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.

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. (C) 2014 Optical Society of Americ

Ultrafast lasers with high repetition rates are of considerable interest in applications such as optical fiber telecommunications, frequency metrology, high-speed optical sampling, and arbitrary waveform generation. For fiber lasers mode-locked at the cavity round-trip frequency, the pulse repetition rate is limited to tens or hundreds of megahertz by the meter-order cavity lengths. Here we report a soliton fiber laser passively mode-locked at a high harmonic (similar to 2 GHz) of its fundamental frequency by means of optoacoustic interactions in the small solid glass core of a short length ( 60 cm) of photonic crystal fiber. Due to tight confinement of both light and vibrations, the optomechanical interaction is strongly enhanced. The long-lived acoustic vibration provides strong modulation of the refractive index in the photonic crystal fiber core, fixing the soliton spacing in the laser cavity and allowing stable mode-locking, with low pulse timing jitter, at gigahertz repetition rates. (C) 2015 Optical Society of Americ