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

Wavelength-tunable broadband lasers based on nanomaterials

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

Two-dimensional material integrated micro-nano fiber, the new opportunity in all-optical signal processing

With the development of all-optical networks, all-optical devices have become a research hotspot in recent years. Two-dimensional materials, represented by graphene and black phosphorus, have attracted great interest in the scientific community due to their excellent optical, electrical, magnetic, and mechanical properties. Bridging the gap between fiber optics and nanotechnology, microfibers can interact with light and matter at the micro or even nanoscale. By combining two-dimensional materials with microfibers, composite waveguides can be formed. They have the advantages of high nonlinear effect, all-fiber structure, and high damage threshold, etc. The composite waveguide can be directly applied to optical fiber communication systems, and plays an important role in the field of all-optical signal processing with a huge application prospect. In this review, the properties of typical 2D materials are first introduced. Next, the preparation methods of the relevant equipments are introduced and compared. Then, the all-optical signal processing technology based on 2D material-integrated microfiber composite waveguide is reviewed. The latest developments of all-optical modulators, all-optical wavelength converters, all-optical logic gates and all-optical thresholding devices are presented. Finally, the challenges and opportunities for the future development of 2D materials-integrated microfiber optoelectronic devices are summarized.

Q-switched and vector soliton pulses from an Er-doped fiber laser with high stability based on a γ-graphyne saturable absorber

As a Dirac material, an allotrope of graphene, namely γ-graphyne (γ-GY), is proved to have excellent nonlinear optical properties. Unfortunately, the saturable absorption properties and ultrafast photonics applications of γ-GY at the 1.5 μm band, which play vital roles in optical communication, have not been reported so far. Herein, γ-GY nanosheets (NSs) are prepared by an improved mechanochemical method, and a saturable absorber (SA) is fabricated by a laser-induced deposition method. The modulation depth (MD) and saturable fluence at 1.5 μm are found to be 5.40% and 23.46 μJ cm^-2, respectively. Consequently, by inserting the as-prepared SA into an Er³⁺-doped fiber laser (EDFL), Q-switching and mode-locking operation with high stability are realized. Also, the mode-locking pulses are verified to be polarization-locked vector solitons (PLVSs) based on further study. With increasing pump power, the phase difference between the two orthogonal components increases, leading to the evolution of state of polarization (SOP). Additionally, the degrees of polarization (DOPs) are measured and all reach more than 97%, meaning high polarization stability. Therefore, this work not only broadens the application scope of γ-GY in ultrafast photonics, but also provides an important foundation for the study of soliton dynamics.

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.

Dispersion Management and Pulse Characterization of Graphene-Based Soliton Mode-Locked Fiber Lasers

This paper presents the generation and characterization of femtosecond pulses utilizing graphene-polymethyl-methacrylate (PMMA) thin-film saturable absorber (SA), which is subjected to different lengths of single-mode fiber (SMF) in an erbium-doped fiber laser cavity. The graphene/PMMA-SA is prepared by using a simple transfer procedure of the thin-film on a fiber ferrule. By increasing the SMF length from 0 to 4 m, the corresponding group velocity dispersion of the entire cavity is estimated to change from −0.033 to −0.121 ps2. Analysis of the pulse performance shows that the pulse width behavior varies from 820 fs to 710 fs against different cavity lengths. Similarly, the pulse repetition rate and the spectral bandwidth can be adjusted from 12.5 to 10.0 MHz, and from 8.2 to 5.6 nm, respectively. A comprehensive discussion on the pulse performance is presented, which can contribute to widening the knowledge on the operation of graphene-based soliton mode-locked erbium-doped fiber lasers based on dispersion management by controlling the cavity length.

Review on Laser Technology in Intravascular Imaging and Treatment

Blood vessels are one of the most essential organs, which nourish all tissues in our body. Once there are intravascular plaques or vascular occlusion, other organs and circulatory systems will not work properly. Therefore, it is necessary to detect abnormal blood vessels by intravascular imaging technologies for subsequent vascular treatment. The emergence of lasers and fiber optics promotes the development of intravascular imaging and treatment. Laser imaging techniques can obtain deep vascular images owing to light scattering and absorption properties. Moreover, photothermal and photomechanical effects of laser make it possible to treat vascular diseases accurately. In this review, we present the research progress and applications of laser techniques in intravascular imaging and treatment. Firstly, we introduce intravascular optical coherent tomography and intravascular photoacoustic imaging, which can obtain various information of plaques. Multimodal intravascular imaging techniques provide more information about intravascular plaques, which have an essential influence on intravascular imaging. Secondly, two laser techniques including laser angioplasty and endovenous laser ablation are discussed for the treatment of arterial and venous diseases, respectively. Finally, the outlook of laser techniques in blood vessels, as well as the integration of laser imaging and treatment are prospected in the section of discussions.

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

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

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.

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.

Raman Spectroscopy Investigation of Graphene Oxide Reduction by Laser Scribing

Laser scribing has been proposed as a fast and easy tool to reduce graphene oxide (GO) for a wide range of applications. Here, we investigate laser reduction of GO under a range of processing and material parameters, such as laser scan speed, number of laser passes, and material coverage. We use Raman spectroscopy for the characterization of the obtained materials. We demonstrate that laser scan speed is the most influential parameter, as a slower scan speed yields poor GO reduction. The number of laser passes is influential where the material coverage is higher, producing a significant improvement of GO reduction on a second pass. Material coverage is the least influential parameter, as it affects GO reduction only under restricted conditions.

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.

Vector quartic solitons in birefringent fibers

We theoretically investigated the vector properties of quartic solitons in a pure fourth-order-dispersion birefringent fiber and a quartic-dispersion-dominant mode-locked fiber laser. We found that, compared with scalar pure quartic solitons, a vector quartic soliton (VQS) in the birefringent fiber still preserves the Gaussian shape, except for the distinctions of reduced peak power, central frequency offset, slight frequency chirp, and mitigated oscillatory tails. We also demonstrated that pulse shaping in the mode-locked laser cavity could explicitly facilitate the formation of Kelly sidebands and distortion of oscillatory tails. Furthermore, dynamical evolutions of quartic group-velocity-locked and polarization-rotating vector solitons were obtained to enrich the nonlinear community of VQSs. We believe that our elaborate findings will bring insights into both the fundamental understanding and potential applications of VQSs.

Stable-, period-N- and multiple-soliton regimes in a mode-locked fiber laser with inconsistently filtered central wavelengths

We systematically study the stable-, period-N- and multiple-soliton regimes in an Erbium-doped fiber laser effectively mode-locked by nonlinear polarization rotation technique. In the stable mode-locked regime, an invariant soliton with 497 fs pulse duration and 6.9 nm optical spectrum are obtained. With a larger pump power of 180 mW, the period-N state (in which the pulse intensity returns to its original value after N cavity-roundtrips) emerges, accompanied by sub-sideband generation on the first Kelly sideband and spectrum shift. Considering the inconsistent central wavelengths between gain and polarization-dependent isolator (PD-ISO) firstly, to our knowledge, the numerical results are in good agreement with the experiment and reveal the composite filtering of gain and PD-ISO takes major responsibility for spectrum shift, which causes group velocity offset simultaneously. Further study shows the continued increase of pump power can lead to the laser operating in the unstable multi-pulse state and the narrow spectral width contributes to stabilizing the multi-pulse state. Our work can promote the understanding of soliton dynamics and filtering in ultrafast fiber lasers.