AgNb7O18 : an ergodic relaxor ferroelectric

AgNb7O18 is an ergodic relaxor ferroelectric at room temperature with an incipient transition to the nonergodic state. Electron diffraction confirms a locally polar symmetry, while X-ray diffraction perceives a nonpolar structure. All ions are repelled away from zones where NbO6 octahedra are edge-sharing

Towards 'smart lasers': self-optimisation of an ultrafast pulse source using a genetic algorithm

Short-pulse fibre lasers are a complex dynamical system possessing a broad space of operating states that can be accessed through control of cavity parameters. Determination of target regimes is a multi-parameter global optimisation problem. Here, we report the implementation of a genetic algorithm to intelligently locate optimum parameters for stable single-pulse mode-locking in a Figure-8 fibre laser, and fully automate the system turn-on procedure. Stable ultrashort pulses are repeatably achieved by employing a compound fitness function that monitors both temporal and spectral output properties of the laser. Our method of encoding photonics expertise into an algorithm and applying machine-learning principles paves the way to self-optimising `smart' optical technologies

Genetic algorithm-based control of birefringent filtering for self-tuning, self-pulsing fiber lasers

Polarization-based filtering in fiber lasers is well-known to enable spectral tunability and a wide range of dynamical operating states. This effect is rarely exploited in practical systems, however, because optimization of cavity parameters is non-trivial and evolves due to environmental sensitivity. Here, we report a genetic algorithm-based approach, utilizing electronic control of the cavity transfer function, to autonomously achieve broad wavelength tuning and the generation of Q-switched pulses with variable repetition rate and duration. The practicalities and limitations of simultaneous spectral and temporal self-tuning from a simple fiber laser are discussed, paving the way to on-demand laser properties through algorithmic control and machine learning schemes.Comment: Accepted for Optics Letters, 12th June 201

Dark solitons in laser radiation build-up dynamics

We reveal the existence of slowly-decaying dark solitons in the radiation build-up dynamics of bright pulses in all-normal dispersion mode-locked fiber lasers, numerically modeled in the framework of a generalized nonlinear Schr\"odinger equation. The evolution of noise perturbations to quasi-stationary dark solitons is examined, and the significance of background shape and soliton-soliton collisions on the eventual soliton decay is established. We demonstrate the role of a restoring force in extending soliton interactions in conservative systems to include the effects of dissipation, as encountered in laser cavities, and generalize our observations to other nonlinear systems

Mode-locked dysprosium fiber laser: picosecond pulse generation from 2.97 to 3.30 {\mu}m

Mode-locked fiber laser technology to date has been limited to sub-3 {\mu}m wavelengths, despite significant application-driven demand for compact picosecond and femtosecond pulse sources at longer wavelengths. Erbium- and holmium-doped fluoride fiber lasers incorporating a saturable absorber are emerging as promising pulse sources for 2.7--2.9 {\mu}m, yet it remains a major challenge to extend this coverage. Here, we propose a new approach using dysprosium-doped fiber with frequency shifted feedback (FSF). Using a simple linear cavity with an acousto-optic tunable filter, we generate 33 ps pulses with up to 2.7 nJ energy and 330 nm tunability from 2.97 to 3.30 {\mu}m (3000--3400 cm^-1)---the first mode-locked fiber laser to cover this spectral region and the most broadly tunable pulsed fiber laser to date. Numerical simulations show excellent agreement with experiments and also offer new insights into the underlying dynamics of FSF pulse generation. This highlights the remarkable potential of both dysprosium as a gain material and FSF for versatile pulse generation, opening new opportunities for mid-IR laser development and practical applications outside the laboratory.Comment: Accepted for APL Photonics, 22nd August 201

Swept-wavelength mid-infrared fiber laser for real-time ammonia gas sensing

The mid-infrared (mid-IR) spectral region holds great promise for new laser-based sensing technologies, based on measuring strong mid-IR molecular absorption features. Practical applications have been limited to date, however, by current low-brightness broadband mid-IR light sources and slow acquisition-time detection systems. Here, we report a new approach by developing a swept-wavelength mid-infrared fiber laser, exploiting the broad emission of dysprosium and using an acousto-optic tunable filter to achieve electronically controlled swept-wavelength operation from 2.89 to 3.25 {\mu}m (3070-3460 cm^-1). Ammonia (NH3) absorption spectroscopy is demonstrated using this swept source with a simple room-temperature single-pixel detector, with 0.3 nm resolution and 40 ms acquisition time. This creates new opportunities for real-time high-sensitivity remote sensing using simple, compact mid-IR fiber-based technologies.Comment: Invited article for APL Photonic