1. Introduction

Stoichiometric laser crystals, in which optically active rare earth ions constitute the crystalline lattice, have been used as one kind of microchip gain media [1–3]. For the Nd³⁺ or Yb³⁺ stoichiometric crystals, high rare earth ion concentrations (generally higher than 5 × 10²¹ ions/cm³) lead to large absorption coefficients at pump wavelength of 0.8 or 0.98 μm respectively. Therefore, incident pump light can be effectively absorbed by the stoichiometric microchips with thickness of 100-200 μm and then continuous-wave (cw) 1.0-1.1 μm laser operations have been realized in NdAl₃(BO₃)₄ [4], KYb(WO₄)₂ [5], and Yb₃Al₅O₁₂ stoichiometric microchips [6].

Compared with the nonlinear frequency conversion technique to obtain 1.5-1.6 μm laser [7–9], LD pumped Er³⁺/Yb³⁺ co-doped material is a simpler method to construct a compact and miniature 1.5-1.6 μm laser. In order to restrain the back energy-transfer from Er³⁺ to Yb³⁺ and upconversion loss of Er³⁺ ions in Yb-Er laser operating at 1.5-1.6 μm, the Er³⁺ ions at the ⁴I_11/2 level must rapidly relax to the upper laser level ⁴I_13/2 through the multiphonon relaxation process. Then, materials with high effective phonon energy, such as borate crystals [10–13] and phosphate glasses [14, 15], are suitable hosts co-doped with Er³⁺ and Yb³⁺ for realizing efficient 1.5-1.6 μm laser operations. YbAl₃(BO₃)₄ (YbAB) as a stoichiometric borate crystal has been grown and some spectroscopic properties have been investigated [16]. When Er³⁺ ions were doped into the YbAB crystal, Er:YbAB crystal should be a potential microchip gain medium of 1.5-1.6 μm laser. Furthermore, due to the large differences in radius and mass between Yb³⁺ and substituted lattice ions (such as Y³⁺ and Gd³⁺) in hosts, ion-substitution-related lattice distortion and the reduction of thermal conductivity of Er³⁺/Yb³⁺ co-doped crystals with high Yb³⁺ concentration become serious [1, 17]. In the Er:YbAB crystal, Yb³⁺ ions are not incorporate as a dopant and the radius and mass of Er³⁺ are close to those of Yb³⁺ ions. Therefore, lattice distortion and the reduction of thermal conductivity of the Er:YbAB crystal with low Er³⁺ concentration will be minimized.

In this paper, cw 1.5-1.6 μm laser realized in a 200-μm-thick Er:YbAB microchip end-pumped by a 976 nm diode laser is reported firstly. Furthermore, 1521 nm pulse laser in the microchip passively Q-switched by a Co²⁺:Mg_0.4Al_2.4O₄ crystal is also reported.

2. Material property and experimental arrangement

An Er:YbAB crystal was grown by the top-seeded solution growth method. The growth process is similar to that of YbAB crystal [16]. The concentrations of Er³⁺ ions in the crystal was measured to be about 1.0 at.% by inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima 2, Jobin-Yvon). The polarized absorption spectra in a range from 875 to 1050 nm of the crystal were recorded with a spectrophotometer (Lambda-900, Perkin-Elmer) at room temperature, as shown in Fig. 1. The full width at half the maximum (FWHM) of the σ-polarized absorption band is 19 nm and the peak absorption coefficient is 184 cm⁻¹ at 976 nm. Then, a c-cut Er:YbAB microchip with thickness of 100-200 μm can absorb more than 85% of incident pump power emitted from a 976 nm diode laser.

 

Fig. 1 Room temperature polarized absorption spectra in 875-1050 nm of the Er:YbAB crystal.

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End-pumped linear resonator was adopted in the experiment and the experimental setup is depicted in Fig. 2. A c-cut Er:YbAB microchip with a dimension of 5 × 5 × 0.2 mm³ was used as gain medium. A 976 nm fiber-coupled diode laser (100 μm diameter core) was used as pump source. After passing a telescopic lens system (TLS), pump beam was focused to a spot with radius of about 65 µm in the microchip. In order to reduce the thermal load at the pump region in the microchip and improve the laser performances [18, 19], the uncoated microchip was pressed into contact with the uncoated rear face of a sapphire crystal with a dimension of 5 × 5 × 1 mm³, which has a high thermal conductivity of 28 W/m·K. The sapphire-Er:YbAB composite-disk with thickness of about 1.2 mm was mounted in a copper heat sink, which was cooled by water at about 20 °C. All the faces of the composite-disk were contacted with the copper and there is a hole with radius of about 1 mm in the center of heat sink to permit the passing of the laser beams. Because the sapphire and Er:YbAB crystals have similar refractive indexes (about 1.75) at 1.5-1.6 μm, the interface reflectivity between them is approximately zero. Input mirror (IM) with 90% transmission at 976 nm and 99.8% reflectivity at 1.5-1.6 μm was directly deposited onto the front face of the sapphire. Four output couplers (OCs) with 5 cm radius of curvature (RoC) and different transmissions (1.0%, 1.5%, 2.0%, and 2.6%) at 1.5-1.6 μm were used respectively. The resonator cavity length was about 5 cm.

 

Fig. 2 Experimental setup of the cw 976 nm diode-end-pumped Er:YbAB laser.

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3. Results and discussion

Figure 3 shows cw output power of the Er:YbAB laser versus incident pump power. When OC transmission was 1.5% and incident pump power was 4.54 W, maximum output power of 220 mW was obtained. The fluctuation of output power was less than ± 3% in an operation period of an hour. Slope efficiency η and threshold with respect to incident pump power were 8.1% and 1.8 W, respectively. The result shows that it is a promising way to use an Er:YbAB microchip as a gain medium to construct a face-cooled 1.5-1.6 μm laser device, such as the microchip and thin-disk lasers. Although the output performances realized presently in the Er:YbAB laser are lower than those (about 1.0 W maximum cw output power and 20-30% slope efficiency) of the Er:Yb:RAl₃(BO₃)₄ (Er:Yb:RAB, R = Y and Gd) lasers [10, 12], they may be enhanced by optimizing the Er³⁺ doping concentration. By measuring the pump thresholds at different OC transmissions [20], optical loss of the Er:YbAB crystal, which originates from defects, impurities and reabsorption, etc, was estimated to be about 1.7%. The optical loss of a 0.7-mm-thick (1.1at.%)Er:(25at.%)Yb:YAB crystal was measured to be about 0.8% under a similar experimental condition. The larger optical loss of the Er:YbAB crystal may be caused by the more impurities introduced by the higher Yb³⁺ doped concentration. Furthermore, thermal focal lengths of the Er:YbAB and Er:Yb:YAB crystals at pump power of 4.54 W were measured to both be about 20 mm [21]. Due to the difference of generated thermal load and cooling between both crystals caused by the different crystal optical quality and thickness, the comparison of their thermal conductivity is difficult at present.

 

Fig. 3 CW output power of the Er:YbAB laser versus incident pump power for different OC transmissions T.

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Spectra of the Er:YbAB laser were recorded with a monochromator (Triax550, Jobin-Yvon) associated with a TE-cooled Ge detector (DSS-G025T, Jobin-Yvon). At incident pump power of 4.54 W, output laser with three lines was centered at about 1543 nm when OC transmission was less than or equal to 2.0%, whereas the laser with five lines was blue-shifted to about 1521 nm when OC transmission was increased to 2.6%, as shown in Figs. 4(a) and 4(b), respectively. Spectra were repeatedly recorded at time interval of 3 min during 30 min of operation. The change of center wavelength of each laser line was less than 0.04 nm and the intensity ratio between different laser lines was slight. The blue-shift of output laser with the increment of OC transmission is originated from the variation of the gain cross-section of the crystal with the inversion ions density (or intracavity loss) [11, 12]. In both spectra, the spacing between the adjacent laser lines is about 0.6 nm and in agreement with the theoretical spacing ∆λ (about 0.57 nm) of laser lines caused by the etalon effect associated to the sapphire-Er:YbAB composite-disk, which can be calculated by $\Delta \lambda \text{=}\frac{{\lambda }^{\text{2}}}{\text{2}nL}$. Here λ is the output laser wavelength, n is refractive index of the composite-disk at λ, and L is the thickness of the composite-disk [20]. When the incident pump power was decreased, the line number of the Er:YbAB laser reduced for all the OCs. However, stable single-line laser oscillations were only observed for 1.5% and 2.0% OC transmissions when incident pump power was lower than 2.4 W. The maximum output power of single-line laser was 40 mW.

 

Fig. 4 Spectra of the Er:YbAB laser at pump power of 4.54W: (a) OC transmission was less than or equal to 2.0%, (b) OC transmission was 2.6%. The inset of left figure shows the laser spectrum at pump power of 2.4 W when OC transmissions were 1.5% and 2.0%.

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In order to analyze beam quality of the Er:YbAB laser, a lens with a 30-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 at various distances from the focusing lens for 1.5% OC transmission was calculated by the 4-sigma method. By fitting these data to the Gaussian beam propagation expression, the beam quality factors M² were estimated to be about 6.8 and 2.5 at pump power of 4.54 and 2.4 W, respectively, as shown in Figs. 5(a) and 5(b). Therefore, as shown in Fig. 5(c), with the decrement of thermal effect of the crystal and the laser line number, the beam quality factor can be improved [22, 23].

 

Fig. 5 Beam quality factor M² of the Er:YbAB laser for the 1.5% OC transmission. (a) and (b) show beam diameter as a function of the distance from the focusing lens at pump power of 4.54 and 2.4 W, respectively. (c) shows the variation of M² with incident pump power.

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When a 1-mm-thick AR-coated Co²⁺:Mg_0.4Al_2.4O₄ crystal with an initial transmission of about 97% around 1530 nm was used as the saturable absorber and placed as close as possible to the Er:YbAB microchip, pulse performances of the passively Q-switched Er:YbAB laser were investigated. When OC transmission was 1.5% and incident pump power was 4.54 W, maximum average output power of 18 mW was obtained, as shown in Fig. 6. Compared with that in the cw laser operation, the laser wavelength was blue-shifted to 1521 nm, which is caused by the higher cavity loss originated from the Co²⁺:Mg_0.4Al_2.4O₄ crystal. The beam quality factor M² of the pulse laser was measured to be about 3.4.

 

Fig. 6 Average output power of the passively Q-switched Er:YbAB laser as a function of incident pump power for the 1.5% OC transmission. Spatial profile of the output beam at pump power of 4.54 W is also shown.

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Pulse profiles of the passively Q-switched Er:YbAB laser were measured by a 2 GHz InGaAs photodiode connected to an oscilloscope with bandwidths of 1 GHz (DSO6102A, Agilent). Pulse train and oscilloscope trace of the laser at pump power of 4.54 W are shown in Figs. 7(a) and 7(b), respectively. Pulse repetition rate was about 96 kHz and pulse duration was about 265 ns. The pulse-to-pulse amplitude fluctuation and interpulse time jittering were about 5% and 6%, respectively. Pulse energy of the laser was about 0.19 μJ.

 

Fig. 7 Pulse train (a) and oscilloscope trace (b) of the passively Q-switched Er:YbAB laser at pump power of 4.54 W. Pulse repetition rate and duration are 96 kHz and 265 ns, respectively.

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4. Conclusion

1.5-1.6 μm cw laser operation was demonstrated in a 200-μm-thick Er:YbAB microchip end-pumped by a 976 nm diode laser. The maximum cw output power of 220 mW with slope efficiency of 8.1% was obtained. The laser line number reduced with the decrement of incident pump power. At incident pump power of 2.4 W and OC transmission of 1.5%, single-line laser with maximum output power of 40 mW was obtained. Using a Co²⁺:Mg_0.4Al_2.4O₄ crystal, 1521 nm passively Q-switched Er:YbAB pulse laser with 0.19 μJ energy, 265 ns duration, and 96 kHz repetition rate was realized. The results show that the Er:YbAB crystal with high absorption coefficient at 976 nm should be a promising gain medium to construct the microchip and thin-disk lasers at 1.5-1.6 μm.