463-MHz fundamental mode-locked fiber laser based on few-layer MoS(2) saturable absorber

Asymmetric Laser Field Interaction with MXene Coated on the Side Surface of Optical Fibers for Ultrafast Nonlinear Switches

In recent years, there has been significant interest in researching ultrafast nonlinear optical phenomena involving light-matter interactions in two-dimensional (2D) materials, owing to their potential applications in optics and photonics. MXene, a recently developed 2D material, has garnered considerable attention due to its graphene-like properties and highly tunable electronic/optical characteristics. Herein, we demonstrate ultrafast all-optical switches based on four-wave-mixing (FWM) utilizing the nonlinear optical property of MXene Ti3C2Tx. In order to realize the device, we deposited multilayered Ti3C2Tx in the form of a supernatant solution onto the polished surface of a side-polished optical fiber, enabling the interaction of Ti3C2Tx with the asymmetric evanescent field of the incident input. We systematically characterized the nonlinear optical responses derived from the Ti3C2Tx layers. The fabricated device exhibits notable performance metrics, an enhancement of the extinction ratio, and a conversion efficiency of the newly generated signal, displaying 5.3 and 5.2 dB, respectively. Additionally, the device operates at high modulation frequencies, reaching up to 20 GHz, and demonstrates high-resolution detuning with channel distances of up to 15 nm. Our findings highlight the potential of MXene-based materials for ultrafast optical data management systems.

Fast linear optical sampling with high repetition-frequency using fiber delay lines

Linear optical sampling (LOS) is one of the most promising techniques for optical modulation analyzers. The LOS system generally adopts a mode-locked fiber laser (MFL) to generate an ultra-stable optical pulse to realize under-sampling for signal under test (SUT). However, it is challenging for MFL to produce a high-repetition-frequency pulse, making more measurement errors of conventional LOS technology, especially for high-speed signals. This paper proposes a dual-pulse mixing (DPM) based LOS system to increase the repetition frequency using fiber delay lines with the multiplied optical pulse. We propose the pulse location and peak extraction algorithms to compensate the time bias and amplitude bias in the DPM-based LOS system, which significantly improves the measurement speed and range. The experiment results show that the DPM-based LOS system can increase the number of sampling points twice compared with the conventional LOS within the same sampling time window. Furthermore, the proposed DPM-based LOS system can achieve less error vector magnitude with a reduction of 9.1% compared with the conventional LOS. Hence, the proposed DPM-based LOS system has great potential for high-speed signal processing.

Novel nanomaterials based saturable absorbers for passive mode locked fiber laser at 1.5μm

Compared with continuous wave lasers, ultrafast lasers have the advantages of ultra-short pulse width and ultra-high peak power, and have significant applications in optical communications, medical diagnostics, and precision machining. Saturable absorber (SA) technology is the most effective technique for the generation of ultra-fast lasers, which are based on artificial SAs and natural SAs. Among them, the semiconductor saturable absorber mirror has become the most commonly used form at present. Recently, basic research and application of nanomaterials such as carbon nanotubes (CNTs) and graphene have been developed rapidly. Researchers have found that nanomaterials exhibit extraordinary characteristics in ultrafast photonics, such as the low saturation intensity of CNTs, zero-band gap of graphene, and extremely high modulation depth of the topological insulator nano-films. Since graphene was first reported as an SA in 2009, many other nanomaterials have been successively explored, resulting in the rapid development of novel nanomaterial-based SAs. In this paper, we classified the nanomaterials used in SA mode-locking technology at 1.5μm and reviewed their research progress with a particular focus on nonlinear optical properties, integration strategies, and applications in the field of ultrafast photonics.

Multistate transformations of a femtosecond fiber laser based on a pure fiber saturable absorber

The graded index multimode-fiber step-index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF) structure was designed as a saturable absorber (SA). To obtain optical pulses that meet the requirements of different applications, the multistate transformations of a femtosecond fiber laser based on GIMF-SIMF-GIMF SA were numerically and experimentally researched. The fiber laser can self-start mode-locking; its fundamental repetition rate of fiber laser is 10.35 MHz. The fiber laser can deliver three different optical pulses, namely, the conventional soliton, second-order bound state, and noise-like pulse. The duration of soliton is 421.2 fs; the energy of noise-like pulse is 197.10 pJ. The experimental and simulated results show that the output states of the fiber laser can be switched by adjusting the pump power.

Ultrafast Fiber Lasers with Low-Dimensional Saturable Absorbers: Status and Prospects

Wide-spectral saturable absorption (SA) in low-dimensional (LD) nanomaterials such as zero-, one-, and two-dimensional materials has been proven experimentally with outstanding results, including low saturation intensity, deep modulation depth, and fast carrier recovery time. LD nanomaterials can therefore be used as SAs for mode-locking or Q-switching to generate ultrafast fiber laser pulses with a high repetition rate and short duration in the visible, near-infrared, and mid-infrared wavelength regions. Here, we review the recent development of emerging LD nanomaterials as SAs for ultrafast mode-locked fiber laser applications in different dispersion regimes such as anomalous and normal dispersion regimes of the laser cavity operating in the near-infrared region, especially at ~1550 nm. The preparation methods, nonlinear optical properties of LD SAs, and various integration schemes for incorporating LD SAs into fiber laser systems are introduced. In addition to these, externally (electrically or optically) controlled pulsed fiber laser behavior and other characteristics of various LD SAs are summarized. Finally, the perspectives and challenges facing LD SA-based mode-locked ultrafast fiber lasers are highlighted.

2.8 µm passively Q-switched Er:ZBLAN fiber laser with an Sb saturable absorber mirror

A Q-switched Er:ZBLAN fiber laser operating at 2.8 µm was realized by employing Sb as the saturable material. The Sb material was deposited on a gold mirror by the magnetron-sputtering deposition method to develop a saturable absorber mirror (SAM). By employing the Sb-SAM in an Er:ZBLAN fiber laser, stable Q-switching operation was achieved at central wavelength of 2799.7 nm with the repetition rates ranging from 33.3 to 58.8 kHz and the pulse duration ranging from 5.7 to 1.7 µs. The Sb-SAM still works stably under the maximum pump power of 5.6 W, with an output power of 59 mW corresponding to the pulse energy of 1.03 µJ. To our knowledge, this was the first demonstration of Sb-based saturable material in Er:ZBLAN fiber laser for mid-infrared Q-switched pulse generation operating in the 2.8 µm regime, indicating its potential applications in the mid-infrared waveband.

Sub-200 fs, 344 MHz mode-locked Tm-doped fiber laser

We demonstrate a compact, self-starting mode-locked thulium-doped fiber laser based on nonlinear polarization evolution (NPE), with a fundamental repetition rate of ∼344MHz and a pulse duration of ∼160fs. The generated pulses centered at ∼1975nm have a maximum output power of ∼560mW, corresponding to a pulse energy of ∼1.63nJ. To the best of our knowledge, the achieved repetition rate represents the highest value of fundamentally NPE mode-locked fiber lasers at ∼2µm, while the average output power is also higher than the previously reported 2 µm ultrafast single-mode fiber oscillators. The timing jitter in the integrated range [5 kHz, 10 MHz] and the integrated relative intensity noise in the range [10 Hz, 10 MHz] reach ∼35fs and ∼0.009%, respectively. Our high-performance laser is an ideal candidate for various applications, including mid-infrared frequency metrology and high-speed optical sampling.

Ultrafast Fiber Lasers: An Expanding Versatile Toolbox

Ultrafast fiber lasers have gained rapid advances in last decades for their intrinsic merits such as potential of all-fiber format, excellent beam quality, superior power scalability, and high single-pass gain, which opened widespread applications in high-field science, laser machining, precision metrology, optical communication, microscopy and spectroscopy, and modern ophthalmology, to name a few. Performance of an ultrafast fiber laser is well defined by the laser parameters including repetition rate, spectral bandwidth, pulse duration, pulse energy, wavelength tuning range, and average power. During past years, these parameters have been pushed to an unprecedented level. In this paper, we review these enabling technologies and explicitly show that the nonlinear interaction between ultrafast pulses and optical fibers plays the essential role. As a result of rapid development in both active and passive fibers, the toolbox of ultrafast fiber lasers will continue to expand and provide solutions to scientific and industrial problems.

Niobium disulfide as a new saturable absorber for an ultrafast fiber laser

Group VB transition metal dichalcogenides (TMDCs) are emerging two-dimensional materials and have attracted significant interests in the fields of physics, chemistry, and material sciences. However, there are very few reports about the optical characteristics and ultrafast photonic applications based on group VB TMDCs so far. In this work, we have calculated the niobium disulfide (NbS2) band structure by the density functional theory (DFT), which has revealed that NbS2 is a metallic TMDC. In addition, we have prepared an NbS2-microfiber device and the nonlinear optical characteristics have been investigated. The modulation depth, saturation intensity and non-saturable loss have been measured to be 13.7%, 59.93 MW cm-2 and 17.74%, respectively. Based on the nonlinear optical modulation effect, the Er-doped fiber (EDF) laser works in the soliton mode-locking state with the pump power of 94-413 mW. The pulse duration of 709 fs and the maximum average output power of 23.34 mW have been obtained at the pump power of 413 mW. The slope efficiency is as high as 6.79%. Compared to the recently reported studies based on TMDCs comprehensively, our experimental results are better. These experimental results demonstrate that NbS2 with excellent nonlinear optical properties can be used as a promising candidate to advance the development of ultrafast photonics.

Single-axis soliton molecule and multiple solitons generation from a vector fiber laser

We investigate various patterns of vector solitons arising in a passively mode-locked fiber laser based on semiconductor saturable absorber mirror (SESAM). By properly adjusting the cavity parameters including the pump power and intra-cavity birefringence, the fundamental vector solitons, vector soliton molecules, and macroscopic vector solitons can be separately observed. In particular, both vector soliton molecule and macroscopic vector solitons exhibit multi-pulse structure along one polarization axis while there occurs single pulse profile at its orthogonal polarization component. Thus, they can be treated as "1 + 2" and "1+n" vector solitons. Moreover, the size of the macroscopic solitons can be manipulated from half of the cavity to even the whole cavity. The generation mechanisms of these vector soliton patterns are also investigated.

Ultrafast nonlinear absorption enhancement of monolayer MoS2 with plasmonic Au nanoantennas

In this work, we experimentally study the nonlinear absorption enhancement of saturable absorption and two-photon absorption on a hybrid structure comprising a monolayer MoS₂ and Au nanoantennas via femtosecond I-scan measurement. Specifically, a 13-fold increment in the linear absorption coefficient is attained at 1.85 eV, along with an 8-fold enhancement of the two-photon absorption coefficient at 1.65 eV, which is attributed to exciton-plasmon coupling resonance and plasmonic hot electron transfer. The exciton-plasmon coupling effect is characterized by stable photoluminescence experiments. Furthermore, the exciton recombination time is extracted from the pump-probe measurement, whose value in the hybrid structure is shortened from 18.5 ps (pure MoS₂) to 1.84 ps. Our findings facilitate a new perspective to modulate the nonlinear optical response and to promote the performance of nonlinear photonic devices.

Nonlinear optical properties of MoS2-WS2 heterostructure in fiber lasers

As a saturable absorption material, the heterostructure with the van der Waals structure has been paid much attention in material science. In general, the heterogeneous combination is able to neutralize, or even exceed, the individual material's advantages in some aspects. In this paper, which describes the magnetron sputtering deposition method, the tapered fiber is coated by the MoS₂-WS₂ heterostructure, and the MoS₂-WS₂ heterostructure saturable absorber (SA) is fabricated. The modulation depth of the prepared MoS₂-WS₂ heterostructure SA is measured to be 19.12%. Besides, the theoretical calculations for the band gap and carrier mobility of the MoS₂-WS₂ heterostructure are provided. By employing the prepared SA, a stable and passively erbium-doped fiber laser is implemented. The generated pulse duration of 154 fs is certified to be the shortest among all fiber lasers based on transition mental dichalcogenides. Results in this paper provide the new direction for the fabrication of ultrafast photon modulation devices.

Mode-Locked Er-Doped Fiber Laser by Using MoS2/SiO2 Saturable Absorber

The two-dimensional (2D) layered material MoS₂ has attracted numerous attentions for electronics and optoelectronics applications. In this work, a novel type of MoS₂-doped sol-gel glass composite material is prepared. The nonlinear optical properties of prepared MoS₂/SiO₂ composite material are measured with modulation depth (ΔT) of 3.5% and saturable intensity (I_sat) of 20.15 MW/cm². The optical damage threshold is 3.46 J/cm². Using the MoS₂/SiO₂ composite material as saturable absorber (SA), a passive mode-locked Er-doped fiber (EDF) laser is realized. Stable conventional soliton mode-locking pulses are successfully generated with a pulse width of 780 fs at the pump power of 90 mW. In the pump power range of 100-600 mW, another stable mode-locking operation is obtained. The pulse width is 1.21 ps and the maximum output power is 5.11 mW. The results indicate that MoS₂/SiO₂ composite materials could offer a new way for optical applications.

α-In2Se3 wideband optical modulator for pulsed fiber lasers

With the magnetron-sputtering deposition method, α phase indium selenide (α-In₂Se₃) was developed into a saturable absorber (SA) with wideband saturable absorption property at 800, 1560, and 1930 nm. After inserting the α-In₂Se₃ SA into erbium-doped fiber laser (EDFL) and thulium-doped fiber laser (TDFL) systems, we can easily obtain stable soliton pulse trains. The pulse duration/pulse energy/slope efficiency for EDFL and TDFL were 276 fs/2.03 nJ/15.8% and 1.02 ps/7.1 nJ/23.5%, respectively. These results showed that the MSD-grown α-In₂Se₃ could be regarded as high efficiency material to be applied in ultrafast photonics.

Sub-200 femtosecond dispersion-managed soliton ytterbium-doped fiber laser based on carbon nanotubes saturable absorber

Ultrafast fiber laser light sources attract enormous interest due to the booming applications they are enabling, including long-distance communication, optical metrology, detecting technology of infra-biophotons, and novel material processing. In this paper, we demonstrate 175 fs dispersion-managed soliton (DMS) mode-locked ytterbium-doped fiber (YDF) laser based on single-walled carbon nanotubes (SWCNTs) saturable absorber (SA). The output DMSs have been achieved with repetition rate of 21.2 MHz, center wavelength of 1025.5 nm, and a spectral width of 32.7 nm. The operation directly pulse duration of 300 fs for generated pulse is the reported shortest pulse width for broadband SA based YDF lasers. By using an external grating-based compressor, the pulse duration could be compressed down to 175 fs. To the best of our knowledge, it is the shortest pulse duration obtained directly from YDF laser based on broadband SAs. In this paper, SWCNTs-SA has been utilized as the key optical component (mode locker) and the grating pair providing negative dispersion acts as the dispersion controller.

Transition-metal dichalcogenides heterostructure saturable absorbers for ultrafast photonics

In this Letter, high-quality WS₂ film and MoS₂ film were vertically stacked on the tip of a single-mode fiber in turns to form heterostructure (WS₂-MoS₂-WS₂)-based saturable absorbers with all-fiber integrated features. Their nonlinear saturable absorption properties were remarkable, such as a large modulation depth (∼16.99%) and a small saturable intensity (6.23  MW·cm^-2). Stable pulses at 1.55 μm with duration as short as 296 fs and average power as high as 25 mW were obtained in an erbium-doped fiber laser system. The results demonstrate that the proposed heterostructures own remarkable nonlinear optical properties and offer a platform for adjusting nonlinear optical properties by stacking different transition-metal dichalcogenides or modifying the thickness of each layer, paving the way for engineering functional ultrafast photonics devices with desirable properties.

5 GHz fundamental repetition rate, wavelength tunable, all-fiber passively mode-locked Yb-fiber laser

A passively mode-locked Yb³⁺-doped fiber laser with a fundamental repetition rate of 5 GHz and wavelength tunable performance is demonstrated. A piece of heavily Yb³⁺-doped phosphate fiber with a high net gain coefficient of 5.7 dB/cm, in conjunction with a fiber mirror by directly coating the SiO₂/Ta₂O₅ dielectric films on a fiber ferrule is exploited for shortening the laser cavity to 2 cm. The mode-locked oscillator has a peak wavelength of 1058.7 nm, pulse duration of 2.6 ps, and the repetition rate signal has a high signal-to-noise ratio of 90 dB. Moreover, the wavelength of the oscillator is found to be continuously tuned from 1056.7 to 1060.9 nm by increasing the temperature of the laser cavity. Simultaneously, the repetition rate correspondingly decreases from 4.945874 to 4.945496 GHz. Furthermore, the long-term stability of the mode-locked operation in the ultrashort laser cavity is realized by exploiting temperature controls. This is, to the best of our knowledge, the highest fundamental pulse repetition rate for 1-μm mode-locked fiber lasers.

The Property, Preparation and Application of Topological Insulators: A Review

Topological insulator (TI), a promising quantum and semiconductor material, has gapless surface state and narrow bulk band gap. Firstly, the properties, classifications and compounds of TI are introduced. Secondly, the preparation and doping of TI are assessed. Some results are listed. (1) Although various preparation methods are used to improve the crystal quality of the TI, it cannot reach the industrialization. Fermi level regulation still faces challenges; (2) The carrier type and lattice of TI are affected by non-magnetic impurities. The most promising property is the superconductivity at low temperature; (3) Magnetic impurities can destroy the time-reversal symmetry of the TI surface, which opens the band gap on the TI surface resulting in some novel physical effects such as quantum anomalous Hall effect (QAHE). Thirdly, this paper summarizes various applications of TI including photodetector, magnetic device, field-effect transistor (FET), laser, and so on. Furthermore, many of their parameters are compared based on TI and some common materials. It is found that TI-based devices exhibit excellent performance, but some parameters such as signal to noise ratio (S/N) are still lower than other materials. Finally, its advantages, challenges and future prospects are discussed. Overall, this paper provides an opportunity to improve crystal quality, doping regulation and application of TI.

Tungsten disulphide for ultrashort pulse generation in all-fiber lasers

Tungsten disulphide (WS₂), which exhibits excellent saturable absorption properties, has attracted much attention in the applications of photonic devices. In this paper, WS₂ is applied for the preparation of a saturable absorber (SA). Using the pulsed laser deposition (PLD) method, WS₂ is deposited on the side surface of the tapered fiber. In order to obtain larger non-linearity of the SAs with evanescent wave interaction, the tapered fiber had a smaller waist diameter and longer fused zone. Gold film was deposited on the fiber-taper WS₂ SAs to improve their reliability and avoid oxidation and corrosion. Employing the balanced twin-detector method, the modulation depth of the fiber-taper WS₂ SAs was measured to be 17.2%. With the fiber-taper WS₂ SA, a generated pulse with 246 fs duration and a 57 nm bandwidth was obtained at 1561 nm. The electrical signal to noise ratio was better than 92 dB. To our knowledge, the pulse duration was the shortest among the reported all-fiber lasers with transition metal dichalcogenide (TMD) SAs. These results indicate that fiber-taper WS₂ SAs with smaller waist diameter and longer fused zone are promising photonic devices for ultrashort pulse generation in all-fiber lasers.

2D Materials for Optical Modulation: Challenges and Opportunities

Owing to their atomic layer thickness, strong light-material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D material-based optical modulation in three categories is reviewed: free-space, fiber-based, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.

Emerging Low-Dimensional Materials for Nonlinear Optics and Ultrafast Photonics

Low-dimensional (LD) materials demonstrate intriguing optical properties, which lead to applications in diverse fields, such as photonics, biomedicine and energy. Due to modulation of electronic structure by the reduced structural dimensionality, LD versions of metal, semiconductor and topological insulators (TIs) at the same time bear distinct nonlinear optical (NLO) properties as compared with their bulk counterparts. Their interaction with short pulse laser excitation exhibits a strong nonlinear character manifested by NLO absorption, giving rise to optical limiting or saturated absorption associated with excited state absorption and Pauli blocking in different materials. In particular, the saturable absorption of these emerging LD materials including two-dimensional semiconductors as well as colloidal TI nanoparticles has recently been utilized for Q-switching and mode-locking ultra-short pulse generation across the visible, near infrared and middle infrared wavelength regions. Beside the large operation bandwidth, these ultrafast photonics applications are especially benefit from the high recovery rate as well as the facile processibility of these LD materials. The prominent NLO response of these LD materials have also provided new avenues for the development of novel NLO and photonics devices for all-optical control as well as optical circuits beyond ultrafast lasers.

Tungsten disulfide saturable absorbers for 67 fs mode-locked erbium-doped fiber lasers

In this paper, we demonstrate 67 fs pulse emitting with tungsten disulfide (WS₂) in mode-locked erbium-doped fiber (EDF) lasers. Using the pulsed laser deposition method, WS₂ is deposited on the surface of the tapered fiber to form the evanescent field. The fiber-taper WS₂ saturable absorber (SA) with the large modulation depth is fabricated to support the ultrashort pulse generation. The influences of the WS₂ SA are analyzed through contrastive experiments on fiber lasers with or without the WS₂ SA. The pulse duration is measured to be 67 fs, which is the shortest pulse duration obtained in the mode-locked fiber lasers with two dimensional (2D) material SAs. Compared to graphene, topological insulator, and other transition metal dichalcogenides (TMDs) SAs, results in this paper indicate that the fiber-taper WS₂ SA with large modulation depth is a more promising photonic device in mode-locked fiber lasers with the wide spectrum and ultrashort pulse duration.

Reference-free, high-resolution measurement method of timing jitter spectra of optical frequency combs

Timing jitter is one of the most important properties of femtosecond mode-locked lasers and optical frequency combs. Accurate measurement of timing jitter power spectral density (PSD) is a critical prerequisite for optimizing overall noise performance and further advancing comb applications both in the time and frequency domains. Commonly used jitter measurement methods require a reference mode-locked laser with timing jitter similar to or lower than that of the laser-under-test, which is a demanding requirement for many laser laboratories, and/or have limited measurement resolution. Here we show a high-resolution and reference-source-free measurement method of timing jitter spectra of optical frequency combs using an optical fibre delay line and optical carrier interference. The demonstrated method works well for both mode-locked oscillators and supercontinua, with 2 × 10-9 fs2/Hz (equivalent to -174 dBc/Hz at 10-GHz carrier frequency) measurement noise floor. The demonstrated method can serve as a simple and powerful characterization tool for timing jitter PSDs of various comb sources including mode-locked oscillators, supercontinua and recently emerging Kerr-frequency combs; the jitter measurement results enabled by our method will provide new insights for understanding and optimizing timing noise in such comb sources.

Vector soliton fiber laser passively mode locked by few layer black phosphorus-based optical saturable absorber

We report on the optical saturable absorption of few-layer black phosphorus nanoflakes and demonstrate its application for the generation of vector solitons in an erbium-doped fiber laser. By incorporating the black phosphorus nanoflakes-based saturable absorber (SA) into an all-fiber erbium-doped fiber laser cavity, we are able to obtain passive mode-locking operation with soliton pulses down to ~670 fs. The properties and dynamics of the as-generated vector solitons are experimentally investigated. Our results show that BP nanoflakes could be developed as an effective SA for ultrashort pulse fiber lasers, particularly for the generation of vector soliton pulses in fiber lasers.

Switchable thulium-doped fiber laser from polarization rotation vector to scalar soliton

We experimentally demonstrate switchable temporal soliton generation from a thulium-doped fiber laser (TDFL), using carbon nanotubes as the mode-locker. With the help of residual polarization dependent loss of a wavelength division multiplexer, a weak nonlinear polarization rotation (NPR) effect can be achieved within the laser cavity, which may provide joint contribution for passive mode-locking operation. By finely adjusting the polarization to alter the strength of NPR-based saturable absorption, the TDFL either approaches the operation regime of scalar soliton with strong NPR effect, or generates polarization rotation locked vector soliton (PRLVS) with weak NPR effect. The scalar solitons and PRLVSs possess 3-dB optical spectrum bandwidth of 2.2 nm and 2 nm, pulse-width of 1.8 ps and 2 ps, respectively. Moreover, the PRLVSs demonstrate a typical energy exchange between two polarized components on optical spectra and a period-doubling feature in time domain. Such operation principle can also be used in 1550 nm band fiber lasers and other nonlinear systems.

Scalar-vector soliton fiber laser mode-locked by nonlinear polarization rotation

We report a passively mode-locked fiber laser by nonlinear polarization rotation (NPR), where both vector and scalar soliton can co-exist within the laser cavity. The mode-locked pulse evolves as a vector soliton in the strong birefringent segment and is transformed into a regular scalar soliton after the polarizer within the laser cavity. The existence of solutions in a polarization-dependent cavity comprising a periodic combination of two distinct nonlinear waves is first demonstrated and likely to be applicable to various other nonlinear systems. For very large local birefringence, our laser approaches the operation regime of vector soliton lasers, while it approaches scalar soliton fiber lasers under the condition of very small birefringence.

High-damage-resistant tungsten disulfide saturable absorber mirror for passively Q-switched fiber laser

In this paper, we demonstrate a high-damage-resistant tungsten disulfide saturable absorber mirror (WS₂-SAM) fabricated by magnetron sputtering technique. The WS₂-SAM has an all-fiber-integrated configuration and high-damage-resistant merit because the WS₂ layer is protected by gold film so as to avoid being oxidized and destroyed at high pump power. Employing the WS₂-SAM in an Erbium-doped fiber laser (EDFL) with linear cavity, the stable Q-switching operation is achieved at central wavelength of 1560 nm, with the repetition rates ranging from 29.5 kHz to 367.8 kHz and the pulse duration ranging from 1.269 μs to 154.9 ns. For the condition of the maximum pump power of 600 mW, the WS₂-SAM still works stably with an output power of 25.2 mW, pulse energy of 68.5 nJ, and signal-noise-ratio of 42 dB. The proposed WS₂-SAM configuration provides a promising solution for advanced pulsed fiber lasers with the characteristics of high damage resistance, high output energy, and wide tunable frequency.

Femtosecond harmonic mode-locking of a fiber laser at 3.27 GHz using a bulk-like, MoSe2-based saturable absorber

We experimentally demonstrate the use of a bulk-like, MoSe₂-based saturable absorber (SA) as a passive harmonic mode-locker for the production of femtosecond pulses from a fiber laser at a repetition rate of 3.27 GHz. By incorporating a bulk-like, MoSe₂/PVA-composite-deposited side-polished fiber as an SA within an erbium-doped-fiber-ring cavity, mode-locked pulses with a temporal width of 737 fs to 798 fs can be readily obtained at various harmonic frequencies. The fundamental resonance frequency and the maximum harmonic-resonance frequency are 15.38 MHz and 3.27 GHz (212th harmonic), respectively. The temporal and spectral characteristics of the output pulses are systematically investigated as a function of the pump power. The output pulses exhibited Gaussian-temporal shapes irrespective of the harmonic order, and even when their spectra possessed hyperbolic-secant shapes. The saturable absorption and harmonic-mode-locking performance of our prepared SA are compared with those of previously demonstrated SAs that are based on other transition metal dichalcogenides (TMDs). To the best of the authors' knowledge, the repetition rate of 3.27 GHz is the highest frequency that has ever been demonstrated regarding the production of femtosecond pulses from a fiber laser that is based on SA-induced passive harmonic mode-locking.

Direct synthesis of large-scale hierarchical MoS2 films nanostructured with orthogonally oriented vertically and horizontally aligned layers

Hierarchical MoS2 thin films nanostructured with orthogonally oriented vertically and horizontally aligned layers were designed and excellent passive Q-switching behavior in a fiber laser was demonstrated. A special solvothermal system containing a small amount of water was applied to synthesize such hierarchical MoS2 nanofilms, in which the reaction rate is carefully controlled by the diffusion rate of the sulfur precursor. Wafer-scale MoS2 thin films with hierarchical structures are formed on various substrates. Moreover, the hierarchical MoS2 thin films consisting of both vertical and horizontal layers can be tuned to possess only horizontally aligned layers by controlling the solvothermal time. To show the potential application proof-of-concept, the nonlinear optical performance of the hierarchical MoS2 was investigated. Superior passive Q-switching behavior in a fiber laser with a minimum pulse width of 2.2 μs was observed.

300-MHz-repetition-rate, all-fiber, femtosecond laser mode-locked by planar lightwave circuit-based saturable absorber

We show the implementation of fiber-pigtailed, evanescent-field-interacting, single-walled carbon nanotube (CNT)-based saturable absorbers (SAs) using standard planar lightwave circuit (PLC) fabrication processes. The implemented PLC-CNT-SA device is employed to realize self-starting, high-repetition-rate, all-fiber ring oscillators at telecommunication wavelength. We demonstrate all-fiber Er ring lasers operating at 303-MHz (soliton regime) and 274-MHz (stretched-pulse regime) repetition-rates. The 303-MHz (274-MHz) laser centered at 1555 nm (1550 nm) provides 7.5 nm (19 nm) spectral bandwidth. After extra-cavity amplilfication, the amplified pulse train of the 303-MHz (274-MHz) laser delivers 209 fs (178 fs) pulses. To our knowledge, this corresponds to the highest repetition-rates achieved for femtosecond lasers employing evanescent-field-interacting SAs. The demonstrated SA fabrication method, which is based on well-established PLC processes, also shows a potential way for mass-producible and lower-cost waveguide-type SA devices suitable for all-fiber and waveguide lasers.

Optical Limiting and Theoretical Modelling of Layered Transition Metal Dichalcogenide Nanosheets

Nonlinear optical property of transition metal dichalcogenide (TMDC) nanosheet dispersions, including MoS₂, MoSe₂, WS₂, and WSe₂, was performed by using Z-scan technique with ns pulsed laser at 1064 nm and 532 nm. The results demonstrate that the TMDC dispersions exhibit significant optical limiting response at 1064 nm due to nonlinear scattering, in contrast to the combined effect of both saturable absorption and nonlinear scattering at 532 nm. Selenium compounds show better optical limiting performance than that of the sulfides in the near infrared. A liquid dispersion system based theoretical modelling is proposed to estimate the number density of the nanosheet dispersions, the relationship between incident laser fluence and the size of the laser generated micro-bubbles, and hence the Mie scattering-induced broadband optical limiting behavior in the TMDC dispersions.

Microfiber-based few-layer black phosphorus saturable absorber for ultra-fast fiber laser

Few-layer black phosphorus (BP), as the most alluring graphene analogue owing to its similar structure as graphene and thickness dependent direct band-gap, has now triggered a new wave of research on two-dimensional (2D) materials based photonics and optoelectronics. However, a major obstacle of practical applications for few-layer BPs comes from their instabilities of laser-induced optical damage. Herein, we demonstrate that, few-layer BPs, which was fabricated through the liquid exfoliation approach, can be developed as a new and practical saturable absorber (SA) by depositing few-layer BPs with microfiber. The saturable absorption property of few-layer BPs had been verified through an open-aperture z-scan measurement at the telecommunication band. The microfiber-based BP device had been found to show a saturable average power of ~4.5 mW and a modulation depth of 10.9%, which is further confirmed through a balanced twin detection measurement. By integrating this optical SA device into an erbium-doped fiber laser, it was found that it can deliver the mode-locked pulse with duration down to 940 fs with central wavelength tunable from 1532 nm to 1570 nm. The prevention of BP from oxidation through the "lateral interaction scheme" owing to this microfiber-based few-layer BP SA device might partially mitigate the optical damage problem of BP. Our results not only demonstrate that black phosphorus might be another promising SA material for ultrafast photonics, but also provide a practical solution to solve the optical damage problem of black phosphorus by assembling with waveguide structures such as microfiber.

Dual-state dissipative solitons from an all-normal-dispersion erbium-doped fiber laser: continuous wavelength tuning and multi-wavelength emission

We propose and experimentally demonstrate switchable operation of dissipative solitons (DSs) in an erbium-doped fiber laser. By managing normal dispersion of laser cavity, the 3-dB spectral bandwidth up to 8.1 nm can be obtained with the help of a semiconductor saturable absorber mirror. Using an inline polarizer, the fiber laser can be separately operated at either wavelength-tunable or multi-wavelength emission. The central wavelength of DS can be continuously tuned from 1554 to 1561 nm with its spectra maintaining standard rectangular shape. Alternatively, triple-wavelength DSs at 1535, 1544, and 1553 nm can be simultaneously obtained.