3 at.% Er:SrF2 laser crystals with high optical quality were successfully grown using the temperature gradient technique (TGT). The intense mid-infrared emission was observed around 2.7 μm with excitation by a 970 nm LD. Based on the Judd–Ofelt theory, the emission cross-sections of the 4I13/2-4I11/2 transition were calculated by using the Fuchtbauer–Ladenburg (FL) method. Efficient continuous-wave laser operation at 2.8 µm was achieved with the lightly-doped 3 at.% Er:SrF2 crystal pumped by a 970 nm laser diode. The laser output power reached up to 1.06 W with a maximum slope efficiency of 26%.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In recent years, 3 µm mid-infrared (MIR) lasers have attracted much attention due to their applications in many fields, such as medical surgery, remote sensor, and pumping source of optical parametric oscillators (OPO) operated in the infrared wavelength range [1–3]. Such lasers can be realized by doping Er3+ or Ho3+ as activator ions [4–7]. However, due to the lack of suitable pump source, the development of Ho-doped lasers have been limited. In contrast, the strong absorption bands of Er3+-doped crystals are around 970 nm, so that they can be pumped by the well developed 970 nm InGaAs LD. Despite Er3+-doped MIR lasers have attracted increasing attention, the transition of this wavelength range is self-terminating because the fluorescence lifetime of the initial laser level is considerably shorter than that of the terminal laser level. To overcome this “bottleneck”, the doping concentration of Er3+ ions in some hosts is also generally required to be higher than 30 at% [8–12]. With highly doped concentration of Er3+, the lifetime of the lower laser level 4I13/2 of the Er3+ ions can be efficiently reduced. However, it causes difficulties in achieving high-quality laser crystals. Moreover, the energy gaps between the initial levels and the just lower levels are very small (less than 3600 cm−1).
In order to obtain high efficiency of the emission, it is necessary to find extremely low phonon energy hosts to reduce the nonradiative transition probability. SrF2 crystal is a famous laser host material with a low phonon energy of 280 cm−1, which is much lower than that of many oxide matrix such as YAG (700cm−1 ), Y2O3 (591 cm−1 ) and fluoride matrix such as YLF (560 cm−1 ). And SrF2 crystals have a key advantage that is rare earth ions tend to form clusters even when the doping concentration is low. Such clusters additional shorten the distance between Er3+ ions and thus enhances the energy transfer process between them, which is beneficial for achieving mid-infrared laser under low doping concentration.
In this paper, we reported spectroscopic properties of Er:SrF2 crystal grown by TGT method. An efficient 2.8µm laser pumped by a 970 nm LD was demonstrated with a lightly (3 at.%) Er3+-doped SrF2 crystal. A maximum average output power of 1.06 W was achieved and the corresponding slope efficiency was 26%.
Er3+ singly doped SrF2 crystal were grown by the temperature gradient technique (TGT) in a resistively heated furnace. The ErF3, SrF2 powders with 99.99% purity have been used as starting materials with the following atomic compositions: 3.0 at.% ErF3, 97.0 at.% SrF2, respectively. To prevent oxidation in the growing process, 1.0 wt % PbF2 was added to the starting materials. And the weighed chemical powders were mixed thoroughly and then placed into the graphite crucibles. The room temperature absorption spectra were measured by a Jasco V-570 UV/VIS/NIR spectro-photometer. The emission spectra were employed on a FLS980 time-resolved fluorescence spectrometer and detected by InAs photodetector. Luminescence decay curves were measured with μF900 microsecond lamp. All the measurements were carried out at room temperature.
3. Results and discussion
3.1 Spectroscopic properties
The absorption spectra of the Er:SrF2 crystal in a wavelength range of 300 nm to 2000 nm are shown in Fig. 1. There are 13 main absorption bands with center wavelengths at 355, 364, 378, 405, 441, 449, 486, 522, 539, 651, 800, 971 and 1510 nm, respectively, which are the typical absorption transitions of Er3+. We are interested in the band located at the 970 nm, corresponding to the transition 4I11/2-4I13/2 of Er3+ ions, which well matches with the mature laser diode (LD) pumping wavelength. As shown in the inset of Fig. 1, the FWHM of absorption band centered at 971 nm is about 19.6 nm and the absorption coefficient is 1.29 cm−1. The absorption cross-section at 970 nm is calculated to be 0.18 × 10−20 cm−2.
The emission spectra of 3at.% Er:SrF2 crystal from 2500 to 2860 nm excited by a 970 nm LD are shown in Fig. 2(a). Numerous fluorescence peaks are observed within 2.5-2.8 µm resulting from the transitions of the stark levels from 4I13/2 to 4I11/2 of Er3+ ions. The emission cross-section is calculated with the measured fluorescence spectrum based on the Fuchtbauer-Ladenburg equation:
The fluorescence decay curve of the Er:SrF2 crystal excited by 970 nm lasers shows a single exponential decay behavior, as shown in Fig. 2(b). By fitting the decay curve, the lifetime of the upper laser level 4I11/2 is obtained to be 10.8 ms.
3.2 Gain cross-section spectra
Based on the emission cross section (σem(λ)), the absorption cross section (σabs(λ)) can be obtained by:Fig. 3, along with the emission cross-sections.
Then, the gain spectra (G(λ)) at 2.7 μm can be calculated by the following equation:Fig. 4. Evidently, the gain cross-section becomes positive once the population inversion level reaches 60%.
3.3 Laser experiments
The laser performance was carried out for 3 at.% Er:SrF2 crystal. It had a length of 10 mm and cross-section of 3 × 3 mm2. The laser setup used in our experiment is showed in Fig. 5. The pump source was a commercial laser diode, delivering a maximum power of 30 W at 970 nm from a multi-mode fiber with a core diameter of 105 μm and a numerical aperture of 0.22. The pump beam was focused into the sample by a 2:3 coupling system. The uncoated Er:SrF2 crystal was mounted in a copper block stabilized at 3 °C by a cooling system. The laser cavity was formed by an input mirror M1 (T>90% @970 nm and R> 99.7% @ 2.8 μm) and an output coupler (OC) (T = 3% @ 2.8 μm). The OC had a radius of curvature of 50 mm.
Figure 6 shows the CW output power from the 3 at.% Er:SrF2 crystal laser pumped at 970 nm. In CW operation mode, the Er:SrF2 laser had a low threshold of 0.28 W. As the incident pump power increased, the output power increased continuously, but not linearly. There are three stages with laser slope efficiencies of 11%, 18%, and 26%, respectively, as shown in Fig. 6 The maximum output power of 1.06 W was obtained under an absorbed pump power of 5.7 W. This is the new breakthrough in the output power for the Er:SrF2 crystals around 2.8 μm wavelength, compared with the previous results of 356 mW  and 814 mW .
In summary, a lightly doped 3 at.% Er-doped SrF2 single crystal was successfully prepared by TGT method. The absorption, emission and gain cross sections spectra of the crystal were investigated. Under 970 nm LD pumping, the CW laser operation of 3at.% Er:SrF2 crystal was demonstrated around 2.8 μm. With pump power increasing, there are three stages for the laser output power, with slope efficiencies 11%, 18%, and 26%, respectively. The maximum CW output power of 1.06 W was obtained, which is a breakthrough for the laser operation of Er:SrF2 crystals in mid-infrared region.
National Key Research and Development Program of China (No. 2016YFB0701002); National Natural Science Foundation of China (Grants Nos. 61635012 and 51432007).
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