Linearly polarized laser oscillation is observed with the isotropic disordered Yb:Ca3(NbGa)2-xGa3O12 crystal, resulting from stress-induced birefringence. At high pump levels, two orthogonal polarization states exist simultaneously. A continuous-wave output power of 2.0 W is obtained with an optical-to-optical efficiency of 32%, the slope efficiency amounts to 40%. In the passively Q-switched regime, the average output power reaches 1.35 W at a pulse repetition frequency of 16.7 kHz, with a slope efficiency of 37%; the laser pulse energy, duration, and peak power amount to 81 μJ, 25 ns, and 3.24 kW, respectively.
© 2008 Optical Society of America
Most of the host materials that are widely used for trivalent rare earth laser ions (Nd3+, Yb3+, Tm3+, Ho3+, Er3+, etc.) fall into two completely distinct categories: crystals and glasses. The crystal medium possesses the advantage of high thermal conductivity, facilitating high-power or high-repetition-rate laser operation, but usually limited by narrow absorption and emission bands; by contrast, the glass medium is characterized by broad spectra but suffers from a low thermal conductivity. As a trade-off, the so-called disordered crystals, which are capable of combining the advantages inherent to both crystals and glasses, are very interesting for applications in tunable, Q-switched, and mode-locked solid-state lasers.
The calcium niobium gallium garnet, Ca3(NbGa)2-xGa3O12 (CNGG), is a promising representative of such disordered crystals. It has a thermal conductivity of 4.7 Wm-1K-1, which amounts to one third of that of YAG, and is much higher than for glasses [1,2]. It has a number of nonequivalent lattice sites to accommodate trivalent dopant ions, providing an effective mechanism for substantial inhomogeneous line broadening. This feature has been confirmed in Nd- and Tm-doped CNGG crystals [2-4]. For the Yb ion, owing to its intrinsic broad absorption and emission spectra in most host crystals, such line broadening effect is not so remarkable in Yb:CNGG. Despite this, the main emission band around 1028 nm of Yb:CNGG, with a FWHM of 21 nm , is still two times wider than the corresponding line (<10 nm of FWHM) in Yb:YAG .
The Yb:CNGG crystal was developed very lately in 2007. Besides its spectroscopic properties, only a preliminary study was conducted on its continuous-wave (cw) laser performance . In this paper, we report efficient laser operation achieved with Yb:CNGG in cw as well as Q-switched regimes, revealing interesting characteristics of the laser oscillation.
2. Experimental laser setup
The Yb:CNGG crystal was grown by the Czochralski method , with the Yb concentration in crystal measured to be 2.3×1020 cm-3 (5.77 at. %) by the X-ray fluorescence method. Several crystal samples of the same Yb concentration were prepared from one boule, cut along the  crystallographic direction, with a square aperture of 3.3 mm × 3.3 mm and thickness of 1.5, 2.0, 2.5, 3.0, and 3.5 mm, respectively. The cw and passively Q-switched Yb:CNGG laser was built by employing a simple plano-concave resonator, as illustrated in Fig. 1. The plane mirror, M1, was coated highly reflecting for 1015–1230 nm (>99.8%) and highly transmitting for 880–990 nm (>97%). Several concave mirrors were utilized as the output coupler (M2), with radius-of-curvature of 50 mm, and transmission (T) in the range of 0.5%–10%. The uncoated Yb:CNGG crystals were held in a water-cooled copper block and placed close to the plane mirror inside the cavity. For passively Q-switched operation, a 0.3 mm thick Cr4+:YAG crystal with an initial transmission (T 0) of 97.6% at 1.06 μm, which was antireflection coated for 1.06 μm, served as the saturable absorber. The pump source used was a 50 W fiber-coupled diode (S50-980-2, Apollo Instruments, Incorporated, fiber core diameter of 200 μm and NA of 0.22) emitting unpolarized infrared radiation at 974–985 nm depending on the output level. Its output beam was focused by a 1:1 reimaging unit and delivered onto the Yb:CNGG crystal with a spot radius of ~100 μm through the plane mirror. The physical cavity length was ~ 23 mm.
3. Results and discussion
Continuous-wave laser operation was achieved at room temperature with all the Yb:CNGG samples having thickness of 1.5–3.5 mm. As a quasi-three-level system suffering from reabsorption losses, the performance of the Yb:CNGG laser was affected considerably by the crystal length. It was found that the 2.5 mm thick crystal was the optimal with respect to the output power generated and the conversion efficiency. Figure 2 shows the output power against the absorbed pump power (P abs), obtained with the 2.5 mm thick crystal for T = 2%, 5%, and 10%, respectively. The most efficient operation was achieved with a coupler of T = 2%. In this case, the lasing threshold was reached at P abs = 0.79 W, above which the output power increased approximately linearly, with a slope efficiency of η = 40%. A maximum output power of 2.0 W was obtained at P abs = 6.32 W, corresponding to an optical-to-optical efficiency of 32%. In the cases of T = 5% and 10%, the highest attainable output power was lower, 1.8 and 1.51 W, respectively. One notes also from Fig. 2 that the slope efficiencies, η = 37%–40%, were actually very close for T = 2%–10%. Only a slight reduction in efficiency is seen at high pump powers, suggesting a capacity for further power scaling. With the present pump diode, however, it was not feasible to achieve this; exceeding the highest P abs = 6.32 W, which corresponded to an incident pump power (P in) of 26 W at ~ 983 nm giving a ratio of P abs to P in of ~ 0.25, the pump efficiency (the fraction of P in absorbed) dropped to such a low level that Pabs no longer increased with P in. A pump diode of emission wavelength in the range of 920–947 nm will be more desirable to overcome this tendency . Nevertheless, the slope efficiencies achieved here are considerably improved in comparison with the previous results , which is attributed to the optimization of both the resonator parameters and the crystal thickness.
Like YAG (Y3Al5O12), the cubic Yb:CNGG is optically isotropic, an unpolarized laser oscillation is therefore expected for this crystal. However, the actual laser oscillation was found to be linearly polarized; at high power levels, two orthogonal polarization states existed simultaneously. Figure 3 depicts the output characteristics for T = 5%, showing the varying output power of the two polarization components with P abs. At threshold (P abs = 1.1 W) the laser started oscillating with polarization direction parallel to one of the diagonals (denoted as diagonal 1) of the square crystal end faces, along which a transversal pressure was applied by the crystal holder. The laser maintained oscillating in this single polarization state up to P abs = 3.02 W, at which the second polarization state appeared, which was orthogonal to the first one, with polarization direction along the other diagonal (denoted as diagonal 2). The output power of the second polarization component (E // diagonal 2) increased monotonically with P abs, reaching 0.84 W at the highest P abs = 6.32 W; the first polarization component (E //diagonal 1), on the other hand, became saturated in the coexistence region, reaching 0.96 W at P abs = 6.32 W. Such kind of polarization features were also exhibited in the laser oscillation achieved with other couplers of T = 2% and 10%. For a coupler of higher transmission, the second polarization state appeared at a higher P abs: at P abs = 2.26 and 3.65 W for T = 2% and 10%, respectively.
The linearly polarized laser oscillation of the cubic Yb:CNGG crystal stems from a stress-induced birefringence in the crystal. The transverse stress field causing birefringence was established by the mechanically applied pressure from the crystal holder. Owing to the existence of such an induced birefringence, two orthogonal eigenpolarization directions were defined in the initially isotropic transverse plane of the crystal, which are parallel to the principal stress axes, i.e., parallel and perpendicular to the direction of the externally applied pressure. With help of a He–Ne laser beam, we examined the Yb:CNGG crystal fixed in its holder, by placing it in between two crossed polarizers. A weak birefringence was observed, with the two eigenpolarization directions parallel to the diagonals 1 and 2. This means the isotropic Yb:CNGG crystal has become uniaxial under mechanical pressure. As in the case of a normal uniaxial crystal, the laser oscillation has one of the eigenpolarizations; under high pump levels, oscillation of the other eigenpolarization is likely to occur, due to the close gain seen by the two polarization modes. A similar situation was also encountered in a Nd:YAG laser .
Efficient passively Q-switched laser operation was achieved with the 2.5 mm thick Yb:CNGG crystal, by inserting the Cr4+:YAG in between the laser medium and the output coupler inside the resonator. To avoid possible damage to the intracavity elements, only the coupler with T = 10% was utilized in the pulsed regime. Figure 4 shows the average output power as a function of P abs, to give a comparison the cw output power is also presented. The pump power absorbed at threshold for Q-switched oscillation was measured to be P abs = 2.74 W. Above this the average output power scaled almost linearly with P abs; at the highest available P abs = 6.32 W, an average output power of 1.35 W was obtained at 1033 nm, resulting in an optical-to-optical efficiency of 21%. As in the case of cw operation, further power scaling is possible since no saturation tendency is seen from the output-input curve shown in Fig. 4. The slope efficiency determined for the Q-switched operation, η = 37%, is actually the same as in the cw regime, indicating a very effective Q-switching action.
In the passively Q-switched laser, the pulse repetition frequency (PRF) increased with P abs, from 2.3 kHz just above threshold to 16.7 kHz at P abs = 6.32 W. From the measured average output power and the corresponding PRF, the laser pulse energy generated at different power levels is calculated to be 74–81 μJ. The duration of the generated laser pulse was almost constant in the whole operational range. Figure 5 illustrates a typical pulse profile, recorded at an output power of 1.07 W, giving a pulse duration of 25 ns (FWHM). From the pulse energy (81 μJ) and duration a peak power of 3.24 kW is estimated. Shown as an inset in Fig. 5 is an oscilloscope trace of the pulse train, taken at the same power level. The amplitude fluctuations and time jitter are estimated to be less than 5% and 10%, respectively.
In conclusion, laser operation of the disordered Yb:CNGG crystal was demonstrated at room temperature by diode pumping in both cw and Q-switched regimes. 2.0 W of cw output power was produced at an absorbed pump power of 6.32 W, leading to an optical-to-optical efficiency of 32%, the slope efficiency was determined to be 40%. The laser oscillation of the isotropic crystal, rather than having no specific polarization, was actually linearly polarized, resulting from the stress-induced birefringence. Two orthogonal polarization states oscillated simultaneously at high pump power levels. Passively Q-switched by a Cr4+:YAG saturable absorber, 1.35 W of average output power at 1033 nm was produced at a PRF of 16.7 kHz; the laser pulse energy, duration, and peak power were 81 μJ, 25 ns, and 3.24 kW, respectively.
This work was supported by the National Natural Science Foundation of China (Nos. 60778013, 10744003, 50672050, 50590401, and 50721002), and the EU project DT-CRYS, NMP3-CT-2003-505580.
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