We demonstrate mode-locking of a thulium-doped fiber laser operating at 1.94 μm, using a graphene-polymer based saturable absorber. The laser outputs 3.6 ps pulses, with ~0.4 nJ energy and an amplitude fluctuation ~0.5%, at 6.46 MHz. This is a simple, low-cost, stable and convenient laser oscillator for applications where eye-safe and low-photon-energy light sources are required, such as sensing and biomedical diagnostics

Ferrari, AC

Hasan, T

Kelleher, EJR

Popa, D

Popov, SV

Sun, Z

Taylor, JR

Torrisi, F

Wang, F

Zhang, M

Optics Express

English

arXiv

Apollo (Cambridge)

arXiv:1208.1804v1  [cond-mat.mes-hall]  9 Aug 2012Tm-doped fiber laser mode-locked by graphene-polymer compositeM. Zhang1, E.J.R. Kelleher1, F. Torrisi2, Z. Sun2, T. Hasan2,D.Popa2, F. Wang2, A.C. Ferrari2, S.V. Popov1 and J.R. Taylor11Femtosecond Optics Group, Department of Physics,Imperial College London, Prince Consort Road, London SW7 2BW, UK2Department of Engineering, University of Cambridge, Cambridge, CB3 0FA, UKWe demonstrate mode-locking of a thulium-doped fiber laser operating at 1.94µm, using agraphene-based saturable absorber. The laser outputs 3.6ps pulses, with∼0.4nJ energy and anamplitude fluctuation∼ 0.5%, at 6.46MHz. This is a simple, low-cost, stable and convenient laseroscillator for applications where eye-safe and low-photon-energy light sources are required, such assensing and biomedical diagnostics.Ultrafast lasers based on thulium (Tm) doped fibers,operating in the 1.7-2.1µm range, are important to ad-dress demands for mid-IR sources necessary for a vari-ety of applications, ranging from molecular spectroscopy1and biomedical diagnostics2, to medicine3 and remotesensing4,5. The 2µm region is important because severalgas molecules (e.g. CO2) have characteristic absorptionlines there6. Since liquid water (main constituent of hu-man tissue) absorbs more strongly at∼2µm (∼100/cm)7than at∼1.5µm (∼10/cm)7 and∼1µm (∼1/cm)7, lasersources emitting at∼2µm are promising for medical di-agnostic and laser surgery7. In addition, Light De-tection And Ranging8 measurements and optical free-space telecommunications8 can be performed within the2-2.5µm atmospheric transparency window9. Further-more, fiber lasers offer advantages compared to solid-state lasers, such as compact geometry, efficient heat dis-sipation and alignment-free operation10,11.2µm fiber lasers have been mainly mode-locked us-ing nonlinear polarization evolution (NPE)1 and semi-conductor saturable absorber mirrors (SESAMs)3. How-ever, these have disadvantages: NPE suffers from bulkyconstruction and environmental sensitivity1, SESAMshave complex fabrication and packaging, as wellas limited bandwidth12. Nanotubes and grapheneare promising saturable absorbers (SA) due to theirfavorable properties: ultrafast recovery time13–15,broadband operation16,17, ease of fabrication16–20 andintegration19,20 into all-fiber configurations. On onehand, broadband operation can be achieved using a dis-tribution of nanotube diameters16. On the other hand,this is an intrinsic property of graphene, due to the gap-less linear dispersion of Dirac electrons17,18,20,21. Nan-otubes have mode-locked fiber16,22–27, waveguide28,29,solid-state30–32 and semiconductor lasers33, coveringfrom∼0.8 to∼2µm19,20. Ultrafast pulse generation at0.834, 135, 1.336, 1.5µm17,19–21,37 was demonstratedby exploiting graphene saturable absorbers (GSAs).Refs.38,39 reported 2µm solid-state lasers mode-lockedwith graphene oxide and chemical vapor deposited(CVD) 1-2 layer graphene. However graphene oxide40,41is fundamentally different from graphene; it is an insu-lating material with a mixture of sp2/sp3 regions40,41,with lots of defects and gap states41. Thus it does notFIG. 1: Scheme of our laser cavity. TDFA-Tm-doped fiberamplifier; ISO-isolator; BPF-bandpass filter; OC-output cou-pler; GSA-graphene-SA; PC-polarization controlleroffer in principle the wideband tunability of graphene.CVD graphene, on the other hand, is normally grown atvery high temperature on Cu42 or Ni43. Therefore, extrasteps are required to transfer graphene to the target sub-strates for photonic applications. Indeed, graphene canbe produced in a variety of ways, ranging from microme-chanical cleavage20, to liquid phase exfoliation(LPE)44,CVD of hydrocarbons42,45, carbon segregation from sili-con carbide46 or metal substrates47 and chemical synthe-sis from polyaromatic hydrocarbons48. LPE has the ad-vantage of scalability, room temperature processing, anddoes not require any growth substrate. This producesdispersions that can be easily embedded into polymersto form composites with novel optoelectronic propertiesto be integrated into various systems20.Here we demonstrate a fiber laser mode-locked using agraphene-polymer composite. It operates at∼2µm, withlow-noise 3.6ps pulses. Our results show the potential ofGSAs for practical fiber lasers in mid-IR.The laser cavity is schematically shown in Fig.1. Itcomprises all-fiber integrated components to create anenvironmentally robust and compact system. A Tm-doped fiber amplifier (TDFA), with integrated opticalisolator (ISO), having∼25dB small signal gain at 1.94µm,and a broad gain bandwidth (full width at half max-imum, FWHM∼60nm) is followed by a fiber pigtailed2airgap (∼80% insertion loss) used to include a band-pass filter (BPF) for pulse stabilization, with 80% max-imum transmission and 11nm transmission bandwidth,centered∼1.94µm. A fused-fiber output coupler (OC) ex-tracts 10% of the light per pass; a polarization controller(PC) allows adjustment of the intra-cavity polarization.The graphene-polymer composite is produced as fol-lows: 120mg of graphite (NGS, Naturgraphit) and 90mgsodium deoxycholate (SDC) are sonicated in at roomtemperature. The unexfoliated particles are allowed tosettle for 10 minutes, followed by 60 min centrifugationat∼17000g. The top 70% of the centrifuged dispersionis then used for the composite fabrication. Drops arealso placed on Transmission Electron Microscope (TEM)grids for analysis in a high resolution TEM (HRTEM).Combined HRTEM and normal-incidence/tilted angleelectron diffraction measurements show that our disper-sion has∼66% ≤ 3-layer flakes (∼26% single layer,∼22%bi-layer and∼18% tri-layer). The remainder have lessthan 10 layers. The dispersion is also drop-cast onSi/SiO2 for Raman measurements with a Renishaw 1000.5ml of dispersion is then mixed with polyvinyl alco-hol (PVA) in water (∼2wt%) and centrifuged at∼4000g.Evaporation at room temperature gives a∼40µm film,then used for Raman and absorption measurements.Fig.2 plots a typical Raman spectrum of a flake de-posited on Si/SiO2. Besides the G and 2D peaks, thishas significant D and D’ intensities49,50. We assignthe D and D’ peaks to the edges of the submicrome-ter flakes, rather than a large amount of disorder withinthe flakes51. This is further supported by analyzingthe G peak dispersion, Disp(G). In disordered carbonsthe G peak position, Pos(G), increases with decreas-ing excitation wavelength, from IR to UV50. Thus,Disp(G)=∆Pos(G)/∆λL, where λL is the laser excita-tion wavelength, increases with disorder50,52. FWHM(G)always increases with disorder53. Hence, combining theintensity ratio of the D and G peaks, I(D)/I(G), withFWHM(G) and Disp(G) allows us to discriminate be-tween edges, and disorder in the bulk of the samples. Inthe latter case, a higher I(D)/I(G) would correspond tohigher FWHM(G) and Disp(G). By analyzing 30 flakes,we find that the distribution of Disp(G), I(D)/I(G) andFWHM(G) are not correlated, indicating that the D peakis mostly due to edges. Also, Disp(G) is nearly zero forall samples (compared to≥0.1cm−1/nm expected for dis-ordered carbons52). Although 2D is broader than in pris-tine graphene, it is still a single Lorentzian. This impliesthat, even if the flakes are multilayers, they are electron-ically decoupled and, to a first approximation, behave asa collection of single layers54. Fig.2 compares a typicalflake with our graphene-PVA composite and pure PVA.We note that the spectrum of the composite (Fig.2) is asuperposition of that of the flake and PVA . Thus, PVAdoes not affect the structure of the embedded flakes.Fig.3 plots the transmittance of graphene-PVA com-pared to pure PVA. The UV peak in graphene-PVA isa signature of the van Hove singularity in the graphene1500 2000 2500 3000 Intensity (a.u.)Raman Shift (cm-1)LPE graphenePure PVAGraphene PVA compositeDGD'2DD+D' 2D'FIG. 2: Raman spectra of flake on Si/SiO2, polyvinyl alcohol(PVA), graphene-PVA composite300 600 900 1200 1500 1800 2100 2400020406080100 Transmittance (%)Wavelength (nm) Pure PVA Graphene PVAFIG. 3: Transmittance of PVA and graphene-composite.density of states55. Strong UV absorption is also ob-served in pure PVA56. By considering the pure PVAabsorption, we can estimate that of the graphene com-ponent to be∼50% in the NIR. Given that a monolayerabsorbs∼2.3%57, we estimate that an average∼21 layerscross the light path.The GSA is then prepared by sandwiching 2mm2 of thecomposite between two fiber connectors, adhered withindex matching gel. The integrated device has∼4dB(∼60%) total insertion loss.The autocorrelation of the output pulse, and the corre-sponding optical spectrum are plotted in Fig.4. Fig.4(a)shows that the pulse temporal profile is well representedby a sech2. The FWHM duration (after deconvolution)is 3.6ps. The corresponding FWHM spectral width is2.1nm, giving a time-bandwidth product∼0.59, indicat-ing low chirp58. The output power is∼2mW. Althoughthe laser operates with negative cavity dispersion, andthe pulses are soliton-like, the typical spectral sidebandsignature of deviation from average soliton operation31930 1940 1950 196010-310-210-1100-20 -10 0 10 200.00.20.40.60.81.0 Intensity (a.u.)Wavelength (nm)(b)(a)  Data Fit Amplitude (a.u.)Time delay (ps)FIG. 4: (a) Autocorrelation, (b) optical spectrum.-500 -250 0 250 500-90-75-60-45-30-4 -2 0 2 4-90-75-60-45-30-15-500 -250 0 250 500-100-90-80-70-60-50-4 -2 0 2 4-110-100-90-80-70-60-50(b) Intensity (dBm)Frequency (f-f1) (KHz) Background Signal(a)(c) Intensity (dBm)Frequency (f-f1) (KHz) Background Signal  Intensity (dBm)Frequency (f-f60) (KHz) Background Signal(d)  Intensity (dBm)Frequency (f-f60) (KHz) Background SignalFIG. 5: RF spectra. (a) Fundamental, (c) 60th harmonicon a long range span (1MHz), with 300Hz resolution; (b)Fundamental, and (d) 60th harmonic on a short range span(8kHz), with 30Hz resolution.is not observed, because the soliton length, given byzsol. =pi2τ20|β2|, is long (zsol. ∼ 300m, based on an esti-mated β2 = 34 ps nm−1 km−1 at 1.94µm) compared tocavity length (31 m ≈ 110zsol.), with τ0 the pulse durationand β2 the group velocity dispersion.The stability and quality of the generated pulses areevaluated via the radio frequency (RF) spectrum59.Fig.5 plots the fundamental and 60th harmonics overlong (1MHz) and short (8kHz) frequency spans. Thelong range spectra indicate that the stability is high, withpeak to noise-floor ratio limited by our 300Hz resolution(the noise floor of the analyzer is plotted in red). Nosidebands at harmonic cavity frequencies are observedover the 1MHz span, suggesting good pulse-train stabilityand no Q-switching instabilities. This is confirmed bya spectral sweep over 100MHz, showing the first fifteenharmonics of the fundamental cavity frequency (Fig.6).The short range spectra (Fig.5(b,d)), spanning 8kHzwith 30Hz resolution, reveal a low level pedestalcomponent∼70dB from the central f0 spike. FollowingRef.59, we estimate the energy fluctuations, defined asoutput pulse energy change divided by average output20 40 60 80 100-70-60-50-40-30Intensity (dB)Frequency (MHz)FIG. 6: (Blue) RF spectrum over 100MHz,with 3kHz resolu-tion. (Red) analyzer backgroundenergy, as ∆E =[∆P∆f∆fRes.]1/2, where ∆P is the powerratio between the central spike at f1 and the peak ofthe noise band, ∆f (Hz) is the frequency width of thenoise component, and ∆fRes. (Hz) is the resolution band-width of the spectrum analyzer. With ∆P = 1 × 10−6,∆f = 730 Hz and ∆fRes. = 30 Hz, give low pulse-to-pulseenergy fluctuation ∆E ≈ 5× 10−3.Similarly, when the amplitude noise is low, the timingjitter can be evaluated as59:∆tT =12pin[∆Pn∆f∆fRes.]1/2, whereT is the cavity period, n is the harmonic order. The low-frequency timing jitter (Fig.5(d)), evaluated at the 60thharmonic with ∆P60 = 1.6 × 10−5, ∆f = 393 Hz and∆fRes. = 30 Hz, is estimated as ∆t/T = 3.9 × 10−5.Given the long cavity period T = 155 ns, this indicatesa low timing jitter ∆t ≈ 6 ps.Our analysis suggests that, despite a very simple cav-ity consisting of non-polarization maintaining (PM) fiber,the laser emits high-quality pulses with low amplitudefluctuations (0.5%) and low timing jitter∼ 6ps. 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Tm-doped fiber laser mode-locked by graphene-polymer composite.

M. Zhang

E. J. R. Kelleher

F. Torrisi

Z. Sun

T. Hasan

D. Popa

F. Wang

A. C. Ferrari

S. V. Popov

J. R. Taylor

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Tm-doped fiber laser mode-locked by graphene-polymer composite

https://www.repository.cam.ac.uk/bitstream/handle/1810/279769/1208.1804v1.pdf?sequence=1&isAllowed=y

Tm-doped fiber laser mode-locked by graphene-polymer composite.

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