Holmium-doped Lu2O3, Y2O3, and Sc2O3 for lasers ... - OSA Publishing

Holmium-doped Lu2O3, Y2O3, and Sc2O3 for lasers above 2.1 µm Philipp Koopmann,1,2,∗ Samir Lamrini,2 Karsten Scholle,2 Michael ¨ Huber1 Schäfer,2 Peter Fuhrberg,2 and Gunter 1 Institute

of Laser-Physics, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany 2 LISA laser products, Max-Planck-Str. 1, 27191 Katlenburg-Lindau, Germany *[email protected]

Abstract: Efficient room-temperature laser operation was obtained in the wavelength range from 2117 nm to 2134 nm with Ho:Lu2 O3 and Ho:Y2 O3 as the active materials. With an FBG-stabilized Tm-doped fiber laser as the pump source, the maximum slope efficiency and output power of the Ho:Y2 O3 laser were 63 % and 18.8 W, respectively. With Ho:Lu2 O3 the respective values were 76 % and 25.2 W. With Ho:Sc2 O3 as the active material the accessible wavelength range could be expanded to 2158 nm in a diode-pumped setup. © 2013 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3580) Lasers, solid-state.

References and links 1. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, “2 µm laser sources and their possible applications” in Frontiers in Guided Wave Optics and Optoelectronics B. Pal, ed. (Intech, 2010), pp. 471–500. 2. J. H. Taylor and H. W. Yates, “Atmospheric transmission in the infrared,” J. Opt. Soc. Am. 47, 223–225 (1957). 3. K. T. Zawilski, P. G. Schunemann, S. D. Setzler, and T. M. Pollak, “Large aperture single crystal ZnGeP2 for high-energy applications,” J. Cryst. Growth 310, 1891–1896 (2008). 4. B. M. Walsh, N. P. Barnes, and B. D. Bartolo, “Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids: application to Tm3+ and Ho3+ ions in LiYF4 ,” J. Appl. Phys. 83, 2772–2787 (1998). 5. S. Lamrini, P. Koopmann, M. Schäfer, K. Scholle, and P. Fuhrberg, “Directly diode-pumped high-energy Ho:YAG oscillator,” Opt. Lett. 37, 515–517 (2012). 6. R. Peters, C. Kraenkel, S. Fredrich-Thornton, K. Beil, K. Petermann, G. Huber, O. Heckl, C. Baer, C. Saraceno, T. Suedmeyer, and U. Keller, “Thermal analysis and efficient high power continuous-wave and mode-locked thin disk laser operation of Yb-doped sesquioxides,” Appl. Phys. B 102, 509–514 (2011). 7. P. Koopmann, S. Lamrini, K. Scholle, P. Fuhrberg, K. Petermann, and G. Huber, “Efficient diode-pumped laser operation of Tm:Lu2 O3 around 2 µm,” Opt. Lett. 36, 948–950 (2011). 8. P. Koopmann, “Thulium- and Holmium-Doped Sesquioxides for 2 µm Lasers,” PhD thesis, University of Hamburg (2012). 9. C. Brandt, N. A. Tolstik, N. V. Kuleshov, K. Petermann, and G. Huber, “Inband pumped Er:Lu2 O3 and (Er,Yb):YVO4 Lasers near 1.6 µm for CO2 LIDAR,” in Advanced Solid-State Photonics (Optical Society of America, 2010), p. AMB15. 10. T. Li, K. Beil, C. Kränkel, and G. Huber, “Efficient high-power continuous wave Er:Lu2 O3 laser at 2.85 µm,” Opt. Lett. 37, 2568–2570 (2012). 11. F. Reichert, M. Fechner, P. Koopmann, C. Brandt, K. Petermann, and G. Huber, “Spectroscopy and laser operation of Nd-doped mixed sesquioxides (Lu1−x Scx )2 O3 ,” Appl. Phys. B 108, 475–478 (2012). 12. G. A. Newburgh, A. Word-Daniels, A. Michael, L. D. Merkle, A. Ikesue, and M. Dubinskii, “Resonantly diodepumped Ho3+ :Y2 O3 ceramic 2.1 µm laser,” Opt. Express 19, 3604–3611 (2011). 13. P. Koopmann, S. Lamrini, K. Scholle, M. Schäfer, P. Fuhrberg, and G. Huber, “Multi-watt laser operation and laser parameters of Ho-doped Lu2 O3 at 2.12 µm,” Opt. Mater. Express 1, 1447–1456 (2011). 14. M. Becker, S. Brückner, M. Leich, E. Lindner, M. Rothhardt, S. Unger, S. Jetschke, and H. Bartelt, “Towards a monolithic fiber laser with deep UV femtosecond-induced fiber Bragg gratings,” Opt. Commun. 284, 5770–5773 (2011).

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Received 21 Dec 2012; revised 26 Jan 2013; accepted 26 Jan 2013; published 8 Feb 2013

11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3926

15. S. Lamrini, P. Koopmann, M. Schäfer, K. Scholle, and P. Fuhrberg, “Efficient high power Ho:YAG laser directly in-band pumped by a GaSb-based laser diode stack at 1.9 µm,” Appl. Phys. B 106, 315–319 (2012). 16. J. A. Caird, S. A. Payne, P. Randall Staver, A. J. Ramponi, L. L. Chase, and W. F. Krupke, “Quantum electronic properties of the Na3 Ga2 Li3 F12 :Cr3+ laser,” IEEE J. Quantum Electron. 24, 1077–1099 (1988).

1.

Introduction

Numerous applications lead to a growing interest in laser systems with wavelengths in the nominally eye-safe 2 µm region. They can be found in several fields of medicine, in LIDAR systems, in gas detection, and as pump sources for mid-IR OPOs, to name a few [1]. Many of these applications demand for wavelengths above 2 µm or even above 2.1 µm, because in this area the absorption of the atmosphere and of the common mid-IR OPO material ZGP is reduced [2, 3]. The most common laser systems for the generation of laser radiation with wavelengths in the 2 µm region are based on thulium- and holmium-doped crystals or glasses. While the wavelengths of most thulium-based lasers are in the area of 1.8 µm to 2.05 µm, the wavelengths above 2.05 µm are typically addressed by holmium-based lasers. However, the wavelengths reached with these systems are commonly not above 2.1 µm, especially when high inversion densities are present, as it is the case for Q-switched lasers [4, 5]. These conditions are different when cubic sesquioxides are chosen as the host material. In consequence of the strong Stark splitting of the rare earth ion manifolds, these materials generally support laser operation at longer wavelengths. Due to the high melting point of these materials (∼ 2500 ◦ C) one has only recently been able to grow high-quality crystals. Efficient laser operation was successfully realized with the dopant ions Yb, Tm, Er, and Nd [6–11]. Recently, laser operation was also obtained with holmium-doped Y2 O3 and Lu2 O3 . While with Ho:Y2 O3 ceramics laser operation could only be obtained at cryogenic temperatures and the output power was limited to 2.5 W at a slope efficiency of 35 % [12], the crystalline Ho:Lu2 O3 laser reached up to 15 W of output power under diode pumping, but the slope efficiency with respect to the absorbed power could only be estimated to be around 50 % [13]. Due to the broad spectra of the pump diode, the absorption efficiency could not be determined precisely. With a narrow-bandwidth thulium fiber laser as the pump source, a slope efficiency of 54 % and an output power of 5.2 W were reached. In this paper we present laser operation of Ho:Lu2 O3 and Ho:Y2 O3 with a fiber-Bragggrating-stabilized thulium fiber laser as the pump source. Slope efficiencies of up to 76 % and 63 % and output powers of 25.2 W and 18.8 W, respectively, were reached. The emission wavelengths of the lasers were between 2117 nm and 2134 nm. Even longer emission wavelengths of up to 2158 nm could be reached with a diode-pumped Ho:Sc2 O3 laser. A detailed description of the crystal growth and spectroscopy of each Ho-doped sesquioxide crystal can be found in [8]. 2. 2.1.

The setup The FBG-stabilized fiber laser

A schematic of the fiber laser is shown in Fig. 1. The fiber laser was cladding-pumped by a fiber-coupled laser diode with a maximum output power of 300 W and a wavelength of 790 nm. The pump light was focused into the fiber with an f = 20 mm lens system through an infrasil substrate, which was welded to the fiber with a CO2 laser. The surface of the substrate was ARcoated for the pump and the laser wavelengths in order to avoid back reflection from the fiber end. The first and last 1 m long pieces of fiber were undoped, permitting direct water cooling of the whole 12 m long thulium-doped fiber, which was spliced to the undoped fiber ends. The cooling water had a temperature of 18 ◦ C. The core of the active fiber had a diameter of 14 µm and an NA of 0.2, the diameter and NA of the cladding were 400 µm and 0.46, respectively. All

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pieces of undoped fiber employed in the setup exhibited the same dimensions. Another piece of undoped fiber was spliced between the active fiber and the undoped fiber end. Into the core of this undoped fiber a fiber Bragg grating (FBG) had been inscribed with a mode-locked and frequency-tripled Ti:sapphire laser at the IPHT Jena (Institute of Photonic Technology, Jena, Germany) [14]. The FBG exhibited a high reflectivity at a wavelength of 1940 nm. Laser diode 790 nm

Infrasil substrate AR @ 790 nm AR @ 1940 nm

Water f = 20 mm 12 m Tm-doped fiber 14 µm/400 µm FBG HR @ 1940 nm

1940 nm Infrasil substrate AR @ 790 nm AR @ 1940 nm

Fig. 1. Setup of the FBG-stabilized fiber laser.

With the fiber laser a maximum output power of 70 W at a wavelength of 1940 nm could be obtained. At the splice between the active fiber and the undoped fiber at the incoupling side the 1.94 µm light could not be coupled into the core of the undoped fiber. This can be attributed to an improper core to core matching of the splice at the incoupling side. Consequently, the pump light for the laser experiments with the holmium-doped rods was extracted from the 400 µm diameter cladding instead of the 14 µm diameter core. 2.2.

The resonator

The resonator was formed by a plane incoupling mirror which was AR coated for the pump and HR coated for the laser wavelengths, a plane folding mirror and a spherical output coupling mirror with r = 100 mm, as shown in Fig. 2. The folding mirror (R = 2 % at 1940 nm and 45◦ ) was used to reduce the back reflection of pump light to the fiber laser. This was essential because, due to the randomly polarized output of the fiber laser, an optical diode was not placed between the fiber laser and the holmium laser resonator. The incoupling mirror was placed as close to the laser rod as possible while the distances from the folding mirror to the rod’s rear facet and the output coupling mirror were 20 mm each. The laser rod was directly water-cooled to 20 ◦ C. Both rods applied in the experiments were 2.5 mm in diameter and 20 mm in length. The pump light was focused onto the laser rod with an f = 80 mm lens. Since the pump light was collimated by an f = 20 mm lens system and the diameter of the fiber was 400 µm, the pump spot inside the laser rod had a diameter of ∼ 1.6 mm. Due to the resonator geometry, the diameter of the TEM00 beam inside the crystal was ∼ 350 µm, which was considerably smaller than the diameter of the pump channel. Consequently, a multimode output of the laser is expected. Since 98 % of the transmitted pump light was transmitted by the resonator folding mirror, the absorbed pump power during laser operation could be derived by simultaneously measuring the transmitted pump power and the laser output power.

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AR @ 1.94 µm HR @ 2.1 µm f = 80 mm

HR @ 2.1 µm R = 2 % @ 1.94 µm

Crystal rod 20 x 2.5 mm²

1.94 µm

AR @ 1.94 µm & 2.1 µm TOC, r = 100 mm 2.1 µm

Fig. 2. Setup of the resonator applied for the laser experiments with holmium-doped laser rods and a fiber laser as the pump source.

3.

Experimental results

Before laser experiments were performed, the transmitted pump power was measured without laser operation. Strong bleaching was evident. For example in the case of the 20 mm long Ho(0.3 at.%):Lu2 O3 rod the absorption at low pump powers was almost 80 % of the incident power while this value decreased to ∼ 30 % for pump powers of 20 W and above. During laser operation the fraction of absorbed pump power was ∼ 55 % for low output coupling and ∼ 35 % for the highest output coupler transmissions. These absorption levels did not change significantly over the whole pump power range. The higher absorption efficiency in the case of low output coupler transmissions can be attributed to the lower inversion in the active medium required to reach the laser threshold. In addition to the dependence of the absorption efficiency on the output coupling also a fluctuation of the transmitted pump power with time could be observed. The time scale of the fluctuations was seconds to several 10 seconds. They correlated to changes of the fiber laser wavelength of ∼ 0.5 nm. Especially in the case of Ho:Lu2 O3 , where the pump wavelength was at the edge of an absorption peak, this led to significant changes of the absorption efficiency. However, since the transmitted pump power could be measured synchronously to the output power of the holmium laser, these fluctuations did not interfere with the determination of the laser efficiency.

Fig. 3. Laser characteristics of the fiber-laser-pumped lasers with (a) Ho(0.3 at.%):Lu2 O3 and (b) Ho(0.4 at.%):Y2 O3 .

For the laser experiments output coupler transmissions between 1.1 % and 23 % were inves#181944 - $15.00 USD

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tigated. The laser characteristics of Ho(0.3 at.%):Lu2 O3 and Ho(0.4 at.%):Y2 O3 are shown in Fig. 3. In the case of Ho:Lu2 O3 the threshold pump power was 1.6 W with TOC = 1.1 % and increased to 3.1 W with TOC = 23 %. The slope efficiencies ranged from 65 % (TOC = 1.1 %) to 76 % (TOC = 8 % and 13 %), the highest optical-to-optical efficiency with respect to the absorbed pump power was 68 % (TOC = 8 %). The maximum output power was 25.2 W which was obtained with 3 % of output coupling. The mentioned wavelength fluctuations are the reason for the higher absorbed pump power in the case of the 3 % output coupling mirror in comparison to 1.1 % of output coupling. The decrease of the slope efficiency to 68 % in the case of 23 % of output coupling can be attributed to upconversion losses, which is a well-known loss mechanism in holmium-doped media at high inversion levels. The emission wavelength of the Ho:Lu2 O3 laser was 2134 nm in the case of TOC = 1.1 % and 2124 nm for all higher transmissions. This is a remarkably long wavelength for a holmiumbased laser at high inversion densities. The well-established laser material Ho:YAG exhibits a laser wavelength of 2090 nm when high inversion densities are present [15]. With the Ho:Y2 O3 laser rod slightly higher threshold pump powers of 1.8 W with TOC = 1.1 % and 3.9 W with TOC = 23 % were measured, which can be attributed to stronger reabsorption due to the higher dopant concentration of the Ho:Y2 O3 rod and higher losses, see below. The slope efficiencies were lower than in the case of Ho:Lu2 O3 , especially for low output coupler transmissions. With 1.1 % of output coupling the slope efficiency was only 34 % as opposed to 65 % with Ho:Lu2 O3 . With Ho:Y2 O3 the highest slope efficiency of 63 % was obtained with 13 % of output coupling. The highest output power was 18.8 W, which was obtained with 8 % of output coupling (not shown in Fig. 3(b)). The highest optical-to-optical efficiency of 52 % was obtained with TOC = 13 %. Also with Ho:Y2 O3 a slight decrease of the slope efficiency with the highest output coupler transmission was observed. The Ho:Y2 O3 laser wavelength was 2127 nm when 1.1 % of output coupling were applied and 2117 nm for higher values of TOC .

Fig. 4. Caird analysis of the 20 mm long Ho(0.3 at.%):Lu2 O3 and Ho(0.4 at.%):Y2 O3 laser rods.

The losses were determined with the Caird analysis [16]. The curves for the analyzed Ho:Lu2 O3 and Ho:Y2 O3 rods are shown in Fig. 4. The data points for TOC = 23 % are not plotted because for these points the impact of upconversion effects was severe. In general the Caird plot is only valid when the laser wavelength is the same for all output coupler transmissions and thus the quantum defect is uniform. In the case of Ho:Lu2 O3 the wavelengths were 2134 nm and 2124 nm, respectively, and in the case of Ho:Y2 O3 the wavelengths were 2127 nm and 2117 nm, respectively. Consequently, the quantum defect is changed by only ∼ 5 % which #181944 - $15.00 USD

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does not have a strong impact on the determination of the losses, therefore, the values for TOC = 1.1 % are also plotted in Fig. 4. The round-trip losses Λ, which were determined for the 20 mm long Ho:Lu2 O3 laser rod, are only 0.22 % which indicates the high quality of this crystal. The losses of the Ho:Y2 O3 laser rod are 1.1 % and therefore clearly higher. This result can be attributed to the intrinsic scattering which is found in Y2 O3 crystals. In comparison to the so far only published laser experiments with Ho:Y2 O3 , the losses of 0.28 % cm−1 , which were obtained in the presented experiments, are an order of magnitude smaller. The losses which were determined for a cryogenic Ho:Y2 O3 ceramic laser by Newburgh et al. were ∼ 7 % cm−1 [12]. Besides the experiments in a fiber-laser-pumped setup, laser experiments were also performed with a GaSb-based laser diode stack as the pump source at a wavelength of ∼ 1.9 µm. The experiments on Ho:Lu2 O3 with an output power of up to 15 W are described in [13]. Under identical conditions and with Ho:Y2 O3 and Ho:Sc2 O3 as active materials output powers of 12.6 W and 18.4 W, respectively, could be obtained [8]. While in the case of Ho:Y2 O3 the laser wavelengths were the same as in the fiber-laser-pumped experiments, the Ho:Sc2 O3 -based laser exhibited wavelengths of 2123 nm, 2145 nm, and 2158 nm, respectively, depending on the output coupling. These very long wavelengths for a holmium-based laser can be attributed to the strong Stark splitting which is present in Sc2 O3 crystals. These experiments give prospect of highly efficient lasers also with Ho:Sc2 O3 when a narrow-bandwidth pump source is used. 4.

Conclusion

With an in-house-built FBG-stabilized thulium fiber laser as the pump source, very high laser efficiencies could be obtained with Ho:Lu2 O3 and Ho:Y2 O3 as the active materials at room temperature. With Ho:Lu2 O3 a maximum slope efficiency of 76 % was reached. This value is among the highest so far obtained with holmium-doped media. The pump-power-limited maximum ouput power was 25.2 W. With crystalline Ho:Y2 O3 laser operation could be obtained for the first time, to the best of our knowledge. The maximum slope efficiency was 63 % and the maximum output power was 18.8 W. The emission wavelengths of the lasers were between 2117 nm and 2134 nm. These remarkably long wavelengths for holmium-based lasers were reached also when high inversion densities were present. Even longer wavelengths of up to 2158 nm were reached with a diode-pumped Ho:Sc2 O3 laser. High laser efficiencies are expected to be attainable also with this material when a narrow-bandwith pump source is used. The results prove the great potential of holmium-doped sesquioxides for highly efficient lasers with emission wavelengths above 2.1 µm.

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