HK1169751B - Graphene-based saturable absorber devices and methods - Google Patents

Abstract

A graphene-based saturable absorber device (22) suitable for use in a ring-cavity fiber laser (200) or a linear-cavity fiber laser (300) is disclosed. The saturable absorber device includes an optical element (10) and a graphene-based saturable absorber material (18) supported by the optical element and comprising at least one of graphene, a graphene derivative and functionalized graphene. An examplary optical element is an optical fiber having an end facet (14) that supports the saturable absorber material. Various forms of the graphene-based saturable absorber materials and methods of forming same are also disclosed.

Description

Graphene-based saturable absorber device and method

Priority requirement

This application claims priority from U.S. provisional patent application 61/168,661 entitled "Optical element" filed on 13.4.2009.

Technical Field

The present invention relates to saturable absorbers for fiber lasers, and in particular to graphene-based saturable absorber devices and methods for mode locking, Q-switching, optical signal processing, etc. in fiber lasers.

Background

Mode-locked fiber lasers have replaced bulk solid-state lasers in many research/industrial areas where high quality optical pulses are required. The advantages include: simple structure, outstanding pulse quality and high efficiency of operation. Recently, development of compact diode pumped ultrafast fiber lasers as replacements for bulk solid state lasers has progressed rapidly.

Currently, short pulse generation has been particularly effective using passive mode locking techniques. The main technology in passively mode-locked fiber lasers is based on semiconductor saturable absorber mirrors (SESAMs) which utilize III-V semiconductor multiple quantum wells grown on Distributed Bragg Reflectors (DBRs).

However, SESAM has a number of disadvantages. SESAM requires complex and expensive clean room-based manufacturing systems such as Metal Organic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE). Furthermore, an additional substrate removal process is required in some cases. High energy heavy ion implantation is required to introduce defect sites to shorten device recovery time (typically nanoseconds) to the picosecond range required for short pulse laser mode locking applications.

Since the SESAM is a reflective device, its use is limited to only a specific type of linear cavity topology. Other laser cavity topologies, such as ring cavity designs, which require transmissive mode devices, have advantages such as doubling the repetition rate for a given cavity length, and are less sensitive to reflection-induced instabilities through the use of optical isolators, are not possible unless optical circulators are employed, which would increase cavity loss and laser complexity. The photodamage threshold of SESAM is also low.

Until recently, there was no alternative saturable absorbing material that competed with SESAMs for passive mode locking of fiber lasers. Recently, the discovery of the saturable absorption properties of single-walled carbon nanotubes (SWCNTs) in the near infrared region and ultra-fast saturation recovery times of-1 picosecond has resulted in a new type of solid saturable absorber that is significantly different from SESAMs in structure and manufacture, and has in fact led to the emergence of picosecond or subpicosecond Erbium Doped Fiber (EDF) lasers. In these lasers, solid SWCNT saturable absorbers have been formed by direct deposition of SWCNT films on flat glass substrates, mirror substrates, or end facets of optical fibers.

But the non-uniform chiral nature of SWCNTs has inherent problems with the precise control of the properties of saturable absorbers. SWCNTs that are not at resonance cause insertion loss when operating at a specific wavelength. Therefore, SWCNT has poor broadband tunability. Furthermore, although the polymer host may to some extent prevent some of these problems and ease device integration, the presence of bundled and tangled SWCNTs, catalyst particles and the formation of bubbles leads to high unsaturation loss in the cavity.

Disclosure of Invention

One aspect of the present invention relates to a novel saturable absorber material composed of graphene or a derivative thereof, and an assembly thereof on an optical element such as an optical fiber to replace SESAMs and SWCNTs as saturable absorbers for short pulse generation.

The present invention overcomes the above-mentioned problems of better performance, lower manufacturing cost, and easier integration with the manufacturing process, compared to conventional methods involving SESAMs or SWCNTs.

Graphene is a mechanically and chemically robust material with high conductivity and advantageous optical properties, such as interband optical transitions and universal optical conductivity. Graphene materials also have low unsaturation loss, high conversion efficiency, and broadband tunability for their use as saturable absorbers.

The ultra-fast recovery time of graphene also facilitates ultra-short pulse generation (picosecond to femtosecond pulses). The optical modulation depth can be tuned over a wide range by using single to multilayer graphene or doping/intercalating with other materials. The present invention uses graphene, graphene derivatives, and graphite composites (e.g., polymer-graphene, graphene gels) as saturable absorber materials for mode locking, Q-switching, optical pulse shaping, optical switching, optical signal processing, etc. in fiber lasers.

Drawings

FIG. 1 is a perspective proximal end view of a saturable absorber device in the form of an optical fiber held within a ferrule, the optical fiber having a distal facet populated with a saturable absorber material comprising an atomic layer of graphene;

FIG. 2 is a perspective proximal end view of a saturable absorber device in the form of an optical fiber clamped within a ferrule, the optical fiber having a distal facet populated with a saturable absorber material comprising several atomic layers of graphene to form a multilayer graphene film;

FIG. 3 is a perspective view of a saturable absorber device in the form of a fiber pigtail having a multilayer graphene film disposed on the end of the fiber pigtail;

FIG. 4 is an optical image of the end of a fiber pigtail having a multilayer graphene film disposed thereon and covering a ferrule pinhole;

FIG. 5 is a perspective near-end view of a saturable absorber device in the form of an optical fiber having a distal facet populated with a saturable absorber material comprising a graphene platelet a monolayer;

FIG. 6 is a perspective proximal end view of a saturable absorber device in the form of an optical fiber clamped within a ferrule, the optical fiber having a distal facet with a saturable absorber material comprising graphene and a polymer composite assembled thereon;

FIG. 7 is a perspective near-end view of a saturable absorber device in the form of an optical fiber clamped within a ferrule, the optical fiber having a distal facet assembled with a saturable absorber material comprising a hybrid film of graphene in combination with other thin film materials;

FIG. 8 is a schematic diagram of an exemplary fiber laser with ring cavity using a graphene-based saturable absorber device; and

fig. 9 is a schematic diagram of a fiber laser with a linear cavity using a graphene-based saturable absorber device.

Detailed Description

Aspects of the present invention relate to the use of graphene and its derivatives, such as graphene oxide or functionalized graphene, as saturable absorber materials carried by optical elements (e.g., optical fibers, glass substrates, mirrors, etc.) to form graphene-based saturable absorber devices. The device is used, for example, in fiber lasers. The graphene-based saturable absorber device can exhibit optical switching operation by a change in light transmittance accompanied by saturable absorption by a graphene-based saturable absorber material. The graphene-based saturable absorber device may also be used for pulse shaping. The graphene may be incorporated as one or more layers of a graphene film, or as a composite of graphene and a polymer, or as a composite of graphene and an organic or inorganic material. The graphene-based saturable absorber device can be used in fiber lasers for optical signal processing, mode locking, Q-switching, pulse shaping, and the like.

In general, a saturable absorber is an optical component with some optical loss, which is reduced at high light intensities. The main application of saturable absorbers is in mode-locking and Q-switching of lasers, i.e. the generation of short pulses. Saturable absorbers are also commonly used in the processing of optical signals. One aspect of the present invention is the use of graphene and its derivatives as saturable absorber materials for graphene-based saturable absorber devices for optical signal processing, mode locking, Q-switching, pulse shaping, etc. in fiber lasers.

The graphene is sp forming a honeycomb lattice²A monoatomic layer of hybridized carbon with a linear spectrum where the electron and hole cones intersect (dirac point) in the band structure. Since the 2+1 dimensional dirac equation governs the dynamics of quasi-particles in graphene, many of its properties are significantly different from other materials. The light transmission of single-layer graphene is completely defined by a fine structure constant alpha ═ e²The/hc limit. The expected absorbance has been calculated and measured to absorb a significant portion (pi α 2.293%) of the infrared to visible incident light independent of frequency. In comparison, a 10nm thick GaAs layer absorbs around 1% of the light near the bandgap. In principle, under strong excitation, the interband absorption of photons in zero band gap graphene can be easily saturated due to pauli blocking, i.e. the photon-generated carriers cool down in subpicosecond to form a new fermi-dirac distribution and the newly generated electron-hole pairs block some of the originally possible optical transitions.

With excitation increasing to a sufficiently high intensity, the photogenerated carriers have a high concentration (well above about 8 x 10 in graphene at room temperature)¹⁰cm^-2Intrinsic electron and hole carrier densities) and may result in filling of states near the conduction and valence band edges to further block absorption, so it is slightly more transparent to light with photon energies than at the band edges. Band filling occurs because no two electrons can fill the same state. Thus, saturable absorption or absorption bleaching is obtained as a result of the pauli occlusion process. In principle, graphene can be a perfect saturable absorber.

The intensity-dependent attenuation allows high intensity components of the light pulse to pass through the graphene film, while lower intensity components of the pulse, such as the pulse tail, pulse base level, or background continuous wave (cw) radiation, cannot.

When placed in the laser cavity, a saturable absorber in the form of a graphene film will facilitate short pulse generation and suppress continuous wave (cw) radiation, which can be used for mode locking. For ultrashort pulse generation applications, graphene has a fast recovery time on the order of 200fs scale or less, which is required for stable laser mode locking, while a slower recovery time on the order of several ps scale may facilitate laser self-start.

The invention is not limited to assembled atomic scale graphene nanoplatelets carried by optical elements (e.g., on the end facet of an optical fiber) as saturable absorbers for laser mode locking, but includes derivatives thereof, such as functionalized graphene or graphene-polymer composites. Advantageously, a graphene film with or without a uniform layer can be assembled onto the end facet of the optical fiber as a saturable absorber. Advantageously, assembling small-sized graphene sheets onto a fiber end facet to form a saturable absorber device is described. Advantageously, the saturable absorber film can include at least one layer of graphene, graphene sheets, or functionalized derivatives thereof on the terminal facet of the optical fiber.

In addition, intercalates of graphene or graphene-functionalized derivatives with other thin film materials (e.g., polymers, organic dyes, inorganic materials) can be assembled on the terminal facets of optical fibers to form saturable absorber devices for mode-locked lasers or related signal processing devices.

The term "graphene" as used herein is defined as single-or multi-layer graphene as described, for example, in publication Novoselov, k.s. et al PNAS, vol.102, No.30, 2005 and publication Novoselov, k.s. et al Science, Vol 306, 2004. Exemplary graphene films contemplated herein include at least one layer of graphene or one or more graphene sheets (e.g., a network or a nanogrid). The graphene considered in the present invention describes the material without being limited by the methods used to prepare the material, including mechanical lift-off, epitaxial growth, chemical vapor deposition and chemical processing (solution processing) methods, and laser ablation and filtered cathodic arc methods.

The graphene is sp forming a honeycomb lattice²-a single atomic layer of hybrid carbon. One atomic layer of graphene absorbs a significant portion (2.293%) of incident light from infrared to visible wavelengths. Under strong excitation, the interband absorption of photons in zero band gap graphene can be easily saturated due to pauli blocking. Thus, graphene can be used as saturable absorber material to form broadband tunable saturable absorber devices for photonic devices such as fiber lasers.

Other features and advantages of the invention are described in the accompanying drawings. In addition to certain alternative embodiments, one or more of the embodiments disclosed above will be described in greater detail below with reference to the accompanying drawings. The present invention is not limited to any particular embodiment disclosed, but is to be defined by the scope of the claims.

The term "graphene-based" is used herein and in the claims as a shorthand for graphene, graphene derivatives, functionalized graphene, or combinations thereof.

Example 1

Fig. 1 is a perspective view of an optical fiber 10, the optical fiber 10 having a terminal facet 14, the terminal facet 14 being populated with a graphene-based saturable absorber material 18 in the form of a single-layer graphene film 20 (i.e., a one-atomic-layer graphene or "graphene monolayer") as a saturable absorber device 22. The saturable absorber device 22 of fig. 1 is suitable for use in mode-locked and Q-switched fiber lasers as described below. Fig. 1 shows the optical fiber 10 clamped within the axial pinhole 4 of a ferrule 6 having an end face 8. The ferrule 6 serves as a fiber holder.

The graphene monolayer 20 may be obtained by methods such as mechanical lift-off, epitaxial growth, chemical vapor deposition and chemical processing (solution processing) methods, as well as laser ablation and filtered cathodic arc methods. After the graphene monolayer 20 has been suitably prepared on the substrate, the monolayer is removed as a graphene film and transferred onto the end facet 14 of the optical fiber 10.

In one example, the graphene structures (e.g., graphene monolayers 20 and graphene multilayers as discussed below) are produced by a Chemical Vapor Deposition (CVD) process. In one exemplary process for growing graphene monolayers, a piece of copper (Cu) foil is loaded into a CVD chamber and held at 10sscm H₂Flow rate. The copper foil is heated to about 1000 c to activate the copper catalyst. CH was then introduced into the chamber at 110sscm₄And lasted for 30 minutes. CH (CH)₄And (3) catalytically decomposing on the surface of Cu, and adsorbing carbon atoms on the surface of Cu to form single-layer graphene after the sample is cooled. At H₂The system was cooled to room temperature at a rate of about 10 deg.c/sec under the protection of the gas flow. The single-layer graphene film grown by this method is continuous, has a uniform thickness, and is as large as the size of the copper foil.

In another experiment to grow graphene multilayers, SiO with 300nm nickel (Ni) films₂the/Si substrate is loaded into the CVD chamber. Then at 100sccm H₂The Ni catalyst was activated under a gas flow at 700 ℃. In a quartz tube with Ar/CH₄/H₂Mixed stream (Ar: CH)₄∶H₂3: 1) was heated to 900-1000 ℃ and reacted for 10 minutes. Finally, the system was rapidly cooled to room temperature at a rate of about 10 ℃/sec under the protection of Ar gas flow. Then, since the solubility of carbon in Ni is temperature dependent, after the sample is cooled, carbon atoms precipitate as graphene layers on the Ni surface. The thickness of the graphene film can be controlled from monolayer to multilayer by the flow rate of the reactants and the growth time. The graphene film produced in this way may be continuous in the dimension of the substrate.

To remove the graphene film from the substrate, iron (III) chloride (FeCl) is used₃) An aqueous solution (about 1M) was used as an oxidizing etchant to remove the Cu/Ni layer. Floating on FeCl in the sample₃The redox process effectively slowly etches the Cu/Ni layer while on the surface of the solution. Before the graphene film is completely separated from the substrate, the sample is gently transferred to Deionized (DI) water and held thereFor at least ten hours. Then, the graphene film was subsequently separated from the Cu/Ni layer by immersing the sample in water using a floating method to obtain a self-supporting film. Before the etching reaction, the dried Cu foil or Ni/SiO₂The substrate is cut into sections to obtain graphene sheets having the desired dimensions.

The transfer process can be tailored to the specific method and substrate used to prepare the graphene film.

Example 2

Fig. 2 is a perspective view, similar to fig. 1, of an optical fiber 10 having a graphene-based saturable absorber material 18 in the form of a multilayer graphene film 30 (i.e., a graphene polyatomic layer or "graphene multilayer") assembled on an optical fiber end facet 14 as a saturable absorber device 22. The saturable absorber device 22 of fig. 2 is incorporated in a mode-locked and Q-switched fiber laser as described below.

Fig. 3 is a photograph of a fiber pigtail 100 having a ferrule 6 with the ferrule 6 holding the fiber 10 and a multilayer graphene film 30 on the end face 8 covering the pinhole 4 and the fiber end facet 14. A fiber pigtail 100 is inserted into a fiber laser to generate mode-locked or Q-switched pulses as described below.

Fig. 4 is an enlarged optical image of the end face of the fiber pigtail 100 showing the graphene-based saturable absorber material 18 in the form of multi-layered graphene 30 on the ferrule surface 8 covering the pinhole 4 and the fiber end facet 14. The fiber pigtail 100 so produced (which can be considered a saturable absorber device) is inserted into a fiber laser to produce mode-locked or Q-switched pulses. The multi-layer graphene 30 can be assembled using, for example, an electrostatic layer-by-layer process, a transfer printing process, or an optical trapping process.

The transfer process varies with the method and substrate used to prepare the graphene film. One example is the use of PDMS stamps to transfer graphene films onto the fiber end facet 14, which is suitable for a wide range of initial substrates in which graphene or its derivatives are prepared. For graphene films produced by epitaxial growth and chemical vapor deposition, the graphene film is separated from the original substrate by a floating method, such as etching the substrate in an acid or salt solution. The graphene film may then adhere thereto due to strong van der waals forces by contacting with the target substrate.

For mechanically stripped graphene, the initially stripped ribbon is adhered directly to the fiber end facet 14 by carefully aligning the graphene with the fiber pin hole 4.

Another example uses an assembly technique that relies on electrostatic interactions, such as layer-by-layer assembly of graphene or its derivatives on the fiber end facet 14, which is suitable for solution processed graphene or graphene dispersed in a solvent.

Yet another example uses optical trapping to adhere graphene to the fiber end facet, where a clean fiber connected to a laser source with tunable optical parameters is immersed in a graphene solution.

Example 3

Fig. 5 is a perspective view, similar to fig. 1, of an optical fiber 10, the optical fiber 10 having a graphene-based saturable absorber material 18 in the form of a graphene film 40. The graphene film 40 is formed from a single layer of graphene sheets 42 assembled on the fiber end facet 14, thereby forming the saturable absorber device 22. The saturable absorber device 22 of fig. 5 is suitable for use in mode-locked and Q-switched fiber lasers as described below.

In one example, the single-layer graphene sheets 42 have a small size, e.g., less than 10 μm. In one example, graphene sheets 42 are assembled onto the end facet of the fiber pigtail as graphene films 40 covering the pinhole 4, and the pigtail 100 is inserted into a fiber laser to generate mode-locked or Q-switched pulses. In one example, the small-sized graphene sheets 42 are obtained by a solution processing route or by post-processing of single-layer graphene on a substrate. Post-treatment methods include, but are not limited to, chemical etching (e.g., acid etching) or physical etching (e.g., electron bombardment) or UV exposure. One example of transferring graphene sheets 42 of pristine small size onto the fiber end facet 14 is using assembly techniques such as layer-by-layer methods, transfer printing, or optical trapping.

Example 4

Fig. 6 is a perspective view, similar to fig. 1, of an optical fiber 10 having assembled thereon a graphene-based saturable absorber material 18 in the form of a graphene film 50 comprising a plurality of graphene sheets 42 as saturable absorber device 22. The saturable absorber device of fig. 6 is suitable for use in mode-locked and Q-switched fiber lasers as described below.

The graphene sheets 42 may have small dimensions (e.g., less than 10 μm). In one example, graphene sheets 42 are assembled on the fiber end facet 14 of the fiber pigtail 100 to cover the pinhole 4, which is inserted into a fiber laser to generate mode-locked or Q-switched pulses. The multilayer graphene film 50 comprises a thin film of small-sized multilayer graphene sheets 42, or alternatively, a thin film 40 of several layers stacked, wherein each layer (film) comprises a single layer of graphene sheets 42 having a small size (e.g., less than 10 μm).

The small-sized graphene sheets 42 are obtained by a solution processing route or by post-processing of a single layer of graphene on a substrate. Post-treatment methods include, but are not limited to, chemical etching (e.g., acid etching) or physical etching (e.g., electron bombardment) or UV exposure. The small-sized graphene sheets 42 may be transferred onto the fiber end facets using assembly techniques such as layer-by-layer methods, transfer printing, or optical trapping.

Example 5

Fig. 7 is a perspective view of an optical fiber 10, the optical fiber 10 having a graphene-based saturable absorber material 18 in the form of a hybrid film 60 assembled on the fiber end facet 14, wherein the hybrid film is formed by intercalation of a graphene film 62 with another material 64, such as an organic material. The saturable absorber device 22 of fig. 7 is suitable for use in mode-locked and Q-switched fiber lasers as described below. In one example, the organic material is a conjugated molecule that may have photochromic properties.

In one example, the intercalation of the different layers of the hybrid film 60 is tuned to optimize the desired properties of the hybrid film. In one example, the above-mentioned techniques, such as layer-by-layer methods, transfer printing, or optical trapping, are used in combination to assemble the hybrid film on the fiber end facet.

Example 6

In example 6, saturable absorber material 18 is provided on the fiber end facet 14, wherein the material comprises functionalized or derivatized graphene, wherein the graphene is derived from organic, inorganic, or organometallic materials to form a composite or hybrid film with improved performance for mode-locking, Q-switching, or optical confinement.

Example 7

Referring again to fig. 7, in example 7, the saturable absorber material 18 including the hybrid film 60 is formed from a graphene-based polymer composite made of graphene or its derivatives (e.g., graphene oxide, or functionalized graphene) 62 embedded in a host polymer 64 to serve as a saturable absorber. The choice of matrix polymer depends on properties such as transparency in the wavelength range of interest, reduction of propagation loss, low refractive index mismatch with graphene materials, and good thermal and environmental stability. A non-exhaustive list of useful matrix polymers includes polyvinyl alcohol (PVA), Polycarbonate (PC), polyimides and poly (phenylene vinylene) (PPV) derivatives, cellulose derivatives, conjugated polymers such as poly (3-hexylthiophene-2, 5-diyl) (P3HT), poly (3,3 ″ -dialkyltetrabithiophene) (PQT).

The graphene material and the polymeric host may be dispersed in an organic solvent such as Dichlorobenzene (DCB) and hexane with, for example, ultrasound or high shear mixing. Exemplary methods of final deposition of the thin film include spin coating, spray coating, drop coating, dip coating, vacuum filtration, and printing, but are not limited to these aforementioned methods.

Fiber laser with ring cavity and graphene-based saturable absorber device

Fig. 8 is a schematic diagram of a fiber laser 200 with a ring cavity 210, the fiber laser 200 designed for mode-locking and Q-switching by using a graphene-based saturable absorber device 22. To gain conclusive evidence of soliton mode locking, i.e., clear soliton sidebands, an additional Single Mode Fiber (SMF)224 is added to compensate for the normal dispersion of graphene, making the net cavity dispersion anomalous. The two interfaced fiber pigtails 100 in the ring cavity 210 constitute a "graphene mode locker" 225 comprising a graphene-based saturable absorber device 22.

In this embodiment, the fibre laser 200 has a ring cavity 210, the ring cavity 210 having a section of 6.4m Erbium Doped Fibre (EDF)230 with Group Velocity Dispersion (GVD) of 10ps/km/nm and 8.3m (6.4m) SMF224 with a GVD of 18 ps/km/nm. Soliton sidebands were observed after adding an additional 100m SMF224 in the cavity, indicating that net cavity dispersion is anomalous in this cavity. The total fiber dispersion was about 1.96 ps/nm. A 10% fiber coupler 250 is used to output the signal (as shown by arrow 252).

The fiber laser 200 is pumped by a high power fiber raman laser source 260(BWC-FL-1480-1) at a wavelength of 1480nm coupled into the laser cavity 210 with a Wavelength Division Multiplexer (WDM) 266. Polarization independent isolator 270 is hinged into laser cavity 210 to facilitate unidirectional operation. An intra-cavity polarization controller 280 is used to change the linear birefringence of the cavity.

Fiber laser with linear cavity and graphene-based saturable absorber device

Fig. 9 is a schematic diagram of a fiber laser 300 having a linear cavity 310, the fiber laser 300 being designed for mode-locking and Q-switching by using a graphene-based saturable absorber device 22. The saturable absorber material 18 (see, e.g., fig. 1) for the graphene-based saturable absorber device 22 includes, e.g., graphene of varying thicknesses, an assembled structure or composition, which is coated as a film onto an optical element in the form of a highly reflective mirror 326 such that the saturable absorber device 22 can operate in a reflective mode.

The mirror 326 is attached, together with the graphene film 30 (see e.g. fig. 3), to the fiber end facet 14 of the optical fiber 10 carried in the pigtail 100, the pigtail 100 being arranged at one end 312 of the wire-shaped cavity 310. The linear cavity 310 contains an SMF 324 and an EDF 330. At the opposite side 314 of the wire-shaped cavity 310, a faraday mirror 336 is connected to the SMF 324. A fiber coupler 350 is used to output a signal, indicated at 352, through an isolator 370.

A high power fiber raman laser source 360(BWC-FL-1480-1) at a wavelength of 1480nm, coupled to the laser cavity 310 through WDM 366, pumps the fiber laser 300. The linear birefringence of the cavity is changed using an intra-cavity polarization controller 380. Bidirectional oscillation is obtained in the laser cavity 310.

Other aspects and embodiments of the invention

According to a first aspect of the present invention, there is provided a saturable absorber material comprising graphene or a graphene derivative. Saturable absorption is a material property in which the absorption of light decreases with increasing light intensity. Saturable absorbers can be used in laser cavities. Key parameters of a saturable absorber are its wavelength range (at what wavelength it absorbs), its dynamic response (how quickly it recovers), and its saturation intensity and flux (at what intensity or pulse energy it saturates). Which is commonly used for passive Q-switching or mode-locking of lasers.

In a first embodiment of the first aspect of the invention, the saturable absorber material comprises graphene or a derivative thereof. Preferably, the derivatives include, but are not limited to, graphene oxide or graphene-polymer composites, hybrids of graphene with inorganic or organic materials.

In a second embodiment of the first aspect of the invention, the saturable absorber material comprises a multilayer (defined as two or more layers) graphene film.

In a third embodiment of the first aspect of the invention, the saturable absorber material comprises one or more single-layer graphene sheets having a small size (defined as less than 10 μm).

In a fourth embodiment of the first aspect of the invention, the saturable absorber material comprises a composite of graphene and an organic molecule. Preferably, the composite of graphene and an organic molecule exhibits photochromism.

In a fifth embodiment of the first aspect of the invention, the saturable absorber material comprises functionalized or derivatized graphene. In this context, the functionalization or derivatization of graphene means the chemical attachment of chemical functional groups or dye molecules on graphene or graphene oxide to alter its solubility, dispersibility, electronic and optical properties. Preferably, the functionalized or derivatized graphene is functionalized or derivatized with, but not limited to, organic, inorganic, or organometallic materials.

In a sixth embodiment of the first aspect of the invention, the saturable absorber material comprises a thin film (defined as 1-30 layers) of a graphene-based polymer composite made of graphene or a derivative thereof embedded in a host polymer. Preferably, the graphene derivative may be, but is not limited to, graphene oxide, or functionalized graphene. Preferably, the host polymer may be, but is not limited to, polyvinyl alcohol (PVA), Polycarbonate (PC), polyimide and poly (phenylene vinylene) (PPV) derivatives, cellulose derivatives, and conjugated polymers such as poly (3-hexylthiophene-2, 5-diyl) (P3HT), poly (3,3 ″ -dialkyltetrabithiophene) (PQT).

According to a second aspect of the present invention there is provided an optical fiber assembly comprising a graphene or graphene derivative based saturable absorber material assembled or deposited on an optical fiber. The optical fiber assembly includes an exemplary embodiment of a graphene-based saturable absorber device.

In a first embodiment of the second aspect of the invention, the optical fiber assembly comprises a layer of graphene or a derivative thereof assembled on the end facet of the optical fiber. Preferably, the graphene derivative includes, but is not limited to, graphene oxide or derivatized graphene assembled on the end facet of the optical fiber.

In a second embodiment of the second aspect of the invention, the optical fiber assembly comprises a multilayer (defined as 1-30 layers) graphene film deposited on the end facet of the optical fiber.

In a third embodiment of the second aspect of the invention, the optical fiber assembly comprises a single layer of graphene sheets of small size (defined as less than 10 μm) deposited on the fiber end facet of the optical fiber.

In a fourth embodiment of the second aspect of the invention, the optical fiber assembly comprises a composite film of graphene and organic molecules built on the end facet of the optical fiber. Preferably, the composite material of graphene and organic molecules has photochromic properties.

In a fifth embodiment of the second aspect of the invention, the optical fiber assembly comprises a functionalized or derivatized graphene film constructed on the end facet of the optical fiber.

Preferably, the functionalized or derivatized graphene is functionalized or derivatized with, but not limited to, organic, inorganic, or organometallic materials.

In a sixth embodiment of the second aspect of the invention, the optical fiber assembly comprises a film made from a composite of graphene or a graphene derivative and a polymer and transferred to the end facet of the optical fiber.

Preferably, the graphene derivative may be, but is not limited to, graphene oxide, or functionalized graphene.

Preferably, the host polymer may be, but is not limited to, polyvinyl alcohol (PVA), Polycarbonate (PC), polyimide and poly (phenylene vinylene) (PPV) derivatives, cellulose derivatives, and conjugated polymers such as poly (3-hexylthiophene-2, 5-diyl) (P3HT), poly (3,3 ″ -dialkyltetrastophenone) (PQT).

According to a third aspect of the present invention, there is provided a method of preparing an optical fiber assembly comprising a graphene-or graphene derivative-based saturable absorber material, the method comprising: a) preparing a graphene-or graphene derivative-based saturable absorber material, and b) transferring the graphene-or graphene derivative-based saturable absorber material to the terminal facet of the optical fiber.

In a first embodiment of the third aspect of the invention, the method of preparing a graphene-based saturable absorber material consists of one of: mechanical lift-off, epitaxial growth, chemical vapor deposition, chemical processing (solution processing), laser ablation, and filtered cathodic arc.

In a second embodiment of the third aspect of the invention, the method of transferring the prepared graphene or graphene derivative based saturable absorber material to the end facet of the optical fiber is: polydimethylsiloxane (PDMS) imprinting is used to transfer the imprinted graphene film onto the fiber end facet, which is suitable for a wide range of initial substrates at which graphene or its derivatives are prepared.

In a third embodiment of the third aspect of the present invention, the method for transferring the prepared saturable absorber material based on graphene or graphene derivatives (wherein the saturable absorber material is a graphene film prepared by epitaxial growth and chemical vapor deposition) to the end facet of the optical fiber is: separated from the original substrate by a float-off process that involves etching the substrate in an acid or salt solution.

In a fourth embodiment of the third aspect of the present invention, the method for transferring the prepared saturable absorber material based on graphene or graphene derivatives (wherein the saturable absorber material is mechanically exfoliated graphene) to the end facet of the optical fiber is: adhering an adhesive tape comprising a graphene surface layer directly onto the fiber end facet by aligning the mechanically exfoliated graphene with the fiber pin holes.

In a fifth embodiment of the third aspect of the invention, the method of transferring the prepared graphene or graphene derivative based saturable absorber material to the terminal facet of the optical fiber is: this is suitable for solution processed graphene or graphene dispersed in a solvent using assembly techniques such as a layer-by-layer approach.

In a sixth embodiment of the third aspect of the invention, the method of transferring the prepared graphene or graphene derivative based saturable absorber material to the end facet of the optical fiber is: optical trapping was used, in which a clean optical fiber with adjustable optical parameters was immersed in a graphene solution.

In a seventh embodiment of the third aspect of the invention, the method of transferring the prepared graphene or graphene derivative based saturable absorber material to the end facet of the optical fiber is: a spin coating technique is used to form a polymer-graphene composite material, which is then applied over the fiber end facet.

In an eighth embodiment of the third aspect of the present invention, the method of transferring the prepared graphene or graphene derivative based saturable absorber material to the end facet of the optical fiber is: graphene-ionic liquid gel is used for application onto the fiber end facet.

According to a fourth aspect of the present invention there is provided a fibre laser comprising a saturable absorber material based on graphene or graphene derivatives. In this context, a fiber laser is a laser in which the active gain medium is an optical fiber doped with rare earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, and thulium.

In a first embodiment of the fourth aspect of the invention, the fibre laser comprises a ring cavity comprising a saturable absorber material based on graphene or graphene derivatives.

In a second embodiment of the fourth aspect of the invention, the fibre laser comprises a wire cavity comprising a saturable absorber material based on graphene or graphene derivatives.

According to a fifth aspect of the present invention there is provided the use of a graphene or graphene derivative based material as a saturable absorber in a fibre laser for mode locking, Q-switching, optical pulse shaping, optical switching, optical signal processing etc. of the laser.

Claims (18)

1. A saturable absorber device for use in a laser cavity, comprising:

an optical element; and

a saturable absorber material carried by the optical element in a manner operable and comprising at least one of graphene, a graphene derivative and functionalized graphene;

wherein the optical element comprises an optical fiber.

2. A saturable absorber device according to claim 1, wherein the saturable absorber material comprises at least one of: at least one layer of graphene; at least one layer of graphene oxide; at least one layer of a graphene-polymer composite; at least one hybrid layer formed of graphene and at least one type of inorganic material; at least one hybrid layer formed of graphene and at least one type of organic material; at least one layer of graphene sheets; at least one layer of a film made of graphene or a graphene derivative embedded in a host polymer; a graphitic alkyl; and oxidized graphitic.

3. A saturable absorber device according to claim 1, wherein the saturable absorber material comprises a combination of graphene and photochromic organic molecules.

4. A saturable absorber device according to claim 1, wherein the saturable absorber material is functionalized or derivatized with at least one of an organic material, an inorganic material, and an organometallic material.

5. A saturable absorber device according to claim 2, wherein the host polymer is at least one of: polyvinyl alcohol (PVA), Polycarbonate (PC), polyimide and poly (phenylene vinylene) (PPV) derivatives, cellulose derivatives, poly (3-hexylthiophene-2, 5-diyl) (P3HT) or poly (3,3 "-dialkyl tetrathiophene) (PQT).

6. A saturable absorber device according to claim 1, wherein the optical element comprises an end facet, and wherein the saturable absorber material is carried by the optical element on the end facet.

7. A saturable absorber device according to claim 1, further comprising a fiber holder holding the optical fiber.

8. A saturable absorber device according to claim 7, wherein the fiber holder and optical fiber comprise a fiber pigtail.

9. A fiber laser comprising:

a ring-shaped or linear laser cavity; and

a saturable absorber device operably disposed within the laser cavity, the saturable absorber device comprising: an optical element and a saturable absorber material carried by the optical element, the saturable absorber material comprising at least one of graphene, a graphene derivative, and functionalized graphene;

wherein the optical element comprises an optical fiber.

10. The fiber laser of claim 9, wherein said saturable absorber device is disposed in said laser cavity to provide at least one of mode locking, Q-switching, optical pulse shaping, optical switching, and optical signal processing.

11. The fiber laser of claim 10, wherein the fiber is clamped within a fiber pigtail.

12. A method of forming a saturable absorber device for use in a laser cavity, comprising:

providing an optical element; and

supporting a saturable absorber material with the optical element, the saturable absorber material comprising at least one of graphene, a graphene derivative, and functionalized graphene;

wherein the optical element comprises an optical fiber.

13. The method of claim 12, further comprising: the saturable absorber material is prepared using at least one of mechanical lift-off, epitaxial growth, chemical vapor deposition, chemical processing, laser ablation, and filtered cathodic arc.

14. The method of claim 12, wherein the optical element has an end facet, the method further comprising: applying the saturable absorber material to the end facet.

15. The method of claim 14, wherein the applying comprises transferring the printed graphene film onto the end facet using a Polydimethylsiloxane (PDMS) stamp.

16. The method of claim 14, wherein the applying comprises at least one of:

a) a floating method;

b) adhesive tape method;

c) applying layer by layer;

d) optical capturing;

e) spin coating;

f) applying a graphene-ionic liquid gel; and

g) and (6) stamping.

17. The method of claim 14, comprising providing the optical element as an optical fiber clamped in a fiber pigtail.

18. The method of claim 12, comprising forming the saturable absorber material as at least one of: at least one layer of graphene; at least one layer of graphene oxide; at least one layer of a graphene-polymer composite; at least one hybrid layer formed of graphene and at least one type of inorganic material; at least one hybrid layer formed of graphene and at least one type of organic material; at least one layer of graphene sheets; at least one layer of a film made of graphene or a graphene derivative embedded in a host polymer; a graphitic alkyl; and oxidized graphitic.