We demonstrate a 968 nm diode end-pumped Er,Pr:GYSGG (Gd1.17Y1.83Sc2Ga3O12) laser at 2.79 μm operated in the pulse and continuous-wave (CW) modes. The lifetimes for the upper laser level 4I11/2 and lower level 4I13/2 are 0.52 and 0.60 ms, respectively. The laser produces 284 mW of power in the CW mode, corresponding to the optical-to-optical efficiency of 14.8% and slope efficiency of 17.4%. The maximum laser energy achieved is 2.4 mJ at a repetition rate of 50 Hz and pulse duration of 0.5 ms, corresponding to a peak power of 4.8 W and slope efficiency of 18.3%. These results suggest that doping deactivator Pr3+ ions can effectively decrease the lower-level lifetime and improve the laser efficiency.
© 2013 Optical Society of America
Solid-state lasers at the wavelength region of 2.7–3 μm are useful for medical and biological applications because of the strong water absorption around this spectral region [1, 2]. In addition, 2.7–3 μm lasers are suitable pump sources for infrared optical parametric oscillation (OPO) or optical parametric generation [3, 4]. Er3+ serves as an active ion emitting 2.7–3 μm by 4I11/2→4I13/2 transition, which is commonly believed to be self-terminating because the lifetime of the upper laser level is less than that of the lower level. Highly doped concentrations (>30 at.%) of Er3+ ions are proposed to overcome this self-terminating “bottleneck” effect by inducing upconversion (UC1) (4I13/2→4I15/2) + (4I13/2→4I9/2→4I11/2) and cross-relaxation (CR) (4S3/2 (2H11/2) →4I15/2) + (4I15/2→4I13/2) + (4I15/2→4I9/2→4I11/2) processes  (Fig. 1). Both processes influence population inversion and produce an even population of the initial (4I11/2) and final (4I13/2) energy levels as a whole. Another upconversion process (UC2) (4I11/2→4I15/2) + (4I11/2→4F7/2→4S3/2) generates green and red emissions and deactivates the initial laser level, but this negative effect can be compensated for by the above mentioned CR process involving 4S3/2 [5, 6]. Another method of reducing the lifetime of the lower laser level is the co-doping of deactivation ions [7–12], which is advantageous to population inversion. Pr3+ is a suitable ion because its energy level 3F4 is adjacent to level 4I13/2 of Er3+. The energy transfer diagram between Er3+ and Pr3+ ions is shown in Fig. 1. AT denotes the pumping absorption transition, and ET1 and ET2 denote energy transfer from 4I11/2→1G4 and 4I13/2→3F4, respectively; the efficiencies of energy transfer are determined as 64.2% and 96.5% in Er,Pr:GGG crystal , respectively. Knowles and Jenssen have reported  in detail the Er:BaY2F8 crystal codoped with Pr ions can efficiently deactivate the terminal laser level and show several advantages, including allowing the laser to be operated at lower Er concentrations which reduces the losses associated with 4I11/2 upconversion, and simplifying the dynamics by allowing quasi-4 level operation. Moreover, the new matrix GYSGG (Gd1.17Y1.83Sc2Ga3O12) that is attracting considerable attention can be obtained by replacing some Gd3+ ions with Y3+ in GSGG (Gd3Sc2Ga3O12). This new matrix exhibits many advantages such as excellent dual-wavelength laser property in Nd:GYSGG crystal [13–15] and radiation resistance ability in Er:GYSGG crystal [11, 16].
In this work, we report for the first time the spectroscopic properties of Er,Pr:GYSGG crystals grown by the Czochralski method. A 968 nm diode end-pumped 2.79 μm laser operated in the CW and pulse modes is also demonstrated.
2. Experimental details
An Er,Pr:GYSGG crystal was grown from a melt of congruent composition containing 20 at.% Er3+ and 0.3 at.% Pr3+ by the Czochralski method. The structural formula can be written as (Er0.6Pr0.009Gd1.17Y1.221)Sc2Ga3O12, which belongs to the space group of Ia3d with a cubic structure, and the lattice parameter is 12.4938 Å. Gd2O3, Ga2O3, and Sc2O3 powders (99.99% purity), as well as Er2O3 and Pr2O3 (99.999% purity), were used as starting raw materials and weighed according to the designed compositions. Ga2O3 was overweighed by 2 wt.% to compensate for evaporation loss during growth. Growth experiments were performed with the aid of an automatic diameter control unit operated in up-weighing mode. The pulling rate was 1 mm/h, and the seed rotation speed was 7 rpm. Er,Pr:GYSGG crystal with a high optical quality and dimensions of about Φ 25 mm × 100 mm was obtained. The photograph of the as-grown Er,Pr:GYSGG crystal is shown in Fig. 2. Sample disks were perpendicularly cut to the growth direction <111> from the post-annealing crystals and then polished on both sides. For spectral measurements, 2 mm-thick samples were used. For laser experiments, 2 mm × 2 mm × 5 mm samples were used. The absorption spectrum was recorded on a lambda 950 spectrophotometer (PerkinElmer, Inc, USA). A FLSP 920 fluorescence spectrometer (Edinburgh instrument Ltd, UK) was used to measure the fluorescence spectrum with a 968 nm laser diode (LD) excitation source, and fluorescence decay curves were obtained by excitation with an Opolette 355 I OPO laser (OPOTEK, Inc, USA).
The configuration for generating laser output is shown in Fig. 3. The pump source was an InGaAs LD emitting up to 40 W at around 968 nm in CW mode, and it also can be operated in pulse mode controlled by the appropriate driving current waveform with the repetition rate of 1-1000 Hz and minimum pulse duration of 0.1 ms. The fiber-coupled pump laser was collimated and focused onto an uncoated 2 mm × 2 mm × 5 mm Er,Pr:GYSGG crystal with parallel and polished end faces. The crystal was enclosed by a copper heat sink with cooling water passage. An indium film was placed between the Er,Pr:GYSGG crystal and copper heat sink for closer contact. A plane–plane cavity was used as a resonator, and the cavity lengths were 12, 15, and 18 mm, respectively. A K9 glass plate with antireflection coating of High transmission (HT) > 95% at 968 nm and reflectivity of 100% at 2.79 μm was used as the input mirror. The output mirrors (CaF2 substrate) with different transmissions of 0.5%, 2%, and 5% at 2.79 μm were used to obtain the optimum laser output.
3. Results and discussion
The absorption spectrum of Er,Pr:GYSGG crystal is shown in Fig. 4. The main absorption bands are found to be centered at around 376, 407, 450, 486, 523, 544, 654, 790, 968, and 1478 nm, 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 inset in Fig. 4 shows the absorption cross section within the wavelength range of 950-990 nm and the maximum absorption cross section at 968 nm is 1.6 × 10−21 cm2. The full width at half-maximum of 968 nm absorption band is about 15 nm, which is relatively suitable and desirable for efficient pumping by high-power InGaAs laser diodes. The Er3+ ions can be directly pumped into upper laser level 4I11/2 by radiation at around 970 nm and can avoid various non-radiative losses and thermal loading .
The fluorescence spectrum of Er,Pr:GYSGG crystal excited by a 968 nm LD is shown in Fig. 5. Many fluorescence peaks (2609, 2636, 2658, 2702, 2794, and 2823 nm) are observed within 2.6–3 μm, and these peaks result from the transitions of stark sub-levels from 4I11/2 to 4I13/2. In addition, we calculate the stimulated emission cross-section based on the Füchtbauer–Ladenburg equation Fig. 5, which is beneficial for obtaining low-threshold and high-efficiency laser output.
The fluorescence decay curves of 2.79 and 1.53 μm excited by OPO show single exponential decay behavior, corresponding to the lifetimes of upper level 4I11/2 and lower level 4I13/2 at 0.52 and 0.60 ms, respectively (Fig. 6). By contrast, the lifetime of upper level 4I11/2 is 1.2 ms and that of the lower level 4I13/2 is 3.9 ms in Er:GYSGG . This finding indicates that Pr3+ can depopulate the Er3+:4I11/2 and 4I13/2 levels by resonant energy transfer to Pr3+: 1G4 (ET1) and 3F4 (ET2) (Fig. 1), respectively. In addition, the energy transfer efficiency of Er→Pr can be calculated from Eq. (2) and the aforementioned lifetime values of Er:GYSGG and Er,Pr:GYSGG, the energy transfer efficiencies of Er→Pr in ET1 and ET2 are 56.7% and 84.6%, respectively. Therefore, the energy transfer rate of ET2 is greater than that of ET1, which is advantageous to the improvement of 2.7–3 µm laser performance. As the efficiency of energy transfer from Er to Pr strongly relies on the concentration of both dopants , a suitable concentration ratio between Er and Pr could keep the 4I11/2 as long as possible while significantly reduce the 4I13/2 lifetime.
Figure 7 shows the laser output power as a function of input pump power for different transmissions of output coupling mirror. The pump laser is operated in the CW mode, and the cavity length is maintained at 12 mm. The output coupler with a transmission of 2% generates the better results than the others. The maximum laser power of 284 mW corresponding to the threshold of 112 mW is obtained. Linear fitting results show 14.8% optical-to-optical efficiency and 17.4% slope efficiency. At 150 mW output power, the far-field divergence of 6.2 mrad and M2 factor of 1.72 were determined using the knife-edge method. This value of slope efficiency (17.4%) of Er,Pr:GYSGG is larger in compared with that of Er:GYSGG (10.1%). The laser threshold is also lower than that of Er:GYSGG, which can be attributed to the doping of deactivator Pr3+ ions that can reduce the lower-level lifetimes of Er3+ by resonant energy transferring. Meanwhile, the maximum laser output power 284 mW is lower than the reported value (348 mW) for Er:GYSGG , which may be due to the fact that the shortened lifetime of upper level 4I11/2 is detrimental to energy storage during diode CW pumping.
The influence of cavity length on laser output power at 2% transmission of the output coupler was investigated, and the results are shown in Fig. 8. Notably, the laser output power decreases with increasing cavity length. This phenomenon is mainly due to the thermal lens effect in Er,Pr:GYSGG crystal, resulting in larger diffraction losses in a longer cavity [17, 20]. Therefore, a shorter cavity and a better cooling setup can improve the laser output power and efficiency. Dinerman et al.  reported a CW mode pumped Er:YSGG crystal with 511 mW output power and slope efficiency up to 26% using two end surfaces of the crystal as resonant cavity. In the future, we will optimize crystal size and cavity structure to improve the laser performance of Er,Pr:GYSGG.
The laser output energy versus the pump energy at different repetition rates of 50, 75, 100,150 and 200 Hz are shown in Fig. 9. Meanwhile, an output coupler with 2% transmission, pulse duration with 0.5 ms and 12 mm-long cavity was used throughout the entire experiment. The laser output energy exhibits moderate difference below the repetition rate of 100 Hz. In addition, it can be seen that at low pump energy the slope efficiency is much higher and falls after a certain roll-over point with increasing energy when the repetition rate is >100 Hz. The phenomenon can be attributed to the fact that a plane-plane resonator is very sensitive against thermal lensing, leading to the slope efficiency measurements distorted dramatically at a higher repetition rate and larger pumping energy. The plane-concave or V-type resonators are more stable and could allow the crystal to be operated at a higher repetition rate. The maximum output energy of 2.4 mJ corresponding to the peak power of 4.8 W is obtained when the repetition rate is 50 Hz. Linear fitting results show optical-to-optical efficiency of 15.5% and slope efficiency of 18.3%. Under similar laser configurations, Liu et al.  reported a maximum peak power of 970 mW with a pulse duration 2 ms and slope efficiency of 7.6% in Cr,Er:YSGG. Dinerman et al.  obtained a peak power of 3.4 W with a pulse duration of 1 ms and slope efficiency of 9.1% on a quasi-CW diode end-pumped Er:YSGG.
The increasing laser efficiency and decreasing laser threshold of Er,Pr:GYSGG crystal are due to the doping of deactivator Pr3+ ions. However, the maximum laser output power and the performance operating at high repetition rate are only close to those of the Er:GYSGG . Therefore, further studies may focus on optimizing the concentration ratio of Pr3+ to Er3+ and cavity parameters to improve the laser performance of Er,Pr:GYSGG crystal entirely.
We report for the first time the spectroscopic and diode end-pumped laser properties of Er,Pr:GYSGG crystal grown by the Czochralski method. The lifetimes for the upper laser level 4I11/2 and lower level 4I13/2 are 0.52 and 0.60 ms, respectively, which are due to the doping of Pr3+ ions. The laser produces 284 mW of power in the CW mode, corresponding to a slope efficiency of 17.4%. The minimum laser threshold is only 112 mW. The maximum laser energy achieved is 2.4 mJ at a repetition rate of 50 Hz and pulse duration of 0.5 ms, corresponding to a peak power of 4.8 W and a slope efficiency of 18.3%. These results suggest that doping deactivator Pr3+ ions can decrease lower-level lifetime and effectively improve laser efficiency.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 91122021, 51272254, 61205173, 51172236, and 50932005).
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