Saturable absorbers incorporating carbon nanotubes directly synthesized onto substrates and fibers and their application to mode-locked fiber lasers

Single-frequency fiber lasers based on metal thiophosphites with sub-kHz linewidths

Metal thiophosphites (CrPX4) (X = S and Se), emerging layered metal chalcogenophosphate materials, have found extensive utility in the fields of advanced electronic devices, magnetic technologies, and electrochemical systems. However, their applications in single-frequency fiber lasers have been relatively limited. Here, liquid phase exfoliation technology is used to exfoliate the CrPX4 into few-layered samples. Subsequently, we explored the application of CrPX4-based saturable absorbers (CrPX4-SA) by precisely depositing CrPX4 nanosheets onto the surface of D-shaped fibers. This led to the successful creation of two stable single-frequency fiber lasers (SFFLs) based on CrPS4 and CrPSe4 with center wavelengths of 1550.068 nm and 1550.092 nm, respectively. The linewidth of CrPSe4 was tested and determined to be roughly 375 Hz, which is the narrowest ever value attained for an SFFL based on its structure, and its signal-to-noise ratio (SNR) was 74.17 dB. The maximum output power of each oscillator was 0.796 mW and 0.963 mW, and their corresponding power fluctuations were 2.5% and 1.4%, respectively.

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.

Buildup of multiple spatiotemporal nonlinear dynamics in an all-fiber multimode laser

Ultrafast lasers based on multimode fibers have attracted extensive attention owing to the large mode-field area and nonlinear tolerance. The high spatial degree of freedom of multimode fibers is significant for spatiotemporal pulses locked both in transverse and longitudinal modes, where the energy of output pulses can be remarkably improved. Herein, the 1.5-μm all-fiber spatiotemporal mode-locked laser was realized based on carbon nanotubes as a saturable absorber. Moreover, by tuning the polarization controller and the pump power carefully, the output wavelengths can be ranged from 1529 to 1565 nm based on the multimode interference filter. In addition, Q-switched mode-locking and spatiotemporal mode-locked dual combs were also observed by further adjusting the polarization controller. Such a kind of an all-fiber multimode laser offers a crucial insight into the spatiotemporal nonlinear dynamics, which is of great significance in scientific research and practical applications.

Mode-locked fiber laser based on a small-period long-period fiber grating inscribed by femtosecond laser

We demonstrate stable mode-locked pulses in an erbium-doped fiber laser (EDFL) using a femtosecond laser-inscribed small-period long-period grating (SP-LPG). The SP-LPG has a period of 25 µm and a length of 2.5 mm. The polarization dependent loss (PDL) of the SP-LPG reaches 20 dB at the wavelength of 1556 nm and 25 dB at the wavelength of 1607 nm, which is sufficient to trigger the mode-locking mechanism. In addition, a mode-locked fiber laser (MLFL) based on the SP-LPG has been demonstrated to generate 1.58-ps pulses at 1577 nm with a bandwidth of 4 nm and a repetition rate of 1.54 MHz. The signal-to-noise ratio (SNR) of 50 dB shows the high stability of this system. This work indicates various potential applications of the SP-LPG in ultra-fast laser technologies due to its simple fabrication, compact structure, and high damage threshold.

Mode-Locked Fiber Laser Sensors with Orthogonally Polarized Pulses Circulating in the Cavity

Intracavity phase interferometry is a powerful phase sensing technique using two correlated, counter-propagating frequency combs (pulse trains) in mode-locked lasers. Generating dual frequency combs of the same repetition rate in fiber lasers is a new field with hitherto unanticipated challenges. The large intensity in the fiber core, coupled with the nonlinear index of glass, result in a cumulative nonlinear index on axis that dwarfs the signal to be measured. The large saturable gain changes in an unpredictable way the repetition rate of the laser impeding the creation of frequency combs with identical repetition rate. The huge amount of phase coupling between pulses crossing at the saturable absorber eliminates the small signal response (deadband). Although there have been prior observation of gyroscopic response in mode-locked ring lasers, to our knowledge this is the first time that orthogonally polarized pulses were used to successfully eliminate the deadband and obtain a beat note.

Numerical analysis of hybrid mode-locking stability in a Ho-doped fiber laser

We numerically study a hybrid mode-locked holmium-doped fiber laser based on the coupled nonlinear Schrödinger equations, by introducing nonlinear polarization rotation and saturable absorber in a same cavity. Maps of mode-locking states related to the intracavity wave-plates are created to evaluate the stability of mode-locking. Nonlinear polarization rotation imposes negative effect of linear absorption and even reverse saturable absorption to the pulses in the hybrid cavity. Saturable absorber compensates the negative effect from nonlinear polarization rotation, then consequently contributes to the stability of single-pulse mode-locking states. The extension of pulse time duration indicates the dynamic synergy between nonlinear polarization rotation and saturable absorber rather than the static combination mechanism of them.

CsPbBr3 Perovskite Nanocrystals for a Q-Switched Pulsed Fiber Laser in the C-Band Region

All-inorganic perovskite nanocrystals have been widely reported as promising light-harvesting and light-emitting semiconductor nanomaterials. However, their nonlinear optical properties and laser applications have rarely been explored, especially for pulse laser modulation in the telecommunication C-band window. Herein, we experimentally demonstrated a passively Q-switched erbium-doped fiber laser (EDFL) operation at the C-band region using perovskite CsPbBr₃ nanocrystals as a saturable absorber (SA). The broadband linear optical absorption in the 300-2000 nm range and the nonlinear optical absorption at the C-band range of around 1560 nm were discovered and investigated in CsPbBr₃ nanocrystals. The CsPbBr₃-based SA exhibited good saturable absorption performance with a modulation depth and saturation intensity equivalent to 19.1% and 10.9 MW/cm², respectively. By integrating the CsPbBr₃ SA into an EDFL cavity, a passively Q-switched operation with a central wavelength of 1560 nm, a threshold pump power of 60 mW, and the shortest pulse duration of 5.96 μs was achieved. In addition, such a Q-switching operation exhibited long-term stability. Our results indicate that the CsPbBr₃ perovskite nanocrystals can serve as an efficient candidate for constructing pulsed lasers in the C-band or even longer NIR wavelength region.

2D BP/InSe Heterostructures as a Nonlinear Optical Material for Ultrafast Photonics

The BP/InSe heterojunction has attracted the attention of many fields in successful combined high hole mobility of black phosphorus (BP) and high electron mobility of indium selenide (InSe), and enhanced the environmental stability of BP. Nevertheless, photonics research on the BP/InSe heterostructure was insufficient, while both components are considered promising in the field. In this work, a two-dimensional (2D) BP/InSe heterostructure was fabricated using the liquid-phase exfoliation method. Its linear and non-linear optical (NLO) absorption was characterized by ultraviolet-visible-infrared and Open-aperture Z-scan technology. On account of the revealed superior NLO properties, an SA based on 2D BP/InSe was prepared and embedded into an erbium-doped fiber laser, traditional soliton pulses were observed at 1.5 μm with the pulse duration of 881 fs. Furthermore, harmonic mode locking of bound solitons and dark-bright soliton pairs were also obtained in the same laser cavity due to the cross-coupling effect. The stable mode-locked operation can be maintained for several days, which overcome the low air stability of BP. This contribution further proves the excellent optical properties of 2D BP/InSe heterostructure and provides new probability of developing nano-photonics devices for the applications of double pulses laser source and long-distance information transmission.

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.

Exploiting the FM-to-AM conversion to generate high-contrast picosecond laser pulses

In this work, we simulate and demonstrate experimentally a new approach to generate picosecond laser pulses. This technique is based on optoelectronic FM-to-AM conversion: a transient radiofrequency phase modulation is applied on a continuous-wave fiber laser otherwise blocked by a bandstop filter. By leveraging the ability of modern electronic amplifiers to generate high voltage swings values on a short timescale, the presented setup induces a large and brief excursion out of the filter, hence generating 18 ps pulses with a time contrast of 38 dB. Detailed simulations pave the road toward pulses shorter than 1 ps by using an optimally shaped RF response.

Performance enhancement of an ultrafast all-fiber laser based on an InN saturable absorber using GRIN coupling

Indium nitride (InN)-based semiconductor saturable absorbers have previously shown advantages for application in near-IR fiber lasers due to their broad modulation depth, ultrafast nonlinear response and thermal stability. However, up to now all demonstrated saturable absorber elements based on InN (either transmissive or reflective) have shown limited performance due to poor coupling and insertion losses. We present here a simple mode-locking device based on a GRIN-rod lens in conjunction with an InN semiconductor saturable absorber mirror (SESAM) for its use in a passively mode-locked all-fiber laser system operating at telecom wavelengths. Our results demonstrate that this coupling element ensures not only a compact, turnkey and alignment-free design but also a highly-stable optical femtosecond pulse train. The reduction of insertion losses (3.5 dB) enables the generation of 90-fs ultrafast pulses with an average power of 40 mW and up to 7 nJ of pulse energy without the need for additional amplification.

MXenes: synthesis, incorporation, and applications in ultrafast lasers

The rapid expansion of nanotechnology and material science prompts two-dimensional (2D) materials to be extensively used in biomedicine, optoelectronic devices, and ultrafast photonics. Owing to the broadband operation, ultrafast recovery time, and saturable absorption properties, 2D materials become the promising candidates for being saturable absorbers in ultrafast pulsed lasers. In recent years, the novel 2D MXene materials have occupied the forefront due to their superior optical and electronic, as well as mechanical and chemical properties. Herein, we introduce the fabrication methods of MXenes, incorporation methods of combining 2D materials with laser cavities, and applications of ultrafast pulsed lasers based on MXenes. Firstly, top-down and bottom-up approaches are two types of fabrication methods, where top-down way mainly contains acid etching and the chief way of bottom-up method is chemical vapor deposition. In addition to these two typical ones, other methods are also discussed. Then we summarize the advantages and drawbacks of these approaches. Besides, commonly used incorporation methods, such as sandwich structure, optical deposition, as well as coupling with D-shaped, tapered, and photonic crystal fibers are reviewed. We also discuss their merits, defects, and conditions of selecting different methods. Moreover, we introduce the state of the art of ultrafast pulsed lasers based on MXenes at different wavelengths and highlight some excellent output performance. Ultimately, the outlook for improving fabrication methods and applications of MXene-based ultrafast lasers is 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.

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.

Compounds Identified from Marine Mangrove Plant (Avicennia alba) as Potential Antiviral Drug Candidates Against WDSV, an In-Silico Approach

Walleye dermal sarcoma virus (WDSV) is a type of retrovirus, which affects most of the adult walleye fishes during the spawning time. The virus causes multiple epithelial tumors on the fish’s skin and fins that are liable for more than 50% of the mortality rate of fish around the world. Till now, no effective antiviral drug or vaccine candidates have been developed that can block the progression of the disease caused by the pathogen. It was found that the 582-amino-acid (aa) residues long internal structural gag polyprotein of the virus plays an important role in virus budding and virion maturation outside of the cell. Inhibition of the protein can block the budding and virion maturation process and can be developed as an antiviral drug candidate against the virus. Therefore, the study aimed to identify potential natural antiviral drug candidates from the tropical mangrove marine plant Avicennia alba, which will be able to block the budding and virion maturation process by inhibiting the activity of the gag protein of the virus. Initially, a homology modeling approach was applied to identify the 3D structure, followed by refinement and validation of the protein. The refined protein structures were then utilized for molecular docking simulation. Eleven phytochemical compounds have been isolated from the marine plant and docked against the virus gag polyprotein. Three compounds, namely Friedlein (CID244297), Phytosterols (CID12303662), and 1-Triacontanol (CID68972) have been selected based on their docking score −8.5 kcal/mol, −8.0 kcal/mol and −7.9 kcal/mol, respectively, and were evaluated through ADME (Absorption, Distribution, Metabolism and Excretion), and toxicity properties. Finally, molecular dynamics (MD) simulation was applied to confirm the binding stability of the protein-ligands complex structure. The ADME and toxicity analysis reveal the efficacy and non-toxic properties of the compounds, where MD simulation confirmed the binding stability of the selected three compounds with the targeted protein. This computational study revealed the virtuous value of the selected three compounds against the targeted gag polyprotein and will be effective and promising antiviral candidates against the pathogen in a significant and worthwhile manner. Although in vitro and in vivo study is required for further evaluation of the compounds against the targeted protein.

MXenes: Synthesis, Optical Properties, and Applications in Ultrafast Photonics

Recently, 2D materials are in great demand for various applications such as optical devices, supercapacitors, sensors, and biomedicine. MXenes as a kind of novel 2D material have attracted considerable research interest due to their outstanding mechanical, thermal, electrical, and optical properties. Especially, the excellent nonlinear optical response enables them to be potential candidates for the applications in ultrafast photonics. Here, a review of MXenes synthesis, optical properties, and applications in ultrafast lasers is presented. First, aqueous acid etching and chemical vapor deposition methods for preparing MXenes are introduced, in which the storage stability and challenges of the existing synthesis techniques are also discussed. Then, the optical properties of MXenes are discussed specifically, including plasmonic properties, optical detection, photothermal effects, and ultrafast dynamics. Furthermore, the typical ultrafast pulsed lasers enabled by MXene-based saturable absorbers operated at different wavelength regions are summarized. Finally, a summary and outlook on the development of MXenes is presented in the perspectives section.

Fe3O4 nanoparticle-enabled mode-locking in an erbium-doped fiber laser

In this paper, we have proposed and demonstrated the generation of passively mode-locked pulses and dissipative soliton resonance in an erbium-doped fiber laser based on Fe3O4 nanoparticles as saturable absorbers. We obtained self-starting mode-locked pulses with fundamental repetition frequency of 7.69 MHz and center wavelength of 1561 nm. The output of a pulsed laser has spectral width of 0.69 nm and pulse duration of 14 ns with rectangular pulse profile at the pump power of 190 mW. As far as we know, this is the firsttimethatFe3O4 nanoparticles have been developed as low-dimensional materials for passive mode-locking with rectangular pulse. Our experiments have confirmed that Fe3O4 has a wide prospect as a nonlinear photonics device for ultrafast fiber laser applications.

Laser-Induced Deposition of Carbon Nanotubes in Fiber Optic Tips of MMI Devices

The integration of carbon nanotubes (CNTs) into optical fibers allows the application of their unique properties in robust and versatile devices. Here, we present a laser-induced technique to obtain the deposition of CNTs onto the fiber optics tips of multimode interference (MMI) devices. An MMI device is constructed by splicing a section of no-core fiber (NCF) to a single-mode fiber (SMF). The tip of the MMI device is immersed into a liquid solution of CNTs and laser light is launched into the MMI device. CNTs solutions using water and methanol as solvents were tested. In addition, the use of a polymer dispersant polyvinylpyrrolidone (PVP) in the CNTs solutions was also studied. We found that the laser-induced deposition of CNTs performed in water-based solutions generates non-uniform deposits. On the other hand, the laser-induced deposition performed with methanol solutions generates uniform deposits over the fiber tip when no PVP is used and deposition at the center of the fiber when PVP is present in the CNTs solution. The results show the crucial role of the solvent on the spatial features of the laser-induced deposition process. Finally, we register and study the reflection spectra of the as-fabricated CNTs deposited MMI devices.

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.

Titanium dioxide fiber saturable absorber for Q-switched fiber laser generation in the 1-micrometer region

A passively Q-switched ytterbium-doped fiber laser (YDFL) operating at 1062 nm was demonstrated by using a segment of 20 cm titanium dioxide-doped fiber saturable absorber (TiO₂DF SA). The Q-switched YDFL emerged stably with tunable repetition rates ranging from 32 kHz to 53 kHz as the pump power rose from 109 mW to 233 mW. Within this range of pump power, a maximum output power of 10.1 mW, maximum peak power of 75 mW, and maximum pulse energy of 191 nJ were obtained. The narrowest pulse width of 2.55 μs was attained at the maximum pump power of 233 mW, while the signal-to-noise ratio of the fundamental frequency was 47 dB. This demonstration reveals that the proposed TiO₂DF SA is feasible for constructing a flexible and reliably stable Q-switched pulsed fiber laser in the 1-micrometer region.

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.

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.

Bulk-structured PtSe2 for femtosecond fiber laser mode-locking

The interest in pulsed fiber laser technology surrounds lots of critical optoelectronic applications ranging from optical communications, remote sensing to industrial material processing. Here, a femtosecond Er-doped fiber laser at 1560 nm have been demonstrated by using bulk-structured transition metal dichalcogenides (TMDs) - PtSe₂ as the pulse modulator. The PtSe₂ modulator was fabricated by using mechanically exfoliated methods from its single crystal and the exfoliated PtSe₂ was directly transferred to the polished surface of the D-shaped fiber. Due to the strong interaction between PtSe₂ and evanescent field, a self-starting Q-switched operation can be obtained. Furthermore, the mode-locking operation can be achieved easily by controlling the intra-cavity polarization states. Stable mode-locked operation can be maintained with pulse duration of 861 fs and signal to noise ratio (SNR) of 61.1 dB. These experimental results can validate the excellent nonlinear optical performance of PtSe₂, and may make inroads for the ultrafast applications of the bulk and low-dimensional TMDs.

Few-Layer Platinum Diselenide as a New Saturable Absorber for Ultrafast Fiber Lasers

A multilayer platinum diselenide (PtSe₂) film was experimentally demonstrated as a new type of saturable absorber with the capability to deliver robust dissipative solitons in a passively mode-locked fiber laser. The PtSe₂ film synthesized by chemical vapor deposition was placed onto the ferule of a single mode optical fiber through a typical dry transfer process. The nonlinear optical measurements reveal efficient saturable absorption characteristics in terms of a large modulation depth (26%) and low saturable intensity (0.346 GW cm^-2) at the wavelength of 1064 nm. An all-fiber ring cavity was built, in which the PtSe₂ film was sandwiched between two ferules as the saturable absorber and Ytterbium-doped fiber was used as the optical gain medium. Robust dissipative soliton pulses with a 3 dB spectral bandwidth of 2.0 nm and a pulse duration of 470 ps centered at 1064.47 nm were successfully observed in the normal dispersion regime. Moreover, our mode-locked lasers also exhibit good long-term stability. Our finding suggests that multilayer PtSe₂ may find potential applications in nonlinear optics and ultrafast photonics.

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

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

Passively Q-switched solid-state Tm:YAG laser using topological insulator Bi2Te3 as a saturable absorber

We demonstrated a passively Q-switched solid-state Tm:YAG laser using topological insulator (TI) Bi₂Te₃ as the saturable absorber (SA) for the first time, to the best of our knowledge. The Q-switched laser pulses were obtained with the minimum pulse width of 382 ns, the maximum pulse energy of 4.8 μJ, the maximum average output power of 272 mW, and a pulse repetition rate of 57.67 kHz. The results indicate that Bi₂Te₃ can be a promising kind of saturable absorber in the 2 μm wavelength region.

Fundamental and harmonic mode-locking at 2.1 μm with black phosphorus saturable absorber

We report mode-locking in holmium-doped all-fiber laser based on black phosphorus saturable absorber. The generated solitons are centered at 2094 nm with bandwidth reaching 4.2 nm and pulse duration of 1.3 ps. In harmonic mode-locking, up to 10th harmonic (290 MHz) was obtained. Properties of black phosphorus saturable absorber are investigated. Our findings validate black phosphorus suitability for ultrafast applications in mid-infrared.

Aluminum oxide nanoparticles as saturable absorber for C-band passively Q-switched fiber laser

We report on aluminum oxide nanoparticles (Al₂O₃ NPs) as a saturable absorber (SA) for passively Q-switched erbium-doped fiber laser (EDFL) operating at a wavelength of 1560.6 nm within the C-band region. A thin film of Al₂O₃-SA was prepared using polyvinyl alcohol (PVA) as a host polymer. Very stable pulses with a 57.8 KHz repetition rate and 5.6 μs pulse width at a threshold pump power of 158 mW were obtained. A 2.8 μs pulse width, 81 kHz pulse repetition rate, with maximum pulse energy of 56.7 nJ at a diode pump power of 330 mW were recorded. To the best of the authors' knowledge, this is the first time that a Al₂O₃-based SA has been used to generate a Q-switched EDFL.

All-fiber mode-locked laser via short single-wall carbon nanotubes interacting with evanescent wave in photonic crystal fiber

We report an all-fiber passively mode-locked laser based on a saturable absorber fabricated by filling short single-wall carbon nanotubes into cladding holes of grapefruit-type photonic crystal fiber. The single-wall carbon nanotube is insensitive to polarization of light for its one-dimensional structure, which suppresses the polarization dependence loss. Carbon nanotubes interact with photonic crystal fiber with ultra-weak evanescent field, which enhances the damage threshold of the saturable absorber and improves the operating stability. In our experiment, conventional soliton with a pulse duration of 1.003 ps and center wavelength of 1566.36 nm under a pump power of 240 mW is generated in a compact erbium-doped fiber laser cavity with net anomalous dispersion of -0.4102 ps². The signal to noise ratio of the fundamental frequency component is ~80 dB. The maximum average output power of the mode-locked laser reaches 9.56 mW under a pump power of 360 mW. The output power can be further improved by a higher pump power.

Polarization domain wall pulses in a microfiber-based topological insulator fiber laser

Topological insulators (TIs), are novel two-dimension materials, which can act as effective saturable absorbers (SAs) in a fiber laser. Moreover, based on the evanescent wave interaction, deposition of the TI on microfiber would create an effective SA, which has combined advantages from the strong nonlinear optical response in TI material together with the sufficiently-long-range interaction length in fiber taper. By using this type of TI SA, various scalar solitons have been obtained in fiber lasers. However, a single mode fiber always exhibits birefringence, and hence can support two orthogonal degenerate modes. Here we investigate experimentally the vector characters of a TI SA fiber laser. Using the saturated absorption and the high nonlinearity of the TI SA, a rich variety of dynamic states, including polarization-locked dark pulses and their harmonic mode locked counterparts, polarization-locked noise-like pulses and their harmonic mode locked counterparts, incoherently coupled polarization domain wall pulses, including bright square pulses, bright-dark pulse pairs, dark pulses and bright square pulse-dark pulse pairs are all observed with different pump powers and polarization states.

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

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

Graphite saturable absorber based on the pencil-sketching method for Q-switching of an erbium fiber laser

We experimentally demonstrate a practical and simple method for the preparation of a graphite-based, fiberized saturable absorber (SA). Our SA is prepared by using a low-cost graphite-core pencil that is commercially available from stationary stores to uniformly shade the flat end of a fiber ferrule. The saturable-absorption performance of the prepared SA was experimentally tested, and the feasibility of using the SA as a passive Q-switch was investigated through its incorporation into an erbium-doped-fiber ring cavity. The modulation depth of the SA is ∼1%, and the Q-switched pulses of a 1.98 μs temporal width were readily obtained at a repetition rate of 46.08 kHz.

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

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

Guided Photoluminescence from Integrated Carbon-Nanotube-Based Optical Waveguides

Thin films and ridge waveguides based on large-diameter semiconducting single-wall carbon nanotubes (s-SWCNTs) dispersed in a polyfluorene derivative are fabricated and optically characterized. Ridge waveguides are designed with appropriate dimensions for single-mode propagation at 1550 nm. Using multimode ridge waveguides, guided s-SWCNT photoluminescence is demonstrated for the first time in the near-infrared telecommunications window.

Mode-locked, 1.94-μm, all-fiberized laser using WS₂ based evanescent field interaction

We demonstrate the use of an all-fiberized, mode-locked 1.94 μm laser with a saturable absorption device based on a tungsten disulfide (WS2)-deposited side-polished fiber. The WS2 particles were prepared via liquid phase exfoliation (LPE) without centrifugation. A series of measurements including Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) revealed that the prepared particles had thick nanostructures of more than 5 layers. The prepared saturable absorption device used the evanescent field interaction mechanism between the oscillating beam and WS2 particles and its modulation depth was measured to be ~10.9% at a wavelength of 1925 nm. Incorporating the WS2-based saturable absorption device into a thulium-holmium co-doped fiber ring cavity, stable mode-locked pulses with a temporal width of ~1.3 ps at a repetition rate of 34.8 MHz were readily obtained at a wavelength of 1941 nm. The results of this experiment confirm that WS2 can be used as an effective broadband saturable absorption material that is suitable to passively generate pulses at 2 μm wavelengths.

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

A cell-type saturable absorber has been demonstrated by filling the single mode photonic crystal fiber (SMPCF) with tungsten disulfide (WS2) nanosheets. The modulation depth, saturable intensity, and non-saturable loss of this SA are measured to be 3.53%, 159 MW/cm(2) and 23.2%, respectively. Based on this SA, a passively mode-locked EDF laser has been achieved with pulse duration of 808 fs and repetition rate of 19.57 MHz, and signal-noise-ratio (SNR) of 60.5 dB. Our results demonstrate that the cell-type WS2 nanosheets SA can serve as a good candidate for short-pulse mode locker.

Pulse dynamics in carbon nanotube mode-locked fiber lasers near zero cavity dispersion

We numerically and experimentally analyze the output characteristics and pulse dynamics of carbon nanotube mode-locked fiber lasers near zero cavity dispersion (from 0.02 to ~-0.02 ps(2)). We focus on such near zero dispersion cavities to reveal the dispersion related transition between different mode-locking regimes (such as soliton-like, stretched-pulse and self-similar regimes). Using our proposed model, we develop a nanotube-mode-locked fiber laser setup generating ~97 fs pulse which operates in the stretched-pulse regime. The corresponding experimental results and pulse dynamics are in good agreement with the numerical results. Also, the experimental results from soliton-like and self-similar regimes exhibit the same trends with simulations. Our study will aid design of different mode-locking regimes based on other new saturable absorber materials to achieve ultra-short pulse duration.

Direct generation of 2 W average-power and 232 nJ picosecond pulses from an ultra-simple Yb-doped double-clad fiber laser

We report the generation of 2.06 W average-power and 232 nJ picosecond mode-locked pulses directly from an ultra-simple Yb-doped fiber laser. A section of Yb-doped double-clad fiber pumped by a 976 nm laser diode provides the large gain, and the linear cavity is simply formed by a 1064 nm highly reflective fiber Bragg grating and a fiber loop mirror (FLM) using a 5/95 optical coupler. The asymmetric FLM not only acts as the output mirror for providing ∼20% optical feedback, but also equivalently behaves as a nonlinear optical loop mirror (NOLM) to initiate the mode-locking operation in this cavity. Stable mode-locking is therefore achieved over a pump power of 3.76 W. The mode-locked pulses show the dissipative soliton resonance (DSR), which has the pulse duration of 695 ps to ∼1  ns, and the almost unchanged peak power of ∼200  W as increasing the pump power. In particular, this laser can emit 232 nJ high-energy DSR pulses with an average output power of >2  W. This is, to the best of our knowledge, the first demonstration of such an ultra-simple, mode-locked fiber laser that enables watt-level, high energy, picosecond DSR pulses.

Femtosecond harmonic mode-locking of a fiber laser based on a bulk-structured Bi(2)Te(3) topological insulator

We experimentally demonstrate femtosecond harmonic mode-locking of a fiber laser using a bulk-structured Bi(2)Te(3) topological insulator (TI)-deposited on a side-polished fiber as a mode-locker. A bulk-structured Bi(2)Te(3) TI film was prepared at a thickness of ~20 μm using a mechanical exfoliation method. Using the mode-locker in an erbium-doped fiber ring cavity, it was experimentally shown that harmonically mode-locked pulses with temporal widths of 630 ~700 fs could readily be generated upto the 55th harmonics. The pulse repetition rate was shown to be tunable from the fundamental resonance frequency of 14.07 MHz to the harmonic frequency of 773.85 MHz with increasing pump power. The pumping efficiency was measured at ~3.36 MHz/mW. The side mode suppression ratio (SMSR) was observed to be more than 27.3 dB over all harmonic orders, while the corresponding signal-to-noise ratio (SNR) ranged from 46.3 to 63 dB.

Automated mode locking in nonlinear polarization rotation fiber lasers by detection of a discontinuous jump in the polarization state

A strategy to align a mode-locked fiber laser with nonlinear polarization rotation is presented. This strategy is based on measurements of the output polarization state. It is shown that, as the angle of a motorized polarization controller inside the cavity is swept, the laser eventually reaches a mode-locked regime and the values of the Stokes parameters undergo an abrupt change. The sensing of this sudden variation is thus used to detect the mode-locking condition and a feedback mechanism drives the alignment of the polarization controller to force mode locking.

A practical topological insulator saturable absorber for mode-locked fiber laser

A novel saturable absorber (SA) was fabricated by coating the topological insulator (TI) film on microfiber using pulsed laser deposition (PLD) method. The TISA device had an insertion loss of ~1.25 dB, a saturable intensity of 26.7 MW/cm², a modulation depth of ~5.7%, and a nonsaturable loss of 20.5%. Upon employing this SA device, we established a passively mode-locked EDFL and achieved nearly free-chirped soliton pulse with 286 fs of pulse duration and >73 dB of signal to noise ratio (SNR). This result clearly evidences that the PLD is an effective scheme for practical SA device fabrication.

Towards low timing phase noise operation in fiber lasers mode locked by graphene oxide and carbon nanotubes at 1.5 µm

We investigate the timing phase noise of fiber lasers mode locked by graphene oxide (GO) and carbon nanotubes (CNTs), respectively, integrated in a linear cavity fiber laser in the reflecting operation. Due to the shorter decay time of the GO and CNTs, weaker slow saturable absorber effects are expected and mode-locked lasers based on these two saturable absorbers exhibit low excess timing phase noise coupled from the laser intensity noise. Compared with a reference laser mode locked by semiconductor saturable absorber mirror (SESAM), GO based laser obtains a timing phase noise reduction of 7 dB at 1 kHz and a timing jitter reduction of 45% experimentally whereas CNTs based laser obtains a timing phase noise reduction of 3 dB and a timing jitter reduction of 29%. This finding suggests that saturable absorbers with short decay time have the potential for achieving mode locking operation with low timing phase noise, which is important for applications including frequency metrology, high-precision optical sampling, clock distribution and optical sensing.

A 2.95 GHz, femtosecond passive harmonic mode-locked fiber laser based on evanescent field interaction with topological insulator film

By utilizing the pulsed laser deposition (PLD) method, we fabricated a kind of microfiber-based topological insulator (TI) saturable absorber (SA) which has inherent merits of effective and robust properties. We also proposed a newly explanation for the impact of nonlinear effect of SA on the harmonic mode-locking (HML) behavior. Upon employing on the SA, we achieved stable fundamental mode-locking (FML) at central wavelength of 1562.4 nm with pulse duration as short as 320 fs. By adjusting the intracavity polarization state at maximum pump power of 395 mW, we obtained stable femtosecond harmonic soliton pulse generation with repetition rate of 2.95 GHz and output power of 45.3 mW. Our results demonstrated that the microfiber-based TI PLD film SA is a promising device for practical multi-GHz ultrashort pulses generation.

Passively mode-locking erbium-doped fiber lasers with 0.3 nm single-walled carbon nanotubes

We demonstrate a passively mode-locked erbium-doped fiber laser (EDFL) by using the smallest single-walled carbon nanotubes (SWNTs) with a diameter of 0.3 nm as the saturable absorber. These ultrasmall SWNTs are fabricated in the elliptical nanochannels of a ZnAPO₄-11 (AEL) single crystal. By placing an AEL crystal into an EDFL cavity pumped by a 980 nm laser diode, stable passive mode-locking is achieved for a threshold pump power of 280 mW, and 73 ps pulses at 1563.2 nm with a repetition rate of 26.79 MHz.

Multiwall carbon nanotube polyvinyl alcohol-based saturable absorber in passively Q-switched fiber laser

In this work, we demonstrated a compact Q-switched erbium-doped fiber laser capable of generating high-energy pulses using a newly developed multiwall carbon nanotube (CNT) polyvinyl alcohol (PVA) thin film based saturable absorber. Q-switched pulse operation is obtained by sandwiching the thin film between two fiber ferrules forming a saturable absorber. A saturable absorber with 1.25 wt. % of PVA concentration shows a consistency in generating pulsed laser with a good range of tunable repetition rate, shortest pulse width, and produces a high pulse energy and peak power. The pulse train generated has a maximum repetition rate of 29.9 kHz with a corresponding pulse width of 3.49 μs as a function of maximum pump power of 32.15 mW. The maximum average output power of the Q-switched fiber laser system is 1.49 mW, which translates to a pulse energy of 49.8 nJ. The proposed method of multiwall CNT/PVA thin film fabrication is low in cost and involves uncomplicated processes.

Widely-tunable, passively Q-switched erbium-doped fiber laser with few-layer MoS2 saturable absorber

We propose and demonstrate a MoS2-based passively Q-switched Er-doped fiber laser with a wide tuning range of 1519.6-1567.7 nm. The few-layer MoS2 nano-platelets are prepared by the liquid-phase exfoliation method, and are then made into polymer-composite film to construct the fiber-compatible MoS2 saturable absorber (SA). It is measured at 1560 nm wavelength, that such MoS2 SA has the modulation depth of ∼ 2% and the saturable optical intensity of ∼ 10 MW/cm(2). By further inserting the filmy MoS2-SA into an Er-doped fiber laser, stable Q-switching operation with a 48.1 nm continuous tuning from S- to C-waveband is successfully achieved. The shortest pulse duration and the maximum pulse energy are 3.3 μs and 160 nJ, respectively. The repetition rate and the pulse duration under different operation conditions have been also characterized. To the best of our knowledge, it is the first demonstration of MoS2 Q-switched, widely-tunable fiber laser.

Mode-locking of Er-doped fiber laser using a multilayer MoS2 thin film as a saturable absorber in both anomalous and normal dispersion regimes

Application of a multilayer Molybdenum Disulfide (MoS2) thin film as a saturable absorber was experimentally demonstrated by realizing a stable and robust passive mode-locked fiber laser via the evanescent field interaction between the light and the film. The MoS2 film was grown by chemical vapor deposition, and was then transferred to a side polished fiber by a lift-off method. Intensity-dependent optical transmission through the MoS2 thin film on side polished fiber was experimentally observed showing efficient saturable absorption characteristics. Using erbium doped fiber as an optical gain medium, we built an all-fiber ring cavity, where the MoS2 film on the side polished fiber was inserted as a saturable absorber. Stable dissipative soliton pulse trains were successfully generated in the normal dispersion regime with a spectral bandwidth of 23.2 nm and the pulse width of 4.98 ps. By adjusting the total dispersion in the cavity, we also obtained soliton pulses with a width of 637 fs in the anomalous dispersion regime near the lasing wavelength λ = 1.55 μm. Detailed and systematic experimental comparisons were made for stable mode locking of an all-fiber laser cavity in both the normal and anomalous regimes.

Sub-130 fs mode-locked Er-doped fiber laser based on topological insulator

In this work we present for the first time, to the best of our knowledge, a stretched-pulse mode-locked fiber laser based on topological insulator. As a saturable absorber (SA) a ~0.5 mm thick lump of antimony telluride (Sb2Te3) deposited on a side-polished fiber was used. Such a SA introduced 6% modulation depth with 43% of non-saturable losses, which is sufficient for supporting stretched-pulse mode-locking. The ring laser resonator based on Er-doped active fiber with managed intracavity dispersion was capable of generating ultrashort optical pulses with full width at half maximum (FWHM) of 30 nm centered at 1565 nm. The pulses with duration of 128 fs were repeated with a frequency of 22.32 MHz.