1. Introduction

In recent years, with the development of laser diode (LD) technology, laser crystals doped with Yb³⁺ ions have drawn much attention. At present, a variety of Yb³⁺ -doped laser crystals have been reported, because trivalent ytterbium (Yb³⁺) doped materials have demonstrated significant potential for application in ultrafast laser, high-power laser and amplifier system. Yb³⁺ ions possess two electronic states of ground state ²F_7/2 and excited state ²F_5/2 that retaining the advantages of low quantum defect, long fluorescence lifetime and high conversion efficiency [1–3].

Compared with oxide host materials, fluorides (CaF₂, SrF₂) have superior properties such as wide transmission wavelength, higher thermal conductivity, lower refractive index, and lower phonon energy [4,5] In 2004, the first CW and tunable laser operation of an Yb³⁺:CaF₂ single crystal was obtained [6]. In 2008, Mathias reported the first time chirped-pulse amplification to the terawatt level in a fully diode-pumped scheme employing Yb³⁺:CaF₂ [7]. However, there are less research on strontium fluoride applied in laser. In 2007, first tunable laser operation of maximum average output power of 270 mW and a tuning range of 73 nm was observed in single crystalline Yb³⁺:SrF₂ [8]. In 2009, Femtosecond mode-locked operation of shortest pulse duration is 143 fs for an average power of 450 mW and highest average power is 620 mW for a pulse duration of 173 fs were demonstrated for the first time with Yb³⁺:SrF₂ crystal [9]. Due to the different valencies of the dopant Yb³⁺ and the substituted alkaline cations (Ca²⁺, Sr²⁺), interstitial fluoride are generated in fluoride, tending to form clusters even in low doping concentrations, which generally act as emission quenching centers. We can use R³⁺(Y³⁺,La³⁺,Gd³⁺,Lu³⁺,etc.) ions as structural modifier, which can break the clusters and change the environment of Yb³⁺ [10–12]. The validity of this method has been proved in CaF₂ system in recent years. In 2014, a 330 fs pulse at a center wavelength of 1049 nm with average output power of 224 mW at a repetition rate of ~83 MHz is obtained in Yb³⁺ doped Ca_1-xY_xF₂ crystal [13].

In this article, for the first time, we study the spectroscopic properties of 3 at.%Yb: Sr_1-xGd_xF_2+x single crystals, grown by the temperature gradient technique (TGT) method, including absorption and emission cross-section. we also report on diode-pumped, true CW laser operation in 3 at.%Yb: Sr_1-xGd_xF_2+x crystal.

2. Experimental

3 at.%Yb: Sr_1-xGd_xF_2+x single crystals are grown by the temperature gradient technique (TGT) method. The starting materials were YbF₃ (4N), GdF₃ (4N) and SrF₂ (4N) powders, and 1 at.% PbF₂ as an oxygen scavenger mixed in SrF₂ powder.

The segregation coefficients of Yb³⁺ ions and R³⁺ ions were detected by inductively coupled plasma atomic emission spectrometry (ICP-AES). The principle of quantitatively determining the element contents is to measure the emission intensity of each element and compare with those in standard solutions. The concentration of the element could be determined accordingly.

The cell parameters of the grown 3 at.%Yb: Sr_1-xGd_xF_2+x crystals were characterized by X-ray Diffraction (Ultima IV diffractometer, Rigaku, Japan) measurement using Cu Kα radiation at a scan width of 0.02° within 2θ = 10 – 80°.The lattice parameters of the crystals were calculated by Jade 5 program.

The spectral measurement was done in polished samples with thickness of 0.5 mm. The absorption spectra were measured by using a UV/VIS/NIR spectrophotometer (VARIAN Cary 5000) with scanning step of 1 nm. The fluorescence spectra and fluorescence lifetime were obtained from InSb detector (Hamamatsu) by using a Time Resolved Fluorimeter (FLS980, Edinburgh Instruments, UK) with pump wavelengths of 915 nm. All measurements were conducted at room temperature.

The laser operation was carried out in a plano-concave cavity. The cavity consisted of a plane mirror (M1, high-reflection (HR) @ 1040 nm) and a concave-plane (r = 200 mm) output coupler (OC) with transmission of 3% @ 1064 nm. The pump source was a fiber-coupled diode laser operating at 976 nm. The laser beam was collimated into the gain medium with a spot waist radius of 105 μm through a 1:2 coupling optics system. An uncoated 3 at.%Yb: Sr_1-xGd_xF_2+x crystal with aperture 3 × 3 mm² and thickness 3 mm was used as gain medium. To remove accumulated heat, the crystal was wrapped with indium foil and mounted in a water-cooled copper block maintaining the temperature at 13°C. The average output power was measured by a 30A-SH-V1 power meter (Israel).

3. Results and discussions

3.1 X-ray diffraction patterns

Figure 1(a) shows the X-ray diffraction patterns of the as-grown 3 at.%Yb: Sr_1-xGd_xF_2+x crystal. The XRD pattern of 3 at.%Yb: Sr_1-xGd_xF_2+x crystal is consistent with standard pattern stemming from PDF standard card(PDF#06-0262). It reveals that the 3 at.%Yb: Sr_1-xGd_xF_2+x crystal maintains a cubic structure Fm3m, which is the same as that in 3 at.%Yb:SrF₂ crystal.

 

Fig. 1 X-ray Diffraction measurement of the as-grown 3 at.%Yb: Sr_1-xGd_xF_2+x single crystal. (a). the whole comparison from 10° to 80° of X-ray Diffraction intensity in 3 at.%Yb: Sr_1-xGd_xF_2+x single crystal; (b). the partial comparison from 25° to 29° of X-ray Diffraction intensity in 3 at.%Yb: Sr_1-xGd_xF_2+x single crystal

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In Fig. 1(b), it is obvious that the diffraction peaks of Yb-doped single or mixed crystals shifted slightly towards the longer angle side and lattice constants decreased compared with standard card. This is mainly ascribed to the replacement of Sr²⁺ with the relatively smaller radius of Gd³⁺ and Yb³⁺. However, it presents a nonlinear relationship between the concentration of Gd³⁺ and the sites of peaks due to the effect of interstitial fluoride ions.

3.2 Segregation coefficients

Table 1 shows that segregation coefficients of Yb³⁺ and Gd³⁺ ions in 3 at.%Yb: Sr_1-xGd_xF_2+x crystal. It is obvious that the segregation coefficients of Yb³⁺ and Gd³⁺ are decreasing with the concentration of Gd³⁺ increasing. It can be contributed to competition on Yb³⁺ and Gd³⁺ ions, when they enter into the lattice of Sr⁴⁺.With the concentration of Gd³⁺ increasing, the Yb³⁺ ions become difficult to enter the lattice, thereby reducing the real concentration of Yb³⁺ ions and reducing the segregation coefficient of Yb³⁺ ions in the crystal.

Table 1. Segregation coefficient of Yb³⁺ and Gd³⁺ ions in 3 at.%Yb: Sr_1-xGd_xF_2+x crystal.

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3.3 Absorption and emission spectra

Figure 2 shows the absorption spectra of 3 at.%Yb: Sr_1-xGd_xF_2+x crystals at room temperature. The absorption coefficient α was determined by the equation $\text{α}=2.303\text{OD}/\text{L }$ and absorption cross-section was calculated from the equation ${\sigma }_{abs}=\alpha /N$. Here, OD stands for optical density. L is sample thickness. N is the concentration of Yb³⁺ ions. According to the absorption spectra depicted in Fig. 2, the absorption peak position of 3 at.%Yb: Sr_1-xGd_xF_2+x disordered crystal were same as compared with that of 3 at.%Yb:SrF₂ crystal. The absorption coefficient and absorption cross section in 3 at.%Yb: Sr_1-xGd_xF_2+x crystal were shown in Fig. 2(a) and Fig. 2(b), respectively. The absorption coefficient of disordered crystals is lower than 3 at.%Yb: SrF₂ crystal, but the absorption cross-section of disordered crystals is higher. Furthermore, in Fig. (b), we can draw that the absorption cross-section of 3 at.%Yb: Sr_1-xGd_xF_2+x crystal is increasing with the concentration of Gd³⁺ increasing, and tends to be saturated under high concentration of Gd³⁺ ions. The absorption cross section in 3 at.%Yb: Sr_0.97Gd_0.03F_2.03, 3 at.%Yb: Sr_0.94Gd_0.06F_2.06 and 3 at.%Yb: Sr_0.91Gd_0.09F_2.09 crystal enhanced to 0.884 × 10⁻²⁰ cm², 0.902 × 10⁻²⁰ cm² and 0.906 × 10⁻²⁰ cm² at 975 nm comparing with that of the absorption cross section of 0.800 × 10⁻²⁰ cm² in 3at%Yb:SrF₂ crystal, respectively. The structural modifier of Gd³⁺ ions affect the local environment of Yb³⁺ ions, resulting in breaking crystal local symmetry, which could enhance the absorption transition oscillation strengthen.

 

Fig. 2 Absorption spectra of 3 at.%Yb: Sr_1-xGd_xF_2+x single crystal. a) absorption coefficient; b) absorption cross-section.

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The emission cross-section σ_em of Yb³⁺ ion was calculated from reciprocity method formula ${\sigma }_{em} (\upsilon )=({\sigma }_{abs}(\upsilon ) Z_l)/Z_u exp[(E_{ZL}-h\upsilon )/kT]$ [14]. Here, σ_abs(v) is the absorption cross section, Z_u is the partition function of the upper energy level, Z_l is the partition function of the lower energy level, h is the Planck's constant, k is Boltzmann's constant, E_ZL is the zero line energy which, is defined to be the energy separation between the lowest components of the upper and lower states, respectively.

Figure 3(a) gives the emission cross section spectra in 3 at.%Yb: Sr_1-xGd_xF_2+x crystals. There are wide emission bands from 940 nm to 1120 nm, which is mainly ascribed to ²F_5/2→²F_7/2 transition process of Yb³⁺. Meanwhile, the emission peaks of 976 nm, 1012 nm, 1029 nm and 1041 nm can be observed, corresponding to transfer from upper level ²F_5/2 to different stark levels. The strong emission at 976 nm reflects obvious self-absorption of Yb³⁺ ions in mixed crystals.

 

Fig. 3 a) The emission cross section spectra for in 3 at.%Yb: Sr_1-xGd_xF_2+x crystals; b) the emission cross-section at 976 nm, 1012 nm, 1029 nm and 1041nm varying with the concentration of Gd³⁺ ions in 3 at.%Yb: Sr_1-xGd_xF_2+x crystals.

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Figure 3(b) shows that the emission cross-section at 976 nm and 1041nm of mixed crystals are enhanced, compared with single doped crystal. However, the emission cross-section at 1029 nm of mixed crystals are higher in the lower concentration of Gd³⁺ ions, smaller in the higher concentration. Specially, the emission cross-section at 1012 nm of 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 is better than 3 at.%Yb: SrF₂ crystal only. After doping regulating ions of Gd³⁺ in lower concentration, fluorescence quenching center of [Yb³⁺-Yb³⁺] clusters is destroyed and changed to be [Yb³⁺-Gd³⁺], resulting in the emission cross section increasing. However, there is an opposite effect that the real concentration of Yb³⁺ ions becomes lower, when Gd³⁺ ions doped in high concentration. In comparison, the highest emission cross section can be obtained in 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 crystal about 1.21 × 10⁻²⁰ cm² at 976 nm, 0.45 × 10⁻²⁰ cm² at 1012 nm, 0.32 × 10⁻²⁰ cm² at 1029 nm and 0.29 × 10⁻²⁰ cm² at 1041 nm. Moreover, the Gd³⁺ ions tend to form [Gd³⁺-Gd³⁺] clusters instead of [Yb³⁺-Gd³⁺] clusters at high concentration of Gd ion, which makes them not play a good role in dispersing Yb³⁺ ions. So, the emission cross-section are slightly decreasing when the concentration of Gd over 3 at.%.

As shown in Table 2, compared with other Yb-doped crystals, Yb:Sr_1-xGd_xF_2+x crystals have relatively high emission cross-section, such as SrF₂, LiYF₄, BaF₂ Y₂SiO₅ and Sc₂SiO₅.

Table 2. The emission cross section σ_em of different Yb-doped crystals.

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3.4 Fluorescence lifetime and laser parameters

As shown in Fig. 4(a), emission decays of ²F_5/2 emitting energy level were recorded for the different 3 at.%Yb: Sr_1-xGd_xF_2+x samples, whose the single-exponential character of decay curves can be observed. Therefore, the decay curves of 3 at.%Yb: Sr_1-xGd_xF_2+x samples were fitting by single-exponential model, and the lifetime τ of 5.41 ms, 2.53 ms, 5.56 ms, 2.37 ms and 2.60 ms are obtained for the different concentrations of Gd³⁺ ions at 0 at%, 1 at%, 3 at%, 6 at% and 9 at%, respectively.

 

Fig. 4 a) The fluorescence lifetime of 3 at.%Yb: Sr_1-xGd_xF_2+x crystals; b) The average output power as a function of input power for CW laser; c) FOM Gain at 1052 nm and optic-optic efficiency in 3 at.%Yb: Sr_1-xGd_xF_2+x crystals; d) The average output power as a function of absorbed pump power for CW laser in 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 crystal.

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Figure 4(b) shows the output power from 3 at.%Yb: Sr_1-xGd_xF_2+x crystals at different input power. The slopes of fitting curves stand for optic-optic efficiency. For 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 sample, we get the best optic-optic efficiency of 46.49%. Moreover, the optic-optic efficiency vary with the concentration of Gd³⁺ ion as shown in Fig. 4(c). Unexpectedly, there is a zigzag variation drawn from the figure, which reveal that doping Gd³⁺ ions have multiple and complex effects on laser performance.

For an amplifier device, the figure-of-merit (FOM) for gain is defined as the product of lifetime and emission cross-section: σ_em × τ [15]. Illustrated in Fig. 4(c), FOM gain at 1052 nm in 3 at.%Yb: Sr_1-xGd_xF_2+x crystals can be calculated to be 1.03 × 10⁻²⁰ ms·cm², 0.51 × 10⁻²⁰ ms·cm², 1.22 × 10⁻²⁰ ms·cm², 0.51 × 10⁻²⁰ ms·cm² and 0.54 × 10⁻²⁰ ms·cm² for the different concentrations of Gd³⁺ ions at 0 at%, 1 at%, 3 at%, 6 at% and 9 at%, respectively. Interestingly, the change regular of FOM gain is consistent with that of optic-optic efficiency, which is a good explanation for the best laser performance in 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 crystal. The higher FOM gain of 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 crystal leads to better laser performance due to its higher emission cross section and longer fluorescence lifetime.

The output power of 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 sample at different absorbed power can be described as follows Fig. 4(d). When the absorbed pump power reached 8.02 W, the maximum output power of 3.8W could be obtained by using an output coupler with transmission T_OC = 3% around 1052 nm, corresponding to an optical conversion efficiency of 46.49%, and a slope efficiency of 53.5%. It could be noted that the condition for maximum slope efficiency was not yet optimized. In the future experiments, we would optimize the transmission value of output coupler as well as the pump wavelength.

4. Conclusions

In summary, 3 at.%Yb: Sr_1-xGd_xF_2+x single crystals were successfully obtained with higher emission cross section as compared with that of 3 at.%Yb:SrF₂ crystal. The 3 at.%Yb: Sr_1-xGd_xF_2+x crystal maintains a cubic structure Fm3m, which is the same as that in 3 at.%Yb:SrF₂ crystal. Specifically, the absorption cross section of 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 crystal was increased by more than 10% when compared with that of 3 at.%Yb:SrF₂. Both the emission cross section σ_em and fluorescence lifetime in 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 crystal were enhanced as compared with that in 3 at.%Yb:SrF₂. A primary but not yet optimized laser experiment produced out power of 3.8 W with a optic-optic efficiency of 46.5% in 3 at.%Yb: Sr_0.97Gd_0.03F_2.03 crystal. So, Yb: Sr_1-xGd_xF_2+x crystals may be a promising laser material in high-power laser system application.