Q-switched Dy:ZBLAN fiber lasers beyond 3 μm: comparison of pulse generation using acousto-optic modulation and inkjet-printed black phosphorus

ASE recirculation effects in pulsed frequency shifted feedback lasers at large frequency shifts

An analysis of the different emission regimes (continuous wave, Q-switched, and different forms of modelocking) of a C-band Er:fiber frequency shifted feedback laser at large frequency shifts is presented. We clarify the role of amplified spontaneous emission (ASE) recirculation in the origin of various spectral and dynamical properties of this type of laser. Specifically, we show that Q-switched pulses are supported by a noisy, quasiperiodic ASE recirculation pattern that univocally identifies the pulses within the sequence, and that these Q-switched pulses are chirped as a consequence of the frequency shift. A specific pattern of ASE recirculation, in the form of a periodic stream of pulses, is identified in resonant cavities, namely, those where the free spectral range and the shifting frequency are commensurable. The phenomenology associated with this pattern is explained through the moving comb model of ASE recirculation. Modelocked emission is induced from both integer and fractional resonant conditions. It is shown that ASE recirculation coexists with modelocked pulses, originates a secondary peak in the optical spectrum, and also drives Q-switched modelocking near resonant conditions. Harmonic modelocking with variable harmonic index is also observed in non-resonant cavities.

Germanate glass for laser applications in ∼ 2.1 μm spectral region: A review

The growing landscape of laser applications in ∼ 2 μm spectral region leads to the development of novel transparent media that provide low attenuation solutions for high power laser applications. This includes investigating glassy media such as heavy metal oxides (e.g. tellurite and germanate), fluoride and chalcogenides as an alternative to the most efficient and widely utilized silica glass. This review article discusses the potential of a heavy metal oxide lead-germanate glass (GeO₂-PbO-Ga₂O₃-Na₂O) for ∼ 2.1 μm laser applications, with a focus on our contribution to the field. Firstly, a comparative study of commercially available silicates and fluorides with germanate glass is presented to reveal germanate to be a favorable material for 2.1 μm laser applications. Secondly, as our contribution to the field, we present the development of the first ∼ 2.1 μm small cavity single frequency laser action in a Ho³⁺ doped GPGN glass that verifies its capability for ∼ 2 μm laser applications and beyond.

Mid-infrared optical switches enabled by metal-organic frameworks for compact high-power nanosecond laser sources at 3 µm

Pulsed lasers operating in the mid-infrared are of great importance for numerous applications in spectroscopy, medical surgery, laser processing, and communications. In spite of recent advances with mid-infrared gain platforms, the lack of a capable pulse generation mechanism hinders the development of compact mid-infrared pulsed laser source. Here we show that MIL-68(Al) and MIL-68(Fe), which are aluminum- and iron- based metal-organic frameworks (MOFs) with ordered atoms distribution and periodic mesoporous structure, constitute exceptional optical switches for the mid-infrared. We fabricated the MIL-68(Al) and MIL-68(Fe) via hydrothermal method and prepared reflection-type MIL-68(Al)- and MIL-68(Fe)- saturable absorber mirrors (SAMs). By employing the as-prepared SAMs in the laser cavities, we achieved high-power nanosecond Q-switched fiber lasers at 2.8 µm. Especially, the average output power and pulse duration of the MIL-68(Al) Q-switched fiber laser reached 809.1 mW and 567 ns, respectively. To the best of our knowledge, this is the first time to demonstrate that MIL-68(M) can be efficient optical switches for 3-µm mid-IR laser pulses generation. Our findings reveal that MIL-68(M) is promising saturable absorber for compact and high-performance mid-infrared pulsed lasers.

Recent developments in lanthanide-doped mid-infrared fluoride fiber lasers [Invited]

Mid-infrared fiber sources, emitting between 2.5 µm and 5.0 µm, are interesting for their great potential in several application fields such as material processing, biomedicine, remote sensing and infrared countermeasures due to their high-power, their diffraction-limited beam quality as well as their robust monolithic architecture. In this review, we will focus on the recent progress in continuous wave and pulsed mid-infrared fiber lasers and the components that bring these laser sources closer to a field deployment as well as in industrial systems. Accordingly, we will briefly illustrate the potential of such mid-infrared fiber lasers through a few selected applications.

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.

Watt-level superfluorescent fiber source near 3 µm

We report a watt-level mid-infrared (mid-IR) superfluorescent fiber source from ${{\rm Er}^{3 +}}$-doped ZBLAN fiber near 3 µm spectral range. With the power amplifier configuration, the mid-IR superfluorescent fiber source with power up to 1.85 W has been delivered successfully with slope efficiency about 18.6%. The experimental results may pave an avenue toward a high-power, high-temporal-stability superfluorescent source for versatile mid-IR applications.

200 µJ, 13 ns Er:ZBLAN mid-infrared fiber laser actively Q-switched by an electro-optic modulator

We report, as far as we know for the first time, on a pulsed 2.7 µm Er:ZBLAN fiber laser Q-switched by an electro-optic modulator. The Q-switched operation was achieved with a repetition rate range of 100 Hz-50 kHz. Pulse energy of 205.7 µJ and pulse width down to 13.1 ns, yielding a peak power of 15.7 kW, were obtained at a repetition rate of 100 Hz. The linewidth of the output spectrum was as narrow as 0.4 nm. The pulse width and the pulse peak power, to the best of our knowledge, are currently the shortest and the highest in the 3-µm-band Q-switched fiber lasers, respectively.

Advancements in Therapeutics via 3D Printed Multifunctional Architectures from Dispersed 2D Nanomaterial Inks

2D nanomaterials (2DNMs) possess fascinating properties and are found in multifarious devices and applications including energy storage devices, new generation of battery technologies, sensor devices, and more recently in biomedical applications. Their use in biomedical applications such as tissue engineering, photothermal therapy, neural regeneration, and drug delivery has opened new horizons in treatment of age-old ailments. It is also a rapidly developing area of advanced research. A new approach of integrating 3D printing (3DP), a layer-by-layer deposition technique for building structures, along with 2DNM multifunctional inks, has gained considerable attention in recent times, especially in biomedical applications. With the ever-growing demand in healthcare industry for novel, efficient, and rapid technologies for therapeutic treatment methods, 3DP structures of 2DNMs provide vast scope for evolution of a new generation of biomedical devices. Recent advances in 3DP structures of dispersed 2DNM inks with established high-performance biomedical properties are focused on. The advantages of their 3D structures, the sustainable formulation methods of such inks, and their feasible printing methods are also covered. Subsequently, it deals with the therapeutic applications of some already researched 3DP structures of 2DNMs and concludes with highlighting the challenges as well as the future directions of research in this area.

1.4 W in-band pumped Dy3+-doped gain-switched fiber laser at 3.24 µm

In this Letter, we report, to the best of our knowledge, the first demonstration of an in-band pumped gain-switched Dy³⁺-doped fiber laser operating at 3.24 µm. The monolithic cavity bounded by two fiber Bragg gratings was pumped by a gain-switched Er³⁺-doped fiber system. It produced stable nanosecond pulses in a single-pulse regime on its entire operating range from 20 kHz to 120 kHz. A record average power of 1.43 W was achieved for a repetition rate of 120 kHz, and a record pulse energy of 19.2 µJ was achieved at 60 kHz. These results represent a significant improvement in Dy³⁺-doped pulsed fiber laser performances and open the way to applications in the fields of remote sensing and material processing.

Experimental Investigation of Actively Q-Switched Er3+:ZBLAN Fiber Laser Operating at around 2.8 µm

A diode-pumped Q-switched Er3+:ZBLAN double-clad, single-transverse mode fiber laser is practically realized. The Q-switched laser characteristics as a function of pump power, repetition rate, and fiber length are experimentally investigated. The results obtained show that the Q-switched operation with 46 µJ pulse energy, 56 ns long pulses, and 0.821 kW peak power is achieved at a pulse repetition rate of 10 kHz. To the best of our knowledge, this is the highest-ever demonstrated peak power emitted from an actively Q-switched, single-transverse mode Er3+:ZBLAN fiber laser operating near 2.8 µm.

Ultrafast nonlinear refraction measurements of infrared transmitting materials in the mid-wave infrared

We utilize the conventional Z-scan technique to provide absolute measurements of third-order nonlinear refraction coefficients (n₂) in the mid-wave infrared at 2 µm and 3.9 µm of common optical materials that have transparency windows spanning this regime. We study a variety of narrow band gap and wide band gap semiconductors, fluoride crystals (BaF₂, CaF₂, LiF, and MgF₂) and optical glasses, and a series of chalcogenide glasses. The n₂ is found to span on the order of ∼10^-15 to ∼10^-12 cm²/W for the semiconductors, ∼10^-16 cm²/W for the fluoride crystals and glasses, and ∼10^-14 to ∼10^-13 cm²/W for the chalcogenides. The experimental results are compared to previous measurements of n₂ conducted in the visible and near-infrared along with empirical and theoretical formulations.

Gain-switched dysprosium fiber laser tunable from 2.8 to 3.1 μm

We demonstrate a tunable gain-switched Dy³⁺-doped ZBLAN fiber laser around 3 μm, for the first time, to the best of our knowledge. The pump source is a homemade actively Q-switched Yb³⁺-doped silica fiber laser at 1.1 μm, yielding a repetition rate of 80 kHz and a pulse width of 300 ns. A plane-ruled grating in a Littrow configuration functions as the tuning element. At the launched pump power of 2.78 W, stable gain-switched pulses tunable within a ~300 nm range of 2800~3095 nm are achieved. When tuning the wavelength to 2946.5 nm, a maximum output power of 218.6 mW with a repetition rate of 80 kHz is obtained. The corresponding pulse width and energy are 530 ns and 2.73 μJ, respectively. The results imply that gain-switching of Dy³⁺-doped ZBLAN fiber laser is an alternative way for mid-infrared nanosecond pulse generation around 3 μm.