Graphene-based Saturable Absorber Research Articles

Hybrid Graphene–Silicon Arrayed Waveguide Gratings for On-Chip Signal–Frequency Conversion

We present the design and simulations of a novel integrated device concept enabling a frequency conversion of a broad signal. The solution is based on a hybrid silicon–graphene photonic chip, which could be used for controlled spectrometry in low-cost devices. The device is based on a silicon-on-insulator (SOI) platform on which an arrayed waveguide grating (AWG) is designed for operation at the center wavelength of λ = 1800 nm. The AWG is spectrally separating one broad input signal to thirty-two-output channels with a channel spacing of 2.72 nm. The output signals are well separated and uniform with the extinction ratio and the standard deviation of 10.00 dB and 0.04, respectively. The 3 dB channel width is 1.34 nm, which is suitable for sensing applications with significant accuracy. After spacial and spectral separation, each output signal is then converted to one signal at 1480 nm wavelength through a graphene-based saturable absorber scheme. Therefore, the device allows the detection of each separated signal with a simple near-infrared camera on which the outputs are imaged using conventional optics, leading to a classical pixel/wavelength correspondence. Crossed-waveguide couplers are designed to combine the controlling signal at 1480 nm to each channel waveguide of the AWG. The combination of the signals saturates the graphene layer at the output waveguides, allowing the pass of the controlling wavelength. This device can be applied as a spectrometer in environmental sensing and monitoring with high efficiency and low cost.

High-performance carbon-based nanomaterial composites for ultrafast lasers based on stretch processing

Carbon-based nanomaterials, and specifically carbon nanotube and graphene, have been widely used as saturable absorber (SA) in ultrafast fiber laser. However, the performance of these SAs is limited due to the low damage threshold or self-starting capability. In our work, we have proposed a novel and straightforward method for enhancing the optical performance of polyvinyl alcohol-based carbon nanotube and graphene composite film. By mechanical stretching the composite films, the damage threshold of carbon nanotube-based SA and the self-starting capability of graphene-based SA can be obviously improved. The distinct nonlinear optical properties of carbon nanotube and graphene are analyzed, and the mode-locking performance is compared. Our finding provides a new insight and experimental basis for the development of high-performance carbon-based nanomaterial mode locker for ultrafast fiber lasers.

Passively Q-Switched Thulium-Holmium Doped Fibre Laser with Electrochemical Exfoliation Graphene in Chitin Based Passive Saturable Absorber

Recent years saw low-dimensional carbon nanostructures like graphene as excellent saturable absorber (SA) for its unique electric and optical properties. Graphene exhibits ultrafast recovery times and high damage thresholds and can be manufactured by relatively simple methods. In this work, Graphene was synthesized using the electrochemical exfoliation (ECE) method. Chitin biopolymer was used to develop graphene-based SA as a more environmentally friendly alternative to conventional synthetic polymers. The fabricated passive SA was then integrated into Thulium-Holmium-doped fibre laser (THDFL) in ring cavity for pulsed laser generation via Q-switching. The optical and physical properties of the graphene-based SA were characterized using Raman spectroscopy, field emission scanning electron microscopy (FESEM) and energy-dispersive x-ray spectroscopy (EDS). Q-switched pulse was successfully demonstrated using the fabricated graphene SAs. ECE graphene-chitin SA generated a Q-switched pulse region at 2 micro-metre with the repetition rate and shortest pulse width of 67.78 kHZ and 10.03 micro-second, respectively. The signal-to-noise ratio (SNR) values obtained was 40 dB indicating high stability of the pulse laser-generated. This study demonstrated the potential of graphene embedded in chitin biopolymer as a sustainable and environmentally friendly SA for a wide range of applications, particularly for pulsed fibre lasers.

SiO2 Passivated Graphene Saturable Absorber Mirrors for Ultrashort Pulse Generation.

Owing to its broadband absorption, ultrafast recovery time, and excellent saturable absorption feature, graphene has been recognized as one of the best candidates as a high-performance saturable absorber (SA). However, the low absorption efficiency and reduced modulation depth severely limit the application of graphene-based SA in ultrafast fiber lasers. In this paper, a single-layer graphene saturable absorber mirror (SG-SAM) was coated by a quarter-wave SiO2 passivated layer, and a significantly enhanced modulation depth and reduced saturation intensity were obtained simultaneously compared to the SG-SAM without the SiO2 coating layer. In addition, long-term operational stability was found in the device due to the excellent isolation and protection of the graphene absorption layer from the external environment by the SiO2 layer. The high performance of the SAM was further confirmed by the construction of a ring-cavity EDF laser generating mode-locked pulses with a central wavelength of 1563.7 nm, a repetition rate of 34.17 MHz, and a pulse width of 830 fs.

Third-order orbital angular momentum pulse generation from a passively Q-switched fiber laser.

We propose and demonstrate an all-fiber passively Q-switched laser generating a third-order orbital angular momentum (OAM) pulse by introducing a few-mode long-period fiber grating (LPFG) into the laser cavity. The LPFG with asymmetric cross structure and strong refractive index modulation overcomes the coupling issue between the fundamental and the third-azimuthal-order (LP31 or OAM3) modes and realizes their direct conversion. A homemade graphene-based saturable absorber is used to realize Q-switched operation. The laser operates at a center wavelength of 1548.2nm, with a 3 dB spectral bandwidth of 0.4nm, and the OAM+3 and OAM-3 beams can achieve the purity of 90.0% and 90.2%, respectively. This all-fiber Q-switched laser has simple and compact structure and high purity of OAM±3 beams, which has potential applications in the fields of optical tweezers and material processing.

Graphene-based Saturable Absorber for Pulsed Fiber Laser Generation

Recently, graphene has been considered as great candidate to be applied as the saturable absorber (SA) with its brilliant optical characteristics such as ultrafast recovery time and ultra-wideband absorption due to its zero bandgap energy and linear dispersion of Dirac electrons. This paper focuses on reviewing the generation of short pulses from passive mode-locked fiber lasers that employ graphene-based saturable absorber (GBSA). Various parameters that make it excellent for generation ultra-short pulsed including modulation depth, nonlinearity, saturation intensity, self-amplitude modulation, its crystal lattice structure, band gap energy distribution are discuss in details. Furthermore, comparison between single layers and multilayer GBSA is made to explain the effect of layers number on the behaviour of SA in ring cavity fiber lasers.

860 femtoseconds mode-locked fiber laser by Gallium co-doped erbium fiber (Ga-EDF)

We proposed and demonstrated a high power mode-locked fiber laser using a new type of gain medium which is called as Erbium Gallium co-doped fiber (Ga-EDF). The mode-locking mechanism is enabled by a graphene-based saturable absorber, which is fabricated by slotting a single layer graphene (SLG) thin film in between two fiber ferrules connected through an adaptor. The Ga-EDF has an absorption rate of 25 dB/m at 980 nm. With a 2-m-long Ga-EDF utilized as the primary gain medium in the laser system, the proposed laser can generate mode-locked solitons, with the central wavelength of 1560 nm, a 3 dB bandwidth of 3.20 nm and an average output power of 18.23 mW. The generated pulse yields a repetition rate of 12.25 MHz with pulse duration and pulse energy of 860 fs and 1.49 nJ respectively. For comparison purpose, the experiment is repeated by replacing the Ga-EDF in the laser cavity with the same length of 2-m-long conventional erbium-doped fiber (EDF), and similar measurement of the mode-locked output performance is undertaken. The result obtained shows in comparison an improvement of the mode-locked Ga-EDF laser output performance to that of the mode-locked EDF laser in terms of the 3 dB bandwidth, pulse width and signal-to-noise ratio. The proposed work is the first time, to the knowledge of the authors, that the application of Ga-EDF as an active gain medium in the development of mode-locked fiber laser incorporating graphene which is a thin layer film as a saturable absorber is explored.

Graphene-based saturable absorber and mode-locked laser behaviors under gamma-ray radiation

We investigate optical and electrical behaviors of a graphene saturable absorber (SA) and mode-locking performance of a graphene-SA-based mode-locked Er fiber laser in gamma-ray radiation. When irradiated up to 4.8 kGy at ∼100 Gy/hr dose rate, the overall nonlinear transmittance in transverse electric mode was increased, while maintaining modulation depth to >10%. The corresponding polarization-dependent loss was reduced at a 1.2-dB/kGy rate. In the electrical properties, the charge carrier mobility was reduced, and the Dirac voltage shift was increased to positive under gamma-ray radiation. The radiation-induced optical and electrical changes turned out to be almost recovered after a few days. In addition, we confirmed that the graphene-SA-based laser showed stable CW mode-locking operation while the inserted graphene SA was irradiated for 2-kGy at a 45-Gy/hr dose rate, which corresponds to >40 years of operation in low Earth orbit satellites. To the best of our knowledge, this is the first evaluation of graphene SAs and graphene-SA-based mode-locked lasers in gamma-ray radiation, and the measured results confirm the high potential of graphene SAs and graphene-SA-based lasers in various outer-space environments as well as other radiation environments, including particle accelerators and radiation-based medical instruments.

Passively Mode-Locked Solid-State Laser With Absorption Tunable Graphene Saturable Absorber Mirror

Two-dimensional layered materials have attracted huge interest in the generation of ultrafast laser for their excellent saturable absorption properties. However, it is still challenging to precisely control their saturable absorption properties. Here, by alternatively changing the electric field intensity on the surface of high-reflection mirror, we successfully control the nonlinear absorption properties (e.g., saturable fluence, modulation depth) of graphene-based saturable absorber mirrors (GSAM) at the optical telecommunication wavelength of 1.3 μm and their applications in solid-state lasers for the first time. Modulation depth of 1.2% is obtained from a GSAM with deposition of a λ/8 (λ = 1.3 μm) thick SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer between the monolayer graphene and a high-reflection mirror, while modulation depth is increased to 4.3% with a λ/4 thick SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer insertion in another GSAM. Pulses with the duration of 20 ps (λ/8 thick SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> insertion) and 7.4 ps (λ/4 thick SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> insertion) are achieved, respectively, based on the two mirrors. Our results indicate that this method is easy and reliable to versatility modulate the saturable absorption properties of other two-dimensional layered materials beyond graphene for the generation of ultrafast solid-state lasers.

Thulium-Doped Silica Fibers with Enhanced Fluorescence Lifetime and Their Application in Ultrafast Fiber Lasers

In this work we report on the thulium-doped silica-based optical fibers with increased fluorescence lifetime of the 3F4 level thanks to the modification of the local environment of thulium ions by high content of alumina. The determination of the cross-relaxation energy-transfer coefficients from the measurements of the fluorescence lifetimes of the 3F4 and 3H4 energy levels of Tm3+ ions in the experimentally prepared optical fiber is provided as well. Preforms of optical fibers were prepared either by conventional solution-doping of Tm3+ and Al3+ ions or by dispersion-doping of Tm3+ ions with alumina nanoparticles. Optical fibers were characterized by means of Tm, Al, and Ge concentrations, refractive index profiles, optical spectral absorption and luminescence, and by time-resolved fluorescence spectroscopy. Highly aluminium-codoped thulium silicate optical fibers exhibited fluorescence lifetimes of over ~500 μs with maximum value of 756 μs, which means a fluorescence lifetime enhancement when compared to the thulium-doped fibers reported elsewhere. We show an application of the thulium-doped fiber in a compact all-fiber ring laser that is passively mode-locked by using graphene-based saturable absorber. The output pulsewidth and repetition rate were 905 fs and 32.67 MHz, respectively.

Sub-100 fs Tm:MgWO4 laser at 2017 nm mode locked by a graphene saturable absorber.

We present the first sub-100fs bulk solid-state laser in the 2-μm spectral range employing the monoclinic Tm3+-dopedMgWO4 crystal as an active medium. By applying a graphene-based saturable absorber and chirped mirrors for dispersion management, stable self-starting mode-locked operation at 2017nm was achieved. Nearly Fourier-limited pulses as short as 86fs featuring a bandwidth of 53nm were generated at a repetition rate of 76MHz. A pulse energy of 1.1nJ was achieved at 87MHz for a pulse duration of 96fs. The mode-locked Tm3+:MgWO4 laser exhibits excellent stability with a fundamental beat note extinction ratio of 80dBc above noise level.

Fast Extinction Ratio Enhancement With a Graphene-Based Device on $1.5~\mu \text{m}$ Optical Signals

Ability of fast intensity discrimination provided by a graphene-based saturable absorber device is demonstrated in the perspective of all-optical signal processing in $1.5~\mu \text{m}$ wavelength range. The dynamic extinction ratio enhancement is measured as more than 3 dB. It is argued that, the device performances are promising enough to be tested further in an efficient all-optical module to reduce crosstalk or intensity noise in high rate signals.

Wavelength tunable ultrafast fiber laser via reflective mirror with taper structure.

Laser sources with a controllable flexible wavelength have found widespread applications in optical fiber communication, optical sensing, and microscopy. Here, we report a tunable mode-locked fiber laser using a graphene-based saturable absorber and a tapered mirror as an end mirror in the cavity. The phase layer in the mirror is precisely etched by focused ion beam (FIB) milling technology, and the resonant wavelength of the mirror shifts correspond to the different etch depths. By scanning the tapered mirror mechanically, the center wavelength of a mode-locked fiber laser can be continuously tuned from 1562 to 1532nm, with a pulse width in the sub-ps level and repetition rate of 27MHz.

Q-switching of a Tm,Ho:KLu(WO4)2 microchip laser by a graphene-based saturable absorber

The first Ho microchip laser passively Q-switched using a graphene-based saturable absorber is demonstrated based on a Tm,Ho:KLu(WO4)2 crystal cut along the Ng-axis. A maximum average output power of 74 mW is extracted from the diode-pumped laser at 2061 nm with a slope efficiency of 4%. Pulses as short as 200 ns with an energy of ~0.2 μJ are obtained at a repetition rate of 340 kHz. The energy transfer (ET), 3F4 (Tm3+) ↔ 5I7 (Ho3+) is studied, yielding ET parameters of P28 = 1.69 and P71 = 0.15 × 10−22 cm3 μs−1, revealing the strong prevalence of direct ET.

Multilayer graphene-based saturable absorbers with scalable modulation depth for mode-locked Er- and Tm-doped fiber lasers

We demonstrate an experimental study on the influence of the parameters of a graphene-based saturable absorber (SA) on the performance of mode-locked Er- and Tm-doped fiber lasers. We have fabricated a set of saturable absorbers with different number of graphene layers: 9, 12, 24, 37 and 48. Each SA was characterized in terms of nonlinear optical parameters (modulation depth, saturation intensity, saturation fluence) and tested in two state-of-the-art, low-power Er- and Tm-doped fiber lasers. Our results show, that in the Er-laser the broadest output spectrum (11 nm) and shortest pulses (345 fs) are generated using 37 layers of graphene in the SA. In case of a Tm-laser, the best performance (737 fs pulses with 5.82 nm bandwidth) was achieved with 24 layers. Additionally, we show that the modulation depth of a 9-layer SA is insufficient to initiate mode-locking in both lasers. This is the first reported comprehensive study on controlling of the parameters of a SA by scaling the number of graphene layers.

Generation of soliton and bound soliton pulses in mode-locked erbium-doped fiber laser using graphene film as saturable absorber

We report an observation of soliton and bound-state soliton in passive mode-locked fibre laser employing graphene film as a passive saturable absorber (SA). The SA was fabricated from the graphene flakes, which were obtained from electrochemical exfoliation process. The graphene flakes was mixed with polyethylene oxide solution to form a polymer composite, which was then dried at room temperature to produce a film. The film was then integrated in a laser cavity by attaching it to the end of a fibre ferrule with the aid of index matching gel. The fibre laser generated soliton pulses with a 20.7 MHz repetition rate, 0.88 ps pulse width, 0.0158 mW average output power, 0.175 pJ pulse energy and 18.72 W peak power at the wavelength of 1564 nm. A bound soliton with pulse duration of ~1.04 ps was also obtained at the pump power of 110.85 mW by carefully adjusting the polarization of the oscillating laser. The formation of bound soliton is due to the direct pulse to pulse interaction. The results show that the proposed graphene-based SA offers a simple and cost efficient approach of generating soliton and bound soliton in mode-locked EDFL set-up.

Graphene Q-Switched Compact Yb:YAG Laser

We describe a compact Yb:YAG laser Q-switched by a graphene-based saturable absorber and pumped by a laser diode at 932 or 969 nm. The compact laser generates a maximum average output power value of 185 mW at 1032 nm with a slope efficiency value of 12%. The shortest duration of the Q-switched pulse achieved is 228 ns at a repetition frequency of 285 kHz. The maximum pulse energy amounts to 0.65 $\mu\mathrm{J}$ .

Polarization insensitive graphene saturable absorbers using etched fiber for highly stable ultrafast fiber lasers.

In this paper, we introduce a graphene-based saturable absorber (GSA) with high damage threshold employing symmetrical evanescent wave interaction for highly stable mode-locking of ultrafast fiber lasers. To enhance the evanescent wave interaction between the graphene layer and the propagating light, graphene flakes are mixed with polydimethylsiloxane (PDMS), and the graphene/PDMS composite is coated onto a chemically etched fiber. The GSA exhibits polarization insensitivity due to its symmetric cross-section, which enables stable operation against environmental disturbance such as stress, bending, and temperature variation. Finally, we demonstrate a fiber laser generating 216 fs pulses with an 80 dB signal-to-noise ratio.

Soliton molecules in a fiber laser mode-locked by a graphene-based saturable absorber

We have experimentally investigated a novel kind of soliton molecule in a graphene-based mode-locking fiber laser with anomalous dispersion. The soliton molecule exhibits a stable rectangular profile on the oscilloscope, whereas it shows randomly distributed peaks in the autocorrelation trace, which indicates that the temporal separation of pulses in the molecule is varying all the time. The optical spectrum is modulated with a depth of about 7 dB over the whole profile, induced by the interaction of the intra-molecule solitons. The experimental results demonstrate that solitons in the pulse molecule oscillate randomly in temporal domain and the neighboring molecules are temporally separated by a fundamental cavity repetition rate, which is very different from that of the multi-solitons or bound-state solitons.

Switchable dual-wavelength fiber laser mode-locked by monolayer graphene on D-shaped fiber

A switchable mode-locking fiber laser is demonstrated by means of a monolayer graphene saturable absorber (SA) based on a D-shaped fiber. The monolayer graphene, which is grown by chemical vapor deposition, is transferred onto the D-shaped fiber and then the light–graphene interaction via the evanescent field of the fiber is enhanced greatly. Using such a graphene-based SA, the single-wavelength mode locking can be switched from 1531.5 to 1559.1 nm by appropriately adjusting the polarization controller (PC). In addition, the stable dual-wavelength mode-locking operation is also observed at the proper state of PC.