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Abstract
We demonstrate a novel Er:LuSGG active gain medium emitting laser wavelength at 2795 nm for the first time. The Er:LuSGG crystal is grown successfully by the Czochralski method with high crystalline and optical quality. The spectra properties, including absorption and fluorescence emission cross-section are presented in contrast with similar Er-doped garnet crystals. The fluorescence lifetimes of the upper (4I11/2) and lower (4I13/2) laser levels are 1.75 and 4.64 ms, respectively. Under 973 nm laser diode pumping, a maximum output power of 789 mW in continuous-wave mode, corresponding to optical-to-optical efficiency of 20.2% and slope efficiency of 24.4%, is achieved with high laser beam quality. The results show that the Er:LuSGG is a promising MIR laser material operated at 2.8 µm.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared (MIR) lasers at around 3 µm have drawn an increasing interest for wide utilization, ranging from biomedicine field to spectroscopic detection, remote sensing and space military due to the strong absorption in vaper, water and biological tissues [1–3]. Meanwhile, this waveband is an ideal pump source for optical parametric oscillator (OPO) systems to obtain far-infrared tunable lasers [4]. The erbium-doped laser materials can be directly pumped into the upper level by the commercial 970 nm laser diodes (LDs) and generate the 2.7-3 µm laser radiation based on 4I11/2 → 4I13/2 luminescent transition. However, the shorter lifetime of the upper level 4I11/2 than lower level 4I13/2 results in the self-terminating “bottleneck” effect, which restricts the development of mid-infrared Er3+-doped crystal laser. To overcome this negative effect, heavily doping Er ions (≥30 at.%) can mitigate the intrinsic self-terminated effect by energy-transfer-upconversion (ETU) process (4I13/2 + 4I13/2 → 4I11/2 + 4I9/2), which leads to a fast depletion of the laser lower level and enables continuous wave (CW) operation.
To date, the Er-doped laser active media based on various matrices have been subjected to numerous researches for generating the laser radiation in the spectral range of 2.7-3 µm, such as Y2O3 [5], Lu2O3 [6], Y3Al5O12 (YAG) [7], Gd3Ga5O12 (GGG) [8], Lu3Ga5O12 (LuGG) [9], Y3Sc2Ga3O12 (YSGG) [10], LiYF4 (LYF) [11], CaF2 [12], and SrF2 [13] etc., among which sesquioxide and fluoride hosts possess a low phonon energy in comparison with oxide garnet hosts, inducing a weaker nonradiative transition rate and consequently a higher laser efficiency. Nevertheless, the sesquioxide crystals are difficult in fabrication owing to their high melting points and fluoride crystals present fragile structure and low laser damage threshold. Therefore, the common garnet crystals are deemed as potential solid-state laser hosts with large size growth due to their outstanding physicochemical characteristics, high thermal conductivity, and excellent laser performance. The garnet family crystals with the chemical formula of A3B2C3O12 belong to the cubic crystallize system and Ia3d space group, where A, B, and C occupy dodecahedral, octahedral, and tetrahedral coordinated lattice sites, respectively. It is well known that the Er:YSGG has a relatively lower phonon energy among the Er-doped garnet crystals, and thus possesses a lower laser threshold and higher laser efficiency. Owing to the similar radius of Er3+ (100.4 pm) and Y3+ (101.9 pm) for a coordination of 8 [14], Er3+ ions substitute for Y3+ ions in YSGG at the dodecahedral A sites with only small changes in the lattice constants.
Even so, high concentration doping of Er3+ ions causes an obvious decrease of the thermal conductivity, from 8.0 Wm−1K−1 for undoped YSGG down to 3.27 Wm−1K−1 for 30 at.% Er:YSGG at 25°C [15]. This will severely reduce the laser performance, not to mention the challenges of the crystal growth. Compared with yttrium compounds, the substitution of lutetium for yttrium in the host materials is more suitable for doping Er3+ ions, because the very small difference of atomic mass between the dopant Er3+ and host Lu3+ ions results in a lower phonon scattering rate besides the similar ionic radii, which in turn can lead to a small decrease of the thermal conductivity for undoped LuSGG. In fact, the effective ionic radius of Lu3+ is 97.7 pm, and it is clear that the atomic mass of Er3+ (167.3 g/mol) is almost same for Lu3+ (175.0 g/mol) and overweighed for Y3+ (88.9 g/mol). This approach has already proved to be beneficial for the development of new materials with resembling crystal structure, e.g. Lu2O3 [16] and LiLuF4 [17]. Therefore, Lu3Sc2Ga3O12 (LuSGG) crystal can be produced with lutetium replacing yttrium as constitutive ions. In addition, the lattice constant of LuSGG (12.26 Å) is slightly smaller than that of YSGG (12.42 Å) [18], thus a tighter lattice gives rise to larger crystal fields. In simple terms this implies that a larger Stark splitting of the energy-level manifold caused by larger crystal field would tend to produce lower thermal occupation factors of both the upper and the lower laser levels, which benefits the population inversion and a reduced lasing threshold at room temperature [19]. Currently, Nd-doped LuSGG crystal is considered to be a promising laser material for broad emission spectrum, small emission cross-section, and excellent pulsed laser performance [20,21]. To the best of our knowledge, the growth of single crystal Er:LuSGG has not yet been reported up till now.
In this work, a high-quality Er:LuSGG single crystal was successfully grown by the Czochralski (Cz) method for the first time. The crystalline quality of the as-grown crystal was examined by the X-ray rocking curve (XRC). The absorption and luminescence spectra, as well as decay behaviors at room temperature were measured, and the laser performances of a diode end-pumped Er:LuSGG laser at 2.8 µm operated in continuous-wave (CW) mode were investigated.
2. Experiment setup
A 30 at.% Er3+-doped LuSGG single crystal was grown along <111 > direction using the Cz technique from a congruent composition melt by a JGD-60 (CETC 26th, China) with an automatic diameter controlled growth system. The raw materials of Er2O3 (5N), Lu2O3 (5N), Sc2O3 (5N), and Ga2O3 (5N) oxide powders were accurately weighed according to the molecular formula of Er0.9Lu2.1Sc2Ga3O12 and mixed adequately. And an extra 1.8 wt.% Ga2O3 was overweighed to compensate for its evaporation loss during the growth progress. The crystal growth was carried out in an iridium crucible and high purity argon atmosphere with a rotation speed of 1 rpm and a pulling rate of 1.4 mm/h. As shown in Fig. 1, the as-grown Er:LuSGG crystal with a dimension of Ф 24 mm × 80 mm was obtained, but the Ga2O3 evaporation of the melt led to a spiral formation in tail position. This phenomenon should be due to a noticeable evaporation during the crystal growth that the Ga2O3 experiences, so the deviation of composition melt would be serious in the tail position of Er:LuSGG crystal. Therefore, the composition deviation causes convex-to-concave interface inversion, which in turn can lead to a spiral formation. The samples with a thickness of 2 mm were cut from the as-grown crystal perpendicular to the growth direction and polished on both sides for spectroscopic experiments.
The crystal structure was identified by X-ray powder diffraction (XRD) using a Philips X'pert PRO X-ray diffractometer equipped with Cu kα radiation. The diffraction data of 2θ were recorded at a scan step of 0.0167° from 10° to 90°. The X-ray rocking curve (XRC) was measured by a high resolution X'pert Pro MPD diffractometer equipped with a Hybrid Kα1 monochromatic. A spectrophotometer (PE Lambada 950 UV/VIS/NTR) was performed to collect absorption spectrum with a spectral interval of 0.5 nm. A fluorescence spectrometer (Edinburgh FLSP 920) was applied to measure the fluorescence spectra and decay curves by using a 972 nm InGaAs LD and an optical parametric oscillation (OPO) laser (Opolette 355 I) as pumping sources, respectively. All the measurements were carried out at room temperature.
The configuration of a simple plane–parallel resonator cavity with a cavity length of 13 mm is shown in Fig. 2. The crystal was wrapped in indium foil and enclosed by a copper heat sink, and its cooling water passage was set in a temperature of 15 ± 0.2 °C. The input mirrors (IM, K9 glass) M1 and M3 were coated for high transmission (>95%) at 967 nm and high reflectivity at 2.8 µm. The output coupler (OC, CaF2 substrate) M2 with different transmissions (TOC) of 0.5%, 2%, 5%, or 10% at 2.8 µm were employed to obtain optimal laser output. The fiber-coupled LD pump laser delivering a maximum power of 50 W at around 973 nm was collimated and focused onto an uncoated 2 × 2 × 7 mm3 Er:LuSGG single crystal with parallel and polished end faces. The laser output power was measured by a power meter (OPHIR 30A-BB-18), and a Pyroelectric Array camera (Ophir-Spiricon PY-III-HR) was used to determine the laser beam profile and M2 factor.
3. Results and discussion
3.1 X-ray powder diffraction and crystal quality
Figure 3 shows the XRD patterns of 30 at.% Er:LuSGG crystal together with the standard pattern of the LuSGG phase (PDF #54-1252) for comparison. All measured diffraction peaks consist well with the standard pattern except the small shift of diffraction angles owing to the high concentration doping of Er3+ ions. Hence, the doped ions do not change the cubic garnet structure of the pure LuSGG crystal with a space group of Ia3d.
As shown in Fig. 4, a wafer with the dimensions of Φ 20 mm × 2 mm was cut and polished for the measurements. The full width at half maximum (FWHM) of the XRC of the <111 > diffraction plane is measured to be 0.012°. The diffraction peak presents a symmetrical shape without split, indicating that the Er:LuSGG crystal possesses a good crystalline quality.
3.2 Absorption and fluorescence spectra
The absorption spectra of the Er:LuSGG crystal in the wavelength range of 0.32-3.0 µm are exhibited in Fig. 5. The characteristic absorption bands of Er3+ centered at around 377, 407, 451, 488, 524, 543, 654, 790, 966, and 1469 (1532) nm could be revealed, which correspond to the transitions from ground state 4I15/2 to excited states 4G11/2, 2H9/2, 4F5/2 + 4F3/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2, and 4I13/2, respectively. The absorption coefficients in the wavelength region of 2.7-3.0 µm are close to zero, which is beneficial to obtain this waveband laser output. From the inset in Fig. 5, it can be seen that the maximum absorption coefficient of 9.23 cm−1 corresponds with the wavelength of 966 nm, which overlaps well with the emission bands of commercially available high power InGaAs LDs. The absorption cross-section σabs at 966 nm can be determined with σabs = α(λ)/Nc, where α(λ) is the absorption coefficient and Nc is the concentration of Er3+ ions in the Er:LuSGG crystal. Thereby, the σabs of Er:LuSGG with peak at 966 nm is calculated to be 2.42× 10−21 cm2. In addition, the FWHM within the absorption band of 950-990 nm is approximately 11 nm, which is beneficial to improve the efficient pumping and decrease the temperature dependence for the LDs.
Fig. 5. Absorption spectra of the Er:LuSGG crystal. Inset: enlarged curve in the range of 950-990 nm.
The Er:LuSGG crystal possesses broadband MIR emission spectrum in the spectral range of 2.6-3.0 µm, which is potentially interesting for generating short pulse laser, and the fluorescence curve excited by 972 nm LD is presented in Fig. 6. A few strong fluorescence peaks are located at wavelengths around 2635, 2703, 2794, and 2820 nm, which are assigned to stark sub-level transitions from 4I11/2 to 4I13/2 in Er3+ ions, indicating the possibility of 2.6-3.0 µm laser generation on the Er:LuSGG crystal.
Fluorescence decay curves of Er:LuSGG at the upper (4I11/2) and lower (4I13/2) laser levels are measured at 1024 and 1532 nm excited by 970 nm OPO pulse lasers, as shown in Fig. 7. The decay curves exhibit two-exponential decay behavior. Using an exponential fit, the fluorescence lifetimes are evaluated to be 1.75 ms (4I11/2) and 4.64 ms (4I13/2), respectively. It has to be noted that the fluorescence lifetime of upper laser level is shorter than that of lower laser level, so the ∼3.0 µm lasing transition of Er:LuSGG can suffer by the self-termination effect. Nonetheless, this phenomenon is probably suppressed by high concentration of Er3+, because the ETU process will make the lower 4I13/2 level upconverted to the 4I9/2 level, and by subsequent multiphoton relaxation, then recycled to the 4I11/2 upper laser level to establish a fast depletion of the lower laser level with energy recycling [22]. Moreover, the Er:LuSGG has an obvious longer upper laser level lifetime in comparison with the similar garnet crystals, which could facilitate the population inversion and provide a low pump threshold. Herein, the spectral parameters of several Er-doped garnet crystals are listed in Table 1.
Fig. 7. Fluorescence decay curves of the Er:LuSGG crystal at (a) upper laser level 4I11/2 and (b) lower laser level 4I13/2.
Table 1. Spectral parameters of several erbium activated garnet crystals
The emission cross-section σem could be estimated by Fuchtbauer-Ladenburg (F-L) formula [23]:
3.3 Laser performance
The MIR laser based on the 30 at.% Er:LuSGG crystal is operated in CW mode. The laser output powers as a function of absorbed pump power at different levels of OC transmittance are illustrated in Fig. 8(a). Using the OC TOC = 5%, a maximum output power of 789 mW, slope efficiency of 24.4% and laser threshold of 561 mW are achieved at the absorbed pump power of 3.9 W. The optical-to-optical efficiency is 20.2%, which is defined as the ratio of output power to absorbed pump power. The emitting laser wavelength of 2795 nm with a FWHM of 2.5 nm is displayed in Fig. 8(b) at the maximum output power of 789 mW. The images of two-dimensional and three-dimensional transverse beam profiles of the Er:LuSGG laser monitored by the pyroelectric array camera reveal that the laser beam is nearly circular with a Gaussian distribution of the electrical field.
Fig. 8. (a) CW laser output power versus absorbed pump power and (b) spectrum of the lasers with insets showing two-dimensional and three-dimensional beam profiles at the maximum output power of 789 mW.
The laser threshold increases with the increase of OC transmittance due to the increased transmission loss, which is confirmed in our laser experiments. Under the same condition, the estimated laser threshold of 242 mW is reached at an OC transmission of 0.5%. Therefore, the round trip loss of Er:LuSGG laser can be calculated to be 2.9% by using the Pth = k(δ + T), where Pth is the laser threshold, k is a constant for the crystal and cavity, T is the transmission of the output coupler, and δ is the round trip loss of crystal mainly induced by absorption, scattering and non-uniformity of the laser crystal [10]. Furthermore, it is worth noting that a higher slope efficiency is fitted to be 26.0% for TOC = 2%, whereas the laser output power has a tendency to saturate at 698 mW with the absorbed pump power of 3.2 W, corresponding to an estimated laser threshold of 342 mW. In this process, we do not find any crystal damage even if the pump power reaches up to 5 W. But under the absorbed pump power of 5 W, the average output powers of Er:LuSGG crystal decrease obviously due to the serious thermal effect.
Compared with other Er3+-doped garnet laser crystals in Table 2, the Er:LuSGG laser has a significant larger CW laser output. Because the Er:LuSGG crystal possesses better laser performance than other garnet crystals on the basis of a larger splitting of the energy-level manifolds by the crystal field, which would tend to produce more favorable thermal occupation factors of both the upper and the lower laser levels [17]. Meanwhile, a lower phonon scattering rate of Er:LuSGG crystal results in a smaller thermal effect in comparison of Er:YSGG and Er:YAG. But a higher efficiency of 30.8% and lower laser threshold of 139 mW were obtained in Er:YSGG and Er:GSGG laser crystals reported in Ref. [10,30]. We mainly ascribe the present results to the mismatch between the wavelength (973 nm) of LD pump source and the center absorption wave (966 nm) in the Er:LuSGG crystal. Further improvement in laser performance might be achievable by rationally designing the cavity structure and considering thermal lensing effect with a thermal bonding LuSGG end-cap.
Table 2. Comparison of ∼2.8 µm laser performance in several erbium activated garnet crystals
The M2 factor of laser beam profile at the maximum output power is determined through a 400-mm focal length K9 lens. The horizontal and vertical diameters are recorded at the positions around the K9 lens focus point, and the hyperbolic fitting lines are adopted to the experimental data as shown in Fig. 9. The quality factor M2 is calculated from the following equation [31]:
where ω is the beam waist diameter, Θ is the far-field divergence, and λ is the wavelength. The laser beam quality M2 factors in the x and y axis are calculated to be Mx2 = 1.30 and My2 = 1.33 in the CW regime, hence the laser beam profile is close to the fundamental transverse electromagnetic (TEM00) mode. All results indicate that the Er:LuSGG crystal is a novel and potential 2.8-µm MIR laser material.4. Conclusion
In conclusion, a new 30 at.% Er:LuSGG laser active medium grown by Cz method was developed for the first time, which possesses the garnet structure with Ia3d space group. This crystal has broader absorption around 966 nm with the FWHM of 11 nm, which is suitable for matching to LD pumping. Additionally, the fluorescence lifetimes of the upper (4I11/2) and lower (4I13/2) laser levels are 1.75 and 4.64 ms, respectively. In the CW regime, the 973 nm LD end-pumped Er:LuSGG laser emits at 2795 nm with a maximum output power of 789 mW, corresponding to optical-to-optical efficiency of 20.2% and slope efficiency of 26%. Besides, a matched 966 nm LD pump source and optimal cavity structure are expected to further improve the performance of end-pumped Er:LuSGG laser. All these features make the Er:LuSGG crystal as a promising and excellent candidate for 2.8-µm solid-state MIR lasers with low laser threshold, high output power, and high slope efficiency compared with other Er-doped garnet crystals.
Funding
National Natural Science Foundation of China (51872290); National Key Research and Development Program of China (2016YFB1102301).
Disclosures
The authors declare no conflicts of interest.
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