Pulse bursts with a controllable number of pulses from a mode-locked Yb-doped all fiber laser system Xingliang Li, Shumin Zhang,* Yanping Hao, and Zhenjun Yang College of Physics Science and Information Engineering, Hebei Advanced Thin Films Laboratory, Hebei Normal University, Shijiazhuang 050024, China * [email protected]
Abstract: Pulse bursts with a controllable number of pulses per burst have been produced directly from a mode-locked Yb-doped fiber laser for the first time. Each output burst contained numerous pulses with a high pulse repetition rate of 29.4 MHz. The duration of a single pulse was 680 ps. The pulse burst had a repetition rate of 251.6 kHz. The pulse burst could easily be further amplified to a total pulse burst energy of ~795 nJ, corresponding to a total average power of 200 mW. ©2014 Optical Society of America OCIS codes: (060.3510) Lasers, fiber; (140.3280) Laser amplifiers; (140.3615) Lasers, ytterbium; (140.7090) Ultrafast lasers.
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Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6699
18. S. Breitkopf, A. Klenke, T. Gottschall, H. J. Otto, C. Jauregui, J. Limpert, and A. Tünnermann, “58 mJ burst comprising ultrashort pulses with homogenous energy level from an Yb-doped fiber amplifier,” Opt. Lett. 37(24), 5169–5171 (2012). 19. J. Körner, J. Hein, H. Liebetrau, R. Seifert, D. Klöpfel, M. Kahle, M. Loeser, M. Siebold, U. Schramm, and M. C. Kaluza, “Efficient burst mode amplifier for ultra-short pulses based on cryogenically cooled Yb3+:CaF2,” Opt. Express 21(23), 29006–29012 (2013). 20. L. Zhao, D. Tang, X. Wu, and H. Zhang, “Dissipative soliton generation in Yb-fiber laser with an invisible intracavity bandpass filter,” Opt. Lett. 35(16), 2756–2758 (2010). 21. W. S. Man, H. Y. Tam, M. S. Demokan, P. K. A. Wai, and D. Y. Tang, “Mechanism of intrinsic wavelength tuning and sideband asymmetry in a passively mode-locked soliton fiber ring laser,” J. Opt. Soc. Am. B 17(1), 28–33 (2000). 22. G. P. Agrawal, Nonlinear Fiber Optics, 4th ed. (Academic, 2007). 23. X. Tian, M. Tang, X. Cheng, P. P. Shum, Y. Gong, and C. Lin, “High-energy wave-breaking-free pulse from allfiber mode-locked laser system,” Opt. Express 17(9), 7222–7227 (2009). 24. L. M. Zhao, D. Y. Tang, H. Zhang, X. Wu, Q. Bao, and K. P. Loh, “Dissipative soliton operation of an ytterbium-doped fiber laser mode locked with atomic multilayer graphene,” Opt. Lett. 35(21), 3622–3624 (2010). 25. X. Tian, M. Tang, P. P. Shum, Y. Gong, C. Lin, S. Fu, and T. Zhang, “High-energy laser pulse with a submegahertz repetition rate from a passively mode-locked fiber laser,” Opt. Lett. 34(9), 1432–1434 (2009). 26. H. Zhang, D. Y. Tang, X. Wu, and L. M. Zhao, “Multi-wavelength dissipative soliton operation of an erbiumdoped fiber laser,” Opt. Express 17(15), 12692–12697 (2009). 27. A. Komarov, H. Leblond, and F. Sanchez, “Multistability and hysteresis phenomena in passively mode-locked fiber lasers,” Phys. Rev. A 71(5), 053809 (2005). 28. W. H. Renninger, A. Chong, and F. W. Wise, “Area theorem and energy quantization for dissipative optical solitons,” J. Opt. Soc. Am. B 27(10), 1978–1982 (2010). 29. J. Liu, J. Xu, K. Liu, F. Tan, and P. Wang, “High average power picosecond pulse and supercontinuum generation from a thulium-doped, all-fiber amplifier,” Opt. Lett. 38(20), 4150–4153 (2013). 30. J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, and A. Tünnermann, “All fiber chirped-pulse amplification system based on compression in air-guiding photonic bandgap fiber,” Opt. Express 11(24), 3332–3337 (2003).
1. Introduction Passively mode-locked fiber lasers have attracted much attention because they are compact, robust, and versatile, and can produce short and ultrashort pulses. Apart from single pulses, when high pump powers are used, multiple pulses can also be formed in the laser cavity. With different cavity parameters, these groups of pulses can exist in various dynamic patterns, such as soliton bound states, harmonic mode-locked states and soliton bunches. In particular, as an intrinsic feature of fiber lasers, soliton bound states have been observed under conditions that are independent of the dispersion. For example, bound states have been theoretically studied or experimentally demonstrated in anomalous dispersion lasers [1–4], in stretched pulse lasers [5], in dispersion-managed cavities with large net normal dispersion lasers [6] and in allnormal dispersion lasers [7]. Another well-known phenomenon in passively mode-locked soliton fiber lasers is passive harmonic mode-locking, in which the pulses are uniformly distributed along the cavity. Low order [8–10] and very high order [11] passive harmonic mode-locking have been observed. These lasers have an immediate advantage in that their soliton pulse repetition rates can be many times the cavity fundamental repetition rate, which allows them to be widely applied in biomedical diagnostics, optical measurement instruments and high-speed optical communication. However, there are many other applications, such as laser systems in accelerators [12], pulsed laser deposition [13], combustion diagnostics [14] and flow measurements in aerodynamics [15], that require high pulse energies at a high repetition rate only for a certain number of pulses and not for a long time. This new mode– burst mode–has attracted a good deal of attention from many researchers [16]. A pulse burst is a group of pulses with a limited number of high-repetition-rate pulses (kHz–MHz) within the burst. The overall repetition-rate of the bursts is at a relatively low level (Hz–kHz). In 2009, a burst-mode Yb-doped fiber laser operating at a repetition rate of several hundred kilohertz and with 0.4 μJ per burst was constructed and used for pulsed laser deposition [13]. In 2012, Kalaycıoğlu demonstrated burst-mode operation in a polarizationmaintaining Yb-doped fiber amplifier capable of generating 60 μJ pulses within bursts of 11 pulses per burst facilitated by a novel feedback mechanism shaping the seed of the burst#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6700
mode amplifier [17]. In the same year, Breitkopf et al. reported an Yb-doped laser system producing 58 mJ pulse bursts comprising 2000 ultrashort pulses at a pulse repetition rate of 10 MHz [18]. Recently, a total output energy in excess of 600 mJ per burst at a repetition rate of 10 Hz has been demonstrated in a amplifier based on cryogenically cooled Yb3+: CaF2 [19]. Up to now, in order to operate in the burst-mode, an acousto-optic modulator (AOM), has been used to select desirable seed pulse groups from the pulse train generated by the pulse laser oscillator. That is to say, to date, the AOM, as a pulse picker, almost has been an essential element, which has significantly limited the laser efficiency. In this article, we report pulse bursts with a controllable number of pulses per burst that have been directly produced from an all-normal-dispersion (ANDi) ring Yb-doped fiber laser. The use of clamped peak power and weak pulse-pulse repulsive interactions between adjacent pulses are at the origin of the burst structure. As the pump power is increased, new pulses within a burst appear one by one from the trailing edge of the burst. To the best our knowledge, this is the first observation of this type of burst operation in an all fiber ANDi Ybdoped fiber laser with an ultralong cavity. The burst laser had a 251.6 kHz overall repetition rate and each burst contained up to 41 pulses limited only by the available pump power. The number of pulses in a burst could be easily controlled by increasing or decreasing the pump power. The bursts could be further amplified to energy ranges of interest for applications using standard master oscillator power amplification technology. 2. Experimental setup The full experimental setup is shown in Fig. 1. In the seed oscillator, a 976/1064 nm wavelength division multiplexer (WDM1) was used to couple 300 mW (maximum value) of pump light at 976 nm from the laser diode (LD) into a 0.45 m long Yb-doped fiber (YDF1) (Yb1200-6/125, LIEKKITM). Also a 976/1064 nm WDM2 was used to remove residual pump light. A 90:10 output coupler (OC) was located after WDM2 to output 10% of the power in the laser cavity into the amplifier. The nonlinear polarization rotation (NPR) technique was used to achieve mode locking. A polarization-dependent isolator (PD-ISO) with a central wavelength of 1064 nm and insertion loss of 1.48 dB was used to ensure unidirectional propagation and linear polarization of light, and two intracavity polarization controllers (PCs) were used to fine tune the polarization state of the cavity. An 800 m long single mode fiber (SMF) (Nufern 1060-XP) was inserted into the cavity to increase the cavity length. The total cavity length was ~800.5 m. An optical spectrum analyzer (Yokogawa AQ6317C) with a maximum resolution of 0.01 nm, a 1 GHz oscillograph (Yokogawa DL9140) with a photodetector with a 1 GHz bandwidth, a 20 GHz oscillograph (Agilent 86100D&86105D module) with a 25 GHz high speed photodetector (Newport 1414), and a radio frequency (RF) spectrum analyzer (Agilent N9020A) with a maximum measurable RF frequency of 26.5 GHz were used to monitor the spectrum, pulse train, pulse width, pulse repetition rate and the stability, respectively. In order to provide enough power for the second double-clad Yb-doped fiber amplifier (DCYDFA) and protect the seed oscillator against instabilities caused by backwardspropagating laser light coming from the amplifiers, a simple preamplifier composed of a polarization-independent isolator (PI-ISO1), a 976 nm LD with a maximum output power of 300 mW, a 976/1064 nm WDM3 and a segment of 0.6 m single-clad, Yb-doped fiber (YDF2) was spliced directly to the front end of the second DCYDFA. In the second DCYDFA, the amplifier was composed of a PI-ISO2, a 976 nm multi-mode pump LD with a maximum output power of 3 W and a gain module (AMP-SM-YDF-5/130, Nufern), consisting of a 6 m long segment of double-clad Yb-doped fiber and a (6 + 1) × 1 multi-mode fiber combiner. The multi-mode fiber had a core diameter of 105 μm and a numerical aperture (NA) of 0.22. The input and output signal fibers were SMFs, with a core diameter of 6 μm and a NA of 0.14. The entire seed laser oscillator and amplifier were designed to have an all-fiber configuration and can be packaged in a small foot print.
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6701
Fig. 1. Schematic setup of the all-fiber laser system. WDM: wavelength division multiplexer, YDF: Yb-doped fiber, OC: 10% output coupler, PC: polarization controller, PD-ISO: polarization-dependent isolator, SMF: single mode fiber. PI-ISO: polarization-independent isolator.
3. Experimental results and discussion 3.1 Single pulse The laser mode locking threshold was about 78 mW. When the pumping power exceeded this threshold, self-starting mode locking could be easily achieved. When the pump power was 80 mW, the single pulse had a full width at half maximum (FWHM) of 726 ps, as confirmed by a high speed oscillograph as shown in Fig. 2(a). The output spectrum was characterized by the steep edges typical of ANDi lasers, as shown in Fig. 2(b). The inset of Fig. 2(b) shows the optical spectrum on a linear scale. The central wavelength was 1070.5 nm, and the 3-dB spectral bandwidth was ~1.7 nm. The high time-bandwidth product (~323.1) indicated that the pulses were strongly chirped due to the gigantic positive dispersion of the ultralong ANDi fiber laser cavity. The large pulse FWHM can be compressed using the anomalous dispersion compensation technique. The RF spectrum shown in Fig. 2(c), indicates that the signal/noise ratio (SNR) was higher than 40 dB, and the fundamental peak was located at the cavity repetition rate of 251.6 kHz, which corresponds to the cavity length. The Fig. 2(d) shows the wideband RF spectrum up to 30 MHz, which confirms stable mode locking and the absence of sidebands or harmonic frequencies. With a pump power of 80 mW, we obtained an output power of 3.1 mW via the 10% output port, corresponding to a single pulse energy of 12.3 nJ. Typically, when a laser is operated in an all-normal-dispersion regime, a external bandwidth-limited bandpass filter is always inserted into the laser resonant cavity to change the loss of the cavity. However, there was no a real external bandpass filter but an artificial bandpass filter in our laser cavity as can be understood in the following manner. Since the NPR technique was used to realize mode locking, that is to say, the PCs were rotated to achieve mode locked operation, the modal birefringence value changed with the rotation of the PC, as did the beat length. As described in Ref [20], in NPR mode-locking fiber lasers, the cavity transmission is a sinusoidal function of the linear and nonlinear phase shift of the light in the cavity. Depending on the linear cavity birefringence, the cavity transmission exhibited different transmission bandwidths, defined as Bw = λcLb/(2L), where Bw is transmission bandwidth, λc is the central wavelength, Lb is beat length and L is the cavity length [20, 21]. Since our cavity length is about 800.5 m, then assuming the beat length has a typical value of 1 m [22], the value of Bw is about 0.66 nm at a wavelength of 1064 nm. Further more, with the rotation of the PC, this transmission bandwidth could also be changed. Therefore, we had reasons to believe that NPR mode locking can naturally introduce an artificial bandpass filter to the laser and that it consequently narrowed the effective gain band width of the laser and
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6702
resulted in dissipative soliton formation. These phenomena frequently appear in fiber lasers with long cavity lengths [23–26].
Fig. 2. Single pulse with pulse width of 726 ps (a), optical spectrum recorded with 0.5 nm resolution (b) (Inset: optical spectrum with a linear scale), a typical RF spectrum of the laser output after optical-to-electrical conversion, with the repetition rate of the fiber oscillator at 251.6 kHz (c), and the wideband RF spectrum up to 30 MHz (d).
3.2 Burst On increasing the pumping power, multiple pulses appeared in the cavity. By tuning the orientation of the PCs and the pump power, different multiple-pulse operation patterns could be formed. The most easily observed state was one in which solitons randomly distributed either in one tight packet or throughout the whole cavity. When several identical pulses were in one tight packet, further tuning of the orientation of the PCs resulted in these pulses becoming uniformly distribution in the packet. Furthermore, the number of the pulses in the packet could be made to increase one by one by increasing the pump power, and that was reversible when decreased the pump power. Because of the pump hysteresis phenomenon [27], the pump power when the power was decreased was slightly lower than the initial power for the same number of pulses. Since the limited number of pulses in the packet had a high repetition rate, but the packets themselves had the cavity fundamental repetition rate, which was much lower, the packet could be called a pulse burst. The process of increasing the number of pulses in a burst can described as follows. When the pump power was increased, a new small pulse, would suddenly pop up near the trailing edge of the burst. The precise position of the new pulse was such as to maintain the equal spacing of all pulses within the burst, including the new pulse. The intensity of the new pulse then gradually increased as the pump power was further increased until it reached an upper limit of energy [28]. The new pulses always appeared at the trailing edge of the burst. Eventually, with the addition of many individual pulses, a very long burst was formed. It was stable, and the phases of all the pulses in the burst were locked and the overall repetition rate was the cavity fundamental repetition rate. The evolution of the burst is shown in Media 1.
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6703
Fig. 3. Burst-mode characteristics. Typical laser output pulse burst trains (a). An expanded version of a single pulse burst (b) (Inset: Video capture of the evolution process of the pulse burst with controllable pulse number. See Media 1). Output pulse burst spectrum, centered at 1068 nm, recorded with 0.01 nm resolution (c) (Inset: output burst spectrum with a linear scale). A typical RF spectrum of a single burst with 19 pulses (d). RF spectrum of the onset of a burst with 6 pulses (Inset: Temporal onset of pulse burst.) (e). RF spectrum of a burst with 6 pulses after optical-to-electrical conversion, with a measurable 30 MHz span and 2 kHz RBW (f).
It is worth noting that in order to protect the passive optical components and the pump LD, the maximal pump power used was 292 mW, and the maximum number of pulses in a burst at this power was 41. However, as confirmed numerically in [8, 27], with increasing pump power, the average burst width is expected to become longer. When the whole laser cavity is filled by such a long train, harmonic passive mode locking would realized. Figures 3(a) and 3(b) show oscilloscope traces of pulse bursts and one expanded version of a single burst with 19 pulses obtained with a pump power of 145 mW. The temporal spacing was 34 ns and the single pulse width (FWHM) was 680 ps. In this case, the FWHM of the burst was 612 ns, and the output power was 8.7 mW. Therefore, the burst energy was 34.6 nJ and each single pulse within this burst had an average energy of 1.8 nJ. The inset of Fig. 3(b) shows a burst with the maximum of 41 pulses (video capture of Media1). Media 1 also records the evolution of a burst with a controllable pulse number. The optical spectrum shown in Fig. 3(c) retains the typical structure of ANDi fiber laser pulses with steep edges, except for a small modulation, and the inset of Fig. 3(c) shows the spectrum structure on a linear scale. The RF spectra for a
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6704
single burst with 19 pulses and the onset of the pulse burst (burst with 6 pulses) are shown in Figs. 3(d) and 3(e), with 68 kHz resolution bandwidth (RBW) and a frequency span of 1 GHz, respectively. The figures indicate that the frequency peaks of the bursts were all located at the repetition rate of 29.4 MHz, which corresponds to a temporal pulse spacing of 34 ns. For a detailed explanation of the RF spectrum of pulse burst, Fig. 3(f) shows the RF spectrum with 2 kHz RBW and 30 MHz frequency span at the onset of the burst. The small interval between adjacent peaks was 251.6 kHz, which also corresponds to the cavity length. The modulation of the RF spectrum as shown in Fig. 3(f) is due to the existence of side frequencies which are caused by the fact that the equal-spaced pulses do not occupy all the available space along the cavity.
Fig. 4. The measured average output power and burst width as a function of the pump power (a). Peak power and burst energy vs pump power (b).
Figure 4(a) shows the burst width and average output power versus the pump power. The burst width varied linearly from 170 ns (6 pulses) to 1360 ns (41 pulses) when the pump power was increased from 78.9 to 292 mW. The maximum output power was 19.8 mW at a pump power of 292 mW, corresponding to an intracavity average power of about 198 mW, given that the output coupler extracted approximately 10% of the cavity power. The intracavity optical-to-optical efficiency was therefore about 67.8%. Figure 4(b) shows the experimentally measured pulse burst energy and peak power as a function of the pump power. Note that the pulse burst energy increased linearly with the pump power, while the peak power of the pulse burst inside the cavity remained almost constant. In our experiment, the peak power of the pulse burst was about 600 mW in the cavity and the maximum pulse burst energy was 78.6 nJ. We further investigated the amplification characteristics of the pulse burst. The pump power of the laser oscillator was set at 145 mW and the powers of the preamplifier and the second master DCYDFA were fixed at 290 mW and 2.8 W, respectively. A good pulse profile was still maintained after the two-stage amplifier. The temporal widths of the initial and amplified single pulse were 680 ps and 682 ps, respectively. The average power of the bursts increased from 8.7 to 200 mW, corresponding to the pulse burst energy increasing from 34.6 to 795 nJ. Higher energies may be reached directly by multistage master oscillator power amplification or the chirped pulse amplification technique [29, 30]. 4. Conclusion We have experimentally investigated a burst-mode, ANDi mode locked fiber laser with the NPR technique. We have obtained a stable single mode-locked pulse with a repetition rate of 251.6 kHz, spectral width of 1.7 nm, pulse width of 726 ps, and pulse energy of 12.3 nJ. By increasing the pump power and tuning the orientation of the PCs, pulse bursts have also been obtained, i.e. each pulse became a burst of pulses still with a overall repetition rate of 251.6 kHz. The pulses in the burst had a uniform pulse interval of 34 ns and FWHM of 680 ps. By adjusting the pump power, we could precisely control the number of pulses in the burst. The
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6705
average output power of the laser system could reach as high as 200 mW, corresponding to a pulse burst energy of ~795 nJ at the 251.6 kHz repetition rate. This fiber laser system has a compact all-fiber construction, flexible operability and low-cost features that make it suitable for practical applications, such as pulsed laser deposition. Extending the laser cavity length to obtain stable pulse bursts with a lower repetition rate, using diffraction gratings to compress seed pulses and using multistage power amplification technology to obtain higher burst energy are the object of further work. Acknowledgments This research was supported by grants from the National Natural Science Foundation of China (Grant Nos. 11074065, 11374089, 61308016), the Hebei Natural Science Foundation (Grant Nos. F2012205076 and A2012205023), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20101303110003) and the Technology Key Project of Colleges and Universities of Hebei Province (Grant Nos. ZH2011107, ZD20131014).
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6706
Abstract: Pulse bursts with a controllable number of pulses per burst have been produced directly from a mode-locked Yb-doped fiber laser for the first time. Each output burst contained numerous pulses with a high pulse repetition rate of 29.4 MHz. The duration of a single pulse was 680 ps. The pulse burst had a repetition rate of 251.6 kHz. The pulse burst could easily be further amplified to a total pulse burst energy of ~795 nJ, corresponding to a total average power of 200 mW. ©2014 Optical Society of America OCIS codes: (060.3510) Lasers, fiber; (140.3280) Laser amplifiers; (140.3615) Lasers, ytterbium; (140.7090) Ultrafast lasers.
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Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6699
18. S. Breitkopf, A. Klenke, T. Gottschall, H. J. Otto, C. Jauregui, J. Limpert, and A. Tünnermann, “58 mJ burst comprising ultrashort pulses with homogenous energy level from an Yb-doped fiber amplifier,” Opt. Lett. 37(24), 5169–5171 (2012). 19. J. Körner, J. Hein, H. Liebetrau, R. Seifert, D. Klöpfel, M. Kahle, M. Loeser, M. Siebold, U. Schramm, and M. C. Kaluza, “Efficient burst mode amplifier for ultra-short pulses based on cryogenically cooled Yb3+:CaF2,” Opt. Express 21(23), 29006–29012 (2013). 20. L. Zhao, D. Tang, X. Wu, and H. Zhang, “Dissipative soliton generation in Yb-fiber laser with an invisible intracavity bandpass filter,” Opt. Lett. 35(16), 2756–2758 (2010). 21. W. S. Man, H. Y. Tam, M. S. Demokan, P. K. A. Wai, and D. Y. Tang, “Mechanism of intrinsic wavelength tuning and sideband asymmetry in a passively mode-locked soliton fiber ring laser,” J. Opt. Soc. Am. B 17(1), 28–33 (2000). 22. G. P. Agrawal, Nonlinear Fiber Optics, 4th ed. (Academic, 2007). 23. X. Tian, M. Tang, X. Cheng, P. P. Shum, Y. Gong, and C. Lin, “High-energy wave-breaking-free pulse from allfiber mode-locked laser system,” Opt. Express 17(9), 7222–7227 (2009). 24. L. M. Zhao, D. Y. Tang, H. Zhang, X. Wu, Q. Bao, and K. P. Loh, “Dissipative soliton operation of an ytterbium-doped fiber laser mode locked with atomic multilayer graphene,” Opt. Lett. 35(21), 3622–3624 (2010). 25. X. Tian, M. Tang, P. P. Shum, Y. Gong, C. Lin, S. Fu, and T. Zhang, “High-energy laser pulse with a submegahertz repetition rate from a passively mode-locked fiber laser,” Opt. Lett. 34(9), 1432–1434 (2009). 26. H. Zhang, D. Y. Tang, X. Wu, and L. M. Zhao, “Multi-wavelength dissipative soliton operation of an erbiumdoped fiber laser,” Opt. Express 17(15), 12692–12697 (2009). 27. A. Komarov, H. Leblond, and F. Sanchez, “Multistability and hysteresis phenomena in passively mode-locked fiber lasers,” Phys. Rev. A 71(5), 053809 (2005). 28. W. H. Renninger, A. Chong, and F. W. Wise, “Area theorem and energy quantization for dissipative optical solitons,” J. Opt. Soc. Am. B 27(10), 1978–1982 (2010). 29. J. Liu, J. Xu, K. Liu, F. Tan, and P. Wang, “High average power picosecond pulse and supercontinuum generation from a thulium-doped, all-fiber amplifier,” Opt. Lett. 38(20), 4150–4153 (2013). 30. J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, and A. Tünnermann, “All fiber chirped-pulse amplification system based on compression in air-guiding photonic bandgap fiber,” Opt. Express 11(24), 3332–3337 (2003).
1. Introduction Passively mode-locked fiber lasers have attracted much attention because they are compact, robust, and versatile, and can produce short and ultrashort pulses. Apart from single pulses, when high pump powers are used, multiple pulses can also be formed in the laser cavity. With different cavity parameters, these groups of pulses can exist in various dynamic patterns, such as soliton bound states, harmonic mode-locked states and soliton bunches. In particular, as an intrinsic feature of fiber lasers, soliton bound states have been observed under conditions that are independent of the dispersion. For example, bound states have been theoretically studied or experimentally demonstrated in anomalous dispersion lasers [1–4], in stretched pulse lasers [5], in dispersion-managed cavities with large net normal dispersion lasers [6] and in allnormal dispersion lasers [7]. Another well-known phenomenon in passively mode-locked soliton fiber lasers is passive harmonic mode-locking, in which the pulses are uniformly distributed along the cavity. Low order [8–10] and very high order [11] passive harmonic mode-locking have been observed. These lasers have an immediate advantage in that their soliton pulse repetition rates can be many times the cavity fundamental repetition rate, which allows them to be widely applied in biomedical diagnostics, optical measurement instruments and high-speed optical communication. However, there are many other applications, such as laser systems in accelerators [12], pulsed laser deposition [13], combustion diagnostics [14] and flow measurements in aerodynamics [15], that require high pulse energies at a high repetition rate only for a certain number of pulses and not for a long time. This new mode– burst mode–has attracted a good deal of attention from many researchers [16]. A pulse burst is a group of pulses with a limited number of high-repetition-rate pulses (kHz–MHz) within the burst. The overall repetition-rate of the bursts is at a relatively low level (Hz–kHz). In 2009, a burst-mode Yb-doped fiber laser operating at a repetition rate of several hundred kilohertz and with 0.4 μJ per burst was constructed and used for pulsed laser deposition [13]. In 2012, Kalaycıoğlu demonstrated burst-mode operation in a polarizationmaintaining Yb-doped fiber amplifier capable of generating 60 μJ pulses within bursts of 11 pulses per burst facilitated by a novel feedback mechanism shaping the seed of the burst#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6700
mode amplifier [17]. In the same year, Breitkopf et al. reported an Yb-doped laser system producing 58 mJ pulse bursts comprising 2000 ultrashort pulses at a pulse repetition rate of 10 MHz [18]. Recently, a total output energy in excess of 600 mJ per burst at a repetition rate of 10 Hz has been demonstrated in a amplifier based on cryogenically cooled Yb3+: CaF2 [19]. Up to now, in order to operate in the burst-mode, an acousto-optic modulator (AOM), has been used to select desirable seed pulse groups from the pulse train generated by the pulse laser oscillator. That is to say, to date, the AOM, as a pulse picker, almost has been an essential element, which has significantly limited the laser efficiency. In this article, we report pulse bursts with a controllable number of pulses per burst that have been directly produced from an all-normal-dispersion (ANDi) ring Yb-doped fiber laser. The use of clamped peak power and weak pulse-pulse repulsive interactions between adjacent pulses are at the origin of the burst structure. As the pump power is increased, new pulses within a burst appear one by one from the trailing edge of the burst. To the best our knowledge, this is the first observation of this type of burst operation in an all fiber ANDi Ybdoped fiber laser with an ultralong cavity. The burst laser had a 251.6 kHz overall repetition rate and each burst contained up to 41 pulses limited only by the available pump power. The number of pulses in a burst could be easily controlled by increasing or decreasing the pump power. The bursts could be further amplified to energy ranges of interest for applications using standard master oscillator power amplification technology. 2. Experimental setup The full experimental setup is shown in Fig. 1. In the seed oscillator, a 976/1064 nm wavelength division multiplexer (WDM1) was used to couple 300 mW (maximum value) of pump light at 976 nm from the laser diode (LD) into a 0.45 m long Yb-doped fiber (YDF1) (Yb1200-6/125, LIEKKITM). Also a 976/1064 nm WDM2 was used to remove residual pump light. A 90:10 output coupler (OC) was located after WDM2 to output 10% of the power in the laser cavity into the amplifier. The nonlinear polarization rotation (NPR) technique was used to achieve mode locking. A polarization-dependent isolator (PD-ISO) with a central wavelength of 1064 nm and insertion loss of 1.48 dB was used to ensure unidirectional propagation and linear polarization of light, and two intracavity polarization controllers (PCs) were used to fine tune the polarization state of the cavity. An 800 m long single mode fiber (SMF) (Nufern 1060-XP) was inserted into the cavity to increase the cavity length. The total cavity length was ~800.5 m. An optical spectrum analyzer (Yokogawa AQ6317C) with a maximum resolution of 0.01 nm, a 1 GHz oscillograph (Yokogawa DL9140) with a photodetector with a 1 GHz bandwidth, a 20 GHz oscillograph (Agilent 86100D&86105D module) with a 25 GHz high speed photodetector (Newport 1414), and a radio frequency (RF) spectrum analyzer (Agilent N9020A) with a maximum measurable RF frequency of 26.5 GHz were used to monitor the spectrum, pulse train, pulse width, pulse repetition rate and the stability, respectively. In order to provide enough power for the second double-clad Yb-doped fiber amplifier (DCYDFA) and protect the seed oscillator against instabilities caused by backwardspropagating laser light coming from the amplifiers, a simple preamplifier composed of a polarization-independent isolator (PI-ISO1), a 976 nm LD with a maximum output power of 300 mW, a 976/1064 nm WDM3 and a segment of 0.6 m single-clad, Yb-doped fiber (YDF2) was spliced directly to the front end of the second DCYDFA. In the second DCYDFA, the amplifier was composed of a PI-ISO2, a 976 nm multi-mode pump LD with a maximum output power of 3 W and a gain module (AMP-SM-YDF-5/130, Nufern), consisting of a 6 m long segment of double-clad Yb-doped fiber and a (6 + 1) × 1 multi-mode fiber combiner. The multi-mode fiber had a core diameter of 105 μm and a numerical aperture (NA) of 0.22. The input and output signal fibers were SMFs, with a core diameter of 6 μm and a NA of 0.14. The entire seed laser oscillator and amplifier were designed to have an all-fiber configuration and can be packaged in a small foot print.
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6701
Fig. 1. Schematic setup of the all-fiber laser system. WDM: wavelength division multiplexer, YDF: Yb-doped fiber, OC: 10% output coupler, PC: polarization controller, PD-ISO: polarization-dependent isolator, SMF: single mode fiber. PI-ISO: polarization-independent isolator.
3. Experimental results and discussion 3.1 Single pulse The laser mode locking threshold was about 78 mW. When the pumping power exceeded this threshold, self-starting mode locking could be easily achieved. When the pump power was 80 mW, the single pulse had a full width at half maximum (FWHM) of 726 ps, as confirmed by a high speed oscillograph as shown in Fig. 2(a). The output spectrum was characterized by the steep edges typical of ANDi lasers, as shown in Fig. 2(b). The inset of Fig. 2(b) shows the optical spectrum on a linear scale. The central wavelength was 1070.5 nm, and the 3-dB spectral bandwidth was ~1.7 nm. The high time-bandwidth product (~323.1) indicated that the pulses were strongly chirped due to the gigantic positive dispersion of the ultralong ANDi fiber laser cavity. The large pulse FWHM can be compressed using the anomalous dispersion compensation technique. The RF spectrum shown in Fig. 2(c), indicates that the signal/noise ratio (SNR) was higher than 40 dB, and the fundamental peak was located at the cavity repetition rate of 251.6 kHz, which corresponds to the cavity length. The Fig. 2(d) shows the wideband RF spectrum up to 30 MHz, which confirms stable mode locking and the absence of sidebands or harmonic frequencies. With a pump power of 80 mW, we obtained an output power of 3.1 mW via the 10% output port, corresponding to a single pulse energy of 12.3 nJ. Typically, when a laser is operated in an all-normal-dispersion regime, a external bandwidth-limited bandpass filter is always inserted into the laser resonant cavity to change the loss of the cavity. However, there was no a real external bandpass filter but an artificial bandpass filter in our laser cavity as can be understood in the following manner. Since the NPR technique was used to realize mode locking, that is to say, the PCs were rotated to achieve mode locked operation, the modal birefringence value changed with the rotation of the PC, as did the beat length. As described in Ref [20], in NPR mode-locking fiber lasers, the cavity transmission is a sinusoidal function of the linear and nonlinear phase shift of the light in the cavity. Depending on the linear cavity birefringence, the cavity transmission exhibited different transmission bandwidths, defined as Bw = λcLb/(2L), where Bw is transmission bandwidth, λc is the central wavelength, Lb is beat length and L is the cavity length [20, 21]. Since our cavity length is about 800.5 m, then assuming the beat length has a typical value of 1 m [22], the value of Bw is about 0.66 nm at a wavelength of 1064 nm. Further more, with the rotation of the PC, this transmission bandwidth could also be changed. Therefore, we had reasons to believe that NPR mode locking can naturally introduce an artificial bandpass filter to the laser and that it consequently narrowed the effective gain band width of the laser and
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6702
resulted in dissipative soliton formation. These phenomena frequently appear in fiber lasers with long cavity lengths [23–26].
Fig. 2. Single pulse with pulse width of 726 ps (a), optical spectrum recorded with 0.5 nm resolution (b) (Inset: optical spectrum with a linear scale), a typical RF spectrum of the laser output after optical-to-electrical conversion, with the repetition rate of the fiber oscillator at 251.6 kHz (c), and the wideband RF spectrum up to 30 MHz (d).
3.2 Burst On increasing the pumping power, multiple pulses appeared in the cavity. By tuning the orientation of the PCs and the pump power, different multiple-pulse operation patterns could be formed. The most easily observed state was one in which solitons randomly distributed either in one tight packet or throughout the whole cavity. When several identical pulses were in one tight packet, further tuning of the orientation of the PCs resulted in these pulses becoming uniformly distribution in the packet. Furthermore, the number of the pulses in the packet could be made to increase one by one by increasing the pump power, and that was reversible when decreased the pump power. Because of the pump hysteresis phenomenon [27], the pump power when the power was decreased was slightly lower than the initial power for the same number of pulses. Since the limited number of pulses in the packet had a high repetition rate, but the packets themselves had the cavity fundamental repetition rate, which was much lower, the packet could be called a pulse burst. The process of increasing the number of pulses in a burst can described as follows. When the pump power was increased, a new small pulse, would suddenly pop up near the trailing edge of the burst. The precise position of the new pulse was such as to maintain the equal spacing of all pulses within the burst, including the new pulse. The intensity of the new pulse then gradually increased as the pump power was further increased until it reached an upper limit of energy [28]. The new pulses always appeared at the trailing edge of the burst. Eventually, with the addition of many individual pulses, a very long burst was formed. It was stable, and the phases of all the pulses in the burst were locked and the overall repetition rate was the cavity fundamental repetition rate. The evolution of the burst is shown in Media 1.
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6703
Fig. 3. Burst-mode characteristics. Typical laser output pulse burst trains (a). An expanded version of a single pulse burst (b) (Inset: Video capture of the evolution process of the pulse burst with controllable pulse number. See Media 1). Output pulse burst spectrum, centered at 1068 nm, recorded with 0.01 nm resolution (c) (Inset: output burst spectrum with a linear scale). A typical RF spectrum of a single burst with 19 pulses (d). RF spectrum of the onset of a burst with 6 pulses (Inset: Temporal onset of pulse burst.) (e). RF spectrum of a burst with 6 pulses after optical-to-electrical conversion, with a measurable 30 MHz span and 2 kHz RBW (f).
It is worth noting that in order to protect the passive optical components and the pump LD, the maximal pump power used was 292 mW, and the maximum number of pulses in a burst at this power was 41. However, as confirmed numerically in [8, 27], with increasing pump power, the average burst width is expected to become longer. When the whole laser cavity is filled by such a long train, harmonic passive mode locking would realized. Figures 3(a) and 3(b) show oscilloscope traces of pulse bursts and one expanded version of a single burst with 19 pulses obtained with a pump power of 145 mW. The temporal spacing was 34 ns and the single pulse width (FWHM) was 680 ps. In this case, the FWHM of the burst was 612 ns, and the output power was 8.7 mW. Therefore, the burst energy was 34.6 nJ and each single pulse within this burst had an average energy of 1.8 nJ. The inset of Fig. 3(b) shows a burst with the maximum of 41 pulses (video capture of Media1). Media 1 also records the evolution of a burst with a controllable pulse number. The optical spectrum shown in Fig. 3(c) retains the typical structure of ANDi fiber laser pulses with steep edges, except for a small modulation, and the inset of Fig. 3(c) shows the spectrum structure on a linear scale. The RF spectra for a
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6704
single burst with 19 pulses and the onset of the pulse burst (burst with 6 pulses) are shown in Figs. 3(d) and 3(e), with 68 kHz resolution bandwidth (RBW) and a frequency span of 1 GHz, respectively. The figures indicate that the frequency peaks of the bursts were all located at the repetition rate of 29.4 MHz, which corresponds to a temporal pulse spacing of 34 ns. For a detailed explanation of the RF spectrum of pulse burst, Fig. 3(f) shows the RF spectrum with 2 kHz RBW and 30 MHz frequency span at the onset of the burst. The small interval between adjacent peaks was 251.6 kHz, which also corresponds to the cavity length. The modulation of the RF spectrum as shown in Fig. 3(f) is due to the existence of side frequencies which are caused by the fact that the equal-spaced pulses do not occupy all the available space along the cavity.
Fig. 4. The measured average output power and burst width as a function of the pump power (a). Peak power and burst energy vs pump power (b).
Figure 4(a) shows the burst width and average output power versus the pump power. The burst width varied linearly from 170 ns (6 pulses) to 1360 ns (41 pulses) when the pump power was increased from 78.9 to 292 mW. The maximum output power was 19.8 mW at a pump power of 292 mW, corresponding to an intracavity average power of about 198 mW, given that the output coupler extracted approximately 10% of the cavity power. The intracavity optical-to-optical efficiency was therefore about 67.8%. Figure 4(b) shows the experimentally measured pulse burst energy and peak power as a function of the pump power. Note that the pulse burst energy increased linearly with the pump power, while the peak power of the pulse burst inside the cavity remained almost constant. In our experiment, the peak power of the pulse burst was about 600 mW in the cavity and the maximum pulse burst energy was 78.6 nJ. We further investigated the amplification characteristics of the pulse burst. The pump power of the laser oscillator was set at 145 mW and the powers of the preamplifier and the second master DCYDFA were fixed at 290 mW and 2.8 W, respectively. A good pulse profile was still maintained after the two-stage amplifier. The temporal widths of the initial and amplified single pulse were 680 ps and 682 ps, respectively. The average power of the bursts increased from 8.7 to 200 mW, corresponding to the pulse burst energy increasing from 34.6 to 795 nJ. Higher energies may be reached directly by multistage master oscillator power amplification or the chirped pulse amplification technique [29, 30]. 4. Conclusion We have experimentally investigated a burst-mode, ANDi mode locked fiber laser with the NPR technique. We have obtained a stable single mode-locked pulse with a repetition rate of 251.6 kHz, spectral width of 1.7 nm, pulse width of 726 ps, and pulse energy of 12.3 nJ. By increasing the pump power and tuning the orientation of the PCs, pulse bursts have also been obtained, i.e. each pulse became a burst of pulses still with a overall repetition rate of 251.6 kHz. The pulses in the burst had a uniform pulse interval of 34 ns and FWHM of 680 ps. By adjusting the pump power, we could precisely control the number of pulses in the burst. The
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6705
average output power of the laser system could reach as high as 200 mW, corresponding to a pulse burst energy of ~795 nJ at the 251.6 kHz repetition rate. This fiber laser system has a compact all-fiber construction, flexible operability and low-cost features that make it suitable for practical applications, such as pulsed laser deposition. Extending the laser cavity length to obtain stable pulse bursts with a lower repetition rate, using diffraction gratings to compress seed pulses and using multistage power amplification technology to obtain higher burst energy are the object of further work. Acknowledgments This research was supported by grants from the National Natural Science Foundation of China (Grant Nos. 11074065, 11374089, 61308016), the Hebei Natural Science Foundation (Grant Nos. F2012205076 and A2012205023), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20101303110003) and the Technology Key Project of Colleges and Universities of Hebei Province (Grant Nos. ZH2011107, ZD20131014).
#204364 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 21 Feb 2014; accepted 23 Feb 2014; published 14 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006699 | OPTICS EXPRESS 6706
