A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

2D Layered Materials: Synthesis, Nonlinear Optical Properties, and Device Applications

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.

Emerging Low-Dimensional Materials for Nonlinear Optics and Ultrafast Photonics.

Ultrafast fiber lasers have significant applications in ultra-precision manufacturing, medical diagnostics, medical treatment, precision measurement and astronomical detection, owing to their ultra-short pulse width and ultra-high peak-power. Since graphene was first explored as an optical saturable absorber for passively mode-locked lasers in 2009, many other 2D materials beyond graphene, including phosphorene, antimonene, bismuthene, transition metal dichalcogenides (TMDs), topological insulators (TIs), metal–organic frameworks (MOFs) and MXenes, have been successively explored, resulting in rapid development of novel 2D materials-based saturable absorbers. Herein, we review the latest progress of the emerging 2D materials beyond graphene for passively mode-locked fiber laser application. These 2D materials are classified into mono-elemental, dual-elemental and multi-elemental 2D materials. The atomic structure, band structure, nonlinear optical properties, and preparation methods of 2D materials are summarized. Diverse integration strategies for applying 2D materials into fiber laser systems are introduced, and the mode-locking performance of the 2D materials-based fiber lasers working at 1–3 μm are discussed. Finally, the perspectives and challenges facing 2D materials-based mode-locked fiber lasers are highlighted.

Emerging 2D materials beyond graphene for ultrashort pulse generation in fiber lasers.

High-performance mode-locked and Q-switched fiber lasers based on novel 2D materials of topological insulators, transition metal dichalcogenides and black phosphorus: review and perspective (invited)

We reported diverse soliton operations in a thulium/holmium-doped fiber laser by taking advantage of a tapered fiber-based topological insulator (TI) Bi2Te3 saturable absorber (SA). The SA had a nonsaturable loss of ∼53.5% and a modulation depth of 9.8%. Stable fundamentally mode-locked solitons at 1909.5 nm with distinct Kelly sidebands on the output spectrum, a pulse repetition rate of 21.5 MHz, and a measured pulse width of 1.26 ps were observed in the work. By increasing the pump power, both bunched solitons with soliton number up to 15 and harmonically mode-locked solitons with harmonic order up to 10 were obtained. To our knowledge, this is the first report of both bunched solitons and harmonically mode-locked solitons in a fiber laser at 2 μm region incorporated with TIs.

Soliton mode-locked fiber laser based on topological insulator Bi 2 Te 3 nanosheets at 2 μm

In this Letter, a high-power 1.9-4.2 μm supercontinuum (SC) laser source with a real all-fiber structure is reported. A 12 m length of ZBLAN fluoride fiber was used as the nonlinear medium, which was pumped by a thulium-doped fiber amplifier through a firm fusion-spliced joint between silica fiber and itself. The obtained SC laser had a high spectral flatness with a maximal 10 dB bandwidth of 2090 nm spanning from 1960 to 4050 nm. A record power of 15.2 W for an all-fiber mid-infrared SC laser was measured. The average spectral power density of this SC laser was as high as 7.2 mW/nm. In particular, the SC power beyond 3.0 and 3.8 μm was 8.1 and 1.08 W which corresponded to a power ratio of 53.2% and 7.1%, respectively. This Letter, to the best of our knowledge, represents the brightest all-fiber mid-infrared SC laser, and provides a promising high-power pump light for SC generation in cascaded chalcogenide fibers.

15.2 W spectrally flat all-fiber supercontinuum laser source with >1 W power beyond 3.8 μm.

We report an all-fiberized 30-W supercontinuum (SC) generation in a piece of ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber. The pump source is a thulium-doped fiber amplifier (TDFA) with broadband output spectrum spanning the 1.9 to ∼2.6  μm region. The used ZBLAN fiber has a core diameter of 10 μm, and was directly fusion-spliced to the pigtail of the TDFA without using a traditional mode field adapter (MFA) or a piece of transition fiber. Such a low-loss and robust fusion splice joint, together with a robust AlF3-fiber-based endcap, enables efficient and high-power SC generation in the ZBLAN fiber. An SC with an average power up to 30.0 W and a spectral coverage of 1.9–3.35 μm with 20-dB bandwidth of 1.92–3.20 μm was obtained. Moreover, an SC with a broader spectrum was achieved by raising the pump pulse peak power (via reducing the duty ratio of the pump laser pulse). An SC with an output power of 27.4 W and a spectral coverage of 1.9–3.63 μm (with 20-dB bandwidth of 1.92–3.47 μm) was obtained, as well as an SC with output power of 24.8 W and a spectral coverage of 1.9–3.70 μm (with 20-dB bandwidth of 1.93–3.56 μm). The power conversion efficiency was measured as >69%. To the best of the authors’ knowledge, this research demonstrates the record output power of SC lasers based on ZBLAN fibers, paving the way for broadband and efficient multi-tens-of-watts SC generation in soft-glass fibers.

30-W supercontinuum generation based on ZBLAN fiber in an all-fiber configuration

A high-power, wavelength-tunable, all-fiber integrated thulium-doped fiber laser (TDFL) at 2 μm is presented The TDFL has a compact configuration which only consists of a low power seed oscillator and a stage of fiber power amplifier The seed oscillator adopts a tunable band-pass filter as the wavelength selective element, matching the gain spectrum of thulium-doped fiber It can provide ~5 W single-mode seed laser with superb spectral characteristics, and the lasing wavelength is adjustable from 1890 to 2050 nm The fiber power amplifier provides a total gain of ~17 dB at 2 μm which boosts the signal power to the 300 W-level The maximum average power reaches 3275 W at 1930 nm with the highest slope efficiency of 574% This TDFL can afford >270 W lasing operation over the whole tuning range of 140 nm spanning from 1910 to 2050 nm, together with high spectral quality and power stability This is the first demonstration, to the best of our knowledge, on an all-fiber integrated wavelength-widely-tunable TDFL at 2 μm with output power at the 300 W-level The results are of great interest for many applications

300 W-level, wavelength-widely-tunable, all-fiber integrated thulium-doped fiber laser

In this Letter, a high-power supercontinuum (SC) laser source which spanned from 19 to 36 μm with an all-fiber configuration was reported This SC laser was obtained by concatenating a thulium-doped fiber amplifier (TDFA) and a 12 cm long highly nonlinear germania fiber A 19–27 μm SC laser from the TDFA was spectrally broadened continuously into the mid-infrared region (>3  μm) in the following germania fiber When the repetition rate was 2 MHz, the obtained SC laser had a maximum output power of 612 W with an optical conversion efficiency of 153% with respect to the TDFA pump power The SC laser had a spectral bandwidth of 1506 nm ranging from 1944 to 3450 nm at the −20  dB level The SC power with wavelengths >3  μm was 29 W, corresponding to a high power ratio of 474% in the mid-infrared region The achieved power ratio in the mid-infrared region, as well as the long wavelength cutoff, to the best of our knowledge, were the best results ever reported in germania fibers

Bin Zhang

Papers

Soliton mode-locked fiber laser based on topological insulator Bi 2 Te 3 nanosheets at 2 μm

15.2 W spectrally flat all-fiber supercontinuum laser source with >1 W power beyond 3.8 μm.

30-W supercontinuum generation based on ZBLAN fiber in an all-fiber configuration

300 W-level, wavelength-widely-tunable, all-fiber integrated thulium-doped fiber laser

1.9-3.6 μm supercontinuum generation in a very short highly nonlinear germania fiber with a high mid-infrared power ratio.