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We have demonstrated the continuous wave and passively Q-switched Tm, Mg: LiTaO3 lasers for the first time. In continuous wave (CW) regime, a maximum CW output power of 1.03 W at 1952 nm was obtained, giving a slope efficiency of 9.5% and a beam quality M2 = 2.2. In passive Q-switching regime, a single walled carbon nanotube (SWCNT) was employed as saturable absorber (SA). The Tm,Mg:LiTaO3 laser has yielded a pulse of 560 ns under repetition rate of 34.2 kHz at 1926 nm, corresponding to a single pulse energy of 10.1 μJ. The results indicate a promising potential of nonlinear crystals in the applications for laser host materials.
© 2014 Optical Society of America
1. Introduction
The lithium tantalate crystal (LiTaO3) is a multifunctional material belonging to the group of ferroelectric crystals. It possesses good electro-optical parameters and nonlinear effect [1–5]. Compared with its isomorph lithium niobate (LiNbO3) crystal, LiTaO3 is considered to be more attractive due to its smaller birefringence and lower susceptibility to optical damage [6–8]. On the other hand, the rare earth (RE) doped ferroelectric crystal has attracted a lot because the laser oscillation combined with nonlinear and ferroelectric effect could be realized by using just one crystal. This advantage makes it play an important role in the field of integrated optoelectronics [9]. To date, the Nd3+ doped LiTaO3 laser has been widely investigated, and both CW and pulsed operations have been demonstrated in Ref [10]- [12]. Other than that, there was no report on the other RE-doped LiTaO3 lasers.
The solid-state laser emission at 2 μm eye-safe region relying on trivalent lanthanide ions Tm3+ have attracted much interest for their significant applications in civil, military and scientific research fields [13,14]. A long-term and on-going effort has been paid in this wavelength region is to explore novel host crystals for doping Tm3+ ions. With Tm3+ doped in LiTaO3, an absorption peak located at 790 nm with a FWHM of 5 nm is obtained [15], that greatly decreases the sensitivity of laser diode for pumping. However, the photorefractive effect in Tm3+:LiTaO3 crystal limits its high power laser operation. To solve this problem, magnesium oxide (MgO) is doped in Tm:LiTaO3 for enhancing the photorefractive damage threshold [16].
In this paper, we have demonstrated the CW and pulsed operation of Tm,Mg:LiTaO3 lasers for the first time. In CW regime, a maximum CW output power of 1.03 W at 1952 nm was obtained, introducing to a slope efficiency of 9.5% and a beam quality M2 = 2.2. By employing a SWCNT-SA, the Tm,Mg:LiTaO3 laser could easily run into Q-switching regime, and the shortest pulse duration of 560 ns at 1926 nm under repetition rate of 34.2 kHz was obtained with OC of T = 0.5%, leading to a single pulse energy of 10.1 μJ. A maximum single pulse energy of 20.4 μJ was obtained at the incident pump power of 13.78 W by using OC of T = 2%.
2. Experimental setup
The employed resonant cavity was formed by two mirrors with a total length of 2.5 cm. Its schematic diagram is shown in Fig. 1. The pump source was a fiber-coupled diode laser with a fiber core size of 400 μm in diameter and its emission wavelength was 790 nm at 25°C. The pump light was focused into the Tm,Mg:LiTaO3 crystal with a pump spot diameter of 400 μm through a 1:1 imaging module. The employed Tm,Mg:LiTaO3 crystal was grown by the Czochralski technique. The crystal was 3 × 3 × 10 mm3 in size and doped with 5 wt.% Tm3+. Both surfaces of the Tm,Mg:LiTaO3 crystal were antireflection coated from 750 to 850 nm (reflectivity < 2%) and 1910-2230 nm (reflectivity < 0.8%). To remove the generated heat while being pumped, the crystal was wrapped in indium foil and mounted in a copper block cooled by a water-cooler to 16°C. M1 was a concave mirror (R = 100 mm) working as input mirror with antireflection coated from 750 to 850 nm (reflectivity < 2%) and high reflectivity coated (reflectivity > 99.9%) from 1910 to 2100 nm. M2 was a flat mirror acting as output coupler (OC). A filter was put behind mirror M2 to remove the pump light. A laser power meter (MAX 500AD, Coherent, USA) was used to measure the average output power. The output spectrums were measured by employing a laser spectrometer which had a resolution bandwidth of 0.4 nm (APE WaveScan, APE Inc.).
Fig. 1 Schematic diagram of diode-pumped Tm,Mg:LiTaO3 laser. Upper: transmission rate of the SWCNT saturable absorber.
3. Experimental results and discussion
The CW operation of Tm,Mg:LiTaO3 laser was investigated first by using three output couplers (OCs) with different transmissions (from 1820 to 2150 nm) of 0.5%, 1%, 2%. The CW output power and spectra are shown in Fig. 2. A maximum CW output power of 1.03 W at 1952 nm was obtained at the incident pump power of 11.6 W and T = 1%, giving a slope efficiency of 9.5%. The maximum CW output powers of 0.91 W at 1953 nm and 0.83 W at 1948 nm were achieved for T = 0.5% and T = 2%, corresponding to slope efficiencies of 8.4% and 8.2%, respectively. We believe that this is the first demonstration of CW watt-level laser with Tm,Mg:LiTaO3 crystal.
Fig. 2 The CW output power with respect to the incident pump power from Tm,Mg:LiTaO3 lasers in CW regime. Inset: the output spectra from Tm,Mg:LiTaO3 lasers .
The SWCNT-SA employed in our experiment was fabricated by using a vertical evaporation technique without the polymer material [17]. Both sides were coated with 150 nm ZnO protection film by atomic layer deposition with a transmission rate of 85% at ~2 μm, as shown in Fig. 1 (upper). By aligning the cavity and optimizing the position of SWCNT-SA carefully, stable passive Q-switching operation was achieved as soon as the incident pump power exceeded the thresholds of 7.73 W, 7.84 W and 8.28 W for T = 0.5%, T = 1%, and T = 2%, respectively, which were higher than the thresholds of CW operation lasers, mainly induced by the additional insertion loss of SWCNT-SA. Figure 3 gives the output powers and spectra of passively Q-switched Tm,Mg:LiTaO3 lasers. A maximum average output power of 0.39 W at 1922 nm was achieved by using OC of T = 1%, giving a slope efficiency of 4.9%. Although the laser powers were far away from saturation, we didn’t further increase the incident pump power to prevent the SWCNT-SA from fragmentation induced by the leakage of incident pump power.
Fig. 3 The average output power with respect to the incident pump power from Q-switched Tm,Mg:LiTaO3 lasers. Inset: the output spectra from Q-switched Tm,Mg:LiTaO3 lasers.
The pulse trains were recorded by a digital oscilloscope (1 GHz bandwidth, Tektronix DPO 7102, USA) and a fast InGaAs photodetector with rise time of 35ps (EOT, ET-5000, USA). The variations of pulse repetition rate and pulse duration as well as single pulse energy with incident pump power are shown in Fig. 4. The pulse repetition rate increased with the augment of incident pump power, while the single pulse energy varied in a different way. It increased with the augment of incident pump power at the beginning, and then was insensitive to the incident pump power levels. We attribute this to the fast increase of pulse repetition rate, which almost grew linearly from threshold to the maximum incident pump power. Besides, Fig. 4 also indicates that higher repetition rate and shorter pulse duration could be obtained when lower transmissions of OCs were employed. However, the single pulse energies were increased with the increase of OC transmissions. At T = 0.5%, the laser generated 560 ns pulses under a repetition rate of 34.2 kHz, and the single pulse energy was correspondingly calculated to be 10.1 μJ. A maximum single pulse energy of 20.4 μJ was obtained at incident pump power of 13.78 W by using OC of T = 2%. The temporal oscilloscope traces shown in Fig. 5 indicates a stable Q-switching laser operation.
Fig. 4 The pulse repetition rate (upper), pulse duration (middle) and single pulse energy (below) versus the incident pump power.
The beam qualities for CW and Q-switched lasers were measured by using the 90.0/10.0 scanning-knife-edge method, and the measured data are shown in Fig. 6. The M2 factors of the laser beam at the highest output power were best-fitted to be 2.2 in CW laser and 2.4 in Q-switched laser. We attribute the terrible beam qualities to the bad quality of the Tm,Mg:LiTaO3 crystal. In our experiment, strain induced stripes could be observed inside the Tm,Mg:LiTaO3 crystal, since it is quite difficult to grow high quality RE-doped nonlinear crystals [2].
Fig. 6 Laser beam radius as a function of distance from the waist location (z = 0) for CW and Q-switched operations at the highest laser output.
4. Conclusion
In conclusion, we demonstrated the CW and Q-switched operations of Tm,Mg:LiTaO3 lasers. A maximum CW output power of 1.03 W at 1952 nm was obtained in CW regime, corresponding to a slope efficiency of 9.5% and a beam quality M2 = 2.2. By using a SWCNT as SA, the Tm,Mg:LiTaO3 laser could easily run into Q-switching regime. With OC of T = 0.5%, a minimum pulse of 560 ns under repetition rate of 34.2 kHz at 1926 nm was achieved, corresponding to single pulse energy of 10.1 μJ. By employing OC of T = 1%, a maximum average output power of 0.39 W was also obtained at 1922 nm, giving a slope efficiency of 4.9%. We believe this is the first report on CW and pulsed operations of Tm,Mg:LiTaO3 laser. On the other hand, the efforts on growing high quality RE-doped nonlinear crystal are still in progress and better experimental results would be expected.
Acknowledgments
The authors acknowledge the financial assistances provided by National Natural Science Foundation of China (61308020, 61008024), Independent Innovation Foundation of Shandong University, IIFSDU (2013HW013, and 2012JC025), Research Award Fund for Outstanding Middle-aged and Young Scientist of Shandong Province (BS2011DX022) and the State Key Program for Basic Research of China (No.2010CB630703).
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