Passively mode-locked fiber laser by a cell-type WS2 nanosheets saturable absorber

Wavelength-tunable broadband lasers based on nanomaterials

Nanomaterials are widely used in the fields of sensors, optoelectronics, biophotonics and ultrafast photonics due to their excellent mechanical, thermal, optical, electrical and magnetic properties. Particularly, owing to their nonlinear optical properties, fast response time and broadband operation, nanomaterials are ideal saturable absorption materials in ultrafast photonics, which contribute to the improvement of laser performance. Therefore, nanomaterials are of great importance to applications in wavelength-tunable broadband pulsed lasers. Herein, we review the integration and applications of nanomaterials in wavelength-tunable broadband ultrafast photonics. Firstly, the two integration methods, which are direct coupling and evanescent field coupling, and their characteristics are introduced. Secondly, the applications of nanomaterials in wavelength-tunable broadband lasers are summarized. Finally, the development of nanomaterials and broadband tunable lasers is reviewed and discussed.

Frontier and Hot Topics of Pulsed Fiber Lasers via CiteSpace Scientometric Analysis: Passively Mode-Locked Fiber Lasers with Real Saturable Absorbers Based on Two-Dimensional Materials

Pulsed fiber lasers, with high peak power and narrow pulse widths, have been proven to be an important tool for a variety of fields of application. In this work, frontier and hot topics in pulsed fiber lasers were analyzed with 11,064 articles. Benefitting from the scientometric analysis capabilities of CiteSpace, the analysis found that passively mode-locked fiber lasers with saturable absorbers (SAs) based on two-dimensional (2D) materials have become a hot research topic in the field of pulsed fiber lasers due to the advantages of self-starting operation, high stability, and good compatibility. The excellent nonlinear optical properties exhibited by 2D materials at nanometer-scale thicknesses have become a particularly popular research topic; the research has paved the way for exploring its wider applications. We summarize the performance of several typical 2D materials in ultrafast fiber lasers, such as graphene, topological insulators (TIs), transition metal dichalcogenides (TMDs), and black phosphorus (BP). Meanwhile, we review and analyze the direction of the development of 2D SAs for ultrafast fiber lasers.

Nanosecond Q-switched laser with PEDOT: PSS saturable absorber

We demonstrate deployment of the nonlinear saturable absorption property of the organic material poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) for pulse generation in the near-infrared region. The saturable absorber (SA) film was made using a straightforward process of depositing a layer of the PEDOT: PSS material onto a polyvinyl alcohol (PVA) film. The prepared SA was inserted into an erbium-doped fiber laser cavity as a Q-switcher to produce laser pulses with a maximum pulse rate of 92.75 kHz, minimum pulse duration of 912 ns, and highest pulse energy of 222.83 nJ. Results showed that PEDOT: PSS/PVA SA could become a promising SA for various laser applications. To our knowledge, this is the first time that PEDOT: PSS/PVA has been utilized as a SA to produce a stable Q-switched laser in 1.55 µm.

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.

Passively Mode-Locked Er-Doped Fiber Laser Based on Sb2S3-PVA Saturable Absorber

In this paper, antimony trisulfide (Sb₂S₃) was successfully prepared with the liquid phase exfoliation method and embedded into polyvinyl alcohol (PVA) as a saturable absorber (SA) in a passively mode-locked Er-doped fiber laser for the first time. Based on Sb₂S₃-PVA SA with a modulation depth of 4.0% and a saturable intensity of 1.545 GW/cm², a maximum average output power of 3.04 mW and maximum peak power of 325.6 W for the stable mode-locked pulses was achieved with slope a efficiency of 0.87% and maximum single pulse energy of 0.81 nJ at a repetition rate of 3.47 MHz under a pump power of 369 mW. A minimum pulse width value of 2.4 ps with a variation range less than 0.1 ps, and a maximum signal to noise ratio (SNR) of 54.3 dB indicated reliable stability of mode-locking, revealing promising potentials of Sb₂S₃ as a saturable absorber in ultrafast all-fiber lasers.

Nonlinear optical property and application of yttrium oxide in erbium-doped fiber lasers

Yttrium oxide (Y₂O₃) has garnered some attention in view of its potential to be integrated into a wide range of high-strength structural components, microelectronic and optoelectronic devices. However, the nonlinear optical research of this promising material has not been implemented yet. In this paper, not only the electronic band structures of Y₂O₃ are theoretically calculated but also the optical nonlinearity of Y₂O₃ is validated by using the fiber laser as a platform. Meanwhile, the influence of sample thickness on laser performance is further explored by using Y₂O₃ saturable absorbers with different thickness. Results indicate that Y₂O₃ not only has impressive optical nonlinearity but also is beneficial to the investigation of ultrafast photons by adjusting the thickness of Y₂O₃. Therefore, Y₂O₃ can be used as a potential saturable absorber candidate for in-depth research and application.

Generation of High Peak Power Mode-Locked Green Pulses Based on WS2 and EOM: Experiment and Theory

Based on an as-prepared high-quality WS₂ film and an electro-optic modulator (EOM), a dual-loss-modulated low repetition rate mode-locking laser at 0.53 μm with high peak power is presented for the first time. The laser characteristics versus the pump power are investigated experimentally and theoretically. At a pump power of 10.67 W, the shortest pulse duration of 305 ps can be measured, corresponding to the highest peak power of 931 kW, which is much higher than those of the single passive modulated lasers with WS₂-SA. A simple rate equation simulation was used to describe this dual-loss-modulated mode-locking green laser based on WS₂ and EOM. The results of the numerical simulation are basically in accordance with the experimental values.

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.

Soliton mode-locked pulse generation with a bulk structured MXene Ti3AlC2 deposited onto a D-shaped fiber

We propose a bulk structured MXene, Ti₃AlC₂ deposited onto D-shaped fiber for soliton generation in an erbium-doped fiber laser (EDFL) cavity. Our saturable absorber (SA) device, based on MAX phase, was prepared by using stirring and ultrasonic vibration, which offer easier sample preparation compared with its 2D counterparts. By means of the polishing wheel technique, we fabricated a D-shaped fiber with a controlled polishing depth and incorporated the MAX phase Ti₃AlC₂ solution onto its polishing region. We obtained a mode-locked soliton pulse with the proposed MAX phase D-shaped (MAX-DS) SA in EDFL cavity. The pulse width, repetition rate, and central wavelength of the pulse train are 2.21 ps, 1.89 MHz, and 1557.63 nm, respectively. The polarization-insensitive EDFL cavity initiated a soliton operation with superior stability as the pump power tuned from 21 to 131 mW; further, the ML laser exhibits an average power of 15.3 mW, peak power of 3.8 kW, and pump efficiency of 12.5%. The MAX-DS SA incorporated inside the EDFL reveals efficient output performance, with a pulse energy of 8.14 nJ, the highest ever reported, to our best knowledge, among D-shaped fiber-based SA.

Nonlinear optical characteristics of ZrSe2 and its application for designing multi-wavelength mode-locked operations

In our work, a ZrSe₂-polyvinyl alcohol film-type saturable absorber (SA) with a modulation depth of 4.99% and a saturable intensity of 12.42MW/cm² was successfully prepared and employed in mode-locked Er-doped fiber laser. The fiber laser can generate stable multi-wavelength mode-locked operations with a threshold power of 224 mW and a maximum average output power of 3.272 mW at the repetition rate of 3.38 MHz for the first time, to the best of our knowledge. Our experimental results fully prove that ZrSe₂ nanosheets were efficient SA candidates for demonstrating multi-wavelength mode-locked operation fiber lasers due to their tunable absorption peak and excellent saturable absorption properties.

Fiber all-optical light control with low-dimensional materials (LDMs): thermo-optic effect and saturable absorption

In this paper, we review the recent studies on all-optical light control based on two main nonlinear mechanisms in LDMs: the thermo-optic effect and saturable absorption. The compactness of LDMs makes them the ideal medium for all-optical control systems. Many all-optical devices are demonstrated based on the properties of thermo-optic effects and saturable absorption. The materials characteristics and fabrication and the future prospects for all-optical control will also be shown.

Output energy enhancement in a mode-locked Er-doped fiber laser using CVD-Bi2Se3 as a saturable absorber

In this study, the output energy in topological insulators (TIs)-based Erbium-doped fiber laser (EDFL) was improved using two strategies: bidirectional pumped laser cavity and saturable absorber (SA) with high damage threshold and large modulation depth. Using the chemical vapor deposition (CVD) method, Bismuth Selenide (Bi₂Se₃) film was synthesized and improved to a SA. Employing this CVD-Bi₂Se₃ SA in an EDFL, bright and bright-dark soliton operations were achieved. The average output power/pulse energy was 82.6 mW/48.3 nJ and 81.2 mW/47.5 nJ, respectively. The results demonstrate that CVD-Bi₂Se₃ can act as an excellent performance material to improve output power performance in TISA-based EDFL.

Noise-like mode-locked Yb-doped fiber laser in a linear cavity based on SnS2 nanosheets as a saturable absorber

In this study, a high-energy noise-like mode-locked Yb-doped fiber laser in a linear cavity was achieved with SnS₂-polyvinyl alcohol film as the saturable absorber. In addition, the nonlinear saturable absorption characteristics of the SnS₂ were investigated experimentally. The saturation intensity and modulation depth were about 6.01  MW/cm² and 8.68%, respectively. Under pump power of 422 mW, stable noise-like mode-locked operation with a maximum output power and largest single pulse energy of 9.50 mW and 18.1 nJ, respectively, was obtained. To the best of our knowledge, this study is the first to observe and experimentally investigate noise-like operation in a linear laser cavity. Our study may provide some valuable design guidelines for noise-like operation and create new directions for advanced photonic devices based on SnS₂.

Wavelength-tunable passively mode-locked all-fiber laser at 1.5 μm

We have experimentally demonstrated a wavelength-tunable passively mode-locked all-fiber laser at 1.5 μm wavelength by using an erbium-doped fiber amplifier, a fiber-pigtailed semiconductor saturable absorber mirror, and a tunable birefringence Sagnac filter. In our work, by properly setting the polarization state of the propagating light in the birefringence Sagnac filter, the mode-locked lasing wavelength can be continuously tuned from 1544.1 to 1560.8 nm, corresponding to a tuning range of 16.7 nm. At a central wavelength of 1548.5 nm, the fiber laser delivers pulses as short as 713.2 fs with a repetition rate of 4.65 MHz, a 3 dB bandwidth of 5.7 nm, and an average output power of 4.86 mW. Our results show that such a mode-locked all-fiber laser has great potential in applications in nonlinear optics at the 1.5 μm band.

Fe3O4 nanoparticles as a saturable absorber for giant chirped pulse generation

Fe₃O₄ nanoparticles (FONPs) are magnetic materials with a small band gap and have well-demonstrated applications in ultrafast photonics, medical science, magnetic detection, and electronics. Very recently, FONPs were proposed as an ideal candidate for pulse generation in fiber-based oscillators. However, the pulses obtained to date are on the order of microseconds, which is too long for real application in communication. Here, we report the use of FONPs synthesized by a sol-hydrothermal method and used as a saturable absorber (SA) to achieve nanosecond pulses in an erbium-doped fiber laser (EDFL) for the first time. The proposed fiber laser is demonstrated to have a narrow spectral width of around 0.8 nm and a fixed fundamental repetition rate (RPR) of 4.63 MHz, whose spectra and pulse dynamics are different from the mode-locked lasers reported previously. It is demonstrated that the proposed fiber laser based on a FONP SA operates in the giant-chirp mode-locked regime. The most important result is the demonstration of a pulse duration of 55 ns at an output power of 16.2 mW, which is the shortest pulse based on FONPs for EDFLs reported to date. Our results demonstrate that the FONP dispersion allows for an excellent photonic material for application in ultrafast photonics devices, photoconductive detectors, and optical modulators.

Versatile Mode-Locked Operations in an Er-Doped Fiber Laser with a Film-Type Indium Tin Oxide Saturable Absorber

We demonstrate the generation of versatile mode-locked operations in an Er-doped fiber laser with an indium tin oxide (ITO) saturable absorber (SA). As an epsilon-near-zero material, ITO has been only used to fashion a mode-locked fiber laser as an ITO nanoparticle-polyvinyl alcohol SA. However, this type of SA cannot work at high power or ensure that the SA materials can be transmitted by the light. Thus, we covered the end face of a fiber with a uniform ITO film using the radio frequency magnetron sputtering technology to fabricate a novel ITO SA. Using this new type of SA, single-wavelength pulses, dual-wavelength pulses, and triple-wavelength multi-pulses were achieved easily. The pulse durations of these mode-locked operations were 1.67, 6.91, and 1 ns, respectively. At the dual-wavelength mode-locked state, the fiber laser could achieve an output power of 2.91 mW and a pulse energy of 1.48 nJ. This study reveals that such a proposed film-type ITO SA has excellent nonlinear absorption properties, which can promote the application of ITO film for ultrafast photonics.

Optical properties and applications of molybdenum disulfide/SiO2 saturable absorber fabricated by sol-gel technique

We investigate a new type of molybdenum disulfide (MoS₂)-doped sol-gel glass saturable absorber (SA) fabricated by sol-gel technique. The reagents used for the sol-gel glass contain Tetraethyl orthosilicate (TEOS), ethanol, water, and hydrochloric acid. Different from the traditional ways of fabricating SAs, the MoS₂ in our method is encapsulated by inorganic sol-gel glass instead of polymer compound with low laser damage resistance, which greatly increases the optical damage threshold of MoS₂ SA. The MoS₂-doped sol-gel glass as an SA is experimentally demonstrated in a passively mode-locked ytterbium-doped fiber laser (YDFL). Stable mode-locked pulse trains are successfully generated in the normal dispersion regime with a pulse width of 13.8 ps and the average output power of 34.6 mW. The fluctuation of the central wavelength and spectral bandwidth is as low as 0.9% in one week, which indicates that the mode-locking state has good environmental stability. To the best of our knowledge, it is the first example of sol-gel glass SA for ultrafast pulses generated in YDFL, which potentially gives a new approach to improve optical damage threshold and long-term working stability for broadband absorbers.

Emerging 2D materials beyond graphene for ultrashort pulse generation in fiber lasers

Ultrafast fiber lasers have significant applications in ultra-precision manufacturing, medical diagnostics, medical treatment, precision measurement and astronomical detection, owing to their ultra-short pulse width and ultra-high peak-power. Since graphene was first explored as an optical saturable absorber for passively mode-locked lasers in 2009, many other 2D materials beyond graphene, including phosphorene, antimonene, bismuthene, transition metal dichalcogenides (TMDs), topological insulators (TIs), metal-organic frameworks (MOFs) and MXenes, have been successively explored, resulting in rapid development of novel 2D materials-based saturable absorbers. Herein, we review the latest progress of the emerging 2D materials beyond graphene for passively mode-locked fiber laser application. These 2D materials are classified into mono-elemental, dual-elemental and multi-elemental 2D materials. The atomic structure, band structure, nonlinear optical properties, and preparation methods of 2D materials are summarized. Diverse integration strategies for applying 2D materials into fiber laser systems are introduced, and the mode-locking performance of the 2D materials-based fiber lasers working at 1-3 μm are discussed. Finally, the perspectives and challenges facing 2D materials-based mode-locked fiber lasers are highlighted.

2D bismuthene fabricated via acid-intercalated exfoliation showing strong nonlinear near-infrared responses for mode-locking lasers

The rediscovery of black phosphorus (BP) has expanded the 2D family into Group 15 (Nitrogen Group) elements, among which bismuthene is the latest member with extraordinary opto-electronic, catalytic and biocompatible properties and potential as a 2D topological insulator. However, bulk Bi is not easily mechanically exfoliated as its counterpart of BP. Thus, to date, the reports on 2D Bi fabrication are rare, and investigations on its nonlinear optical properties are even less. Herein, we rationally designed a new strategy combining acid-interaction and liquid exfoliation to successfully transform metal bulk Bi into few-layer semiconductor, which resulted in unseen opto-electronic properties, such as tunable nonlinear responses all the way to the near-infrared (NIR) region. This band is critical for telecommunication and military purposes, but currently, functioning materials are extremely scarce. The origin of this strong saturable absorption was thoroughly explored through time-resolved spectroscopy spanning from the fs to μs timescale, which indicated ultrafast fs to ps carrier dynamics in the early stage and long exciton bleaching recovery up to μs. As a proof-of-concept application, the as-prepared 2D Bi was employed as a saturable absorber to mode-lock a Tm-doped fiber laser and successfully realized a 2 μm NIR-wavelength output. This study not only offers an effective and scalable method to fabricate the new 2D family member bismuthene with extraordinary stability, but also explores its strong and broad nonlinear responses extending into the NIR region and fundamental photoinduced dynamics, which demonstrate the full potential of 2D Bi for application in opto-electronic devices and nonlinear optics.

Various large-energy soliton operations within an Er-doped fiber laser with bismuth selenide as a saturable absorber

Different large-energy mode-locked operations were successfully obtained within a Bi₂Se₃-based Er-doped fiber laser. First, mode-locked operation with maximum pulse energy of 17.2 nJ and pulse width of 187 ns under a pulse repetition rate of 537.6 kHz was obtained under the pump power of 680 mW. In addition, the characteristics of dark solitons and soliton rains, which also exhibit large pulse energies, have been investigated experimentally. Our results fully proved that Bi₂Se₃ was an excellent candidate for investigating various mode-locked operations with large pulse energy due to its high nonlinear effect and high damage threshold.

Sensitive, reproducible, and stable 3D plasmonic hybrids with bilayer WS2 as nanospacer for SERS analysis

The highly enhanced local electromagnetic field occurring through nanometer gap between the plamonic nanostructures provides the dominant contribution in surface enhancement Raman scattering (SERS) enhancement. Thence, we designed the remarkable SERS platform (AuNPs/WS₂@AuNPs hybrids) by introducing bilayer WS₂ film as the precise nanospacer. Bilayer WS₂ film can realize the facile and tight combination with AuNPs via the thermal decomposition approach. Dense three-dimension (3D) hot spots provided by this hybrid plasmonic nanostructures are responsible for the extremely satisfying SERS performances. Using rhodamine 6G (R6G) as the probe molecules, the AuNPs/WS₂@AuNPs hybrids perform the excellent sensitivity with the minimum detectable concentration as low as 10^-11 M. Uniform and reproducible SERS signals illustrate that the synthesized SERS hybrids perform the splendid spot-to-spot reproducibility (RSD~5.4%) and substrate-to-substrate reproducibility (RSD~5.7%). The stability of AuNPs and the protection of WS₂ film endow this hybrid plasmonic nanostructures with the brilliant anti-oxidation stability. Moreover, the enhanced electric field distribution simulated with the COMSOL software proves the remarkable SERS performance in theory. Therefore, AuNPs/WS₂@AuNPs substrate not only widens the SERS research filed of WS₂, but also shows vast potential as excellent SERS sensor for practical applicability.

Dual-wavelength, mode-locked erbium-doped fiber laser employing a graphene/polymethyl-methacrylate saturable absorber

Mode-locked fiber laser incorporating a saturable absorber is an attractive configuration due to its stability and simple structure. In this work, we demonstrate a dual-wavelength passively mode-locked erbium-doped fiber laser employing a graphene/polymethyl-methacrylate saturable absorber. A laser resonator is developed based on dual cavity architecture with unidirectional signal oscillation, which is connected by a fiber branch sharing a common gain medium and saturable absorber. Dual wavelength mode-locked fiber lasers are observed at approximately 1530 and 1560 nm with 22.6 mW pump power threshold. Soliton pulse circulates in the laser cavity with pulse duration of 900 and 940 fs at shorter and longer wavelengths, respectively. This work presents a viable option in developing a low threshold mode-locked laser source with closely spaced dual wavelength femtosecond pulses in the C-band wavelength region.

Tungsten diselenide for mode-locked erbium-doped fiber lasers with short pulse duration

In this paper, a WSe₂ film prepared by chemical vapor deposition (CVD) is transferred onto a tapered fiber, and a WSe₂ saturable absorber (SA) is fabricated. In order to measure the third-order optical nonlinearity of the WSe₂, the Z-scan technique is applied. The modulation depth of the WSe₂ SA is measured as being 21.89%. Taking advantage of the remarkable nonlinear absorption characteristic of the WSe₂ SA, a mode-locked erbium-doped fiber laser is demonstrated at 1557.4 nm with a bandwidth of 25.8 nm and signal to noise ratio of 96 dB. To the best of our knowledge, the pulse duration of 163.5 fs is confirmed to be the shortest compared with previous mode-locked fiber lasers based on transition-metal dichalcogenides SAs. These results indicate that WSe₂ is a powerful competitor in the application of ultrashort pulse lasers.

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.

Mode-locked Er-doped fiber laser based on PbS/CdS core/shell quantum dots as saturable absorber

Previously, PbS/CdS core/shell quantum dots with excellent optical properties have been widely used as light-harvesting materials in solar cell and biomarkers in bio-medicine. However, the nonlinear absorption characteristics of PbS/CdS core/shell quantum dots have been rarely investigated. In this work, PbS/CdS core/shell quantum dots were successfully employed as nonlinear saturable absorber (SA) for demonstrating a mode-locked Er-doped fiber laser. Based on a film-type SA, which was prepared by incorporating the quantum dots with the polyvinyl alcohol (PVA), mode-locked Er-doped operation with a pulse width of 54 ps and a maximum average output power of 2.71 mW at the repetition rate of 3.302 MHz was obtained. Our long-time stable results indicate that the CdS shell can effectively protect the PbS core from the effect of photo-oxidation and PbS/CdS core/shell quantum dots were efficient SA candidates for demonstrating pulse fiber lasers due to its tunable absorption peak and excellent saturable absorption properties.

Large-area and highly crystalline MoSe2 for optical modulator

Transition metal dichalcogenides (TMDs) have been successfully used as broadband optical modulator materials for pulsed fiber laser systems. However, the nonlinear optical absorptions of exfoliated TMDs are strongly limited by their nanoflakes morphology with uncontrollable lateral size and thickness. In this work, we provide an effective method to fully explore the nonlinear optical properties of MoSe₂. Large-area and high quality lattice MoSe₂ grown by chemical vapor deposition method was adopted as an optical modulator for the first time. The large-area MoSe₂ shows excellent nonlinear optical absorption with a large modulation depth of 21.7% and small saturable intensity of 9.4 MW cm^-2. After incorporating the MoSe₂ optical modulator into fiber laser cavity as a saturable absorber, a highly stable Q-switching operation with single pulse energy of 224 nJ is achieved. The large-area MoSe₂ possessing superior nonlinear optical properties compared to exfoliated nanoflakes affords possibility for the larger-area two-dimensional materials family as high performance optical devices.

D-shaped photonic crystal fiber refractive index sensor based on surface plasmon resonance

A type of D-shaped photonic crystal fiber sensor based on surface plasmon resonance (SPR) is proposed for refractive index sensing and analyzed by the finite element method. The SPR effect between surface plasmon polariton modes and fiber core modes of the designed D-shaped photonic crystal fiber is used to measure the refractive index of the analyte. Numerical results show that the sensor can detect a range of refractive index ranging from 1.33 to 1.38. When the thickness of metal film is t=20  nm, the maximum sensitivity of 10,493  nm/RIU is obtained with a very high resolution of 9.53×10^-6  RIU. The good sensing performance makes the proposed sensor a competitive candidate for environmental, biological, and biochemical sensing applications.

Enhanced nonlinear optical response of Se-doped MoS2 nanosheets for passively Q-switched fiber laser application

An enhanced nonlinear optical (NLO) performance was observed in Se-doped MoS₂ nanosheets synthesized through a facile annealing process. Se-doped MoS₂ nanosheets with a large saturable intensity and high modulation depth generated stable passively Q-switched fiber laser pulses at 1559 nm. In comparison with the Q-switched fiber laser utilizing the pristine MoS₂ nanosheets as a saturable absorber, the passive Q-switching operation based on Se-doped MoS₂ nanosheets could be conducted at a lower threshold power of 50 mW, a wider range of repetition rates from 28.97 to 130 kHz, and a higher SNR of 56 dB. More importantly, the shortest pulse duration of 1.502 μs was realized and the output power and pulse energy reached 17.2 mW and 133.07 nJ, respectively. These results indicate that tailoring the chemical composition of optical nanomaterials by introducing a dopant is a feasible method of improving the NLO response of the MoS₂ nanosheets and realizing excellent ultrafast pulse generation.

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.

State distributions in two-dimensional parameter spaces of a nonlinear optical loop mirror-based, mode-locked, all-normal-dispersion fiber laser

We present the results of numerical simulations of dissipative soliton generation using nonlinear Schrödinger equations in an all-normal-dispersion (ANDi) mode-locked fiber laser based on a nonlinear optical loop mirror (NOLM). Firstly, systematic and computationally intensive analysis of the pulse state distributions in two-dimensional parameter spaces of an ANDi fiber laser was conducted. In addition, we determined that unstable non-vanishing regions including pulsation and noise-like pulses are directly related to the saturable absorptions of NOLMs and that two critical filter bandwidths separate those regions from stable ones. Finally, we found that the multi-pulsing power threshold can be maximized by using an optimal optical filter bandwidth.

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.

MoS2-clad microfibre laser delivering conventional, dispersion-managed and dissipative solitons

Molybdenum disulfide (MoS₂), whose monolayer possesses a direct band gap, displays promising applications in optoelectronics, photonics, and lasers. Recent researches have demonstrated that MoS₂ has not only a significant broadband saturable absorption performance, but also a higher optical nonlinear response than graphene. However, MoS₂ shows much lower optical damage threshold owing to the poorer thermal conductivity and mechanical property. Here, we exploit a MoS₂-clad microfibre (MCM) as the saturable absorber (SA) for the generation of ultrashort pulses under different dispersion conditions. The improved evanescent field interaction scheme can overcome the laser-induced thermal damage, as well as take full advantage of the strong nonlinear effect of MoS₂. With the MCM SA, conventional, dispersion-managed, and dissipative solitons are generated around 1600 nm in Er-doped fibre lasers with anomalous, near-zero, and normal cavity dispersions, respectively. Our work paves the way for applications of 2D layered materials in photonics, especially in laser sources.

Black phosphorus: a two-dimension saturable absorption material for mid-infrared Q-switched and mode-locked fiber lasers

Black phosphorus (BP) as a novel class of two-dimension (2D) materials has recently attracted enormous attention as a result of its unique physical and chemical features. The remarkably strong light-matter interaction and tunable direct band-gap at a wide range make it an ideal candidate especially in the mid-infrared wavelength region as the saturable absorber (SA). In this paper, the simple and effective liquid phase exfoliation (LPE) method was used to fabricate BP. By introducing the same BP SA into two specifically designed rare earth ions doped fluoride fiber lasers at mid-infrared wavebands, Q-switching with the pulse energy of 4.93 μJ and mode-locking with the pulse duration of 8.6 ps were obtained, respectively. The operation wavelength of ~2970 nm for generated pulse is the reported longest wavelength for BP SA based fiber lasers.

Stable Similariton Generation in an All-Fiber Hybrid Mode-Locked Ring Laser for Frequency Metrology

Ultrashort pulse lasers constitute an important tool in the emerging field of optical frequency metrology and are enabling unprecedented measurement capabilities and new applications in a wide range of fields, including precision spectroscopy, atomic clocks, ultracold gases, and molecular fingerprinting. We demonstrate the generation of stable 127-fs self-similar pulses at a central wavelength of 1560 nm with 7.14-mW average output power. Similariton lasers have a low repetition rate deviation in the averaging time interval [Formula: see text], a low relative intensity noise [Formula: see text] (30 Hz to 10 kHz), a narrow single comb line width of 32 kHz, and high reliability. Thus, such lasers are highly promising for further development of the stabilized combs and open up a robust and substantially simplified route to synthesizing low-noise microwaves.

Diversified pulse generation from frequency shifted feedback Tm-doped fibre lasers

Pulsed fibre lasers operating in the eye-safe 2 μm spectral region have numerous potential applications in areas such as remote sensing, medicine, mid-infrared frequency conversion, and free-space communication. Here, for the first time, we demonstrate versatile 2 μm ps-ns pulses generation from Tm-based fibre lasers based on frequency shifted feedback and provide a comprehensive report of their special behaviors. The lasers are featured with elegant construction and the unparalleled capacity of generating versatile pulses. The self-starting mode-locking is initiated by an intra-cavity acousto-optical frequency shifter. Diversified mode-locked pulse dynamics were observed by altering the pump power, intra-cavity polarization state and cavity structure, including as short as 8 ps single pulse sequence, pulse bundle state and up to 12 nJ, 3 ns nanosecond rectangular pulse. A reflective nonlinear optical loop mirror was introduced to successfully shorten the pulses from 24 ps to 8 ps. Beside the mode-locking operation, flexible Q-switching and Q-switched mode-locking operation can also be readily achieved in the same cavity. Up to 78 μJ high energy nanosecond pulse can be generated in this regime. Several intriguing pulse dynamics are characterized and discussed.

Passively Q-switched erbium-doped fiber laser at C-band region based on WS₂ saturable absorber

We demonstrate a Q-switched erbium-doped fiber laser using tungsten disulfide (WS₂) as a saturable absorber. The WS₂ is deposited onto fiber ferrules using a drop-casting method. Passive Q-switched pulses operating in the C-band region with a central wavelength of 1560.7 nm are successfully generated by a tunable pulse repetition rate ranging from 27.2 to 84.8 kHz when pump power is increased from 40 to 220 mW. At the same time, the pulse width decreases from a maximum value of 3.84 μs to a minimum value of 1.44 μs. The signal-to-noise ratio gives a stable value of 43.7 dB. The modulation depth and saturation intensity are measured to be 0.99% and 36.2  MW/cm², respectively.

Black phosphorus crystal as a saturable absorber for both a Q-switched and mode-locked erbium-doped fiber laser

We report a simple way to generate Q-switched and mode-locked pulses by incorporating black phosphorus (BP) as a saturable absorber (SA) in an erbium-doped fiber laser (EDFL) cavity.

High-energy, sub-100 fs, all-fiber stretched-pulse mode-locked Er-doped ring laser with a highly-nonlinear resonator

We report on ultra-short stretched pulse generation in an all-fiber erbium-doped ring laser with a highly-nonlinear germanosilicate fiber inside the resonator with a slightly positive net-cavity group velocity dispersion (GVD). Stable 84 fs pulses were obtained with a 12 MHz repetition rate at a central wavelength of 1560 nm with a 48.1 nm spectral pulse width (full width at half maximum, FWHM) and 30 mW average output power; this corresponds to the 29.7 kW maximum peak power and 2.5 nJ pulse energy obtained immediately from the oscillator.

Investigation on pulse shaping in fiber laser hybrid mode-locked by Sb(2)Te(3) saturable absorber

We report a study on a hybrid mode-locked fiber laser with two saturable absorbers: slow and fast, integrated in a single device. Amorphous antimony telluride (Sb(2)Te(3)) layer was deposited on side-polished fiber to form the slow saturable absorber due to the third order nonlinear susceptibility of Sb(2)Te(3). Additionally, an unsymmetrical design of the device causes polarization-dependent losses and together with polarization controller allows to use a nonlinear polarization evolution to form the artificial fast saturable absorber. Sub-200 fs soliton pulses with 0.27 nJ of pulse energy were generated in the hybrid mode-locked Er-doped fiber laser. Differences in the dynamics of mode-locked laser are further investigated with the use of slow and fast saturable absorbers solely, and compared with the hybrid device. Joint operation of two saturable absorbers enhances the laser performance and stability. The conducted experiments allowed to define roles of each mechanism on the pulse shaping in the laser cavity.

WS(2)/fluorine mica (FM) saturable absorbers for all-normal-dispersion mode-locked fiber laser

The report firstly propose a new WS(2) absorber based on fluorine mica (FM) substrate. The WS(2) material was fabricated by thermal decomposition method. The FM was stripped into one single layer as thin as 20 μm and deposited WS(2) on it, which can be attached to the fiber flank without causing the laser deviation. Similar to quartz, the transmission rate of FM is as high as 90% at near infrared wavelength from one to two micrometers. Furthermore, FM is a highly elastic material so that it is not easy to break off even its thickness was only 20 μm. On the contrary, quartz is hard to be processed and easy to break off when its thickness is less than 100 μm. Compared to organic matrix such as polyvinyl alcohol (PVA), FM has higher softening temperature, heat dissipation and laser damage threshold than those of organic composites. In our work, the modulation depth (MD) and non-saturable losses (NLs) of this kind of saturable absorber were measured to be 5.8% and 14.8%, respectively. The WS(2)/FM absorber has a high damage threshold of 406 MW/cm(2), two times higher than that of WS(2)/PVA. By incorporating the saturable absorber into Yb-doped fiber laser cavity, a mode-locked fiber laser was achieved with central wavelength of 1052.45 nm. The repetition rate was 23.26 MHz and the maximum average output power was 30 mW. The long term stability of working was proved to be good too. The results indicate that WS(2)/FM film is a practical nonlinear optical material for photonic applications.