October 15, 2014 / Vol. 39, No. 20 / OPTICS LETTERS

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Spectroscopic properties and continuous-wave laser operation of Yb:LuPO4 crystal Junhai Liu,1,2,* Wenjuan Han,1,2 Xiaowen Chen,1,2 Degao Zhong,1,2 Bing Teng,1,2 Chao Wang,1,2 and Yuyi Li1,2 1

2

College of Physics, Qingdao University, 308 Ning-Xia Load, Qingdao 266071, China Key Laboratory of Photonics Materials and Technology in Universities of Shandong (Qingdao University), 308 Ning-Xia Road, Qingdao 266071, China *Corresponding author: [email protected] Received July 4, 2014; revised September 7, 2014; accepted September 12, 2014; posted September 15, 2014 (Doc. ID 216305); published October 9, 2014

For the first time, and to the best of our knowledge, we report a continuous-wave (cw) laser operation of Yb:LuPO4 crystal, demonstrated at room temperature in a compact plano-concave resonator end pumped by a diode laser. 1.61 W of cw output power around 1039 nm were generated with 2.40 W of pump power at 976 nm absorbed in a 0.3 mm thick crystal, leading to an optical-to-optical efficiency of 67%, whereas the slope efficiency was 75%. Polarized absorption and emission cross-section spectra are also presented and discussed in detail. © 2014 Optical Society of America OCIS codes: (140.3615) Lasers, ytterbium; (140.3380) Laser materials; (140.3580) Lasers, solid-state. http://dx.doi.org/10.1364/OL.39.005881

As host crystals for the trivalent neodymium (Nd) ion, the orthovanadates TVO4 (T  Y, Gd, Lu), which possess the tetragonal zircon structure (space group I41 ∕amd, and point group 4∕mmm), occupy a unique position among a wide variety of inorganic dielectric crystals that can serve as host materials. In reality, Nd:TVO4 crystals have a dominant role in Nd-ion solid-state lasers that operate in a low-to-medium power regime. During the past two decades, numerous studies have been conducted on these Nd:TVO4 crystals, including their mixtures [1]. The great success in applying the orthovanadates as host media for the Nd ion might encourage the search for promising new host materials from those crystals that possess the same tetragonal zircon structure. The rareearth orthophosphates, RePO4 , where Re represents the heavy lanthanide elements from holmium (Ho) to lutetium (Lu), form one group of such crystals that belong to the space group I41 ∕amd and point group 4∕mmm. These orthophosphates can allow large amounts of other rareearth ions to be incorporated into their lattice, substituting the compositional rare-earth ions that occupy sites with a local symmetry of D2d . They are also known to have extremely high chemical stability [2]. Therefore, these tetragonal orthophosphates are potential ideal host crystals for rare-earth active ions. However, because the orthophosphates do not melt congruently, they cannot be grown by the Czochralski method, making it difficult to produce high-quality single crystals with large sizes [2]. Presently, only very limited work has been conducted on developing laser crystals based on these tetragonal orthophosphates. Most of the relevant work was on LuPO4 . The spectroscopy of both erbium (Er) and Nd ions doped in LuPO4 crystals was studied thoroughly by Rapaport et al. [3,4]; laser operation was also achieved in their experiment with Nd:LuPO4 , producing 0.3 W of output power [4]. To the best of our knowledge, this is the only laser action ever realized with orthophosphates for the past several years. In addition to Er- and Nd-doped LuPO4 , some preliminary results on the spectroscopy of Yb:LuPO4 were reported in the early days of Yb lasers [5]. 0146-9592/14/205881-04$15.00/0

Recently, our group grew plate crystals of Yb:LuPO4 by employing the high-temperature solution method [6]. The crystals produced were approximately 0.2–0.5 mm thick along the crystallographic a axis, with transverse sizes of at most 6 mm × 2 mm. In this Letter, and to the best of our knowledge, we report for the first time laser operation of Yb:LuPO4 , achieved with such thin crystal plates in a simple plano-concave resonator under diode end pumping. The polarized absorption and emission spectra of Yb:LuPO4 are also presented in terms of cross sections. The Yb:LuPO4 crystal sample utilized in the experiment was 0.3 mm thick (along a axis), with an aperture of 5.0 mm ==c axis × 2.0 mm==a axis. The crystallographic axes of the sample were determined by use of the x-ray diffraction method. The Yb concentration was 10 at. % (corresponding to 1.23 × 1021 cm−3 in the crystal), which was essentially that in the solution where the crystal was grown (assuming a unity segregation coefficient for the crystal growing process). Because of the limited amount of the Yb:LuPO4 crystal and its extremely small dissolubility, measuring an accurate Yb concentration in the crystal employing frequently used methods, such as the x-ray fluorescence analysis or the inductively coupled plasma atomic emission spectroscopy, was not feasible. For laser operation, a simple plano-concave resonator was used, as illustrated schematically in Fig. 1. M1 was a plane mirror coated for high reflectance at 1020– 1200 nm and high transmittance at 808–980 nm; M2 was a concave mirror with a radius-of-curvature of 25 mm that

Fig. 1. Schematic diagram of the experimental laser setup. Attached on the front face of the heat sink is the thin Yb:LuPO4 crystal sample. © 2014 Optical Society of America

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acted as output coupler for the resonator. The cavity length was chosen at 23 mm. The uncoated Yb:LuPO4 crystal sample was attached to a copper heat sink, and was positioned close to the plane mirror inside the resonator. A high-brightness fiber-coupled diode laser was employed to pump the Yb:LuPO4 crystal, whose coupling fiber was 100 μm in core diameter, with a NA of 0.2. The unpolarized pump radiation produced by the diode laser, with an emission wavelength of 976 nm and a bandwidth of less than 0.5 nm, was coupled through a focusing optics into the crystal sample, with a pump spot radius of 70 μm. First, we studied the spectroscopic properties of the Yb:LuPO4 crystal because previously reported data on its spectroscopy was still incomplete, in that reliable polarized absorption and emission spectra were lacking [5,6]. In the current work, the absorption spectra for E==c (π polarization) and for E⊥c (σ polarization) were measured at room temperature over a wavelength (λ) range from 850 to 1100 nm, from which the polarized emission spectra were calculated using the reciprocity method [5]. In the calculation, a value of 0.96 was obtained for Z l ∕Z u , the partition function ratio [5]. Shown in Fig. 2 are the room temperature π- and σ-polarized absorption and emission spectra for Yb:LuPO4 , represented in terms of σ abs λ (absorption cross section) and σ em λ (emission cross section). One sees that for either absorption or emission, the spectral features for σ and π polarizations prove to be quite different, revealing the strong anisotropy that exists in the spectroscopic properties of this crystal. The π-polarized absorption spectrum is dominated by a main absorption band with the peak located at 985 nm, at which the absorption cross section amounts to σ abs  2.1 × 10−20 cm2 , whereas the bandwidth is 8.5 nm (FWHM). However, for σ polarization, the absorption spectrum comprises three well-resolved absorption peaks, with the strongest occurring at 975 nm, which corresponds to the zero-phonon line of the 2 F7∕2 → 2 F5∕2 transition [5]. The maximum absorption cross section at 975 nm is σ abs  2.7 × 10−20 cm2 . One notes that the peak at 985 nm that appears in the π-polarized spectrum as the strongest absorption is almost masked in the σ-polarized absorption spectrum.

Similar to the situation of absorption, the polarized emission spectrum differs significantly for different polarizations, as can be seen clearly from Fig. 2. For π polarization, the strongest emission occurs at 985 nm, with the maximum cross section of σ em  3.0 × 10−20 cm2 . Because of overlapping with the absorption peak at 985 nm, this emission peak seems to be less important for laser action; of more practical importance is the emission that occurs at wavelengths in its trailing edge (approximately 1000–1060 nm), where σ em falls in a range of 1.9 − 0.4 × 10−20 cm2 in which several emission peaks exist. From the σ-polarized emission spectrum, one sees that at the strongest emission peak located at 975 nm, σ em reaches a maximum value of 2.4 × 10−20 cm2 . Over the wavelength range of approximately 1000– 1050 nm where practical laser oscillation is expected, the emission cross section is found to be σ em  1.8 − 0.3 × 10−20 cm2 . It is also instructive to compare Yb:LuPO4 with Yb:LuVO4 , an isomorphic orthovanadate crystal. Table 1 lists the main spectroscopic parameters for the two Ybdoped crystals, which have exactly the same crystal structure, crystal symmetry, and local symmetry for the lattice site occupied by the Yb ion. In this table, the undefined symbols are peak wavelength λpeak ; absorption bandwidth Δλabs ; and fluorescence lifetime τf . Attempts were also made in the present work to measure the fluorescence lifetime of the 2 F5∕2 upper state of the Yb ion in the host crystal of LuPO4 . Unfortunately, however, given the limited transverse size of the crystal sample, the fluorescence signal resulted significantly weak to allow reliable data acquisition with the measurement equipment available for our experiment. Cw laser operation was realized at room temperature with the 0.3 mm thick Yb:LuPO4 crystal employing the simple plano-concave resonator (as shown in Fig. 1.). The laser oscillation was linearly polarized with E==c (π-polarized). Figure 3 shows the output power as a function of absorbed pump power (P abs ), which was measured with an output coupling of T  1.3%, 3.0%, and 11%, respectively. The absorbed pump power was determined from P in , the pump power incident into the Yb:LuPO4 crystal, by P abs  ηp P in ; here, ηp represents the small signal or unsaturated absorption. For the 0.3 mm thick crystal, Table 1. Comparison of Spectroscopic Properties between Yb:LuPO4 and Yb:LuVO4 Crystals Spectroscopic Parameters σ abs π10−20 cm2  λpeak πnm Δλabs πnm σ abs σ10−20 cm2  λpeak σnm Δλabs σnm σ em π10−20 cm2  λpeak πnm σ em σ10−20 cm2  λpeak σnm τf ms

Fig. 2. Room-temperature polarized absorption and emission spectra of the Yb:LuPO4 crystal.

a

Data cited from Ref. 7. From Ref. 5.

b

Yb:LuPO4

Yb:LuVO4 a

2.1 985 8.5 2.7 975 14 3.0 985 2.4 975 0.83b

8.4 985 7.3 2.0 969 45 11.8 985 3.5 985 0.26

October 15, 2014 / Vol. 39, No. 20 / OPTICS LETTERS

Fig. 3. Output power versus P abs , measured for the Yb:LuPO4 laser under different output coupling conditions.

ηp was measured as 0.26. In the case of the lowest output coupling (T  1.3%), the lasing threshold was reached at P abs  0.17 W, above which the output power increased linearly with P abs , reaching 1.61 W at P abs  2.40 W, which corresponds to an optical-to-optical efficiency of 67%, whereas the slope efficiency, determined for P abs > 0.5 W, amounts to 75%. With the output coupling increased, the absorbed pump power required to arrive at the laser threshold also increased to 0.32 and 0.51 W for T  3.0% and T  11%, respectively. The output power produced at P abs  2.40 W, the highest pumping level applied in the experiment, decreased to 1.39 and 1.01 W, respectively, in the two cases of T  3.0% and T  11%. Obviously, because of the very limited gain that can be obtained from the thin Yb:LuPO4 crystal, the optimum output coupling for the laser must be very low. Therefore, it is expected that the output power and laser efficiency can be further increased with the optimum output coupling that proves to be lower than 1.3%, the lowest available in our experiment. Under certain output coupling conditions, the wavelengths at which laser oscillation occurred were found to vary only slightly with pump power, but they shifted significantly as the output coupling of the laser changed. Figure 4 shows the laser emission spectra measured at an

Fig. 4. Emission spectrum of the Yb:LuPO4 laser, measured at P abs  1.6 W for different output couplings.

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intermediate pump level of P abs  1.6 W for the three output couplings of T  1.3%, 3.0%, and 11%. One sees that the laser oscillation shifted from 1035.7–1039.5 to 1009.6–1013.0 nm as the output coupling increased from T  1.3% to T  11%. For a quasi-three-level system such as Yb:LuPO4 , the laser action in free-running mode is determined by the effective gain cross section, σ g λ, rather than simply by σ em λ. σ g λ is related to σ em λ and to σ abs λ through the relationship σ g λ  βσ em λ − 1 − βσ abs λ, where the parameter β denotes the fraction of excited active (Yb) ions. Depicted in Fig. 5 are a group of curves for π-polarized σ g λ versus λ, calculated using σ abs λ and σ em λ (given in Fig. 2) with the excitation parameter changed from β  0.10 to β  0.45. One sees that the maximum σ g λ tends to shift toward the short-wavelength side as the excitation level (β), or equivalently, the output coupling of a laser, is increased. For β  0.10, the net gain maximum is reached at approximately 1036 nm, which corresponds to the case of T  1.3%, as can be noted from Fig. 4. As the excitation level increases to β  0.33, the net gain for several emission bands, whose peaks are located approximately at 1003, 1010, 1026, and 1034 nm, results very close; this provides a qualitative explanation for the emission spectrum measured in the case of T  3.0%, which consists of four discrete emission bands (Fig. 4). With the excitation level increased further, e.g., to β  0.45, the wavelength at which the net gain reaches its maximum shifts to the broad main emission band at approximately 1001 nm. The evolution behavior of laser emission spectrum with increasing output coupling illustrated in Fig. 4 reflects, to a large extent, the tendency of oscillation wavelength shifting, which is predicted by the σ g λ curves plotted in Fig. 5. In conclusion, efficient cw laser operation was demonstrated at room temperature with a 0.3 mm thick Yb:LuPO4 crystal under diode-end-pumping conditions, generating an output power of 1.61 W with an opticalto-optical efficiency of 67%, whereas the slope efficiency was as high as 75%. Polarized absorption and emission cross-section spectra were also determined for the Yb:LuPO4 crystal. Given the very promising laser performance demonstrated here, and the desirable advantages

Fig. 5. π-polarized effective gain cross section versus wavelength, calculated for different excitation levels.

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of LuPO4 as host crystal, one can predict that Yb:LuPO4 could become a new Yb crystal of great potential. This work was supported in part by the National Natural Science Foundation of China (grant 11374170). References 1. H. Yu, J. Liu, H. Zhang, A. A. Kaminskii, Z. Wang, and J. Wang, “Advances in vanadate laser crystals at a lasing wavelength of 1 micrometer,” Laser Photonics Rev., doi: 10.1002/lpor.201400022 (posted online March 27, 2014). 2. L. A. Boatner, Rev. Mineral. Geochem. 48, 87 (2002).

3. A. Rapaport, V. David, M. Bass, C. Deka, and L. A. Boatner, J. Lumin. 85, 155 (1999). 4. A. Rapaport, O. Moteau, M. Bass, L. A. Boatner, and C. Deka, J. Opt. Soc. Am. B 16, 911 (1999). 5. L. D. DeLoach, S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, IEEE J. Quantum Electron. 29, 1179 (1993). 6. D. G. Zhong, B. Teng, L. F. Cao, C. Wang, L. X. He, J. H. Li, S. M. Zhang, and Y. Y. Li, Cryst. Res. Technol. 48, 369 (2013). 7. J. Liu, X. Mateos, H. Zhang, J. Wang, M. Jiang, U. Griebner, and V. Petrov, Opt. Lett. 30, 3162 (2005).

Spectroscopic properties and continuous-wave laser operation of Yb:LuPO₄ crystal.

For the first time, and to the best of our knowledge, we report a continuous-wave (cw) laser operation of Yb:LuPO₄ crystal, demonstrated at room tempe...
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