1992
OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
Deep red diode-pumped Pr3+:KY3F10 continuous-wave laser Alberto Sottile1,* and Philip W. Metz2 1
2
Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, Pisa 56127, Italy Institut für Laser-Physik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany *Corresponding author: [email protected] Received February 12, 2015; revised March 31, 2015; accepted April 3, 2015; posted April 3, 2015 (Doc. ID 234589); published April 24, 2015
We report the first observation of continuous-wave laser emission at 720 nm, in the deep red region, of a Pr3 : KY3 F10 (Pr:KYF) single crystal, pumped with a blue InGaN-based laser diode at 446 nm. We employed a hemispherical cavity with three different output coupling mirrors transmitting 0.7%, 1.4%, and 2.7% of laser radiation. We obtained a maximum output power of 207 mW with a slope efficiency of 24.3%, comparable for the first time to what has been reported for other praseodymium-doped fluoride crystals. The round-trip cavity losses for our sample were estimated to be between 0.3% and 0.6%, a remarkably small value for this material. © 2015 Optical Society of America OCIS codes: (140.3580) Lasers, solid-state; (140.3480) Lasers, diode-pumped; (140.7300) Visible lasers; (160.5690) Rare-earth-doped materials. http://dx.doi.org/10.1364/OL.40.001992
0146-9592/15/091992-04$15.00/0
the green [20], orange [21], and red [22] regions. In addition to that, Pr:KYF crystals have been recently employed to achieve tunable laser operation with a wide range around the emission laser lines, although in quasicontinuous-wave mode [20]. In this report, we report the first observation, to the best of our knowledge, of continuous-wave laser emission at 720 nm from a Pr:KYF single crystal. Figure 1 shows absorption cross-sections, in the blue region, and emission cross-sections, in the deep red region, for this material. These properties have been previously studied and reported for this host [1,22]. The actual value of the transition cross-sections varies between the publications; here we follow the data reported in [1]. Absorption data indicate that blue, InGaN-based, laser diodes can efficiently pump this crystal at about 445 nm, exciting the transition between the ground level 3 H4 and the 3 P2 level. The emission cross-section shows three distinct areas, with two peaks located at about 700 nm, one peak at 720 nm, and three peaks around 730 nm. The
10
20
cm2
430
440
450
460
470
480
490
2.5 2.0 1.5
abs
1.0 0.5 0
10
20
cm2
5.0
em
Trivalent praseodymium (Pr) ions have been studied over the past few years as dopant for visible laser generation. This ion is particularly appealing because of its absorption band in the blue region, which makes it suitable for pumping with blue InGaN laser diodes [1]. Emission in the green, orange, red, deep red, and near-infrared regions have been reported in several materials so far [2,3]. Sources in the deep red region (around 720 nm) have been employed for several studies, from spectroscopy of water vapor [4], to characterization of bismuth-doped materials [5], to entrapment of Bose–Einstein condensates [6], and laser cooling of Fermi gases [7]. These experiments used complex and large systems as pump sources, like dye, gas, or Ti:sapphire lasers, because of the lack of laser diodes at this wavelength, which is barely covered by the high-energy spectrum of AlGaAs-based diodes. Pr-doped materials can provide a solid-state alternative for laser sources at 720 nm, more compact than all systems previously available and with high quality of the output beam. A source with these characteristics could be also employed to generate radiation at 360 nm via second-harmonic generation, providing a laser emission that could be of interest for metrology [8]. Duplications like this have been reported with other Pr-based lasers [9–11]. Among all the available hosts, Pr has shown the best performances in fluoride crystals, because of its low phonon energies that reduce the probability of non-radiative decays [12]. To date, the highest laser efficiencies with this ion have been achieved in LiYF4 [13,14], LiLuF4 [15], and BaY2 F8 [16,17], at several visible and infrared lines. Also lasers in oxide crystals like YAlO3 were successfully operated in the wavelength region around 720 nm [18]. However, these crystals have anisotropic structures. Thus, their absorption and emission properties are polarization dependent. KY3 F10 (KYF) instead has a cubic crystalline lattice [19], therefore is completely isotropic, which means that lasers generated in this material can in principle be unpolarized, and have a high-quality output mode. Continuous-wave laser emission with Pr-doped KYF (Pr:KYF) has been reported in
4.0 3.0 2.0 1.0 0
700
710
720
730
740
Wavelength (nm)
Fig. 1. Absorption and emission cross-sections of Pr:KYF in blue and deep red regions, respectively. © 2015 Optical Society of America
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
cross-section value is different from zero in the left part of the plot because of vibrational side bands at about 670 nm. The line at 720 nm is caused by the decay from the 3 P0 level to the 3 F3 level of the Pr ion. This emission has the highest cross-section in this region, and is the one studied in this work. The setup employed for our laser experiments is described in Fig. 2. The pump source was a multimode blue InGaN laser diode (LD), which was thermally tuned to emit at 446 nm with an output power of 1.6 W. The emission from this diode was linearly polarized. After collimation by a high numerical aperture lens (CL), the pump beam was further conditioned by a pair of cylindrical lenses (CYL) to improve its shape. We employed a combination of a λ∕2-waveplate (HW) and a polarizing beam splitter cube (PBS) to continuously alter the amount of power sent to the cavity. The pump light was finally focused into the crystal using a lens (FL) with a focal length of 40 mm. The maximum power incident to the cavity was 1.3 W. We employed a hemispherical linear resonator, made of two mirrors, one plane (M1), placed right before the crystal (Pr:KYF), and the other plane-concave (M2) with a radius of curvature approximately equal to the length of the cavity. The input mirror was anti-reflective coated between 430 and 550 nm T > 95% and highly reflective between 680 and 790 nm R > 99.9%. We used alternatively three plane-concave mirrors as output couplers, all with a radius of curvature of 50 mm. The output coupling mirrors were reflective from 660 to 750 nm, with the maximum reflectivity at 720 nm. This narrow bandwidth coating was necessary to exclude the other visible emission lines of the Pr ion. The output mirrors had respectively T 0.7%, T 1.4%, and T 2.7% at 720 nm. We used a 5.9-mm-long sample of 1.3% at. doped Pr:KYF, with a cross area of 3 × 4 mm2 . The concentration of the dopant ions in the sample was determined via transmission measurements. Details about the growth procedures for this crystal can be found in [20]. We polished the two smallest facets to make them plane and parallel. No coating was applied on these facets. We determined the fraction of single-pass pump power absorbed by the crystal to be about 85% of incident power. Considering reflection of the transmitted pump beam
1993
from the output mirror M2, this fraction increases to about 87% of incident power. The crystal was placed right behind the M1 mirror, mounted on a copper holder with water cooling at 14°C to keep it refrigerated during the measurements. This holder was fixed on a goniometric head that allowed rotation and translation of the sample along the two horizontal axes. The output mirror M2 was mounted on a translation stage employed to change and optimize the cavity length, which was varied around 50 mm. A 540 nm longpass filter was placed after the mirror M2 to remove the residual pump laser transmitted through the cavity. We used two silicon-based power meters (PM) to determine at the same time the laser output power and the incident pump power. The wavelength of the deep red emission was determined with an Ocean Optics HR2000 portable spectrophotometer, with a resolution of 1 nm. Figure 3 shows the output power as a function of the amount of pump power absorbed by the crystal, for different transmittances of the output mirror, with the relative slope efficiencies (ηabs ). For these calculations, we considered double-pass absorption of the pump beam in the crystal. With the 0.7% mirror, we measured a maximum output power of 174 mW, with a slope efficiency of 19.2% and a threshold power of 57 mW. The 1.4% mirror led to a maximum output power of 187 mW, a slope efficiency of 20.7%, and a threshold power of 75 mW. We obtained the best results with the 2.7% mirror with a maximum output power of 207 mW, a slope efficiency of 24.3%, and a threshold power of 138 mW. With all the mirrors, the laser output had a wavelength of 720 nm and a measurement performed with a Glan– Taylor polarizer revealed that was unpolarized. We calculated the round-trip cavity losses with the Findlay–Clay [23] and Caird [24] methods using the laser thresholds and the slope efficiencies measured with the three output mirrors. With these analyses, we estimated a fraction of losses of 0.6% and 0.3%, respectively. These values are comparable to the losses measured in Pr-doped LiYF4 (0.6%) [25] and LiLuF4 (0.3%) [15] before, and lower than what has been reported for other
Output power (mW)
200
150
T
0.7
— abs
19.2
T
1.4
— abs
20.7
T
2.7
— abs
24.3
100
50
0
Fig. 2. Layout of the experimental setup employed for laser measurements. For description refer to the text.
200
400 600 800 Absorbed power (mW)
1000
Fig. 3. Output power at 720 nm as a function of absorbed power, for three output coupler transmittances (T). Slope efficiencies (ηabs ) are also shown.
1994
OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
Beam diameter ( µ )
500
400
300
200
100 -60
X Y -40 -20 0 20 40 Distance to focal plane (mm)
60
Fig. 4. Evolution of the output beam diameter, measured with a propagation analyzer along two orthogonal directions parallel to the edges of the crystal. Inset, far-field output beam profile acquired with a CCD camera. Both measurements were conducted at maximum output power with T 2.7%.
Pr:KYF crystals so far (0.9% [21], 2% [20], and 5% [22]). Calculations performed for the Findlay–Clay analysis also allow to estimate the optimum output coupling, for which the cavity should deliver the highest output power at the given pump power [26], leading to a value of about 4%, which was not available in our laboratories. Despite that, the Caird method indicates a limiting slope efficiency for our current pumping setup of 26.2%, which is close to what we achieved with the 2.7% output coupler. We also measured the M2 factor of the output laser beam to further check its quality. We employed a Spiricon M2 -200s beam propagation analyzer. We used the 2.7% output coupler and adjusted the cavity to emit at maximum power. The diameter of the beam as a function of the distance to the focal plane is shown in Fig. 4 for two orthogonal directions parallel to the edges of the crystal. The M 2 parameter of the emission was 1.1 along the horizontal axis and 1.2 along the vertical axis. A profile of the output intensity is also shown in the inset of Fig. 4. All these measurements highlight the high quality of the output beam and its symmetric shape, close to a diffraction-limited Gaussian beam. In comparison with other Pr-doped fluoride crystals, the highest slope efficiency of our sample (24.3%) is comparable with what has been reported for diode-pumped Pr3 :LiYF4 (30%) and Pr3 :LiLuF4 (24%) [27] at 720 nm. These results also evidence the high quality of the crystal characterized in this work, and show that this material can compete for the first time with the other, wellestablished fluorides on laser performances, at least in the deep red region. We still expect that the efficiencies of Pr:KYF in this wavelength region could be further improved by increasing the output coupler transmittance, by optimizing the doping concentration and by changing the length of the sample. In summary, we report the first observation of continuous-wave laser emission at 720 nm with Pr3 :KY3 F10 . After growing the crystal, we prepared a sample for laser experiments in the deep red region,
pumping it with a laser diode emitting at 446 nm. We employed a hemispherical resonant cavity with three different mirrors as output couplers and characterized the laser properties for three transmittance values. We obtained a maximum output power of 207 mW with a slope efficiency of 24.3%, which is comparable with those reported for LiYF4 and LiLuF4 for the same transition. Because of the high absorption efficiency of 87%, the optical-to-optical efficiency with respect to the pump power incident to the crystal reaches a value of almost 20%, which is close to the maximum slope efficiency achieved. Moreover, we determined that the total round-trip resonator losses were lower than in previously reported Pr:KYF crystals, with an improvement of at least a factor of two. Those losses are also comparable to the values reported for other Pr-doped fluorides, showing the high optical quality of this sample. The maximum slope efficiency and the round-trip losses we determined display that Pr:KYF can have remarkable optical characteristics and laser properties, as good as LiYF4 and LiLuF4 crystals. The output intensity profile and the beam propagation were also measured, resulting in a quasidiffraction-limited output beam with an average M2 value lower than 1.2. Because the Pr ion is prone to non-radiative lifetime quenching effects by cross-relaxation [1,28], detailed investigation on energy transfer mechanisms with respect to the Pr concentration might allow to find an optimum doping concentration that keeps a reasonably short absorption length, conserves a long lifetime, and enhances the laser performances. In addition to that, secondharmonic generation from this transition could be investigated to achieve laser operation at 360 nm. Finally, Pr:KYF could also be used to explore potential laser emissions at the other wavelengths available in the deep red region, around 700 and 740 nm. We would like to thank the AIT-DAAD Vigoni program “Solid state laser in the visible spectral range” for financial support. We would also like to express our gratitude to Prof. Günter Huber for supporting us with the facilities in his laboratories at the University of Hamburg and to Prof. Mauro Tonelli for his assistance and help during the preparation of this work. References 1. A. Richter, Laser Parameters and Performance of Pr3+Doped Fluorides Operating in the Visible Spectral Region (Cuvillier Verlag, 2008). 2. J. Y. Allain, M. Monerie, and H. Poignant, Electron. Lett. 27, 189 (1991). 3. M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, Appl. Phys. B 97, 363 (2009). 4. B. E. Grossmann and E. V. Browell, J. Mol. Spectrosc. 136, 264 (1989). 5. Y. Arai, T. Suzuki, Y. Ohishi, S. Morimoto, and S. Khonthon, Appl. Phys. Lett. 90, 261110 (2007). 6. J. Dziarmaga and M. Lewenstein, Phys. Rev. A 72, 033606 (2005). 7. Z. Idziaszek, L. Santos, M. Baranov, and M. Lewenstein, J. Opt. B 5, S190 (2003). 8. H. Hachisu, K. Miyagishi, S. G. Porsev, A. Derevianko, V. D. Ovsiannikov, V. G. Pal’chikov, M. Takamoto, and H. Katori, Phys. Rev. Lett. 100, 053001 (2008).
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS 9. V. Ostroumov, W. Seelert, L. Hunziker, C. Ihli, A. Richter, E. Heumann, and G. Huber, Proc. SPIE 6451, 645103 (2007). 10. A. Richter, N. Pavel, E. Heumann, G. Huber, D. Parisi, A. Toncelli, M. Tonelli, A. Diening, and W. Seelert, Opt. Express 14, 3282 (2006). 11. Y. Dong, S. T. Li, and X. H. Zhang, Laser Phys. Lett. 9, 116 (2012). 12. L. Esterowitz, R. Allen, M. Kruer, F. Bartoli, L. S. Goldberg, H. P. Jenssen, A. Linz, and V. O. Nicolai, J. Appl. Phys. 48, 650 (1977). 13. T. Gün, P. Metz, and G. Huber, Opt. Lett. 36, 1002 (2011). 14. P. W. Metz, F. Reichert, F. Moglia, S. Müller, D.-T. Marzahl, C. Kränkel, and G. Huber, Opt. Lett. 39, 3193 (2014). 15. F. Cornacchia, A. Richter, E. Heumann, G. Huber, D. Parisi, and M. Tonelli, Opt. Express 15, 992 (2007). 16. D. Pabœuf, O. Mhibik, F. Bretenaker, P. Goldner, D. Parisi, and M. Tonelli, Opt. Lett. 36, 280 (2011). 17. A. Sottile, D. Parisi, and M. Tonelli, Opt. Express 22, 13784 (2014). 18. M. Fibrich and H. Jelínková, Laser Phys. Lett. 10, 035801 (2013).
1995
19. K. Friese, H. Krüger, V. Kahlenberg, T. Balić-Žunic, H. Emerich, J.-Y. Gesland, and A. Grzechnik, J. Phys. Condens. Matter 18, 2677 (2006). 20. P. W. Metz, S. Müller, F. Reichert, D.-T. Marzahl, F. Moglia, C. Kränkel, and G. Huber, Opt. Express 21, 31274 (2013). 21. B. Xu, P. Camy, J.-L. Doualan, Z. Cai, and R. Moncorgé, Opt. Express 19, 1191 (2011). 22. P. Camy, J. L. Doualan, R. Moncorgé, J. Bengoechea, and U. Weichmann, Opt. Lett. 32, 1462 (2007). 23. D. Findlay and R. A. Clay, Phys. Lett. 20, 277 (1966). 24. J. A. Caird, S. A. Payne, P. R. Staver, A. J. Ramponi, L. L. Chase, and W. F. Krupke, IEEE J. Quantum Electron. 24, 1077 (1988). 25. A. Richter, E. Heumann, G. Huber, V. Ostroumov, and W. Seelert, Opt. Express 15, 5172 (2007). 26. O. Svelto, Principles of Lasers (Springer, 2010). 27. F. Cornacchia, A. Di Lieto, M. Tonelli, A. Richter, E. Heumann, and G. Huber, Opt. Express 16, 15932 (2008). 28. J. Hegarty, D. L. Huber, and W. M. Yen, Phys. Rev. B 25, 5638 (1982).
OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
Deep red diode-pumped Pr3+:KY3F10 continuous-wave laser Alberto Sottile1,* and Philip W. Metz2 1
2
Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, Pisa 56127, Italy Institut für Laser-Physik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany *Corresponding author: [email protected] Received February 12, 2015; revised March 31, 2015; accepted April 3, 2015; posted April 3, 2015 (Doc. ID 234589); published April 24, 2015
We report the first observation of continuous-wave laser emission at 720 nm, in the deep red region, of a Pr3 : KY3 F10 (Pr:KYF) single crystal, pumped with a blue InGaN-based laser diode at 446 nm. We employed a hemispherical cavity with three different output coupling mirrors transmitting 0.7%, 1.4%, and 2.7% of laser radiation. We obtained a maximum output power of 207 mW with a slope efficiency of 24.3%, comparable for the first time to what has been reported for other praseodymium-doped fluoride crystals. The round-trip cavity losses for our sample were estimated to be between 0.3% and 0.6%, a remarkably small value for this material. © 2015 Optical Society of America OCIS codes: (140.3580) Lasers, solid-state; (140.3480) Lasers, diode-pumped; (140.7300) Visible lasers; (160.5690) Rare-earth-doped materials. http://dx.doi.org/10.1364/OL.40.001992
0146-9592/15/091992-04$15.00/0
the green [20], orange [21], and red [22] regions. In addition to that, Pr:KYF crystals have been recently employed to achieve tunable laser operation with a wide range around the emission laser lines, although in quasicontinuous-wave mode [20]. In this report, we report the first observation, to the best of our knowledge, of continuous-wave laser emission at 720 nm from a Pr:KYF single crystal. Figure 1 shows absorption cross-sections, in the blue region, and emission cross-sections, in the deep red region, for this material. These properties have been previously studied and reported for this host [1,22]. The actual value of the transition cross-sections varies between the publications; here we follow the data reported in [1]. Absorption data indicate that blue, InGaN-based, laser diodes can efficiently pump this crystal at about 445 nm, exciting the transition between the ground level 3 H4 and the 3 P2 level. The emission cross-section shows three distinct areas, with two peaks located at about 700 nm, one peak at 720 nm, and three peaks around 730 nm. The
10
20
cm2
430
440
450
460
470
480
490
2.5 2.0 1.5
abs
1.0 0.5 0
10
20
cm2
5.0
em
Trivalent praseodymium (Pr) ions have been studied over the past few years as dopant for visible laser generation. This ion is particularly appealing because of its absorption band in the blue region, which makes it suitable for pumping with blue InGaN laser diodes [1]. Emission in the green, orange, red, deep red, and near-infrared regions have been reported in several materials so far [2,3]. Sources in the deep red region (around 720 nm) have been employed for several studies, from spectroscopy of water vapor [4], to characterization of bismuth-doped materials [5], to entrapment of Bose–Einstein condensates [6], and laser cooling of Fermi gases [7]. These experiments used complex and large systems as pump sources, like dye, gas, or Ti:sapphire lasers, because of the lack of laser diodes at this wavelength, which is barely covered by the high-energy spectrum of AlGaAs-based diodes. Pr-doped materials can provide a solid-state alternative for laser sources at 720 nm, more compact than all systems previously available and with high quality of the output beam. A source with these characteristics could be also employed to generate radiation at 360 nm via second-harmonic generation, providing a laser emission that could be of interest for metrology [8]. Duplications like this have been reported with other Pr-based lasers [9–11]. Among all the available hosts, Pr has shown the best performances in fluoride crystals, because of its low phonon energies that reduce the probability of non-radiative decays [12]. To date, the highest laser efficiencies with this ion have been achieved in LiYF4 [13,14], LiLuF4 [15], and BaY2 F8 [16,17], at several visible and infrared lines. Also lasers in oxide crystals like YAlO3 were successfully operated in the wavelength region around 720 nm [18]. However, these crystals have anisotropic structures. Thus, their absorption and emission properties are polarization dependent. KY3 F10 (KYF) instead has a cubic crystalline lattice [19], therefore is completely isotropic, which means that lasers generated in this material can in principle be unpolarized, and have a high-quality output mode. Continuous-wave laser emission with Pr-doped KYF (Pr:KYF) has been reported in
4.0 3.0 2.0 1.0 0
700
710
720
730
740
Wavelength (nm)
Fig. 1. Absorption and emission cross-sections of Pr:KYF in blue and deep red regions, respectively. © 2015 Optical Society of America
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
cross-section value is different from zero in the left part of the plot because of vibrational side bands at about 670 nm. The line at 720 nm is caused by the decay from the 3 P0 level to the 3 F3 level of the Pr ion. This emission has the highest cross-section in this region, and is the one studied in this work. The setup employed for our laser experiments is described in Fig. 2. The pump source was a multimode blue InGaN laser diode (LD), which was thermally tuned to emit at 446 nm with an output power of 1.6 W. The emission from this diode was linearly polarized. After collimation by a high numerical aperture lens (CL), the pump beam was further conditioned by a pair of cylindrical lenses (CYL) to improve its shape. We employed a combination of a λ∕2-waveplate (HW) and a polarizing beam splitter cube (PBS) to continuously alter the amount of power sent to the cavity. The pump light was finally focused into the crystal using a lens (FL) with a focal length of 40 mm. The maximum power incident to the cavity was 1.3 W. We employed a hemispherical linear resonator, made of two mirrors, one plane (M1), placed right before the crystal (Pr:KYF), and the other plane-concave (M2) with a radius of curvature approximately equal to the length of the cavity. The input mirror was anti-reflective coated between 430 and 550 nm T > 95% and highly reflective between 680 and 790 nm R > 99.9%. We used alternatively three plane-concave mirrors as output couplers, all with a radius of curvature of 50 mm. The output coupling mirrors were reflective from 660 to 750 nm, with the maximum reflectivity at 720 nm. This narrow bandwidth coating was necessary to exclude the other visible emission lines of the Pr ion. The output mirrors had respectively T 0.7%, T 1.4%, and T 2.7% at 720 nm. We used a 5.9-mm-long sample of 1.3% at. doped Pr:KYF, with a cross area of 3 × 4 mm2 . The concentration of the dopant ions in the sample was determined via transmission measurements. Details about the growth procedures for this crystal can be found in [20]. We polished the two smallest facets to make them plane and parallel. No coating was applied on these facets. We determined the fraction of single-pass pump power absorbed by the crystal to be about 85% of incident power. Considering reflection of the transmitted pump beam
1993
from the output mirror M2, this fraction increases to about 87% of incident power. The crystal was placed right behind the M1 mirror, mounted on a copper holder with water cooling at 14°C to keep it refrigerated during the measurements. This holder was fixed on a goniometric head that allowed rotation and translation of the sample along the two horizontal axes. The output mirror M2 was mounted on a translation stage employed to change and optimize the cavity length, which was varied around 50 mm. A 540 nm longpass filter was placed after the mirror M2 to remove the residual pump laser transmitted through the cavity. We used two silicon-based power meters (PM) to determine at the same time the laser output power and the incident pump power. The wavelength of the deep red emission was determined with an Ocean Optics HR2000 portable spectrophotometer, with a resolution of 1 nm. Figure 3 shows the output power as a function of the amount of pump power absorbed by the crystal, for different transmittances of the output mirror, with the relative slope efficiencies (ηabs ). For these calculations, we considered double-pass absorption of the pump beam in the crystal. With the 0.7% mirror, we measured a maximum output power of 174 mW, with a slope efficiency of 19.2% and a threshold power of 57 mW. The 1.4% mirror led to a maximum output power of 187 mW, a slope efficiency of 20.7%, and a threshold power of 75 mW. We obtained the best results with the 2.7% mirror with a maximum output power of 207 mW, a slope efficiency of 24.3%, and a threshold power of 138 mW. With all the mirrors, the laser output had a wavelength of 720 nm and a measurement performed with a Glan– Taylor polarizer revealed that was unpolarized. We calculated the round-trip cavity losses with the Findlay–Clay [23] and Caird [24] methods using the laser thresholds and the slope efficiencies measured with the three output mirrors. With these analyses, we estimated a fraction of losses of 0.6% and 0.3%, respectively. These values are comparable to the losses measured in Pr-doped LiYF4 (0.6%) [25] and LiLuF4 (0.3%) [15] before, and lower than what has been reported for other
Output power (mW)
200
150
T
0.7
— abs
19.2
T
1.4
— abs
20.7
T
2.7
— abs
24.3
100
50
0
Fig. 2. Layout of the experimental setup employed for laser measurements. For description refer to the text.
200
400 600 800 Absorbed power (mW)
1000
Fig. 3. Output power at 720 nm as a function of absorbed power, for three output coupler transmittances (T). Slope efficiencies (ηabs ) are also shown.
1994
OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
Beam diameter ( µ )
500
400
300
200
100 -60
X Y -40 -20 0 20 40 Distance to focal plane (mm)
60
Fig. 4. Evolution of the output beam diameter, measured with a propagation analyzer along two orthogonal directions parallel to the edges of the crystal. Inset, far-field output beam profile acquired with a CCD camera. Both measurements were conducted at maximum output power with T 2.7%.
Pr:KYF crystals so far (0.9% [21], 2% [20], and 5% [22]). Calculations performed for the Findlay–Clay analysis also allow to estimate the optimum output coupling, for which the cavity should deliver the highest output power at the given pump power [26], leading to a value of about 4%, which was not available in our laboratories. Despite that, the Caird method indicates a limiting slope efficiency for our current pumping setup of 26.2%, which is close to what we achieved with the 2.7% output coupler. We also measured the M2 factor of the output laser beam to further check its quality. We employed a Spiricon M2 -200s beam propagation analyzer. We used the 2.7% output coupler and adjusted the cavity to emit at maximum power. The diameter of the beam as a function of the distance to the focal plane is shown in Fig. 4 for two orthogonal directions parallel to the edges of the crystal. The M 2 parameter of the emission was 1.1 along the horizontal axis and 1.2 along the vertical axis. A profile of the output intensity is also shown in the inset of Fig. 4. All these measurements highlight the high quality of the output beam and its symmetric shape, close to a diffraction-limited Gaussian beam. In comparison with other Pr-doped fluoride crystals, the highest slope efficiency of our sample (24.3%) is comparable with what has been reported for diode-pumped Pr3 :LiYF4 (30%) and Pr3 :LiLuF4 (24%) [27] at 720 nm. These results also evidence the high quality of the crystal characterized in this work, and show that this material can compete for the first time with the other, wellestablished fluorides on laser performances, at least in the deep red region. We still expect that the efficiencies of Pr:KYF in this wavelength region could be further improved by increasing the output coupler transmittance, by optimizing the doping concentration and by changing the length of the sample. In summary, we report the first observation of continuous-wave laser emission at 720 nm with Pr3 :KY3 F10 . After growing the crystal, we prepared a sample for laser experiments in the deep red region,
pumping it with a laser diode emitting at 446 nm. We employed a hemispherical resonant cavity with three different mirrors as output couplers and characterized the laser properties for three transmittance values. We obtained a maximum output power of 207 mW with a slope efficiency of 24.3%, which is comparable with those reported for LiYF4 and LiLuF4 for the same transition. Because of the high absorption efficiency of 87%, the optical-to-optical efficiency with respect to the pump power incident to the crystal reaches a value of almost 20%, which is close to the maximum slope efficiency achieved. Moreover, we determined that the total round-trip resonator losses were lower than in previously reported Pr:KYF crystals, with an improvement of at least a factor of two. Those losses are also comparable to the values reported for other Pr-doped fluorides, showing the high optical quality of this sample. The maximum slope efficiency and the round-trip losses we determined display that Pr:KYF can have remarkable optical characteristics and laser properties, as good as LiYF4 and LiLuF4 crystals. The output intensity profile and the beam propagation were also measured, resulting in a quasidiffraction-limited output beam with an average M2 value lower than 1.2. Because the Pr ion is prone to non-radiative lifetime quenching effects by cross-relaxation [1,28], detailed investigation on energy transfer mechanisms with respect to the Pr concentration might allow to find an optimum doping concentration that keeps a reasonably short absorption length, conserves a long lifetime, and enhances the laser performances. In addition to that, secondharmonic generation from this transition could be investigated to achieve laser operation at 360 nm. Finally, Pr:KYF could also be used to explore potential laser emissions at the other wavelengths available in the deep red region, around 700 and 740 nm. We would like to thank the AIT-DAAD Vigoni program “Solid state laser in the visible spectral range” for financial support. We would also like to express our gratitude to Prof. Günter Huber for supporting us with the facilities in his laboratories at the University of Hamburg and to Prof. Mauro Tonelli for his assistance and help during the preparation of this work. References 1. A. Richter, Laser Parameters and Performance of Pr3+Doped Fluorides Operating in the Visible Spectral Region (Cuvillier Verlag, 2008). 2. J. Y. Allain, M. Monerie, and H. Poignant, Electron. Lett. 27, 189 (1991). 3. M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, Appl. Phys. B 97, 363 (2009). 4. B. E. Grossmann and E. V. Browell, J. Mol. Spectrosc. 136, 264 (1989). 5. Y. Arai, T. Suzuki, Y. Ohishi, S. Morimoto, and S. Khonthon, Appl. Phys. Lett. 90, 261110 (2007). 6. J. Dziarmaga and M. Lewenstein, Phys. Rev. A 72, 033606 (2005). 7. Z. Idziaszek, L. Santos, M. Baranov, and M. Lewenstein, J. Opt. B 5, S190 (2003). 8. H. Hachisu, K. Miyagishi, S. G. Porsev, A. Derevianko, V. D. Ovsiannikov, V. G. Pal’chikov, M. Takamoto, and H. Katori, Phys. Rev. Lett. 100, 053001 (2008).
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS 9. V. Ostroumov, W. Seelert, L. Hunziker, C. Ihli, A. Richter, E. Heumann, and G. Huber, Proc. SPIE 6451, 645103 (2007). 10. A. Richter, N. Pavel, E. Heumann, G. Huber, D. Parisi, A. Toncelli, M. Tonelli, A. Diening, and W. Seelert, Opt. Express 14, 3282 (2006). 11. Y. Dong, S. T. Li, and X. H. Zhang, Laser Phys. Lett. 9, 116 (2012). 12. L. Esterowitz, R. Allen, M. Kruer, F. Bartoli, L. S. Goldberg, H. P. Jenssen, A. Linz, and V. O. Nicolai, J. Appl. Phys. 48, 650 (1977). 13. T. Gün, P. Metz, and G. Huber, Opt. Lett. 36, 1002 (2011). 14. P. W. Metz, F. Reichert, F. Moglia, S. Müller, D.-T. Marzahl, C. Kränkel, and G. Huber, Opt. Lett. 39, 3193 (2014). 15. F. Cornacchia, A. Richter, E. Heumann, G. Huber, D. Parisi, and M. Tonelli, Opt. Express 15, 992 (2007). 16. D. Pabœuf, O. Mhibik, F. Bretenaker, P. Goldner, D. Parisi, and M. Tonelli, Opt. Lett. 36, 280 (2011). 17. A. Sottile, D. Parisi, and M. Tonelli, Opt. Express 22, 13784 (2014). 18. M. Fibrich and H. Jelínková, Laser Phys. Lett. 10, 035801 (2013).
1995
19. K. Friese, H. Krüger, V. Kahlenberg, T. Balić-Žunic, H. Emerich, J.-Y. Gesland, and A. Grzechnik, J. Phys. Condens. Matter 18, 2677 (2006). 20. P. W. Metz, S. Müller, F. Reichert, D.-T. Marzahl, F. Moglia, C. Kränkel, and G. Huber, Opt. Express 21, 31274 (2013). 21. B. Xu, P. Camy, J.-L. Doualan, Z. Cai, and R. Moncorgé, Opt. Express 19, 1191 (2011). 22. P. Camy, J. L. Doualan, R. Moncorgé, J. Bengoechea, and U. Weichmann, Opt. Lett. 32, 1462 (2007). 23. D. Findlay and R. A. Clay, Phys. Lett. 20, 277 (1966). 24. J. A. Caird, S. A. Payne, P. R. Staver, A. J. Ramponi, L. L. Chase, and W. F. Krupke, IEEE J. Quantum Electron. 24, 1077 (1988). 25. A. Richter, E. Heumann, G. Huber, V. Ostroumov, and W. Seelert, Opt. Express 15, 5172 (2007). 26. O. Svelto, Principles of Lasers (Springer, 2010). 27. F. Cornacchia, A. Di Lieto, M. Tonelli, A. Richter, E. Heumann, and G. Huber, Opt. Express 16, 15932 (2008). 28. J. Hegarty, D. L. Huber, and W. M. Yen, Phys. Rev. B 25, 5638 (1982).
