We report on the results from a comparative study on the continuous-wave laser performance of Yb:YGG, Yb:LuGG, and Yb:GGG gallium garnets grown by the optical floating zone technique, conducted under identical experimental conditions. 5−7 W of output powers were generated with the three garnet crystals in a compact linear resonator longitudinally pumped by a diode laser emitting at 970−974 nm. The optical-to-optical and slope efficiencies measured for Yb:YGG amounted to 68% and 80%, respectively, which proved to be much higher than the corresponding efficiencies for Yb:LuGG (49% and 60%) or Yb:GGG (52% and 64%).
©2013 Optical Society of America
Y3Al5O12 (YAG), the yttrium aluminum garnet, is a well-known crystal possessing excellent chemical, mechanical, and thermal properties. Since the 1960s it has been widely utilized as a host medium for trivalent lanthanide rare-earth active ions, in particular for the Nd ion. Nowadays, Nd:YAG has become the most popular laser crystal for high-power solid-state laser systems.
Compared to traditional Nd ion lasers, ytterbium (Yb) ion lasers are advantageous in some aspects such as low quantum defect leading to a reduction in thermal load; the absence of both excited-state absorption and concentration quenching; and wide absorption and emission bands that not only facilitate diode pumping but also enable ultrashort laser-pulse generation through a mode-locking technique. During the past two decades, great attention has been paid to the exploration and development of Yb laser crystals.
When utilized as a host crystal for the Yb ion, YAG was found to exhibit a serious drawback––with increasing Yb doping level, the thermal conductivity of Yb:YAG is likely to decrease significantly , a detrimental feature especially for applications in microchip or thin-disk lasers. As a consequence, much effort has been made to search for other promising Yb crystals in the garnet family. Yb:Lu3Al5O12 (Yb:LuAG) is the one into which extensive investigations have been carried out in recent years [2–5]. Apart from the aluminum garnets, crystals in the class of gallium garnets are also expected to be promising candidates for Yb ion hosts due to their identical crystal structure and similar general properties common to all garnets. As an early instance, Yb:Gd3Ga5O12 (Yb:GGG) has been developed, demonstrating continuous-wave (cw) laser performance comparable to Yb:YAG  and also the potential capability of generating pulsed-laser radiation . Recently, Yb:Y3Ga5O12 (Yb:YGG) and Yb:Lu3Ga5O12 (Yb:LuGG), another two crystals belonging to the gallium garnets, have been grown by employing the optical floating zone technique [7,8], and laser action has been demonstrated in different operational modes [9–11].
Given the similar physical and spectroscopic properties of Yb:YGG, Yb:LuGG, and Yb:GGG , it is instructive to compare, under identical experimental conditions, the basic laser performance of the three Yb-doped gallium garnets and to give an evaluation of their laser capacity, which is of importance for developing practical laser devices based on these Yb garnet crystals. This paper is an experimental study of such a direct comparison of cw laser performance conducted with a simple linear resonator that was longitudinally pumped by a high-power diode laser.
2. Description of experiment
All three of the gallium garnets, Yb:YGG, Yb:LuGG, and Yb:GGG, studied in the current experiment were grown in an oxygen atmosphere by the optical floating zone method, and the detailed growing procedure was the same as reported previously [7,8]. The nominal Yb concentration in melt was 7.5 at. % for the three crystals. Due to the distinction in segregation coefficients, the actual Yb concentrations were different, which were determined to be 7.35 at. % (Yb:YGG); 7.13 at. % (Yb:LuGG); and 8.55 at. % (Yb:GGG). For each garnet a crystal sample with a square aperture of 3 mm × 3 mm and a thickness of 4 mm was cut along the  crystallographic direction from the annealed crystal boule.
To study the cw laser performance of the three Yb-doped garnets, we employed a simple compact linear resonator formed with a plane reflector and a concave mirror of radius-of-curvature of 25 mm. The plane reflector was coated for high reflectance at 1030−1200 nm and high transmittance at 820−990 nm. As the output coupler, a group of concave mirrors was used with transmissions (output couplings) in a range of T = 0.5−30%. To generate laser oscillation, the uncoated crystal sample was held in a water-cooled copper block and was placed close to the plane reflector inside the resonator. The pump source used was a high-power fiber-coupled diode laser (fiber core diameter of 200 μm and NA of 0.22) producing unpolarized radiation at 970−974 nm, depending on the output power level. The pump radiation was focused first by a focusing optics and then delivered through the plane reflector onto the laser crystal with a beam spot radius of ~100 μm.
3. Results and discussion
The spectroscopic properties of the Yb ion in the three gallium garnets, including absorption and emission spectra and fluorescence lifetime, have been studied in previous work [1,7,8]. Table 1 summarizes the relevant parameters that are helpful for understanding the laser performance demonstrated in the current experiment.
For the three Yb-doped gallium garnets, there is an absorption peak around 970 nm in their absorption spectra, which corresponds to the zero-phonon transition . In our experiment, the crystals were pumped with the pumping wavelengths coinciding with this narrow absorption band. Due to the output-level dependence of the pumping wavelength, which was varying from 970 to 974 nm, the absorption of pump power for each garnet crystal was likely to be variable. Figure 1 shows the absorption, i.e., the fraction of pump power absorbed in the crystal (ηa), as a function of the incident pump power, determined by measuring the residual unabsorbed pump power with the output coupler removed from the cavity, and taking into account the Fresnel reflection on the crystal surface. Due to the strong divergence of the pump beam, it was not feasible to measure the absorption of the laser crystal under lasing conditions. Indeed, the absorption measured in this way would differ to some extent from the real absorption when the laser was operating. Nevertheless, it could serve as an approximation for ηa, in order to evaluate the laser performance, especially for comparing different crystals. From Fig. 1 one sees a similar variation behavior of ηa with pump power for the three garnet crystals, a result arising from the similarity in the absorption band around 970 nm for these crystals . With the incident pump power increasing from ~0.3 to ~25 W, ηa was measured to be varying from 0.82−0.38 (Yb:YGG); 0.91−0.44 (Yb:LuGG); and 0.92−0.52 (Yb:GGG). The absorption dropping with increasing pump power was related to two factors: the deviation of pumping wavelength from the absorption peak and the saturation of absorption. One can also note the stronger absorption of Yb:GGG than the other two garnets; this resulted mainly from the higher Yb concentration in the Yb:GGG crystal.
Efficient cw laser operation was achieved with all three garnets at room temperature, with the output coupling ranging from 0.5% to 30%. Figure 2 shows the output power versus the absorbed pump power (Pabs) measured with different output couplers for (a) Yb:YGG, (b) Yb:LuGG, and (c) Yb:GGG. For the Yb:YGG crystal, the lasing threshold was reached at Pabs = 0.32 W in the case of T = 0.5%, but the operation above threshold proved to be less efficient. Nevertheless, a maximum output power of 4.67 W could be produced in this case. By a comparison of the results obtained with different output couplings, one can see that the most efficient operation was achieved with a coupler of T = 1%, generating a maximum output power of 6.75 W at Pabs = 9.93 W, which was the highest available absorbed pump power; the corresponding optical-to-optical efficiency was 68%, whereas the slope efficiency was as high as 80%, as determined for Pabs > ~4 W. Compared to the previously reported cw laser results of this crystal , both the output power and the optical-to-optical efficiency obtained in the current experiment were much higher, while the slope efficiency was comparable.
For the Yb:YGG crystal, the laser performance, demonstrated with different output couplers of T = 1%, 3%, and 10%, proved to be considerably different. By contrast, however, the output characteristics of either Yb:LuGG or Yb:GGG laser obtained with an output coupler of T = 1%, 3%, 5%, or 10% were found to be very close over an operational range from Pabs ≈2 W to Pabs ≈9 W. This point can be clearly seen from Fig. 2, in which, for clarity, the results corresponding to T = 3% and T = 5% are not presented for Yb:LuGG and Yb:GGG crystals. The maximum output power produced with the Yb:LuGG crystal was 5.67 W achieved with the coupler of T = 10%, which was the optimum output coupling, and the corresponding optical-to-optical and slope efficiencies were 49% and 60%, respectively. These results were very close to those previously reported . Like the case of Yb:LuGG, the optimum output coupling for the Yb:GGG laser was also T = 10%. The highest output power, 7.1 W, was generated at Pabs = 13.6 W, leading to an optical-to-optical efficiency of 52%, whereas the slope efficiency was determined to be 64%. In comparison with a previously reported Yb:GGG laser pumped with a similar diode, the efficiencies were identical, while the output power generated in the current experiment was much higher .
As can be seen from Fig. 2, the laser performance corresponding to the lowest output coupling, T = 0.5%, turns out to be quite different for Yb:YGG in comparison with that for Yb:LuGG or Yb:GGG. With either Yb:LuGG or Yb:GGG, the output power would eventually become saturated with increasing pump power, with the output power limited to less than 2.6 W (Yb:LuGG) and 3.6 W (Yb:GGG). By contrast, the output power produced with the Yb:YGG crystal could continue to increase, reaching 4.67 W at the highest available Pabs, with a slope efficiency of 55% that was only slightly reduced at the highest pump power.
It is also worth noting from Fig. 2 that for the three gallium garnets, even with an output coupling as high as T = 30%, the output power could reach about 4−6 W with slope efficiencies in excess of 50%. This is significantly different from the laser performance of Yb doped vanadates, for which the output coupling is usually not higher than 5% . This feature for the gallium garnets seems to be advantageous for generating Q-switched laser operation.
It is known that internal (or intracavity) losses also have significant influence on the performance of a laser. For the garnet lasers studied in our experiment, the internal losses originated mainly from scattering, absorption, and diffraction of the laser medium due to the inhomogeneity, impurity, and defects existing in the crystal. Such passive losses are usually represented by introducing a loss coefficient, δ, for the laser crystal. This loss coefficient can be determined, on the basis of measured slope efficiencies for different output couplings, by a method developed by Caird et al. . According to this method, the inverse slope efficiency is related linearly to the inverse output coupling by ηs−1 = η0−1(1 + LT−1), where L denotes the round-trip internal losses, ηs is the slope efficiency measured under conditions of output coupling T, while η0 represents the intrinsic or limiting slope efficiency that is achievable in the ideal case of L = 0.
To estimate the amounts of the loss coefficients for Yb:YGG, Yb:LuGG, and Yb:GGG crystals involved in the comparative study, we performed a Caird analysis. As can be noted in Fig. 2, the relationships between the output power and the absorbed pump power exhibit more or less some nonlinearity, which is true even for the low-power region where thermal effects are negligible. In fact, such nonlinearities turn out to be an inherent feature for quasi-three-level lasers, which are likely to be frequently encountered with Yb lasers . Accordingly, the exact slope efficiency for a given output coupling is seen, from Fig. 2, to be dependent on the absorbed pump power for the three garnet crystal lasers. As a consequence, the average slope efficiencies determined for a low-to-mediate pump power range are utilized in this analysis, and the output couplings used are limited to T ≤ 15%.
Listed in Table 2 are the results obtained from this analysis. One sees that the round-trip internal losses determined for the three gallium garnet lasers prove to be very close, falling in a range of 0.020−0.024. This implies that the distinctions in laser performance achieved with the three garnets, as illustrated in Fig. 2, are attributed to their different spectroscopic and thermal properties. Taking into account the crystal thickness of 4 mm, the loss coefficient can be calculated to be δ = 0.030, 0.026, and 0.025 cm−1 for the Yb:YGG, Yb:LuGG, and Yb:GGG crystals, respectively. Such a close similarity in loss coefficient reveals a comparable optical quality of the three gallium garnets. The intrinsic slope efficiency determined for Yb:YGG, η0 = 90%, is considerably higher than that for either Yb:LuGG (η0 = 74%) or for Yb:GGG (η0 = 76%), in consistency with the highest slope efficiencies (ηmax) achieved experimentally, as listed also in Table 2. The parameter ξ, defined by ξ = ηmax/η0, representing the extent to which the laser operates efficiently, is also presented in this table for the three garnet crystals.
As quasi-three-level lasers, the oscillation wavelengths of the three garnets depended not only on the output coupling but on the pump power as well. Figure 3 illustrates the laser emission spectra for different output couplings measured at an intermediate pump power level. For each crystal, the emission spectrum shifted toward the short-wavelength side with increasing output coupling, which is a common behavior for a quasi-three-level laser. Generally, the emission spectra measured for the three garnets under similar operational conditions were found to be quite close; e.g., as shown in Fig. 3, for the case of T = 10%, the oscillation wavelengths covered a range of 1035−1037 nm (Yb:YGG); 1037−1040 nm (Yb:LuGG); and 1038−1040 nm (Yb:GGG). However, the case of T = 0.5% seems to be an exception. As can be seen from Fig. 3, while the emission spectra for Yb:LuGG and Yb:GGG were very close, they were completely different from that for Yb:YGG, whose main emission band shifted to the short-wavelength side by ~30 nm. In order to further examine the laser emission feature of the Yb:YGG crystal in the case of T = 0.5%, Fig. 4 illustrates the evolution of the emission spectrum with absorbed pump power. One can see from this figure that at low pump power levels, the laser oscillated around 1040 nm; in excess of Pabs ≈5.82 W, however, a second emission band started to occur in a wavelength range of ~1060−1075 nm. At high pump power levels, the Yb:YGG laser actually was operating simultaneously in two separate wavelength ranges; e.g., at the highest absorbed pump power of 9.93 W, the oscillating wavelength ranges were 1042−1051 and 1063−1074 nm. In contrast to this complex evolution behavior, the emission spectrum of either Yb:LuGG or Yb:GGG was found to change only slightly with pump power, with the oscillating wavelength range varying from 1069−1076 to 1066−1080 nm for Yb:LuGG, and from 1070−1079 to 1069−1077 nm for Yb:GGG when Pabs was increased from threshold to the highest pump power level.
For a free-running quasi-three-level laser, its oscillation will occur at wavelengths at which the effective gain cross section, σg(λ) = βσem(λ) − (1−β)σabs(λ), reaches its maximum. Here σabs and σem are absorption and stimulated emission cross sections, respectively, whereas β is the fraction of the active ions excited to the upper manifold. For a given σg(λ), the round-trip gain can be calculated by g(λ) = 2σg(λ)Ntl, with Nt and l being the density of the active ions and the thickness of the laser crystal, respectively.
Figure 5 shows the calculated round-trip gain of the Yb:YGG laser as a function of wavelength. The calculation was made by use of the data for σabs(λ) and σem(λ) previously reported for Yb:YGG . In the calculation, l = 0.4 cm and Nt = 9.5 × 1020 cm−3, corresponding to 7.35 at. % of Yb ion concentration, were also used. Using the round-trip passive losses for the Yb:YGG laser, L = 0.024 (as listed in Table 2), Fig. 5 also shows two overall loss levels: T + L = 0.029 and T + L = 0.124, corresponding to T = 0.5% and T = 10%, respectively. One sees that the round-trip gain for β = 0.05 in the two wavelength ranges around 1045 and 1070 nm can compensate exactly for the overall losses. This is the physical reason for the two laser emission bands existing simultaneously at high pump power levels in the case of T = 0.5% (Fig. 4). Due to the strong gain competition between the two emission bands, which depends sensitively on the amount of β, lasing in one emission band may suppress that in the other band upon changing the pump power that will result in small perturbations to the level of β. It is also clearly seen from Fig. 5 that the round-trip gain curve for β = 0.072 corresponds to the laser action for T = 10% with a lasing band around 1036 nm. This is in agreement with the actual oscillation wavelengths, 1035−1037 nm, as measured for the Yb:YGG laser operating under conditions of T = 10% [Fig. 3(a)]. It is also worthwhile to note that for β = 0.045, the gain can balance an overall loss of T + L = 0.024 (T = 0) over a wide range of ~1065−1080 nm, which suggests that laser action can be achieved in this wavelength region for a very small output coupling such as T = 0.2%.
To give a direct comparison, Table 3 summarizes the main parameters characterizing the cw laser performance of the three gallium garnet crystals obtained in the current experiment. The symbols appearing in this table are defined as follows: Pth is absorbed pump power measured at lasing threshold; Pabs,max is highest absorbed pump power; Pout,max is maximum output power; ηopt is optical-to-optical efficiency; ηs is slope efficiency; ηa is fraction of incident pump power absorbed by the crystal; and λlasing is laser oscillating wavelength.
In summary, we have conducted, under identical experimental conditions, a comprehensive investigation into the continuous-wave laser performance of Yb:YGG, Yb:LuGG, and Yb:GGG, the three Yb doped gallium garnets, which were grown by the optical floating zone technique. With a compact linear resonator longitudinally pumped by a diode laser emitting at 970−974 nm, output powers of 5−7 W were generated with the three garnet crystals. While the laser properties of Yb:LuGG and Yb:GGG prove to be very similar, the Yb:YGG turns out to be distinguished from the other two garnets in terms of optimum output coupling, laser emission spectrum for the case of T = 0.5%, and in particular, the optical-to-optical and slope efficiencies amounting respectively to 68% and 80%, significantly higher than those for Yb:LuGG (49% and 60%) or Yb:GGG (52% and 64%).
This work was supported by the National Natural Science Foundation of China (grants 60978023, 51272131, and 51025210).
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