We present a novel, simple and highly effective method for generating multi-GHz regular pulse trains in lasers. This method relies on self-sustaining cross-gain modulation achieved by incorporating negative optical feedback (NOF) into a cavity equipped with a semiconductor optical amplifier (SOA). Notably, this approach not only facilitates pulse formation without the need for active modulation or saturable absorption but also enables effortless multiplexing of the laser system to achieve diverse pulse repetition rates. The feasibility of the method was confirmed by the stable generation of sub-nanosecond pulses at $1.35 \mathrm{GHz}, 1.57 \mathrm{GHz}$, and 1.79 GHz repetition rates in an SOA-based laser with a simple ring all-fiber all-PM cavity.

Introducing negative optical feedback into a semiconductor optical amplifier-based laser presents a straightforward and effective approach for generating high-repetition-rate pulses through the mechanism of self-sustaining cross-gain modulation. In this work, we demonstrate, for the first time, the tunability of high-frequency pulse generation over a wide wavelength range by adjusting the central transmission wavelength of a tunable filter. Our laser system achieved a full tuning range from 1480 nm to 1556 nm (76 nm) while maintaining pulsed lasing. Remarkably, the laser exhibited a maximum pulse contrast exceeding 85% over a tuning range of 43 nm, from 1505.7 nm to 1548.7 nm. At any wavelength within this range, stable, self-starting pulse generation was consistently achieved, with a pulse repetition rate of 1.3 GHz.

We report on synchronized dual-wavelength (1.07 μm and 1.24 μm) pulsed lasing driven by a quasi-synchronous primary pumping (at 0.98 μm) of an Yb-doped fiber laser, which incorporates also a P2O5-doped fiber as an intracavity Raman converter. The original method developed for such lasing does not require saturable absorbers (or optical modulators) and dispersion management. We demonstrated that the mechanism of the quasi-synchronous pumping enables the aforesaid stationary lasing in spite of significant differential group delay (DGD) inevitably acquired by light pulses with such different wavelengths during an intracavity round trip due to large normal chromatic dispersion. This DGD can be actively compensated at every round trip by the forced “acceleration” of the pulses at 1.07 μm in the Yb-doped active fiber due to the overrated frequency of the quasi-synchronous pumping at 0.98 μm. This mechanism is related to the particular pulse amplification dynamics in a such gain-modulated active fiber. The demonstrated approach to synchronized dual-wavelength pulsed lasing in a single-cavity fiber laser features remarkable simplicity and reliability. Our proof-of-concept setup enabled the stable two-wavelength generation of regular trains of nanosecond pulses with energy up to 34 nJ at equal repetition rates.

We demonstrate the feasibility of triggering stationary high-energy pulse generation in Er-doped fiber lasers at ~1.5 µm via quasi-synchronous gain modulation. This simple method relies upon the sine-wave modulation of pump power at a frequency slightly surpassing the intrinsic frequency spacing of longitudinal modes in the laser cavity. This was previously implemented only in Yb-doped fiber lasers at ~1.1 µm. Here, for the first time, we experimentally validate the pulse shaping capabilities of this method also in Er fiber lasers, which, unlike Yb fiber lasers, have a three-level laser energy diagram (when pumped at 0.98 µm) with a very long-lived (10 ms) upper laser level. The feasibility of the method was validated both for normal and anomalous intracavity dispersion, which was not available in previous implementations in Yb fiber lasers at ~1.1 µm. Thus, the stable generation of a regular train of discrete nanosecond pulses with an energy of up to 180 nJ was achieved in our test-bed Er fiber laser upon the quasi-synchronous sine-wave modulation of the pump power at 0.98 µm. The results of our study testify to the general applicability of this affordable and reliable method for high-energy pulse generation in various rare-earth-doped fiber lasers.

We propose and study a novel type of fiber laser cavities which features dynamically transformable topology and enables pulse-to-pulse directional switching of the intracavity laser power flow. The key element of the proposed fiber cavity configuration is a fast 2 × 2 waveguide electrooptic switch (WEOS) which links a Sagnac-like (bidirectional) passive fiber loop with a unidirectional fiber loop incorporating an optical amplifier (OA) (rare-earth- or semiconductor-based). Electronic control of the WEOS port-to-port transmittance enables reversable gradual transformation of the laser cavity topology. This transformation results in directional switching of the laser radiation circulation in the Sagnac fiber loop. Proper dynamics of such transformation allows obtaining of complementary pulse trains from the counter-directional outputs in the Sagnac fiber loop. We validated this capability in a proof-of-concept transformable fiber cavity employing a semiconductor OA. The proposed cavity configuration enables switchable bidirectional pulse generation in various regimes including mode-locked ones.

Primary methods for generating short pulses in lasers require intracavity elements or physical mechanisms for modulation or the saturable absorption of radiation. This often complicates laser design and limits capabilities, particularly beyond single-wavelength operation. We propose and explore a method for the synchronous generation of bicolor, high-repetition-rate pulses that combines stimulated emission from Yb rare-earth ions and Raman scattering in a shared all-fiber laser cavity, without employing saturable absorbers or modulators. The proposed mechanism for pulsed lasing is analogous to an optical shift register, with two pulse trains shifting relative to each other by one period after every round trip. This naturally solves the critical problem of compensating for the dispersion-induced differential delay of bi-chromatic pulses during an intracavity round trip. The shift register inherently enables stationary generation of bi-chromatic pulses with a common relatively high repetition rate that is inversely proportional to the differential delay. We have demonstrated the feasibility of the proposed technique through the stable generation of sub-nanosecond bi-chromatic (1066 and 1241 nm) pulses with a repetition rate exceeding 166 MHz. The proposed approach is rather general, and we anticipate that it can facilitate more affordable bi-chromatic pulse generation in a variety of laser systems.

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.

We demonstrate the possibility of the accurate direct laser synthesis of high-energy arbitrary optical waveforms by the programmable driving of partial cavity dumping in a specific continuous-wave fiber laser. To this effect we have developed an original hybrid laser configuration which integrates two different active media. The first medium, a semiconductor optical amplifier (SOA), acts as a saturated lumped preamplifier. It features a relatively fast (sub-nanosecond) gain recovery, and thus effectively suppresses the intracavity power fluctuations induced by cavity dumping. The second active medium, an erbium-doped fiber amplifier (EDFA), acts mainly as a booster amplifier. This distributed inertial amplifying medium effectively accumulates pump energy, thereby providing an enhancement of output energy upon cavity dumping. Our simple proof-of-concept laser setup has allowed the synthesis of nanosecond arbitrary optical waveforms with an energy up to 40 nJ and arbitrarily tunable repetition rate. The proposed combination of a slow (EDFA) and fast (SOA) amplifying stages prevents the laser from strong relaxation oscillations and power flux fluctuations which essentially restrict cavity dumping in conventional rare-earth-doped fiber lasers. The applied two-stage intracavity spectral filtering ensures spectral purity of a rather narrowband (⩽0.1 nm) laser output. For the purpose considered, the integrated SOA-EDFA laser configuration is preferable to a conventional architecture ‘master oscillator—power amplifier’ whose nonlinear gain can obstruct the accurate synthesis of high-energy optical waveforms.

The results of R&D of electrochemical components of an energy system based on hydrogen–air open cathode fuel cells with proton-exchange membrane are presented. The scheme is shown being capable of realizing electrical power system with high specific energies (up to 700 W h/kg) on the condition that it contains no humidifiers and heaters, light metals are used as the material of bipolar plates, and the fuel cell operates in the mode of self-humidification of the membrane using only the reaction water therefor. Under these conditions, at operating temperatures up to 50°C, the air consumption is 50–100 times higher than the stoichiometric value; there appears a danger of the membrane drying-out. To improve the current–voltage characteristics, a combined method of manufacturing membrane–electrode assembles is used, according to which the catalytic layer was applied by screen printing, and the membrane is formed by direct application of an ionomer to the electrode. The properties of C–Pt- and TiN-based protective coatings on the surface of a titanium bipolar plate are also investigated. The dynamics of changes in the potentials of the electrodes is investigated at “critical” modes of the fuel cell operation and the process stabilization at the nominal mode. Using the experimental data for a fuel cell stack with a power of 1.2 kW and the specific enthalpy–temperature–air humidity diagram, the fuel-cell-operating temperature limits are calculated, at which the process of the membrane self-humidification with reaction water is maintained. The improving of electrochemical components of an open-cathode fuel cell stack is shown to allow achieving a specific power of the power modulus as high as 1 kW/kg.

Generation of two synchronized pulses with broadly different wavelengths in the same fiber laser cavity opens up promising possibilities of entering the mid-IR wavelength range by difference frequency generation (DFG). One of the ways to generate synchronous pulses with very different wavelengths is Raman conversion in a P 2 O 5 -doped fiber incorporated into the laser cavity in addition to an Yb-doped fiber. This work demonstrates, for the first time, efficient generation of synchronized pulses at wavelengths of 1064 and 1240 nm in such an all-fiber nested-cavity laser. An external DFG stage allows conversion of the laser output into pulsed radiation at ~7.5 µm.

We report on research into the properties of supercontinuum (SC) generated from low-coherence bursts of different duration in a 1-km long P2O5 fibre. It was found out that SC with a spectral width of ~135–150 nm is formed within the ~900–1200-nm range mostly due to cascaded Raman scattering of noise-like pulses with the variable envelope duration of 36–153 ps (sub-pulse duration was ~ 300 fs) and average power of 560 mW at 1080 nm. It was discovered that the spectral width of SC is predominantly affected by the duration of interaction between the pumping and Raman pulses.

A unique variability of modern fiber and integrated optics as well as the development of physics of complex nonlinear optical phenomena and nanophotonics have contributed to the development of qualitatively new approaches to the implementation and control of pulsed lasing in fiber lasers in the last decade. This article provides a structured review and comparative analysis of current methods for static and dynamic control of the regimes and parameters of lasing in mode-locked fiber lasers.

We present a new convenient method for generation of various pulse bunches in synchronously pumped linearly polarized Yb-fibre lasers. This method uses a slight mismatch between the pumping pulse repetition rate and that of the generated pulses [1] . It is shown that this method not only shortens the generated pulses substantially relative to the pumping pulses, but also can be used to generate bunches of such shortened pulses and precisely set the number of pulses per bunch. The advantages of the proposed technique are full electronic control, scalability, and possibility of high-energy (in excess of 150 nJ) bunches.

We explored possibility to enhance spectroscopic capabilities of mid-IR optical parametric oscillators (OPOs). Gas analysis based on the principles of photoacoustic spectroscopy (PAS) [1] is known to be an efficient approach to measure concentration of certain gases (e.g., methane) in the atmosphere. Laser radiation source for PAS gas analyzer should provide high-energy pulsed output with narrow optical spectrum. There should be also possibility for fine tuning of its wavelength to a strong absorption line of the detected gas. Herein, we report an original high-energy pulsed OPO, in which the fan-out periodically poled lithium niobate (PPLN) and injection seeding technique were employed for spectral narrowing and fine wavelength tuning.

The authors demonstrate the possibilities of supercontinuum formation due to cascaded stimulated Raman scattering in a 1-km long optical fibre with standard transverse dimensions doped with phosphorus pentoxide P2O5. For supercontinuum pumping were used noise-like pulses with variable envelope duration (35 to 150 ps) and different coherence degree (with different levels of stochastic fluctuations which corresponded to different amplitudes of the autocorrelation coherence peak). At the average pumping radiation power of 600 mW, the generated supercontinuum width reached 140 nm. Dependencies of supercontinuum properties upon the noise-like pulse parameters are also presented. It is shown that even upon significant increase of the envelope duration of double-scale pulses (from 36 to 153 ps), the efficiency of nonlinear conversion does not decrease. It is also shown that the width of the emission spectrum can reach 225 nm upon reducing the envelope duration down to 25 ps and increasing the coherence peak up to 40%.

Additional capabilities of the method of quasi-synchronous pump power modulation developed by the authors for nanosecond high-energy pulsed oscillation of fibre lasers with a long-lived (about 1 ms) upper laser level are investigated. Using an Yb fibre laser as an example, it is shown that quasi-synchronous pump power modulation makes it possible to generate not only a periodic sequence of single nanosecond pulses, but also regular pulse clusters with a controlled number of nanosecond subpulses that make up a cluster. In addition, the feasibility of scaling the energy of laser pulses obtained by the method of quasi-synchronous modulation of the pump power is studied when proceeding to the use of active double-clad fibres and higher-power multimode pump sources. Pulses with energies up to 430 nJ are obtained in a laser configuration maintaining linear polarisation of radiation. The results obtained significantly expand the possibilities of applying the method of quasi-synchronous modulation of the pump power in conventional fibre lasers based on stimulated emission.

High-efficiency harmonic generation is observed in a semiconducting polymer RR-P3HT [regioregular poly(3-hexylthiophene)], which exhibits a high nonlinear susceptibility [ χ (2) > 10 −6 m V −1 ] under pumping by a pulsed fibre laser. The harmonic generation efficiency in RR-P3HT is comparable or higher than that observed for liquid crystals, which are used in reference experiments to estimate the susceptibility [the quadratic susceptibility of a nematic liquid crystal NLC 1289 is χ (2) ≈ 2 × 10 −6 m V −1 ]. No generation is observed in polythiophene with a random structure, RRa-P3HT [regiorandom poly(3-hexylthiophene)], at the same pump power. It is experimentally demonstrated that a necessary condition for generating the second and third harmonics in a polymer, along with a high pump power density, is the presence of a large power density gradient (exceeding 10 13 W m −3 ). Based on a preliminary theoretical analysis, we can suggest that the quadrupole mechanism, which is a consequence of the regularity of RR-P3HT structure in the thin near-wall layer, may contribute significantly to the nonlinear radiation conversion in semiconducting polymers.

A new mode locking fibre laser configuration is proposed that delivers record-high energy (150 nJ) of short linearly polarised pulses (~ 200 ns) without external amplification. It is further shown that such performance may be achieved by elongation of the laser cavity with a standard PM-fibre.