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Abstract
An Er:Yb:Ba3Gd(PO4)3 crystal was grown by the Czochralski method. Spectral properties of the crystal associated with 1.5–1.6 µm laser operation were investigated, including the absorption and fluorescence spectra and the fluorescence decay at room temperature. The fluorescence quantum efficiency of the 4I13/2 level of Er3+ ions and energy transfer efficiency from Yb3+ to Er3+ ions were calculated. A CW diode-pumped Er:Yb:Ba3Gd(PO4)3 laser at 1.5–1.6 µm with output power of about 73.9 mW and slope efficiency of 16.3% was achieved for the first time in the Er3+/Yb3+ co-doped phosphate crystal.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Er3+ laser at 1.5–1.6 µm has many practical applications, such as optical communication, range finding, remote sensing, and medical treatment [1–3]. Considering that Yb3+ has large absorption cross-section around 980 nm, i.e. the emission wavelength of InGaAs diode laser, it is often 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. For Er3+ laser at 1.5–1.6 µm, a crystal with phonon energy higher than 1300 cm−1 is ascribed to a high phonon energy crystal empirically, and a crystal with phonon energy higher than 1000 cm−1 but lower than 1300 cm−1 is classified as a moderate phonon energy crystal.
Er3+/Yb3+ co-doped borate crystals, especially YAl3(BO3)4 (YAB) [3], have been demonstrated as the most efficient gain media for the 1.5–1.6 µm laser due to the short fluorescence lifetime of the 4I11/2 level of Er3+, which is originated from its high effective phonon energy (about 1400 cm−1) and makes the energy transfer from Yb3+ to Er3+ ions efficient. However, the YAB crystal can only be grown by the flux method with a long growing period. Furthermore, the short fluorescence lifetime (about 325 µs) of the upper laser level 4I13/2 of Er3+ in the crystal, also originated from the high effective phonon energy, limits its energy storage capacity [3], which is unfavorable to the realization of pulsed laser with high energy. Er3+/Yb3+ co-doped phosphate glass (PG) with moderate phonon energy (about 1300 cm−1) combines a long fluorescence lifetime (7–8 ms) of the 4I13/2 level with a short fluorescence lifetime of the 4I11/2 level (2–3 µs) [4]. However, due to the low thermal conductivity of about 0.85 Wm−1K−1, high power laser cannot be realized in the Er:Yb:PG [4]. The reported maximal continuous-wave (CW) output power did not exceed 350 mW with a slope efficiency of 27% [5]. Generally speaking, crystal has higher thermal conductivity than glass. Therefore, Er3+/Yb3+ co-doped phosphate crystals, such as Er:Yb:RPO4 (R = Lu and La) and Er:Yb:KGd(PO3)4 [6–8], have been investigated as gain media for 1.5–1.6 µm laser because they have similar phonon energy as that of PG. However, the above crystals with large size and high optical quality are difficult to be obtained by the flux method. Therefore, only the spectral properties of the above crystals have been reported.
Recently, a 1.5–1.6 µm quasi-continuous-wave (QCW) laser operation in Er:Yb:Sr3Y(PO4)3 (Er:Yb:SYP) crystal, which was grown by the Czochralski method, has been reported [9]. However, the optical quality of the Er:Yb:SYP crystal is poor, which should be further improved for realizing effective 1.5–1.6 µm CW laser. The SYP crystal belongs to the family of eulytite-type rare-earth phosphates M3R(PO4)3 (where M = alkaline-earth metal, and R = lanthanide) and the cubic system with a space group of I$\bar{4}$3d. They have attracted attention on their use as luminescence and laser materials in recent years [10–15]. For example, the luminescence properties of Eu3+:Ba3Tb(PO4)3 and Eu3+:Ba3Gd(PO4)3 phosphors have been investigated as light-emitting diodes [10,11]. The luminescence properties of Eu2+:Ba3Y(PO4)3 have been investigated as a novel yellow-emitting phosphor [12]. The spectral properties of Ba3Tb(PO4)3 single crystal have been studied for visible laser application [13]. The spectral properties of Ba3Yb(PO4)3 single crystal have been studied as a laser crystal for tunable or ultrashort pulsed lasers [14]. The thermal properties and damage threshold of Ba3Y(PO4)3 (BYP) crystal have been investigated for potential opto-electric application [15].
The thermal conductivity of the BYP crystal is 1.39 Wm−1K−1 at room temperature (RT), and reaches up to 3.27 Wm−1K−1 when the temperature is approaching 400°C, which is due to the glasslike behavior of the disordered crystal structure [15]. To enrich 1.5–1.6 µm crystal materials, the Ba3Gd(PO4)3 (BGP) host crystal was continuously explored for the following reasons. First, the BGP has moderate phonon energy as that of commercial PG. Second, both the BGP and BYP crystals belong to the family of eulytite-type rare-earth phosphates M3R(PO4)3 and so the BGP has similar thermal properties to those of the BYP crystal. Third, the pure or rare earth doped BGP crystal has not been reported to date. Therefore, the spectral properties and diode-pumped 1.5–1.6 µm CW laser performance of the Er:Yb:BGP crystal are investigated in this paper.
2. Spectral properties
An Er:Yb:BGP crystal with high optical quality was successfully grown by the Czochralski method. In the crystal growth, the raw materials of Er:Yb:BGP, synthesized by means of solid-state reaction, had been placed in an iridium crucible protected by N2 atmosphere and heated up to a temperature of about 50°C higher than the melting point for 2 h in order to melt homogeneously. The crystal was grown at a pulling rate of 0.5–1.5 mm/h and a rotating rate of 15–25 rpm. After the growth, the crystal was drawn above the surface of the melt and cooled down to the RT at a rate of about –20°C/h. The XRD pattern of the crystal was recorded by an X-ray diffractometer (Miniflex600, Rigaku) with a Cu Kα radiation (λ=0.15405 nm). The rocking curve was measured by another X-ray diffractometer (SmartLab Rigaku, 45 kV, 200 mA) equipped with a Ge(220) monochromator. Using a spectrophotometer (Lambda 950, Perkin-Elmer), RT absorption spectrum of the crystal was recorded. The Er3+ and Yb3+ concentrations were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima2, Jobin-Yvon). Using a microsecond flash lamp (µF900, Edinburgh) as the exciting source, RT fluorescence decay curves of the 2F5/2 level of Yb3+ ions at 1060 nm and the 4I13/2 level of Er3+ ions at 1537 nm were recorded by the spectrometer (FLS1000, Edinburgh) when the exciting wavelength were set at 911 and 975 nm, respectively.
The obtained Er:Yb:BGP crystal is shown in the inset of Fig. 1. The Er3+ and Yb3+ concentrations in the grown crystal, selected based on the ones of other crystals that have achieved good laser performance of 1.5–1.6 µm, were determined to be 1.85 at.% (0.65 × 1020 cm−3) and 23.95 at.% (8.40 × 1020 cm−3), respectively. Then the segregation coefficients of the Er3+ and Yb3+ ions in the crystal were calculated to be 0.92 and 0.80, respectively, after the initial 2.0 at.% Er3+ and 30.0 at.% Yb3+ ions in the melt were taken into account.
Fig. 1. RT absorption spectra of the Er:Yb:BGP crystal in a range of 250–1675 nm and the Er:BYbP crystal in a range of 250–600 nm. The inset shows the grown Er:Yb:BGP single crystal.
The X-ray diffraction (XRD) pattern of the Er:Yb:BGP crystal is shown in Fig. 2(a). It can be found that the pattern of the as-grown Er:Yb:BGP crystal matches well with that of the pure BGP on the powder standard (JCPDS, No.29-0163), which indicates that the doping of Er3+ and Yb3+ does not change the structure of the BGP crystal. The crystal quality of Er:Yb:BGP crystal was also investigated by the rocking curve, shown in Fig. 2(b). The result indicates that the as grown crystal has good quality with a FWHM of 57.6″ from the (510) reflection.
Fig. 2. (a) X-ray diffraction patterns of the grown Er:Yb:BGP crystal and standard BGP (JCPDS 29-0163). (b) Pattern of rocking curve of the Er:Yb:BGP crystal.
RT absorption spectra of the Er:Yb:BGP crystal in a range of 250–1675 nm and the Er:Ba3Yb(PO4)3 (BYbP) crystal grown by the Czochralski method in a range of 250–600 nm are shown in Fig. 1. The band around 975 nm is consisted of the transitions of 2F7/2→2F5/2 of Yb3+ ions and 4I15/2→4I11/2 of Er3+ ions. It can be found from the figure that the bands around 312 and 274 nm are consisted of the transitions of 8S7/2→6P3/2+6P5/2+6P7/2 and 8S7/2→6I7/2+6I9/2+6I11/2+6I13/2+6I15/2+6I17/2 of Gd3+ ions, respectively [16]. The other bands are related to the transitions originating from the 4I15/2 ground level to different excited levels of Er3+ ions. As particularly shown in Fig. 3(a) for the band around 975 nm, the peak absorption wavelength and cross-section are 975 nm and 0.80×10−20 cm2, respectively. Considering the smaller absorption cross-section at 975 nm and much lower doping concentration of Er3+, the contribution of Er3+ is ignored in the calculation of absorption cross-section of Yb3+. The full width at half the maximum (FWHM) for this band is 6 nm. A RT absorption cross-section spectrum of the Er:Yb:BGP crystal in a range of 1425–1675 nm is also particularly shown in Fig. 3(b). The peak absorption cross-section is 0.51×10−20 cm2 and located at 1534.5 nm.
Fig. 3. RT absorption spectrum from 850 to 1070 nm (a) and absorption and fluorescence spectra from1425 to 1675 nm (b) of the Er:Yb:BGP crystal.
The Judd-Ofelt (J-O) theory [17,18] is an effective method for estimating spectral parameters of rare earth ions in crystals and glasses. Since the application of the J-O theory has been reported widely, only the calculation results about the upper laser level 4I13/2 are presented and the detailed calculation procedure is similar to that reported in [19]. The calculated J–O intensity parameters Ωt (t = 2, 4, 6) are 3.02×10−20, 0.78×10−20, and 0.54×10−20 cm2, respectively. Then the spontaneous emission probability for the 4I13/2→4I15/2 transition and radiative lifetime ${\tau _r}$ of the 4I13/2 level can be calculated to be 98.89 s−1 and 10.11 ms, respectively.
Based on the fluorescence spectrum, the stimulated emission cross-section can be estimated according to the Fuchtbauer-Ladenburg (F-L) formula [20]:
To avoid the influence of re-absorption [21], all the decay curves were recorded from powder sample as reported in [22] and are shown in Fig. 4. The fluorescence decay curve of the 4I13/2→4I15/2 transition in the Er:Yb:BGP crystal is single exponential and the fitted fluorescence lifetime of the 4I13/2 level is 9.91 ms. Combining with the radiative lifetime ${\tau _r}$ mentioned above, the fluorescence quantum efficiency of the 4I13/2 level is close to 98%. For achieving efficient 1.5–1.6 µm laser operation, the fluorescence lifetime of the 4I11/2 level is a key factor. However, it can not be measured directly in the Er:Yb:BGP crystal due to the spectral overlap of Er3+ and Yb3+ ions around 975 nm. Therefore, a 1.46 at.% Er3+ single doped BGP crystal was also grown. The RT fluorescence decay curve of the 4I11/2 level of Er3+ ions at 980 nm was recorded by another spectrometer (FLS920, Edinburgh) when an OPO pulse laser at 521 nm was used as the exciting source. The fitted fluorescence lifetime of the 4I11/2 level is 4.89 µs.
Fig. 4. RT fluorescence decay curves of Yb3+ ions in the Er:Yb:BGP crystal (a) and Yb:BGP polycrystalline powder (b). RT fluorescence decay curves of the 4I13/2 and 4I11/2 levels of Er3+ ions in the Er:Yb:BGP (c) and Er:BGP (d) crystals, respectively.
The efficiency of energy transfer from Yb3+ to Er3+ ions can be estimated by ${\eta _{ET}} = 1 - {\tau _f}/{\tau _0}$, where ${\tau _f}$ and ${\tau _0}$ are the fluorescence lifetimes of Yb3+ ions in the crystals co-doped with Er3+ ions and singly doped, respectively [23]. The fitted fluorescence lifetime ${\tau _f}$ of the 2F5/2 level of Yb3+ ions in the Er:Yb:BGP crystal is 197.1 µs. The fitted fluorescence lifetime of a 23.95 at.% Yb3+ singly doped BGP powder sample, which was obtained by solid state synthesis, is about 1.95 ms. Therefore, the efficiency of energy transfer in the Er:Yb:BGP crystal doped with 1.85 at.% Er3+ and 23.95 at.% Yb3+ is about 90%.
For comparison, the key spectral parameters of the Er:Yb:BGP crystal and other 1.55 µm phosphate laser materials are listed in Table 1. The fluorescence lifetime of the 4I13/2 level in the Er:Yb:BGP crystal is a little longer than those of the Er:Yb:SYP crystal and Er:Yb:PG. The long lifetime of the upper laser level is favorable to achieving pulsed laser operation with high energy. The fluorescence lifetime of the 4I11/2 level in the Er:Yb:BGP crystal is short and comparable with those of the Er:Yb:SYP crystal and Er:Yb:PG. The short lifetime of the 4I11/2 is favorable to reducing back energy transfer from Er3+ to Yb3+ and up-conversion losses. The peak absorption and emission cross-sections, located at 975 and 1537 nm, respectively, are smaller than those of the Er:Yb:SYP crystal and Er:Yb:PG. This phenomenon can be explained by the difference of the odd crystal field strength between BGP and SYP crystals, which determines the electric dipole transition. With the same crystal structure, the odd crystal field strength on the doped Er3+ ions increases with a decrease in bond length generally. Because the correlation between the odd crystal field strength and bond length are Atp∝1/R(t+1) (t = 1, 3 5, and 7), where Atp are the odd crystal field strength parameters and R is the bond length between the central ion and its ligands [17,24]. Considering that the ionic radii of Ba2+ and Gd3+ are larger than those of Sr2+ and Y3+, respectively, therefore, the odd crystal field strength of the BGP is weaker than that of the SYP crystal. The same phenomenon has been observed in Nd3+:Ba3Gd2(BO3)4 and Nd3+:Sr3Y2(BO3)4 crystals [25,26]. The energy transfer efficiency in the Er:Yb:BGP crystal is comparable with those of Er:Yb:SYP crystal and Er:Yb:PG.
Table 1. Comparison of the key parameters of spectra and laser for Er3+ and Yb3+ co-doped BGP crystal and other1.5–1.6 µm phosphate laser materials
3. Laser experiments
The experimental setup is shown in Fig. 5. An end-pumped hemispherical laser cavity was adopted. A 975 nm CW diode laser coupled by a fiber with a 100 µm diameter core was used as the pumping source. A 2.65-mm-thick uncoated Er:Yb:BGP crystal was used as a gain medium in the laser experiment. The input mirror was coated on a sapphire. Then, a 1.0-mm-thick sapphire and the Er:Yb:BGP crystals were optically contacted and placed in the direction of the light. Both the sapphire and Er:Yb:BGP crystals were mounted in a copper chamber, which was cooled by water at about 20 °C. The absorption coefficient of the Er:Yb:BGP crystal at 975 nm is 6.66 cm−1; therefore, about 83% of the incident pump power is absorbed in a single pass of the crystal without lasing. After passing a simple telescopic lens system, the pumping beam was focused to a spot with radius of about 50 µm in the crystal. The flat input mirror (IM) has 89% transmission at 975 nm and 99.7% reflectivity at 1.5–1.6 µm. Three output mirrors (OMs) made of K9 glass with identical curvature radius of 100 mm and different transmissions of 0.6%, 1.7% and 3.0% at 1.5–1.6 µm were used. The cavity length was kept at about 100 mm.
Laser input-output characteristics of the Er:Yb:BGP crystal at 1.5–1.6 µm are shown in Fig. 6, where the absorption pump power was not further increased in our preliminary laser experiment to avoid crystal cracking. The lowest laser threshold of about 108.3 mW was achieved for the output mirror with 0.6% transmission. For this output mirror, the slope efficiency was 9.9% and output power was 46.4 mW when the absorbed pump power was 789.2 mW. When the output mirror transmission increased to 1.7%, the best laser performance was obtained. For this output mirror, the maximum output power up to 73.9 mW was achieved when the absorbed pump power was 789.2 mW. The absorbed pump threshold was about 136.3 mW and slope efficiency was 16.3% when the absorbed pump power was higher than 478 mW. 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 [29].
Fig. 6. Input-output characteristics of the CW diode-pumped Er:Yb:BGP laser. Output laser spectra at absorbed pump power of 789.2 mW are also shown.
As also shown in Fig. 6, output laser spectra for different output mirror transmissions were measured by a monochromator (Triax550, Jobin-Yvon) equipped with a Ge detector when the absorbed pump power was set at 789.2 mW. The output laser is centered at about 1567 nm for T = 0.6% and T = 1.7%. When the output mirror transmission increased to 3.0%, the output laser is centered at about 1542 nm. The output laser wavelengths of 1567 nm and 1542 nm were not detected simultaneously for the three output mirrors. The variation of output laser wavelength can be explained by the gain cross-section spectra calculated by $\sigma^{g}(\lambda)=\beta \sigma^{e m}(\lambda)-(1-\beta) \sigma^{a b s}(\lambda)$ [30], where β is the ratio of the number of Er3+ ions at the upper laser level 4I13/2 to the total number of Er3+ ions, σabs and σem are the absorption and emission cross-sections, respectively. The gain curves in a range from 1425 to 1675 nm are shown in Fig. 7. Because the cavity loss increases with the increasing transmission of the output mirror, a larger value of population of Er3+ ions at the upper laser level is required at higher output mirror transmission. Therefore, the blue shift of the output laser is happened with the increment of the inversion ratio β [31].
Fig. 7. Gain curve of the 4I13/2→4I15/2 transition of Er3+ ions in the Er:Yb:BGP crystal with different values of β.
For comparison, 1.5–1.6 µm CW laser performances of the Er:Yb:BGP crystal and other phosphate gain media are also listed in Table 1. Compared with the Er:Yb:SYP crystal, lower threshold, comparable slope efficiency and CW laser operation were achieved in the Er:Yb:BGP crystal. However, the obtained laser performance in the Er:Yb:BGP crystal is inferior to that of Er:Yb:PG. We think there may be three reasons. First, the Er:Yb:BGP crystal has smaller absorption cross-section at 975 nm and emission cross-section at 1537 nm. Second, the Er3+ and Yb3+ doping concentrations of Er:Yb:BGP crystal are much lower than those of Er:Yb:PG. Third, only preliminary laser experiment has been achieved in the Er:Yb:BGP crystal. Therefore, the laser performance of the Er:Yb:BGP crystal can be improved by optimizing Er3+/Yb3+ doping concentrations and laser cavity in the further experiment.
4. Conclusion
The spectral properties of a 1.85 at.% Er3+ and 23.95 at.% Yb3+ co-doped BGP crystal grown by the Czochralski method were investigated. The fluorescence lifetimes of the 4I13/2 and 4I11/2 levels of Er3+ ions in the crystal are 9.91 ms and 4.89 µs, respectively. The fluorescence quantum efficiency of the 4I13/2 level is close to 98%. The efficiency of the energy transfer from Yb3+ to Er3+ ions is about 90%.
A CW diode-pumped Er:Yb:BGP laser at 1.5–1.6 µm with output power of about 73.9 mW and slope efficiency of 16.3% was achieved for the first time in the Er3+/Yb3+ co-doped phosphate crystal to our knowledge. Lasing was achieved at wavelengths of 1567 and 1542 nm. Combining with the long fluorescence lifetime and high quantum efficiency of the upper laser level 4I13/2, the Er:Yb:BGP crystal may be a suitable gain medium for pulsed laser at 1.5–1.6 µm.
Funding
Scientific Instrument Developing Project of the Chinese Academy of Sciences (YZLY202001); Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZR119, 2021ZZ118); Natural Science Foundation of Fujian Province (2019J01127).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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