Open Access
Add to BibSonomyAbstract
We report on the first experimental observation of dark-bright and dark-dark vector solitons in a highly birefringent cavity of all-normal-dispersion ytterbium-doped fiber laser. With the usage of different length of polarization maintaining fibers in the cavity, totally four types of vector solitons with different features were observed.
© 2013 Optical Society of America
1. Introduction
Pulse propagation in birefringent fiber has been theoretically investigated in past years [1]. Due to the soliton trapping, the two orthogonal polarization components of the pulse can propagate with the same group velocity in birefringent optical fibers while they form a type of vector solitons called group velocity locked vector solitons (GVLVSs) [2–4]. Besides, Akhmediev theoretically predicted another type of vector solitons in birefringent optical fibers, which propagate with maintained polarization and fixed phase differences between the two orthogonal polarization components [5–7]. This type of vector solitons called phase (or polarization) locked vector solitons (PLVSs) was later observed by Cundiff and Collings [8–10]. Moreover, Sheppard and Kivshar et al. predicted the vector optical solitons composed of coupled bright-bright, dark-bright, and dark-dark pulses in nonlinear dispersive media [11–15].
Vector solitons can also form in fiber laser resonating cavity. Compare to birefringent optical fibers, the formation of vector solitons in a fiber laser cavity is not only decided by the fiber birefringence and nonlinear Kerr effect, but also the cavity gain, losses and dispersion, as well as the cavity boundary condition. In other words, the vector solitons formed in fiber laser cavity is dissipative. Anyway, the formation of vector solitons in fiber laser cavity has caused widely concern in recent years [16, 17].
Experimentally, a series of vector solitons have been observed in weakly birefringent cavities of fiber laser cavity [18–21]. Once the polarization maintaining fibers being used, the cavity of a fiber laser is considered highly birefringent. Different from a weakly birefringent cavity of fiber laser, the group velocity mismatch between the two orthogonal polarization components cannot be ignored in a highly birefringent cavity. Thus, the formation of vector solitons is more difficult. To form a stable vector soliton under the cavity situation of highly birefringent, the coupling of the two polarized components must be strong enough to overcome the group velocity mismatch. Therefore, the observation is difficult and challenging. To the best of our knowledge, there has not been any experimental report of vector solitons in a highly birefringent cavity of fiber laser.
In this paper, we first report an experimental observation of vector solitons in a highly birefringent cavity of all-normal-dispersion ytterbium-doped fiber laser. With the usage of polarization maintaining fibers, we set up fiber lasers with different cavity birefringence by replacing part of intracavity single mode fibers (SMFs) with the same length of polarization maintaining fibers (PMFs) piece by piece. Totally four types of vector solitons with different features were observed.
2. Experimental setup
The cavity is constructed with all fiber components with a total length of 6.26 m as shown in Fig. 1. A 980/1064 nm wavelength-division multiplexing (WDM) is used for delivering the 976 nm laser diode (LD) pump power. A ytterbium highly-doped phosphate glass fiber with 23 mm length and a net gain coefficient of 5.7 dB/cm [22] is used as the gain medium, followed by a band pass filter with 3 dB bandwidth of 12 nm. A polarization insensitive isolator (PII) pigtailed of 1 m PMFs (Coring PM980) is set to force the unidirectional operation and an in-line polarization controller (PC) is inserted to fine tune the cavity birefringence. With the usage of PMFs, the cavity is considered highly birefringent. The rest part of the cavity is consisted of 5 m SMFs (Nufern 1060-XP SMFs and Coring HI 1060 SMFs). The net cavity dispersion is calculated 0.138 ps2. With an output coupler (90:10), 10% of the laser is delivered for analysis.
Outside the cavity, a polarization independent isolator is used to avoid the laser reflecting into the cavity, and an external PC is followed to fine tune the output polarization for the optimal analysis. The output component is split into two equal signals by a 3 dB coupler. One is analyzed by an optical spectrum analyzer (YOKOGAWA AQ6370B) and the other is analyzed by an oscilloscope (TDS3012B, 100MHz, 1.25Gs/s) and an RF spectrum analyzer (Agilent N9320A, 3.0GHz). Using a fiber-pigtailed isolator polarization beam splitter (PBS), the signal can be separated into two orthogonal principal polarization beams by altering the external PC. The isolator PBS also acts as an external polarizer outside the cavity. Finally, with the help of a photodetector (Thorlabs DET01CFC, 1.2GHz), the output pulse is displayed on the oscilloscope and the frequency of pulse is displayed on the RF spectrum analyzer.
3. Experimental results and discussions
When the pump power was increased to 150 mW, a bright GVLVS (dark-bright, Fig. 2) and a dark PLVS (dark-dark, Fig. 3) were observed by altering the PCs, respectively.
Fig. 2 Dual-peak bright GVLVS. (a) optical spectra, (b) output pulse train without PBS, (c) dark-bright pulse trains with PBS, (d) RF spectrum after PBS.
Fig. 3 Sidebands dark PLVS. (a) optical spectra, (b) output pulse train without PBS, (c) dark-dark pulse trains with PBS, (d) RF spectrum after PBS.
The GVLVS exhibits a typical character of dual-peak spectrum as shown in Fig. 2(a). The two orthogonal polarization components have different central wavelengths and spectral distributions, showing that the coupling between them is incoherent. Due to the soliton trapping, the two orthogonal components with different central wavelengths propagate with the same group velocity in the cavity. The 3 dB spectral bandwidth of the bright pulse and the dark pulse is both ~0.17 nm. Besides, the wavelength difference and the intensity dip could be changed by slightly adjusting the intracavity PC or changing the pump power.
Figure 2(b) shows the total laser emission in the form of bright pulse train with a fundamental repetition frequency of 31.96 MHz. By fine altering the external PC outside the cavity, the two orthogonal polarization components are coupled into the orthogonal axis of the isolator PBS and separated to two pulse traces displaying on the oscilloscope [Fig. 2(c)]. The horizontal polarization component is bright pulse while the vertical polarization component is dark pulse. Each of them shows equal intensity in time domain [Fig. 2(c)] and exhibits no polarization evolution frequency (PEF) clearly in the RF spectrum [Fig. 2(d)] after passing the isolator PBS. The isolator PBS also acts as an external polarizer outside the cavity. According to Ref [23], a PLVS exhibits no PEF on the RF spectrum after passing a linear polarizer. Therefore, we confirm that the bright GVLVS (dark-bright, Fig. 2) is a PLVS as well.
At the same pump power, a dark PLVS (dark-dark, Fig. 3) was obtained just by adjusting the PCs. The spectra exhibits clear sidebands as shown in Fig. 3(a). The two orthogonal components have the same central wavelength and spectral distribution with the total component. The clear spectral sidebands are caused by the polarization modulation instability, which well match the previous observation in a weakly birefringent cavity of normal dispersion fiber laser [21]. The 3 dB spectral bandwidth of the dark pulse is ~0.04 nm. The total laser emission exhibits a dark pulse train [Fig. 3(b)] while the two orthogonal polarization components exhibit dark pulse on the oscilloscope [Fig. 3(c)]. Furthermore, no PEF is observed on the RF spectrum after passing the isolator PBS [Fig. 3(d)], suggesting that the soliton is a PLVS.
Further, we gradually replaced the intracavity SMFs with PMFs piece by piece. When the length of intracavity PMFs was up to 3.4 m, by adjusting the PC, a new type of vector soliton, bright PLVS (dark-bright, Fig. 4) characterized by its single-peak spectrum [Fig. 4(a)] appeared. The 3 dB spectral bandwidth of the horizontal component and the vertical component is both ~0.09 nm. The two orthogonal polarization components exhibit dark-bright pulses on the oscilloscope [Fig. 4(c)] while its total emission is bright pulse train [Fig. 4(b)]. The RF spectrum after passing the isolator PBS is given as shown in Fig. 4(d). There is clearly no PEF on the RF spectrum, thus we confirm that this type of vector soliton is a PLVS, too.
Fig. 4 Single-peak bright PLVS. (a) optical spectra, (b) output pulse train without PBS, (c) dark-bright pulse trains with PBS, (d) RF spectrum after PBS.
Moreover, another type of dark GVLVS (dark-dark, Fig. 5) similar to the bright GVLVS (dark-bright, Fig. 2) was observed. The dark GVLVS [Fig. 5(a)] exhibits dual-peak spectrum but the wavelength difference is bigger and the dip is much deeper than the bright GVLVS [Fig. 2(a)]. While increasing the length of PMFs in the cavity, the wavelength difference was getting smaller and the dip was getting shallower [Fig. 5(a)]. However, the dark GVLVS is unstable. The total emission is given as shown in Fig. 5(b).
Fig. 5 Dual-peak dark GVLVS. (a) optical spectra of different length of intracavity PMFs, (b) total dark pulse train (unstable).
During the process of replacing SMFs with PMFs piece by piece, we noticed that the spectral bandwidth was getting smaller when the cavity birefringence becoming more highly for all of the experimentally observed vector solitons. The dual-peak bright GVLVS (dark-bright, Fig. 2) was stably supported in lower cavity birefringence. On the contrary, the sidebands dark PLVS (dark-dark, Fig. 3) was more stable in higher cavity birefringence. However, the single-peak bright PLVS (dark-bright, Fig. 4) was only observed in the cavity composed of almost equal fiber length of SMFs and PMFs. Anyway, the dual-peak dark GVLVS (dark-dark, Fig. 5) was always unstable whether in lower or higher cavity birefringence.
Finally, when the length of intracavity PMFs were up to 4.7 m and the rest part of pigtailed SMFs were 1.2 m, no stable dual-peak bright GVLVS or single-peak bright PLVS was observed by altering the PC. Nevertheless, only the sidebands dark PLVS (dark-dark) formed by the polarization modulation instability, was stably observed in the high cavity birefringence.
4. Conclusions
In conclusion, we have generally demonstrated four types of vector solitons in a highly birefringent cavity of ytterbium-doped fiber laser with different features: dual-peak bright GVLVS (dark-bright), sidebands dark PLVS (dark-dark), single-peak bright PLVS (dark-bright) and dual-peak dark GVLVS (dark-dark). By replacing the intracavity SMFs with PMFs piece by piece, the stability of all the experimentally observed vector solitons has been discussed in different cavity birefringence. Our experimental results provide an important way to investigate the formation and propagation of many types of vector solitons in a highly birefringent cavity of fiber laser.
Acknowledgment
This research was partly supported by the China State 863 Hi-tech Program (2012AA041203, 2011AA030203), the National Natural Science Foundation of China (NSFC) (11174085, U0934001, and 60977060), the Guangdong Province and Hong Kong Invite Public Bidding Program (TC10BH07-1), the Science and Technology Project of Guangdong (cgzhzd0903, 2011B090400055), Natural Science Fund of Guangdong Province (S2011030001349), the Fundamental Research Funds for the Central Universities (2012ZZ0002, 2012ZB0002), and the Open Research Fund of State Key Laboratory of Transient Optics and Photonics, Chinese Academy of Sciences (SKLST201101).
References and links
1. C. R. Menyuk, “Nonlinear pulse propagation in birefringent optical fibers,” IEEE J. Quantum Electron. 23(2), 174–176 (1987). [CrossRef]
2. C. R. Menyuk, “Stability of solitons in birefringent optical fibers. I: Equal propagation amplitudes,” Opt. Lett. 12(8), 614–616 (1987). [CrossRef] [PubMed]
3. C. R. Menyuk, “Stability of solitons in birefringent optical fibers. II. Arbitrary amplitudes,” J. Opt. Soc. Am. B 5(2), 392–402 (1988). [CrossRef]
4. M. N. Islam, C. D. Poole, and J. P. Gordon, “Soliton trapping in birefringent optical fibers,” Opt. Lett. 14(18), 1011–1013 (1989). [CrossRef] [PubMed]
5. N. N. Akhmediev, A. V. Buryak, J. M. Soto-Crespo, and D. R. Andersen, “Phase-locked stationary soliton states in birefringent nonlinear optical fibers,” J. Opt. Soc. Am. B 12(3), 434–439 (1995). [CrossRef]
6. N. Akhmediev and J. M. Soto-Crespo, “Dynamics of solitonlike pulse propagation in birefringent optical fibers,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 49(6), 5742–5754 (1994). [CrossRef] [PubMed]
7. J. M. Soto-Crespo, N. Akhmediev, and A. Ankiewicz, “Stationary solitonlike pulses in birefringent optical fibers,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 51(4), 3547–3555 (1995). [CrossRef] [PubMed]
8. S. T. Cundiff, B. C. Collings, N. N. Akhmediev, J. M. Soto-Crespo, K. Bergman, and W. Knox, “Observation of polarization-locked vector solitons in an optical fiber,” Phys. Rev. Lett. 82(20), 3988–3991 (1999). [CrossRef]
9. B. C. Collings, S. T. Cundiff, N. N. Akhmediev, J. M. Soto-Crespo, K. Bergman, and W. H. Knox, “Polarization-locked temporal vector solitons in a fiber laser: theory,” J. Opt. Soc. Am. B 17(3), 354–372 (2000). [CrossRef]
10. B. C. Collings, S. T. Cundiff, N. N. Akhmediev, J. M. Soto-Crespo, K. Bergman, and W. H. Knox, “Polarization-locked temporal vector solitons in a fiber laser: experiment,” J. Opt. Soc. Am. B 17(3), 354–365 (2000). [CrossRef]
11. M. Haelterman and A. P. Sheppard, “Polarization domain walls in diffractive or dispersive Kerr media,” Opt. Lett. 19(2), 96–98 (1994). [CrossRef] [PubMed]
12. A. P. Sheppard and Y. S. Kivshar, “Polarized dark solitons in isotropic Kerr media,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 55(4), 4773–4782 (1997). [CrossRef]
13. D. N. Christodoulides and R. I. Joseph, “Vector solitons in birefringent nonlinear dispersive media,” Opt. Lett. 13(1), 53–55 (1988). [CrossRef] [PubMed]
14. Y. S. Kivshar, “Stable vector solitons composed of bright and dark pulses,” Opt. Lett. 17(19), 1322–1324 (1992). [CrossRef] [PubMed]
15. Y. S. Kivshar and S. K. Turitsyn, “Vector dark solitons,” Opt. Lett. 18(5), 337–339 (1993). [CrossRef] [PubMed]
16. L. M. Zhao, D. Y. Tang, H. Zhang, X. Wu, and N. Xiang, “Soliton trapping in fiber lasers,” Opt. Express 16(13), 9528–9533 (2008). [CrossRef] [PubMed]
17. H. Zhang, D. Y. Tang, L. M. Zhao, and H. Y. Tam, “Induced solitons formed by cross-polarization coupling in a birefringent cavity fiber laser,” Opt. Lett. 33(20), 2317–2319 (2008). [CrossRef] [PubMed]
18. L. M. Zhao, D. Y. Tang, X. Wu, H. Zhang, and H. Y. Tam, “Coexistence of polarization-locked and polarization-rotating vector solitons in a fiber laser with SESAM,” Opt. Lett. 34(20), 3059–3061 (2009). [CrossRef] [PubMed]
19. L. M. Zhao, D. Y. Tang, H. Zhang, and X. Wu, “Polarization rotation locking of vector solitons in a fiber ring laser,” Opt. Express 16(14), 10053–10058 (2008). [CrossRef] [PubMed]
20. H. Zhang, D. Y. Tang, L. M. Zhao, and R. J. Knize, “Vector dark domain wall solitons in a fiber ring laser,” Opt. Express 18(5), 4428–4433 (2010). [CrossRef] [PubMed]
21. H. Zhang, D. Y. Tang, L. M. Zhao, and X. Wu, “Observation of polarization domain wall solitons in weakly birefringent cavity fiber lasers,” Phys. Rev. B 80(5), 052302 (2009). [CrossRef]
22. S. H. Xu, Z. M. Yang, W. N. Zhang, X. M. Wei, Q. Qian, D. D. Chen, Q. Y. Zhang, S. X. Shen, M. Y. Peng, and J. R. Qiu, “400 mW ultrashort cavity low-noise single-frequency Yb3+-doped phosphate laser,” Opt. Lett. 36(18), 3708–3710 (2011).
23. S. T. Cundiff, B. C. Collings, and W. H. Knox, “Polarization locking in an isotropic, modelocked soliton Er/Yb fiber laser,” Opt. Express 1(1), 12–21 (1997). [CrossRef] [PubMed]
