Widely-pulsewidth-tunable ultrashort pulse generation from a birefringent carbon nanotube mode-locked fiber laser Ya Liu, Xin Zhao, Jiansheng Liu, Guoqing Hu, Zheng Gong, and Zheng Zheng* School of Electronic and Information Engineering, Beihang University, 37 Xueyuan Rd, Beijing 100191, China *[email protected]
Abstract: We demonstrate the generation of soliton pulses covering a nearly one order-of-magnitude pulsewidth range from a simple carbon nanotube (CNT) mode-locked fiber laser with birefringence. A polarizationmaintaining-fiber-pigtailed, inline polarization beam splitter and its associated birefringence is leveraged to either enable additional nonlinear polarization evolution (NPE) mode-locking effect or result in a bandwidthtunable Lyot filter, through adjusting the intracavity polarization settings. The large pulsewidth tuning range is achieved by exploiting both the nonlinear CNT-NPE hybrid mode-locking mechanism that narrows the pulses and the linear filtering effect that broadens them. Induced vector soliton pulses with pulsewidth from 360 fs to 3 ps can be generated, and their time-bandwidth products indicate they are close to transform-limited. ©2014 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (320.7090) Ultrafast lasers.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
M. E. Fermann and I. Hartl, “Ultrafast fiber laser technology,” IEEE J. Sel. Top. Quantum Electron. 15(1), 191– 206 (2009). J. Squier and M. Müller, “High resolution nonlinear microscopy: A review of sources and methods for achieving optimal imaging,” Rev. Sci. Instrum. 72(7), 2855–2867 (2001). L. E. Nelson, D. J. Jones, K. Tamura, H. A. Haus, and E. P. Ippen, “Ultrashort-pulse fiber ring lasers,” Appl. Phys. B 65(2), 277–294 (1997). X. Yang, K. Hammani, D. J. Richardson, and P. Petropoulos, “Passively mode-locked fiber laser incorporating adaptive filtering and dispersion management,” in CLEO: Science and Innovations, June 9 - 14, 2013, (Optical Society of American, 2013), paper CM1I.1. P. L. Huang, S.-C. Lin, C.-Y. Yeh, H.-H. Kuo, S.-H. Huang, G.-R. Lin, L.-J. Li, C.-Y. Su, and W.-H. Cheng, “Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber,” Opt. Express 20(3), 2460–2465 (2012). D. Churin, K. Kieu, and N. Peyghambarian, “The role of the saturable absorber in a mode-locked fiber laser,” Proc. SPIE 8237, 823722 (2012). J.-C. Chiu, Y.-F. Lan, C.-M. Chang, X.-Z. Chen, C.-Y. Yeh, C.-K. Lee, G.-R. Lin, J.-J. Lin, and W.-H. Cheng, “Concentration effect of carbon nanotube based saturable absorber on stabilizing and shortening mode-locked pulse,” Opt. Express 18(4), 3592–3600 (2010). S. Yamashita, Y. Inoue, S. Maruyama, Y. Murakami, H. Yaguchi, T. Kotake, and S. Y. Set, “Mode-locked fiber lasers using adjustable saturable absorption in vertically aligned carbon nanotubes,” Jpn. J. Appl. Phys. 45(1), L17–L19 (2006). H. Jeong, S. Y. Choi, F. Rotermund, and D.-I. Yeom, “Pulse width shaping of passively mode-locked soliton fiber laser via polarization control in carbon nanotube saturable absorber,” Opt. Express 21(22), 27011–27016 (2013). V. A. Lazarev, S. S. Sazonkin, A. B. Pniov, K. P. Tsapenko, A. A. Krylov, and E. D. Obraztsova, “Hybrid modelocked ultrashort-pulse erbium-doped fiber laser,” J. Phys. Conf. Ser. 486, 012004 (2014). G. E. Villanueva and P. Pérez-Millán, “Dynamic control of the operation regimes of a mode-locked fiber laser based on intracavity polarizing fibers: experimental and theoretical validation,” Opt. Lett. 37(11), 1971–1973 (2012). X. Li, W. Zou, and J. Chen, “41.9 fs hybridly mode-locked Er-doped fiber laser at 212 MHz repetition rate,” Opt. Lett. 39(6), 1553–1556 (2014). S. Kim, Y. Kim, J. Park, S. Han, S. Park, Y.-J. Kim, and S.-W. Kim, “Hybrid mode-locked Er-doped fiber femtosecond oscillator with 156 mW output power,” Opt. Express 20(14), 15054–15060 (2012).
#215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21012
14. N. S. Shahabuddin, H. Mohamad, M. A. Mahdi, Z. Yusoff, H. Ahmad, and S. W. Harun, “Passively modelocked soliton fiber laser using a combination of saturable absorber and nonlinear polarization rotation technique,” Microw. Opt. Technol. Lett. 54(6), 1430–1432 (2012). 15. Z.-C. Luo, A.-P. Luo, and W.-C. Xu, “Multiwavelength picosecond and single wavelength femtosecond pulses emission in a passively mode-locked fiber laser using a semiconductor saturable absorber mirror and a contrast ratio tunable comb filter,” Appl. Opt. 50(18), 2831–2835 (2011). 16. Y. Fukuchi, J. Maeda, and Ieee, “Widely wavelength-tunable and pulsewidth-variable harmonically modelocked short-cavity fiber ring laser using a bismuth-oxide-based highly nonlinear erbium-doped fiber,” in 35th European Conference on Optical Communication, ECOC 2009, SEP 20–24, 2009 (IEEE, 2009). 17. P. S. Liang, Z. X. Zhang, Q. Q. Kuang, and M. H. Sang, “All-fiber birefringent filter with fine tunability and changeable spacing,” Laser Phys. 19(11), 2124–2128 (2009). 18. Z.-C. Luo, A.-P. Luo, W.-C. Xu, H. S. Yin, L. Jia-Rui, Q. Ye, and Z. J. Fang, “Tunable multiwavelength passively mode-locked fiber ring laser using intracavity birefringence-induced comb filter,” IEEE Photon. J. 2(4), 571–577 (2010). 19. Z. R. Cai, W. J. Cao, A. P. Luo, Z. B. Lin, Z. C. Luo, and W. C. Xu, “A wide-band tunable femtosecond pulse fiber laser based on an intracavity-birefringence induced spectral filter,” Laser Phys. 23(3), 035107 (2013). 20. Y. S. Fedotov, S. M. Kobtsev, R. N. Arif, A. G. Rozhin, C. Mou, and S. K. Turitsyn, “Spectrum-, pulsewidth-, and wavelength-switchable all-fiber mode-locked Yb laser with fiber based birefringent filter,” Opt. Express 20(16), 17797–17805 (2012). 21. Z. X. Zhang, Z. Q. Ye, M. H. Sang, and Y. Y. Nie, “Nonlinear-polarization-rotation based multiwavelength erbium-doped fiber lasers with highly nonlinear fiber,” Laser Phys. 21(10), 1820–1824 (2011). 22. M. L. Dennis and I. N. Duling III, “Intracavity dispersion measurement in modelocked fibre laser,” Electron. Lett. 29(4), 409–411 (1993). 23. J. W. Nicholson, R. S. Windeler, and D. J. Digiovanni, “Optically driven deposition of single-walled carbonnanotube saturable absorbers on optical fiber end-faces,” Opt. Express 15(15), 9176–9183 (2007). 24. A. Martinez, K. Fuse, B. Xu, and S. Yamashita, “Optical deposition of graphene and carbon nanotubes in a fiber ferrule for passive mode-locked lasing,” Opt. Express 18(22), 23054–23061 (2010). 25. D. Y. Tang, L. M. Zhao, B. Zhao, and A. Q. Liu, “Mechanism of multisoliton formation and soliton energy quantization in passively mode-locked fiber lasers,” Phys. Rev. A 72(4), 043816 (2005). 26. 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). 27. X. Zhao, Z. Zheng, L. Liu, Y. Liu, Y. Jiang, X. Yang, and J. Zhu, “Switchable, dual-wavelength passively modelocked ultrafast fiber laser based on a single-wall carbon nanotube modelocker and intracavity loss tuning,” Opt. Express 19(2), 1168–1173 (2011).
1. Introduction Both femtosecond and picosecond mode-locked fiber lasers have been extensively studied in the past couple of decades, as they cover a wide range of interesting applications, such as high-precision metrology, nonlinear imaging, nanofabrication and optical signal processing [1]. The difference in their pulsewidths could be leveraged to access different excitation regimes in applications like bioimaging [2]. Even though, by extra-cavity pulse shaping or spectral filtering of transform-limited laser pulses, their pulsewidths could be significantly increased over a large range, it comes at the cost of increased system complexity or poor energy efficiency. Thus, it would be useful to develop lasers with pulsewidth-tunable outputs. Through studies on the pulse formation dynamics, it’s well-known that the cavity configuration and mode-locking regime determined by the intracavity dispersion and nonlinearity distribution could dictate the range of the output pulsewidth. In the soliton regime we discuss here, the continuum generation sets a limit on the minimal pulsewidth under certain cavity dispersion [3]. Varying the net cavity dispersion is relatively complicated to implement. Recently, using a programmable spectral phase filter to impose different dispersion profiles, it was shown that passively mode-locked picosecond pulses with different pulsewidths could be generated [4]. For those lasers with saturable absorbers (SAs), for example those fabricated with carbon nano-material like single-walled carbon nanotube (SWNT) or graphene, the output pulsewidth could also be significantly affected by the nonlinear characteristics of the SA’s, such as the modulation depth, saturation intensity and nonsaturable losses [5–7]. Though it is very challenging to vary such SA’s characteristics in the cavity, interesting approaches leveraging the polarization-dependent variations in the nonlinear response of SWNT [8] or the waveguide modal distribution of the SA [9] have realized the tuning of the pulsewidth by up to a factor of ~4. Besides tuning the SA, the introduction of additional pulse narrowing mechanisms such as the nonlinear polarization evolution (NPE) effect could enable hybrid #215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21013
mode-locking [10–14]. This could generate significantly shorter pulses than those by the SA scheme alone, while keeping many advantages of the latter. On the other hand, intracavity optical bandwidth affected by the gain profile of the gain media and the spectral width of any optical filters in the cavity also plays a critical role in determining the pulsewidth. Thus, filters with a tunable pass bandwidth could be applied in mode-locked lasers to vary the spectral bandwidth as well as the oscillating wavelengths [15, 16]. Among the filtering schemes, all-fiber Lyot filters consisting of birefringent fiber and polarization-dependent loss components [17] have been often used in mode-locked fiber lasers, due to its fiber-optic compatibility. Based on the principle of polarization interferometry, a transmission comb filter with a well-defined frequency spacing and tunable contrast ratio can be realized [18–21]. Either the inherent weak intracavity birefringence or additional high-birefringence fiber can provide the birefringence needed. Tuning the spectral shape and peak positions of the Lyot filtering profile has been leveraged to not only control the bandwidth and pulsewidth but also modify the lasering wavelength [19, 20]. Yet, the tunable range of the pulse bandwidth is often limited before the laser stops mode-locking as the pulse gets too long. In this paper, we demonstrate a scheme that effectively leverages the nonlinear, NPEenabled hybrid mode-locking effect as well as the linear, polarization interferometric filtering [20] to significantly increase the pulsewidth tuning range of a passively mode-locked laser. It is achieved by simply introducing an inline polarization beam splitter (PBS) with polarization-maintaining fiber (PMF) pigtails into a fiber ring laser with a SWNT SA modelocker. Ultrashort pulses covering a nearly one order-of-magnitude pulsewidth range from 360 fs to 3 ps can be generated from the same cavity by tuning the intracavity polarization state and slightly optimizing the pumping condition. 2. Experimental setup and principle Figure 1 shows the schematic of the laser cavity. It consists of a 2-meter-long piece of highly doped Erbium-doped fiber (EDF, Changfei 1022) pumped by a 980-nm pump laser through a 980/1550 wavelength-division multiplexer (WDM), 4.4-meter-long single mode fiber (SMF, including the pigtail fiber of all components), and a fiber inline polarization beam splitter (PBS) with 1.2-meter-long polarization maintaining fiber (PMF) pigtails at each of its two output ports. A polarization-independent isolator (ISO) is used to ensure the unidirectional operation. To better monitor the pulse characteristics at different positions of the cavity, two 10/90 optical couplers (OC’s) are placed before and after the PBS. The output Port 2 (working at fast axis) of the PBS is used as a monitor port of the laser, while the output Port 1 (working at slow axis) is connected back into the loop. The optical spectrum of the laser output is measured by an optical spectrum analyzer (Agilent 86142B) and the temporal pulse shape by a home-built, second-harmonic intensity autocorrelator. The repetition rate of the output is measured by an Agilent N9320B RF spectrum analyzer following a 1-GHz photodetector (New Focus 1611).
Fig. 1. Schematic diagram of the laser cavity.
The absorption coefficient of the EDF is ~18 dB/m at 980 nm and its dispersion is estimated as −6 ps/km/nm at 1560 nm. The beat length Lb of the PMF is ~4 mm and its dispersion is estimated as ~18.4 ps/km/nm at 1560 nm. The total net cavity dispersion is estimated to be −0.11 ps2, which is in good agreement with the estimated −0.12 ps2 from the #215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21014
separations of the Kelly sidebands on the mode-locked spectrum and the pulsewidth [22]. The carbon nanotube mode locker is fabricated on an FC/APC ferrule using the optical deposition method [23, 24] with ~1 dB insertion loss at 1550 nm. Besides a fiber-squeezer-type polarization controller (PC) in the cavity to adjust the state of polarization, an additional fiber polarization controller with only one 4-coil ring is applied as roughly an in-line half-wave plate (HWP) to provide more quantifiable control of the state of polarization. Adjusting the polarization rotation introduced by HWP would change the transmission ratio of the light through PBS. While ideally the PMF after the PBS should not introduce much birefringent effect if its slow axis is perfectly aligned with the direction of polarization of the transmitted light, the inline PBS with a finite extinction ratio (ER) may result in a weak Lyot filter effect, as we observe here. To analyze the effect brought by its finite ER, separate tests of the PBS is carried out. An amplified spontaneous emission (ASE) light source between 1530 and 1560 nm is connected to the PBS used in the cavity followed by the HWP and another inline PBS. The transmission spectrum is measured under different polarization rotation introduced by the HWP. With certain residual birefringence introduced by the PM fiber pigtail, the output spectrum from the second PBS would possess the corresponding spectral interference pattern. This also emulates the situation in the laser cavity where the light repeatedly passes through the PBS, possibly leading to interference effect in the presence of polarization rotation. By assuming a 0.7° misalignment between the direction of polarization of the transmitted light from the PBS component and the slow axis of the PMF pigtail (derived from the 39 dB extinction ratio at Port 1 of the in-line PBS device), the transmission can be theoretically calculated as in [11, 18, 25]. The normalized experimental results in Fig. 2 show weak Lyot filtering effect when HWP is adjusted to different positions. Theoretical curves based on the estimated phase rotation angles θ caused by HWP match the experimental ones reasonably well. Noticeable changes in the average transmission and positions of the transmission peaks are seen for different θ [20].
Fig. 2. Transmission spectra of the PBS at different input polarization orientation.
In order to further investigate the polarization characteristics of the pulse in this birefringent cavity, the output from OC 1 can be sent through an external PBS after a PC to resolve its field components in different polarizations. 3. Experimental results and discussions When the pump power of the 980-nm pump laser is set at 10 mW and the intracavity loss, i.e. the power at port 2, is minimized by adjusting the PC, the mode-locking can self-start at a repetition rate of 27.2 MHz. The PC’s orientation is then fixed at this condition in the subsequent experiments. In this case, θ is considered as ~0°, and the pulse experiences little linear polarization rotation after each round trip and is nearly linear polarization along the slow axis of the PMF before entering PBS. The spectra at the output ports of OC 1 and OC 2 are measured and shown in Fig. 3(a). The corresponding 3-dB bandwidths in Fig. 3(a) are ~4.3 nm. The autocorrelation traces of the outputs of OC 1 and OC 2 are shown in Fig. 3(b) with full-width-at-half-maximum (FWHM) of 600 and 663 fs, if sech2-shape is assumed. The time-bandwidth products (TBPs) are 0.32 and 0.35, respectively, suggesting nearly transformlimited solitons, with small chirps in the latter possibly introduced by the PBS. #215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21015
Additional measurements of the pulse at OC 1 are done to more thoroughly examine its polarization characteristics. By adjusting the PC before the external PBS so that the pulse intensity measured in one port is maximized, it is observed that at the other port there exists a weak but non-negligible spectrum. This polarization-resolved minor component of the spectrum as seen in Fig. 3(a) indicates that the pulse is a vector soliton with a weak orthogonal polarization component, e.g. an induced soliton [26]. The modulation in the spectrum of the induced weak soliton could be caused by the Lyot filter effect.
Fig. 3. (a) Optical spectra of the outputs of OC 1 and OC 2 and polarization-resolved spectrum of the induced weak soliton at OC 1; (b) autocorrelation traces of the outputs, when θ = 0°.
When rotating the HWP to increase θ and keeping the pump power steady, the output power at port 2 significantly increases, while the output power of OC 2 is reduced. Yet, the bandwidth of the laser output gradually increases with decreased separation between the Kelly sidebands. This shows typical signs of hybrid mode-locking with NPE, where the nonlinear polarization rotation could offset some of the linear polarization rotation at higher instantaneous powers. When the polarization rotation resulted from the HWP reaches 37°, the pump power needs to be increased to above 10 mW in order to maintain the mode-locking and compensate the increased loss. Maximal bandwidth is reached at θ = 51° and 17 mW of pump power, beyond which condition the laser stops mode-locking even under higher pump powers. We note that the laser cannot be mode-locked solely by NPE without an SWNT SA under any pumping conditions and the nonlinear-phase-shift-involved phenomenon is only seen when the mode-locking is initiated by the SA as described above. Based on the comparison of the spectra at different θ, as shown in Fig. 3(a) and Fig. 4, one could see an obvious bandwidth broadening and a slight blue-shift of the spectra. The blueshift is likely caused by the changes in the EDF’s gain tilt resulted from the varied intracavity loss [27]. The 3dB bandwidth reaches ~7.5 nm, which is far larger than that of a laser configuration using the same SWNT mode-locker, similar total dispersion but without the PBS. This further verifies the effect of hybrid mode-locking mechanism. The magnitude of the weaker portion of the vector soliton also significantly increases at larger θ.
Fig. 4. Optical spectra of the outputs of OC 1 and OC 2 and polarization-resolved spectrum of the induced weak soliton at OC 1, when (a) θ = 37° and (b) θ = 51°.
On the other hand, when the HWP is rotated in the opposite direction, the spectral width quickly narrows, as seen in Fig. 5(a). The pump power needs to be slightly reduced, in order to avoid CW peaking and harmonic mode-locking. The minimal θ to maintain mode-locking
#215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21016
is −40°. The output spectra are shifted to the shorter wavelength side, due to the likely gain profile changes and, later, the peak positions of the Lyot filter. The fast narrowing of the spectra could be attributed to the increased contrast ratio, i.e. the reduced effective bandwidth, of the Lyot filter as described in Section 2, and the lower intracavity power and nonlinearity.
Fig. 5. (a) Optical spectra and (b) autocorrelation traces of the outputs of OC 1 with different θ.
Fig. 6. (a) Pulse durations, spectral widths and TBPs of the outputs of OC 1 and (b) pump power, output power of the outputs of OC 1 and the monitored power with different θ.
The autocorrelation traces of the outputs of OC 1 at different polarization orientations are shown in Fig. 5(b). Pulses with FWHM durations from 3 ps to 360 fs are generated, and the corresponding 3-dB spectral bandwidths vary from 0.9 to 7.5 nm. For the shortest pulse with a small sidelobe, the peak-to-valley ratio of the autocorrelation trace is ~100. The trend of pulse durations and spectral bandwidths at differentθ are summarized in Fig. 6(a). Their TBPs are labeled for each case, indicating that the pulses are close to transform-limited. The pump power, the output power at OC 1 and port 2 also shown in Fig. 6(b) indicate that the increase in the pump power under the hybrid mode-locking regime is to mostly compensate the sharp increase in the loss at the PBS. Due to the constant soliton area rule [3], the output power drops for the longer pulses under lower pump powers, yet sufficient to introduce self-phase modulation for soliton generation. 4. Conclusion We show that through incorporating SWNT mode-locker with an inline fiber PBS, soliton pulses covering nearly one order-of-magnitude pulsewidth range from femtosecond to picosecond could be generated by mostly tuning the intracavity state of polarization setting. Helped with both the linear Lyot filter and the nonlinear NPE effect generated by the same physical components, the induced vector soliton pulses can be either broadened or narrowed, respectively. The transition of the effect from nonlinear to linear is clearly observed by varying the polarization ‘bias’ in the cavity. Acknowledgments This work at Beihang University was supported by 973 Program (2012CB315601) and NSFC (61221061/61107057).
#215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21017
Abstract: We demonstrate the generation of soliton pulses covering a nearly one order-of-magnitude pulsewidth range from a simple carbon nanotube (CNT) mode-locked fiber laser with birefringence. A polarizationmaintaining-fiber-pigtailed, inline polarization beam splitter and its associated birefringence is leveraged to either enable additional nonlinear polarization evolution (NPE) mode-locking effect or result in a bandwidthtunable Lyot filter, through adjusting the intracavity polarization settings. The large pulsewidth tuning range is achieved by exploiting both the nonlinear CNT-NPE hybrid mode-locking mechanism that narrows the pulses and the linear filtering effect that broadens them. Induced vector soliton pulses with pulsewidth from 360 fs to 3 ps can be generated, and their time-bandwidth products indicate they are close to transform-limited. ©2014 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (320.7090) Ultrafast lasers.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
M. E. Fermann and I. Hartl, “Ultrafast fiber laser technology,” IEEE J. Sel. Top. Quantum Electron. 15(1), 191– 206 (2009). J. Squier and M. Müller, “High resolution nonlinear microscopy: A review of sources and methods for achieving optimal imaging,” Rev. Sci. Instrum. 72(7), 2855–2867 (2001). L. E. Nelson, D. J. Jones, K. Tamura, H. A. Haus, and E. P. Ippen, “Ultrashort-pulse fiber ring lasers,” Appl. Phys. B 65(2), 277–294 (1997). X. Yang, K. Hammani, D. J. Richardson, and P. Petropoulos, “Passively mode-locked fiber laser incorporating adaptive filtering and dispersion management,” in CLEO: Science and Innovations, June 9 - 14, 2013, (Optical Society of American, 2013), paper CM1I.1. P. L. Huang, S.-C. Lin, C.-Y. Yeh, H.-H. Kuo, S.-H. Huang, G.-R. Lin, L.-J. Li, C.-Y. Su, and W.-H. Cheng, “Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber,” Opt. Express 20(3), 2460–2465 (2012). D. Churin, K. Kieu, and N. Peyghambarian, “The role of the saturable absorber in a mode-locked fiber laser,” Proc. SPIE 8237, 823722 (2012). J.-C. Chiu, Y.-F. Lan, C.-M. Chang, X.-Z. Chen, C.-Y. Yeh, C.-K. Lee, G.-R. Lin, J.-J. Lin, and W.-H. Cheng, “Concentration effect of carbon nanotube based saturable absorber on stabilizing and shortening mode-locked pulse,” Opt. Express 18(4), 3592–3600 (2010). S. Yamashita, Y. Inoue, S. Maruyama, Y. Murakami, H. Yaguchi, T. Kotake, and S. Y. Set, “Mode-locked fiber lasers using adjustable saturable absorption in vertically aligned carbon nanotubes,” Jpn. J. Appl. Phys. 45(1), L17–L19 (2006). H. Jeong, S. Y. Choi, F. Rotermund, and D.-I. Yeom, “Pulse width shaping of passively mode-locked soliton fiber laser via polarization control in carbon nanotube saturable absorber,” Opt. Express 21(22), 27011–27016 (2013). V. A. Lazarev, S. S. Sazonkin, A. B. Pniov, K. P. Tsapenko, A. A. Krylov, and E. D. Obraztsova, “Hybrid modelocked ultrashort-pulse erbium-doped fiber laser,” J. Phys. Conf. Ser. 486, 012004 (2014). G. E. Villanueva and P. Pérez-Millán, “Dynamic control of the operation regimes of a mode-locked fiber laser based on intracavity polarizing fibers: experimental and theoretical validation,” Opt. Lett. 37(11), 1971–1973 (2012). X. Li, W. Zou, and J. Chen, “41.9 fs hybridly mode-locked Er-doped fiber laser at 212 MHz repetition rate,” Opt. Lett. 39(6), 1553–1556 (2014). S. Kim, Y. Kim, J. Park, S. Han, S. Park, Y.-J. Kim, and S.-W. Kim, “Hybrid mode-locked Er-doped fiber femtosecond oscillator with 156 mW output power,” Opt. Express 20(14), 15054–15060 (2012).
#215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21012
14. N. S. Shahabuddin, H. Mohamad, M. A. Mahdi, Z. Yusoff, H. Ahmad, and S. W. Harun, “Passively modelocked soliton fiber laser using a combination of saturable absorber and nonlinear polarization rotation technique,” Microw. Opt. Technol. Lett. 54(6), 1430–1432 (2012). 15. Z.-C. Luo, A.-P. Luo, and W.-C. Xu, “Multiwavelength picosecond and single wavelength femtosecond pulses emission in a passively mode-locked fiber laser using a semiconductor saturable absorber mirror and a contrast ratio tunable comb filter,” Appl. Opt. 50(18), 2831–2835 (2011). 16. Y. Fukuchi, J. Maeda, and Ieee, “Widely wavelength-tunable and pulsewidth-variable harmonically modelocked short-cavity fiber ring laser using a bismuth-oxide-based highly nonlinear erbium-doped fiber,” in 35th European Conference on Optical Communication, ECOC 2009, SEP 20–24, 2009 (IEEE, 2009). 17. P. S. Liang, Z. X. Zhang, Q. Q. Kuang, and M. H. Sang, “All-fiber birefringent filter with fine tunability and changeable spacing,” Laser Phys. 19(11), 2124–2128 (2009). 18. Z.-C. Luo, A.-P. Luo, W.-C. Xu, H. S. Yin, L. Jia-Rui, Q. Ye, and Z. J. Fang, “Tunable multiwavelength passively mode-locked fiber ring laser using intracavity birefringence-induced comb filter,” IEEE Photon. J. 2(4), 571–577 (2010). 19. Z. R. Cai, W. J. Cao, A. P. Luo, Z. B. Lin, Z. C. Luo, and W. C. Xu, “A wide-band tunable femtosecond pulse fiber laser based on an intracavity-birefringence induced spectral filter,” Laser Phys. 23(3), 035107 (2013). 20. Y. S. Fedotov, S. M. Kobtsev, R. N. Arif, A. G. Rozhin, C. Mou, and S. K. Turitsyn, “Spectrum-, pulsewidth-, and wavelength-switchable all-fiber mode-locked Yb laser with fiber based birefringent filter,” Opt. Express 20(16), 17797–17805 (2012). 21. Z. X. Zhang, Z. Q. Ye, M. H. Sang, and Y. Y. Nie, “Nonlinear-polarization-rotation based multiwavelength erbium-doped fiber lasers with highly nonlinear fiber,” Laser Phys. 21(10), 1820–1824 (2011). 22. M. L. Dennis and I. N. Duling III, “Intracavity dispersion measurement in modelocked fibre laser,” Electron. Lett. 29(4), 409–411 (1993). 23. J. W. Nicholson, R. S. Windeler, and D. J. Digiovanni, “Optically driven deposition of single-walled carbonnanotube saturable absorbers on optical fiber end-faces,” Opt. Express 15(15), 9176–9183 (2007). 24. A. Martinez, K. Fuse, B. Xu, and S. Yamashita, “Optical deposition of graphene and carbon nanotubes in a fiber ferrule for passive mode-locked lasing,” Opt. Express 18(22), 23054–23061 (2010). 25. D. Y. Tang, L. M. Zhao, B. Zhao, and A. Q. Liu, “Mechanism of multisoliton formation and soliton energy quantization in passively mode-locked fiber lasers,” Phys. Rev. A 72(4), 043816 (2005). 26. 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). 27. X. Zhao, Z. Zheng, L. Liu, Y. Liu, Y. Jiang, X. Yang, and J. Zhu, “Switchable, dual-wavelength passively modelocked ultrafast fiber laser based on a single-wall carbon nanotube modelocker and intracavity loss tuning,” Opt. Express 19(2), 1168–1173 (2011).
1. Introduction Both femtosecond and picosecond mode-locked fiber lasers have been extensively studied in the past couple of decades, as they cover a wide range of interesting applications, such as high-precision metrology, nonlinear imaging, nanofabrication and optical signal processing [1]. The difference in their pulsewidths could be leveraged to access different excitation regimes in applications like bioimaging [2]. Even though, by extra-cavity pulse shaping or spectral filtering of transform-limited laser pulses, their pulsewidths could be significantly increased over a large range, it comes at the cost of increased system complexity or poor energy efficiency. Thus, it would be useful to develop lasers with pulsewidth-tunable outputs. Through studies on the pulse formation dynamics, it’s well-known that the cavity configuration and mode-locking regime determined by the intracavity dispersion and nonlinearity distribution could dictate the range of the output pulsewidth. In the soliton regime we discuss here, the continuum generation sets a limit on the minimal pulsewidth under certain cavity dispersion [3]. Varying the net cavity dispersion is relatively complicated to implement. Recently, using a programmable spectral phase filter to impose different dispersion profiles, it was shown that passively mode-locked picosecond pulses with different pulsewidths could be generated [4]. For those lasers with saturable absorbers (SAs), for example those fabricated with carbon nano-material like single-walled carbon nanotube (SWNT) or graphene, the output pulsewidth could also be significantly affected by the nonlinear characteristics of the SA’s, such as the modulation depth, saturation intensity and nonsaturable losses [5–7]. Though it is very challenging to vary such SA’s characteristics in the cavity, interesting approaches leveraging the polarization-dependent variations in the nonlinear response of SWNT [8] or the waveguide modal distribution of the SA [9] have realized the tuning of the pulsewidth by up to a factor of ~4. Besides tuning the SA, the introduction of additional pulse narrowing mechanisms such as the nonlinear polarization evolution (NPE) effect could enable hybrid #215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21013
mode-locking [10–14]. This could generate significantly shorter pulses than those by the SA scheme alone, while keeping many advantages of the latter. On the other hand, intracavity optical bandwidth affected by the gain profile of the gain media and the spectral width of any optical filters in the cavity also plays a critical role in determining the pulsewidth. Thus, filters with a tunable pass bandwidth could be applied in mode-locked lasers to vary the spectral bandwidth as well as the oscillating wavelengths [15, 16]. Among the filtering schemes, all-fiber Lyot filters consisting of birefringent fiber and polarization-dependent loss components [17] have been often used in mode-locked fiber lasers, due to its fiber-optic compatibility. Based on the principle of polarization interferometry, a transmission comb filter with a well-defined frequency spacing and tunable contrast ratio can be realized [18–21]. Either the inherent weak intracavity birefringence or additional high-birefringence fiber can provide the birefringence needed. Tuning the spectral shape and peak positions of the Lyot filtering profile has been leveraged to not only control the bandwidth and pulsewidth but also modify the lasering wavelength [19, 20]. Yet, the tunable range of the pulse bandwidth is often limited before the laser stops mode-locking as the pulse gets too long. In this paper, we demonstrate a scheme that effectively leverages the nonlinear, NPEenabled hybrid mode-locking effect as well as the linear, polarization interferometric filtering [20] to significantly increase the pulsewidth tuning range of a passively mode-locked laser. It is achieved by simply introducing an inline polarization beam splitter (PBS) with polarization-maintaining fiber (PMF) pigtails into a fiber ring laser with a SWNT SA modelocker. Ultrashort pulses covering a nearly one order-of-magnitude pulsewidth range from 360 fs to 3 ps can be generated from the same cavity by tuning the intracavity polarization state and slightly optimizing the pumping condition. 2. Experimental setup and principle Figure 1 shows the schematic of the laser cavity. It consists of a 2-meter-long piece of highly doped Erbium-doped fiber (EDF, Changfei 1022) pumped by a 980-nm pump laser through a 980/1550 wavelength-division multiplexer (WDM), 4.4-meter-long single mode fiber (SMF, including the pigtail fiber of all components), and a fiber inline polarization beam splitter (PBS) with 1.2-meter-long polarization maintaining fiber (PMF) pigtails at each of its two output ports. A polarization-independent isolator (ISO) is used to ensure the unidirectional operation. To better monitor the pulse characteristics at different positions of the cavity, two 10/90 optical couplers (OC’s) are placed before and after the PBS. The output Port 2 (working at fast axis) of the PBS is used as a monitor port of the laser, while the output Port 1 (working at slow axis) is connected back into the loop. The optical spectrum of the laser output is measured by an optical spectrum analyzer (Agilent 86142B) and the temporal pulse shape by a home-built, second-harmonic intensity autocorrelator. The repetition rate of the output is measured by an Agilent N9320B RF spectrum analyzer following a 1-GHz photodetector (New Focus 1611).
Fig. 1. Schematic diagram of the laser cavity.
The absorption coefficient of the EDF is ~18 dB/m at 980 nm and its dispersion is estimated as −6 ps/km/nm at 1560 nm. The beat length Lb of the PMF is ~4 mm and its dispersion is estimated as ~18.4 ps/km/nm at 1560 nm. The total net cavity dispersion is estimated to be −0.11 ps2, which is in good agreement with the estimated −0.12 ps2 from the #215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21014
separations of the Kelly sidebands on the mode-locked spectrum and the pulsewidth [22]. The carbon nanotube mode locker is fabricated on an FC/APC ferrule using the optical deposition method [23, 24] with ~1 dB insertion loss at 1550 nm. Besides a fiber-squeezer-type polarization controller (PC) in the cavity to adjust the state of polarization, an additional fiber polarization controller with only one 4-coil ring is applied as roughly an in-line half-wave plate (HWP) to provide more quantifiable control of the state of polarization. Adjusting the polarization rotation introduced by HWP would change the transmission ratio of the light through PBS. While ideally the PMF after the PBS should not introduce much birefringent effect if its slow axis is perfectly aligned with the direction of polarization of the transmitted light, the inline PBS with a finite extinction ratio (ER) may result in a weak Lyot filter effect, as we observe here. To analyze the effect brought by its finite ER, separate tests of the PBS is carried out. An amplified spontaneous emission (ASE) light source between 1530 and 1560 nm is connected to the PBS used in the cavity followed by the HWP and another inline PBS. The transmission spectrum is measured under different polarization rotation introduced by the HWP. With certain residual birefringence introduced by the PM fiber pigtail, the output spectrum from the second PBS would possess the corresponding spectral interference pattern. This also emulates the situation in the laser cavity where the light repeatedly passes through the PBS, possibly leading to interference effect in the presence of polarization rotation. By assuming a 0.7° misalignment between the direction of polarization of the transmitted light from the PBS component and the slow axis of the PMF pigtail (derived from the 39 dB extinction ratio at Port 1 of the in-line PBS device), the transmission can be theoretically calculated as in [11, 18, 25]. The normalized experimental results in Fig. 2 show weak Lyot filtering effect when HWP is adjusted to different positions. Theoretical curves based on the estimated phase rotation angles θ caused by HWP match the experimental ones reasonably well. Noticeable changes in the average transmission and positions of the transmission peaks are seen for different θ [20].
Fig. 2. Transmission spectra of the PBS at different input polarization orientation.
In order to further investigate the polarization characteristics of the pulse in this birefringent cavity, the output from OC 1 can be sent through an external PBS after a PC to resolve its field components in different polarizations. 3. Experimental results and discussions When the pump power of the 980-nm pump laser is set at 10 mW and the intracavity loss, i.e. the power at port 2, is minimized by adjusting the PC, the mode-locking can self-start at a repetition rate of 27.2 MHz. The PC’s orientation is then fixed at this condition in the subsequent experiments. In this case, θ is considered as ~0°, and the pulse experiences little linear polarization rotation after each round trip and is nearly linear polarization along the slow axis of the PMF before entering PBS. The spectra at the output ports of OC 1 and OC 2 are measured and shown in Fig. 3(a). The corresponding 3-dB bandwidths in Fig. 3(a) are ~4.3 nm. The autocorrelation traces of the outputs of OC 1 and OC 2 are shown in Fig. 3(b) with full-width-at-half-maximum (FWHM) of 600 and 663 fs, if sech2-shape is assumed. The time-bandwidth products (TBPs) are 0.32 and 0.35, respectively, suggesting nearly transformlimited solitons, with small chirps in the latter possibly introduced by the PBS. #215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21015
Additional measurements of the pulse at OC 1 are done to more thoroughly examine its polarization characteristics. By adjusting the PC before the external PBS so that the pulse intensity measured in one port is maximized, it is observed that at the other port there exists a weak but non-negligible spectrum. This polarization-resolved minor component of the spectrum as seen in Fig. 3(a) indicates that the pulse is a vector soliton with a weak orthogonal polarization component, e.g. an induced soliton [26]. The modulation in the spectrum of the induced weak soliton could be caused by the Lyot filter effect.
Fig. 3. (a) Optical spectra of the outputs of OC 1 and OC 2 and polarization-resolved spectrum of the induced weak soliton at OC 1; (b) autocorrelation traces of the outputs, when θ = 0°.
When rotating the HWP to increase θ and keeping the pump power steady, the output power at port 2 significantly increases, while the output power of OC 2 is reduced. Yet, the bandwidth of the laser output gradually increases with decreased separation between the Kelly sidebands. This shows typical signs of hybrid mode-locking with NPE, where the nonlinear polarization rotation could offset some of the linear polarization rotation at higher instantaneous powers. When the polarization rotation resulted from the HWP reaches 37°, the pump power needs to be increased to above 10 mW in order to maintain the mode-locking and compensate the increased loss. Maximal bandwidth is reached at θ = 51° and 17 mW of pump power, beyond which condition the laser stops mode-locking even under higher pump powers. We note that the laser cannot be mode-locked solely by NPE without an SWNT SA under any pumping conditions and the nonlinear-phase-shift-involved phenomenon is only seen when the mode-locking is initiated by the SA as described above. Based on the comparison of the spectra at different θ, as shown in Fig. 3(a) and Fig. 4, one could see an obvious bandwidth broadening and a slight blue-shift of the spectra. The blueshift is likely caused by the changes in the EDF’s gain tilt resulted from the varied intracavity loss [27]. The 3dB bandwidth reaches ~7.5 nm, which is far larger than that of a laser configuration using the same SWNT mode-locker, similar total dispersion but without the PBS. This further verifies the effect of hybrid mode-locking mechanism. The magnitude of the weaker portion of the vector soliton also significantly increases at larger θ.
Fig. 4. Optical spectra of the outputs of OC 1 and OC 2 and polarization-resolved spectrum of the induced weak soliton at OC 1, when (a) θ = 37° and (b) θ = 51°.
On the other hand, when the HWP is rotated in the opposite direction, the spectral width quickly narrows, as seen in Fig. 5(a). The pump power needs to be slightly reduced, in order to avoid CW peaking and harmonic mode-locking. The minimal θ to maintain mode-locking
#215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21016
is −40°. The output spectra are shifted to the shorter wavelength side, due to the likely gain profile changes and, later, the peak positions of the Lyot filter. The fast narrowing of the spectra could be attributed to the increased contrast ratio, i.e. the reduced effective bandwidth, of the Lyot filter as described in Section 2, and the lower intracavity power and nonlinearity.
Fig. 5. (a) Optical spectra and (b) autocorrelation traces of the outputs of OC 1 with different θ.
Fig. 6. (a) Pulse durations, spectral widths and TBPs of the outputs of OC 1 and (b) pump power, output power of the outputs of OC 1 and the monitored power with different θ.
The autocorrelation traces of the outputs of OC 1 at different polarization orientations are shown in Fig. 5(b). Pulses with FWHM durations from 3 ps to 360 fs are generated, and the corresponding 3-dB spectral bandwidths vary from 0.9 to 7.5 nm. For the shortest pulse with a small sidelobe, the peak-to-valley ratio of the autocorrelation trace is ~100. The trend of pulse durations and spectral bandwidths at differentθ are summarized in Fig. 6(a). Their TBPs are labeled for each case, indicating that the pulses are close to transform-limited. The pump power, the output power at OC 1 and port 2 also shown in Fig. 6(b) indicate that the increase in the pump power under the hybrid mode-locking regime is to mostly compensate the sharp increase in the loss at the PBS. Due to the constant soliton area rule [3], the output power drops for the longer pulses under lower pump powers, yet sufficient to introduce self-phase modulation for soliton generation. 4. Conclusion We show that through incorporating SWNT mode-locker with an inline fiber PBS, soliton pulses covering nearly one order-of-magnitude pulsewidth range from femtosecond to picosecond could be generated by mostly tuning the intracavity state of polarization setting. Helped with both the linear Lyot filter and the nonlinear NPE effect generated by the same physical components, the induced vector soliton pulses can be either broadened or narrowed, respectively. The transition of the effect from nonlinear to linear is clearly observed by varying the polarization ‘bias’ in the cavity. Acknowledgments This work at Beihang University was supported by 973 Program (2012CB315601) and NSFC (61221061/61107057).
#215192 - $15.00 USD Received 2 Jul 2014; revised 9 Aug 2014; accepted 9 Aug 2014; published 22 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.021012 | OPTICS EXPRESS 21017
