Fehim Babic

Orbital-angular-momentum-preserving helical Bloch modes in twisted photonic crystal fiber

In optical fiber telecommunications, there is much current work on the use of orbital angular momentum (OAM) modes for increasing channel capacity. Here we study the properties of a helically twisted photonic crystal fiber (PCF) that preserves the chirality of OAM modes of the same order, i.e., it inhibits scattering between an order +1 mode to an order −1 mode. This is achieved by thermally inducing a helical twist in a PCF with a novel three-bladed Y-shaped core. The effect is seen for twist periods of a few millimeters or less. We develop a novel scalar theory to analyze the properties of the twisted fiber, based on a helicoidal extension to Bloch wave theory. It yields results that are in excellent agreement with full finite element simulations. Since twisted PCFs with complex core structures can be produced in long lengths from a fiber drawing tower, they are of potential interest for increasing channel capacity in optical telecommunications, but the result is also of interest to the photonic crystal community, where a new kind of guided helical Bloch mode is sure to excite interest, and among the spin–orbit coupling community.

Single-mode hollow-core photonic crystal fiber made from soft glass

We demonstrate the first soft-glass hollow core photonic crystal fiber. The fiber is made from a high-index lead-silicate glass (Schott SF6, refractive index 1.82 at 500 nm). Fabricated by the stack-and-draw technique, the fiber incorporates a 7-cell hollow core embedded in a highly uniform 6-layer cladding structure that resembles a kagomé-like lattice. Effective single mode guidance of light is observed from 750 to 1050 nm in a large mode area (core diameter ~30 µm) with a low loss of 0.74 dB/m. The underlying guidance mechanism of the fiber is investigated using finite element modeling. The fiber is promising for applications requiring single mode guidance in a large mode area, such as particle guidance, fluid and gas filled devices.

Five-ring hollow-core photonic crystal fiber with 1.8 dB/km loss

A 19-cell hollow-core photonic crystal fiber reaching 1.8±0.5 dB/km loss at 1530 nm is reported. Despite expanded corner holes in the first ring adjacent to the core, and only five cladding rings, the minimum loss is close to the previously published record of 1.7 dB/km at a comparable wavelength, achieved in a fiber with seven cladding rings. Since each additional cladding ring requires a significant increase in fabrication time and complexity, it is highly desirable to use as few as possible while still achieving low loss. Modeling results confirm that further reducing cladding deformations would yield only a small decrease in loss. This demonstrates that loss comparable to the previously demonstrated lowest-loss bandgap fibers can be achieved with fiber structures that are significantly simpler and faster to fabricate.

High-resolution wavefront shaping with a photonic crystal fiber for multimode fiber imaging

We demonstrate that a high-numerical-aperture photonic crystal fiber allows lensless focusing at an unparalleled resolution by complex wavefront shaping. This paves the way toward high-resolution imaging exceeding the capabilities of imaging with multi-core single-mode optical fibers. We analyze the beam waist and power in the focal spot on the fiber output using different types of fibers and different wavefront shaping approaches. We show that the complex wavefront shaping technique, together with a properly designed multimode photonic crystal fiber, enables us to create a tightly focused spot on the desired position on the fiber output facet with a subwavelength beam waist.

Supercontinuum generation in ZBLAN glass photonic crystal fiber with six nanobore cores

Photonic crystal fibers (PCFs) made from ZBLAN glass are of great interest for generating broadband supercontinua extending into the ultraviolet and mid-infrared regions. Precise sub-micrometer structuring makes it possible to adjust the modal dispersion over a wide range, making the generation of new frequencies more efficient. Here we report a novel ZBLAN PCF with six cores, each containing a central nanobore of a diameter ∼330  nm. Each nanobore core supports several guided modes, and the presence of the nanobore significantly modifies the dispersion, strongly influencing the dynamics and the extent of supercontinuum generation. Spectral broadening is observed when a single core is pumped in the fundamental and first higher order core modes with 200 fs long pulses at a wavelength of 1042 nm. Frequency-resolved optical gating is used to characterize the output pulses when pumping in the lowest order mode. The results are verified by numerical simulations.

Reducing losses in solid-core photonic crystal fibers using chlorine dehydration

The fabrication of photonic crystal fibers (PCFs) involves the stacking of multiple preform elements, providing many opportunities for contamination by water vapor or dust particles and causing increased fiber loss. Even after manufacture, diffusion of water vapor into the hollow channels is known to cause a slow increase in loss if the fibers are stored in a humid environment. In this paper we report a systematic study of three methods to reduce OH-related loss in solid-core PCFs: (1) treating the stack (primary preform) with chlorine or oxygen; (2) treating the cane (intermediate preform) with chlorine or oxygen; and (3) using a dry gas for pressurization of the hollow channels during the final step of fiber drawing. Each treatment is independently found effective in reducing OH-related loss, although stack treatment alone is not sufficient if the canes are subsequently stored for a longer time. On the other hand, chlorine-treatment of the canes and/or using a suitably dry gas using fiber drawing significantly lowers the loss even when the canes have been stored for more than two years in a closed tube at room temperature and at relative humidities in the range ~20% to ~50%.

Five-ring hollow-core photonic bandgap fiber with 1.8 dB/km loss

A 19-cell hollow-core photonic bandgap fiber reaching 1.8±0.5 dB/km loss is reported. Despite expanded corner-holes in the first ring adjacent to the core, and only five cladding rings, the minimum loss is close to the previous record of 1.2 dB/km.

Soft-glass photonic crystal fibers: From advanced fabrication techniques to novel applications

We report on recent advances in soft-glass photonic crystal fibers that make use of improved stack-and-draw and extrusion techniques, and on their use in supercontinuum generation, photochemistry and triplet photon generation.

Supercontinuum generation in microstructured ZBLAN fibre with six nanobore cores

We report fabrication of a microstructured ZBLAN fibre with six cores each containing a nanobore of diameter ~330 nm. Spectral broadening is observed when pumped by 1042 nm pulses in both fundamental and higher order modes.

Novel microstructured fibres for supercontinuum generation

We report recent progress on the fabrication of photonic crystal fibre from ZBLAN and tellurite glasses and their application to generating broadband supercontinua.