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Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker

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Due to its unique electronic property and the Pauli blocking principle, atomic layer graphene possesses wavelength-independent ultrafast saturable absorption, which can be exploited for the ultrafast photonics application. Through chemical
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  Large energy soliton erbium-doped fiber laser with a graphene-polymercomposite mode locker Han Zhang, 1 Qiaoliang Bao, 2 Dingyuan Tang, 1,a  Luming Zhao, 1 and Kianping Loh 2 1 School of Electrical and Electronic Engineering, Nanyang Technological University,Singapore 639798, Singapore 2  Department of Chemistry, National University of Singapore, Singapore 117543, Singapore  Received 4 August 2009; accepted 15 September 2009; published online 5 October 2009  Due to its unique electronic property and the Pauli blocking principle, atomic layer graphenepossesses wavelength-independent ultrafast saturable absorption, which can be exploited for theultrafast photonics application. Through chemical functionalization, a graphene-polymernanocomposite membrane was fabricated and first used to mode lock a fiber laser. Stable modelocked solitons with 3 nJ pulse energy, 700 fs pulse width at the 1590 nm wavelength have beendirectly generated from the laser. We show that graphene-polymer nanocomposites could be anattractive saturable absorber for high power fiber laser mode locking. © 2009 American Institute of Physics .  doi:10.1063/1.3244206  Recently, passive mode locking of fiber lasers usingsingle-wall carbon nanotubes  SWCNTs  as saturableab-sorbers has attracted considerable attention. Set et al. 1 firstreported fiber laser mode locking using SWCNTs. It wasshown that SWCNTs have broadband saturable absorptionand a fast absorption recovery time, which could be used asa saturable absorber for laser mode locking. Conventionally,passive mode locking of a laser is achieved with a semicon-ductor saturable absorption mirror  SESAM  . However, fab-rication of SESAMs requires expensive and complex epitax-ial growth technique. In contrast, saturable absorbers based on carbonnanotubes could be easily fabricated. 1–4 FollowingSet et al. , 1 mode locking of fiber lasers using SWCNTs or itspolymer composites has been intensivelyinvestigated. Worthmentioning are the works by Song et al. , 5 who have achievedin an erbium-doped fiber laser picosecond pulse emissionwitha record high pulse energy of   6.5 nJ, and Wang et al. , 6 who have demonstrated wideband tuning of a modelocked erbium-doped fiber laser. In addition, Kieu and Wise 7 have shown mode locking of an all-normal dispersion Ybfiber laser using a SWCNTs-based absorber, and generatedpulses with 1.5 ps pulse duration and 3 nJ pulse energy.Recently, mode locking of solid-state lasers using SWCNTshas also been demonstrated. 8 A drawback of SWCNTs-based saturable absorbers isthat SWCNTs tend to form bundled and entangled morphol-ogy, which causes strong scattering and thus strong nonsat-urable losses. In addition, under large energy, ultrashort pulseradiation multiphoton effect induced oxidation occurs, 9 which degrades the long-term stability of the absorber. Inthis letter, we report on the soliton mode locking of anerbium-doped fiber laser using a graphene-polymer nano-composite membrane as the mode locker. Graphene is asingle two-dimensional  2D  atomic layer of carbon atomarranged in a hexagonal lattice. Although as an isolated filmit is a zero bandgap semiconductor, we found that lik e theSWCNTs, graphene also possesses saturable absorption. 10 Inparticular, as it has no bandgap, its saturable absorption fea-ture is wavelength independent. It is potentially possible touse graphene or graphene-polymer composites to make awideband saturable absorber for laser mode locking. Further-more, comparing with the SWCNTs, as graphene has a 2Dstructure, it should have less surface tension, therefore, muchhigher damage threshold. Indeed, in our experiments with anerbium-doped fiber laser we have first demonstrated self-started mode locking of the laser with a grapheme-polymermembrane as the saturable absorber, and achieved stablesoliton pulses with 700 fs pulse width and 3 nJ pulse energy.Our fiber laser is schematically shown in Fig.1  a  . Apiece of 5.0 m, 2880 ppm erbium-doped fiber  EDF  withgroup velocity dispersion  GVD  of   32  ps/nm   /km wasused as the gain medium, and 23.5 m single mode fiber  SMF  with GVD 18  ps/nm   /km was employed in the cavityto compensate the normal dispersion of the EDF and ensurethat the cavity has net anomalous GVD. The net cavity dis-persion is estimated −0.3419 ps 2 . A 30% fiber coupler wasused to output the signal, and the laser was pumped by a highpower fiber Raman laser source  KPS-BT2-RFL-1480–60-FA  of wavelength 1480 nm. The maximum pump power canbe as high as 5 W. A polarization independent isolator wasspliced in the cavity to force the unidirectional operation of the ring cavity, and an intracavity polarization controller  PC  was used to fine tune the linear cavity birefringence. Anoptical spectrum analyzer  Ando AQ-6315B  and a 350 MHzoscilloscope  Agilent 54641A  combined with a 2 GHz pho-todetector was used to simultaneously monitor the spectraand pulse train, respectively. A graphene-polymer nanocom-posite membrane with a thickness of   10  m inserted be-tween two ferules was used as the saturable absorber for thelaser mode locking. To make the graphene-polymer nano-composite, graphene nanosheets were produced by chemi-cally reducingthe oxidized graphene exfoliated from graph-ite flakes. 11 The graphene nanosheets were thennoncovalently functionalized with 1-pyrenebutanoic acid,succinimidyl ester to improve their solubility in organic sol-vents  i.e., ethanol and acetone  and compatibility with poly-mers  i.e., polyvinylidene fluoride  PVDF  . The as-producedgraphene  2 mg  was further dispersed in ethanol  3 ml  andmixed with PVDF solution comprising 1.5 g PVDF dis- a  Author to whom correspondence should be addressed. Electronic mail:edytang@ntu.edu.sg. APPLIED PHYSICS LETTERS 95 , 141103  2009  0003-6951/2009/95  14   /141103/3/$25.00 © 2009 American Institute of Physics 95 , 141103-1 Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp  solved in 10 ml dimethylacetamide/acetone  2:3  . Thegraphene-PVDF solution was then stirred for 24 h at 60 °Cin sealed bottle to form wet paste for electrospinning. Theelectrospinning was carried out in a MECC NANON ELEC-TROSPINNING SETUP at a bias voltage of 30 kV and feed-ing rate of 0.5 ml/h. Figure1  b  is a scanning electron mi-croscopy  SEM  image of the membrane. It shows that themembrane mainly comprises networks of the graphene-filledpolymer nanofibers. The inset of Fig.1  b  is the photo of afreestanding membrane. Figure1  c  shows the transmissionelectron microscopy  TEM  image of a graphene-PVDFnanofiber. It reveals that the graphene nanosheets are welldispersed in the polymer matrix without obvious aggrega-tion.We have also measured the linear and nonlinear absorp-tion of the graphene-polymer membrane. The linear absorp-tion spectra of both graphene-based PVDF nanocompositeand pure PVDF were compared in Fig.2  a  ,which showsthat pure PVDF has a relatively low absorbance of   35 % inthe L-band of the optical communication windows whilegraphene-based PVDF nanocomposite has an enhanced ab-sorbance of   52 % . The nonlinear absorption curve of Fig.2  b  measured at the wavelength of   =1590 nm shows thatthe graphene-based PVDF nanocomposite has a normalizedmodulation depth of   28.3 % and a saturable fluence of 0.75 MW / cm 2 , which is about an order of magnitudesmaller than that of the SWCNTs based saturable absorber.Moreover, the insertion loss of the graphene-based PVDFnanocomposite was as low as  1.5 dB. Previous studieshave also shown that graphene and graphite thin films haveboth an ultrafast absorption recovery time constant of   200 fs and a slower recovery constant of 2.5–5 ps. 12 Mode locking of the laser self-started at a pump powerof   400 mW. Figure3  a  shows the typical optical spectraof the mode locked laser emissions. The spectra have a broadspectral bandwidth with obvious Kelly spectral sidebands,characterizing that the mode locked pulses are optical soli-tons. The central wavelength of the spectra is at 1589.68 nm,which is in the L-band of the optical communication win-dows. The 3 dB bandwidth of the spectra is about 5.0 nm.Figure3  b  shows the measured autocorrelation trace of thesolitons. It has a Sech-profile with a FWHM width of about1.07 ps, which, divided by the decorrelation factor of 1.54,corresponds to a pulse width of 694 fs. The time-bandwidthproduct of the pulses is 0.412, showing that the solitons areslightly chirped. Figure3  c  shows an oscilloscope trace of the laser emission. We also measured the radio-frequency  rf   spectrum of the mode locking state. Figure3  d  shows ameasurement made at a span of 10 kHz and a resolutionbandwidth of 10 Hz. The fundamental peak located at thecavity repetition rate of 6.95 MHz has a signal-to-noise ratioof 65 dB. The Fig.3  e  shows the wideband rf spectrum upto 1 GHz. The absence of spectral modulation in RF spec-trum indicates that the laser operates well in the cw mode-locking regime.Different from the soliton operation of the erbium-dopedfiber lasers mode locked with the nonlinear polarization ro-tation technique or a SESAM, no multiple solitons wereformed in the cavity immediately after the mode locking.Instead only one soliton was always initially formed. This FIG. 1.  Color online   a  Schematic of the soliton fiber laser. WDM denoteswavelength division multiplexer, EDF denotes erbium-doped fiber, PC de-notes polarization controller, and SMF denotes single mode fiber.  b  SEMimage of the graphene-polymer nanofiber networks. Inset: a photograph of the freestanding graphene-polymer composite membrane.  c  TEM image of a graphene-PVDF nanofiber.FIG. 2.  Color online   a  UV-visible-NIR absorption spectra of graphene-based PVDF nanocomposites and pure PVDF. The inset shows the chemicalstructure of the functionalized graphene.  b  Power dependent nonlinearsaturable absorption of graphene-based PVDF nanocomposites. 141103-2 Zhang et al. Appl. Phys. Lett. 95 , 141103  2009  Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp  experimental result shows that the laser has a much lowereffective mode locking threshold than those mode lockedwith SESAMs, which is traced back to the much smallersaturation fluence of the grapheme-polymer nanocompositethan the SESAMs. The single soliton operation could bemaintained in the laser as the pump power was graduallyincreased to 2 W. Further increasing pump power, pulsebreaking occurred. Eventually multiple solitons were formedin the laser. Under multiple soliton operations occasionallyharmonic mode locking was also observed. We have focusedon the single soliton operation of the laser. The energy of thesoliton increased with the pump power. A maximum outputpower of   13.1 dBm had been obtained under the pumppower of   2 W, which indicates the single soliton energy ashigh as  3 nJ. Experimentally we found that the pumppower could be increased to as high as 3.2 W and the laseroutput power could be as large as 17 dBm. Under the pumpand laser operation condition the mode locking of the lasercould still maintain for hours, which indicates that thegraphene-polymer composite could endure at least an opticalfluency as high as 21.4 mJ / cm 2 . After the operation we hadalso checked the graphene-polymer composite film using theoptical microscopy and found that its morphology was keptintact, which verified its strong thermal stability.In order to investigate the long-term stability of themode locking, we have recorded the soliton spectra of thelaser in a 4 h interval over 2 days, as shown in Fig.3  a  . Itshows that the soliton parameters, these are the central wave-length, 3 dB spectral bandwidth, Kelly sideband positionsand the spectral peak powers, have kept reasonably un-changed. Experimentally, it was also found that the solitonpulse width could be compressed to  524 fs after passingthrough a 10 m dispersion compensation fiber of GVD  −4  ps / nm  / km. The result shows that the output solitonswere negatively chirped. We note that the current experimen-tal results were obtained by the first try of the mode lockingtechnique. It is expected that through further careful designof the laser cavity and optimization on the saturable absorp-tion parameters, mode locked pulses with even larger pulseenergy and narrower pulse width could be generated.In conclusion, we have reported an erbium-doped solitonfiber laser with a graphene-polymer nanocomposite mem-brane as the mode locker. Self-started mode locking of thelaser was first experimentally demonstrated, and stable soli-ton pulses at 1590 nm with 3 nJ pulse energy and 700 fspulse width were directly generated from the laser. Our ex-perimental results have clearly shown that a graphene-polymer composite membrane has not only the desired satu-rable absorption for laser mode locking, but also a largedamage threshold. It could be a cost-effective saturable ab-sorber for large energy fiber laser mode locking.K.P.L. wishes to acknowledge funding support fromNRF-CRP Graphene Related Materials and Devices  GrantNo. R-143-000-360-281  . 1 S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski,J. Lightwave Technol. 22 , 51  2004  . 2 K. Kieu and M. Mansuripur,Opt. Lett. 32 , 2242  2007  . 3 Z. Sun, A. G. Rozhin, F. Wang, V. Scardaci, W. I. Milne, I. H. White, F.Hennrich, and A. C. Ferrari,Appl. Phys. Lett. 93 , 061114  2008  . 4 S. Yamashita, Y. Inoue, S. Maruyama, Y. Murakami, H. Yaguchi, M.Jablonski, and S. Y. Set,Opt. Lett. 29 , 1581  2004  . 5 Y. W. Song, S. Yamashita, and S. Maruyama,Appl. Phys. Lett. 92 , 021115  2008  . 6 F. Wang, A. Rozhin, V. Scardaci, Z. Sun, F. Hennrich, I. H. White, W. I.Milne, and A. C. Ferrari,Nat. Nanotechnol. 3 , 738  2008  . 7 K. Kieu and F. W. Wise,Opt. Express 16 , 11453  2008  . 8 K. H. Fong, K. Kikuchi, C. S. Goh, S. Y. Set, R. Grange, M. Haiml, A.Schlatter, and U. Keller,Opt. Lett. 32 , 38  2007  . 9 T. R. Schibli, K. Minoshima, H. Kataura, E. Itoga, N. Minami, S. Kazaoui,K. Miyashita, M. Tokumoto, and Y. Sakakibara,Opt. Express 13 , 8025  2005  . 10 Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Lohand D. Y. Tang, Adv. Funct. Mater. 19 , 3077  2009  . 11 W. S. Hummers and R. E. Offeman,J. Am. Chem. Soc. 80 , 1339  1958  . 12 R. W. Newson, J. Dean, B. Schmidt, and H. M. van Driel,Opt. Express 17 , 2326  2009  .FIG. 3.  Color online  Soliton operation of the fiber laser.  a  Multiplesoliton spectra measured at a 4 h interval over 2 days.  b  Autocorrelationtraces of the solitons.  c  An oscilloscope trace of the laser emission.  d  Thefundamental rf spectrum of the laser output.  e  Wideband rf spectrum up to1 GHz. 141103-3 Zhang et al. Appl. Phys. Lett. 95 , 141103  2009  Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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