1 Introduction

Er3+-doped materials have caused much attention as an important method to obtain the mid-infrared lasers near 3 μm. High efficiency that beyond the stokes limit can be achieved by the InGaAs diode laser (LD) pumping and the cooperative up-conversion process of Er3+ [1, 2]. The LD-pumped solid-state lasers have the features of high efficiency, high beam quality, compact structure, long service life and high reliability [3]. According to the different laser working materials, the Er3+-doped solid-state lasers can be divided into fiber lasers and crystal lasers. The Er3+-doped fiber lasers are mainly represented using the Er:ZBLAN and Er:ZrF4 glass fibers as the laser gain medium [4, 5]. The Er3+-doped crystal lasers are characterized using the Er:YAG (Y3Al5O12) [6], Er:YSGG (Y3Sc2Ga3O12) [7], Er:YLF (YLiF4) [8] and other crystals as the laser materials. Compared with the fiber lasers, the crystal lasers possess the main advantages of low nonlinear effects and large mode area, which is more suitable for achieving large-energy and high-peak-power ultrashort pulses laser [9].

Up to now, the Er3+-doped solid-state lasers have been realized in many crystal materials, including the oxide crystals (YAG [10], Gd3Ga5O12 (GGG) [11], YGG [12], YSGG [7], YAlO3 (YAP) [13], Y2O3 [14], and Lu2O3 [15]) and fluoride crystal (CaF2 [16], SrF2 [17], YLF [8], etc.). Thereinto, the oxide crystals with garnet structure possess the advantages of relatively high thermal conductivity, stable physical and chemical properties, and easy grow to obtain large size single crystal. Watt-level mid-infrared continuous wave (CW) laser has been achieved in LD end-pumping Er:YAG (1.15 W) [7], Er:YGG (1.38 W) [12], and Er:YSGG (1.37 W) [18] crystals, with slope efficiency of 34%, 35.4% and 23.6%, respectively. As we all know, the Er:YAG (857 cm−1) crystal has a much higher phonon energy, and the YSGG (6.5 W·m·K−1) crystal has a lower thermal conductivity, which is harmful for the improvement of laser power and efficiency. By contrast, the YGG crystal has a relatively lower phonon energy (752 cm−1) and higher thermal conductivity (9 W·m·K−1) [19]. Therefore, the Er:YGG crystal is an excellent gain medium to achieve the 3 μm laser operations.

In the recent years, Li et.al reported a high-quality Er:YGG crystal grown by optical floating zone method, the maximum output power of 1.38 W is obtained with slope efficiency of 35.4% [12]. This work indicated that the high-power and high-efficiency mid-infrared laser can be achieved in the Er:YGG crystal, but the laser out power has not been further improved due to the LD end-pumped method and the limited size of laser element.

In this work, a high-quality Er:YGG crystal was grown with size of Φ 25 × 70 mm3 by Czochralski (CZ) method, and the LD side-pumped way was used to increase laser power. The crystal phase structure, spectroscopic properties, and CW laser performance were investigated. A maximum output power of 7.25 W was achieved in the LD side-pumped Er:YGG crystal.

2 Methods and experiments

2.1 Crystal growth

Using the Czochralski (CZ) method, a high-quality 30 at% Er:YGG crystal with dimensions of about Φ 25 mm × 70 mm was successfully grown along the crystal orientation of < 111 > , as shown in Figs. 1 and 2. High-purity Er2O3 (5N), Y2O3 (5N) and Ga2O3 (5N) were used as raw materials, the Ga2O3 was overweighed by 1.8 wt. % to compensate the volatilization loss during crystal growth. During the crystal grown process, the pulling rate and rotation speed were maintained with 0.8–2 mm/h and 0.5–2 rpm, respectively.

Fig. 1
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Photograph of as-grown Er:YGG laser crystal

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Fig. 2
figure 2

Schematic diagram of LD side-pumped Er:YGG laser

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2.2 Characterization

The inductively coupled plasma mass spectrometer (ICP-MS, ICAP-QC, Thermo Fisher Scientific) was used to measure the concentrations of Y3+ and Er3+ ions in the Er:YGG crystal. Using a Philips X'pert PRO X-ray diffractometer, the crystal phase was determined from 10° to 90°. Single crystal XRD data of the Er:YGG crystal were collected using a Bruker D8 VENTURE diffractometer, which was equipped with graphite monochromatic Mo-Kα radiation (λ = 0.071073 nm) at 193 K. The thermal conductivity of the Er:YGG samples with a size of Φ 12.7 mm × 2.5 mm was tested in the range of 295 to 500 K by employing a laser thermal conductivity meter (LFA467 LT). The absorption spectra were performed by a spectrophotometer (PE lambda 1050+) in the wavelength range of 250–2500 nm and 950–985 nm with step of 1 nm and 0.1 nm, respectively. The transmission spectrum was achieved by a Fourier Transform Infrared Spectrometer (Nicolet iS50R AM FTIR) with the wavelength range of 2.5–10 μm. With an exciting source of 973 nm LD, the fluorescence spectrum from 2.6 to 3.0 μm was recorded using an Edinburgh fluorescence spectrometer (FLSP 920). The fluorescence decay curves were obtained by adopting FLSP 920 with the excitation source of Opolette (OPO) 355 I lasers.

2.3 Laser experiment setup

For the laser experiment, a Φ 3 mm × 66 mm Er:YGG crystal rod was cut and processed as the gain medium. The two end-faces of the crystal rod were coated with an antireflection film in the region of 2.65∼3.0 µm, which was placed into a close-coupled and LD side-pumped photospot chamber. The cavity of pump source was cooled with circulating water and maintained at a temperature of 22 ℃. The center emission wavelength of LD side-pumping source was located at 978 nm. The resonant cavity is a flat–flat cavity structure with a geometric length of 80 mm. The high-reflection (HR) mirror was coated with a HR film (> 99.5%) in the region of 2.7 ~ 3.0 μm, the output coupler (OC) mirrors with transmissions of 0.5%, 2% and 5% at 2.79 μm were used, respectively. The laser output power and laser beam profile were determined by a power meter (Ophir 30A-BB-18) and pyroelectric array camera (Ophir-Spiricon PY-III-HR). The laser spectra were recorded using a Fourier Transform Infrared Spectrometer (Nicolet iS50R AM FTIR) with resolution of 0.125 cm−1, corresponding to the step width of 0.015 cm−1.

3 Results and discussion

3.1 Crystal growth, structure, and thermal properties

The actual concentrations of Y3+ and Er3+ in the top section of Er:YGG crystal were surveyed by the ICP-OES method, as shown in Table 1. The values are determined to be 69.7 at% and 30.3 at%, respectively. The segregation coefficients (keff) Y3+ and Er3+ can be calculated to be 0.996 and 1.01 with formula of keff = Cs/Cl, in which Cs and Cl are the concentrations in the as grown crystal and melt. Both two values are close to 1, which should due to the ionic radii of Y3+ and Er3+ are relatively close.

Table 1 Segregation coefficients of Er and Y ions
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The powder XRD data of Er:YGG crystal and standard card of YGG phase (PDF#43-0512) are shown in Fig. 3a. The diffraction peaks in the range of 10–90° are sharp and consistent with the standard cards of pure YGG crystal, indicating the Er:YGG crystal with garnet structure is successfully prepared. Besides the detailed crystallographic data obtained by single-crystal XRD, the three-dimensional crystal structure diagram of Er:YGG crystal is shown in Fig. 3 (b). Other detailed crystallographic data are summarized in Tables S1–S6, respectively. The results indicate that the Er:YGG crystal is crystallized in the cubic system with Ia-3d space group, the unit cell parameters are a = b = c = 12.2379(4) Å, α = β = γ = 90°, cell volume V = 1832.82(18) Å3, and Z = 8, respectively.

Fig. 3
figure 3

a XRD patterns of Er:YGG crystal, b crystal structure of Er:YGG crystal along (010) orientation

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The thermal diffusivity, specific heat and thermal conductivity of Er:YGG crystal are tested and shown in the Table S7. Thereinto, the thermal conductivity as function of temperatures is shown in Fig. 4. The thermal conductivity of Er:YGG crystal is 4.95 W·m−1·K−1 at 295 K and it decreases with increasing temperature. This value is much larger than that of the Er:YSGG crystal (3.27 W·m−1·K−1) [20], which is beneficial to decrease the thermal effect during the laser oscillation.

Fig. 4
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Thermal conductivity of Er:YGG crystal at different temperatures

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3.2 Absorption and transmission spectrum

The room temperature (RT) absorption spectrum of the 30 at% Er:YGG crystal in the range of 250–2500 nm is shown in Fig. 5. The characteristic absorption bands of Er3+ are exhibited, ascribing to the transitions from ground state to excited states. A wide absorption band with full width at half maximum (FWHM) of 12.5 nm is observed, corresponding to the transition of 4I15/2 → 4I11/2, as shown in the inset a of Fig. 5. The maximum absorption coefficient of 7.33 cm−1 is located at 965.8 nm, which is overlapped with the emission bands of commercially InGaAs LD. Besides, the transmission of Er:YGG crystal in the 2.5 ~ 10 μm waveband is shown in the inset b of Fig. 5. The Er:YGG crystal possesses a high transmission of ≥ 80% in the 2.5–4.5 μm.

Fig. 5
figure 5

Absorption spectrum of Er:YGG crystal; inset: a enlarged absorption curve in the range of 950 ~ 985 nm; b transmission curve of Er:YGG crystal in the range of 2.5–10 μm

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3.3 Fluorescence spectrum and level lifetime

The mid-infrared fluorescence spectrum of the 30 at% Er:YGG crystal is exhibited in Fig. 6 (a), which is corresponding to the energy transition from 4I11/2 to 4I13/2. Several strong fluorescence peaks are located at wavelengths around 2634, 2702, 2796, 2820 and 2922 nm, respectively, suggesting that the Er:YGG crystal is potential candidate for the mid-infrared laser near 3 μm pumped by the 970 nm LD. By tuning the emission wavelength of OPO pulse lasers to 970 nm, the fluorescence decay curves of the Er:YGG crystal are measured at 1012 and 1625 nm, corresponding to the transitions from upper (4I11/2) and lower (4I13/2) laser levels to ground state level (4I15/2). The lifetimes of 4I11/2 and 4I13/2 are fitted to be 0.83 and 3.79 ms, and the level lifetimes ratio is 4.57. This value is smaller than those of the 50 at% Er:YAG (4.55/0.33 = 13.79), 30 at% Er:GGG (7.55/0.9 = 8.39) and 20 at% Er:LuGG (10/0.7 = 14.29) crystals [21,22,23], which is beneficial to implementing particle number inversion and decreasing the laser threshold.

Fig. 6
figure 6

a Fluorescence spectrum of the Er:YGG crystal excited by 973 nm LD; b fluorescence decay curves of the 4I11/2 and 4I13/2 levels in the Er:YGG crystal

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3.4 Laser performance

The experiment setup is shown in Fig. 2, the laser performance of Er:YGG crystal is investigated under 978 nm LD side-pumped method in CW mode. As shown in Fig. 7a, the laser performance was studied with OCs of different transmission for 0.5%, 2% and 5%, respectively. The maximum output power of 7.25 W is achieved with the OC of T = 2%, corresponding the slope efficiency of 5.74%. Under the OC of T = 0.5% and 5%, the maximum output powers are 5.38 and 0.18 W, respectively. With the increase of OC transmittance, the laser threshold is gradually increased due to the reduction of the light in the resonator.

Fig. 7
figure 7

a Laser output power as function of input power, b output power (under input power of 228 W) as function of OC transmittance

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Usually, the increased OC transmission (Toc) will enhance the light strength output from the resonator but decrease the light in the resonator. Therefore, the optimal Toc is existed to obtain a maximum output power, the output power in a laser resonator can be expressed as

$${P}_{out}=A\left(\frac{{T}_{OC}}{2-{T}_{OC}}\right){I}_{s}\left(\frac{2{g}_{0}l}{\delta -\mathrm{ln}\left(1-{T}_{OC}\right)}-1\right)$$
(1)

where A is the beam cross-section, the cross-section (= 0.07065 cm2) of the crystal was used in this work. Is, g0, and l are denoting the pump saturation intensity, unsaturated gain coefficient, and medium length (l = 6.6 cm), respectively. δ is the resonator loss caused by Toc and light diffraction. The output power measured at 228 W under different Toc is shown in Fig. 7b, and it is fitted according to the Eq. (1). And then the Is, g0, and δ can be determined by the fitted result, as listed in Table 2, the optimized Toc can be calculated to be 1.3%.

Table 2 Parameters of the laser system
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Compared with other Er3+-doped mid-infrared lasers operated in the CW mode, the LD side-pumped Er:YGG laser exhibits a higher laser output power, but the laser efficiency is much lower (Table 3). The laser output power and laser efficiency are promising to be further improved using the 969 nm LD (match with the central of absorption peak) and further optimizing the Toc.

Table 3 Comparison of the Er3+-doped mid-infrared solid-state laser operated in the CW mode
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The beam quality and laser wavelength are studied and shown in Fig. 8. The beam profiles are recorded after through a CaF2 lens with focal length of 300 mm. The beam diameters as function of the propagation distance after the lens are shown in Fig. 8a, and the 2D and 3D beam profiles near the focus point are also given. And the laser beam quality M2 factors in the x and y axes are fitted and calculated to be 5.23 and 5.24, corresponding to the far-field divergence angle of 17.61 and 17.74 mrad, respectively. The laser emitting wavelength of LD side-pumped Er:YGG laser is shown in Fig. 8b. No other laser wavelengths can be observed when the output power increase from 0.36 to 6.06 W. The central wavelength is located at 2823.9 nm (3541.2 cm−1) with a FWHM of 0.12 nm (0.15 cm−1).

Fig. 8
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a Beam quality and profiles under the output power of 5.4 W, b laser emitting wavelength

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4 Conclusion

High-quality Er:YGG crystal was grown successfully by the CZ method. The results of XRD and thermal conductivity indicate the crystal possesses high crystalline quality and excellent thermal properties. The absorption spectrum exhibits the crystal has broader absorption around 965.8 nm with the FWHM of 12.5 nm, which is match well with the InGaAs LD. The mid-infrared fluorescence spectra and energy level lifetimes suggest that the Er:YGG crystal is an promising medium to achieve 2.7–3 μm laser. A LD side-pumped Er:YGG CW laser is demonstrated with a maximum output power of 7.25 W and a slope efficiency of 5.74%. The beam quality factors are determined to be 5.23/5.24, the central laser wavelength is located at 2823.9 nm. The laser output power and efficiency are promising to be further improved by optimizing the Er3+ concentration, resonant cavity structure (using the flat concave cavity and double concave cavity), crystal size, OC transmittance, LD wavelength and enhancing thermal management on the laser crystal (thermal boding and concave end face crystal element).