentally observed spectrum.

Ultrahigh-resolution optical coherence tomography by broadband continuum generation from a photonic crystal fiber

We have developed an ultrahigh-resolution optical coherence tomographic system in which broadband continuum generation from a photonic crystal fiber is used to produce high longitudinal resolution. Longitudinal resolution of 1.3-microm has been achieved in a biological tissue by use of continuum light from 800 to 1400 nm. The system employed a dynamic-focusing tracking method to maintain high lateral resolution over a large imaging depth. Subcellular imaging is demonstrated.

Cross-correlation frequency resolved optical gating analysis of broadband continuum generation in photonic crystal fiber: simulations and experiments

Numerical simulations are used to study the temporal and spectral characteristics of broadband supercontinua generated in photonic crystal fiber. In particular, the simulations are used to follow the evolution with propagation distance of the temporal intensity, the spectrum, and the cross-correlation frequency resolved optical gating (XFROG) trace. The simulations allow several important physical processes responsible for supercontinuum generation to be identified and, moreover, illustrate how the XFROG trace provides an intuitive means of interpreting correlated temporal and spectral features of the supercontinuum. Good qualitative agreement with preliminary XFROG measurements is observed.

Grating resonances in air-silica microstructured optical fibers.

We report what is believed to be the first demonstration of optical fiber gratings written in photonic crystal fibers. The fiber consists of a germanium-doped photosensitive core surrounded by a hexagonal periodic air-hole lattice in a silica matrix. The spectra of these gratings allow for a detailed characterization of the fiber. In particular, the gratings facilitate coupling to higher-order leaky modes. We show that the spatial distribution and the effective index of these modes are determined largely by the design of the lattice and that the grating spectra are unaffected by the refractive index surrounding the fiber. We describe these measurements and corresponding simulations and discuss their implications for the understanding of such air-hole structures.

Optimal wavelength for ultrahigh-resolution optical coherence tomography

The influence of depth dependent dispersion by the main component of biological tissues, water, on the resolution of OCT was studied. Investigations showed that it was possible to eliminate the influence of depth dependent dispersion by water in tissue by choosing a light source with a center wavelength near 1.0 microm. Ultrahigh resolution ophthalmic imaging was performed at this wavelength range with a microstructure fiber light source.

Optically driven deposition of single-walled carbon-nanotube saturable absorbers on optical fiber end-faces.

Optical radiation propagating in a fiber is used to deposit commercially available, single-walled carbon nanotubes on cleaved optical fiber end faces and fiber connectors. Thermophoresis caused by heating due to optical absorption is considered to be a likely candidate responsible for the deposition process. Single-walled carbon nanotubes have a fast saturable absorption over a broad wavelength range, and the demonstrated technique is an extremely simple and inexpensive method for making fiber-integrated, saturable absorbers for passive modelocking of fiber lasers. Pulse widths of 247 fs are demonstrated from an erbium-doped fiber laser operating at 1560 nm, and 137 fs pulses are demonstrated from an amplified Yb-doped fiber laser at 1070 nm.