Mode-Locking Thulium-Doped Fiber Laser With 1.78 ... - IEEE Xplore

Open Access

Mode-Locking Thulium-Doped Fiber Laser With 1.78-GHz Repetition Rate Based on Combination of Nonlinear Polarization Rotation and Semiconductor Saturable Absorber Mirror Volume 9, Number 3, June 2017 Jia Qingsong Wang Tianshu Ma Wanzhuo Wang Zhen Su Qingchao Bo Baoxue Jiang Huilin

DOI: 10.1109/JPHOT.2017.2690690 1943-0655 © 2016 IEEE

IEEE Photonics Journal

Mode-Locking Thulium-Doped Fiber Laser With 1.78-GHz

Mode-Locking Thulium-Doped Fiber Laser With 1.78-GHz Repetition Rate Based on Combination of Nonlinear Polarization Rotation and Semiconductor Saturable Absorber Mirror Jia Qingsong,1 Wang Tianshu,2 Ma Wanzhuo,1 Wang Zhen,3 Su Qingchao,3 Bo Baoxue,4 and Jiang Huilin2 1 College

of Optoelectronic Engineering, Changchun University of Science and Technology, Changchun 130022, China 2 National and Local Joint Engineering Research Center of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun 130022, China 3 College of Science, Changchun University of Science and Technology, Changchun 130022, China 4 Institute of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun 130022, China

DOI:10.1109/JPHOT.2017.2690690 C 2016 IEEE. Translations and content mining are permitted for academic research only. 1943-0655 Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received January 25, 2017; revised March 30, 2017; accepted March 31, 2017. Date of current version April 21, 2017. This work was supported in part by the National Natural Science Foundation of China under Grant 60907020, in part by the Nature Science Fund of Jilin Province (20150101044JC), and in part by the Science and Technology Project of Jilin Province (20170414041GH). Corresponding author: Wang Tianshu (e-mail: [email protected]).

Abstract: We experimentally demonstrate a passively mode-locking thulium-doped fiber laser based on the combination of the nonlinear polarization rotation (NPR) and the semiconductor saturable absorber mirror (SESAM). In contrast to SESAM mode-locking merely, the fiber lasers based on NPR working together with SESAM have more advantages. By using two mode-locking mechanisms, the nonlinear effect can be enhanced tremendously; then, the threshold of high-order harmonic mode-locking (HML) pulse trains can be decreased, and the pulses width can be compressed. The shapes of the pulses can also be reshaped better, and the HML pulse trains stability can be improved. The high repetition rate of 1.78 GHz is obtained by both NPR and SESAM in the experiment. Because of the variation of the gain-and-loss distribution in the cavity, the output wavelength of mode-locking fiber laser can be tuned from 1839 to 1890 nm, and the multiwavelength HML is also observed in the experiment. Index Terms: Thulium-doped fiber laser, nonlinear polarization rotation, semiconductor saturable absorber mirror, harmonic mode-locking, high repetition rate.

1. Introduction Thulium (Tm)-doped mode-locking fiber lasers operated in the 2 μm eye safe region have attracted great research interests due to their potential applications in laser medicine, optoelectronic countermeasure, LIDAR, and mid-IR supercontinuum generation [1]–[3]. Furthermore, high repetition rate mode-locking fiber lasers have been intensively studied because of their numerous applica-

Vol. 9, No. 3, June 2017

1502808



Fig. 1. Configuration of Tm-doped mode-locking fiber laser. BP, Brillouin pump; EYDFA, erbium ytterbium co-doped optical fiber amplifier; PC, polarization controller; SMF, single-mode fiber; OC, optical circulator; TDF, thulium-doped fiber; WDM, wavelength division multiplexer; PD, photodetector; OSA, optical spectrum analyzer; ESA, electrical spectrum analyzer.

tions in metrology, spectroscopy, and telecommunication [4], [5]. High repetition rate mode-locking pulse trains generation by HML is considered to be effective. High-order harmonic mode-locking (HML) pulse trains can be easily obtained by using passively mode-locking techniques. The passively mode-locking fiber lasers can adopt mainly NPR, SESAM, nonlinear optical/amplifying mirror (NOLM/NALM), etc, and the combination of two mechanisms has become a hotspot research [6]– [9]. In these experiments, mode-locking can be started by one mechanism and the other mechanism is usually used to optimize the mode-locking pulses including pulse shape and threshold. Peng et al. found that the combination of NPR and NOLM can shorten the length of cavity, and the output pulses also can be improved [7]. Li et al. found that SESAM and nonlinear polarization evolution hybrid mode-locking mechanisms can realize stable mode-locking operation under low power density inside the cavity [8]. Budnicki et al. combined NPR and NALM mode-locking mechanisms, and bound state of solitons were obtained by adjusting polarization controllers (PCs) [9]. Nevertheless, none of above researchers analyzed the effect of hybrid mode-locking on the generation of HML pulse trains. HML pulse trains are reported basing on NPR and SESAM hybrid mode-locking mechanisms in this experiment. The SESAM is used to generate the mode-locking, and NPR is applied in pulse shaping and enhancement of nonlinear effect. Therefore, NPR and SESAM hybrid mode-locking can enhance the nonlinear effect in the cavity which is benefit to generate HML pulse trains. It is a very simple way to improve the repetition rate more than GHz without reducing length of the fiber cavity for the passive mode-locking fiber laser [10], [11]. In this paper, we show that stable high-order HML pulse trains can be obtained experimentally by exploiting the combination of SESAM and NPR mode-locking mechanisms. The nonlinear effect in the cavity can be greatly increased suitable for generating HML pulses trains, and high-order HML pulse trains can be generated. It can not only realize stable high-order HML pulse trains under the lower pump power, but also can reshape pulses. The pulses width of merely SESAM mode-locking is 360ps at the repetition rate of 91.91 MHz, and it can be compressed to 190ps by adopting NPR and SESAM hybrid mode-locking. What’s more, a 1.78 GHz HML pulse train is obtained due to the combination of NPR and SESAM mode-locking mechanisms. And the central wavelength of HML can be varied in a wide range of 1839 to 1890 nm. In addition, by carefully adjusting the PCs, multi-wavelength HML operation is also obtained in our structure.

2. Experimental Setup and Operation Principle Fig. 1 shows the schematic of the experiment setup. The laser is configured in a ring cavity. A piece of 3 m length Tm-doped fiber is pumped by a 1565 nm laser through a 1570/1900 nm wavelength division multiplexer (WDM), and an Erbium-ytterbium co-doped fiber amplifier (EYDFA) with a maximum output power of 2W is used to enhance the pump power. Tm-doped fiber, with an absorption coefficient of 13 dB/m at 1570 nm, and the dispersion parameter β2 of −0.0707 ps2 /m at 1990 nm, is used as the gain medium. Amplified spontaneous emission can be observed with

Vol. 9, No. 3, June 2017

1502808



Fig. 2. (a) Kelly sidebands, (b) fundamental repetition rate pulse train, (c) HML pulse train, and (d) spectrum of multi-wavelength output obtained by NPR mode-locking.

Tm-doped fiber pumped by the pump laser at 1565 nm. A 2 μm band isolator is used to ensure unidirectional operation. The output from the isolator is connected to PC1, along with a 2 μm band polarizer and PC2. The NPR effect can be induced by two PCs and a 2 μm band polarizer in the cavity. The linear polarization light can be formed by the polarizer, and the linearly polarized light can be changed to elliptically polarized light by PC2. Then, the polarization state of the beam inside the laser ring cavity starts rotating due to the nonlinearity effects of the cavity. By adjusting PC1, the central part of the pulse can pass through the polarizer, so the intensity of pulse wings can be suppressed and the intensity of the pulse center can be enhanced. As the pulse signals become narrower, the main nonlinear effects are self-phase modulation and cross phase modulation. A segment of 50 m length SMF is used to enhance the NPR effect [12]. The fiber pigtails of optical component in the cavity are SMF with the dispersion parameter β2 of −0.0679 ps2 /m at 1900 nm. The overall length of SMF in the cavity is 63 m. A SESAM is connected into the ring cavity by an optical circulator, its mode-locking mechanism uses mainly intensity-dependent loss to narrow the pulses. Peak of pulses can pass through SESAM with low loss while wings are cut off due to higher loss, therefore, the pulses can be narrowed to generate mode-locking pulse trains. The modulation depth, the recovery time and the saturation fluence of SESAM are 1.2 %, 30 ps, and 70 μJ/cm2 , respectively. It is clear that there are two mode-locking mechanisms in the cavity. The 80% port of the 80/20 coupler is engaged to feedback and the 20% port is output port. The output optical signal is divided by a 3 dB coupler, one portion is observed by optical spectrum analyzer (AQ6375) and the other is sent into oscilloscope (Agilent DSO-X 93204A) after a photodetector with 10 GHz bandwidth. The total cavity length is about 66m and the net cavity dispersion is calculated to be −4.4898 ps2 at 1900 nm.

3. Results and Discussions First, we only used NPR mode-locking in the cavity. We use a 3 m long single mode fiber (SMF) instead of OC for keeping the length of cavity. When the pump power increases to 800 mW, which is the mode-locked threshold of the laser, we can get Kelly sidebands by adjusting PCs, as shown in Fig. 2(a). The laser is operated in passively mode-locked state, single pulse train is shown in Fig. 2(b), and the repetition rate is 3.06 MHz. The repetition rate consistent with the cavity length of 66 m, it can be calculated by the equation f = c/nL [13], where c is the speed of the light in fibers, n is the refractive index of the fibers, and L is the length of the ring cavity. When the pump power

Vol. 9, No. 3, June 2017

1502808



Fig. 3. (a) Kelly sidebands, (b) fundamental repetition rate pulse train, (c) single pulse time-domain waveform, and (d) RF spectrum of fundamental repetition rate pulse train obtained by SESAM modelocking.

Fig. 4. (a) HML spectrum, (b) HML pulse train, (c) HML single pulse time-domain waveform, and (d) RF spectrum of HML pulse train obtained by SESAM mode-locking.

is 950 mW, the maximum 133 order HML pulse train is obtained by adjusting PCs, as shown in Fig. 2(c). The NPR reached saturation when the pump power continues increasing, the transmission degrades with the further increase of the light intensity, and multi-wavelength output is obtained because of the intensity-dependent loss induced by NPR effect [14], [15]. The mode-locking state is evolved into multi-wavelength output, as shown in Fig. 2(d). Second, we only used a SESAM in the cavity without polarizer. We use a 2 m SMF instead of polarizer for keeping the length of cavity. Mode-locking pulses train can be observed by increasing the pump power to SESAM mode-locking threshold. A stable mode-locking pulse train with fundamental repetition rate appears when the pump power is 900 mW. Fig. 3(a) shows the output spectrum of Kelly sidebands, and the repetition rate is 3.06 MHz, as shown in Fig. 4(b). Both pulse width and shape are degraded shown in Fig. 4(c) because of the relatively low modulation depth of

Vol. 9, No. 3, June 2017

1502808



Fig. 5. (a) Splitting optical spectrum and (b) multiple pulse train obtained by SESAM mode-locking.

the SESAM in our experiment. As the figure shows, the edge of pulse signals is not compressed effectively, and the pulse width is about 580 ps using Lorentz fitting. The radio frequency (RF) spectrum of the output pulses is measured by an electrical spectrum analyzer (N9030A) shown in Fig. 3(d), and signal-to-noise ratio (SNR) is 40 dB. Increasing the pump power slowly, the output spectrum shrinks obviously, and Kelly sidebands disappear. The mode-locking pulse train with fundamental repetition rate is split into high-order HML pulse trains. When the pump power increases to 950 mW, the HML pulse train with 91.91 MHz repetition rate can be obtained by carefully adjusting PCs. The spectrum of HML pulse train is shown in Fig. 4(a). The HML pulse train is shown in Fig. 4(b), and the HML pulse width is about 360 ps using Lorentz fitting, as shown in Fig. 4(c). Fig. 4(d) shows the RF spectrum of the HML, and the SNR is more than 35 dB. With further increasing the pump power to 1W, the light becomes unstable since the peak power of solitons is clamped by SESAM mode-locking mechanism and the pulses are split. If the pump power is too high, the SESAM absorption would not reach the balance of the gain in the cavity. In order to increase the SESAM absorption to achieve balance, a monopulse signal will split into multiplepulse signals and the pulse amplitude will decrease [16]. The optical spectrum and timedomain waveform are observed in Fig. 5(a) and Fig. 5(b), respectively. In order to prevent the damage of the SESAM, the pump power can not be increased continuously. Finally, we use a polarizer between PC1 and PC2, it means there are two mode-locking mechanisms in the cavity. The minimum pump power for mode-locking is 900 mW which is the SESAM mode-locking threshold. A stable HML mode-locking pulse train can self start with a repetition rate of 91.91 MHz. In the experiment, fundamental repetition rate pulse train is not observed at any position of the two PCs in contrast to SESAM mode-locking at the same pump power. Hybrid modelocking can increase the nonlinear effect in the cavity, and it helps to generate HML pulse trains even if the pump power is just to the SESAM mode-locking threshold. In Fig. 6(a), 3 dB linewidth of the spectrum is 0.086 nm which is narrower than that of only using SESAM. The repetition rate is 91.91 MHz as shown in Fig. 6(b). Fig. 6(c) shows that the pulse width can be compressed to 190 ps and the pulse has been improved obviously. Fig. 6(d) presents the RF spectrum and shows that the SNR can be enhanced to 40 dB. Because there are two mode-locking mechanisms in the laser, the pulse experience twice amplitude modulation per round trip. Thus, the pulse wings are cut down more effectively than a single mode-locking mechanism, which results in the pulses width become narrower. Slowly increasing the pump power in the experiment, the harmonic order increases stepwise as the pump power increases. The passive HML can keep operating stably. The maximum repetition rate is 1.78 GHz corresponding to 588th-order HML pulse train with 1W pump power, the time-domain waveform and RF spectrum are observed in Fig. 7(a) and (b). Neither multi-wavelength output nor pulse split is appeared. Indicating that SESAM plays a leading role in the generation of mode-locking, and NPR is mainly used for pulse shaping and enhancement of nonlinear effects. In the experiment, we did not observe the split of wavelength and pulse because the intensity-dependent loss induced by NPR can suppress the mode competition and guarantee the stable HML operation [17]–[19]. In order to prevent the damage of the SESAM, the pump power is not increased to obtain higher order

Vol. 9, No. 3, June 2017

1502808



Fig. 6. (a) HML spectrum, (b) HML pulse train, (c) HML single pulse time-domain waveform, and (d) RF spectrum of HML pulse train obtained by hybrid mode-locking.

Fig. 7. (a) The 1.78GHz HML pulse train and (b) RF spectrum of HML pulse train obtained by hybrid mode-locking.

HML pulse trains. The pulse repetition rate is mainly affected by the ellipticity of mode-locking fiber laser. By increasing the pump power, in conjunction with minor adjustments to the PCs, resulting in a wider ellipticity range that can accommodate mode-locking operation [20], [21]. Therefore, high-order HML pulse trains are obtained by increasing the pump power and adjusting the PCs In the experiment, adjusting PCs can lead to the variation of gain-and-loss distribution [22], which makes the peak of the spectral modulation shifts to other wavelength. The central wavelength of HML can be varied in a wide range of 1839 to 1890 nm, as shown in Fig. 8. The wavelength is quite sensitive to the loss in the cavity, therefore the laser should be in a stable external environment in order to achieve stable output wavelength. In addition, by carefully adjusting the PCs, multi-wavelength HML can also be observed because of the polarization-dependent loss and birefringent of the SESAM. By combining the polarizationdependent loss with the cavity birefringence, a comb filter can be produced, it can be used to achieve the multi-wavelength laser operation [23]. Fig. 9(a) shows the optical spectrum of dualwavelength HML and the corresponding pulse train is shown in Fig. 9(b). It indicates that the dual-wavelength HML pulse trains have same fixed soliton separation and different intensities. However, such unstable state can disappear in a short time. Tri-wavelength HML is also obtained by carefully adjusting the PCs, the optical spectrum of tri-wavelength HML is shown in Fig. 9(c), and the corresponding pulse train is shown in Fig. 9(d). The comb filter wavelength spacing is given by λ = λ2 /(LB), where λ is the central wavelength of the laser, B is the birefringence of the cavity and L is the length of the cavity [24], [25]. The wavelength spacing of the comb filter is inversely proportional to the birefringence in the cavity according to the formula. When the birefringence of the cavity is increased, the comb filter has

Vol. 9, No. 3, June 2017

1502808



Fig. 8. Tunable range of the output optical signal.

Fig. 9. (a) Dual-wavelength HML spectrum, (b) dual-wavelength HML pulse train, (c) tri-wavelength HML spectrum, and (d) tri-wavelength HML pulse train.

narrow bandwidth, within the effective laser gain bandwidth range, multi-wavelength HML pulse trains can be obtained. When the birefringence of the cavity is decreased, the bandwidth of the comb filter become so broad that only a single wavelength HML pulse train can form in the cavity. Therefore, the number of different wavelengths HML pulse trains can be observed in our experiment.

4. Conclusion We present a mode-locking fiber laser with high repetition rate by high-order HML pulse trains using NPR and SESAM hybrid mode-locking. Stable high-order HML pulse trains can be produced under lower pump power in this configuration. The repetition rate of HML pulse train is 1.78 GHz. The central wavelength of HML can be varied in a wide range of 1839 to 1890 nm. And multi-wavelength HML operation is also obtained. Furthermore, HML pulse trains with higher repetition-rate and pulses with high-quality can be obtained according to the proposed method by using a SESAM with deeper modulation depth and less recovery time.

Vol. 9, No. 3, June 2017

1502808



References [1] P. Zhang, T. Wang, W. Ma, K. Dong, and H. Jiang, “Tunable multi-wavelength Tm-doped fiber laser based on the multimode interference effect,” Appl. Opt., vol. 54, no. 15, pp. 4667–4671, 2015. [2] M. Eckerle et al., “Actively Q-switched and mode-locked Tm3+ -doped silicate 2 μm fiber laser for supercontinuum generation in fluoride fiber,” Opt. Lett., vol. 37, no. 4, pp. 512–514, 2012. [3] C. R. Phillips et al., “Supercontinuum generation in quasi-phase-matched LiNbO3 waveguide pumped by a Tm-doped fiber laser system,” Opt. Lett., vol. 36, no. 19, pp. 3912–3914, 2011. [4] G. Sobon, J. Sotor, and K. M. Abramski, “Passive harmonic mode-locking in Er-doped fiber laser based on grapheme saturable absorber with repetition rates scalable to 2.22 GHz,” Appl. Phys. Lett., vol. 100, no. 16, pp. 161109-1–1611094, 2012. [5] X. Zhang, H. Hu, W. Li, and N. K. Dutta, “High-repetition-rate ultrashort pulsed fiber ring laser using hybrid mode locking,” Appl. Opt., vol. 55, no. 28, pp. 7885–7891, 2016. [6] Y. C. Meng, S. Zhang, X. Li, H. Li, J. Du, and Y. P. Hao, “Multiple soliton dynamic patterns in a graphene mode-locked fiber laser,” Opt. Exp., vol. 20, no. 6, pp. 6685–6692, 2012. [7] J. Peng, L. Zhan, Z. Gu, K. Qian, S. Luo, and Q. Shen, “Direct generation of 128-fs Gaussian pulses from a compensation-free fiber laser using dual mode-locking mechanisms,” Opt. Commun., vol. 285, no. 5, pp. 731–733, 2012. [8] P. Li et al., “Subpicosecond SESAM and nonlinear polarization evolution hybrid mode-locking ytterbium-doped fiber oscillator,” Appl. Phys. B, vol. 118, no. 4, pp. 561–566, 2015. [9] A. Budnicki, L. Szpak, M. Maternia, and K. M. Abramski, “Passively mode-locked erbium doped laser with combination of NPR and NALM effects,” in Proc. Int. Conf. Transp. Opt. Netw., 2006, pp. 181–184. [10] F. Amrani et al., “Passively mode-locked erbium-doped double-clad fiber laser operating at the 322nd harmonic,” Opt. Lett., vol. 34, no. 14, pp. 2120–2122, 2009. [11] Z.-C. Luo, A.-P. Luo, and W.-C. Xu, “Tunable and switchable multiwavelength passively mode-locked fiber laser based on SESAM and inline birefringence comb filter,” Photon. J., vol. 3, no. 1, pp. 64–70, 2011. [12] X. Feng, H.-Y. Tam, and P. K. A. Wai, “Stable and uniform multiwavelength erbium doped fiber laser using nonlinear polarization rotation,” Opt. Exp., vol. 14, no. 18, pp. 8205–8510, 2006. [13] Y. K. Chen, M. C. Wu, T. Tanbun-Ek, R. A. Logan, and M. A. Chin, “Subpicosecond monolithic colliding-pulse modelocked multiple quantum well lasers,” Appl. Phys. Lett., vol. 58, no. 12, pp. 1253–1255, 1991. [14] Z. Zhang, L. Zhan, K. Xu, J. Wu, Y. Xia, and J. Lin, “Multiwavelength fiber laser with fine adjustment, based on nonlinear polarization rotation and birefringence fiber filter,” Opt. Lett., vol. 33, no. 4, pp. 324–326. 2008. [15] X. Feng, H.-Y. Tam, and P. K. A. Wai, “Stable and uniform multiwavelength erbium doped fiber laser using nonlinear polarization rotation,” Opt. Exp., vol. 14, no. 18, pp. 8205–8510, 2006. [16] G. P. Agrawal, Nonlinear Fiber Optics, 2nd ed. New York, NY, USA: Academic, 1995. [17] A. P. Luo, Z. C. Luo, and W. C. Xu, “Tunable and switchable multi-wavelength erbium-doped fiber ring laser based on amodified dual-pass Mach-Zehnder interferometer,” Opt. Lett., vol. 34, no. 14, pp. 2135–2137, 2009. [18] S. L. Pan and C. Y. Lou, “Stable multiwavelength dispersion-tuned actively mode-locked erbium-doped fiber laser using nonlinear polarization rotation,” IEEE Photon. Technol. Lett., vol. 18, no. 13, pp. 1451–1453, Jul. 2006. [19] X. Feng, H. Tam, and P. K. A. Wai, “Stable and uniform multiwavelength erbium-doped fiber laser using nonlinear polarization rotation,” Opt. Exp., vol. 14, no. 18, pp. 8205–8210, 2006. [20] H. Ahmad et al., “Investigation of ellipticity and pump power in a passively mode-locked fiber laser using the nonlinear polarization rotation technique,” Chin. Opt. Lett., vol. 15, no. 5, pp. 051402-1–051402-5, 2017. [21] R. I. Woodward and E. J. R. Kelleher, “Towards ‘smart lasers’: Self optimization of an ultrafast pulse source using a genetic algorithm,” Sci. Rep., vol. 6, no. 37616, pp. 1–9, 2016. [22] X.-H. Li et al., “Wavelength-switchable and wavelength-tunable all-normal-dispersion mode-locking Yb-Doped fiber laser based on single-walled carbon nanotube wall paper absorber,” Photon. J., vol. 4, no. 1, pp. 234–241, 2012. [23] M. Liu et al. “Dual-Wavelength harmonically mode-locked fiber laser with topological insulator saturable absorber,” Photon. Technol. Lett., vol. 26, no. 10, pp. 983–986, 2014. [24] H. Zhang, D. Y. Tang, X. Wu, and L. M. Zhao, “Multi-wavelength dissipative soliton operation of an erbium-doped fiber laser,” Opt. Exp., vol. 17, no. 15, pp. 12692–12697, 2009. [25] X. Jin, X. Wang, X. Wang, and P. Zhou, “Tunable multiwavelength mode-locked Tm/Ho-doped fiber laser based on anonlinear amplified loop mirror,” Appl.Opt. vol. 54, no. 28, pp. 8260–8264, 2015.

Vol. 9, No. 3, June 2017

1502808