We report the spectroscopic characteristics and the laser performances of a low-doped 1% at. Yb:Sc2O3 ceramic sample. Under end- pumping at 933 nm and 968 nm in quasi-CW mode, at 1040.5 nm the laser delivers a maximum output power of 4.3 W and 1.77 W, respectively with a corresponding slope efficiency of 74% and 80%, which are, to the best of our knowledge, the highest value reported in literature for ceramics. We explored the tuning range of the sample, which spans from 1005 nm to 1050.5 nm, and finally we characterized the low losses tunable cavity at 1032 nm.
© 2012 OSA
Among the rare earths used as dopants in gain media for solid-state laser sources emitting in the infrared region, ytterbium (Yb3+) is currently having an unquestioned success. Its main advantages are largely known: the simple structure of the energy levels with a long lifetime of the upper state involved in the laser action (from 0.99 msec in YAG to 2.2 msec in CaF2 ), the low quantum defect, the absence of non-radiative channels or up conversion processes and the broad emission spectrum. On the other hand, Yb3+ is a quasi-three level system, thus at room temperature a consistent fraction of population is present on the higher level of the lower manifold causing ground level absorption of the laser radiation. Moreover, this population fraction increases with the host temperature. The requirement for an efficient removal of the heat generated in the material by the pumping process is therefore mandatory. In this frame, the role played by the host matrix becomes extremely important and hosts with high thermal conductivity are preferred. In recent years different materials have been investigated and sesquioxides as Y2O3 [2,3], Lu2O3 [4–11], and Sc2O3 [11–13], were found interesting because they comply this need. However, sesquioxides in crystalline forms are characterized by high melting point temperatures (~2400° C) and sophisticated growing techniques as the heat exchange , the Bridgman or the micro-pulling down methods  are required. So far, crystalline samples are grown [14,15] but with quite limited sizes. The improvement of the mentioned techniques could really pave the way to a new class of diode-pumped solid-state lasers with short pulses [16,17] and high intensity. But an alternative pathway to these targets is represented by the fabrication of other innovative host materials, i.e. transparent polycrystalline ceramics [18–20]. Ceramics are studied for several reasons. First, if compared with the corresponding crystal matrices, they support higher level of doping with more controlled distributions, and they can withstand higher thermo-mechanical stresses . Secondly, they are characterized by a lower sintering temperature (~1700° C) which allows an easier fabrication of sesquioxides with large sizes and good optical quality and, at the same time, with a reduced cost.
The thermal conductivity of the host is affected by the difference in the atomic weight of the dopant and of the substituted ion, as the phonons transporting the heat are scattered at the mass defects. High concentrations of dopants, which are required in the case of Yb3+ due to its low absorption cross section, enhance the mentioned effect decreasing the thermal conductivity, K, of the sample. To focus on Sc2O3, K decreases from 15.5 W/Km (undoped) to 6.4 W/Km (doping 3% at.) . The preservation of the high initial value can be obtained only by using reasonably low concentration of dopants.
The aim of this work is an in-depth investigation of a low-doped 1% at. Yb:Sc2O3 ceramic sample, which has a thermal conductivity as high as K = 9.2 W/mK  (corresponding to 0.7% at. Yb:YAG). We present a spectroscopic characterization and the laser performance obtained by a longitudinal pumping at λp = 933 nm and at λp = 968 nm, in quasi-CW at room temperature. Emitting at λl = 1040.5 nm the laser delivers Pout = 4.3 W and Pout = 1.77 W, respectively, with an output coupler of Toc = 10.50%. The corresponding slope efficiencies are ηslope = 74% and ηslope = 80%, while the optical to optical efficiencies are ηopt = 52.5% and ηopt = 60.8%, respectively. We investigated the influence of the thermally induced losses on the sample behavior by setting different Duty Factors (DF), from 20% to 40%, and measuring the output power. Finally, we explored the tunability of the sample pumped at 933 nm and characterized the low losses cavity at λl = 1032 nm (corresponding to the main emission wavelength of the Yb:YAG).
2. Experimental set-up
Figure 1 reports the schematic of the laser cavity. The gain sample, fabricated by Baikowski Japan Co. Ltd., is an antireflection coated Yb:Sc2O3 ceramic sample with a level of doping of 1% at.. It has a parallelepiped shape with a surface of 5x5mm2, length 3 mm. The ceramic is soldered with Indium on a copper heat sink, water-cooled at 18°C. In the first campaign of measurements the sample was pumped at 933 nm by a laser diode delivering 25 W, coupled with a fiber (200 μm core diameter, numerical aperture 0.22). The focused pump beam inside the sample has an almost Gaussian intensity distribution with 150 μm of radius at 1/e2. In the second set of measurements the ceramic was pumped at 968 nm by using a laser diode coupled to a fiber (100 μm core diameter, numerical aperture 0.15). The distribution of intensity of the laser diode in the focal plane is fairly Gaussian with a spot radius around 67 μm at 1/e2. In both cases the sample was pumped in uasi-Continuous Wave (QCW) at room temperature. Different DF were obtained by modulating the pump laser with square pulses at low repetition frequency, i.e. 10 Hz, and with a pulse lengths going from 20 ms (DF = 20%) to 40 ms (DF = 40%). The pump wavelength, which is monitored on-line, is kept constant at the different pumping powers by adjusting the cooling temperature of the laser diode. However, as it will be shown in the following, the absorption of the sample at 933 nm and 968 is relatively insensitive to the small variation of the pump wavelength. This reduces the requirement of a tight stabilization of the pump emission wavelength over a wide range of laser diode input current levels, which instead would be required by pumping on the zero-line absorption peak at 975 nm with a FWHM of 3 nm which is even narrower than the pump laser linewidth (~5 nm FWHM).
The resonator is V-shaped with a total length of 24 cm. The radiation coming out from the fiber is collimated and focused into the sample by a pair of achromatic doublets with a magnification of 1:1. The End Mirror (EM) is flat with a dichroic coating with high transmission around the pump wavelength and high reflectivity in the laser emission band. The Folding Mirror (FM) has a curvature radius of 100 mm. The convergent lens, L, placed behind the FM, is employed during the absorption measurements to collect the residual pump power, which is sent to the power meter M2. The output couplers (OCs) are flat with different transmissions, i.e. Toc = 1.8%, Toc = 5.2%, Toc = 10.5%. The short pass filter, F, placed behind the OC mirror, prevents that the residual pump radiation reaches the power meter M1. The laser wavelength was measured with a fiber coupled spectrometer with 60 cm focal length equipped with a multichannel detector, with a spectral resolution of 0.4 nm.
A low losses tunable cavity is obtained by substituting the output coupler by a gold coated ruled grating (1800 grooves/mm) at the Littrow’s configuration, using the zero order for the output coupling. The zero-order diffraction efficiency at around 1040 nm is 7.0% while the absorption and scattering are around 5.0%. At 1032 nm the zero order diffraction efficiency is about 6.2%.
3. Experimental results
3.1 Optical and luminescence characteristics
Figures 2(a) and 2(b) report the optical absorption spectrum of the ceramic sample at room temperature. The edge of charge transition (ligand-to-Yb3+) is found at about 235 nm while at least two color center absorption bands (of unclear origin) are noted at about 270 nm and 520 nm. The absorption band measured at 370 nm is ascribed to the F+ center (electron in oxygen vacancy)  while the band at about 600 nm is due to the Yb2+ ion [25,26]. The absorption lines between 850 and 1000 nm come from the 4f-4f transitions of Yb3+.
Excitation within the Yb3+ charge transfer transition band at 225 nm in Yb:Lu2O3 at low temperatures  shows the well-known double peak luminescence band with the peaks at about 350 nm and 500 nm. These features are similar in several oxide compounds due to similar positioning of the top of valence band (oxygen states) and ground state of Yb2+ in the forbidden gap [28,29]. The band gap edge in undoped Sc2O3 was established at 5.9 eV (210 nm)  and excitation in this region or under X-ray shows the emission at about 3.6 eV (345 nm) in the undoped sample [31,32]. In the present 1% at. Yb:Sc2O3 ceramic, the excitation at Ex = 225 nm, see Fig. 3(a) , provides leading emission band at about 360 nm with the shoulder at about 290 nm and a broad structureless wing within 400-600 nm. Such a complex emission is most probably a mixture of the host and Yb3+ charge transfer emission bands with possible participation of another defect- and host-related emission centers. Photoluminescence excitation spectrum for the dominant emission wavelength at 360 nm is reported in Fig. 3(b).
The excitation spectrum shows the leading edge within 200-210 nm which is perfectly correlated with the Sc2O3 host absorption edge  and the shoulder around 230 nm which agrees with the position of the Yb3+ charge transfer absorption and excitation  spectra in RE2O3 hosts and confirms the interpretation of the emission spectrum above.
3.2 Laser characteristics
Figure 4 reports the measured laser output power as a function of the absorbed pump power, Pabs, obtained with three OCs pumping the sample at 933 nm (a) and at 968 nm (b) in quasi-CW with a Duty Factor of 20%. In both cases the maximum output power is reached by using the OC mirror with the highest transmission (Toc = 10.5%) at 1040.5 nm. In particular, when pumping at 933 nm the laser delivers Pout = 4.3 W with a corresponding slope efficiency as high as ηslope = 74% (ηopt = 52.5%) while it delivers Pout = 1.77 W with a slope efficiency of ηslope = 80% (ηopt = 60.8%) at 968 nm. The laser thresholds range from around Pabs = 0.55 W (Toc = 1.8%) to Pabs = 0.77W (Toc = 10.5%). It is worth to note that the slope and the optical efficiencies are estimated by using the real absorbed pump power as the sample absorption during the laser action is taken into account. Figure 4(c) reports the absorbed fraction of the pump power, ABS, as a function of the pump power, Pp, pumping at 933 nm with the three OCs used in the experiment. The absorption of the ceramic decreases from 37.5% (unsaturated absorption) to 29.2% when the laser action is blocked. Conversely, when the sample is lasing the absorption decreases at low incident pump power (from ~1.5 W to ~11 W) while the trend is reversed at higher values.
This behavior is due to the balance between the saturation of the absorption for the pump beam, which is due to the transfer of the population from the lower to the upper laser level, and the effect of the intracavity laser beam. This latter induces a fast recycling of the population toward the lower laser level, due to the stimulated emission. For high intracavity laser intensity levels, this second process compensates almost completely the saturation of the pump absorption , as it has been experimentally observed for instance also in Yb:YAG .
The variations of ABS are estimated around 2.7% with Toc = 1.8%, 1.9% by using Toc = 5.2%, and 1.8% with Toc = 10.5%. Pumping at 968 nm the unsaturated absorption of the sample is 15.6%.
The variation of the maximum output power, which is around 58.8%, is due to the higher pump absorption at 933 nm with respect to 968 nm (the absorption cross sections are respectively σabs(933 nm) = 0.38 x10−20 cm2 and σabs(968 nm) = 0.24x10−20 cm2).
The difference of the performance in term of slope efficiency is ascribed to a combination of different factors. For a fixed lasing wavelength and pumping geometry, the increase in the pumping wavelength entails a corresponding increase in the slope efficiency because of the reduction of the energy defect between the absorbed and the emitted photon. In our case the variation in the quantum defect shifts from 6.7% at 968 nm to 10.3% at 933 nm, with an expected enhancement of about 2.5% in the slope efficiency. Moreover, the smaller spot size radius and the better M2 factor of the pump diode (respectively from 150 μm and 40 at 933 nm to 67 μm and 24 at 968 nm), permit a better matching of the fundamental cavity mode radius, which reasonably pushes for a further increase of the slope efficiency. By performing the Caird analysis , see Table 1 , which allowed an estimation of the intrinsic slope efficiency, ηI, and the cavity losses, ξ, we calculated a mode-matching efficiency  of around ηm = 98% pumping at 968 nm and around ηm = 85% pumping at 933 nm. The corresponding loss per mm is, in both cases, less than 0.37%
Conversely, the thermal loads experienced by the sample in the two pumping configurations are almost comparable, so that they are not expected to affect significantly the measured slope efficiencies. This is exemplified by the results of the thermal simulations reported in Figs. 5(a) and 5(b), which show the distributions of the temperature difference ΔT with respect to the heat sink, calculated with the geometry used in our experiments, occurring at the end of the of the 50th pumping pulse. The two simulations feature the same value of absorbed pump power. It can be seen that despite the weaker focusing, pumping at 933 nm (a) produces a slightly stronger peak temperature increase, that is 5.3 K, with respect to the value of 4.8 K found when pumping at 968 nm (b). The simulation takes into account the different thermal load due to spontaneous emission (occurring at the average wavelength λ = 1022 nm) and to laser emission (occurring at 1040.5 nm) Further details of the simulation method and software platform can be found elsewhere .
Basically the Quasi-Continuous Wave (QCW) pumping configuration is used for attenuating the negative effect on the laser efficiency due to the thermally induced losses. In particular, the thermal conductivity is reduced as the temperature experienced by the sample increase. However, in some applications a CW pumping is required and the knowledge of the thermal behavior of gain media becomes important. We measured the output power by varying the Duty factor from 20% to 40%. Figure 6 shows the results obtained pumping at the shorter pump wavelength because the thermal loads are higher. From our data we can conclude that this ceramic sample is quite robust and it could be employed in high power laser systems. In fact the output power, the slope and optical to optical efficiencies, as well as the laser threshold, are almost constant.
Although the measurements here described were carried out under QCW operation, this material can efficiently operate in CW as well. Preliminary measurements in CW pumping conditions, performed with the same sample in a similar experimental set-up, have shown a maximum output power of 4.1 W under pumping at 940 nm (ηopt = 40%) and 1.85 W (ηopt = 55%) under pumping at 968 nm.
In our experiments the maximum output power was primarily limited by the small absorption of the pump power. With the same available pump power, higher output levels could be obtained by increasing the absorbed fraction of the pump power. This can be done either by optimizing the pumping geometry (e.g. increasing the sample length or using multipass pumping geometries ) or the matching between the pumping wavelength and the absorption peaks of the material, provided that suitable cooling methods are implemented in order to handle the increased power dissipation in the pumped volume.
The beam quality factor was measured using a CCD camera and a laser beam analysis software. At the maximum pump power levels reported in Figs. 4(a) and 4(b) it resulted M2 = 2.6 (in the direction of the folding plane of the cavity) and M2 = 2.3 (in the perpendicular plane) when pumping at 933 nm, while at 968 nm we obtained M2 = 2.3 in both directions.
The range of tunability of the gain medium, see Fig. 7(a) , is measured by using a gold coated ruled grating placed at the Littrow’s angle, which acts as a flat OC with a transmission of 7.0% at 1040 nm. Pumping at 933 nm the sample is tuned from 1005.0 nm to 1050.5 nm. The maximum output power, Pout = 2.1 W, is achieved at 1040 nm with a pump power of Pp = 22.6 W. The laser line width ranges from 1.3 nm to 1.5 nm across the whole range of the tunability. The measurements obtained at 968 can be found in .
Finally, we have characterized the tunable cavity at 1032 nm, which is close to the main emission wavelength of Yb:YAG samples. Figure 7(b) reports the output power as a function on the absorbed pump power. The maximum output power is 900 mW with a corresponding slope efficiency of ηslope = 21%. However, the absorbed pump power is calculated taking into account the unsaturated value of the absorption, 37.5%, and in consequence we expect that it is slightly underestimated.
We presented an in-depth investigation of the spectroscopic and laser properties of Yb:Sc2O3 ceramic sample with a doping level of 1% at..
Concerning the spectroscopic investigation on the Yb3+ in Sc2O3 host the absorption spectrum reports the features of 4f-4f transitions spanning in the region of 880-1040 nm and charge transfer transitions of Yb3+ below 240 nm. Moreover, two unknown color centers show their absorption bands at about 270 nm and 520 nm. When the sample is excited either in the Sc2O3 absorption edge (at around 210 nm), or in the absorption band of the Yb3+ charge transfer transition (at about 230 nm), the emission is composed by two broads and overlapping bands situated in the range 260-600 nm, due to emission of the host and of the Yb3+ charge transfer transition.
To test the laser performance, the sample was longitudinally pumped at 933 nm and 968 nm in quasi-CW (10 Hz, DF = 20%), at room temperature. We measured the output power as a function of the absorbed pump power by three output couplers with different transmission. At the longest pump wavelength the maximum output powers is Pout = 4.3 W with a corresponding slope efficiency of 74% while at the shortest pump wavelength the laser delivers Pout = 1.77 W with a slope efficiency of 80% (optical to optical efficiencies 52.5% and 60.8%, respectively). A low losses tunable cavity allowed us to explore the range of tunability of the gain medium, which exceeds 45 nm, and a characterization of the laser cavity around the wavelength of 1032 nm.
The research was supported by Regione Toscana, project “CTOTUS-Progetto integrato per lo sviluppo della Capacità Tecnologica e Operativa della Toscana per l’Utilizzo dello Spazio” (POR FESR 2007-2013 Attività 1.1 Linea d'intervento D); by the Consiglio Nazionale delle Ricerche, CNR-RSTL “Ricerca Spontanea a Tema libero”, id. 959; by the joint project of ASCR and CNR and Czech GA AV project M100100910.
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