1. Introduction

During the past few years the ytterbium ion has been recognized as a very attractive dopant for solid-state laser and especially in the high-power and ultrafast laser systems in the emission region near 1 μm. The very simple electronic-level scheme based on only two manifolds of ytterbium ions leads to a low intrinsic quantum defect (generally less than 10%) eliminating undesired effects such as up-conversion, excited-state absorption, cross relaxation, and concentration quenching. The intense and broad absorption bands of ytterbium-doped media in the 900–980 nm range are covered by high-brightness or high-power InGaAs commercial laser diodes. Furthermore, ytterbium-doped crystals typically exhibit a broad emission bandwidth. Efficient laser operations have already been demonstrated in numerous Yb-doped materials, such as garnet Yb:YAG and Yb:GGG^1,2, borate Yb:GdCOB and Yb:BOYS^3,4, fluoride Yb:Ca${{\mathrm{F}}_2}^5$, oxyorthosilicates Yb:Y₂SiO₅ (Yb:YSO), Yb:Lu₂SiO₅ (Yb:LSO)^6,7 and Yb:Gd₂SiO₅ (Yb:GSO)⁸, tungstates Yb:KYW and Yb:KGW⁹, or sesquioxides Yb:Y₂O₃, Yb:Lu₂O₃, and Yb:Sc₂O₃^10,11. A large number of ytterbium-doped crystals have been recognized in recent years as very interesting active media for diode-pumped femtosecond laser oscillators and amplifiers¹².

Recently, ytterbium-doped oxyorthosilicates Yb:YSO and Yb:LSO have been demonstrated to exhibit large ground-state splittings, high thermal conductivities, and broad emission spectra⁶. The crystal structure of Y₂SiO₅ and Lu₂SiO₅ belongs to the end-centered monoclinic I2/a space group. Both YSO and LSO have two non-equivalent crystallographic sites coordinated with 7 and 6 oxygen atoms, respectively. Depending on the substitution site, ytterbium ions in Yb:YSO and Yb:LSO exhibit different but large overall splittings of the ground-state manifold ²F_7/2. And ytterbium ions in Yb:YSO and Yb:LSO have separate emission bands, corresponding to transitions from the lowest levels of ²F_5/2 manifold to the split levels of ²F_7/2 manifold. Separate terminal laser sub-levels may cause multi-wavelength and thus instable laser oscillation, which limits the continuous tunability. In this letter, we demonstrate that a new ytterbium-doped alloyed oxyorthosilicate crystal Yb:LuYSiO₅ (Yb:LYSO) can be used as a promising laser gain medium in generating broadband continuously tunable solid-state lasers. In Yb:LYSO, multi-type of substitutional sites provide an inhomogeneous strong crystal field for ytterbium ions, which causes inhomogeneous splittings of ²F_7/2 manifold. As a consequence, the split levels, which function as terminal laser levels, cover broad bands in average. This may support continuous tunability of Yb:LYSO laser in a broad range of a smooth tuning curve. As a whole, the alloyed oxyorthosilicate crystal Yb:LYSO exhibits combined advantages of both Yb:LSO and Yb:YSO crystals, such as large ground-state splittings and broad emission spectra. In addition, the terminal laser sublevels that are inhomogeneously broadened with inhomogeneous fundamental splittings of ytterbium ions in the alloyed host matrices suppress multi-wavelength oscillation and bring about a smooth continuous tunability in a broad range. Efficient continuous-wave (cw) laser actions are obtained under high-power diode-pumping of Yb:LYSO crystals, which exhibit not only high optical conversions but also a large and continuous tunability between 1014 and 1091 nm.

2. Spectral properties of Yb:LYSO

The alloyed Yb:LYSO crystal was grown by the Czochralski method from a solution of Y₂SiO₅ (YSO) and Lu₂SiO₅ (LSO) in inductively heated iridium crucibles under nitrogen ambient atmosphere. The alloyed LYSO crystal has the same crystal structure with the simple oxyorthosilicate crystal (YSO or LSO), where the (OY₄) and (OLu₄) tetrahedra share edges with the (SiO₄) tetrahedra and form chains interconnected by isolated (SiO₄) tetrahedra. High quality Yb:LYSO crystal exhibits good mechanical properties. Optical absorption spectra of the as-grown alloyed Yb:LYSO crystal was recorded by a Jasco V-570 UV/VIS/NIR spectrophotometer at room temperature. The luminescence spectra were obtained with a Triax550 spectrofluorimeter under 940 nm laser diode excitation. The room-temperature unpolarised absorption and emission spectra of 5 at.% ytterbium-doped LYSO crystal are shown in Fig. 1. The absorption spectrum is mainly composed of three strong bands around 899, 923, and 977 nm. The absorption peak of 977 nm belongs to the zero-line transition between the lowest levels of ²F_7/2 and ²F_5/2 manifolds. Other absorption bands correspond to typical transitions from the ground state ²F_7/2 to other ²F_5/2 sublevels of Yb³⁺. The absorption bandwidth of Yb³⁺ in LYSO host is much greater than that in Yb:YAG. Its broadband absorption is well-adapted for diode-pumping with commercially available high-power InGaAs laser diodes. The emission spectrum mainly includes bands around 1005, 1033, 1058 and 1082 nm. Those emission bands exhibit very small quantum defect as the zero-line absorption transition is used for pumping, which reduces the heat deposition into the crystal. The emission band at the longest wavelength around 1082 nm corresponds to the transition from the lowest levels of ²F_5/2 manifold to the highest levels of ²F_7/2 manifold. We can therefore estimate the maximum splitting of the ²F_7/2 manifold of Yb:LYSO as 993 cm^-1. Furthermore, Yb:LYSO has a rather high thermal conductivity about 5 Wm^-1K^-1 for 5 at.% Yb doping. The combination of structural, absorption and emission spectral characterization indicate that the Yb:LYSO crystal is a promising material for high-power laser application.

 

Fig. 1. The room-temperature absorption and emission spectra of Yb:LYSO laser crystal.

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According to the emission spectrum, laser action may occur around the strong emission bands of 1005, 1033, 1058 and 1082 nm. Thermal populating of the terminal laser level produces strong reabsorption losses and detrimentally affects the laser action around the 1005 nm. The second and third bands around 1033 and 1058 nm correspond to an energy scheme of medium emission cross-sections where the terminal level is a little populated. Efficient laser action is possible. Finally, the last band around 1082 nm exhibits a terminal laser level very few populated but the emission cross-section is small, however, it makes low threshold and efficient laser action. Interestingly, Yb:YSO has a large emission cross-section at the short-wavelength band around 1003 nm, while Yb:LSO exhibits relatively large emission cross-sections at the long-wavelength bands. The alloyed crystal Yb:LYSO inherits emission properties of both Yb:LSO and Yb:YSO, with a large emission cross-section at the short-wavelength band around 1005 nm, and relatively large emission cross-sections at the intermediate-wavelength bands. The overall emission bandwidth may support tunable or ultrashort laser oscillation of a broad band.

3. Experimental setup and results

In the first part of the experiments, we tested cw laser operation of 5 at.% doped Yb:LYSO with a simple laser cavity consisting of two flat mirrors (M₁ and OC), where dichroic mirror M₁ is antireflection coated at 976 nm and high-reflection coated in a broad band from 1020 to 1120 nm, and OC is the output coupler with different transmission (T=2.5% and T=5.5%). The Yb:LYSO crystal is antireflection coated with the gain length of 2 mm (5 mm×6 mm in size). The crystal was end-pumped by high-power and high-brighness fiber-coupled laser diodes at 976 nm, with the core diameters of 50 and 400 μm, respectively. The pump laser beam was focused by a series of lenses with an imaging ratio of 1:1. To efficiently remove the generated heat in the laser crystal, we wrapped the crystal with indium foil and fixed it tightly in a water-cooled copper heat sink. The temperature of the laser crystal was controlled at 14°C.

 

Fig. 2. Yb:LYSO cw laser output power as a function of absorbed pump power. The core-diameters of the pump source are 50 μm (a) and 400 μm (b), respectively.

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Fig. 3. Experimental setup for continuously tunable Yb:LYSO laser cavity.

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A 5 W fiber-coupled laser diode with a numerical aperture of 0.22 and a core diameter of 50 μm was at first used for pumping. The pump spot was about 50 μm on the Yb:LYSO crystal. The cw Yb:LYSO lasers were operated in TEM₀₀ mode at 1086 nm. Figure 2(a) compares the cw Yb:LYSO laser outputs with different output couplers (T=2.5% and T=5.5%) under different absorbed pump power. The best performance has been obtained at 1086 nm with a T=2.5% output coupler. Under an unsaturated absorbed pump power of 3.4 W (the incident pump power is 4.6 W), the output power reached 2.1 W. The threshold absorbed pump power was approximately 1.1 W. The corresponding slope efficiency was 85%. The highest slope efficiency was about 96% with a 5.5% transmission output coupler. The maximum optical-to-optical efficiency was about 62%. Then, we replaced the pump source with a high-power fiber-coupled laser diode with with a numerical aperture of 0.22 and a core-diameter of 400 μm. The pump spot was about 400 μm on the Yb:LYSO crystal. The best performance was also obtained at 1086 nm with a T=2.5% output coupler. Maximum output power of 7.8 W was obtained under an absorbed pump power of 15.3 W. The maximum optical-to-optical efficiency was about 51%. The corresponding slope efficiency is 64% (see Fig. 2(b)).

 

Fig. 4. Tuning curve obtained with an intracavity dispersive prism for Yb:LYSO

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The Yb:LYSO laser tunability was demonstrated at an incident pump power of 14.2 W with a 2.5% transmission output coupler. We inserted an SF 14 dispersive prism into the resonator as the tuning element between the folded mirror and the output coupler at the Brewster’s angle (see Fig. 3). We achieved a quasi-continuous and smooth tuning range broader than 77 nm, from 1014.4 to 1091.7 nm, with a maximum output power near 1060 nm (Fig. 4). The tuning range is limited on the short wavelengths by the dichroic coating of both input and folding mirrors (high-reflection coating from 1020 to 1120 nm). This indicates that Yb:LYSO exhibits large emission cross-section and small reabsorption losses at the emission band around 1005 nm. By employing appropriate input and folding mirrors with high-reflection coating below 1020 nm or off-axis diode pumping near 940 nm, we expect to reach even shorter wavelengths. In comparison with the output power of 6.6 W for the cavity without dispersive tuning element, the maximum output power of the laser with an SF14 prism was 1.4 W. The relatively poor efficiency is mainly due to the relatively large losses of moistureproof coating of the intracavity SF14 prism. The laser efficiency can be improved by optimizing the intracavity tuning element.

4. Conclusions

In conclusion, we have demonstrated efficient continuous-wave laser action of a new ytterbium-doped alloyed laser crystal that can be continuously tunable in a broadband range. Under the high-power diode pump with a fiber core diameter of 400 μm, we abtained a cw output power up to 7.8 W at 1086 nm with an absorbed pump power of 15.3 W. The corresponding optical-to-optical efficiency was 51%. Under high-brightness diode pump with a fiber core diameter of 50 μm, an output power of 2.1 W was obtained at the absorbed pump power of 3.3 W. The corresponding optical-to-optical efficiency was 62%. Yb:LYSO exhibits a large ground-state splitting and broad emission spectrum with inhomogeneously broadened terminal laser sublevels to support a smooth continuous tunability in a broad range. Using an SF14 prism as the intracavity tuning element, a continuously tunability was obtained in the range of 1014–1091 nm. Broadband tunable Yb:LYSO laser may find promising applications in helium magnetometers, neutron spin filters, and polarized electron sources.