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Abstract
The Tm:CaF2 single-crystal fibers were successfully grown by modified temperature gradient technique method. The J-O intensity parameters, spontaneous radiative transition rates, radiative lifetimes and fluorescence branching ratios of Tm3+ were calculated with Judd-Ofelt theory. A systematic study of the fluorescence characteristics has been carried out. Simulated emission cross-sections of the 3F4 → 3H6 transition were calculated to be 6.68×10−21 cm−2 and 4.65×10−21cm−2 for crystal doped with 3 at.% and 4 at.% Tm3+. The 64.4% slope efficiency with output power of 2.23W was achieved in 3 at.% Tm:CaF2 single-crystal fiber. The slope efficiency decreased to 44.5% and maximum output power decreased to 1.65W in 4 at.% Tm:CaF2 single-crystal fiber. Obtained results show that Tm-doped CaF2 single-crystal fibers are promising materials for IR laser action generation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over past decades, solid-state laser operating at ∼2 µm received great attention due to their extensive use in remote sensing, environment monitoring, and coherent radar applications [1–3]. A strong absorption of liquid water in this spectral region allows those lasers to be applied in modern medical field as laser surgery equipment [3,4]. Lasers operating at ∼2µm are considered as a promising pump source for optical parametric oscillations delivering laser radiation [5,6].
Thulium-based solid-state lasers have already shown tunable laser emission around 2 µm [7–9]. Among available host materials, calcium fluoride (CaF2) shows multiple outstanding laser properties. The thermal conductivity of pure CaF2 crystal is as high as 10 W/(cm·K) [10], which is important for maintaining laser generation stability. The maximum phonon energy is only 322 cm−1 [11]. Low phonon energy reduces the energy loss caused by the non-radiative relaxation. So that calcium fluoride was used as one of the first laser hosts in the early 1960s [12,13]. CaF2 belongs to cubic system with the lattice constant of 5.46 Å [14]. When a trivalent rare-earth-ions (RE3+) introduce in this fluorite-structure crystal, the divalent calcium ions are easily substituted by the RE3+. The charge imbalance derived from this nonequivalent substitution is compensated by the introduction of a near-site interstitial F- anions, which form an electric dipole. Coulomb interaction between these electric dipoles facilitates RE3+ cations clustering even in the case of lightly doping. The spontaneous aggregation shortens the distance between RE3+ ions, significantly enhancing the process of cross relaxation between active ions. Figure 1 shows that two Tm ions could be excited from ground state 3H6 to the upper level 3F4 using only one photon thanks to the cross relaxation: 3H4 (Tm3+) + 3H6 (Tm3+) → 3F4 (Tm3+) + 3F4 (Tm3+). The interionic distances between two Tm3+ ions in clusters are short enough to enhance efficiency of a cross-relaxation processes [15,16]. Recently, X. Liu demonstrated the diode-pumped CW laser operation in Tm,Y:CaF2 crystal with the slope efficiency of 21.5% and a tunable wavelength range of 190 nm [17]. Z. Zhang achieved the CW laser operation with as high slope efficiency as 67.8% and a tunable wavelength range of 192nm in Tm,La:CaF2 crystal [18]. Such high slope efficiency is close to twice the theoretical quantum efficiency due to cross relaxation. C. Zhang achieved a pulse laser in Tm,Y:CaF2 crystal by mode-locking and passive Q-switching methods [19].
The single-crystal fibers (SCFs) received great attention as novel laser materials due to theirs advantages such as small size, low weight and favorable pump guidance [20–22]. Aforementioned advantages allowed to obtain better laser generation performance compared with corresponding single crystal. SCFs are a kind of intermediate material between the bulk single crystals and the glass fibers. SCFs possess lower stimulated brillouin scattering (SBS) cross-sections and higher thermal conductivities compared to glass fibers. This means that SCF lasers could achieve to much higher power levels. Therefore, it’s possible to obtain greater gain bandwidth of mode spacing by using SCF for its function to shorten the cavity length in the laser operations [23]. Given those advantages, SCFs are promising materials for satisfying the urgent demands in long-wavelength lasers [24,25]. There are several ways to obtain SCFs, such as laser heated pedestal growth (LHPG) [26,27], micro-pulling-down (µ-PD) method [28,29] and edge-defined film-fed growth (EFG) method [30,31]. Over past several years, LHPG developed a lot. Burrus et al. have successfully synthesized Nd:YAG single-crystal fibers with the diameter smaller than 250 µm [32]. Shen et al. grew Nd:YAG fiber with the diameter of 0.6-1.2 mm [33]. In 1988, Feigelson et al. synthesised Nb2O5 single-crystal fibers of Φ700-1700 nm and SrBaNb2O6 of Φ600-1700 nm [34]. Mimura et al. successfully grew CsBr single-crystal fibers with the diameter between 1 mm and 2 mm [35,36]. Nowadays, Shao Wang has achieved a CW laser operation in temperature gradient technique (TGT) grown Er:SrF2 SCFs with a slope efficiency of 34.9% and 860 mW output power [37]. However there are no reports about Tm:CaF2 SCFs.
In this paper, we report on growth procedure, spectroscopic properties and laser performance of high-quality 3 at.% Tm:CaF2 and 4 at.% Tm:CaF2 SCFs. The laser-diode (LD)-pumped CW laser action generation was obtained in these SCFs. We achieved the maximum laser output power of 2.23 W and the corresponding slope efficiency of 64.4% in 3% Tm:CaF2 SCF. This value of slope efficiency is higher than Tm3+:∼2 µm stokes theoretical quantum efficiency. In addition, laser operations in 4% Tm:CaF2 SCF was also achieved with high slope efficiency of 49.6% and a maximum laser output power of 1.65 W.
2. Experiment
2.1 Experiments for crystal growth and materials properties
The thulium-doped CaF2 single-crystal fibers were grown by modified temperature gradient technique (TGT) method. The raw materials were TmF3 (99.99%) and CaF2 (99.99%). The intended Tm3+ concentration levels were set to 3 at.% and 4 at.%. 1.0 wt.% PbF2 were added as an oxygen scavenger. The furnace used was vacuum-tight and the maintained pressure was below 5×10−3 Pa during the entire progress of crystal growth. Compared with other methods, TGT allows to obtain CaF2 SCFs of higher optical quality with lower-costs. The obtained Tm:CaF2 SCFs are free of crack, as is shown in Fig. 2(b).
Fig. 2. (a) Photograph of the as-grown Tm:CaF2 SCFs; (b) Photograph of high-quaily and crackless Tm:CaF2 SCF.
The crystalline phases of SCFs were investigated by powder X-ray diffraction (PXRD) using a Rigaku Ultima IV diffractometer. The actual concentration of Tm3+ ions in these SCFs was measured by inductively coupled plasma optical emission spectrometer (ICP-OES) and Energy Dispersive Spectrometer (EDS). UV-VIS-NIR spectrophotometer was used to measure the optical absorption spectra. For absorption measurements, the samples are fabricated into the dimension of Φ1.9 mm × 1.0 mm, with both ends polished. The emission spectra were collected by the time-resolved fluorimeter equipped with an InGaAs detector. For emission measurements samples were optically excited by 796 nm laser diode. The samples used for emission spectra measurements are fabricated into the dimension of Φ1.9 mm × 2.0 mm, with both ends polished. All the measurements mentioned above were carried out at room temperature.
2.2 Laser experiments
In order to ensure the high absorption rate of laser experiment, prepared samples were cylinders with the dimension of Φ1.9 mm × 10 mm with both ends polished. The scheme of CW laser experiment set-up is shown in Fig. 3. We used 792 nm LD as a pumping source with a numerical aperture (NA) of 0.22 and a fiber core diameter of 105 µm. The pump light was focused into the Tm:CaF2 SCFs by a coupling system of 1:2. In order to minimize the thermal effect, the SCFs were wrapped by indium foil and installed in a copper block whose temperature was maintained at 12.5 °C by the cooling water. The input mirror (IM) M1 was anti-reflection coated for 780-810 nm range and high-reflection coated for 1.9-2.0 µm range. The output couplers (OC) M2 have a curvature radius of 100 mm and transmissions of 2%, 5% and 10% for 1.9-2.0 µm range.
Fig. 3. The experiment set-up of CW laser operation of Tm:CaF2 SCFs with the dimension of Φ1.9mm×10 mm.
3. Results and discussion
3.1 Real concentrations and segregation phenomenon
The real concentrations of Tm3+ ions were measured by ICP-OES. The obtained values were 2.85 at.% and 4.14 at.% in the 3 at.% Tm and 4 at.% Tm samples, respectively. In order to analysis the uniformity of as-grown SCFs, four samples were cut from different positions in one rod of 3 at.% Tm:CaF2 fiber for EDS experiment. These samples were numbered to be 1# ∼ 4#. Figure 4 reveals that there is almost no segregation phenomenon in 3 at.% Tm:CaF2 SCF. According to EDS results that are shown in Table 1 and Fig. 4, Tm3+ ions are uniformly-distributed along fiber, which means we have successfully obtained the desired materials by TGT method. On the basis of Eq. (1), the segregation coefficients of Tm3+ in those SCFs were calculated to be 1.02 in 3 at.% Tm:CaF2 SCF and 1.03 in 4 at.% Tm:CaF2 SCF SCFs. In Eq. (1), cs denotes dopant concentration at the initial crystallization part, and cl denotes starting dopant concentration.
Table 1. The EDS result of 3 at.% Tm:CaF2 SCF.
3.2 XRD results
The X-ray diffraction (XRD) patterns shows in Fig. 5(a). All the diffraction peaks match well with the standard pattern of CaF2 (PDF#35-0816). We could easily find out that Tm:CaF2 SCFs maintain their original cubic system of Fm$\bar{3}$m. Figure 5(b) shows that there are two peaks corresponding to (111) crystal plane. The peak to the right of main peak is related to Cu Kα2 radiation, since X-ray beam of equipment used was not filtrated by monochromator. It could be easily found that the main peaks of (111) crystal plane are shifted slightly towards higher angles in both samples. The ionic radius of Tm3+ is much smaller than Ca2+ ionic radius, which means the lattice constant becomes smaller when the replacement takes place. When the trivalent Tm3+ take the place of divalent Ca2+, the existence of interstitial F- makes lattice constant bigger thanks to the lattice distortions. From our experiment seems that the ionic radius mismatch is more influential to lattice constants than interstitial F-.
Fig. 5. (a) X-ray diffraction pattern of as-grown Tm:CaF2 SCFs; (b) (111) crystal plane X-ray diffraction pattern of these SCFs.
3.3 Absorption properties
The absorption spectra of Tm:CaF2 SCFs are shown in Fig. 6(a). There are six obvious absorption bands in the range of 300-2000 nm, related to the energy transitions from 3H6 ground state to higher energy levels. Figure 6(b) shows the absorption spectra in 720-900 nm range. The band in this spectral region is related to 3H6 → 3H4 transition. Knowing the shape, structure and lines FWHM of this band is important, since this transition is used for optical pumping of Tm based lasers operating at ∼2 µm range. The strongest absorption line is located at 767 nm with the FWHM of 13.95 nm. There is a relatively strong absorption around 796 nm, matching very well with the emission wavelength of available commercial AlGaAs laser diodes (LDs). The maximum absorption coefficient equals 3.48 cm−1 for 4 at.% Tm:CaF2 SCF at 767 nm, much higher than 2.53 cm−1 for 3 at.% Tm:CaF2 SCF. The FWHM at 796 nm corresponding to 3H6 → 3H4 transition is calculated to be 13.33 nm and 14.51 nm for 3 at.% Tm:CaF2 SCF and 4 at.% Tm:CaF2 SCF, respectively. The absorption cross-sections of 3H6 → 3H4 transition are 8.37×10−21 cm−2 and 8.69×10−21 cm−2 at 767 nm respectively in 3 at.% Tm:CaF2 SCF and 4 at.% Tm:CaF2 SCF
Fig. 6. (a) Absorption spectra of Tm:CaF2 SCFs with the dimension of Φ1.9mm×1.0 m; (b) Absorption spectra of Tm:CaF2 SCFs with range of 720-900 nm.
3.4 J-O theory calculation and emission properties
The Judd-Ofelt (J-O) theory could be used to calculate several optical parameters such as metastable levels radiative lifetimes and branching ratios. But J-O theory emphasizes more on Forced Electric-Dipole (ED) transition. It’s worth noting that Magnetic-Dipole (MD) transition matters in transitions that meet the conditions of ΔJ = 0 or ±1, ΔS = 0 and ΔL = 0. Taking our absorption spectra into accounts, SMD≠0 in 3F2 → 3F3, 3F3 → 3F4, 3H4 → 3H5 and 3H5 → 3H6 transitions.
The reduced matrix elements are stable in different hosts materials, which could refer to [38]. Table 2 presents the least square fitting results. As is known to all, Ω2 is the parameter which is sensitive to the environment around doped ions and is associated with the asymmetry and covalency of rare-earth ions sites [39]. The Ω2 of 3 at.% Tm:CaF2 SCF is much lower than 4 at.% Tm:CaF2 SCF, which means that there are different clusters between those two samples. The parameter X=Ω4/Ω6 is usually regarded as the spectroscopic quality factor [40] and a larger X value means higher ability of intense laser transition [41]. The X value of those samples are 0.613 and 0.590, respectively in 3 at.% Tm:CaF2 SCF and another one. According to our calculation, the laser ability of 3 at.% sample is stronger than 4 at.% sample.
Table 2. The calculated J-O parameters of Tm:CaF2 SCFs.
The spontaneous radiative transition rates, radiative lifetimes and fluorescence branching ratios could also be calculated and the results are shown in the Table 3.
Table 3. Spontaneous transition rates, fluorescence branching ratios and radiative lifetimes of Tm3+ in the as-grown SCFs.
The RMSΔS is calculated to be 1.014×10−21 cm−2 and 1.047×10−21 cm−2, respectively in 3 at.% Tm:CaF2 SCF and 4 at.% Tm:CaF2 SCF. And the relative root-mean-square-errors (RMSEΔS) of the calculations are 6.30% for 3 at.% Tm:CaF2 SCF and 8.36% for 4 at.% Tm:CaF2. The emission spectra of 3% Tm:CaF2 and 4% Tm:CaF2 SCFs corresponding to the 3F4 → 3H6 transition are shown in Fig. 7. There are three sharp peaks around 1611 nm, 1669 nm and 1825 nm. The FWHM of main peak at 1825nm equals 144.31 nm in 3 at.% Tm:CaF2 SCF and 149.73 nm in 4 at.% Tm:CaF2 SCF, respectively. The large FWHM parameters suggest that these SCFs could be used for tunable laser operations. The emission cross-section could be calculated by the Fuchtbauer-Ladenburg (F-L) formula as shown in Eq. (2), where n is the refractive index, I(λ) is the fluorescence intensity at wavelength λ.
Fig. 7. (a) Emission spectrum of 3 at.% Tm:CaF2 SCF with dimension of Φ1.9mm×2.0 mm pumped at 796 nm; (b) Emission spectrum of 4 at.% Tm:CaF2 SCF with dimension of Φ1.9mm×2.0 mm pumped at 796 nm.
The gain cross-section could be calculated by Eq. (3). In the Eq. (3), β=Ne/Ng means the excited state population fraction, Ne is the electron population density of excited state, Ng is the electron population density of ground state. Laser gain is expected to occur only at σg>0 [42]. Figures 9(a) and 9(b) show the calculated gain cross-section of 3F4 → 3H6 transition by assuming a set of β values ranging from 0 to 1. It is obvious that there is a shift towards longer wavelength of maximum gain cross-section with the decreasing of β, according with the schematic shown in Fig. 1. For these as-grown SCFs, a positive gain appears with a broad tunable wavelength range from 1784 to more than 2000 nm in 3 at.% Tm:CaF2 SCF, while 1830 to over 2000 nm in 4 at.% Tm:CaF2 SCF when β=0.2. When β=0.4, the tunable wavelength range is practicable from 1730 to over 2000 nm in 3 at.% Tm:CaF2 SCF and 1765 to over 2000 nm in 4 at.% Tm:CaF2 SCF. 3 at.% Tm:CaF2 SCF performs better than 4 at.% in the tunable laser experiment. Such large tunable ranges indicate that the SCFs obtained are favorable candidates for tunable and ultrashort pulse lasers [43,44], which might be our further efforts.
Fig. 9. (a) Calculated gain cross-section of Tm3+ in 3 at.% Tm:CaF2 SCF; (b) Calculated gain cross-section of Tm3+ in 4 at.% Tm:CaF2 SCF.
3.5 CW Laser operation results
Pumping at 792 nm, the high slope efficiency and large output power of 3 at.% Tm:CaF2 and 4 at.% Tm:CaF2 SCFs are obtained, as shown in Figs. 10(a) and 10(b). 3% Tm:CaF2 SCFs prove to show much better laser properties than 4% Tm:CaF2 SCFs. For 3 at.% Tm:CaF2 SCF, we have achieved absorption rate of 44.6% of 792 nm diode pumping laser, while 57.5% in 4 at.% Tm:CaF2 SCF.
Fig. 10. (a) Output power versus absorbed pump power at 792 nm of 3 at.% Tm:CaF2 SCFs for CW laser operation with T = 2%, 5% and 10% OCs; (b) Output power versus absorbed pump power at 792 nm of 4 at.% Tm:CaF2 SCFs for CW laser operation with T = 2%, 5% and 10% OCs. Both of η in those pictures mean the slope efficiency.
For 3 at.% Tm:CaF2 SCFs, the maximum laser output power of 2.23 W and slope efficiency of 64.4% with a low laser threshold of 0.32 W has been achieved with output coupler of T = 2% in single-crystal fibers for the first time. Those parameters are higher than the slope efficiency of 41% of bulk Tm:CaF2 crystal reported by Camy [45]. Additionally, slope efficiency of 64.4% is much higher than the theoretical quantum efficiency (ηtheor=λex/λem=792/1950≈41%). The achieved laser beam wavelength was centered at 1963.87 nm and the most suitable cavity length for 3 at.% Tm:CaF2 SCF was 12 mm. A CW laser output power of 2.06 W with a slope efficiency of 62.9% with T = 5% OC was also achieved, with the laser threshold of 0.51W. The laser wavelength was centered at 1924.64 nm. When T = 10%, the maximum output power of CW laser decreased to 1.40 W. The slope efficiency decreased to 55.9% and the laser threshold increased to 0.85 W. The center wavelength of this laser was 1899.29nm.
For 4% Tm:CaF2, the best found cavity length was 85 mm. The maximum laser output power of 1.65 W and the slope efficiency of 49.6% have been obtained with T = 5% OC, higher than 1.65 W of 44.5% in T = 2% and 1.12 W of 43.2% in T = 10%. All laser threshold of those laser operations of 4 at.% Tm:CaF2 SCF are higher than 3 at.% Tm:CaF2 SCF with the same conditions. The threshold are 0.63 W in T = 2%, 1.04 W in T = 5% and 1.76 W in T = 10%. When T = 2%, the laser wavelength is centered at 1977.96 nm. When T = 5%, the laser wavelength is centered at 1942.56 nm. When T = 10%, the laser wavelength is centered at 1917.42nm. Obtained results show that 3 at.% Tm:CaF2 SCF performs better than 4 at.% Tm:CaF2 SCF in CW laser operation. The details of CW laser operation results show in Table 4.
Table 4. CW laser performance of the as-grown SCFs.
3.6 Comparison and discussion
Thanks to the cluster established in CaF2 single-crystal fibers, there is a relatively strong cross-relaxation (CR): 3H4 (Tm3+) + 3H6 (Tm3+) → 3F4 (Tm3+) + 3F4 (Tm3+), which could explain that the slope efficiency of those as-grown SCFs are almost twice their theoretical quantum efficiency. There is increasing number of reports for co-doping optically inactive rare-earth ions as Y3+, Lu3+, La3+ and Gd3+ for the purpose of center compositions controlling [17,18,46,47], which attributes to the higher slope efficiency. Table 5 shows that the slope efficiency is much higher than other Tm-doped bulk single crystals. Compared with bulk single crystal, SCF performs better in CW laser operation due to their better thermal conductivity. The result demonstrates that the as-grown SCFs are the promising material for future laser operation.
Table 5. Diode-pumped laser performance of Tm3+ ion doped materials.
4. Conclusions
In conclusion, the Tm:CaF2 SCFs were grown by temperature gradient technique (TGT) method for the first time. The ICP and EDS results indicate that the dopants in the as-grown SCFs are evenly-distributed and their real concentration match the intended one. The XRD results show that there are no significant structure changes in those SCFs. J-O parameters, spontaneous radiative transition rates, fluorescence branching ratios and radiative lifetimes are obtained with use of the Judd-Ofelt theory. The difference of Ω2 suggests that there are totally different environment, asymmetry and covalence of Tm3+ ions. Absorption cross-sections were calculated to be 8.37×10−21 cm−2 in 3 at.% Tm:CaF2 SCF and 8.69×10−21 cm−2 in 4 at.% Tm:CaF2 SCF. The stimulated emission cross-sections for the 3F4 → 3H6 transition were estimated to be 6.68×10−21 cm−2 in 3 at.% Tm:CaF2 SCF and 4.65×10−21 cm−2 in 4 at.% Tm:CaF2 SCF. The value of X equals to be 0.613 in 3 at.% Tm:CaF2 SCF and 0.590 in 4 at.% Tm:CaF2 SCF. Pumped by 792 nm LDs, the maximum laser output power of 2.23 W and slope efficiency of 64.4% with a low laser threshold of 0.32 W have been achieved with output coupler of T = 2% in 3 at.% Tm:CaF2 single-crystal fibers, higher than 1.65W output power and 49.6% slope efficiency in 4 at.% Tm:CaF2 single-crystal fibers.
Funding
National Natural Science Foundation of China (11974220, 61635012); Science and Technology Commission of Shanghai Municipality (18511109700); National Key Research and Development Program of China (2016YFB0402101); Strategic Priority Program of the Chinese Academy of Sciences (XDB16030000); Chinese Academy of Sciences Interdisciplinary Innovation Team.
Disclosures
The authors declare no conflicts of interests.
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