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Mode‐locked lasers are prone to exhibit a wide range of dynamics, including the coherent dissipative soliton with high robustness or the possible emission of incoherent pulse bursts. Herein, it is proposed to expand the notion of dissipative breathing solitons to incoherent localized formation, which is illustrated experimentally by reporting original coherent and incoherent breathing solitons generated in a 2 μm bidirectional ultrafast fiber laser. A real‐time spectral characterization of the breathing soliton dynamics with different coherence is provided, highlighting salient features of the self‐organization. The switching between coherent breathing soliton and noise‐like pulse is also experimentally demonstrated, corresponding to the multiple transient decoherence and coherence recovery. High behavior similarity exists in bidirectional solitons evolution owing to the common gain and loss modulation. Numerical simulations using a discrete laser model validate the experimental findings. This research generalizes previous observations of breathing solitons made in the coherent case and opens up new possibilities for generating breathers in various dissipative systems. [ABSTRACT FROM AUTHOR]

Period-doubling bifurcation, as an intermediate state between order and chaos, is ubiquitous in all disciplines of nonlinear science. However, previous experimental observations of period doubling in ultrafast fiber lasers are mainly restricted to self-sustained steady state, controllable manipulation and dynamic switching between period doubling and other intriguing dynamical states are still largely unexplored. Here, we propose to expand the vision of dissipative soliton periodic doubling, which we illustrate experimentally by reporting original spontaneous, collisional, and controllable spectral period doubling in a polarization-maintaining ultrafast fiber laser. Specifically, the spontaneous period doubling can be observed in both single- and double-pulses. The mechanism of the switchable state and periodic doubling was revealed by numerical simulation. Moreover, state transformation of individual solitons can be resolved during the collision of triple solitons involving stationary, oscillating, and period doubling. Further, controllable deterministic switching between period doubling and other dynamical states, as well as exemplifying the application of period-doubling-based digital encoding, is achieved under programmable pump modulation. Our results open a new window for unveiling complex Hopf bifurcation in dissipative systems and bring useful insights into nonlinear science and applications. [ABSTRACT FROM AUTHOR]

Abstract Period-doubling bifurcation, as an intermediate state between order and chaos, is ubiquitous in all disciplines of nonlinear science. However, previous experimental observations of period doubling in ultrafast fiber lasers are mainly restricted to self-sustained steady state, controllable manipulation and dynamic switching between period doubling and other intriguing dynamical states are still largely unexplored. Here, we propose to expand the vision of dissipative soliton periodic doubling, which we illustrate experimentally by reporting original spontaneous, collisional, and controllable spectral period doubling in a polarization-maintaining ultrafast fiber laser. Specifically, the spontaneous period doubling can be observed in both single- and double-pulses. The mechanism of the switchable state and periodic doubling was revealed by numerical simulation. Moreover, state transformation of individual solitons can be resolved during the collision of triple solitons involving stationary, oscillating, and period doubling. Further, controllable deterministic switching between period doubling and other dynamical states, as well as exemplifying the application of period-doubling-based digital encoding, is achieved under programmable pump modulation. Our results open a new window for unveiling complex Hopf bifurcation in dissipative systems and bring useful insights into nonlinear science and applications.

Trapped bosonic atoms can be cooled down to temperatures where the atomic cloud experiences Bose–Einstein condensation. Almost all atoms in a dilute gaseous system can be Bose-condensed, which implies that this system is in a coherent state. The coherent atomic system enjoys many properties typical of coherent optical systems. It is possible to generate different condensate coherent modes similarly to the generation of optical modes. Several effects can be observed, such as interference patterns, interference current, Rabi oscillations, harmonic generation, parametric conversion, Ramsey fringes, mode locking, dynamic transition between Rabi and Josephson regimes, and atomic squeezing. [ABSTRACT FROM AUTHOR]

Mode‐locked lasers are prone to exhibit a wide range of dynamics, including the coherent dissipative soliton with high robustness or the possible emission of incoherent pulse bursts. Herein, it is proposed to expand the notion of dissipative breathing solitons to incoherent localized formation, which is illustrated experimentally by reporting original coherent and incoherent breathing solitons generated in a 2 μm bidirectional ultrafast fiber laser. A real‐time spectral characterization of the breathing soliton dynamics with different coherence is provided, highlighting salient features of the self‐organization. The switching between coherent breathing soliton and noise‐like pulse is also experimentally demonstrated, corresponding to the multiple transient decoherence and coherence recovery. High behavior similarity exists in bidirectional solitons evolution owing to the common gain and loss modulation. Numerical simulations using a discrete laser model validate the experimental findings. This research generalizes previous observations of breathing solitons made in the coherent case and opens up new possibilities for generating breathers in various dissipative systems.

Since the beginning of this millennium, frequency comb generators have reshaped frequency metrology and related areas. After more than two decades since their first realization, several other ways to generate frequency combs, in any spectral region, have been demonstrated, each way with its peculiar features. This trend has triggered the need to quantitatively assess how close the new comb realizations are to an ideal comb, a feature that will be called combness throughout this paper. We will briefly review the very dynamic area of novel frequency comb sources and we will describe the techniques that have been recently developed to quantitatively assess the key parameters of old and new frequency combs, in view of the specific applications. Finally, we will try to sketch future steps in this recently born research area.

Extreme wave events or rogue waves (RWs) are both statistically rare and of exceptionally large amplitude. They are observed in many complex systems ranging from oceanic and optical environments to financial models and Bose–Einstein condensates. As they appear from nowhere and disappear without a trace, their emergence is unpredictable and non‐repetitive, which makes them particularly challenging to control. Here, the use of genetic algorithms (GAs), which are exclusively designed for searching and optimizing stationary or repetitive processes in nonlinear optical systems, is extended to the active control of extreme events in a fiber laser cavity. Feeding real‐time spectral measurements into a GA controlling the electronics to optimize the cavity parameters, the wave events are able to be triggered in the cavity that have the typical statistics of RWs in the frequency domain. The intensity of the induced RWs can also be tailored. This accurate control enables the generation of optical RWs with a spectral peak intensity 32.8 times higher than the significant intensity threshold. A rationale is proposed and confirmed by numerical simulations of the laser model for the related frequency up‐ and downshifting of the optical spectrum that are experimentally observed. [ABSTRACT FROM AUTHOR]

We present sub-50 fs soliton pulse generation from a diode-pumped Kerr-lens mode-locked laser based on an Yb3+-doped BaF2 crystal. Utilizing a spatially single-mode, fiber-coupled InGaAs laser diode at 976 nm as a pump source, the Yb:BaF2 laser generates pulses as short as 46 fs at 1060.1 nm with an average output power of 45 mW at a pulse repetition rate of ~65.6 MHz via soft-aperture Kerr-lens mode locking. To the best of our knowledge, this represents the first demonstration of Kerr-lens mode-locked operation of the Yb:BaF2 crystal, as well as the shortest pulse duration ever achieved from any diode-pumped mode-locked laser based on an Yb3+-doped alkaline-earth fluoride crystal. [ABSTRACT FROM AUTHOR]

In this work, an all-fiber saturable absorber (SA) based on nonlinear multimode interference in a short few-mode fiber (FMF) was proposed. Saturable absorption in this all-fiber device was demonstrated with an absorption modulation depth and a saturation fluence of 14.3% and 55.6 μ J cm−2, respectively. A stable mode-locking operation was observed in a thulium-doped passively mode-locked fiber laser using the FMF-based FMF SA. Ultrafast pulses were generated at 1911 nm with a pulse width of 1.96 ps and a 15.69 MHz repetition rate, respectively. The output average power was 2.8 mW, corresponding to a pulse energy of 178 pJ, which was significantly improved compared to graded-index multimode fiber-based SA. The experimental results demonstrate that FMF SA has great potential in eye-safe ultrafast photonics. [ABSTRACT FROM AUTHOR]

In this study, we have investigated the evolution process and dynamic characteristics of a multi-pulse regime in an erbium-doped fiber ring laser based on a single-mode fiber–graded-index multimode fiber–single-mode fiber (SMF-GIMF-SMF) structure as an optical modulator. By utilizing the excellent nonlinear optical absorption of the SMF-GIMF-SMF (SMS) device with a modulation depth of ~8.68%, stable single-pulse mode locking at the frequency of 9.84 MHz can be readily observed at low pump power. In addition, the single-pulse operation can evolve into a multiple-pulse regime on account of the peak-power-clamping effect via suitably raising the pump power and carefully regulating the polarization state. Further, the single-shot temporal evolution of multiple pulses is monitored, indicating that this state shows unique and interesting temporal characteristics with variable pulse separations and inconsistent pulse intensities, which, as far as we know, is the first such observation in ultrafast fiber lasers. Additionally, this study, based on the time-stretch dispersive Fourier transformation method, suggests that these multiple pulses consist of chaotic wave envelopes with erratic intensities and changeable pulse energy. We believe that these findings have profound implications for revealing fascinating nonlinear pulse dynamics in ultrafast fiber optics. [ABSTRACT FROM AUTHOR]

In this work, we proposed and experimentally demonstrated a combination of single mode fibre (SMF) with graded indexed multimode fibre (GIMF) to form an artificial saturable absorber. A SMF-GIMF-SMF structure is incorporated into Erbium-doped fibre laser (EDFL) system to induce mode-locking operation with very low threshold pump power of 41.6 mW. The highest pulse energy obtained is 406.65 pJ at pump power of 84.7 mW. With a relatively short GIMF used as artificial saturable absorber, high fundamental repetition rate with low threshold power is achieved. The proposed artificial saturable absorber is potentially to contribute to the development of sustainable industry, innovation, and infrastructure.

We theoretically introduce an effective Kerr lens mode locking concept to a polarization maintaining (PM) Er3+:ZBLAN fiber laser and generate a stable femtosecond pulse train at 2.76 µm. Phase-mismatched periodically poled lithium niobate (PPLN) is utilized to implement cascaded second order nonlinearities (CSN) for the effective Kerr lens mode locking. Furthermore, the CSN in the PPLN can be tuned to balance the huge fiber anomalous dispersion for femtosecond soliton formation in the mid-infrared regime. High coupling efficiency of light into fiber, shorter effective focal length of PPLN and larger effective nonlinear refractive index are essential for enhancing intracavity peak power and shortening pulse duration. The pulse could be compressed down to 311 fs with output pulse energy of 1.36 nJ and peak power of 3.6 kW. This mode locking method provides deep insight into direct generation of femtosecond fiber laser in mid-infrared regime.

We propose and demonstrate a passively mode-locked all-fiber laser utilizing a multimode interference effect in graded-index multimode fiber (GIMF). The mode-locking is obtained due to the Kerr effect in GIMF, which induces the intensity modulation inside the ring cavity laser. We obtain stable soliton mode-locked pulses at a center wavelength of 1574.1 nm, with a fixed repetition rate of 22 MHz and a pulse duration of 650 fs, when the pump power is changed in the range from 66.2 to 140.0 mW. The maximum average output power and pulse energy are recorded at 9.82 mW and 448 pJ, respectively, at a pump power of 140 mW. This proves that a nonlinear optical response in GIMF can be used as a new modulation mechanism for obtaining ultra-short pulses based on all-fiber ring cavity. [ABSTRACT FROM AUTHOR]

Formation of ultrashort pulses in a fiber laser cavity with a slow saturated absorber and spectral filter is studied. It is found that the finite relaxation time of the saturable absorber in a cavity with full normal chromatic dispersion leads to the spectral shift of generated pulses with respect to the central wavelength of the spectral filter. The numerical results are verified using two experimental laser sources: a fiber ring laser with a saturable semiconductor mirror and a fiber laser with a nonlinear amplifying loop mirror. The obtained results are topical for designing sources of ultrashort pulses in applications for which spectral properties of the radiation are essential. [ABSTRACT FROM AUTHOR]

We present sub-50 fs soliton pulse generation from a diode-pumped Kerr-lens mode-locked laser based on an Yb3+-doped BaF2 crystal. Utilizing a spatially single-mode, fiber-coupled InGaAs laser diode at 976 nm as a pump source, the Yb:BaF2 laser generates pulses as short as 46 fs at 1060.1 nm with an average output power of 45 mW at a pulse repetition rate of ~65.6 MHz via soft-aperture Kerr-lens mode locking. To the best of our knowledge, this represents the first demonstration of Kerr-lens mode-locked operation of the Yb:BaF2 crystal, as well as the shortest pulse duration ever achieved from any diode-pumped mode-locked laser based on an Yb3+-doped alkaline-earth fluoride crystal.

In this work, we have demonstrated passive Q-switched and mode-locked erbium-doped fiber lasers (EDFLs) by using metal salt of cadmium chloride hydrate (CdCl2.xH2O) as a saturable absorber (SA). The mechanical exfoliation method was utilized to fabricate the CdCl2.xH2O-SA. The fabricated SA showed a modulation depth of 20% with a saturation intensity of 80 mW/cm2. At a pump power of 167 mW, the proposed Q-switched laser operated at 1542 nm with a maximum repetition rate of 127 kHz and a narrowest pulse width of 1.76 µs. The maximum output power was 23 mW with a signal to noise ratio (SNR) of 76 dB. On the other hand, the mode-locked pulses were generated within a pump power range of 91–167 mW, achieving a repetition rate of 990 kHz. The maximum pulse energy and output power were 7.3 nJ and 7.2 mW, respectively. The SA film demonstrated reliability and simplicity in the fabrication process. [ABSTRACT FROM AUTHOR]

Short pulses are showing increasing importance in various industrial and scientific applications. Here, we exploit the saturable absorption of a ternary layered MAX-phase compound of Vanadium Aluminum carbide (V4AlC3) to produce mode-locked pulses in Erbium-doped fiber-laser (EDFL) cavity. The V4AlC3 composite thin film with a modulation depth of 24% is successfully obtained by embedding the commercial V4AlC3 powder into polyvinyl alcohol (PVA). It is integrated into an EDFL cavity, as a saturable absorber (SA), to generate a highly-stable mode-locked pulse, which operates at the 1559.8 nm wavelength. We successfully obtain the mode-locked pulse train with a fixed repetition rate of 1.8 MHz and a pulse width of 3.66 ps, as the pump power is set within a range from 73.1 to 108.1 mW. At the maximum pump power equal to 108.1 mW, the average output power, pulse energy, and peak power are 10.2 mW, 5.4 nJ, and 1.5 W, respectively. Overall, these results show the potential of V4AlC3 MAX-phase material to be used in ultrafast generation. The proposed approach is straightforward and can also be applied to operate in other wavelength regions. [ABSTRACT FROM AUTHOR]

Mode‐locked fiber lasers are applied in versatile scientific fields and exhibit a rich diversity of nonlinear dynamics. However, the nontrivial coexistence of solitons and embedded dispersive wave radiation prevents unveiling the real nonlinear dynamics in mode‐locked fiber lasers, such as multiple solitons interaction. Here, nonlinear Fourier transform (NFT) is applied as a signal processing tool to reveal nonequilibrium multiple soliton dynamics. It is feasible to isolate solitons from the continuous wave background in a fiber laser. The real‐time coherent homodyne detection methodology is used to measure the full‐field dynamic evolution of multiple solitons, including multiple solitons buildup and sequentially nonequilibrium evolution with complex splitting, drifting, and collision processes. With the approach of inverse NFT, the corresponding various pure solitons buildup and collision are reconstructed. The eigenvalue probability distributions are used to classify different lasing regimes. Moreover, the controllable multiple solitons drifting is achieved and characterized by using all‐optical methods. Experimental results suggest that NFT can be used to identify localized soliton nonequilibrium evolution excluding dispersive wave radiation influence, which provides a new window into the physics of the underlying laser dynamics and uncovers real soliton interaction in dissipative nonlinear systems. [ABSTRACT FROM AUTHOR]

Actively mode-locked fiber ring lasers (AMLLs) with loss modulators are used to generate approximately 100 ps pulses with 100 MHz repetition. RF detuning around the fundamental frequency, f 0 , causes a loss in phase lock (unlocking) of cavity modes and partial mode locking. Multiple RF inputs are shown, theoretically, to relock and extend the locking range of cavity modes in a detuned partially mode-locked AMLL. A custom-built Yb 3 + -doped AMLL with f 0 = 26 MHz , and operating wavelength of 1064 nm , was used to experimentally verify the theoretical predictions. Two RF sinusoidal signals with constant phase and equal amplitude resulted in an extension of the range by X n = 6.4 kHz in addition to the range R n = 14.34 kHz with single input for the mode n = 10 . An increase in locking range was also observed for higher modes. Pulsewidth reduction to approximately 205 ps from about 2 ns was also observed in the AMLL. [ABSTRACT FROM AUTHOR]

A 30-GHz pulse-train is generated using the rational harmonic mode-locking technique, experimentally, using a Mach–Zehnder Lithium Niobate modulator. The width of the pulses is then reduced from 5.8-ps to 1.9-ps by incorporating nonlinear polarization rotation. This phenomenon arises due to the very high nonlinear behavior of the photonic crystal fiber (PCF) added to the ring laser cavity. Numerically solving the Generalized Nonlinear Schrödinger Equation provided insights into the pulse evolution behavior. The relative polarization angle and length of the PCF were varied to study their effects on the pulse-width. [ABSTRACT FROM AUTHOR]

Optical pulses at high repetition rates are generated using rational harmonic mode locking and saturable absorber made of graphene nanoparticles in a fiber laser. The pulse generation from the fiber laser is modeled by solving the Generalized Nonlinear Schrodinger Equation. The computation involved varying the various saturable absorption parameters, such as linear and nonlinear absorption coefficients. Experimentally stable pulse trains at 20 GHz and 50 GHz are generated with a pulse width of ∼ 2.7 ps. This result agrees with the simulation. [ABSTRACT FROM AUTHOR]

Results of experiments on the observation of frequency locking by resonant transitions of cesium vapor, a cell with which was placed in the cavity of a titanium–sapphire laser, are reported. The radiation frequency of the titanium–sapphire laser was tuned near the DII absorption line of Cs at 852 nm. In this case, simultaneous generation on the absorption line in the coherent mode locking regime and generation at the frequency specified by the spectral selector were demonstrated. Passive losses of the laser cavity at the selector frequency were less than the losses at the frequency of the Cs absorption transition. This unusual result was discussed from the viewpoint of the effect of intracavity narrow-band absorption on the emission spectra of broadband lasers and the effect of spectrum condensation. [ABSTRACT FROM AUTHOR]

In this study, we have investigated the evolution process and dynamic characteristics of a multi-pulse regime in an erbium-doped fiber ring laser based on a single-mode fiber–graded-index multimode fiber–single-mode fiber (SMF-GIMF-SMF) structure as an optical modulator. By utilizing the excellent nonlinear optical absorption of the SMF-GIMF-SMF (SMS) device with a modulation depth of ~8.68%, stable single-pulse mode locking at the frequency of 9.84 MHz can be readily observed at low pump power. In addition, the single-pulse operation can evolve into a multiple-pulse regime on account of the peak-power-clamping effect via suitably raising the pump power and carefully regulating the polarization state. Further, the single-shot temporal evolution of multiple pulses is monitored, indicating that this state shows unique and interesting temporal characteristics with variable pulse separations and inconsistent pulse intensities, which, as far as we know, is the first such observation in ultrafast fiber lasers. Additionally, this study, based on the time-stretch dispersive Fourier transformation method, suggests that these multiple pulses consist of chaotic wave envelopes with erratic intensities and changeable pulse energy. We believe that these findings have profound implications for revealing fascinating nonlinear pulse dynamics in ultrafast fiber optics.

High repetition frequency (HRF) ultrashort pulse fiber laser has been widely used in laser cold processing. The technical solutions such as short cavity length fiber laser have been proposed to achieve HRF ultrashort pulse output recently. However, the application of material-based saturable absorbers in this field has been astricted due to the low modulation depth, low damage resistance threshold, and high saturation fluence. Here, we designed a one-dimensional asymmetric photonic crystal with defect layer (1D-APCDL) as a novel saturable absorber, where the defect layer is Bi1.6Sb0.4Te3 with high modulation depth. The harmonic pulse with 3.82 GHz repetition frequency is achieved at the wavelength of 1562 nm, which is the highest repetition frequency of the topological insulator-based ring fiber laser so far to the best of our knowledge. The research provides a new saturable absorber solution, and provides a new idea for the application of material-based nonlinear optical chip in high-repetition frequency ultrashort pulse fiber lasers.

Dual‐wavelength mode‐locking lasers delivering asynchronous dual‐color solitons are excellent sources for dual‐comb spectroscopies. Up to now, diverse approaches have been developed to implement dual‐wavelength lasers. However, the buildup dynamics of dual‐color solitons have not been well revealed. Here, by means of the time‐stretch dispersive Fourier transform technique, real‐time observation of the buildup process and propagation dynamics of asynchronous dual‐color solitons in a mode‐locked fiber laser are reported. In addition to the typical phases that have been well discussed on buildup dynamics of conventional solitons, such as raised relaxation oscillation and spectral beating behavior, it is observed that optical or solitons collision plays a key role in facilitating the buildup of dual‐ or multi‐wavelength mode locking. Specifically, when the dual‐wavelength solitons carrying the same color photons encounter in the time domain, their collision induces the solitons explosion and brings about the new evolved mode‐locking pulse at another waveband. Experimental results agree well with numerical predictions and bring insights to the understanding of nonlinear dynamics in fiber lasers.

Carbon nanotube (CNT)–polymer composite films are key elements in widespread photonic and optoelectronic applications. One particular successful application is to produce ultrashort optical pulses in the laser cavity. Nevertheless, such composite film may suffer from severe humidity/water degradation, from which the laser may even stop generating regular pulses. Here, surfactant‐free CNT–styrene methyl‐methacrylate polymer composite film is fabricated. Based on such film‐type saturable absorber, a self‐starting passively mode‐locked erbium‐doped fiber laser with the capability of generating ≈1 ps pulses is demonstrated. From the perspective of experimental measurements and numerical simulations, the effectiveness of a saturable absorber in generating ultrashort pulses is confirmed. The operating composite film can survive under fully immersed water condition for 30 days. More importantly, after the non‐operating composite film has been fully soaked in the water tank for 2 days, it can still mode‐lock the laser as it is in dry air condition, confirming the humidity resistance capability. In addition, it is experimentally found that the mechanical packaging of the composite film has an impact on the survival time of composite film under water immersion. [ABSTRACT FROM AUTHOR]

We analyse the spectral properties of an optical frequency comb emitted by a III-V-on-silicon ring laser operated in the hybrid mode-locking regime. We demonstrate that the mode-locked laser has disparate responses to an external RF modulation depending on the saturable absorber bias and pump current. Firstly, we show that, when the mode-locking is fully achieved, the hybrid operation maintains the full mode-locking characteristics across the whole locking region. Secondly, starting from an incomplete passive mode-locking regime, and using the modulation frequency as a control parameter, we demonstrate the gradual improvement of the mode-locking characteristics within the locking tongue. This effect is maximized at the RF locking tongue boundary and manifests in an increase of the spectral width and a diminution of the modal linewidth dispersion. [ABSTRACT FROM AUTHOR]

Recently, optoelectronic oscillators (OEOs) have shown good behavior in microwave signal generation. In this letter, we propose a rational harmonic mode-locking OEO (RHML-OEO) to generate microwave frequency combs (MFCs) with tunable comb intervals. By embedding an extra Mach–Zehnder modulator (MZM) into the OEO cavity, the intensity of the oscillating mode can be modulated by the signal applied on the MZM, and sidebands having fixed phase differences are produced in addition to the oscillating mode. A main feature of the RHML-OEO is that there is a frequency detuning between the modulation signal and longitudinal mode spacing of the OEO. Consequently, MFCs with large comb intervals can be generated even with a low-frequency modulation signal. In our experiment, third-order, fifth-order, and tenth-order RHML-OEOs are realized to generate MFCs with different frequency intervals of 1.128, 1.88, and 3.76 MHz when the modulation frequencies are 564, 940 kHz, and 1.253 MHz, respectively. [ABSTRACT FROM AUTHOR]

Actively mode-locked fiber ring lasers (AMLLs) with loss modulators are used to generate approximately 100ps pulses with 100MHz repetition. RF detuning around the fundamental frequency, f0, causes a loss in phase lock (unlocking) of cavity modes and partial mode locking. Multiple RF inputs are shown, theoretically, to relock and extend the locking range of cavity modes in a detuned partially mode-locked AMLL. A custom-built Yb3+-doped AMLL with f0=26MHz, and operating wavelength of 1064nm, was used to experimentally verify the theoretical predictions. Two RF sinusoidal signals with constant phase and equal amplitude resulted in an extension of the range by Xn=6.4kHz in addition to the range Rn=14.34kHz with single input for the mode n=10. An increase in locking range was also observed for higher modes. Pulsewidth reduction to approximately 205ps from about 2ns was also observed in the AMLL.

We present our recent experimental results on the pulsed regimes of Raman conversion of highly multimode laser diode (LD) pump radiation into the 1st and higher order Stokes radiation in multimode graded-index fibers. Three different linear cavities of Raman fiber laser with the modulation of losses (by acousto-optic modulator, AOM) or gain (by LD current) are explored and compared. An LD with wavelength of 976 nm is used for pumping enabling Raman lasing at wavelength of the 1st (1018 nm) and 2nd (1064 nm) Stokes orders. At ~27.2-kHz repetition rate corresponding to the laser cavity round-trip frequency (i.e., in the mode-locking regime), nanosecond pulses have been observed for both Stokes orders having the highest peak power of ~300 W in the scheme with bulk AOM and the shortest duration of 5–7 ns in the scheme with fiber-pigtailed AOM. At the same time, the beam quality of generated pulses is greatly improved as compared to that for pump diode (M2 > 20) reaching the best value (M2 = 2.05) for the 2nd order Stokes beam in the scheme with the gain modulation and demonstrating also the most stable regime. [ABSTRACT FROM AUTHOR]

The current phase retrieval methods are challenging to retrieve the internal phase of soliton molecules (SMs) in the normal-dispersion regime of mode-locked lasers because the interference-like spectrum of such SMs is extremely dense. Here, we first retrieve phase evolutions of such SMs, by virtue of a novel phase retrieval method (Nat. Commun. 10, 830 (2019)). Three types of phase evolutions of SMs are revealed. Our work paves the way towards revealing various SM dynamics in ultrafast fibe lasers. [ABSTRACT FROM AUTHOR]

• Topological insulator NbAs nanosheets are prepared through liquid phase exfoliation. • The modulation depth of NbAs nanosheets is 3.92 % through balanced twin detector measurement systems. • For the first time, wely semimetal NbAs nanosheets as a saturable absorber obtained a pulse width of 640 fs. For the first time, NbAs nanosheets are used as a saturable absorber for fiber laser mode locking, achieving high efficiency saturation absorption. NbAs nanosheets are prepared through a liquid-phase exfoliation method, and the following nonlinear optical testing is performed with a self-built twin balanced detection device, with a modulation depth of 3.92 %. Additionally, NbAs nanosheets are embedded in erbium-doped fiber laser as saturable absorber to produce stable laser output with excellent performance. NbAs nanosheets based saturable absorber can achieve a pulse width of 640 fs. Compared with other topological materials, weyl semi-metal NbAs nanosheets based saturable absorber shows narrower pulses and broader linewidths. Those results hold significant importance for exploring the potential applications of NbAs in ultrafast fiber lasers. [ABSTRACT FROM AUTHOR]

We propose a MXene Ti3C2Tx thin-film and demonstrate as saturable absorber (SA) to generate mode-locked laser pulses. In constructing the SA, titanium carbide (Ti3C2Tx) powder obtained through a selective etching process is dispersed in polyvinyl alcohol (PVA) solution before the mixture is dried to form a thin film. A small piece of the film is placed between two fiber ferrules to form a SA device and added to an erbium-doped fiber laser (EDFL) cavity with a length of 106 m to produce mode-locked pulses. The mode-locked laser operates at 1561.2 nm with a repetition rate of 1.89 MHz and pulse width of 154 ns as the pump power is fixed within a range from 40.2 to 87.2 mW. At 87.2 mW pump power, the maximum average output power, pulse energy, and peak power were measured to be 11.85 mW, 6.24 nJ, and 40.7 mW respectively. [ABSTRACT FROM AUTHOR]

Quantum dot lasers are an attractive option for light sources in silicon photonic integrated circuits. Thanks to the three-dimensional charge carrier confinement in quantum dots, high material gain, low noise and large temperature stability can be achieved. This paper discusses, both theoretically and experimentally, the advantages of silicon-based quantum dot lasers for passive mode-locking applications. Using a frequency domain approach, i.e., with the laser electric field described in terms of a superposition of passive cavity eigenmodes, a precise quantitative description of the conditions for frequency comb and pulse train formation is supported, along with a concise explanation of the progression to mode locking via Adler's equation. The path to transform-limited performance is discussed and compared to the experimental beat-note spectrum and mode-locked pulse generation. A theory/experiment comparison is also used to extract the experimental group velocity dispersion, which is a key obstacle to transform-limited performance. Finally, the linewidth enhancement contribution to the group velocity dispersion is investigated. For passively mode-locked quantum dot lasers directly grown on silicon, our experimental and theoretical investigations provide a self-consistent accounting of the multimode interactions giving rise to the locking mechanism, gain saturation, mode competition and carrier-induced refractive index. [ABSTRACT FROM AUTHOR]

As typical soliton pulse sources, ultrashort-pulse fiber lasers offer excellent platforms for the investigation of soliton dynamics. On the basis of dispersive Fourier transform, we report the pulse evolution and dynamics of a pulsed erbium-doped fiber laser with plasmonic titanium nitride nanoparticles under different pump powers in two states: mode-locking and Q-switched mode-locking (QS-ML). In the QSML regime, the real-time evolution spectra exhibit a regular periodic change that corresponds to the Q-switching rate. The results of this study may stimulate further exploration of metal nitride materials for pulse generation in fiber lasers, which have widespread applications in science and technology. [ABSTRACT FROM AUTHOR]

Mode locked fiber ring laser using graphene nanoparticles as saturable absorbers to compress the pulse width has been studied. An experimental method to demonstrate the generation of pulse train with 50 GHz repetition rate with an ultrashort pulse width has been demonstrated. This method uses a combination of rational harmonic mode-locking (RHML) and a saturable absorber in the fiber ring laser. The pulse width using saturable absorbers is shorter by about a factor of 2 compared to that without a saturable absorber. Pulse generation using fiber ring laser with a saturable absorber has been analyzed. The experimental results are in agreement with the results from the simulation. [ABSTRACT FROM AUTHOR]

Breathing optical solitons propagating in a dissipative nonlinear system can interact and bind stably, forming an optical soliton molecule that presents striking molecule‐like dynamics. To date, the breathing soliton pair has been mainly observed in the microcavity platform, and the peculiar dynamic evolution of breathing soliton molecules (BSMs) remains largely unexplored in mode‐locked fiber lasers. Herein, the transient switching dynamics of BSMs in a bidirectional ultrafast fiber laser is revealed, specifically triggered at different parameter spaces of saturable absorption with manipulation polarizations that maintain constant pulse separation or undergo strong repulsion and kink after switching. All‐optical switching of breather molecules and significantly increasing multiple soliton molecules’ switching of two or three solitons between states with different binding separations by applying a strong stimulus with periodic pump modulation is demonstrated. The instantaneous pulse breakup can be induced by the collision of bidirectional breathing solitons in each switching, which is characteristic of the breathing solitons in a bidirectional fiber laser and further corroborated by numerical simulation. The study unveils new perspectives into the ultrafast transient process of BSMs in various dissipative systems.

The trend of data center transceivers is to increase bandwidth while simultaneously decreasing their size, power consumption, and cost. While data center links have previously relied on vertical-cavity surface-emitting lasers (VCSELs) or in-plane lasers using coarse wavelength division multiplexing (WDM) to encode data, recently, dense WDM (DWDM) has moved to the forefront for next-generation links. Several approaches exist as light sources for DWDM links; DFB arrays, nonlinear microcombs, and semiconductor mode-locked lasers (MLLs). This paper focuses on quantum dot MLLs (QDMLLs), which currently offer the best efficiency, simplicity, and size. The efficiency of optical combs generated by QDMLLs is analyzed in depth in this study. [ABSTRACT FROM AUTHOR]

Harnessing pulse generation from an ultrafast laser is a challenging task as reaching a specific mode‐locked regime generally involves adjusting multiple control parameters, in connection with a wide range of accessible pulse dynamics. Machine‐learning tools have recently shown promising for the design of smart lasers that can tune themselves to desired operating states. Yet, machine‐learning algorithms are mainly designed to target regimes of parameter‐invariant, stationary pulse generation, while the intelligent excitation of evolving pulse patterns in a laser remains largely unexplored. Breathing solitons exhibiting periodic oscillatory behavior, emerging as ubiquitous mode‐locked regime of ultrafast fiber lasers, are attracting considerable interest by virtue of their connection with a range of important nonlinear dynamics, such as exceptional points, and the Fermi‐Pasta‐Ulam paradox. Here, an evolutionary algorithm is implemented for the self‐optimization of the breather regime in a fiber laser mode‐locked through a four‐parameter nonlinear polarization evolution. Depending on the specifications of the merit function used for the optimization procedure, various breathing‐soliton states are obtained, including single breathers with controllable oscillation period and breathing ratio, and breather molecular complexes with a controllable number of elementary constituents. This work opens up a novel avenue for exploration and optimization of complex dynamics in nonlinear systems. [ABSTRACT FROM AUTHOR]

We report, to the best of our knowledge, the first visible-wavelength all-fiber passively mode-locked vortex laser by a figure-9 cavity in combination with $\sim$ 635 nm mode selective coupler, which can deliver picosecond optical vortex pulses with topological charges of OAM $_{\pm 1}$. The mode-locked vortex laser emits stable rectangular pulses with pulse duration tunable range from 85 to 510 ps and a 0.16-nm narrow linewidth at 634.36 nm. The maximum output power of the vortex laser reaches 1.3 mW with a high purity $\sim 97.2\%$. This work demonstrated the generation of picosecond pulsed vortex in visible passively mode-locked fiber laser, showing their potential for applications in particle trapping, optical tweezers, and high-resolution microscopy. [ABSTRACT FROM AUTHOR]

We demonstrated high power all-fiber $2\sim 3~\mu \text{m}$ supercontinuum (SC) generation from a short piece of germania-doped fiber (GDF) pumped by a thulium-doped fiber amplifier (TDFA). As 39 W 2 $\mu \text{m}$ picosecond pulse launched into GDF, a mid-infrared SC laser with maximum average power of 33.6 W and broadband spectrum of $1.8\sim 3.0~\mu \text{m}$ was achieved, which was the record SC power in $2\sim 3~\mu \text{m}$ region based on GDF to date, to the best of the authors’ knowledge. The spectral broadening mechanism of the high-power SC generation were theoretical investigated by solving the generalized nonlinear Schrödinger equation (GNLSE). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Vanadium diselenide (VSe2), as a transition metal dichalcogenides (TMDC), is an important class of graphene‐like 2D layered materials. It has excellent optical absorption nonlinearity, high thermal damage threshold, and large modulation depth, which can make it to be applied in the field of ultrashort pulse. Here, the nonlinear optical absorption properties of VSe2 are investigated along with in‐depth exploration of its ultrafast photonics performance. VSe2 is used as a saturable absorber (SA) in an erbium‐doped fiber ring laser to successfully generate a mode‐locked laser pulse with a central wavelength of 1565.3 nm, a repetition rate of 5.882 MHz, and a pulse width of 1.2 ps. What is noticeable is that the pulse has a high signal‐to‐noise ratio, which ensures that the mode‐locked pulse can remain stable for a long time. The results suggest that VSe2 as TMDC has potential applications in optical communications. Furthermore, this study expands the field of SA materials in fiber lasers. [ABSTRACT FROM AUTHOR]

In this work, we analyze numerically the dynamics of one-dimensional gaseous detonations propagating in a mixture with periodically varying properties such as density, temperature, or mixture composition. It is found that periodic upstream variations can lead to resonant amplification of intrinsic oscillations and to mode locking whereby the detonation oscillations are controlled more by the imposed mixture nonuniformities than by the detonation intrinsic instability. The results are consistent with predictions made in the recent work (Kasimov and Gonchar, Proceedings of 27th ICDERS, 2019; Proc Combust Inst 38(3):3725–3732, 2021) that analyzed the same problem in the framework of reactive Burgers' equation. [ABSTRACT FROM AUTHOR]

Mode-locked lasers only emit stationary ultrashort pulses in a narrow parameter space. Generally, they exhibit instabilities, outputting non-stationary yet localized structures. Soliton explosions and noise-like pulsing are two representative instabilities regularly encountered in mode-locked lasers. Enabled by emerging fast detection techniques, these laser instabilities can now be readily accessed. However, in-depth knowledge on these instabilities is extremely challenging as standard statistical tools such as correlation analysis is only sensitive to linear relationship between variables, overlooking the nonlinear parts. Here, we employ the mutual information to analyze soliton explosions and noise-like pulsing in mode-locked fiber lasers. We show that the mutual information is more sensitive compared to the standard correlation analysis. While the correlation between a signal and itself returns a value of 1 independent of the signal structure, mutual information has varying values depending on the structure. Both Pearson correlation and mutual information reveal that noise-like pulses thought-to-be stochastic radiation are generally correlated and some of them even has a high degree of correlation. Our work is conducive to understand complexities in laser systems. [ABSTRACT FROM AUTHOR]

An all-fiber frequency shifted feedback laser is developed to export mode-locked pulses with a narrow spectral bandwidth (238 MHz). A simple cavity structure is established by employing a fiber Bragg grating-based acousto-optic frequency shifter. An exotic reflection peak in a large spectral separation from the Bragg reflection peak is generated by applying flexural acoustic waves, and is utilized for narrow-band filtering and frequency shifting in the laser cavity. Stable mode-locked pulse output can be obtained with the cavity length as short as 16.5 m, whereas increasing the cavity length can reduce the pump power to 37 mW for mode-locked pulse output. A maximum power conversion efficiency of 17% with an average output power of 103 mW were obtained experimentally at a pump power of 600 mW, with corresponding pulse width, pulse energy and peak power of 2 ns, 88 nJ and 4.5 W, respectively. A time-bandwidth product of mode locked pulses of 0.476 denotes nearly Fourier-transform-limited performance. This work provides a new technique to implement efficient, compact and low-cost all-fiber FSFLs for narrow spectral bandwidth mode-locked pulse output. [ABSTRACT FROM AUTHOR]