Nanotube and graphene saturable absorbers for fibre lasers

COMMENTARY FOCUS Nanotube and graphene saturable absorbers for fibre lasers Amos Martinez and Zhipei Sun Nanotubes and graphene have emerged as promising materials for use in ultrafast fibre lasers. Their unique electrical and optical properties enable them to be used as saturable absorbers that have fast responses and broadband operation and that can be easily integrated in fibre lasers. Owing to their inherent technological advantages, ultrafast fibre lasers are increasingly becoming the light source of choice for a wide range of industrial and scientific applications. When they were first developed, rare-earthdoped fibre gain media were identified as being excellent laser platforms, capable of providing high beam quality and efficient heat dissipation in alignmentfree, compact and simple configurations 1. At that time, the focus was exclusively on telecommunication applications. The development of the ytterbium-doped fibre along with continuous advances in fibre design and fabrication went a long way towards alleviating early reservations regarding the potential to scale up the delivered power. Nowadays, fibre lasers are a mainstay in industrial micromachining for which they offer unrivalled wall-plug efficiencies and easy beam delivery 2. The design flexibility and variety of gain media of fibre lasers permit the micromachining of materials as diverse as metals, dielectrics and soft tissue. The tight mode confinement and long interactive length provided by fibres are exploited in various ultrafast nonlinear frequency conversion sources such as supercontinuum sources and optical parametric oscillators. Because these sources have high efficiencies and are alignment free, they are increasingly being considered for applications in medical diagnosis and treatment and for scientific applications such as spectroscopy, microscopy and imaging 3,4. In addition to being used as a technological tool, ultrafast fibre lasers are an effective platform for studying nonlinear phenomena (such as self-similarity 5, rogue waves 6 and turbulence 7 ) to advance our understanding of general nonlinear processes in nature. These applications, however, represent only the tip of the iceberg after the furious pace of development of ultrafast fibre laser a Adapter b c connector technology in the past few years. Current research continues to extend the limits of available average and peak powers and pulse duration of ultrafast fibre lasers, and to extend the range of emission wavelengths towards the mid-infrared 8 and ultraviolet 9 regions. These advances have important practical implications. It may not be an overstatement to say that ultrafast fibre lasers will gradually become ubiquitous in everyday life. Current state-of-the-art saturable absorbers Ultrashort pulses are typically generated using a passive mode-locking technique that involves inserting a nonlinear optical device called a saturable absorber () into the laser cavity. In passive mode locking, pulsed operation originates from noise fluctuations in the laser cavity. A noise spike of sufficient intensity will begin to saturate the absorber, reducing its losses. The noise spike will be further amplified in subsequent round trips, and this increased d e f Coated intensity will further saturate the losses until a stable train of pulses eventually forms. Within these pulses, the central peak intensity saturates the more strongly than the pulse wings; this can lead to a further reduction in the pulse duration. The absorption of a decreases with increasing input optical intensity. This favours the generation of high-intensity pulses over continuous-wave operation 10. For all materials, typical macroscopic parameters are the operating wavelength range, the saturable fluence (or intensity), the non-saturable loss, the modulation depth and the recovery time 11. In principle, most light-absorbing materials can be used as s in their resonant absorption wavelength range. Indeed, over the past couple of decades, a large range of materials have been demonstrated, including dyes, colour filter glasses, dye- or ion-doped crystals and glasses, metal nanoparticles and semiconductors. Unfortunately, all these s have their own drawbacks, and are thus unable to Gain fibre Grating Figure 1 Various CNT-/graphene- integration methods for fibre devices. a, Sandwiched device, b, in-fibre microfluidic channels, c, photonic-crystal fibres, d, D-shaped and e, tapered fibres. f, Fully integrated monolithic fibre laser. The s represented in this figure could be either CNT-s or graphene s. Pump 842 NATURE PHOTONICS VOL 7 NOVEMBER 2013 www.nature.com/naturephotonics

FOCUS COMMENTARY satisfy the key requirements for ultrafast fibre lasers, such as fast response time, strong nonlinearity, broad bandwidth, low loss, high power handling, low cost, and simplicity of fabrication and integration into various optical fibre systems. Current commercial ultrafast lasers generally use semiconductor mirrors (SEMs) 11. The excellent performance of SEMs is mainly credited to the welldeveloped semiconductor technologies for electronics (for example, bandgap and defect engineering and growth techniques) 11. This allows good control over the parameters, and thus SEMs currently are the primary s employed for mode locking of not only solidstate lasers for which they were first developed 11 but also of most other types of lasers (including fibre, waveguide and semiconductor lasers). However, the fabrication of SEMs generally involves complex, highly specialized equipment and either post-growth ion implantation or low-temperature growth to reduce the device response time 11. In addition to being expensive, they are an inflexible and bulky solution for fibre lasers. Also, a typical SEM design functions only in a narrow (< ~100 nm) wavelength range, centred between 800 nm and 2,000 nm (ref. 11). The disadvantages of traditional s have spurred researchers to develop artificial s based on nonlinear effects in fibres, such as nonlinear optical loop mirrors and nonlinear polarization evolution 10. However, it is challenging to experimentally quantify and reproduce the performance of these artificial s, and they are typically sensitive to environmental changes. Furthermore, they often involve free-space optical components, and thus cannot meet the alignment-free requirement of commercial applications. It is thus crucial to identify s that are fully compatible with all-polarization-maintaining fibre configurations and that exhibit ultrafast responses and broadband operation while having low fabrication and integration costs and complexity. Such s will facilitate the transition of new types and applications of ultrafast fibre lasers to the market place. Carbon-nanotube- and graphenebased saturable absorbers s based on single-walled carbon nanotubes (CNTs) were first developed in 2003 12, and have subsequently been rapidly adopted by several groups because they are relatively simple and inexpensive to fabricate 13. CNT-s are particularly advantageous for fibre lasers a b Optical output (dbm) 30 10 10 30 50 70 -coated mirror 1 cm cavity mirror Pump 1,558.5 1,559.5 1,560.5 1,561.5 90 1,545 1,555 1,565 1,575 1,585 because they can be easily integrated into various fibre configurations while preserving an alignment-free, all-fibre format. For example, CNTs or their polymer composites can be sandwiched between two fibre connectors (Fig. 1a), injected into in-fibre microfluidic channels (Fig. 1b) and photonic-crystal fibres (Fig. 1c), and coated on the surfaces of D-shaped (Fig. 1d) or tapered fibres (Fig. 1e). The versatility of CNT-s has inspired various fully integrated, fibrebased s (Fig. 1b e), which cannot be realized using SEMs. In addition, CNT- s have a sub-picosecond response time, making them suitable for ultrafast pulse generation. As-produced CNTs typically have a broad diameter distribution, which can be beneficial for ultrafast pulse generation as it leads to broadband operation 14. For example, a single CNT- can mode lock fibre lasers covering all the major wavelengths between 1 μm and 2 μm (ref. 15). This broadband operation is undoubtedly a key advantage of CNT-s over SEMs. The main challenge associated with CNT-s is their relatively large nonsaturable losses, which are partly caused by the broad diameter distribution. Resonant absorption occurs efficiently only in semiconducting CNTs whose diameter corresponds to a resonance c Normalized spectral intensity 1,520 1,540 1,560 1,580 Figure 2 Representative performance of ultrafast fibre lasers mode locked by CNT-s and graphene s. a, Photograph of a high-repetition-rate CNT- mode-locked fibre laser; b, its output spectrum. c, spectrum of a widely tunable fibre laser mode locked by graphene. Figure reproduced with permission from: a,b, ref. 36, 2013 SPIE; c, ref. 21, 2010 NPG. at the photon energy (Box 1). The large number of CNTs (which are generally bundled together) that are not in resonance with the operational wavelength contribute to the fast recovery time of, but also give rise to the high non-saturable losses of CNT-s via scattering 16. The ratio of the modulation depth to the non-saturable loss of CNT-s is typically less than 1 (ref. 16). In contrast, SEMs can exhibit an excellent ratio of the modulation depth to the non-saturable loss a recent study reported non-saturable losses as low as 0.1% and a ratio in excess of 40 (ref. 17). The high non-saturable loss of CNT-s is a major drawback for their application in ultrafast solid-state lasers but not in fibre lasers, which can tolerate high losses because of the high single-pass cavity gain of fibres. Consequently, most efforts to exploit CNT-s have focused on various fibre laser applications, exhibiting the advantages of miniature size and broadband operation. After the first demonstrations of graphene s in 2009 18,19, research into them has been progressing at a fast pace. Laser scientists were attracted by their fabrication methods, which do not require a large capital investment 20 21, and by the possibility of adopting techniques previously developed for CNT- lasers (Fig. 1a e). Graphene s have ultrafast response times NATURE PHOTONICS VOL 7 NOVEMBER 2013 www.nature.com/naturephotonics 843

COMMENTARY FOCUS of a few hundred femtoseconds, making them suitable for ultrafast pulse generation. They typically have a ratio of the modulation depth to the non-saturable loss of around 1 (refs 22,23), which is slightly better than that of CNT-s. Compared to SEMs and CNT-s, graphene has the major advantage of intrinsic wideband operation, which can extend from the ultraviolet to the far-infrared region, owing to the linear energy dispersion relation of graphene (Box 1) 21. Graphene s have been used to produce pulses from 0.8 μm (ref. 22) to 2.9 μm (ref. 24). Such broadband operation does not imply that the performance of graphene s is wavelength independent; the saturation fluence of graphene s is lower at longer wavelengths (for example, 66 μj cm 2 at 800 nm (ref. 22), compared with 14 μj cm 2 at 1,500 nm (ref. 23)). This wavelength dependence of the saturation fluence generally favours the use of graphene s in the mid-infrared region. Another challenge for graphene s is the low modulation depth of singlelayer graphene; it is typically around 1% (refs 22,23), which is too low for ultrafast fibre lasers. However, the modulation depth of graphene s can be improved by stacking multiple single layers of graphene, but this also increases the saturation fluence 21. The characteristics of graphene s can be further engineered by, for example, modulating its optical absorption by electrical gating 25, providing a combined active and passive modulation function. In conclusion, the key benefits of CNT- s and graphene s, namely broadband operation and simple, cost-effective fabrication and integration, outweigh their relatively high losses. Current trends Since their recent introduction, the performances of CNT-s and graphene s have steadily improved, and their unique features have resulted in the development of various novel modelocked fibre lasers. However, to confirm the suitability of these s for future applications, it is necessary to demonstrate that they can handle higher optical powers and shorter pulse durations, and that they can provide saturable absorption over a wider wavelength range (from the ultraviolet to the mid-infrared region). Below, we consider the specific requirements for s and approaches that are being considered to enable CNT-s and graphene s to meet these requirements. power: Most early studies produced few-milliwatt soliton pulses by employing erbium-doped fibres as the gain medium Box 1: Graphene and carbon nanotubes Graphene is a monolayer of hexagonally arranged carbon atoms, and is the building block for graphitic materials of every other dimensionality. Its energy momentum relation is linear at low energy near the six corners of the two-dimensional hexagonal Brillouin zone (Fig. B1a). Graphene can absorb ~2.3% of white light, despite being only a single atom thick (~0.3 nm). The optical absorption of few-layer graphene is proportional to the number of layers. Interband excitation in graphene by ultrafast optical pulses produces a nonequilibrium carrier population in the valence and conduction bands. Saturable absorption is observed as a consequence of Pauli blocking. Ultrafast responses down to 100 fs can be observed by different relaxation channels in timeresolved experiments 21. The linear dispersion of the Dirac electrons ensures that, for any excitation, there will always be an electron hole pair in resonance 21. Thus, graphene is an ultrafast and ultrawideband material for ultrafast pulse generation. CNTs are rolled-up graphene sheets that form seamless cylinders. Their electronic properties depend on their diameter and chirality (that is, the twist angle along the tube axis). The density of states of single-walled CNTs is dominated by a series of characteristic van Hove singularities because of the one-dimensional nature of the electronic bands. Saturable absorption occurs when strong excitation depletes the electron population of the valence band and enhances the electron occupation of the conduction band. Semiconducting CNTs have very fast recovery times that are of the order of picoseconds. CNT bundles, with the naturally occurring and operating fibre lasers in the anomalous dispersion regime at 1.55 μm (see ref. 13 and references therein). In such soliton fibre lasers, mode locking relies on the balance between dispersion and self-phase modulation. In the past few years, the focus has shifted towards the generation of dissipative solitons and similaritons to overcome the limits imposed by conservative soliton propagation. In these fibre lasers, in addition to the nonlinearity and dispersion, the cavity gain and losses need to be carefully balanced. These lasers generally produce highly chirped pulses and require spectral filtering to compensate a E k Energy b 1 0.0 semiconducting to metallic ratio, exhibit even faster dynamics (sub-picosecond) because of excited states relaxing through the metallic tubes. To a first-order approximation, the bandgap of CNTs, which is directly related to their optical absorption peak positions, varies inversely with their diameter. Therefore, broadband operation is possible by using CNTs with a broad diameter distribution. The fabrication methods of CNT-s and graphene s can be categorized into two classes: dry processing and solution processing. Dry processing typically involves directly incorporating freestanding, as-grown CNTs or mechanically exfoliated graphene into the optical system. Solution processing generally entails dispersing CNTs or graphene into various solvents to either spray coat a substrate or embed the CNTs or graphene in a host polymer. In general, solutionprocessing methods offer a simple, scalable and cost-effective approach to fabricate and integrate various CNT-s and graphene s. for the dispersion 26. The use of these novel regimes has resulted in a rapid increase in the pulse energies directly achievable from a fibre oscillator 26. Most of the work in this area has relied on nonlinear polarization evolution for mode locking. However, most nonlinear polarization evolution devices employ free-space optics and non-polarization-maintaining fibres. For practical applications and environmental stability, all-polarizationmaintaining, all-fibre lasers 27 are desirable. Certainly, there are significant advantages of using CNT-s and graphene s in these new mode-locking regimes 28 : not 1 0 Conduction Valence 0.5 1.0 1.5 Electronic DOS Figure B1 Electronic density of states. a,b Plots of energy momentum relation of graphene (a) and of energy against electronic density of states (DOS) of a semiconducting nanotube (b). 844 NATURE PHOTONICS VOL 7 NOVEMBER 2013 www.nature.com/naturephotonics

FOCUS COMMENTARY only are all-polarization-maintaining fibre configurations attainable, but also the ultrasmall dimensions and the easy integration of these s offer more flexibility in cavity design. To meet the increasingly stringent requirements for fibre lasers, it is necessary to further engineer the parameters of CNT-s and graphene s (specifically the modulation depth, saturation fluence and non-saturable loss) and to increase their damage resistance by using, for example, evanescent field interaction 29. wavelength: The key performance advantage of CNT-s and graphene s over SEMs is their broadband operation (Fig. 2c). This aspect will become increasingly important with the growing attention that the mid- and far-infrared regions are attracting because of their suitability for diverse applications in spectroscopy and chemical sensing 8. CNT-s 30 and graphene s 31 have been used to modulate thulium- and holmium-doped fibre lasers operating at about 2 μm (and even up to 2.9 μm (ref. 24)) with a wide tunability 30. These s are also capable of operating at shorter wavelengths, including the visible spectral range. The recent demonstration of broadband Raman gain and wideband CNT-s/graphene s shows the possibility of obtaining broader output spectra than ever before 32. Pulse duration: The shortest fibre lasers that have been mode locked using CNT-s and graphene s to date are based on the stretched-pulse design 33. It is not necessary to generate short pulses and high powers directly in the oscillator, because the pulse energy can be amplified and the pulse duration can be compressed externally 13,34. Repetition rate: There is a strong demand for all-fibre mode-locked lasers with a high repetition rate of the order of gigahertzs for applications in telecommunications, metrology and optical coherence tomography. Most fibre lasers mode locked by CNT-s or graphene s have a few-metre-wide ring configuration and have repetition rates of the order of megahertzs 13. An effective approach for increasing the repetition rate is to reduce the cavity length in a linear cavity design. CNT-s and graphene s are particularly well suited for such miniature laser cavities, as they can be easily deposited on a fibre end, a mirror or a fibre surface (Fig. 1f). An early demonstration of such all-fibre, linear-cavity lasers mode locked with CNT-s or graphene s (Fig. 2a,b) could provide a fundamental repetition rate as high as ~20 GHz (ref. 35). Outlook New applications in photonics for graphene and CNTs are being proposed on an almost daily basis. Although some of these proposed applications will fade from view over time, others are likely to become a reality. Both materials have been utilized for achieving passive mode locking in all major laser systems ranging from fibre, solid-state and waveguide lasers to vertical-external-cavity surfaceemitting lasers (VECSELs). Their viability for this application has been confirmed by the successfully commercialization of s based on these materials. We believe that research on graphene- and CNT-based s will continue to grow and that such s will be used, along with other s (for example, SEMs and s based on nonlinear polarization evolution), to provide ultrafast fibre lasers for various applications. These new s are particularly suitable for a large range of emerging applications that require broad operation bandwidths, small dimensions and easy integration. To fully exploit the advantages of CNT-s and graphene s, it is necessary to improve control over the modulation depth, saturation fluence, non-saturable loss and optical damage threshold (important for highpower operation). One way to achieve this is to control the CNT diameter distribution, length and bundling during the fabrication process and to optimize the light material interaction conditions by using evanescent field interaction during the integration process. It is worth emphasizing that the easy fabrication and integration of s based on CNTs and graphene and their broad operation bandwidths are extremely valuable for various new fibre modelocking techniques operating at broad wavelength ranges. Amos Martinez is at the Research Center for Advance Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan. Zhipei Sun is at the Department of Microand Nanosciences, Aalto University, PO Box 13500, FI-00076 Aalto, Finland. e-mail: martinea@cntp.t.u-tokyo.ac.jp, zhipei.sun@aalto.fi References 1. Poole, S. B., Payne, D. N. & Fermann M. E. Electron. Lett. 21, 737 738 (1985). 2. Richardson, D. J., Nilsson, J. & Clarkson. W. A. J. Opt. Soc. Am. B 27, B63 B92 (2010). 3. 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Correction In the Commentary entitled Nanotube and graphene saturable absorbers for fibre lasers (Nature Photon. 7, 842 845; 2013), electrical gating 24 on page 844 should have been electrical gating 25, compressed externally 19,34 on page 845 should have been compressed externally 13,34 and 23. Davide Di Dio Cafiso, S. Opt. Lett. 38, 1745 1747 (2013) in the reference section should have been 23. Davide Di Dio Cafiso, S. et al. Opt. Lett. 38, 1745 1747 (2013). These errors have been corrected in both the HTML and PDF versions.