We experimentally demonstrated a wavelength switchable passively mode-locked Yb:LuAG laser. The mode locked laser wavelength could be switched between 1031 nm and 1046 nm by a slit. With a coupler of 2% transmission, mode locked pulses with pulse duration of 1.15 ps and average output power of 1.3 W were generated at 1031 nm. By simply translating the slit position, 1046-nm mode locked pulses were generated with pulse duration of 252 fs and average output power of 676mW. With the output coupler of 8% transmission, higher average output power of 2.2 W was generated at 1031 nm with pulse duration of 1.8 ps, which is the highest average output power ever reported for Yb:LuAG mode-locked lasers.
© 2014 Optical Society of America
Ytterbium (Yb3+)-doped crystals have been proved to be excellent gain media for efficient ultrafast lasers. Yb3+-doped materials generally have low quantum defect, and they do not suffer from upconversion and excited-state absorption. Besides, the wide luminescence band of Yb3+ materials makes it possible for generation of ultrashort pulses. Up to now, passive mode locking of various Yb3+-doped gain media have been reported, such as Yb:KGW [1,2], Yb:YVO4 , Yb:CYB , Yb:YCOB , etc. Especially, Yb3+-doped garnets have attracted wide attention because of the good thermo-mechanical property of garnet host. Besides, the ability of high Yb3+ doping in garnets is a big advantage for high power lasers. The mode locking results of Yb:YAG lasers were frequently reported in the past years [6,7]. Yb:YAG has shown its superiority in high-average-power ultrafast laser. However, compared with other Yb-doped garnets, Yb:YAG has a relative low emission cross section (2 × 10−20 cm2).
Yb3+-doped Lu3Al5O12 (Yb:LuAG) is another garnet crystal. Compared to Yb:YAG crystal, Yb:LuAG crystal has a larger emission cross section of 3 × 10−20 cm2 . In addition, Yb:LuAG remains high thermal conductivity at high Yb3+-doping concentration. Because of the similar mass of Lu3+ (175 g/mol) and Yb3+ (173 g/mol), Lu3+ ion can be perfectly replaced by Yb3+ ion in LuAG lattices. Thus, the thermal conductivity of Yb:LuAG almost remains unchanged at high Yb3+-doping concentration. The thermal conductivity of Yb:LuAG crystal is ~15% higher than that of Yb:YAG for the same Yb3+ doping of 10%, and the difference is even larger at higher Yb3+-doping concentration . Therefore, it is expected that Yb:LuAG is a better gain medium over Yb:YAG for high-average-power operation.
Up to now, the growth, spectroscopic characteristics and continuous-wave (CW) laser performances of Yb:LuAG have been extensively investigated [11–13]. It is noteworthy that 5-kW CW output power from a Yb:LuAG thin disk laser has been achieved in 2010 . However, for mode locked operation, only single wavelength mode-locked Yb:LuAG lasers have been reported, which generated 7.63 ps pulses with 610 mW average output power  and 699 fs pulses with 200 mW average output power .
In this paper we report on a wavelength-switchable passively mode-locked Yb:LuAG laser. With an output coupler of 2% transmission, the mode locked laser wavelength could be switched between 1031 nm and 1046 nm by tuning the slit position in the cavity. 1.15-ps pulses at 1031 nm and 252-fs pulses at 1046 nm were obtained, and the corresponding average output powers were 1.3W and 676 mW, respectively. When the output coupler with 8% transmission was used, 1.8-ps pulses centered at 1031 nm were generated with the average output power as high as 2.2 W. Since the laser emits picosecond pulses at 1031 nm or femtosecond pulses at 1046 nm, the wavelength-switchable laser provides a dreamful picosecond and femtosecond source for pumping mid-infrared optical parameter oscillator (OPO).
The setup of the mode locked laser is shown in Fig. 1. An 8 W single-emitter laser diode at 940 nm was used as the pump source (nLight Laser). The pump beam was focused into the crystal with a waist spot size of 112 μm × 40 μm. A 10 at.% Yb-doped LuAG crystal with dimensions of 3 mm × 3 mm × 3 mm was employed as gain medium in the experiment. The Yb:LuAG sample was supplied by Shanghai Institute of Ceramics. The Yb:LuAG rod was wrapped with indium foil and mounted in a water-cooled copper block. Temperature of circulating water was maintained at 12°C. An X-folding cavity was adopted, with laser mode diameters of ~50 μm in the crystal and ~100 μm on the SESAM. The SESAM (BATOP GmbH) employed in the experiment has a saturation fluence of 70 μJ/cm2, modulation depth of 1%, and relaxation time of 10 ps. A pair of SF10 prisms with tip to tip distance of 460 mm was inserted in the cavity for dispersion compensation. The laser beam after the prism pair exhibited a spatial chirp, so a slit could be inserted to act for wavelength switching between 1031nm and 1046 nm. The whole cavity length was ~1.74 m.
3. Result and discussion
From the spectroscopic characteristics of Yb:LuAG crystal , it shows that Yb:LuAG has two emission peaks near 1031 nm and 1046 nm, respectively. The two emission lines share the same upper stark level of Yb3+, thus introduce the gain competition between 1031 nm and 1046 nm. Oscillation at 1031 nm or 1046 nm depends on whose net gain is dominant, and the net gain is related to the gain of laser medium, the loss introduced by slit, SESAM and output coupler. For Yb:LuAG, the stimulated emission cross section at 1031 nm is much larger that at 1046 nm. However, there exists a significant reabsorption loss at 1031 nm. The gain cross section spectra of Yb:LuAG can be expressed as σg(λ) = βσe(λ)-(1-β)σα(λ), Here σe denotes stimulated emission cross section and σα denotes absorption cross section. β = N2/N0, N2 is the inversion population and N0 is the total population of Yb3+-ions in Yb:LuAG crystal . Otherwise, since the slit was inserted into the beam with spatial chirp, the slit could be used to adjust the loss of a specific wavelength. Also, the SESAM would introduce a loss including the linear absorption and saturable absorption relating to intracavity pulse energy. The oscillating wavelength of the Yb:LuAG laser could be switched by controlling the relative loss of 1031 line and 1046 line. In our experiment we could switch the laser wavelength between 1031 nm and 1046 nm by simply adjusting the slit position in the dispersion arm.
Firstly, we investigated the laser output performance under an output coupler with 2% transmission. We first adjusted the cavity to get stable mode-locking at 1031 nm and suppress oscillation at 1046 nm by adjusting the slit position. The average output power versus absorbed pump power in mode-locking operation is shown in Fig. 2. For 1031 nm operation, the pump power threshold for CW mode-locking was 1.4 W. The maximum mode locking output power of 1.3 W was achieved under an absorbed pump power of 3.7 W, with a slope efficiency of 43%. Higher output power was limited by the available pump power. The CW mode locking was very stable and could be sustained for some hours. The CW mode locked pulse trains, captured by an oscilloscope (DPO 3054, Tektronix), are shown in Fig. 3. The mode locked pulses had a repetition rate of ~86 MHz, corresponding to the cavity length of 1.74 m.
The mode-locked pulse duration was measured by a commercial autocorrelator (APE, PluseCheck 50). The autocorrelation trace and spectrum (measured by USB 2000, Ocean Optics) for 1031 nm oscillation are shown in Fig. 4. Assuming a sech2 pulse shape, the mode locked pulse duration was 1.15 ps. The spectrum had a FWHM width of 1.1 nm, and the time-bandwidth product was calculated to be 0.357, which was close to the Fourier transform limit value of 0.315.
Mode-locking operation at 1046 nm could also be obtained by simply tuning the slit position. For single 1046 nm operation, pump power threshold for CW mode-locking was also 1.4 W, as shown in Fig. 2. The maximum mode locking output power was 676 mW under an absorbed pump power of 2.3 W, with a slope efficiency of 36.5%. Beyond 2.3 W of absorbed pump power, the oscillation at 1031 nm arose and disturbed the mode locking stability at 1046 nm. The mode locked laser generated femtosecond pulses at 1046 nm, as shown in Fig. 5. Assuming a sech2 pulse shape, the mode locked pulse duration was 252 fs. The spectrum had a FWHM width of 5.1 nm, and the time-bandwidth product was calculated to be 0.35.
To achieve higher mode locking output power, we also tried an output coupler with 8% transmission in the experiment. With the 8% output coupler, the laser could only oscillate at 1031 nm even in the highest available pump power. In stable CW mode locked operation, the maximum average output power was as high as 2.2 W under an absorbed pump power of 3.7 W (Fig. 2), and no multiple pulsing was observed. Higher mode locking output power was only limited by the available pump power. The autocorrelation and spectrum measurements show that the mode locked pulse had a pulse duration of 1.8 ps and spectrum width of 1.8 nm centered at 1031 nm. Up to now, 2.2 W is the highest average output power ever reported for mode-locked Yb:LuAG lasers, which will be an excellent pump source for synchronously pumped mid-infrared optical parametric oscillators.
It is noticed that there have been literatures reported on dual-wavelength mode locked lasers with Yb-doped gain media, including the independent dual-wavelength mode locking of Yb:YAG laser , synchronously dual-wavelength mode locking of Yb:LYSO  and Yb:YAG lasers . The laser performance comparison for Yb-based dual-wavelength mode locked lasers is shown in Table 1. We achieved much higher pulse peak power in this paper. Indeed, the synchronously dual-wavelength mode-locked lasers could be used to generate THz wave by frequency mixing. Our laser with high pulse peak power provides an excellent ultrashort laser source for pumping mid-infrared OPO, such as MgO:PPLN-based OPO. In the laser, picosecond pulse at 1031 nm or femtosecond pulses at 1046 nm could be generated by simply tuning the slit. Thus the wavelength-switchable mode locked laser provides a picosecond and femtosecond pump source for mid-infrared OPO. Accordingly, it is possible to generate mid-infrared picosecond or femtosecond pulses from the OPO by simply switching the pump laser wavelength.
In conclusion, we experimentally demonstrated a passively mode-locked Yb:LuAG laser with switchable wavelengths between 1031 nm and 1046 nm. When output coupler with 2% transmission was employed, the laser could emit mode-locked pulses at 1031 nm or 1046 nm by adjusting the slit position in the cavity. For 1031 nm operation, the maximum output power was 1.3 W, with pulse duration of 1.15 ps and repetition rate of ~86 MHz. And for 1046 nm operation, maximum average output power of 676 mW was obtained with pulse duration of 252 fs and repetition rate of ~86 MHz. In addition, we optimized the output coupler with a transmission of 8%, the laser emitted mode locked pulses at 1031 nm with pulse duration of 1.8 ps and average output power of 2.2 W, which is the highest mode-locked output power ever reported for Yb:LuAG lasers. The experimental results show that Yb:LuAG crystal is an excellent gain medium for high-average-power ultrashort pulses lasers.
The work is partially supported by the National Natural Science Foundation of China (No. 61008018 and 11121504) and the National Basic Research Program of China (Grant No. 2013CBA01505).
References and links
2. G. Paunescu, J. Hein, and R. Sauerbrey, “100-fs diode-pumped Yb:KGW mode-locked laser,” Appl. Phys. B 79(5), 555–558 (2004). [CrossRef]
3. A. A. Lagatsky, A. R. Sarmani, C. T. A. Brown, W. Sibbett, V. E. Kisel, A. G. Selivanov, I. A. Denisov, A. E. Troshin, K. V. Yumashev, N. V. Kuleshov, V. N. Matrosov, T. A. Matrosova, and M. I. Kupchenko, “Yb3+-doped YVO4 crystal for efficient Kerr-lens mode locking in solid-state lasers,” Opt. Lett. 30(23), 3234–3236 (2005). [CrossRef] [PubMed]
4. J. L. Xu, J. L. He, H. T. Huang, S. D. Liu, F. Q. Liu, J. F. Yang, B. T. Zhang, K. J. Yang, C. Y. Tu, Y. Wang, and F. G. Yang, “Generation of 244-fs pulse at 1044.7 nm by a diode-pumped mode-locked Yb:Y2Ca3(BO3)4 laser,” Laser Phys. Lett. 8(1), 24–27 (2011). [CrossRef]
5. A. Yoshida, A. Schmidt, V. Petrov, C. Fiebig, G. Erbert, J. H. Liu, H. J. Zhang, J. Y. Wang, and U. Griebner, “Diode-pumped mode-locked Yb:YCOB laser generating 35 fs pulses,” Opt. Lett. 36(22), 4425–4427 (2011). [CrossRef] [PubMed]
6. B. B. Zhou, Z. Y. Wei, Y. W. Zou, Y. D. Zhang, X. Zhong, G. L. Bourdet, and J. L. Wang, “High-efficiency diode-pumped femtosecond Yb:YAG ceramic laser,” Opt. Lett. 35(3), 288–290 (2010). [CrossRef] [PubMed]
7. S. Uemura and K. Torizuka, “Sub-40-fs Pulses from a Diode-Pumped Kerr-Lens Mode-Locked Yb-Doped Yttrium Aluminum Garnet Laser,” Jpn. J. Appl. Phys. 50, 010201 (2011). [CrossRef]
8. A. Brenier, Y. Guyot, H. Canibano, G. Boulon, A. Rodenas, D. Jaque, A. Eganyan, and A. G. Petrosyan, “Growth, spectroscopic, and laser properties of Yb3+-doped Lu3Al5O12 garnet crystal,” J. Opt. Soc. Am. B 23(4), 676–683 (2006). [CrossRef]
9. M. E. Wieser and T. B. Coplen, “Atomic weights of the elements 2009 (IUPAC Technical Report),” Pure Appl. Chem. 83(2), 359–396 (2011). [CrossRef]
10. K. Beil, S. T. Fredrich-Thornton, F. Tellkamp, R. Peters, C. Kränkel, K. Petermann, and G. Huber, “Thermal and laser properties of Yb:LuAG for kW thin disk lasers,” Opt. Express 18(20), 20712–20722 (2010). [CrossRef] [PubMed]
12. J. Dong, K. Ueda, and A. A. Kaminskii, “Laser-diode pumped efficient Yb:LuAG microchip lasers oscillating at 1030 and 1047 nm,” Laser Phys. Lett. 7(10), 726–733 (2010). [CrossRef]
13. C. W. Xu, D. W. Luo, J. Zhang, H. Yang, X. P. Qin, W. D. Tan, and D. Y. Tang, “Diode pumped highly efficient Yb:Lu3Al5O12 ceramic laser,” Laser Phys. Lett. 9(1), 30–34 (2012). [CrossRef]
14. J. P. He, X. Y. Liang, J. F. Li, H. B. Yu, X. D. Xu, Z. W. Zhao, J. Xu, and Z. Z. Xu, “LD pumped Yb:LuAG mode-locked laser with 7.63ps duration,” Opt. Express 17(14), 11537–11542 (2009). [CrossRef] [PubMed]
17. Q. Yang, Y. G. Wang, D. H. Liu, J. Liu, J. H. Zheng, L. B. Su, and J. Xu, “Dual-wavelength mode-locked Yb: LuYSiO5 laser with a double-walled carbon nanotube saturable absorber,” Laser Phys. Lett. 9(2), 135–140 (2012). [CrossRef]
18. W. Z. Zhuang, M. T. Chang, K. W. Su, K. F. Huang, and Y. F. Chen, “High-power terahertz optical pulse generation with a dual-wavelength harmonically mode-locked Yb: YAG laser,” Laser Phys. 23(7), 075803 (2013). [CrossRef]