1. Introduction

Eye-safe laser around 1.55 μm realized in Er³⁺/Yb³⁺ co-doped materials can be used in many applications, such as lidar, laser ranging, three-dimensional imaging, and environmental sensing [1–3]. The high phonon energy (about 1400 cm⁻¹) of borate crystals can enhance the multi-phonon relaxation from ⁴I_11/2 to ⁴I_13/2 multiplet of Er³⁺, and restrain the upconversion from ⁴I_11/2 multiplet and back energy-transfer from Er³⁺ to Yb³⁺. Therefore, Er³⁺/Yb³⁺ co-doped borate crystals can be used as good gain media for lasers around 1.55 μm with slope efficiencies higher than 20% [4–8]. However, the high phonon energy also decreases fluorescence quantum efficiency (generally 10–20%) of ⁴I_13/2 multiplet of Er³⁺ in the borate crystals. As a result, a large amount of pump power is converted to heat for Er³⁺/Yb³⁺ co-doped borate crystals, especially, when a high quantum defect (37%) of Er³⁺/Yb³⁺ laser is also taken into account [8]. Therefore, in order to reduce the thermal effects of gain medium and improve the laser performance, it is necessary to find borate crystals with a high thermal conductivity. At present, except for Er³⁺:Yb³⁺:RAl₃(BO₃)₄ (Er:Yb:RAB, R = Y, Gd and Lu) crystals with a thermal conductivity of about 4.7 Wm⁻¹K⁻¹, in which 1.0–2.0 W continuous-wave (cw) lasers have been realized [5,6,8], other reported Er³⁺/Yb³⁺ co-doped borate crystals, such as LaSc₃(BO₃)₄ [9], RCa₄O(BO₃)₃ [4,10,11] and Sr₃R₂(BO₃)₄ [7,12], etc., have lower thermal conductivities (1.2–2.7 Wm⁻¹K⁻¹) and are prone to be fractured at a high cw pumping power. Thus, only low power cw (lower than 260 mW) or quasi-cw laser operations have been realized in these Er³⁺/Yb³⁺ co-doped borate crystals until now [4,7,9–12].

LaMgB₅O₁₀ (LMB) crystal belongs to the monoclinic system with the space group P2₁/c [13]. Its thermal conductivity is 5.0 Wm⁻¹K⁻¹ at room temperature [14]. Nd³⁺ and Yb³⁺ singly-doped LMB crystals have been demonstrated to be good gain media for 1.0 µm lasers. Cw orthogonally polarized dual-wavelength laser at 1051.8 and 1081.4 nm with a maximum output power of 5.1 W and a slope efficiency of 42.5% has been achieved in a Nd:LMB crystal [15], and cw dual-wavelength laser at 1053 and 1057 nm with a maximum output power of 2.76 W and a slope efficiency of 64.5% has also been obtained in a Yb:LMB crystal [16]. Recently, an Er:Yb:LMB crystal has been successfully grown by the top-seeded solution growth method and its spectroscopic properties have been investigated in detail [17]. Fluorescence lifetime of ⁴I_13/2 multiplet and stimulated emission cross section at 1518 nm for E//Z polarization of the Er:Yb:LMB crystal are reported to be 538 µs and 0.88 × 10⁻²⁰ cm², respectively [17]. Compared with those of Er:Yb:RAB crystals (310–350 µs and around 2.1 × 10⁻²⁰ cm² at 1530 nm, respectively) [5,6], it implies that the Er:Yb:LMB crystal may have a larger energy storage capacity, which is favorable for pulse laser generation with higher output energy. However, the reported maximum output power and slope efficiency are only 160 mW and 10.1%, respectively, for the initial cw laser experiment of the crystal [17].

In this work, absorption and gain characteristics of an Er:Yb:LMB crystal related to laser around 1.55 μm were investigated in detail. End-pumped by a 976 nm laser diode (LD), efficient 1.57 μm laser operation was successfully demonstrated in an Er:Yb:LMB crystal.

2. Material property and experimental arrangement

A 2.0-mm-thick, X-cut Er(0.68 at.%):Yb(7.51 at.%):LMB crystal with cross section of 5 × 5 mm² was cut from a single crystal grown by the top-seeded solution growth method [17]. As mentioned in [17], the increment of Yb³⁺ concentration in the crystal will enhance the energy transfer efficiency from Yb³⁺ to Er³⁺ and then improve the laser performance. However, for the present Er:Yb:LMB crystal growth technique, optical quality of the crystal with higher Yb³⁺ concentration is reduced. Therefore, an Er:Yb:LMB crystal with the same Er³⁺ and Yb³⁺ concentrations as [17] was still used as a gain medium in this work, although the used doping concentrations are not the optimal ones. Polarized absorption spectra in 850–1050 nm of the crystal were recorded at room temperature by an UV-VIS-NIR spectrophotometer (Lambda-950, Perkin-Elmer), and are shown in Fig. 1. Absorption coefficients peaked at 976 nm for both E//Z and E//Y polarizations are 7.8 and 7.1 cm⁻¹, respectively. The full widths at half the maximum (FWHMs) of this absorption band for both polarizations are 5 nm. Furthermore, it can be seen that there is another absorption band peaked at 940 nm with absorption coefficient of 4 cm⁻¹ and FWHM of 20 nm for both polarizations. The larger FWHM for the absorption band at 940 nm, which is consistent with the output wavelength of the LD frequently-used to pump Er:Yb:YAG [18], may be favorable to realize a stable laser operation, when the bandwidth and temperature-dependent wavelength-shift of the LD are taken into account.

 

Fig. 1 Room-temperature polarized absorption spectra between 850 and 1050 nm of an X-cut Er:Yb:LMB crystal.

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An end-pumped linear plano-concave resonator was adopted and the experimental setup is depicted in Fig. 2. The uncoated Er:Yb:LMB crystal was closely contacted with a 1.0-mm-thick sapphire crystal with cross section of 5 × 5 mm², which has been demonstrated to be an efficient heat sink for reducing the thermal effects of the gain medium and then improving output laser performance [8]. Both the Er:Yb:LMB and sapphire crystals were mounted in a copper holder, which was cooled by water at 20 °C. There is a hole with radius of 1 mm in the center of the holder to permit the passing of laser beams. A cw 976 nm fiber-coupled LD with core diameter of 100 μm from Dilas Inc., in which the output wavelength is stabilized by the volume Bragg grating (VBG) technique, was used as the pumping source. By measuring the powers before and after the Er:Yb:LMB crystal with a power meter (PM100D, Thorlabs) at low power density, about 75% incident pump power was absorbed by the crystal, which is consistent with the result of the spectral experiment. After passing a telescopic lens system (TLS) consisted of two convex lenses with the same focal length of 45 mm, pumping beam with diameter of about 100 µm was focused in the crystal. An input mirror (IM) with 90% transmission at 976 nm and 99.8% reflectivity in 1.5–1.6 μm was directly deposited on the outside surface of the sapphire crystal. Three output mirrors (OMs) with identical radius curvature of 100 mm and different transmissions (1.8%, 4% and 6.5%) in 1.5–1.6 μm were used. The resonator length was close to 100 mm. Laser spectrum was recorded by a monochromator (Triax550, Jobin-Yvon) with a Ge detector. Using a convex lens with a 10-cm focal length to focus output laser beam, spatial profiles of the focused beam at various distances from the focusing lens were recorded with a Pyrocam III camera (Ophir Optronics).

 

Fig. 2 Experimental setup of cw 976 nm-diode-pumped Er:Yb:LMB 1.57 μm laser.

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3. Results and discussion

Figure 3(a) shows cw output power realized in the Er:Yb:LMB crystal versus absorbed pump power for different OM transmissions. When the OM transmission was 4%, a maximum output power of 0.61 W was obtained at an absorbed pump power of 4.0 W, which is limited by the available output power of the used LD in our lab. The slope efficiency η was 23% and the absorbed pump threshold was about 0.96 W. Although the laser performance realized in the Er:Yb:LMB crystal at present is still inferior to that of Er:Yb:YAB crystal (maximum cw output power of 1 W and slope efficiency of 35%) [5], the obtained maximum output power is much higher than those of other reported Er³⁺/Yb³⁺ co-doped materials, such as 0.255 W for Er:Yb:YCOB crystal [4] and 0.35 W for Er:Yb:phosphate glass [19,20]. Meanwhile, the obtained slope efficiency is close to those of Er:Yb:YCOB crystal (27% [4]) and Er:Yb:phosphate glass (generally 20–30% [19,20]). By comparing the pump thresholds at different OM transmissions for three-level laser [21], internal optical loss of the 2.0-mm-thick Er:Yb:LMB crystal originating from defects, impurities, and reabsorption, etc., was estimated to be 4%, and is higher than the 0.8% reported in an 0.7-mm-thick Er:Yb:YAB crystal [22]. The effective reflectance, in which the Fresnel reflection originating from the uncoated crystal (refractive index of about 1.4 [17]) and reflections of both cavity mirrors in 1500-1600 nm are taken into account, was adopted in the above calculating process [23]. It implies that when optical quality of the Er:Yb:LMB crystal is improved, its laser performance can be enhanced. A nearly circular output beam with ellipticity of 0.93 was observed, as shown in Fig. 3(b). The beam radius of output laser is calculated by the 4-sigma method and the beam quality factor M² can be estimated by fitting these data to the Gaussian beam propagation expression. The factors M² of the Er:Yb:LMB laser in the horizontal and vertical directions were similar and about 2.5 at an absorbed pump power of 4.0 W, as shown in Fig. 3(b).

 

Fig. 3 (a) Output power realized in the Er:Yb:LMB crystal as a function of absorbed pump power. (b) Squared beam radius ω² of output laser as a function of the distance Z from the focusing lens at an absorbed pump power of 4.0 W. (c) Laser spectra for different OM transmissions at an absorbed pump power of 4.0 W.

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Spectra of output lasers at an absorbed pump power of 4.0 W for different OM transmissions are shown in Fig. 3(c). For the OM transmissions of 1.8% and 4%, the laser wavelengths were centered at 1566 nm. When the OM transmission was increased to 6.5%, laser wavelength was in 1556–1566 nm. Polarization of the Er:Yb:LMB laser was measured to be totally linear and parallel to the crystalline Z axis. Room-temperature polarized absorption cross section spectra σ_abs and emission cross section spectra σ_em calculated by the Füchtbauer-Ladenburg formula [17] of the X-cut Er:Yb:LMB crystal in 1450–1650 nm are shown in Fig. 4(a). Then, the polarized gain cross section spectra σ_g can be calculated by ${\sigma }_g\left(\lambda \right)=\beta {\sigma }_{em}\left(\lambda \right)-\left(1-\beta \right){\sigma }_{abs}\left(\lambda \right)$ [5]. Here, the inversion parameter β is the ratio of the number of Er³⁺ ions in the upper laser level to the total number of Er³⁺ ions. Gain cross section spectra for E//Y and E//Z polarizations at β = 0.4 are presented in Fig. 4(b). It can be seen that the value of σ_g for E//Z polarization is larger than that for E//Y polarization around 1566 nm, which causes the generation of the E//Z polarized laser. Furthermore, when β is increased to 0.55, which corresponds to a higher cavity losses L, the value of σ_g at 1556 nm is similar to that at 1566 nm for E//Z polarization, as shown in Fig. 4(c). Here, cavity losses mainly originating from the crystal and mirror transmission are nearly a certain constant in 1500–1600 nm. Therefore, when the OM transmission is 6.5% corresponding to L₂ depicted in Fig. 4(c), the laser can oscillate between 1556 and 1566 nm.

 

Fig. 4 (a) Room-temperature absorption (black line) and emission (red line) cross section spectra of the Er:Yb:LMB crystal in 1450–1650 nm for different polarizations. (b) Room-temperature gain spectra of the Er:Yb:LMB crystal in 1500–1600 nm for different polarizations when the inversion parameter β is 0.4. (c) Gain spectra of the Er:Yb:LMB crystal in 1500–1600 nm for E//Z polarization and different β. L is the cavity losses.

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When a flat OM made of another sapphire crystal with thickness of 1.0 mm and cross section of 5 × 5 mm² was closely pressed to the Er:Yb:LMB crystal through the use of screws, a micro-laser was constructed and the relevant schematic diagram is shown in Fig. 5(a). The OM has a transmission of 3% around 1.57 μm. Resonator length of the micro-laser was 4 mm. End-pumped by the 976 nm LD, a maximum cw output power of 0.47 W was obtained at an absorbed pump power of 4.0 W and the slope efficiency was 16%, as shown in Fig. 5(b). Laser wavelength was around 1566 nm and only two longitudinal modes were observed. At an absorbed pump power of 4.0 W, a nearly circular output beam with M² value of about 1.3 was obtained, as shown in Fig. 5(c). Compared with that realized in the plano-concave cavity, the higher beam quality of the micro-laser may be caused by the less longitudinal modes and the using of two sapphire crystals on each face of the gain medium as heat sinks to weaken the thermal effects.

 

Fig. 5 (a) Experimental setup of cw 976 nm-diode-pumped Er:Yb:LMB 1.57 μm micro-laser. (b) Output power realized in the micro-laser as a function of absorbed pump power. The inset shows the spectrum of the micro-laser. (c) Squared beam radius ω² of the output laser as a function of the distance Z from the focusing lens at an absorbed pump power of 4.0 W.

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4. Conclusion

End-pumped by a 976 nm LD, 1.57 μm cw polarized laser with a maximum output power of 0.61 W and a slope efficiency of 23% was realized in an Er:Yb:LMB crystal. With the improvement of the crystal optical quality, output laser performance of the Er:Yb:LMB crystal can be further enhanced. Combined with a high thermal conductivity of 5.0 Wm⁻¹K⁻¹, the Er:Yb:LMB crystal may be considered as a good gain medium for laser around 1.55 μm.