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Dual-wavelength synchronous operation of a mode-locked 2-μm Tm:CaYAlO4 laser L. C. Kong,1 Z. P. Qin,1 G. Q. Xie,1,* X. D. Xu,2 J. Xu,2 P. Yuan,1 and L. J. Qian1 1
Key Laboratory for Laser Plasmas (Ministry of Education), IFSA Collaborative Innovation Center, Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
2
Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China *Corresponding author: [email protected] Received October 28, 2014; revised December 21, 2014; accepted December 22, 2014; posted December 23, 2014 (Doc. ID 225931); published January 26, 2015
We experimentally demonstrated dual-wavelength synchronous operation of a high-power passively mode-locked 2-μm Tm:CaYAlO4 (Tm:CYA) disordered crystal laser with semiconductor saturable absorber mirror (SESAM) as mode locker. The mode-locked laser emitted an average output power as high as 830 mW with pulse duration of 35.3 ps and repetition rate of 145.4 MHz. The mode-locking dual wavelengths were centered at 1958.9 nm and 1960.6 nm, respectively. Autocorrelation trace clearly shows beating pulses with pulse width of 3.5 ps and repetition rate of 0.13 THz. © 2015 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (140.3580) Lasers, solid-state; (140.7090) Ultrafast lasers. http://dx.doi.org/10.1364/OL.40.000356
Dual-wavelength synchronously mode-locked lasers are promising for generation of coherent terahertz radiation by difference frequency [1,2], and also have potential applications in optical communication, pump-probe experiments, optical frequency up-conversion, and so on. So far, such lasers have been investigated in Ti:sapphire lasers [3,4] at 0.8-μm wavelength and in Nd-doping or Ybdoping bulk lasers at 1-μm wavelength [5–12]. Compared to the above sources, dual-wavelength lasers at longer wavelengths have the advantage of much higher conversion efficiency in the generation of coherent terahertz radiation since their wavelengths are closer to terahertz frequency. Up to now, dual-wavelength continuous-wave (CW) and Q-switched lasers at 2 μm [13] and even longer wavelength [14] have been demonstrated. However, dualwavelength synchronously mode-locked lasers at such wavelengths will be more attractive due to their merits of much shorter pulses and much higher peak power when used as pump sources for generating terahertz radiation. Mode-locked Tm-doped bulk lasers offer a simple and low-cost method to generate ultrafast laser at 2 μm, which are expected to be applied in molecular spectroscopy, laser microsurgery, remote sensing, optical communications, and as pump sources of optical parametric oscillators to generate mid-infrared laser. So far, Tm-doped mode-locked lasers have been realized on diverse media by use of semiconductor saturable absorber mirrors (SESAMs), carbon nanotuber, and graphene, etc. [15–22]. Tm:CaYAlO4(Tm:CYA) belongs to the tetragonal ABCO4 compounds with Tm3 substituted for Y3 as doped ions [23,24]. Tm:CYA has attracted increasing interest as an excellent 2-μm laser crystal because it possesses large emission cross-section, considerable cross-relaxation, and broad gain linewidth due to structural disorder [25]. Also it has high mechanical strength, chemical stability, and high thermal conductivity [26]. The spectroscopic characteristics and laser performance of the Tm:CYA have been investigated. In 1997, 0146-9592/15/030356-03$15.00/0
Moncorgke et al. reported on a CW laser operation of Tm:CYA with 50-mW output power by pumping of Ti:sapphire laser [27]. Recently, a maximum output power of 4.3 W with a slope efficiency as high as 46.7% in diode-pumped Tm:CYA laser has been realized in our group, which indicates the potential of Tm:CYA crystal as high-power laser medium at 2-μm wavelength [28]. In this report, we demonstrated a dual-wavelength synchronously mode-locked Tm:CYA laser at 2-μm wavelength. By employing a semiconductor saturable absorber mirror (SESAM) as mode locker and pumping of laser diode, the Tm:CYA laser emitted stable dualcolor mode-locked pulses with pulse duration of 35.3 ps, repetition rate of 145.4 MHz, and the average output power of 830 mW. By optical beating of the dual-color pulses at 1958.9 and 1060.6 nm wavelength, a train of beat pulses was observed on the autocorrelation trace with 3.5-ps pulse duration and 0.13-THz pulse repetition rate. Figure 1 shows the schematic of mode-locked Tm:CYA laser. A typical five-mirror X-folded resonator was adopted with a SESAM as mode locker. The pump source was a single-emitter AlGaAs laser diode at around 790 nm. The pump light was collimated and focused into
Fig. 1. Schematic of the dual-wavelength synchronously mode-locked Tm:CYA laser. L1, L2: convex lens with the same focal length of 100 mm; M1, M2, and M3: plano–concave mirrors with radius of curvature (ROC) of −100 mm; OC: output coupler. © 2015 Optical Society of America
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the Tm:CYA crystal by two coupling convex lenses with the same focal length of 100 mm. The focused pump spot radii were about 25 μm × 86 μm in the crystal. The Tm: CYA crystal employed in the experiment was grown by Czochralski technique with Tm concentration of 6 at. %. The Tm:CYA sample had a size of 9 mm in length and 4 mm × 4 mm in cross-section. Both end faces of the Tm:CYA crystal were optically polished and placed with Brewster angle to compensate the astigmatism in the cavity and also minimize transmission losses for p-polarization light. To remove the generated heat, the crystal was wrapped with indium foil and tightly mounted in a water-cooled copper block, and the circulating water temperature was sustained at 24°C. According to the ABCD propagation matrix method, the laser mode size in the crystal was calculated to be about 45 μm × 72 μm in radius for the sagittal and tangential planes, respectively. The three plano-concave mirrors M1, M2, and M3 have the same radius of curvature of −100 mm, and were all highly reflectively coated for laser wavelength (reflectivity >99.7% from 1850 nm to 2100 nm). M1 and M2 were also anti-reflectively coated for pumping wavelength (transmission >95% at 790 nm). The plano–plano output coupler has a transmission of 8% from 1850 to 2100 nm. The commercial SESAM (BATOP, SAM-2000-2-1ps-25) was designed to operate from 1700 to 2150 nm with a modulation depth of 1.2% and a relaxation time of 1 ps. After careful alignment of the cavity, stable dualwavelength continuous wave (CW) mode locking was realized in the laser when the incident pump power was beyond 6.34 W. Figure 2 shows a typical modelocked pulse train measured with a high-speed detector (EOT, ET-5000) and a 500-MHz-bandwidth oscilloscope (Tektronix, DPO3054). The mode-locked pulse period was about 7 ns. And in a cavity cycle, only a mode-locked pulse was observed. In dual-wavelength CW mode locking operation, the maximum average output power reached 830 mW under an incident pump power of 6.97 W, corresponding to an optical-optical efficiency of 11.9%. At the maximum output power, the laser output beam had a round mode with a measured M 2 factor of ∼1.1. The radio-frequency (RF) spectrum of the mode locking was recorded by a RF spectrum analyzer with a bandwidth of 3 GHz and a resolution of 3 kHz (E4402B, Agilent). The RF spectrum obtained in Fig. 3 shows a clean peak at the fundamental repetition rate
of 145.4 MHz, corresponding to the laser cavity length of 1.03 m. The signal-to-noise ratio of the RF spectrum reached 67 dB, indicating a clear CW mode locking in the Tm:CYA laser. The optical spectrum of the mode-locked pulses was measured with a mid-infrared (mid-IR) optical spectrum analyzer (Ocean Optics, SIR5000) with a resolution of 0.22 nm, as shown in Fig. 4. The laser spectrum shows dual peaks with nearly equal intensity, and the wavelength peaks were at 1958.9 nm and 1960.6 nm, respectively. The dual-wavelength laser emission is a typical characteristic of disordered crystal, in which multiple emission centers exist, and they have different emission wavelengths [6]. With a commercial intensity autocorrelator (APE, PulseCheck50), we measured the autocorrelation trace of the dual wavelength mode-locked pulses, as shown in Fig. 5. Optical beating between two carrier-wave frequencies of the dual-wavelength mode-locked pulses was clearly observed in the autocorrelation trace, which suggests the dual-wavelength mode-locked pulses were temporally synchronous. The pulses synchronization is mainly attributed to saturable absorption of SESAM. For synchronous dual-color pulses, SESAM will see higher pulse energy fluence compared with that of two temporally separated pulses, thus resulting in lower saturable absorption loss. Consequently, the laser will operate in dual-wavelength synchronous-mode locking state. The beating pulses typically show a cosine pulse shape with pulse duration of 3.5 ps and beating period of 7.5 ps. For optical beating, it should meet the relation:
Fig. 2. Typical mode-locked pulse trains in nanosecond and microsecond time scales.
Fig. 4. Spectrum of the dual-wavelength mode-locked pulses.
Fig. 3.
Radio-frequency spectrum of the mode locked pulses.
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Fig. 5. Autocorrelation trace of the dual-wavelength synchronously mode-locked pulses.
Δν T 1, where Δν is the frequency difference of dual wavelengths, and T is beating period. In our case, the frequency difference of dual wavelengths at 1958.9 nm and 1960.6 nm is ∼0.13 THz, which is in good agreement with the above relation. From the autocorrelation trace, we can find that the dual-color pulse has an FWHM pulse duration of 35.3 ps, assuming a Gaussian pulse shape. What should be mentioned is that the measurement window of our autocorrelator is only 50 ps, which is not enough to display the total autocorrelation trace of the mode-locked pulses. In the experiment, we, respectively, measured three parts of the autocorrelation trace by adjusting the time delay screw of the autocorrelator, and then jointed to a whole autocorrelation trace. In conclusion, a high-power dual-wavelength synchronously mode-locked Tm:CYA laser was experimentally demonstrated for the first time. Benefitting from the dual-wavelength emission of Tm:CYA disordered crystal, the mode-locked laser generated temporally synchronous dual-color pulses with pulse duration of 35.3 ps and average output power as high as 830 mW at 2-μm wavelength. Optical beating pulses were clearly observed with 3.5-ps pulse width and 0.13-THz repetition rate. The dual-wavelength synchronously mode-locked laser at 2-μm wavelength will provide an excellent pump source for efficient generation of coherent teraherz radiation by optical difference frequency. The work is partially supported by the National Natural Science Foundation of China (Grant Nos. 61008018 and 11421064) and the National Basic Research Program of China (Grant No. 2013CBA01505). References 1. A. Majkic, M. Zgonik, A. Petelin, M. Jazbinsek, B. Ruiz, C. Medrano, and P. Gunter, Appl. Phys. Let. 105, 141115 (2014).
2. J. H. Chang, T. T. Wang, C. Zhang, Y. X. Ge, and Z. H. Tao, Chin. Phys. Lett. 30, 114206 (2013). 3. W. H. Knox and F. A. Beisser, Opt. Lett. 17, 1012 (1992). 4. Z. G. Zhang and T. Yag, Opt. Lett. 18, 2126 (1993). 5. G. Q. Xie, D. Y. Tang, H. Luo, H. J. Zhang, H. H. Yu, J. Y. Wang, X. T. Tao, M. H. Jiang, and L. J. Qian, Opt. Lett. 33, 1872 (2008). 6. G. Q. Xie, D. Y. Tang, W. D. Tan, H. Luo, S. Y. Guo, H. H. Yu, and H. J. Zhang, Appl. Phys. B 95, 691 (2009). 7. H. Yoshioka, S. Nakamura, T. Ogawa, and S. Wada, Opt. Express 18, 1479 (2010). 8. Z. H. Cong, D. Y. Tang, W. D. Tan, J. Zhang, C. W. Xu, D. W. Luo, X. D. Xu, D. Z. Li, J. Xu, X. Y. Zhang, and Q. P. Wang, Opt. Express 19, 3984 (2011). 9. J. L. Xu, S. Y. Guo, J. L. He, B. Y. Zhang, Y. Yang, H. Yang, and S. D. Liu, Appl. Phys. B 107, 53 (2012). 10. Y. Yang, Z. T. Jia, J. L. He, J. L. Xu, B. T. Zhang, R. H. Wang, X. M. Liu, J. Hou, F. Lou, Z. W. Wang, H. Yang, and X. T. Tao, Laser Phys. Lett. 10, 1612 (2013). 11. H. W. Chu, S. Z. Zhao, Y. F. Li, K. J. Yang, G. Q. Li, D. C. Li, J. Zhao, W. C. Qiao, T. Li, C. S. Feng, and H. J. Zhang, Opt. Commun. 315, 8 (2014). 12. Y. J. Huang, Y. S. Tzeng, and C. Y. Tang, Opt. Lett. 39, 1477 (2014). 13. W. J. Peng, F. P. Yan, Q. Li, S. Liu, T. Feng, S. Y. Tan, and S. C. Feng, Appl. Opt. 52, 4601 (2013). 14. J. F. Li, T. Hu, and S. D. Jackson, Opt. Lett. 37, 2208 (2012). 15. A. A. Lagatsky, F. Fusari, S. Calvez, J. A. Gupta, V. E. Kisel, N. V. Kuleshov, C. T. A. Brown, M. D. Dawson, and W. Sibbett, Opt. Lett. 34, 2587 (2009). 16. J. Ma, G. Q. Xie, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, J. Y. Wang, and D. Y. Tang, Opt. Lett. 37, 1376 (2012). 17. A. A. Lagatsky, P. Koopmann, P. Fuhrberg, G. Huber, C. T. A. Brown, and W. Sibbett, Opt. Lett. 37, 437 (2012). 18. A. A. Lagatsky, O. L. Antipov, and W. Sibbett, Opt. Express 20, 19349 (2012). 19. K. J. Yang, D. Heinecke, J. Paajaste, C. Kölbl, T. Dekorsy, S. Suomalainen, and M. Guina, Opt. Express 21, 4311 (2014). 20. A. Schmidt, S. Y. Choi, D. Yeom, F. Rotermund, X. Mateos, M. Segura, F. Diaz, V. Petrov, and U. Griebner, Appl. Phys. Express 5, 092704 (2012). 21. J. Liu, Y. G. Wang, Z. S. Qu, L. H. Zheng, L. B. Su, and J. Xu, Laser Phys. Lett. 9, 15 (2012). 22. J. Ma, G. Q. Xie, J. Zhang, P. Yuan, D. Y. Tang, and L. J. Qian, IEEE J. Sel. Top. Quantum Electron. 21, 1 (2015). 23. M. Yamaga, T. Yosida, Y. Naitoh, and N. Kodama, J. Phys.-Condes. Matter 6, 4381 (1994). 24. W. Y. Wang, X. L. Yan, X. Wu, Z. G. Zhang, B. Q. Hu, and J. F. Zhou, J. Cryst. Growth 219, 56 (2000). 25. J. A. Hutchinsona, H. R. Verdunb, B. H. T. Chaic, B. Zandid, and L. D. Merkled, Opt. Mater. 3, 287 (1994). 26. E. F. Kustov, V. P. Petrov, D. S. Petrova, and J. P. Udalov, Phys. Status Solidi. A 41, 379 (1977). 27. R. Moncorgk, N. Garnier, P. Kerbrat, C. Wyon, and C. Borel, Opt. Commun. 141, 29 (1997). 28. Z. P. Qin, J. G. Liu, G. Q. Xie, J. Ma, W. L. Gao, L. J. Qian, P. Yuan, X. D. Xu, J. Xu, and D. H. Zhou, Laser Phys. 23, 105806 (2013).
OPTICS LETTERS / Vol. 40, No. 3 / February 1, 2015
Dual-wavelength synchronous operation of a mode-locked 2-μm Tm:CaYAlO4 laser L. C. Kong,1 Z. P. Qin,1 G. Q. Xie,1,* X. D. Xu,2 J. Xu,2 P. Yuan,1 and L. J. Qian1 1
Key Laboratory for Laser Plasmas (Ministry of Education), IFSA Collaborative Innovation Center, Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
2
Key Laboratory of Transparent and Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China *Corresponding author: [email protected] Received October 28, 2014; revised December 21, 2014; accepted December 22, 2014; posted December 23, 2014 (Doc. ID 225931); published January 26, 2015
We experimentally demonstrated dual-wavelength synchronous operation of a high-power passively mode-locked 2-μm Tm:CaYAlO4 (Tm:CYA) disordered crystal laser with semiconductor saturable absorber mirror (SESAM) as mode locker. The mode-locked laser emitted an average output power as high as 830 mW with pulse duration of 35.3 ps and repetition rate of 145.4 MHz. The mode-locking dual wavelengths were centered at 1958.9 nm and 1960.6 nm, respectively. Autocorrelation trace clearly shows beating pulses with pulse width of 3.5 ps and repetition rate of 0.13 THz. © 2015 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (140.3580) Lasers, solid-state; (140.7090) Ultrafast lasers. http://dx.doi.org/10.1364/OL.40.000356
Dual-wavelength synchronously mode-locked lasers are promising for generation of coherent terahertz radiation by difference frequency [1,2], and also have potential applications in optical communication, pump-probe experiments, optical frequency up-conversion, and so on. So far, such lasers have been investigated in Ti:sapphire lasers [3,4] at 0.8-μm wavelength and in Nd-doping or Ybdoping bulk lasers at 1-μm wavelength [5–12]. Compared to the above sources, dual-wavelength lasers at longer wavelengths have the advantage of much higher conversion efficiency in the generation of coherent terahertz radiation since their wavelengths are closer to terahertz frequency. Up to now, dual-wavelength continuous-wave (CW) and Q-switched lasers at 2 μm [13] and even longer wavelength [14] have been demonstrated. However, dualwavelength synchronously mode-locked lasers at such wavelengths will be more attractive due to their merits of much shorter pulses and much higher peak power when used as pump sources for generating terahertz radiation. Mode-locked Tm-doped bulk lasers offer a simple and low-cost method to generate ultrafast laser at 2 μm, which are expected to be applied in molecular spectroscopy, laser microsurgery, remote sensing, optical communications, and as pump sources of optical parametric oscillators to generate mid-infrared laser. So far, Tm-doped mode-locked lasers have been realized on diverse media by use of semiconductor saturable absorber mirrors (SESAMs), carbon nanotuber, and graphene, etc. [15–22]. Tm:CaYAlO4(Tm:CYA) belongs to the tetragonal ABCO4 compounds with Tm3 substituted for Y3 as doped ions [23,24]. Tm:CYA has attracted increasing interest as an excellent 2-μm laser crystal because it possesses large emission cross-section, considerable cross-relaxation, and broad gain linewidth due to structural disorder [25]. Also it has high mechanical strength, chemical stability, and high thermal conductivity [26]. The spectroscopic characteristics and laser performance of the Tm:CYA have been investigated. In 1997, 0146-9592/15/030356-03$15.00/0
Moncorgke et al. reported on a CW laser operation of Tm:CYA with 50-mW output power by pumping of Ti:sapphire laser [27]. Recently, a maximum output power of 4.3 W with a slope efficiency as high as 46.7% in diode-pumped Tm:CYA laser has been realized in our group, which indicates the potential of Tm:CYA crystal as high-power laser medium at 2-μm wavelength [28]. In this report, we demonstrated a dual-wavelength synchronously mode-locked Tm:CYA laser at 2-μm wavelength. By employing a semiconductor saturable absorber mirror (SESAM) as mode locker and pumping of laser diode, the Tm:CYA laser emitted stable dualcolor mode-locked pulses with pulse duration of 35.3 ps, repetition rate of 145.4 MHz, and the average output power of 830 mW. By optical beating of the dual-color pulses at 1958.9 and 1060.6 nm wavelength, a train of beat pulses was observed on the autocorrelation trace with 3.5-ps pulse duration and 0.13-THz pulse repetition rate. Figure 1 shows the schematic of mode-locked Tm:CYA laser. A typical five-mirror X-folded resonator was adopted with a SESAM as mode locker. The pump source was a single-emitter AlGaAs laser diode at around 790 nm. The pump light was collimated and focused into
Fig. 1. Schematic of the dual-wavelength synchronously mode-locked Tm:CYA laser. L1, L2: convex lens with the same focal length of 100 mm; M1, M2, and M3: plano–concave mirrors with radius of curvature (ROC) of −100 mm; OC: output coupler. © 2015 Optical Society of America
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the Tm:CYA crystal by two coupling convex lenses with the same focal length of 100 mm. The focused pump spot radii were about 25 μm × 86 μm in the crystal. The Tm: CYA crystal employed in the experiment was grown by Czochralski technique with Tm concentration of 6 at. %. The Tm:CYA sample had a size of 9 mm in length and 4 mm × 4 mm in cross-section. Both end faces of the Tm:CYA crystal were optically polished and placed with Brewster angle to compensate the astigmatism in the cavity and also minimize transmission losses for p-polarization light. To remove the generated heat, the crystal was wrapped with indium foil and tightly mounted in a water-cooled copper block, and the circulating water temperature was sustained at 24°C. According to the ABCD propagation matrix method, the laser mode size in the crystal was calculated to be about 45 μm × 72 μm in radius for the sagittal and tangential planes, respectively. The three plano-concave mirrors M1, M2, and M3 have the same radius of curvature of −100 mm, and were all highly reflectively coated for laser wavelength (reflectivity >99.7% from 1850 nm to 2100 nm). M1 and M2 were also anti-reflectively coated for pumping wavelength (transmission >95% at 790 nm). The plano–plano output coupler has a transmission of 8% from 1850 to 2100 nm. The commercial SESAM (BATOP, SAM-2000-2-1ps-25) was designed to operate from 1700 to 2150 nm with a modulation depth of 1.2% and a relaxation time of 1 ps. After careful alignment of the cavity, stable dualwavelength continuous wave (CW) mode locking was realized in the laser when the incident pump power was beyond 6.34 W. Figure 2 shows a typical modelocked pulse train measured with a high-speed detector (EOT, ET-5000) and a 500-MHz-bandwidth oscilloscope (Tektronix, DPO3054). The mode-locked pulse period was about 7 ns. And in a cavity cycle, only a mode-locked pulse was observed. In dual-wavelength CW mode locking operation, the maximum average output power reached 830 mW under an incident pump power of 6.97 W, corresponding to an optical-optical efficiency of 11.9%. At the maximum output power, the laser output beam had a round mode with a measured M 2 factor of ∼1.1. The radio-frequency (RF) spectrum of the mode locking was recorded by a RF spectrum analyzer with a bandwidth of 3 GHz and a resolution of 3 kHz (E4402B, Agilent). The RF spectrum obtained in Fig. 3 shows a clean peak at the fundamental repetition rate
of 145.4 MHz, corresponding to the laser cavity length of 1.03 m. The signal-to-noise ratio of the RF spectrum reached 67 dB, indicating a clear CW mode locking in the Tm:CYA laser. The optical spectrum of the mode-locked pulses was measured with a mid-infrared (mid-IR) optical spectrum analyzer (Ocean Optics, SIR5000) with a resolution of 0.22 nm, as shown in Fig. 4. The laser spectrum shows dual peaks with nearly equal intensity, and the wavelength peaks were at 1958.9 nm and 1960.6 nm, respectively. The dual-wavelength laser emission is a typical characteristic of disordered crystal, in which multiple emission centers exist, and they have different emission wavelengths [6]. With a commercial intensity autocorrelator (APE, PulseCheck50), we measured the autocorrelation trace of the dual wavelength mode-locked pulses, as shown in Fig. 5. Optical beating between two carrier-wave frequencies of the dual-wavelength mode-locked pulses was clearly observed in the autocorrelation trace, which suggests the dual-wavelength mode-locked pulses were temporally synchronous. The pulses synchronization is mainly attributed to saturable absorption of SESAM. For synchronous dual-color pulses, SESAM will see higher pulse energy fluence compared with that of two temporally separated pulses, thus resulting in lower saturable absorption loss. Consequently, the laser will operate in dual-wavelength synchronous-mode locking state. The beating pulses typically show a cosine pulse shape with pulse duration of 3.5 ps and beating period of 7.5 ps. For optical beating, it should meet the relation:
Fig. 2. Typical mode-locked pulse trains in nanosecond and microsecond time scales.
Fig. 4. Spectrum of the dual-wavelength mode-locked pulses.
Fig. 3.
Radio-frequency spectrum of the mode locked pulses.
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Fig. 5. Autocorrelation trace of the dual-wavelength synchronously mode-locked pulses.
Δν T 1, where Δν is the frequency difference of dual wavelengths, and T is beating period. In our case, the frequency difference of dual wavelengths at 1958.9 nm and 1960.6 nm is ∼0.13 THz, which is in good agreement with the above relation. From the autocorrelation trace, we can find that the dual-color pulse has an FWHM pulse duration of 35.3 ps, assuming a Gaussian pulse shape. What should be mentioned is that the measurement window of our autocorrelator is only 50 ps, which is not enough to display the total autocorrelation trace of the mode-locked pulses. In the experiment, we, respectively, measured three parts of the autocorrelation trace by adjusting the time delay screw of the autocorrelator, and then jointed to a whole autocorrelation trace. In conclusion, a high-power dual-wavelength synchronously mode-locked Tm:CYA laser was experimentally demonstrated for the first time. Benefitting from the dual-wavelength emission of Tm:CYA disordered crystal, the mode-locked laser generated temporally synchronous dual-color pulses with pulse duration of 35.3 ps and average output power as high as 830 mW at 2-μm wavelength. Optical beating pulses were clearly observed with 3.5-ps pulse width and 0.13-THz repetition rate. The dual-wavelength synchronously mode-locked laser at 2-μm wavelength will provide an excellent pump source for efficient generation of coherent teraherz radiation by optical difference frequency. The work is partially supported by the National Natural Science Foundation of China (Grant Nos. 61008018 and 11421064) and the National Basic Research Program of China (Grant No. 2013CBA01505). References 1. A. Majkic, M. Zgonik, A. Petelin, M. Jazbinsek, B. Ruiz, C. Medrano, and P. Gunter, Appl. Phys. Let. 105, 141115 (2014).
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