A stable mode-locked fiber laser (MLFL) employing multi-layer graphene as saturable absorber (SA) is presented. The multi-layer graphene were grown by chemical vapor deposition (CVD) on Ni close to A-A stacking. Linear absorbance spectrum of multi-layer graphene was observed without absorption peak from 400 to 2000 nm. Optical nonlinearities of different atomic-layers (7-, 11-, 14-, and 21- layers) graphene based SA are investigated and compared. The results found that the thicker 21-layer graphene based SA exhibited a smaller modulation depth (MD) value of 2.93% due to more available density of states in the band structure of multi-layer graphene and favored SA nonlinearity. A stable MLFL of 21-layer graphene based SA showed a pulsewidth of 432.47 fs, a bandwidth of 6.16 nm, and a time-bandwidth product (TBP) of 0.323 at fundamental soliton-like operation. This study demonstrates that the atomic-layer structure of graphene from CVD process may provide a reliable graphene based SA for stable soliton-like pulse formation of the MLFL.
© 2012 OSA
Ultrafast lasers possess several applications, such as optical fiber communications, ultrafast probing, nonlinear microscopy, optical coherent tomography, and frequency comb generation [1,2]. A passively mode-locked erbium-doped fiber laser (MLEDFL) is able to generate pulses ranging from picosecond (ps) to femtosecond (fs). The pulse producing mechanism is initiated from noise filtering by saturable absorber (SA) with nonlinear absorption properties . The SA widely used in passive mode-locked lasers is semiconductor saturable absorber mirror (SESAM). However, the drawbacks of SESAM are cost-ineffective and a time-consuming fabricated process. Recently, single-wall carbon nanotubes (SWCNTs) of 1D and graphene of 2D carbon allotrope have been noticed due to their large optical nonlinearity and low saturation intensity [4–7]. The first passive mode-locked fiber laser (MLFL) based on SWCNT-SA was reported by S. Y. Set et al. in 2003 . Recently, the atomic layer graphene as SA for ultrafast pulsed lasers were also demonstrated by Q. Bao et al. [9–12]. Graphene has excellent optical properties, such as optically visualized, high transparency, and linear absorption. It also has ultra-fast relaxation time and the SA is not limited by band gap because of its point band gap structure. Therefore, graphene can be used as fast SA with wide spectral operated range [13–16]. However, mono-atomic layer graphene have relatively high nonlinearity that makes a laser cavity not easy to form a stable soliton pulse . According to our previous study, it was found that lower layers of graphene, such as mono-layer and double layers based SA had difficulty forming a stable soliton-like pulse even with extra SMF was added. In our nonlinear optical transmission experiments, we recognized that in addition to saturable absorption, the inverse saturation absorption (ISA) could also be formed at a higher intensity level. The ISA could be caused by two photon absorption (TPA) which was similar to the phenomena reported in the SESAM SA . The ISA from a thinner layer graphene could destroy the stability of mode locked pulse formation. Consequently, a thicker layer of graphene with less nonlinearity was identified as the mode locker to reduce the TPA and suppress the ISA. Furthermore, atomic-layer graphene showed high nonlinear absorption which implied high nonlinear dispersion from Kramers-Kronig relationship. Total dispersion was contributed from all the optical elements in the cavity including linear and nonlinear dispersion [9, 18, 19]. The nonlinear dispersion, such as self phase modulation (SPM) was contributed from SMF and high-order dispersion of graphene. The total nonlinear dispersion inside the laser cavity could be compensated by anomalous linear dispersion from SMF to generate stable soliton pulses .
In this study, optical nonlinearities of different atomic-layers (7-, 11-, 14-, and 21- layers) graphene based SA are investigated and compared. It was found that the thicker 21-layer graphene based SA exhibited a small MD value of 2.93%. Compared with the thinner 7-, 11-, and 14- layers, the results showed that a better stable MLFL with the thicker 21-layer graphene based SA exhibited a pulsewidth of 432.47 fs, a bandwidth of 6.16 nm, and a time-bandwidth product (TBP) of 0.323 at fundamental soliton-like operation. This study demonstrates that the atomic-layer structure of graphene from CVD process may provide a reliable graphene based SA for stable soliton-like pulse formation of the MLFL.
Different layers of graphene were produced by using a CVD method [20,21]. For the CVD process of graphene on Ni substrate, the substrate structure of Ni(300 nm)/SiO2(300 nm)/Si was put on a quartz plate and then loaded into the center of a tubular furnace. The chamber was evacuated to ~5 mTorr and the temperature was increased to 1000°C during the process. Prior to growth, a pretreatment step was performed under a H2 atmosphere with 400 sccm flow at 2.8 Torr for 10 minutes. In the growth step, a gas mixture of methane and hydrogen (CH4 = 80 sccm and H2 = 40 sccm) was introduced for 10 minutes. The system was then cooled down to room temperature to complete the growth. To transfer the as-grown graphene onto the substrate, the Ni substrate after the CVD growth was coated with a layer of Poly (methyl methacrylate) (PMMA) by spinning-coating method, followed by baking at 90°C for 1 minute. Then the PMMA-caped Ni substrate was immerged into a diluted HCl solution (HCl/Water = 1:3) for 20 minutes to etch away the Ni thin layer. The PMMA-caped graphene film was floated on the solution surface, and then it was transferred to a deionized (DI) water to dilute and remove the etchant and residues. The PMMA/graphene was transferred to the receiving substrate and dried on a hot-plate. The PMMA was removed by warm acetone (90°C), and then the sample was rinsed with isopropyl alcohol and DI water. To strip off the graphene film from the quartz substrate, the graphene was covered by an aqueous solution of polyurethane (PVA). After water evaporation, graphene with a supporting layer of PVA film was laminated from the quartz substrate. The composite film of graphene/PVA was then obtained.
Figure 1(a) shows an all-fiber passive MLFL system. An 85 cm highly doped erbium fiber (LIEKKITM Er80-4/125) was used as the gain medium. It was pumped by a 980 nm diode laser via a wavelength division multiplexer (WDM). The graphene films were inserted between two FC/APC fiber connectors as a SA to generate the mode-lock pulses. An isolator was employed to ensure the unidirectional operation, a polarization controller was utilized to optimize mode-locking. The emission light from EDF gain passed the graphene films then fed back into ring laser with partial transmission by 40/60 output coupler. The 60% port was connected to a 10/90 coupler for separating the laser output to optical spectrum analyzer, power meter, autocorrelator, oscilloscope, and radio-frequency spectrum analyzer. Figure 1(b) shows the photos of 7-, 11-, 14-, and 21 layers of the graphene samples, the dimension of the quartz substrate are around 15 mm by 15 mm.
3. Results and discussions
The linear absorption spectra of various layers of graphene were measured and all traces showed featureless from 400 to 1800 nm as theoretically expected . The linear absorption spectrum of the 21-layer graphene was showed in Fig. 2 . The CVD-deposited graphene was well-formatted close to A-A stacked structure. Figure 2 (inset) plots the Raman spectrum of graphene-PVA film with two typical Raman peaks G (~1580 cm−1, line width 24 cm−1) and 2D (~2726 cm−1, line width 63 cm−1). The G-band was a doubly degenerate phonon mode at the Brillouin zone (BZ) center that was Raman active for sp2-hybridized carbon-carbon bonds in graphene. The 2D-band was originated from a double-resonance process of crystalline graphite. The broaden line width of the 2D-band was mainly due to the multi-layer stacks. An increase in the number of defects among graphene would result in an increase of the D-band (~1350 cm−1) intensity. In this case, the D-band was not observed in the Raman spectrum, suggesting a low defect-level of graphene was prepared [23,24].
The nonlinear transmission characteristics were measured using a SWCNT based SA MLFL. The laser was operated at a central wavelength of 1558.88 nm with a repetition rate of 25.51 MHz and pulse duration of 483 fs. Through a broadband attenuator, the laser output was able to provide intensity up to 80 MW/cm2. A coupler was connected after the attenuator so that the output power levels with and without passing the SAs could be measured simultaneously. The single-pass optical transmission then was derived . The MD of the 7-, 11-, 14- and 21- layers graphene based SA were measured at 3.98%, 3.50%, 3.28% and 2.93%, respectively. The nonsaturable loss of the 7-, 11-, 14- and 21- layers graphene based SA were also measured at 18.40%, 29.50%, 35.14% and 53.05%, respectively. Figure 3 shows the nonlinear transmission characteristics of the 21-layer graphene SA.
The performance of MLFL using the 7-, 11-, 14- and 21- layers graphene based SA with different SMF fiber lengths were investigated and compared. The thinner 7-, 11- and 14- layers of graphene based SA were difficult to form a stable soliton-like pulse unless extra SMFs were added. The reason is the thinner layer graphene samples have relatively high MD that makes it difficult for a laser cavity to form a stable soliton pulse and it may need extra SMF to compensate the dispersion. In comparison with the thicker 21-layer graphene as SA, a stable mode locking is easy to form. This may be due to more available density of states in the band structure of stacking-layer graphene than the thinner layer and favored low order nonlinear optics control of graphene inside the cavity. The comparison of passively MLFL performance based on graphene SA is shown in Table 1 .
For a passive MLFL using a 21-layer graphene as SA; the threshold pump power in continuous wave (cw) lasing was about 33 mW. The mode-locked pulses were self-started as the pumping power increased to 53.30 mW. The optical spectrum of the mode locked pulse is shown in Fig. 4(a) . The spectrum was centered at 1559.12 nm with 3 dB spectral bandwidth of 6.16 nm. In Fig. 4(b), the output pulse train of MLEDF exhibited a repetition rate at about 25.51 MHz and the pulse width was measured of 433 fs from the autocorrelator trace. Further increasing the pumping power to 73.78 mW, the harmonic mode locking was observed which could be confirmed by pulse train with a repetition rate about two times of the fundamental mode locking. The TBP was calculated to be 0.323 which was close to the bandwidth limited case. All optical spectra reveal the Kelly sideband indicating that a soliton-like pulse was generated. The laser cavity included 0.85 m of EDF (GVD:-0.02 ps2/m), 1.35 m of corning flexcor 1060 (GVD:-0.007 ps2/m), and 5.4 m of SMF28 (GVD:-0.023 ps2/m). Based on the Kelly sideband location the total cavity dispersion was estimated to be 0.2124 ps/nm .
The RF spectrum of ML pulses was measured by connecting a high sensitivity photo detector to a RF spectrum analyzer (HP8563E). As shown in Fig. 5 , the major peak was the cavity repetition rate of 25.67 MHz with a signal-to-noise ratio of 31 dB. In this work, the stability measurement was similar to the previous graphene-based works [9,10], the power stability performance was monitored for 8 hours a day and repeated measurements after 12 hours for two weeks within 2% variation.
The soliton pulse laser performance of fundamental mode locking with 21-layer graphene based SA was shown in Table 2 . Table 2 indicated that the stable soliton-like operation was achieved at a pumping level from 53.30 to 63.79 mW. Second-order harmonic soliton-like was achieved at a higher pumping level from 73.78 to 83.26 mW.
In summary, the 7-, 11-, 14- and 21- layers graphene based SA with different SMF fiber lengths for the generation of ultrafast laser pulse were comprehensively studied and compared. It was found that the thinner 7-, 11- and 14- layers of graphene based SA had difficulty in forming a stable soliton-like pulse unless extra SMFs were added. The reason was the thinner layer graphene samples exhibited relatively high MD with ISA made it difficult for a laser cavity to form a stable soliton pulse and needed additional length of SMF to compensate the dispersion. In comparison with the thicker 21-layer graphene as SA, a stable mode locking pulse train was easier to establish. This might be due to more available density of states in the band structure of stacking-layer graphene than the thinner layer and favored nonlinear optics control of graphene inside the laser cavity.
The results showed that the optical nonlinearity of the thick 21-layer graphene based SA exhibited a smaller MD value of 2.93% and a higher saturation intensity of 53.25 MW/cm2. A stable MLFL of 21-layer graphene based SA showed a pulsewidth of 432.47 fs, a bandwidth of 6.16 nm, and a TBP of 0.323 at fundamental soliton-like operation. This study demonstrated that the atomic-layer structure of graphene from a CVD process provided a reliable graphene based SA for stable soliton-like pulse formation of the MLFL.
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