IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER/OCTOBER 2014 1100705

Broadband Graphene Saturable Absorber forPulsed Fiber Lasers at 1, 1.5, and 2 μm

Bo Fu, Yi Hua, Xiaosheng Xiao, Member, IEEE, Hongwei Zhu, Zhipei Sun, and Changxi Yang

Abstract—Ultrafast fiber lasers with broad spectral coverage arein great demand for a variety of applications, such as spectroscopy,and biomedical diagnosis. Graphene is an ideal ultrawide-band sat-urable absorber. We report broadband (up to ∼1000 nm) ultrafastpulse generation from three fiber lasers mode-locked by a singlegraphene saturable absorber device. The mode-locked pulses werebased on Yb-, Er- and Tm:Ho-doped fiber lasers at the centralwavelength of 1035, 1564, and 1908 nm, respectively. The maxi-mum output energy is up to 16.2 nJ at 1908 nm. It is the first timethat ultrafast fiber lasers covering 1, 1.5, and 2 μm spectral re-gion are mode-locked with a single graphene device. Our resultsvalidate the intrinsic broadband operation property of graphenedevices for all major fiber laser wavelengths from 1 to 2 μm.

Index Terms—Graphene saturable absorber, mode-locked fiberlasers, Yb-doped fiber laser, Er-doped fiber laser, Tm-doped fiberlaser.

I. INTRODUCTION

PASSIVELY mode-locked fiber lasers have been exten-sively investigated due to their inherent advantages, such

as compactness, high pulse quality, and alignment-free. In par-ticular, ultrafast fiber lasers with broad spectral coverage are ingreat demand for a variety of applications, such as spectroscopy,and biomedical diagnosis. Ultrafast fiber lasers are generallyimplemented with various saturable absorbers, such as nonlin-ear polarization evolution (NPE) [1], semiconductor saturableabsorber mirrors (SESAMs) [2], [3], and single-wall carbonnanotubes (SWNTs) [4]–[6]. However, these saturable absorber

Manuscript received November 15, 2013; revised January 16, 2014; acceptedJanuary 17, 2014. This work was supported by the National Natural ScienceFoundation of China under Grants 61077032 and 50972067 and by the Re-search Fund for Doctoral Program of Ministry of Education of China underGrant 20100002110087. The work of B. Fu was supported by the China Schol-arship Council and H. Yang for plotting the laser setup. The work of X. Xiaowas supported by Tsinghua University Initiative Scientific Research Program.The work of Z. Sun was supported by Teknologiateollisuus TT-100, the Eu-ropean Union’s Seventh Framework Program (REA grant agreement 631610),and Aalto University, Finland.

B. Fu, Y. Hua, X. Xiao, and C. Yang are with the State KeyLaboratory of Precision Measurement Technology and Instruments, De-partment of Precision Instruments, Tsinghua University, Beijing 100084,China (e-mail: fub10@mails.tsinghua.edu.cn; huay12@mails.tsinghua.edu.cn;xsxiao@tsinghua.edu.cn; cxyang@tsinghua.edu.cn).

H. Zhu is at the School of Materials Science and Engineering, Tsinghua Uni-versity, Beijing 100084, China, and also with the Center for Nano and MicroMechanics, Tsinghua University, Beijing 100084, China (e-mail: hongweizhu@tsinghua.edu.cn).

Z. Sun is at the Department of Micro- and Nanosciences, Aalto University,FI-00076 Aalto, Finland (e-mail: zhipei.sun@aalto.fi).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2014.2302361

technologies still suffer from drawbacks. For example, it is chal-lenging to experimentally quantify and reproduce the perfor-mance of these artificial NPE based saturable absorbers. Theyare typically sensitive to environmental changes [6]. SESAMstypically have a narrow operation bandwidth (<∼100 nm) andexpensive due to the need of complex, highly specialized growthequipment [2], [3]. Broadband spectral coverage is possible us-ing SWNTs with a wide diameter distribution [6]–[11]. How-ever, when operating at a particular wavelength, SWNTs not inresonance are not used and give insertion losses [5], [6]. Re-cently, graphene-based saturable absorbers have attracted muchattention for ultrafast pulse generation [11]–[28]. Broadbandoperation is an intrinsic property of graphene due to the gap-less linear dispersion of Dirac electrons [16]. Since the firstdemonstration of graphene-based passively mode-locked fiberlasers [11], graphene has been used to mode-lock fiber lasersat 1 [22], [23], 1.5 [11]–[21], and 2 μm regions [24]–[28].Tunable and multi-wavelength ultrafast output also have beendemonstrated [17], [27]. However, broadband (e.g., >500 nm)saturable absorption operation of graphene has not been demon-strated. In this paper, we experimentally demonstrate that allmajor fiber lasers (i.e., Yb-, Er-, and Tm-doped fiber lasers)are mode-locked by a single broadband graphene sample. Tothe best of our knowledge, it is the first time that such broad-band (up to ∼1000 nm) operation property of a single graphenesaturable absorber is demonstrated to enable ultrafast pulse gen-eration covering three key wavelengths (i.e., 1, 1.5, and 2 μm).

II. FABRICATION AND CHARACTERIZATION OF

GRAPHENE DEVICE

The graphene film was synthesized by chemical vapor depo-sition (CVD) [29]–[31]. As observed from the high-resolutiontransmission electron microscopy (HRTEM) in Fig. 1(a),the graphene film is predominantly multi-layered (>5-layer).Raman spectroscopy measurement result is shown in Fig. 1(b).The G and 2-D peaks are at 1584 and 2698 cm−1 , respec-tively. The ratio of 2-D and G is 2.39, which shows a typicalsingle-layer behavior. However, the mis-oriented layers in ourgraphene are different from AB stacked graphite and each layermaintains the integrity of single layer graphene, resulting inan effective decoupling of adjacent layers [32]. As shown ina typical scanning electron microscopic image [see Fig. 1(c)]of the graphene film on the copper, folds and wrinkles are fre-quently observed on the film surface. Although the film hasa polycrystalline feature, it still maintains good integrity andcontinuity [32]. Similar results were reported in Ref. [12]. Be-sides, we have measured the transmittance of graphene film[see Fig. 1(d)], which further confirms the number of graphene

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1100705 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER/OCTOBER 2014

Fig. 1. (a) HRTEM image. (b) Raman spectroscopy. (c) SEM image.(d) Transmittance of the graphene film. Three operation wavelengths are in-dicated. (e) Nonlinear transmittance as a function of average pump power.

layer (5–6 layers). The small D peak (∼1350 cm−1) indicatesnegligible defects existing in the graphene film. The transmit-tance is featureless in the near infrared region, showing thebroadband property of our graphene device. The transmittanceis larger at longer wavelength range possibly due to the dop-ing [33], [34].

For ultrafast fiber lasers, graphene was then integrated bysandwiching three graphene films between two fiber ferrule con-nectors, which were directly transferred after CVD growth [30].The transmittance of the integrated graphene fiber device isabout 64% at 1550 nm. Fig. 1(e) shows the nonlinear transmit-tance of our integrated graphene device as a function of averagepump power at the wavelength of 1550 nm [11]. The repeti-tion rate of the input pulses is 24.97 MHz, and pulse durationis 220 fs. The transmittance increases by ∼1% when the inputaverage power is raised to 7.2 mW. Further increase in trans-mittance is feasible but limited to our maximum available peakpower. The modulation depth is estimated to be ∼2.7% [35],which is much larger than what is typically reported for singlelayer graphene devices [6], [36], [37] (e.g., 0.5% in [36]).

III. LASER SETUP

To test the broadband operation of our integrated graphenedevice, ring lasers operated at different wavelength ranges areutilized with identical layout (see Fig. 2). It consists of gainfiber (0.5 m Yb- (Nufern SM-YSF-HI, 250 dB/m absorptionat 975 nm, second-order cut-off wavelength is 860 ± 70 nm,

Fig. 2. Setup of the fiber lasers.

the GVD is about 22 ps2 /km at 1030 nm), 0.5 m Er- (LIEKKI,Er110–4/125, cut-off wavelength is 890 ± 90 nm, 110 dB/m ab-sorption at 1530 nm, the GVD is about 12 ps2 /km at 1550 nm)and 3.4 m Tm:Ho-doped fibers (CorActive TH512, 23 dB/m ab-sorption at 1570 nm, the GVD is about –55 ps2 /km at 1950 nm)for 1, 1.5 and 2 μm, respectively), a wavelength-division mul-tiplexer to couple the pump source (980 nm laser diodes for1 and 1.5 μm, A 1550 nm fiber laser for 2 μm) into thegain fiber, a polarization-insensitive isolator (PI-ISO) to en-sure unidirectional operation, a polarization controller (PC) toadjust the cavity polarization (The operating wavelength of thePI-ISO is 1060, 1550, and 1950 nm for Yb-, Er- and Tm:Ho-doped fibers, respectively), our integrated graphene saturableabsorber (GSA), and a fiber coupler (20%, 10% and 50% outputratio for 1, 1.5, and 2 μm, respectively). The rest of cavity ismade by standard single-mode fiber (SMF28). Note that we usethe same graphene integrated device during all measurements.

The output was monitored by an optical spectrum analyzer(1 and 1.5 μm lasers with Agilent 86142B, 2 μm laser withYokogawa AQ6375), a 2 GHz oscilloscope (Agilent InfiniiumDSO80204B) via a 1 GHz photodetector, a second-harmonic-generation autocorrelator (FR-103XL), and a signal analyzer(Agilent N9020 A). To fully verify that the mode-locking ofthese three fiber lasers are resulted from the graphene saturableabsorber, we removed our graphene saturable absorber from thelaser cavities. Then, the pulse train was not observed any morewith no phenomenon of mode-locking. This proves that ourpulse operation is mainly contributed by the graphene saturableabsorber.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

A. 1.5 μm Er-Doped Fiber Laser (EDFL)

In EDFL working at 1.5 μm, the most common wavelengthfor fiber telecommunications, mode-locking self-started from47 mW pump power. Fig. 3 shows the typical experimental re-sults of the mode-locked fiber laser. The output optical spectrumis shown in Fig. 3(a). The central wavelength is 1564 nm with

FU et al.: BROADBAND GRAPHENE SATURABLE ABSORBER FOR PULSED FIBER LASERS AT 1, 1.5, AND 2 μm 1100705

Fig. 3. Experimental results at 1.5 μm. (a) Optical spectrum. (b) Temporalwaveform. (c) Autocorrelation trace. (d) RF spectrum with 2-Hz resolution.

the full width at half maximum (FWHM) of 3.15 nm. The netdispersion of the EDFL was about −0.23 ps2 , which was set tofavor soliton-like operation. The soliton-like operation is alsoconfirmed by the Kelly sidebands [see Fig. 3(a)]. The typicaloutput pulse energy is 10.4 pJ. The repetition rate of the pulsetrain [see Fig. 3(b)] is 19.30 MHz corresponding to our cav-ity length of 10.7 m. Fig. 3(c) shows the autocorrelation traceof the mode-locked pulses. The pulse duration is 870 fs. Thetime-bandwidth product is 0.336, which is close to the theoreti-cal value (0.315) for the transform-limited sech2-shaped pulses.The signal-to-noise ratio (SNR) of the radio-frequency (RF)spectrum [see Fig. 3(d)] is 64 dB at the fundamental repetitionrate (f1), which indicates the stable mode-locking.

B. 2 μm Tm:Ho-Doped Fiber Laser (THDFL)

In THDFL, a 1570 nm home-made continuous wave lasersource was used as the pump source. The total dispersion ofthe cavity is estimated to be −7.68 ps2 , also facilitating thesoliton-like operation [also confirmed by the Kelly sidebands,in Fig. 4(a)]. The sub-ns pulse mode-locking of the fiber laserstarted from the pump power around 580 mW. With the in-crease of the pump power, the fiber laser can operate in theaverage-soliton regime by adjusting the PC (such as multipulse,bunched pulse and higher-order harmonic mode-locking). Whenthe pump power was above ∼720 mW, the stable nanosecond-pulse mode-locking was achieved by adjusting the PC. Fig. 4shows the experimental results of the THDFL at the pump powerof 1120 mW. As shown in Fig. 4(a), the central wavelength is1908 nm with 0.26 nm FWHM. It is worth noting that the outputwavelength can be tuning from 1912 to 1918 nm by adjusting thecavity polarization. Fig. 4(b) and (c) shows the temporal wave-form and pulse profile of the mode-locked pulses. Longer cavity(compared to our 1 and 1.5 μm lasers) is selected to decreasethe repetition rate, and thus increase the output pulse energy.The fundamental repetition rate of our laser is 1.82 MHz corre-sponding to the cavity length of 114 m. The maximum output

Fig. 4. Experimental results at 2 μm. (a) Optical spectrum. (b) Temporalwaveform. (c) Pulse profile. (d) RF spectrum with 10-Hz resolution (inset:100 MHz span).

Fig. 5. Experimental results at 1 μm. (a) Optical spectrum. (b) Temporalwaveform. (c) Pulse profile. (d) RF spectrum with 2-Hz resolution.

pulse energy (at 1120 mW pump power) is 16.2 nJ. The pulseduration [see Fig. 4(c)] is ∼65 ns. In Fig. 4(d), the SNR is 60 dBat the fundamental repetition rate. The inset in Fig. 4(d) is RFspectrum with 100 MHz span.

C. 1 μm Yb-Doped Fiber Laser (YDFL)

In YDFL, the cavity length is about 12.7 m, and the total dis-persion of the cavity is set to be 0.307 ps2 for dissipative solitonmode-locking [6]. Mode-locking started from the pump powerof 240 mW. Fig. 5 shows the experimental results of the mode-locked fiber laser at the maximum pump power of 513 mW.The typical center wavelength is 1035 nm with the FWHM of0.18 nm. The spectrum has steep sides, typical of mode-lockedfiber lasers with significant net normal dispersion [38], [39].The pulses shaping mechanism in such lasers is typically depen-dent on dissipative processes (e.g., gain [22], [40] or loss [41],

1100705 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER/OCTOBER 2014

and nonlinear saturable absorption [42]). Fig. 5(b) shows thetemporal waveform. The repetition rate is 16.29 MHz corre-sponding to the cavity length of 12.7 m. The pulse duration is6.5 ns [see Fig. 5(c)] and the output pulse energy is 0.81 nJ. InFig. 5(d), the SNR of RF spectrum is 55 dB at the fundamentalrepetition rate.

V. CONCLUSION

We experimentally demonstrate mode-locking of three majorfiber lasers (i.e., YDFL, EDFL, and THDFL) with a broadbandgraphene device. Wideband spectral coverage is achieved from∼1 to 2 μm. The central wavelength is 1035, 1564, and 1908 nm,respectively. Our results exhibit the intrinsic broadband (up to∼1000 nm) operation property of graphene device, which pavesthe way to a broadband (e.g., fully covering all wavelengths from1 to 2 μm) and compact ultrafast all-fiber laser [27], [43], [44]with a graphene saturable absorber.

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Bo Fu was born in Jiamusi City, China, in 1983.He received the B.S. and M.S. degrees in physicsfrom Yanshan University, Qinhuangdao, China, in2007 and 2010, respectively. Since 2010, he has beenworking toward the Ph.D. degree in optical engineer-ing at Tsinghua University, Beijing, China. Now, hehas been awarded a scholarship under the State Schol-arship Fund to pursue his study in Aalto University(Department of Micro- and Nanosciences) as a jointPh.D. student from September 2013 to August 2014.His research interests include fiber lasers and ultra-

fast fiber lasers enabled by graphene and carbon nanotubes.

Yi Hua received the B.S. degree in optoelectron-ics from Nankai University, China. She is currentlyworking toward the M.S. degree in optical engineer-ing at Tsinghua University, Beijing, China. Her re-search interests include the nonlinear optics and ul-trafast pulse propagation phenomena.

Xiaosheng Xiao (M’10) received the B.E. and Ph.D.degrees from Tsinghua University, Beijing, China,in 2002 and 2007, respectively. From 2007 to 2008,he was with the Network Technology Research Cen-tre (NTRC), Nanyang Technological University, Sin-gapore, as a Research Fellow. Since 2009, he hasbeen an assistant professor with the Department ofPrecision Instruments, Tsinghua University, Beijing,China. His research interests include fiber laser, non-linear fiber optics, and fiber sensor.

Hongwei Zhu received the B.S. degree in mechan-ical engineering from Tsinghua University in 1998.He started his doctoral studies then and received thePh.D. degree in materials processing engineering in2003. After postdoctoral research in Japan (2003.9–2004.8) and USA (2004.8–2008.3), he began his in-dependent career as a faculty member at the Depart-ment of Mechanical Engineering, Tsinghua Univer-sity, where he is currently a Professor of MaterialsScience and Engineering. His research results in-clude continuous assembly and controllable growth

of monolithic nanocarbon macrostructures (1-D fibers, 2-D films, and 3-Dsponges); atom-resolved imaging of graphene and nanotubes and their nucle-ation points, offering further implications for understanding catalytic reactionsand device quality synthesis; applications of nanomaterials in energy harvesting,conversion/storage, composites and environmental areas. Dr. Zhu received theNew Century Excellent Talents Award in University, National Natural ScienceAward (second-class), the Education Ministry Nominated Scientific and Tech-nology Achievements Award (1st-class), and the National Excellent DoctoralDissertation Award.

Zhipei Sun, photograph and biography not available at the time of publication.

Changxi Yang received the B.S. and M.S. degreesin physics from Nankai University of China, Tian-jin, China, in 1986, and 1989, respectively, and thePh.D. degree in applied physics from the Instituteof Physics, Chinese Academy of Sciences, Beijing,China, in 1992. After graduate degree in 1992, heworked as an Assistant Research Scientist at the In-stitute of Physics, Chinese Academy of Sciences. Hewas a Postdoctoral Scientist at the University of Cal-ifornia, Santa Barbara, CA,USA; STA Fellow at theNational Laboratory of Metrology, Tsukuba, Japan;

Research Associate at the University of Exeter, U.K.; Senior Research Associateat Bell Labs, Murray Hill, NJ, USA, and at the University of Arkansas, Arkansas,USA. Since March 2001, he has been working at the Department of PrecisionInstruments at the Tsinghua University as a Professor. He was a Visiting Pro-fessor at the School of Applied and Engineering Physics, Cornell University, inSeptember 2010. His current research interests include nonlinear fiber optics,femtosecond fiber lasers, optical fiber sensors, and surface-enhanced vibrationalspectroscopy.