Jesper Lægsgaard

Single-Mode Regime in Large-Mode-Area Rare-Earth-Doped Rod-Type PCFs

In this paper, large-mode-area, double-cladding, rare-earth-doped photonic crystal fibers are investigated in order to understand how the refractive index distribution and the mode competition given by the amplification can assure single-mode propagation. Fibers with different core diameters, i.e., 35, 60, and 100 mum, are considered. The analysis of the mode effective index, overlap, effective area, gain, and power evolution along the doped fiber provides clear guidelines on the fiber physical characteristics to be matched in the fabrication process to obtain a truly or effectively single-mode output beam.

Suppression of Higher-Order Modes by Segmented Core Doping in Rod-Type Photonic Crystal Fibers

A large mode area Yb-doped rod-type photonic crystal fiber design with a low refractive index ring in the core is proposed to provide an improved suppression of the first higher-order mode compared to the case of uniform core doping, in a way which is more robust against fluctuations in the refractive index value. After applying a scalar step-index model for a first parameter optimization of the proposed design, a full-vector modal solver based on the finite element method has been exploited to analyze the guided mode overlap and effective area for the most promising fibers identified. Finally, a spatial and spectral amplifier model has been considered to study the gain competition among the fundamental and the first higher-order mode guided in the Yb-doped rod-type fibers. Results have demonstrated the effectiveness of the low refractive index ring in suppressing the higher-order mode, thus providing an effectively single-mode behavior for the rod-type fibers.

Low-Noise Operation of All-Fiber Femtosecond Cherenkov Laser

We investigate the noise properties of a femtosecond all-fiber Cherenkov radiation source with emission wavelength 600 nm, based on an Yb-fiber laser and a highly nonlinear photonic crystal fiber. A relative intensity noise as low as 103 dBc/Hz, corresponding to 2.48% pulse-to-pulse fluctuation in energy, is observed at the Cherenkov radiation output power of 4.3 mW, or 150 pJ-pulse energy. This pulse-to-pulse fluctuation is at least 10.6-dB lower compared to spectrally sliced supercontinuum sources traditionally used for ultrafast fiber-based generation at visible wavelengths. Low noise makes all-fiber Cherenkov sources promising for biophotonics applications such as multiphoton microscopy, where minimum pulse-to-pulse energy fluctuation is required. We present the dependency of the noise figure on both the Cherenkov radiation output power and its spectrum.

Tuning quadratic nonlinear photonic crystal fibers for zero group-velocity mismatch

A nonlinear index-guiding silica PCF is optimized for efficient second-harmonic generation through dispersion calculations. Zero group-velocity mismatch is possible for any pump wavelength above 780 nm. Very high conversion efficiencies and bandwidths are found.

Photonic crystal fibers; fundamental properties and applications within sensors

Since the first experimental demonstration of a photonic crystal fiber (PCF) in 1996 by Knight et al. the optical properties and the fabrication of such fibers have attracted significant attention. The fiber structure with a lattice of air holes running along the length of the fiber provides a large variety of novel optical properties and improvements compared to standard optical fibers. The stack-and-pull procedure used to manufacture PCFs is a highly flexible method offering a large degree of freedom in the fabrication of PCFs with specific characteristics. A few of the remarkable optical properties of silica based PCFs are described and their applications within sensors are summarized.

Guided mode cutoff in rare-earth doped rod-type PCFs

Guided mode properties of rare-earth doped photonic crystal fibers are investigated as a function of the core refractive index, showing the possibility to obtain cutoff at low normalized wavelength.

Guiding and amplification properties of rod-type photonic crystal fibers with sectioned core doping

Rod-type photonic crystal fibers are large mode area double-cladding fibers with an outer diameter of few millimeters which can provide important advantages for high-power lasers and amplifiers. Numerical studies have recently demonstrated the guidance of higher-order modes in these fibers, which can worsen the output beam quality of lasers and amplifiers. In the present analysis a sectioned core doping has been proposed for Ybdoped rod-type photonic crystal fibers, with the aim to improve the higher-order mode suppression. A full-vector modal solver based on the finite element method has been applied to properly design the low refractive index ring in the fiber core, which can provide an increase of the differential overlap between the fundamental and the higher-order mode. Then, the gain competition among the guided modes along the Yb-doped rod-type fibers has been investigated with a spatial and spectral amplifier model. Simulation results have shown the effectiveness of the sectioned core doping in worsening the higher-order mode overlap on the doped area, thus providing an effective single-mode behavior of the Yb-doped rod-type photonic crystal fibers.

Biased liquid crystal photonic bandgap fiber

We simulate the director structure of all capillaries in a biased photonic crystal fiber infiltrated with liquid crystals. Various mode simulations for different capillaries show the necessity to consider the entire structure.

Hybrid photonic crystal fiber components and amplifiers

We present recent development of hybrid photonic crystal fiber amplifiers and components providing enhanced spectral and modal filtering.

Modeling of High-Power Pulse Compression and Soliton Formation in Hollow-Core Photonic Bandgap Fibers

We model the formation and self-frequency shift of solitons in a hollow-core photonic bandgap fiber. Extraction of clean femtosecond pulses by long-pass filtering in fibers with different air pressures is investigated.