Wide wavelength tunability and green laser operation of diode-pumped Pr3+:KY3F10 Philip W. Metz,1,* Sebastian Müller,1 Fabian Reichert,1 Daniel-Timo Marzahl1, Francesca Moglia,1 Christian Kränkel,1,2 and Günter Huber1,2 1
Institut für Laser-Physik, Universität Hamburg,Luruper Chaussee 149, 22761 Hamburg, Germany 2 The Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany * [email protected]
Abstract: We report on wide spectral tunability of a quasi-continuous wave Pr3+:KY3F10 laser under InGaN laser diode excitation at 445 nm. The total tuning range exceeded 100 nm in the visible spectral range on several intervals between 521 nm and 737 nm. The broadest continuously tunable region of almost 50 nm extended from 688 nm to 737 nm. Furthermore we present what is to the best of our knowledge the first demonstration of continuous wave laser operation in Pr3+:KY3F10 on three transitions in the green spectral region. The highest output power of 121 mW was achieved at an emission wavelength of 554 nm with a slope efficiency of 27%. ©2013 Optical Society of America OCIS codes: (140.3480) Lasers, diode-pumped; (140.3600) Lasers, tunable; (140.7300) Visible lasers.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
A. Richter, E. Heumann, E. Osiac, G. Huber, W. Seelert, and A. Diening, “Diode pumping of a continuous-wave Pr3+-doped LiYF4 laser,” Opt. Lett. 29(22), 2638–2640 (2004). Y. Fujitomo, M. Murakami, J. Nakanishi, T. Yamada, O. Ishii, and M. Yamazaki, “Visible lasers in waterproof fluoro-aluminate glass fibers excited by GaN laser diodes,” Advanced Solid State Lasers Conference, AM2A.2, Paris, (2013). M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009). F. Reichert, D.-T. Marzahl, P. Metz, M. Fechner, N.-O. Hansen, and G. Huber, “Efficient laser operation of Pr3+, Mg2+:SrAl12O19.,” Opt. Lett. 37(23), 4889–4891 (2012). K. Hashimoto and F. Kannari, “High-power GaN diode-pumped continuous wave Pr3+-doped LiYF4 laser,” Opt. Lett. 32(17), 2493–2495 (2007). B. Xu, P. Camy, J. L. Doualan, Z. Cai, and R. Moncorgé, “Visible laser operation of Pr3+-doped fluoride crystals pumped by a 469 nm blue laser,” Opt. Express 19(2), 1191–1197 (2011). T. Gün, P. Metz, and G. Huber, “Power scaling of laser diode pumped Pr3+:LiYF4 cw lasers: efficient laser operation at 522.6 nm, 545.9 nm, 607.2 nm, and 639.5 nm,” Opt. Lett. 36(6), 1002–1004 (2011). Z. Liu, Z. Cai, S. Huang, C. Zeng, Z. Meng, Y. Bu, Z. Luo, B. Xu, H. Xu, C. Ye, F. Stareki, P. Camy, and R. Moncorgé, “Diode-pumped Pr3+:LiYF4 continuous-wave deep red laser at 698 nm,” J. Opt. Soc. Am. B 30(2), 302–305 (2013). V. Ostroumov and W. Seelert, “1 W of 261 nm generation in Pr:LiYF4 laser pumped by an optically pumped semiconductor at 479 nm,” Proc. SPIE 6871, 68711K (2008). P. W. Metz, F. Moglia, F. Reichert, S. Müller, D.-T. Marzahl, N.-O. Hansen, C. Kränkel, and G. Huber, “Novel Rare Earth Solid State Lasers with Emission Wavelengths in the Visible Spectral Range,” CLEO/Europe 2013, CA-2.5 SUN, (2013). V. Lupei, N. Pavel, and T. Taira, “Basic enhancement of the overall efficiency of intracavity frequency-doubling devices for 1 µm continuous-wave Nd:Y3Al5O12 laser emission,” Appl. Phys. Lett. 83(18), 3653 (2003). H. Okamoto, K. Kasuga, I. Hara, and Y. Kubota, “Visible-NIR tunable Pr3+-doped fiber laser pumped by a GaN laser diode,” Opt. Express 17(22), 20227–20232 (2009). F. Reichert, F. Moglia, D.-T. Marzahl, P. Metz, M. Fechner, N.-O. Hansen, and G. Huber, “Diode pumped laser operation and spectroscopy of Pr3+:LaF3.,” Opt. Express 20(18), 20387–20395 (2012). J. M. Sutherland, P. M. W. French, J. R. Taylor, and B. H. T. Chai, “Visible continuous-wave laser transitions in Pr3+:YLF and femtosecond pulse generation,” Opt. Lett. 21(11), 797–799 (1996). S. Khiari, M. Velazquez, R. Moncorgé, J. L. Doualan, P. Camy, A. Ferrier, and M. Diaf, “Red-luminescence analysis of Pr3+ doped fluoride crystals,” J. Alloy. Comp. 451(1-2), 128–131 (2008). P. Camy, J. L. Doualan, R. Moncorgé, J. Bengoechea, and U. Weichmann, “Diode-pumped Pr3+:KY3F10 red laser,” Opt. Lett. 32(11), 1462–1464 (2007).
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31274
17. Y. Dong, S. T. Li, and X. H. Zhang, “All-solid-state blue laser pumped Pr:KY3F10-BBO ultraviolet laser at 305 nm,” Laser Phys. Lett. 9(2), 116–119 (2012). 18. R. Nacken, “Über das Wachsen von Kristallpolyedern in ihrem Schmelzfluß,” Neues Jahrb. Geol. Palaeontol. 2, 133 (1915). 19. A. Richter, “Laser parameters and performance of Pr3+-doped fluorides operating in the visible spectral region,” PhD-thesis, University of Hamburg, (Cuvillier, Hamburg 2008). 20. A. Braud, P. Y. Tigreat, J. L. Doualan, and R. Morcorgé, “Spectroscopy and cw operation of a 1.85 μm Tm:KY3F10 laser,” Appl. Phys. B 72(8), 909–912 (2001). 21. P. W. Metz, D. Parisi, K. Hasse, N.-O. Hansen, C. Kränkel, M. Tonelli, and G. Huber, “Room Temperature Cyan Pr3+:BaY2F8 Laser at 495 nm,” Advanced Solid State Lasers Conference, AF2A.7, Paris, 2013.
1. Introduction Applications of lasers in the visible spectral region exist in many fields such as display technology and fluorescence microscopy. In particular devices for fluorescence microscopy could dramatically benefit from compact widely tunable laser systems which allow addressing a variety of different transitions in various specimens. Nowadays laser systems which utilize nonlinear conversion processes can generate coherent radiation covering the whole visible spectral range. However, most of these systems are either complex, expensive, not very efficient, and/or in many cases sensitive with respect to alignment issues. In this context the trivalent praseodymium ion is an extremely interesting active ion. When excited by cheap and compact InGaN laser diodes it enables the direct generation of laser oscillation on a variety of transitions covering large parts of the visible spectral range [1]. High power laser operation has already been demonstrated in various praseodymium doped materials, among them fibers [2] as well as oxide [3,4] and fluoride crystals [5–8]. The highest efficiencies so far were obtained using fluoride crystalline materials as the host. With praseodymium doped LiYF4 almost 3 W of output power at 523 nm and 640 nm in the green and red spectral range, respectively, could be obtained at optical-to-optical efficiencies of 70% [9,10], comparable to those of infrared emitting neodymium lasers [11]. Very broad tuning of praseodymium lasers is so far only realized using glass fibers as the host material [12]. However, also the emission spectra of praseodymium doped single crystals reveal broad phonon assisted emission bands which should enable broad wavelength tuning as well. Experiments using Pr3+:LaF3 in this respect were recently published [13]. Although moderate wavelength tuning around many transitions between the orange and the deep red spectral region could be also demonstrated in the well investigated Pr3+:LiYF4 [14], this material has a drawback in this respect. The emission cross sections of the sharp zero-phonon lines provide too much gain compared to the lower cross sections of the phonon sidebands. Therefore the suppression of the high-gain transitions during tuning experiments becomes very challenging. In contrast, the difference between the highest emission cross sections and the phonon sidebands is lower in Pr3+:KY3F10 (Pr:KYF) which makes this system more promising for broadly tunable lasers [15]. The first Pr:KYF laser was operated at an emission wavelength of 645 nm [16] and lasing at 610 nm was also subject of investigations in the past years [6,17]. Though the efficiencies of these lasers were lower compared to those obtained with Pr3+:LiYF4, the spectroscopic features are outstandingly suitable for broad wavelength tuning applications. In this work we present wavelength tuning of an InGaN laser diode pumped Pr:KYF laser covering different intervals over a total of 100 nm between 520 nm and 740 nm in quasicontinuous wave (q-cw) mode. The widest continuous tuning range of almost 50 nm was obtained between 688 nm and 737 nm. The experiments include the first demonstration of cw laser operation in Pr:KYF on three different transitions with emission wavelengths in the green spectral region. The strongest of these transitions at a wavelength of 554 nm was also characterized regarding its laser parameters. A maximum output power of 121 mW could be obtained with a slope efficiency of 27%. 2. Crystal growth and spectroscopic properties KYF crystallizes at 1030 °C in the cubic Fm-3m structure, which remains stable in the whole temperature range down to ambient temperature. We fabricated the crystal used in our #198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31275
experiments by the Nacken-Kyropoulos-method [18]. In this type of growth technique a seed crystal is put in contact to the surface of the molten raw material. While slowly cooling down the setup a single crystal grows from the seed into the melt. The raw materials were 5N grade KY3F10 and PrF3, and 4N grade KF. The components were mixed according to a stoichiometric composition including 1.4 mol.% of PrF3 and filled into a 38 mm high conical vitreous carbon crucible (HTW Germany, GAK2) with an upper diameter of 35 mm. Before starting the growth, the furnace was evacuated to a pressure of 5 × 10−5 mbar. Afterwards, the chamber was charged with a mixture of 60% Ar and 40% CF4 gas with a pressure of 1.6 bar. As no seed crystal was available, an iridium wire was initially dipped into the molten material and pulled up with a velocity of 0.7 mm/h and a rotation speed of 5 rpm. That way, a crystal with a diameter of 4 mm and a length of 15 mm was grown, which served as a seed crystal. Subsequently, the pulling and the rotation were stopped and the Nacken-Kyropoulos-growth was initialized by cooling down the melt at a rate of approximately 0.7 K/h with the seed crystal still dipped into the melt. The cooling rate was kept constant until the complete melt was crystallized. Since the crystal was fabricated by slowly cooling down the melt, the shape of the resulting boule equals the inner walls of the crucible. The optical quality was inhomogeneous with the majority of the clear material located in the lower half of the crystal. Due to the cubic crystal matrix no effort was done to orientate the crystal with respect to the crystallographic axes. A 3 × 3 × 3 mm3 piece was cut out of the resulting boule and polished. By transmission measurements the dopant concentration of this sample was determined to be 0.9 at.%. The deviation from the composition of the melt is a result of the segregation coefficient of 0.5 for praseodymium in KY3F10 [19]. Consequently different parts of the boule exhibited different dopant concentrations.
Fig. 1. Ground state absorption and emission cross sections for transitions originating from the 3 PJ manifold of Pr:KYF.
The spectroscopic properties of Pr:KYF single crystals have been studied previously [16,19]. Figure 1 shows the absorption and emission characteristics of this material in the spectral range relevant for this work. It can be seen, that the highest absorption cross sections are in the spectral range around 445 nm where blue emitting InGaN laser diodes are commercially available. The excitation into the 3PJ manifold is followed by an emission on several narrow lines, but also on some broad phonon assisted sidebands. The effective lifetime of the upper laser level of the above mentioned sample was about 40 µs. 3. Wavelength tunability of the Pr3+:KY3F10 laser 3.1 Experimental setup The laser setup for the wavelength tuning experiments is depicted in Fig. 2. It had to fulfill two main requirements. First, it should provide a collimated arm to introduce a birefringent
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31276
quartz crystal under Brewster’s angle. Secondly, the laser mode in the active material should match the pump channel with a diameter of 90 µm, which in prior experiments proved to be a good choice. To match these conditions, a v-type cavity consisting of three mirrors M1, M2, and M3 was set up. The input coupling mirror M1 was plane and coated highly reflective for the respective laser wavelengths while it was highly transmittive for the pump light. The 0.9 at.% doped 3 mm long Pr:KYF crystal was placed in a water cooled copper mount (T = 10 °C) and placed directly behind M1. A highly reflective mirror M2 with a radius of curvature of 100 mm was placed about 50 mm behind M1. The inclusion of the plane end mirror M3 resulted in an almost collimated resonator mode between M2 and M3. Hence, the resonator was not very sensitive with respect to the length of this resonator arm which was about 220 mm. The full opening angle of the resonator mode was about 15-20°. A 2 mm thick quartz crystal in a rotation mount was placed into this resonator arm under Brewster’s angle to serve as a birefringent wavelength filter. A total of 6 sets of mirrors M1, M2, and M3 (A-F in Fig. 3) was used to keep the cavity highly reflective at the respective laser wavelength and to suppress laser operation on a neighboring transmission maximum of the quartz plate and/or one of the pronounced emission maxima of Pr:KYF. The output coupling was provided by residual reflections at the wavelength filter. The excitation source was an InGaN laser diode with an emission wavelength of 445 nm, a maximum output power of 1.2 W, and an M2 of approximately 5. Its output beam was collimated and beam shaped by a combination of an aspheric lens and a pair of cylindrical lenses. Finally the excitation beam was focused into the laser crystal by a lens with a focal length of 40 mm. The small signal absorption in this configuration was about 65%.
Fig. 2. Schematic setup for the wavelength tuning of the q-cw Pr:KYF laser.
Initially, the experiments were performed under cw excitation. However, the spectral tuning range under these conditions was limited by strong thermal depolarization effects which increased the output coupling losses at the birefringent filter. Once the laser threshold is reached, the stimulated emission leads to an efficient removal of the excitation energy from the crystal. At wavelengths with low emission cross sections, the laser threshold becomes high and thus the crystal heats up more, further increasing the laser threshold. As a consequence those wavelengths that already exhibit a high threshold were affected by higher thermal depolarization losses, impeding laser operation. To reduce the thermal load, we positioned a mechanical chopper into the excitation beam, which strongly increased the tuning range of the system. In all experiments presented in this section the pump beam was modulated at a frequency of 500 Hz with a duty cycle of 0.5. Thus the pump period was 1 ms, which exceeds the effective lifetime of the upper laser level by a factor of about 25, ensuring true q-cw operation. All output powers that will be given in this section represent the output to both sides of the birefringent filter and denote the q-cw power. 3.2 Results of the wavelength tuning experiments The results of the wavelength tuning experiments are depicted in Fig. 3 together with a gain spectrum of Pr:KYF. The widest continuous tuning range was achieved in the deep red spectral region around 715 nm with a full width of almost 50 nm. Another broad tuning band
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31277
is located around 620 nm with a full width of almost 23 nm. For comparison, in true cw operation this tuning range was narrowed to about 17 nm. The widest tuning range in the green spectral region is located around 523 nm with a full width of 5 nm. We believe that these results represent the widest tuning of any praseodymium doped crystalline material so far. The exact on- and offset wavelengths λmin and λmax as well as the full tuning widths Δλ are listed in Table 1 together with the highest output power Pmax that was observed within the respective tuning range.
Fig. 3. Output power versus wavelength of the q-cw Pr:KYF laser. The grey line represents a gain spectrum for the maximum expected inversion level in our setup (β = 0.02). The capital characters A-F denote the different sets of mirrors. The colored bars in the lower part visualize the areas where continuous tuning was achieved.
3.3 Summary and discussion of the wavelength tuning experiments In the wavelength tuning experiments we were able to address almost every transition in four level schemes with emission wavelengths in the visible spectral region originating from the 3 PJ multiplet. We showed tuning ranges > 10 nm around three laser transitions as well as the first demonstration of laser operation on eight transitions in the green and the deep red spectral range in Pr:KYF to the best of our knowledge. A total wavelength tuning range exceeding 100 nm was realized between 520 nm and 737 nm. In wavenumbers this corresponds to 42% of the total range of 5640 cm−1. This is the broadest tuning range that has ever been obtained with a crystalline praseodymium doped gain material. It is noteworthy that while the tuning curve depicted in Fig. 3 follows the gain spectrum (gray curve) in almost all spectral ranges, as expected for a four level laser, it does not in the region around 620 nm. Here, a ground state absorption transition into the 1D2 level does not allow for a description of this laser by the conventional four level theory. The result is a very smooth tuning curve (ΔνFWHM ≈ 420 cm−1). This makes the Pr:KYF laser at 620 nm very promising for ultra-short pulse generation. At emission wavelengths far off the emission maxima, the laser suffered from thermal depolarization which could so far only be overcome by operating the laser in q-cw mode. In order to realize the tuning ranges demonstrated in this work in true cw mode, the growth of crystals with higher optical quality, which could reduce the laser thresholds and thus the thermal load, should be investigated. Additionally, the application of longer crystals with a lower dopant concentration would reduce the heat injection per volume. Though this approach is limited by the beam quality of the pump source, a twice as long crystal with half the doping concentration should still allow for an adequate overlap between pump and laser mode at the same absorption efficiency. The lower doping concentration would furthermore lead to an increased effective lifetime of the upper laser level [19] which decreases the laser threshold. Moreover, applying tuning mechanisms that are less sensitive to the polarization,
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31278
e.g. prisms or gratings, can be expected to be more suitable for the Pr:KYF system if the polarization of the output beam is not important for the desired application. Table 1. List of realized tuning ranges. λmin and λmax denote on- and offset wavelengths, respectively. Δλ is the full width of the tuning range. Pmax represents the highest output power achieved within the respective interval. λmin (nm)
λmax (nm)
Δλ (nm)
Pmax (mW)
520.8
525.9
5.1
45.2
538.0
539.9
1.9
38.4
551.4
555.9
4.5
72.8
606.7
629.5
22.8
41.2
638.4
638.8
0.4
2.8
641.3
656.5
15.2
133.2
670.7
670.9
0.2
0.5
672.4
673.1
0.7
1.2
676.5
677.1
0.6
3.2
687.6
737.3
49.7
111.0
4. Green laser operation of Pr3+:KY3F10 The tuning experiments revealed that laser operation in Pr:KYF is possible on several wavelengths that have not been investigated so far. Especially the transitions in the green spectral region are interesting because they offer low Stokes losses which should allow for efficient laser operation. Therefore we investigated these transitions further in true cw mode without any tuning element inside the resonator. 4.1 Experimental setup A schematic of the setup used for the laser experiments described in this section is depicted in Fig. 4. The hemispheric linear resonator consisted of the 3 mm long 0.9 at.% doped Pr:KYF crystal placed in a water cooled copper mount (T = 10 °C), and two mirrors M1 and M2. M1 was plane, highly transmittive coated at the excitation wavelength, and highly reflective at the respective laser wavelengths. The mirror M2 had a radius of curvature of 50 mm and was placed about 48 mm behind M1. For wavelength selection different mirrors with appropriate transmission characteristics for each desired laser wavelength were chosen for M2.
Fig. 4. Schematic setup for the cw laser experiments in a free running mode.
The radiation of the laser diode was focused into the laser crystal by a lens with a focal length of 40 mm. The laser diode was operated at maximum output power and attenuated by a combination of a λ/2-waveplate and a polarizing beam splitter cube. The absorbed pump power was calculated from the measured fraction of pump power absorbed at the laser threshold and accounts also for the reflectivity of M2 for the pump wavelength.
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31279
4.2 Results of the cw laser experiments In the cw setup we were able to demonstrate several tens of mW of output power at 523 nm, 539 nm, and 554 nm for the first time with Pr:KYF. The output spectra are depicted on the left hand side of Fig. 5. Due to a lack of suitable output coupling mirrors, a thorough characterization was only performed for the laser at 554 nm. The right hand side of Fig. 5 shows the laser characteristics with respect to the absorbed pump power for different output coupler transmissions at 554 nm.
Fig. 5. Output spectra of the realized green emitting cw Pr:KYF lasers (left) and output characteristics of the laser at 554 nm for different output coupler transmissions TOC (right).
The highest slope efficiency of ηsl = 27% was obtained at an output coupler transmission of TOC = 0.8% yielding a maximum output power of Pmax = 121 mW at an absorbed pump power of 686 mW. For comparison we performed further cw laser experiments at λlas = 610 nm and 645 nm.
Fig. 6. Comparison between the output characteristics of cw Pr:KYF lasers operated at the wavelengths 554 nm, 610 nm, and 645 nm.
The characteristics of the red and orange lasers realized at optimized output coupler transmissions are shown in Fig. 6 together with the input-output curve at 554 nm which delivered the highest output power. Both, the efficiency of the red laser at 645 nm and that of the orange laser at 610 nm are comparable to previously published results [6]. All cw laser results are summarized in Table 2. 4.3 Summary and discussion of the cw experiments The continuous wave experiments include the first characterization of a green emitting Pr:KYF laser to the best of our knowledge as well as a comparative characterization of the
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31280
lasers at 610 nm and 645 nm. From all these lasers output powers in the 100 mW regime could be extracted. From the difference between the obtained slope efficiencies and the Stokes-limit the maximum resonator roundtrip losses were estimated to be in the order of 2%. Comparing these losses to the 0.5% which were estimated in [7] for Pr3+:LiYF4 leads to the conclusion that, as long as no fundamental problems such as excited state absorption are present in Pr:KYF, there is still room for improvement of the crystal quality. In principle KYF is suitable for the Czochralski-growth technique [20] which is the standard growth technique for high quality LiYF4 crystals. Thus, the fabrication of crystals with significantly improved optical quality seems possible. Further investigations should be carried out regarding the effects of varying dopant concentrations. Table 2. Summary of the cw laser parameters. λlas (nm)
TOC (nm)
ηsl (nm)
Pmax (mW)
Pthr (mW)
554
0.8
27
121
166
610
6.4
18
97
162
645
2.3
38
268
30
5. Summary and outlook We reported on laser diode pumped wavelength tuning experiments with Pr:KYF. In the deep red spectral range around 715 nm a full width of almost 50 nm could be continuously addressed. To the best of our knowledge, this represents the widest tuning range for any praseodymium doped crystalline material. Wide tuning was also possible around 5 other wavelengths in the green, orange, and red spectral range. Due to thermal depolarization the full tuning range could only be achieved in q-cw operation. True cw operation throughout the whole range shown in these experiments seems feasible when applying tuning elements which are not sensitive to the polarization. Furthermore we have demonstrated what are to the best of our knowledge the first green emitting continuous wave Pr:KYF lasers at three different transitions. At 554 nm a maximum output power of 121 mW could be obtained with a slope efficiency of 27%. However, the efficiencies of all lasers are still almost a factor of two lower than in the standard material Pr3+:LiYF4 [7]. A significant improvement of the performance can be expected by the use of crystals with higher optical quality and an optimization of the dopant concentration. With these improvements we are confident, that Pr:KYF has the potential for reasonably efficient and competitive lasers with a broad tuning range in the visible. Recent results obtained with Pr3+:BaY2F8 reveal that praseodymium doped crystals can be suitable for cw laser operation in the cyan-blue spectral range below 500 nm [21]. Also the spectroscopic features of Pr:KYF seem suitable for an extension of the tuning range into the cyan-blue if the crystal quality can be significantly improved. Acknowledgments We are very grateful for the financial support by the Deutsche Forschungsgemeinschaft within the graduate school 1355 “Physics with new advanced coherent light sources” and the Joachim Herz Stiftung.
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31281
Institut für Laser-Physik, Universität Hamburg,Luruper Chaussee 149, 22761 Hamburg, Germany 2 The Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany * [email protected]
Abstract: We report on wide spectral tunability of a quasi-continuous wave Pr3+:KY3F10 laser under InGaN laser diode excitation at 445 nm. The total tuning range exceeded 100 nm in the visible spectral range on several intervals between 521 nm and 737 nm. The broadest continuously tunable region of almost 50 nm extended from 688 nm to 737 nm. Furthermore we present what is to the best of our knowledge the first demonstration of continuous wave laser operation in Pr3+:KY3F10 on three transitions in the green spectral region. The highest output power of 121 mW was achieved at an emission wavelength of 554 nm with a slope efficiency of 27%. ©2013 Optical Society of America OCIS codes: (140.3480) Lasers, diode-pumped; (140.3600) Lasers, tunable; (140.7300) Visible lasers.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
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#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31274
17. Y. Dong, S. T. Li, and X. H. Zhang, “All-solid-state blue laser pumped Pr:KY3F10-BBO ultraviolet laser at 305 nm,” Laser Phys. Lett. 9(2), 116–119 (2012). 18. R. Nacken, “Über das Wachsen von Kristallpolyedern in ihrem Schmelzfluß,” Neues Jahrb. Geol. Palaeontol. 2, 133 (1915). 19. A. Richter, “Laser parameters and performance of Pr3+-doped fluorides operating in the visible spectral region,” PhD-thesis, University of Hamburg, (Cuvillier, Hamburg 2008). 20. A. Braud, P. Y. Tigreat, J. L. Doualan, and R. Morcorgé, “Spectroscopy and cw operation of a 1.85 μm Tm:KY3F10 laser,” Appl. Phys. B 72(8), 909–912 (2001). 21. P. W. Metz, D. Parisi, K. Hasse, N.-O. Hansen, C. Kränkel, M. Tonelli, and G. Huber, “Room Temperature Cyan Pr3+:BaY2F8 Laser at 495 nm,” Advanced Solid State Lasers Conference, AF2A.7, Paris, 2013.
1. Introduction Applications of lasers in the visible spectral region exist in many fields such as display technology and fluorescence microscopy. In particular devices for fluorescence microscopy could dramatically benefit from compact widely tunable laser systems which allow addressing a variety of different transitions in various specimens. Nowadays laser systems which utilize nonlinear conversion processes can generate coherent radiation covering the whole visible spectral range. However, most of these systems are either complex, expensive, not very efficient, and/or in many cases sensitive with respect to alignment issues. In this context the trivalent praseodymium ion is an extremely interesting active ion. When excited by cheap and compact InGaN laser diodes it enables the direct generation of laser oscillation on a variety of transitions covering large parts of the visible spectral range [1]. High power laser operation has already been demonstrated in various praseodymium doped materials, among them fibers [2] as well as oxide [3,4] and fluoride crystals [5–8]. The highest efficiencies so far were obtained using fluoride crystalline materials as the host. With praseodymium doped LiYF4 almost 3 W of output power at 523 nm and 640 nm in the green and red spectral range, respectively, could be obtained at optical-to-optical efficiencies of 70% [9,10], comparable to those of infrared emitting neodymium lasers [11]. Very broad tuning of praseodymium lasers is so far only realized using glass fibers as the host material [12]. However, also the emission spectra of praseodymium doped single crystals reveal broad phonon assisted emission bands which should enable broad wavelength tuning as well. Experiments using Pr3+:LaF3 in this respect were recently published [13]. Although moderate wavelength tuning around many transitions between the orange and the deep red spectral region could be also demonstrated in the well investigated Pr3+:LiYF4 [14], this material has a drawback in this respect. The emission cross sections of the sharp zero-phonon lines provide too much gain compared to the lower cross sections of the phonon sidebands. Therefore the suppression of the high-gain transitions during tuning experiments becomes very challenging. In contrast, the difference between the highest emission cross sections and the phonon sidebands is lower in Pr3+:KY3F10 (Pr:KYF) which makes this system more promising for broadly tunable lasers [15]. The first Pr:KYF laser was operated at an emission wavelength of 645 nm [16] and lasing at 610 nm was also subject of investigations in the past years [6,17]. Though the efficiencies of these lasers were lower compared to those obtained with Pr3+:LiYF4, the spectroscopic features are outstandingly suitable for broad wavelength tuning applications. In this work we present wavelength tuning of an InGaN laser diode pumped Pr:KYF laser covering different intervals over a total of 100 nm between 520 nm and 740 nm in quasicontinuous wave (q-cw) mode. The widest continuous tuning range of almost 50 nm was obtained between 688 nm and 737 nm. The experiments include the first demonstration of cw laser operation in Pr:KYF on three different transitions with emission wavelengths in the green spectral region. The strongest of these transitions at a wavelength of 554 nm was also characterized regarding its laser parameters. A maximum output power of 121 mW could be obtained with a slope efficiency of 27%. 2. Crystal growth and spectroscopic properties KYF crystallizes at 1030 °C in the cubic Fm-3m structure, which remains stable in the whole temperature range down to ambient temperature. We fabricated the crystal used in our #198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31275
experiments by the Nacken-Kyropoulos-method [18]. In this type of growth technique a seed crystal is put in contact to the surface of the molten raw material. While slowly cooling down the setup a single crystal grows from the seed into the melt. The raw materials were 5N grade KY3F10 and PrF3, and 4N grade KF. The components were mixed according to a stoichiometric composition including 1.4 mol.% of PrF3 and filled into a 38 mm high conical vitreous carbon crucible (HTW Germany, GAK2) with an upper diameter of 35 mm. Before starting the growth, the furnace was evacuated to a pressure of 5 × 10−5 mbar. Afterwards, the chamber was charged with a mixture of 60% Ar and 40% CF4 gas with a pressure of 1.6 bar. As no seed crystal was available, an iridium wire was initially dipped into the molten material and pulled up with a velocity of 0.7 mm/h and a rotation speed of 5 rpm. That way, a crystal with a diameter of 4 mm and a length of 15 mm was grown, which served as a seed crystal. Subsequently, the pulling and the rotation were stopped and the Nacken-Kyropoulos-growth was initialized by cooling down the melt at a rate of approximately 0.7 K/h with the seed crystal still dipped into the melt. The cooling rate was kept constant until the complete melt was crystallized. Since the crystal was fabricated by slowly cooling down the melt, the shape of the resulting boule equals the inner walls of the crucible. The optical quality was inhomogeneous with the majority of the clear material located in the lower half of the crystal. Due to the cubic crystal matrix no effort was done to orientate the crystal with respect to the crystallographic axes. A 3 × 3 × 3 mm3 piece was cut out of the resulting boule and polished. By transmission measurements the dopant concentration of this sample was determined to be 0.9 at.%. The deviation from the composition of the melt is a result of the segregation coefficient of 0.5 for praseodymium in KY3F10 [19]. Consequently different parts of the boule exhibited different dopant concentrations.
Fig. 1. Ground state absorption and emission cross sections for transitions originating from the 3 PJ manifold of Pr:KYF.
The spectroscopic properties of Pr:KYF single crystals have been studied previously [16,19]. Figure 1 shows the absorption and emission characteristics of this material in the spectral range relevant for this work. It can be seen, that the highest absorption cross sections are in the spectral range around 445 nm where blue emitting InGaN laser diodes are commercially available. The excitation into the 3PJ manifold is followed by an emission on several narrow lines, but also on some broad phonon assisted sidebands. The effective lifetime of the upper laser level of the above mentioned sample was about 40 µs. 3. Wavelength tunability of the Pr3+:KY3F10 laser 3.1 Experimental setup The laser setup for the wavelength tuning experiments is depicted in Fig. 2. It had to fulfill two main requirements. First, it should provide a collimated arm to introduce a birefringent
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31276
quartz crystal under Brewster’s angle. Secondly, the laser mode in the active material should match the pump channel with a diameter of 90 µm, which in prior experiments proved to be a good choice. To match these conditions, a v-type cavity consisting of three mirrors M1, M2, and M3 was set up. The input coupling mirror M1 was plane and coated highly reflective for the respective laser wavelengths while it was highly transmittive for the pump light. The 0.9 at.% doped 3 mm long Pr:KYF crystal was placed in a water cooled copper mount (T = 10 °C) and placed directly behind M1. A highly reflective mirror M2 with a radius of curvature of 100 mm was placed about 50 mm behind M1. The inclusion of the plane end mirror M3 resulted in an almost collimated resonator mode between M2 and M3. Hence, the resonator was not very sensitive with respect to the length of this resonator arm which was about 220 mm. The full opening angle of the resonator mode was about 15-20°. A 2 mm thick quartz crystal in a rotation mount was placed into this resonator arm under Brewster’s angle to serve as a birefringent wavelength filter. A total of 6 sets of mirrors M1, M2, and M3 (A-F in Fig. 3) was used to keep the cavity highly reflective at the respective laser wavelength and to suppress laser operation on a neighboring transmission maximum of the quartz plate and/or one of the pronounced emission maxima of Pr:KYF. The output coupling was provided by residual reflections at the wavelength filter. The excitation source was an InGaN laser diode with an emission wavelength of 445 nm, a maximum output power of 1.2 W, and an M2 of approximately 5. Its output beam was collimated and beam shaped by a combination of an aspheric lens and a pair of cylindrical lenses. Finally the excitation beam was focused into the laser crystal by a lens with a focal length of 40 mm. The small signal absorption in this configuration was about 65%.
Fig. 2. Schematic setup for the wavelength tuning of the q-cw Pr:KYF laser.
Initially, the experiments were performed under cw excitation. However, the spectral tuning range under these conditions was limited by strong thermal depolarization effects which increased the output coupling losses at the birefringent filter. Once the laser threshold is reached, the stimulated emission leads to an efficient removal of the excitation energy from the crystal. At wavelengths with low emission cross sections, the laser threshold becomes high and thus the crystal heats up more, further increasing the laser threshold. As a consequence those wavelengths that already exhibit a high threshold were affected by higher thermal depolarization losses, impeding laser operation. To reduce the thermal load, we positioned a mechanical chopper into the excitation beam, which strongly increased the tuning range of the system. In all experiments presented in this section the pump beam was modulated at a frequency of 500 Hz with a duty cycle of 0.5. Thus the pump period was 1 ms, which exceeds the effective lifetime of the upper laser level by a factor of about 25, ensuring true q-cw operation. All output powers that will be given in this section represent the output to both sides of the birefringent filter and denote the q-cw power. 3.2 Results of the wavelength tuning experiments The results of the wavelength tuning experiments are depicted in Fig. 3 together with a gain spectrum of Pr:KYF. The widest continuous tuning range was achieved in the deep red spectral region around 715 nm with a full width of almost 50 nm. Another broad tuning band
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31277
is located around 620 nm with a full width of almost 23 nm. For comparison, in true cw operation this tuning range was narrowed to about 17 nm. The widest tuning range in the green spectral region is located around 523 nm with a full width of 5 nm. We believe that these results represent the widest tuning of any praseodymium doped crystalline material so far. The exact on- and offset wavelengths λmin and λmax as well as the full tuning widths Δλ are listed in Table 1 together with the highest output power Pmax that was observed within the respective tuning range.
Fig. 3. Output power versus wavelength of the q-cw Pr:KYF laser. The grey line represents a gain spectrum for the maximum expected inversion level in our setup (β = 0.02). The capital characters A-F denote the different sets of mirrors. The colored bars in the lower part visualize the areas where continuous tuning was achieved.
3.3 Summary and discussion of the wavelength tuning experiments In the wavelength tuning experiments we were able to address almost every transition in four level schemes with emission wavelengths in the visible spectral region originating from the 3 PJ multiplet. We showed tuning ranges > 10 nm around three laser transitions as well as the first demonstration of laser operation on eight transitions in the green and the deep red spectral range in Pr:KYF to the best of our knowledge. A total wavelength tuning range exceeding 100 nm was realized between 520 nm and 737 nm. In wavenumbers this corresponds to 42% of the total range of 5640 cm−1. This is the broadest tuning range that has ever been obtained with a crystalline praseodymium doped gain material. It is noteworthy that while the tuning curve depicted in Fig. 3 follows the gain spectrum (gray curve) in almost all spectral ranges, as expected for a four level laser, it does not in the region around 620 nm. Here, a ground state absorption transition into the 1D2 level does not allow for a description of this laser by the conventional four level theory. The result is a very smooth tuning curve (ΔνFWHM ≈ 420 cm−1). This makes the Pr:KYF laser at 620 nm very promising for ultra-short pulse generation. At emission wavelengths far off the emission maxima, the laser suffered from thermal depolarization which could so far only be overcome by operating the laser in q-cw mode. In order to realize the tuning ranges demonstrated in this work in true cw mode, the growth of crystals with higher optical quality, which could reduce the laser thresholds and thus the thermal load, should be investigated. Additionally, the application of longer crystals with a lower dopant concentration would reduce the heat injection per volume. Though this approach is limited by the beam quality of the pump source, a twice as long crystal with half the doping concentration should still allow for an adequate overlap between pump and laser mode at the same absorption efficiency. The lower doping concentration would furthermore lead to an increased effective lifetime of the upper laser level [19] which decreases the laser threshold. Moreover, applying tuning mechanisms that are less sensitive to the polarization,
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31278
e.g. prisms or gratings, can be expected to be more suitable for the Pr:KYF system if the polarization of the output beam is not important for the desired application. Table 1. List of realized tuning ranges. λmin and λmax denote on- and offset wavelengths, respectively. Δλ is the full width of the tuning range. Pmax represents the highest output power achieved within the respective interval. λmin (nm)
λmax (nm)
Δλ (nm)
Pmax (mW)
520.8
525.9
5.1
45.2
538.0
539.9
1.9
38.4
551.4
555.9
4.5
72.8
606.7
629.5
22.8
41.2
638.4
638.8
0.4
2.8
641.3
656.5
15.2
133.2
670.7
670.9
0.2
0.5
672.4
673.1
0.7
1.2
676.5
677.1
0.6
3.2
687.6
737.3
49.7
111.0
4. Green laser operation of Pr3+:KY3F10 The tuning experiments revealed that laser operation in Pr:KYF is possible on several wavelengths that have not been investigated so far. Especially the transitions in the green spectral region are interesting because they offer low Stokes losses which should allow for efficient laser operation. Therefore we investigated these transitions further in true cw mode without any tuning element inside the resonator. 4.1 Experimental setup A schematic of the setup used for the laser experiments described in this section is depicted in Fig. 4. The hemispheric linear resonator consisted of the 3 mm long 0.9 at.% doped Pr:KYF crystal placed in a water cooled copper mount (T = 10 °C), and two mirrors M1 and M2. M1 was plane, highly transmittive coated at the excitation wavelength, and highly reflective at the respective laser wavelengths. The mirror M2 had a radius of curvature of 50 mm and was placed about 48 mm behind M1. For wavelength selection different mirrors with appropriate transmission characteristics for each desired laser wavelength were chosen for M2.
Fig. 4. Schematic setup for the cw laser experiments in a free running mode.
The radiation of the laser diode was focused into the laser crystal by a lens with a focal length of 40 mm. The laser diode was operated at maximum output power and attenuated by a combination of a λ/2-waveplate and a polarizing beam splitter cube. The absorbed pump power was calculated from the measured fraction of pump power absorbed at the laser threshold and accounts also for the reflectivity of M2 for the pump wavelength.
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31279
4.2 Results of the cw laser experiments In the cw setup we were able to demonstrate several tens of mW of output power at 523 nm, 539 nm, and 554 nm for the first time with Pr:KYF. The output spectra are depicted on the left hand side of Fig. 5. Due to a lack of suitable output coupling mirrors, a thorough characterization was only performed for the laser at 554 nm. The right hand side of Fig. 5 shows the laser characteristics with respect to the absorbed pump power for different output coupler transmissions at 554 nm.
Fig. 5. Output spectra of the realized green emitting cw Pr:KYF lasers (left) and output characteristics of the laser at 554 nm for different output coupler transmissions TOC (right).
The highest slope efficiency of ηsl = 27% was obtained at an output coupler transmission of TOC = 0.8% yielding a maximum output power of Pmax = 121 mW at an absorbed pump power of 686 mW. For comparison we performed further cw laser experiments at λlas = 610 nm and 645 nm.
Fig. 6. Comparison between the output characteristics of cw Pr:KYF lasers operated at the wavelengths 554 nm, 610 nm, and 645 nm.
The characteristics of the red and orange lasers realized at optimized output coupler transmissions are shown in Fig. 6 together with the input-output curve at 554 nm which delivered the highest output power. Both, the efficiency of the red laser at 645 nm and that of the orange laser at 610 nm are comparable to previously published results [6]. All cw laser results are summarized in Table 2. 4.3 Summary and discussion of the cw experiments The continuous wave experiments include the first characterization of a green emitting Pr:KYF laser to the best of our knowledge as well as a comparative characterization of the
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31280
lasers at 610 nm and 645 nm. From all these lasers output powers in the 100 mW regime could be extracted. From the difference between the obtained slope efficiencies and the Stokes-limit the maximum resonator roundtrip losses were estimated to be in the order of 2%. Comparing these losses to the 0.5% which were estimated in [7] for Pr3+:LiYF4 leads to the conclusion that, as long as no fundamental problems such as excited state absorption are present in Pr:KYF, there is still room for improvement of the crystal quality. In principle KYF is suitable for the Czochralski-growth technique [20] which is the standard growth technique for high quality LiYF4 crystals. Thus, the fabrication of crystals with significantly improved optical quality seems possible. Further investigations should be carried out regarding the effects of varying dopant concentrations. Table 2. Summary of the cw laser parameters. λlas (nm)
TOC (nm)
ηsl (nm)
Pmax (mW)
Pthr (mW)
554
0.8
27
121
166
610
6.4
18
97
162
645
2.3
38
268
30
5. Summary and outlook We reported on laser diode pumped wavelength tuning experiments with Pr:KYF. In the deep red spectral range around 715 nm a full width of almost 50 nm could be continuously addressed. To the best of our knowledge, this represents the widest tuning range for any praseodymium doped crystalline material. Wide tuning was also possible around 5 other wavelengths in the green, orange, and red spectral range. Due to thermal depolarization the full tuning range could only be achieved in q-cw operation. True cw operation throughout the whole range shown in these experiments seems feasible when applying tuning elements which are not sensitive to the polarization. Furthermore we have demonstrated what are to the best of our knowledge the first green emitting continuous wave Pr:KYF lasers at three different transitions. At 554 nm a maximum output power of 121 mW could be obtained with a slope efficiency of 27%. However, the efficiencies of all lasers are still almost a factor of two lower than in the standard material Pr3+:LiYF4 [7]. A significant improvement of the performance can be expected by the use of crystals with higher optical quality and an optimization of the dopant concentration. With these improvements we are confident, that Pr:KYF has the potential for reasonably efficient and competitive lasers with a broad tuning range in the visible. Recent results obtained with Pr3+:BaY2F8 reveal that praseodymium doped crystals can be suitable for cw laser operation in the cyan-blue spectral range below 500 nm [21]. Also the spectroscopic features of Pr:KYF seem suitable for an extension of the tuning range into the cyan-blue if the crystal quality can be significantly improved. Acknowledgments We are very grateful for the financial support by the Deutsche Forschungsgemeinschaft within the graduate school 1355 “Physics with new advanced coherent light sources” and the Joachim Herz Stiftung.
#198051 - $15.00 USD Received 20 Sep 2013; revised 13 Nov 2013; accepted 21 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031274 | OPTICS EXPRESS 31281
