Transition state to mode locking in a passively mode-locked erbium-doped fibre ring laser

The transition state between the continuous wave region and the mode-locked region in a passively mode-locked erbium-doped fibre ring laser has been experimentally observed by utilizing the nonlinear polarization rotation technique. When the pump power reaches the mode-locked threshold, the metastable pulse train with a tunable repetition rate is obtained in the transition from the continuous wave state to the passive mode-locked state via proper adjustment of the polarization controller. A simpie model has been established to explain the experimental observation.

Subharmonic mode locking of a Q-switched Nd:YAG laser

A Q-switched Nd:YAG laser has been actively mode-locked at a subharmonic frequency for the first time,to the authors’knowledge.The laser operation mode is provided by a combination of a traveling wave acousto-optic modulator and a spherical cavity mirror.The dynamics of laser generation is investigated.Pulses with a duration of 70 ps and a peak power of about 10 MW were obtained.Also presented are new results on obtaining high-power[~60 kW]picosecond tunable radiation in the~620 nm region based on frequency conversion of a superluminescent parametric generator pumped by such a laser.

On the generation of high-quality Nyquist pulses in mode-locked fiber lasers

Nyquist pulses have wide applications in many areas,from electronics to optics.Mode-locked lasers are ideal platforms to generate such pulses.However,how to generate high-quality Nyquist pulses in mode-locked lasers remains elusive.We address this problem by managing different physical effects in mode-locked fiber lasers through extensive numerical simulations.We find that net dispersion,linear loss,gain and filter shaping can affect the quality of Nyquist pulses significantly.We also demonstrate that Nyquist pulses experience similariton shaping due to the nonlinear attractor effect in the gain medium.Our work may contribute to the design of Nyquist pulse sources and enrich the understanding of pulse shaping dynamics in mode-locked lasers.

Dissipative soliton resonance within different dispersion regimes in a single mode-locked laser

Dissipative soliton resonance(DSR) was previously studied in separated mode-locked fiber lasers within different dispersion regimes including anomalous, near-zero and normal dispersion. Here we propose a method to study DSR in a single mode-locked laser in these different dispersion regimes. This is achieved by virtue of a waveshaper which can control the laser dispersion readily using software, avoiding the usual tedious cutback method. We find that dispersion has a negligible effect on DSR since the pulse duration keeps constant while dispersion is varied. Moreover, we examine the dynamics of DSR on the parameters of the SA including modulation depth and saturation power, and find that the pulse duration can be changed in a large range when the saturation power is decreased. Our numerical simulations could be important to guide relative experimental studies.

Second harmonic generation of laser beams in transverse mode locking states

Nonlinear frequency conversion of structured beams has been of great interest recently.We present an intracavity second harmonic generation(SHG)of laser beams in transverse mode locking(TML)states with a specially designed sandwich such as a microchip laser.The intracavity nonlinear frequency conversion process of a laser beam in a TML state to its second harmonic is theoretically and experimentally investigated,considering different relative phase and weight parameters between the basic modes in the TML beam.Comparison between the far-field SHG beam patterns of fundamental frequency transverse modes in coherently locked and incoherently superposed states demonstrates that the SHG of TML beams can carry more information.Various rarely observed far-field SHG beam patterns are obtained,and they are consistent with the theoretical analysis and numerical simulations.With the obtained SHG beams,the characteristics of the structured fundamental frequency beams can also be conversely investigated or predicted.This work may have important applications in optical 3D printing,optical trapping of particles,and free-space optical communication areas.

Pulse-train nonuniformity in an all-fiber ring laser passively mode-locked by nonlinear polarization rotation

This paper reports the periodic power variation of the pulse-train in a passively mode-locked soliton fiber ring laser. It can obtain either the uniform or nonuniform pulse-train output by simply rotating the polarization controllers. The experimental results show that the pulse-train nonuniformity is caused by the interaction between the nonuniform polarization states of the soliton pulses and the passive polarizer in the cavity.

Passively mode-locked erbium-doped fiber laser via a D-shape-fiber-based MoS_2 saturable absorber with a very low nonsaturable loss

We report on the generation of conventional and dissipative solitons in erbium-doped fiber lasers by the evanescent field interaction between the propagating light and a multilayer molybdenum disulfide（MoS_2） thin film. The MoS_2 film is fabricated by depositing the MoS_2 water–ethanol mixture on a D-shape-fiber（DF） repetitively. The measured nonsaturable loss, saturable optical intensity, and the modulation depth of this device are 13.3%, 110 MW/cm^2, and 3.4% respectively.Owing to the very low nonsaturable loss, the laser threshold of conventional soliton is as low as 4.8 mW. The further increase of net cavity dispersion to normal regime, stable dissipation soliton pulse trains with a spectral bandwidth of 11.7 nm and pulse duration of 116 ps are successfully generated. Our experiment demonstrates that the MoS_2-DF device can indeed be used as a high performance saturable absorber for further applications in ultrafast photonics.

Operation of Kerr-lens mode-locked Ti:sapphire laser in the non-soliton regime

A Kerr-lens mode-locked Ti：sapphire laser operating in a non-soliton regime is demonstrated. Dispersive wave generation is observed as a result of third order dispersion in the vicinity of zero dispersion. The characteristics of the Ti：sapphire l^ser operating in a positive dispersion regime are presented, where the oscillator directly generates pulses with duration continuously tunable from 0.37 ps to 2.11 ps, and 36 fs pulses are achieved atter extracavity compression. The oscillation is numerically simulated with an extended nonlinear Schr6dinger equation, and the simulation results are in good agreement with the experimental results.

An all-polarization-maintaining repetition-tunable erbium-doped passively mode-locked fiber laser

An environmentally stable, repetition rate tunable, all-polarization-maintaining, Er-doped pulse fiber laser with a single-wall carbon nanotubes saturated absorber is demonstrated. The ring laser cavity includes a delay line enabling a tunable repetition rate to vary from 35.52 MHz to 35.64 MHz with continuous mode-locked operation. The laser output parameters confirm that the tunable mode-locked operations are stable. High environmental stability is also confirmed by the -130 dBc/Hz low phase noise, a 70-dB signal-to-noise ratio of radio frequency signals, a low amplitude fluctuation of 5.76 × 10-4, and a low fluctuation of reoetition rate of 12 Hz. The laser shows a high de^ree of oolarization of 93%.

Passively mode-locked Nd：GdVO4 laser at 912nm

This paper demonstrates the passively mode-locked Nd：GdVO4 laser operating on the ^4F3/2-^4I9/2 transition at 912 nm by using a semiconductor saturable-absorber mirror for passive mode locking, stable continuous wave modelocked 912nm laser was achieved with a repetition rate of 176 MHz. At the incident pump power of 17.7W, 22.6mW average output power of stable mode-locked laser was obtained with a slope efficiency of 0.3%.

Observation of 550 MHz passively harmonic mode-locked pulses at L-band in an Er-doped fiber laser using carbon nanotubes film

We demonstrate a passively harmonic mode-locked（PHML） fiber laser operating at the L-band using carbon nanotubes polyvinyl alcohol（CNTs-PVA） film. Under suitable pump power and an appropriate setting of the polarization controller（PC）, the 54^（th） harmonic pulses at the L-band are generated with the side mode suppression ratio（SMSR） better than 44 dB and a repetition frequency of 503.37 MHz. Further increasing the pump power leads to a higher frequency of 550 MHz with compromised stability of 38.5 dB SMSR. To the best of our knowledge, this is the first demonstration on the generation of L-band PHML pulses from an Er-doped fiber laser based on CNTs.

Bound soliton pulses in a passively mode-locked fibre ring laser

The bound solitons in a passively mode-locked fibre ring laser are observed and their formation mechanism is summarized in this paper. In order to obtain stable bound solitons, a strong CW laser field at the centre of the soliton spectral is necessary to suppress and synchronize the random soliton phase variations.

Sb_2Te_3 mode-locked ultrafast fiber laser at 1.93μm

Ultrafast pulse generation was demonstrated in thulium doped fiber laser mode locked by magnetron sputtering de- posited Sb2Te3 with the modulation depth, non-saturable loss, and saturable intensity of 38%, 31.2%, and 3.3 MW/cm2, respectively. Stable soliton pulses emitting at 1930.07 nm were obtained with pulse duration of 1.24 ps, a 3-dB spectral bandwidth of 3.87 nm, an average output power of 130 mW, and signal-to-noise ratio （SNR） of 84 dB. To our knowledge, this is the first demonstration of Sb2Te3-based SA in fiber lasers at 2-p.m regime. Ultrafast pulse generation was demonstrated in thulium doped fiber laser mode locked by magnetron sputtering de- posited Sb2Te3 with the modulation depth, non-saturable loss, and saturable intensity of 38%, 31.2%, and 3.3 MW/cm2, respectively. Stable soliton pulses emitting at 1930.07 nm were obtained with pulse duration of 1.24 ps, a 3-dB spectral bandwidth of 3.87 nm, an average output power of 130 mW, and signal-to-noise ratio （SNR） of 84 dB. To our knowledge, this is the first demonstration of Sb2Te3-based SA in fiber lasers at 2-p.m regime.

A brief review:effects of resonant magnetic perturbation on classical and neoclassical tearing modes in tokamaks

This paper reviews the effects of resonant magnetic perturbation(RMP)on classical tearing modes(TMs)and neoclassical tearing modes(NTMs)from the theory,experimental discovery and numerical results with a focus on four major aspects:(i)mode mitigation,where the TM/NTM is totally suppressed or partly mitigated by the use of RMP;(ii)mode penetration,which means a linearly stable TM/NTM triggered by the externally applied RMP;(iii)mode locking,namely an existing rotating magnetic island braked and finally stopped by the RMP;(iv)mode unlocking,as the name suggests,it is the reverse of the mode locking process.The key mechanism and physical picture of above phenomena are revealed and summarized.

Neural Network Prediction of Disruptions Caused by Locked Modes on J-TEXT Tokamak

Prediction of disruptions caused by locked modes using the Back-Propagation （BP） neural network is completed on J-TEXT tokamak. The network, which is based on the BP neural network, uses Mirnov coils and locked mode coils signals as input data, and outputs a signal including information of prediction of locked mode. The rate of successful prediction of locked modes is more than 90%. For intrinsic locked mode disruptions, the network can give a prewarning signal about 1 ms ahead of the locking-time. For the disruption caused by resonant magnetic perturbation （RMPs） locked modes, the network can give a prewarning signal about 10 ms ahead of the locking-time.

Magnetic diagnostics for magnetohydrodynamic instability research and the detection of locked modes in J-TEXT

Magnetohydrodynamic(MHD)instabilities are widely observed during tokamak plasma operation.Magnetic diagnostics provide important information which supports the understanding and control of MHD instabilities.This paper presents the current status of the magnetic diagnostics dedicated to measuring MHD instabilities at the J-TEXT tokamak;the diagnostics consist of five Mirnov probe arrays for measuring high-frequency magnetic perturbations and two saddle-loop arrays for low-frequency magnetic perturbations,such as the locked mode.In recent years,several changes have been made to these arrays.The structure of the probes in the poloidal Mirnov arrays has been optimized to improve their mechanical strength,and the number of in-vessel saddle loops has also been improved to support better spatial resolution.Due to the installation of high-field-side(HFS)divertor targets in early 2019,some of the probes were removed,but an HFS Mirnov array was designed and installed behind the targets.Owing to its excellent toroidal symmetry,the HFS Mirnov array has,for the first time at J-TEXT,provided valuable new information about the locked mode and the quasi-static mode(QSM)in the HFS.Besides,various groups of magnetic diagnostics at different poloidal locations have been systematically used to measure the QSM,which confirmed the poloidal mode number m and the helical structure of the QSM.By including the HFS information,the 2/1 resonant magnetic perturbation(RMP)-induced locked mode was measured to have a poloidal mode number m of~2.

Stable watt-level mode-locked noise-like pulse from an all-PM fiber oscillator at 2 μm

We have experimentally presented a watt-level noise-like (NL) pulse mode-locked all-polarization-maintaining(PM) fiber laser centered at ~1995 nm, which can directly generate stable NL pulses with a maximum output power of ~1.017 W and pulse energy of ~0.61 μJ, representing the highest output power of mode-locked NL pulse at 2 μm from any fiber oscillators,to the best of our knowledge. The mode-locked NL pulse laser exhibits an excellent stability with a power fluctuation of~0.1% in 8 h of monitoring, and a signal-to-noise ratio of ~83 dB at a fundamental frequency of ~1.662 MHz. Moreover, the pulse envelope and coherence spike width of the NL pulse can be widely tuned from ~4.5 ns to ~16 ns, and ~364 fs to~323 fs, respectively, with the enhancement of the pump power. Such an all-PM fiber oscillator is the ideal seed source for the implementation of a high-power NL pulse laser and has potential valuable applications in mid-infrared spectroscopy and industrial processing.

Comparative studies of semiconductor saturable absorber mirror mode-locking dynamics in pulsed diode-end-pumped picosecond Nd:GdVO_4 and Nd:YAG lasers

Ultrashort pulses were generated in passively mode-locked Nd：YAC and Nd：CdVO4 lasers pumped by a pulsed laser diode with 10-Hz repetition rate. Stable mode-locked pulse trains were produced with the pulse width of 10 ps. The evolution of the mode-locked pulse was observed in the experiment and was discussed in detail. Comparing the pulse evolutions of Nd：YAG and Nd：GdVO4 lasers, we found that the buildup time of the steady-state mode-locking with semiconductor saturable absorber mirrors （SESAMs） was relevant to the upper-state lifetime and the emission cross-section of the gain medium.

Generation of all-fiber femtosecond vortex laser based on NPR mode-locking and mechanical LPG

An all-fiber femtosecond vortex laser based on common fiber components is constructed. It can produce femtosecond orbital angular momentum modes whose time pulse width is 398 fs. The topological charge of output orbital angular momentum （OAM） modes from this laser can be adjusted among 0, ＋1, and -1 easily while it is also easy to convert between continuous OAM modes and pulse OAM modes.

Experimental investigation on Q-switching and Q-switched mode-locking operation in gold nanorods-based erbium-doped fiber laser

We report on the generation of Q-switched and Q-switched mode-locked （QML） pulses in an erbium-doped fiber ring laser by using a polyvinyl alcohol （PVA） -based gold nanorod （GNR） saturable absorber （SA）. The PVA- based GNR SA has a modulation depth of -4.8% and a non-saturable loss of -26.9% at 1.5 μm. A Q-switched pulse train with a repetition rate varying from 18.70 to 39.85 kHz and a QML pulse train with an envelope repetition rate tuning from 20.31 to 31.50 kHz are obtained. Moreover, the lasing wavelengths of the Q-switched pulses can be flexibly tuned by introducing a narrow bandwidth, tunable filter into the laser cavity. The results demonstrate that the GNRs exhibit good optical performance and can find a wide range of applications in the field of laser technology.