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Long-cavity passively mode-locked fiber ring laser with high-energy rectangular-shape pulses in anomalous dispersion regime Xiaohui Li, Xueming Liu,* Xiaohong Hu, Leirang Wang, Hua Lu, Yishan Wang, and Wei Zhao State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China *Corresponding author:
[email protected] Received June 29, 2010; revised August 17, 2010; accepted September 6, 2010; posted September 9, 2010 (Doc. ID 130899); published September 24, 2010 We report on a long-cavity passively mode-locked fiber laser in the anomalous dispersion regime. The nonlinear polarization rotation technique is employed to achieve mode locking. The output pulse from the fiber laser has a rectangular shape and a corresponding Gaussian-shape spectral profile. Stable mode-locked pulses at a repetition rate of 278 kHz with single pulse energy as high as 715 nJ are obtained under equal bidirectional pumping power of 500 mW in cavity. The experimental results demonstrate that the passively mode-locked fiber laser operating in the anomalous regime can also realize a high-energy pulse, which is different from the conventional low-energy soliton pulse. © 2010 Optical Society of America OCIS codes: 140.3510, 140.4050, 190.5530, 060.2310.
Long-cavity fiber ring lasers with ultra-high-energy pulses and low repetition rates can be widely applied in micromachining, material processing, remote sensing, and light detection systems [1–5]. Recently, many experiments have been done to generate high-energy pulses by using a long-fiber cavity in the normal dispersion regime [6,7]. In the anomalous dispersion regime, soliton formation is mainly due to the interplay between the anomalous cavity dispersion and the fiber nonlinear optical Kerr effect [8,9]. The output pulse energy of the fiber ring laser is limited because of energy quantization [10]. However, we investigate a long-cavity passively modelocked fiber ring laser operating in the anomalous dispersion regime. The nonlinear effect can be very strong for the long-cavity fiber ring laser, and it is difficult for the anomalous dispersion to balance the nonlinearity in this case. Some new features will be formed in the long cavity. The experimental results show that the proposed fiber ring laser can also realize high-energy pulses. Chang et al. theoretically predicted that high-energy pulses can be generated by fiber lasers operating in the anomalous dispersion regime [11]. For a dissipative system, we have to admit the complex balance among dispersion, nonlinearity, gain, and loss. Zhang et al. mode locked the fiber ring laser to achieve a high-energy pulse with a graphene–polymer composite [12,13]. Square pulse emissions were first observed in anomalous dispersion fiber lasers by Matsas et al. [14]. They discussed only the phenomenon of square pulses and did not consider the pulse energy of the fiber laser. In this Letter, a 700-m-long single-mode fiber (SMF) is incorporated into the cavity of a fiber ring laser. The nonlinear polarization rotation (NPR) technique is employed to passively mode lock the fiber laser. The bidirectional pumping configuration is used to enhance the output power. Stable mode locking is achieved without using any dispersion-compensation components or spectral filters in the anomalous dispersion regime. The long-cavity fiber laser covers the C-band and a large part of the Lband; the total dispersion is negative. The experiment re0146-9592/10/193249-03$15.00/0
sults show that the duration of the output pulse increases linearly with the pump powers, while the pulse intensity is kept almost the same. High-energy rectangular pulses have been obtained in our experiment. After propagating in three sections of SMF with different lengths, the steep edges of the pulses become slow. Our experimental results are qualitative evidence of the existence of dissipative soliton resonance [15,16]. The experimental schematic diagram setup of the proposed fiber laser is shown in Fig. 1(a). The total length of the laser cavity is 720 m, which comprises an 11-m-long erbium-doped fiber (EDF), which is used as the gain medium, with group velocity dispersion (GVD) of −9 ps=nm= km at 1550 nm, a 700-m-long SMF with GVD of 17 ps=nm= km, and a 9-m-long pigtailed SMF of passive components. The EDF is from the CorActive Company, and its type is C600 with an absorption coefficient of 6 dB=m at 980 nm. The 700 m SMF is standard G. 652B fiber. Each splicing loss is less than 0:01 dB. A polarization-dependent
Fig. 1. (Color online) Schematic diagram of the long-cavity fiber ring laser and transmission system. © 2010 Optical Society of America
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Fig. 4. (Color online) Typical oscilloscope trace in the bidirectional pumping case; the bidirectional pump powers are all set as 500 mW.
Fig. 2. (Color online) (a), (c), (e) Different spectra and (b), (d), (f) corresponding pulse evolution of the proposed longcavity passively mode-locked fiber laser in different pumping configuration [(a), (b), forward pumping; (c), (d), backward pumping; (e), (f), bidirectional pumping].
isolator (PD-ISO) and two polarization controllers (PCs) are responsible for the mode locking of the fiber laser. A 10% fiber coupler is used as the output port, and two 980/ 1550 wavelength-division multiplexed (WDM) couplers are employed to couple the pump power into the ring cavity. A PD-ISO was used to force the unidirectional operation of the ring and to generate the linear polarization light in the cavity. The isolator at the output is used to prevent the light from reflecting back into the cavity. The two 976 nm laser diodes, which both have maximum output power of 525 mW, are used to pump the EDF. An optical spectrum analyzer (Yokogawa AQ-6370) and a 70 GHz digital sampling oscilloscope (Tektronix TDS8200), together with a 50 GHz photodetector, were used to simultaneously monitor the spectra and the mode-locked pulse train, respectively. The NPR technique is employed to mode lock the longcavity fiber ring laser. Depending on the net cavity birefringence, various laser operation modes are observed. The fiber laser can operate in the multipulse regime,
Fig. 3. (Color online) Measured pulse durations and pulse energy versus the pump strengths under three different pump configurations.
and the corresponding spectra show a sideband structure. Pulse bunching and higher-order harmonic mode locking can also be observed in the cavity by increasing the pump power. However, a novel operation state can be realized when the forward pump power reaches 54 mW. The typical spectra with respect to different forward and backward pump strengths are shown in Figs. 2(a), 2(c), and 2(e). There are no sidebands, as seen from the figures, which indicates distinct characteristics from the conventional soliton fiber lasers in the conservative system. When we increase the pump power, the proposed fiber ring laser can still operate at the fundamental cavity repetition rates. Figure 2(a) shows the spectra with respect to different forward pump powers from 100 to 500 mW. The central wavelength is 1563 nm, and the spectral widths fluctuate from 5.84 to 5:96 nm when the pump powers increase. Figures 2(c) and 2(e) show the typical spectra under backward and bidirectional pumping configurations, respectively. Compared with the situation of forward pumping, the spectra widths in the cases of backward and bidirectional pumping are narrower. Figures 2(b), 2(d), and 2(f) show the corresponding evolution of an output rectangular pulse with the pump strengths in three different pump configurations. The injected pump powers are selected as 100, 200, 300, 400, and 500 mW. The corresponding pulse durations are (1) forward pumping, 17.7, 34.5, 50.1, 66.5, and 82:2 ns; (2) backward pumping, 12.8, 29.1, 43.1, 57.3, and 70:8 ns; and (3) bidirectional pumping, 33.3, 67.3, 98.2, 127.8, and 155:4 ns.
Fig. 5. (Color online) Corresponding rf spectrum.
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locked by the NPR technique is employed to investigate the novel pulses. The pulse duration and the output power increase linearly with the pump power, as evidence of the dissipative soliton resonant in the anomalous dispersion regime. Such a long-cavity fiber ring laser with ultrahigh energy and a low repetition rate can be widely applied in micromachining, material processing, remote sensing, and light detection systems.
Fig. 6. (Color online) Output pulse shapes after propagating in SMFs with different lengths.
Figure 3 exhibits the details of pulse durations and pulse energies versus the pump strengths. As shown in the figure, the pulse duration increases linearly with the pump strengths. The slope under the bidirectional pumping case is higher than that in the other two cases. Because of the insertion and transmission losses of the fiber system, the maximum output power is 22:16 mW, corresponding to the pulse energy of 79:5 nJ. Considering that a 10/90 optical coupler is used as output port, the pulse energy is 715 nJ in cavity. Figure 4 shows the typical oscilloscope trace when the bidirectional pump powers are all 500 mW. The rf spectrum of the fiber laser is measured by a 42:98 GHz rf spectrum analyzer, as shown in Fig. 5. It is clear that the fiber laser can be mode locked at the fundament repetition rate of 278:79 kHz, and the signal-to-noise ratio is larger than 70 dB, as shown in the inset of Fig. 5. The envelope of the rf spectrum is periodically varied, and the period is 6:44 MHz, corresponding to pulse duration of about 155:4 ns in the temporal domain. We further investigate output pulse duration of the proposed fiber ring laser when the forward pump powers are designated as 140 mW. The output pulse train is launched into a section of standard SMF (G. 652) with different lengths. Figure 1(b) shows the transmission system. After transmitting in 10-, 35-, and 60-km-long SMFs, the corresponding oscilloscope traces are shown in Fig. 6. It can be seen that the steep leading and trailing edges of the pulses become slow with increasing length of the fiber. The rectangular-shape pulse is generated independent of the characteristics of the cavity dispersion [14,17]. The features of the proposed fiber laser have some characteristics that are quite different from the noiselike pulses [18,19]. In conclusion, we have experimentally observed a high-energy pulse with a rectangular shape in the anomalous dispersion regime. A long-cavity fiber laser mode
This work was supported by the “Hundreds of Talents Programs” of the Chinese Academy of Sciences and by the National Natural Science Foundation of China (NSFC) under grants 10874239 and 10604066. The authors thank Zhi Yang, Cunxiao Gao, Wei Zhang, Dingkang Tang, and Yuanshan Liu for useful suggestions and discussions. References 1. A. Killi, J. Dorring, U. Morgner, M. J. Lederer, J. Frei, and D. Kopf, Opt. Express 13, 1916 (2005). 2. R. Evans, S. Camacho-Lopez, F. G. Perez-Gutierrez, and G. Aguilar, Opt. Express 16, 7481 (2008). 3. I. Jovanovic, C. A. Ebbers, and C. P. J. Barty, Opt. Lett. 27, 1622 (2002). 4. U. Sharma, C.-S. Kim, and J. U. Kang, IEEE Photonics Technol. Lett. 16, 1277 (2004). 5. M. Yan, W. Li, Q. Hao, Y. Li, K. Yang, H. Zhou, and H. Zeng, Opt. Lett. 34, 3331 (2009). 6. S. Kobtsev, S. Kukarin, and Y. Fedotov, Opt. Express 16, 21936 (2008). 7. X. Tian, M. Tang, P. P. Shum, Y. Gong, C. Lin, S. Fu, and T. Zhang, Opt. Lett. 34, 1432 (2009). 8. K. Tamura, H. A. Haus, and E. P. Ippen, Electron. Lett. 28, 2226 (1992). 9. L. E. Nelson, D. J. Jones, K. Tamura, H. A. Haus, and E. P. Ippen, Appl. Phys. B 65, 277 (1997). 10. A. B. Grudinin, D. J. Richardson, and D. N. Payne, Electron. Lett. 28, 67 (1992). 11. W. Chang, J. M. Soto-Crespo, A. Ankiewicz, and N. Akhmediev, Phys. Rev. A 79, 033840 (2009). 12. H. Zhang, Q. Bao, D. Tang, L. Zhao, and K. Loh, Appl. Phys. Lett. 95, 141103 (2009). 13. H. Zhang, D. Tang, R. J. Knize, L. Zhao, Q. Bao, and K. P. Loh, Appl. Phys. Lett. 96, 111112 (2010). 14. V. J. Matsas, T. P. Newson, and M. N. Zervas, Opt. Commun. 92, 61 (1992). 15. X. Liu, Phys. Rev. A 81, 053819 (2010). 16. A. Komarov and F. Sanchez, Phys. Rev. E 77, 066201 (2008). 17. X. Wu, D. Y. Tang, H. Zhang, and L. M. Zhao, Opt. Express 17, 5580 (2009). 18. M. Horowitz, Y. Barad, and Y. Silberberg, Opt. Lett. 22, 799 (1997). 19. S. Kobtsev, S. Kukarin, S. Smirnov, S. Turitsyn, and A. Latkin, Opt. Express 17, 20707 (2009).
