Passively Mode-Locked Tm -Doped Fiber Laser With ... - IEEE Xplore

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 24, NO. 1, JANUARY/FEBRUARY 2018

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Passively Mode-Locked Tm3+-Doped Fiber Laser With Gigahertz Fundamental Repetition Rate Huihui Cheng, Wei Lin, Zhengqian Luo, Senior Member, IEEE, and Zhongmin Yang

Abstract—We propose and demonstrate a compact passively mode-locked Tm3+ –doped fiber laser with a fundamental repetition rate up to 1.6 GHz. A 5.9 cm home-made, heavily Tm3+ –doped barium gallo-germanate glass fiber with a gain coefficient of 2.3 dB/cm at 1950 nm is employed as gain medium. The compact Fabry-Pérot (FP) laser cavity contains a fiber mirror by directly coating the SiO2 /Ta2 O5 dielectric films on a fiber ferrule. The miniature FP laser pumped by a 793-nm laser diode is passively mode-locked by a semiconductor saturable absorber mirror. Stable self-started mode-locking is successfully achieved at a low pump threshold of 107 mW. The mode-locked operation at the central wavelength of 1959.7 nm has a spectral bandwidth of 12.2 nm, and the pulse duration of 7.2 ps. In particular, the pulse repetition frequency of the fundamental mode-locking can be as high as 1.6 GHz. Moreover, numerical simulation for the 2-µm mode-locked Tm-doped fiber lasers is performed using the lumped model, and the numerical results are in good agreement with the experimental ones. This is, to the best of our knowledge, the highest fundamental repetition frequency for the 2-µm mode-locked fiber lasers. Index Terms—Fiber lasers, mode locking, nonlinear fiber optics, ultrafast optics, thulium.

I. INTRODUCTION IGH repetition rate (>1 GHz) mode-locked Tm-doped fiber lasers (TDFLs) operating at the 2 μm eye-safe wavelength are of great interest for various applications, including high speed optical sampling, high-precision frequency metrology, biomedical imaging, particle accelerators and range finding [1]–[3]. Although harmonic mode locking can easily obtain a higher repetition rate, the large timing jitter and amplitude fluctuation maybe make such harmonically mode-locked lasers

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Manuscript received December 13, 2016; revised December 30, 2016; accepted January 19, 2017. This work was supported in part by the China National Funds for Distinguished Young Scientists under Grant 61325024, in part by the High-level Personnel Special Support Program of Guangdong Province under Grant 2014TX01C087, in part by the Fundamental Research Funds for the Central Universities under Grant 2015ZP019, in part by the China State 863 Hi-tech Program under Grant 2014AA041902, in part by the National Natural Science Foundation of China under Grant 61535014, and in part by China Postdoctoral Science Foundation under Grant 2016M602462. H. Cheng, W. Lin, and Z. Yang are with the State Key Laboratory of Luminescent Materials and Devices and the Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, China, with the Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices, and also with the Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, Guangdong 510640, China (e-mail: [email protected]; [email protected]; [email protected]). Z. Luo is with the Department of Electronic Engineering, Xiamen University, Xiamen 361005, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2017.2657489

unsuitable for practical applications. In contrast, fundamental mode locking is preferred because it often provides a better spectral purity, which is important for high-precision ultrafast photonic applications. Over the past decade, fundamentally mode-locked Er3+ , Yb3+ , and Er3+ /Yb3+ -doped fiber lasers with multi-gigahertz repetition rate have been extensively studied [4]–[11], and subsequently have been applied in frequency combs, optical communication and metrology [1], [12]. Martinez et al. [8] have successfully presented a passively modelocked Er3+ /Yb3+ co-doped fiber laser with a repetition rate of 19.45 GHz, which is the highest fundamental repetition rate at 1.5 μm wavelength range. In the 1 μm spectral region, Chen et al. have demonstrated the 3 GHz fundamentally mode-locked laser using a 1 cm heavily Yb-doped phosphate glass fiber [9]. In order to obtain a high-repetition rate of fundamental mode-locking, the use of high-gain fibers for shortening laser cavity is a key factor extremely. Unfortunately, Tm3+ –doped gain fibers have been suffering from the restrictions of the Tm3+ high-doping concentration and the low OH- content of the glass fibers [13]. Therefore, a gigahertz fundamentally mode-locked TDFLs at 2 μm wavelength has not yet been obtained and reported. Even for the repetition rate of several hundreds of megahertz, very few researches has been reported previously [14]–[17]. Kuan et al. [14] have obtained a 535 MHz fundamental repetition rate in a passively mode-locked Tm-doped double-cladding laser. Wang et al. have realized the mode-locking operation at 982 MHz repetition rate using a piece of highly Tm3+ -doped silicate glass fiber [15]. The sources have demonstrated the advantage of compact all-fiber structure and low noise. However, their spectral 3 dB spectral bandwidths of less than 1 nm are relatively narrow, it could not fully exploit the Tm3+ broad emission of ∼300 nm, for femtosecond pulse generation in mode-locked TDFLs [18]–[26]. Moreover, the mode-locking thresholds in these works were not optimized well. Very recently, our group has proposed and fabricated a highly Tm-doped barium gallo-germanate (BGG) glass fiber, which can provide the outstanding gain coefficient at 2 μm wavelength [27]. The highly Tm-doped BGG glass fiber can be expected as an ideal alternative because of the significant advantages, including high rare-earth solubility, low phonon energy, superior IR transparency, high damage threshold and strong mechanical strength, overcoming these inherent drawbacks of fiber glasses such as silica, silicate and tellurite. Therefore, the highly Tmdoped BGG fiber could be very promising for developing 2 μm mode-locked fiber laser with gigahertz fundamental repetition rate.

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Fig. 1. Experimental setup of the fundamentally mode-locked Tm3+ -doped BGG fiber laser. (a) Optical transmission spectrum of the dielectric films. (b) Highly-reflective dielectric films coated on a fiber ferrule. (c) The semiconductor saturable absorber mirror (SESAM) was put onto the end of fiber ferrule.

In this letter, using a 5.9 cm heavily Tm3+ -doped BGG fiber, we experimentally demonstrated a fundamentally mode-locked TDFLs with the repetition rate up to 1.6 GHz. To the best of our knowledge, this is the first report of gigahertz repetitionrate pulses at the 2 μm wavelength range. The mode-locking threshold is as low as 107 mW by optimizing the coupling ratio of dielectric mirrors in the compact all-fiber resonator. The output optical spectrum has a 3 dB linewidth of 12.2 nm. Moreover, the integrated relative intensity noise and the time jitter for mode locking with high repetition rate are optimized to be 0.04 % and 1.705 ps, respectively. II. FABRICATIONS OF THE BGG THULIUM-DOPED FIBER AND EXPERIMENTAL SETUP Heavily Tm3+ -doped single mode BGG glass fibers used in the experiment were fabricated by the rod-in-tube technique [27]. The Tm3+ doping concentration reaches 4.5 × 1020 cm−3 ions/cm3 , and the gain coefficient of the fiber is 2.3 dB/cm at 1950 nm. La2 O3 and Y2 O3 were added into BGG glass to enhance the glass anti-crystallization ability. Additionally, the optimized Reaction Atmosphere Procedure (RAP) was utilized to minimize OH- content. The TDF has 8.6/125 μm core/cladding diameters with the numerical aperture of 0.145. The group velocity dispersion of the fiber is estimated to be ∼ −100 fs2 /cm. A schematic of the experimental setup is shown in Fig. 1. A piece of 5.9 cm Tm3+ -doped fiber (TDF) as a gain medium was pumped by a 793 nm/250 mW LD through a wavelength division multiplexer (WDM). The TDF was inserted a size-matched ceramic ferrules, both end facets of which were perpendicularly polished. A common port of the WDM was spliced to a fiber pigtail dielectric mirror which was butt-coupled to one end of TDF. The mirror was fabricated by directly coating multiplelayer SiO2 /Ta2 O5 dielectric films onto a fiber ferrule using a plasma sputter deposition system. As seen in Fig. 1(a), the dielectric films has a high transmittance of 91% at the pump wavelength of 793 nm as well as a high reflectivity of ∼90% at the wavelength range of 1.8–2.1 μm (e.g. T = 7.4%@1960 nm). A photograph of the fiber ferrule with the uniform multiple-layer dielectric films is shown in Fig. 1(b).

Fig. 2.

Low intensity spectral reflectance and dispersion of the SESAM.

A semiconductor saturable absorber mirror (SESAM) is exploited with a modulation depth of 12%, a recovery time of 10 ps and a saturation fluence of 65 μJ/cm2 (Batop GmbH). Low intensity reflectance and group delay dispersion (GDD) of the SESAM are showed in Fig. 2. At the wavelength of 1960 nm, the corresponding unsaturated reflectance and the GDD are 85.5% and −28.1 fs2 , respectively. In order to realize compact structure for passive mode-locking at 2 μm, a key step is to compatibly insert the SESAM into the laser cavity. As shown in Fig. 1(c), the SESAM with a chip area of 1.0 × 1.0 mm and the thickness of 450 μm, was put onto the end of a fiber ferrule and then sandwiched between the TDF and the fiber ferrule. Therefore, the length of the Tm3+ -doped fiber of 5.9 cm represents the entire cavity length. The laser is output from the signal port of the WDM. The optical spectrum of the output laser was measured by using an optical spectrum analyzer (YOKOGAWA AQ6375. The pulsed characteristics were detected by using a 12.5 GHz photodetector (PD), together with a 13 GHz bandwidth digital oscilloscope (Keysight DSA91304A), an autocorrelator (APE Pulsecheck USB 50) and a 50 GHz bandwidth radio-frequency (RF) spectrum analyzer (Keysight N9030B). The noise characteristics were detected by using a phase noise analyzer (Rohde&Schwarz FSWP). III. RESEARCH AND DISCUSSIONS Fig. 3 shows the output power of the mode-locked TDFL as increasing the launched pump power. The continuous wave (CW) laser oscillation starts at the pump power of 46 mW. In the 46 ࣘ P ࣘ 107 mW range, the laser operates in Q-switched mode locking and the output power rises linearly with increasing pump power. Above injected pump power P ∼ 107 mW, a continuous-wave mode locking can self-start and the function dependence keeps linear but with slightly higher slope. The maximum power extracted from this laser is 4.5 mW with the 793 nm pump power of 130 mW. Above the pump power, the pulsation can be observed from the oscillator. Compared with the high mode-locked pump threshold in the schemes reported previously [14], [15], the pump-threshold in our experiment is significantly reduced when one also considers the difficulty

CHENG et al.: PASSIVELY MODE-LOCKED TM3+ -DOPED FIBER LASER WITH GIGAHERTZ FUNDAMENTAL REPETITION RATE

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Fig. 3. Variation of the laser output power with the pump power. The modelocked pump threshold observed was as low as 107 mW.

arising from fewer numbers of longitudinal modes in such a short resonant cavity. Fig. 4(a) illustrates the optical spectrum of the CW modelocking as the pump power is increased to 110 mW. Having a width of 12.2 nm (FWHM) and a central wavelength of 1959.7 nm, the mode-locked TDFL in the wavelength range of 1890-2010 nm can be efficiently initiated in the experiment, which mostly covers the emission bandwidth of 3 F4 to 3 H6 level of the Tm3+ ions. Moreover, a distribution of the spectrum shows a quasi-triangle measured with a spectrometer resolution of 0.05 nm, and simulation performed by the lumped mode produce good quantitative agreement with the measurement, revealing how the robust mode-locking with slow saturable absorber here (recovery time of 10 ps for the SESAM). Correspondingly, the system of equation consisted of a generalized nonlinear Schrödinger equation and a rate equation for SESAM that are solved read [28]: β2 ∂ 2 u g ∂2 u g ∂u = −i + iγu2 u + u + 2 ∂z 2 ∂t 2 2Ω2 ∂t2 ∂q q − Δq u2 = −q ∂t τA EA where u is the electric field amplitude, while z and t represent the propagation distance and the retarded time, respectively. β2 and γ denote the dispersion and nonlinearity coefficient of the TDF. g is the saturable gain which is expressed as g = 2 g0 e−( |u | dt)/E S . In addition, g0 and Es account for the small signal gain coefficient and gain saturation energy, respectively. With regard to the equation for the time dependent absorbance q of the SESAM, τA is the relaxation time, Δq is the modulation depth and EA is the saturation energy. The parameters utilized in the simulation are: β2 = − 100 fs2 / cm, γ = 0.8 W−1 km−1 , g0 = 80 m−1 , Es = 90 pJ, Ω = 6.3π THz, q0 = 8%, Δq = 12%, τA = 10 ps, EA = 100 pJ. In order to characterize the details, a portion of the spectra both experiment and simulation within a wavelength range of 1934–1960 nm are magnified in Fig. 4(b). The measurement

Fig. 4. (a) Experimental and simulated optical spectra of the fundamental mode-locking operation measured in a wide span of 200 nm; (b) magnified view of a portion of the spectrum of (a), showing a dip-type sideband to have a wavelength of 1952 nm in the fine structure.

shows a mass of absorption peaks due to the water absorption. In addition, it is interesting to notice that, a dip-type sideband at a wavelength of ∼1952 nm, which is indicated by a black arrow and distinct from the others induced by water absorption, can be clearly observed both in experiment and simulation. This dip-type sideband will disappear unless the model of slow absorber is used in the simulation, resulting from the recovery time of 10 ps for the saturable absorber. This can be attributed to the relaxation dynamics of the slow absorber instead of the parametric four-wave-mixing effect [29]. The pulse train of the oscilloscope trace revealing the laser pulses with a repetition rate of ∼1.6 GHz is depicted in Fig. 5(a), which matches the cavity length of 5.9 cm and indicates that the oscillator operated in fundamental mode-locking. It needs to be emphasized that, the measurement of autocorrelation profile faces a challenge for the current situation with 1.6 GHz pulse repetition rate at 2 μm wavelength range. Because the pulse peak power of the 1.6 GHz high repetition-rate pulses is lower than two orders of one in typical mode locked fiber laser with tens of megahertz repetition rate, it is difficult to be efficiently detected

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Fig. 6. Measured RF spectrum of the fundamental mode-locking operation. Inset: the broad-span RF output spectrum.

Fig. 5. (a) Typical pulse train of fundamental mode-locked TDFL with 1.6 GHz repetition rate; (b) Measured autocorrelation trace of individual pulse (blue square) and Gauss fit trace (red line). In measuring the autocorrelation trace, the average power of output pulses was amplified from 4.5 to 200 mW; (c) Calculated pulse profile and the absorbance response of the SESAM, and the inset shows the calculated autocorrelation trace.

by 2 μm InGaAs photodetector (low sensitivity of 0.1 W2 ) in the autocorrelator. In order to measure the autocorrelation trace, the average output power from the TDFL has to be amplified for reaching the photodetector’s sensitivity of the autocorrelation. In our experiment, the average power of the 1.6 GHz mode-locked pulses was amplified to >200 mW using a Tm-doped doublecladding fiber amplifier. As plotted in Fig. 5(b), the measured autocorrelation trace shows the Gaussian profile, and the pulse duration is estimated to be 7.2 ps with a deconvolution factor of 1.54 together with a significant pedestal. As a comparison, the transform-limited width of the pulse is calculated to be 331 fs

from the optical spectrum of emission line of 12.2 nm. The pulse broadening and the pedestal could be attributed to the amplification process. The Tm-doped double-cladding amplifier could not perfectly amplify the wide mode-locked spectrum, and induces some CW part in the >200 mW average power, leading to the pedestal. Moreover, the amplified pulses with high peak power could experience self-phase modulation and strong dispersion in the long gain fiber of the amplifier. In addition, with respect to the mode-locking started by the slow saturable absorber here, a trailing wing is easily to arise [30]. The simulated pulse and the absorbance response of the SESAM are displayed in Fig. 5(c), and the corresponding autocorrelation trace is showed in the inset. The typical asymmetric absorber response leads to formation of a platform at the trailing edge of the pulse, further resulting in the pulse broadening in the amplification process. Radio-frequency spectrum of the mode-locked Tm-doped fiber laser with 1.6 GHz repetition rate is displayed in Fig. 6. The fundamental frequency is located at the repetition rate of 1.589 GHz with a signal-to-noise ratio (SNR) of up to 70 dB. The SNR is considerably high among previous highrepetition rate mode-locked fiber laser, and illustrate that the noise in the oscillator was well suppressed. Inset to Fig. 6 shows the fundamental repetition frequency and its harmonics out to a frequency range of 12 GHz (limited by the bandwidth of the PD). The clear intensity demonstrated that the laser was well operation at CW mode-locking, without spurious Q-switched instabilities. Without any enclosure of the laser or any other means to prevent ambient disturbance (e.g., air current, vibration, temperature fluctuation), we measured the relative intensity noise (RIN) of the 1.589 GHz laser running at the input pump power of 110 mW, shown as the blue curve in Fig. 7. Also plotted in the same figure (green dashed curve) is the integrated RIN. It can be seen that, integration between 10 Hz and 10 MHz results in an integrated RIN of 0.04%. Moreover, the phase noise of the mode-locked TDFL is also plotted in the Fig. 8. The phase noise gradually decreases from −50 dBc/Hz to −150 dBc/Hz

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2 μm mode-locked TDFL were performed using the lumped model, and the numerical results were in good agreement with the experimental ones. For future work, we will use solid-state amplifier system to decrease the nonlinear effect for optimizing the autocorrelation profile. The laser described here is readily incorporated into existing laser systems of frequency comb with 1.6 GHz line spacing.

REFERENCES

Fig. 7. Relative intensity noise (RIN) (blue curve) of the mode-locked TDFL with 1.6 GHz repetition rate and integrated RIN (green dashed curve).

Fig. 8. The measurement of the phase noise (PN) of the mode-locked TDFL with 1.6 GHz repetition rate.

with increasing the offset frequency from 10 Hz to 10 MHz. Meanwhile, it can be noted that, the timing jitter integrated from 10 MHz down to 10 Hz in the TDFL with high repetition rate could be obtained to be 1.705 ps. IV. CONCLUSION In summary, we have demonstrated a passively mode-locked TDFL with the highest repetition rate of 1.6 GHz among reported Tm3+ –doped lasers. A 5.9 cm home-developed heavily Tm-doped BGG glass fiber was used as the gain medium in the proposed miniature FP laser cavity. The mode-locked TDFL could self-start at a low pump threshold of 107 mW. The optical spectrum with a central wavelength of 1959.7 nm covers a wide wavelength bandwidth, and the pulse width after amplification was measured to be 7.2 ps. The mode locking operation for the all-fiber scheme can keep for a 10 hours stability and long term reliable repeatability. The integrated RIN and timing jitter for the oscillator were measured to be 0.04 % and 1.705 ps, respectively. Moreover, numerical simulations for the

[1] J. Kim, M. J. Park, M. H. Perrott, and F. X. Kärtner, “Photonic subsampling analog-to-digital conversion of microwave signals at 40-GHz with higher than 7-ENOB resolution,” Opt. Express, vol. 16, no. 21, pp. 16509–16515, 2008. [2] A. Schliesser, N. Picqué, and T. W. Hänsch, “Mid-infrared frequency combs,” Nature Photon., vol. 6, no. 7, pp. 440–449, 2012. [3] C. W. Rudy, M. J. F. Digonnet, and R. L. Byer, “Advances in 2-μm Tm-doped mode-locked fiber lasers,” Opt. Fiber Technol., vol. 20, no. 6, pp. 642–649, 2014. [4] K. Wu, X. Zhang, J. Wang, and J. Chen, “463-MHz fundamental modelocked fiber laser based on few-layer MoS2 saturable absorber,” Opt. Lett., vol. 40, no. 7, pp. 1374–1377, 2015. [5] J. J. McFerran, L. Nenadovic, W. C. Swann, J. B. Schlager, and N. R. Newbury, “A passively mode-locked fiber laser at 1.54 μm with a fundamental repetition frequency reaching 2 GHz,” Opt. Express, vol. 15, no. 20, pp. 13155–13166, 2007. [6] X. Wei, S. Xu, H. Huang, M. Peng, and Z. Yang, “Compact all-fiber ring femtosecond laser with high fundamental repetition rate,” Opt. Express., vol. 20, no. 22, pp. 24607–24613, 2012. [7] A. Martinez and S. Yamashita, “10 GHz fundamental mode fiber laser using a graphene saturable absorber,” Appl. Phys. Lett., vol. 101, no. 4, 2012, Art. no. 041118. [8] A. Martinez and S. Yamashita, “Multi-gigahertz repetition rate passively mode locked fiber lasers using carbon nanotubes,” Opt. Express, vol. 19, no. 7, pp. 6155–6163, 2011. [9] H. –W. Chen, G. Chang, S. Xu, Z. Yang, and F. X. Kärtner, “3 GHz, fundamentally mode-locked, femtosecond Yb-fiber laser,” Opt. Lett., vol. 37, no. 17, pp. 3522–3524, 2012. [10] H. Byun et al., “Compact, stable 1 GHz femtosecond Er-doped fiber lasers,” Appl. Opt., vol. 49, no. 29, pp. 5577–5582, 2010. [11] R. Thapa, D. Nguyen, J. Zong, and A. Chavez-Pirson, “All-fiber fundamentally mode-locked 12 GHz laser oscillator based on an Er/Yb-doped phosphate glass fiber,” Opt. Lett., vol. 39, no. 6, pp. 1418–1421, 2014. [12] C. -H. Li et al., “A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s−1 ,” Nature, vol. 452, no. 7187, pp. 610–612, 2008. [13] S. D. Jackson, “Cross relaxation and energy transfer upconversion processes relevant to the functioning of 2 μm Tm3+ -doped silica fiber lasers,” Opt. Commun., vol. 230, no. 1-3, pp. 197–203, 2004. [14] P. -W. Kuan et al., “0.5-GHz repetition rate fundamentally Tm-doped mode-locked fiber laser,” IEEE Photon. Technol. Lett., vol. 28, no. 14, pp. 1525–1528, Jul. 2016. [15] Q. Wang, J. Geng, T. Luo, and S. Jiang, “2 mode-locked fiber lasers,” in Proc. SPIE, 2012, vol. 8237, pp. 82371N-1–8237N-8. [16] J. Liu, J. Xu, K. Liu, F. Tan, and P. Wang, “High average power picosecond pulse and supercontinuum generation from a thulium-doped, all-fiber amplifier,” Opt. Lett., vol. 38, no. 20, pp. 4150–4153, 2013. [17] J. Jiang et al., “500 MHz, 58 fs highly coherent Tm fiber soliton laser,” in Proc. Conf. Lasers Electro-Opt., 2012, pp. 1–2. [18] R. C. Sharp, D. E. Spock, N. Pan, and J. Elliot, “190-fs passively modelocked thulium fiber laser with a low threshold,” Opt. Lett., vol. 21, no. 12, pp. 881–883, 1996. [19] M. Zhang et al., “Tm-doped fiber laser mode-locked by graphene-polymer composite,” Opt. Express, vol. 20, no. 22, pp. 25077–25084, 2012. [20] Z. Yan et al., “Widely tunable Tm-doped mode-locked all-fiber laser,” Sci. Rep., vol. 6, 2016, Art. no. 27245. [21] C. W. Rudy, K. E. Urbanek, M. J. F. Digonnet, and R. L. Byer, “Amplified 2-μm Thulium-doped all-fiber mode-locked figure-eight laser,” J. Lightw. Technol., vol. 31, no. 11, pp. 1809–1812, Jun. 2013. [22] F. Haxsen, A. Wienke, D. Wandt, J. Neumann, and D. Kracht, “Tm-doped mode-locked fiber lasers,” Opt. Fiber Tech., vol. 20, no. 6, pp. 650–656, 2014.

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[23] B. Sun, J. Luo, B. P. Ng, and X. Yu, “Dispersion-compensation-free femtosecond Tm-doped all-fiber laser with a 248 MHz repetition rate,” Opt. Lett., vol. 41, no. 17, pp. 4052–4055, 2016. [24] G. Sobon et al., “260 fs and 1 nJ pulse generation from a compact, mode-locked Tm-doped fiber laser,” Opt. Express, vol. 23, no. 24, 2015, Art. no. 031446. [25] Y. Tang, A. Chong, and F. W. Wise, “Generation of 8 nJ pulses from a normal-dispersion thulium fiber laser,” Opt. Lett., vol. 40, no. 10, pp. 2361– 2364, 2015. [26] J. Ma et al., “Graphene mode-locked femtosecond laser at 2 μm wavelength,” Opt. Lett., vol. 37, no. 11, pp. 2085–2087, 2012. [27] X. Wen et al., “Tm3+ doped barium gallo-germanate glass single-mode fibers for 2.0 μm laser,” Opt. Express, vol. 23, no. 6, pp. 7722–7731, 2015. [28] H. Kotb, M. Abdelalim, and H. Anis, “Generalized analytical model for dissipative soliton in all-normal-dispersion mode-locked fiber laser,” IEEE J. Sel. Topics Quantum Electron., vol. 22, no. 2, Mar./Apr. 2016, Art. no. 1100209. [29] L. M. Zhao et al., “Observation of dip-type sideband in a soliton fiber laser,” Opt. Commun., vol. 283, no. 2, pp. 340–343, 2010. [30] F. X. Kärtner, J. A. der Au, and U. Keller, “Mode-locking with slow and fast saturable absorbers-what’s the difference?,” IEEE J. Sel. Topics Quantum Electron., vol. 4, no. 2, pp. 159–168, Mar./Apr. 1998.

Huihui Cheng was born in Henan, China, in 1986. He received the B.S. degree in electronic science and technology from PLA information Engineering University, Zhengzhou, China, in 2009, and the Ph.D. degrees in electronic engineering from Xiamen University, Xiamen, China, in 2015. In 2013, he received the Chinese Government Overseas Study Scholarship and joined the Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, IL, USA. He is currently a Postdoctoral Researcher in South China University of Technology, Guangzhou, China. His research interests include the ultrafast fiber lasers with high repetition rate and their practical applications.

Wei Lin was born in Jiangxi, China, in 1988. He received the M.S. degree in material physics from Wuhan University of Technology, Wuhan, China. He is currently working toward the Ph.D. degree in optics at South China University of Technology, Guangzhou, China. His research interests include mode locked fiber laser and nonlinear dynamics, e.g., dissipative soliton, dissipative soliton resonance, and so on.

Zhengqian Luo (M’11–SM’15) was born in Hubei, China, in 1982. He received the B.S. degree in applied physics from Harbin Institute of Technology, Harbin, in 2004, and the Ph.D. degree in electronic and communication engineering from Xiamen University, Xiamen, China, in 2009. From 2007 to 2009, he received the Chinese Government Overseas Study Scholarship and joined the Network Technology Research Center, Nanyang Technological University, Singapore. He is currently an Associate Professor in the School of Information Science and Technology, Xiamen University, China. He is currently working in Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA as a one-year Visiting Professor. His research interests include nonlinear optics, optical fiber amplifiers and lasers, nanomaterial photonics, and on-chip waveguides. He is an author or coauthor of more than 100 international journal and conference papers.

Zhongmin Yang received the M.Sc. and Ph.D. degrees from the Wuhan University of Technology, Wuhan, China, in 1999 and 2002, respectively. From 2002 to 2004, he was a Postdoctoral Researcher in the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai. He is currently a Professor in the Institute of Optical Communication Materials and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, China. His current research interests include areas of luminescent materials and laser physics.