1. Introduction

For laser materials used in the generation of ultra-short pulsed radiation through the mode-locking technique, one of the most important requirements is having a broad band emission spectrum. Generally, a glass host can provide much wider emission spectra than a crystal one in the cases of trivalent rare earth active ions like Nd³⁺, Yb³⁺, Tm3⁺, Er³⁺, etc. However, the thermal properties, in particular, the thermal conductivities of glass hosts are usually poor, limiting their application in high-power or high repetition rate systems.

Between the two distinct categories (glass and crystal) of host materials, there are so-called disordered crystals, which are, to some extent, capable of having the advantages possessed by both a crystal and a glass. Among such disordered crystals, calcium niobium gallium garnet (CNGG), Ca₃(NbGa)₂-xGa₃O₁₂, is a promising one. It is an isotropic material similar to YAG and hence potentially interesting also for laser ceramics. Its thermal conductivity is 4.7 Wm^-1K^-1, about one third of that of YAG which is much larger than for glasses [1, 2]. The disordered nature of the CNGG crystal provides several different lattice positions for the trivalent dopant ions, giving rise to significant inhomogeneous broadening in the absorption and emission spectra. This feature has been demonstrated in Nd- and Tm-doped CNGG crystals [2–4]. The broad absorption band around the diode-pumping wavelength of about 808 nm greatly reduces the restraint on temperature controlling of the diode in Nd:CNGG lasers, leading to improved laser performance in the low-power continuous-wave operation regime in comparison with Nd:YAG [3]. The potential of Nd:CNGG in generating ultra-short pulses was demonstrated in a lamp-pumped passively mode-locked laser producing 5–10 ps laser pulses [5].

The favorable properties and promising laser performance of Nd:CNGG make it attractive to exploit the CNGG crystal as a host for the Yb ion. In this paper, we report for the first time the spectroscopic properties and continuous-wave (cw) laser operation of Yb:CNGG.

2. Experiments

The Yb:CNGG crystal was grown in a platinum crucible by using the Czochralski method. Chemical compounds Yb₂O₃, Ga₂O₃, Nb₂O₅ and CaCO₃ with 99.99% purity were used. The mixture was prepared according to the chemical formula (Yb₃Ga₅O₁₂)_0.05(Ca₃Nb_1.6875Ga_3.1875O₁₂)_0.95 and 1–2 wt % Ga₂O₃ was added to the polycrystalline material to compensate the volatilization of Ga2O₃ in the process of synthesizing the poly-crystalline and crystal growth. First the weighted starting materials were ground, mixed and heated in the Pt crucible at 1000 °C for 10 h to decompose CaCO3 and form Yb:CNGG polycrystalline material through solid-state reaction. Then the polycrystalline material was ground, mixed again and pressed into disks, put into Pt crucible and heated to 1050 °C for 15 h to form the polycrystalline material for Yb:CNGG crystal growth. As a seed for crystal growth, a Nd:YAG rod with dimensions of Φ4×30 mm, cut along the [111] crystallographic direction, was used. The pulling rate was 1–2 mm/h during the crystal growing process, and the rotating rate was 10–15 rpm. A high-quality Yb:CNGG boule of Φ30×35 mm was obtained. The as-grown crystal boule was annealed in air at a temperature of 1000 °C for 10 hours, in order to remove the thermal stress inside the crystal. The annealed crystal was processed into crystal samples for spectra and laser experiments.

We measured the absorption spectrum at room temperature by use of two Yb:CNGG samples of thickness of 3 and 8 mm.

Employing a simple plano-concave resonator, we realized cw laser operation with the Yb:CNGG crystal at room temperature. The plane mirror, which acted as a total reflector, was made highly reflecting for 1015–1230 nm (>99.8%) and highly transmitting for 880–990 nm (>97%). Several concave mirrors of radius of curvature of 50 mm with transmission in the range of 0.5%–10% were used as the output coupler. The physical cavity length was 49 mm. The Yb:CNGG crystal used in the laser experiment was cut along the [111] crystallographic direction. It was uncoated, 3 mm thick with a 3.3 mm × 3.3 mm aperture. The laser crystal was held in a water-cooled copper block maintained at a temperature of 12 °C, and was positioned close to the plane cavity mirror. The pump source used was a high-power high-brightness fiber-coupled diode laser (S50-980-2, Apollo Instruments, Incorporated) with a fiber core diameter of 200 µm and a NA of 0.22. Its emission wavelength varied from 974 to 981 nm depending on the output power level. The unpolarized pump radiation was focused by a 1:1 reimaging unit and delivered onto the crystal through the plane mirror.

3. Results and discussion

The Yb concentration in the crystal was measured to be 5.77 at. % by X-ray fluorescence method. The fluorescence lifetime of Yb:CNGG was 791±19 µs, determined by employing the pin-hole technique that can effectively eliminate the radiation trapping effect [6]. The crystal sample used in this determination was 1.5 mm thick with an aperture of 3.3 mm × 3.3 mm.

 

Fig. 1. Room temperature absorption and emission spectra of Yb:CNGG

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Based on the measured absorption data, the emission spectrum was calculated by the modified reciprocity method [7], the refractive index and density of the crystal used in this calculation are 1.92 and 4.69 gcm-3, respectively.

Figure 1 shows the absorption and emission spectra in terms of their cross sections. The maximum absorption occurs at 973.5 nm, giving an absorption cross section of σ_abs=3.3×10^-20 cm², the bandwidth (FWHM) is 3.5 nm. Beyond this sharp absorption peak there is a broad absorption band composed of three overlapping peaks located at 921 nm (1.2×10^-20 cm²), 934 nm (1.6×10^-20 cm²), and 946 nm (1.5×10^-20 cm²). Having a FWHM of 43 nm, this absorption band provides great flexibility for diode pumping. One can see in Fig. 1 that the stimulated emission cross section reaches its peak value of σ _em=3.2×10^-20 cm² at 974 nm. For this zero-zero transition, the emission and absorption peaks completely overlap and obviously it cannot be used for lasing due to the overwhelming reabsorption losses. The second emission band centered at 1028 nm is more important in practice, it has a maximum cross section of σ _em=2.5×10^-20 cm², a FWHM bandwidth of 21 nm, and a slowly decreasing shoulder extending to as far as 1075 nm.

Figure 2 shows the output power versus the absorbed pump power for four different output couplings (T). The Yb:CNGG laser reached threshold at absorbed pump power of P_abs=0.7 W, 0.95 W, 1.2 W, and 1.85 W for T=0.5%, 2%, 5%, and 10%, respectively. In the whole operational range involved in the experiment, the most efficient operation was achieved by using the coupler of T=2%. The maximum output power was 1.9 W, reached at P_abs=7.65 W, resulting in an optical conversion efficiency of 25%, and a slope efficiency of 28.5%. In the cases of T=0.5%, 5%, and 10%, the highest attainable output powers were lower than obtained with T=2% coupler; the corresponding slope efficiencies were determined to be 23%, 26.4%, and 24%. In comparison with other Yb-doped crystals, the slope efficiencies achieved with the Yb:CNGG are lower. Several reasons are believed to be responsible for this: the optical quality of the crystal was not as good as others; the Yb concentration and thickness of the crystal were not optimal; the crystal was lack of antireflection (AR) coatings on its end faces. So the slope efficiency has sufficient room to increase by improving the crystal quality, by optimizing the Yb doping level and the crystal length, and by making AR coatings on the crystal.

 

Fig. 2. Continuous-wave output power versus absorbed pump power for different output couplings.

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Fig. 3. Emission spectra of the Yb:CNGG laser at P_abs=4.3 W, showing the output coupling dependence

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From the input-output characteristics presented in Fig. 2, one can see that no tendency of output saturation appears except for the case of T=0.5%, indicating an insignificant thermal losses at the pump levels applied. A substantial thermal lensing was likely to occur inside the crystal; however, a purely lensing effect has very limited influence on the resonator employed here. Due to the more than two times larger pump spot radius compared to the fundamental laser mode radius, the out beam contained higher-order transverse modes, with the beam quality factor (M²) estimated in the range of 2.0~3.0. The output power suffered no saturation also suggests that further power scaling with the Yb:CNGG laser can be expected. However, it was not possible to achieve this with the present pump diode, because the narrow absorption band around 973 nm led to rapid reduction in P_abs when the pump wavelength got longer with increasing pump power. Therefore, a diode laser emitting in the wavelength range of 920–947 nm will be more favorable.

Like in most Yb lasers, the emission wavelength of the Yb:CNGG laser varied with both the output coupling and the operational power level. Figure 3 gives the laser emission spectra recorded at P_abs=4.3 W for different output couplers. One can see that the emission wavelength decreased with increasing output coupling, shifting from 1055.5 to 1036.8 nm when the output coupling increased from T=0.5% to 10%.

 

Fig 4. Wavelength dependence of the gain cross section for different fractions of population inversion.

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The wavelength dependence of gain cross section, calculated by using g(λ)=βσ_em-(1-β)σ_abs where β denotes the fraction of population inversion, is given in Fig. 4 for β=0.04, 0.06, 0.08, and 0.1, respectively. It is noted that the wavelength at which g(λ) reaches its maximum is shifted to shorter values with increasing magnitude of β, from beyond 1050 nm for β=0.04 to 1033 nm for β=0.1. This explains qualitatively the behavior of the emission spectra shown in Fig. 3, if one bears in mind the fact that the laser gain will be clamped at a higher level for a larger output coupling.

4. Conclusions

The absorption and emission cross sections of a newly developed Yb:CNGG crystal were measured. Continuous-wave laser oscillation was realized at room temperature through diode end pumping, yielding an output power of 1.9 W with an optical-to-optical efficiency of 25% and a slope efficiency of 28.5%. It can be expected that the output power of the Yb:CNGG laser can be further increased provided a 920–947 nm diode laser is used as pump source.