Abstract
A mode-locked fiber laser using Semiconductor Saturable Absorber Mirrors (SESAMs) is one of the popular candidate seed light sources for the construction of picosecond pulse fiber amplifier. In this paper, the influence of the fiber length of a gain fiber, the reflectance of the Fiber Bragg Grating (FBG), the modulation depth, the unsaturated loss, and the saturation flux of SESAMs, the mode field radii of single-mode transmission fibers and a single-mode gain fiber, on the output pulse characteristics, have been theoretically analyzed using the nonlinear Schrodinger equations. The characteristics of the pulse and the spectrum of an outputted laser have also been investigated theoretically. According to the simulation results, we built an ytterbium-doped mode-locking fiber laser system based on the non-polarization-maintaining linear cavity and a SESAM. Without any compensation for intra-cavity dispersion and external polarization control, a stable mode-locked pulse laser output has been achieved with the center wavelength of 1.06 μm, the pulse width of less than 12.51 ps, the spectral width of 0.32 nm, the repetition rate of 37 MHz, and the output power of 2 mW, respectively. The spectral edges of laser pulses appear smooth in our experiment, and the spectral distribution is close to the Gaussian shape. Finally, the overall structure of the near-infrared mode-locked fiber laser has been optimized by the systematic simulation. The mode-locked fiber laser introduced in this paper has a compact non-polarization-maintaining fiber structure, simple intra-cavity configuration with fewer components, high-quality output pulse correlation characteristics, which might provide a practical seed light source with the excellent performance for the next-generation picosecond pulse fiber lasers.
SESAMs, composed of a Saturable Absorber (SA) and a Bragg reflector, are the new mode-locked devices developed by the procedure of Metal-Organic Chemical Vapor Deposition (MOCVD). Compared with the mode-locking technology with a S
Since 1992, Keller et al., first realized the passive mode-locking of solid-state lasers by using a Fabry-Perot etalon SA, SESAMs have undergone several innovation
In order to make the correlation characteristics of outputted pulses for a mode-locked fiber laser with the compact linear cavity more ideal, and to further reduce the development cost, we build a SESAM-based passively mode-locked fiber laser with the non-polarization maintaining linear cavity by the use of the theoretical simulation in this paper. The cavity length can be further shortened by using the ytterbium-doped gain fiber with the relatively high absorption coefficient, where any polarization controllers have not been employed. Without compensating the dispersion in the cavity, we obtained the stable mode-locked pulse output with the center wavelength of 1.06 μm, the pulse width of less than 12 ps, the 3 dB spectral width of 0.32 nm, the repetition rate of 37 MHz, and the output power of 2 mW, respectively. The spectral edge of the pulse is smooth, and the spectral shape is very close to the Gaussian distribution. The similar research reports have been proved to be very rare. This paper might put forward to a valuable route for the commercialization of the next-generation SESAM mode-locked fiber lasers.
The structure of a linear-cavity fiber laser using the SESAM-based mode-locking is shown in

Fig. 1 The schematic illustration of a SESAM mode-locked fiber laser with the linear cavity
图1 线型腔SESAM锁模光纤激光器示意图
When the pulse is transmitted in a single-mode fiber, the relationship is followed by the nonlinear Schrodinger equation as expressed by
, | (1) |
where A is the pulse intensity, T is time, β2 is the second-order group velocity dispersion, β3 is the third-order group velocity dispersion, γ is the nonlinear coefficient, α is the absorption coefficient, respectively. When the pulse is transmitted in the Yb-doped fiber, the variation of refractive index n2 is specified by the electric polarization rate of doped ions χd :
, | (2) |
where E is energy. After considering the bandwidth limitation caused by the gain, the group velocity dispersion, β
, | (3) |
where T2 is the dipole relaxation time, g is the gain coefficient following
, | (4) |
where Ep is the instantaneous pulse energy and Esat is the gain saturation energy, respectively.
Therefore, when the pulse is transmitted in the fiber doped with rare earth ions, the physical relationship can be given by the following Ginzburg-Landau equation:
. | (5) |
Two terms on the right side of the equation characterize the nonlinear effect and gain-loss relationship, respectively.
The radius of mode field is a parameter to describe the energy transmission concentration of a single-mode fiber, which is important in estimating the fiber loss and dispersion.

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(b)
Fig. 2 Influence of the mode field radius of a single-mode fiber on characteristics in both the time domain (a) and the frequency domain (b) for different linear cavity mode-locked laser pulses
图2 单模光纤的模场半径对不同线型腔锁模激光脉冲的时域(a)和频域(b)特性的影响

Fig. 3 Influence of the mode field radius of a single-mode fiber on the pulse width and spectral width
图3 单模光纤的模场半径对脉冲宽度和光谱宽度的影响
In

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Fig. 4 Influence of the gain fiber length on characteristics in both the time domain (a) and the frequency domain (b) of different linear cavity mode-locked laser pulses
图4 增益光纤的长度对不同线型腔锁模激光脉冲的时域(a)和频域(b)特性的影响

Fig. 5 Effect of the gain fiber length on the pulse width and spectral width
图5 增益光纤的长度对脉冲宽度和光谱宽度的影响
The reflectance of a FBG influences the energy returned to the resonant cavity and outputted pulse characteristics. The output features of mode-locked pulses are shown in

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Fig. 6 Influence of the FBG output reflectance on characteristics in both the time domain (a) and the frequency domain (b) of different linear cavity mode-locked laser pulses
图6 光纤布拉格光栅的反射率对不同线型腔锁模激光脉冲的时域(a)和频域(b)特性的影响

Fig. 7 Influence of the reflectance of a FBG on the pulse width and spectral width
图7 光纤布拉格光栅的反射率对脉冲宽度和光谱宽度的影响
The greater the modulation depth of a SESAM is, the stronger the ability of a SESAM to absorb the weak signal becomes, and the easier the realization of mode-locked self-starting is.

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Fig. 8 Influence of the SESAM modulation depth on characteristics in both the time domain (a) and the frequency domain (b) of different linear cavity mode-locked laser pulses
图8 SESAM的调制深度对不同线型腔锁模激光脉冲的时域(a)和频域(b)特性的影响

Fig. 9 Effect of the modulation depth of a SESAM on the pulse width and spectral width
图9 SESAM的调制深度对脉冲宽度和光谱宽度的影响
Unsaturated loss is one kind of depletions which exists even in the saturated state of a SESAM. Insufficient reflectivity, rough surface, impurities, etc., of a SESAM can all lead to unsaturated loss. In

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Fig. 10 Influence of the SESAM unsaturated loss on characteristics in both the time domain (a) and the frequency domain (b) of different linear cavity mode-locked laser pulses
图10 SESAM的非饱和损耗对不同线型腔锁模激光脉冲的时域(a)和频域(b)特性的影响
In

Fig. 11 Effect of unsaturated loss of SESAM on the pulse width and spectral width
图11 SESAM的非饱和损耗对脉冲宽度和光谱宽度的影响
The saturation flux is the photon energy per absorption cross section when the reflectance of a SESAM is 1/e of its modulation depth, which is directly related to the pump power required to achieve mode-locking self-starting. In

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Fig. 12 Influence of SESAM saturation flux on characteristics in both the time domain (a) and the frequency domain (b) of different linear cavity mode-locked laser pulses
图12 SESAM的饱和通量对不同线型腔锁模激光脉冲的时域(a)和频域(b)特性的影响

Fig. 13 Effects of saturation flux of SESAM on the pulse width and spectral width
图13 SESAM的饱和通量对脉冲宽度和光谱宽度的影响
Using theoretical simulation results analyzed by an approach of control variation, we finally determined all suitable parameters of an Yb-doped fiber laser. Next, we undertook the experimental verification using a system as shown in

Fig. 14 Structural diagram of the experimental setup
图14 实验装置的结构图
When the power of the 976 nm pump LD was gradually increased from 0 to 24 mW, the unstable pulse lasing can be observed through an 8 GHz oscilloscope. When the pump power is increased to 27 mW, a pulse cluster is generated. When the pump power is increased to 40 mW, a stable mode-locked pulse output can be achieved with the repetition rate of 37 MHz. The output average power was measured as 0.45 mW by a power meter (Newport1830-R). The spectral width of the mode-locked pulse was measured as 0.22 nm@3 dB at the center wavelength of 1.06 μm by using a spectrometer (AQ6373-10-H, YOKOGAWA Inc.). The spectral width is much narrower than the theoretical results because the reflection bandwidth of the selected FBG is as narrow as 0.31 nm. The measured pulse sequence and spectrum are shown in

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Fig. 15 Pulse train (a) and spectrum (b) of a mode-locked laser with the pump power of 40 mW
图15 泵浦功率为40 mW的锁模激光器的脉冲序列(a)和光谱(b)

Fig. 16 Output power as a function of the input power
图16 输入-输出功率特性曲线
Next, the pump power was gradually increased to 65 mW, and the output power is increased to 2 mW. After working for 5 hours, we observed that the mode-locked pulse maintained stably, and the output power has no obvious fluctuation. When the pump power was increased to 66 mW, the regular single pulse output changed to an irregular double pulse one for our mode-locked laser, and the time interval between two mode-locked pulses is about 0.027 μs, as shown in

Fig. 17 Generation of mode-locked pulses with the pump power of 66 mW
图17 泵浦功率为66 mW时,产生多脉冲锁模
The spectra of the mode-locked pulses are also compared for different pump power as shown in

Fig. 18 Spectra of mode-locked pulses with the pump power of 40, 50, and 60 mW
图18 泵浦功率为40、50和60 mW的锁模脉冲光谱
Because the output power of the seed source is low and the peak power is not high enough, the pulse width cannot be detected by the autocorrelation instrument. In our study, the pre-amplifier module was added to enlarge the outputted power on purpose, and the pulse width can be therefore measured. The pump power of the seed and pre-amplifier modules are 60 and 100 mW, respectively. The output pulse width was measured as 12.51 ps by use of an autocorrelator (A·P·E Ltd.) as shown in

Fig. 19 Mode-locked pulse measured by the autocorrelation procedure
图19 通过自相关仪测量的锁模脉冲
The mode field radius of a single-mode fiber, the length of a gain fiber, the reflectivity of a FBG, and the main parameters of a SESAM in the linear cavity are theoretically analyzed by the numerical simulation for a mode-locked fiber laser. A subtraction optimization of the laser structure has been realized based on the theoretical results, and a mode-locked fiber laser with the non-polarization maintaining linear cavity has been built for experimental verification. Finally, we obtained a satisfactory mode-locked laser source with the center wavelength of 1.06 μm, the pulse width of less than 12.51 ps, the 3 dB spectral width of 0.32 nm, the repetition rate of 37 MHz, and the output power of 2 mW, respectively. It is believed that such a mode-locked fiber laser might be valuable as a seed source for the construction of an industrial picosecond pulse laser system in the near future.
References
Sathiyan S, Velmurugan V, Senthilnathan K, et al. All-normal dispersion passively mode-locked Yb-doped fiber laser using MOS2-PVA saturable absorber[J]. Laser Physics, 2016, 26(5). 10.1088/1054-660x/26/5/055103 [Baidu Scholar]
Chi Jun-Jie, Li Ping-Xue, Yang Chun, et al. Theoretical and experimental study on all-normal-dispersion Yb-doped mode-locked fiber lasers[J]. Chinese Physics B, 2013, 22(4):44204-044204. 10.1088/1674-1056/22/4/044204 [Baidu Scholar]
Jiang Tong-Xiao, Cui Yi-Fan, Lu Pei, et al. All PM fiber laser mode locked with a compact phase biased amplifier loop mirror[J]. IEEE Photonics Technology Letters, 2016, 28(16):1786-1789. 10.1109/lpt.2016.2572167 [Baidu Scholar]
Keller U, Miller DA, Boyd GD, et al. Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers: an A-FPSA[J]. Optics Letters, 1992, 17(7):505-507. 10.1364/ol.17.000505 [Baidu Scholar]
Lai Xue, Li Jian-Feng, Luo Hong-Yu, et al. High power passively Q-switched E
Wang Zhao-Kun, Zou Feng, Wang Zi-Wei, et al. Tunable and switchable narrow bandwidth semiconductor-saturable absorber mirror mode-locked Yb-doped fiber laser delivering different pulse widths[J]. Chinese Physics Letters, 2016, 33(3):034202. 10.1088/0256-307x/33/3/034202 [Baidu Scholar]
Gaponenko, Maxim, Metz, et al. SESAM mode-locked red praseodymium laser[J]. Optics Letters, 2014, 39(24):6939-6941. 10.1364/ol.39.006939 [Baidu Scholar]
Gong M, Yu Hai-Juan, Wushouer X, et al. Passively mode‐locked Nd:YVO4 picosecond laser with oblique incidence on SESAM[J]. Laser Physics Letters, 2010, 5(7). [Baidu Scholar]
Kim J W, Park S, Kim G H, et al. A 1030 nm all-PM SESAM mode-locked dissipative soliton fiber oscillator and its amplification with Yb-doped fiber and a Yb:YAG thin rod[J]. Laser Physics, 2022, 32(10). 10.1088/1555-6611/ac92dc [Baidu Scholar]
Bai Yang-Bo, Xiang Wang-Hua, Zu Peng, et al. Tunable dual-wavelength passively mode-locked Yb-doped fiber laser using SESAM[J]. Chinese Optics Letters, 2012, 10(11). 10.3788/col201210.111405 [Baidu Scholar]
Wang Yong-Gang, Ma Xiao-Yu, Fu Sheng-Gui, et al. Passive Q-switched mode locking of double-clading Yb fiber laser with ion-implanted GaAs[J]. Acta Physica Sinica, 2004, 53(6): 1810-1814. 10.7498/aps.53.1810 [Baidu Scholar]
Li Ping-Xue, Yao Yi-Fei, Chi Jun-Jie, et al. 980-nm all-fiber mode-locked Yb-doped phosphate fiber oscillator based on semiconductor saturable absorber mirror and its amplifier[J]. Chinese Physics B, 2016(8):84207-084207. 10.1088/1674-1056/25/8/084207 [Baidu Scholar]
Ori Katz, Yoav Sintov. Strictly all-fiber picosecond ytterbium fiber laser utilizing chirped-fiber-bragg-gratings for dispersion control[J]. Optics Communications, 2008, 281(10), 2874–2878. 10.1016/j.optcom.2008.01.048 [Baidu Scholar]
Wang Zi-Wei, Wang Zhao-Kun, Zhou Feng, et al. High-peak-powerrod-type photonic crystal fiber amplifier for picosecond pulses[J]. Chinses Journal Of Lasers, 2016, 43(10):17-23. [Baidu Scholar]
王子薇,王兆坤,邹峰,等.高峰值功率皮秒脉冲棒状光子晶体光纤放大器[J].中国激光,2016,43(10):17-23. [Baidu Scholar]
Sun-Jiang, Hou Lei, Lin Qi-Meng, et al. All-polarization maintaining mode-loched Yb-doped fiber laser with chirped fiber bragg grating[J]. Acta Photonica Sinica, 2018, 47(01):119-124. [Baidu Scholar]
孙江,侯磊,林启蒙,等.基于啁啾光纤布拉格光栅的掺镱保偏锁模光纤激光研究[J].光子学报,2018,47(01):119-124. [Baidu Scholar]