200-fs mode-locked Erbium-doped fiber laser by using mechanically exfoliated MoS2 saturable absorber onto D-shaped optical fiber

CrPS4 Nanoflakes as Stable Direct-Band-Gap 2D Materials for Ultrafast Pulse Laser Applications

Two-dimensional (2D) materials have attracted considerable attention due to their potential for generating ultrafast pulsed lasers. Unfortunately, the poor stability of most layered 2D materials under air exposure leads to increased fabrication costs; this has limited their development for practical applications. In this paper, we describe the successful preparation of a novel, air-stable, and broadband saturable absorber (SA), the metal thiophosphate CrPS₄, using a simple and cost-effective liquid exfoliation method. The van der Waals crystal structure of CrPS₄ consists of chains of CrS₆ units interconnected by phosphorus. In this study, we calculated the electronic band structures of CrPS₄, revealing a direct band gap. The nonlinear saturable absorption properties, which were investigated using the P-scan technique at 1550 nm, revealed that CrPS₄-SA had a modulation depth of 12.2% and a saturation intensity of 463 MW/cm². Integration of the CrPS₄-SA into Yb-doped fiber and Er-doped fiber laser cavities led to mode-locking for the first time, resulting in the shortest pulse durations of 298 ps and 500 fs at 1 and 1.5 µm, respectively. These results indicate that CrPS₄ has great potential for broadband ultrafast photonic applications and could be developed into an excellent candidate for SA devices, providing new directions in the search for stable SA materials and for their design.

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.

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.

Vanadium disulfide for ultrafast photonic application

The investigation of two-dimensional (2D) nonlinear optical materials offers a promising way to construct the high-performance optical devices in fundamental and industrial applications because of their rich distinct optoelectronic properties. Herein, by utilizing the liquid exfoliation method, vanadium disulfide (VS₂) nanosheets are prepared and the thickness is measured to be 3.16 nm. In addition, we have fabricated the VS₂-based optical device and the nonlinear optical property is characterized with modulation depth of 23.97%. By using VS₂ as saturable absorber, a high stable passively mode-locking Er-doped fiber laser is obtained with pulse duration of 169 fs and the largest average output power of 70.5 mW. The slope efficiency is up to 7.9%. In comparison to recent results of mode-locking fiber lasers with 2D materials, the VS₂-based fiber laser demonstrates better performance. To the best of our knowledge, this is the first example of using VS₂ for generating femtosecond mode-locked laser pulse. Our experimental results not only reveal VS₂ ultrafast photonics application, but also advance the high-performance applications for information science and nonlinear optics.

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.

Graphene/WS2 heterostructure saturable absorbers for ultrashort pulse generation in L-band passively mode-locked fiber lasers

Graphene/WS₂ (G/WS₂) van der Waals (vdW) heterostructures are utilized as saturable absorbers (SAs) in compact mode-locked fiber lasers operating in the telecommunication L-band for the first time. The interlayer coupling is confirmed by Raman and photoluminescence spectra. In comparison with pure WS₂, the heterostructure exhibits excellent nonlinear optical properties in terms of larger modulation depth and lower saturation intensity due to the strong interlayer coupling. By incorporating the G/WS₂-based SA into an all-anomalous-dispersion fiber laser, stable conventional-soliton pulses with a pulse duration down to 660 fs can be realized at 1601.9 nm, manifesting better output performance compared to pure WS₂. In addition, through shifting the cavity dispersion to the net-normal dispersion, the G/WS₂ SA can also be applied for dissipative-soliton generation. Resultant output pulses feature the central wavelength of 1593.5 nm and the pulse duration of 55.6 ps. Our results indicate that the G/WS₂ vdW heterostructure is a promising candidate as SA for pulsed laser applications, which pave the way for the development of novel ultrafast photonic devices with desirable performance.

Ultrafast fiber laser based on HfSe2 saturable absorber

We demonstrate the HfSe₂ saturable absorber (SA) for the generation of ultrafast pulse laser. The HfSe₂ SA device is fabricated by integrating HfSe₂ nanosheets (NSs) with a microfiber. The material and optical characteristics of HfSe₂ NSs show their high quality. The nonlinear optical absorption of HfSe₂ SA is measured with a modulation depth of 5.8%. Stable soliton mode-locked laser based on HfSe₂ SA is realized at the central wavelength of 1561.43 nm with pulse duration of 297 fs and the maximum pulse energy of 2.68 nJ. Our soliton fiber laser has a maximum output power of 48.5 mW with a high slope efficiency of 12.8%, which indicate that HfSe₂ is a good candidate of SA for high efficient ultrashort pulses generation.

Synthesis of WS1.76Te0.24 alloy through chemical vapor transport and its high-performance saturable absorption

Layered transitional metal dichalcogenides (TMDs) are drawing significant attentions for the applications of optics and optoelectronics. To achieve optimal performances of functional devices, precisely controlled doping engineering of 2D TMDs alloys has provided a reasonable approach to tailor their physical and chemical properties. By the chemical vapor transport (CVT) method and liquid phase exfoliation technique, in this work, we synthesized WS_1.76Te_0.24 saturable absorber (SA) which exhibited high-performance of nonlinear optics. The nonlinear saturable absorption of the WS_1.76Te_0.24 SA was also measured by the open aperture Z-scan technique. Compared to that of the binary component WS₂ and WTe₂, WS_1.76Te_0.24 SA has shown 4 times deeper modulation depth, 28% lower saturable intensity and a much faster recovery time of 3.8 ps. The passively Q-switched laser based on WS_1.76Te_0.24 was more efficient, with pulse duration narrowed to 18%, threshold decreased to 28% and output power enlarged by 200%. The promising findings can provide a method to optimize performances of functional devices by doping engineering.

In2Se3 nanosheets with broadband saturable absorption used for near-infrared femtosecond laser mode locking

Indium selenide (In₂Se₃) has attracted tremendous attention due to its favorable electronic features, broad tunable bandgap, high stability and other attractive properties. However, its further applications for nonlinear optics have not yet been fully explored. In this work, we demonstrate that few-layer α-In₂Se₃ nanosheets exhibit strong saturable absorption properties over a wide wavelength range covering 800, 1064 and 1550 nm. The few-layer α-In₂Se₃ nanosheets used for this experiment are fabricated via a simple ultrasonic exfoliation in liquid. Stable ultrafast mode-locking laser pulses are obtained from both ytterbium-doped and erbium-doped fiber laser systems operating at 1064 and 1550 nm, respectively. A pulse duration as short as 215 fs was achieved in the Er-doped fiber laser system. Stable output pulses over 6 h of operation were obtained in both laser systems. The pulse energy and peak power of the laser output pulses were increased by reducing the In₂Se₃ thickness. These results indicate that α-In₂Se₃ nanosheets with low layer numbers are promising candidates for broad ultrafast photonics devices, such as optical switchers, Q-switchers and mode lockers.

Excessively tilted fiber grating based Fe3O4 saturable absorber for passively mode-locked fiber laser

A novel approach to saturable absorber (SA) formation is presented by taking advantage of the mode coupling property of excessively tilted fiber grating (Ex-TFG). Stable mode-locked operation can be conveniently achieved based on the interaction between Ex-TFG coupled light and deposited ferroferric-oxide (Fe₃O₄) nanoparticles. The central wavelength, bandwidth and single pulse duration of the output are 1595 nm, 4.05 nm, and 912 fs, respectively. The fiber laser exhibits good long-term stability with signal-to-noise ratio (SNR) of 67 dB. For the first time, to the best of our knowledge, Ex-TFG based Fe₃O₄ SA for mode-locked fiber laser is demonstrated.

MXene-based saturable absorber for femtosecond mode-locked fiber lasers

We report simple and compact all-fiber erbium-doped soliton and dispersion-managed soliton femtosecond lasers mode-locked by the MXene Ti₃C₂Tx. A saturable absorber device fabricated by optical deposition of Ti₃C₂Tx onto a microfiber exhibits strong saturable absorption properties, with a modulation depth of 11.3%. The oscillator operating in the soliton regime produces 597.8 fs-pulses with 5.21 nm of bandwidth, while the cavity with weak normal dispersion (~0.008 ps²) delivers 104 fs pulses with 42.5 nm of bandwidth. Our results contribute to the growing body of work studying the nonlinear optical properties of MXene that underpin new opportunities for ultrafast photonic technology.

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.

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.

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.