Modelling of long-wave mid-infrared ultrashort pulse generation via difference frequency generation.

In this paper, a model of generating mid-infrared (MIR) ultrashort laser pulse through difference frequency generation (DFG) is established. The pulse evolution relationship among the pump, signal, and idler pulses during the DFG process, as well as the effects of crystal length, pulse energy of the pump and signal lights, pulse width, and other factors on the characteristics of the MIR pulse are explored. Furthermore, through simulations from the time domain to the frequency domain, the spectral characteristics and angular distribution of MIR were analyzed.

Vector magnetometer based on the effect of coherent population trapping.

The method is presented for converting a coherent population trapping (CPT) atomic clock into a CPT vector (compass-) magnetometer without mechanically moving parts through a relatively simple add-on. The system of 3D Helmholtz coils was used for compensation of the external magnetic field, allowing measurement of both strength and direction of a magnetic field at the sensitivity level of -/ within a 10-Hz bandwidth. Angular resolution of the developed vector magnetometer amounts to about 10 degrees.

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 LiNbO 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.

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.

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.

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.

Raman-converted high-energy double-scale pulses at 1270 nm in PO-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.

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.

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.

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.

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.

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.

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%.

Stochasticity, periodicity and localized light structures in partially mode-locked fibre lasers.

Physical systems with co-existence and interplay of processes featuring distinct spatio-temporal scales are found in various research areas ranging from studies of brain activity to astrophysics. The complexity of such systems makes their theoretical and experimental analysis technically and conceptually challenging. Here, we discovered that while radiation of partially mode-locked fibre lasers is stochastic and intermittent on a short time scale, it exhibits non-trivial periodicity and long-scale correlations over slow evolution from one round-trip to another.

Linear compression of chirped pulses in optical fibre with large step-index mode area.

The possibilities and limitations of linear compression of positively chirped pulses in the negative-dispersion region of a step-index large mode-area single-mode optical fibre are investigated for the first time. Analytical formulae for critical values of radiation power are found, below which pedestal-free pulse compression is possible down to the Fourier limit. It is demonstrated that at radiation powers exceeding these critical values, there exists an optimal compressing fibre length, over which laser pulses reach the minimum of the time-bandwidth product, and beyond which irreversible pulse deformation occurs.

High average power mode-locked figure-eight Yb fibre master oscillator.

For the first time, we demonstrate the possibility to generate record-high output radiation power exceeding 1 W in an all-fibre figure-eight mode-locked Yb fibre master oscillator based on non-linear amplifying loop mirror. Generated at the repetition rate of 25 MHz clusters of sub-pulses with duration of no more than 200 fs have envelope width of 23 ps or less. Two typical mode-locked laser regimes with substantially different generation spectra are identified and results of their study presented.

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.

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.

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.

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.

Spectrum-, pulsewidth-, and wavelength-switchable all-fiber mode-locked Yb laser with fiber based birefringent filter.

We examined methods of controlling the pulse duration, spectral width and wavelength of the output from an all-fiber Yb laser mode-locked by carbon nanotubes. It is shown that a segment of polarization maintaining (PM) fiber inserted into a standard single mode fiber based laser cavity can function as a spectral selective filter. Adjustment of the length of the PM fiber from 1 to 2 m led to a corresponding variation in the pulse duration from 2 to 3.

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.

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.

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.