1. Introduction

Er³⁺ ions laser radiation at 1.5–1.6 μm has many applications, such as range finding, optical communication, remote sensing, and medical treatment [1–5]. Yb³⁺ ions with large absorption cross-section around 980 nm, i.e. the emission wavelength of InGaAs diode laser, are generally co-doped as a sensitizer to improve the performance of the 1.5–1.6 μm laser via the efficient energy transfer from Yb³⁺ to Er³⁺ ions [1–3].

At present, Er³⁺ and Yb³⁺ co-doped borate crystals, such as YAl₃(BO₃)₄ (YAB), YCa₄O(BO₃)₃ (YCOB), and Sr₃Y₂(BO₃)₄ (SYB) crystals [2–4], have been demonstrated as a kind of the most efficient gain media for the 1.5–1.6 μm laser. However, the YAB crystal can only be obtained by the flux method with a long growing period. Although the Er:Yb:YCOB crystal can be obtained by the short-period and low-cost Czochralski method, it has small absorption cross-section (0.9 × 10⁻²⁰ cm²) and narrow full width at half the maximum (FWHM) of the absorption band (4 nm) around 976 nm [2, 6]. The narrow absorption band makes precise temperature control of the pumping diode laser necessary and the small absorption cross-section implies that a thicker medium is required for a certain doping concentration, which will increase the internal loss and reduce the laser performance. Although the Er:Yb:SYB crystal, which combines the advantages of the YAB and YCOB crystals, can be grown by the Czochralski method and has large absorption cross-section and broad absorption band around 976 nm, the segregation coefficient of Yb³⁺ ions is only 0.75 in this crystal [7]. The segregation coefficient less than 1 may be originated from the difference of ionic radii between Y³⁺ (0.900 Å) and Yb³⁺ (0.868 Å) ions [8] and the high Yb³⁺ doping concentration in the crystal. Thus, lattice distortion and defects may exist in the Er:Yb:SYB crystal after the introduction of high Yb³⁺ doping and then affect the performance of the 1.5–1.6 μm laser.

It has been reported that 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 [9–12]. This is because that the radius of Lu³⁺ ions (0.861 Å) is much closer to that of Yb³⁺ ions (0.868 Å) [8]. Therefore, the crystal Sr₃Lu₂(BO₃)₄ (SLuB) may provide preferable condition for the doping of Er³⁺ and Yb³⁺ ions. To our knowledge, polarized spectral properties of the Er:SLuB crystal has been reported in [13]. However, the detailed spectral properties of the Er:Yb:SLuB related to the 1.5–1.6 μm laser have not been reported accurately. Furthermore, laser performance of rare-earth-doped SLuB crystal has not been investigated to date. Therefore, spectral properties and quasi-cw laser performance of the Er:Yb:SLuB crystal are reported in this paper.

2. Spectral properties

An Er:Yb:SLuB crystal with good optical quality, shown in Fig. 1, was grown by the Czochralski method. The Er³⁺ and Yb³⁺ concentrations in the grown crystal were determined to be 0.81 at.% (6.3 × 10¹⁹ cm⁻³) and 24.2 at.% (19.2 × 10²⁰ cm⁻³), respectively, by inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima2, Jobin-Yvon). Then the segregation coefficients of the Er³⁺ and Yb³⁺ ions in the crystal were calculated to be 1.13 and 0.97, respectively, after the initial 0.72 at.% Er³⁺ and 25.0 at.% Yb³⁺ ions in the melt were taken into account.

 

Fig. 1 Photograph of the grown Er:Yb:SLuB crystal. Inset shows the polished sample used in the laser experiment.

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Room temperature (RT) polarized absorption spectra recorded with a spectrophotometer (Lambda 900, Perkin-Elmer) in a range from 280−1670 nm are shown in Fig. 2.In the figure X, Y, and Z represent the three principal axes of the optical indicatrix in order of increasing refractive index in these three directions n_X<n_Y<n_Z. The absorption spectra display weak polarization dependence although the crystal is biaxial. Except the band around 977 nm consisted by the ²F_7/2→²F_5/2 transition of Yb³⁺ ions and the ⁴I_15/2→⁴I_11/2 transition of Er³⁺ ions, the other bands are related to the transitions originated from the ⁴I_15/2 ground level to different excited levels of Er³⁺ ions. For the absorption band around 977 nm, the peak absorption wavelength and FWHM are 977 and 9 nm, respectively, for all the three polarizations. The peak absorption cross-sections are 1.69 × 10⁻²⁰, 1.92 × 10⁻²⁰, and 1.52 × 10⁻²⁰ cm² for E//X, E//Y, and E//Z, respectively.

 

Fig. 2 RT polarized absorption spectra of the Er:Yb:SLuB crystal in a range from 280 to 1670 nm.

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The Judd-Ofelt (J-O) theory [14, 15] is an effective method for estimating spectral parameters of rare earth ions in crystals and glasses. Since the applications of the J-O theory have been reported widely, only the calculation results about upper laser level ⁴I_13/2 are presented and the detailed calculation procedure is similar to that reported in [16]. The calculated spontaneous emission probabilities A for the ⁴I_13/2→⁴I_15/2 transition are 217.86 s⁻¹, 238.96 s⁻¹, and 212.17 s⁻¹ for E//X, E//Y, and E//Z, respectively and then the radiative lifetime of the ⁴I_13/2 level can be calculated to be 4.48 ms.

Under excitation at 930 nm, i.e. exciting the Yb³⁺ ions to the ²F_5/2 level, RT polarized fluorescence spectra in a range from 940 to 1670 nm were measured using a spectrometer (FLS920, Edinburgh). Considering the similarity of the fluorescence spectra for the three polarizations, only the spectrum for E//Y polarization is shown in the inset of Fig. 3 for brevity. It can be found that the fluorescence band around 1030 nm mainly originated from the ²F_5/2→²F_7/2 transition of Yb³⁺ ions is very weak, and the fluorescence band around 1533 nm originated from ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions is very strong. It can be concluded that most of the pump energy 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 upper laser level ⁴I_13/2 of Er³⁺ ions by the rapid multi-phonon relaxation. Polarized emission cross-section spectra, which were calculated from the recorded fluorescence spectra by the Fuchtbauer-Ladenburg (FL) method [17], are shown in Fig. 3. The peak emission cross-sections are 0.78 × 10⁻²⁰, 0.88 × 10⁻²⁰, and 0.77 × 10⁻²⁰ cm² for E//X, E//Y, and E//Z, respectively, and all located at 1533 nm.

 

Fig. 3 RT polarized emission cross-sections of the Er:Yb:SLuB crystal in a range from 1400 to 1670 nm. The inset shows the E//Y polarization emission spectrum of the Er:Yb:SLuB crystal in a range from 940 to 1670 nm.

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From the absorption and emission cross-sections, denoted as σ^abs and σ^em, respectively, a gain cross-section can be calculated by ${\sigma }^{\text{g}}(\lambda )=\beta {\sigma }^{em}(\lambda )-(1-\beta ){\sigma }^{abs}(\lambda )$, where β is the ratio of the number of Er³⁺ ions in the upper laser level ⁴I_13/2 to the total number of Er³⁺ ions. Due to the similarity of the gain curves for the three polarizations, only the curves for E//Y with different values of β and the curves for E//X and E//Y with β = 0.5, which are useful for the analysis of the output laser wavelength of a Z-cut Er:Yb:SLuB crystal used in the following experiment, are shown in Fig. 4 for clearness and comparison. The gain curve for E//Y polarization with β = 0.5 is flat with FWHM of 72 nm.

 

Fig. 4 Gain curves of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions in the Er:Yb:SLuB crystal for E//X and E//Y when β is 0.5 (upper) and for E//Y with different β (lower).

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Using a microsecond flash lamp (μF900, Edinburgh) as the exciting source, RT fluorescence decay curves at wavelengths of 1040 and 1533 nm corresponding to the ²F_5/2→²F_7/2 transition of Yb³⁺ ions and the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions were recorded by the spectrometer (FLS920, Edinburgh) when the exciting wavelength were set at 930 and 521 nm respectively. The signals were detected with an NIR PMT (R5509, Hamamatsu). To avoid the influence of re-absorption on the measured fluorescence lifetime [18], all the decay curves were recorded from the powder sample of the crystal as reported in [19] and are not shown here for brevity. The fitted fluorescence lifetime of the ⁴I_13/2 level of Er³⁺ ions in the Er:Yb:SLuB crystal is 0.67 ms. Combining with the radiative lifetime calculated above, the fluorescence quantum efficiency of the ⁴I_13/2 level is estimated to be 15%.

The efficiency of energy transfer from Yb³⁺ to Er³⁺ ions could be estimated by ${\eta }_{ET}=1-{\tau }_f/{\tau }_0$, where and are the fluorescence lifetimes of Yb³⁺ ions in the crystal co-doped with Er³⁺ ions and not, respectively [2]. The fitted fluorescence lifetime of the ²F_5/2 level of Yb³⁺ ions in the Er:Yb:SLuB crystal is 27.9 μs. The fluorescence decay curve of a 24.2 at.% Yb³⁺ doped SLuB powder sample, which was obtained by solid state synthesis, was also recorded and the fitted fluorescence lifetime is about 606.5 μs. Therefore, the efficiency of energy transfer in the Er:Yb:SLuB crystal is about 95%. The high efficiency of energy transfer from Yb³⁺ to Er³⁺ ions further demonstrates that the pump energy absorbed by Yb³⁺ ions can be efficiently transferred to the ⁴I_11/2 level of Er³⁺ ions.

For comparison, spectral parameters of the Er:Yb:SRB (R = Y, Lu) crystals are listed in Table 1. It can be found that the spectral properties of the Er:Yb:SLuB crystal are similar to those of the Er:Yb:SYB crystal. However, the higher segregation coefficients of the Er³⁺ and Yb³⁺ ions in the Er:Yb:SLuB crystal imply that this crystal with high Yb³⁺ doping may have higher optical quality. It has been reported that the spectral properties of the Er:Yb:YAB crystal are superior to those of the Er:Yb:SYB crystal and similar to those of the Er:Yb:LuAl₃(BO₃)₄ (LuAB) crystal [12, 20]. Therefore, the comprehensive spectral properties of the Er:Yb:SRB (R = Y, Lu) crystals are inferior to those of the Er:Yb:RAB (R = Y, Lu) crystals. However, it is worth noting that the longer fluorescence lifetimes and higher fluorescence quantum efficiencies of the upper laser level ⁴I_13/2 in the Er:Yb:SRB (R = Y, Lu) crystals are beneficial to realizing a lower threshold 1.5–1.6 μm laser [3, 12, 21].

Table 1. Comparison of the spectral parameters for the Er:Yb:SRB (R = Y, Lu) crystals.

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3. Laser experiments

A Z-cut 1.08-mm-thick Er:Yb:SLuB crystal was used as gain medium in the laser experiment. The uncoated sample was held in an aluminum slab. An end-pumped hemispherical laser cavity was adopted in the laser experiment. The flat input mirror has 90% transmission at 970 nm and 99.8% reflectivity at 1.5–1.6 μm. Three output couplers with the same radius curvature of 50 mm and different transmissions of 1.0%, 1.5% and 2.9% at 1.5–1.6 μm were used. The reflectivities of the three output couplers at 970 nm were higher than 98%. The cavity length was kept at about 50 mm. A 970 nm diode laser coupled by a fiber with 800 μm diameter core (FAP-980, Coherent) 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. Considering no special device was used to control the cooling of the sample, the diode laser operated in the pulse mode for reducing the influence of the pump-induced thermal load on the laser performance and avoiding possible fracture of the sample. The pump pulse duration was 2 ms and the duty cycle was 2%.

Figure 5 shows the measured fundamental laser output power as a function of the absorbed pump power for the three output couplers. Because the duty cycle of the quasi-cw pulse laser was 2%, the values in the figure are the measured ones multiplied by 50. The lowest laser threshold of about 3.48 W was achieved for the output coupler with 1.0% transmission. For this output coupler, the slope efficiency was 18% and up to 1.38 W output power was realized when the absorbed pump power was 12.19 W. When the output coupler transmission increased to 1.5%, the best laser performance was obtained. For this output coupler, the maximum output power up to 1.45 W was achieved when the absorbed pump power was 12.19 W. The absorbed pump threshold was about 3.9 W and slope efficiency was 20% when the absorbed pump power was higher than 6.22 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 [22].

 

Fig. 5 Laser output power at 1.5–1.6 μm as a function of absorbed pump power at 970 nm. The power shown in the figure was the measured value multiplied by 50, because the duty cycle of the used diode laser was 2%.

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The laser emission spectra for the three output couplers, recorded with a monochromator (Triax 550, Jobin-Yvon) when the absorbed pump power was 12.19 W, are shown in Fig. 6. All the laser beams are linear polarized with E//Y. This is due to the larger gain cross-section for E//Y at a certain β (see Fig. 4). The laser wavelengths are around 1566, 1562, and 1533 nm when the transmissions of three couplers are 1.0%, 1.5%, and 2.9%, respectively, which are agree with those estimated by the gain curves shown in Fig. 4. The blue shift of laser wavelength with the increment of output coupler transmission can be explained by the E//Y gain curves with different values of the population inversion β [23].

 

Fig. 6 Spectra of the Er:Yb:SLuB laser at 1.5–1.6 μm when the absorbed pump power is 12.19 W and output coupler transmissions are: (a) T = 1.0%, (b) T = 1.5%, (c) T = 2.9%.

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For comparison, the laser performance of the Er:Yb:SRB and Er:Yb:RAB (R = Y, Lu) crystals are listed in Table 2. Compared with those of the Er:Yb:SYB crystal obtained in the same experimental conditions including the gain medium thickness and laser cavity, lower threshold, higher output power and comparable slope efficiency were achieved in the Er:Yb:SLuB crystal. The improvement of the laser performance may be attributed to the higher optical quality of the Er:Yb:SLuB crystal. The improvement of the laser performance also exists in the Er:Yb:RAB (R = Y, Lu) crystals (see Table 2). The larger improvement of the laser performance between the Er:Yb:YAB and Er:Yb:LuAB crystals may be originated from a larger improvement of the crystal quality. Compared with those of Er:Yb:RAB (R = Y, Lu) crystals obtained in the similar experimental condition, the comprehensive laser performance of the Er:Yb:SRB (R = Y, Lu) crystals are somewhat inferior. However, compared with the Er:Yb:RAB (R = Y, Lu) crystals grown by the flux method, one advantage of the Er:Yb:SRB (R = Y, Lu) crystals for 1.5–1.6 μm laser is that they can be grown by the short-period and low-cost Czochralski method. Furthermore, a 0.75 W laser at 1.5–1.6 μm with slope efficiency of only 7% has been obtained in an Er:Sr₃Yb₂(BO₃)₄ crystal after the substitution of Yb³⁺ ions for Lu³⁺ ions [24]. Comparing the laser performance of the Er:Yb:SLuB and Er:Sr₃Yb₂(BO₃)₄ crystals with different Yb³⁺ doping concentrations indicates that there is scope to further optimize the ratio and absolute doping concentrations of the Er³⁺ and Yb³⁺ ions in the SLuB crystal. Generally, the Er³⁺ doping concentration in the Er:Yb:SRB (R = Y, Gd, and Lu) crystals should be about 0.6-0.8 × 10²⁰ cm⁻³ [3, 20]. Then, a suitable Yb³⁺ doping concentration can be determined for both absorbing pump power and energy transfer from Yb³⁺ to Er³⁺ ions effectively. The low slop efficiency obtained in the Er:Sr₃Yb₂(BO₃)₄ crystal reveals that most of the pump power absorbed by Yb³⁺ ions cannot be transferred to Er³⁺ ions when the Yb³⁺ doping concentration is too high. For the Er:Yb:SRB (R = Y, Gd and Lu) crystals, the Yb³⁺ doping concentration in the Er:Yb:SLuB crystal should be about 15-20 × 10²⁰ cm⁻³ [3, 20]. However, the optimal ratio and absolute doping concentrations of Er³⁺ and Yb³⁺ would be determined in the further work.

Table 2. Comparison of the 1.5–1.6 μm laser performance for the Er:Yb:SRB and Er:Yb:RAB (R = Y, Lu) crystals in the similar experimental condition.

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4. Conclusion

Detailed spectral properties of the 0.81 at.% Er³⁺ and 24.2 at.% Yb³⁺ co-doped SLuB crystal, grown by the Czochralski method, were investigated. The results show that the spectral properties of the Er:Yb:SLuB crystal are similar to those of the Er:Yb:SYB crystal.

End-pumped by a diode laser at 970 nm in a hemispherical cavity, 1.45 W quasi-cw laser at 1.5–1.6 μm with slope efficiency of 20% and absorbed pump threshold of 3.9 W were achieved in a 1.08-mm-thick Z-cut Er:Yb:SLuB crystal. It is worth noting that the absorption cross-section of the Er:Yb:SLuB crystal at 977 nm is nearly three times as big as that at the pump wavelength of 970 nm in this work (see Fig. 2). Therefore, if a pump source around 977 nm is used, the laser performance of a thinner Er:Yb:SLuB medium may be improved. Compared with the Er:Yb:RAB (R = Y, Lu) crystals, the larger Er:Yb:SRB (R = Y, Lu) crystals can be grown more easily by the Czochralski method. Compared with the Er:Yb:SYB crystal, the Er:Yb:SLuB crystal have similar properties, higher segregation coefficients for the Er³⁺ and Yb³⁺ ions, and better laser performance. Therefore, the Er:Yb:SLuB crystal may also be a potential gain medium for 1.5–1.6 μm laser.