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Abstract
Good-quality single crystals of Yb:YPO4 were grown in a high-temperature solution. Polarized absorption and emission cross-section spectra were determined by use of a plate-shaped crystal of 1.8 mm in thickness. Efficient continuous-wave laser action was achieved under diode end-pumping conditions, with output coupling changed in a wide range of 0.5%−40%. A maximum output power of 3.62 W was produced with a 1.0 mm thick crystal, with an optical-to-optical efficiency of 36%.
© 2017 Optical Society of America
1. Introduction
The trivalent ytterbium (Yb) ion doped crystals have attracted consistent attention as laser media in the 1-μm spectral range. In some sense, the Yb ion exhibits more advantages in comparison with neodymium (Nd) ion as active center. The Yb ion is of a very simple energy level scheme, having only two manifolds, the 2F7/2 ground state and the 2F5/2 excited state. The simple level scheme enables the elimination of excited-state absorption as well as some degree of mitigation of concentration quenching. Additionally, the small quantum defect in the emission process of the Yb ion, which leads to low thermal load, is beneficial to high-power laser operation [1]. Among the wide variety of Yb-ion host crystals that have been known so far, the orthophosphates, RePO4, where Re represents the heavy lanthanide rare-earths from holmium (Ho) to lutetium (Lu) and also yttrium (Y) and scandium (Sc), seem very interesting to be explored. These crystals possess the tetragonal zircon structure (space group of I41/amd and point group of 4/mmm) [2], which is identical to that of orthovanadates (TVO4 with T being Y, Gd, or Lu). These orthovanadates have been recognized as one important class of host crystals for trivalent rare-earth active ions, in particular for the Nd ion [3]. As host materials, the orthophosphates are also desirable for their capability of accommodating large amounts of dopant ions, as well as for their extremely high chemical stability [2].
Despite the suitability of the orthophosphates as host crystals for rare-earth active ions, the development of laser crystals based on these orthophosphates still remains very limited. This situation is most probably attributed to the difficulty in crystal growing. Unlike most of other laser crystals, these orthophosphates, including their mixed members, cannot be grown by the Czochralski method because of their incongruently melting character [2, 4]. Up to now, the orthophosphate crystals, with which laser action have been realized, include only Nd:LuPO4 and Yb:LuPO4 [5, 6]. With Nd:LuPO4, continuous-wave (cw) output power of 0.3 W was generated from an end-pumped laser oscillator [5]. The research work conducted on Yb:LuPO4 has been more intensive, plate-shaped crystals of high optical quality have been grown successfully, with dimensions being at most 6.0 mm × 2.0 mm × 0.5 mm [7]; cw laser output power of 1.61 W was produced with a 0.3 mm thick Yb:LuPO4 crystal [6]. Furthermore, miniature columnar crystals of Yb:LuPO4 were developed, making it possible to scale the cw laser output power to a 5−10 W level [8]. With such miniature Yb:LuPO4 crystal rods, efficient laser operation at about 1000−1010 nm has been achieved in actively as well as passively Q-switched modes [9–11].
Given the promising laser performance of the Yb:LuPO4 crystal, it is also of interest to investigate other Yb-doped orthophosphates, for instance, the Yb:YPO4 crystal. The relevant work on this crystal, which was carried out thus far, involved mainly crystal field analysis and temperature- and pressure-dependent spectroscopic properties [12, 13]. Apart from this, charge transfer luminescence of Yb ions in YPO4 was also studied [14].
In this paper we report, for the first time to our knowledge, on the polarized absorption and emission cross-section spectra, as well as efficient cw laser action of the Yb:YPO4 crystal. Plate-shaped crystals, with aperture areas of up to 30 mm2 and thicknesses of 0.5−2.0 mm, have been successfully grown by high-temperature solution method.
2. Description of experiment
The crystal of Yb:YPO4 was grown by high-temperature solution method using Pb2P2O7 as the solvent. The flux was synthesized through solid-state reactions, with high-purity (99.99%) Y2O3 and Yb2O3 employed as raw materials. The stoichiometric amounts of starting materials were thoroughly mixed, dissolved and reacted in molten lead pyrophosphates (PbHPO4) in a platinum crucible, whose diameter was 4.0 cm, 6.0 cm high, having a volume of 50 cm3. The crucible was placed in muffle furnace and was slowly heated to 1250 °C. Having been kept at this temperature for 24 hours, the crucible was slowly cooled down to 900 °C, with a cooling rate of about 1 °C/hour. After that, the crucible was removed from the muffle furnace and inverted right away. Most of the crystals grown resided at the bottom of the crucible, they were removed by boiling in nitric acid. Figure 1 shows a picture of one of the as-grown crystals, with a size of about 6.0 mm × 4.0 mm × 0.5 mm.
Fig. 1 A picture of an as-grown plate-shaped crystal of Yb:YPO4, obtained by the high-temperature solution method.
The crystal sample utilized for the laser experiment was 1.0 mm thick (along the crystallographic a axis), with an aperture of approximately 5.0 mm × 3.0 mm. The Yb ion concentration of the crystal was 5 at. % (6.9 × 1020 cm−3). To study the laser properties of the Yb:YPO4 crystal, a plano-concave resonator was employed, which is illustrated schematically in Fig. 2.
Fig. 2 A diagram of the experimental laser setup. The Yb:YPO4 sample was held on a heat sink of copper. The left lower part shows the reflectivity curve for mirror M1.
The plane mirror (M1) was coated highly reflecting for 1020−1200 nm, and highly transmitting for 808−980 nm, whose reflectivity curve for 960−1060 nm is presented in Fig. 2. As the output coupler, the concave mirror (M2) of a radius-of-curvature of 25 mm was used, whose transmission (output coupling) could be chosen in a range of T = 0.5%−40%. The uncoated Yb:YPO4 crystal sample was fixed on a copper heat sink, and was placed close to the plane mirror inside the resonator. The pump source was a high-brightness fiber-coupled diode laser (fiber core diameter of 100 μm and NA of 0.22), whose emission wavelength was 976 nm and the bandwidth was less than 0.5 nm. The pump radiation was re-imaged by a focusing unit and then was delivered into the Yb:YPO4 crystal.
3. Results and discussion
Figure 3 shows the X-ray powder diffraction (XRPD) pattern of the Yb:YPO4 crystal, which was recorded by using a D8 Advance diffractometer with CuKα radiation (λ = 1.5406 Å). One sees that all peaks in the pattern can be indexed according to a pure tetragonal phase of YPO4 crystal. The structures of the YPO4 and YbPO4 were identical under the same conditions, the patterns of which could be assigned to tetragonal zircon structure, belonging to the space group I41/amd (Z = 4) [2]. Table 1 presents the parameters of the crystal cell determined for the Yb:YPO4, the data for the crystals of YPO4 and YbPO4 are also given for comparison. One can notice that the lattice constants of Yb:YPO4 crystal are smaller than those of the YPO4, arising from the substitution of Yb ions (ionic radius of 0.858 Å) for Y ions (ionic radius of 0.893 Å) in the Yb:YPO4 lattice. The XRPD spectrum also confirms the absence of unwanted impurities, and the incorporation of Yb ions into YPO4.
Table 1. The Lattice Constants of YPO4, YbPO4 and Yb:YPO4 Crystals
The polarized absorption spectra of Yb:YPO4 were measured with a spectral resolution of 0.2 nm at room temperature, with a crystal sample having a thickness of 1.8 mm. The results are shown in Fig. 4, in terms of absorption cross-section (σabs). From this figure one can notice the presence of significant polarization dependence of σabs versus λ (wavelength), indicating considerable anisotropy of Yb:YPO4 crystal in its absorption in the near infrared region. The absorption spectrum for σ polarization consists of three absorption peaks at about 950.9, 974.7, and 982.3 nm, respectively; the strongest absorption line is observed at 974.7 nm, with an absorption cross-section of σabs = 2.30 × 10−20 cm2 and a full-width at half-maximum (FWHM) line-width of 6.5 nm. In contrast, the π-polarized absorption spectrum is made up of two principal bands with their peaks locating, respectively, at 958.7 and 982.3 nm. The maximum absorption cross-section for π polarization, σabs = 1.16 × 10−20 cm2, is reached at 982.3 nm (bandwidth of 8.2 nm). One sees that the absorption peak occurring at 974.7 nm, which is the strongest for σ polarization, becomes much less pronounced in the π-polarized absorption spectrum.
Fig. 4 Polarized absorption and emission cross-section spectra of the Yb:YPO4 crystal determined at room temperature.
The polarized emission cross-section spectra (σem(λ)) were calculated based on the absorption spectra by use of the integral method of reciprocity [15]. For the calculation the refractive indexes were taken to be nσ = 1.72, nπ = 1.82 [16]. While the radiative lifetime, which was also required in the calculation, was taken to be τrad = 0.90 ms, the fluorescence lifetime measured at room-temperature [13]. We assumed here a quantum efficiency of unity, ignoring the effects of both quenching and radiation trapping. In situations where the accurate radiative lifetime is unknown, this seems to be a reasonable, useful practice [17]. Similar to the situation of absorption, strong anisotropy can readily be seen in the polarized emission spectra, which are also shown in Fig. 4. For σ polarization, the strongest emission occurs at 974.7 nm corresponding to the zero-phonon line, with the maximum emission cross-section determined to be σem = 1.51 × 10−20 cm2. For π polarization, however, the emission peak corresponding to the zero-phonon transition is almost masked by the longer emission band peaked at 982.5 nm, where the emission cross-section amounts to σem = 1.09 × 10−20 cm2. One notices in the π-polarized emission spectrum that besides this emission peak at 982.5 nm, there also exists a much wider emission band that is peaked at about 1001 nm, with a maximum emission cross-section of σem = 1.0 × 10−20 cm2. This wide emission band, which extends up to a wavelength of about 1040 nm, turns out to be the most important because actual laser action usually occurs, under free-running conditions, over the wavelength range covered by this emission band (as will be indicated clearly in Fig. 6). Table 2 lists the main spectroscopic parameters of the Yb:YPO4, along with those for Yb:LuPO4 for comparison.
Table 2. Comparison of Spectroscopic Parameters between Yb:YPO4 and Yb:LuPO4 Crystals
Continuous-wave laser action of the Yb:YPO4 crystal was achieved at room temperature employing the simple plano-concave resonator (Fig. 2), with the output coupling changed from T = 0.5% to T = 40%. In all cases, the laser oscillation was linearly polarized with E//c (π-polarized). Figure 5 shows the output power versus the absorbed pump power (Pabs) measured under different output coupling conditions. The amount of Pabs was determined from the incident pump power (Pin) by Pabs = ηpPin, here ηp, the small-signal or unsaturated absorption fraction, was measured to be 0.65 for the 1.0 mm thick Yb:YPO4 crystal sample.
Fig. 5 Output power versus absorbed pump power, produced with the Yb:YPO4 crystal laser under different output coupling conditions.
In the case of the lowest output coupling (T = 0.5%), the lasing threshold was reached at Pabs = 0.17 W. With the output coupling increased to T = 40%, the lasing threshold also increased to Pabs = 1.12 W, due to the high overall losses of the laser oscillator introduced by the output coupler. As can be seen from Fig. 5, the laser action became more efficient when the output coupling was increased from the very low value of T = 0.5%; over an intermediate output coupling range of T = 5%−20%, the Yb:YPO4 laser was found to operate almost equally efficiently, with slope efficiencies of 38%−39%. The maximum output power of 3.62 W was generated at Pabs = 10.01 W in the case of T = 5%, resulting in an optical-to-optical efficiency of 36%. It is worthwhile to notice that even with an output coupling as high as T = 40%, the output power could still reach a 3 W level with the 1-mm thick crystal sample, this may imply the potential of the Yb:YPO4 crystal plate in applications of making passively Q-switched microchip lasers.
Figure 6 presents three various laser emission spectra which were recorded at a pump level of Pabs = 3.13 W, with the output coupling being, respectively, T = 0.5%, 5%, and 40%. These emission spectra consisting of several lasing lines were typical of free-running quasi-three-level lasers with wide emission bands. The laser oscillation shifted toward the short-wavelength side as the output coupling was increased from T = 0.5% to T = 40%. Under the condition of T = 0.5%, the laser oscillation occurred at 1027.4−1035.2 nm; with the highest output coupling of T = 40% utilized, the oscillation wavelength range was measured to be 1009.7−1010.9 nm.
Fig. 6 Laser emission spectra measured at Pabs = 3.13 W for the Yb:YPO4 crystal laser operating under different output coupling conditions.
For a quasi-three-level laser operating in free-running mode, such as the current Yb:YPO4 crystal laser, the wavelengths where the actual laser action occurs are determined by the effective gain cross section, σg(λ) = βσem(λ) − (1 − β)σabs(λ), with β denoting the fraction of Yb ions that have been excited to the upper manifold (2F5/2). Figure 7 depicts a number of σg(λ) curves for different values of the parameter β ranging from 0.15 to 0.45. Since the overall resonator losses become increased with the increase of the output coupling, accordingly, in order to meet the conditions of laser oscillation, a higher gain and hence a greater β is required for a larger output coupling. One notes that, with the parameter β increased from 0.15 to 0.45, the maximum gain cross-section tends to move to short-wavelength side, this provides a qualitative explanation for the emission wavelength shifting with the variation of the output coupling (Fig. 6). For example, the maximum gain occurs at about 1035 nm for a low excitation level of β = 0.15, this roughly accounts for the case of T = 0.5%; for a higher excitation level of β = 0.35, the maximum gain is reached at 1009.2 nm, this could interpret the lasing wavelengths observed in the case of T = 40%.
Fig. 7 π-polarized effective gain cross section curves for excitation levels ranging from β = 0.15 to β = 0.45.
It is worth pointing out that in our discussion of the laser performance, the absorbed pump power was estimated using the unsaturated absorption fraction (ηp), which is, in general, applicable to the cases of very low overall resonator losses where the upper-manifold population (or inversion) is negligible. For laser oscillation achieved under a higher output coupling, the fraction of pump power absorbed in the crystal would depend on the inversion level, ηp = ηp(β) = 1 − exp[σg(λp)Ntl], with Nt denoting the Yb ion concentration, l the crystal thickness, and λp the pumping wavelength. As mentioned above, the parameter β, for T = 0.5% and T = 40%, amounts respectively to 0.15 and 0.35; while a rough gain estimation yields β = 0.23 for T = 5%. Taking into account the un-polarized nature of the pump radiation at λp = 976 nm, σg(λp) should take its average value over the σ and π polarizations. From the polarized absorption and emission spectra (Fig. 4) one obtains: σg(λp) = −0.95, −0.76, and −0.53 × 10−20 cm2, for β = 0.15, 0.23, and 0.35, respectively. Given the numbers of Nt and l for the Yb:YPO4 crystal sample used, Nt = 6.9 × 1020 cm−3 and l = 0.1 cm, one can thus calculate the magnitude of ηp(β), yielding the results: ηp(β) = 0.49 (T = 0.5%); 0.42 (T = 5%); and 0.31 (T = 40%). Obviously, for T = 3%, the amount of ηp(β) should fall in a range of 0.49−0.42; while for T = 10% and T = 20%, the corresponding ηp(β) ranges from 0.42 to 0.31. Compared to the measured value of the unsaturated ηp (0.65), one sees that the actual absorption under lasing conditions could be greatly reduced, suggesting that the laser efficiencies given in Fig. 5 might be underestimated considerably.
4. Conclusions
In summary, good-quality Yb:YPO4 crystals were successfully grown by the flux process. With a 1.0 mm thick crystal sample longitudinally pumped by a 976-nm diode laser in a plano-concave resonator, efficient continuous-wave laser action was achieved at wavelengths of about 1010−1035 nm, with output coupling changed from 0.5% to 40%. An output power of 3.62 W was generated with an optical-to-optical efficiency of 36%. In addition, polarized absorption and emission cross-sections of the Yb:YPO4 crystal were determined for a wavelength range from 850 to 1100 nm. The spectral features in absorption as well as in emission spectra prove to be largely dependent on the polarization direction, showing strong anisotropy.
Funding
National Natural Science Foundation of China (Grant nos. 11374170 and 11204148); China Postdoctoral Science Foundation (Grant no. 2015M580573); Applied Basic Research Programs for Youths of Qingdao (Grant no. 15-9-1-52-JCH); Qingdao Postdoctoral Application Research Project (Grant no. 2015127); Open Project of State Key Laboratory of Rare Earth Resource Utilization (RERU2016015).
Acknowledgments
The author also would like to thank the Taishan Scholar Program of Shandong Province, China.
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