End-pumped by a 970 nm diode laser, 1534 nm pulse laser with about 16 μJ energy, 48 ns duration, and 21 kHz repetition rate was obtained at absorbed pump power of 11.8 W in a Z-cut 1.08-mm-thick Er3+:Yb3+:Sr3Lu2(BO3)4 crystal passively Q-switched by a Co2+:Mg0.4Al2.4O4 crystal. The effects of absorbed pump power and resonator cavity length on output performances of the pulse laser were investigated. Compared with that of the Er3+:Yb3+:LuAl3(BO3)4 laser in a similar experimental condition, higher pulse energy realized in the Er3+:Yb3+:Sr3Lu2(BO3)4 crystal may be originated from its smaller stimulated-emission cross section at 1534 nm and longer fluorescence lifetime of the 4I13/2 upper laser level of Er3+ ions.
© 2014 Optical Society of America
Passive Q-switching of Er-Yb solid-state laser is an attractive technique to construct a compact and low-cost 1.5-1.6 μm pulse laser source [1–3], which is eye-safe and can be used in some applications such as laser-range-finding, lidar, and medicine.
At present, continuous-wave (cw) and pulse laser operations at 1.5-1.6 μm have been demonstrated in some Er3+ and Yb3+ co-doped glass [4, 5], ceramic  and crystalline hosts, such as YAG , YVO4 , tungstate  and borate crystals [2, 9–11]. Among these materials, Er3+:Yb3+:RAl3(BO3)4 (Er:Yb:RAB, R = Y, Gd and Lu) crystals have been considered as one kind of good 1.5-1.6 μm laser gain media, because they have moderate thermal conductivity and high cw laser operation efficiency (up to 35%) [10–12]. Acousto-optic actively Q-switched, passively Q-switched and mode-locked 1.5-1.6 μm pulse laser operations have also been realized in the crystals [13–15]. However, short fluorescence lifetime (about 0.3 ms [10–12]) and low fluorescence quantum efficiency (below 10%) of the upper laser level 4I13/2 of Er3+ ions in the Er:Yb:RAB crystals limit their energy storage capacity and result in high thermal load during laser operation. Furthermore, flux growth method of RAB crystals also causes a long growth period and non-uniform of crystal optical quality.
Sr3R2(BO3)4 (SRB, R = Y, Gd and Lu) crystals belong to the orthorhombic system and can be easily grown by the Czochralski method in a short period [16, 17]. Compared with those of Y3+ and Gd3+, the radius and mass of Lu3+ ions are closer to those of Yb3+ ions. Therefore, the part substitution of Yb3+ ions for Lu3+ ions in lutetium crystals will have the higher optical quality and thermal conductivity than those of Yb3+-doped yttrium and gadolinium crystals . Recently, Er3+ and Yb3+ co-doped SLuB (Er:Yb:SLuB) crystal has been grown and 1.45 W quasi-cw laser at 1.5-1.6 μm with slope efficiency of 20% has been realized . Furthermore, fluorescence lifetime (about 0.67 ms ) of the 4I13/2 level of Er3+ ions in the crystal is two times larger than that in the Er:Yb:RAB crystal, which implies that the crystal has higher energy storage capacity and is beneficial to generating higher pulse output energy.
In this work, a 970 nm diode-pumped 1534 nm pulse laser operation in an Er:Yb:SLuB crystal passively Q-switched by a Co2+:Mg0.4Al2.4O4 spinel crystal is reported. The effects of absorbed pump power and resonator cavity length on output performances of the pulse laser have been investigated.
2. Laser experimental arrangement
A Z-cut, 1.08-mm-thick Er3+ (0.81 at.%):Yb3+ (24.2 at.%):SLuB crystal was investigated and the laser experiments were carried out on a plane-concave cavity, as shown in Fig. 1. The pump source was a fiber-coupled diode laser with core diameter of 800 μm and center wavelength of 970 nm. The pump light was re-imaged into the Er:Yb:SLuB crystal with a spot size of about 440 µm in diameter by a simple telescopic lens system (TLS). The crystal was uncoated and attached on an aluminum slab. In the center of the slab, there was a hole with diameter of 3 mm to permit the passing of the laser beams. In order to avoid the fracture of the crystal at high pump power, the diode laser was operated in quasi-cw mode. The pump pulse width was 2 ms and pulse period was 100 ms. Because the used pump pulse width was longer than the fluorescence lifetime (about 0.67 ms) of the 4I13/2 upper laser level of Er3+ ions in the crystal, the pump mode in this experiment can be considered as quasi-cw pumping. The absorption coefficient of the crystal at pump wavelength of 970 nm was about 10 cm−1 and then about 65% of incident pump power was absorbed by the crystal. A 1-mm-thick Co2+:Mg0.4Al2.4O4 crystal with an initial transmission of about 97% around 1.53 μm was used as the saturable absorber. It was antireflection coated for 1.53 μm on both faces and placed as close as possible to the Er:Yb:SLuB crystal. Flat input mirror (IM) of the laser resonator had 90% transmission at 970 nm and 99.8% reflectivity around 1.53 μm. An output mirror (OM) with 10 cm radius of curvature (RoC) and 3.0% transmission around 1.53 μm was used to complete the resonator. The length of the plano-concave cavity was about 10 cm.
3. Results and discussion
Figure 2 shows average output power of the passively Q-switched Er:Yb:SLuB laser as a function of absorbed pump power at 970 nm. Because laser pulse train was occurred only during the pumping time of 2 ms, the value in the figure is the measured power multiplied by fifty due to the 2% duty cycle of the quasi-CW diode laser. At absorbed pump power of 11.8 W, pulse laser with maximum average output power of 0.33 W and slope efficiency of 5.7% was achieved. The absorbed pump threshold was about 6 W. Spectra of the pulse laser recorded at various pump powers were similar. Then, only that recorded at the maximum absorbed pump power of 11.8 W is shown in Fig. 2 for the sake of brevity. It can be seen that laser wavelength is centered at about 1534 nm, which is consisted with the peak fluorescence wavelength of the Er:Yb:SLuB crystal . Furthermore, the polarization of output laser was measured to be totally linear and parallel to the Y axis of the crystal, which is similar to that observed in the quasi-cw Er:Yb:SLuB laser and caused by the larger stimulated-emission cross section at E//Y polarization direction .
Pulse profiles of the passively Q-switched Er:Yb:SLuB laser were measured by a 2 GHz InGaAs photodiode connected to a digital oscilloscope with bandwidths of 1 GHz (DSO6102A, Agilent). Pulse train and oscilloscope trace of the laser at absorbed pump power of 11.8 W are shown in Figs. 3(a) and 3(b), respectively. Pulse repetition rate was about 21 kHz and pulse duration was about 136 ns. The pulse-to-pulse amplitude fluctuation and interpulse time jittering were about 12% and 10%, respectively. Figure 4 shows pulse repetition rate and energy of the passively Q-switched Er:Yb:SLuB laser as functions of absorbed pump power. With decrement of absorbed pump power from 11.8 to 7.7 W, pulse repetition rate decreased from 21 to 5 kHz and pulse energy almost kept invariant at various pump powers . Pulse energy and output peak power of the laser were estimated to be about 16 μJ and 0.12 kW. The obtained energy is higher than that (about 9.9 μJ at 1520 nm) for the passively Q-switched Er:Yb:LuAB laser in a plano-concave cavity with a similar OM transmission . This may be a result of the smaller stimulated-emission cross section (about 0.88 × 10−20 cm2 at 1534 nm ) and longer fluorescence lifetime (about 0.67 ms) of 4I13/2 upper laser level of Er:Yb:SLuB crystal than those (about 1.7 × 10−20 cm2 at 1520 nm and 0.3 ms, respectively ) of Er:Yb:LuAB crystal, which implies the higher energy storage capacity of Er:Yb:SLuB crystal . Furthermore, the pulse energy obtained in the Er:Yb:SLuB laser is also higher than those realized in the passively Q-switched Er:Yb:YVO4 (about 4.3 μJ at 1604 nm ) and Er:Yb:GdCa4O(BO3)3 (about 2.8 μJ at 1538 nm ) lasers. Furthermore, because some important spectroscopic parameters, such as energy transfer rate, upconversion coefficient, and fluorescence lifetimes of some related manifolds, have not been accurately measured and reported until now, the comparison between theoretical and experimental pulse parameters of the Er:Yb:SLuB laser cannot be made at present.
In order to analyze the beam quality of the pulse laser, a lens with 10-cm focal length was used to focus the output beam and then the spatial profiles of the focused beam were recorded with a Pyrocam III camera (Ophir Optronics Ltd.). The beam diameter, which is calculated by the 4-sigma method, at various distances from the focusing lens at absorbed pump power of 11.8 W was measured and is shown in Fig. 5. By fitting these data to the Gaussian beam propagation expression, the quality factors and of output beam for the horizontal and vertical directions were estimated roughly to be about 4.8 and 3.6, respectively. Then, an optimal design of the resonator cavity is necessary in the future to improve the beam quality.
Previous investigation has shown that better quasi-cw laser performances of the Er:Yb:SLuB crystal can be realized when the OM transmissions were 1.0% and 1.5% . However, when above OMs were used in the passively Q-switched laser, dual-wavelength laser oscillations around 1534 and 1552 nm were observed, which causes the generation of multi-pulse and instability of the output pulse laser . Furthermore, when an OM with transmission higher than 3.0% was used, performances of the passively Q-switched Er:Yb:SLuB laser was degraded. Therefore, the OM with transmission of 3.0%, which is optimal and available in our lab, was adopted in this experiment. Moreover, when the cavity length was nearing the hemispherical condition and set closer to about 10 cm, i.e. the RoC of OM, better pulse performances of the Er:Yb:SLuB laser were obtained. When the cavity length was shortened and away from the hemispherical condition, output laser power decreased and output beam quality became worse.
It has been demonstrated that the pulse with shorter pulse duration and higher peak power can be obtained in a shorter cavity . Therefore, another OM with RoC of 3 cm and same transmission of 3.0% around 1.53 μm was also investigated. The cavity length was shortened to about 3 cm. At absorbed pump power of 11.8 W, pulse duration of the laser reduced to about 48 ns, as shown in Fig. 6. However, average output power, pulse repetition rate and energy still kept at about 0.33 W, 21 kHz and 16 μJ, respectively. Output peak power of the pulse laser increased to about 0.33 kW. Spatial profile and beam diameter of the laser at absorbed pump power of 11.8 W were also recorded and both are shown in Fig. 7. With the decrement of the cavity length, output beam quality became worse and M2 increased to 6.2.
970 nm diode-end-pumped passively Q-switched 1534 nm pulse laser was realized in an Er:Yb:SLuB crystal. At absorbed pump power of 11.8 W and cavity length of 3 cm, 1534 nm pulse laser with 16 μJ energy, 48 ns duration, 21 kHz repetition rate, and 0.33 kW peak power was obtained. When the cavity length increased to 10 cm, pulse energy and repetition rate kept invariant. However, pulse duration increased to 136 ns and peak power decreased to 0.12 kW. Compared with that of the passively Q-switched Er:Yb:LuAB laser in a similar experimental condition, higher pulse energy was achieved in the Er:Yb:SLuB crystal.
This work has been supported by the National Natural Science Foundation of China (grant 91122033), Chunmiao Project of Haixi Institute of Chinese Academy of Sciences (CMZX-2013-005), and the Knowledge Innovation Program of the Chinese Academy of Sciences (grant KJCX2-EW-H03-01).
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