2684
Letter
Vol. 40, No. 12 / June 15 2015 / Optics Letters
Dual-state dissipative solitons from an all-normaldispersion erbium-doped fiber laser: continuous wavelength tuning and multi-wavelength emission ZHICHAO WU,1 SONGNIAN FU,1,2,* CHANGXIU CHEN,2 MING TANG,1,2 PERRY SHUM,3
AND
DEMING LIU2
1
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China National Engineering Laboratory for Next Generation Internet Access System, School of Optics and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China 3 Centre for Optical Fibre Technology, Nanyang Technological University, Singapore 637553, Singapore *Corresponding author: [email protected] 2
Received 2 April 2015; revised 7 May 2015; accepted 14 May 2015; posted 15 May 2015 (Doc. ID 237171); published 2 June 2015
We propose and experimentally demonstrate switchable operation of dissipative solitons (DSs) in an erbium-doped fiber laser. By managing normal dispersion of laser cavity, the 3-dB spectral bandwidth up to 8.1 nm can be obtained with the help of a semiconductor saturable absorber mirror. Using an inline polarizer, the fiber laser can be separately operated at either wavelength-tunable or multi-wavelength emission. The central wavelength of DS can be continuously tuned from 1554 to 1561 nm with its spectra maintaining standard rectangular shape. Alternatively, triple-wavelength DSs at 1535, 1544, and 1553 nm can be simultaneously obtained. © 2015 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.4050) Mode-locked lasers; (060.5530) Pulse propagation and temporal solitons. http://dx.doi.org/10.1364/OL.40.002684
Wavelength-tunable and multi-wavelength mode-locked fiber lasers have attracted considerable research interest in fiber sensing, optical signal processing, and telecommunication systems. Various approaches have been exploited to achieve tunable and multiple wavelengths mode-locked fiber lasers. In order to obtain wavelength-tunable mode-locking operation, several wavelength-selective elements are inserted into the laser cavity, such as a Mach–Zehnder interferometer [1], chirped fiber Bragg grating [2], Sagnac fiber filter [3], and tunable bandpass filter [4]. However, these additional components extend the cavity length, reduce the repetition rate of output pulse, and increase the implementation cost. Alternatively, the mechanism of fiber cavity birefringence paves another way to achieve tunable mode-locking operation [5–7]. In particular, we notice that traditional solitons have been widely demonstrated and comprehensively discussed in erbium-doped fiber laser (EDFL). Actually, DSs in normal dispersion cavity possess advantages of large normal chirp, rectangular-shaped spectra, and high output pulse energy, compared with traditional solitons [8–10]. 0146-9592/15/122684-04$15/0$15.00 © 2015 Optical Society of America
Recently, continuously wavelength-tunable DSs over a wide range of 1570–1600 nm have been demonstrated in an EDFL [11]. However, the spectral profiles can’t keep standard rectangular shape, and 3-dB bandwidth is varied during wavelength tuning. In terms of multi-wavelength mode-locking operation, the nonlinear polarization rotation (NPR) and nonlinear amplifying loop mirror (NALM) techniques are widely utilized [12,13]. However, NPR- and NALM-based technologies generally need high pump power and careful adjustment in order to realize mode locking. Thus, semiconductor saturable absorber mirror (SESAM) serves as a better choice. Stable dualand triple-wavelength DSs formed in EDFL have been reported, using SESAM as mode-locker [14]. However, the wavelength spacing is only 4 nm, which may limit the practical applications as multi-wavelength light source. In this Letter, we experimentally demonstrate switchable DSs operation of both wavelength-tunable and multiwavelength within the same EDFL configuration. To the best of our knowledge, it is the first time that both wavelengthtunable and multiple wavelengths passive-mode locking have been realized using an all-normal-dispersion EDFL, with the help of SESAM. Without additional optical filters, the optical spectrum can be continuously tuned from 1554 to 1561 nm with stable rectangular shape. Moreover, we also obtain dualwavelength (1553 nm and 1562 nm) and triple-wavelength (1535 nm, 1544 nm, and 1553 nm) DSs, respectively, with a wide wavelength spacing of 9 nm. The proposed fiber laser is schematically showed in Fig. 1. The self-starting mode-locking operation is achieved by a commercial SESAM (BATOP, SAM-1550-22-12ps-x), which is deposited on the end surface of a standard FC/PC fiber connector. The SESAM possesses low-intensity absorption of 22% at the wavelength of 1550 nm, modulation depth of 13%, saturation fluence of 40 μJ∕cm2 , nonsaturable loss of 9%, damage threshold of 800 μJ∕cm2 , and relaxation time of 12 ps. The circulator is used to incorporate the SESAM into the cavity, as well as guarantee unidirectional propagation and suppress detrimental reflections. A 1.3-m EDF with a group velocity dispersion (GVD) parameter of −18.72 ps∕nm∕km is used
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Fig. 1. Experimental configuration.
as the gain medium, which is pumped by a 980-nm laser diode (LD) with maximum output power of 400 mW. The absorption coefficient of the EDF is 21.9 dB/m at 980 nm. A 2.4-m dispersion compensation fiber (DCF) with GVD parameter of ∼ − 38 ps∕nm∕km is used to achieve dispersion management. The fiber pigtails of optical components are 6.2-m standard single-mode fiber (SMF) with GVD parameter of 17 ps∕nm∕km. Thus, the net dispersion of the ring cavity is estimated as 0.13 ps2 . The total optical cavity length is estimated around 9.9 m. Two polarization controllers (PCs) are used to manage the state of polarization (SOP) within the ring cavity. Note that, due to the use of inline polarizer, we can form an artificial birefringent filter in the cavity [11]. A 10:90 fiber optical coupler (OC) is utilized as the output port of the fiber laser. Finally, an optical spectrum analyzer (OSA, Yokogawa AQ6370C-20) with a resolution of 0.02 nm and a 5-GHz oscilloscope (OSC, Tektronix DPO 4104) together with a 40-GHz photodetector (PD) are applied to monitor the optical spectrum and the output pulse-train, respectively. The radiofrequency (RF) spectrum is measured by a 40-GHz electrical spectrum analyzer (ESA, Agilent E4447A). Additionally, the pulse profile is measured by a commercial autocorrelator (APE PulseCheck 50). First, we focus on the generation of DSs in the absence of the polarizer. Continuous wave (CW) laser emission at the central wavelength of 1559 nm is observed when the pump power is above 20 mW. When the pump power is further increased to 63 mW, self-starting mode locking at 1559 nm can be observed, as shown in Fig. 2, with an average output power of 1 mW. Figure 2(a) presents the rectangular-shaped spectrum with steep spectral edges, which proves the generation of DSs in the normal dispersion region. The 3-dB spectrum bandwidth is 8.1 nm, and the repetition rate of the pulse-train is 20.3 MHz, corresponding to the cavity length of 9.9 m, as shown in Fig. 2(b). The RF spectrum is illustrated in Fig. 2(c), and the inset corresponds to the fundamental frequency with a signal-to-noise ratio of more than 60 dB when the resolution of ESA is set 100 Hz. The full-width at half maximum (FWHM) of the pulse is 19.8 ps, as shown in Fig. 2(d). Therefore, the time-bandwidth product of the pulses is ∼20, which is larger than the transformed limited products. We can conclude that the mode-locked pulses are strongly chirped. Next, in order to obtain the wavelength-tunable operation, we insert an inline polarizer into the cavity between DCF and PC1. The central wavelength is shifted to 1555 nm, and 3-dB spectral bandwidth is reduced to ∼2 nm, due to the artificial
Fig. 2. DS generation at 1559 nm: (a) optical spectrum; (b) oscilloscope trace of the pulse-train; (c) RF spectrum and fundamental frequency signal (inset); (d) autocorrelation trace.
birefringent filter induced by the polarizer [7]. Although the EDF has a broad gain profile, considering the bandwidth limitation of the birefringent filter, the bandwidth of effective gain becomes narrow. In order to investigate the effect of pump power, we fix the PCs on the condition of mode locking. With the growing of pump power from 60 to 230 mW, the central wavelength stays unchanged. As for the 3-dB spectral bandwidth, it becomes wider first and drops back to 1.75 nm at 110 mW, then turns wider again and falls to 2.16 nm at 190 mW. The entire process is shown in Figs. 3(a) and 3(b). The drops can be explained as pulse broken, due to the pulse peak clamping effect [15]. Moreover, pulse broadening in time domain gives rise to spectrum narrowing. Under the same principle, triple solitons can be generated in case the pump power is further increased to 190 mW. Figures 3(c) and 3(d) present the pulse-trains of dual solitons and triple solitons, respectively. Then, under the pump power of 63 mW, the central wavelength of DS can be continuously tuned by proper adjustment of the PCs. If the PCs are varied to the initial state, the mode
Fig. 3. (a) 3-dB spectral bandwidth at different pump power; (b) optical spectra at pump power of 70 mW, 130 mW, and 190 mW; (c) dual solitons pulse-train; (d) triple solitons pulse-train.
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Fig. 5. (a) Peak power; (b) output power; (c) repetition rate of pulse-train; and (d) 3-dB spectral bandwidth vary with respect to the tunable wavelength, respectively. Fig. 4. Continuously tunable wavelength from 1554 to 1561 nm with the insets of corresponding pulse-trains.
locking can be achieved at 1555 nm again. Meanwhile, the DS with steep spectral edges can be always generated in our fiber ring cavity. As a result, wavelength-tunability of normal dispersion mode-locked operation is successfully obtained from 1554 to 1561 nm. Different from previous wavelength-tunable mechanism using additional optical filter, our PCs together with the inline polarizer provide a cavity birefringent filter in the designated band. As long as we rotate the orientation of the PCs, the central wavelength of the invisible birefringent filter can be continuously tuned [11]. Figure 4 specifically illustrates the optical spectra in four designated wavelengths. The 3-dB spectral bandwidths are 1.83 nm, 1.87 nm, 1.86 nm, and 1.81 nm, respectively, corresponding to the central wavelengths of 1553, 1556, 1558, and 1561 nm. It is obvious that the rectangular shapes of the spectra remain almost unchanged, and the characteristics of pulse-trains have no significant difference as well, as shown in four insets. Those characteristics of proposed fiber laser output are ideal for the application of optical sampling systems [16]. Since both polarization and intra-cavity loss are varied, the soliton parameters including 3-dB spectral bandwidths, output powers, peak powers, and repetition rates of tunable DS are characterized with small fluctuations simultaneously, as shown in Figs. 5(a)–5(d). Next, we further rotate the orientation of PCs to change the cavity birefringent filter. In Fig. 6(a), two rectangular-shaped spectra at central wavelengths of 1553 and 1562 nm appear simultaneously, indicating of stable dual-wavelength mode locking. The 1553-nm and 1562-nm DSs have 3-dB spectral bandwidths of 1.62 and 1.57 nm, and peak powers of −31.68 dBm and −31.66 dBm, respectively. In case we continuously increase pump power to
80 mW and further adjust the PC properly, triple-wavelength DSs at 1535, 1544, and 1553 nm can be achieved. The optical spectrum is shown in Fig. 6(c). Three DSs have 3-dB spectral bandwidth of 1.54, 1.86, and 1.61 nm, peak powers of −33.82 dBm, −33.83 dBm, and −33.98 dBm, respectively. Since the wavelength spacing between two DSs is 9 nm, we don’t observe additional peak from the RF spectrum of dualwavelength DSs, due to the limited bandwidth of PD and ESA. As for the time domain characterization, Figs. 6(b) and 6(d) show the snapshots of multi-wavelength DS pulse trains. Each soliton has different pulse energies that are observed with different amplitude on the oscilloscope screen. Due to the
Fig. 6. Dual- and triple-wavelength DSs: (a),(b) optical spectrum and pulse-train of dual-wavelength; (c),(d) optical spectrum and pulse-train of triple-wavelength.
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Fig. 7. Repeating scans of the multi-wavelength DSs output with 10-min intervals: (a) dual-wavelength operation, (b) triple-wavelength operation.
difference of group velocity within the cavity, two soliton pulses have different repetition rates. Therefore, when we trigger with one soliton pulse, the other one will move randomly on the oscilloscope screen [14]. The long-term stability of the multiwavelength DSs is shown in Figs. 7(a) and 7(b), and we carry out repeating scans of 7 times with 10-min intervals for the dual- and triple-wavelength operations, respectively. No significant wavelength drift is observed, and the power fluctuation is less than 0.1 dB within one-hour monitoring. Consequently, multi-wavelength DSs are verified with good stability at room temperature. To further investigate the dual-state DSs, we consider using some novel mode lockers including either graphene [17] or topological insulator [18–20], instead of the SESAM. We believe that both wavelength-tunable operation and multiwavelength DS generation can be certainly observed as long as the cavity birefringence is well managed. Moreover, it is expected that both the range of wavelength tuning and the number of multi-wavelength can be further improved using novel mode-locker with broadband bandwidth of saturable absorption. In conclusion, we experimentally investigate dual-state DSs in passively mode-locked EDFL using SESAM. As for the wavelength-tunable operation, the tunable wavelength ranges from 1554 to 1561 nm with the spectra maintaining flat tops and steep edges. In terms of the multi-wavelength operation, triple-wavelength DSs at 1535, 1544, and 1553 nm can be simultaneously obtained, when the pump power is 80 mW. Such flexible fiber laser source may find versatile applications in fiber sensing, optical signal processing, and fiber communication systems. National Key Scientific Instrument and Equipment Development Project (2013YQ16048702); National Natural Science Foundation of China (NSFC) (61275069, 61331010).
REFERENCES 1. S. M. Zhang, Q. S. Meng, and G. Z. Zhao, Eur. Phys. J. D 60, 383 (2010). 2. X. He, Z. Liu, and D. N. Wang, Opt. Lett. 37, 2394 (2012). 3. B. Ibarra-Escamilla, O. Pottiez, J. W. Haus, E. A. Kuzin, M. BelloJimenez, and A. Flores-Rosas, J. Eur. Opt. Soc. 3, 08036 (2008). 4. Z. P. Sun, D. Popa, T. Hasan, F. Torrisi, F. Q. Wang, E. J. R. Kelleher, J. C. Travers, V. Nicolosi, and A. C. Ferrari, Nano Res. 3, 653 (2010). 5. K. Tamura, C. R. Doerr, H. A. Haus, and E. P. lppen, IEEE Photon. Technol. Lett. 6, 697 (1994). 6. Z. C. Luo, A. P. Luo, and W. C. Xu, Photon. J. 3, 64 (2011). 7. Z. C. Luo, A. P. Luo, W. C. Xu, H. S. Yin, J. R. Liu, Q. Ye, and Z. J. Fang, Photon. J. 2, 571 (2010). 8. L. M. Zhao, D. Y. Tang, and J. Wu, Opt. Lett. 31, 1788 (2006). 9. W. H. Renninger, A. Chong, and F. W. Wise, Phys. Rev. A 77, 023814 (2008). 10. A. Chong, W. H. Renninger, and F. W. Wise, Opt. Lett. 32, 2408 (2007). 11. H. Zhang, D. Y. Tang, R. J. Knize, L. M. Zhao, Q. L. Bao, and K. P. Loh, Apply. Phys. Lett. 96, 111112 (2010). 12. Y. D. Gong, X. L. Tian, M. Tang, P. Shum, M. Y. W. Chia, V. Paulose, J. Wu, and K. Xu, Opt. Commun. 265, 628 (2006). 13. D. U. Noske, M. J. Guy, K. Rottwitt, R. Kashyap, and J. R. Taylor, Opt. Commun. 108, 297 (1994). 14. H. Zhang, D. Y. Tang, and L. M. Zhao, Opt. Express 17, 12692 (2009). 15. L. M. Zhao, D. Y. Tang, T. H. Cheng, H. Y. Tam, and C. Lu, Opt. Lett. 32, 1581 (2007). 16. H. Sunnerud, M. Skold, M. Westlund, and P. A. Andrekson, J. Lightwave Technol. 30, 3747 (2012). 17. Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, Adv. Mater. 19, 3077 (2009). 18. H. Zhang, S. B. Lu, J. Zheng, J. Du, S. C. Wen, D. Y. Tang, and K. P. Loh, Opt. Express 22, 7249 (2014). 19. K. Wu, X. Y. Zhang, J. Wang, and J. P. Chen, Opt. Lett. 40, 1374 (2015). 20. Q. K. Wang, Y. Chen, L. L. Miao, G. B. Jiang, S. Q. Chen, J. Liu, X. Q. Fu, C. J. Zhao, and H. Zhang, Opt. Express 23, 7681 (2015).
Letter
Vol. 40, No. 12 / June 15 2015 / Optics Letters
Dual-state dissipative solitons from an all-normaldispersion erbium-doped fiber laser: continuous wavelength tuning and multi-wavelength emission ZHICHAO WU,1 SONGNIAN FU,1,2,* CHANGXIU CHEN,2 MING TANG,1,2 PERRY SHUM,3
AND
DEMING LIU2
1
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China National Engineering Laboratory for Next Generation Internet Access System, School of Optics and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China 3 Centre for Optical Fibre Technology, Nanyang Technological University, Singapore 637553, Singapore *Corresponding author: [email protected] 2
Received 2 April 2015; revised 7 May 2015; accepted 14 May 2015; posted 15 May 2015 (Doc. ID 237171); published 2 June 2015
We propose and experimentally demonstrate switchable operation of dissipative solitons (DSs) in an erbium-doped fiber laser. By managing normal dispersion of laser cavity, the 3-dB spectral bandwidth up to 8.1 nm can be obtained with the help of a semiconductor saturable absorber mirror. Using an inline polarizer, the fiber laser can be separately operated at either wavelength-tunable or multi-wavelength emission. The central wavelength of DS can be continuously tuned from 1554 to 1561 nm with its spectra maintaining standard rectangular shape. Alternatively, triple-wavelength DSs at 1535, 1544, and 1553 nm can be simultaneously obtained. © 2015 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.4050) Mode-locked lasers; (060.5530) Pulse propagation and temporal solitons. http://dx.doi.org/10.1364/OL.40.002684
Wavelength-tunable and multi-wavelength mode-locked fiber lasers have attracted considerable research interest in fiber sensing, optical signal processing, and telecommunication systems. Various approaches have been exploited to achieve tunable and multiple wavelengths mode-locked fiber lasers. In order to obtain wavelength-tunable mode-locking operation, several wavelength-selective elements are inserted into the laser cavity, such as a Mach–Zehnder interferometer [1], chirped fiber Bragg grating [2], Sagnac fiber filter [3], and tunable bandpass filter [4]. However, these additional components extend the cavity length, reduce the repetition rate of output pulse, and increase the implementation cost. Alternatively, the mechanism of fiber cavity birefringence paves another way to achieve tunable mode-locking operation [5–7]. In particular, we notice that traditional solitons have been widely demonstrated and comprehensively discussed in erbium-doped fiber laser (EDFL). Actually, DSs in normal dispersion cavity possess advantages of large normal chirp, rectangular-shaped spectra, and high output pulse energy, compared with traditional solitons [8–10]. 0146-9592/15/122684-04$15/0$15.00 © 2015 Optical Society of America
Recently, continuously wavelength-tunable DSs over a wide range of 1570–1600 nm have been demonstrated in an EDFL [11]. However, the spectral profiles can’t keep standard rectangular shape, and 3-dB bandwidth is varied during wavelength tuning. In terms of multi-wavelength mode-locking operation, the nonlinear polarization rotation (NPR) and nonlinear amplifying loop mirror (NALM) techniques are widely utilized [12,13]. However, NPR- and NALM-based technologies generally need high pump power and careful adjustment in order to realize mode locking. Thus, semiconductor saturable absorber mirror (SESAM) serves as a better choice. Stable dualand triple-wavelength DSs formed in EDFL have been reported, using SESAM as mode-locker [14]. However, the wavelength spacing is only 4 nm, which may limit the practical applications as multi-wavelength light source. In this Letter, we experimentally demonstrate switchable DSs operation of both wavelength-tunable and multiwavelength within the same EDFL configuration. To the best of our knowledge, it is the first time that both wavelengthtunable and multiple wavelengths passive-mode locking have been realized using an all-normal-dispersion EDFL, with the help of SESAM. Without additional optical filters, the optical spectrum can be continuously tuned from 1554 to 1561 nm with stable rectangular shape. Moreover, we also obtain dualwavelength (1553 nm and 1562 nm) and triple-wavelength (1535 nm, 1544 nm, and 1553 nm) DSs, respectively, with a wide wavelength spacing of 9 nm. The proposed fiber laser is schematically showed in Fig. 1. The self-starting mode-locking operation is achieved by a commercial SESAM (BATOP, SAM-1550-22-12ps-x), which is deposited on the end surface of a standard FC/PC fiber connector. The SESAM possesses low-intensity absorption of 22% at the wavelength of 1550 nm, modulation depth of 13%, saturation fluence of 40 μJ∕cm2 , nonsaturable loss of 9%, damage threshold of 800 μJ∕cm2 , and relaxation time of 12 ps. The circulator is used to incorporate the SESAM into the cavity, as well as guarantee unidirectional propagation and suppress detrimental reflections. A 1.3-m EDF with a group velocity dispersion (GVD) parameter of −18.72 ps∕nm∕km is used
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Fig. 1. Experimental configuration.
as the gain medium, which is pumped by a 980-nm laser diode (LD) with maximum output power of 400 mW. The absorption coefficient of the EDF is 21.9 dB/m at 980 nm. A 2.4-m dispersion compensation fiber (DCF) with GVD parameter of ∼ − 38 ps∕nm∕km is used to achieve dispersion management. The fiber pigtails of optical components are 6.2-m standard single-mode fiber (SMF) with GVD parameter of 17 ps∕nm∕km. Thus, the net dispersion of the ring cavity is estimated as 0.13 ps2 . The total optical cavity length is estimated around 9.9 m. Two polarization controllers (PCs) are used to manage the state of polarization (SOP) within the ring cavity. Note that, due to the use of inline polarizer, we can form an artificial birefringent filter in the cavity [11]. A 10:90 fiber optical coupler (OC) is utilized as the output port of the fiber laser. Finally, an optical spectrum analyzer (OSA, Yokogawa AQ6370C-20) with a resolution of 0.02 nm and a 5-GHz oscilloscope (OSC, Tektronix DPO 4104) together with a 40-GHz photodetector (PD) are applied to monitor the optical spectrum and the output pulse-train, respectively. The radiofrequency (RF) spectrum is measured by a 40-GHz electrical spectrum analyzer (ESA, Agilent E4447A). Additionally, the pulse profile is measured by a commercial autocorrelator (APE PulseCheck 50). First, we focus on the generation of DSs in the absence of the polarizer. Continuous wave (CW) laser emission at the central wavelength of 1559 nm is observed when the pump power is above 20 mW. When the pump power is further increased to 63 mW, self-starting mode locking at 1559 nm can be observed, as shown in Fig. 2, with an average output power of 1 mW. Figure 2(a) presents the rectangular-shaped spectrum with steep spectral edges, which proves the generation of DSs in the normal dispersion region. The 3-dB spectrum bandwidth is 8.1 nm, and the repetition rate of the pulse-train is 20.3 MHz, corresponding to the cavity length of 9.9 m, as shown in Fig. 2(b). The RF spectrum is illustrated in Fig. 2(c), and the inset corresponds to the fundamental frequency with a signal-to-noise ratio of more than 60 dB when the resolution of ESA is set 100 Hz. The full-width at half maximum (FWHM) of the pulse is 19.8 ps, as shown in Fig. 2(d). Therefore, the time-bandwidth product of the pulses is ∼20, which is larger than the transformed limited products. We can conclude that the mode-locked pulses are strongly chirped. Next, in order to obtain the wavelength-tunable operation, we insert an inline polarizer into the cavity between DCF and PC1. The central wavelength is shifted to 1555 nm, and 3-dB spectral bandwidth is reduced to ∼2 nm, due to the artificial
Fig. 2. DS generation at 1559 nm: (a) optical spectrum; (b) oscilloscope trace of the pulse-train; (c) RF spectrum and fundamental frequency signal (inset); (d) autocorrelation trace.
birefringent filter induced by the polarizer [7]. Although the EDF has a broad gain profile, considering the bandwidth limitation of the birefringent filter, the bandwidth of effective gain becomes narrow. In order to investigate the effect of pump power, we fix the PCs on the condition of mode locking. With the growing of pump power from 60 to 230 mW, the central wavelength stays unchanged. As for the 3-dB spectral bandwidth, it becomes wider first and drops back to 1.75 nm at 110 mW, then turns wider again and falls to 2.16 nm at 190 mW. The entire process is shown in Figs. 3(a) and 3(b). The drops can be explained as pulse broken, due to the pulse peak clamping effect [15]. Moreover, pulse broadening in time domain gives rise to spectrum narrowing. Under the same principle, triple solitons can be generated in case the pump power is further increased to 190 mW. Figures 3(c) and 3(d) present the pulse-trains of dual solitons and triple solitons, respectively. Then, under the pump power of 63 mW, the central wavelength of DS can be continuously tuned by proper adjustment of the PCs. If the PCs are varied to the initial state, the mode
Fig. 3. (a) 3-dB spectral bandwidth at different pump power; (b) optical spectra at pump power of 70 mW, 130 mW, and 190 mW; (c) dual solitons pulse-train; (d) triple solitons pulse-train.
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Fig. 5. (a) Peak power; (b) output power; (c) repetition rate of pulse-train; and (d) 3-dB spectral bandwidth vary with respect to the tunable wavelength, respectively. Fig. 4. Continuously tunable wavelength from 1554 to 1561 nm with the insets of corresponding pulse-trains.
locking can be achieved at 1555 nm again. Meanwhile, the DS with steep spectral edges can be always generated in our fiber ring cavity. As a result, wavelength-tunability of normal dispersion mode-locked operation is successfully obtained from 1554 to 1561 nm. Different from previous wavelength-tunable mechanism using additional optical filter, our PCs together with the inline polarizer provide a cavity birefringent filter in the designated band. As long as we rotate the orientation of the PCs, the central wavelength of the invisible birefringent filter can be continuously tuned [11]. Figure 4 specifically illustrates the optical spectra in four designated wavelengths. The 3-dB spectral bandwidths are 1.83 nm, 1.87 nm, 1.86 nm, and 1.81 nm, respectively, corresponding to the central wavelengths of 1553, 1556, 1558, and 1561 nm. It is obvious that the rectangular shapes of the spectra remain almost unchanged, and the characteristics of pulse-trains have no significant difference as well, as shown in four insets. Those characteristics of proposed fiber laser output are ideal for the application of optical sampling systems [16]. Since both polarization and intra-cavity loss are varied, the soliton parameters including 3-dB spectral bandwidths, output powers, peak powers, and repetition rates of tunable DS are characterized with small fluctuations simultaneously, as shown in Figs. 5(a)–5(d). Next, we further rotate the orientation of PCs to change the cavity birefringent filter. In Fig. 6(a), two rectangular-shaped spectra at central wavelengths of 1553 and 1562 nm appear simultaneously, indicating of stable dual-wavelength mode locking. The 1553-nm and 1562-nm DSs have 3-dB spectral bandwidths of 1.62 and 1.57 nm, and peak powers of −31.68 dBm and −31.66 dBm, respectively. In case we continuously increase pump power to
80 mW and further adjust the PC properly, triple-wavelength DSs at 1535, 1544, and 1553 nm can be achieved. The optical spectrum is shown in Fig. 6(c). Three DSs have 3-dB spectral bandwidth of 1.54, 1.86, and 1.61 nm, peak powers of −33.82 dBm, −33.83 dBm, and −33.98 dBm, respectively. Since the wavelength spacing between two DSs is 9 nm, we don’t observe additional peak from the RF spectrum of dualwavelength DSs, due to the limited bandwidth of PD and ESA. As for the time domain characterization, Figs. 6(b) and 6(d) show the snapshots of multi-wavelength DS pulse trains. Each soliton has different pulse energies that are observed with different amplitude on the oscilloscope screen. Due to the
Fig. 6. Dual- and triple-wavelength DSs: (a),(b) optical spectrum and pulse-train of dual-wavelength; (c),(d) optical spectrum and pulse-train of triple-wavelength.
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Fig. 7. Repeating scans of the multi-wavelength DSs output with 10-min intervals: (a) dual-wavelength operation, (b) triple-wavelength operation.
difference of group velocity within the cavity, two soliton pulses have different repetition rates. Therefore, when we trigger with one soliton pulse, the other one will move randomly on the oscilloscope screen [14]. The long-term stability of the multiwavelength DSs is shown in Figs. 7(a) and 7(b), and we carry out repeating scans of 7 times with 10-min intervals for the dual- and triple-wavelength operations, respectively. No significant wavelength drift is observed, and the power fluctuation is less than 0.1 dB within one-hour monitoring. Consequently, multi-wavelength DSs are verified with good stability at room temperature. To further investigate the dual-state DSs, we consider using some novel mode lockers including either graphene [17] or topological insulator [18–20], instead of the SESAM. We believe that both wavelength-tunable operation and multiwavelength DS generation can be certainly observed as long as the cavity birefringence is well managed. Moreover, it is expected that both the range of wavelength tuning and the number of multi-wavelength can be further improved using novel mode-locker with broadband bandwidth of saturable absorption. In conclusion, we experimentally investigate dual-state DSs in passively mode-locked EDFL using SESAM. As for the wavelength-tunable operation, the tunable wavelength ranges from 1554 to 1561 nm with the spectra maintaining flat tops and steep edges. In terms of the multi-wavelength operation, triple-wavelength DSs at 1535, 1544, and 1553 nm can be simultaneously obtained, when the pump power is 80 mW. Such flexible fiber laser source may find versatile applications in fiber sensing, optical signal processing, and fiber communication systems. National Key Scientific Instrument and Equipment Development Project (2013YQ16048702); National Natural Science Foundation of China (NSFC) (61275069, 61331010).
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