Resonantly Pumped Er:YAG and Er:YAP Lasers

Resonantly Pumped Er:YAG and Er:YAP Lasers Michal Němec a*, Helena Jelínková a, Jan Šulc a Karel Nejezchleb b, Václav Škoda b a Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague, Břehová 7, 115 19 Prague 1, Czech Republic b Crytur,Ltd. Turnov, Palackého 175, 511 01 Turnov, Czech Republic ABSTRACT Resonant pumping by a solid state Er:glass laser was successfully examined for Er:YAG and for the first time also for Er:YAP laser. The maximal incident pumping energy on the wavelength 1535 nm was 640 mj with a repetition rate of 0.5 Hz; the corresponding pulse length was 1.9 ms (FWHM). The Er:glass laser radiation was focused into the active crystal by a CaF 2 lens with 70 mm focal length. The measured beam diameter in focal plane was ~ 400 μm. The Er:YAG and Er:YAP rods had 10 mm in length and 5 mm in diameter. Various concentrations of Er 3+ ions were used: 0.5 at.% for YAG and 1 at.% for YAP crystal. The resonator consisted of pumping and output dielectric mirrors. For both cases, the pumping dielectric mirror with high transmittance at pumping wavelength (T > 95 % @ 1532 nm) and maximal reflectance at the oscillating wavelength (around 1640 nm) was used. The output coupler reflectance was 85 % and 90 % for 1532 nm and 1640 nm, respectively. The advantage of resonantly pumped lasers is low thermal load corresponding to low quantum defect, and, therefore, it was not necessary to cool the active crystals. The output generated energy for the Er:YAG laser medium was 45 mj at 1648 nm for 465 mj incident pumping energy. For Er:YAP crystal the energy reached was 20 mj at the lasing wavelength 1623 nm. The incident pumping was 640 mj. For both resonantly pumped laser systems other characteristics i.e., spatial beam structure, divergence, and efficiency were investigated. Keywords: resonantly pumped lasers, Er:YAG, Er:YAP. 1. INTRODUCTION Erbium lasers generating at 1.5-1.6 μm wavelength are very interesting for applications such as satellite communication, range finding, and atmospheric sounding. For many of these applications, high efficiency and good beam quality are required. As pumping systems, standard diodes or fiber lasers could be used, but then a problem arises with heating the bulk laser material, consecutively with strong thermal lensing. These factors can decrease laser beam quality and efficiency. For the fiber laser, a long device length and small core size implicate some nonlinear effect, which can also reduce efficiency 1. An alternative method is resonant pumping. The advantage of resonantly pumped lasers is a small quantum defect 2 and low thermal stress, and thus it reduced significantly the necessary of active crystals cooling 3-5. Low doping crystal with Er 3+ reduces the deleterious effects of ground state reabsorption and up-conversion and provides better thermal management for good beam quality 2. We designed and constructed laser systems pumped by Er:glass laser radiation in an end-pumped arrangement. As active materials, Er:YAG and Er:YAP crystals were utilized. From our knowledge, this is the first time that the Er:YAP crystal resonantly pumped by Er:glass laser has been investigated. * michal.nemec@fjfi.cvut.cz; phone +420 224 358 672; fax +420 222 512 735; Solid State Lasers XVIII: Technology and Devices, edited by W. Andrew Clarkson, Norman Hodgson, Ramesh K. Shori Proc. of SPIE Vol. 7193, 71932P 2009 SPIE CCC code: 0277-786X/09/$18 doi: 10.1117/12.808925 Proc. of SPIE Vol. 7193 71932P-1

2. MATERIALS AND METHODS 2.1. Er:glass pumping laser system As pumping source, the Er:glass laser operating at wavelength 1535 nm was designed and constructed. The Cr:Yb:Er:glass rod (3.9 x 76 mm) was placed into a ceramic diffuse cavity LMI together with an Xe-flashlamp. The plan-parallel resonator consisted of a total-reflecting copper mirror and output dielectric mirror (R = 42 % @ 1530 nm, Crytur Ltd.). The output characteristics were investigated for repetition rate 0.5 Hz in a free-running long-pulse regime. The maximal output energy and pulse length were 640 mj and 1.9 ms (FWHM), respectively. The generated wavelength 1535 nm was measured by an optical fiber and spectrometer StellarNet EPP 2000. The emitted linewidth was 3 nm FWHM. 2.2. Er:YAG and Er:YAP laser crystal The laser action in the Er:YAG and Er:YAP crystals could be demonstrated. To derive the right concentration, the transmission and fluorescence measurements of Er:YAG and Er:YAP plates (Crytur Ltd.) with various Er 3+ ion concentrations (5 %, 2 %, 1 %, 0.5 %, 0.2 % for Er:YAG, and 5 %, 3 %, 2 %, 1.5 %, 1 % for Er:YAP crystals) were performed. The thicknesses of plates were 4.85 mm and 4.5 mm for Er:YAG and for Er:YAP, respectively. The plates were without any coating. The summary of 1535 nm radiation transmission by plates is in Table 1. Table 1 Summary of 1535 nm radiation transmission (including Fresnel losses) by Er:YAG (4.85 mm) and Er:YAP (4.5 mm) plates. Concentration Er 3+ in YAG crystal [%] 5 2 1 0.5 0.2 Transmission at 1535 nm [%] 27.678 52.976 65.459 75.005 81.316 Concentration Er 3+ in YAP crystal [%] 5 3 2 1.5 1 Transmission at 1535 nm [%] 35.935 46.504 56.046 63.891 67.026 The designed Er:YAG and Er:YAP rods (Crytur Ltd.) were 10 mm in length and 5 mm in diameter, and had different concentrations of Er 3+ ions: 0.5 at.% for YAG and 1 at.% for YAP crystals. The faces of crystals had anti-reflection layers, the reflectance (1.5-1.7 μm) being lower than 1%. The measured lifetime was 6.0 ms and 4.6 ms for Er:YAG and Er:YAP active crystals, respectively. 2.3 Measuring instruments and methods The laser systems were characterized from the point of the output energy, pulse length, generated wavelength, and beam spatial structure. The output energy was investigated by two-channel Molectron JD2000 Joulemeter/Ratiometer with probes J25LP-YAG and J25 (Coherent Inc). The time characteristics of the generated pulses were measured by infrared detector No.83614 in connection with the Tektronix 3052B oscilloscope (500 MHz, 5 GS/s). The radiation spectrum was measured by Oriel monochromator 77250 (50 μm wide slit) or spectrometer StellarNet EPP 2000 with optical fiber. A pyroelectric camera (Spiricon Pyrocam III, sensor dimension 12.4 x 12.4 mm) was used to investigate the laser beam spatial structure. The transmission spectrum of crystal plates, crystals, and mirror was obtained by Shimadzu UV-3600 UV-VIS-NIR spectrophotometer. For measuring the output energy dependence, a filter set was utilized; in the process, pumping beam spatial profile was kept and only its energy was changed. The absorbed energy was calculated from the input energy into an active crystal and measured transmission of pumping radiation by the crystal. The input energy was obtained from the incident pumping energy including transmission parameters of mirrors and the focusing lens. The measurement of transmission by crystal was carried out without output coupler (i.e., without laser generation). We studied the dependence of output energy from crystal on the incident pumping energy. Then, the transmission value for threshold energy of generation was considered and an absorption coefficient was set for this value. Hence, it was the upper estimate of differential efficiency Proc. of SPIE Vol. 7193 71932P-2

of the output energy on absorbed energy because the absorption during lasing could be only higher than without radiation generation. 3. RESULTS 3.1 Er:glass laser radiation The Er:glass laser was working with 0.5 Hz repetition rate. The spatial structure was recorded and the mode is shown in Figure 1. The maximal incident pumping energy was 640 mj in free-running regime; the corresponding pulse length was 1.9 ms (FWHM). It was noted that this laser operating at 1535 nm served as a coherent source for resonant pumping of Er:YAG a Er:YAP lasers. For that reason, the Er:glass laser radiation was focused into active crystals by a CaF 2 lens with a focal length of 70 mm. The measured diameter in the focal plane was less than 400 μm. Fig. 1 Output beam spatial structure of 1535 nm Er:glass laser radiation a) 362 mj b) 493 mj c) 617 mj(2d and 3D image, Spiricon Pyrocam III). 3.2 Er:YAG and Er:YAP laser resonator For both cases, the resonator consisted of a concave pumping dielectric mirror with high transmittance at the pumping wavelength (less 5 % @ 1535 nm) and maximal reflectance at the oscillating wavelength (about 1640 nm). The parallel output coupler reflectance was 85 % and 90 % for 1535 nm and 1640 nm, respectively. Transmission of the lens and pumping mirror was 86.7 %, therefore the intensity in the focal plane for maximal pumping energy was 442 J/cm 2. 3.2.1 Resonantly pumped Er:YAG laser The dependence of output energy on absorbed energy for Er:YAG laser is given Fig.2. The differential efficiency and pumping threshold energy into crystal were 60.7 % and 31 mj, respectively. The maximum output energy reached up to 45 mj. For this energy, the time dependence of output Er:YAG together with pumping Er:glass radiations was recorded - oscillogram in Fig.3. The generated wavelength measured by Oriel monochromator was 1648 nm. The output beam spatial structure of Er:YAG laser radiation for 40 mj generated energy (2D and 3D image, Spiricon camera) is shown in Fig.4. Proc. of SPIE Vol. 7193 71932P-3

50 -:1 Er:YAG laser - 1648 nm - E(max) 45mJ DiLefficiency = 60.7 % 0 10 20 30 40 50 60 70 80 Absorbed energy [rnj] Fig. 2 Dependence of output energy on absorbed energy for Er:YAG laser. Ch2 rstw p.033m C 302.311 Chi OrsOW p 2.1 5am a 202.111 -_I...,..:Il..lJ1l., I I.r. %k'.ia U I! Ii.52000ms I Fig. 3 Time dependence of pumping Er:glass (Ch1) and output Er:YAG (Ch2) radiations (40 mj output Er:YAG laser energy).!cca.a.s-- :'ri....;. Fig. 4. Output beam spatial structure of 1648 nm Er:YAG laser radiation for 40 mj generated energy (2D and 3D image, Spiricon camera). 3.2.2 Resonantly pumped Er:YAP laser Analogous measurements as for the Er:YAG laser were made also for the Er:YAP laser. The output energy versus absorbed energy is in Fig.5. Its differential efficiency was 32.1 % and pumping threshold energy into crystal was 160 mj. The maximal value of output energy reached was 20 mj. The time dependence of the output Er:YAP pulse and pumping Er:glass radiation is seen in Fig.6. The measured wavelength was 1632 nm; the output beam spatial profile for generated 1632 nm wavelength is in Fig.7. Proc. of SPIE Vol. 7193 71932P-4

25 -:1 L20 Er:YAP laser - 1623 nm - E(max) = 20 rnj DiLefficiency = 32.1 % 0 10 20 30 40 50 60 70 80 Absorbed energy [rnj] Fig. 5 Dependence of output energy on absorbed energy for Er:YAP laser. Tekre Ch2 hrstw p 2.120m C 753.Op ChlhrstW p.607m C 783.Sp Chi 20.OmV 20.OmV MLq}4J a Ch2 J 22.4mV H, SSS.000ps 14:31:35 Fig. 6 Time dependence of pumping Er:glass (Ch1) and output Er:YAP (Ch2) radiations (20 mj output Er:YAP laser energy). Fig. 7 Output beam spatial structure of 1623 nm Er:YAP laser radiation for 20 mj generated energy (2D and 3D image, Spiricon camera). 4. DISCUSSION AND CONCLUSION In this work, the Er:YAG and Er:YAP laser systems resonantly pumped by Er:glass laser radiation were designed and constructed. The lasers worked in free-running regime and generated close to fundamental TEM 00 mode. From our knowledge, this is the first time that the Er:YAP crystal resonantly pumped by Er:glass laser radiation has been investigated. The Er:YAG laser generated energy up to 45 mj at 1648 nm for the 465 mj pumping energy; the corresponding absorbed energy was 78.5 mj. For the Er:YAP crystal the lasing wavelength was 1623 nm. The energy reached was up to 20 mj for the 640 mj pumping value and 91.1 mj absorbed energy. The differential efficiency was 60.7 % and 32.1 % for Er:YAG and ErYAP system, respectively. The spatial beam structures were close to fundamental mode TEM 00 for both systems. Proc. of SPIE Vol. 7193 71932P-5

ACKNOWLEDGEMENT This research has been supported by the Grant of the Czech Ministry of Education No.MSM6840770022 "Laser systems, radiation and modern optical applications". REFERENCES [1] Kim, J., W., Sahu, J., K. and Clarkson W., A., "Impact of energy-transfer-upconversion on the performance of hybrid Er:YAG lasers," Proc. SPIE 6871, 68710W (2008). [2] Chen, D.,-W., Rose T., S. and Beck, S., M., "High Performance 1645-nm Er:YAG Laser," in Advanced Solid-State Photonics 2008 on CD-ROM (The Optical Society of America, Washington, DC, 2008), WE46 (2008). [3] Spariosu, K., Birnbaum, M. and Viana, B., "Er 3+ :Y 3 Al 5 O 12 laser dynamics: effect of upconversion," J. Opt. Soc. Am. B. 11, 894 (1994). [4] Clarkson W., A., Shen, D. and Sahu, J., K.,"High-Power Fiber-Bulk Hybrid Lasers," Proc. SPIE 6100, 61000A (2006). [5] Setzler, S., D., Konves, J., R. and Chicklis, E., P., "Resonantly Diode-Pumped Eyesafe Er:YAG Lasers," Proc. SPIE 5707, 117 (2005). Proc. of SPIE Vol. 7193 71932P-6