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Abstract
We present a robust simultaneous multiwavelength bulk laser based on Yb:LaMgB5O10 (Yb:LMB) crystal, including its continuous-wave (CW) and ultrafast pulsed regimes. CW dual wavelengths at 1077 nm and 1091 nm with an average output power of 2.67 W were achieved with a 1% output coupling (OC). A maximum output power of 4.42 W with triple wavelengths of 1056, 1077, and 1091 nm was generated with 3% OC. In the case of 25% OC, the Yb:LMB laser can operate with four wavelengths (1025.1, 1031.4, 1033.2, and 1053.3 nm), producing an average output power of up to 4.44 W. Furthermore, 568 fs pulses with an average power of 470 mW were obtained at 1057.4, 1079.2, and 1092.4 nm from a synchronous tri-wavelength mode-locked Yb:LMB laser. These pulses are the shortest ever reported from a synchronous tri-wavelength mode-locked bulk laser. The detected frequency beating pulses had a primary interval of 0.11 ps and a full width at half maximum of 77 fs. Results indicated that equal spectral separation between each wavelength is not an essential factor for establishing a synchronous tri-wavelength mode-locking operation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Simultaneous multiwavelength solid-state lasers have many applications in atmospheric detection, lidar system, and terahertz (THz) wave generation [1–3]. In a 1 µm region, multiwavelength lasers play an important role in medical treatment. Combined with the frequency doubling technique, multiwavelength lasers can be used as sources in the treatment of the fundus oculi disease, facial telangiectasia, and nevus flammeus [4,5]. Nd:YAG and Nd:YVO4 crystals are commonly used in 1 µm multiwavelength lasers due to their good optical and thermal properties. However, understanding simultaneous multiwavelength laser operation in 1010–1100 nm without additional optical elements or special coating design is difficult because the gain of 1064 nm is considerably higher than those of other wavelengths [6,7]. Therefore, the motivation to explore gain crystals with evident spectral splitting, comparable gain cross sections at lasing wavelengths, and reasonable thermal conductivity is strong to realize a low-cost, robust, and compact multiwavelength laser system of approximately 1 µm that can address the requirements of optical treatment via frequency doubling.
RMgB5O10 (R = La, Gd) recently emerged as a potential laser host material for simultaneous multiwavelength generation [8–13]. An average output power of 5.1 W was achieved based on Nd:LaMgB5O10 (LMB) crystal with an optical-to-optical efficiency of 34.5% working at 1051.8 nm and 1081.4 nm [14]. Using Er,Yb:LMB crystal, continuous-wave (CW) and acousto-optic Q-switched lasers were produced at 1556 nm and 1568 nm, respectively [15]. In addition, a spectroscopic study on Yb:GMB crystal demonstrated that its emission curve has three primary peaks (1011, 1035, and 1059 nm) [11]. In an acousto-optic Q-switched Yb:GMB laser, 2.5 W with 36 ns simultaneous dual-wavelength pulses were obtained at 1059.2 nm and 1060.8 nm [16]. For Yb-doped LMB crystal, growth, spectral properties, and single-wavelength mode-locking operation were reported in our previous work [10,17]. Compared with Yb:GMB, achieving a high-quality Yb:LMB crystal is more difficult because a relatively larger difference in ionic radius between La3+(1.216 Å) and Yb3+(0.985 Å) ions exists. However, this discrepancy will cause a flexible splitting of the emission spectra of Yb:LMB. Simultaneously, Yb-doped LMB crystal has comparable gain cross sections of approximately 1.06 µm and a considerably good thermal conductivity (∼5 W/m/K at 300 K). Therefore, diode-pumped solid-state Yb:LMB laser is a promising design for achieving a robust and compact multiwavelength laser.
In the current study, the multiwavelength performance of Yb:LMB laser was investigated in detail, including its CW and ultrafast pulsed regimes. The results indicate the high potential of Yb:LMB crystal in achieving a multiwavelength laser source.
2. Laser experimental setup
CW laser performance was achieved with a simple concave–plano resonator. A concave mirror (radius-of-curvature of 200 mm) coated for high transmission (HT) at 900–990 nm and high reflection (HR) at 1010–1100 nm was used as an input mirror. A group of plano-mirrors with different transmittances of 1%, 3%, 10%, 15%, and 25% served as the output coupler. A 3.5 mm-thick Z-cut Yb (5 at.%):LMB crystal was used in the laser experiments. Due to the strong volatility of MgO and B2O3 in the growing process, it is very difficult to obtain high quality and large size LMB crystal by the conventional Czochralski method. In this work, the Yb:LMB crystal was grown by the top-seed solution growth (TSSG) method from Li2O-B2O3-LiF flux. The uncoated Yb:LMB crystal was tightly mounted onto a copper block that was maintained at 10 degrees Celsius. The pump source was a 977 nm fiber-coupled laser diode with a core diameter of 200 µm. The pump beam was focused onto Yb:LMB with a radius of 100 µm by a 1:1 optical collimation system.
Using the same gain crystal as a CW laser, the ultrafast multiwavelength laser characteristic of Yb:LMB crystal was explored in a 3.5 m-long folded cavity (Fig. 1). A 977 nm pump beam with a diameter of 110 µm was reimaged into the crystal with a radius of 30 µm by a 1.8:1 optical collimation system. The laser beam inside the Yb:LMB crystal was calculated to be 29 µm × 31 µm, which matched the pump beam. The transmittance (T) of the output mirror (M3) was 1%. A commercial semiconductor saturable absorber mirror (SESAM) with a modulation depth of 0.8% and a saturation fluence of 90 µJ/cm2 was used to initiate and stabilize the mode-locking operation (BATOP, GmbH). The laser mode was focused on the SESAM by a folded mirror with a radius of curvature of 0.2 m, corresponding to a laser beam radius of 35 µm × 37 µm. A plane Gires–Tournois interferometer (GTI) mirror (LAYERTEC, GmbH) with a dispersion compensation of –550 ± 100 fs2 at 1040–1090 nm was inserted into the cavity to compensate for intracavity dispersion. The laser spectra were detected by a spectrometer with a resolution of 0.3 nm (AvaSpec-3648, Avantes B. V.). Moreover, the pulse duration was measured using a commercial autocorrelator (Pulse Check USB, APE). A digital oscilloscope with a bandwidth of 1 GHz was used to monitor laser pulses (AQ6370C, Yokogawa). The radio frequency (RF) spectra were recorded using an RF spectrum analyzer (N9000A, Agilent).
Fig. 1. Configuration of the Yb:LMB laser for synchronous tri-wavelength mode-locking operation. M1, input mirror: flat mirror coated for HT at 900–990 nm and HR in 1010–1100 nm; M2 and M4, HR fold mirrors. GTI: HR mirror with dispersion of –550 ± 100 fs2 per reflection; M3: output mirror.
3. Simultaneous multiwavelength CW laser performance
The CW laser operation of Yb:LMB crystal was realized in a 20 mm-long linear cavity without additional optical elements. Figure 2 depicts the average output power versus absorbed pump power (Pabs) for five output couplers (OCs) of T=1%, 3%, 10%, 15%, and 25%. The lowest lasing threshold was reached at 1 W in the case of T=1%. The most efficient laser oscillation was obtained with an OC of T=15%, generating a maximum output power of 5.29 W at an absorbed pump power of 11 W, with an associated slope efficiency of 63%. In the cases of T=3%, 10%, and 25%, the maximum output power produced was 4.42, 4.61, and 4.44 W, respectively, at an absorbed pump power of 11 W. The slope efficiency (η) measured for T=3%, 10%, and 25% was 48%, 51%, and 62%, respectively. For the coupling of T=1%, the absorbed pump power was limited to 7.5 W to prevent the thermal damage of Yb:LMB. This value corresponded to a maximum output power of 2.67 W. For all OCs, no saturation trend was observed in the experiment. However, with the OC of T=1%, the resulting laser oscillation became less efficient, producing the lowest slope efficiency of 41%. The optical-to-optical efficiencies at the maximum output power were determined to be 35.6%, 40.2%, 42%, 48.1%, and 40.4% for T=1%, 3%, 10%, 15%, and 25%, respectively.
The typical emission spectra of the CW Yb:LMB laser under different OC conditions are illustrated in Figs. 3 and 4. In the case of T=1%, the laser operated in a dual-wavelength regime centered at 1077 nm and 1091 nm. However, the power ratio between 1077 nm and 1091 nm ranged from 10:1 to 2:1 with increasing absorbed pump power. The ratio of output powers between the individual peaks were computed by integrating the photon energies over the spectral bands corresponding to the peaks. This phenomenon can account for thermal accumulation in the relatively low sublevels of the ground state 2F7/2 under high pump power, causing a rapid reduction in the gain of short wavelengths. The peak intensity of two emissions were approximately equal to the pump power range from 4 W to 7.5 W, corresponding to an output power of 1.1 W to 2.67 W. The dual wavelengths centered at 1077 nm and 1091 nm were linearly polarized parallel to the crystalline X- and Y-axes, respectively. An orthogonally polarized dual-wavelength laser has an important application in self-sensing metrology and THz wave generation.
For the T=3% case, triple-wavelength emissions centered at 1056, 1077, and 1091 nm were generated. At the absorbed pump power of 11 W, the output power proportion of the three wavelengths was 1:0.9:0.7, corresponding to the output power of 1.7, 1.53, and 1.19 W, respectively. The Yb:LMB laser always worked in a tri-wavelength regime with the wavelength centered at 1056, 1077, and 1091 nm, except near the laser threshold (dual-wavelength at 1054.5 nm and 1077 nm). The polarization directions of 1056 nm and 1077 nm were along the X-axis, and that of the 1091 nm emission was parallel to the Y-axis.
Using OCs of 10% and 15%, Yb:LMB laser radiation was X polarized with a single wavelength centered at 1054.1 and 1053.5 nm, respectively. No other wavelength was observed as the absorbed pump power increased. An efficient multiwavelength Yb:LMB laser was also achieved with an OC of T=25%. In the pump power range of Pabs<∼5.5 W, laser radiation was Y polarized, with the spectra comprising three emission peaks at approximately 1024.8, 1031.1, and 1033 nm. At high pump levels of Pabs>∼5.5 W, in addition to the three wavelengths of 1025.1, 1031.4, and 1033.2 nm, another emission at 1053.3 nm with polarization parallel to the X-axis survived in the gain competition process.
The laser oscillation wavelengths, including their polarization performance, were determined by the gain cross section of the Yb crystal for the free-running quasi-three-level laser. The polarized gain cross section [σg(λ)] of Yb:LMB was calculated using σg(λ)=βσem(λ)−(1−β)σabs(λ) (Fig. 5), where β denotes the fraction of Yb3+ ions excited to the upper manifold; σabs(λ) and σem(λ) are the absorption and emission cross sections at wavelength λ, respectively. Notably, the maximum gain occurred at approximately 1077 nm and 1091 nm for β=0.05, accounting for the case of T=1%. When β=0.07, the maximum points of the gain curve were found at approximately 1055, 1077, and 1091 nm, corresponding to the case of T=3%. When β increased to 0.09–0.15, the maximum gain reached approximately 1055 nm, which conformed to the spectra observed in the cases of T=10% and 15%. In addition, the values of σg at 1025, 1032, and 1055 nm were nearly equal for an excitation level of β=0.18, corresponding to the case of T=25%. Moreover, two intersections at 1052 nm and 1081 nm were found between the σg(λ) curves of E//X and E//Y polarizations. The Z-cut Yb:LMB laser will operate at linear polarization, and the polarization direction will be converted after the intersection point. For example, in accordance with the gain curve, the emission lines at approximately 1055 nm will operate along the E//X direction in which the wavelength gain of E//X is higher than that of E//Y. Meanwhile, the situation of 1032 nm or 1091 nm is the reverse (Fig. 5). The numerical results are in good agreement with the corresponding laser experiments.
4. Synchronous tri-wavelength mode-locking laser performance
By carefully aligning the folded cavity (Fig. 1), the laser can operate in a stable CW mode-locking (CWML) regime when the absorbed pump power exceeds 7.2 W. A maximum output power of 470 mW (for twin laser beam) was obtained with an OC of T=1% at an absorbed pump power of 8.2 W. Figure 6(a) shows the pulse trains at a pulse repetition rate of 42.8 MHz with the time span of 20 ns and 1 ms, demonstrating good amplitude stability. In addition, multipulse operation was not detected. In the radio frequency spectra, as shown in Fig. 6(b), (a) high extinction ratio of 60 dB was detected at the fundamental beat note, indicating the stable mode-locking operation of Yb:LMB laser. The intensity of higher order beat notes (>500 MHz) presented an obvious falling tendency, which was mainly attributed to the larger loss for higher frequencies in the employed RG-58 type coaxial cable for connecting the photodetector and RF analyzer.
Fig. 6. (a) The mode-locked pulse train recorded with time scale of 20 ns/div and 1 ms/div. (b) RF spectrum of the mode-locked laser [resolution bandwidth (RBW): 8 KHz]. Inset: 1 GHz wide-span spectrum (RBW:110 KHz).
The laser spectrum, shown in Fig. 7(a), comprises triple peaks centered at λ1=1057.4 nm (E//X), λ2=1079.2 nm (E//X), and λ3=1092.4 nm (E//Y) with an intensity ratio of 1:0.8:0.8. The full width at half maximum (FWHM) spectral bandwidths of the three bands were 3.8, 3.1, and 3 nm, respectively. The frequency differences among the three wavelengths were Δν12=5.731 THz (1057.4 nm–1079.2 nm), Δν23=3.359 THz (1079.2 nm–1092.4 nm), and Δν13=9.09 THz (1057.4 nm–1092.4 nm), and the corresponding time interval was Δt12=0.174 ps, Δt23=0.3 ps, and Δt13=0.11 ps. The autocorrelation (AC) trace of the tri-wavelength mode-locked laser was measured under the maximum output power, as shown in Fig. 7(b), from which a frequency beating was observed. The mode-locked pulses had a pulse duration of 568 fs with an assumed sech2 shape, and the beat pulse width was 77 fs. In addition, the AC trace primarily showed a beating period of 0.11 ps, except for the field of the pulse edge.
In the experiment, the CWML regime was only sustained within a narrow pumping range (Pabs=7.2–8.2 W). This finding was a result of the tough stabilization condition for a synchronous tri-wavelength mode-locking laser. The intensity ratio, pulse width ratio, and similarity degree among the oscillation wavelengths and the nonlinear properties of SESAM will exert an impact on the establishment time and shape of a laser pulse. However, although many factors can affect the operation of a synchronous tri-wavelength mode-locked laser, the present work demonstrated that a uniform wavelength interval is not a requirement. In the previously reported studies, all the synchronous tri-wavelength mode-locking operations were achieved at three wavelengths with equal spectral separation (Δλ). In a research conducted by Liu et al., tri-wavelengths with Δλ=1.9 nm were successfully synchronously locked in a Yb, Y:CaF2 laser, corresponding to the emission lines centered at 1045.7, 1047.6, and 1049.5 nm [18]. With a Nd:SYSO crystal, our group reported a synchronous mode-locked laser emitting triple wavelengths with a Δλ=1.3 nm [19]. In the current work, however, Δλ had different values of 21.8 nm and 13.2 nm, as shown in Fig. 7(a).
A numerical simulation was conducted to understand the AC trace of a synchronous tri-wavelength mode-locked laser. Assuming that the three wavelengths (λ1, λ2, λ3) have a Gaussian shape, I1, I2, and I3 denote the corresponding pulse intensity, and w1, w2, and w3 denote the pulse width of each pulses. The total intensity of the interference pattern is calculated using the following relation:
Fig. 8. The calculated autocorrelation traces with different ratios of w1: w2: w3 and a constant ratio of .I1:I2:I3=1:0.8:0.8.[Gray area: Δt (time interval of adjacent peaks)=Δt13=0.11 ps; Yellow area: Δt =Δt23=0.3 ps; Green area: Δt =Δt12=0.174 ps; Red cross-court zone: transition area].
Table 1 lists the state-of-the-art results from synchronous tri-wavelength mode-locked bulk lasers. This type of lasers have been previously reported only with Nd:SYSO and Yb,Y:CaF2 crystals. Among them, Yb:LMB laser provides the shortest pulse.
Table 1. Reported synchronous tri-wavelength mode-locked bulk lasers.
5. Conclusion
Multiwavelength laser performance in CW and mode-locking regimes with Yb:LMB crystal was reported in the current paper. By analyzing the experimental and numerical results, the following conclusions were drawn.
- (1) In the CW regime, Yb:LMB laser can naturally operate with multiwavelengths without additional optical elements and special coating design of the cavity mirror. By using output couplers with transmittances ranging from 1% to 25%, versatile wavelengths were generated efficiently and powerfully with a simple linear cavity. With an output coupler of T=1%, orthogonally polarized dual wavelengths were obtained at 1077 nm and 1091 nm with a maximum average output power of 2.67 W. Triple-wavelength oscillation at 1056, 1077, and 1091 nm was obtained with an output coupler of T=3%, producing a maximum average output power of 4.42 W. In the case of T=25%, four wavelengths within the range of 1020–1060 nm were simultaneously realized with a maximum average output power of 4.44 W. Nevertheless, under T=15% and 25% conditions, only a single wavelength at approximately 1054 nm was detected. The experimental results, including the laser polarization behavior, exhibited good agreement with the calculated gain cross section of the Yb:LMB crystal. Therefore, the output couplers of T=1%, 3%, and 25% should be selected preferentially to produce multiwavelengths for Z-cut Yb:LMB laser in a given application.
- (2) With a folding cavity, a synchronous tri-wavelength mode-locked bulk laser centered at 1057.4, 1079.2, and 1092.4 nm was achieved based on Yb:LMB crystal. The pulse duration was measured as 568 fs by assuming a sech2 pulse shape. Frequency beating was clearly observed from the AC trace, and the beat pulses presented an evident time interval of 0.11 ps and an FWHM width of 77 fs. The results indicated that equal spectral separation between each oscillation wavelengths was not a necessary condition for achieving a synchronous tri-wavelength mode-locked bulk laser. Furthermore, the calculation results showed that various time intervals of beat pulses can be obtained in the AC curves by assuming different ratios of w1:w2:w3. The three primary values of Δt12=0.174, Δt23=0.3, and Δt13=0.11 ps corresponded to the laser wavelength separation of 21.8, 13.2, and 35 nm, respectively. The dependency relationship between pulse widths of lasing wavelengths and time interval of beat pulses are summarized in Table 2.
Table 2. Relationship between pulse widths of the synchronous lasing wavelengths and time interval of beat pulses (I1:I2:I3=1:0.8:0.8).
In summary, the current study presented a promising route for implementing multiwavelength laser sources that require high power, high efficiency, a compact structure, and low cost. Through simple frequency doubling, this robust simultaneous multiwavelength Yb:LMB laser is an important and convenience source for the optical treatment of facial telangiectasia and nevus flammeus, among others.
Funding
National Natural Science Foundation of China (61705231); Science and Technology Project of Qingdao (19-6-2-5-cg); Science and Technology Project for the Universities of Shandong Province (J17KA180); Doctoral Foundation of QUST (210/010022849); Natural Science Foundation of Shandong Province (ZR2018BF028, ZR2019MF061).
Disclosures
The authors declare no conflicts of interest.
References
1. W. J. Bloss, T. J. Gravestock, D. E. Heard, T. Ingham, G. P. Johnson, and J. D. Lee, “Application of a compact all solid-state laser system to the in situ detection of atmospheric OH, HO2, NO and IO by laser-induced fluorescence,” J. Environ. Monit. 5(1), 21–28 (2003). [CrossRef]
2. T. Chuang, P. Burns, E. B. Walters, T. Wysocki, T. Deely, A. Losse, K. Le, B. Drumheller, T. Schum, M. Hart, K. Puffenburger, B. Ziegler, and F. Hovis, “Space-based, multi-wavelength solid-state lasers for NASA's Cloud Aerosol Transport System for International Space Station (CATS-ISS),” Proc. SPIE 8599, 85990N (2013). [CrossRef]
3. R. Guo, K. Akiyama, H. Minamide, and H. Ito, “All-solid-state, narrow linewidth, wavelength-agile terahertz-wave generator,” Appl. Phys. Lett. 88(9), 091120 (2006). [CrossRef]
4. P. H. Morse, “Laser treatment for retinal vascular diseases,” Ann Ophthalmol. 17(2), 156–162 (1985).
5. L. J. Chen, Z. P. Wang, H. H. Yu, S. D. Zhuang, S. Han, Y. G. Zhao, and X. G. Xu, “High-Power Single- and Dual-Wavelength Nd:GdVO4 Lasers with Potential Application for the Treatment of Telangiectasia,” Appl. Phys. Express 5(11), 112701 (2012). [CrossRef]
6. L. J. Chen, Z. P. Wang, S. D. Zhuang, H. H. Yu, Y. G. Zhao, L. Guo, and X. G. Xu, “Dual-wavelength Nd:YAG crystal laser at 1074 and 1112 nm,” Opt. Lett. 36(13), 2554–2556 (2011). [CrossRef]
7. X. Z. Wang, Z. F. Wang, Y. K. Bu, Z. Liu, L. J. Chen, G. X. Cai, and Z. P. Cai, “A 2014 nm, 1085 nm dual-wavelength Nd:YVO4 laser using Fabry–Perot filters as output couplers,” IEEE Photonics Technol. Lett. 26(19), 1983–1985 (2014). [CrossRef]
8. Y. S. Huang, H. B. Chen, S. J. Sun, F. F. Yuan, L. Z. Zhang, Z. B. Lin, G. Zhang, and G. F. Wang, “Growth, thermal, spectral and laser properties of Nd3+:LaMgB5O10 crystal-A new promising laser material,” J. Alloys Compd. 646, 1083–1088 (2015). [CrossRef]
9. Y. S. Huang, S. J. Sun, F. F. Yuan, L. Z. Zhang, and Z. B. Lin, “Spectroscopic properties and continuous-wave laser operation of Er3+:Yb3+:LaMgB5O10 crystal,” J. Alloys Compd. 695, 215–220 (2017). [CrossRef]
10. Y. S. Huang, W. W. Zhou, S. J. Sun, F. F. Yuan, L. Z. Zhang, W. Zhao, G. F. Wang, and Z. B. Lin, “Growth, structure, spectral and laser properties of Yb3+:LaMgB5O10-a new laser material,” CrystEngComm 17(38), 7392–7397 (2015). [CrossRef]
11. Y. S. Huang, F. Lou, S. J. Sun, F. F. Yuan, L. Z. Zhang, Z. B. Lin, and Z. Y. You, “Spectroscopy and laser performance of Yb3+:GdMgB5O10 crystal,” J. Lumin. 188, 7–11 (2017). [CrossRef]
12. Y. J. Chen, Q. Hou, Y. S. Huang, Y. F. Lin, J. H. Huang, X. H. Gong, Z. D. Luo, Z. B. Lin, and Y. D. Huang, “Efficient continuous-wave diode-pumped Er3+:Yb3+:LaMgB5O10 laser with sapphire cooling at 1.57 µm,” Opt. Express 25(16), 19320–19325 (2017). [CrossRef]
13. Y. J. Chen, Y. S. Huang, Z. B. Lin, and Y. D. Huang, “Passively Q-switched Er:Yb:GdMgB5O10 pulse laser at 1567 nm,” OSA Continuum 2(12), 3598–3603 (2019). [CrossRef]
14. H. B. Chen, Y. S. Huang, B. X. Li, W. B. Liao, G. Zhang, and Z. B. Lin, “Efficient orthogonally polarized dual-wavelength Nd:LaMgB5O10 laser,” Opt. Lett. 40(20), 4659–4662 (2015). [CrossRef]
15. Y. J. Chen, Y. S. Huang, Z. B. Lin, and Y. D. Huang, “Polarization switching realized in the continuous-wave and acousto-optic Q-switched pulse Er:Yb:LaMgB5O10 lasers at1556 and 1568 nm,” Opt. Express 26(15), 19037–19042 (2018). [CrossRef]
16. Z. Y. You, Z. J. Zhu, Y. J. Sun, Y. S. Huang, C. K. Lee, Y. Wang, J. F. Li, C. Y. Tu, and Z. B. Lin, “Simultaneous Q-switched orthogonally polarized dual-wavelength Yb3+:GdMgB5O10 laser,” Opt. Mater. Express 7(8), 2760–2766 (2017). [CrossRef]
17. F. Lou, B. T. Zhang, Y. S. Huang, B. Teng, J. Y. Jiang, G. H. Guo, S. Y. Zhang, S. J. Sun, X. Wang, J. L. He, and Z. B. Lin, “Exploring the mode-locking laser performance of Yb:LaMgB5O10 crystal,” Opt. Mater. Express 7(11), 4183–4191 (2017). [CrossRef]
18. C. Li, J. Liu, L. B. Su, D. P. Jiang, X. B. Qian, and J. Xu, “Diode-pumped tri-wavelength synchronously mode-locked Yb, Y:CaF2 laser,” Appl. Opt. 54(32), 9509–9512 (2015). [CrossRef]
19. J. Hou, L. H. Zheng, J. L. He, J. Xu, B. T. Zhang, Z. W. Wang, F. Lou, R. H. Wang, and X. M. Liu, “A tri-wavelength synchronous mode-locked Nd:SYSO laser with semiconductor saturable absorber mirror,” Laser Phys. Lett. 11(3), 035803 (2014). [CrossRef]
