Ultrafast fiber lasers with short pulses and broad bandwidth are in great
demand for a variety of applications, such as spectroscopy, biomedical
diagnosis and optical communications. In particular sub-200fs pulses are
required for ultrafast spectroscopy with high temporal resolution. Graphene is
an ideal ultra-wide-band saturable absorber. We report the generation of 174fs
pulses from a graphene-based fiber lase

Ferrari, AC

Hasan, T

Popa, D

Sun, Z

Torrisi, F

Wang, F

English

arXiv

Apollo (Cambridge)

arXiv:1010.1329v1  [cond-mat.mes-hall]  7 Oct 2010Sub 200fs pulse generation from a graphene mode-locked fiber laserD. Popa, Z. Sun, F. Torrisi, T. Hasan, F. Wang, A. C. Ferrari∗Department of Engineering, University of Cambridge,Cambridge CB3 0FA, UKUltrafast fiber lasers with short pulses and broad bandwidth are in great demand for a variety ofapplications, such as spectroscopy, biomedical diagnosis and optical communications. In particularsub-200fs pulses are required for ultrafast spectroscopy with high temporal resolution. Grapheneis an ideal ultra-wide-band saturable absorber. We report the generation of 174fs pulses from agraphene-based fiber laser.Ultrafast fiber lasers have many applications, rang-ing from basic research to materials processing andmedicine[1, 2]. Compared to other types of lasers,they have a simple and compact design, efficient heatdissipation and high-quality pulse generation[3]. Thelatter is typically achieved through a nonlinear opti-cal device called saturable absorber (SA)[4, 5]. TheSA technology is currently dominated by semiconduc-tor SA mirrors[2, 3, 6]. However these require complexand expensive fabrication and packaging and have lim-ited bandwidth[2, 3, 6]. Single wall carbon nanotubes(SWNTs) and graphene are ideal SAs, having fast re-covery times, low saturation intensity, low cost and easyfabrication[7–22]. Broadband operation can be achievedusing a distribution of tube diameters[7]. However, thisis an intrinsic property of graphene, due to the gaplesslinear dispersion of Dirac electrons[8, 9, 18, 23–25].Soliton-like mode-locking is the conventional methodto generate ultrashort pulses in fiber lasers, down to sev-eral hundred fs[7, 9–12, 26]. In this regime, the pulse isshaped by the cavity dispersion and nonlinearity. Usingthis technique,∼400fs pulses were reported for SWNT-based SAs[17, 19] and graphene-based SAs[9, 27]. Thearea theorem[28] for solitons states that the product ofpulse energy (E) and duration (τ) is fixed by the cav-ity dispersion and nonlinearity[29]: E × τ ∝ β2/n2,with β2 the group velocity dispersion (GVD) coefficientand n2 the nonlinear refractive index of the propagat-ing medium. Thus, for a given system design, there isa trade-off between E and τ . One approach to mitigatethese effects is to alternate segments of large normal (pos-itive) and anomalous (negative) GVD fiber. In this way,the average pulse width in one cavity round trip can in-crease by an order of magnitude or more[29], significantlydecreasing the intracavity average peak power, comparedto soliton-like operation[29]. This decreases nonlinear ef-fects, reducing pulse duration[30].Here, we report a dispersion-managed fiber laser mode-locked by a graphene-based SA (GSA). The cavity com-prises sections with positive and negative dispersion, pro-vided by an erbium doped (EDF) and a single mode fiber(SMF). We get∼174fs pulses with 15.6nm spectral width,much shorter than reported thus far for GSAs[9, 27].Graphite flakes are exfoliated by mild ultrasonica-tion with sodium deoxycholate (SDC) surfactant[9, 31].500 1000 1500 20000.00.51.01.5 a) Compositeb) PVAAbsorbanceWavelength (nm)a)b) 2D.u.)FIG. 1: Absorption spectra of a) Composite and b) PVA.1500 2000 2500 3000Intensity (a. u)Raman shift (cm-1)DGD'2DD+D'2D'a)b)c)FlakeCompositePVAFIG. 2: Raman spectra of a) Flake; b) Composite and c) PVAA dispersion enriched with single (SLG) and few layergraphene (FLG) is then mixed with an aqueous solutionof polyvinyl alcohol (PVA). After water evaporation, agraphene-PVA composite is obtained[9, 13]. Absorptionmeasurements show a featureless spectrum from 500 to2000nm, Fig.1. The UV peak is a signature of the vanHove singularity in the graphene density of states[32].The PVA only shows significant absorption for shorterwavelengths[33]. Fig.2a plots a typical Raman spectrumof an exfoliated flake on Si/SiO2. Besides the G and2D peaks, it has D, D’, D+D’ bands. The G peak corre-sponds to the E2g phonon at the Brillouin zone centre[34].The D peak is due to the breathing modes of sp2 ringsand requires a defect for its activation by double reso-20.01 0.1 1 10 10027.027.528.028.5 Transmittance (%)Peak intensity (MW/cm2)~2%FIG. 3: Transmittance as a function of peak intensity.nance (DR)[34, 35]. The 2D peak is the second orderof the D peak. This is a single band in SLG, whereasit splits in multi-layer graphite[34], reflecting the evolu-tion of the band structure. The 2D peak is always seen,even when no D peak is present, since no defects are re-quired for the activation of two phonons with the samemomentum, one backscattering from the other. DR canalso happen intra-valley, i.e. connecting two points be-longing to the same cone around K or K’. This gives riseto the D’ peak. The 2D’ is the second order of the D’peak. We do not assign the D and D’ intensity in Fig.2ato the presence of a large amount of structural defects,otherwise they would be much broader, and G, D’ wouldmerge[35]. We rather ascribe them to the edges of oursub-micrometer flakes[36]. Although 2D is broader thanin pristine graphene, is still a single Lorentzian. Thus,even if the flakes are multi-layers, they behave, to a firstapproximation, as a SLG ensemble. Fig.2c is a referencePVA. The spectrum of the graphene-PVA composite isa superposition of Figs.2(a,c): embedding into PVA pre-serves the structure.Power-dependent absorbtion is measured with aSWNT-based mode-locked fiber laser, delivering 450fspulses at 1558nm with 38MHz repetition rate[17]. Thesignal is directed to the sample via an attenuator. Adouble-channel power meter measures both input andoutput power. The GSA is then placed between twofiber connectors. The output is monitored while the in-put power is varied, Fig.3. The maximum peak inten-sity Ipeak ∼337MW/cm2 is reached for an average pumppower∼5.15mW. The insertion loss is∼5dB (i.e.∼28%transmittance), acceptable for the single-pass gain of afiber laser[7]. The transmittance increases by 2% whenthe SA saturates at Ipeak ∼100MW/cm2. The modula-tion depth is 2%, as in SWNT-SAs[7, 15, 16].Fig.4a is a schematic laser setup. We use 1.25m highly-doped EDF as gain medium, pumped by a 980nm laserdiode (LD) through a fused wavelength division multi-plexer (WDM). The laser output is directed through the20% port of a coupler. The total cavity length is∼7.6m.We use a EDF with β2=48ps2/km, estimated by insert-ing it into the SWNT-mode-locked soliton-like fiber laserLDPCGSAoutputcouplerISOWDMEDFEDFSMFcavity positiondispersiona)b)SMFdispersionAcademic Use OnlyFIG. 4: a) Laser setup. LD (laser diode), WDM (wavelengthdivision multiplexer), SMF (single mode fiber), EDF (erbium-doped fiber), ISO (isolator), GSA (graphene saturable ab-sorber), PC (polarization controller). b) Dispersion map.Positive and negative dispersion provided by EDF and SMF.described above, and measuring the shift (∆λ) betweensidebands and central wavelength of the soliton pulsespectrum[37]. The rest of the cavity consists of a combi-nation of SMF Flexcor 1060 and SMF-28 with anomalousGVD, as indicated in the dispersion map of Fig.4b. Themeasured total intracavity GVD is∼-0.052ps2, typical fordispersion-managed cavities[29]. The laser operation ismonitored with a second harmonic generation (SHG) au-tocorrelator and an oscilloscope. An optical spectrumanalyzer with 0.07nm resolution probes the output.Continuous wave (CW) operation starts at∼18mWpump power, single-pulse mode-locking at∼25mW. Therepetition rate is 27.4MHz, as determined by the cav-ity length. The output power is∼1.2mW for∼28mWpump, with pulse energy∼44pJ and Ipeak ∼282MW/cm2.Fig.5a shows that the sidebands, expected for solitonoperation, are small, as a result of temporal pulsestretching/compression[38]. The full width at half max-imum (FWHM) bandwidth is∼15.6nm, larger than pre-vious GSA-fiber lasers[9, 18, 27, 39]. Fig.5b plotsthe SHG trace. For a sech2 profile, we get a pulseduration∼174fs, much shorter than previous GSA-mode-locked lasers[9, 18, 27, 39]. The time-bandwidth productis∼0.335, slightly higher than 0.315, expected in the caseof transform-limited sech2 pulses, possibly due to uncom-pensated third-order dispersion distorting the intracavitypulse, thus limiting the minimum pulse width[38]. Toachieve even shorter pulses, SAs with higher modulationdepth would be required[2]. Higher power could be feasi-ble by evanescent field interaction with the GSAs[18, 39].Stability is important for applications. We measuredthe radio frequency (rf) spectrum using a photodetectorconnected to a rf spectrum analyzer around the funda-31520 1540 1560 1580 1600-60-50-40-30-20-400 -200 0 200 4000.00.20.40.60.81.0 Intensity (dBm)Wavelength (nm)∆λ= 15.6nma)   Experimental data sech2 fitNormalized intensityTime delay (fs)~268fsτ = ~174fsb)FIG. 5: a) Optical spectrum with bandwidth ∆λ=15.6nm. b)Autocorrelation trace with sech2 fit.-400 -200 0 200 400-150-120-90-60-30  rf signal background Intensity (dBm)Frequency (f-f1) (Hz)~87.4dBf1=27.4 MHzFIG. 6: rf spectrum, measured around the fundamental rep-etition rate f1=27.4MHz with 10Hz resolution.mental repetition rate (f1=27.4MHz), Fig.6. The signal-to-noise ratio is> 80dB, highlighting the low-amplitudefluctuations of our laser, thus stable mode-locking[40].In conclusion, we demonstrated the use of a graphene-based SA as a mode locker for generation of pulses shorterthan 200fs. The flexibility offered by the fiber laser de-sign, along with the easy SA preparation, can lead tonovel, low-cost ultrafast light sources.We acknowledge funding from grants EPSRC-GR/S97613/01, EP/E500935/1, ERC-NANOPOTS,Royal Society Brian Mercer Award for Innovation andWolfson Research Merit Award, CIKC in Advanced Man-ufacturing Technology, and Kings College.∗ Electronic address: acf26@eng.cam.ac.uk[1] F. Dausinger, F. Lichtner, H. Lubatschowski, Femtosec-ond technology for technical and medical applications,(Springer, 2004).[2] U. Keller, Nature 424, 831 (2003).[3] O. Okhotnikov et al. New J. Phys. 6,177 (2004)[4] M. E. Fermann, M. J. Andrejco, Y. Silberberg, M. L.Stock, Opt. Lett. 18, 894 (1993).[5] T.Brabec et al. Opt. Lett. 17, 1292 (1992)[6] G. Steinmeyer, D. H. Sutter, L. Gallmann, N. Matuschek,U. Keller, Science 286, 1507 (1999).[7] F. Wang et al. Nature Nano. 3, 738 (2008).[8] A. K. Geim, K. S. Novoselov, Nat. Mater. 6, 183 (2007).[9] Z. Sun et al. ACS Nano 4, 803 (2010)[10] S. Y. Set, H. Yaguchi, Y. Tanaka, M. Jablonski, IEEE J.Sel. Top. Quant. 10, 137 (2004).[11] A.V.Tausenev et al. Appl. Phys. Lett.92,171113 (2008)[12] Z. Sun, T. Hasan, F. Wang, A. G. Rozhin, I. H. White,and A. C. Ferrari, Nano Res. 3, 404 (2010).[13] T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P. H. Tan, A.G. Rozhin, A. C. Ferrari, Adv. Mater. 21, 3874 (2009).[14] G. Della Valle, R. Osellame, G. Galzerano, N. Chiodo, G.Cerullo, P. Laporta, O. Svelto, A. G. Rozhin, V. Scar-daci,A.C. Ferrari, Appl. Phys. Lett. 89, 231115 (2006).[15] Z. Sun et al. Appl. Phys. Lett. 95, 253102 (2009).[16] V. Scardaci et al. Adv. Mater. 20, 4040 (2008).[17] Z. Sun et al. Appl. Phys. Lett. 93, 061114 (2008)[18] F.Bonaccorso et al. Nature Phot. 4, 611 (2010)[19] S. Kivisto¨ et al. Opt. Expr. 17, 2358 (2009).[20] M. Breusing et al. Phys. Rev. Lett. 102, 086809 (2009)[21] D. Sun et al. Phys. Rev. Lett. 101, 157402 (2008).[22] T. Kampfrath, L. Perfetti, F. Schapper, C. Frischkorn,M. Wolf, Phys. Rev. Lett. 95, 187403 (2005).[23] Z. Sun et al. Nano Res. 3, 653 (2010).[24] C. Casiraghi et al. Nano Lett. 7, 2711 (2007).[25] R. R. Nair et al. Science 320, 1308 (2008)[26] L. F. Mollenauer, R. H. Stolen, J. P. Gordon, Phys. Rev.Lett. 45, 1095 (1980).[27] H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, K. P. Loh,Opt. Express 17, 17630 (2009).[28] S.L.McCall,E.L.Hahn, Phys. Rev. Lett. 18,908 (1967)[29] L. E. Nelson, D. J. Jones, K. Tamura, H. A. Haus, E. P.Ippen, Appl. Phys. B 65, 277 (1997).[30] F. Shohda, M. Nakazawa, J. Mata, J. Tsukamoto, Opt.Express 18, 9712 (2010).[31] Y. Hernandez et al. Nature Nanotech. 3, 563 (2008).[32] V.G. Kravets et al. Phys. Rev. B 81,155413 (2010)[33] K. M. Abd El-Kader, J. Appl. Polym. Sci. 88, 589 (2003).[34] A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M.Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov,S. Roth, A. K. Geim, Phys. Rev. Lett. 97, 187401 (2006).[35] A.C.Ferrari,J.Robertson, Phys. Rev. B 61,14095 (2000)[36] C. Casiraghi et al. Nano Lett. 9, 1433 (2009).[37] M.L.Dennis,I.N.Duling, IEEE J. Quant. Elect. 30, 1469(1994).[38] K. Tamura, L. E. Nelson, H. A. Haus, E. P. Ippen, Appl.Phys. Lett. 64, 149 (1994).[39] Y. W. Song, S. Y. Jang, W. S. Han, and M. K. Bae,Appl. Phys. Lett. 96, 051122 (2010).[40] D. von der Linde, Appl. Phys. B 39, 201 (1986).

Sub 200 fs pulse generation from a graphene mode-locked fiber laser

CUED - Cambridge University Engineering Department

D. Popa

Z. Sun

F. Torrisi

T. Hasan

F. Wang

A. C. Ferrari

Crossref

Applied Physics Letters

https://www.repository.cam.ac.uk/bitstream/handle/1810/286122/1010.1329v1.pdf?sequence=1&isAllowed=y

Sub 200 fs pulse generation from a graphene mode-locked fiber laser

Abstract

Similar works

Full text

Available Versions

Apollo (Cambridge)

CUED - Cambridge University Engineering Department

Crossref

CUED - Cambridge University Engineering Department

CUED - Cambridge University Engineering Department