Spectroscopic properties and laser performance of resonantly-pumped cryo-cooled Er 3 + : GdVO 4

We report a highly efficient cryo-cooled eye-safe laser operation of a resonantly-pumped (in-band) Er 3+ :GdVO4 single crystal. The maximum continuous wave (CW) power of 10.3 W with 84% slope efficiency was achieved at 1598.7 with pumping at 1538.6 nm by a spectrally-narrowed Er-fiber laser. Under the 1529 nm resonant pumping by a commercially available diode bar stack operating in a quasi-CW (QCW) mode, the laser delivered 37 W of output power with 68% slope efficiency. This is believed to be the first reported cryo-cooled Er 3+ :GdVO4 laser, resonantly-pumped into the 4 I15/2 → 4 I13/2 transition. ©2012 Optical Society of America OCIS codes: (140.3480) Lasers, diode-pumped; (140.3580) Lasers, solid-state; (140.3500) Lasers, erbium. References and links 1. S. D. Setzler, M. J. Shaw, M. J. Kukla, J. R. Unternahrer, K. M. Dinndorf, J. A. Beattie, and E. P. Chicklis, “A 400 W cryogenic Er:YAG slab laser at 1645 nm,” Proc. SPIE 7686, 76860C, C7 (2010). 2. N. Ter-Gabrielyan, L. D. Merkle, A. Ikesue, and M. Dubinskii, “Ultralow quantum-defect eye-safe Er:Sc2O3 laser,” Opt. Lett. 33(13), 1524–1526 (2008). 3. M. Dubinskii, V. Fromzel, N. Ter-Gabrielyan, M. D. Serrano, D. E. Lahera, C. Cascales, and C. Zaldo, “Spectroscopic characterization and laser performance of resonantly diode-pumped Er()-doped disordered NaY(WO4)2.,” Opt. Lett. 36(16), 3263–3265 (2011). 4. N. Ter-Gabrielyan, V. Fromzel, T. Lukasiewicz, W. Ryba-Romanowski, and M. Dubinskii, “High power resonantly diode-pumped σ-configuration Er:YVO4 laser at 1593.5 nm,” Laser Phys. Lett. 8(7), 529–534 (2011). 5. N. Ter-Gabrielyan, V. Fromzel, T. Lukasiewicz, W. Ryba-Romanowski, and M. Dubinskii, “Nearly quantumdefect-limited efficiency, resonantly pumped, Er3+:YVO4 laser at 1593.5 nm,” Opt. Lett. 36(7), 1218–1220 (2011). 6. N. Ter-Gabrielyan, V. Fromzel, L. D. Merkle, and M. Dubinskii, “Resonant in-band pumping of cryo-cooled Er:YAG laser at 1532, 1534 and 1546 nm: a comparative study,” Opt. Mater. Express 1(2), 223–233 (2011). 7. A. I. Zagumenny, V. G. Ostroumov, I. A. Shcherbakov, T. Jensen, J.-P. Meyn, and G. Huber, “The Nd:GdVO4 crystal: a new material for diode-pumped laser,” Sov. J. Quantum Electron. 22(12), 1071–1072 (1992). 8. A. I. Zagumenny, P. A. Popov, F. Zerouk, Yu. D. Zavartsev, S. A. Kutovoi, and I. A. Scherbakov, “Heat conduction of laser vanadate crystals,” Quantum Electron. 38(3), 227–232 (2008). 9. Y. Sato and T. Taira, “The studies of thermal conductivity in GdVO4, YVO4, and Y3Al5O12 measured by quasione-dimensional flash method,” Opt. Express 14(22), 10528–10536 (2006). 10. J. Didierjean, E. Herault, F. Balembois, and P. Georges, “Thermal conductivity measurements of laser crystals by infrared thermography. Application to Nd:doped crystals,” Opt. Express 16(12), 8995–9010 (2008). 11. J. Sulc, H. Jelinkova, W. Ryba-Romanowski, and T. Lukasiewicz, “1.6 um microchip laser,” Laser Phys. Lett. 6(3), 207–211 (2009). 12. C. Brandt, V. Matrosov, K. Petermann, and G. Huber, “In-band fiber-laser-pumped Er:YVO4 laser emitting around 1.6 μm,” Opt. Lett. 36(7), 1188–1190 (2011). 13. N. Ter-Gabrielyan, V. Fromzel, W. Ryba-Romanowski, T. Lukasiewicz, and M. Dubinskii, “Efficient, Resonantly Pumped, Room Temperature Er:GdVO4 Laser,” Opt. Lett.: posted 01/27/2012; Doc. ID 160739 14. W. Fowler and D. Dexter, “Relation between absorption and emission probabilities in luminescent centers in ionic solids,” Phys. Rev. 128(5), 2154–2165 (1962). 15. C. Bertini, A. Toncelli, M. Tonelli, E. Cavalli, and N. Magnani, “Optical spectroscopy and laser parameters of GdVO4:Er ,” J. Lumin. 106(3-4), 235–242 (2004). #159726 $15.00 USD Received 9 Dec 2011; revised 16 Feb 2012; accepted 21 Feb 2012; published 29 Feb 2012 (C) 2012 OSA 12 March 2012 / Vol. 20, No. 6 / OPTICS EXPRESS 6080 Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.


Form Approved OMB No. 0704-0188
Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information.Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302.Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.operation [1][2][3][4][5].Cryogenic cooling is known to improve thermo-mechanical, thermo-optical, and spectroscopic properties for the majority of laser hosts.While absorption and emission cross-sections become stronger with cooling, the absorption lines also become much narrower and require narrow-linewidth pump sources -the issue which can hinder pumping efficiency [6].Disordered laser hosts, such as Er:NaY(WO 4 ) 2 , even at cryogenic temperatures exhibit very broad absorption transitions convenient for resonant pumping, but their thermal conductivity is usually low, which limits power scaling [3].Uniaxial Erbium-doped yttrium vanadate material (Er:YVO 4 ) offers a compromise: it exhibits large absorption and emission cross-sections while its absorption lines remain relatively broad.This feature helps in achieving efficient laser operation with InP diode-based pump sources [4].Similarly high efficiency can potentially be expected for Er:GdVO 4 laser (which is a close analog to Er:YVO 4 ).Rare earth-doped gadolinium vanadate (GdVO 4 ) was introduced in 1992 [7] as a close analog to commercially available YVO 4 .Additional interest to GdVO 4 is also driven by contradictory literature data on its thermal conductivity relative to that of YAG and especially of YVO 4 [8][9][10].Most of the latest reported data indicates that the thermal conductivity of GdVO 4 along its crystallographic c-axis is higher than that of YAG and at least similar to that of YVO 4 .While efficient resonantly-pumped cryogenic and room temperature Er:YVO 4 lasers have been reported earlier [4,5,11,12], the laser potential of the similarly pumped Er 3+ :GdVO 4 , to the best of our knowledge, has only been evaluated at room temperature [13].The maximum CW output power of 3.5 W with slope efficiency of 56% was achieved with resonant pumping by an Er-fiber laser at 1538.6 nm.With pumping by a commercial diode laser bar stack, a quasi-CW (QCW) output of 7.7 W and a maximum slope efficiency of ~53% with respect to the absorbed pump power were obtained [13].
In this paper, we present spectroscopic characteristics of Er 3+ :GdVO 4 in the 1450-1650 nm wavelength range defined by the
The polarization-resolved absorption spectra of the Er 3+ :GdVO 4 crystal, cryogenicallycooled to 77 K, were measured using Cary 6000i spectrophotometer, operating in the fixedslit-width mode with the 0.1 nm resolution.The cross-sections of Er 3+ :GdVO 4 , derived from the measured absorbance, are presented in Fig. 1(a).The absorption bands are generally stronger for σ-polarization, with the exception of a broad and strong transition at ~1502 nm (cross-section σ abs ~2.2•10 −19 cm 2 ).The zero-zero absorption line at 1529.3 nm, which corresponds to the transition from the lowest Stark components of the 4 I 15/2 and the 4 I 13/2 manifolds, also exhibits a large cross-section of ~1.5-1.6•10−19 cm 2 .This transition has approximately equal strength for both polarizations with the linewidth of ~1.3 nm full width half maximum (FWHM) and, thus, is suitable for pumping by an unpolarized source.The fluorescence lifetime of the 4 I 13/2 level was measured on a 0.5%-doped Er 3+ :GdVO 4 crystal sample, which was pulverized in order to minimize effects of reabsorption and radiation trapping on experimental results.The sample was excited by 100 µsec pulses of an InP diode laser tuned to the 1529 nm absorption line.The fluorescence signal was detected with a Germanium photodiode and the sampled kinetic was processed by a digital oscilloscope (Tektronix, model TDS 2022B).A room temperature lifetime was measured to be ~3.06 ms and versus ~3.5 ms measured at 77 K.The results for the entire temperature range of 77-300K, presented in Fig. 2(a), were consistent with those measured by Bertini et al for a 1% Er:GdVO 4 sample [15].
The simplified energy level diagram of the Er 3+ :GdVO 4 crystal for the 4 I 13/2 and the 4 I 15/2 manifolds obtained from the analysis of the absorption and emission spectra and major transitions between their Stark sublevels are shown in Fig. 2(b).This diagram is also consistent with that reported by Bertini et al [15].

Cryogenic laser experiments
Laser experiments were carried out on an anti-reflection coated 10 mm long, 7 mm wide, and 3 mm thick Er 3+ (0.7%):GdVO 4 single crystal (N Er = 8.47•10 19 cm −3 ).Its crystallographic caxis was normal to the axis of the laser cavity, thus, the crystal could be longitudinally pumped in any chosen polarization.The crystal was mounted on a copper plate inside the liquid nitrogen cryostat.The setup was almost identical to that described in [4].
Two different pump sources spectrally matching one of the major absorption lines of Er 3+ :GdVO 4 were used in laser experiments.One of them was a 20 W, CW, unpolarized, single mode Er-fiber laser with a ~0.3 nm FWHM emission bandwidth, which fits inside the 1538.6 nm absorption line.The motivation for using a narrow-linewidth, single mode fiber laser as a pump source was to avoid both spectral and spatial mismatches in pumping in order to achieve maximum efficiency of the Er 3+ :GdVO 4 laser in a low QD operation.
The pump beam was focused into the crystal by the F = 100 mm spherical lens through a flat dichroic mirror (T > 98% @ 1520 −1540 nm, R > 99.5% @ 1590 −1650 nm).The pumped volume was cylindrical in shape with a diameter of ~330 µm (at 1/e 2 level) along the entire crystal length, as measured by IR CCD camera (Spiricon, model LW230).
The laser cavity was formed by the flat dichroic mirror and a concave output coupler with the radius of curvature (R CC ) of 100 mm and reflectivities (R OC ) within the 70% -90% range.The cavity length (L cav ) was chosen to be about 80 mm to provide the best pump-cavity mode spatial matching.
The CW performance of the cryogenic Er 3+ :GdVO 4 laser, resonantly pumped into the 1538.6 nm absorption line by the Er-fiber laser, is shown in Fig. 3(a).Without any wavelength selective elements inside the cavity, the laser operated in π-polarization at 1598.7 nm with 3.9% QD.The emission bandwidth strongly depended on the pump power and its full value approached 4 nm at the maximum pump.The fraction of the absorbed pump varied due to the saturation effect [16] from ~0.85 at the threshold to ~0.65 at full 20 W of the incident pump and was predictably dependent on the output coupling.In order to avoid inconsistencies in laser efficiency measurement, we determined the absorbed pump when the laser was in the "operational" mode (above the threshold), measuring the incident pump, the transmitted pump and the laser output power simultaneously.
As seen in Fig. 3(a), the maximum achieved slope efficiency was 84% with R OC = 85%.The maximum obtained CW output power was 10.3 W for the R OC = 70%.
The second pump source was a commercial spectrally narrowed (~2 nm FWHM), both fast-and slow-axis collimated (FAC/SAC), InGaAsP/InP 13-bar diode laser stack, rated at 1532 nm (QPC Lasers).This source is much more relevant for practical laser designs, which was a motivation for this part of the study.In our experiments, it was operated in a quasi-CW regime with a 5% duty cycle (τ pulse = 10 ms).The incident pump beam was π-polarized and its wavelength was temperature-tuned to the absorption band around 1529 nm by varying the coolant temperature of the stack.In order to nearly equalize the divergence of the pump beam in the vertical and the horizontal directions, an additional 4 X cylindrical telescope was placed after the diode stack.A variable pump attenuator, consisting of a polarizing cube and a halfwave plate, was inserted between the telescope and the stack.This setup allowed us to maintain a constant pumping wavelength while varying the pump power.The collimated pump beam was focused into the crystal by a spherical lens (F = 75 mm) through the same flat dichroic mirror used in the previous experiment.This time, the pump formed a cone inside the crystal with the 1/e 2 diameter varying from 960 µm in the center to approximately 1200 µm at the crystal ends.
As in the previous case, the laser cavity was formed by the dichroic high reflector and a concave output coupler with R OC varying from 90% to 70%.This time, however, in order to accommodate a larger pump mode and to achieve a better pump-cavity mode matching, the radius of curvature was chosen to be 250 mm with the L cav = 80 mm.
Figure 3(b) shows the QCW performance of the Er 3+ :GdVO 4 laser with the laser diode stack pumping.It also operated in a π-polarization at 1598.7 nm (4.6% QD).The best slope efficiency of 68% was achieved with the R OC = 85%.The maximum obtained QCW laser output was 37.3 W. The fraction of the QCW absorbed pump varied from 0.73 -0.83 at the threshold to 0.66-0.77at full 96 W of the pump power (for R OC = 85% and 70% respectively).The difference in efficiencies of fiber laser-and the diode bar stack-pumped Er:GdVO 4 lasers can be explained by much better pump-cavity mode matching in the former case.

Conclusions
We reported what is believed to be the first resonantly-pumped cryo-cooled laser based on an Er 3+ doped GdVO 4 single crystal.Spectroscopic characteristics and laser performance of this laser utilizing transitions between the 4 I 15/2 and 4 I 13/2 manifolds (1450-1650 nm) at cryogenic temperatures were investigated.A slope efficiency as high as 84% and a maximum CW output power of 10.3 W at 1598.7 nm have been obtained for resonant pumping into the 1538.6 nm absorption line by a narrow-linewidth Er-fiber laser.With the resonant pumping into the 1529 nm absorption band by a commercial InGaAsP/InP FAC/SAC laser diode bar stack, we achieved the maximum slope efficiency of ~68% and the maximum QCW output power of ~37 W.

Fig. 2 .
Fig. 2. (a) Temperature dependence of the 4 I13/2 lifetime of Er 3+ in GdVO4.(b) A simplified energy level diagram of the 4 I13/2 and the 4 I15/2 manifolds with Boltzmann populations at 77 K.The major pump transitions are shown with blue arrows; red arrows represent the fluorescence transitions of relevance to laser operation around 1.6 µm.