Absorption and emission spectra of Er3+:LuVO4 for the 4I15/2 ↔ 4I13/2 transitions have been investigated at room and cryogenic temperatures. A corrected energy level diagram was built. An efficient, resonantly pumped Er3+:LuVO4 laser operating at 1609 nm (π-polarization) or 1597.5 nm (σ-polarization) was demonstrated. For CW pumping into the 1532 nm absorption line by a narrow-bandwidth Er-fiber laser, the maximum output power was ~4.6 W and the maximum slope efficiency was 64% - the best reported efficiency of Er-doped vanadate lasers at room temperature. Resonant pumping into the 1529 nm absorption band with a fiber-coupled diode laser yielded a 62% slope efficiency. To the best of our knowledge, this is the first report on an Er:LuVO4 laser.
© 2014 Optical Society of America
Er3+-doped uniaxial orthovanadates YVO4 and GdVO4 have recently been shown to be among the most promising gain materials for eye-safe lasers operating in the 1.6 μm wavelength range [1–3]. Their major advantages over conventional Er3+-doped YAG are in their stronger and broader absorption lines. This feature makes resonant pumping by 15XX-nm InGaAsP/InP laser diodes easier and allows for higher laser efficiency with good beam quality in simple laser designs. Both Er3+:YVO4 and Er3+:GdVO4 have similar spectroscopic features and have recently demonstrated excellent, low quantum defect (QD) laser performance with slope efficiencies as high as 54-58% at room temperature [3, 4]. It is reasonable to expect that other orthovanadates can be equally, if not more, attractive for high power eye-safe laser development. Indeed, recent studies of Yb- and Nd-doped Lutetium Orthovanadate (LuVO4), a newer member of the vanadate family, show that this laser host has significant potential for laser power scaling. On the other hand, while thermal properties and CW laser performance of Yb- and Nd-doped LuVO4 have been sufficiently explored [5–8], Er3+-doped LuVO4 has not been adequately studied. In fact, we are aware of only one publication, which had an emphasis strictly on spectroscopic features of this crystal . To the best of our knowledge, the actual laser potential of the Er3+:LuVO4 crystal has never been demonstrated.
This paper presents the results of a detailed spectroscopic characterization as well as the first laser performance of resonantly pumped Er3+:LuVO4. Polarized output power of 4.6 W has been achieved with slope efficiencies of 62-64% at the 1609 or 1597.5 nm wavelength - the highest laser efficiency reported for Er-doped orthovanadate lasers at room temperature.
Er3+:LuVO4 single crystals were grown by the Czochralski technique. The Er3+ doping concentration of 0.59 ± 0.03% (NEr = 7.7 x 1019 cm−3) was determined by Galbraith Laboratories. Spectroscopic characterization was performed in the 1450 – 1650 nm wavelength range corresponding to the 4I13/2 ↔ 4I15/2 transitions of Er3+. The polarization resolved absorption spectra collected with a Cary 6000i spectrophotometer are shown in Figs. 1(a) and 1(b). The spectral resolution for these measurements was set to 0.02 nm.
The emission spectra of Er:LuVO4 were obtained by illuminating the crystal with a 970 nm diode laser and collecting the luminescence with an Optical Spectrum Analyzer (Yokogawa AQ6370C, spectral resolution of 0.1 nm). A polarization beam splitter was inserted between the sample and the collecting optics to separate π- and σ-polarized emission. The measured fluorescence of the 4I13/2 manifold was found to be a single exponential with the lifetime of 2.98 ms – the same as reported by Lisiecki et al for a 1% Er:LuVO4 sample . The emission cross-sections calculated using the Fuchtbauer-Landenburg method , are depicted in Figs. 2(a) and 2(b).
Generally, the σ-polarized absorption is stronger except for the π-polarized lines around 1490 nm. Absorption cross-sections for both polarizations in the 1529 nm band are approximately equal for both polarizations. Thus, this particular band is convenient for resonant pumping with unpolarized sources. Also, the absorption peaks are 3 – 4 times stronger than those in Er3+-doped YVO4 and GdVO4 . Since higher absorption facilitates better spatial overlap between the pumped volume and the laser cavity mode over shorter crystal lengths, Er3+:LuVO4 has an advantage over other studied Er3+-doped vanadates for applications demanding compact and high efficiency lasers.
An energy level diagram of the 4I13/2 and 4I15/2 manifolds is shown in Fig. 3. It was derived from the collected polarization resolved absorption and emission spectra at 10, 77 and 300K. Our diagram differs from the previous one , in energies of the five lowest Stark energy levels Zi and the upper Y1 sub-level.
Our spectroscopic analysis also shows that Er3+:LuVO4 has the two most promising transitions for laser operation at room temperature with approximately equal cross-sections: at ~1609 nm (π-polarization) with the cross-section of σ ~0.5 × 10−20 cm2, and at ~1597.5 nm (σ - polarization) with the cross-section of σ ~0.45 × 10−20 cm2. Despite the unusual high absorption strength of Er3+ ions in LuVO4, the strength of its emission transitions is about the same as that of similar transitions in Er3+:GdVO4 and Er3+:YVO4 .
3. Laser experiments
3.1. Experimental setup
Laser experiments were performed with anti-reflection (AR) coated Er3+:LuVO4 slabs with transverse dimensions of 3.5 x 6 mm. The c-axis of the crystal was normal to the laser cavity axis. Thus, the Er3+:LuVO4 could be longitudinally pumped in any chosen polarization. The slabs were wrapped in indium foil, and clamped between water-cooled copper plates. The water temperature was maintained at ~14° C. A simplified experimental laser setup is shown in Fig. 4.
As in our previous laser experiments with Er3+-doped orthovanadates [1, 2], we used two different pump sources. A single-mode, narrow bandwidth (~0.3 nm), CW Er-fiber laser with collimated output was configured to pump into the (somewhat weaker) 1532 nm absorption band of Er3+:LuVO4. The unpolarized pump beam was focused into the crystal by a spherical lens L2 (F2 = 100 mm) through a flat dichroic mirror and formed a cylindrically-shaped pumped volume with a diameter of ~330 μm (at 1/e2) along the entire crystal length.
The second pump source was a spectrally narrowed, InGaAsP/InP, fiber-coupled laser diode module (FCLDM) emitting at ~1529 nm with the output spectral bandwidth of ~2 nm. The fiber core diameter was 105 μm with the NA of 0.15. The pump beam was collimated by a spherical lens L1 (F1 = 60 mm) and focused into the crystal by another lens L2 (F2 = 75 mm) forming a conically-shaped pumped volume with a waist diameter of ~250 μm.
The laser cavity was formed by a flat dichroic mirror (T > 90% at 1520-1540 nm, R > 99.5% at 1580-1650 nm) and a plano-concave output coupler (OC) with an optimized radius of curvature (RoC) of 100 mm. Spectral transmission of the OC in the 1580-1620 nm range was almost flat.
The pump focusing lens L2 and the concave output coupler were spaced approximately F +RoC apart (with proper correction for laser material index at the pump wavelength). This optically conjugated configuration allowed for the double-passing of the pump beam through the crystal as well as for providing the best spatial overlap between the pumped volume and the cavity mode.
In both cases, when measuring the absorbed pump, we took into account the contribution of the residual pump reflected back into the crystal from the concave OC. Here we assumed that the absorption coefficient is the same for both pump directions. The ratio of the absorbed pump power to the incident pump power in the case of pumping with the Er-fiber laser was 0.5 – 0.55 (measured for three output couplers with reflectivity of 85, 90% and 95%). With the diode pumping the same ratio decreased by about 5%. It is worth mentioning that the contribution of the second-pass pumping in the FCLDM case was small.
3.2. Laser performance
Figure 5(a) shows the output power of the CW Er:LuVO4 laser pumped by the Er-fiber laser at 1532 nm. During laser optimization the ROC varied between 85% and 95% and the cavity length ZCAV ~102 mm. The best slope efficiency of 64% by the absorbed pump power was achieved when the ROC = 85%. To the best of our knowledge, this is the highest reported efficiency for Er-doped orthovanadate lasers at room temperature. The maximum achieved output power in this case was ~4.6 W.
Figure 5(b) shows the output power of a similar Er:LuVO4 laser pumped by the FCLDM. The best slope efficiency of 62% with respect to the absorbed pump power was obtained when the ROC = 90%. The maximum achieved output power was almost the same ~4.5 W. It should be noted that the available Er:LuVO4 crystal possessed a relatively high passive loss. We measured it at ~0.04-0.05 cm−1 (at 1618 nm wavelength where ground-state absorption does not affect attenuation of the probe beam). Thus, even higher laser efficiency can be achieved with better quality crystals.
The observed laser output was single transverse mode, which is consistent with the cavity geometry described above and the small diameter of the pumped volume.
Our Er:LuVO4 laser output consisted of two spectral components: π-polarized 1608.9 nm and σ-polarized 1597.5 nm, see Fig. 6(a). We had to run the laser in the QCW mode in order to accumulate spectral data with the same Yokogawa OSA. Time-resolved spectral studies revealed that in most cases lasing was π-polarized, with a full bandwidth of 0.4 - 1.76 nm (depending on the pump power). Typical wavelength switching between these two closely-spaced spectral components is shown in Fig. 6(b). However, by manipulating the parameters of the cavity and pump power, it was possible to observe lasing only at a σ-polarized wavelength of 1597.5 nm with the bandwidth of 0.56 – 1.4 nm.
As can be seen from the emission spectra of Er:LuVO4 (see Fig. 2), two major laser transitions form relatively wide (~10 nm FWHM) emission bands centered at the 1609 nm and the 1597.5 nm wavelengths. In addition, their peak cross-sections are approximately equal. Both bands originate from the tightly spaced Stark sublevels Y1 and Y2 of the 4I13/2 manifold with nearly equal energies of 6543 and 6547 cm−1, see Fig. 3. Two lasing bands terminate at slightly different upper Stark sublevels Z7 and Z8 of the ground state 4I15/2 manifold (energies 286 and 332 cm−1, respectively). At room temperature the latter levels substantially overlap (kT ~207 cm−1 whereas the energy gap between them is only ~46 cm−1). Thus, any variation in wavelength- or polarization-dependent losses in the cavity significantly impacts the laser output spectrum. This situation could cause major spectral competition. An example of such time-resolved spectral behavior of the Er3+:LuVO4 laser in the QCW regime is shown in Fig. 6(b).
A more detailed investigation of these peculiarities of the Er:LuVO4 laser spectrum would be desirable but it was outside the scope of this paper.
We report what is believed to be the first laser operation based on Er3+:LuVO4 single crystal. Absorption and emission spectra of Er3+:LuVO4 were investigated to yield a corrected energy level diagram of 4I15/2 and 4I13/2 manifolds. A highly efficient resonantly (in-band) pumped laser emitting at 1609 nm (π-polarization) or 1597.5 nm (σ-polarization) has been demonstrated. With pumping by a single-mode, narrow bandwidth (~0.3 nm FWHM), CW Er-fiber laser at 1532 nm, the maximum output power of 4.6 W at 1609 nm was achieved with a maximum slope efficiency of 64% – the highest efficiency among the lasers based on Er3+-doped orthovanadates at room temperature. With pumping by a spectrally-narrowed InGaAsP/InP fiber-coupled laser diode module emitting at ~1529 nm the maximum CW output power of 4.5 W was obtained with the slope efficiency of 62%. Such high efficiencies were possible to achieve due to the much stronger absorption of Er in Lutetium Orthovanadate comparatively to the other known Er-doped vanadates.
References and Links
1. N. Ter-Gabrielyan, V. Fromzel, T. Lukasiewicz, W. Ryba-Romanowski, and M. Dubinskii, “Nearly quantum-defect-limited efficiency, resonantly pumped, Er3+:YVO₄ laser at 1593.5 nm,” Opt. Lett. 36(7), 1218–1220 (2011). [CrossRef] [PubMed]
2. N. Ter-Gabrielyan, V. Fromzel, W. Ryba-Romanowski, T. Lukasiewicz, and M. Dubinskii, “Efficient, resonantly pumped, room-temperature Er3+:GdVO4 laser,” Opt. Lett. 37(7), 1151–1153 (2012). [CrossRef] [PubMed]
4. N. Ter-Gabrielyan, V. Fromzel, W. Ryba-Romanowski, T. Lukasiewicz, and M. Dubinskii, “Spectroscopic and laser properties of resonantly (in-band) pumped Er:YVO4 and Er:GdVO4 crystals: a comparative study,” Opt. Mater. Express 2(8), 1040–1049 (2012). [CrossRef]
8. Y. Cheng, H. J. Zhang, Y. G. Yu, J. Y. Wang, X. T. Tao, J. H. Lio, V. Petrov, Z. C. Long, H. R. Xia, and M. H. Jiang, “Thermal properties and continuous-wave laser performance of Yb:LuVO4 crystal,” Appl. Phys. B 86(4), 681–685 (2007). [CrossRef]
9. R. Lisiecki, G. Dominiak-Dzik, P. Solarz, A. Strzep, W. Ryba-Romanowski, and T. Lukasiewicz, “Spectroscopic characterization of Er-doped LuVO4 single crystals,” Appl. Phys. B 101(4), 791–800 (2010). [CrossRef]
10. W. B. Fowler and D. L. Dexter, “Relation between absorption and emission probabilities in luminescent centers in ionic solids,” Phys. Rev. 128(5), 2154–2165 (1962). [CrossRef]