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.

Layout of NALM fiber laser with adjustable peak power of generated pulses.

The Letter proposes a new layout of a passively mode-locked fiber laser based on a nonlinear amplifying loop mirror (NALM) with two stretches of active fiber and two independently controlled pump modules. In contrast with conventional NALM configurations using a single piece of active fiber that yields virtually constant peak power, the proposed novel laser features larger than a factor of 2 adjustment range of peak power of generated pulses. The proposed layout also provides independent adjustment of duration and peak power of generated pulses as well as power-independent control of generated pulse spectral width impossible in NALM lasers with a single piece of active fiber.

Mode-locked long fibre master oscillator with intra-cavity power management and pulse energy > 12 µJ.

Combined lengthening of the cavity of a passive mode-locked fibre master oscillator and implementation of a new concept of intra-cavity power management led to achievement of a record-high pulse energy directly at the output of the mode-locked fibre master oscillator (without any subsequent amplification) exceeding 12 µJ. Output powers at the level of > 12 µJ obtainable from a long-cavity mode-locked fibre master oscillator open new possibilities of application of all pulse types that can be generated in such oscillators.

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.

Efficiency of different methods of extra-cavity second harmonic generation of continuous wave single-frequency radiation.

This work presents for the first time to the best of our knowledge a comparative efficiency analysis among various techniques of extra-cavity second harmonic generation (SHG) of continuous-wave single-frequency radiation in nonperiodically poled nonlinear crystals within a broad range of power levels. Efficiency of nonlinear radiation transformation at powers from 1 W to 10 kW was studied in three different configurations: with an external power-enhancement cavity and without the cavity in the case of single and double radiation pass through a nonlinear crystal. It is demonstrated that at power levels exceeding 1 kW, the efficiencies of methods with and without external power-enhancement cavities become comparable, whereas at even higher powers, SHG by a single or double pass through a nonlinear crystal becomes preferable because of the relatively high efficiency of nonlinear transformation and fairly simple implementation.

Synchronously pumped picosecond all-fibre Raman laser based on phosphorus-doped silica fibre.

Reported for the first time is picosecond-range pulse generation in an all-fibre Raman laser based on P2O5-doped silica fibre. Employment of phosphor-silicate fibre made possible single-cascade spectral transformation of pumping pulses at 1084 nm into 270-ps long Raman laser pulses at 1270 nm. The highest observed fraction of the Stokes component radiation at 1270 nm in the total output of the Raman laser amounted to 30%. The identified optimal duration of the input pulses at which the amount of Stokes component radiation in a ~16-m long phosphorus-based Raman fibre converter reaches its maximum was 140-180 ps.

240-GHz continuously frequency-tuneable Nd:YVO4/LBO laser with two intra-cavity locked etalons.

This work reports for the first time on study of a single-frequency Nd:YVO4 laser with intra-cavity frequency doubling, in which quasi-continuous frequency tuning over an ultra-broad range is implemented with two intra-cavity locked etalons and automatic stitching between ranges of smooth scanning of the laser's output frequency. The proposed and demonstrated method allowed to continuously tune the output frequency of the studied 1.5-W Nd:YVO4/LBO laser over a record-breaking broad range of 240 GHz at 532 nm.

Cascaded SRS of single- and double-scale fiber laser pulses in long extra-cavity fiber.

This work presents, for the first time, the results of studies of stimulated Raman scattering (SRS) in 1.2-km P2O5-doped silica fiber of radiation of single- and double-scale picosecond pulses generated in a fiber master oscillator and amplified in a one-stage fiber amplifier. Shown are differences in supercontinuum spectra composed of several Stokes components when pumped with pulses of different structure. More efficient Raman transformation of double-scale pulses was identified, leading to broader supercontinuum spectra.

Efficiency of non-linear frequency conversion of double-scale pico-femtosecond pulses of passively mode-locked fiber laser.

For the first time we report the results of both numerical simulation and experimental observation of second-harmonic generation as an example of non-linear frequency conversion of pulses generated by passively mode-locked fiber master oscillator in different regimes including conventional (stable) and double-scale (partially coherent and noise-like) ones. We show that non-linear frequency conversion efficiency of double-scale pulses is slightly higher than that of conventional picosecond laser pulses with the same energy and duration despite strong phase fluctuations of double-scale pulses.

Variable-wavelength second harmonic generation of CW Yb-fibre laser in partially coupled enhancement cavity.

This work for the first time proposes and studies a method of frequency doubling of CW non-single-frequency fibre lasers with a high-Q resonator partially coupled to the fibre laser cavity. The proposed new approach resulted in the following parameters: laser's maximal output power 880 mW at 536 nm when pumped with 6.2 W at 976 nm, wavelength tuneability range 521-545 nm with the output power at the extreme ends of this range 420 and 220 mW correspondingly. The proposed configuration allows efficient non-linear transformation of both CW and pulsed radiation in a partially coupled enhancement cavity.

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.

Generation of double-scale femto/pico-second optical lumps in mode-locked fiber lasers.

We observed generation of stable picoseconds pulse train and double-scale optical lumps with picosecond envelope and femtosecond noise-like oscillations in the same Yb-doped fiber laser with all-positive-dispersion cavity mode-locked due to the effect of non-linear polarization evolution. In the noise-like pulse generation regime the auto-correlation function has a non-usual double (femto- and picosecond) scale shape. We discuss mechanisms of laser switching between two operation regimes and demonstrate a good qualitative agreement between experimental results and numerical modeling based on modified nonlinear Schrödinger equations.

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.

Ultra-low repetition rate mode-locked fiber laser with high-energy pulses.

This paper reports on the results of research into passively modelocked fiber laser with a record-setting optical length of the resonant cavity amounting to 3.8 km. Significant elongation of the laser resonator led to more than two orders of magnitude increase in the output pulse energy at the same pump radiation power. At ultra-low (for mode-locked lasers) pulse repetition rate (77 kHz) and pulse duration of 3 ns the energy per pulse reached 3.9 microJ. At this moment this is the highest pulse energy on record generated directly from a mode-locked laser without Q-switching, cavity dumping techniques, or additional optical amplifiers.

Influence of noise amplification on generation of regular short pulse trains in optical fibre pumped by intensity-modulated CW radiation.

For the first time the influence of noise amplification on decay of modulated continuous-wave pumping into a pulse series in an optical fiber is considered. Dependence of noise-to-signal ratio in pulse train at fibre exit on initial modulation depth obtained both analytically and by means of numerical simulations. The minimum modulation frequency is estimated which leads to a regular pulse train formation from CW pumping.

Long-term frequency stabilization of a continuous-wave tunable laser with the help of a precision wavelengthmeter.

For the first time to our knowledge we experimentally demonstrate an efficient method for the reduction of long-term radiation line drift in single-frequency cw Ti:sapphire and dye lasers that relies on a fast and precise wavelengthmeter together with a digital-analog feedback system. Generation line drift of lasers is reduced approximately by an order of magnitude down to 40 MHz/h, which corresponds to the residual drift in readings of the wavelengthmeter itself. The implemented automatic frequency control system allows us to lock the laser generation frequency to a specified absolute value. This approach may be used in single-frequency lasers of different types (solid-state, fiber, diode, dye lasers, etc.) and allows reduction by an order of magnitude or more of the long-term generation line drift in lasers that are not equipped with other systems for long-term stabilization of output radiation frequency.