Erbium-doped fiber laser passively mode locked with few-layer WSe2/MoSe2 nanosheets

Ternary ReS2(1-x)Se2xalloys of different composition for Q-switched and mode-locked all-fiber laser

This paper systematically studied the composition-controlled nonlinear optical properties and pulse modulation of ternary ReS2(1-x)Se2xalloys for the first time. The compositionally modulated characteristics of ReS2(1-x)Se2xon the band gap were simulated based on the first principles. We investigated the effect of the band gap on the saturable absorption properties. In addition, we demonstrated the modulation characteristics of different components ReS2(1-x)Se2xon 1.5μm Q-switched pulse performance. The Q-switched threshold, repetition rate, and pulse duration increase as the S(sulfur)-element composition rise. And pulse energy also was affected by the S(sulfur)-element composition. The ReS0.8Se1.2SA was selected to realize a conventional soliton with high energy in the all-fiber mode-locked laser. The pulse was centered at 1562.9 nm with a pulse duration of 2.26 ps, a repetition rate of 3.88 MHz, and maximum pulse energy of 1.95 nJ. This work suggests that ReS2(1-x)Se2xhas great potential in laser technology and nonlinear optics, and widely extends the material applications in ultrafast photonics.

Single and Bunch Soliton Generation in Optical Fiber Lasers Using Bismuth Selenide Topological Insulator Saturable Absorber

In this work, we present the generation of two distinct types of soliton pulses using a Bismuth Selenide (Bi₂Se₃) saturable absorber (SA) synthesized in our laboratory. The soliton pulses were generated in two different laser cavity configurations, resulting in two types of solitons: a soliton pulse with Kelly sidebands and a bunched soliton pulse with peak-dip sidebands. Both solitons operated at the fundamental repetition rate-23.3 MHz (for the soliton with Kelly sidebands) and 13 MHz (for the bunched soliton with peak-dip sidebands). We observed that the accumulation of nonlinear phase shift from the added single mode fiber (SMF) split the single soliton pulse into 44 pulses in a bunched oscillation envelope. At the same time, peak-dip sidebands were imposed on the bunched soliton spectrum due to constructive and destructive interferences between soliton pulse and dispersive waves. The measured pulse width for both solitons were 0.63 ps (for the soliton with Kelly sidebands) and 1.52 ps (for the bunched soliton with peak-dip sidebands), respectively. Our results demonstrate the potential of Bi₂Se₃ SAs in generating different types of soliton pulses, which could have potential applications in various areas of optical communication and spectroscopy.

Low-Threshold, Multiple High-Order Harmonics Fiber Laser Employing Cr2Si2Te6 Saturable Absorber

Abundant research findings have proved the value of two-dimensional (2D) materials in the study of nonlinear optics in fiber lasers. However, there remains two problems: how to reduce the start-up threshold, and how to improve the damage threshold, of fiber lasers based on 2D materials. A 15.1 mW low-threshold mode-locked fiber laser, based on a Cr₂Si₂Te₆ saturable absorber (SA) prepared by the liquid-phase exfoliation method, is demonstrated successfully in this work. This provides a useful and economical method to produce SAs with low insertion loss and low saturation intensity. Besides, multiple high-order harmonics, from the fundamental frequency (12.6 MHz) to the 49th-order harmonic (617.6 MHz), mode-locked operations are recorded. The experimental results indicate the excellent potential of Cr₂Si₂Te₆ as an optical modulator in exploring the soliton dynamics, harmonic mode locking, and other nonlinear effects in fiber lasers.

Co-MOFs as Emerging Pulse Modulators for Femtosecond Ultrafast Fiber Laser

The metal organic framework (MOF) has attracted more and more attention due to its unique morphology, functional linkers, and orderly network structure. Zeolitio imidazolata frameworks (ZIFs), which are formed by bivalent transition metals (Zn, Co, etc.) and nitrogen-containing heterocyclic imidazole or purine organic ligands, are a very attractive subclass of MOFs. ZIF-67, obtained by the nucleation growth of dimethylimidazole and Co 2p, has been developed as a precursor for porous nanostructured cobalt-based metal oxides. During material preparation we add rGO because it can be used as a basic element to construct macroscopic three-dimensional carbon structural materials, which self-assemble into a 3D network structure with ZIF-67 through simple van der Waals forces or hydrogen bonds, and some samples contain specific functional groups that are added to the precursor. In this paper, we employ liquid-phase synthesis to generate rGO-ZIF-67 and calcine it at the temperature of 350 °C to obtain rGO-Co₃O₄. Then we fabricate rGO-Co₃O₄ and rGO-ZIF-67 modulators based on microfibers and test their nonlinear optical absorption in 1.5 μm range. The modulation depths of rGO-Co₃O₄ and rGO-ZIF-67 are measured as 10.41% and 6.61%, respectively. By using microfiber-based rGO-Co₃O₄ modulator, we have obtained a conventional soliton and a soliton molecule in Er³⁺-doped fiber lasers. The conventional soliton has a pulse width of 793.4 fs and a spectral width of 3.3 at 1558.9 nm, respectively. The obtained soliton molecule has a spectral modulation period of 1.65 nm and temporal separation of 4.94 ps at 1563.2 nm. By employing a microfiber-based rGO-ZIF-67 modulator, we obtain conventional solitons with a spectral width of 1.9 nm at the central wavelength of 1529.8 nm. Our research may expand the MOF-based materials for ultrafast photonics, blazing a new path for fiber laser, optical communications, and optoelectronics, etc.

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.

Femtosecond Pulsed Fiber Laser by an Optical Device Based on NaOH-LPE Prepared WSe2 Saturable Absorber

We report on all-optical devices prepared from WSe2 combined with drawn tapered fibers as saturable absorbers to achieve ultrashort pulse output. The saturable absorber with a high damage threshold and high saturable absorption characteristics is prepared for application in erbium-doped fiber lasers by the liquid phase exfoliation method for WSe2, and the all-optical device exhibited strong saturable absorption characteristics with a modulation depth of 15% and a saturation intensity of 100.58 W. The net dispersion of the erbium-doped fiber laser cavity is ~-0.1 ps2, and a femtosecond pulse output with a bandwidth of 11.4 nm, a pulse width of 390 fs, and a single-pulse capability of 42 pJ is obtained. Results indicate that the proposed WSe2 saturable absorbers are efficient, photonic devices to realize stable fiber lasers. The results demonstrate that the WSe2 saturable absorber is an effective photonic device for realizing stable fiber lasers, which have a certain significance for the development of potential photonic devices.

Ultrafast fiber laser at 1570 nm based on organic material as saturable absorber

In this work, we demonstrated Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) as a saturable absorber (SA) to produce mode-locking operation in different length of Erbium-doped fiber laser (EDFL). The PEDOT: PSS was embedded into polyvinyl alcohol to form a thin film that acts as an absorber into the laser setup. The three different mode-locked EDFL were successfully demonstrated with different cavity length and output coupler ratio. The pulse repetition rate/width of 3.417 MHz/710 fs, 4.831 MHz/510 fs, and 6.049 MHz/460 fs were obtained by utilizing optical coupler/ cavity length of 20:80/60.7 m, 10:90/42.7 m, and 5:95/33.7 m, respectively. All experiments generated a stable and mode-locked operation at a central wavelength of 1570.76 nm, 1570.3 nm, and 1569.95 nm with 3 dB bandwidth of 4.8 nm, 5.6 nm, and 6.5 nm, respectively. The long-time stability of the ultrafast fiber lasers was investigated for each setup via 120 min. The proposed PEDOT: PSS has proven as a promising material to induce mode-locking operation in different fiber laser setup.

Mode-locked ytterbium-doped fiber laser with zinc phthalocyanine thin film saturable absorber

A stable mode-locked laser was demonstrated using a newly developed zinc phthalocyanine (ZnPc) thin film as passive saturable absorber (SA) in ytterbium-doped fiber laser (YDFL). The ZnPc thin film was obtained using a casting method and then inserted between the two fiber ferrules of a YDFL ring cavity to generate mode-locked pulses. The resulting pulsed laser operated at a wavelength of 1034.5 nm having a repetition rate of 3.3 MHz. At pump power of 277 mW, the maximum output power and pulse energy are achieved at 4.92 mW and 1.36 nJ, respectively. ZnPc has a high chemical and photochemical stability, and its significance for use as a potential SA in a mode-locked laser is reported in this work.

MoS2/MXene pillared nanocomposite for ultrafast photonics applications

In this work, we used nanocomposite saturable absorbers (SAs) in order to precisely design and modulate the process of compositing the light absorption by band gap engineering. Due to the higher absorption intensity of our MoS₂/MXene nanocomposite, we have successfully shortened the pulse duration (1.2μs) of SA with enhancing saturable absorption intensity (7.22 MW cm^-2), and the ultra-fast fiber laser based on this nanocomposite SA has shown wider Q-switching stable range in the case of high pump power. This strategy can efficiently improve the performance of SA and shows the potential application prospect of nanocomposites in nonlinear optics.

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.

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.

Highly Stable Passively Q-Switched Erbium-Doped All-Fiber Laser Based on Niobium Diselenide Saturable Absorber

A saturable absorber (SA) based on niobium diselenide (NbSe₂), which is a layered transition metal dichalcogenide (TMD) in the VB group, is fabricated by the optically driven deposition method, and the related nonlinear optical properties are characterized. The modulation depth, saturable intensity, and nonsaturable loss of the as-prepared NbSe₂ nanosheet-based SA are measured to be 16.2%, 0.76 MW/cm², and 14%, respectively. By using the as-fabricated NbSe₂ SA, a highly stable, passively Q-switched, erbium-doped, all-fiber laser is realized. The obtained shortest pulse width is 1.49 μs, with a pulse energy of 48.33 nJ at a center wavelength of 1560.38 nm. As far as we know, this is the shortest pulse duration ever obtained by an NbSe₂ SA in a Q-switched fiber laser.

Recent Progress of Two-Dimensional Materials for Ultrafast Photonics

Owing to their extraordinary physical and chemical properties, two-dimensional (2D) materials have aroused extensive attention and have been widely used in photonic and optoelectronic devices, catalytic reactions, and biomedicine. In particular, 2D materials possess a unique bandgap structure and nonlinear optical properties, which can be used as saturable absorbers in ultrafast lasers. Here, we mainly review the top-down and bottom-up methods for preparing 2D materials, such as graphene, topological insulators, transition metal dichalcogenides, black phosphorus, and MXenes. Then, we focus on the ultrafast applications of 2D materials at the typical operating wavelengths of 1, 1.5, 2, and 3 μm. The key parameters and output performance of ultrafast pulsed lasers based on 2D materials are discussed. Furthermore, an outlook regarding the fabrication methods and the development of 2D materials in ultrafast photonics is also presented.

Three-dimensional Ag2S cubes for switchable multi-wavelength ultrashort pulse application

Three-dimensional (3D) materials are widely used in optoelectronics, thermodynamics and ultrafast fiber lasers because of their excellent nonlinear optical properties. Silver sulfide (Ag₂S) is a kind of 3D material with a unique cubic structure and large absorption coefficient. In this paper, a double-balance detection system is used to measure the saturation absorption intensity of Ag₂S as 226.6 MW cm^-2and the modulation depth as 13.9%. In the ring fiber laser, Ag₂S is used as a saturable absorber (SA) to obtain a stable dual-wavelength mode locking. The center wavelengths of the mode locking are 1536.9 and 1544.5 nm, and the corresponding 3 dB bandwidths are 1.3 and 1.5. nm. By adjusting the polarization controller, a tuning process from two wavelengths to multiple wavelengths is realized, and the tunable width is 13.1 nm. This phenomenon is due to the combined effect of birefringence and nonlinear effects in the cavity. To our knowledge, this is the first report of a multiplexed fiber laser with Ag₂S as a SA. The emergence of this result provides a valuable reference information for the multifunctional compact fiber laser, and the formed system can be applied in the fields of fiber sensing, telecommunications and optical communication.

SnS nanosheets for harmonic pulses generation in near infrared region

Two-dimensional materials have attracted increasing attention because of their excellent mechanical, thermodynamic, magnetic, electrical and optical properties. Here, a new two-dimensional material of tin sulfide (SnS) is experimentally prepared. It is layered like black phosphorus and owns distinct optoelectronic properties, but eliminates the disadvantage of instability. The nonlinear saturable absorption characteristics of the SnS nanosheets is investigated at 1563.3 nm by the double-balanced detection method. The obtained modulation depth and saturation intensity are 5.4% and 66.3 MW/cm², respectively. A passively harmonic mode-locked erbium-doped fiber laser based on the SnS saturable absorber (SA) has been demonstrated. The results show that mode-locking with fundamental frequency of 5.47 MHz is realized at pump power of 28.38 mW. With the increase of pump power, the laser can operate from fundamental frequency to high-order harmonic mode-locking. The maximum repetition rate of 412.73 MHz has been obtained, which is equivalent to the 76th harmonic mode-locking. This work reveals that SnS nanosheets is a novel and efficient SA with high damage threshold, which will find potential applications in optical communication, photoelectric detection, laser medicine, etc.

Electrochemical Peeling Few-Layer SnSe2 for High-Performance Ultrafast Photonics

In recent years, the photoelectric properties and nonlinear optical properties of layered metal chalcogenides (LMCs) have attracted extensive attentions. Because of lower phonon thermal conductivity, larger energy storage rate, and larger electron mobility, LMCs are widely studied in the fields of thermoelectric energy conversion, battery electrode materials, and semiconductor devices. As 2D LMCs, SnSe₂ nanosheets (Ns) are connected to each other by van der Waals force, which makes it possible to use electrochemical methods to help peel off the thin layer structure. Two-dimensional SnSe₂ has obvious adjustable band gap characteristics. Its thickness can be controlled to keep it on the desired band gap. In this article, we prepared a thin layer of SnSe₂ by electrochemical methods and detected its nonlinear optical characteristics. It shows that our prepared materials have good optical absorption characteristics; it has a modulation depth of 15% and a saturation intensity of 61 MW/cm². To investigate the nonlinear effects of SnSe₂ in short and long cavities, the Q-mode-locking phenomenon was first achieved in a fiber laser with cavity length of 6 m. After increasing the cavity length to 56 m, the pump power is adjusted to achieve an adjustable repetition frequency from MHz to GHz in turn in an Er-doped fiber laser through utilizing an SnSe₂ incorporating a tapered fiber as a saturable absorber (SA). The nonlinear optical properties of thin layer SnSe₂ are fully proven, which opens a new way for advanced photonics, optical communication, laser measurement, and other fields.

Nonlinear optical properties of arsenic telluride and its use in ultrafast fiber lasers

We report the first investigation results of the nonlinear optical properties of As2Te3. More specifically, the nonlinear optical absorption properties of the prepared α-As2Te3 were investigated at wavelengths of 1.56 and 1.9 μm using the open-aperture (OA) Z-scan technique. Using the OA Z-scan technique, the nonlinear absorption coefficients (β) of α-As2Te3 were estimated in a range from (- 54.8 ± 3.4) × 104 cm/GW to (- 4.9 ± 0.4) × 104 cm/GW depending on the irradiance of the input beam at 1.56 μm, whereas the values did from (- 19.8 ± 0.8) × 104 cm/GW to (- 3.2 ± 0.1) × 104 cm/GW at 1.9 μm. In particular, the β value at 1.56 μm is an order of magnitude larger than the previously reported values of other group-15 sesquichalcogenides such as Bi2Se3, Bi2Te3, and Bi2TeSe2. Furthermore, this is the first time report on β value of a group-15 sesquichalcogenide at a 1.9-μm wavelength. The density functional theory (DFT) calculations of the electronic band structures of α-As2Te3 were also conducted to obtain a better understanding of their energy band structure. The DFT calculations indicated that α-As2Te3 possess sufficient optical absorption in a wide wavelength region, including 1.5 μm, 1.9 μm, and beyond (up to 3.7 μm). Using both the measured nonlinear absorption coefficients and the theoretically obtained refractive indices from the DFT calculations, the imaginary parts of the third-order optical susceptibilities (Im χ(3)) of As2Te3 were estimated and they were found to vary from (- 39 ± 2.4) × 10-19 m2/V2 to (- 3.5 ± 0.3) × 10-19 m2/V2 at 1.56 μm and (- 16.5 ± 0.7) × 10-19 m2/V2 to (- 2.7 ± 0.1) × 10-19 m2/V2 at 1.9 μm, respectively, depending on the irradiance of the input beam. Finally, the feasibility of using α-As2Te3 for SAs was investigated, and the prepared SAs were thus tested by incorporating them into an erbium (Er)-doped fiber cavity and a thulium-holmium (Tm-Ho) co-doped fiber cavity for both 1.5 and 1.9 μm operation.

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.

Conventional Soliton and Noise-Like Pulse Generated in an Er-Doped Fiber Laser with Carbon Nanotube Saturable Absorbers

Conventional soliton (CS) and noise-like pulse (NLP) are two different kinds of pulse regimes in ultrafast fiber lasers, which have many intense applications. In this article, we experimentally demonstrate that the pulse regime of an Er-doped fiber laser could be converted between conventional soliton and noise-like pulse by using fast response saturable absorbers (SA) made from different layers of single-wall carbon nanotubes (CNT). For the monolayer (ML) single-wall CNT-SA, CS with pulse duration of 439 fs at 1560 nm is achieved while for the bilayer (BL) single-wall CNT, NLP at 1560 nm with a 1.75 ps spike and a 98 ps pedestal is obtained. The transition mechanism from CS to NLP is investigated by analyzing the optical characteristics of ML and BL single-wall CNT. The further theoretical simulation illustrates that CNT-SA enables the switching between CS and NLP in anomalous dispersion regime in Er-doped fiber lasers.

Solution-processed two-dimensional materials for ultrafast fiber lasers (invited)

Since graphene was first reported as a saturable absorber to achieve ultrafast pulses in fiber lasers, many other two-dimensional (2D) materials, such as topological insulators, transition metal dichalcogenides, black phosphorus, and MXenes, have been widely investigated in fiber lasers due to their broadband operation, ultrafast recovery time, and controllable modulation depth. Recently, solution-processing methods for the fabrication of 2D materials have attracted considerable interest due to their advantages of low cost, easy fabrication, and scalability. Here, we review the various solution-processed methods for the preparation of different 2D materials. Then, the applications and performance of solution-processing-based 2D materials in fiber lasers are discussed. Finally, a perspective of the solution-processed methods and 2D material-based saturable absorbers are presented.

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.

P-type laser-doped WSe2/MoTe2 van der Waals heterostructure photodetector

Van der Waals heterostructures (vdWHs) based on two-dimensional (2D) materials are being studied extensively for their prospective applications in photodetectors. As the pristine WSe₂/MoTe₂ heterostructure is a type I (straddling gap) structure, it cannot be used as a photovoltaic device theoretically, although both WSe₂ and MoTe₂ have excellent photoelectric properties. The Fermi level of p-doped WSe₂ is close to its valence band. The p-doped WSe₂/MoTe₂ heterostructure can perform as a photovoltaic device because a built-in electric field appears at the interface between MoTe₂ and p-doped WSe₂. Here, a 633 nm laser was used for scanning the surface of WSe₂ in order to obtain the p-doped WSe₂. x-ray photoelectron spectroscopy (XPS) and electrical measurements verified that p-type doping in WSe₂ is produced through laser treatment. The p-type doping in WSe₂ includes substoichiometric WO_x and nonstoichiometric WSe_x. A photovoltaic device using p-doped WSe₂ and MoTe₂ was successfully fabricated. The band structure, light-matter reactions, and carrier-transport in the p-doped WSe₂/MoTe₂ heterojunction were analyzed. The results showed that this photodetector has an on/off ratio of ≈10⁴, dark current of ≈1 pA, and response time of 72 μs under the illumination of 633 nm laser at zero bias (V _ds = 0 V). The proposed p-doping method may provide a new approach to improve the performance of nanoscale optoelectronic devices.

High-efficiency second-order nonlinear processes in an optical microfibre assisted by few-layer GaSe

The centrosymmetric nature of silica fibre precludes the realisation of second-order nonlinear processes in optical fibre systems. Recently, the integration of 2D materials with optical fibres has opened up a great opportunity to develop all-fibre active devices. Here, we demonstrate high-efficiency second-order nonlinear frequency conversions in an optical microfibre assisted with few-layer gallium selenide (GaSe) nanoflakes. Attributed to the strong evanescent field of the microfibre and ultrahigh second-order nonlinearity of the GaSe nanoflakes, second harmonic generation (SHG) and sum-frequency generation (SFG) are effectively achieved with only sub-milliwatt continuous-wave (CW) lasers in the wavelength range of 1500-1620 nm, covering the C and L telecom bands. The SHG intensity from the microfibre is enhanced by more than four orders of magnitude with the assistance of the GaSe nanoflakes on fibre nonlinear processes. Moreover, in the SFG process, the intensity transfer between different frequencies can be effectively manipulated by changing the wavelengths and powers of two pump lasers. The realised strong second-order nonlinearity in the GaSe-integrated microfibre might expand the applications of all-fibre devices in all-optical signal processing and new light source generation at awkward wavelengths.

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.

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.

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.

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.

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.

Synthesis of high quality silver nanowires and their applications in ultrafast photonics

Silver nanowires are widely used in catalysts, surface enhanced Raman scattering, microelectronic equipment, thin film solar cells, microelectrodes and biosensors for their excellent conductivity, heat transfer, low surface resistance, high transparency and good biocompatibility. However, the optical nonlinearity of silver nanowires has not been further explored yet. In this paper, three silver nanowire samples with different concentrations are produced via a typical hydrothermal method. Their applications to fiber lasers are implemented to prove the optical nonlinearity of silver nanowires for the first time. Based on three kinds of silver nanowires, the mode-locked operation of fiber lasers is successfully realized. Moreover, the fiber laser based on the silver nanowire with a concentration of 2 mg/L demonstrates the shortest pulse duration of 149.3 fs. The experiment not only proves the optical nonlinearity of silver nanowires, but also has some enlightenment on the selection of the optimum concentration of silver nanowires in the consideration of ultrashort pulse output.

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.

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.

Nonlinear optical properties of MoS2-WS2 heterostructure in fiber lasers

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

A pulsewidth measurement technology based on carbon-nanotube saturable absorber

We demonstrate a proof-of-concept saturable absorption based pulsewidth measurement (SAPM) by exploring the intensity dependent nonlinear transmission (i.e., saturable absorption) of low-dimensional material (LDM) carbon nanotubes. A minimum pulse energy of 75 fJ is experimentally detected with an average-power-peak-power product (P_av⋅ P_pk) of 5.44×10^-7 W² near 1550 nm. A minimum detectable pulse energy of 10 fJ with a P_av⋅ P_pk of 1.3×10^-9 W² is estimated with further optimization. The nanometer-level thickness and femtosecond-level decay time of LDMs allow ultrafast light interaction on a very small footprint, which potentially supports chip-scale characterization of ultrafast pulses with minimum distortion.

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.

CVD-grown MoSe2 with high modulation depth for ultrafast mode-locked erbium-doped fiber laser

Two-dimensional materials have been widely used as optical modulator materials in mode-locked fiber lasers. In terms of the performance of the fiber laser, one with an ultrashort pulse and high stability has great commercial value. Herein, the MoSe₂ grown by the chemical vapor deposition (CVD) method with high modulation depth, quality lattice structure and uniformity is successfully applied in a mode-locked erbium-doped fiber laser. The pulse duration and signal-to-noise ratio of the laser are 207 fs and 85 dB, respectively. The multifarious performance comparisons indicate that the CVD-based MoSe₂ saturable absorber with the tapered fiber structure has unique advantages not only in the generation of ultrashort pulses, but also in the optimization of laser stability.

Mechanically exfoliated In2Se3 as a saturable absorber for mode-locking a thulium-doped fluoride fiber laser operating in S-band

The rise of 2D materials since the discovery of graphene has been exponential. Their mechanical, electrical and optical properties are exceptional, similar to their 3D counterparts. In this paper, an α-In₂Se₃ crystal is mechanically exfoliated and transferred directly onto a fiber ferrule to serve as a saturable absorber (SA). A thulium-doped fluoride fiber is used as a gain medium to generate mode-locked pulses together with the In₂Se₃-based SA. The SA has a modulation depth of 14.6% and a saturation intensity of 0.4  kW/cm². The passively generated mode-locked pulses have a repetition rate of 6.93 MHz and a pulse width of 5.79 ps. The mode-locked pulses also have a signal-to-noise ratio of 65.4 dB and a time-bandwidth product of 0.36. The pulse energy and peak power are 0.179 nJ and 27.2 W, respectively.

Size effect of WSe2 on red passively Q-switched fiber laser output performance

Transition metal dichalcogenides (TMDs) possess a direct bandgap in the visible frequency range and can be applied as attractive visible saturable absorbers (SAs). In this paper, a new TMDs tungsten selenide (WSe₂)-based red Q-switcher is fabricated and successfully used for Q-switched pulse generation in a red Pr³⁺-doped ZBLAN fiber laser. The passive Q-switching fiber laser at 635.2 nm generates a stable pulse with pulse duration of 504 ns and average output power of 4.91 mW as well as a tunable pulse repetition rate of 131.9-260.4 kHz. Furthermore, the size effect of WSe₂ on the 635 nm passive Q-switching output performance is also investigated. Our work will provide certain significance for the optimization of passive Q-switching red fiber lasers based on TMDs.

Tungsten diselenide for all-fiber lasers with the chemical vapor deposition method

Two-dimensional materials have become the focus of research for their photoelectric properties, and are employed as saturable absorption materials. Currently, the challenge is how to further improve the modulation depth of saturable absorbers (SAs) based on two-dimensional materials. In this paper, three kinds of WSe2 films with different thicknesses are prepared using the chemical vapor deposition method. The nonlinear optical responses of the WSe2 films including the nonlinear saturable absorption and nonlinear refractive index are characterized by the double-balanced detection method and Z-scan experiments. Different modulation depths are successfully obtained by controlling the thickness of the WSe2 films. We further incorporate them into an all-fiber laser to generate mode-locked pulses. The mode-locked fiber lasers with a pulse duration of 185 fs, 205.7 fs and 230.3 fs are demonstrated when the thickness of the WSe2 films is measured to be 1.5 nm, 5.7 nm and 11 nm, respectively. This work provides new prospects for WSe2 in ultrafast photonic device applications.

Mode-locked thulium-doped fiber laser with chemical vapor deposited molybdenum ditelluride

A passively mode-locked thulium-doped fiber (TDF) laser was realized by employing chemical vapor deposited few-layer molybdenum ditelluride (MoTe₂) as a saturable absorber (SA). The few-layer MoTe₂ film was transferred onto the waist of a microfiber and then incorporated into a TDF laser with a typical all-fiber ring cavity configuration. Stable soliton pulses emitting at 1930.22 nm were obtained with a 3 dB bandwidth of 4.45 nm, a pulse duration of 952 fs, and an average power of 36.7 mW.

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.

Electrostatic Functionalization and Passivation of Water-Exfoliated Few-Layer Black Phosphorus by Poly Dimethyldiallyl Ammonium Chloride and Its Ultrafast Laser Application

Few-layer black phosphorus (BP) which exhibits excellent optical and electronic properties, has great potential applications in nanodevices. However, BP inevitably suffers from the rapid degradation in ambient air because of the high reactivity of P atoms with oxygen and water, which greatly hinders its wide applications. Herein, we demonstrate the electrostatic functionalization as an effective way to simultaneously enhance the stability and dispersity of aqueous phase exfoliated few-layer BP. The poly dimethyldiallyl ammonium chloride (PDDA) is selected to spontaneously and uniformly adsorb on the surface of few-layer BP via electrostatic interaction. The positive charge-center of the N atom of PDDA, which passivates the lone-pair electrons of P, plays a critical role in stabilizing the BP. Meanwhile, the PDDA could serve as hydrophilic ligands to improve the dispersity of exfoliated BP in water. The thinner PDDA-BP nanosheets can stabilize in both air and water even after 15 days of exposure. Finally, the uniform PDDA-BP-polymer film was used as a saturable absorber to realize passive mode-locking operations in a fiber laser, delivering a train of ultrafast pulses with the duration of 1.2 ps at 1557.8 nm. This work provides a new way to obtain highly stable few-layer BP, which shows great promise in ultrafast optics application.