1. Introduction

Because the radius and mass of Lu³⁺ ions are the closest to those of Yb³⁺ ions, the part substitution of Yb³⁺ ions for Lu³⁺ ions in some lutetium crystals will affect the optical quality and thermal conductivity more lightly than those for Yb³⁺-doped yttrium and gadolinium crystals [1–3]. Therefore, the Yb³⁺-doped lutetium crystals are of special interest for solid state lasers at present. Growth, spectroscopic and laser properties have been investigated widely for some Yb³⁺-doped lutetium crystals, such as Lu₃Al₅O₁₂ (LuAG), LuVO₄, and KLu(WO₄)₂ (KLuW) [1–3].

Among the solid state laser hosts, the huntite-like borate crystals with a general formula RAl₃(BO₃)₄ (R = Y, La-Lu) are interesting as polyfunctional materials because of their good chemical and physical properties and possibility of wide isomorphous substitutions [4,5]. For example, YAl₃(BO₃)₄ (YAB) and GdAl₃(BO₃)₄ (GAB) crystals possess rather large nonlinear optical coefficient and the crystals singly doped with Nd³⁺ and Yb³⁺ ions have been proved as good laser and nonlinear laser media [6–9]. Benefitting from their high effective phonon energy and good thermal property, YAB and GAB crystals co-doped with Yb³⁺ and Er³⁺ ions have been also demonstrated as efficient gain media for 1.5–1.6 μm laser [10–14], which has many potential applications, such as optical fiber communication, medicine, laser-range-finding, and lidar. However, except for the recent report about the growth and optical properties of Yb³⁺-doped LuAl₃(BO₃)₄ (LuAB) crystal [15], the investigation about the laser property of LuAB crystal doped with rare earth ions is scanty. In this paper, spectroscopic property of Yb³⁺ and Er³⁺ co-doped LuAB crystal is investigated and efficient 1.5–1.6 μm laser oscillation in the crystal is reported.

2. Spectroscopic property of Er³⁺:Yb³⁺:LuAB crystal

An Er³⁺:Yb³⁺:LuAB crystal was grown by the flux method. The growth process is similar to that of Yb³⁺:LuAB crystal [15]. The concentrations of the Yb³⁺ and Er³⁺ ions in the crystal were measured to be 24.1 and 1.1 at.%, respectively, by the inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima 2, Jobin-Yvon), corresponding to ion densities of 13.5 × 10²⁰ and 0.62 × 10²⁰ cm⁻³ (the density of LuAB crystal used is 4.569 g/cm³ [16]), respectively. The polarized absorption spectra of the crystal were recorded with a spectrophotometer (Lambda-900, Perkin-Elmer) at room temperature. Figure 0.1 shows the polarized absorption spectra in a range from 900 to 1050 nm. For the σ-polarized absorption spectrum, the full width at half the maximum (FWHM) of the band is 19 nm and the peak absorption coefficient is about 45 cm⁻¹ at 976 nm, which are similar to those (19 nm and 43 cm⁻¹, respectively) of YAB crystal doped with the similar Yb³⁺ and Er³⁺ concentrations [11].

Because the magnetic–dipole has a significant contribution to the transition between ⁴I_15/2 and ⁴I_13/2 of Er³⁺ ions [12,17], the complete polarized absorption spectra of the Er³⁺:Yb³⁺:LuAB crystal in a range from 1450 to 1620 nm, i.e. α (E⊥c, k//c), σ (E⊥c, k⊥c), and π (E//c, k⊥c) polarizations [12,17], were recorded and are shown in Fig. 2 . Here, E is electric field and k is wave vector. In order to avoid the re-absorption effect [18], the polarized stimulated emission spectra of the crystal were calculated by the reciprocity method [19] and are also shown in Fig. 2. Because the crystal field level structure of Er³⁺ ions in LuAB crystal is unavailable, the energy-level diagram of Er:YAB crystal is adopted approximately in the calculation [13]. It can be seen from the figure that the absorption and emission spectra in a range from 1450 to 1620 nm of Er³⁺:Yb³⁺:LuAB crystal are similar to those of Er³⁺:Yb³⁺:YAB and Er³⁺:Yb³⁺:GAB crystals [10,12]. The peak stimulated emission cross section of Er³⁺:Yb³⁺:LuAB crystal is about 1.7 × 10⁻²⁰ cm² at 1530 nm for σ polarization. Furthermore, the polarized fluorescence spectra in a range from 1000 to 1700 nm were also measured at room temperature using a monochromator (Triax550, Jobin-Yvon) with a TE-cooled PbS detector (DSS-PS020T, Jobin-Yvon) when the crystal was excited at 976 nm. The fluorescence around 1000 nm originated from the ²F_5/2→²F_7/2 transition of Yb³⁺ ions and the ⁴I_11/2→⁴I_15/2 transition of Er³⁺ ions was not observed, and only an intensive fluorescence band around 1550 nm originated from the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions was recorded. Combining with the very weak upconversion green fluorescence observed in the spectroscopic and laser experiments, it can be deduced that in this Er³⁺:Yb³⁺:LuAB crystal the pump energy mainly absorbed by Yb³⁺ ions can be efficiently transferred to the ⁴I_11/2 level of Er³⁺ ions by the resonant energy-transfer, then most of the excited ions populate the ⁴I_13/2 upper laser level of Er³⁺ ions by the rapid multiphonon relaxation [13].

 

Fig. 2 Room temperature polarized absorption (solid lines) and emission (dashed lines) spectra of Er³⁺:Yb³⁺:LuAB crystal in a range from 1450 to 1620 nm.

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Based on the above absorption and emission spectra, the polarized gain curves of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺:Yb³⁺:LuAB crystal were calculated for different values of population version β and only the α-polarized gain spectra in a range from 1500 to 1620 nm, which is useful for the analysis of the output laser wavelength of a c-cut crystal used in the following experiment, are shown in Fig. 3 for the sake of brevity. Using a microsecond flash lamp (μF900, Edinburgh) as the exciting source, the fluorescence decay curve at 1550 nm was recorded by a spectrophotometer (FL920, Edinburgh) when the exciting wavelength was 976 nm and the result is shown in the inset of Fig. 3 in a semilog scale. The linear relationship in the figure displays the single exponential behavior of the fluorescence decay and the fluorescence lifetime of the ⁴I_13/2 level of Er³⁺ ions in the crystal was estimated to be about 0.31 ms, which is similar to those (about 0.3-0.32 ms) of Er³⁺ and Yb³⁺ co-doped YAB and GAB crystals [10,12].

 

Fig. 3 Room temperature α-polarized gain spectra of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺:Yb³⁺:LuAB crystal in a range from 1500 to 1620 nm. The inset shows the fluorescence decay curve at 1550 nm of Er³⁺:Yb³⁺:LuAB crystal excited by 976 nm

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3. Laser experimental arrangement

A c-cut, 0.7-mm-thick Er³⁺:Yb³⁺:LuAB crystal was used as gain medium and an end-pumped linear resonator was adopted in the laser experiment. A 970 nm fiber-coupled diode laser (800 μm diameter core) from Coherent Inc. was used as the pump source. After passing a simple telescopic lens system, the pump beam was focused to a spot with waist diameter of about 290 µm in the crystal. The uncoated sample was attached on an aluminum slab with heat-conducting adhesive and there is a hole in the center of the slab for the passing of the pump and fundamental laser beams. Because no other device was used to control the cooling of the sample, for reducing the influence of the pump-induced thermal load on the laser performance and avoiding the fracture of sample at high pump power, the diode laser operated in the pulse mode. The pump pulse width was 2 ms and the duty cycle was 2%. The flat input mirror of the laser cavity had 90% transmission at 970 nm and 99.8% reflectivity at 1.5–1.6 μm. For the hemispherical cavity, three output couplers with a fixed 100 mm radius of curvature (RoC) and different transmissions (1.0%, 1.5%, and 2.9%) at 1.5–1.6 μm were used. In order to reduce the cavity loss, the length of the hemispherical cavity was set near to the RoC of the output couplers. For the plano-plano cavity, three flat output couplers with different transmissions (0.8%, 2.8%, and 5.3%) at 1.5–1.6 μm were used. The cavity length was kept at about 5 mm. The reflectivities of all the output couplers at 970 nm were higher than 98%. It can be seen from Fig. 1 that the σ-polarized absorption coefficient of the crystal is about 29 cm⁻¹ at the pump wavelength of 970 nm. Then, about 85% of the incident pump power was absorbed by the 0.7-mm-thick, c-cut sample. The laser spectrum was recorded with a monochromator (Triax550, Jobin-Yvon) associated with a TE-cooled Ge detector (DSS-G025T, Jobin-Yvon). The resolution of the recorded spectrum was about 0.02 nm.

 

Fig. 1 Room temperature polarized absorption spectra of Er³⁺:Yb³⁺:LuAB crystal in a range from 900 to 1050 nm

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4. Results and discussion

Figure 4 shows the output peak power of the Er³⁺:Yb³⁺:LuAB laser as a function of the absorbed pump peak power at 970 nm for the hemispherical cavity. Because the duty cycle of the quasi-CW laser was 2%, the values in the figure are the measured average ones multiplied by fifty. For the low output coupler transmissions (1.0% and 1.5%), the output laser wavelengths were located around 1598 nm in the range of the absorbed pump powers used in this work. The laser spectrum recorded at the highest absorbed pump peak power is also shown in the figure. For the output coupler transmission of 1.5%, the maximum output peak power of 2.43 W was achieved when the absorbed pump peak power was 15.7 W. The absorbed pump threshold was about 3.9 W and slope efficiency η was 23% when absorbed pump peak power was higher than 7.0 W. The lower slope efficiency near the threshold is caused by the quasi-three-level nature of Er³⁺ laser operating at 1.5–1.6 μm, in which the population of the lower laser level acts as a saturable loss and decreases with the increment of the fundamental laser intensity in the cavity [20]. Compared with those (maximum output peak power of 2.0 W, threshold of 4.7 W, and slope efficiency of 21%) of Er³⁺:Yb³⁺:YAB laser obtained in the similar experimental condition [11], the higher maximum output peak power, lower threshold, and higher slope efficiency were achieved in the Er³⁺:Yb³⁺:LuAB crystal. The improvement of the laser performance may be caused by the higher optical quality of Er³⁺:Yb³⁺:LuAB crystal, which is originated from the less lattice distortion between dopant and Lu³⁺ ions.

 

Fig. 4 Output peak power of the Er³⁺:Yb³⁺:LuAB laser as a function of absorbed pump peak power at 970 nm in a hemispherical cavity. The laser spectra at the highest absorbed pump peak powers are also shown.

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As also shown in Fig. 4, when the output coupler transmission increased to 2.9% in the hemispherical cavity, the output laser wavelength was blue-shifted to 1541 nm, which is another gain peak of the Er³⁺:Yb³⁺:LuAB crystal (see Fig. 3). The dependence of output laser wavelength of the crystal on the output coupler transmission is caused by variation of the intracavity passive losses and the thermal effect of the crystal, which is typical for three-level laser [13,21]. For this output coupler, the maximum output peak power decreased to 1.57 W when the absorbed pump peak power was 15.7 W. The absorbed pump threshold increased to about 6.6 W and slope efficiency was 18% when absorbed pump peak power was higher than 7.0 W.

By replacing the concave mirrors with flat output couplers and shortening the cavity length from about 100 to 5 mm, the laser properties of the Er³⁺:Yb³⁺:LuAB crystal in a plano-plano cavity were investigated. As shown in Fig. 5 , the output laser wavelengths were about 1598 and 1541 nm in the range of the absorbed pump powers in the experiment, respectively, for the output couplers with 0.8% and 5.3% transmissions. Compared with those recorded in the hemispherical cavity, the FWHM of each longitudinal mode became narrower. For the output coupler transmission of 0.8%, the maximum output peak power of 1.6 W was achieved when the absorbed pump peak power was 15.7 W. The absorbed pump threshold was about 4.0 W and slope efficiency was 17% when absorbed pump peak power was higher than 9.0 W, whereas the corresponding values for the similar output coupler transmission (1.0%) obtained in the hemispherical cavity were 2.35 W, 2.8 W and 21%, respectively (see Fig. 4). For the output coupler transmission of 5.3%, the 1541 nm laser with maximum output peak power of 1.5 W was achieved when the absorbed pump peak power was 15.7 W. The absorbed pump threshold was about 6.9 W and slope efficiency was 20% when absorbed pump peak power was higher than 9.0 W.

 

Fig. 5 Output peak power of the Er³⁺:Yb³⁺:LuAB laser as a function of absorbed pump peak power at 970 nm for the low and high output coupler transmissions (0.8% and 5.3%) in a plano-plano cavity. The laser spectra at the highest absorbed pump peak powers are also shown.

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For the medium output coupler transmission of 2.8% in the plano-plano cavity, laser oscillations at dual wavelengths of about 1544 and 1556 nm were always observed in the range of the absorbed pump powers in the experiment, as shown in Fig. 6 . At the low absorbed pump power, the number of the oscillating longitudinal modes reduced. Because the frequency difference between these two mode bands is close to 1.5 THz, the Er³⁺:Yb³⁺:LuAB crystal may be expected as a promising laser medium for generating the terahertz wave [22,23], which is interest for a variety of applications in basic and applied physics, communication, and life science, etc. When the absorbed pump peak power was 15.7 W, the maximum output peak power was 1.7 W. The absorbed pump threshold was about 6.0 W and slope efficiency was 20% when absorbed pump peak power was higher than 9.0 W.

 

Fig. 6 Output peak power of the Er³⁺:Yb³⁺:LuAB laser as a function of absorbed pump peak power at 970 nm for a medium coupler transmission of 2.8% in a plano-plano cavity. The laser spectra at different absorbed pump peak powers are also shown.

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The laser experimental results of diode-pumped Er³⁺:Yb³⁺:LuAB crystal in different cavities are summarized in Table 1 . Similar to those of Er³⁺:Yb³⁺:YAB and Er³⁺:Yb³⁺:GAB crystals [10,12], 1580 and 1520 nm are also the gain peaks of the Er³⁺:Yb³⁺:LuAB crystal, as shown in Fig. 3. However, the laser oscillations at 1580 and 1520 nm, which have been realized in the Er³⁺:Yb³⁺:YAB and Er³⁺:Yb³⁺:GAB crystals [11–14], were not observed in the Er³⁺:Yb³⁺:LuAB crystal for the present experimental conditions. This phenomenon may be explained by the difference of thermal effect of the crystals originated from the difference of the laser experimental conditions and the thermal conductivity between these crystals. Because the thermal load in the crystal induced by the pump power can change the Boltzmann populations of the crystal field levels in the ⁴I_13/2 and ⁴I_15/2 multiplets and then the gain characteristics at different wavelengths [14,24], the oscillating wavelength of Er-Yb laser is strongly affected by the thermal effect of the gain medium. By using a polarizing device, the ratios of output powers of Er³⁺:Yb³⁺:LuAB crystal between horizontal and vertical polarizations were measured to be close to unity for all the output coupler transmissions in the hemispherical and plano-plano cavities and in the range of the absorbed pump powers in the experiment. Therefore, the output laser beams were unpolarized for the c-cut Er³⁺:Yb³⁺:LuAB uniaxial crystal because of the isotropic nature of the plane perpendicular to the optical axis c and the uniform dissipation of the generating heat in this plane. At the same time, the output beams were observed to be circular symmetry by naked eyes in an upconversion fluorescence card. Furthermore, when the pump wavelength is changed from 970 to 976 nm, at which the absorption coefficient of the crystal is larger (see Fig. 1), the laser performances of the Er³⁺:Yb³⁺:LuAB crystal can be further improved, because for absorbing the same pump power, the crystal can be thinned about 36% and then the internal loss of the sample will be decreased.

Table 1. Laser experimental results of Er³⁺:Yb³⁺:LuAB crystal in different cavities

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5. Conclusion

An Er³⁺:Yb³⁺:LuAB crystal doped with 24.1 at.% Yb³⁺ and 1.1 at.% Er³⁺ ions was grown by the flux method. The investigations show that the spectroscopic properties of the Er³⁺:Yb³⁺:LuAB crystal are similar to those of the Er³⁺:Yb³⁺:YAB crystal with the similar dopant concentrations. Diode-pumped laser properties of a 0.7-mm-thick, c-cut crystal were investigated in the hemispherical and plano-plano cavities, respectively. Laser oscillations at the different wavelengths from 1541 to 1598 nm with output peak powers of 1.5-2.43 W and slope efficiencies of 17-23% were realized. Compared with that of Er³⁺:Yb³⁺:YAl₃(BO₃)₄ crystal obtained at the similar experimental condition, better laser performance was achieved in the Er³⁺:Yb³⁺:LuAB crystal, which may be partly attributed to the higher optical quality of the crystal originated from the less lattice distortion between dopant and Lu³⁺ ions. Furthermore, simultaneous laser oscillations at dual wavelengths of about 1544 and 1556 nm were also observed in the Er³⁺:Yb³⁺:LuAB crystal at a specific experimental condition. Therefore, the Er³⁺:Yb³⁺:LuAB crystal may be expected as a promising gain medium of the solid-state 1.5–1.6 μm and terahertz wave lasers.