Vector Soliton Generation in a Tm Fiber Laser - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 8, APRIL 15, 2014

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Vector Soliton Generation in a Tm Fiber Laser Yong Wang, Siming Wang, Jiaolin Luo, Yanqi Ge, Lei Li, Dingyuan Tang, Deyuan Shen, Shumin Zhang, Frank W. Wise, and Luming Zhao, Senior Member, IEEE

Abstract— Vector soliton generation is experimentally demonstrated in an all-fiber Tm/Ho-doped fiber laser operated at 1951 nm. To the best of our knowledge, it is the first clear evidence of group-velocity-locked vector soliton formation around 2 µm. Numerical simulation well reproduces the experimental observation and suggests that the central wavelength shift between the two orthogonal polarized components of the vector soliton is determined by the cavity birefringence. The central wavelength shift increased with the increasing of the cavity birefringence. However, the vector solitons become linearly polarized if the cavity birefringence is too strong. Experimental observation and numerical simulations both suggest that the vector soliton generation is wavelength independent. Index Terms— Vector soliton, optical fiber lasers, ultrafast optics.

I. I NTRODUCTION ECTOR solitons refer to solitons with multiple components trapped together and propagating with same group velocity in the media. Except the polarization maintaining fiber, generally speaking, fibers have weak birefringence. In another word, there are two orthogonal polarization directions in a fiber. Therefore, vector solitons could be generated in fibers. Curtis R. Menyuk first predicted the existence of vector solitons in optical fibers [1], [2]. The central frequency shift between orthogonal polarizations is able to lock the two polarizations together. That is, regardless of the intrinsic group velocity difference caused by the fiber birefringence, two solitons formed along each polarization direction of a weakly birefringent fiber can trap each other and propagate as a non-dispersive unit.

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Manuscript received January 10, 2014; revised February 6, 2014; accepted February 10, 2014. Date of publication February 12, 2014; date of current version March 20, 2014. This work was supported in part by the National Natural Science Foundation of China under Grant 61275109 and in part by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Y. Wang and D. Shen are with the School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China, also with the Jiangsu Key Laboratory of Advanced Laser Materials and Devices, Jiangsu 212013, China, and also with the Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China (e-mail: [email protected]; [email protected]). S. Wang, J. Luo, Y. Ge, L. Li, D. Tang, and L. Zhao are with the School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China, and also with the Jiangsu Key Laboratory of Advanced Laser Materials and Devices, Jiangsu 212013, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). S. Zhang is with the College of Physics Science and Information Engineering, Hebei Advanced Thin Films Laboratory, Hebei Normal University, Shijiazhuang 050024, China (e-mail: [email protected]). F. W. Wise is with the Department of Applied Physics, Cornell University, Ithaca, NY 14853 USA (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2305988

Soliton trapping effect has been experimentally demonstrated either in optical fibers [3], [4], or in fiber lasers [5]–[7]. Solitons generated in fiber lasers are different from those formed in fibers. Apart from the balanced interaction between the cavity dispersion effect and the fiber nonlinear effect imposed on the pulses propagating in the fiber laser cavity, solitons generated in fiber lasers are subject to the dynamic balance between the cavity gain and loss, as well as the cavity boundary condition. If the fiber laser is operated in the anomalous dispersion regime, characteristic sideband structure appears on the optical spectrum due to the periodic discrete perturbations of the cavity components [8]. The sideband position on the soliton spectrum is determined by the soliton parameters and the net cavity dispersion. Generally the sidebands are narrow and pair-wise symmetric to the soliton peak wavelength. Therefore, the sidebands could function as a good indicator to suggest the wavelength/frequency shift between the solitons along the two orthogonal polarizations. Zhao et al have experimentally observed and numerically confirmed that depending on the net cavity birefringence, there may exist two sets of soliton sidebands on the vector soliton spectrum in an erbium-doped fiber laser operated in the anomalous dispersion regime around 1.55 μm [5]. The wavelength shift between the same order sidebands depends on the cavity birefringence. For fiber lasers operated in the normal dispersion regime, wavelength shift as-well exists [6] even though there are no sidebands on the soliton spectrum and the solitons are chirped. Recently the experimental observation of vector solitons with locked and precessing states of polarization for fundamental and multipulse soliton operations [9], and soliton molecules [10] are reported in a carbon nanotube mode-locked fiber laser with anomalous dispersion laser cavity. The wavelength regime around 2 μm is part of the so-called “eye safe” wavelength region. Laser systems operated in this region have exceptional advantage for free space applications, as well as for medical applications due to the strong water absorption. As the fiber birefringence is caused by deviations of the core shape from circularity, by transverse internal stress, or by residual twist especially in a fiber laser cavity, it is desired to know whether the vector solitons could be generated in this wavelength regime or not. In this letter we report the vector soliton generation in an all-fiber Tm/Ho-doped fiber laser operated around 1950 nm. To the best of our knowledge, it is the first clear evidence of group-velocity-locked vector soliton formation around 2 μm. We numerically demonstrate that the central wavelength shift between the two orthogonally polarized components of the vector soliton is determined by the cavity birefringence. With moderate fiber birefringence,

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Fig. 1. Schematic of the fiber laser. SESAM: semiconductor saturable absorber mirror; WDM: wavelength-division multiplexer; Tm/Ho-DF: Thulium/Holmium-doped fiber; PC: polarization controller.

two sets of soliton sidebands appear on the vector soliton spectrum. Much stronger birefringence would cause the vector soliton evolved into linear polarization. Experimental observation and numerical simulations both suggest that the vector soliton generation is wavelength independent. II. E XPERIMENTAL S ETUP AND R ESULTS Fig. 1 shows the fiber laser used. Linear cavity scheme is used due to the practical lack of the isolator for ring cavity design. A semiconductor saturable absorber mirror (SESAM from Batop) is exploited to achieve mode locking. The nonsaturable loss of the SESAM is 8% and the modulation depth is about 10%. The saturation fluence is 35 uJ/cm2 . The relaxation time is about 10 ps. The reflection band is from 1900 to 2060 nm. The Thulium/Holmium-doped fiber (Tm/Ho-DF) is from Coractive (TH512). We use 2.8m TH512 as the gain medium, all the other fibers used are the standard single mode fibers (SMFs). An optical loop mirror made of a 50:50 fiber coupler is used as both the partially reflected mirror and the output. A fiber-based polarization controller is inserted in the loop mirror to modify the output ratio. A single frequency fiber laser (NKT Koheras Boostik) is used to pump the laser with pump power up to 5 W. The reverse pumping theme is adopted to avoid the overdriving of the SESAM resulted from the residual pump power. The pump is introduced into the cavity by a wavelength-division-multiplexer (WDM) and the SESAM is butt-coupled to the other fiber branch of the WDM. With appropriate PC setting, self-started mode-locking of the laser is achieved by simply increasing the pump power above the mode-locking threshold. Or the mode locking could be obtained by tuning the PC under certain pump power (generally around 1.5 W). Generally, the so-called “noise-like” pulses [11], [12] are obtained: there is only one bunch existing in the cavity. The bunch consists of developing pulses with random pulse width and pulse intensity but trapping together. The spectrum of the “noise-like” pulse is broad and unstable as shown in Fig. 2(a). With carefully tuning of the paddles of the PC after the “noise-like” pulse is obtained, vector solitons as shown in Fig. 2(b) are achieved. Once the vector solitons are obtained, they are self-started. Fig. 2(c) shows the pulse train recorded by an EOT photodetector (ET-5000) with bandwidth of 9 GHz. There is fluctuation on the pulse intensity. Experimentally we tried to reduce the pump power

Fig. 2. Optical spectrum of (a) noise-like pulse; (b) vector soliton; (c) oscilloscope trace; (d) autocorrelation trace of the vector soliton (inset: RF spectrum).

to make the pulses more stable. However, the pulses always died out before a more stable pulse train could be obtained. Increasing the pump power leads the vector soliton into the state of “noise-like” pulse. Fig. 2(d) shows the inteferometric autocorrelation of the vector soliton. The pulse duration is about 526 fs. The inset of Fig. 2(d) shows the RF spectrum. The vector soliton feature is self-elaborated by its optical spectrum as shown in Fig. 2(b): the sideband structure appears on the optical spectrum while each sideband has two peaks. That is, there are two sets of sideband structure existing on the soliton spectrum. The separation between the two sets of sideband structure is about 1 nm. The 3-dB bandwidth of the vector soliton is 4.3 nm, the output power is about 40 mW, and the pulse repetition rate is 25.9 MHz.

WANG et al.: VECTOR SOLITON GENERATION IN A Tm FIBER LASER

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III. N UMERICAL S IMULATIONS AND R ESULTS Due to the lack of necessary components, for example, the polarization beam splitter at 2 μm, we could not experimentally resolve the polarizations of the vector soliton. Therefore, we focus on the numerical simulation to reproduce the experimental observation. Based on the coupled Ginzburg-Landau equations (GLEs) and the pulse tracing technique described in [15], we carry out the numerical simulations on the laser performance. In principle, the light propagation in the birefringent fiber segments is determined by the GLEs. To simplify the simulation, we consider the loop mirror as the combination of pulse propagation and an output coupler. The performance of the SESAM is described by the rate equation [14]: 2 2 u + v ls − l0 ∂ls =− − ls ∂t Trec E sat where Trec is the absorption recovery time, l0 is the initial saturable absorption of the SESAM and E sat is the absorber saturation energy. Numerically the following parameters are used: E sat = 80 pJ, l0 = 0.12, Trec = 10 ps, the cavity length L = 7.8 m, the cavity dispersion is −0.89 ps2 , and the output ratio is 10%. Numerically we find that similar to the performance of vector soliton generated at the wavelength around 1.55 μm [5], the mode-locked pulse could have different optical spectra depending on the cavity birefringence. Fig. 3 shows the vector soliton obtained when the cavity beat length (Lb ) is 0.156 m. The total optical spectrum is shown in Fig. 3(a), where two sets of sidebands appear. Fig. 3(b) shows the optical spectrum of the vector soliton along different polarization directions. The soliton along different polarization directions has its own sidebands and different central wavelength, therefore, resulting in double sets of sidebands on the vector soliton spectrum. There are small peak-dip structure appearing between different sideband orders, which is due to the sub-sideband generation [15] and agrees well with the experimental observation as shown in Fig. 2(b). Fig. 3(c) shows the temporal profile of the vector soliton and its components along different polarization directions. Although the two components along different polarization directions have different central wavelength, they are trapped together and propagate as a unit in the cavity. The wavelength shift is about 1 nm, which agrees well with the experimental observation. Fig. 4 shows the optical spectrum of the vector soliton obtained with same simulation parameters as that of Fig. 3 except the beat length. Fig. 4(a)–(c) show the optical spectrum of the vector soliton along different polarization directions when the beat length is 3.9 m, 0.39 m, and 0.039 m, respectively. It is clear that for weak birefringence there is tiny wavelength shift or even no wavelength shift between the orthogonal components of a vector soliton, that is, the generated vector soliton is a coherent vector soliton. For larger fiber birefringence, the incoherent vector soliton appears and the wavelength shift between the components of the vector soliton increase with the birefringence.

Fig. 3. (a) Optical spectrum of the numerically obtained vector soliton; (b) optical spectrum of the numerically obtained vector soliton along different polarization directions; (c) temporal profile of the numerically obtained vector soliton and its components along different polarization directions when Lb = 0.156 m.

IV. D ISCUSSION Due to the lack of necessary measurement device at 2 μm, we can’t distinguish whether there is only one pulse in the cavity or not. The pulse energy would be about 1.5 nJ if there is only one pulse in the cavity. However, the soliton peak power of 2852 W is far larger than the fundamental soliton peak power of 431 W. Therefore, there are multiple solitons existing in the cavity but appearing as one bunch. As the central wavelength is around 1950 nm, which is away from the water absorption regime of less than 1900 nm, there are no water absorption peaks on the spectrum. The fluctuation of the pulse train is caused by the coexisting background. We suspect that the SESAM is not good at extinguishing the background while it still could achieve mode locking. The typical fiber beat length is about 20 m at 2 μm and the typical birefringence for a polarization-maintained fiber is about 10−4 , which corresponds to ∼20 mm beat length at 2 μm. Numerically we also simulate the case for beat length of 3.9 mm, the output is a linearly polarized pulse along the horizontal polarization direction. Therefore, the simulation results suggest that: for a general fiber there is tiny wavelength shift [as shown in Fig. 4(a)] or even no wavelength shift between the two components of the vector soliton generated; for a PM fiber or fiber with strong birefringence, the

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as we know. Numerically we reproduced the experimental observations and found that, similar to the performance of vector solitons generated in the 1.55 μm spectral range, the wavelength shift between the two orthogonal polarizations of the vector soliton generated in the 2 μm spectral range is as-well determined by the cavity birefringence. Clear wavelength shift could only be achieved with moderate cavity birefringence. The experimental observation and numerical simulations both suggest that the vector soliton generation is an intrinsic property of fiber lasers mode locked by SESAM. Carefully cavity birefringence design should be taken into account for specific applications if the vector soliton generation is an issue. R EFERENCES

Fig. 4. Optical spectrum of the numerically obtained vector soliton along different polarization directions (a) Lb = 3.9 m; (b) Lb = 0.39 m; (c) Lb = 0.039 m.

wavelength shift is too large, i. e. the group velocity difference caused by the cavity birefringence can’t be compensated by the wavelength-difference-induced group velocity difference. Hence only linear polarized pulse could be generated and no trapping between different polarizations could be achieved. The vector solitons could only be generated under moderate birefringence, here corresponds to beat length between 0.39 m and 0.039 m, for example. Vector soliton bunch was observed in a Thulium-Holmium fiber laser mode locked by a saturable absorber in 2 μm spectral range recently [16]. However, due to the coexistence of slow and fast absorption of the saturable absorber, the solitons generated tend to oscillate within the bunch [17]. Therefore, there are no wavelength shift could be distinguished from the optical spectrum of the generated vector soliton bunch. To the best of our knowledge, the results reported in this letter are the first evidential observation of group-velocity-locked vector soliton generation in lasers operated around 2 μm. V. C ONCLUSION Vector solitons with characteristic wavelength shift, indicated by the appearance of two sets of sidebands, are observed for the first time in a fiber laser operated around 2 μm, so far

[1] C. R. Menyuk, “Stability of solitons in birefringent optical fibers. I: Equal propagation amplitudes,” Opt. Lett., vol. 12, no. 8, pp. 614–616, Aug. 1987. [2] C. R. Menyuk, “Stability of solitons in birefringent optical fibers. II. Arbitrary amplitudes,” J. Opt. Soc. Amer. B, Opt. Phys., vol. 5, no. 2, pp. 392–402, Feb. 1988. [3] M. N. Islam, C. D. Poole, and J. P. Gordon, “Soliton trapping in birefringent optical fibers,” Opt. Lett., vol. 14, no. 18, pp. 1011–1013, Sep. 1989. [4] E. Korolev, V. N. Nazarov, D. A. Nolan, and C. M. Truesdale, “Experimental observation of orthogonally polarized time-delayed optical soliton trapping in birefringent fibers,” Opt. Lett., vol. 30, no. 2, pp. 132–134, Feb. 2005. [5] L. M. Zhao, D. Y. Tang, H. Zhang, X. Wu, and N. Xiang, “Soliton trapping in fiber lasers,” Opt. Express, vol. 16, no. 13, pp. 9528–9533, Jun. 2008. [6] L. M. Zhao, D. Y. Tang, X. Wu, and H. Zhang, “Dissipative soliton trapping in normal dispersion-fiber lasers,” Opt. Lett., vol. 35, no. 11, pp. 1902–1904, Jun. 2010. [7] X. Yuan, et al., “Experimental observation of vector solitons in a highly birefringent cavity of ytterbium-doped fiber laser,” Opt. Express, vol. 21, no. 20, pp. 23866–23872, Oct. 2013. [8] S. M. J. Kelly, “Characteristic sideband instability of periodically amplified average soliton,” Electron. Lett., vol. 28, no. 8, pp. 806–807, Apr. 1992. [9] S. V. Sergeyev, C. Mou, A. Rozhin, and S. K. Turitsyn, “Vector solitons with locked and precessing states of polarization,” Opt. Express, vol. 20, no. 24, pp. 27434–27440, Nov. 2012. [10] V. Tsatourian, et al., “Polarisation dynamics of vector soliton molecules in mode locked fibre laser,” Sci. Rep., vol. 3, pp. 1–8, Nov. 2013. [11] M. Horowitz, Y. Barad, and Y. Silberberg, “Noiselike pulses with a broadband spectrum generated from an erbium-doped fiber laser,” Opt. Lett., vol. 22, no. 11, pp. 799–801, Jun. 1997. [12] Q. Wang, T. Chen, B. Zhang, A. P. Heberle, and K. P. Chen, “All-fiber passively mode-locked thulium-doped fiber ring oscillator operated at solitary and noiselike modes,” Opt. Lett., vol. 36, no. 19, pp. 3750–3752, Oct. 2011. [13] D. Y. Tang, L. M. Zhao, B. Zhao, and A. Q. Liu, “Mechanism of multisoliton formation and soliton energy quantization in passively modelocked fiber lasers,” Phys. Rev. A, vol. 72, no. 4, pp. 043816-1–043816-9, Oct. 2005. [14] N. N. Akhmediev, A. Ankiewicz, M. J. Lederer, and B. Luther-Davies, “Ultrashort pulses generated by mode-locked lasers with either a slow or a fast saturable-absorber response,” Opt. Lett., vol. 23, no. 4, pp. 280–282, Feb. 1998. [15] D. Y. Tang, S. Fleming, W. S. Man, H. Y. Tam, and M. S. Demokan, “Subsideband generation and modulational instability lasing in a fiber soliton laser,” J. Opt. Soc. Amer. B, Opt. Phys., vol. 18, no. 10, pp. 1443–1450, Oct. 2001. [16] R. Gumenyuk, M. S. Gaponenko, K. V. Yumashev, A. A. Onushchenko, and O. G. Okhotnikov, “Vector soliton bunching in thulium-holmium fiber laser mode locked with PbS quantum-dot-doped glass absorber,” IEEE J. Quantum Electron., vol. 48, no. 7, pp. 903–907, Jul. 2012. [17] L. M. Zhao, D. Y. Tang, H. Zhang, and X. Wu, “Bunch of restless vector solitons in a fiber laser with SESAM,” Opt. Express, vol. 17, no. 10, pp. 8103–8108, May 2009.