1. Introduction

Yb³⁺ ion has attracted much attention because it has only two multiplets, i.e. ²F_7/2 ground state and ²F_5/2 excited state, with a separation of about 10000 cm⁻¹. As a result, Yb³⁺-doped laser materials have some advantages [1], such as no upconversion losses and weak concentration dependent fluorescence quenching.

The bismuth silicate Bi₄Si₃O₁₂ (BSO) crystal belongs to cubic symmetry with space group I-43d and has been investigated as an alternative of bismuth germinate (Bi₄Ge₃O₁₂) scintillator applied in high-energy physics [2, 3]. Compared with other silicate crystals, such as Gd₂SiO₅ (GSO) [4], Y₂SiO₅ (YSO) [5] and SrY₄(SiO₄)₃O (SYS) [6], BSO crystal is isotropic and has a lower melt point of about 1025 °C, which reduce the difficult of crystal growth and orientation. Furthermore, rare earth doped BSO crystal has also been investigated as a gain medium of self stimulated Raman scattering laser [2]. An Yb³⁺:BSO single crystal with Yb³⁺ concentration of 0.7 at% has been grown by the modified vertical Bridgman method and some spectroscopic properties have been studied [7]. However, to our knowledge, laser performance of the Yb³⁺:BSO crystal has not been reported till now.

In this work, an Yb³⁺:BSO single crystal with high Yb³⁺ doped concentration was grown successfully by the Czochralski method. The spectroscopic properties and diode-pumped continuous-wave (cw) laser performances of the Yb:BSO crystal were investigated in detail.

2. Spectral properties

Yb³⁺:BSO single crystal was grown by the Czochralski method. During the growth, the pulling rate was 0.5-1.5 mm/h and the rotating rate was 10-20 rpm. A transparent and crackless sample was cut from the grown crystal for spectral experiments, as shown in the inset of Fig. 1. A ZYGO GPI optical interferometer was used to examine the optical quality of the crystal. As shown in Fig. 1, the interference fringes of the Yb³⁺:BSO crystal had obvious wavefront distortions, which indicates that the optical quality of the crystal needs further improvement. In order to obtain accurate Yb³⁺ doping concentration, the part close to the crystal sample for the spectral measurement was used. Considering the relative error of the instrument, the Yb³⁺ concentration in the crystal for spectral analysis was measured to be about (5.7 ± 0.1) at.%, i.e. (8.35 ± 0.15) × 10²⁰ ion/cm³, by inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima2, Jobin-Yvon) and Yb³⁺ ions occupy Bi³⁺ sites with a C₃ local symmetry in the crystal [2]. Room temperature absorption spectrum in a range from 850 to 1150 nm was measured by an UV-VIS spectrophotometer (Lambda-900, Perkin-Elmer) and is shown in Fig. 2(a). Excited at 970 nm, fluorescence spectrum between 850 and 1150 nm was recorded by a monochromator (Triax550, Jobin-Yvon) equipped with a cooled PbS detector (DSS-PS020T, Jobin-Yvon) at room temperature and is shown in Fig. 2(b).

 

Fig. 1 Interference fringes of Yb:BSO crystal sample.

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Fig. 2 Room temperature absorption cross-section spectrum (a) and fluorescence spectrum (b) of Yb:BSO crystal. The energy level diagram of Yb³⁺ in BSO crystal is shown at the top right.

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As shown in Fig. 2(a), there are three main absorption peaks centered at 908, 958, and 976 nm, respectively, which are associated with the transitions from the lowest level of ground multiplet ²F_7/2 to the levels of excited multiplet ²F_5/2. As depicted in Fig. 2(b), a Lorentz fitting was applied to the fluorescence spectrum and four peaks were picked at 976, 1008, 1033 and 1063 nm, which are attributed to the transitions from the lowest level of excited multiplet ²F_5/2 to the levels of ground multiplet ²F_7/2. Then, the level positions of Yb³⁺ ions in BSO crystal were obtained and the level diagram is also shown in Fig. 2(a). Furthermore, some other peaks were confirmed in both absorption and fluorescence spectra, which proved the correctness of the obtained level positions. A large splitting of the ground multiplet ²F_7/2 can reduce the thermal population of the terminal laser level, which is favorable for a quasi-three-level laser operation [8]. The splitting of ground multiplet ²F_7/2 of Yb³⁺ ions in BSO crystal is 839 cm⁻¹ and larger than those in YAG (783 cm⁻¹) and SYS (810 cm⁻¹), but smaller than those in YSO (964 cm⁻¹) and GSO (1067 cm⁻¹) [8, 9], which means the re-absorption loss of Yb:BSO may be between those of Yb:YAG and Yb:GSO.

As shown in Fig. 2, the peak absorption cross-section σ_abs at 976 nm is (0.98 ± 0.02) × 10⁻²⁰ cm² when the uncertainties of both the measured Yb³⁺ concentration and absorption spectrum were taken into account, and much smaller than the 2.92 × 10⁻²⁰ cm² reported previously for a 0.7 at.% Yb:BSO crystal [7]. The difference may be caused by the baseline of the spectrum and the low signal-noise ratio for the low doping concentration and the thin sample in [7]. The stimulated emission cross-section σ_em of the Yb:BSO crystal was calculated by the reciprocity method (RM) [10] and is shown in Fig. 3. After considering the uncertainties of the absorption cross-section, the value of σ_em at 1035 nm is (0.57 ± 0.07) × 10⁻²⁰ cm². Based on the absorption and emission cross-sections, the gain cross-section σ_g versus different population inversion ratio β, i.e. the ratio of the number of Yb³⁺ ions in the upper laser level to the total number of Yb³⁺ ions, can be calculated by ${\sigma }_g\left(\lambda \right)=\beta {\sigma }_{em}\left(\lambda \right)-\left(1-\beta \right){\sigma }_{abs}\left(\lambda \right)$ and is shown in the inset of Fig. 3.

 

Fig. 3 Room temperature emission cross-section spectrum of Yb:BSO crystal. The inset shows the gain curves of the Yb:BSO crystal at different population inversion ratio β.

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In order to weaken the influence of radiation trapping effect [11, 12], powders of the Yb:BSO crystal immersed in refractive index-matching fluid monochlorobenzene (refractive index n = 1.5240) were used to measure the fluorescence decay curve [13]. In principle, fluid with a refractive index similar to the n = 1.96 of BSO crystal should be used, but the fluid with higher refractive index is toxic and difficult to be found. Fluorescence decay curve at 1035 nm was recorded using a spectrophotometer (FL980, Edinburgh) when a microsecond flashlamp (μF900, Edinburgh) was used as the exciting source and the exciting wavelength was 976 nm. As shown in Fig. 4, the measured fluorescence decay curve has a single exponential behavior and the fitted lifetime τ_f is about (1.26 ± 0.01) ms. Because the refractive index of the matching fluid is less than that of the BSO crystal, the radiation trapping effect cannot be eliminated completely and the measured lifetime would be longer than the true one. The more severe radiation trapping effect caused by higher Yb³⁺ concentration in this work is also the major reason for the lifetime longer than the 1.01 ms for a 0.2 at.% Yb:BSO crystal reported previously [7].

 

Fig. 4 Room temperature fluorescence decay curves of the Yb:BSO powders at 1035 nm under excitation at 976 nm.

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Some spectroscopic parameters of the Yb:BSO crystal and other Yb³⁺-doped laser crystals are listed in Table 1. The σ_abs of the Yb:BSO crystal at pump wavelength is larger than those of Yb:GSO and Yb:YAG, but smaller than those of Yb:SYS and Yb:YSO. The full-width at half maximum (FWHM) of absorption band around 976 nm in the Yb:BSO crystal is 2 nm, which means that more precise control for the emitting wavelength of LD pump source is necessary. Furthermore, the Yb:BSO crystal has a larger emission cross-section than those of other Yb³⁺-doped silicate crystals, and the fluorescence lifetime of the excited multiplet ²F_5/2 of Yb³⁺ in the BSO crystal is at the millisecond level and similar to those of the Yb³⁺ doped GSO and YAG crystals, which is favorable for the laser operation.

Table 1. Spectroscopic parameters of Yb:BSO crystal and other Yb³⁺-doped laser crystals.

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3. Laser experiment

An end-pumped hemispherical cavity was adopted in the laser experiment. A 5.7at.% Yb:BSO crystal with thickness of 1.5 mm was used as the gain medium. The crystal was mounted in a copper heat sink, which was cooled by water at about 20 °C. All the faces of the crystal were contacted with the copper and there is a hole with radius of about 1 mm in the center of heat sink to permit the passing of the laser beams. The absorption coefficient at 976 nm of the Yb:BSO crystal is 8.1 cm⁻¹, resulting in about 70% absorption of the incident pump power in a single pass of the sample. A 976 nm diode laser coupled by a fiber with 100 μm diameter core was used as the pump source and the linewidth of the diode laser is about 3 nm. After passing a simple telescopic lens system, pump beam was focused to a spot with waist radius of about 54 μm in the crystal. Flat input mirror had 90% transmission at 976 nm and 99.8% reflectivity at 1000-1100 nm. Three output couplers with the same radius curvature of 50 mm and different transmissions (1.6%, 2.6% and 4.8%) at 1000-1100nm were used. The cavity length was kept at about 50 mm.

Figure 5 shows the measured continuous-wave laser output power versus the incident pump power. For the output coupler with transmission of 1.6%, the incident pump threshold was 1.6 W, and the maximum output power was 180 mW when the incident pump power was 3.26 W. For the output coupler with transmission of 4.8%, the incident pump threshold, maximum output power and slope efficiency were 2.06 W, 240 mW and 27%, respectively.

 

Fig. 5 Continuous-wave output power of the Yb:BSO laser as a function of incident pump power.

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The output laser spectrum at various output coupler transmissions were recorded with the monochromator (Triax550, Jobin-Yvon) equipped with a TE-cooled Ge detector (DSS-G025T, Jobin-Yvon) and are shown in Fig. 6. A dual-wavelength laser oscillation centered at about 1040 and 1070 nm was observed when the output coupler transmission was 1.6%. For the 2.6% and 4.8% output couplers, the laser wavelength is about 1038 nm. The blue-shift of output laser with the increment of output coupler transmission is originated from the variation of the gain cross-section of the crystal with the inversion ions density (or intracavity loss). The higher intracavity loss implies that larger population inversion ratio of Yb³⁺ ions, i.e. higher gain in the cavity, is required for achieving laser oscillation. As shown in the inset of Fig. 3, for a small population inversion ratio β, the gain cross-section around 1040 nm is comparable to that around 1070 nm, which offers the possibility of achieving dual-wavelength laser oscillation. However, for a larger β, the gain cross-section around 1038 nm is much larger than that around 1070 nm, which leads to the disappearance of the laser oscillation around 1070 nm.

 

Fig. 6 Spectra of the Yb:BSO laser when the incident pump power was 3.26 W and output coupler transmissions were:(a)T = 1.6%, (b)T = 2.6%, (c)T = 4.8%.

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At present, the laser performance obtained in the Yb:BSO crystal is inferior to those of well-known silicate crystals, such as the slope efficiency of Yb:YSO crystal is 53% [5] and the slope efficiency of Yb:GSO crystal with respect to the absorbed pump power reaches 86% [4]. However, a lower growth temperature and the isotropic property make the Yb:BSO crystal attractive as a laser gain medium. For its narrow absorption bandwidth around 976 nm, a novel pump source, such as the narrowband distributed Bragg reflector tapered diode lasers (DBR-TDL) with a narrow spectral linewidth of less than 13 pm (FWHM) [16], should be used rather than the diode laser with a linewidth of 3 nm used presently in this work. In addition, the optimization of the concentration of Yb³⁺ ions and crystal optical quality may improve the laser performance of the Yb:BSO crystal in the future.

4. Conclusion

An Yb:BSO single crystal with Yb³⁺ concentration of 5.7 at% was grown successfully by the Czochralski method. Compared with other Yb³⁺-doped silicate crystals, Yb:BSO crystal has a larger emission cross-section. Diode-pumped continuous-wave laser operation of the Yb:BSO crystal was demonstrated. For a output coupler with transmission of 4.8%, a laser at about 1038 nm with a slope efficiency of about 27% and maximum output power of 240mW was realized. The results indicate that the Yb:BSO crystal is one kind of promising laser materials.