Open Access
Add to BibSonomyAbstract
We report on a novel polarization switching laser from a bidirectional passively mode-locked thulium(Tm)-doped fiber oscillator, which was characterized by the periodical change of polarization state of every pulse. The switching laser was created by combing two orthogonally stable vector solitons, which were found to be wave-breaking-free pulses in the all-anomalous-dispersion regime. The measured repetition rates of switching laser and the corresponding vector solitons were 49.596 MHz, 24.798 MHz, and 24.798MHz. By controlling wave plates, either of the polarized pulse trains can be switched on or off. To our knowledge, this is the first report of polarization switching laser with vector solitons in Tm fiber oscillators.
©2013 Optical Society of America
1. Introduction
Mode-locked thulium (Tm)-doped fiber lasers operating at ~2 μm have attracted considerable attention [1–5]due to their numerous applications in eye-safe laser radar, medicine [6], remote sensing, atmosphere spectroscopy [7], and pump sources of mid-infrared supercontinuum (SC) [8]. In order to reduce the spurious cavity reflections and decrease the self-starting mode-locking threshold in passively mode-locked Tm fiber lasers, an optical isolator was conventionally employed in ring cavities [9–12]. However, the isolator makes these lasers less compact and these lasers can offer only unidirectional operation of pulse trains. Recently, bidirectional mode-locked lasers without isolators can simultaneously generate two pulse trains, which are attractive for various sensing applications. Buholz and Chodorow demonstrated the first bidirectional mode-locked laser gyroscopic sensor [13]. Kieu and Mansuripur presented the first all-fiber bidirectional passively mode-locked ring laser at 1.55 μm [14]. Braga et al. reported the operation and gyro response of a bidirectional mode-locked Er-doped fiber ring laser, of which the crossing point can be electronically controlled [15]. More recently, Ouyang and his associates presented a novel 1.56 μm all-fiber soliton laser design that can implement bidirectional pulsed operation with distinct output characteristics [16]. They also believed that their low-cost, compact, bidirectional pulsed laser would find more potential applications. So far, to our knowledge, the bidirectional pulsed lasers around 2 μm have never been reported.
In this paper, we report on a novel polarization switching fiber laser with bidirectional outputs. The laser was characterized by the periodical change of polarization state of every pulse, which was created by combing two orthogonally stable linearity polarized vector solitons. The measured repetition rates of switching laser and the corresponding vector solitons were 49.596 MHz, 24.798 MHz, and 24.798 MHz. The laser was found to be wave-breaking-free pulses with central wavelength of 2001.9 nm, bandwidth of 0.9 nm, output average power of 220 mW, and pulse energy of ~4.4 nJ. The output intensity ratio of orthogonally polarized components can be continuously changed by tuning wave plates. To our knowledge, this is the first report of polarization switching and vector solitons in Tm fiber laser.
2. Experimental setup
The schematic diagram of the Tm-doped fiber laser is shown in Fig. 1(a). A 1.5 m-long Tm-doped double-cladding single mode fiber (Nufern Inc, 10 μm and 0.15 NA) was chosen for the active element. The laser was pumped by diode laser at 793 nm and a corresponding pump combiner ((2 + 1) × 1, ITF company) with 1.5 m undoped single mode fiber pigtail was used to deliver pump light into the active fiber. All the fibers were angle cleaved to eliminate parasitic oscillation. A piece of commercial semiconductor saturable absorption mirrors (SESAMs) with high modulation depth (26%) and large saturation fluence (130μJ/cm2) was used to start and maintain mode-locking operation. A polarization beam splitter (PBS) was employed to control the directions of the light traveling in the cavity. The cavity had an overall optical path length of ~6 m, corresponding to the round trip time of ~20 ns. The overall cavity group-velocity dispersion (GVD) was estimated to be −0.24 ps2. In order to orthogonal-polarization resolve the output pulses and control the relative output ratio of the beams with different directions, we created the tuning system including a PBS, two flat mirrors and two quarter-wave-plates. The temporal behavior of output pulses was monitored by a high-speed Oscilloscope (bandwidth of 6 GHz) and a fast photodiode detector (EOT Inc, ET-5000 and band width of 10 GHz). The output spectrum was monitored by the 0.55 m monochromator with the resolution of 0.9 nm at 2 μm.
Fig. 1 (a) Schematic diagram of Tm-doped fiber laser. SESAMs, semiconductor saturable absorption mirrors; PBS1, PBS2, polarization beam splitters; WP1, WP2, quarter-wave-plates; Combiner, pump combiner with passive fiber pigtail; LD, laser diode centered at 793 nm; DM, dichroic mirror (HR 2000 nm/AR 793 nm); L1, L2, lens with f = 40 mm; L3, lens with f = 20 mm; M1,M2, flat mirrors (HR 2000 nm). (b) Light transmission system. Ix, the polarization component transmitted by PBS1; Iy, the polarization component reflected by PBS1.
Different from the conventional ring-shaped and linear lasers, the laser was constructed by folding two “overlapped” linear lasers, which simultaneously generated lasers with two directions of clockwise (CW) and counterclockwise (CCW). Figure 1(b) gives the map of light traveling in the cavity. The blue line and green line refer to the two polarization components of Ix and Iy, which resulted from the fiber birefringence [17]. The symbols of “m” and “n” in Fig. 1(b) are the positions of two angle-cleaved fiber ends in Fig. 1(a), respectively. The light in the cavity propagates at CW direction during the first roundtrip time and then at CCW direction during the second roundtrip. With circulating in every cavity roundtrip, it gives rise to a pulse of corresponding direction and polarization state. This predicts that the interval of the output adjacent pulses of Itotal (including Ix and Iy) should be the roundtrip time of ~20 ns, corresponding to the repetition rate of ~50 MHz. The pulse-to-pulse durations of both polarization states Ix and Iy are ~40 ns, corresponding to the repetition rate of ~25 MHz.
3. Results and discussions
The total efficiency of the pump coupling was measured to be ~60%. The low efficiency was caused by the large insertion loss of Combiner and fusion-splicing losses from the mismatch between fibers. When the pump power reached 1.6 W, corresponding to 0.96W pump power launched into active fiber, the continuous wave operation started. With the increase of pump strength, Q-switching and Q-switched mode-locking were sequentially observed. At the pump power of 3 W, we obtained stable mode-locked pulses. By reducing the pump power to below 3 W, stable mode-locked pulses were still observed and finally changed into Q-switched pulses at the pump power of 2.1 W. The pump power of initiating the mode-locking operation is higher than the lowest pump power of maintaining the mode-locking, which was called pump power hysteresis [18,19]. Once the mode-locking was initiated, the pulse trains could be very stable for many hours without any perturbations such as Q-switching or Q-switched mode-locking. When the pump power was switched on/off or inserting/removing a light barrier in the cavity at the mode-locking operation, the laser could quickly recover to the previous state without tuning the incident beam size on the SESAMs or the pump strength. This indicates that the mode-locked laser was stable and self-starting.
The typical output pulse-trains of different polarization states are shown in Fig. 2. Pulses in Figs. 2(a)-2(f) and Figs. 2(g)-2(h) were recorded by Oscilloscope in the span of 200 ns and 2 μs, respectively. It can be seen that behaviors of Ix and Iy (Figs. 2(a)-2(d)) are very different from the Itotal (Figs. 2(e)-2(h)). For the case (see Figs. 2(a), 2(c), 2(e) and 2(g)) of equal ratio of Ix and Iy, the measured doubling-time of pulses in Ix or Iy was 40.30 ns which was twice of the value (20.15 ns) of pulses in Itotal. This is different from the conventional vector solitons with their polarization components and the total pulses sharing the same value of pulse-to-pulse durations [20]. The corresponding pulse-amplitudes of Itotal keep unchanged comparing with Ix and Iy, which is also distinct from the conventional vector solitons of amplitude-adding. This indicates that there are no interactions between pulses in Ix and Iy. For the case of unequal ratio, Figs. 2(b), 2(d), 2(f), and 2(h) show a typical case of intensity-relationships of Ix>Iy by adjusting the wave plates. We can easily find that polarization components and total pulses have the similar behaviors of pulse period as the case above.
Fig. 2 Mode-locked pulse-trains at different intensity ratios and time spans. Figures 2(a) and 2(b) are for the polarization components of Ix and Figs. 2(c) and 2(d) are for the Iy; Figs. 2(e)-2(h) refer to the Itotal; Figs. 2(a), 2(c), 2(e) and 2(g) are for the equal intensity ratio of Ix and Iy; Figs. 2(b), 2(d), 2(f) and 2(h) are for the unequal ratio of Ix > Iy.
It should be noted that pulses in the case of Ix>Iy are not the period-doubling pulses reported in [21–23]. although they shared many features such as the doubled pulse-period. The period-doubling laser rooted from the nonlinear dynamic systems transiting from a stable state to a chaotic state, which the laser took on period-two state, period-three state, period-four state and finally came into chaotic state with the increasing pump power [21]. However, pulses of doubled period in our experiment were originated from the combination of two polarization beams with different amplitude. By increasing the pump strength, the laser had been operating at the stable mode-locking without the emergence of period-doubled, chaotic state and multi-pulsing. The observed self-starting operation was also distinct from the so-called period-doubling vector solitons bunch which cannot self-start [23].
The output laser without passing the PBS2 tended to operate at the case of Ix>Iy rather than the Ix = Iy. This may be due to the weak fiber birefringence and the two different orthogonal polarization directions of PBS1 in cavity. However, the output intensity ratio of Ix and Iy can be continuously changed just by adjusting wave plates in the tuning system (see Fig. 1(a)), and the results are shown in Fig. 3 and the corresponding video (Media 1). Although the conventional isolator was not inserted in front of the DM, the instability or losing of mode-locking caused by the reflected laser from the tuning system was not observed. This further confirms the stability of the laser.
The polarization degree of the outputs was measured by the above tuning system. According to the definition, the polarization degree was denoted as P = (Imax – Imin)/(Imax + Imin), where Imax is the maximum of light intensity in one direction (i.e., x-axis output of PBS), and Imin is the minimum intensity in the orthogonal direction (i.e., y-axis output of PBS). The maximum value of P = 1 corresponds to linear polarized light and the minimum value P = 0 indicates unpolarized light. By adjusting wave plates, the maximum and minimum intensities for two polarization components were recorded by power meter, respectively. The measured power of Imax and Imin of both polarization components were 31.2 mW, 0.5 mW, 29.5 mW and 0.3 mW, corresponding to the polarization degree of 0.97 and 0.98, respectively. This indicates that the output pulse-trains were composed of a row of linear polarized pulses with periodical change of polarization state, which we call it polarization switching laser.
In order to obtain further details of the laser in frequency domain, the radio frequency (RF) spectrum was recorded by the spectrum analyzer (Agilent E447A) with bandwidth of 42.98 GHz and resolution of 3 Hz. Figure 4 shows the RF spectrum of the two polarization components at the span of 0.05 MHz and resolution bandwidth (RBW) of 470 Hz. The fundamental peaks of Ix, Iy and Itotal were located at ~24.798 MHz, ~24.798 MHz, and ~49.596 MHz. According to the measured repetition rates, the corresponding pulse-to-pulse separations were calculated to be 40.3 ns, 40.3 ns and 20.15 ns, which were in good agreement with the results by Oscilloscope and predictions by our unique design of light-traveling-way in Fig. 1(b). The measured greater than 60 dB signal-to-noise ratios indicate a relatively low timing jitter. When increasing the span of spectrum analyzer, no frequency sidebands denoting the polarization evolution frequency (PEF) [24] were observed, this indicates that the polarization states of the output pulses were locked and very stable in time domain.
As the above observations showed that the output pulses were very stable when changing the pump strength, the corresponding evolutions with the pump power were recorded. In the mode-locking operation monitored by the high-speed detector, the minimum output average power was 30 mW. As the pump power continually increased, the mode-locking operation kept stable and the output average power became higher. At the maximum available pump power of 5 W, the mode-locking was still stable with the amplitude intensity fluctuation of being less than 3%, generating the maximum output average power of ~220 mW and pulse energy of ~4.4 nJ. The slope efficiency was estimated to be ~6.6%. The low efficiency was attributed to the relatively large loss of the free-space components (such as spheric lenses, PBS and Combiner), low pump couple ratio, and the relatively low absorption coefficient of active fiber at 793 nm (nominal absorption of 3dB/m). Figure 5 illustrates the optical spectrum of polarization switching laser at the maximum average power. As can be seen, the spectra of Ix, Iy and Itotal nearly share the same central wavelength of ~2002 nm and the 3-dB bandwidth of ~0.9 nm. The relatively smooth peaks in spectrum indicate a stable mode-locking operation. Tests also showed that the output spectrums at other average powers or output-intensity-ratios still had the same central wavelength and bandwidth, showing the spectrum stability in pump strength and polarization states.
The high-speed Oscilloscope recorded the pulse evolutions with output average power, which are shown in Fig. 6. As can be seen from Fig. 6(a), with the increase of output average power, pulses of Ix always kept a single pulse without pulse-splitting but the intensities and durations almost increased linearly, yielding the minimum pulse duration of 390 ps and maximum pulse duration of 690 ps. The pulses of Iy shared the similar behaviors with Ix and generating the pulse widths changing from 350 ps to 540 ps, which can be seen in Fig. 6(b).
In order to further confirm the single-pulse operation, the autocorrelation trace was measured by a commercial autocorrelator (PulseCheck, APE company) with maximum scale-range of 1.6 ns and resolution of 10 fs. We measured the autocorrelation trace of Ix at the maximum output average power in the maximum scale-range, which was shown in Fig. 7. The single peak without a large pedestal or fine structure (In other scale ranges fine structures were also not observed) in trace confirms a single-pulse operation of our laser. The trace was best fitted to be a Gaussian shape and the pulse duration was estimated to be ~691 ps, which agreed with the results by high-speed Oscilloscope. The corresponding peak power was calculated to be ~4W. The obtained polarization switching pulses without fine structures greatly enlarges the pump range of keeping stable mode-locking and helps to get a very stable mode locking operation. The typical time-bandwidth product (TBP) was calculated to be ~46, which is far from the typical value of transform-limited pulse waveform (0.441) for Gaussian pulses. This indicates the output pulses were strongly chirped, which was due to a large amount of cavity dispersion.
The pulse shaping is obviously different from the conventional solitons in pure negative dispersion cavity due to the very low peak power, long pulse duration, wave-break-free evolution, and suppressing of Kelly band. According to the power-output-ratio (estimated to be ~50%), the maximum peak power of pulse circulating in cavity was estimated to be ~4 W, corresponding to a very weak fiber nonlinearity. Hence, the solitons formation by the balance of nonlinearity and dispersion can be ruled out and the multiple pulsing can also be avoided, as well as the Kelly bands in optical spectrum. Actually, we tried a longer fiber of 10 m and obtained similar pulses with larger pulse width of ns-levels, indicating that the fiber dispersion played an important role. Therefore, the fiber dispersion and pulse shortening of SESAMs contributed to the pulse shaping [20].
4. Conclusion
We have demonstrated a novel polarization switching Tm fiber laser with the periodical changing of polarization state of every pulse. The measured repetition rates of switching laser and the corresponding vector solitons were 49.596 MHz, 24.798 MHz, and 24.798 MHz. The two orthogonally polarized components can be well controlled by tuning wave plates. To our knowledge, this is the first report of ultra-fast polarization switching and vector solitons in Tm fiber laser, which should be attractive for polarization multiplexing in free-space and fiber optics communications by employing a piece of highly Tm-doped fiber with short length [25,26].
Acknowledgments
This work was supported by the “Hundreds of Talents Programs” of the Chinese Academy of Sciences. The authors thank Jingyu Long, Ying An, Jing Bai, Cunxiao Gao, Zhi Yang, and Jia Yu for their help in experiments. The authors also thank the valuable discussions with Yong Wang in Jiangsu Normal University.
References and links
1. Q. Wang, J. H. Geng, T. Luo, and S. B. Jiang, “Mode-locked 2 μm laser with highly thulium-doped silicate fiber,” Opt. Lett. 34(23), 3616–3618 (2009). [CrossRef] [PubMed]
2. K. Kieu and F. W. Wise, “Soliton thulium-doped fiber laser with carbon nanotube saturable absorber,” IEEE Photon. Technol. Lett. 21(3), 128–130 (2009). [CrossRef] [PubMed]
3. R. Gumenyuk, I. Vartiainen, H. Tuovinen, and O. G. Okhotnikov, “Dissipative dispersion-managed soliton 2 μm thulium/holmium fiber laser,” Opt. Lett. 36(5), 609–611 (2011). [CrossRef] [PubMed]
4. Q. Wang, J. H. Geng, Z. Jiang, T. Luo, and S. B. Jiang, “Mode-locked Tm–Ho-codoped fiber laser at 2.06 μm,” IEEE Photon. Technol. Lett. 23(11), 682–684 (2011). [CrossRef]
5. L. M. Yang, P. Wan, V. Protopopov, and J. Liu, “2 µm femtosecond fiber laser at low repetition rate and high pulse energy,” Opt. Express 20(5), 5683–5688 (2012). [CrossRef] [PubMed]
6. G. Hüttmann, C. Yao, and E. Endl, “New concepts in laser medicine: towards a laser surgery with cellular precision,” Med. Laser Appl. 20(2), 135–139 (2005). [CrossRef]
7. P. Kadwani, R. A. Sims, M. Baudelet, L. Shah, and M. C. Richardson, “Atmospheric propagation testing using broadband thulium fiber systems,” in Conference on Fiber Lasers Applications (FILAS), (Optical Society of America, 2011), paper FWB3.
8. M. Eckerle, C. Kieleck, J. Świderski, S. D. Jackson, G. Mazé, and M. Eichhorn, “Actively Q-switched and mode-locked Tm3+-doped silicate 2 μm fiber laser for supercontinuum generation in fluoride fiber,” Opt. Lett. 37(4), 512–514 (2012). [CrossRef] [PubMed]
9. Q. Wang, T. Chen, B. Zhang, A. P. Heberle, and K. P. Chen, “All-fiber passively mode-locked thulium-doped fiber ring oscillator operated at solitary and noiselike modes,” Opt. Lett. 36(19), 3750–3752 (2011). [CrossRef] [PubMed]
10. M. Zhang, E. J. R. Kelleher, F. Torrisi, Z. Sun, T. Hasan, D. Popa, F. Wang, A. C. Ferrari, S. V. Popov, and J. R. Taylor, “Tm-doped fiber laser mode-locked by graphene-polymer composite,” Opt. Express 20(22), 25077–25084 (2012). [CrossRef] [PubMed]
11. R. Kadel and B. R. Washburn, “All-fiber passively mode-locked thulium/holmium laser with two center wavelengths,” Appl. Opt. 51(27), 6465–6470 (2012). [CrossRef] [PubMed]
12. A. Wienke, F. Haxsen, D. Wandt, U. Morgner, J. Neumann, and D. Kracht, “Ultrafast, stretched-pulse thulium-doped fiber laser with a fiber-based dispersion management,” Opt. Lett. 37(13), 2466–2468 (2012). [CrossRef] [PubMed]
13. N. Buholz and M. Chodorow, “3.2 - Acoustic wave amplitude modulation of a multimode ring laser,” IEEE J. Quantum Electron. 3(11), 454–459 (1967). [CrossRef]
14. K. Kieu and M. Mansuripur, “All-fiber bidirectional passively mode-locked ring laser,” Opt. Lett. 33(1), 64–66 (2008). [CrossRef] [PubMed]
15. A. Braga, J. C. Diels, R. Jain, R. Kay, and L. Wang, “Bidirectional mode-locked fiber ring laser using self-regenerative, passively controlled, threshold gating,” Opt. Lett. 35(15), 2648–2650 (2010). [CrossRef] [PubMed]
16. C. Ouyang, P. Shum, K. Wu, J. H. Wong, H. Q. Lam, and S. Aditya, “Bidirectional passively mode-locked soliton fiber laser with a four-port circulator,” Opt. Lett. 36(11), 2089–2091 (2011). [CrossRef] [PubMed]
17. D. Y. Tang, H. Zhang, L. M. Zhao, and X. Wu, “Observation of high-order polarization-locked vector solitons in a fiber laser,” Phys. Rev. Lett. 101(15), 153904 (2008). [CrossRef] [PubMed]
18. M. Nakazawa, E. Yoshida, and Y. Kimura, “Low threshold, 290 fs erbium-doped fiber laser with a nonlinear amplifying loop mirror pumped by InGaAsP laser diodes,” Appl. Phys. Lett. 59(17), 2073–2075 (1991). [CrossRef]
19. 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). [CrossRef]
20. G. P. Agrawal, Nonlinear Fiber Optics, Fourth Edition & Applications of Nonlinear Fiber Optics, Second Edition (Academic Press, 2007).
21. L. M. Zhao, D. Y. Tang, F. Lin, and B. Zhao, “Observation of period-doubling bifurcations in a femtosecond fiber soliton laser with dispersion management cavity,” Opt. Express 12(19), 4573–4578 (2004). [CrossRef] [PubMed]
22. L. M. Zhao, D. Y. Tang, H. Zhang, X. Wu, C. Lu, and H. Y. Tam, “Period-doubling of vector solitons in a ring fiber laser,” Opt. Commun. 281(22), 5614–5617 (2008). [CrossRef]
23. L. M. Zhao, D. Y. Tang, H. Zhang, and X. Wu, “Bunch of restless vector solitons in a fiber laser with SESAM,” Opt. Express 17(10), 8103–8108 (2009). [CrossRef] [PubMed]
24. S. T. Cundiff, B. C. Collings, and W. H. Knox, “Polarization locking in an isotropic, mode locked soliton Er/Yb fiber laser,” Opt. Express 1(1), 12–20 (1997). [CrossRef] [PubMed]
25. M. Schmidt, M. Witte, F. Buchali, E. Lach, and H. Bülow, “Adaptive PMD compensation for 170 Gbit/s RZ transmission systems with alternating polarisation,” in Conference on Optical Fiber Communication (OFC), (Optical Society of America, 2005), paper JWA38. [CrossRef]
26. S. Appathurai, V. Mikhailov, R. I. Killey, and P. Bayvel, “Suppression of intra-channel nonlinear distortion in 40Gbit/s transmission over standard single mode fibre using alternate-phase RZ and alternate-polarisation,” in Conference on Optical Fiber Communication (OFC), (Optical Society of America, 2004), paper ThE5.
