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Abstract
We report a compact, simultaneous actively Q-switched orthogonally polarized dual-wavelength laser realized with a Yb3+:GdMgB5O10 (Yb:GMB) crystal for the first time. The two wavelengths of the laser were 1059.2 nm and 1060.8 nm. No additional optical elements were needed to balance the gain-to-loss of the dual-wavelength emissions. A maximum output power of 2.5 W was obtained at a repetition rate of 30 kHz, where the output powers of wavelengths at 1059.2 nm and 1060.8 nm were 1.23 W and 1.27 W, respectively. For a repetition rate of 10 kHz, the narrowest pulse width of 36 ns was achieved, corresponding to the highest peak power of 4.59 kW. The advantages of the good absorption efficiency, the natural balanced output powers of two wavelengths and the ideal average output power show that the Yb:GMB is an efficient and promising gain medium for generating the orthogonally polarized dual-wavelength laser.
© 2017 Optical Society of America
Corrections
15 August 2017: A typographical correction was made to Ref. 12.
1. Introduction
Simultaneous orthogonally polarized dual-wavelength lasers exhibit versatile applications, such as precision laser spectroscopy, precision metrology, terahertz (THz) radiation generation, differential absorption lidar (DIAL) and so on [1–5]. THz radiation generated by utilizing nonlinear difference frequency mixing or a photoconductive switching should employ two lasers with different but close wavelengths [6]. Moreover, adopting a dual-wavelength laser with a small wavelength separation is conducive to improve the accuracy of the DIAL [7]. For these reasons, the dual-wavelength lasers generated from the same energy transition with a small separation of wavelength have attracted great interests. Since this type of dual-wavelength lasers is the result of the splitting of stark levels of the same transition line, the interval of two laser wavelengths is very small. Thus, it is hard to design the cavity coating to balance the gain and loss of dual wavelengths. Currently, there are two major approaches to realize the dual-wavelength laser with a small wavelength interval. The first approach is to insert a birefringent element inside the laser cavity to balance the gain-to-loss of dual wavelengths. With this method, dual-wavelength lasers have been achieved in Yb:KGW, Nd:YAG, Nd:YVO4, Nd:LuVO4 and so on [8–13]. However, it also brings the laser cavity much round-trip loss and makes the structure complicated, resulting in a decrease of the efficiency and instability of the laser system. The second approach is to employ some laser crystals, whose emission cross-sections of the dual wavelengths are quite close. As a result, the emissions of such two wavelengths have equal laser thresholds and similar slope efficiencies. Without employing any additional optical elements, the ratios of output powers at two individual wavelengths can approach 1. To some extent, the second approach with the merits of compact structure and low loss in the laser cavity is more desirable and convenient than the former. Nowadays, some Nd3+ and Yb3+-doped anisotropic crystals have been evaluated for generating stable dual-wavelength lasers at about 1.0 μm [14–16]. Compared to the Nd3+ lasers, Yb3+ lasers possess lower thermal loading due to lower quantum defect during the laser operation. Besides, the Yb3+ lasers features excellent wavelength tunability, which is capable of realizing a tunable THz radiation [17]. In spite of so many advantages in the Yb3+-doped anisotropic crystal laser, few Yb3+-doped laser crystal emitting a simultaneous orthogonally polarized dual-wavelength Q-switched laser without additional optical elements have been reported [16,18,19]. Therefore, it is essential to seek some novel laser gain media owning such features.
Yb:GMB crystal is birefringent and belongs to the monoclinic system with the space group P21/c. Owing to the small difference of ionic radius between Yb3+ ion and Gd3+ ion, a high doping concentration of Yb3+ ions is permitted in the crystal without bringing a few defects. At the same time, the high Yb3+ ions doped crystal can retain a good thermal conductivity close to the un-doped crystal. Therefore, it is capable of enduring a high incident pump power without cracking. In a recent work, we have studied its main spectroscopic properties, and a laser with the output power of 5.35 W in continuous-wave (CW) mode was realized, which testifies that the Yb:GMB is a promising material for solid state lasers [20].
In this work, a diode-end-pumped actively Q-switched dual-wavelength Yb:GMB laser was demonstrated. A pulsed laser with simultaneous orthogonally polarized dual-wavelength at 1059.2 nm and 1060.8 nm was achieved. The minimum pulse width of 36 ns and the maximum output power of 2.5 W were also demonstrated.
2. Experimental details
A schematic of the experimental setup for the actively simultaneous dual-wavelength generation with orthogonal polarizations is presented in Fig. 1. A compact concave-plano cavity was employed for the laser operation. The pump source was a fiber-coupled laser diode emitting at 976 nm with a radius of 100 μm and a numerical aperture of 0.22. By using a 1:1 focusing system, the pump beam was focused into the Yb:GMB crystal with a spot size of about 200 μm in diameter. The cavity length was about 45 mm. The input mirror (IM) was a flat mirror coated with highly reflection (> 99.9%) at 1030-1200 nm and high transmission (> 95%) at 940–990 nm. The output coupler (OC) was a concave mirror having a radius of 50 mm and coated with the transmittance of 5% at 1020-1080 nm.
Fig. 1 Schematic of the experimental setup for the orthogonally polarized dual-wavelength Q-switched Yb:GMB laser.
The Yb:GMB crystal was grown by using the top seeded solution growth (TSSG) method from a flux of K2Mo3O10. A X-cut Yb:GMB crystal with the dimensions of 3 × 3 × 2 mm3 was employed with both sides polished and no anti-reflection coated. The concentration of Yb3+ ions in the crystal was about 13.05 at.%. In order to reduce the influence of thermal lens effects, the laser crystal was wrapped with indium foil and mounted in a copper block cooled by water at 18 °C. A 35-mm-long acousto-optic (AO) Q-switcher (Gooch & Housego Company) was placed between the Yb:GMB crystal and the OC. The laser pulse signal was recorded by an Agilent digital oscilloscope. Meanwhile, the laser spectra were recorded with a Laser Spectrometer (WaveScan, APE).
3. Results and discussions
To begin with, the absorption behavior of the Yb:GMB crystal was studied under lasing conduction at different incident pump powers. Figure 2 shows the dependence of the absorbed pump power and absorption efficiency on the incident pump power. It can be seen that, benefits from the high concentration of Yb3+ ions in the crystal, the entire absorption efficiency is more than 87%. When the incident pump power is below 6.0 W, the efficiency is about 96%. However, the absorption efficiency decreased dramatically from 96% to 87% when the incident pump power was increased from 6.0 to 11.7 W, corresponding to an increase in absorbed pump power from 5.8 to 10.2 W. Considering the ground state population can be continuously replenished by the stimulated emission of up-state populations when lasing generated, it appears a non-depletion characteristic [21]. The impact of the depletion of the ground state on the absorption behavior can be neglected. Thus, we think the decrease of pump absorption is mainly attributed to the thermal effect in the crystal.
Fig. 2 Absorbed pump power and absorption efficiency as a function of the incident pump power for Yb:GMB crystal.
As shown in Fig. 3(a), the absorption of 976 nm corresponds to the energy transition from the lowest sublevel of ground state 2F7/2 (G1) to the lowest sublevel of excited state 2F5/2 (E1). A large amount of heat is generated by non-radiative transfer under a high incident pump power. With a rise in the temperature of the crystal, the population of the ground state sublevels will be redistributed according to the Boltzmann distribution.
where N and Ni are a total number of particles and the number of particles on ith sublevel, respectively. Ei is the values of the ith sublevel energy [20], k is the Boltzmann's constant and T is the temperature. From Fig. 3(b), we can see that the particle population on the sublevel G1 decreases with increasing temperature. Therefore, the absorption efficiency of pump power is also reduced. We believe that a better thermal management in the laser crystal will alleviate this problem.Fig. 3 (a) Stark splitting of the 2F5/2 and 2I7/2 levels of Yb3+ in GMB crystal. (b) The particle populations of ground-state sublevels of the Yb3+ ion versus different temperatures.
In addition, actively Q-switched laser properties were also investigated. For the sake of contrast, the pulse repetition rates were setted to be 10, 20 and 30 kHz, respectively. When the absorbed pump power reached to 6.67 W, a maximum average output power of 2.5 W and a pulse energy of 0.226 mJ were achieved at the repetition rates of 30 and 10 kHz, respectively, as shown in Fig. 4. The slope efficiencies were about 42.78%, 45.81% and 47.45% for the repetition rates of 10, 20 and 30 kHz, respectively. Figure 5 displays the pulse width as a function of the absorbed pump power at different repetition rates. The narrowest pulse width of 36 ns was obtained at the repetition rate of 10 kHz under the absorbed pump power of 5.34 W, corresponding to the maximum pulse peak power of 4.59 kW.
Fig. 4 (a) Average output power and (b) Pulse energy of laser versus absorbed pump power for different pulse repetition.
Fig. 5 Dependence of the pulse width on the absorbed pump power. The inset shows the temporal pulse profile of the 36 ns pulse.
The laser spectra were recorded with a spectrum analyzer at the absorbed pump power of 6.67 W. As shown in Fig. 6(a), there are dual wavelengths centered at 1059.2 nm and 1060.8 nm. By using a Glan–Taylor polarizer, we found that the polarization directions of the 1059.2 nm and 1060.8 nm were along the Y- and Z- axes, respectively. The realization of orthogonally polarized dual-wavelength emission should due to the close peak emission cross-sections for E//Y (2.1 × 10−20 cm2 at 1059 nm) and E//Z (1.9 × 10−20 cm2 at 1060 nm). In Q-switching operation, the gain in the crystal is much higher than in CW operation. The high population inversion rate makes the slightly weaker emission line not be depressed by gain competition, thus two polarizations could both reach the laser thresholds. As far as we know, such a wavelength interval of 1.6 nm is roughly equivalent to a frequency difference of 0.43 THz, which is the smallest frequency provided by the orthogonally polarized dual-wavelength laser without additional optical elements. Meanwhile, the average output powers for individual wavelengths at a repetition rate of 30 kHz were measured, as shown in Fig. 6(b). At first, the output power of the 1060.8-nm line is larger than the 1059.2-nm line at the same absorbed pump power. When the pump power was increased, the output power of the 1059.2-nm line gets close to the 1060.8-nm line. At the absorbed pump power of 6.67 W, the output powers of 1059.2 nm and 1060.8 nm line are 1.23 W and 1.27 W, respectively. There is no need of any additional optical elements inside the laser cavity, the ratio of output powers of the two wavelengths approaches 1 at a total output power of 2.5 W. Comparing with the ratio of output powers of dual wavelengths in the report [16], which is difficult to approach 1, Yb:GMB have the advantage that it is more easier to achieve balanced output powers of two wavelengths. Meanwhile, because the realization of a balanced power of two wavelengths is of crucial importance in producing THz waves by utilizing nonlinear difference frequency mixing or a photoconductive switching, Yb:GMB crystal shows a great naturally advantage in achieving a compact, high power and balanced orthogonally polarized dual-wavelength Q-switched laser, which is desirable for the development of terahertz sources with the frequency less than 0.5 THz.
Fig. 6 (a) Optical spectrum of orthogonally polarized dual-wavelength laser at the absorbed pump power of 6.67 W. (b) The output powers versus the incident pump power for the dual-wavelengths at 1059.2 and 1060.8 nm.
Furthermore, combining with the stimulated Raman scattering technique, this dual-wavelength laser also can be adopted to realize simultaneous orthogonally polarized multi-wavelength laser. By utilizing a KGW crystal located inside the laser cavity, having two strong orthogonally polarized Raman lines at 768 and 901 cm−1, a simultaneous orthogonally polarized dual-wavelength pulse laser at 1157.2 nm and 1173.0 nm were achieved. The spectra of multi-wavelength lasers were shown in Fig. 7. The total output powers of the Raman lines were of about 200 mW using a non-optimized output coupler.
4. Conclusion
In summary, an orthogonally polarized dual-wavelength Q-switched laser at 1059.2 nm and 1060.8 nm in Yb:GMB crystal was demonstrated. At the absorbed pump power of 6.67 W, the maximum output pulse energy of 0.226 mJ and power of 2.5 W (1.23 W at 1059.2 nm and 1.27 W at 1060.8 nm) were obtained at the repetition rate of 10 kHz and 30 kHz, respectively. The narrowest pulse width of 36 ns was obtained at the repetition rate of 10 Hz with the maximum peak power of 4.59 kW. To our knowledge, the wavelength separation of 1.6 nm, corresponding to the frequency difference of 0.43 THz, is the smallest in the orthogonally polarized dual-wavelength laser which needs no additional optical elements. Furthermore, an orthogonally polarized multi-wavelength laser was also achieved by adopting the SRS technique. Considering the advantages of the good absorption efficiency, the natural balanced output powers of dual wavelengths and the ideal average output power, the Yb:GMB is a promising gain medium for diode-pumped dual-wavelength lasers.
Funding
National Natural Science Foundation of China (51472240, 61078076, and 11304313); the Strategic Priority Research Program of the Chinese Academy of Science (Grant No. XDB20010200); Science and Technology Plan Cooperation Project of Fujian Province (2015I0007); Natural Science Foundation of Fujian Province (2015J05134, and 2016J01274).
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