2 GHz passively harmonic mode-locked fiber laser ... - Semantic Scholar
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OPTICS LETTERS / Vol. 38, No. 24 / December 15, 2013
2 GHz passively harmonic mode-locked fiber laser by a microfiber-based topological insulator saturable absorber Zhi-Chao Luo,1 Meng Liu,1 Hao Liu,1 Xu-Wu Zheng,1 Ai-Ping Luo,1,3 Chu-Jun Zhao,2 Han Zhang,2 Shuang-Chun Wen,2 and Wen-Cheng Xu1,4 1
Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, Guangdong 510006, China 2 Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, College of Physics and Microelectronic Science, Hunan University, Changsha 410082, China 3 e-mail: [email protected] 4
e-mail: [email protected]
Received October 16, 2013; revised November 5, 2013; accepted November 5, 2013; posted November 6, 2013 (Doc. ID 199624); published December 3, 2013 We report on the generation of passive harmonic mode locking of a fiber laser using a microfiber-based topological insulator (TI) Bi2 Te3 saturable absorber (SA). The optical deposition method was employed to fabricate the microfiber-based TISA. By virtue of the excellent nonlinear optical property of the proposed TISA, the fiber laser could operate at the pulse repetition rate of 2.04 GHz under a pump power of 126 mW, corresponding to the 418th harmonic of fundamental repetition frequency. The results demonstrate that the microfiber-based TI photonic device can operate as both the high nonlinear optical component and the SA in fiber lasers, and could also find other applications in the related fields of photonics. © 2013 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (140.4050) Mode-locked lasers; (140.3510) Lasers, fiber; (250.5530) Pulse propagation and temporal solitons. http://dx.doi.org/10.1364/OL.38.005212
Passively mode-locked fiber lasers, as an excellent platform for ultrafast pulse generation, possess widespread applications ranging from industrial purposes to fundamental research, such as material processing, optical communications, medicine, and sensing [1]. To achieve passive mode locking in a fiber laser, the key component is the saturable absorber (SA). To date, several approaches have been proposed to achieve the saturable absorption effect in a mode-locked fiber laser, i.e., nonlinear polarization rotation [2,3], nonlinear amplifying loop mirror [4,5], and real SAs [6,7]. Among them, incorporating a real SA into the laser cavity is expected to be a more efficient way to generate a mode-locked pulse compared with the other two techniques, because it does not require the fine tuning of polarization states. Therefore, there is always a strong motivation to search for high-performance SAs. With the development of novel materials, the SA is fabricated with materials evolving from semiconductor toward nanomaterials [8–10]. Indeed, nanomaterials such as carbon nanotube and graphene were found to be good candidates for fabricating excellent SAs. In particular, the graphene SA [11–13], a type of Dirac material, possesses some advantages such as wavelength-independent saturable absorbing characteristics, low saturable absorbing threshold, and large modulation depth. Therefore, much attention has been paid to the graphene SA-based ultrafast fiber lasers in recent years. On the other hand, topological insulators (TIs), as a rising material, have attracted much interest in the field of condensed-matter physics [14,15]. Similar to graphene, TIs are also Dirac materials, exhibiting Dirac-like linear band dispersion [16,17]. Recently, Bernard et al. found that TIs show saturable absorption behavior around 0146-9592/13/245212-04$15.00/0
the 1550 nm wavelength range [18]. Taking advantage of the saturable absorption behavior of TI-based SAs, Zhao et al. further demonstrated the achievement of ultrafast fiber laser mode locking by inserting the TIbased SA into the laser cavity [19,20]. In addition to the saturable absorption effect, it was found that the TIs also possess large nonlinear refractive index with the Z-scan technique [21]. Thus, the TIs could serve as both the high nonlinear photonic device and the SA in the laser system. As we know, high repetition rate pulse fiber lasers are regarded as useful light sources due to such wide applications as astronomical frequency combs [22] and optical fiber communication systems [23]. A direct approach to achieve a high repetition rate pulse is to make the fiber laser operate in the harmonic mode-locking state, which could increase the repetition rate up to the order of GHz without reducing the laser cavity length [24–26]. It was demonstrated that introducing proper high nonlinear effect into the laser cavity is favorable for generating harmonic mode locking (HML) pulses [27]. Therefore, considering the large nonlinear refractive index of TIs, it would be interesting to know whether the high-order passive HML could be obtained in a fiber laser with a TISA. However, so far, the reported TISA is fabricated with a quartz plate [19,20] or onto the fiber end facet [28], making the interaction length between the light and the TI very short. Thus, the impact of nonlinear effect of the TISA on the pulse shaping is not evident and no HML operation was observed in the reported fiber laser with a TISA. Nevertheless, it could be improved by depositing TI onto the microfiber, where the deposited amount and length of TI along the microfiber could be controlled by adjusting the deposition time. In this case, it could effectively increase the interaction length between light © 2013 Optical Society of America
December 15, 2013 / Vol. 38, No. 24 / OPTICS LETTERS
and the TI. Therefore, with a microfiber-based TISA, a question naturally arises as to whether the high-order HML could be achieved in fiber lasers. In this Letter, we will address this issue. The passive HML in a fiber laser by using a microfiber-based TISA was demonstrated. With the proper adjustment of cavity parameters, 2.04 GHz repetition rate pulse could be achieved, which corresponds to the 418th harmonic of fundamental cavity frequency. The obtained results provided the first demonstration of the simultaneous applications of both high nonlinear and saturable absorption effects of TIs. The TI Bi2 Te3 was synthesized by the cost-effective hydrothermal intercalation and exfoliation method. Figure 1 shows the scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the Bi2 Te3 nanosheets. It can be seen clearly that the nanomaterial has a quasi-two-dimensional sheet-like structure with intact surface texture. After preparing the Bi2 Te3 nanosheets, they were dispersed in the acetone solution and ultrasonicated for 30 min. The concentration of TI acetone solution is ∼0.08 mg∕ml in this experiment. The experimental setup for the fabrication of microfiber-based TISA was shown in Fig. 2(a). To obtain stronger evanescent field, the input amplified spontaneous emission (ASE) light source was amplified by an erbium-doped fiber amplifier (EDFA), where the output power can reach up to 34 mW. The microfiber was fabricated with the method similar to that of [29]. After preparing the microfiber, we could observe the evanescent field through injecting the visible light, as shown in Fig. 2(b). Generally, the waist diameter of
Fig. 1. (a) SEM and (b) TEM images of the Bi2 Te3 nanosheets prepared by hydrothermal intercalation/exfoliation method.
Fig. 2. (a) Experimental setup for the fabrication of microfiber-based TISA; (b) evanescent field of microfiber observed by the visible light; (c) microscopy image of fabricated microfiber-based TISA.
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the fiber could be tapered down to a few micrometers to tens of micrometers. In this experiment, the fiber was tapered down to about 12 μm. First, the TI acetone solution was dropped onto the glass slide which covered the microfiber. Then the ASE light source was turned on, and the optical deposition process started. The process of optical deposition was in situ observed by using a microscope with a magnification of 100-fold. As mentioned previously, the deposition length of TI could be roughly adjusted by altering the deposition time. Thus, to increase the interaction length between the TI and propagating light, we turn off the ASE light source once the deposition length became sufficiently long, i.e., ∼1.2 mm for this experiment. The remaining TI acetone solution was taken out by an injector. Finally, the fabricated TISA was evaporated at room temperature, as presented in Fig. 2(c). In order to further investigate characteristics of the fabricated microfiber-based TISA, we measured the nonlinear absorption of TISA by using an in-house made femtosecond pulse pump source (center wavelength, 1554.4 nm; repetition rate, 26 MHz; pulse duration, ∼500 fs). The experimental setup is shown in Fig. 3(a). To increase the output power, the output femtosecond pulse from the fiber laser is amplified by a commercial EDFA. A variable optical attenuator was used to control the input optical power of the TISA. Figure 3(b) provides the saturable absorption data of the microfiber-based TISA and the corresponding fitting curve as a function of average pump power. As can be seen here, the modulation depth is ∼1.7% and the nonsaturable loss is ∼69.9%. Correspondingly, the insertion loss of the microfiberbased TISA is about 5.23 dB. Moreover, the modulation depth is a little low, which could be improved by optimizing the fabrication quality of microfiber and the deposition amount of TI. It should be noted that we have measured the nonlinear optical response of the same microfiber-based TISA twice under the same experimental conditions. Both the transmission curves give similar modulation depths and nonsaturable losses. After the measurement, the microfiber-based TISA was examined again under an optical microscope. It was found that the TI was still well deposited along the microfiber. These results demonstrated that the measured SA effect is indeed from the fabricated microfiber-based TISA rather than other artifices such as the optical damage of TISA and the measurement errors. After we complete the preparation of the microfiberbased TISA, we incorporated the TISA device into the laser cavity. The schematic of the proposed fiber laser
Fig. 3. (a) Experimental setup for nonlinear absorption measurement of microfiber-based TISA; (b) measured transmission curve and the corresponding fitting curve.
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Fig. 4. Schematic of the passive HML fiber laser with a microfiber-based TISA.
was shown in Fig. 4. A ∼5 m erbium-doped fiber (EDF) was used as the gain medium. Two polarization controllers (PCs) were employed to adjust the polarization state of the propagation light. Unidirectional operation of the fiber laser was ensured by a polarization-independent isolator. To enhance the interaction between the TI and the light, the microfiber-based TISA was spliced right after the EDF. The laser output was taken out by a 10% coupler. An optical spectrum analyzer (Anritsu MS9710C) and an oscilloscope (LeCroy WaveRunner 620Zi, 2 GHz) with a photodetector (New Focus P818-BB-35F, 12.5 GHz) were used to study the laser spectrum and output pulse train, respectively. Moreover, the pulse duration was measured using a commercial autocorrelator. In the experiment, apart from the HML operation, the bunched solitons were also occasionally observed when the PCs were adjusted. However, we only focused on the HML operation in this work. With the proper rotations of the PCs, the self-started HML operation could be achieved at a pump power of 26.5 mW. Due to the high nonlinear effect caused by the microfiber-based TISA, the proposed fiber laser always tends to operate in HML (multipulse) state. However, by carefully adjusting the pump power, the fiber laser could still emit modelocked pulse at the fundamental repetition rate because of the pump hysteresis phenomenon [30,31]. Figure 5 shows the typical mode-locked operation with the fundamental repetition rate at a pump power of 7.5 mW. As can be seen in Fig. 5(a), the mode-locked spectrum centered at 1558.5 nm has a 3 dB spectral bandwidth of 0.95 nm. It should be noted that the low pump power level could result in an imperfect mode-locking state. Thus, the cw component coexists with the mode-locked component in the optical spectrum. Figure 5(b) presents the mode-locked pulse train. The pulse repetition rate,
Fig. 5. Mode-locked operation at the fundamental repetition rate. (a) Mode-locked spectrum; (b) mode-locked pulse-train; inset, autocorrelation trace.
which is determined by the cavity length, is 4.88 MHz. Correspondingly, the pulse duration was measured to be 1.22 ps, as shown in the inset of Fig. 5(b). A typical characteristic of the passive HML in fiber lasers is that the pulse repetition rate increases with the increasing of pump power. In our laser cavity, the fiber laser achieved ∼2 GHz repetition rate at the pump power of 126 mW, corresponding to 418th harmonic of fundamental repetition frequency. Figure 6(a) shows the mode-locked spectrum of the HML at the repetition rate of 2.04 GHz. Here, the center wavelength and the 3 dB spectral bandwidth are 1558.5 and 1.08 nm, respectively. In this case, the cw component vanished because the pump power level was higher than that of the fundamental repetition rate. Note that there is a spectral sideband on the right side of the mode-locked spectrum. In the experimental observation, the wavelength position of the spectral sideband is related to the PC settings. Therefore, it was believed that the spectral sideband was formed by four-wave-mixing effect [32]. The pulse train was presented in Fig. 6(b). The time interval between the HML pulses is ∼0.489 ns, corresponding to a repetition rate of 2.04 GHz. The pulse duration was shown in Fig. 6(c), which gives a pulse duration of 2.49 ps if a hyperbolic secant pulse profile is assumed. Correspondingly, the time–bandwidth product is 0.332, indicating that the output pulse is almost transform limited. In order to better show the performance of HML fiber laser, we have plotted the relation between the harmonic number and the output power with respect to the pump power, as shown in Fig. 7. It can be clearly seen that the harmonic number linearly scales with the pump power level. The measured maximum repetition rate of 2.04 GHz corresponds to the 418th harmonic order. Moreover, the output pump power varies from 0.32 to 5.02 mW when the pump power was adjusted from 14.68 to 126 mW. The previously observed results are the typical characteristics of HML fiber lasers. In the experiment, it is worth noting that the pulse repetition rate could be further increased if the pump power was further increased. Due to the bandwidth limitation of
Fig. 6. HML operation at the repetition rate of 2.04 GHz. (a) Mode-locked spectrum; (b) recorded pulse-train; inset, pulse train with 300 ns span; (c) corresponding autocorrelation trace.
December 15, 2013 / Vol. 38, No. 24 / OPTICS LETTERS
Fig. 7. Harmonic number and output power versus pump power.
the oscilloscope used, we only showed the results with ∼2 GHz pulse repetition rate here. It should be also noted that the optical damage of the proposed TISA could be the major limitation to further scaling in harmonic number. However, in our experiment, the optical damage of the proposed TISA was not observed because of the limited output power (∼250 mW) of the pumping laser. In addition, we have verified whether the TISA contributed to the passive mode locking. To this end, we removed the microfiber-based TISA from the fiber ring laser. In this case, even if the PCs were rotated and the pump power was adjusted in a large range, neither the mode-locked pulse nor the HML operation could be observed. The comparative results demonstrated that the high-order HML operation of the proposed fiber laser was indeed generated by the combination of the saturable absorption and high nonlinear effects of TI. In summary, we have demonstrated a high-order passively HML fiber laser by using a microfiber-based TISA. Taking advantage of both the high nonlinear and saturable absorption effects generated by the microfiber-based TISA, HML operation could be effectively achieved. At a pump power of 126 mW, the fiber laser operates in 418th harmonic mode-locking state, which corresponds to a repetition rate of 2.04 GHz. The observed results provided the first demonstration of the simultaneous applications of both high nonlinear and saturable absorption effects of TIs, showing that the proposed TISA could find important applications in the fields of nonlinear and ultrafast photonics. This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 11074078, 61378036, 61307058, and 11304101); the PhD Start-up Fund of the Natural Science Foundation of Guangdong Province, China (Grant No. S2013040016320); and the Key Program for Scientific and Technological Innovations of Higher Education Institutes in Guangdong Province, China (Grant No. cxzd1011). References 1. U. Keller, Nature 424, 831 (2003).
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2. L. E. Nelson, D. J. Jones, K. Tamura, H. A. Haus, and E. P. Ippen, Appl. Phys. B 65, 277 (1997). 3. B. Oktem, C. Ülgüdür, and F. Ö. Ilday, Nat. Photonics 4, 307 (2010). 4. D. J. Richardson, R. I. Laming, D. N. Payne, V. Matsas, and M. W. Phillips, Electron. Lett. 27, 542 (1991). 5. S. K. Wang, Q. Y. Ning, A. P. Luo, Z. B. Lin, Z. C. Luo, and W. C. Xu, Opt. Express 21, 2402 (2013). 6. O. Okhotnikov, A. Grudinin, and M. Pessa, New J. Phys. 6, 177 (2004). 7. H. Zhang, D. Y. Tang, R. J. Knize, L. Zhao, Q. Bao, and K. P. Loh, Appl. Phys. Lett. 96, 111112 (2010). 8. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, J. Lightwave Technol. 22, 51 (2004). 9. F. Wang, A. G. Rozhin, V. Scardaci, Z. Sun, F. Hennrich, I. H. White, W. I. Milne, and A. C. Ferrari, Nat. Nanotechnol. 3, 738 (2008). 10. Q. L. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z. Shen, K. P. Loh, and D. Y. Tang, Adv. Funct. Mater. 19, 3077 (2009). 11. H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, and K. P. Loh, Opt. Express 17, 17630 (2009). 12. Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, ACS Nano 4, 803 (2010). 13. Z. Luo, M. Zhou, J. Weng, G. Huang, H. Xu, C. Ye, and Z. Cai, Opt. Lett. 35, 3709 (2010). 14. M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010). 15. X. L. Qi and S. C. Zhang, Rev. Mod. Phys. 83, 1057 (2011). 16. D. Hsieh, D. Qian, L. Wray, Y. Xia, Y. S. Hor, R. J. Cava, and M. Z. Hasan, Nature 452, 970 (2008). 17. H. Zhang, C. X. Liu, X. L. Qi, X. Dai, Z. Fang, and S. C. Zhang, Nat. Phys. 5, 438 (2009). 18. F. Bernard, H. Zhang, S. P. Gorza, and P. Emplit, in Nonlinear Photonics, OSA Technical Digest (online) (Optical Society of America, 2012), paper NTh1A.5. 19. C. J. Zhao, H. Zhang, X. Qi, Y. Chen, Z. T. Wang, S. C. Wen, and D. Y. Tang, Appl. Phys. Lett. 101, 211106 (2012). 20. C. J. Zhao, Y. H. Zou, Y. Chen, Z. T. Wang, S. B. Lu, H. Zhang, S. C. Wen, and D. Y. Tang, Opt. Express 20, 27888 (2012). 21. S. Lu, C. Zhao, Y. Zou, S. Chen, Y. Chen, Y. Li, H. Zhang, S. Wen, and D. Tang, Opt. Express 21, 2072 (2013). 22. J. J. McFerran, Appl. Opt. 48, 2752 (2009). 23. B. Mikulla, L. Leng, S. Sears, B. C. Collings, M. Arend, and K. Bergman, IEEE Photon. Technol. Lett. 11, 418 (1999). 24. S. Zhou, D. G. Ouzounov, and F. W. Wise, Opt. Lett. 31, 1041 (2006). 25. G. Sobon, J. Sotor, and K. M. Abramski, Appl. Phys. Lett. 100, 161109 (2012). 26. C. Lecaplain and P. Grelu, Opt. Express 21, 10897 (2013). 27. C. S. Jun, S. Y. Choi, F. Rotermund, B. Y. Kim, and D. I. Yeom, Opt. Lett. 37, 1862 (2012). 28. Z. Q. Luo, Y. Z. Huang, J. Weng, H. H. Cheng, Z. Q. Lin, Z. P. Cai, and H. Y. Xu, “1.06 um Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi2Se3 as a saturable absorber,” arXiv:1307.4979 (2013). 29. K. Kashiwagi and S. Yamashita, Opt. Express 17, 18364 (2009). 30. A. Komarov, H. Leblond, and F. Sanchez, Phys. Rev. A 71, 053809 (2005). 31. D. Y. Tang, L. M. Zhao, B. Zhao, and A. Q. Liu, Phys. Rev. A 72, 043816 (2005). 32. H. Zhang, D. Y. Tang, L. M. Zhao, and N. Xiang, Opt. Express 16, 12618 (2008).
OPTICS LETTERS / Vol. 38, No. 24 / December 15, 2013
2 GHz passively harmonic mode-locked fiber laser by a microfiber-based topological insulator saturable absorber Zhi-Chao Luo,1 Meng Liu,1 Hao Liu,1 Xu-Wu Zheng,1 Ai-Ping Luo,1,3 Chu-Jun Zhao,2 Han Zhang,2 Shuang-Chun Wen,2 and Wen-Cheng Xu1,4 1
Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, Guangdong 510006, China 2 Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, College of Physics and Microelectronic Science, Hunan University, Changsha 410082, China 3 e-mail: [email protected] 4
e-mail: [email protected]
Received October 16, 2013; revised November 5, 2013; accepted November 5, 2013; posted November 6, 2013 (Doc. ID 199624); published December 3, 2013 We report on the generation of passive harmonic mode locking of a fiber laser using a microfiber-based topological insulator (TI) Bi2 Te3 saturable absorber (SA). The optical deposition method was employed to fabricate the microfiber-based TISA. By virtue of the excellent nonlinear optical property of the proposed TISA, the fiber laser could operate at the pulse repetition rate of 2.04 GHz under a pump power of 126 mW, corresponding to the 418th harmonic of fundamental repetition frequency. The results demonstrate that the microfiber-based TI photonic device can operate as both the high nonlinear optical component and the SA in fiber lasers, and could also find other applications in the related fields of photonics. © 2013 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (140.4050) Mode-locked lasers; (140.3510) Lasers, fiber; (250.5530) Pulse propagation and temporal solitons. http://dx.doi.org/10.1364/OL.38.005212
Passively mode-locked fiber lasers, as an excellent platform for ultrafast pulse generation, possess widespread applications ranging from industrial purposes to fundamental research, such as material processing, optical communications, medicine, and sensing [1]. To achieve passive mode locking in a fiber laser, the key component is the saturable absorber (SA). To date, several approaches have been proposed to achieve the saturable absorption effect in a mode-locked fiber laser, i.e., nonlinear polarization rotation [2,3], nonlinear amplifying loop mirror [4,5], and real SAs [6,7]. Among them, incorporating a real SA into the laser cavity is expected to be a more efficient way to generate a mode-locked pulse compared with the other two techniques, because it does not require the fine tuning of polarization states. Therefore, there is always a strong motivation to search for high-performance SAs. With the development of novel materials, the SA is fabricated with materials evolving from semiconductor toward nanomaterials [8–10]. Indeed, nanomaterials such as carbon nanotube and graphene were found to be good candidates for fabricating excellent SAs. In particular, the graphene SA [11–13], a type of Dirac material, possesses some advantages such as wavelength-independent saturable absorbing characteristics, low saturable absorbing threshold, and large modulation depth. Therefore, much attention has been paid to the graphene SA-based ultrafast fiber lasers in recent years. On the other hand, topological insulators (TIs), as a rising material, have attracted much interest in the field of condensed-matter physics [14,15]. Similar to graphene, TIs are also Dirac materials, exhibiting Dirac-like linear band dispersion [16,17]. Recently, Bernard et al. found that TIs show saturable absorption behavior around 0146-9592/13/245212-04$15.00/0
the 1550 nm wavelength range [18]. Taking advantage of the saturable absorption behavior of TI-based SAs, Zhao et al. further demonstrated the achievement of ultrafast fiber laser mode locking by inserting the TIbased SA into the laser cavity [19,20]. In addition to the saturable absorption effect, it was found that the TIs also possess large nonlinear refractive index with the Z-scan technique [21]. Thus, the TIs could serve as both the high nonlinear photonic device and the SA in the laser system. As we know, high repetition rate pulse fiber lasers are regarded as useful light sources due to such wide applications as astronomical frequency combs [22] and optical fiber communication systems [23]. A direct approach to achieve a high repetition rate pulse is to make the fiber laser operate in the harmonic mode-locking state, which could increase the repetition rate up to the order of GHz without reducing the laser cavity length [24–26]. It was demonstrated that introducing proper high nonlinear effect into the laser cavity is favorable for generating harmonic mode locking (HML) pulses [27]. Therefore, considering the large nonlinear refractive index of TIs, it would be interesting to know whether the high-order passive HML could be obtained in a fiber laser with a TISA. However, so far, the reported TISA is fabricated with a quartz plate [19,20] or onto the fiber end facet [28], making the interaction length between the light and the TI very short. Thus, the impact of nonlinear effect of the TISA on the pulse shaping is not evident and no HML operation was observed in the reported fiber laser with a TISA. Nevertheless, it could be improved by depositing TI onto the microfiber, where the deposited amount and length of TI along the microfiber could be controlled by adjusting the deposition time. In this case, it could effectively increase the interaction length between light © 2013 Optical Society of America
December 15, 2013 / Vol. 38, No. 24 / OPTICS LETTERS
and the TI. Therefore, with a microfiber-based TISA, a question naturally arises as to whether the high-order HML could be achieved in fiber lasers. In this Letter, we will address this issue. The passive HML in a fiber laser by using a microfiber-based TISA was demonstrated. With the proper adjustment of cavity parameters, 2.04 GHz repetition rate pulse could be achieved, which corresponds to the 418th harmonic of fundamental cavity frequency. The obtained results provided the first demonstration of the simultaneous applications of both high nonlinear and saturable absorption effects of TIs. The TI Bi2 Te3 was synthesized by the cost-effective hydrothermal intercalation and exfoliation method. Figure 1 shows the scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the Bi2 Te3 nanosheets. It can be seen clearly that the nanomaterial has a quasi-two-dimensional sheet-like structure with intact surface texture. After preparing the Bi2 Te3 nanosheets, they were dispersed in the acetone solution and ultrasonicated for 30 min. The concentration of TI acetone solution is ∼0.08 mg∕ml in this experiment. The experimental setup for the fabrication of microfiber-based TISA was shown in Fig. 2(a). To obtain stronger evanescent field, the input amplified spontaneous emission (ASE) light source was amplified by an erbium-doped fiber amplifier (EDFA), where the output power can reach up to 34 mW. The microfiber was fabricated with the method similar to that of [29]. After preparing the microfiber, we could observe the evanescent field through injecting the visible light, as shown in Fig. 2(b). Generally, the waist diameter of
Fig. 1. (a) SEM and (b) TEM images of the Bi2 Te3 nanosheets prepared by hydrothermal intercalation/exfoliation method.
Fig. 2. (a) Experimental setup for the fabrication of microfiber-based TISA; (b) evanescent field of microfiber observed by the visible light; (c) microscopy image of fabricated microfiber-based TISA.
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the fiber could be tapered down to a few micrometers to tens of micrometers. In this experiment, the fiber was tapered down to about 12 μm. First, the TI acetone solution was dropped onto the glass slide which covered the microfiber. Then the ASE light source was turned on, and the optical deposition process started. The process of optical deposition was in situ observed by using a microscope with a magnification of 100-fold. As mentioned previously, the deposition length of TI could be roughly adjusted by altering the deposition time. Thus, to increase the interaction length between the TI and propagating light, we turn off the ASE light source once the deposition length became sufficiently long, i.e., ∼1.2 mm for this experiment. The remaining TI acetone solution was taken out by an injector. Finally, the fabricated TISA was evaporated at room temperature, as presented in Fig. 2(c). In order to further investigate characteristics of the fabricated microfiber-based TISA, we measured the nonlinear absorption of TISA by using an in-house made femtosecond pulse pump source (center wavelength, 1554.4 nm; repetition rate, 26 MHz; pulse duration, ∼500 fs). The experimental setup is shown in Fig. 3(a). To increase the output power, the output femtosecond pulse from the fiber laser is amplified by a commercial EDFA. A variable optical attenuator was used to control the input optical power of the TISA. Figure 3(b) provides the saturable absorption data of the microfiber-based TISA and the corresponding fitting curve as a function of average pump power. As can be seen here, the modulation depth is ∼1.7% and the nonsaturable loss is ∼69.9%. Correspondingly, the insertion loss of the microfiberbased TISA is about 5.23 dB. Moreover, the modulation depth is a little low, which could be improved by optimizing the fabrication quality of microfiber and the deposition amount of TI. It should be noted that we have measured the nonlinear optical response of the same microfiber-based TISA twice under the same experimental conditions. Both the transmission curves give similar modulation depths and nonsaturable losses. After the measurement, the microfiber-based TISA was examined again under an optical microscope. It was found that the TI was still well deposited along the microfiber. These results demonstrated that the measured SA effect is indeed from the fabricated microfiber-based TISA rather than other artifices such as the optical damage of TISA and the measurement errors. After we complete the preparation of the microfiberbased TISA, we incorporated the TISA device into the laser cavity. The schematic of the proposed fiber laser
Fig. 3. (a) Experimental setup for nonlinear absorption measurement of microfiber-based TISA; (b) measured transmission curve and the corresponding fitting curve.
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Fig. 4. Schematic of the passive HML fiber laser with a microfiber-based TISA.
was shown in Fig. 4. A ∼5 m erbium-doped fiber (EDF) was used as the gain medium. Two polarization controllers (PCs) were employed to adjust the polarization state of the propagation light. Unidirectional operation of the fiber laser was ensured by a polarization-independent isolator. To enhance the interaction between the TI and the light, the microfiber-based TISA was spliced right after the EDF. The laser output was taken out by a 10% coupler. An optical spectrum analyzer (Anritsu MS9710C) and an oscilloscope (LeCroy WaveRunner 620Zi, 2 GHz) with a photodetector (New Focus P818-BB-35F, 12.5 GHz) were used to study the laser spectrum and output pulse train, respectively. Moreover, the pulse duration was measured using a commercial autocorrelator. In the experiment, apart from the HML operation, the bunched solitons were also occasionally observed when the PCs were adjusted. However, we only focused on the HML operation in this work. With the proper rotations of the PCs, the self-started HML operation could be achieved at a pump power of 26.5 mW. Due to the high nonlinear effect caused by the microfiber-based TISA, the proposed fiber laser always tends to operate in HML (multipulse) state. However, by carefully adjusting the pump power, the fiber laser could still emit modelocked pulse at the fundamental repetition rate because of the pump hysteresis phenomenon [30,31]. Figure 5 shows the typical mode-locked operation with the fundamental repetition rate at a pump power of 7.5 mW. As can be seen in Fig. 5(a), the mode-locked spectrum centered at 1558.5 nm has a 3 dB spectral bandwidth of 0.95 nm. It should be noted that the low pump power level could result in an imperfect mode-locking state. Thus, the cw component coexists with the mode-locked component in the optical spectrum. Figure 5(b) presents the mode-locked pulse train. The pulse repetition rate,
Fig. 5. Mode-locked operation at the fundamental repetition rate. (a) Mode-locked spectrum; (b) mode-locked pulse-train; inset, autocorrelation trace.
which is determined by the cavity length, is 4.88 MHz. Correspondingly, the pulse duration was measured to be 1.22 ps, as shown in the inset of Fig. 5(b). A typical characteristic of the passive HML in fiber lasers is that the pulse repetition rate increases with the increasing of pump power. In our laser cavity, the fiber laser achieved ∼2 GHz repetition rate at the pump power of 126 mW, corresponding to 418th harmonic of fundamental repetition frequency. Figure 6(a) shows the mode-locked spectrum of the HML at the repetition rate of 2.04 GHz. Here, the center wavelength and the 3 dB spectral bandwidth are 1558.5 and 1.08 nm, respectively. In this case, the cw component vanished because the pump power level was higher than that of the fundamental repetition rate. Note that there is a spectral sideband on the right side of the mode-locked spectrum. In the experimental observation, the wavelength position of the spectral sideband is related to the PC settings. Therefore, it was believed that the spectral sideband was formed by four-wave-mixing effect [32]. The pulse train was presented in Fig. 6(b). The time interval between the HML pulses is ∼0.489 ns, corresponding to a repetition rate of 2.04 GHz. The pulse duration was shown in Fig. 6(c), which gives a pulse duration of 2.49 ps if a hyperbolic secant pulse profile is assumed. Correspondingly, the time–bandwidth product is 0.332, indicating that the output pulse is almost transform limited. In order to better show the performance of HML fiber laser, we have plotted the relation between the harmonic number and the output power with respect to the pump power, as shown in Fig. 7. It can be clearly seen that the harmonic number linearly scales with the pump power level. The measured maximum repetition rate of 2.04 GHz corresponds to the 418th harmonic order. Moreover, the output pump power varies from 0.32 to 5.02 mW when the pump power was adjusted from 14.68 to 126 mW. The previously observed results are the typical characteristics of HML fiber lasers. In the experiment, it is worth noting that the pulse repetition rate could be further increased if the pump power was further increased. Due to the bandwidth limitation of
Fig. 6. HML operation at the repetition rate of 2.04 GHz. (a) Mode-locked spectrum; (b) recorded pulse-train; inset, pulse train with 300 ns span; (c) corresponding autocorrelation trace.
December 15, 2013 / Vol. 38, No. 24 / OPTICS LETTERS
Fig. 7. Harmonic number and output power versus pump power.
the oscilloscope used, we only showed the results with ∼2 GHz pulse repetition rate here. It should be also noted that the optical damage of the proposed TISA could be the major limitation to further scaling in harmonic number. However, in our experiment, the optical damage of the proposed TISA was not observed because of the limited output power (∼250 mW) of the pumping laser. In addition, we have verified whether the TISA contributed to the passive mode locking. To this end, we removed the microfiber-based TISA from the fiber ring laser. In this case, even if the PCs were rotated and the pump power was adjusted in a large range, neither the mode-locked pulse nor the HML operation could be observed. The comparative results demonstrated that the high-order HML operation of the proposed fiber laser was indeed generated by the combination of the saturable absorption and high nonlinear effects of TI. In summary, we have demonstrated a high-order passively HML fiber laser by using a microfiber-based TISA. Taking advantage of both the high nonlinear and saturable absorption effects generated by the microfiber-based TISA, HML operation could be effectively achieved. At a pump power of 126 mW, the fiber laser operates in 418th harmonic mode-locking state, which corresponds to a repetition rate of 2.04 GHz. The observed results provided the first demonstration of the simultaneous applications of both high nonlinear and saturable absorption effects of TIs, showing that the proposed TISA could find important applications in the fields of nonlinear and ultrafast photonics. This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 11074078, 61378036, 61307058, and 11304101); the PhD Start-up Fund of the Natural Science Foundation of Guangdong Province, China (Grant No. S2013040016320); and the Key Program for Scientific and Technological Innovations of Higher Education Institutes in Guangdong Province, China (Grant No. cxzd1011). References 1. U. Keller, Nature 424, 831 (2003).
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