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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 21 — Oct. 21, 2013
  • pp: 25184–25196
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Self-amplitude and self-phase modulation of the charcoal mode-locked erbium-doped fiber lasers

Yung-Hsiang Lin, Jui-Yung Lo, Wei-Hsuan Tseng, Chih-I Wu, and Gong-Ru Lin  »View Author Affiliations


Optics Express, Vol. 21, Issue 21, pp. 25184-25196 (2013)
http://dx.doi.org/10.1364/OE.21.025184


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Abstract

With the intra-cavity nano-scale charcoal powder based saturable absorber, the 455-fs passive mode-locking of an L-band erbium-doped fiber laser (EDFL) is demonstrated. The size reduction of charcoal nano-particle is implemented with a simple imprinting–exfoliation–wiping method, which assists to increase the transmittance up to 0.91 with corresponding modulation depth of 26%. By detuning the power gain from 17 to 21 dB and cavity dispersion from −0.004 to −0.156 ps2 of the EDFL, the shortening of mode-locked pulsewidth from picosecond to sub-picosecond by the transformation of the pulse forming mechanism from self-amplitude modulation (SAM) to the combining effect of self-phase modulation (SPM) and group delay dispersion (GDD) is observed. A narrower spectrum with 3-dB linewidth of 1.83-nm is in the SAM case, whereas the spectral linewidth broadens to 5.86 nm with significant Kelly sideband pair can be observed if the EDFL enters into the SPM regime. The mode-locking mechanism transferred from SAM to SPM/GDD dominates the pulse shortening procedure in the EDFL, whereas the intrinsic defects in charcoal nano-particle only affect the pulse formation at initial stage. The minor role of the saturable absorber played in the EDFL cavity with strongest SPM is observed.

© 2013 OSA

1. Introduction

Since the first demonstration of graphene saturable absorber for passively mode-locked erbium-doped fiber lasers (EDFLs) by Bao et al. in 2009, versatile graphene samples in different forms have been developed, including single-layer graphene [1

1. Q. L. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]

6

6. S. Yamashita, “A tutorial on nonlinear photonic applications of carbon nanotube and graphene,” J. Lightwave Technol. 30(4), 427–447 (2012). [CrossRef]

], few-layer graphene [7

7. H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, K. P. Loh, B. Lin, and S. C. Tjin, “Compact graphene mode-locked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion to all normal dispersion,” Laser Phys. Lett. 7(8), 591–596 (2010). [CrossRef]

13

13. G. Sobon, J. Sotor, I. Pasternak, W. Strupinski, K. Krzempek, P. Kaczmarek, and K. M. Abramski, “Chirped pulse amplification of a femtosecond Er-doped fiber laser mode-locked by a graphene saturable absorber,” Laser Phys. Lett. 10(3), 035104 (2013). [CrossRef]

], multi-layer graphene [14

14. Y. M. Chang, H. Kim, J. H. Lee, and Y.-W. Song, “Multilayered graphene efficiently formed by mechanical exfoliation for nonlinear saturable absorbers in fiber mode-locked lasers,” Appl. Phys. Lett. 97(21), 211102 (2010). [CrossRef]

17

17. G. Sobon, J. Sotor, and K. M. Abramski, “All-polarization maintaining femtosecond Er-doped fiber laser mode-locked by graphene saturable absorber,” Laser Phys. Lett. 9(8), 581–586 (2012). [CrossRef]

], graphene polymer [18

18. T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P. H. Tan, A. G. Rozhin, and A. C. Ferrari, “Nanotube–polymer composites for ultrafast photonics,” Adv. Mater. 21, 3874–3899 (2009). [CrossRef]

23

23. H. Kim, J. Cho, S. Y. Jang, and Y. W. Song, “Deformation-immunized optical deposition of graphene for ultrafast pulsed lasers,” Appl. Phys. Lett. 98(2), 021104 (2011). [CrossRef]

], graphene composite [24

24. J. Xu, J. Liu, S. Wu, Q. H. Yang, and P. Wang, “Graphene oxide mode-locked femtosecond erbium-doped fiber lasers,” Opt. Express 20(14), 15474–15480 (2012). [CrossRef] [PubMed]

26

26. J. Xu, S. Wu, H. Li, J. Liu, R. Sun, F. Tan, Q.-H. Yang, and P. Wang, “Dissipative soliton generation from a graphene oxide mode-locked Er-doped fiber laser,” Opt. Express 20(21), 23653–23658 (2012). [CrossRef] [PubMed]

], graphene solution [27

27. Z. B. Liu, X. He, and D. N. Wang, “Passively mode-locked fiber laser based on a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution,” Opt. Lett. 36(16), 3024–3026 (2011). [CrossRef] [PubMed]

] and graphite nano-particle [28

28. G.-R. Lin and Y.-C. Lin, “Directly exfoliated and imprinted graphite nano-particle saturable absorber for passive mode-locking erbium-doped fiber laser,” Laser Phys. Lett. 8(12), 880–886 (2011). [CrossRef]

30

30. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

] etc., which progressively show the capabilities on initiating the ultrafast saturable absorption in the EDFL cavity. In particular, Zhang et al. investigated a dissipative soliton mode-locking laser with a wavelength tuning range of 1570-1600 nm by using few-layer graphene [8

8. H. Zhang, D. Y. Tang, R. J. Knize, L. Zhao, Q. L. Bao, and K. P. Loh, “Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser,” Appl. Phys. Lett. 96(11), 111112 (2010). [CrossRef]

]. Sun et al. incorporated the exfoliated graphene flakes into the polyvinyl alcohol (PVA) film to obtain a 460-fs passively mode-locked EDFL [20

20. Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, “Graphene mode-locked ultrafast laser,” ACS Nano 4(2), 803–810 (2010). [CrossRef] [PubMed]

]. Sobon et al. used graphene oxide to stabilize a mode-locked EDFL with soliton pulsewidth of sub-400 fs [25

25. G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, P. Paletko, J. Boguslawski, L. Lipinska, and K. M. Abramski, “Graphene oxide vs. reduced graphene oxide as saturable absorbers for Er-doped passively mode-locked fiber laser,” Opt. Express 20(17), 19463–19473 (2012). [CrossRef] [PubMed]

]. Liu et al. integrated the graphene-oxide with hollow-core photonic crystal fiber to demonstrate the passive mode-locking in nanosecond regime [27

27. Z. B. Liu, X. He, and D. N. Wang, “Passively mode-locked fiber laser based on a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution,” Opt. Lett. 36(16), 3024–3026 (2011). [CrossRef] [PubMed]

]. To simplify the production, Lin et al. directly brushed the polished graphite nano-particles on the fiber patchcord end-face in EDFL and achieve 400-fs mode-locking [28

28. G.-R. Lin and Y.-C. Lin, “Directly exfoliated and imprinted graphite nano-particle saturable absorber for passive mode-locking erbium-doped fiber laser,” Laser Phys. Lett. 8(12), 880–886 (2011). [CrossRef]

30

30. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

]. Up to now, almost all graphene, graphene oxide and graphite materials can initiate mode-locking of EDFL with ultrafast recovery time, broadband wavelength tunability, ultrahigh nonlinearity and large optical damage threshold.

Not long ago, Singh et al. preliminarily demonstrated a green and simple method to synthesis the graphene nano-sheets from a pencil by using the electrochemical exfoliation [31

31. V. V. Singh, G. Gupta, A. Batra, A. K. Nigam, M. Boopathi, P. K. Gutch, B. K. Tripathi, A. Srivastava, M. Samuel, G. S. Agarwal, B. Singh, and R. Vijayaraghavan, “Greener electrochemical synthesis of high quality graphene nanosheets directly from pencil and its SPR sensing application,” Adv. Funct. Mater. 22(11), 2352–2362 (2012). [CrossRef]

]. The bulk charcoal structure in a pencil is confirmed to be similar with graphite that contains multi-layer graphene. In the meantime, Lin's group also obtained the charcoal nano-particles by simply polishing the pencil [32

32. Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

], and presented that even the unprocessed charcoal nano-particles possess the ability of saturable absorption to passively mode-lock the EDFL. However, charcoal is a graphene contained raw material with plenty of structural defects, which can only achieve passive mode-locking of EDFL at picosecond regime due to its small modulation depth and large absorption loss [32

32. Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

]. Although the pulse compression into femtosecond regime is limited by the natural characteristic of charcoal nano-particles, which can essentially be improved via the parametric tuning of the EDFL cavity. In this work, the passively mode-locked EDFL at L-band is demonstrated with charcoal nano-particle based saturable absorber, and the mode-locking laser pulse can be shortened to <500 fs. The scanning electron microscope (SEM), X-ray photoelectron spectrum (XPS), Raman scattering spectroscopy and X-ray diffraction (XRD) diagnosis are utilized to investigate the structural properties of charcoal nano-particles. By detuning the power gain and cavity dispersion of the EDFL, the transformation of the pulse forming mechanism from self-amplitude modulation (SAM) to the effects of self-phase modulation (SPM) and group delay dispersion (GDD) is achieved in the EDFL. This eventually strengthens the mode-locking and pulse-shortening force. Although the structural defects degrade the nonlinear modulation behavior, the improved gain and dispersion of the EDFL cavity starts the high-order SPM effect to further compress the mode-locked pulsewidth afterwards.

2. Experiment setup

Figure 2
Fig. 2 The schematic diagram of the passively mode-locked EDFL system. LD: laser diode, WDM: wavelength-division multiplexer, PC: polarization controller.
illustrates the schematic diagram of a passively mode-locked EDFL system. In the beginning, a low-gain EDFA is employed as the gain medium, which uses 12-m long erbium-doped fiber (EDF) with a peak absorption of 1.5 dB/m (SDO, EFAS1B1438002A) to provide a power gain of 17 dB. The other SMF based components including the input/output couplers, the bi-directional pumping laser diodes (LDs), an isolator and a polarization controller (PC) are employed to form the EDFL system with a total length of 21 m. The cavity group delay dispersion (GDD) is minimized to a nearly dispersion free value of −0.004 ps2. Owing to the low-gain operation, such a low-gain EDFL system can only be operated in the self-amplitude-modulation (SAM) regime. For comparison, a homemade EDFA is established. To improve the intra-cavity gain and dispersion for operating the EDFL in the self-phase-modulation (SPM) regime with an enlarged negative GDD, a 2-m long high-gain erbium-doped fiber (HGEDF, nLIGHT Liekki Er80-8/125) with a peak absorption of 80 dB/m and a dispersion coefficient β2 of −20 ps2/km is used as the gain medium, which is bi-directionally pumped by two LDs (forward: 980-nm LD, backward: 1480-nm LD). A 980/1550 wavelength division multiplexer (WDM) and a 1480/1550 WDM are involved to deliver the pumping powers. An isolator set behind the EDF is used to decide the circulation direction and avoid feedback. A PC is inserted in front of the saturable absorber for controlling the intra-cavity polarization [33

33. K. H. Lin, J. J. Kang, H. H. Wu, C. K. Lee, and G.-R. Lin, “Manipulation of operation states by polarization control in an erbium-doped fiber laser with a hybrid saturable absorber,” Opt. Express 17(6), 4806–4814 (2009). [CrossRef] [PubMed]

]. The charcoal nano-particles are confined between two SMF patchcord connectors. An output optical coupler provides 95% feedback ratio and 5% output coupling ratio. The pulse shape, optical spectrum and pulse train are measured by an autocorrelator (Femtochrome FR-103XL), an optical spectrum analyzer (Ando AQ6317B) and an oscilloscope (Tektronix TDS 2022), respectively.

3. Results and discussions

3.1 Characterization of charcoal nano-particles and EDFAs

The Stokes Raman scattering spectrum of charcoal nano-particle shown in Fig. 5(a)
Fig. 5 The (a) Raman spectrum and (b) XRD spectrum of charcoal nano-particle [32].
demonstrates a prominent G band located at 1570 cm−1 corresponding to the existence of sp2 carbon bonds [32

32. Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

, 37

37. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, “Raman spectrum of graphene and graphene Layers,” Phys. Rev. Lett. 97(18), 187401 (2006). [CrossRef] [PubMed]

39

39. V. V. Singh, G. Gupta, A. Batra, A. K. Nigam, M. Boopathi, P. K. Gutch, B. K. Tripathi, A. Srivastava, M. Samuel, G. S. Agarwal, B. Singh, and R. Vijayaraghavan, “Greener electrochemical synthesis of high quality graphene nanosheets directly from pencil and its SPR sensing application,” Adv. Funct. Mater. 22(11), 2352–2362 (2012). [CrossRef]

], a distinct D peak at 1328 cm−1 caused by the breathing mode of k-point phonons from structural defects [32

32. Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

], and a weak 2D peak at 2771 cm−1 due to the second-order double resonant process. The structural defects also lead to a broadened G band with 3-dB linewidth of 81 cm−1 and an intensity ratio of 2D band over G band (I2D/IG) as high as 0.67. The 2D peak intensity attenuates whereas its bandwidth broadens by increasing the layer number of graphene, which correlate well with the highly disordered carbon structure in charcoal nano-particles. This results in the intensity ratio of 2D band over G band (I2D/IG) decreasing to 0.13. As evidence, the XRD analysis of charcoal nano-particle shown in Fig. 5(b) reveals a relatively broadened diffraction peak at 25.2° from the {002}-oriented lattice, indicating the bad crystallinity of the hexagonal graphite plane [29

29. Y. H. Lin and G.-R. Lin, “Free-standing nano-scale graphite saturable absorber for passively mode-locked erbium doped fiber ring laser,” Laser Phys. Lett. 9(5), 398–404 (2012). [CrossRef]

, 32

32. Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

]. In comparison with the {002} peak of nature graphite (26.54°), the small angle shift of −0.34° and the broadened linewidth of charcoal nano-particle elucidates the existence of structural defects including curvatures and distortions of graphene layers [40

40. Z. Q. Li, C. J. Lu, Z. P. Xia, Y. Zhou, and Z. Luo, “X-ray diffraction patterns of graphite and turbostratic carbon,” Carbon 45(8), 1686–1695 (2007). [CrossRef]

].

The saturable absorbance of charcoal nano-particle containing crystalline graphene structure is characterized by femtosecond laser illumination, in which the optical absorption of charcoal nano-particle is reduced under high optical power illumination, because the carrier transition from valence band to conduction band is forbidden by the Pauli blocking effect [32

32. Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

]. The optical absorbance α of charcoal nano-particle is correlated with the linear absorbance (qlin), the nonlinear (saturbale) absorbance (qnon), as described by [6

6. S. Yamashita, “A tutorial on nonlinear photonic applications of carbon nanotube and graphene,” J. Lightwave Technol. 30(4), 427–447 (2012). [CrossRef]

,30

30. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

]:
α=qlin+qnon1+Pin/Psatqin+qnonqnonPsatPinqlin+qnon+3ωIm(χ(3))2ε0c2n02Pin,
(1)
where Pin denotes the input power, Psat the saturation power of charcoal nano-particle, ω the optical angular frequency, Imχ(3) the imaginary part (the extinction constant) of third-order nonlinear refractive index, ε0 the dielectric permittivity, c the speed of light and n0 the refractive index. The modulation coefficient γ is defined as qnon/Psat, and the saturable absorbance can be simplified as qlin + qnon + γPin, which represents the SAM effect. After derivation, the saturable absorbance is expressed as a function of the Imχ(3).

The triturated charcoal nano-particles with larger sizes exhibit lower transmittance changing from 0.66 to 0.72 with enlarged illuminating power, which provide a modulation depth (MD) of 22%, as shown in Fig. 6
Fig. 6 The (a) Saturable transmittance and (b) saturable absorbance of charcoal nano-particle [32].
. Fitting the curve of saturable absorbance obtains the linear loss of qlin = 0.292, the nonlinear loss of qnon = 0.13, and the saturation power of Psat = 12 mW. After multiple imprinting-exfoliation-wiping process, the charcoal nano-particle with smaller sizes show the transmittance increasing from 0.87 to 0.91 with corresponding modulation depth enlarging up to MD = 26%. Not only the linear loss decreases to qlin = 0.1, but also the saturable absorbance attenuates to qnon = 0.036, and the saturation power reduces to Psat = 3.5 mW. The size shrinkage of charcoal nano-particle reduces the linear/nonlinear absorption loss, which concurrently enlarges the nonlinear modulation depth and scales down the saturation power for initiating the saturable absorption. Both the two factors are significant for improving the passively mode-locked EDFL performances.

The power and gain characteristics of the high-gain and low-gain EDFAs are demonstrated in Fig. 7(a)
Fig. 7 (a) The Pout vs. Pin curves in logarithm scale of the high-gain and low-gain EDFAs. (b) The Gain vs. Pin curves in linear scale of the high-gain and low-gain EDFAs.
. To shorten the EDFL pulse, a sufficiently large amount of the longitudinal modes must be phase-locked over a wide frequency range limited by gain bandwidth [41

41. E. P. Ippen, “Principles of passive mode locking,” Appl. Phys. B 58(3), 159–170 (1994). [CrossRef]

, 42

42. H. A. Haus, “Mode-locking of lasers,” IEEE J. Sel. Top. Quantum Electron. 6(6), 1173–1185 (2000). [CrossRef]

]. The enhanced cavity gain helps the circulating pulse to overcome the saturated absorption threshold so as to reduce the passively mode-locked EDFL pulsewidth. The bi-directionally pumped high-gain EDFL operated at maximum current conditions (900 mA for both pumping LDs) provides forward and backward pumping powers of 290 mW at 980 nm and 200 mW at 1480 nm, respectively. The equation G = g0/(1 + Pin/Psat) is utilized to simulate the gain of two EDFLs in linear scale, as shown in Fig. 7(b), where g0 is the small signal gain and Psat is the saturation power of the EDFA. With an input power Pin as low as −10 dBm, the high-gain EDFL provide a power gain of up to 20 dB, which is nearly 4-dB larger than that provided by the low-gain EDFL (16 dB). The g0 = 8.62 for high-gain EDFA is larger than g0 = 7.3 for low-gain EDFA. Even at the output power saturated condition (at Pin>0 dBm), the high-gain EDFL still provides a 21-dB gain that is 3.1 dB higher than gain provided by the low-gain EDFL.

3.2 SAM and SPM dominated passive mode-locking of EDFLs with charcoal nano-particles

Figure 8(a)
Fig. 8 The (a) auto-correlated pulses and (b) optical spectra of the high-gain(upper) and low-gain (lower) EDFLs mode-locked by charcoal nano-particles (solid: measured; dashed: fitting) [32].
and 8(b) present the autocorrelation traces and optical spectra of the high-gain and low-gain EDFLs passively mode-locked by charcoal nano-particles. Without using high-gain EDF [43

43. J.-C. Chiu, C.-M. Chang, B.-Z. Hsieh, S.-C. Lin, C.-Y. Yeh, G.-R. Lin, C.-K. Lee, J.-J. Lin, and W.-H. Cheng, “Pulse shortening mode-locked fiber laser by thickness and concentration product of carbon nanotube based saturable absorber,” Opt. Express 19(5), 4036–4041 (2011). [CrossRef] [PubMed]

] or externally high-order soliton compression [44

44. Y.-T. Lin and G.-R. Lin, “Dual-stage soliton compression of a self-started additive pulse mode-locked erbium-doped fiber laser for 48 fs pulse generation,” Opt. Lett. 31(10), 1382–1384 (2006). [CrossRef] [PubMed]

, 45

45. G.-R. Lin, C.-L. Pan, and Y.-T. Lin, “Self-steepening of prechirped amplified and compressed 29-fs fiber laser pulse in large-mode-area erbium-doped fiber amplifier,” J. Lightwave Technol. 25(11), 3597–3601 (2007). [CrossRef]

] approach, the low-gain EDFL only delivers a pulsewidth of 1.36 ps at central wavelength of 1573 nm accompanied with spectral linewidth of 1.83 nm. The time-bandwidth product (TBP) of the low-gain EDFL is about 0.32, which is a nearly transform limited pulse but still operated at SAM condition. The pulse formation is thus determined by the saturable absorber when operating at SAM mode. The passively mode-locked EDFL enters into the SPM operation after enhancing the cavity gain and reducing the absorption loss of saturable absorber, which shortens its pulsewidth to 455 fs with corresponding linewidth of 5.86 nm. Moreover, the red-shift of central wavelength to 1577 nm is observed due to the enlarged intra-cavity gain [46

46. G.-R. Lin, J. Y. Chang, Y. S. Liao, and H. H. Lu, “L-band erbium-doped fiber laser with coupling-ratio controlled wavelength tunability,” Opt. Express 14(21), 9743–9749 (2006). [CrossRef] [PubMed]

]. The TBP of the improved passively mode-locked EDFL is about 0.322. The length and dispersion coefficient β2,EDF of EDF are 2 m and −20 ps2/km; For SMF used in the EDFL cavity, the length and β2,SMF are 5.8 m and −20 ps2/km. The resonant cavity is in the anomalous dispersion regime with group delay dispersion (GDD) of −0.156 ps2. Obviously, the high-gain EDFL is operated at strong SPM and negative GDD regime, which greatly improves the pulse shortening mechanism.

Figure 9(a)
Fig. 9 (a) The improved passively mode-locked EDFL pulse train. (b) The original passively mode-locked EDFL pulse train [32].
demonstrates the pulse-train of the high-gain EDFL passively mode-locked by charcoal nano-particles, which indicates the repetition time and frequency of 35 ns and 28.5 MHz, corresponding to the cavity length of 7.8 m (EDF: 2 m, SMF: 5.8 m). Even with such short cavity length, an extremely large GDD is caused by the high-gain EDF with a large negative dispersion. In contrast, Fig. 9(b) reveals pulse-train of the low-gain EDFL under passive mode-locking, the repetition time and frequency of 11 ns and 8.9 MHz corresponds to the cavity length of 21 m (EDF: 8.5 m, SMF: 12.5 m). The high-gain EDFL not only shortens the mode-locked pulsewidth but also shrinks the cavity length to increases the repetition rate by three times. In addition, the fluctuation of pulse peak power as well as the quality of pulse amplitude equalization is characterized by measuring the carrier amplitude jitter, which is defined as (σ/Iave) x 100%, where σ denotes the standard deviation of peak pulse intensity, Iave the average pulse intensity [49

49. G.-R. Lin, J. J. Kang, and C. K. Lee, “High-order rational harmonic mode-locking and pulse-amplitude equalization of SOAFL via reshaped gain-switching FPLD pulse injection,” Opt. Express 18(9), 9570–9579 (2010). [CrossRef] [PubMed]

]. The CAJ value of the high-gain EDFL is 1.74%, which is better than the low-gain EDFL of 1.89%. Table 1

Table 1. Parametric comparisons of the high-gain and low-gain EDFL mode-locked bt charcoal nano-particles.

table-icon
View This Table
summarizes the parametric comparison on both EDFLs mode-locked by charcoal nano-particle based saturable absorber, in which the pulse shortening by three times can be attributed to the increased cavity gain (by EDF) and the reduced cavity loss (by small charcoal nano-particles).

With the aforementioned parameters, the pulsating dynamics of passively mode-locked EDFL is simulated by Haus master equation. By considering a pulse with hyperbolic secant shape A(T,t) = A0sech(t/τ) propagating in the resonant cavity, the master equation is written as [42

42. H. A. Haus, “Mode-locking of lasers,” IEEE J. Sel. Top. Quantum Electron. 6(6), 1173–1185 (2000). [CrossRef]

, 50

50. F. X. Kurtner, J. A. der Au, and U. Keller, “Mode-locking with slow and fast saturable absorbers—what’s the difference?” IEEE J. Sel. Top. Quantum Electron. 4(2), 159–168 (1998). [CrossRef]

]:
TRA(T,t)T=[gl0+Dg,f2t2+γ|A0|2]A(T,t)+j[D2t2δ|A0|2]A(T,t),
(2)
where g denotes the cavity gain, l0 the insertion loss, Dg,f the gain and filter dispersions, γ the modulation coefficient, D and δ are the intra-cavity GDD and SPM coefficient, respectively. The right-hand side is divided into two terms to distinguish the SAM from the SPM effect. The first square bracket in the right hand side of Eq. (2) leads to a self-amplitude modulation (SAM) mechanism, whereas the second square bracket is contributed by the self-phase modulation (SPM) mechanism. Without considering the effects of GDD and SPM, the pulse pulsewidth is only determined by the charcoal nano-particle based saturable absorber. In SAM case, the fast saturable absorber benefits from the short pulse generation with a pulsewidth given by [51

51. F. X. Kaertner, “Mode-locked Laser Theory,” physics.gatech.edu, (2006).

]

τ=2Dg,fγ|A0|2.
(3)

When considering the effects of GDD and SPM at high power condition, the phase perturbation induces an additional chirp to distort the pulse shape. However, the strong SPM mechanism can compensate the pulse distortion by the GDD in anomalous dispersion regime. The charcoal nano-particle behaves like rather a mode-locking starter than a pulse compressor. Under the effects of GDD and SPM, the pulsewidth τ' is modified as:
τ'=τ2(2β23βDN)=τ2[2β(β3DN)],
(4)
β=32(1+δNDNδN+DN)±[32(1+δNDNδN+DN)]2+2,
(5)
where β is the chirp correlated with the normalized dispersion of DN = D/Dg,f and the normalized nonlinearity δN = δ/γ . When the chirp is larger than 3DN under a strong soliton operation with δN >> 1, DN >> 1 and δN≈-DN, the soliton pulse can be further compressed by a factor of 2~3. The strong SPM and GDD dominate the pulse shaping, whereas the saturable absorber only plays the role to start and stabilize the pulse-train. That is, the gain enhancement not only enlarges the intra-cavity power to shorten the pulse, but also enhances the SPM effect to form the soliton mode-locking. However, a huge cavity gain could further induce giant SPM (ie. δN >> DN) to induce the extremely large phase instability and pulse amplitude jitter.

Figure 10
Fig. 10 The (a) simulated autocorrelation traces and (b) optical spectra of the passively mode-locked EDFLs without (upper) and with (lower) the GDD and SPM effects.
shows the simulated passively mode-locked EDFL pulse shapes and optical spectra with (and without) the effects of GDD and SPM. These results are in good agreement with our experimental results. In our case, the soliton mode-locking is operated with the cavity GDD of −0.156 ps2 and the SPM coefficient of 1x10−2 W−1. With only the SAM effect inside the cavity, the charcoal nano-particles can mode-lock the EDFL to deliver a pulsewidth of 820 fs and a spectral linewidth of 3.21 nm. With a relatively strong SPM, the charcoal nano-particle only starts the pulse-train, and the SPM dominates the compression of the EDFL pulsewidth down to 460 fs with corresponding linewidth of 5.82 nm. Therefore, no matter the fast or slow saturable absorber, the mode-locked EDFL can stably deliver femtosecond pulse with the mechanism transferring from SAM to SPM.

4. Conclusion

Acknowledgment

This work was supported by National Science Council and National Taiwan University under grants NSC101-2622-E-002-009-CC2, NSC101-2221-E-002-071-MY3 and NTU102R89083.

References and links

1.

Q. L. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]

2.

H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, and K. P. Loh, “Large energy mode locking of an erbium-doped fiber laser with atomic layer graphene,” Opt. Express 17(20), 17630–17635 (2009). [CrossRef] [PubMed]

3.

F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010). [CrossRef]

4.

H. Zhang, D. Y. Tang, L. Zhao, Q. L. Bao, and K. P. Loh, “Vector dissipative solitons in graphene mode locked fiber lasers,” Opt. Commun. 283(17), 3334–3338 (2010). [CrossRef]

5.

Q. L. Bao, H. Zhang, Z. Ni, Y. Wang, L. Polavarapu, Z. Shen, Q. H. Xu, D. Y. Tang, and K. P. Loh, “Monolayer graphene as saturable absorber in mode-locked laser,” Nano Res. 4(3), 297–307 (2011). [CrossRef]

6.

S. Yamashita, “A tutorial on nonlinear photonic applications of carbon nanotube and graphene,” J. Lightwave Technol. 30(4), 427–447 (2012). [CrossRef]

7.

H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, K. P. Loh, B. Lin, and S. C. Tjin, “Compact graphene mode-locked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion to all normal dispersion,” Laser Phys. Lett. 7(8), 591–596 (2010). [CrossRef]

8.

H. Zhang, D. Y. Tang, R. J. Knize, L. Zhao, Q. L. Bao, and K. P. Loh, “Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser,” Appl. Phys. Lett. 96(11), 111112 (2010). [CrossRef]

9.

Y. W. Song, S. Y. Jang, W. S. Han, and M. K. Bae, “Graphene mode-lockers for fiber lasers functioned with evanescent field interaction,” Appl. Phys. Lett. 96(5), 051122 (2010). [CrossRef]

10.

A. Martinez, K. Fuse, B. Xu, and S. Yamashita, “Optical deposition of graphene and carbon nanotubes in a fiber ferrule for passive mode-locked lasing,” Opt. Express 18(22), 23054–23061 (2010). [CrossRef] [PubMed]

11.

Z. Sun, D. Popa, T. Hasan, F. Torrisi, F. Wang, E. J. R. Kelleher, J. C. Travers, V. Nicolosi, and A. C. Ferrari, “A stable, wideband tunable, near transform-limited, graphene-mode-locked, ultrafast laser,” Nano Res. 3(9), 653–660 (2010). [CrossRef]

12.

B. V. Cunning, C. L. Brown, and D. Kielpinski, “Low-loss flake-graphene saturable absorber mirror for laser mode-locking at sub-200-fs pulse duration,” Appl. Phys. Lett. 99(26), 261109 (2011). [CrossRef]

13.

G. Sobon, J. Sotor, I. Pasternak, W. Strupinski, K. Krzempek, P. Kaczmarek, and K. M. Abramski, “Chirped pulse amplification of a femtosecond Er-doped fiber laser mode-locked by a graphene saturable absorber,” Laser Phys. Lett. 10(3), 035104 (2013). [CrossRef]

14.

Y. M. Chang, H. Kim, J. H. Lee, and Y.-W. Song, “Multilayered graphene efficiently formed by mechanical exfoliation for nonlinear saturable absorbers in fiber mode-locked lasers,” Appl. Phys. Lett. 97(21), 211102 (2010). [CrossRef]

15.

P. L. Huang, S. C. Lin, C. Y. Yeh, H. H. Kuo, S. H. Huang, G.-R. Lin, L. J. Li, C. Y. Su, and W. H. Cheng, “Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber,” Opt. Express 20(3), 2460–2465 (2012). [CrossRef] [PubMed]

16.

J. Sotor, G. Sobon, and K. M. Abramski, “Scalar soliton generation in all-polarization-maintaining, graphene mode-locked fiber laser,” Opt. Lett. 37(11), 2166–2168 (2012). [CrossRef] [PubMed]

17.

G. Sobon, J. Sotor, and K. M. Abramski, “All-polarization maintaining femtosecond Er-doped fiber laser mode-locked by graphene saturable absorber,” Laser Phys. Lett. 9(8), 581–586 (2012). [CrossRef]

18.

T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P. H. Tan, A. G. Rozhin, and A. C. Ferrari, “Nanotube–polymer composites for ultrafast photonics,” Adv. Mater. 21, 3874–3899 (2009). [CrossRef]

19.

H. Zhang, Q. L. Bao, D. Y. Tang, L. Zhao, and K. P. Loh, “Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker,” Appl. Phys. Lett. 95(14), 141103 (2009). [CrossRef]

20.

Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, “Graphene mode-locked ultrafast laser,” ACS Nano 4(2), 803–810 (2010). [CrossRef] [PubMed]

21.

Q. L. Bao, H. Zhang, J. Yang, S. Wang, D. Y. Tang, R. Jose, S. Ramakrishna, C. T. Lim, and K. P. Loh, “Graphene–polymer nanofiber membrane for ultrafast photonics,” Adv. Funct. Mater. 20(5), 782–791 (2010). [CrossRef]

22.

D. Popa, Z. Sun, F. Torrisi, T. Hasan, F. Wang, and A. C. Ferrari, “Sub 200 fs pulse generation from a graphene mode-locked fiber laser,” Appl. Phys. Lett. 97(20), 203106 (2010). [CrossRef]

23.

H. Kim, J. Cho, S. Y. Jang, and Y. W. Song, “Deformation-immunized optical deposition of graphene for ultrafast pulsed lasers,” Appl. Phys. Lett. 98(2), 021104 (2011). [CrossRef]

24.

J. Xu, J. Liu, S. Wu, Q. H. Yang, and P. Wang, “Graphene oxide mode-locked femtosecond erbium-doped fiber lasers,” Opt. Express 20(14), 15474–15480 (2012). [CrossRef] [PubMed]

25.

G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, P. Paletko, J. Boguslawski, L. Lipinska, and K. M. Abramski, “Graphene oxide vs. reduced graphene oxide as saturable absorbers for Er-doped passively mode-locked fiber laser,” Opt. Express 20(17), 19463–19473 (2012). [CrossRef] [PubMed]

26.

J. Xu, S. Wu, H. Li, J. Liu, R. Sun, F. Tan, Q.-H. Yang, and P. Wang, “Dissipative soliton generation from a graphene oxide mode-locked Er-doped fiber laser,” Opt. Express 20(21), 23653–23658 (2012). [CrossRef] [PubMed]

27.

Z. B. Liu, X. He, and D. N. Wang, “Passively mode-locked fiber laser based on a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution,” Opt. Lett. 36(16), 3024–3026 (2011). [CrossRef] [PubMed]

28.

G.-R. Lin and Y.-C. Lin, “Directly exfoliated and imprinted graphite nano-particle saturable absorber for passive mode-locking erbium-doped fiber laser,” Laser Phys. Lett. 8(12), 880–886 (2011). [CrossRef]

29.

Y. H. Lin and G.-R. Lin, “Free-standing nano-scale graphite saturable absorber for passively mode-locked erbium doped fiber ring laser,” Laser Phys. Lett. 9(5), 398–404 (2012). [CrossRef]

30.

Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett. 10(4), 045109 (2013). [CrossRef]

31.

V. V. Singh, G. Gupta, A. Batra, A. K. Nigam, M. Boopathi, P. K. Gutch, B. K. Tripathi, A. Srivastava, M. Samuel, G. S. Agarwal, B. Singh, and R. Vijayaraghavan, “Greener electrochemical synthesis of high quality graphene nanosheets directly from pencil and its SPR sensing application,” Adv. Funct. Mater. 22(11), 2352–2362 (2012). [CrossRef]

32.

Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett. 10(5), 055105 (2013). [CrossRef]

33.

K. H. Lin, J. J. Kang, H. H. Wu, C. K. Lee, and G.-R. Lin, “Manipulation of operation states by polarization control in an erbium-doped fiber laser with a hybrid saturable absorber,” Opt. Express 17(6), 4806–4814 (2009). [CrossRef] [PubMed]

34.

K. Nishimiya, T. Hata, Y. Imamura, and S. Ishihara, “Analysis of chemical structure of wood charcoal by X-ray photoelectron spectroscopy,” J. Wood Sci. 44(1), 56–61 (1998). [CrossRef]

35.

H. A. Becerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and Y. Chen, “Evaluation of solution-processed reduced graphene oxide films as transparent conductors,” ACS Nano 2(3), 463–470 (2008). [CrossRef] [PubMed]

36.

X. Dong, C.-Y. Su, W. Zhang, J. Zhao, Q. Ling, W. Huang, P. Chen, and L.-J. Li, “Ultra-large single-layer graphene obtained from solution chemical reduction and its electrical properties,” Phys. Chem. Chem. Phys. 12(9), 2164–2169 (2010). [CrossRef] [PubMed]

37.

A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, “Raman spectrum of graphene and graphene Layers,” Phys. Rev. Lett. 97(18), 187401 (2006). [CrossRef] [PubMed]

38.

Z. H. Ni, T. Yu, Y. H. Lu, Y. Y. Wang, Y. P. Feng, and Z. X. Shen, “Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening,” ACS Nano 2(11), 2301–2305 (2008). [CrossRef] [PubMed]

39.

V. V. Singh, G. Gupta, A. Batra, A. K. Nigam, M. Boopathi, P. K. Gutch, B. K. Tripathi, A. Srivastava, M. Samuel, G. S. Agarwal, B. Singh, and R. Vijayaraghavan, “Greener electrochemical synthesis of high quality graphene nanosheets directly from pencil and its SPR sensing application,” Adv. Funct. Mater. 22(11), 2352–2362 (2012). [CrossRef]

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Z. Q. Li, C. J. Lu, Z. P. Xia, Y. Zhou, and Z. Luo, “X-ray diffraction patterns of graphite and turbostratic carbon,” Carbon 45(8), 1686–1695 (2007). [CrossRef]

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J.-C. Chiu, C.-M. Chang, B.-Z. Hsieh, S.-C. Lin, C.-Y. Yeh, G.-R. Lin, C.-K. Lee, J.-J. Lin, and W.-H. Cheng, “Pulse shortening mode-locked fiber laser by thickness and concentration product of carbon nanotube based saturable absorber,” Opt. Express 19(5), 4036–4041 (2011). [CrossRef] [PubMed]

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Y.-T. Lin and G.-R. Lin, “Dual-stage soliton compression of a self-started additive pulse mode-locked erbium-doped fiber laser for 48 fs pulse generation,” Opt. Lett. 31(10), 1382–1384 (2006). [CrossRef] [PubMed]

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G.-R. Lin, C.-L. Pan, and Y.-T. Lin, “Self-steepening of prechirped amplified and compressed 29-fs fiber laser pulse in large-mode-area erbium-doped fiber amplifier,” J. Lightwave Technol. 25(11), 3597–3601 (2007). [CrossRef]

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G.-R. Lin, J. Y. Chang, Y. S. Liao, and H. H. Lu, “L-band erbium-doped fiber laser with coupling-ratio controlled wavelength tunability,” Opt. Express 14(21), 9743–9749 (2006). [CrossRef] [PubMed]

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G.-R. Lin, I.-H. Chiu, and M. C. Wu, “1.2 ps mode-locked semiconductor optical amplifier fiber laser pulses generated by 60 ps backward dark-optical comb injection and soliton compression,” Opt. Express 13(3), 1008–1014 (2005). [CrossRef] [PubMed]

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G.-R. Lin, J. J. Kang, and C. K. Lee, “High-order rational harmonic mode-locking and pulse-amplitude equalization of SOAFL via reshaped gain-switching FPLD pulse injection,” Opt. Express 18(9), 9570–9579 (2010). [CrossRef] [PubMed]

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F. X. Kurtner, J. A. der Au, and U. Keller, “Mode-locking with slow and fast saturable absorbers—what’s the difference?” IEEE J. Sel. Top. Quantum Electron. 4(2), 159–168 (1998). [CrossRef]

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OCIS Codes
(140.4050) Lasers and laser optics : Mode-locked lasers
(140.7090) Lasers and laser optics : Ultrafast lasers
(160.4236) Materials : Nanomaterials
(060.3510) Fiber optics and optical communications : Lasers, fiber

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: April 26, 2013
Manuscript Accepted: May 28, 2013
Published: October 15, 2013

Citation
Yung-Hsiang Lin, Jui-Yung Lo, Wei-Hsuan Tseng, Chih-I Wu, and Gong-Ru Lin, "Self-amplitude and self-phase modulation of the charcoal mode-locked erbium-doped fiber lasers," Opt. Express 21, 25184-25196 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-21-25184


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References

  1. Q. L. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater.19(19), 3077–3083 (2009). [CrossRef]
  2. H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, and K. P. Loh, “Large energy mode locking of an erbium-doped fiber laser with atomic layer graphene,” Opt. Express17(20), 17630–17635 (2009). [CrossRef] [PubMed]
  3. F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics4(9), 611–622 (2010). [CrossRef]
  4. H. Zhang, D. Y. Tang, L. Zhao, Q. L. Bao, and K. P. Loh, “Vector dissipative solitons in graphene mode locked fiber lasers,” Opt. Commun.283(17), 3334–3338 (2010). [CrossRef]
  5. Q. L. Bao, H. Zhang, Z. Ni, Y. Wang, L. Polavarapu, Z. Shen, Q. H. Xu, D. Y. Tang, and K. P. Loh, “Monolayer graphene as saturable absorber in mode-locked laser,” Nano Res.4(3), 297–307 (2011). [CrossRef]
  6. S. Yamashita, “A tutorial on nonlinear photonic applications of carbon nanotube and graphene,” J. Lightwave Technol.30(4), 427–447 (2012). [CrossRef]
  7. H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, K. P. Loh, B. Lin, and S. C. Tjin, “Compact graphene mode-locked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion to all normal dispersion,” Laser Phys. Lett.7(8), 591–596 (2010). [CrossRef]
  8. H. Zhang, D. Y. Tang, R. J. Knize, L. Zhao, Q. L. Bao, and K. P. Loh, “Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser,” Appl. Phys. Lett.96(11), 111112 (2010). [CrossRef]
  9. Y. W. Song, S. Y. Jang, W. S. Han, and M. K. Bae, “Graphene mode-lockers for fiber lasers functioned with evanescent field interaction,” Appl. Phys. Lett.96(5), 051122 (2010). [CrossRef]
  10. A. Martinez, K. Fuse, B. Xu, and S. Yamashita, “Optical deposition of graphene and carbon nanotubes in a fiber ferrule for passive mode-locked lasing,” Opt. Express18(22), 23054–23061 (2010). [CrossRef] [PubMed]
  11. Z. Sun, D. Popa, T. Hasan, F. Torrisi, F. Wang, E. J. R. Kelleher, J. C. Travers, V. Nicolosi, and A. C. Ferrari, “A stable, wideband tunable, near transform-limited, graphene-mode-locked, ultrafast laser,” Nano Res.3(9), 653–660 (2010). [CrossRef]
  12. B. V. Cunning, C. L. Brown, and D. Kielpinski, “Low-loss flake-graphene saturable absorber mirror for laser mode-locking at sub-200-fs pulse duration,” Appl. Phys. Lett.99(26), 261109 (2011). [CrossRef]
  13. G. Sobon, J. Sotor, I. Pasternak, W. Strupinski, K. Krzempek, P. Kaczmarek, and K. M. Abramski, “Chirped pulse amplification of a femtosecond Er-doped fiber laser mode-locked by a graphene saturable absorber,” Laser Phys. Lett.10(3), 035104 (2013). [CrossRef]
  14. Y. M. Chang, H. Kim, J. H. Lee, and Y.-W. Song, “Multilayered graphene efficiently formed by mechanical exfoliation for nonlinear saturable absorbers in fiber mode-locked lasers,” Appl. Phys. Lett.97(21), 211102 (2010). [CrossRef]
  15. P. L. Huang, S. C. Lin, C. Y. Yeh, H. H. Kuo, S. H. Huang, G.-R. Lin, L. J. Li, C. Y. Su, and W. H. Cheng, “Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber,” Opt. Express20(3), 2460–2465 (2012). [CrossRef] [PubMed]
  16. J. Sotor, G. Sobon, and K. M. Abramski, “Scalar soliton generation in all-polarization-maintaining, graphene mode-locked fiber laser,” Opt. Lett.37(11), 2166–2168 (2012). [CrossRef] [PubMed]
  17. G. Sobon, J. Sotor, and K. M. Abramski, “All-polarization maintaining femtosecond Er-doped fiber laser mode-locked by graphene saturable absorber,” Laser Phys. Lett.9(8), 581–586 (2012). [CrossRef]
  18. T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P. H. Tan, A. G. Rozhin, and A. C. Ferrari, “Nanotube–polymer composites for ultrafast photonics,” Adv. Mater.21, 3874–3899 (2009). [CrossRef]
  19. H. Zhang, Q. L. Bao, D. Y. Tang, L. Zhao, and K. P. Loh, “Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker,” Appl. Phys. Lett.95(14), 141103 (2009). [CrossRef]
  20. Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, “Graphene mode-locked ultrafast laser,” ACS Nano4(2), 803–810 (2010). [CrossRef] [PubMed]
  21. Q. L. Bao, H. Zhang, J. Yang, S. Wang, D. Y. Tang, R. Jose, S. Ramakrishna, C. T. Lim, and K. P. Loh, “Graphene–polymer nanofiber membrane for ultrafast photonics,” Adv. Funct. Mater.20(5), 782–791 (2010). [CrossRef]
  22. D. Popa, Z. Sun, F. Torrisi, T. Hasan, F. Wang, and A. C. Ferrari, “Sub 200 fs pulse generation from a graphene mode-locked fiber laser,” Appl. Phys. Lett.97(20), 203106 (2010). [CrossRef]
  23. H. Kim, J. Cho, S. Y. Jang, and Y. W. Song, “Deformation-immunized optical deposition of graphene for ultrafast pulsed lasers,” Appl. Phys. Lett.98(2), 021104 (2011). [CrossRef]
  24. J. Xu, J. Liu, S. Wu, Q. H. Yang, and P. Wang, “Graphene oxide mode-locked femtosecond erbium-doped fiber lasers,” Opt. Express20(14), 15474–15480 (2012). [CrossRef] [PubMed]
  25. G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, P. Paletko, J. Boguslawski, L. Lipinska, and K. M. Abramski, “Graphene oxide vs. reduced graphene oxide as saturable absorbers for Er-doped passively mode-locked fiber laser,” Opt. Express20(17), 19463–19473 (2012). [CrossRef] [PubMed]
  26. J. Xu, S. Wu, H. Li, J. Liu, R. Sun, F. Tan, Q.-H. Yang, and P. Wang, “Dissipative soliton generation from a graphene oxide mode-locked Er-doped fiber laser,” Opt. Express20(21), 23653–23658 (2012). [CrossRef] [PubMed]
  27. Z. B. Liu, X. He, and D. N. Wang, “Passively mode-locked fiber laser based on a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution,” Opt. Lett.36(16), 3024–3026 (2011). [CrossRef] [PubMed]
  28. G.-R. Lin and Y.-C. Lin, “Directly exfoliated and imprinted graphite nano-particle saturable absorber for passive mode-locking erbium-doped fiber laser,” Laser Phys. Lett.8(12), 880–886 (2011). [CrossRef]
  29. Y. H. Lin and G.-R. Lin, “Free-standing nano-scale graphite saturable absorber for passively mode-locked erbium doped fiber ring laser,” Laser Phys. Lett.9(5), 398–404 (2012). [CrossRef]
  30. Y. H. Lin and G.-R. Lin, “Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles,” Laser Phys. Lett.10(4), 045109 (2013). [CrossRef]
  31. V. V. Singh, G. Gupta, A. Batra, A. K. Nigam, M. Boopathi, P. K. Gutch, B. K. Tripathi, A. Srivastava, M. Samuel, G. S. Agarwal, B. Singh, and R. Vijayaraghavan, “Greener electrochemical synthesis of high quality graphene nanosheets directly from pencil and its SPR sensing application,” Adv. Funct. Mater.22(11), 2352–2362 (2012). [CrossRef]
  32. Y. H. Lin, Y. C. Chi, and G.-R. Lin, “Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser,” Laser Phys. Lett.10(5), 055105 (2013). [CrossRef]
  33. K. H. Lin, J. J. Kang, H. H. Wu, C. K. Lee, and G.-R. Lin, “Manipulation of operation states by polarization control in an erbium-doped fiber laser with a hybrid saturable absorber,” Opt. Express17(6), 4806–4814 (2009). [CrossRef] [PubMed]
  34. K. Nishimiya, T. Hata, Y. Imamura, and S. Ishihara, “Analysis of chemical structure of wood charcoal by X-ray photoelectron spectroscopy,” J. Wood Sci.44(1), 56–61 (1998). [CrossRef]
  35. H. A. Becerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and Y. Chen, “Evaluation of solution-processed reduced graphene oxide films as transparent conductors,” ACS Nano2(3), 463–470 (2008). [CrossRef] [PubMed]
  36. X. Dong, C.-Y. Su, W. Zhang, J. Zhao, Q. Ling, W. Huang, P. Chen, and L.-J. Li, “Ultra-large single-layer graphene obtained from solution chemical reduction and its electrical properties,” Phys. Chem. Chem. Phys.12(9), 2164–2169 (2010). [CrossRef] [PubMed]
  37. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, “Raman spectrum of graphene and graphene Layers,” Phys. Rev. Lett.97(18), 187401 (2006). [CrossRef] [PubMed]
  38. Z. H. Ni, T. Yu, Y. H. Lu, Y. Y. Wang, Y. P. Feng, and Z. X. Shen, “Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening,” ACS Nano2(11), 2301–2305 (2008). [CrossRef] [PubMed]
  39. V. V. Singh, G. Gupta, A. Batra, A. K. Nigam, M. Boopathi, P. K. Gutch, B. K. Tripathi, A. Srivastava, M. Samuel, G. S. Agarwal, B. Singh, and R. Vijayaraghavan, “Greener electrochemical synthesis of high quality graphene nanosheets directly from pencil and its SPR sensing application,” Adv. Funct. Mater.22(11), 2352–2362 (2012). [CrossRef]
  40. Z. Q. Li, C. J. Lu, Z. P. Xia, Y. Zhou, and Z. Luo, “X-ray diffraction patterns of graphite and turbostratic carbon,” Carbon45(8), 1686–1695 (2007). [CrossRef]
  41. E. P. Ippen, “Principles of passive mode locking,” Appl. Phys. B58(3), 159–170 (1994). [CrossRef]
  42. H. A. Haus, “Mode-locking of lasers,” IEEE J. Sel. Top. Quantum Electron.6(6), 1173–1185 (2000). [CrossRef]
  43. J.-C. Chiu, C.-M. Chang, B.-Z. Hsieh, S.-C. Lin, C.-Y. Yeh, G.-R. Lin, C.-K. Lee, J.-J. Lin, and W.-H. Cheng, “Pulse shortening mode-locked fiber laser by thickness and concentration product of carbon nanotube based saturable absorber,” Opt. Express19(5), 4036–4041 (2011). [CrossRef] [PubMed]
  44. Y.-T. Lin and G.-R. Lin, “Dual-stage soliton compression of a self-started additive pulse mode-locked erbium-doped fiber laser for 48 fs pulse generation,” Opt. Lett.31(10), 1382–1384 (2006). [CrossRef] [PubMed]
  45. G.-R. Lin, C.-L. Pan, and Y.-T. Lin, “Self-steepening of prechirped amplified and compressed 29-fs fiber laser pulse in large-mode-area erbium-doped fiber amplifier,” J. Lightwave Technol.25(11), 3597–3601 (2007). [CrossRef]
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