Tunable laser operation of ytterbium in disordered single crystals of Yb : NaGd ( WO 4 ) 2

Lasing of Yb in a disordered single crystal host, NaGd(WO4)2, is reported. Pump efficiencies as high as 20% and slope efficiencies as high as 30% are achieved for both σand π-polarizations with Ti:sapphire laser pumping. The emission of Yb:NaGd(WO4)2 is centered near 1030 nm. Tunability between 1016 and 1049 nm is obtained with a Lyot filter. 2004 Optical Society of America OCIS codes: (140.5680) Rare earth and transition metal solid-state lasers; (160.5690) Rare earth doped materials.


Introduction
Solid-state lasers based on rare-earth doped crystals operating in the 1 µm spectral region are nowadays widely spread as compact and reliable sources for various applications due to the possibility for highly efficient diode pumping.The potential of the trivalent ytterbium ion, Yb 3+ , as a dopant was recognized in the very early years of diode pumping because in comparison with Nd 3+ this ion possesses longer energy-storage lifetime and smaller quantum defect, and can be pumped by the optically more robust InGaAs laser diodes operating in the 900-1000 nm spectral range [1].The relatively simple two-manifold structure of Yb 3+ has the advantage that excited state absorption, up-conversion and cross-relaxation processes are absent but the operation in a quasi-three-level laser scheme poses a significant design and implementation challenge.An additional advantage of the Yb 3+ over the Nd 3+ ion is related to the broader absorption and emission linewidths which on the one hand reduces the requirements to the pump laser diodes and on the other hand holds a greater promise for the generation of shorter light pulses by mode-locking in the high-power regime.
The monoclinic potassium double tungstates KR(WO 4 ) 2 with R=Y, Bi, Ln (Ln=Sm…Lu) [2] occupy a special position among the laser crystals due to their extremely large absorption and emission cross sections which is related also to their strong anisotropy and biaxiality [3].In addition, the separation of adjacent R 3+ ions in these ordered hosts is relatively large and therefore less concentration quenching occurs for increased doping levels.Highly efficient laser operation has already been demonstrated for Yb:KY(WO 4 ) 2 and Yb:KGd(WO 4 ) 2 [4], and more recently for Yb:KLu(WO 4 ) 2 [5].Continuous wave (cw) room-temperature laser operation was possible even with the 100% Yb-doped (stoichiometric) KYb(WO 4 ) 2 [6].
Sodium double tungstates NaR(WO 4 ) 2 and molybdates NaR(MoO 4 ) 2 where R=Y, La, Bi and Ln (Ln=Ce…Lu) are also related to the Ca 2+ ion substitution in the CaWO 4 tetragonal scheelite structure, space group 6 h 4 C (I4 1 /a), by a pair of Na + and R 3+ ions [7].For these compounds only weak phase transformations are observed upon cooling from high temperature.As a consequence the tetragonal crystalline symmetry is preserved at room temperature although some distortions towards lower symmetry space groups may appear as observed in the Bi-containing compounds [8,9].
In the tetragonal double tungstates and molybdates an almost random distribution of Na + and trivalent metal R 3+ ions over two lattice sites occurs which holds also for the active laser dopant.As a consequence, a locally variable crystal field around the dopant ion is present and the linewidths of the electronic transitions for the rare earth elements are found to be broader than in ordered crystals (see Fig. 10 in ref. [10]).Such disordered crystals with their inhomogeneous line broadening [11] have an intermediate position with respect to their optomechanical and spectroscopic properties between ordered laser hosts [12] and glasses [13].Moreover, the uniaxial sodium double tungstates and molybdates possess an important technological advantage over their monoclinic potassium analogues.Some of them (e.g.R= La, Nd…Er, Bi) melt congruently and can be pulled in large sizes by the Czochralski method.
The present work is devoted to cw laser studies of Yb 3+ -doped disordered NaGd(WO 4 ) 2 or shortly Yb:NaGdW.In order to determine the optimum pump conditions for the two polarizations we used in this initial experiment a tunable Ti:sapphire laser for pumping and in order to assess the mode-locking potential of Yb:NaGdW tunable operation for the σ and π polarizations using a Lyot filter was studied.

Crystal growth and characterization of Yb:NaGdW
Single crystals of NaGd 1-x Yb x (WO 4 ) 2 with x=0.1 in the melt were grown in air by the Czochralski method using a pulling equipment supplied with a crystal diameter control system.The raw materials, Gd 2 O 3 (99.99%),Yb 2 O 3 (99.99%),Na 2 CO 3 (99.9%)and WO 3 (99.8%)from Alfa Aesar, were mixed in the appropriate ratios and the desired phase was synthesized in a two step process: firstly the mixture was heated up to 750ºC at a rate of 50ºC/h, held at this temperature for 18 h and then cooled down to room temperature.The product was ground and in a second step heated to 850ºC for 24 h to complete the synthesis.The phase obtained was assessed by X-ray powder diffraction (XRPD) and then heated to 25ºC above the melting temperature in a 75 cc Pt crucible.The melt was kept at this temperature for one hour for homogenization and then the optimum crystallization temperature was determined by monitoring the crucible weight when seeding with a (001)oriented NaLa(WO 4 ) 2 seed.The rotation and pulling rates were 10 rpm and 2.3 mm/h, respectively.The grown crystals were cooled to room temperature at a rate of 10ºC/h.The crystals obtained were colourless.The dimensions of the pulled boule were 20 mm in diameter and 50 mm in length.Some crystal cracking appeared likely upon cooling.The refractive index was measured for this composition only at 1047 nm: n o =1.948 and n e =1.943 indicate that the NaGdW host is a negative crystal with relatively low birefringence.
The crystal phase obtained was further studied by XRPD and the Yb content determined from ICP emission measurements.A tetragonal crystal structure was found.The lattice parameters derived from a Rietveld refinement of the XRPD profile were a=5.2370(2)Å, c=11.3636(6)Å [V=311.66(3)Å 3 ].The molar Yb content in the crystal was 8% giving a distribution coefficient of 0.8.Hence the Yb density in the crystal is 5.1×10 20 cm -3 .The laser sample prepared was a 3.273 mm thick plate with a clear aperture of ≈10×15 mm 2 (Fig. 1) allowing polarization either parallel or perpendicular to the optical axis c to be used.
The resonant transition of Yb 3+ is actually also magnetic-dipole allowed and three spectra have to be measured for a complete description in an uniaxial crystal: α (E⊥c, H⊥c), π (E//c) and σ (H//c).In the case of Yb:NaGdW, however, we recorded very similar optical densities for the α and σ polarizations (Fig. 1).The maximum absorption cross sections near 974 nm calculated for the π and σ polarizations are 1.51 and 1.21×10 -20 cm 2 , respectively; very close to the estimations in [22].For the Brewster angle geometry used in the laser the small signal absorption amounts to 94.4% and 89.4% for the π and σ polarizations, respectively.The Yb segregation coefficient, crystal lattice parameters and optical absorption properties so far reported for our sample agree rather well with those previously reported for Yb:NaGdW crystals grown in N 2 /O 2 reductive atmosphere [22][23][24][25], despite the fact that the latter are supposed to contain W and O lattice vacancies and exhibit coloration in the visible and infrared spectral range.This suggests that the interaction of such lattice defects with the active Yb 3+ ions can be neglected at least for the doping level considered here, x<10 molar %.The maximum emission cross sections calculated near 996 nm amount to ≈2×10 -20 cm 2 for the π polarization and ≈1.5×10 -20 cm 2 for the σ polarization [22].Nevertheless the reabsorption makes it difficult to predict which polarization will have better laser performance and calculations of the gain cross sections (substracted emission and absorption cross sections assuming some inversion level) provide reliable information basically on the lasing wavelength at threshold only [22].
Measurements of fine powders in an immersion liquid in [22] provided a value of 320 µs for the fluorescence decay time independent of the Yb-doping level in the 0.5 to 5% range.This value is close to the radiative lifetime of 397 µs calculated from the absorption spectra [22], and it is also comparable to the lifetimes estimated for the monoclinic potassium double tungstates [5].The pump saturation intensity we calculated with it is equal to 42.3 kW/cm 2 (π) and 52.7 kW/cm 2 (σ), respectively.Note that both the emission and the absorption cross sections of Yb:NaGdW are almost an order of magnitude lower than the cross sections observed for the E//N m polarization in the Yb-doped monoclinic double tungstates [4][5][6].That is why it is normal to expect lower efficiencies and higher laser thresholds for Yb:NaGdW than those found with the ordered double tungstates.

Experimental results
The experimental set-up used in the present work is shown in Fig. 2. The astigmatically compensated three-mirror cavity consisted of a total reflector M 1 for the laser radiation with 50 mm radius of curvature, a folding totally reflecting mirror M 2 with 100 mm radius of curvature and a plane output coupler M 3 whose transmission was varied between 3 and 10%.Both M 1 and M 2 were highly transmitting for the pump radiation which ensured single side pumping and helped to avoid any feedback to the pump laser.The antireflection coated pump lens L (focal length 62.8 mm) and M 2 introduced only negligible reduction of the pump power so that the incident power P in on the crystal could be conveniently measured outside the resonator.The total cavity length amounted to 73 cm.The pump and the laser spot diameters were almost equal in the position of the crystal and we estimated from the Brewster reflection a Gaussian pump waist of w pump ≈22 µm.Thus the pump beam size is πw p 2 /2=7.6×10-6 cm 2 .The peak incident on-axis intensity for the maximum incident power of P in =2 W used in the present experiment is 260 kW/cm 2 .Averaging across the cross section by taking a factor of 0.5 means that we are approximately 3 times (π) and 2.5 times (σ) above the saturation intensity.For both polarizations, however, our measurements showed that the absorption depletion effect is partially compensated at high pump levels by the recycling effect (Fig. 3).Nonetheless, the absorbed power is always higher for the πpolarization.Both the thresholds (at P abs ≈0.5 W) and the slope efficiencies obtained with three different output couplers were essentially the same for the two polarizations (Fig. 4).In both cases the optimum pump wavelengths λ P indicated in the figures are very close to the absorption maximum.Such a similar performance for E//c and H//c can be attributed not only to the different reabsorption effect but also to the comparable emission cross sections at the actual laser wavelengths (Fig. 4) which are far from the corresponding maxima.Thermal effects were not observed up to P in =2 W although no special cooling was provided to the crystal.Fig. 5. Output power P out versus laser wavelength λ L for a maximum incident pump power of P in =2 W and T OC = 5% near 1030 nm.The tuning performance by a Lyot filter for both the π (filled triangles) and σ (empty triangles) polarizations is shown.
The tuning operation of the laser was investigated inserting a two-plate Lyot filter under Brewster angle in the M 2 -M 3 arm (Fig. 2) close to the output coupler.Under optimum alignment the power reduction with the filter inside the cavity did not exceed 10%.The tuning curve obtained for the π-polarization is slightly broader than for the σ-polarization (Fig. 5).Both curves seem to be slightly truncated at shorter wavelengths by the reflection characteristics of M 1 and M 2 .Tunability extending from 1016 to 1049 nm at the zero-level was obtained.The FWHM for the π-polarization corresponds to ∆ν=6.5×10 12 Hz.For a sech 2shaped mode-locked pulse this means than sub-50-fs pulses can be expected from this laser in the mode-locking regime.This is about two times less than the shortest pulses typically achieved nowadays with femtosecond lasers based on the monoclinic double tungstates for similar T OC values [26].

Conclusion
In conclusion cw and tunable room temperature laser operation of Yb 3+ :NaGd(WO 4 ) 2 under Ti:sapphire laser pumping was demonstrated.The performance for the two possible polarizations was very similar achieving an output power of about 300 mW for 1.5 W of absorbed pump radiation.The optimum pump wavelength corresponds to the maximum absorption and is suitable for InGaAs laser diodes.Initial tuning experiments indicate interesting potential for mode-locking with this disordered Yb-host.We note that in comparison to the recent laser demonstration of Yb:CaF 2 [27] the disorder and the inhomogeneous line broadening in the sodium double tungstates is rather due to occupation with some probability of two possible sites (each with multiple environments) than to lattice defects.
The mode-locking experiments planned for the near future will reveal the actual advantages of this new Yb host.Besides this we intend to investigate the spectroscopic and laser properties for higher doping levels which could be interesting for highly absorbing thin layer laser designs.

Fig. 2 .
Fig. 2. Scheme of the three-mirror laser cavity of the Yb:NaGdW laser.

Fig. 3 .Fig. 4 .
Fig.3.Measured single pass absorption with lasing and with lasing interrupted in the arm containing the output coupler versus incident pump power P in for T OC = 10%.