Dissipative solitons in a passively mode-locked Er ... - OSA Publishing

Dissipative solitons in a passively mode-locked Er-doped fiber with strong normal dispersion A. Cabasse1, B. Ortaç2, G. Martel1, A. Hideur1, and J. Limpert2 1

Groupe d’Optique et d’Optronique, CORIA UMR 6614, Université de Rouen, Avenue de l’université Boîte Postale 12, 76801 Saint Etienne du Rouvray CEDEX, France 2 Institute of Applied Physics, Friedrich Schiller University Jena, Albert-Einstein-Strasse 15, D-07745 Jena, Germany [email protected]

Abstract: We report on ultrashort pulse generation from a passively modelocked erbium fiber laser operating in the highly positive dispersion regime. Highly-chirped pulses with 5.3 ps duration and spectral bandwidth of 8.3 nm are generated. They are extra-cavity compressed down to 757 fs. Numerical simulations confirm the experimental results and show that these pulses could be interpreted as dissipative solitons. ©2008 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators (140.3510) Lasers, fiber; (140.4050) Mode-locked lasers.

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B. Ortaç, O. Schmidt, T. Schreiber, J. Limpert, A. Tünnermann, and A. Hideur, "High-energy femtosecond Yb-doped dispersion compensation free fiber laser," Opt. Express 15, 10725-10732 (2007). C. Lecaplain, C. Chédot, A. Hideur, B. Ortaç, and J. Limpert, "High-power all-normal-dispersion femtosecond pulse generation from an Yb-doped large-mode-area microstructure fiber laser," Opt. Lett. 32, 2738-2740 (2007) F. Ö. Ilday, J. R. Buckley, W. G. Clark, and F. W. Wise, "Self-Similar Evolution of Parabolic Pulses in a Laser," Phys. Rev. Lett. 92, 213902 (2004). T. Schreiber, B. Ortaç, J. Limpert, and A. Tünnermann, "On the study of pulse evolution in ultra-short pulse mode-locked fiber lasers by numerical simulations," Opt. Express 15, 8252-8262 (2007). J. An, D. Kim, J. W. Dawson, M. J. Messerly, and C. P. J. Barty, "Grating-less, fiber-based oscillator that generates 25 nJ pulses at 80 MHz, compressible to 150 fs," Opt. Lett. 32, 2010-2012 (2007). A. Chong, W. H. Renninger, and F. W. Wise, "All-normal-dispersion femtosecond fiber laser with pulse energy above 20 nJ," Opt. Lett. 32, 2408-2410 (2007). A. Ruehl, V. Kuhn, D. Wandt, and D. Kracht, "Normal dispersion erbium-doped fiber laser with pulse energies above 10 nJ," Opt. Express 16, 3130-3135 (2008). R. Herda and O. G. Okhotnikov, "Dispersion compensation-free fiber laser mode-locked and stabilized by high-contrast saturable absorber mirror," IEEE J. Quantum Electron. 40, 893-899 (2004). L. M. Zhao, D. Y. Tang, and J. Wu, "Gain-guided soliton in a positive group-dispersion fiber laser," Opt. Lett. 31, 1788-1790 (2006). L. M. Zhao, D. Y. Tang, T. H. Cheng, and C. Lu, "Gain-guided soliton in dispersion-managed fiber lasers with large net cavity dispersion," Opt. Lett. 31, 2957-2959 (2006). L. M. Zhao, D. Y. Tang, H. Zhang, T. H. Cheng, H. Y. Tam, and C. Lu, "Dynamics of gain-guided solitons in an all-normal-dispersion fiber laser," Opt. Lett. 32, 1806-1808 (2007). A. Chong, J. Buckley, W. Renninger, and F. Wise, "All-normal-dispersion femtosecond fiber laser," Opt. Express 14, 10095-10100 (2006). A. Chong, W. H. Renninger, and F. W. Wise, "Properties of normal-dispersion femtosecond fiber lasers," J. Opt. Soc. Am. B 25, 140-148 (2008). W. H. Renninger, A. Chong, and F. Wise, “Dissipative solitons in normal-dispersion fiber lasers”, Phys. Rev. A 77, 023814 (2008). D. Y. Tang, L. M. Zhao, G. Q. Xie, and L. J. Qian, "Coexistence and competition between different solitonshaping mechanisms in a laser," Phys. Rev. A 75, 063810 (2007). see e.g. Dissipative Solitons: From Optics to Biology and Medicine, N. Akhmediev; A. Ankievicz Eds, Springer (2008). G. Martel, C. Chédot, V. Réglier, A. Hideur, B. Ortaç, and Ph. Grelu, "On the possibility of observing bound soliton pairs in a wave-breaking-free mode-locked fiber laser," Opt. Lett. 32, 343-345 (2007). 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. 23, 280-282 (1998).

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Received 28 Jul 2008; revised 6 Sep 2008; accepted 10 Sep 2008; published 7 Nov 2008

10 November 2008 / Vol. 16, No. 23 / OPTICS EXPRESS 19322

19. P. A. Belanger, L. Gagnon, and C. Pare, "Solitary pulses in an amplified nonlinear dispersive medium," Opt. Lett. 14, 943-945 (1989). 20. L. M. Zhao, D. Y. Tang, X. Wu, H. Zhang, C. Lu, H. Y. Tam, "Dynamics of gain-guided solitons in a dispersion-managed fiber laser with large normal cavity dispersion" Opt. Commun. 281, 3324-3326 (2008).

1. Introduction Many practical as well as fundamental research applications request compact, reliable, efficient and low-cost laser sources capable of delivering highly energetic ultrashort pulses. Fiber-based lasers are promising systems for fulfilling these requirements in the very near future. Recent demonstration of such sources operating in the 1-µm range with deliverable energy per pulse approaching or even exceeding 100 nJ [1,2] have made them henceforth available now and fully competitive with their bulk-based counterparts like the more standard Ti-Sa based femtosecond oscillators. The main concept that allows energy scaling in mode-locked fiber lasers depends on group velocity dispersion (GVD) management to operate with large net normal cavity GVD. In particular, it has been demonstrated that pulse shaping in normal GVD fibres is favourable for the generation of self-similar pulses in fibre oscillators [3, 4]. Indeed, the interplay between normal dispersion, self-phase modulation (SPM) and gain leads to the propagation of linearly chirped pulses which are more resistant to the restrictions imposed by the fibre nonlinearities [3,4]. These demonstrations have attracted much interest in the development of purely normal dispersion fibre lasers in the last few years leading to a significant improvement in the energy extracted from fibre oscillators [5-7]. Herda et al. [8] have reported a first demonstration of picoseconds pulse generation from a purely normal GVD fiber laser. This laser consisted in a short Yb-fiber with positive GVD and used a high-modulation depth saturable absorber mirror (SAM) for mode-locking. The second important contribution was the report of Zhao et al. [911] of the so-called gain-guided soliton formation in an all-normal dispersion fiber laser. The all-normal dispersion fiber laser concept has been significantly improved by Chong et al. [12, 13] by adding a spectral filter inside the cavity leading to the achievement of more than 20 nJ energy in step index fiber lasers [6]. Pulse shaping in these lasers arises from the spectral filter, which converts frequency chirp to self-amplitude modulation. A wide variety of pulse shapes are obtained with variation of the filter bandwidth [13] and have been interpreted in terms of highly chirped dissipative solitons in the frame of the cubic-quintic Ginzburg-Landau equation [14]. The spectral filter bandwidth being much narrower than the gain bandwidth, gain dispersion did not play a key role in pulse shaping in such lasers [13]. The situation seems different in the laser of Zhao et. al. where pulse shaping is dominated by the saturable absorber nonlinearity whereas the intra-cavity pulse dynamics and its output characteristics are highly dependent on the gain saturation and dispersion [11]. Such gainguided solitons have been observed in purely positive dispersion fiber lasers as well as in dispersion managed fiber lasers [9, 10] and are often associated with gain-bandwidth limited pulses. In addition, such solitons seemed to suffer from low dechirped capabilities [10, 11]. Recently, it has been shown that these gain-guided solitons could be obtained in a dispersion managed erbium laser with nearly zero net cavity dispersion [15]. In particular, it has been shown that these gain-guided solitons can coexist with the dispersion-managed solitons in the same cavity configuration. The transition from one regime to another can be obtained by adjusting the intra-cavity pulse peak power. In addition, recent experimental and numerical studies of all-normal dispersion large mode area fiber lasers show that the gain dispersion effect could manifest even for mode-locking regimes with spectral widths much narrower than the gain bandwidth [1]. So, one wonders if gain-guided solitons with spectral widths much narrower than the gain bandwidth could exist. In this contribution, we report on a passively mode-locked dispersion-managed erbiumfiber laser with large net cavity dispersion. The laser generates positively chirped 5.3 ps output pulses that are compressed down to 757 fs. The optical spectrum is characterized by #99506 - $15.00 USD

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steep edges and a narrow width of 8.3 nm, which is far below the gain-bandwidth of erbium. Numerical simulations show that in addition to the SAM nonlinearity, the gain and gain dispersion play a key role in the formation of these pulses. Then our results confirm the tendency that gain-guided solitons belong to a more general class of so-called ‘dissipative solitons’ as highlighted in recent publication [14] or book [16]. Such term is used hereafter in order to generalize discussion. 2. Experiments and results The experimental set-up of the laser is drawn on Fig. 1. It consists in a short heavily doped erbium fiber (HDEF) laser presenting high normal GVD. The net cavity dispersion is highly positive thanks to the minimization in length of all other passive fibered components which are restricted, in our case, solely to the pigtails of a wavelength division multiplexer (WDM). The laser cavity is mounted in a sigma configuration using a polarization-sensitive optical isolator. The 1.15 m HDEF presents an unpumped absorption of 80 dB/m at 1530 nm. Its GVD has been estimated at -48 (ps/nm)/km at 1550 nm. Pigtails of the WDM are made up of Hi1060 fibers with a measured GVD of 8.7 (ps/nm)/km. The net positive cavity dispersion is β2 ≈ + 0.063 ps² around 1550 nm. Rest of the cavity comprises bulk isolator, wave-plates, coupling lenses and commercial SAM (Batop 1550-23). The total optical cavity length is about 3.26 m leading to a pulse repetition rate of 92 MHz. The SAM presents a low intensity reflectivity of 77%, modulation depth of 14% and saturation fluence of 25 µJ/cm². Temporal relaxation of the SAM is roughly estimated around 2 ps but clearly present a bi-temporal response time with a fast sub-picosecond component and a slower part approaching 10 ps at high fluence. One should note that up to a fluence of 458 µJ/cm², no reverse saturable absorption (free carrier absorption-based, FCA) has been observed for this SAM structure. A negligible positive dispersion (+ 1000 fs²) has been evaluated for the SAM at the laser central operating wavelength of 1560 nm. Output SAM

λ/4

L1 Pump Diode @980 nm

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Fig. 1. Experimental set-up. WDM : 980/1550 nm multiplexer; L1, L2, L3: coupling lenses; λ/2, λ/4: half- and quarter- wave plates.

For an optimized setting of the wave-plates, the laser starts in a CW regime for a pump power of 40 mW and self-starts in a pure mode-locking regime when pump power reaches 220 mW. It should be noted here that, even if we have carefully checked that mode-locking could not be experimentally obtained without the SAM, we expect that nonlinear polarization evolution (NPE) mechanism participates to pulse shaping. Indeed, the laser operation is very sensitive to the wave-plates orientation and mode-locking can not be obtained for all settings. Numerical simulations reported on next section will provide further information on such hypothesis. The single-pulse mode-locking operation is sustained up to the maximum of available pump power of 450 mW. At maximum pump level, the measured output average power is 31 mW. The positively chirped output pulses are well fitted with a Gaussian shape with pulse duration of 5.3 ps (Fig. 2). The optical spectrum of Fig. 2 is characterized by steep edges similarly to the dissipative solitons regime and more generally to mode-locked fiber lasers operating in the highly positive dispersion regime [14]. However, the narrow spectral width of 8.3 nm #99506 - $15.00 USD

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obtained in our case is different from that of the dissipative solitons reported by Zhao et al. which present a broad gain-bandwidth limited spectrum [9-11]. Interestingly, in our configuration the output pulses are extra-cavity dechirped down to 757 fs using bulk gratings (Fig. 3(a)). It is about 15 % higher than the pulse duration obtained from Fouriertransformation of the experimental optical spectrum. The average power after extra-cavity compression is 24 mW. 1.2

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Fig. 2. Right: Typical output spectrum on a linear scale. Inset presents the spectrum on a logarithmic scale. Left: Autocorrelation trace of the output pulse on linear and logarithmic (inset) scales.

To evaluate the quality of the mode-locked pulse-train, we performed amplitude noise measurements using the radio-frequency (RF) power spectrum obtained with a microwave spectrum analyzer via a high-speed photodetector (8-GHz bandwidth). Spectra were taken at different spectral ranges from 100 MHz to 10 kHz at the fundamental harmonic around 92 MHz. The different spectra presents only one noise-substructure at low-frequencies