P. Steinvurzel

Tapered photonic crystal fibers

We demonstrate the tapering of a photonic crystal fiber to achieve a microstructure pitch of less than 300 nm. We probe the tapered fiber in the transverse geometry to demonstrate the scaling of the photonic bandgaps associated with the microstructure. We show that the fundamental gap can be shifted down to the communications wavelengths, or even further to the visible spectrum. Our optical measurements are correlated with band structure calculations.

Microstructured optical fiber photonic wires with subwavelength core diameter

We demonstrate fabrication of robust, low-loss silica photonic wires using tapered microstructured silica optical fiber. The fiber is tapered by a factor of fifty while retaining the internal structure and leaving the air holes completely open. The air holes isolate the core mode from the surrounding environment, making it insensitive to surface contamination and contact leakage, suggesting applications as nanowires for photonic circuits . We describe a transition between two different operation regimes of our photonic wire from the embedded regime, where the mode is isolated from the environment, to the evanescent regime, where more than 70% of the mode intensity can propagate outside of the fiber. Interesting dispersion and nonlinear properties are identified.

Highly tunable birefringent microstructured optical fiber.

We demonstrate a method for introducing and dynamically tuning birefringence in a microstructured optical fiber. Waveguide asymmetry in the fiber is obtained by selective filling of air holes with polymer, and tunability is achieved by temperature tuning of the polymers index. The fiber is tapered such that the mode field expands into the cladding and efficiently overlaps the polymer that has been infused into the air holes, ensuring enhanced tunability and low splice loss. Experimental results are compared with numerical simulations made with the beam propagation method and confirm birefringence tuning that corresponds to a phase change of 6pi for a 1-cm length of fiber.

Long period grating resonances in photonic bandgap fiber

We demonstrate the formation of stress-induced long period gratings (LPGs) in fluid-filled photonic bandgap fiber (PBGF). Based on our experimental results, simulations, and theoretical understanding of LPGs, we identify coupling to a guided LP(11)-like mode of the core and lossy LP1x-like modes of cladding microstructure for a single grating period. The periodic modal properties of PBGFs allow for coupling to the same mode at multiple wavelengths without a dispersion turning point. Simulations identify inherent differences in the modal structure of even and odd bands.

Tuning properties of long period gratings in photonic bandgap fibers.

We investigate the thermal tuning properties of long period gratings (LPGs) in a fluid-filled photonic bandgap fiber (PBGF). The combination of strong, resonant waveguide dispersion, characteristic of all PBGF modes, and the large thermo-optic coefficients of fluids yields highly tunable grating resonances. We measure grating resonances in three transmission bands with large tuning coefficients of up to -1.58 nm/degrees C, which match numerical results. We derive an analytic model for the PBGF LPG tuning coefficient to show how it depends on both the shift of the transmission bands and the dispersion of the coupled modes.

Long wavelength anti-resonant guidance in high index inclusion microstructured fibers.

Microstructured optical fibers consisting of a low refractive index core surrounded by high index inclusions guide by anti-resonant reflection. Previous experiments considered only wavelengths that are short compared to microstructure dimensions. We experimentally investigate a microstructured fiber with high index inclusions and demonstrate antiresonant guidance at long wavelengths. We also numerically simulate these structures, including coupling loss, propagation loss, and structural disorder, and compare with the experimental results.

Nonlinear propagation effects in antiresonant high-index inclusion photonic crystal fibers.

We experimentally and numerically investigate femtosecond-pulse propagation in a microstructured optical fiber consisting of a silica core surrounded by airholes that are filled with a high-index fluid. This fiber combines the resonant properties of hollow-core bandgap fibers and the high nonlinearity of index-guiding waveguides. A range of nonlinear optical effects can be observed, including soliton propagation, dispersive wave generation, and a Raman self-frequency shift. Tuning the center wavelength of the laser and varying the refractive index of the fluid lead to different propagation effects, mediated by the strongly wavelength-dependent group-velocity dispersion in these photonic bandgap confining structures.

Nonlinear pulse propagation at zero dispersion wavelength in anti-resonant photonic crystal fibers

We experimentally and numerically investigate femtosecond pulse propagation in a microstructured optical fiber consisting of a silica core surrounded by air holes which are filled with a high index fluid. Such fibers have discrete transmission bands which exhibit strong dispersion arising from the scattering resonances of the high index cylinders. We focus on nonlinear propagation near the zero dispersion point of one of these transmission bands. As expected from theory, we observe propagation of a red-shifted soliton which radiates dispersive waves. Using frequency resolved optical gating, we measure the pulse evolution in the time and frequency domains as a function of both fiber length and input power. Experimental data are compared with numerical simulations.

Tunable acoustic gratings in solid-core photonic bandgap fiber

We investigate acousto-optic long period grating resonances in a fluid-filled solid-core photonic bandgap fiber (PBGF). The acoustic grating design enables electrically tunable notches in each of the PBGF transmission bands, where both the center frequency and depth of the resonances can be varied. The measured intermodal beat length and resonance bandwidth are in good agreement with numerical simulations based on multipole method. We show that the highly dispersive nature of PBGF modes results in very narrow-band rejection for a given grating pitch.

Transverse characterization of tapered photonic crystal fibers

We demonstrate the tapering of photonic crystal fibers to create an all-silica two-dimensional photonic crystal with a submicron pitch and holes smaller than 200nm. We characterize the fundamental partial band gaps of this structure as a function of the taper diameter by probing the fiber in the transverse geometry along both symmetry axes. In tapering the fiber, we are able to shift the fundamental partial gap from the midinfrared to 1100nm, corresponding to a reduction in diameter by 2.5 times. A complete description of the tapering process is provided. We produced a number of tapers that either maintained the cross-sectional aspect ratio or collapse the holes relative to the photonic crystal structure. From these, we show that the strength of the band gap can be continuously varied by collapsing the holes. For tapers which maintain the cross-sectional aspect ratio, we show that there is a good correlation between the experiment and the band-structure calculations.