Graphene Q-switched 2.78 μm Er3+-doped fluoride fiber laser

VTe2: Broadband Saturable Absorber for Passively Q-Switched Lasers in the Near- and Mid-Infrared Regions

In this study, we demonstrate the fabrication of a novel 2D transition metal dichalcogenide, VTe2, into a saturable absorber (SA) by using the liquid phase exfoliation method. Furthermore, the first-principles calculations were conducted to elucidate the electronic band structures and absorption spectrum. The nonlinear optical absorption properties of VTe2 at 1.0, 2.0, and 3.0 μm were measured using open-aperture Z-scan and P-scan methods, which showed saturation intensities and modulation depths of 95.57 GW/cm2 and 9.24%, 3.11 GW/cm2 and 7.26%, and 15.8 MW/cm2 and 17.1%, respectively. Furthermore, in the realm of practical implementation, the achievement of stable passively Q-switched (PQS) lasers employing SA composed of few-layered VTe2 nanosheets has manifested itself with broadband operating wavelengths from 1.0 to ∼3.0 μm. Specifically, PQS laser operations from near-infrared to mid-infrared with pulse durations of 195 and 563 ns for 1.0 and 2.0 μm solid-state lasers, respectively, and 749 ns for an Er3+-doped fluoride fiber laser at 3.0 μm were obtained. Our experimental results demonstrate that VTe2 is a potential broadband SA device for achieving PQS lasers. To the best of our knowledge, this is the first demonstration of using VTe2 as an SA in PQS lasers in the near- and mid-infrared regions, which highlights the potential of VTe2 for future research and applications in optoelectronic devices.

Broadband saturable absorption of indium tin oxide nanocrystals toward mid-infrared regime

We experimentally demonstrate the ultrabroadband optical nonlinearity of indium tin oxide nanocrystals (ITO NCs) in the mid-infrared regime. Especially, the ITO NCs show considerable saturation absorption behavior with large modulation depth covering the spectral range from 2-µm to 10-µm wavelength. We also demonstrate the application of the optical nonlinearity to successfully modulate the erbium-doped fluoride fiber laser to deliver a nanosecond pulse with a signal-to-noise ratio over 43 dB at 2.8-µm wavelength. The results provide a promising platform for the development of ITO-based broadband and robust optoelectronic devices toward the deep mid-infrared spectral range.

Mid-Infrared Optoelectronic Devices Based on Two-Dimensional Materials beyond Graphene: Status and Trends

Since atomically thin two-dimensional (2D) graphene was successfully synthesized in 2004, it has garnered considerable interest due to its advanced properties. However, the weak optical absorption and zero bandgap strictly limit its further development in optoelectronic applications. In this regard, other 2D materials, including black phosphorus (BP), transition metal dichalcogenides (TMDCs), 2D Te nanoflakes, and so forth, possess advantage properties, such as tunable bandgap, high carrier mobility, ultra-broadband optical absorption, and response, enable 2D materials to hold great potential for next-generation optoelectronic devices, in particular, mid-infrared (MIR) band, which has attracted much attention due to its intensive applications, such as target acquisition, remote sensing, optical communication, and night vision. Motivated by this, this article will focus on the recent progress of semiconducting 2D materials in MIR optoelectronic devices that present a suitable category of 2D materials for light emission devices, modulators, and photodetectors in the MIR band. The challenges encountered and prospects are summarized at the end. We believe that milestone investigations of 2D materials beyond graphene-based MIR optoelectronic devices will emerge soon, and their positive contribution to the nano device commercialization is highly expected.

High Modulation Depth Enabled by Mo2Ti2C3Tx MXene for Q-Switched Pulse Generation in a Mid-Infrared Fiber Laser

Two-dimensional (2D) materials show great promise as saturable absorbers (SAs) for ultrafast fiber lasers. However, the relatively low modulation depth and poor stability of some 2D materials, such as graphene and black phosphorus, restrict their applications in the mid-infrared pulse generation. Herein, we first report a novel 2D double transition metal carbide, denoted as Mo₂Ti₂C₃T_x MXene, as the saturable absorber (SA) for a passively Q-switched mid-infrared fiber laser. Due to the unique four-metal atomic layer structure, the Mo₂Ti₂C₃T_x exhibits superior saturable absorption properties, particularly with a higher modulation depth (40% at 2796 nm) than most of the other reported 2D SA materials. After incorporating the MXene SA with an erbium-doped fiber system, the passively Q-switched pulses were achieved with a repetition rate of 157.3 kHz, the shortest pulse width of 370 ns, and single-pulse energy of 1.92 μJ, respectively. Such results extend the MXene-based SAs as promising candidates for advanced photonic devices.

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 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.

Room-temperature watt-level and tunable ∼3 µm lasers in Ho3+/Pr3+ co-doped AlF3-based glass fiber

A room-temperature watt-level continuous-wave-output power mid-infrared fiber laser operating at $\lambda\sim 3\; \unicode{x00B5}{\rm m}$ is demonstrated using a ${{\rm Ho}^{3 +}}/{{\rm Pr}^{3 +}}$ co-doped ${{\rm AlF} _3}$ based glass fiber as a gain fiber. This fixed-wavelength laser had maximum output power of 1.13 W with a slope efficiency of 10.3% and a long-term operating stability of ${\gt}{40}\;{\min }$ without any additional packaging or active thermal management. A fiber laser with tunability from 2.842 to 2.938 µm showed a maximum output power of 110 mW.

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.

Long-term stable platinum diselenide for nanosecond pulse generation in a 3-µm mid-infrared fiber laser

In this paper, we fabricate the bulk-like multilayer platinum diselenide (PtSe₂) and employ it as saturable absorber (SA) for a passively Q-switched fiber laser operating at 2865 nm for the first time, to the best of our knowledge. The nonlinear optical measurements of the bulk-like multilayer PtSe₂ reveal efficient saturable absorption property at around 3 µm showing a modulation depth of 8.54% and a saturation intensity of 0.074 GW/cm². By introducing the bulk-like PtSe₂-SA into the Ho³⁺/Pr³⁺ co-doped ZBLAN fiber laser, stable Q-switched pulses with a duration as short as 620 ns are achieved at the pulse repetition rate of 238.1 kHz. The maximum average power is 93 mW, corresponding to a peak power of 0.63 W. The excellent long-term stability of the PtSe₂-SA was also verified utilizing the same experimental setup after 40 days of ambient storage of the PtSe₂ sample. The results not only validate the excellent nonlinear optical performance of PtSe₂, but also indicate that the bulk-like PtSe₂ is a promising long-term stable SA material under ambient conditions for nanosecond pulse generation in the 3-µm mid-infrared spectral region.

2166 nm all-fiber short-pulsed Raman laser based on germania-core fiber

We report a compact 2166 nm germania-fiber short-pulsed Raman laser based on the cavity matching scheme. The all-fiber Raman cavity is formed by a pair of 2166 nm fiber Bragg gratings. High-power noise-like pulses from a 1981 nm fiber laser are used to pump a 22 m germania-core fiber for providing Raman gain at ∼2166 nm, and readily realizes the Raman-cavity synchronization with high mismatching tolerance. Stable Raman pulses at 2166 nm are therefore generated with the tunable pulse width of 0.9-4.4 ns and the large pulse energy up to 12.15 nJ. This is, to the best of our knowledge, the first demonstration of all-fiber short-pulsed Raman laser in the mid-infrared region.

Au nanocages saturable absorber for 3-µm mid-infrared pulsed fiber laser with a wide wavelength tuning range

Au nanocages (Au-NCs) have attracted wide attention as low-dimensional materials with broadband absorption, ultrafast optical response, large third-order optical nonlinearity coefficient, and high photothermal stability and thermal tolerance. By employing Au-NCs as saturable absorbers, we demonstrate a widely tunable passively Q-switched erbium-doped fluoride fiber laser at the wavelength of 2.8 µm. When operates at 2778.0 nm, this laser delivers stable Q-switched pulses with a maximum average power of 584.6 mW at a pulse repetition rate of 80.6 kHz. The minimum pulse duration attained was 1.16 µs corresponding with the single pulse energy of 7.25 µJ. Our results present onefold increase in pulse energy over previously published values achieved from Au nanoparticles based 3-µm passively Q-switched fiber lasers. By introducing a plane ruled grating, a tuning rage of 57.0 nm from 2753.0 to 2810.0 nm is achieved, while maintaining stable Q-switched operation. To our knowledge, this is the first time to demonstrate that Au-NCs can realize mid-infrared pulsed laser. Our research results show that Au-NCs are promising broadband nonlinear modulators for mid-infrared pulse generation.

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

We report high-energy mid-infrared pulse generation by Q-switching of dysprosium-doped fiber lasers for the first time. Two different modulation techniques are demonstrated. Firstly, using active acousto-optic modulation, pulses are produced with up to 12 μJ energy and durations as short as 270 ns, with variable repetition rates from 100 Hz to 20 kHz and central wavelengths tunable from 2.97 to 3.23 μm. Experiments are supported by numerical modeling, identifying routes for improved pulse energies and to avoid multi-pulsing by careful choice of modulator parameters. Secondly, we demonstrate passive Q-switching by fabricating an inkjet-printed black phosphorus saturable absorber, simplifying the cavity and generating 1.0 μJ pulses with 740 ns duration. The performance and relative merits of each modulation approach are then critically discussed. These demonstrations highlight the potential of dysprosium as a versatile gain medium for high-performance pulsed sources beyond 3 μm.

Large aspect ratio gold nanorods (LAR-GNRs) for mid-infrared pulse generation with a tunable wavelength near 3 μm

We report a tunable passively Q-switched fiber laser at the wavelengths near 3 μm, using large aspect ratio gold nanorods (LAR-GNRs) as a saturable absorber (SA) for the first time. The GNRs with a large average aspect ratio of up to ~20 were prepared using the seed-mediated growth method, which yielded a strong absorption band of 2.2-3 μm with a peak at ~2600 nm, stemming from longitudinal surface plasmon resonance (SPR). The corresponding nonlinear absorption was characterized using 2.87 μm ultrafast pulses, giving the modulation depth of 8.89%, saturation intensity of 14.9 MW/cm², and nonsaturation loss of 39.9%. When introducing the material into a tunable Ho³⁺/Pr³⁺ codoped ZBLAN fiber laser as a SA, stable Q-switched pulses with a tunable wavelength within 2.83-2.88 μm were achieved. The largest output power of 30.8 mW, repetition rate of 78.12 kHz, and narrowest pulse width of 2.18 μs were simultaneously attained when tuned to ~2.865 μm at the pump power of 307.2 mW, while the largest pulse energy of 0.48 μJ was obtained at the longest tuning edge of 2.88 μm. Our work indicates that LAR-GNRs are a type of versatile broadband SA material available for the mid-infrared region.

Passively Q-switched fiber lasers based on pure water as the saturable absorber

We propose and demonstrate a passively Q-switched Er-doped fiber laser based on pure water as the saturable absorber (SA). The SA is made of two optical ferrules matched with a cannula, and the gap between the end-facets is filled with pure water. The nonlinear response of this SA has been characterized, and stable Q-switching operation at 1558.03 nm has been achieved. The maximum output power is 21.1 mW with 65.0 kHz repetition rate. The duration is 1.44 μs, and the pulse energy reaches 324.8 nJ. To the best of our knowledge, this is the first demonstration of the passively Q-switched laser with pure water as the SA. It provides further evidence of the possibility of liquid as an effective SA for pulsed lasers.

Ultrafast Dy3+:fluoride fiber laser beyond 3 μm

We report a passively mode-locked Dy³⁺:fluoride fiber laser emitting around 3.1 μm based on the nonlinear polarization evolution technique in a ring configuration, using in-band pumping at 2.8 μm. Transform-limited and self-starting mode-locked pulses as short as 828 fs with a center wavelength around 3.1 μm and repetition rates up to 60 MHz are obtained. In the single-pulse regime, a maximum average output power of 204 mW is measured, corresponding to a peak power of 4.2 kW and a pulse energy of 4.8 nJ. This first demonstration, to the best of our knowledge, of a femtosecond mode-locked fiber laser emitting directly beyond 3 μm paves the way for frequency comb synthesis in the molecular fingerprint region.

Compact self-Q-switched, tunable mid-infrared all-fiber pulsed laser

A compact self-Q-switched wavelength-tunable Ho³⁺:ZBLAN fiber laser around 3 μm is experimentally demonstrated for the first time. The mid-IR all-fiber cavity is formed by a pair of fiber end-facet mirrors. A 2 m-long heavily-doped Ho³⁺:ZBLAN fiber pumped by a homemade 1.15 μm fiber laser not only provides mid-IR optical gain, but also functions as an equivalent saturable absorber. Stable self-Q-switched pulses around 2.9 μm are generated at a low threshold of 36.6 mW, and the maximum average power obtained is 3.17 mW, corresponding to a pulse width of 1.54 μs and repetition rate of 67.8 kHz, respectively. By simply increasing the incident pump power, the mid-IR laser wavelength can be continuously tunable from 2927 nm to 2960 nm. Furthermore, when the pump power is fixed at 207.7 mW, a 42 nm wavelength tuning (2923 ∼ 2965 nm) from the self-Q-switched all-fiber laser is also achieved by applying the novel loss-adjusting technique.

Silicon-nanoparticle-based broadband optical modulators for solid-state lasers

Motivated by the tremendous success of carbon nanomaterials in acting as optical nonlinear modulators, in this Letter, the optical nonlinearity of their counterpart, silicon nanoparticles (SiNPs), is investigated. For the first time, to the best of our knowledge, the nonlinear optical property of SiNPs in 1 μm and 2 μm wavelength bands is observed. Its practical modulation performance is investigated by employing SiNPs as a saturable absorber (SA) in pulsed lasers, and the fabrication process, surface morphology, and linear and nonlinear optical response properties of the prepared SiNPs-SA are presented. Based on the SiNPs-SA, the formed Q-switched Nd:LuAG laser can generate laser pulses with the shortest duration of 490 ns at ∼1  μm and ∼2  μm laser pulses with the shortest duration of 453 ns are delivered from a Q-switched Tm:YAP laser, which shows that the SiNPs can be employed as a promising broadband SA for near- and mid-infrared spectral regions.

Gold nanostars as a Q-switcher for the mid-infrared erbium-doped fluoride fiber laser

We demonstrated passively Q-switched mid-infrared (mid-IR) erbium-doped fiber lasers by using gold nanostars (GNSs) as the Q-switcher. The nonlinear optical responses of the GNSs synthesized via the seed-mediated method have been characterized via a Z-scan technique, and the modulation depth and saturation intensity of the GNSs are measured to be 25% and 15.75  kW/cm², respectively. The Q-switched fiber laser can deliver a maximum average power of 454 mW with corresponding pulse energy of 3.6 μJ and pulse duration of 536 ns at a repetition rate of 125 kHz under the incident pump power 3.5 W. To the best of our knowledge, this is the first Letter that reports that the GNSs can act as a Q-switcher for the mid-IR erbium-doped ZBLAN fiber lasers. This Letter can deepen the understanding of the nonlinear optical behavior of the gold nanomaterials and may make inroads for the excellent mid-infrared optoelectronic devices.

An Ultrabroadband Mid-Infrared Pulsed Optical Switch Employing Solution-Processed Bismuth Oxyselenide

Pulsed lasers operating in the mid-infrared (3-25 µm) are increasingly becoming the light source of choice for a wide range of industrial and scientific applications such as spectroscopy, biomedical research, sensing, imaging, and communication. Up to now, one of the factors limiting the mid-infrared pulsed lasers is the lack of optical switch with a capability of pulse generation, especially for those with wideband response. Here, a semiconductor material of bismuth oxyselenide (Bi₂ O₂ Se) with a facile processibility, constituting an ultrabroadband saturable absorber for the mid-infrared (actually from the near-infrared to mid-infrared: 0.8-5.0 µm) is exhibited. Significantly, it is found that the optical response is associated with a strong nonlinear character, showing picosecond response time and response amplitude up to ≈330.1% at 5.0 µm. Combined with facile processibility and low cost, these solution-processed Bi₂ O₂ Se materials may offer a scalable and printable mid-infrared optical switch to open up the long-sought parameter space which is crucial for the exploitation of compact and high-performance mid-infrared pulsed laser sources.

Continuous-wave and chemical vapor deposition graphene-based passively Q-switched Er:Y2O3 ceramic lasers at 2.7 μm

We report on 976-nm diode-pumped Er:Y₂O₃ ceramic lasers in continuous-wave and passively Q-switched regimes. The maximum output power of continuous-wave laser operation is about 0.78 W with slope efficiency of about 11.8% at 2.7 μm. Passively Q-switched Er:Y₂O₃ ceramic laser operation with chemical vapor deposition (CVD) graphene as the saturable absorber is also demonstrated for the first time, to our knowledge. Using a monolayer CVD graphene, the achieved shortest pulse width is about 408 ns, while the shortest pulse width reduces to about 296 ns with pulse energy of 2.59 μJ and peak power of 8.77 W by using a three-layer CVD graphene. The results reveal that graphene is a very promising saturable absorber operating in the middle infrared spectral region.

State-switchable and wavelength-tunable gain-switched mid-infrared fiber laser in the wavelength region around 2.94 μm

We demonstrate a gain-switched singly Ho³⁺-doped ZBLAN fiber laser for the first time in the wavelength region around 2.94 μm which circumvents the strong water vapor lines. Four switchable gain-switched temporal states with 1/n (n = 4,3,2,1) pump repetition rates are first observed. The influences of pump power (pulse energy), repetition rate, duty cycle (pulse duration), and laser wavelength on their characteristics are studied, respectively. The results indicate that high pump repetition rate, large pump duty cycle, and short laser wavelength are beneficial for obtaining more gain-switched temporal states. For the case (n = 1), the increased pump repetition rate contributes to the increased pulse duration while decreased pulse energy and peak power. While μs-level pump pulse duration variation has an almost negligible effect on them. By introducing a plane ruled grating, the wavelength tuning was performed yielding a tuning range of 105 nm from 2895.5 nm to 3000.5 nm which just overlays the peak region of liquid water absorption. Finally, further optimizing of laser performances is discussed as well. This demonstration is helpful for preliminarily designing, prior to constructing a mid-infrared gain-switched laser which can find direct applications in laser surgery.

Emerging Low-Dimensional Materials for Nonlinear Optics and Ultrafast Photonics

Low-dimensional (LD) materials demonstrate intriguing optical properties, which lead to applications in diverse fields, such as photonics, biomedicine and energy. Due to modulation of electronic structure by the reduced structural dimensionality, LD versions of metal, semiconductor and topological insulators (TIs) at the same time bear distinct nonlinear optical (NLO) properties as compared with their bulk counterparts. Their interaction with short pulse laser excitation exhibits a strong nonlinear character manifested by NLO absorption, giving rise to optical limiting or saturated absorption associated with excited state absorption and Pauli blocking in different materials. In particular, the saturable absorption of these emerging LD materials including two-dimensional semiconductors as well as colloidal TI nanoparticles has recently been utilized for Q-switching and mode-locking ultra-short pulse generation across the visible, near infrared and middle infrared wavelength regions. Beside the large operation bandwidth, these ultrafast photonics applications are especially benefit from the high recovery rate as well as the facile processibility of these LD materials. The prominent NLO response of these LD materials have also provided new avenues for the development of novel NLO and photonics devices for all-optical control as well as optical circuits beyond ultrafast lasers.

Diode-end-pumped Ho, Pr:LiLuF4 bulk laser at 2.95 μm

A diode-end-pumped continuous-wave (CW) and passively Q-switched Ho, Pr:LiLuF₄ (Ho, Pr:LLF) laser operation at 2.95 μm was demonstrated for the first time, to the best of our knowledge. The maximum CW output power was 172 mW. By using a monolayer graphene as the saturable absorber, the passively Q-switched operation was realized, in which regimes with the highest output power, the shortest pulse duration, and the maximum repetition rate were determined to be 88 mW, 937.5 ns, and 55.7 kHz, respectively. The laser beam quality factor M² at the maximum CW output power were measured to be Mx2=1.48 and My2=1.47.

Graphitic C3N4 as a new saturable absorber for the mid-infrared spectral range

The saturable absorption properties of few-layer graphitic carbon nitride (g-C₃N₄) nanosheets near 3 μm were investigated. A stable Q-switched Er:Lu₂O₃ laser at 2.84 μm was realized by using a home-made g-C₃N₄ saturable absorber (SA), generating a pulse duration of 351 ns and an average output power of 1.09 W at a repetition rate of 99 kHz, corresponding to a pulse energy of 11.1 μJ. Our result indicates a great potential of g-C₃N₄ as a new SA in the 3 μm wavelength range.

High average power and energy microsecond pulse generation from an erbium-doped fluoride fiber MOPA system

We reported a high average power and energy microsecond pulse erbium-doped fluoride fiber MOPA system centered at 2786.8 nm. The master oscillator was a passively Q-switched erbium-doped fluoride fiber laser based on SESAM in a linear cavity. Then a one-stage erbium-doped fluoride fiber amplifier was used to boost its average output power to 4.2 W and pulse energy to 58.87 μJ. The pulse duration and repetition rate were 2.29 µs and 71.73 kHz, respectively. To the best of our knowledge, the achieved average output power and pulse energy are the recorded levels for the passively Q-switched fiber lasers at 3 μm wavelength region.

Passively synchronized Q-switched and mode-locked dual-band Tm3+:ZBLAN fiber lasers using a common graphene saturable absorber

Dual-band fiber lasers are emerging as a promising technology to penetrate new industrial and medical applications from their dual-band properties, in addition to providing compactness and environmental robustness from the waveguide structure. Here, we demonstrate the use of a common graphene saturable absorber and a single gain medium (Tm³⁺:ZBLAN fiber) to implement (1) a dual-band fiber ring laser with synchronized Q-switched pulses at wavelengths of 1480 nm and 1840 nm, and (2) a dual-band fiber linear laser with synchronized mode-locked pulses at wavelengths of 1480 nm and 1845 nm. Q-switched operation at 1480 nm and 1840 nm is achieved with a synchronized repetition rate from 20 kHz to 40.5 kHz. For synchronous mode-locked operation, pulses with full-width at half maximum durations of 610 fs and 1.68 ps at wavelengths of 1480 nm and 1845 nm, respectively, are obtained at a repetition rate of 12.3 MHz. These dual-band pulsed sources with an ultra-broadband wavelength separation of ~360 nm will add new capabilities in applications including optical sensing, spectroscopy, and communications.

Black phosphorus: a two-dimension saturable absorption material for mid-infrared Q-switched and mode-locked fiber lasers

Black phosphorus (BP) as a novel class of two-dimension (2D) materials has recently attracted enormous attention as a result of its unique physical and chemical features. The remarkably strong light-matter interaction and tunable direct band-gap at a wide range make it an ideal candidate especially in the mid-infrared wavelength region as the saturable absorber (SA). In this paper, the simple and effective liquid phase exfoliation (LPE) method was used to fabricate BP. By introducing the same BP SA into two specifically designed rare earth ions doped fluoride fiber lasers at mid-infrared wavebands, Q-switching with the pulse energy of 4.93 μJ and mode-locking with the pulse duration of 8.6 ps were obtained, respectively. The operation wavelength of ~2970 nm for generated pulse is the reported longest wavelength for BP SA based fiber lasers.

Watt-level passively Q-switched heavily Er(3+)-doped ZBLAN fiber laser with a semiconductor saturable absorber mirror

A diode-cladding pumped mid-infrared passively Q-switched Er³⁺-doped ZBLAN fiber laser with an average output power of watt-level based on a semiconductor saturable absorber mirror (SESAM) is demonstrated. Stable pulse train was produced at a slope efficiency of 17.8% with respect to launched pump power. The maximum average power of 1.01 W at a repetition rate of 146.3 kHz was achieved with a corresponding pulse energy of 6.9 μJ, from which the maximum peak power was calculated to be 21.9 W. To the best of our knowledge, the average power and the peak power are the highest in 3 μm region passively Q-switched fiber lasers. The influence of gain fiber length on the operation regime of the fiber laser has been investigated in detail.

Watt-level passively Q-switched Er:Lu₂O₃ laser at 2.84 μm using MoS₂

Efficient diode-pumped passively Q-switched Er:Lu2O3 laser operation at 2.84 μm was realized. A few-layer MoS2 nanosheet film on a YAG substrate, was fabricated and employed as saturable absorber (SA) in a short plane-plane cavity. Under an absorbed diode laser pump power of 7.61 W, an average output power of 1.03 W was generated with a pulse duration of 335 ns and a repetition rate of 121 kHz, resulting in a pulse energy of 8.5 μJ.

Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers

Passive Q-switching or mode-locking by placing a saturable absorber inside the laser cavity is one of the most effective and popular techniques for pulse generation. However, most of the current saturable absorbers cannot work well in the visible spectral region, which seriously impedes the progress of passively Q-switched/mode-locked visible pulsed fibre lasers. Here, we report a kind of visible saturable absorber-two-dimensional transition-metal dichalcogenides (TMDs, e.g. WS2, MoS2, MoSe2), and successfully demonstrate compact red-light Q-switched praseodymium (Pr(3+))-doped all-fibre lasers. The passive Q-switching operation at 635 nm generates stable laser pulses with ∼200 ns pulse duration, 28.7 nJ pulse energy and repetition rate from 232 to 512 kHz. This achievement is attributed to the ultrafast saturable absorption of these layered TMDs in the visible region, as well as the compact and all-fibre laser-cavity design by coating a dielectric mirror on the fibre end facet. This work may open a new route for next-generation high-performance pulsed laser sources in the visible (even ultraviolet) range.

Black phosphorus as saturable absorber for the Q-switched Er:ZBLAN fiber laser at 2.8 μm

Black phosphorus, a newly emerged two-dimensional material, has attracted wide attention as novel photonic material. Here, multilayer black phosphorus is successfully fabricated by liquid phase exfoliation method. By employing black phosphorus as saturable absorber, we demonstrate a passively Q-switched Er-doped ZBLAN fiber laser at the wavelength of 2.8 μm. The modulation depth and saturation fluence of the black phosphorus saturable absorber are measured to be 15% and 9 μJ/cm(2), respectively. The Q-switched fiber laser delivers a maximum average power of 485 mW with corresponding pulse energy of 7.7 μJ and pulse width of 1.18 μs at repetition rate of 63 kHz. To the best of our knowledge, this is the first time to demonstrate that black phosphorus can realize Q-switching of 2.8-μm fiber laser. Our research results show that black phosphorus is a promising saturable absorber for mid-infrared pulsed lasers.

Tunable Fe(2+):ZnSe passively Q-switched Ho(3+)-doped ZBLAN fiber laser around 3 μm

We report a tunable passively Q-switched Ho(3+)-doped ZBLAN fiber laser at 3 μm waveband using a Fe(2+):ZnSe crystal as saturable absorber (SA) and a plane ruled grating in Littrow configuration as wavelength tuning element. Stable pulse trains with ~85 nm tuning range from 2919.1 nm to 3004.2 nm and spectrum bandwidths of ~1 nm were achieved for the Fe(2+):ZnSe crystal with an initial transmission (IT) of 69%. Pulse duration increased from 1.23 μs to 2.35 μs and repetition rate decreased from 96.1 kHz to 43.56 kHz with the extension towards long wavelength direction. With the IT increasing to 79% and then 89%, though the available tuning range was slightly shortened, higher output power, pulse energy and slope efficiency were obtained with the slightly increased pulse duration and repetition rate. Maximum output power of 337 mW at a slope efficiency of 11.44% and pulse energy of 5.64 μJ were achieved at ~2970 nm and ~2991 nm, respectively. High signal noise ratio (SNR) of over 50 dB across the whole tuning range for the three ITs Fe(2+):ZnSe crystals indicated the stable Q-switching. To our knowledge, this is the first reported wavelength tunable passively Q-switched ZBLAN fiber laser.

3-μm Mid-infrared pulse generation using topological insulator as the saturable absorber

We report an 1150-nm diode-pump passively Q-switched Ho3+-doped ZBLAN fiber laser using topological insulator (TI): Bi2Te3 as the saturable absorber (SA). The TI: Bi2Te3 prepared using the cost-effective hydrothermal intercalation/exfoliation method was dropped onto a CaF2 substrate to fabricate the free-space SA component. It has a low saturable peak intensity of 2.12  MW/cm2 and a high modulation depth of 51.3% measured at 2 μm. Inserting this component into a linear-cavity Ho3+-doped ZBLAN fiber laser, stable Q-switched pulses at 2979.9 nm were obtained with the repetition rate of 81.96 kHz and pulse duration of 1.37 μs. The achieved maximum output power and pulse energy were 327.4 mW at a slope efficiency of 11.6% and 3.99 μJ, respectively, only limited by the available pump power. Our work reveals that the TIs are absolutely a class of promising and reliable SAs for pulse generation at 3-μm mid-infrared waveband.

Passive Q-switching of an all-fiber laser induced by the Kerr effect of multimode interference

A novel passively Q-switched all-fiber laser using a single mode-multimode-single mode fiber device as the saturable absorber based on the Kerr effect of multimode interference is reported. Stable Q-switched operation of an Er(3+)/Yb(3+) co-doped fiber laser at 1559.5 nm was obtained at a pump power range of 190-510 mW with the repetition rate varying from 14.1 kHz to 35.2 kHz and the pulse duration ranging from 5.69 μs to 3.86 μs. A maximum pulse energy of 0.8 μJ at an average output power of 27.6 mW was achieved. This demonstrates a new modulation mechanism for realizing Q-switched all-fiber laser sources.

Mid-infrared passively switched pulsed dual wavelength Ho(3+)-doped fluoride fiber laser at 3 μm and 2 μm

Cascade transitions of rare earth ions involved in infrared host fiber provide the potential to generate dual or multiple wavelength lasing at mid-infrared region. In addition, the fast development of saturable absorber (SA) towards the long wavelengths motivates the realization of passively switched mid-infrared pulsed lasers. In this work, by combing the above two techniques, a new phenomenon of passively Q-switched ~3 μm and gain-switched ~2 μm pulses in a shared cavity was demonstrated with a Ho³⁺-doped fluoride fiber and a specifically designed semiconductor saturable absorber (SESAM) as the SA. The repetition rate of ~2 μm pulses can be tuned between half and same as that of ~3 μm pulses by changing the pump power. The proposed method here will add new capabilities and more flexibility for generating mid-infrared multiple wavelength pulses simultaneously that has important potential applications for laser surgery, material processing, laser radar, and free-space communications, and other areas.

Towards ten-watt-level 3-5 µm Raman lasers using tellurite fiber

Raman lasers based on mid-infrared fibers operating at 3-5 µm atmospheric transparency window are attractive sources for several applications. Compared to fluoride and chalcogenide fibers, tellurite fibers are more advantageous for high power Raman fiber laser sources at 3-5 µm because of their broader Raman gain bandwidth, much larger Raman shift and better physical and chemical properties. Here we report on our simulations for the development of 10-watt-level 3-5 µm Raman lasers using tellurite fibers as the nonlinear gain medium and readily available continuous-wave (cw) and Q-switched erbium-doped fluoride fiber lasers at 2.8 µm as the pump sources. Our results show that a watt-level or even ten-watt-level fiber laser source in the 3-5 µm atmospheric transparency window can be achieved by utilizing the 1st- and 2nd-order Raman scattering in the tellurite fiber. The presented numerical study provides valuable guidance for future 3-5 um Raman fiber laser development.

Graphene Q-switched Ho(3+)-doped ZBLAN fiber laser at 1190 nm

We report Q-switched pulse operation of holmium (Ho(3+))-doped ZrF(4)-BaF(2)-LaF(3)-AlF(3)-NaF (ZBLAN) at ∼1190  nm in an all-fiber ring laser by using a fiber-optic graphene saturable absorber, which was fabricated by depositing graphene onto the flat surface of a side-polished D-shaped fiber. Stable Q-switched operation was established at a pump power of 180 mW with a repetition rate of 24 kHz and pulse width of 5.7 μs. When the pump power was increased to 1125 mW, 0.44 μJ Q-switched pulses with a repetition rate of 111 kHz and a pulse width of 0.8 μs were generated.

Fiber lasers and their applications [Invited]

Fiber lasers have seen progressive developments in terms of spectral coverage and linewidth, output power, pulse energy, and ultrashort pulse width since the first demonstration of a glass fiber laser in 1964. Their applications have extended into a variety of fields accordingly. In this paper, the milestones of glass fiber laser development are briefly reviewed and recent advances of high-power continuous wave, Q-switched, mode-locked, and single-frequency fiber lasers in the 1, 1.5, 2, and 3 μm regions and their applications in such areas as industry, medicine, research, defense, and security are addressed in detail.

Large-energy, wavelength-tunable, all-fiber passively Q-switched Er:Yb-codoped double-clad fiber laser with mono-layer chemical vapor deposition graphene

We demonstrate a large-energy, wavelength-tunable, all-fiber passively Q-switched Er:Yb-codoped laser using a mono-layer chemical vapor deposition (CVD) graphene saturable absorber (SA). By exploiting the large laser gain of Er:Yb double-clad fiber and optimizing the coupling ratio of the output coupler, not only can the mono-layer CVD graphene SA be protected from oversaturation and thermal damage, but also a large pulse energy up to 1.05 μJ (corresponding to the average output power of 25.6 mW) is thus achieved. Using a tunable fiber Fabry-Perot filter, stable Q-switched pulses can operate with a tunable range from 1530.97 to 1546.92 nm, covering a wavelength range of ∼16  nm. The Q-switching states at the different lasing wavelengths have been observed and recorded. The Q-switched repetition rate and the pulse duration (with the minimum one of 2.6 μs) have been characterized as well. This is, to the best of our knowledge, the largest pulse energy from an all-fiber graphene Q-switched laser.