Mid-infrared Raman-soliton continuum pumped by a nanotube-mode-locked sub-picosecond Tm-doped MOPFA

Photonic device combined optical microfiber coupler with saturable-absorption materials and its application in mode-locked fiber laser

We demonstrated a mode-locked fiber laser based on a novel photonic device that combined optical microfiber coupler (OMC) and saturable absorption materials. The stable ultrafast laser was formed based on the interaction between the deposited Indium Antimonide (InSb) and the evanescent field on OMC. Different from optical microfiber (OM), OMC can directly output the mode-locked laser without additional beam splitting devices, which further improves the integrated characteristics of the fiber laser. The pulse duration of the output pulse is 405 fs at the central wavelength of 1560 nm. To the best of our knowledge, this is the first time that optical microfiber coupler based saturable absorber (OMC-SA) for mode-locked fiber laser is demonstrated.

Study on soliton self-frequency shift in a Tm-doped fiber amplifier seeded by a Kelly-sideband-suppressed conventional soliton

We experimentally present mid-infrared Raman soliton self-frequency shift (SSFS) process in a Tm-doped fiber amplifier using sideband-suppressed conventional solitons as seed pulses. The strong Kelly sidebands of the soliton oscillator were efficiently suppressed (more than 21 dB) using a home-made all-fiber Lyot filter (AFLF). As a result, the Raman solitons with a continuously tunable wavelength of 1.95-2.34 µm were achieved, with a high soliton energy conversion of >93% over the range of 1.95-2.24 µm. The conversion efficiency and tunable range of Raman solitons were both significantly improved, comparing to the same amplifier seeded with sideband-unsuppressed pulses.

Mid-infrared Raman lasers and Kerr-frequency combs from an all-silica narrow-linewidth microresonator/fiber laser system

Mid-infrared (mid-IR) lasers have great applications in bio-molecular sensing due to strong vibrational fingerprints in this wavelength range. However, it is a huge challenge to realize mid-IR lasers in conventional silica materials. Here, we demonstrate the generation of mid-IR Raman lasers and Kerr-frequency combs from an all-silica microresonator/fiber laser system. A single wavelength narrow-linewidth laser at ∼2 µm is first realized by using an ultrahigh Q-factor silica whispering-gallery-mode (WGM) microresonator as mode-selection mirror, and thulium-doped silica fiber as gain medium. Due to the strong intensity enhancement in the microresonator itself, multiple third-order nonlinear optical effects are observed, which include stimulated Stokes and anti-Stokes Raman scattering, and (cascaded) four-wave-mixing (FWM). The stimulated Stokes and anti-Stokes Raman scattering shift the initial 2 µm narrow-linewidth laser to as far as ∼2.75 µm and ∼1.56 µm, respectively. While the cascaded FWM helps to form a Kerr-frequency comb with a broad bandwidth of ∼900 nm and a mode spacing of twice of the microresonator free-spectral-range. This work offers a simple and effective route to realize all-silica mid-IR lasers based on enhanced optical nonlinearity in WGM microresonators.

Compact all-fiber 2.1-2.7 μm tunable Raman soliton source based on germania-core fiber

Although ultrafast rare-earth-doped fiber lasers mode-locked at near-infrared and ∼3 μm wavelengths have been well developed, it is relatively difficult to achieve ultrafast fiber laser emitting in the 2.1-2.7 μm spectral gap between ∼2 μm (Tm fiber) and ∼2.8 μm (Er or Ho fluoride fiber). In this paper, we report the generation of 2.1-2.7 μm tunable femtosecond Raman solitons from a compact fusion-spliced all-fiber system using a home-made 1.96 μm ultrafast pump source and a MIR-available germania-core fiber. At first, a Tm-doped double-clad fiber amplifier is used to not only boost up the power of 1957 nm femtosecond seed laser, but also to generate the first-order soliton self-frequency shift (SSFS). The first-order Raman solitons can be tuned from 2.036 to 2.152 μm, have a pulse duration of ∼480 fs and can reach a pulse energy of 1.07 nJ. The first-order Raman solitons are further injected into a 94 mol.% germania-core fiber to excite the second-order SSFS. The second-order solitons can be tuned to longer wavelengths, i.e. from 2.157 μm up to 2.690 μm. Our work could provide an effective way to develop compact, all-fiber ultrafast MIR laser sources with the continuous wavelength tuning of 2.1-2.7 μm.

Nonlinear Optics with 2D Layered Materials

2D layered materials (2DLMs) are a subject of intense research for a wide variety of applications (e.g., electronics, photonics, and optoelectronics) due to their unique physical properties. Most recently, increasing research efforts on 2DLMs are projected toward the nonlinear optical properties of 2DLMs, which are not only fascinating from the fundamental science point of view but also intriguing for various potential applications. Here, the current state of the art in the field of nonlinear optics based on 2DLMs and their hybrid structures (e.g., mixed-dimensional heterostructures, plasmonic structures, and silicon/fiber integrated structures) is reviewed. Several potential perspectives and possible future research directions of these promising nanomaterials for nonlinear optics are also presented.

Wavelength and pulse duration tunable ultrafast fiber laser mode-locked with carbon nanotubes

Ultrafast lasers with tunable parameters in wavelength and time domains are the choice of light source for various applications such as spectroscopy and communication. Here, we report a wavelength and pulse-duration tunable mode-locked Erbium doped fiber laser with single wall carbon nanotube-based saturable absorber. An intra-cavity tunable filter is employed to continuously tune the output wavelength for 34 nm (from 1525 nm to 1559 nm) and pulse duration from 545 fs to 6.1 ps, respectively. Our results provide a novel light source for various applications requiring variable wavelength or pulse duration.

Fabrication of ultra-high numerical aperture GeO2-doped fiber and its use for broadband supercontinuum generation

We report the fabrication, characterization, and application (broadband supercontinuum [SC] generation) of ultra-high numerical-aperture heavily (50 mol. %) GeO₂-doped optical fiber, obtained through a modified chemical vapor deposition process and rod-in-tube method. The formation of Ge-related diamagnetic defect centers, such as germanium oxygen defect centers (GeODC) with nonbridging lone electron pairs, confirmed by x-ray photoelectron spectroscopy and optical absorption studies, inducing hypolarizable local dipoles, may be responsible in boosting the nonlinear effects and enhancing stimulated Raman scattering at pumping with high-power pulses, culminating in generation of broadband SC generation. The SC spans toward the Stokes side up to 2.4 μm, under the action of ns-range pulses launched from a smartly Q-switched erbium-doped fiber laser with operation wavelength (1.56 μm) matching the zero-dispersion wavelength of the high GeO₂-doped fiber.

High-energy and efficient Raman soliton generation tunable from 1.98 to 2.29 µm in an all-silica-fiber thulium laser system

We demonstrate a compact, all-fiber-integrated laser system that delivers Raman solitons with a duration of ∼100  fs and pulse energy of up to 13.3 nJ, continuously wavelength tunable from 1.98 to 2.29 µm via Raman-induced soliton self-frequency shift (SSFS) in a thulium-doped fiber amplifier. We realize a >90% efficiency of Raman conversion, the highest reported value from SSFS-based sources, to the best of our knowledge. This enables us to achieve >10  nJ soliton energy from a 2.16 to 2.29 µm range, the highest energy demonstrated above 2.22 µm from an SSFS-assisted, all-fiber tunable single-soliton-pulse source, to the best of our knowledge. Our simple and compact all-fiber tunable laser could serve as an efficient ∼2  µm femtosecond source for a wide range of mid-infrared applications.

Double-Wall Carbon Nanotube Hybrid Mode-Locker in Tm-doped Fibre Laser: A Novel Mechanism for Robust Bound-State Solitons Generation

The complex nonlinear dynamics of mode-locked fibre lasers, including a broad variety of dissipative structures and self-organization effects, have drawn significant research interest. Around the 2 μm band, conventional saturable absorbers (SAs) possess small modulation depth and slow relaxation time and, therefore, are incapable of ensuring complex inter-pulse dynamics and bound-state soliton generation. We present observation of multi-soliton complex generation in mode-locked thulium (Tm)-doped fibre laser, using double-wall carbon nanotubes (DWNT-SA) and nonlinear polarisation evolution (NPE). The rigid structure of DWNTs ensures high modulation depth (64%), fast relaxation (1.25 ps) and high thermal damage threshold. This enables formation of 560-fs soliton pulses; two-soliton bound-state with 560 fs pulse duration and 1.37 ps separation; and singlet+doublet soliton structures with 1.8 ps duration and 6 ps separation. Numerical simulations based on the vectorial nonlinear Schr¨odinger equation demonstrate a transition from single-pulse to two-soliton bound-states generation. The results imply that DWNTs are an excellent SA for the formation of steady single- and multi-soliton structures around 2 μm region, which could not be supported by single-wall carbon nanotubes (SWNTs). The combination of the potential bandwidth resource around 2 μm with the soliton molecule concept for encoding two bits of data per clock period opens exciting opportunities for data-carrying capacity enhancement.

Mid-infrared transmitter and receiver modules for free-space optical communication

We report on the experimental implementation of single-frequency fiber-laser pumped mid-infrared (mid-IR) transmitter and receiver modules for free-space communications. These modules enable frequency upconversion and downconversion between the 1550-nm telecom wavelength and the mid-IR, thus providing essential free-space transmission links with mid-IR single-frequency lasers in the 3.6 μm region. Specifically, based on difference frequency generation (DFG) in MgO-doped periodically poled LiNbO₃ (MgO:PPLN), the mid-IR transmitter produces 9.3-mW power at 3594 nm with 5-W pump power at 1083 nm (<10  kHz linewidth) and 3-W signal power at 1550 nm (<10  kHz linewidth), and the mid-IR receiver reproduces 12-μW power at 1550 nm with 4.7-W pump power at 1083 nm and 5-mW laser at 3594 nm. The whole modules are integrated into portable and compact devices by incorporating single-frequency fiber lasers, fiber amplifiers, DFG units, and related electronic circuits. In addition, the uses of all polarization-maintaining fiber configuration and well-controlled heat dissipation make the mid-IR transmitter and receiver exhibit a long-term stability.

Gain-switched monolithic fiber laser with ultra-wide tuning range at 2 μm

We demonstrate a gain-switched thulium-doped fiber laser (TDFL) built in an all-fiber format producing nanosecond pulses with variable wavelength in the 2 μm waveband. The laser features tunable operation in an ultra-wide spectral region of 1765 - 2055 nm (24 THz). The nearly 300 nm tunability doubles the record tuning range of existing gain-switched fiber lasers, and to the best of our knowledge, presents the broadest tuning range that has been reported for a monolithic pulsed rare earth doped fiber laser to date. The TDFL can operate at a repetition rate of 2.5 - 100 kHz with a pulse width as short as ~200 ns. Influences of various system parameters on the laser performance are investigated in detail.

Record power, ultra-broadband supercontinuum source based on highly GeO2 doped silica fiber

We demonstrate highly germania doped fibers for mid-infrared supercontinuum generation. Experiments ensure a highest output power of 1.44 W for a broadest spectrum from 700 nm to 3200 nm and 6.4 W for 800 nm to 2700 nm from these fibers, while being pumped by a broadband Erbium-Ytterbium doped fiber based master oscillator power amplifier. The effect of repetition frequency of pump source and length of germania-doped fiber has also been investigated. Further, germania doped fiber has been pumped by conventional supercontinuum source based on silica photonic crystal fiber supercontinuum source. At low power, a considerable broadening of 200-300 nm was observed. Further broadening of spectrum was limited due to limited power of pump source. Our investigations reveal the unexploited potential of germania doped fiber for mid-infrared supercontinuum generation. These measurements ensure the potential of germania based photonic crystal fiber or a step-index fiber supercontinuum source for high power ultra-broad band emission being by pumped a 1060 nm or a 1550 nm laser source. To the best of our knowledge, this is the record power, ultra-broadband, and all-fiberized supercontinuum light source based on silica and germania fiber ever demonstrated to the date.

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

In this Letter, a high-power supercontinuum (SC) laser source which spanned from 1.9 to 3.6 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 1.9-2.7 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 6.12 W with an optical conversion efficiency of 15.3% 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 2.9 W, corresponding to a high power ratio of 47.4% 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.

Ultrahigh-brightness, spectrally-flat, short-wave infrared supercontinuum source for long-range atmospheric applications

Fiber based supercontinuum (SC) sources with output spectra covering the infrared atmospheric window are very useful in long-range atmospheric applications. It is proven that silica fibers can support the generation of broadband SC sources ranging from the visible to the short-wave infrared region. In this paper, we present the generation of an ultrahigh-brightness spectrally-flat 2-2.5 μm SC source in a cladding pumped thulium-doped fiber amplifier (TDFA) numerically and experimentally. The underlying physical mechanisms behind the SC generation process are investigated firstly with a numerical model which includes the fiber gain and loss, the dispersive and nonlinear effects. Simulation results show that abundant soliton pulses are generated in the TDFA, and they are shifted towards the long wavelength side very quickly with the nonlinearity of Raman soliton self-frequency shift (SSFS), and eventually the Raman SSFS process is halted due to the silica fiber's infrared loss. A spectrally-flat 2-2.5 μm SC source could be generated as the result of the spectral superposition of these abundant soliton pulses. These simulation results correspond qualitatively well to the following experimental results. Then, in the experiment, a cladding pumped large-mode-area TDFA is built for pursuing a high-power 2-2.5 μm SC source. By enhancing the pump strength, the output SC spectrum broadens to the long wavelength side gradually. At the highest pump power, the obtained SC source has a maximum average power of 203.4 W with a power conversion efficiency of 38.7%. It has a 3 dB spectral bandwidth of 545 nm ranging from 1990 to 2535 nm, indicating a power spectral density in excess of 370 mW/nm. Meanwhile, the output SC source has a good beam profile. This SC source, to the best of our knowledge, is the brightest spectrally-flat 2-2.5 μm light source ever reported. It will be highly desirable in a lot of long-range atmospheric applications, such as broad-spectrum LIDAR, free space communication and hyper-spectral imaging.

152 fs nanotube-mode-locked thulium-doped all-fiber laser

Ultrafast fiber lasers with broad bandwidth and short pulse duration have a variety of applications, such as ultrafast time-resolved spectroscopy and supercontinuum generation. We report a simple and compact all-fiber thulium-doped femtosecond laser mode-locked by carbon nanotubes. The oscillator operates in slightly normal cavity dispersion at 0.055 ps², and delivers 152 fs pulses with 52.8 nm bandwidth and 0.19 nJ pulse energy. This is the shortest pulse duration and the widest spectral width demonstrated from Tm-doped all-fiber lasers based on 1 or 2 dimensional nanomaterials, underscoring their growing potential as versatile saturable absorber materials.

0.6-3.2 μm supercontinuum generation in a step-index germania-core fiber using a 4.4 kW peak-power pump laser

An ultra-broadband supercontinuum was generated in a short piece of step-index germania-core fiber using a fiber laser with a peak power of 4.4 kW. The pure germania core made this fiber capable of propagating light towards the desirable mid-infrared region. The spectral broadening characteristics towards the mid-infrared region under different lengths of germania-core fiber were investigated using pump pulses of 4.4 kW and 1.1 ns at 1550 nm. The large nonlinear refractive index of germania and the small core size of germania-core fiber produced a nonlinear coefficient as high as 11.8 (W km)^-1 at 1550 nm, which was beneficial for supercontinuum generation. The pump wavelength was located in the anomalous dispersion regime and close to the zero dispersion wavelength of this germania-core fiber, 1.426 μm. Eventually, an ultra-broadband supercontinuum source with a spectrum spanning from 0.6 to 3.2 μm was obtained and had a total output power of 350 mW at an optimized germania-core fiber length of 0.8 m. This work is the first demonstration, to the best of our knowledge, of a germania-core fiber-based ultra-broadband supercontinuum source that spans from the visible region to the mid-infrared region.

High Power Q-Switched Thulium Doped Fibre Laser using Carbon Nanotube Polymer Composite Saturable Absorber

We have proposed and demonstrated a Q-switched Thulium doped fibre laser (TDFL) with a 'Yin-Yang' all-fibre cavity scheme based on a combination of nonlinear optical loop mirror (NOLM) and nonlinear amplified loop mirror (NALM). Unidirectional lasing operation has been achieved without any intracavity isolator. By using a carbon nanotube polymer composite based saturable absorber (SA), we demonstrated the laser output power of ~197 mW and pulse energy of 1.7 μJ. To the best of our knowledge, this is the highest output power from a nanotube polymer composite SA based Q-switched Thulium doped fibre laser.

1 mJ nanosecond all-fiber thulium-doped fiber laser at 2.05 μm

A high energy all-fiber format nanosecond thulium-doped fiber laser at 2050 nm with a master oscillator power amplifier (MOPA) configuration is presented in this paper. The seed oscillator is a linearly polarized gain-switched fiber laser pumped by a 1550 nm fiber laser. The output pulse of the seed has a polarization extinction ratio (PER) better than 16 dB with a maximal output power of 470 mW. After two-stage double- cladding fiber amplifiers, the average power at 40 kHz was boosted up to 40.5 W. The output pulse has a maximum pulse energy of 1 mJ with a pulse width of 100 ns, which corresponds to a peak power of 10 kW. To the best of our knowledge, it is the highest single pulse energy ever reported for a nanosecond thulium-doped all-fiber MOPA system at 2050 nm.

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

All-fiber multiwavelength thulium-doped laser assisted by four-wave mixing in highly germania-doped fiber

An all-fiber multiwavelength Tm-doped laser assisted by four-wave mixing (FWM) in highly Germania-doped highly nonlinear fiber (HG-HNLF) has been experimentally demonstrated. Benefiting from the high nonlinearity of the HG-HNLF, intensity-dependent gain caused by FWM is introduced into the laser cavity to mitigate the gain competition in Tm-doped fiber. Thanks to a 50-m HG-HNLF, 9, 22, and 36 lasing lines with considering 10-dB, 20-dB, and 30-dB bandwidth, respectively is obtained at room temperature with wavelength spacing of 0.86 nm. More than 30-nm broad-band lasing can be obtained. The stability of the proposed fiber laser has also been studied. Repeat measurements show the power fluctuations and wavelength drifts of the lasing lines are less than 1.6 dB and 0.05 nm, respectively. The laser performances without the assistance of HG-HNLF have fewer center wavelengths lasing, which indicates that FWM in HG-HNLF plays an important role for the multiwavelength laser operation.

Generating femtosecond optical pulses tunable from 2 to 3 μm with a silica-based all-fiber laser system

Femtosecond pulses with broad tunability in the range of 2-3 μm are generated in a germanate-glass core silica-glass cladding fiber with a driving pulse at 2 μm produced by an all-fiber laser system consisting of an Er:fiber source at 1.6 μm, a Raman fiber shifter, and a Tm:fiber amplifier. We demonstrate optical pulses with a duration of the order of 100 fs that are the shortest ones reported in the 2.5-3 μm range obtained by fiber laser systems.

A femtosecond pulse fiber laser at 1935 nm using a bulk-structured Bi2Te3 topological insulator

We experimentally demonstrate a femtosecond mode-locked, all-fiberized laser that operates in the 2 μm region and that incorporates a saturable absorber based on a bulk-structured bismuth telluride (Bi(2)Te(3)) topological insulator (TI). Our fiberized saturable absorber was prepared by depositing a mechanically exfoliated, ~30 μm-thick Bi(2)Te(3) TI layer on a side-polished optical fiber platform. The bulk crystalline structure of the prepared Bi(2)Te(3) layer was confirmed by Raman and X-ray photoelectron spectroscopy measurements. The modulation depth of the prepared saturable absorber was measured to be ~20.6%. Using the saturable absorber, it is shown that stable, ultrafast pulses with a temporal width of ~795 fs could readily be generated at a wavelength of 1935 nm from a thulium/holmium co-doped fiber ring cavity. This experimental demonstration confirms that bulk structured, TI-based saturable absorbers can readily be used as an ultra-fast mode-locker for 2 μm lasers.