N. N. Edavalath

Broadband robustly single-mode hollow-core PCF by resonant filtering of higher-order modes.

We report a hollow-core photonic crystal fiber that is engineered so as to strongly suppress higher-order modes, i.e., to provide robust LP01 single-mode guidance in all the wavelength ranges where the fiber guides with low loss. Encircling the core is a single ring of nontouching glass elements whose modes are tailored to ensure resonant phase-matched coupling to higher-order core modes. We show that the resulting modal filtering effect depends on only one dimensionless shape parameter, akin to the well-known d/Λ parameter for endlessly single-mode solid-core PCF. Fabricated fibers show higher-order mode losses some ∼100 higher than for the LP01 mode, with LP01 losses <0.2  dB/m in the near-infrared and a spectral flatness ∼1  dB over a >110  THz bandwidth.

Generation of broadband mid-IR and UV light in gas-filled single-ring hollow-core PCF

We report generation of an ultrafast supercontinuum extending into the mid- infrared in gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF) pumped by 1.7 µm light from an optical parametric amplifier. The simple fiber structure offers shallow dispersion and flat transmission in the near and mid-infrared, enabling the generation of broadband spectra extending from 270 nm to 3.1 µm, with a total energy of a few µJ. In addition, we demonstrate the emission of ultraviolet dispersive waves whose frequency can be tuned simply by adjusting the pump wavelength. SR-PCF thus constitutes an effective means of compressing and delivering tunable ultrafast pulses in the near and mid-infrared spectral regions.

Higher-order mode suppression in twisted single-ring hollow-core photonic crystal fibers

A hollow-core single-ring photonic crystal fiber (SR-PCF) consists of a ring of capillaries arranged around a central hollow core. Spinning the preform during drawing introduces a continuous helical twist, offering a novel means of controlling the modal properties of hollow-core SR-PCF. For example, twisting geometrically increases the effective axial propagation constant of the LP01-like modes of the capillaries, providing a means of optimizing the suppression of HOMs, which occurs when the LP11-like core mode phase-matches to the LP01-like modes of the surrounding capillaries. (In a straight fiber, optimum suppression occurs for a capillary-to-core diameter ratio d/D=0.682.) Twisting also introduces circular birefringence (to be studied in a future Letter) and has a remarkable effect on the transverse intensity profiles of the higher-order core modes, forcing the two-lobed LP11-like mode in the untwisted fiber to become three-fold symmetric in the twisted case. These phenomena are explored by means of extensive numerical modeling, an analytical model, and a series of experiments. Prism-assisted side-coupling is used to measure the losses, refractive indices, and near-field patterns of individual fiber modes in both the straight and twisted cases.

Deep-Ultraviolet to Mid-Infrared Supercontinuum in Single-Ring Hollow-Core Photonic Crystal Fiber

We report the first supercontinuum generated in a gas-filled single-ring hollow-core PCF. When pumped with ultrashort pulses at 1500 nm, the emitted spectrum spans three octaves from the deep ultraviolet to the mid-infrared.

A broad-band robustly single-mode hollow-core PCF by resonant filtering of higher order modes

We propose and theoretically analyse a novel hollow-core photonic crystal fibre (PCF) that is engineered so as to strongly suppress higher order modes, i.e., to provide robust LP

Fabrication and Side-Coupling Characterization of Hexagonal Lattice Single-Ring Hollow-Core PCFs

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Excitation of modes in twisted single-ring PCF by prism-grating-coupling

single-mode guidance in all the wavelength ranges where the fibre guides with low loss. Encircling the core is a single ring of non-touching glass elements whose modes are tailored to ensure resonant phase-matched coupling to higher-order core modes, causing them to leak at a very high rate into the supporting solid glass sheath. Using a model based on coupled capillary waveguides, as well as full vectorial finite element modelling, we show that this modal filtering effect depends on only one dimensionless geometrical parameter, akin to the well-known

Single-circular-polarisation twisted single-ring hollow-core PCF

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Twist-tuning of higher-order mode suppression in single-ring hollow-core photonic crystal fibers

parameter for endlessly single-mode solid-core PCF. The design is scalable up to large core sizes and is predicted to deliver LP

Strong circular dichroism in twisted single-ring hollow-core photonic crystal fiber

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