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We report on a systematic study of the absorption and emission spectral properties of (Yb0.1Y0.9)3(Sc1.5Ga0.5)Ga3O12 (Yb:YSGG) crystals. The broad fluorescence spectral lines indicate great potential of Yb:YSGG for tunable and ultrafast laser applications. Efficient continuous-wave (cw) laser oscillation was also demonstrated at room temperature (RT), generating an output power of 6.11 W with an optical-to-optical efficiency of 64.2%, and a slope efficiency of 80.1% with respect to absorbed pump power. The laser emission spectrum shifts to shorter wavelengths as the transmission of the output coupler varies from 3% to 20%, a result that can be explained based on the effective gain cross-sections of Yb:YSGG.
©2013 Optical Society of America
1. Introduction
Garnets have attracted a great deal of attention in laser research due to their stable structure, and remarkable thermal and optical properties. Numerous studies on yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG), and yttrium gallium garnet (YGG), three typical garnet host materials for rare earth active ions such as Nd3+, Tm3+, Yb3+, Er3+, have been performed and many excellent results already achieved [1–4]. Randomly substituting scandium for gallium in the octahedral sites of the YGG crystal, a partly disordered crystal of YSGG can be grown [5]. Compared to traditional YAG, YSGG shows lower phonon energies, which obviously reduce multiphonon decay rates so that the active ions exhibit a longer fluorescence lifetime of the upper laser level [6]. Moreover, since the radius of Sc is larger than Ga in the octahedra (rSc = 0.745 Å and rGa = 0.620 Å for CN = 6) [7], introducing Sc3+ ions into YSGG can increase the distance between dodecahedral sites, which can be beneficial to reduce the relatively strong ion-ion interaction among active ions [1, 8, 9]. Therefore, YSGG should be quite suitable for high-efficiency and Q-switched laser operation, and it has already shown great laser potential for active ions such as Ho3+, Tm3+ and Er3+ [10–12]. For example, the 3 μm Er:YSGG laser has been developed for laser therapy, because of the strong water absorption around its emission wavelength. In addition, Nd:Gd:YSGG has also been studied both for continuous-wave (cw) and Q-switched laser operation [13].
Due to its high melting point (1877 °C) [14], YSGG is usually grown by the Czochralski (Cz) method in protective atmosphere free of oxygen, which has some inevitable consequences, such as a change in the Ga valence, production of oxygen vacancies and crucible contamination. Using the optical floating zone (OFZ) method [15], however, in which the crystal growth can be kept in a high-oxygen atmosphere without any crucible, such problems are avoided and high-quality YSGG crystals can readily be grown.
In the last two decades, Yb-doped materials have attracted much attention because of their remarkable advantages compared to their Nd counterparts, such as low quantum defect, broad absorption and emission bandwidths, and weak concentration quenching. In the exploration of promising laser hosts for the Yb3+ ion, many Yb-doped garnets have been developed, including Yb:YAG [1], Yb:YGG [16], Yb:GGG [3], Yb:YSAG [9], Yb:GSGG [4], etc. Up to now, however, Yb3+ ion doping of the YSGG crystal has only been investigated as a sensitizing mechanism and its spectroscopic properties have not been systematically studied while the laser performance has never been exploited [6, 17, 18]. In this paper, we report on the spectroscopic properties and efficient cw laser operation of Yb:YSGG crystals grown by the OFZ method.
2. Experimental details
A Yb:YSGG single crystal with a centimeter-level diameter was grown by the OFZ method with a four-ellipsoidal-mirror furnace (FZ-T-12000-X-I-S-SU-Crystal Systems, Inc). By x-ray fluorescence (XRF) analysis, the effective segregation coefficient Yb3+ ions in the as-grown crystal was determined to be 1.01 (corresponding to a dopant concentration of 1.26 × 1021 cm−3), a value that is slightly larger than that of Yb:YGG [16].
The room temperature (RT) absorption and transmission spectra were measured using a near-IR spectrophotometer (JASCO model V-570) on double-sided polished samples (3 × 3 × 0.5 mm3). The RT fluorescence lifetime and the emission spectrum at 78 K were measured on 0.5 mm thick polished samples with an Edinburgh Instruments FLS920 fluorescence spectrometer equipped with an ANDO Shamrock SR-303i high-resolution optical spectrum analyzer and a tunable Opolette (HE) 355 II (5 ns, 20 Hz) pump source.
The cw laser performance of the Yb:YSGG was studied in a plano-concave resonator. The laser experimental setup is shown schematically in Fig. 1. A 40 W fiber–coupled diode laser (200 µm fiber core diameter and 0.22 numerical aperture (NA)) with an output wavelength of ~970 nm was used as a pump source. The plane reflector (M1) was coated for high transmittance at 820-990 nm, and high reflectance at 1020-1200 nm. Concave mirrors (M2), with a radius of curvature of 25 mm and different transmissions (T = 3% and 20%) for 1030 nm, were used as output couplers. The uncoated Yb:YSGG sample used in the laser experiment was cut along the [111] crystallographic direction, with dimensions of 3 × 3 × 5 mm3. It was mounted on a water-cooled copper block, and the temperature of the cooling water was maintained at 5 °C. The physical cavity length was about 25 mm.
3. Results and discussion
The RT absorption and the 78 K emission spectra of Yb:YSGG recorded over a wavelength range of 850 to 1100 nm are shown in Fig. 2. The strongest absorption peak is located at 930.4 nm, in contrast to those of Yb:YAG (940 nm) [2], Yb:YGG (970 nm) [18], and Yb:GGG (971 nm) [3]. The absorption cross-section at this wavelength is 0.54 × 10−20 cm2, and the full-width at half-maximum (FWHM) amounts to 32 nm. It should be outlined that such large bandwidth is quite suitable and desirable for efficient pumping by high-power InGaAs laser diodes. Apart from the wide absorption band, there is a relatively weak absorption peak at 969.8 nm, corresponding to the zero phonon line, whose absorption cross-section amounts to 0.36 × 10−20 cm2.
Electron-phonon coupling has been widely studied in the corresponding literature because it is responsible for such phenomena as vibronic transitions, spectral-line broadening, and energy-transfer. According to the analysis of Ellens et al. [19, 20], the electron-phonon coupling strength is large in the beginning (Ce3+, Nd3+) and the end (Tm3+, Yb3+) of the trivalent lanthanide series, but weak at the center (Gd3+, Tb3+). Due to strong electron-phonon coupling in Yb3+-doped materials, which can disturb the Yb3+ energy transition and generate some additional emission peaks, it is difficult to interpret the information from emission spectra recorded at RT. Measuring the low-temperature fluorescence spectrum can avoid these problems and consequently provide the accurate energy level scheme. Based on charge transfer luminescence of Yb3+ [21], the emission spectrum at 78 K was measured with an excitation wavelength of 275 nm and is shown also in Fig. 2. The strongest emission peak occurs at 1024 nm. Compared to that of Yb:YAG (1030 nm) at the same temperature [22], this peak is blue-shifted. Following the analysis of the energy levels of Yb3+ in Yb:YGG [22–24], the Yb3+ energy level diagram of Yb:YSGG has been identified and can be seen in Fig. 3. The ground state splitting is slightly larger than in Yb:YGG suggesting that the Yb3+ crystal field becomes stronger by doping with Sc3+ ions.
As shown in Fig. 3, the RT stimulated-emission cross-section of Yb:YSGG is determined by use of the reciprocity method [25]:
where σabs(λ) is the absorption cross-section at wavelength λ, h is the Planck constant, k is the Boltzmann constant, c is the velocity of light, and λZL is the zero phonon line wavelength. Zl and Zu are the lower and upper manifold partition functions, respectively, a = l or u, and is the degeneracy of the energy level. The partition functions Zl and Zu are calculated to be 1.375 and 1.385, respectively, based on the energy-level diagram in Fig. 3. The largest emission cross-section occurs at 1025.4 nm with a value of 1.52 × 10−20 cm2, which is slightly lower compared to Yb:YAG (2.1 × 10−20 cm2) [2], Yb:YGG (2.56 × 10−20 cm2) [18], and Yb:GGG (2.0 × 10−20 cm2) [3]. The FWHM of the emission band around 1025.4 nm amounts to 11.4 nm, which is comparable to Yb:YAG (10 nm), Yb:YGG (11 nm), and Yb:GGG (10 nm) [18]. The RT fluorescence lifetime of the YSGG crystal was measured by the time-correlated single-photon counting (TCSPC) method giving 1.22 ms, a value that is slightly larger compared to Yb:YAG (0.95 ms) [2], Yb:YGG (1.1 ms) [26], and Yb:GGG (0.8 ms) [3]. Considering the lower emission cross-section and the longer fluorescence lifetime, it can be expected that the improved energy storage capacity of Yb:YSGG will make it a promising laser material for Q-switched operation.Figure 4 shows the effective gain cross-section of Yb:YSGG, calculated from [27]:
here β is the fraction of Yb3+ ions excited to the upper manifold. The minimum inversion rate βmin (1025.4 nm) is determined to be 0.065, which is slightly larger than the value obtained for Yb:YAG (0.055) [25], but smaller than that of Yb:YGG (0.083) [18]. It can be seen from Fig. 4 that the maximum of σg(λ) shifts to shorter wavelengths with increasing β, from around 1046 nm for β = 0.04 to 1028 nm for β = 0.075, which suggests that, above laser threshold, the laser wavelength will shift towards shorter wavelengths with increasing cavity loss. However, after β exceeds 0.2, the wavelength remains fixed at 1025.4 nm. Similar to Yb:CaGB, Yb:CYB and Yb:YAG [27], Yb:YSGG possesses a broad spectral range of positive gain with a maximum related to the strongest emission peak when β exceeds 0.065. Therefore, Yb:YSGG is likely to be interesting also for tunable and ultrafast lasers.Efficient RT laser oscillation of Yb:YSGG was achieved by using a simple plano-concave resonator. In consideration of the RT thermal conductivity of Yb:YSGG was measured to be 4.57 W·m−1·K−1, which shows smaller than that of Yb:YAG (7 W·m−1·K−1) [3], so the maximum pump power was limited to 15.5 W. The optimum transmission of the output coupler was T = 3%. The relationship between the output power and the absorbed pump power for this output coupler is depicted in Fig. 5. The absorbed pump power for reaching the laser threshold was 1.08 W. Above an absorbed pump power (Pabs) of about 4 W, the output power increased linearly with pump power. The output power of 6.11 W was achieved under an absorbed pump power of 9.51 W, leading to an optical-to-optical efficiency of 64.2% and a slope efficiency of 80.1%.
In order to investigate the shift of the laser wavelength, an output coupler with T = 20% was also used for comparison. Shown in Fig. 6 is the laser emission spectrum measured at Pabs = 2.19 W for T = 3% and T = 20%. In both cases, multi-wavelength oscillation was observed, a typical behavior for Yb3+ lasers operating in the free-running mode. In the T = 3% case, the laser oscillated in the range of 1045-1048 nm; the oscillation wavelength shifted to 1030-1031.5 nm when the output coupling was increased to T = 20%, moving closer to the maximum emission wavelength of 1025.4 nm. The spectral shift confirms the relationship between the emission wavelength and effective gain cross-section discussed in the previous section. The Pabs (2.19 W) value is only slightly above threshold, and in the absence of thermal effects, the inversion rate β does not change with pump power for a constant output coupling. With the change of output couplers from T = 3% to 20%, the cavity loss and the threshold become larger. In order to satisfy the laser oscillation condition, the β value should increase, and the maximum of σg(λ) will shift to a shorter wavelength. In the case of multi-mode oscillation, the laser mode with the lowest loss will oscillate first and the wavelength can be determined from the location of the σg(λ) peak value. Therefore, the observed spectral shift from 1046 nm (β = 0.04) for T = 3% to 1031 nm (β = 0.065) for T = 20% is reasonable.
4. Conclusion
In conclusion, the spectroscopic properties and cw laser performance of Yb:YSGG crystals have been studied for the first time, to our knowledge. Maximum absorption occurs at 930.4 nm, with a peak cross-section of 0.54 × 10−20 cm2, while the emission peak is located at 1025.4 nm, and the highest emission cross-section amounts to 1.52 × 10−20 cm2. An output power of 6.11 W was generated, with optical-to-optical and slope efficiencies of 64.5% and 80.1%, respectively, calculated with respect to absorbed pump power. The laser emission spectrum shifts towards shorter wavelengths with increasing transmission of the output coupler which is qualitatively explained on the basis of the spectroscopic data. The results demonstrate the potential of Yb:YSGG for application in tunable and ultrafast lasers.
Acknowledgments
The authors wish to thank Prof. R. I. Boughton, for the discussion and linguistic advice. This work was supported by the National Natural Science Foundation of China (Nos. 51025210, 51102156, 51272131, 51032004) and the Program of Introducing Talents of Discipline to Universities in China (111 program).
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