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Abstract
We report on the crystal growth, spectroscopic properties and laser performance of Tm3+-doped Bi4Si3O12 (BSO) crystal. The crystal was grown by the vertical Bridgeman method. The spectroscopic properties are investigated based on absorption and luminescence spectroscopy. Judd - Ofelt (JO) analysis is performed to calculate the spontaneous emission probabilities, branching ratio and the radiative lifetimes. The absorption spectrum, emission spectrum and gain cross-section spectra of Tm3+: BSO crystal are determined for the 2 μm transition. Luminescence decay kinetic of 3F4 upper level was analysed in detail. The continuous-wave 2 μm laser with a maximum output power of 650 mW and a slope efficiency of 29.7% is demonstrated for the first time. The beam quality factor (M2) of Tm3+: BSO laser was about 1.03 at the maximum output level.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrafast solid-state lasers operating at about 2 μm are of great interesting due to their variety of applications in medicine, military, optical communication and as a pump source for OPO’s [1–3]. Tm3+- doped crystals are attractive candidate materials in this wavelength range via 3F4 → 3H6 transition. The conventional pumping scheme of Tm3+-doped solid-state lasers is diode pumping at wavelengths around 0.8 μm based on the 3H6 → 3H4 transition of Tm3+ ion. And the effective cross-relaxation cooperative mechanism for the adjacent Tm3+ ions leads to high quantum efficiency with high Tm3+-doping concentration. However, a high quantum defect and nonradiative relaxation accompany with high doping concentration, which may limit the laser output power and beam quality. Direct in-band pumping to the upper laser level 3F4 at around 1.6 μm is an anther attractive scheme for Tm3+-doped lasers. In this pumping scheme, high activator concentration is not necessary. Thus, it can avoid the drawbacks mentioned above and realize high power and highly efficient laser output in Tm-doped lasers. Recently, in-band pumped Tm3+ solid state lasers in the range around 2 μm have been demonstrated [4–8].
Bismuth silicate (Bi4Si3O12, BSO) is derived from the Bi2O3 - SiO2 system. It crystallizes in a eulytine structure with space group I-43d and a = 10.2867(5) Å, V=1088.5 Å3, Z = 4 [9]. The Bi3+ site is coordinated by a distorted octahedron of oxygens. This distortion may be attributed to the stereochemical activity of the Bismuth (III) 6s-orbita lone pair. This certain distorted structure gives rise to inhomogeneous spectral lines broadening, and the broad emission band is particularly appealing for the generation of ultrashort laser pulse. Since trivalent rare earths enter the BSO lattice substitutionally at Bi3+ sites, it has been considered and used as a laser host crystal [10–15]. The thermal conductivity of pure BSO is 3.29 W·m-1K-1 at 298 K [16], which is larger than 2.59 W·m-1K-1 for Bi4Ge3O12 [17], and similar to that of Lu2SiO5 (3.67 W·m-1K-1) at 298 K [18]. Continuous-wave (CW) and passively mode-locked laser operation at 1 µm for Yb3+: BSO crystal has been demonstrated [19,20]. However, laser action based on 3F4 → 3H6 Tm3+ transition in BSO host in the 2 μm wavelength region has not been reported up to now. In this paper, we have grown successfully the Tm3+: BSO single crystals by the modified vertical Bridgman method. The crystalline quality was evaluated through rocking curve measurements. The spectroscopic properties of Tm3+: BSO crystal were detailedly investigated. The spectroscopic parameters of Tm3+ ions in BSO crystal were calculated based on Judd - Ofelt (JO) theory. Furthermore, the radiative transition probabilities, fluorescence branching ratio, and radiative lifetime were also estimated. Investigation of laser properties of the crystal was carried out. Fluorescence decay curve of the 3F4 level was measured and the related energy transfer mechanism between Tm3+ ions in the crystal was discussed in detail. Finally, a room-temperature CW laser operation at 2 μm based on Tm3+: BSO crystal in-band pumped by Er: YAG laser at 1617 nm has been realized.
2. Crystal growth
The BSO crystal doped with 2 at. % Tm was grown using the vertical Bridgeman method. The raw materials were commercially available Tm2O3, Bi2O3 and SiO2 powders with purity of 4N. These raw materials were blended, ground and pressed into pellets, and then calcined in a resistance furnace at 800°C for 10 h. The charge was put into the cylindrical platinum crucible with a seed pocket. Tm-doped BSO crystal was grown along the [001] crystallographic axis using a undoped BSO seed crystal with dimension Φ5 × 30 mm2. The growth was carried out in a self-made resistive heating furnace with MoSi2 heating elements, and the temperature distribution in the furnace was divided into three regions: the high-temperature zone, gradient zone, and low-temperature zone three zones. In high-temperature zone, the charge melted at 30 °C above the melting point of BSO crystal and the solution was homogenized for 20 h. The crucible was then lowered into the gradient zone for crystal growth. A temperature gradient of about 10 - 25 °C/cm was maintained at the growth interface, and the pulling-down rate was about 0.5 - 1 mm/day. After growth was complete, the as-grown crystal was in situ annealed. The as-grown Tm3+: BSO crystal was kept at 900 °C for 6 h in the annealing process, and then was cooled down to room temperature with a rate of 5 - 15 °C/h. Figure 1 shows as-grown crystal with dimension of Φ25×100 mm2 (a) and the polished boule (b) of Tm3+: Bi4Si3O12. The translucent surface of the crystal is due to the segregation phases which is mainly composed of Bi12SiO20, SiO2 and Bi2O3 [21]. Bi12SiO20 is formed by a peritectic reaction L + SiO2 → Bi12SiO20 at 1030 °C. These segregated phased can be restrained by optimized growth procedure, such as low growth rate, Bi2O3-rich melting and suitable temperature gradient [22]. The samples cut from the top, middle and bottom parts of as-grown crystal were ground and mixed for the measurement of concentrations of Tm3+ in the crystal and X-ray powder diffraction (XRD). The content of Tm3+ in BSO crystal was measured by ICP-OES method (Ultima2, Jobin Yvon Horiba). The actual Tm concentration measured is 0.82%. Taking into account cell volume of BSO crystal (V=1088.5 Å3), the Tm density in the crystal used in this work is 1.21 × 1020 cm-3. The initial concentrations of Tm in melt were 2.0 at%. Thus, the effective segregation coefficient of Tm in BSO crystal can be obtained by well-known formula Keff = [Tm]crystal/[Tm]melt to be 0.41. The relative different in ionic radii between thulium ions and host component bismuth ions (rTm3+ = 1.02 Å, rBi3+ = 1.17 Å) is responsible for such low segregation coefficient.
The XRD pattern of the as-grown Tm3+: BSO crystal was determined using using a TD-3500 X-ray power diffractometer, as shown in Fig. 2 (a). The diffraction peaks were indexed by Fullpro program. It can be seen from Fig. 2 (a) that all the diffraction peaks match well with those of pure BSO (PDF#33-0215), and no diffraction peak corresponding to second phase was observed. It is also indicated that the compound indeed crystallizes in the cubic system with lattice parameters: a = 10.2894(7) Å, Z = 4. In order to estimate the crystalline quality of the as-grown crystal, rocking curve measurements were performed using a high-resolution X-ray diffractometer (X’ Pert Pro, Philips). The rocking curve of (004) crystallographic plane was recorded at room temperature and shown in Fig. 2. As seen in the figure, the diffraction peak of (004) is sharp and symmetric without any shoulder peaks. The FWHM value of (004) plane of the crystal has been found as 19″. These results indicate that the as-grown Tm3+: BSO crystal has good crystal quality and excellent structural integrality.
Fig. 2. XRD pattern of Tm3+: BSO crystal (a) and X-ray rocking curve for the (004) Tm3+: BSO plane (b)
3. Spectroscopic characterization
For measuring the spectral properties of the Tm3+: BSO crystal, the samples with the size of 10 mm × 10 mm × 6.1 mm were cut parallel to the (001) planes of an as-grown crystal. The absorption spectrum of Tm3+: BSO crystal was measured between 300 and 2200 nm by a Agilent Cary 5000 spectrophotometer with a resolution of 1 nm. The fluorescence spectra were recorded by Edinburgh Instruments FLS920 spectrophotometer with an external tunable Ti: sapphire laser (Model 3900s, Spectra-Physics) as the excitation source. A liquid nitrogen cooled InAs detector (Hamamatsu P7165) was used to measure the mid-infrared fluorescence spectra for the transition of 3F4 → 3H6 around 1.9 μm. The resolution of the fluorescence spectrum was 1 nm in the range 1550 - 2300 nm. The fluorescence decay curve at 1.9 μm was measured by using an Edinburgh FLSP 920-C spectrophotometer detector equipped with a tunable mid-band OPO pulse laser as the excitation source (Vibrant 355II, OPOTEK). The fluorescence decay signal was detected with an InGaAs detector.
3.1 Optical absorption and Judd-Ofelt analysis
The absorption spectrum of Tm3+: BSO crystal in the range of 300 - 2200 nm was shown in Fig. 3 (a). It can be seen from Fig. 3 (a) that six absorption bands located at about 357, 474, 679, 790, 1205, and 1727nm have been observed, which correspond to the transitions from the 3H6 ground state to the excited multiplets of Tm3+ ion. The absorption spectrum for 3H6 → 3F4 transition was measured with a resolution of 0.5 nm. The absorption cross-sections, which is calculated using σabs = α/[Tm] based on the absorption spectrum, are shown in Fig. 3 (b). The absorption band related to 3H6 → 3F4 transition is quite broad and covers a wavelength range from 1530 to 2000nm. The absorption cross-section at 1617 nm is about 1.27 × 10−20 cm2 and the full widths at half-maximum (FWHM) of this absorption peak is about 10 nm. The broad absorption band is suitable for commercially available pump sources in the 1.6 - 1.7 μm, such as Yb, Er: YAG laser, Tm-doped fiber laser, Yb, Er-doped fiber laser.
Fig. 3. Absorption spectrum of Tm3+: BSO crystal (a) and absorption cross-section spectra for the 3H6 → 3F4 transition (b).
The standard JO analysis [23,24] was performed based on the absorption spectrum to investigate the radiative transition properties of Tm3+ ions in BSO crystal. In JO analysis, the contribution of magnetic-dipole transition has been taken into account. The reduced matrix elements of unit tensor operators used in the calculations were taken from Ref. [25]. The refractive index of Tm3+: BSO crystal was taken as 2.06 [26]. The experimental and calculate oscillator strength (fexp and fcal), and the experimental and calculate line strength (Sexp and Scalc) are given in Table 1. The root-mean-square deviation (RMS) and RMS error between the experimental and calculated values have been examined and present in Table 1. The small value of RMS deviation indicates good reliability of the calculations. By using a least-square fitting of experimental and calculated absorption oscillator strengths, the JO intensity parameters were determined to be Ω2 = 2.1203 × 10−20 cm2, Ω4 = 0.5488 × 10−20 cm2, and Ω6 = 0.6134 × 10−20 cm2 with root mean square (RMS) of 0.828 × 10−21 cm2. Thus, the spontaneous emission probabilities, the fluorescence branching ratios and the radiative lifetimes were determined using the JO intensity parameters, shown in Table 2.
Table 1. The experimental and calculated strengths parameters for Tm3+: BSO crystal.a
Table 2. Spontaneous emission probabilities, the fluorescence branching ratios and the radiative lifetimes of Selected Transitions for Tm3+ in BSO crystal.
3.2 Fluorescence and gain cross section
The normalized fluorescence spectrum associated with the 3F4 → 3H6 transition of Tm3+ ions in BSO crystal was measured by exciting at 790 nm, as shown in Fig. 4. A wide emission band is observed in the wavelength region from 1560 to more than 2230 nm. The stimulated emission cross section of the 3F4 → 3H6 laser transition was calculated according to the Fuchtbauer - Ladenburg formula [27]:
As a quasi-three level system, the laser oscillated wavelength and tunable range of Tm-ion laser can be estimated by gain cross section. The gain cross section can be expressed as:
3.3 Fluorescence dynamics
The fluorescence lifetime of the upper 3F4 level of Tm3+ in Tm3+: BSO crystal was measured by exciting at 790 nm. The decay curve shows an exponential decay character, as shown in Fig. 6. Thus, the fluorescence lifetime can be obtained by an exponential fitting, which is 2.08 ms for the 0.82 at% Tm3+: BSO crystal. The fluorescence lifetime is longer than some Tm3+-doped crystals with good 1.9 μm laser performance, such as Tm3+: YVO4 [32], Tm3+: Y2SiO5 [33] and Tm3+: KGd(WO4)2 [34]. As is well-known that a long upper-state lifetime is favorable for energy storage. Thus, the Tm3+: BSO crystal with a longer upper state lifetime will making it suitable for a continuously pumped Q-switched laser with high pulse energy. Based on the radiative lifetime τrad for the 3F4 level obtained by JO theory analysis, the fluorescence quantum efficiency (η = τf /τrad) of the 3F4 multiplet for the 0.82 at. % Tm3+: BSO crystal is about 65.4%, which compares to a fluorescence quantum efficiency of 63% in the 0.5 at% Tm: LuAG crystal [35].
The measured lifetime of 3F4 level for Tm3+: BSO is shorter than the calculated lifetime, which implies the existence of the nonradiative transition processes in the Tm3+: BSO crystal. Due to large energy gap between the 3F4 and 3H6 multiplets (about 6000 cm-1) as well as maximum phonon energy of the BSO crystal (991 cm-1) [36], it may be assumed that the multiphonon relaxation from the 3F4 upper level is negligible. At relatively low Tm3+ doping concentration (0.82 at%), the interaction between Tm3+ ions is very weak. The 3F4 level is the first excited state, and it is difficulty to quench by cross relaxation processes between other levels. Thus, the reduction of the fluorescence lifetime of 3F4 multiplets in the 0.82 at. % Tm3+: BSO crystal stems mainly from quenching by impurities or structural defects. Moreover, the disagreement between theory and experiment lifetime can also be caused by an intrinsic error of the Judd - Ofelt theory.
4. Laser performance
The laser schematic diagram is shown in Fig. 7. A 10W diode-pumped Er: YAG laser with an emission wavelength of 1617 nm and a beam quality parameter of M2 ∼1.1 was utilized as a pump source. The Tm3+: BSO crystal was 20 mm in length and 3 × 3 mm2 in cross section, and antireflection coated for the 1617 nm and 1900nm. The pump radius was about 0.18 mm in the middle of laser crystal, resulting in a pump Rayleigh length (πωp2n/(λpMp2)) of 118 mm inside the laser crystal. To cool the Tm3+: BSO crystal, it was wrapped in indium foil and clamped in a copper crystal-holder held at a temperature of 15°C with a thermoelectric cooler. The Tm3+: BSO laser cavity consists of a flat mirror M1 with highly reflecting for 1.9 µm and high transmission for pump wavelength, and a concave output coupler M2 with a radius of curvature of 100 mm. The physical cavity length was 30 mm. A dichroic mirror M3 was used to reflect the residual pump light and transport the Tm laser radiation. With ABCD matrix method, the resonant beam radius in the middle of laser crystal was calculated to be about 0.16 mm. Moreover, the calculating result also indicates that the cavity is stable when the thermal focal length from Tm:BSO crystal is more than 11 mm.
The output power of Tm3+: BSO laser as a function of the absorbed pump power is illustrated in Fig. 8. The maximum output power of 0.65 W was achieved at 3.32 W of the incident pump power with a 2.8% transmittance output coupler. A slope efficiency of 29.7% and a pump threshold of 1.14 W were obtained relative to absorbed pump. A lower slope efficiency of 14.5% but a low pump threshold of 0.98 W was obtained with the 1.6% transmittance output coupler. With a transmissivity of 5.1% and at the same incident pump power, Tm3+: BSO laser yielded a CW output power of 0.51 W and a slope efficiency of 25.4%. Figure 9 shows the laser spectra for different output coupler with transmittances of 1.6%, 2.8%, and 5.1%. In addition, the round-trip loss in the crystal of about 2% was estimated by method in Ref. [37]. The Table 3 summarizes the reported 2-µm laser performance of in-band pumped Tm-doped materials. Compared with traditional Tm3+:YAG or Tm3+:YLF laser, the output power and slope efficiency of Tm3+:BSO laser is lower. But the pump threshold is acceptable. With improving of optical quality of crystal and optimizing of dopant parametric, the output performance of Tm3+: BSO laser could be improved. The laser emission center wavelength shifts from 1897.3 to 1921.6 nm with increasing output coupling, which is a typical of the quasi-three-level laser system. The shift of laser wavelength matches well with the behavior of the gain spectra (see Fig. 5). The relatively large shift of the laser emission wavelength provides a wide wavelength tuning range. This makes Tm3+: BSO crystal a good candidate for tunable lasers. The beam quality factor (M2) of Tm3+: BSO laser was estimated by 90/10 knife-edge method, as shown in Fig. 10. At maximum output level, the M2 value was calculated to be 1.03. The inset of Fig. 10 shows the laser beam profile of the Tm3+: BSO crystal, which indicates that the output beam has a good TEM00 transversal profile.
Fig. 10. Beam quality measurement of Tm3+: BSO laser under maximum pump power. The spatial profile has been included as an inset.
Table 3. Summary of lasing performance of in-band pumped Tm-doped materials
5. Conclusion
We reported on the crystal growth, spectral properties and first CW laser operation at 2 μm with Tm3+: BSO crystal. The absorption cross-section at 1617 nm is about 1.27 × 10−20 cm2 and the FWHM of this absorption peak is about 10 nm. The maximum emission cross section is obtained at 1885nm with a value of 0.67 × 10−20 cm2. The gain spectra of 3F4 → 3H6 laser transition were obtained and a tunable laser in a range larger than 200 nm could be expected for Tm3+: BSO laser system. A long upper laser level lifetime (2.08 ms) is desirable for a continuously pumped Q-switched laser. The maximum output power of 65 mW for the CW laser around 1.9 μm was achieved with an output transmittance of 2.8%, and the slope efficiency and a pump threshold were 29.7% and 1.14 W, respectively. At the maximum output level, the M2 value was calculated to be 1.03 for the Tm3+: BSO laser. These results indicate that the Tm3+: BSO crystal is a promising gain medium for solid state lasers operating around 2.0 μm.
Funding
National Natural Science Foundation of China (51572053, 51572175, 51802196); Natural Science Foundation of Shanghai (16ZR1435900).
Acknowledgments
This work has received funding from the Collaborative Innovation Foundation of Shanghai Institute of Technology No. XTCX2018-6.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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