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Widely tunable all-fiber SESAM mode-locked Ytterbium laser with a linear cavity
Optics & Laser Technology 92 (2017) 133–137
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Widely tunable all-fiber SESAM mode-locked Ytterbium laser with a linear cavity
MARK
⁎
Feng Zoua,b, Zhaokun Wanga,b, Ziwei Wanga,b, Yang Baia,b, Qiurui Lia, Jun Zhoua,
a Shanghai Key Laboratory of All Solid-State Laser and Applied Techniques, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China b University of Chinese Academy of Sciences, Beijing 100039, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Tunable fiber lasers SESAM Mode-locked lasers Ytterbium
We present a widely tunable all-fiber mode-locked laser based on semiconductor saturable absorber mirror (SESAM) with a linear cavity design. An easy-to-use tunable bandpass filter based on thin film cavity technology is employed to tune the wavelength. By tuning the filter and adjusting the polarization controller, mode-locked operation can be achieved over the range of 1023 nm–1060 nm. With the polarization controller settled, modelocked operation can be preserved and the wavelength can be continuously tuned from 1030 nm to 1053 nm. At 1030 nm, the laser delivers 9.6 mw average output power with 15.4 ps 10.96 MHz pulses at fundamental modelocked operation.
1. Introduction Wavelength-tunable mode-locked lasers have attracted a lot of interest owing to their versatile applications in laser micromachining, optical fiber sensing, fiber device testing, spectroscopy, and biomedical researches, etc. Especially, ytterbium (Yb)-doped silica fiber is an ideal gain medium for the generation and amplification of wavelengthtunable ultrashort optical pulses owing to its broad gain band-width, high optical conversion efficiency, and large saturation fluence [1]. So far, diverse mode-locking mechanisms combined with tunable techniques have been developed to achieve flexible wavelength. On the one hand, wavelength-selective devices like tunable bandpass filters or gratings are used in laser cavity to realize different wavelengths. By tilting a spatially inserted birefringent plate, Xiao et al. reported a Ybdoped mode-locked laser based on nonlinear polarization rotation (NPR) with a 40 nm tuning range (1020–1060 nm) [2]. By using external grating configuration, Agnesi et al. achieved more widely tunable passively mode-locked fiber laser oscillators based on semiconductor-saturable absorber mirror (SESAM) [3–5]. Moreover, using a phase-shifted long-period fiber grating as spectral filter in the laser cavity, a 10 nm-tuning-range and multi-wavelength mode-locked operation was demonstrated by NPR [6]. For the sake of all-fiber structure, new wavelength-tunable techniques with multimode interference (MMI) filter [7,8], high birefringence (HiBi) fiber Sagnac loop [9] were developed. However, corresponding spatially discrete components [3–5] or troublesome tuning processes like stretching [8] and
⁎
bending fibers [6,7] were used for spectral filtering that destroyed the integrability and simplicity of the fiber laser. On the other hand, the cavity's equivalent filter effect also works. An intrinsic artificial birefringent filter in NPR mode-locked fiber lasers has been investigated [10]. One can take advantage of the artificial birefringent filter to change the central wavelength in Yb-doped mode-locked fiber lasers [11,12]. However, the wavelength’ tuning range of the birefringent filter is around 15 nm. Zhang et al. demonstrated a tunable and switchable all-fiber Yb-doped mode-locked laser with a polarization mode fiber section inserted as a Lyot filter based on NPR [13,14]. Nevertheless, the reported performances are still defective mainly because the following two reasons. First, limited tuning range around 10 nm is below that can be expected. Second, the tuning processes of rotating the polarization controller are in absence of simplicity and repeatability. In this letter, we experimentally demonstrate an all-fiber SESAM mode-locked ytterbium laser with a linear cavity that is spectrally tunable over the range of as much as 37 nm and emits 15.4 ps pulses with little chirp. A loop embedded with a commercial tunable filter based on thin film cavity plays a role of selecting the lasing wavelengths. With the polarization controller settled, we obtain 20 nm tuning range without interrupting pulsed operation by adjusting the filter's tuning knob. The simple, compact and all-fiber structure of the tunable mode-locked fiber laser can meet diverse application needs.
Corresponding author. E-mail address: [email protected] (J. Zhou).
http://dx.doi.org/10.1016/j.optlastec.2016.12.012 Received 21 September 2016; Received in revised form 10 November 2016; Accepted 17 December 2016 0030-3992/ © 2017 Elsevier Ltd. All rights reserved.
Optics & Laser Technology 92 (2017) 133–137
F. Zou et al.
The output spectra and mode-locked pulse train were monitored by an optical spectrum analyzer (YOKOGAWA AQ6370B) and a 6 GHz oscilloscope (Tektronix DPO70604C) together with a 1.2-GHz photodetector (Thorlabs DET01CFC). 3. Results and discussion Experimentally, the tunability of wavelength was realized by slowly tuning the adjustment knob of TF, which changed the inside F-P cavity's length [15]. At first, we set the TF's transmission wavelength around 1030 nm to study the mode-locking properties. Continuous wavelength (CW) emission could be obtained when the pump power was above 40 mW. Stable single-pulse mode-locked operation could be obtained by carefully adjusting the PC for pump powers above 70 mW. Continuously increasing the pump power to 105 mW, we could obtain a maximum average output power of 9.6 mW at single-pulse operation. Fig. 3(a) shows the corresponding output spectra centered at 1030.1 nm with the FWHM of 0.23 nm. Compared with a ringcavity-based mode-locked fiber laser which output changeable pulse width and peak power [16], our oscillator delivers a narrow-band spectrum and it may suitable for nonlinear frequency conversion. Fig. 3(b) shows the pulse train with a repetition rate of 10.96 MHz and the period is about 91 ns, so the calculated equivalent cavity length is 18.8 m according to f=c/nL, where n is 1.455. The corresponding maximum pulse energy is 0.87 nJ. The pulse duration measured with a commercial autocorrelator (APE SM 1200) is 15.4 ps (assuming a Gaussian pulse shape), as is illustrated in Fig. 3(c). So the timebandwidth product is about 0.97, which indicates that the pulses have small chirp. To evaluate the quality of the mode-locked pulse trains, the radio frequency (RF) spectra were characterized by a 1.2 GHz photodetector and a high resolution RF-spectrum analyzer. The fundamental and high-order harmonics signals are shown in Fig. 3(d) and (e) with corresponding resolutions of 10 Hz and 3 kHz respectively. In the RF spectrum of Fig. 3(d), there is no side-lob caused by Q-switched modelocking. A 70 dB signal-to-noise ratio and a narrow spectral width indicate stable mode-locking with low pulse timing jitter. The output power at 1030 nm as function of the incident pump power is shown in Fig. 4(a) with the PC settled. As pump power exceeds 105 mW, the operation state of the laser changed into multiple-pulse mode-locking due to largely accumulated nonlinearity. The maximum output power of 41.8 mW was obtained at the pump power of 300 mW with a slope efficiency of 16% and up to 17 pulses per cavity round-trip were obtained. Note that the considerable total 4.5 dB insertion loss of the TF and circulator adds the cavity loss, which increases the lasing threshold and lowers the slope efficiency. Passive harmonic modelocking (HML) can be generated by scaling up the pump power and adjusting the PC [17]. In our experiment, the 2 nd, 4 th, and 8 th HMLs were obtained under pump powers of 107, 129 and 195 mW and the output powers were 10.06, 13.85 and 24.90 mW. Fig. 4(b) shows the oscilloscope traces of the fundamental mode locking and HML at the repetition rates of 10.96, 21.95, 43.86 and 87.62 MHz respectively. The wavelength tuning of the mode-locked fiber laser was implemented by adjusting the TF's knob as previously mentioned. The results are illustrated in Fig. 5. Because polarization-maintaining fibers and devices' loss is polarization sensitive, PC's paddles also need to be rotated in order to select a proper polarization state. Up to 37 nmtuning-range (1023–1060 nm) was achieved experimentally under optimal settings of PC. The output spectrum's FWHMs of different wavelengths vary from 0.22 nm (at 1060 nm) to 0.25 nm (at 1035 nm). What’ more, the mode-locking threshold increases from 70 mW to 130 mW while the wavelength shifts from 1030 nm towards both ends of the range. Besides, the frequencies (f) measured with RF spectra analyzer are 10.9648, 10.9657 and 10.9664 MHz at 1030, 1040 and 1050 nm respectively. The refractive index n=n (ω) is wavelengthdependent, and in the fiber n decreases with wavelength. So according to f=c/nL, the repetition rate of pulses increases slightly with wave-
Fig. 1. Experimental configuration of the wavelength-tunable Yb-doped fiber laser with linear cavity. SESAM: semiconductor saturable absorber mirror. WDM:980/1030 nm wavelength-division multiplex. YDF:Yb-doped fiber. OC:output coupler. PC:polarization controller. TF:tunable filter.
Fig. 2. Typical transmission spectra at different wavelengths from 1022 nm to 1060 nm of the TF.
2. Experimental setup The wavelength-tunable SESAM mode-locked Yb-doped all fiber laser is depicted in Fig. 1. It has a linear cavity configuration made of pure normal dispersion fibers. A piece of 80 cm-long single cladding single mode YDF (dispersion parameter of 27 ps2/km) with 250 dB/m core absorption was pumped by a single mode laser diode (LD) emitting at 976 nm through a 980/1030 nm WDM. The fiber-pigtailed SESAM (BATOP GmbH, sam-1040-60-500 fs) with 500 fs relaxation time, 35% modulation depth and 30 μJ/cm2 saturation fluence at 1040 nm initiates and sustains the mode locking. On the other side of the laser, a special loop acted as a cavity end. In the loop, a circulator forced the laser to propagate clockwise. A tunable filter based on thin film cavity was employed to select the lasing wavelengths, which offers a tunable spectral range of 1020–1090 nm, a tuning resolution less than 0.1 nm. Fig. 2 shows typical different transmission spectra from 1022 nm to 1060 nm measured with a supercontinuum source (NKT SuperK compact). The transmission spectra’ profiles are similar and their FWHMs are 0.92, 0.92, 0.89, 0.81, and 0.82 nm corresponding to the wavelengths of 1022, 1030, 1040, 1049 and 1060 nm respectively. The insertion loss of the filter gradually decreases from 3.99 dB (at 1020 nm) to 3.57 dB (at 1060 nm). Wavelength dependence of other components were analyzed by the supercontinuum source. The circulator's transmission range covers 1000–1090 nm, and the WDM's covers 1010–1200 nm. So all the wavelength-sensitive components limit the wavelength range to 1020–1090 nm. To fine adjust the net birefringence of the cavity and further minimize the intracavity loss, a polarization controller was inserted into the loop. Except that the PC and circulator were employed with HI1060 fiber, all other devices were pigtailed with PM980 fiber. The total length of passive fiber is 10.6 m with dispersion parameter of 24 ps2/km. Since pulse circles the loop once and passes other fibers twice during a roundtrip, it experiences a passive fiber length of 17.2 m resulting net cavity dispersion of 0.456 ps2. Then the mode-locked pulses were coupled out via the 20% port of a 20:80 coupler. 134
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Fig. 3. (a) Output spectra of single-pulse mode-locked operation (at 105 mW of pump power). Corresponding (b) pulse train at a repetition rate of 10.96 MHz and (c) autocorrelation trace. The RF spectrum at the (d) fundamental and (e) high-order harmonics signals of the mode-locked laser.
wavelength redshifts from 1030 nm to 1060 nm. Moreover, during 1023–1030 nm and 1050–1060 nm, the mode-locking threshold increases dramatically, and the PC must be re-adjusted to achieve stable mode-locking operation. Wider-tuning–range mode-locking is not feasible in this oscillator. Low intensity spectral reflectance versus wavelength of the SESAM is a “V” shape and its valley point is at 1030 nm. When the wavelength comes to 1065 nm, SESAM's low intensity reflectance increases above 80%, which makes modulation depth ΔR too small to initiate mode-locking according to ΔR=A0-Ans (A0: absorption, Ans: non-saturable loss). As for 1020 nm, cavity's gain becomes not enough. So the tuning range is limited in 1023–1060 nm to realize stable mode-locking and avoid damage on the SESAM caused by too intense pump. In the all-fiber laser, the SESAM provides both robust self-starting and strong pulse shaping mechanisms to ensure mode-locked operation [19]. The all-normal dispersion structure is free of any intra-cavity dispersion control and pulses are broadened by the interaction of normal group velocity dispersion and self-phase modulation. As a
length due to chromatic dispersion. The pump power range of fundamental mode-locking changed from 70 to 105 mW at 1030 nm to 81–105 mW at 1050 nm. The wavelength could be continuously tuned at a pump power above the threshold of 81 mW, but the feasible power range is very limited. Considering the tuning stability, we chose a moderate pump power of 85 mW and investigated on the tuning characteristics with the PC settled. Fig. 6 shows the corresponding wavelengths and output powers. The FWHMs of separate wavelength are 0.24, 0.26, 0.23, 0.23, 0.22, 0.22 nm from 1029.68 to 1052.99 nm. At the same time, no significant changes on the mode-locked laser's pulse widths were observed during whole tuning range. Along with wavelength's red shift, we find that the ASE increases while the output power decreases. These phenomena mainly attribute to two main reasons. One is that the employed devices such as circulator are designed for 1030 nm. The cavity loss increases when its operating wavelength goes far away from 1030 nm. The other is that Yb-doped fiber' emission wavelength depends on the length [18]. For a given 80 cm Yb-doped fiber, gain will decrease when the propagating 135
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Fig. 6. Tuning characteristics with the PC settled at 85 mW pump power.
4. Conclusions In conclusion, we have demonstrated a wavelength tunable SESAM mode-locked all-fiber laser with a linear cavity. An easy-to-use tunable filter is applied in the simple all-fiber setups. Stable pulses with little chirp can be obtained from 1023 to 1060 nm by adjusting the PC, and mode-locked operation can be preserved tuning from 1030 nm to 1053 nm with the PC settled. Such a simple and cost-effective tunable laser is very attractive for practical applications. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (No. 2014AA041901), NSAF Foundation of National Natural Science Foundation of China (No. U1330134), and National Natural Science Foundation of China (No. 61308024). References Fig. 4. (a) Output power at 1030 nm as function of pump power. (b) Harmonic modelocking operation.
[1] J. Limpert, F. Roser, T. Schreiber, A. Tunnermann, High-power ultrafast fiber laser systems, IEEE J. Sel. Top. Quantum Electron. 12 (2006) 233. [2] X.S. Xiao, Y. Hua, B. Fu, C.X. Yang, Experimental investigation of the wavelength tunability in all-normal-dispersion ytterbium-doped mode-locked fiber lasers, IEEE Photonics J. 5 (2013). [3] A. Agnesi, L. Carra, C.D. Marco, R. Piccoli, G. Reali, Fourier-limited 19 ps Yb-fiber seeder stabilized by spectral filtering and tunable between 1015 and 1085 nm, IEEE Photonics Technol. Lett. 24 (2012) 927. [4] O.G. Okhotnikov, L. Gomes, N. Xiang, T. Jouhti, A.B. Grudinin, Mode-locked ytterbium fiber laser tunable in the 980–1070 nm spectral range, Opt. Lett. 28 (2003) 1522. [5] Y. Bai, W. Xiang, P. Zu, G. Zhang, Tunable dual-wavelength passively mode-locked Yb-doped fiber laser using SESAM, Chin. Opt. Lett. 10 (2012). [6] Z. Xiaojun, W. Chinhua, Z. Guoan, X. Ruji, Tunable dual- and triple-wavelength mode-locked all-normal-dispersion Yb-doped fiber laser, Appl. Phys. B Lasers Opt. 118 (2015) 69. [7] T. Walbaum, C. Fallnich, Multimode interference filter for tuning of a mode-locked all-fiber erbium laser, Opt. Lett. 36 (2011) 2459. [8] L. Zhang, J. Hu, J. Wang, Y. Feng, Tunable all-fiber dissipative-soliton laser with a multimode interference filter, Opt. Lett. 37 (2012) 3828. [9] O. Chunmei, S. Ping, W. Honghai, F. Songnian, C. Xueping, W. Jia Haur, T. Xiaolong, Wavelength-tunable, IEEE J. Quantum Electron. 47 (2011) 198. [10] W.S. Man, H.Y. Tan, M.S. Demokan, P.K.A. Wai, 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-Opt. Phys. 17 (2000) 28. [11] J.L. Luo, Y.Q. Ge, D.Y. Tang, S.M. Zhang, D.Y. Shen, L.M. Zhao, Mechanism of spectrum moving, narrowing, broadening, and wavelength switching of dissipative solitons in all-normal-dispersion Yb-fiber lasers, IEEE Photonics J. 6 (2014). [12] S. Huang, Y. Wang, P. Yan, J. Zhao, H. Li, R. Lin, Tunable and switchable multiwavelength dissipative soliton generation in a graphene oxide mode-locked Ybdoped fiber laser, Opt. Express 22 (2014) 11417. [13] Z.X. Zhang, Z.W. Xu, L. Zhang, Tunable and switchable dual-wavelength dissipative soliton generation in an all-normal-dispersion Yb-doped fiber laser with birefringence fiber filter, Opt. Express 20 (2012) 26736. [14] Z. Wang, S. Du, J. Wang, F. Zou, Z. Wang, W. Wu, J. Zhou, All fiber tunable- or
Fig. 5. Wavelength tuning of the mode-locked fiber laser by adjusting the TF's knob.
result, the peak power decreases and the nonlinearity weakens, by which the pulse avoids splitting, and gets higher single pulse energy. In the future, it is worthy trying a wider-spectral-range SESAM with flatter reflection and optimizing wavelength-sensitive components with wider operation range to achieve continuous tuning over the entire spectral gain bandwidth in one compact laser configuration.
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F. Zou et al. dual-wavelength Yb-doped fiber laser covering from dissipative soliton to dissipative soliton resonance, Chin. Opt. Lett. 14 (2016). [15] S. Meister, D. Schweda, M. Dziedzina, A. Al-Saadi, B.A. Franke, C. Scharfenorth, B. Grimm, D. Dufft, S.K. Schrader, H.J. Eichler, Single cavity filters on end-faces of optical fibers, Laser Reson. Beam Control Xii 7579 (2010). [16] S.B. Cho, H. Song, S. Gee, C.S. Park, D.Y. Kim, Pulse width and peak power optimization in a mode-locked fiber laser with a semiconductor saturable absorber mirror, Microw. Opt. Technol. Lett. 54 (2012) 2256.
[17] H.F. Li, S.M. Zhang, J. Du, Y.C. Meng, Y.P. Hao, X.L. Li, Passively harmonic modelocked fiber laser with controllable repetition rate based on a carbon nanotube saturable absorber, Opt. Commun. 285 (2012) 1347. [18] R. Paschotta, J. Nilsson, A.C. Tropper, D.C. Hanna, Ytterbium-doped fiber amplifiers, IEEE J. Quantum Electron. 33 (1997) 1049. [19] M. Guina, N. Xiang, O.G. Okhotnikov, Stretched-pulse fiber lasers based on semiconductor saturable absorbers, Appl. Phys. B-Lasers Opt. 74 (2002) S193.
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Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Widely tunable all-fiber SESAM mode-locked Ytterbium laser with a linear cavity
MARK
⁎
Feng Zoua,b, Zhaokun Wanga,b, Ziwei Wanga,b, Yang Baia,b, Qiurui Lia, Jun Zhoua,
a Shanghai Key Laboratory of All Solid-State Laser and Applied Techniques, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China b University of Chinese Academy of Sciences, Beijing 100039, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Tunable fiber lasers SESAM Mode-locked lasers Ytterbium
We present a widely tunable all-fiber mode-locked laser based on semiconductor saturable absorber mirror (SESAM) with a linear cavity design. An easy-to-use tunable bandpass filter based on thin film cavity technology is employed to tune the wavelength. By tuning the filter and adjusting the polarization controller, mode-locked operation can be achieved over the range of 1023 nm–1060 nm. With the polarization controller settled, modelocked operation can be preserved and the wavelength can be continuously tuned from 1030 nm to 1053 nm. At 1030 nm, the laser delivers 9.6 mw average output power with 15.4 ps 10.96 MHz pulses at fundamental modelocked operation.
1. Introduction Wavelength-tunable mode-locked lasers have attracted a lot of interest owing to their versatile applications in laser micromachining, optical fiber sensing, fiber device testing, spectroscopy, and biomedical researches, etc. Especially, ytterbium (Yb)-doped silica fiber is an ideal gain medium for the generation and amplification of wavelengthtunable ultrashort optical pulses owing to its broad gain band-width, high optical conversion efficiency, and large saturation fluence [1]. So far, diverse mode-locking mechanisms combined with tunable techniques have been developed to achieve flexible wavelength. On the one hand, wavelength-selective devices like tunable bandpass filters or gratings are used in laser cavity to realize different wavelengths. By tilting a spatially inserted birefringent plate, Xiao et al. reported a Ybdoped mode-locked laser based on nonlinear polarization rotation (NPR) with a 40 nm tuning range (1020–1060 nm) [2]. By using external grating configuration, Agnesi et al. achieved more widely tunable passively mode-locked fiber laser oscillators based on semiconductor-saturable absorber mirror (SESAM) [3–5]. Moreover, using a phase-shifted long-period fiber grating as spectral filter in the laser cavity, a 10 nm-tuning-range and multi-wavelength mode-locked operation was demonstrated by NPR [6]. For the sake of all-fiber structure, new wavelength-tunable techniques with multimode interference (MMI) filter [7,8], high birefringence (HiBi) fiber Sagnac loop [9] were developed. However, corresponding spatially discrete components [3–5] or troublesome tuning processes like stretching [8] and
⁎
bending fibers [6,7] were used for spectral filtering that destroyed the integrability and simplicity of the fiber laser. On the other hand, the cavity's equivalent filter effect also works. An intrinsic artificial birefringent filter in NPR mode-locked fiber lasers has been investigated [10]. One can take advantage of the artificial birefringent filter to change the central wavelength in Yb-doped mode-locked fiber lasers [11,12]. However, the wavelength’ tuning range of the birefringent filter is around 15 nm. Zhang et al. demonstrated a tunable and switchable all-fiber Yb-doped mode-locked laser with a polarization mode fiber section inserted as a Lyot filter based on NPR [13,14]. Nevertheless, the reported performances are still defective mainly because the following two reasons. First, limited tuning range around 10 nm is below that can be expected. Second, the tuning processes of rotating the polarization controller are in absence of simplicity and repeatability. In this letter, we experimentally demonstrate an all-fiber SESAM mode-locked ytterbium laser with a linear cavity that is spectrally tunable over the range of as much as 37 nm and emits 15.4 ps pulses with little chirp. A loop embedded with a commercial tunable filter based on thin film cavity plays a role of selecting the lasing wavelengths. With the polarization controller settled, we obtain 20 nm tuning range without interrupting pulsed operation by adjusting the filter's tuning knob. The simple, compact and all-fiber structure of the tunable mode-locked fiber laser can meet diverse application needs.
Corresponding author. E-mail address: [email protected] (J. Zhou).
http://dx.doi.org/10.1016/j.optlastec.2016.12.012 Received 21 September 2016; Received in revised form 10 November 2016; Accepted 17 December 2016 0030-3992/ © 2017 Elsevier Ltd. All rights reserved.
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The output spectra and mode-locked pulse train were monitored by an optical spectrum analyzer (YOKOGAWA AQ6370B) and a 6 GHz oscilloscope (Tektronix DPO70604C) together with a 1.2-GHz photodetector (Thorlabs DET01CFC). 3. Results and discussion Experimentally, the tunability of wavelength was realized by slowly tuning the adjustment knob of TF, which changed the inside F-P cavity's length [15]. At first, we set the TF's transmission wavelength around 1030 nm to study the mode-locking properties. Continuous wavelength (CW) emission could be obtained when the pump power was above 40 mW. Stable single-pulse mode-locked operation could be obtained by carefully adjusting the PC for pump powers above 70 mW. Continuously increasing the pump power to 105 mW, we could obtain a maximum average output power of 9.6 mW at single-pulse operation. Fig. 3(a) shows the corresponding output spectra centered at 1030.1 nm with the FWHM of 0.23 nm. Compared with a ringcavity-based mode-locked fiber laser which output changeable pulse width and peak power [16], our oscillator delivers a narrow-band spectrum and it may suitable for nonlinear frequency conversion. Fig. 3(b) shows the pulse train with a repetition rate of 10.96 MHz and the period is about 91 ns, so the calculated equivalent cavity length is 18.8 m according to f=c/nL, where n is 1.455. The corresponding maximum pulse energy is 0.87 nJ. The pulse duration measured with a commercial autocorrelator (APE SM 1200) is 15.4 ps (assuming a Gaussian pulse shape), as is illustrated in Fig. 3(c). So the timebandwidth product is about 0.97, which indicates that the pulses have small chirp. To evaluate the quality of the mode-locked pulse trains, the radio frequency (RF) spectra were characterized by a 1.2 GHz photodetector and a high resolution RF-spectrum analyzer. The fundamental and high-order harmonics signals are shown in Fig. 3(d) and (e) with corresponding resolutions of 10 Hz and 3 kHz respectively. In the RF spectrum of Fig. 3(d), there is no side-lob caused by Q-switched modelocking. A 70 dB signal-to-noise ratio and a narrow spectral width indicate stable mode-locking with low pulse timing jitter. The output power at 1030 nm as function of the incident pump power is shown in Fig. 4(a) with the PC settled. As pump power exceeds 105 mW, the operation state of the laser changed into multiple-pulse mode-locking due to largely accumulated nonlinearity. The maximum output power of 41.8 mW was obtained at the pump power of 300 mW with a slope efficiency of 16% and up to 17 pulses per cavity round-trip were obtained. Note that the considerable total 4.5 dB insertion loss of the TF and circulator adds the cavity loss, which increases the lasing threshold and lowers the slope efficiency. Passive harmonic modelocking (HML) can be generated by scaling up the pump power and adjusting the PC [17]. In our experiment, the 2 nd, 4 th, and 8 th HMLs were obtained under pump powers of 107, 129 and 195 mW and the output powers were 10.06, 13.85 and 24.90 mW. Fig. 4(b) shows the oscilloscope traces of the fundamental mode locking and HML at the repetition rates of 10.96, 21.95, 43.86 and 87.62 MHz respectively. The wavelength tuning of the mode-locked fiber laser was implemented by adjusting the TF's knob as previously mentioned. The results are illustrated in Fig. 5. Because polarization-maintaining fibers and devices' loss is polarization sensitive, PC's paddles also need to be rotated in order to select a proper polarization state. Up to 37 nmtuning-range (1023–1060 nm) was achieved experimentally under optimal settings of PC. The output spectrum's FWHMs of different wavelengths vary from 0.22 nm (at 1060 nm) to 0.25 nm (at 1035 nm). What’ more, the mode-locking threshold increases from 70 mW to 130 mW while the wavelength shifts from 1030 nm towards both ends of the range. Besides, the frequencies (f) measured with RF spectra analyzer are 10.9648, 10.9657 and 10.9664 MHz at 1030, 1040 and 1050 nm respectively. The refractive index n=n (ω) is wavelengthdependent, and in the fiber n decreases with wavelength. So according to f=c/nL, the repetition rate of pulses increases slightly with wave-
Fig. 1. Experimental configuration of the wavelength-tunable Yb-doped fiber laser with linear cavity. SESAM: semiconductor saturable absorber mirror. WDM:980/1030 nm wavelength-division multiplex. YDF:Yb-doped fiber. OC:output coupler. PC:polarization controller. TF:tunable filter.
Fig. 2. Typical transmission spectra at different wavelengths from 1022 nm to 1060 nm of the TF.
2. Experimental setup The wavelength-tunable SESAM mode-locked Yb-doped all fiber laser is depicted in Fig. 1. It has a linear cavity configuration made of pure normal dispersion fibers. A piece of 80 cm-long single cladding single mode YDF (dispersion parameter of 27 ps2/km) with 250 dB/m core absorption was pumped by a single mode laser diode (LD) emitting at 976 nm through a 980/1030 nm WDM. The fiber-pigtailed SESAM (BATOP GmbH, sam-1040-60-500 fs) with 500 fs relaxation time, 35% modulation depth and 30 μJ/cm2 saturation fluence at 1040 nm initiates and sustains the mode locking. On the other side of the laser, a special loop acted as a cavity end. In the loop, a circulator forced the laser to propagate clockwise. A tunable filter based on thin film cavity was employed to select the lasing wavelengths, which offers a tunable spectral range of 1020–1090 nm, a tuning resolution less than 0.1 nm. Fig. 2 shows typical different transmission spectra from 1022 nm to 1060 nm measured with a supercontinuum source (NKT SuperK compact). The transmission spectra’ profiles are similar and their FWHMs are 0.92, 0.92, 0.89, 0.81, and 0.82 nm corresponding to the wavelengths of 1022, 1030, 1040, 1049 and 1060 nm respectively. The insertion loss of the filter gradually decreases from 3.99 dB (at 1020 nm) to 3.57 dB (at 1060 nm). Wavelength dependence of other components were analyzed by the supercontinuum source. The circulator's transmission range covers 1000–1090 nm, and the WDM's covers 1010–1200 nm. So all the wavelength-sensitive components limit the wavelength range to 1020–1090 nm. To fine adjust the net birefringence of the cavity and further minimize the intracavity loss, a polarization controller was inserted into the loop. Except that the PC and circulator were employed with HI1060 fiber, all other devices were pigtailed with PM980 fiber. The total length of passive fiber is 10.6 m with dispersion parameter of 24 ps2/km. Since pulse circles the loop once and passes other fibers twice during a roundtrip, it experiences a passive fiber length of 17.2 m resulting net cavity dispersion of 0.456 ps2. Then the mode-locked pulses were coupled out via the 20% port of a 20:80 coupler. 134
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Fig. 3. (a) Output spectra of single-pulse mode-locked operation (at 105 mW of pump power). Corresponding (b) pulse train at a repetition rate of 10.96 MHz and (c) autocorrelation trace. The RF spectrum at the (d) fundamental and (e) high-order harmonics signals of the mode-locked laser.
wavelength redshifts from 1030 nm to 1060 nm. Moreover, during 1023–1030 nm and 1050–1060 nm, the mode-locking threshold increases dramatically, and the PC must be re-adjusted to achieve stable mode-locking operation. Wider-tuning–range mode-locking is not feasible in this oscillator. Low intensity spectral reflectance versus wavelength of the SESAM is a “V” shape and its valley point is at 1030 nm. When the wavelength comes to 1065 nm, SESAM's low intensity reflectance increases above 80%, which makes modulation depth ΔR too small to initiate mode-locking according to ΔR=A0-Ans (A0: absorption, Ans: non-saturable loss). As for 1020 nm, cavity's gain becomes not enough. So the tuning range is limited in 1023–1060 nm to realize stable mode-locking and avoid damage on the SESAM caused by too intense pump. In the all-fiber laser, the SESAM provides both robust self-starting and strong pulse shaping mechanisms to ensure mode-locked operation [19]. The all-normal dispersion structure is free of any intra-cavity dispersion control and pulses are broadened by the interaction of normal group velocity dispersion and self-phase modulation. As a
length due to chromatic dispersion. The pump power range of fundamental mode-locking changed from 70 to 105 mW at 1030 nm to 81–105 mW at 1050 nm. The wavelength could be continuously tuned at a pump power above the threshold of 81 mW, but the feasible power range is very limited. Considering the tuning stability, we chose a moderate pump power of 85 mW and investigated on the tuning characteristics with the PC settled. Fig. 6 shows the corresponding wavelengths and output powers. The FWHMs of separate wavelength are 0.24, 0.26, 0.23, 0.23, 0.22, 0.22 nm from 1029.68 to 1052.99 nm. At the same time, no significant changes on the mode-locked laser's pulse widths were observed during whole tuning range. Along with wavelength's red shift, we find that the ASE increases while the output power decreases. These phenomena mainly attribute to two main reasons. One is that the employed devices such as circulator are designed for 1030 nm. The cavity loss increases when its operating wavelength goes far away from 1030 nm. The other is that Yb-doped fiber' emission wavelength depends on the length [18]. For a given 80 cm Yb-doped fiber, gain will decrease when the propagating 135
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Fig. 6. Tuning characteristics with the PC settled at 85 mW pump power.
4. Conclusions In conclusion, we have demonstrated a wavelength tunable SESAM mode-locked all-fiber laser with a linear cavity. An easy-to-use tunable filter is applied in the simple all-fiber setups. Stable pulses with little chirp can be obtained from 1023 to 1060 nm by adjusting the PC, and mode-locked operation can be preserved tuning from 1030 nm to 1053 nm with the PC settled. Such a simple and cost-effective tunable laser is very attractive for practical applications. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (No. 2014AA041901), NSAF Foundation of National Natural Science Foundation of China (No. U1330134), and National Natural Science Foundation of China (No. 61308024). References Fig. 4. (a) Output power at 1030 nm as function of pump power. (b) Harmonic modelocking operation.
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Fig. 5. Wavelength tuning of the mode-locked fiber laser by adjusting the TF's knob.
result, the peak power decreases and the nonlinearity weakens, by which the pulse avoids splitting, and gets higher single pulse energy. In the future, it is worthy trying a wider-spectral-range SESAM with flatter reflection and optimizing wavelength-sensitive components with wider operation range to achieve continuous tuning over the entire spectral gain bandwidth in one compact laser configuration.
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