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

Rapid Prototyping for Nanoparticle-Based Photonic Crystal Fiber Sensors

The advent of nanotechnology has motivated a revolution in the development of miniaturized sensors. Such sensors can be used for radiation detection, temperature sensing, radio-frequency sensing, strain sensing, and more. At the nanoscale, integrating the materials of interest into sensing platforms can be a common issue. One promising platform is photonic crystal fibers, which can draw in optically sensitive nanoparticles or have its optical properties changed by specialized nanomaterials. However, testing these sensors at scale is limited by the the need for specialized equipment to integrate these photonic crystal fibers into optical fiber systems. Having a method to enable rapid prototyping of new nanoparticle-based sensors in photonic crystal fibers would open up the field to a wider range of laboratories that could not have initially studied these materials in such a way before. This manuscript discusses the improved processes for cleaving, drawing, and rapidly integrating nanoparticle-based photonic crystal fibers into optical system setups. The method proposed in this manuscript achieved the following innovations: cleaving at a quality needed for nanoparticle integration could be done more reliably (≈100% acceptable cleaving yield versus ≈50% conventionally), nanoparticles could be drawn at scale through photonic crystal fibers in a safe manner (a method to draw multiple photonic crystal fibers at scale versus one fiber at a time), and the new photonic crystal fiber mount was able to be finely adjusted when increasing the optical coupling before inserting it into an optical system (before, expensive fusion splicing was the only other method).

Germanene saturable absorber for mode-locked operation in an all-fiber laser with multiple dispersion environments

In this paper, a high-quality germanene-polyvinyl alcohol (PVA) saturable absorber (SA) with a modulation depth of 3.05% and a saturation intensity of 17.95M W/c m 2 was prepared. Stable conventional mode-locking and harmonic mode-locking (HML) were achieved in germanene-based Er-doped fiber lasers (EDFL) using dispersion management techniques. In a cavity with a net dispersion value of -0.22p s 2, the conventional soliton had a center wavelength of 1558.2 nm, a repetition frequency of 19.09 MHz, and a maximum 3 dB spectrum bandwidth of 3.5 nm. The highest repetition frequencies achieved in cavities with net dispersion values of -2.81p s 2, -1.73p s 2, and -1.09p s 2 were 9.48 MHz, 12.75 MHz, and 12.10 MHz for HML, respectively. Furthermore, the effects of dispersion, power, and the polarization state on HML were systematically investigated. Our research results fully demonstrate the capability of germanene as an optical modulator in generating conventional mode-locked and harmonic mode-locked solitons. This provides meaningful references for promoting its application in ultrafast fiber lasers.

Photoelectrochemical biosensor based on DNA aptamers and dual nano-semiconductor heterojunctions for accurate and selective sensing of chloramphenicol

Nanosheets of anatase TiO₂ and CdS quantum dots modified with thioglycolic acid (TGA-CdS QDs) were prepared and hierarchically modified on the indium tin oxides (ITO) electrodes. The heterojunction structure is formed to improve the light capture ability and carrier migration, significantly enhancing the sensitivity of photoelectrochemical (PEC) biosensors. Specific DNA sequences labeled with TGA-CdS QDs were placed on the electrodes to prepare a biosensor for the detection of chloramphenicol with ultrahigh selectivity. In addition, the heterojunction structure and the principle of photocurrent signal amplification on the electrode are described in detail. Under the optimal conditions, the photoelectrochemical biosensors showed good reproducibility and stability for chloramphenicol with a linear response in the range 10-10,000 pM and a limit of detection (LOD) of 0.23 pM. Due to the specific recognition of base pairs, the sensor has excellent anti-interference ability in practical applications. An effective method was developed for the accurate detection of antibiotics with far reaching prospects.

Passively mode-locked fiber laser based on GeTe as a saturable absorber

In this work, we fabricate a saturable absorber based on GeTe with saturation intensity and modulation depth of 12.6M W/c m ² and 7%, respectively. We obtain stable conventional soliton and stretched soliton mode-locking operation. For the conventional soliton state, the average output power increased from 0.93 to 8.70 mW with the increase of pump power, and the fundamental repetition rate was 7.8351 MHz. Its central wavelength and 3 dB bandwidth were 1564.72 and 4.78 nm, respectively. For the stretched soliton state, when the pump power was increased from 87.4 to 420.3 mW, the average output power increased from 2.05 to 10.46 mW. When the maximum average output power reached 10.46 mW, the maximum average single-pulse energy was 0.86 nJ. The experimental results show that GeTe nanosheets will have broad application potential in the field of ultrafast photonics.

Wavelength-tunable noise-like fiber laser based on PbS quantum dot saturable absorber film

A wavelength-tunable noise-like pulse (NLP) erbium-doped fiber laser incorporating PbS quantum dot (QD) polystyrene (PS) composite film as a saturable absorber (SA) is experimentally demonstrated. The wavelength tuning is implemented via a Lyot filter consisting of a segment of polarization-maintaining fiber (PMF) and a 45° tilted fiber grating. By adjusting the polarization state of the ring cavity, the laser can deliver NLP with a continuous wavelength-tunable range from 1550.21 to 1560.64 nm. During continuous wavelength tuning, the output power varies between a range of 30.88-48.8 mW. Worthwhile noting is that the output power of 48.8 mW is the reported highest output power for wavelength-tunable NLP operation in an erbium-doped fiber laser using composite film as a saturable absorber.

Third-order harmonic mode-locked and Q-switched Er-doped fiber laser based on a Cr2Ge2Te6 saturable absorber

This paper reports the generation of fundamental solitons and third-order solitons in an erbium-doped fiber laser (EDFL) by a Cr₂Ge₂Te₆-polyvinyl alcohol (CGT-PVA) saturable absorber (SA). Stable fundamental solitons at 1559.09 nm at a repetition frequency of 5.1 MHz were detected, and third-order solitons with a maximum output power of 6.807 mW and narrowest monopulse duration of 615.2 fs were obtained under a repetition frequency of 15.3 MHz by changing pump power. To the best of our knowledge, it is the first time to achieve a Q-switched pulse with a minimum pulse duration of 2.2 µs and maximum single pulse energy of 12.11 nJ in EDFL based on CGT-PVA SA after reducing the cavity length. Its repetition rate monotonically increased from 18.8 kHz to 61.8 kHz with a tuning range of about 43 kHz. The experimental results sufficiently demonstrate that CGT has enormous potential as an ultrafast photonics device.

Generation of bright-dark soliton pairs based on a ferromagnetic insulator Cr2Si2Te6 as a modulator in an Er-doped fiber laser

In this work, a saturable absorber (SA) based on Cr₂Si₂Te₆ (CST), with a modulation depth of 14.90% and saturation intensity of 4.81MW/cm², was prepared by a liquid phase stripping method. Its nonlinear optical properties and application in obtaining mode-locked pulse output of bright-dark solitons are studied. When the pump power was 1250 mW, the maximum output power was 26.60 mW; the energy of the single pulse was 15.02 nJ, and the repetition rate was 1.77 MHz. Our results provided evidence that CST can be used as an excellent SA for a mode-locked laser and demonstrated that ferromagnetic insulators can be used for the study of bright-dark soliton pairs.

Ultrafast photonics applications of emerging 2D-Xenes beyond graphene

Driven by new two-dimensional materials, great changes and progress have taken place in the field of ultrafast photonics in recent years. Among them, the emerging single element two-dimensional materials (Xenes) have also received much attention due to their special physical and photoelectric properties including tunable broadband nonlinear saturable absorption, ultrafast carrier recovery rate, and ultrashort recovery time. In this review, the preparation methods of Xenes and various integration strategies are detailedly introduced at first. Then, we summarize the outcomes achieved by Xenes-based (beyond graphene) fiber lasers and make classifications based on the characteristics of output pulses according to the materials characterization and nonlinear optical absorption properties. Finally, an outlook of the future opportunities and challenges of ultrafast photonics devices based on Xenes and other 2D materials are highlighted, and we hope this review will promote their extensive applications in ultrafast photonics technology.

Efficient Saturable Absorber Based on Ferromagnetic Insulator Cr2Ge2Te6 in Er-Doped Mode-Locked Fiber Laser

A ferromagnetic insulator Cr₂Ge₂Te₆ as a saturable absorber in an Er-doped fiber laser (EDFL) was demonstrated. In this work, a CGT-PVA composite film was successfully fabricated using the liquid-phase exfoliation method and employed in an EDFL. The modulation depth and saturation intensity of the SA are 4.26% and 89.40 MW/cm², respectively. Stable pulses with a minimum pulse width of 978.5 fs when the repetition rate was 3.25 MHz were recorded experimentally. Furthermore, stable solitons still need to be obtained when the pulse energy in the cavity is as high as 11.6 nJ. The results fully suggest that CGT has outstanding nonlinear absorption properties, which may have broad potential applications in ultrafast photons.

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.

Passively mode-locked Er-doped fiber laser based on a ferromagnetic insulator Cr2Si2Te6 as a saturable absorber

In our work, a new-type, to the best of our knowledge, ferromagnetic insulator and its nonlinear optical absorption characteristics and related ultrafast modulation applications were investigated. Cr₂Si₂Te₆ saturable absorbers (SAs) with a modulation depth and a saturable intensity of 9.7% and 3.5MW/cm² were fabricated. By adjusting the pump power to 120 mW and optimizing the polarization state, traditional soliton operations were obtained successfully; the corresponding duration of pulse and the fundamental repetition rate were ∼1.33ps and 6.70 MHz, and the signal-to-noise ratio was 50 dB. The experimental results reveal that Cr₂Si₂Te₆ with excellent saturable absorption characteristics can be used as a SA to obtain ultrafast pulse lasers.

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.

Advances in engineering near-infrared luminescent materials

Near-infrared (NIR) luminescent materials have emerged as a growing field of interest, particularly for imaging and optics applications in biology, chemistry, and physics. However, the development of materials for this and other use cases has been hindered by a range of issues that prevents their widespread use beyond benchtop research. This review explores emerging trends in some of the most promising NIR materials and their applications. In particular, we focus on how a more comprehensive understanding of intrinsic NIR material properties might allow researchers to better leverage these traits for innovative and robust applications in biological and physical sciences.

WxNb(1-x)Se2 nanosheets for ultrafast photonics

Ternary transition metal chalcogenides (TTMDCs), a novel type of two-dimensional (2D) three-element materials, possess multiple physical and chemical properties and have promising potentials in basic physics and devices. Herein, the usage of W_xNb_(1-x)Se₂ nanosheets as a rising ultrafast photonic device to generate high power mode-locked and Q-switched pulses in a fiber laser is demonstrated. The W_xNb_(1-x)Se₂ nanosheets were successfully prepared by the liquid exfoliation method with thickness less than 3 nm. The nonlinear optical absorption of the W_xNb_(1-x)Se₂-based device was investigated with the saturable intensity of 40.93 MW cm^-2 and modulation depth of 5.43%. After integrating the W_xNb_(1-x)Se₂-based device into an Er-doped fiber (EDF) laser cavity, mode-locking and Q-switching laser pulses were formed. In the mode-locked mechanism output, the pulse width is as narrow as 131 fs and the output power is 52.93 mW. In Q-switched operation, the shortest pulse duration is 1.47 μs with the largest pulse energy of 257 nJ. Compared to recent studies, our results showed some improvements. This study suggests that 2D TTMDC-based devices could be developed as efficient ultrafast photonics candidates and widely used in nonlinear optical applications.

Titanium Disulfide Based Saturable Absorber for Generating Passively Mode-Locked and Q-Switched Ultra-Fast Fiber Lasers

In our work, passively mode-locked and Q-switched Er-doped fiber lasers (EDFLs) based on titanium disulfide (TiS₂) as a saturable absorber (SA) were generated successfully. Stable mode-locked pulses centred at 1531.69 nm with the minimum pulse width of 2.36 ps were obtained. By reducing the length of the laser cavity and optimizing the cavity loss, Q-switched operation with a maximum pulse energy of 67.2 nJ and a minimum pulse duration of 2.34 µs was also obtained. Its repetition rate monotonically increased from 13.17 kHz to 48.45 kHz with about a 35 kHz tuning range. Our experiment results fully indicate that TiS₂ exhibits excellent nonlinear absorption performance and significant potential in acting as ultra-fast photonics devices.

CVD-Bi2Te3 as a saturable absorber for various solitons in a mode-locked Er-doped fiber laser

In this work, we report about high energy and various solitons' operation by using high-efficiency topological insulator bismuth telluride (Bi₂Te₃) nanofilms as broadband saturable absorbers in the passively mode-locked Er-doped fiber laser. The Bi₂Te₃ film was successfully synthesized by chemical vapor deposition (CVD). Excellent characteristics of the dark-bright pulse pairs, bright pulses, and multiharmonics have been investigated experimentally by adjusting the polarization state. At the same time, the maximum average output power was 40.18 mW, and the single-pulse energy was 20.91 nJ. As we all know, it is the various solitons of the first generation with large pulse energy in an Er-doped fiber laser with Bi₂Te₃ as saturable absorber. The experimental results show that CVD Bi₂Te₃ can be used as an excellent candidate in mode-locked fiber lasers.

Conventional solitons and bound-state solitons in an erbium-doped fiber laser mode-locked by TiSe2-based saturable absorber

Conventional solitons (CSs) as well as bound-state solitons in a passively mode-locked erbium-doped fiber (EDF) laser based on 1 T-phase titanium diselenide (1 T-TiSe₂) saturable absorber (SA) have been systematically demonstrated for the first time. The mode locker is assembled by sandwiching the 1 T-TiSe₂ film between two fiber ferrules to improve compatibility with the all-fiber-integrated ring cavity configuration. The modulation depth, saturation intensity and nonsaturable loss of the prepared 1 T-TiSe₂ SA are 14.36%, 1.33 MW cm^-2 and 9.44%, respectively. The system is switchable between two states: CS and bound-state CS, by carefully adjusting the orientations of the polarization controller (PC). In the CS mode-locked regime, the oscillating wavelength is centered at 1558.294 nm with a pulse duration of 1.74 ps, a pulse repetition rate of 3.23 MHz and a maximum average output power of 2.904 mW. In the bound-state CS regime, two identical solitons form the bound-state pulses with a temporal separation of 6.1 ps, and the bound-state pulses are equally distributed at a repetition rate of 3.23 MHz, corresponding to the fundamental cavity repetition rate. The experimental results further indicate that 1 T-TiSe₂ SA is competitive with the existing SAs explored so far and will promote the applications of 1 T-TiSe₂-based SAs in the field of ultrafast lasers.

ZrSe2 nanosheet as saturable absorber for soliton operations within an Er-doped passive mode-locked fiber laser

Two-dimensional materials have extensively promoted the development of ultrafast photonics in the past decade. In our work, the saturable absorption properties of ZrSe₂ were presented. The saturation intensity, modulation depth, and nonlinear absorption coefficient of the ZrSe₂ saturable absorber (SA) are about 13.14MW/cm², 6.09%, and 1.85∗10^-1cm/GW. In the experiment, based on the ZrSe₂ SA, two types of solitons were recorded. A conventional soliton with a pulse width of 985 fs and a three-pulse bound state soliton have been obtained. Our experiment reveals that ZrSe₂ can be employed for generating multiple ultrafast soliton generations and possess promising application in ultrafast photonics.

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.

Passively Q-switched and mode-locked erbium-doped fiber lasers based on tellurene nanosheets as saturable absorber

Various two-dimensional (2D) materials show unique optical properties and excellent performance in acting as saturable absorber (SA) for demonstrating all-fiber ultra-fast lasers. Tellurene, as a new-fashioned few-layer 2D monoelemental material, was designed as an excellent saturable absorber to achieve Q-switched and mode-locked operations within erbium-doped fiber (EDF) lasers in our experiment. High-quality tellurene-based SA with a modulation depth of 0.97% was obtained by blending few-layer tellurene nanosheet solution prepared by liquid phase exfoliation method and the polyvinyl alcohol (PVA) solution. Inserting the SA into the EDF laser cavity by sandwiching the tellurene-PVA film between two fiber ferrules, either the passively Q-switched or the passively mode-locked operations can be obtained. The repetition rate varies from 15.92 to 47.61 kHz, and the pulse duration decreases from 8.915 to 5.196 µs in the passively Q-switched operation. To the best of our knowledge, this is the first demonstration focusing on the modulation application of tellurene in designing Q-switched pulsed laser operations. Additionally, mode-locked operations were also achieved by adjusting the polarization state. The obtained results fully indicate that tellurene can be developed as an efficient SA for pulsed fiber lasers.

Tellurium-nanorod-based saturable absorber for an ultrafast passive mode-locked erbium-doped fiber laser

In this paper, we propose and demonstrate ultrafast Te nanorods as a saturable absorber (SA) for producing mode locking from an erbium-doped fiber laser for the first time, to the best of our knowledge. The Te nanorods were fabricated by a simple green chemical method with energy conservation and without a purification process. The morphology and structure measurements confirm uniform Te nanorods with a constant aspect ratio. The synthesized SA has a saturation intensity and modulation depth of $25.44\, {\rm MW/cm}^{2} $25.44MW/cm² and 4%, respectively. By integrating the proposed SA into an erbium-doped all fiber-based ring cavity, the mode-locked fiber laser was readily generated. The conventional soliton pulses of ${3.56}\;{\rm ps}$3.56ps pulse width were obtained at 1566.7 nm central wavelength and a pulse repetition rate of 1.87 MHz. The results show that the moderate saturable-absorption characteristics of Te nanorods have superior performance in the ultrafast optics field, which is eligible in many applications, such as optical communications.

Nonlinear optical absorption properties of zirconium selenide in generating dark soliton and dark-bright soliton pairs

Our work reports the preparation of zirconium selenide (ZrSe₂)-polyvinyl alcohol (PVA) film-type saturable absorber (SA) and its nonlinear absorption performance in obtaining dark soliton and dark-bright soliton pairs in an Er-doped fiber (EDF) laser for the first time, to the best of our knowledge. The saturation intensity and modulation depth of the ZrSe₂-PVA SA were ∼12.72MW/cm² and 2.3%, respectively. Due to the modulation of the SA, under a pump power of 525.2 mW, stable dark soliton operation with an average output power of 9.75 mW, and a pulse repetition frequency of 20.84 MHz, a pulse width of 3.85 ns was attained successfully. By adjusting the state of the polarization controllers, dark-bright soliton pairs were also observed. To the best of our knowledge, this was the first demonstration focusing on the nonlinear optical absorption applications of ZrSe₂ in obtaining dark soliton and dark-bright soliton pairs. Our results show that ZrSe₂ is a good two-dimensional SA material for acting as an ultrafast optical device due to its suitable nonlinear optical absorption properties.

Ultra-wideband and flat-gain optical properties of the PbS quantum dots-doped silica fiber

We investigate the microstructural characteristics and optical properties of PbS quantum dots-doped silica fiber (PQDF), prepared by atomic layer deposition (ALD) doping technique. The fiber exhibits ultra-wideband luminescence and flat-gain with 3 dB bandwidth of 300 nm. The on-off gain and net gain can reach to 7.1-15.0 dB and 6.0-9.2 dB at 1050-1350 nm, respectively. The results of high-resolution transmission electron microscopy (HRTEM) further reveal the effects of PbS QDs doping in PQDF. The broadband luminescence spectrum originating from three active centers (1086, 1179, and 1304 nm), can be attributed to the dimension effect of PbS QDs (3.7, 4.0, and 4.3 nm, respectively). Moreover, the calculation results indicate that the multi-sized PbS QDs concentrated at 3.65-4.45 nm make the 3 dB gain bandwidth increase, which is six times wider than that of traditional erbium-doped fiber (EDF). Therefore, this type of PQDF is a promising gain medium for optical amplifiers and broadband light sources.

Ferromagnetic insulator Cr2Ge2Te6 as a modulator for generating near-infrared bright-dark soliton pairs

Novel two-dimensional (2D)-materials-based ultra-fast modulators exhibit significance in extending the fundamental investigations and practical applications of mode-locked fiber lasers. In our work, employing the liquid-phase exfoliation method, a ${{\rm Cr}_2}{{\rm Ge}_2}{{\rm Te}_6}$Cr₂Ge₂Te₆ (CGT) optical modulator with a modulation depth and a saturable intensity of 1.64% and ${6.31}\,\,{{\rm MW/cm}^2}$6.31MW/cm² was fabricated. Due to its suitable modulation properties and high nonlinear coefficient, a stable bright-dark soliton pair was successfully achieved within an Er-doped fiber laser. Under the pump power of 560 mW, the maximum average output power was 5.36 mW with a pulse repetition rate of 1.835 MHz. Our results fully present the capacity of CGT in designing bright-dark soliton operations and provide a meaningful reference for promoting the ultra-fast modulation applications of ferromagnetic insulators.

Nonlinear Optical Properties of Zirconium Diselenide and Its Ultra-Fast Modulator Application

Recently, two-dimensional (2D) materials have been widely studied by researchers due to their exceptional 2D structure and excellent optical characteristics. As one of the typically-layered 2D transition metal dichalcogenide (TMD) semiconductors from group IVB with a bandgap value of 0.9–1.2 eV (bulk to monolayer), the characteristics of zirconium diselenide (ZrSe₂) have already been extensively investigated in many fields. However, the nonlinear absorption properties of ZrSe₂ in ultra-fast lasers have not been previously demonstrated. In this work, we measured various parameters in order to investigate the characteristics of the nonlinear saturable absorption of ZrSe₂. A ZrSe₂–polyvinyl alcohol (PVA) film was successfully prepared, which was employed as a saturable absorber (SA) to demonstrate, for the first time, an erbium (Er)-doped passive mode-locking fiber laser with a ring cavity. The saturation intensity of the ZrSe₂–PVA film-type SA is 12.72 MW/cm², while its modulation depth is 2.3%. The stable soliton state with a maximum output power of 11.37 mW and a narrowest monopulse duration of 12.5 ps at a repetition frequency of 21.22 MHz was detected. The experimental results conclusively proved that ZrSe₂, with its suitable bandgap value and excellent nonlinear absorption properties, as well as its high damage threshold, should have extensive potential applications within the field of ultra-fast pulse lasers.

Generation of high-energy rectangular pulses in a nonlinear polarization rotation mode-locked ring fiber laser

In this paper, a novel high-energy mode-locked fiber laser based on the nonlinear polarization rotation (NPR) technique is presented to generate 331 nJ rectangular pulses. When pump power was 2659 mW, the maximum output power would be 102.3 mW; the maximum peak power was 41.74 W under the pump power of 1766 mW. In this study, the use of two homemade laser diodes and other common fiber devices was a vital step to achieve the low-cost and high-efficiency NPR mode-locked fiber laser. Based on these results, a novel approach could be developed to realize a high-energy rectangular pulse and promote the practical applications of the NPR mode-locked fiber laser in the field of ultrafast photonics.

High-Power Large-Energy Raman Soliton Generations Within a Mode-Locked Yb-Doped Fiber Laser Based on High-Damage-Threshold CVD-MoS2 as Modulator

In our work, based on a high-damage-threshold MoS₂ saturable absorber (SA), high-power intra-cavity Raman solitons within a passively mode-locked Yb-doped fiber laser were demonstrated successfully for the first time. The damage threshold of the MoS₂ SA was as high as ~0.48 J/cm². By adjusting the polarization states, stable single- or dual-pulse Raman soliton operations were obtained. The maximum average output power for single-pulse and dual-pulse Raman soliton operations was 80.11 and 89.33 mW, respectively. Our experiment results show significant enhancement in comparison with previous works, which provides fundamental guidance for future designs of high-power, large-energy, intra-cavity Raman soliton generations based on two-dimensional materials as SAs.

Improved Laser Damage Threshold of In2Se3 Saturable Absorber by PVD for High-Power Mode-Locked Er-Doped Fiber Laser

In this study, a double-end pumped high-power passively mode-locked erbium-doped fiber laser (EDFL) was realized by employing a few-layered In₂Se₃ flakes as a saturable absorber (SA). Herein, the uniform large-scale In₂Se₃ flakes were synthesized by the physical vapor deposition (PVD) method. The PVD-In₂Se₃ SA exhibited a remarkable damage threshold of higher than 24 mJ/cm². Meanwhile, the PVD-In₂Se₃ SA had a modulation depth and saturable intensity of 18.75% and 6.8 MW/cm², respectively. Based on the In₂Se₃ SA, the stable bright pulses emitting at 1559.4 nm with an average output power/pulse energy/pulse duration of 122.4 mW/5.8 nJ/14.4 ns were obtained successfully. To our knowledge, 122.4 mW was the new major breakthrough of mode-locked Er-doped fiber lasers. In addition, this is the first demonstration of the dark-bright pulse pair generation based on In₂Se₃ SA. The maximum average output power of the dark-bright pulse reached 121.2 mW, which also showed significant enhancement in comparison with previous works. Our excellent experiment results fully prove the superiority of our experimental design scheme and indicate that the PVD-In₂Se₃ could operate as a promising highly-nonlinear photonic material for a high-power fiber laser.

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.

Nanosecond passively Q-switched fibre laser using a NiS2 based saturable absorber

Q-switched pulse laser generation is successfully demonstrated in both Erbium-doped fibre laser (EDFL) and Thulium-doped fibre laser (TDFL) cavities by employing Nickel disulfide (NiS₂) as a saturable absorber (SA). Q-switched pulse laser operation at 1.55 μm and 2.0 μm is obtained at low pump power levels of 37 mW and 48 mW, respectively. For the EDFL, stable passively Q-switched laser output at a wavelength of 1561.86 nm is achieved, with a minimum pulse duration of 237 ns and a repetition rate of 243.9 kHz. For the TDFL, the centre wavelength of the laser output is 1915.5 nm, with a minimum pulse duration of 505 ns and a repetition rate of 214.68 kHz. NiS₂ is used as SA for Q-switched laser generation over a broadband wavelength for the first time.

Near-Infrared Optical Modulation for Ultrashort Pulse Generation Employing Indium Monosulfide (InS) Two-Dimensional Semiconductor Nanocrystals

In recent years, metal chalcogenide nanomaterials have received much attention in the field of ultrafast lasers due to their unique band-gap characteristic and excellent optical properties. In this work, two-dimensional (2D) indium monosulfide (InS) nanosheets were synthesized through a modified liquid-phase exfoliation method. In addition, a film-type InS-polyvinyl alcohol (PVA) saturable absorber (SA) was prepared as an optical modulator to generate ultrashort pulses. The nonlinear properties of the InS-PVA SA were systematically investigated. The modulation depth and saturation intensity of the InS-SA were 5.7% and 6.79 MW/cm2, respectively. By employing this InS-PVA SA, a stable, passively mode-locked Yb-doped fiber laser was demonstrated. At the fundamental frequency, the laser operated at 1.02 MHz, with a pulse width of 486.7 ps, and the maximum output power was 1.91 mW. By adjusting the polarization states in the cavity, harmonic mode-locked phenomena were also observed. To our knowledge, this is the first time an ultrashort pulse output based on InS has been achieved. The experimental findings indicate that InS is a viable candidate in the field of ultrafast lasers due to its excellent saturable absorption characteristics, which thereby promotes the ultrafast optical applications of InX (X = S, Se, and Te) and expands the category of new SAs.

Nonlinear Absorption Properties of Cr2Ge2Te6 and Its Application as an Ultra-Fast Optical Modulator

In this manuscript, the nonlinear absorption properties of Cr₂Ge₂Te₆ and its application in ultra-fast optical modulation are investigated. Typical parameters, namely, nonlinear absorption coefficient (β), saturation intensity, and modulation depth are measured to be ~1.66 × 10⁻⁹ m/W, 15.3 MW/cm², and 5.8%, respectively. To investigate the feasibility of using the Cr₂Ge₂Te₆ as an ultra-fast optical modulator, a ring-cavity passively mode-locked Er-doped fiber laser has been constructed. The output power/pulse, duration/pulse, and repetition rate/signal-to-noise ratios for the stable mode-locked operation are 2.88 mW/881 fs/19.33 MHz/48 dB, respectively, which proves that the Cr₂Ge₂Te₆ has outstanding nonlinear optical properties and advantages in performing as an ultra-fast optical modulator. Further, the experimental results provide valuable references and open new avenues for developing two-dimensional, material-based, ultra-fast optical modulators and advanced photonic devices based on Cr₂Ge₂Te₆.

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.

Tunable passively Q-switched Dy3+-doped fiber laser from 2.71 to 3.08 μm using PbS nanoparticles

We present the first widely tunable passively Q-switched Dy³⁺-doped ZBLAN fiber laser around 3 μm pumped at 1.1 μm using PbS nanoparticles as the saturable absorber (SA) to our knowledge. At 2.87 μm, the modulation depth and saturation intensity of the SA were measured to be 12.5% and 1.10  MW/cm², respectively. Stable Q-switching was achieved over a wavelength range of 2.71-3.08 μm (∼370  nm), which is a record tuning range from a pulsed rare-earth-doped fiber laser, to our knowledge. A maximum output power of 252.7 mW was obtained, with a pulse energy of 1.51 μJ, a pulse width of 795 ns, and a repetition rate of 166.8 kHz. This demonstration implies that Dy³⁺ is a promising gain medium for tunable pulsed sources in the 3 μm band and shows for the first time, to the best of our knowledge, the potential of PbS as a mid-infrared SA.

Passively Q-switched Nd-doped fiber laser based on PbS/CdS core/shell quantum dots as a saturable absorber

In our work, PbS/CdS core/shell quantum dots with an absorption peak of 1043 nm were successfully employed as a modulator for achieving a passively Q-switched Nd-doped fiber laser. The saturation intensity and modulation depth of the film-type modulator were 7.6  MW/cm² and 4.1%, respectively. Due to the protection of the CdS shell, the PbS core exhibited good photo-chemical stability, which led to the generation of stable passively Q-switched operation. The maximum average output power was 7.88 mW with a minimum pulse width of 235.7 ns. To our knowledge, this was the first demonstration focusing on the combination of quantum dot and Nd-doped fiber laser; in addition, 235.7 ns was the narrowest pulse width of a passively Q-switched Nd-doped fiber laser. Our results highlighted the excellent nonlinear absorption properties of PbS/CdS core/shell quantum dots and give significant guidance for future optical applications of quantum dots and demonstrations of pulsed Nd-doped fiber lasers.

Large-energy mode-locked ytterbium-doped linear-cavity fiber laser based on chemical vapor deposition-Bi2Se3 as a saturable absorber

We reported on the generation of pulse bunch and large-energy dark pulses in a mode-locked ytterbium-doped linear-cavity fiber laser based on Bi₂Se₃ as a saturable absorber (SA). Bi₂Se₃ nanosheets were successfully synthesized by the chemical vapor deposition (CVD) method and transferred to the end facet of a fiber connector for the proposed SA. Its saturation intensity and modulation depth were measured to be 52  MW/cm² and 14.5%, respectively. By inserting the Bi₂Se₃-based SA into the Yb-doped all-fiber linear cavity, stable pulse bunches were observed. In addition, dark soliton operation with a maximum average output power of 32.6 mW and a pulse energy of 61.8 nJ were also achieved. To the best of our knowledge, this is the first demonstration of a dark soliton within a linear cavity with much larger pulse energy than previous works. Our study fully indicated that CVD-Bi₂Se₃ could be an excellent SA for achieving large-energy pulse operations.

Tailoring optical nonlinearities of LiNbO3 crystals by plasmonic silver nanoparticles for broadband saturable absorbers

We report on the synthesis of plasmonic Ag nanoparticles (NPs) embedded in a LiNbO₃ crystal (AgNP:LN) by ion implantation and its application as an efficient broadband saturable absorber (SA) to realize Q-switched pulsed laser generation at both visible and near-infrared wavelength bands. The nonlinear optical response of AgNP:LN is considered as a synergistic effect between Ag NPs and LiNbO₃. We apply the AgNP:LN as visible-near-infrared broadband saturable absorbers (SAs) into Pr:LuLiF₄ bulk and Nd:YVO₄ waveguide laser cavity, achieving efficient passively Q-switched laser at 639 nm and 1064 nm, respectively. This work paves a new way to tailor the nonlinear optical response of LiNbO₃ crystals by using plasmonic Ag NPs, manifesting the significant potential as broadband SAs in the aspect of pulsed lasing.