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Abstract
In this work, we report on investigations of structure, spectroscopic properties and laser performances of, what we believe to be, a novel Er:YGGAG laser crystal. High crystalline quality is proved by an FWHW of XRC of 0.019°. Thermal conductivity of a 30 at.% Er:YGGAG crystal is determined as 4.98 W/(m·K). The refractive index is measured in the range of 400 to 1000 nm and fitting with Sellmeier equation is done. A broad fluorescence emission band is located at 2786∼2819 nm, suggesting that this crystal is favorable to realize tunable and ultrafast laser. Under the pump at 969 nm with a fiber-coupled diode laser, at 400 Hz repetition rate and 600 µs pulse duration, the 30 at.% Er:YGGAG delivered maximum average power of 506 mW with overall optical-to-optical efficiency of 12.4% and slope efficiency of 16.9%. The laser beam quality was characterized by M2 factors of 1.53 and 1.39 in horizontal and vertical directions, respectively.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, many studies have been focused on mid-infrared spectroscopy and laser generations [1–4]. Among them, the 2.7∼3 µm mid-infrared laser is located in the strong absorption band of OH vibrations in water [5], which has important applications in the fields of optical remote sensing and dentistry. In addition, lasers in this band can also be used as the pump sources for optical parametric oscillators, and the 3∼5 and 8∼12 µm long-wavelength lasers can be obtained by pumping the nonlinear frequency conversion crystals, such as ZnGeP2, BaGa4Se7 or CdSe [6–8].
Presently, Er3+ solid-state lasers are receiving a lot of attention for the 2.7∼3 µm laser generation. This is due to the special energy level structure of Er3+, which allows ions in lower laser levels to jump to higher laser levels through the cooperative upconversion process (ETU1, 4I13/2→4I15/2, 4I13/2→4I9/2→4I11/2) between Er3+ ions, thereby increasing the number of particles in the upper laser level [9]. However, this process heavily relies on the doping concentration of Er3+ ions, and lower doping concentrations are not conducive to the generation of synergistic upconversion. A high Er3+ doping concentration can enhance the ETU1 probability between Er3+ ions and reduce the lifetime of the lower laser level.
In addition, there is also an upconversion process (ETU2, 4I11/2→4I15/2, 4I11/2→4F7/2→4S3/2→4I15/2) between the upper laser levels of Er3+ [10], which will reduce the number of particles in the upper laser level and is unfavorable to realizing the inversion of particle numbers. Jenssen et al. reported [11] that the ETU1 and ETU2 exist at the same time and compete with each other in the Er:YLF crystal. After the optimal doping concentrations, as the doping concentration increases, the ETU2 will gradually occupy a dominant position, ultimately leading to a decrease in the probability of ETU1, which is not conducive to laser output. Moreover, when the doping concentration is excessive, cascade energy transfer will occur because the distance between Er3+ is lower than the critical value until the fluorescence quenching finally occurs [12]. Jenssen et al. found that the preferred Er3+ doping concentration has certain differences in different host crystals, for example, the optimal Er3+ doping concentration for YSGG and YAG are ∼35 at. % and ∼50 at. %, respectively. It can be noted that the optimal doping concentration is closely related to the phonon energy of host, and the higher the phonon energy, the higher the corresponding optimal doping concentration [11]. However, the high doping concentration of active ions will affect the thermal properties of crystal, and the difficulty of growing high-quality laser crystal will also increase.
The Er3+ has been relatively well studied in a variety of hosts. Among them, sesquioxide has the advantages of low phonon energy and high thermal conductivity, but its high melting point makes it difficult to prepare. Although fluoride has a low melting point, it is prone to deliquescence and difficult to process [13]. In contrast, garnet is considered to be an excellent matrix material due to its outstanding thermal and optical properties, such as Er:YAG, Er:GGG and Er:YSGG, and etc [14–17]. In Er:YGGAG (Gd3-xYxGa5-yAlyO12) crystal, partial replacement of Gd3+ and Ga3+ by Y3+ and Al3+ can lead to a non-uniform broadening of spectrum [18]. Moreover, Moulton et al. [19] concluded that tunable laser systems with large gain cross-sections and line widths require a short fluorescence decay time. Therefore, it is anticipated that the Er:YGGAG not only has similar physicochemical properties to Er:YAG and Er:GGG, but also is an excellent potential candidate for tunable and ultrashort lasers.
In this paper, the novel Er0.9Y1.6Gd0.5Ga3Al2O12 crystal was grown for the first time and its structure, physical, spectral, and laser properties were investigated in detail. The results of this study can provide a reference for the development of mid-infrared laser crystals with a broadened spectroscopy.
2. Experiment setup
The Er2O3, Y2O3, Gd2O3, Ga2O3, and Al2O3 powders with 5N purity were dried and weighed accurately according to the molecular formula of Er0.9Y1.6Gd0.5Ga3Al2O12. In addition, an extra 1.6 wt.% Ga2O3 was overweighed to compensate for its evaporation loss during the crystal growth. The whole process was performed in a JGD-80 furnace (CETC 26th, China) with a nitrogen atmosphere to prevent oxidation of the iridium crucible. The pulling rate and rotation speed were 1∼2 mm/h and 0.5∼1 rpm, respectively. Finally, a 30 at. % Er:YGGAG crystal with a size of Φ 26 mm × 105 mm was grown successfully, as shown in Fig. 1. The as-grown Er:YGGAG crystal was annealed in air at 1450 °C for 72 h to reduce the internal stress.
The X-ray diffraction (XRD) data was determined on a Philips X'pert PRO X-ray diffractometer equipped with a Cu-Kα radiation instrument. The X-ray rocking curve (XRC) was measured by a high-resolution X'pert Pro MPD diffractometer equipped with Hybrid Kα1 monochromatic. The Er:YGGAG crystal was processed into a triangular prism, and its refractive index was obtained by a goniometer (DMAT-300IR), which was equipped with a 400∼1100 nm xenon-lamp light source and a 360° rotatable carrier table. The thermal property of the Er:YGGAG samples with a size of 10 mm × 10 mm × 1 mm was tested in the range of 295 to 500 K by laser flash method on a laser thermal conductivity meter (LFA467 LT). The fluorescence spectrum and level lifetime were performed on a fluorescence spectrometer (FLSP-920). The transmission spectrum was measured by a spectrophotometer (PE Lambaba-1050+ UV/VIS/NTR).
The schematic diagram of the laser experiment is shown in Fig. 2. The Er:YGGAG sample size for laser experiment was 2 mm × 2 mm × 8 mm. A 969 nm LD pulsed laser was used as the pumping source. The pump laser was turned into parallel light and then focused on the Er:YGGAG crystal by the coupler lens. The IM was an input mirror with high reflectivity (>99.95%) at 2.7∼3 µm and high transmission at ∼970 nm, and the OC was an output coupler mirror with a transmittance of 5% at 2.7∼3 µm. The Er:YGGAG crystal was wrapped in indium foil and then placed in a cooling fixture of brass with 20 ± 0.5 °C circulating cooling water. The output power was measured and recorded by a power meter (Ophir 30A-BB-18). The laser wavelength was monitored by a fluorescence spectrometer (Edinburgh FLSP 920). The laser beam profile and M2 factor were determined by a pyroelectric array camera (Ophir-Spiricon PY-III-HR).
3. Results and discussion
3.1 Crystal structure and crystalline quality
The XRD pattern of Er:YGGAG crystal with a step size of 0.033492° and standard patterns of GGAG and GGG are shown in Fig. 3. There are no additional impurity peaks can be observed and the doped Er3+ and Y3+ ions do not change the cubic garnet structure of the pure GGG or GGAG crystal. So the Er:YGGAG still belongs to the garnet structure with a space group of Ia3d. Among them, Er3+, Y3+ and Gd3+ occupy the dodecahedral site, and Ga3+ and Al3+ occupy tetrahedral and octahedral sites. Compared with the lattice parameter of 12.381 Å for GGG, the decreased value of 12.148 Å for Er:YGGAG should be due to the ionic radius of Y3+ (90 pm) and Er3+ (88.1 pm) are less than that of Gd3+ (93.8 pm) and the ionic radius of Al3+ (50 pm) is also less than that of Ga3+ (60 pm).
The XRC of the Er:YGGAG crystal with a step of 0.005° is shown in Fig. 4. The diffraction peak corresponding to the (444) crystalline plane is located at 25.85°. The FWHM of the curve is 0.019°, indicating the Er:YGGAG crystal has a high crystalline quality.
3.2 Physical properties
3.2.1 Refractive index
Using the principle of minimum deviation angle to measure the refractive index of a trigonal prism, the schematic diagram and sample are shown in Fig. 5 [20]. The refractive index at different wavelengths can be calculated and the experimental results are listed in Table 1. Moreover, the data were fitted using the Origin software based on the Sellmeier dispersion equation [21]:
According to fittint results, the constants of S is 2.4885 and λ0 is 120.19756, respectively. The refractive index curve for Er:YGGAG in the range of 400∼3000 nm is shown in Fig. 6. The refractive index of the crystal can be found to decrease with the increase of wavelength. The value of the refractive index is 1.868 near 2.8 µm, which is an important parameter for laser crystal coating film.
Table 1. Refractive index of the Er:YGGAG crystal at different wavelengths.
3.2.2 Thermal properties
For laser crystals, the thermal properties are very important because they have a profound impact on the growth process parameters and laser performance of the crystal. The results of the thermal properties are shown in Fig. 7. It can be noted that the thermal diffusivity (λ) decreases with increasing temperature, while the specific heat (Cp) increases with increasing temperature. The thermal conductivity (κ) can be calculated by the following equation [22]:
where ρ is crystal density. The thermal conductivity of Er:YGGAG is 4.981 W·m−1·K−1 at 295 K, which is smaller than that of Er:YAG (6.94 W·m−1·K−1) [23] and larger than that of Er:YSGG (3.27 W·m−1·K−1) [24].
Fig. 7. Thermal properties of the Er:YGGAG at different temperatures. (a) Temperature-dependent thermal diffusivity and specific heat capacity; (b) Temperature-dependent thermal conductivity.
3.3 Optical properties
3.3.1 Absorption spectrum
The absorption spectrum of Er:YGGAG crystal in the range of 320∼3000 nm is shown in Fig. 8. The peaks of 377, 407, 450, 488, 524, 543, 654, 789, 966, and 1468 nm are the characteristic absorption of Er3+, corresponding to the transitions from the ground state (4I15/2) to the excited states of 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 broadband absorption characteristic can be observed at 950∼980 nm, with a maximum absorption coefficient of 5.43 cm−1 at 966 nm and an FWHM of approximately 20 nm, as shown in the inset of Fig. 8. Therefore, the wide and strong spectral band is favorable to match the wavelength of the LD pumping source. In addition, the crystal has strong absorption peaks at 1468 nm and 1530 nm, but there is no absorption peak at 2700∼3000 nm, which makes the crystal suitable for 2.7-3 µm laser output.
Fig. 8. Absorption spectrum of Er:YGGAG crystal; Inset: amplified curve in the range of 940∼1000 nm.
3.3.2 Fluorescence spectrum and lifetime
The fluorescence spectrum of the Er:YGGAG crystal is shown in Fig. 9. The highest intensity peak is located at 2918 nm. There is a wider fluorescence emission band at 2786∼2819 nm, which is beneficial for achieving tunable and ultrafast ∼2.8 µm mid-infrared laser.
The fluorescence decay curves of the Er:YGGAG crystal powders at 1018 nm and 1572 nm were measured using a 966 nm OPO pulsed laser as the excitation source, corresponding to the emission processes of 4I11/2→4I15/2 and 4I13/2→4I15/2. The upper and lower laser level lifetimes of Er:YGGAG were calculated by double exponential fitting to be 358 µs and 3.187 ms, respectively, as shown in Fig. 10. The upper and lower level lifetime ratio for Er:YGGAG is 0.112, which is reasonable and equivalent, compared with the values for Er:YAG and Er:GGAG are 0.101 and 0.137, respectively [25].
Fig. 10. Fluorescence decay curves of the Er:YGGAG crystal at (a) upper laser level 4I11/2; (b) lower laser level 4I13/2.
3.3.3 Laser performance
The laser output powers of the Er:YGGAG crystal pumped by 969 nm LD with different pulse widths are shown in Fig. 11. By using the knife-edge method, the pump spot size was measured to be 190 µm. The resonator length was 145 mm. Under the conditions of 400 Hz repetition rate and different pulse durations with 300, 400, 500, 600, and 700 µs, the maximum output powers are 393, 414, 428, 506 and 474 mW for 5% transmittance, the corresponding slope efficiencies are 20.1, 19.6, 18.2, 16.9 and 16.4% and optical-to-optical efficiencies are 13.7, 13.6, 12.3, 12.4 and 11.8%, respectively. Among them, the pulse duration with 600 µs has best performance than the others. Furthermore, It can be noted from the Fig. 11 that the maximum output power increases gradually until 600 µs because the large pulse duration results in a rapid increase of the pump power, which has a greater impact on the laser output power than the negative effect of the thermal effect. The slope efficiency is highest at a pulse duration of 300 µs and decreases with increasing pulse duration. It should be due to a lot of waste heat generated when the pump pulse duration exceeds the laser upper level lifetime (358 µs), which would aggravate the thermal effects of Er:YGGGA crystal and further affect the laser performance. Richard Švejkar et al. obtained a laser output of 350 mW and slope efficiency of 15.3% at a frequency of 100 Hz in their study of Er:GGAG [26]. Compared with the Er:GGAG, the laser performance of Er:YGGAG has been improved to some degree.
The laser spectrum was measured by a fluorescence spectrometer, as shown in Fig. 12. The peak of the laser is located at 2822 nm and the FWHM is 1.9 nm. The result suggests that the Er:YGGAG crystal can acquire a single wavelength laser with a narrow linewidth.
The beam quality and far-field divergence of the laser were measured in both horizontal and vertical directions at the output power of 300 mW with a frequency of 400 Hz and pulse width of 600 µs. A focusing lens with f = 300 mm was located behind the resonant cavity to focus the laser. The pyroelectric array camera was placed in suitable positions behind the focusing lens for measuring the horizontal and vertical diameters of the beam spot. The data were fitted non-linearly by using Origin software and the M2 factor and far-field divergence were finally calculated by the following equation [27]:
where ω is waist diameter, θ is the far-field divergence, and λ is the laser wavelength. The fitted curve is shown in Fig. 13, including a schematic of the laser spots in two and three dimensions. The M2 factor in the horizontal and vertical directions are 1.53 and 1.39, corresponding to the far-field divergences of 18.8 and 19.3 mrad, respectively.4. Conclusions
In this work, the 30 at. % Er:YGGAG laser crystal with high quality was grown by the CZ method for the first time. The XRD results indicate the Er:YGGAG crystal has a garnet structure, and the XRC proves it has a high crystalline quality. The refractive indices were measured in the range of 400∼1000 nm and fitted extension to 3000 nm. The thermal conductivity is 4.981 W·m−1·K−1 at room temperature. A wide fluorescence band in the range of 2786∼2819 nm, which will be benefit for tunable and ultrafast laser. The lifetimes of Er3+ for 4I11/2 and 4I13/2 energy levels are 358 µs and 3.187 ms, respectively. A maximum output power of 506 mW at 400 Hz and 600 µs is achieved by 966 nm LD end-pumping, and the laser wavelength is located at 2822 nm with a narrow linewidth of 1.9 nm. The relative high beam quality is measured in the horizontal and vertical directions as 1.53 and 1.39, respectively. Therefore, the Er:YGGAG crystal with a fluorescence band is a promising tunable and ultrafast ∼2.8 µm laser gain medium. It is anticipated that the laser performance can be further improved by optimizing the Al/Ga ratio, Er3+ doping concentration, and crystal growth parameters.
Funding
National Natural Science Foundation of China (52102012); Natural Science Foundation of Anhui Province (2208085QF217); Hefei Institutes of Physical Science, Chinese Academy of Sciences (HFIPS) Director's Fund (YZJJ2022QN08).
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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