High-repetition-rate pulse generation via self-sustaining cross-gain modulation in a semiconductor-based laser.

We present a simple and efficient method for generating regular pulse trains with GHz pulse repetition rates in lasers based on semiconductor optical amplifiers (SOAs). This method enables pulse formation without active modulation or saturable absorption of the generated radiation. The method relies upon the self-sustaining cross-gain modulation which is achieved by adding the negative optical feedback (NOF) to a ring laser configuration. The resulting modulation of laser gain is shown to be restricted to the frequencies which match both the spacing of longitudinal laser modes and the highest peaks in the NOF-induced instability gain spectrum. This enables the reproducible stationary pulse generation at the strictly defined repetition rates. The feasibility of the method was confirmed by the stable generation of sub-nanosecond pulses at repetition rates up to 1.79 GHz in a SOA-based laser with a simple fiber cavity.

Triggering of different pulsed regimes in fiber cavity laser by a waveguide electro-optic switch.

A novel practical method for electronic triggering of essentially different pulsed regimes in fiber cavity lasers is introduced. The method relies on electronic control of complementary transmission characteristics of a fiber-coupled LiNbO3 waveguide electro-optic switch (WEOS) which plays the role of the variable output coupler in a fiber cavity. The method was studied using a testbed laser configuration comprised of a semiconductor optical amplifier (SOA) and an all-fiber cavity. Modulation of the WEOS-based output coupling in the fast gain recovery configuration allowed not only high-quality mode locking and harmonic mode-locking at certain pulse repetition rates determined by the cavity round trip time, but it also allowed nanosecond pulsed output of the same quality to be yielded by cavity dumping at widely and continuously tunable repetition rate (ranging from kHz to MHz). Thus, WEOS-based electronically variable output coupling allows uniquely high flexibility for lasing regimes and characteristics within a single all-fiber cavity configuration.

Ionic Liquid Gated Carbon Nanotube Saturable Absorber for Switchable Pulse Generation.

Materials with electrically tunable optical properties offer a wide range of opportunities for photonic applications. The optical properties of the single-walled carbon nanotubes (SWCNTs) can be significantly altered in the near-infrared region by means of electrochemical doping. The states' filling, which is responsible for the optical absorption suppression under doping, also alters the nonlinear optical response of the material. Here, for the first time we report that the electrochemical doping can tailor the nonlinear optical absorption of SWCNT films and demonstrate its application to control pulsed fiber laser generation. With a pump-probe technique, we show that under an applied voltage below 2 V the photobleaching of the material can be gradually reduced and even turned to photoinduced absorption. Furthermore, we integrated a carbon nanotube electrochemical cell on a side-polished fiber to tune the absorption saturation and implemented it into the fully polarization-maintaining fiber laser. We show that the pulse generation regime can be reversibly switched between femtosecond mode-locking and microsecond Q-switching using different gate voltages. This approach paves the road toward carbon nanotube optical devices with tunable nonlinearity.

Machine learning-based pulse characterization in figure-eight mode-locked lasers.

By combining machine learning methods and the dispersive Fourier transform we demonstrate, to the best of our knowledge, for the first time the possibility to determine the temporal duration of picosecond-scale laser pulses using a nanosecond photodetector. A fiber figure of eight lasers with two amplifiers in a resonator was used to generate pulses with durations varying from 28 to 160 ps and spectral widths varied in the range of 0.75-12 nm. The average power of the pulses was in the range from 40 to 300 mW. The trained artificial neural network makes it possible to predict the pulse duration with the mean agreement of 95%. The proposed technique paves the way to creating compact and low-cost feedback for complex laser systems.

Raman-converted high-energy double-scale pulses at 1270 nm in P2O5-doped silica fiber.

This work presents implementation of a new approach to single-cascade Raman conversion of laser pulses from the spectral range around 1.1 µm into the 1.3-µm wavelength region. The proposed conversion technique relies on double-scale pico-femtosecond pulses for synchronous pumping of an external cavity made of phosphosilicate fiber with high-precision adjustment of pulse repetition rate to the inter-mode frequency of the external cavity. This enabled generation of double-scale pulses centered at 1270 nm featuring a record energy of 63 nJ and the pulse envelope duration of 88-180 ps with the sub-pulse duration of 200 fs. The fraction of the radiation that was converted into the 1270 nm range amounted to 47 percent of the total Raman-converted radiation power. The generated results offer promising possibilities for new spectral ranges to be developed in the field of high-energy pulsed sources with unique double-scale temporal structure.

Ultrafast all-fibre laser mode-locked by polymer-free carbon nanotube film.

This work for the first time reports the results on study of a polymer-free carbon nanotube (CNT) films used as a saturable absorber in an all-fibre laser. It is demonstrated that free-standing single-walled CNT films fabricated by an aerosol method are able to ensure generation of transform-limited pulses in an Er all-fibre ring laser with duration of several picoseconds and high quality of mode locking. The optimal average output power levels are identified, amounting to 0.4-0.5 mW depending on the linear transmission of the studied samples (60% or 80%). Application of polymer-free CNT films solves problems related to degradation of conventional polymer matrices of CNT-based saturable absorbers and paves the way to longer-lasting and more reliable saturable absorbers compatible with all-fibre laser configurations.

Automatic electronic-controlled mode locking self-start in fibre lasers with non-linear polarisation evolution.

The present work demonstrates a fibre-laser system with automatic electronic-controlled triggering of dissipative soliton generation mode. Passive mode locking based on the effect of non-linear polarisation evolution has been achieved through a polarisation controller containing a single low-voltage liquid crystal plate whose optimal wave delay was determined from analysis of inter-mode beat spectrum of the output radiation.

Three key regimes of single pulse generation per round trip of all-normal-dispersion fiber lasers mode-locked with nonlinear polarization rotation.

We show experimentally and numerically new transient lasing regime between stable single-pulse generation and noise-like generation. We characterize qualitatively all three regimes of single pulse generation per round-trip of all-normal-dispersion fiber lasers mode-locked due to effect of nonlinear polarization evolution. We study spectral and temporal features of pulses produced in all three regimes as well as compressibility of such pulses. Simple criteria are proposed to identify lasing regime in experiment.

Machine Learning Methods for Control of Fibre Lasers with Double Gain Nonlinear Loop Mirror.

Many types of modern lasers feature nonlinear properties, which makes controlling their operation a challenging engineering problem. In particular, fibre lasers present both high-performance devices that are already used for diverse industrial applications, but also interesting and not yet fully understood nonlinear systems. Fibre laser systems operating at high power often have multiple equilibrium states, and this produces complications with the reproducibility and management of such devices. Self-tuning and feedback-enabled machine learning approaches might define a new era in laser science and technology. The present study is the first to demonstrate experimentally the application of machine learning algorithms for control of the pulsed regimes in an all-normal dispersion, figure-eight fibre laser with two independent amplifying fibre loops. The ability to control the laser operation state by electronically varying two drive currents makes this scheme particularly attractive for implementing machine learning approaches. The self-tuning adjustment of two independent gain levels in the laser cavity enables generation-on-demand pulses with different duration, energy, spectral characteristics and time coherence. We introduce and evaluate the application of several objective functions related to selection of the pulse duration, energy and degree of temporal coherence of the radiation. Our results open up the possibility for new designs of pulsed fibre lasers with robust electronics-managed control.