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Herein, the efficiently high‐power pulsed Raman lasers with wavelength above 2.1 μm are experimentally demonstrated relying on the stimulated Raman scattering. A tailored high‐power noise‐like pulse (NLP) fiber laser system centered at ≈1953 nm with a maximum output power of ≈10.9 W is served as the pump source. By directly pumping a section of highly Ge‐doped silica fiber, the first pulsed Raman laser (centered at≈2139 nm) and the second pulsed Raman laser (centered at ≈2353 nm) with maximum output powers of ≈3.8 and ≈0.25 W are obtained, respectively, which represent the highest output powers of NLP at these wavelength regions, to the best of knowledge. Moreover, a high spectral purity of ≈94.3% of the first Raman laser is obtained, which indicates the significantly potential application of NLP in pulsed Raman laser. The midinfrared NLP fiber laser source will have potential applications in transparent polymer materials processing and midinfrared spectroscopy. [ABSTRACT FROM AUTHOR]

Tapered fiber (TF) and D-shaped fiber (DF) are two types of widely investigated devices in facilitating evanescent-field interactions with external materials. Although they have been found to be particularly useful in various ultrafast regimes, to date there is still no clear or systematic investigation on their local nonlinearities as well as the exerted influences on ultrafast behaviors. Herein, we present such thorough investigation through local nonlinearity engineering on TF and then in contrast with a DF as a reference. Optically deposited black phosphorus quantum dots (BPQDs) are used for saturable absorption. The nanometer-scale extremely small sizes of the BPQDs helpfully eliminate size-induced uncertainties or distortions during both device fabrication and the latter light–matter interaction. For the TF, in the experiment, it is found that the local nonlinear effect starts to be rather appreciable as the TF shrinks to a moderate thickness. Remarkably in comparison, the local nonlinearity of the DF itself can even be neglected reasonably, but after coating with BPQDs, it possesses a much larger modulation depth than any of the used BPQDs-coated TFs with different thicknesses/lengths. Further, we theoretically analyze the related locally nonlinear effects and reveal, for the first time, the direct origin of saturable absorption with evanescent-field-based general structures. [ABSTRACT FROM AUTHOR]

Nonlinear amplifying-loop mirror (NALM) and nonlinear optical loop mirror (NOLM) are two basic Sagnac interferometers widely used in mode-locked fiber lasers. Here we construct a variant by replacing the amplifier in an NALM with an absorber, which forms a type of nonlinear absorbing-loop mirror (NAbLM). The optical waves counter-propagating within an ideal NAbLM possess equal optical powers when they return and interfere inside the used fiber coupler owing to the equal absorptions. Meanwhile, their nonlinear phase shifts can still differ adequately if an asymmetric distribution of the absorber is employed. We theoretically predict that, in comparison to typical NALM and NOLM, the NAbLM can achieve relatively higher extinction ratio and larger modulation depth, resting on the unique equal-power interference. We then first used it as a saturable absorber in a holmium-doped fiber laser and achieved passively mode-locked operations in both normal and anomalous dispersion regimes. In the normal dispersion regime, h-shaped pulse was generated; whereas in the anomalous dispersion regime, both noise-like and dissipative-soliton-resonance-like pulses were achievable with polarization manipulation. NAbLM enables more options in nonlinear fiber optics especially when a high extinction ratio or large modulation depth is required. [ABSTRACT FROM AUTHOR]

By incorporating a house-made microfiber knot resonator (MKR), mode-locking regime transformation in an erbium-doped fiber (EDF) laser was achieved. With typical nonlinear polarization rotation (NPR) as the mode-locker, ∼890 fs soliton pulses could be directly produced from the EDF laser. However, once the MKR was further incorporated in it, the emitted pulse duration fell into the nanosecond regime, and meanwhile the spectral bandwidth dramatically narrowed down around 0.05 nm. This resulted from that the longitudinal laser modes were greatly eliminated by the MKR. The MKR exhibits tightly-packed, extremely narrow transmission teeth, acting as a comb-like, sharp-rolloff bandpass filter. The mode elimination factor could reach ∼98.81%. Further manipulating the intra-cavity polarization state, the emission wavelength could be switchable from one of the transmission teeth of the MKR to another, due to the intrinsic polarization-sensitivity of the NPR. Our results demonstrated that MKR could be a useful device in fiber lasers to enable some artificially pre-defined spectral and temporal characteristics through pre-designing on the related parameters of the MKR. This could also provide a route to transform the temporal characteristics through tailoring the spectral profile, i.e., manipulating the permitted longitudinal laser modes, in fiber lasers. [ABSTRACT FROM AUTHOR]

We experimentally report the novel evolutions of dissipative-soliton-resonance (DSR) pulses with respect to the pump power in an all-normal-dispersion fiber laser based on a nonlinear optical loop mirror. By solely increasing the pump power, pulse breaking of the DSR pulses was observed apart from the typical pulse broadening at a fixed pulse peak power. Instead of arbitrarily broadening with the increase in the pump power or directly losing the mode-locking state, the process of pulse breaking was accompanied by the multipulse state and harmonic mode-locking pulses evolving from the original DSR pulse. In addition, to the best of our knowledge, a narrowing of the DSR pulse with the increase in the pump power was observed for the very first time. Further results show that these unusual evolutions of DSR pulses could be attributed to the changes in several laser parameters resulting from the increasing pump power under specific operating conditions. [ABSTRACT FROM AUTHOR]

We report on a type of peak-power-clamped passive Q-switching in a thulium/holmium co-doped fiber laser, for the first time to the best of our knowledge. Nonlinear polarization rotation was used as an artificial saturable absorber to initiate the Q-switched operation. The peak-power-clamping (PPC) effect was realized by incorporating a piece of high nonlinear fiber in the fiber resonator and launching high-enough pump power. Normal Q-switching behavior could be observed with the pump power within the range of 0.9–0.95 W, where the pulse peak power monotonically rose with the pump power increasing. However, once the pump power reached a threshold-like level, i.e., ∼1 W, the delivered Q-switched pulse peak power started to be clamped at a nearly constant value of ∼0.4 W, corresponding to an intracavity total peak power of ∼2.22 W. It indicates that, besides typical dissipative soliton resonances in mode-locked fiber lasers, PPC effect can also occur in the passively Q-switched regime with appropriate pump power and cavity nonlinearity managements. [ABSTRACT FROM AUTHOR]

We demonstrate what is believed to be the first dissipative-soliton-resonance mode-locking of a holmium-doped fiber (HDF) laser. A telecommunication band fiber source was used to pump a piece of thulium-doped fiber, resulting in 1.9- $\mu \text{m}$ band lasing, which was subsequently used to intra-cavity-pump another piece of HDF, resulting in lasing beyond $2~\mu \text{m}$. A nonlinear optical-loop mirror incorporating two pieces of high nonlinear fiber (HNLF) was employed as an artificial saturable absorber, and, meanwhile, to enhance the cavity nonlinearity. By varying the pump power from ~2.1 to ~4 W, the produced pulse could be tuned from ~4.85 to ~20.8 ns in duration while maintaining a nearly constant peak power, i.e., peak power clamping (PPC) effect. By further incorporating different lengths of HNLF, it was found that the longer the HNLF, the larger the pulse duration and the lower the pulse peak power, due to the expected stronger PPC effect. Meanwhile, spectral blue-shifting effect has occurred due to the increased fiber attenuation loss. Based on our preliminary experiments using the HDF laser as a test bed, a higher pulse energy at a longer wavelength could be expected with further enhanced pumping and gain-tailoring designs. [ABSTRACT FROM AUTHOR]

In this paper, an all fiber self-mode-locked Tm-doped double-clad fiber ring laser is experimentally demonstrated. By inserting a piece of ∼95 m SMF-28 into the cavity to enhance the nonlinearity and bring in a large net anomalous dispersion, stable self-mode-locked operation with mode-locked pulse bundles has been observed. About ∼8 sub-pulses with unequal amplitude and identical separation are contained in each produced pulse bundle. The width and adjacent separation of each sub-pulse are about ∼1 ns and ∼63 ns, respectively. With increasing the pump power to ∼2.17 W, the maximum output power of ∼121 mW is achieved at central wavelength of ∼1962.6 nm. [ABSTRACT FROM AUTHOR]

We give a systematic experimental study of multipulse bunches in a graphene oxide saturable absorber (GOSA) passively mode-locked all-normal dispersion ytterbium-doped fiber laser (YDFL). Some special phenomena such as harmonic multipulse bunches, harmonic mode-locking, and chaotic multipulse states are also obtained. Our experiment reveals that the inserted 2.5-nm narrow bandwidth filter plays an important role in the formation of multipulse in all-normal dispersion fiber lasers. Because of the effective gain bandwidth depends on both the 2.5-nm narrow bandwidth filter and the artificial fiber birefringence filter, the multipulse operation states are sensitive to the polarization. It is the first demonstration of multipulse evolution in a GOSA passively mode-locked all-normal dispersion YDFL. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate an ytterbium-doped fiber (YDF) laser being able to generate dark pulses and Q-switched pulses by using a graphene oxide (GO) based composite as saturable absorber (SA). These operation regimes are realized for the first time in a 1-μm band fiber laser as far as our best knowledge. In the fabrication of GO based saturable absorber (GOSA), a vertical evaporation process is included, which results in a GO concentration and thickness variable SA. The generated dark pulse has pulse duration in ns-scale. In the Q-switched operation, pulses with durations from 5.321μs to 2.655μs and repetition rates from 28.47kHz to 58.95kHz can be obtained as the launched pump power is changed. The transition from the dark pulse to Q-switched operations can be clearly seen as the pump power is increased to a certain value. The radio frequency (RF) and optical spectrum variations with respect to the change of pump power have also been analyzed in detail. [Copyright &y& Elsevier]

We investigate on a multi-wavelength operation erbium-doped fiber laser Q-switched by a graphene-based saturable absorber. Stable pulses were generated with the widths from 6.9 to 1.5 µs, energies from 40.4 to 130.2 nJ and repetition rates from 68.32 to 132.9 kHz, when the pump power increased from 142.32 to 441.86 mW. A fiber Bragg grating with five reflective peaks was inserted into the fiber ring through an optical circulator, resulting in a stable output of five-lasing-wavelength output. The laser can perform as a low-cost and easy-built all fiber light source, and has potential applications in the fields where pulses at multi-wavelength operation are needed, i.e., temperature or strain fiber sensors. [ABSTRACT FROM AUTHOR]

• A quasi-continuous wave thulium-doped fiber laser is constructed with output power reached 70 W and peak power at 700 W, and then applied in laser lithotripsy. • It is proved that the laser-induced bubble channel is a prerequisite for TFL noncontact effective lithotripsy, with bubble length and width markedly influencing stone ablation rates. • Thulium fiber laser with higher peak power performs higher ablation rate compared to previous research in laser lithotripsy. • With no restrictions on the stone, the maximum ablation rate obtained are 8.9 mg/s for uric acid stones and 7.5 mg/s for calcium oxalate monohydrate stones. Despite its widespread use in laser lithotripsy, the Ho:YAG solid-state laser suffers from limited parameter combinations, poor beam quality, and low photoelectric conversion efficiency. The development of 2.0 μm fiber laser technology has positioned the thulium-doped fiber laser (TFL) as a superior alternative to the Ho:YAG laser in lithotripsy. The TFL excels because of its high-water absorption coefficient, all-fiber structure and pump modulation flexibility. However, the peak power reported for TFL in lithotripsy remains at most 500 W. This study presented a quasi-continuous wave (QCW) TFL featuring a wide tunable range of pulse width and pulse repetition rates, and a peak power of 700 W to realize enhanced ablation efficiency. A high-speed camera was used to acquire the laser-induced cavitation bubble images for the analysis of the interaction mechanism of a high peak power QCW TFL pulse with water and stones. While the synergistic action of photothermal and photomechanical effects constituted the mechanism of TFL lithotripsy, the laser-induced bubble channel was a prerequisite for TFL noncontact effective lithotripsy, with bubble length and width markedly influencing stone ablation rates. Ablation experiments on uric acid and calcium oxalate monohydrate stones conducted with various parameter combinations at a peak power of 700 W yielded rates of 8.9 mg/s for uric acid stones and 7.5 mg/s for calcium oxalate monohydrate stones. A comparison with prior findings confirmed that a high peak power TFL could notably improve stone ablation rates. [ABSTRACT FROM AUTHOR]

• The nearly transform-limited pulse with a narrow spectral width at 2 μm is presented. • Stable pulses with a duration of ∼105.6 ps and a linewidth of ∼86 pm were obtained. • The pulse laser system can deliver ∼19.42 W output power with a narrow linewidth. • The maximum output power of ∼100.7 W was obtained. Herein, stable mode-locked pulse trains with an ultra-narrow linewidth at the 2 μm region were obtained in a compact all polarization-maintaining fiber oscillator, which can directly deliver nearly transform-limited pulse with a duration of ∼105.6 ps and a spectral width of ∼86 pm. Relying on the multistage Tm-doped fiber amplifier chains, the output power of ∼19.42 W with a narrow linewidth and the maximum output power of ∼100.7 W were obtained, respectively. The narrow linewidth pulse laser may have the promising potential applications in mid-infrared spectroscopy and polymer processing. [ABSTRACT FROM AUTHOR]

• All-fiber high-power NL pulse laser system at 2 μm was experimentally presented. • The NL pulse with highest output power of ∼72 W was obtained. • Based on the NL pulse, both the mid-IR Raman lights and SC were investigated. • The potential application of the NL pulse in mid-IR spectroscopy was revealed. Herein, a high-power noise-like (NL) pulse fiber laser system in the 2 μm region and its applications in mid-IR Raman stokes light and flat supercontinuum (SC) were experimentally demonstrated. An all-PM fiber configuration oscillator was constructed as the seed source which can deliver stable NL mode-locked pulse trains centered at ∼1950 nm wavelength with ∼5.37 MHz repetition rate. The pulse envelope and coherence spike widths of the NL pulse can be tunable from ∼2.5 ns to ∼3.4 ns and ∼397 fs to ∼375 fs, respectively. Relying on the magnified NL pulse, the cascaded Raman shifted stokes lights centered at ∼2148 nm and ∼2359 nm were separately achieved. Meanwhile, a flat SC with a 10 dB bandwidth of ∼480 nm (from ∼1940 nm to ∼2420 nm) was obtained after boosting in a large-mode-area fiber amplifier stage, and the average output power of ∼72 W represents the highest output power of the NL pulse in the 2 μm region, to the best of our knowledge. Moreover, we noted that the NL pulse-based spectrum broadening is much more effective than that the picosecond pulse-based. Our experimental results strongly imply that the NL pulse can be severed as an ideal laser source for the application of mid-IR spectroscopy. [ABSTRACT FROM AUTHOR]