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2 Optics & Laser Technology 41 (2009) Contents lists available at ScienceDirect Optics & Laser Technology journal homepage: Maximizing TEM 00 solar laser power by a light guide assembly-elliptical cavity Dawei Liang, Rui Pereira CEFITEC, Departamento de Física, FCT, Universidade Nova de Lisboa, 2825, Campus de Caparica, Portugal article info abstract Article history: Received 23 September 2008 Received in revised form 29 January 2009 Accepted 29 January 2009 Available online 13 March 2009 Keywords: Light guide Elliptical cavity Solar laser For maximizing TEM 00 solar laser power, a modified light guide assembly-elliptical cavity pumping approach is proposed. By observing refractive and total internal reflection principles, highly concentrated solar radiations from a primary parabolic mirror are firstly collected by a near-hexagonal input face and then transmitted, at low propagation angles, to a near-rectangular output end of the modified assembly. A sharp elliptical pump cavity with an intervening optics is utilized to further couple and concentrate the radiations from the assembly to a thin Nd:YAG rod. Optimum pumping parameters are found through ZEMAX TM non-sequential ray-tracing and LASCAD TM laser cavity analysis. Compared with the output performances of the parallel-packed light guide assemblyelliptical-cylindrical cavity, 45% increase in TEM 00, 10% in multimode solar laser powers are attained by the proposed approach. Experimental results on both the enhanced transmission efficiency of the assembly and the improved focusing capacity of the elliptical cavity with are finally provided. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction Since the natural sunlight does not provide power density sufficient enough for lasing, additional focusing systems are usually required to convert solar power into laser radiation. Since the report of the first solar-pumped solid-state laser [1], researchers have been exploiting parabolic mirrors and heliostats to attain enough solar flux at the focal point. The utilization of two-dimensional compound parabolic concentrator (2D-CPC) and three-dimensional compound parabolic concentrator (3D-CPC) secondary pumping cavities further boosted up the solar laser power level [2,3]. To increase the spectral match between the emission spectrum of the pump radiation and the absorption spectrum of the laser medium, crystals of different active materials were tested [4]. Several pumping architectures have been proposed for solar-pumped lasers [5 8]. Although the most efficient laser systems have end-pumping approach, side-pumping is an effective configuration for power scaling as it spreads the absorbed pump power along the laser rod, reducing the associated thermal loading problems [9]. Space-based communication and power transmission are typical objectives for solar laser research. High-quality laser beam are indispensable for these applications. The recent progress in high-efficiency and economical solar-pumped laser with Fresnel Corresponding author. Tel.: ; fax: addresses: dl@fct.unl.pt (D. Liang), r.p.pereira@gmail.com (R. Pereira). lens and chromium co-doped ceramic laser medium has, in our opinion, revitalized the solar laser researches, revealing promising futures for solar-pumped lasers [10]. Researchers, including us, are now strongly motivated to build a powerful solar-pumped laser for the efficient reduction of magnesium (Mg) from magnesium oxide (MgO). Large amounts of heat and hydrogen are given off from the reaction of magnesium with water. Since there is the huge Mg resources of tons in the ocean, Mg can be an alternative to fossil fuel. But for a magnesium combustion engine to function as a practical source of energy, the lasers need to be powered by a renewable energy source, such as solar power. Highquality solar laser beam is crucial for tight focusing, indispensable for attaining the high-reduction temperature of 4000 K. Maximizing TEM 00 power and lowering the M 2 factors of solar laser beam are, therefore, our first research objective. An alternative light guide assembly two-dimensional ellipticalcylindrical (2D-EL-CYL) cavity pumping approach was proposed [11]. The parallel packing of the curved light guides hampers, however, further efficiency improvement. For maximizing TEM 00 solar laser power, the modified light guide assembly-elliptical cavity pumping approach is proposed here. It provides a natural way in both guiding and redistributing the highly concentrated solar radiation, transforming a circular focal spot into a nearrectangular shape light column. Due to the low propagation angles through the assembly, the divergence angles from the output end are reduced, ensuring consequently the efficient concentration of pump radiations within thin Nd:YAG rod by the sharp elliptical cavity. Non-sequential ray-tracing is firstly /$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi: /j.optlastec

3 688 D. Liang, R. Pereira / Optics & Laser Technology 41 (2009) performed to analyze the absorbed pump distributions of the Nd:YAG rods. Optimum pumping parameters are found by both ZEMAX TM non-sequential ray-tracing and LASCAD TM laser cavity analysis. Significant improvement in TEM 00 laser power is attained. The laser output performance of the modified approach is finally analyzed and compared to that of the parallel-packed assembly 2D-EL-CYL scheme. Both the enhanced transmission efficiency of the assembly and the improved focusing capacity of the sharp elliptical cavity with the intervening optics are finally tested. 2. Modified fused silica light guide assembly-elliptical cavity with the intervening optics 2.1. Primary-stage solar radiation collection and concentration A heliostat tracks the Sun continuously and reflects the solar radiation to a stationary primary parabolic mirror. The combined reflectivity of the two reflective mirrors are assumed as 76%. The primary mirror has 601 rim angle, 655 mm focal distance and 1.82 m 2 collection area. The semi-angle of is assumed for the circular source to simulate the incoming solar radiation in ZEMAX TM ray-tracing software. Considering 900 W/m 2 terrestrial solar irradiation, the concentrated light spot at the focal spot of the primary mirror reaches 1217 W. Since that only 14% of the useful solar power falls within the absorption band of the Nd:YAG material [2], 230 W is consequently assumed in ray-tracing software to simulate the effective solar pump power Modified fused silica light guide assembly Similar to the concept of the compact packing of 19 optical fibers with hexagonal input ends [12], the polished input columns of the fused silica rods can now be spatially packed together to form the near-hexagonal inward input face, as shown in Fig. 1. The Top-view Near-hexagonal input face Light guide assembly Concentrated focal spot concentrated solar power is firstly collected by the input face and then coupled to its near-rectangular output end, mounted along the first focal line of the sharp elliptical cavity. The laser crystal positioned along the second focal line can then be pumped when the input face is positioned to the focal point of the primary parabolic concentrator, the highly concentrated solar radiations are firstly collected through the input face and then guided to the output end. Due to the rotational symmetry around the central guide, the spatial angle a subtended by the central and each one of the external guides, as indicated in Fig. 1, is equal in amplitude and has strong influence on both the transmission efficiency and the output divergence angles from the assembly. As a consequence, angle a is studied in ZEMAX TM ray-tracing analysis. Each alteration in angle a implies redesigning the whole assembly. For 8 mm diameter (full width at half maximum (FWHM)) focal spot with 601 rim angle, a ¼ 381 is found to ensure the maximum transmission through the assembly. The parallel packing of square light guides corresponds to only a ¼ 01 spatial angle, resulting inevitably in low transmission efficiency. The absorption spectrum of the fused silica material is included to account for absorption losses, and all the fused silica light guides are modeled as uncoated in ZEMAX TM analysis. Optimized L/D ratio of 7.0 is also found. Compared to 76.9% transmission efficiency by the parallel-packed assembly, 83.6% is achieved by the modified assembly. The relationship between output laser power and input aperture D will be discussed accordingly in Section Sharp elliptical pump cavity with the intervening optics For the pump radiations from the modified assembly, 431 rim angle is measured in ZEMAX TM ray-tracing analysis. As compared to the 721 rim angle from the parallel-packed assembly, where only large compound 2D-EL-CYL cavity can be used to couple the pump radiation into the laser rod, the 431 rim angle permits using a sharp elliptical cavity. The transfer efficiency of the elliptical cavity and its dependency upon the cross-sectional dimension of the reflector, the radiant source, the laser rod, the wall reflectivity and the ellipse eccentricity were extensively discussed in early works [13 16]. The sharp elliptical cavity shown in Fig. 2 is composed of a two-dimensional elliptical reflector, closed by two Pump light distribution Y D α Modified light guide assembly X L Elliptical cavity Intervening optics Elliptical cavity Thin Nd:YAG rod Thin laser rod Intervening optics Water Flow tube Absorbed pump light distribution Fig. 1. Modified fused silica light guide assembly is coupled to the elliptical cavity with the intervening optics. Enlarged top view of the near-hexagonal input face is shown along with the concentrated focal spot. Fig. 2. Sharp elliptical pump cavity with the intervening optics is used to couple the pump radiations from the modified light guide assembly to the thin Nd:YAG rod.

4 D. Liang, R. Pereira / Optics & Laser Technology 41 (2009) end plates (not shown in Fig. 2). The elliptical cavity has a major semi-axis of a ¼ 20 mm and a minor semi-axis of b ¼ 15 mm. The elliptical cross-section is then extruded to 50 mm cavity length. For the easy installation of the assembly output end, an 8.5 mm slit is opened from the upper part of the cavity, as shown in Fig. 2. Direct light coupling from the output end to the rod is also accomplished through the intervening optics a two-dimensional fused silica biconic lens with 6.4 mm width, 2 mm thickness, 4 mm front surface curvature, 13 mm rear surface curvature and 50 mm length. To enhance the maximum pump light concentration within the rod, there exists a 2.1 mm gap between the intervening optics and the 9 mm diameter flow tube. The thin Nd:YAG rod is efficiently cooled by water. Hexagonal input face with focal spot Y X 15.4 mm 16.0 mm Pump light distribution 1.53W/mm Influence of tracking error on both the pump light and the absorbed pump light distributions 7mm 49 mm Due to its relatively large input face, the modified pumping approach diminishes the influence of tracking error on output laser power. For zero tracking error, the circular light spot is located at the central position of the input face. The pump light distribution from the output end has symmetrical shape and varies strongly along the near-rectangular output end. The absorbed pump light distribution along the laser rod has symmetrical shape also, as shown in Fig. 3a. For typical tracking errors of 1 mrad along both X- and Y-axis, which is equivalent to 1.13 mm dislocation along each axis, the focal spot is now positioned near the central and the upper-right light guides, resulting in shifted pump light distributions and slightly shifted absorbed pump light distributions, as indicated in Fig. 3b. Strong intensity variation along the output end is observed. The peak pump intensity is moved from the central guide to the upper-right guide. Since the laser rod receives the pump radiations from all the light guides, the absorbed pump light distribution reveals only slight variation along the rod axis. To achieve most favorable pump absorption within the laser medium, ZEMAX TM non-sequential ray-tracing is carried out to find the optimum mounting positions for the modified assembly, the laser rod and the intervening optics. Maximum absorption efficiency and optimum absorption distribution can subsequently be analyzed, where the best pump absorption distribution within the gain medium serve as our design criteria. All the optical components such as the intervening optics and the flow tube are modelled as uncoated. The pump cavity is smooth, specular and 95% reflectivity is assumed. The standard solar spectral intensity distribution and the known absorption spectrum of Nd:YAG material are included in the ray-tracing analysis. The gray-scale absorption distributions at central cross-section C, as indicated in Fig. 3a, of the 2.5 mm diameter Nd:YAG rod is shown in Fig. 4a. Black signifies near maximum absorption for these plots, whereas white signifies little or no absorption. Symmetric absorption distribution is achieved by the modified approach, concentrating more radiation into the central core region of the rod. The absorption profiles can also be evaluated by the absorbed flux/ volume distributions along both the central cross-section row and the central cross-section column. The absorbed flux peaks along the central core region of the rod, favoring the generation of TEM 00 laser power. For the tracking errors of 1 mrad along both Y- and Y-axis, the gray-scale absorption distribution in the central cross-section C, as indicated in Fig. 3b, of the 2.5 mm diameter rod is presented in Fig. 4b. Excellent absorbed flux/volume distributions are also observed. Only some very small peaks are noticed along both the central cross-section row and the central cross-section column. The peak absorption flux/volume values are, nevertheless, Φ Φ 7mm Y Absorbed pump light distribution 70mm 0.34W/mm 3 Hexagonal input face with shifted focal spot X 15.4mm noticeably reduced from to W/mm 3. The longitudinal shift of the highest absorbed pump flux/volume value along the rod, as shown in Fig. 3b, explains this reduction at 16.0 mm Shifted pump light distribution 49 mm Slightly shifted absorbed pump light distribution C 70mm 1.83W/mm W/mm 3 Fig. 3. Tracking error-dependent pump light distributions along the assembly output end and the absorbed pump light distributions along the thin laser rod. (Fig. 3a) 0 mrad tracking error, (Fig. 3b) 1 mrad tracking errors along both X- and Y-axis.

5 690 D. Liang, R. Pereira / Optics & Laser Technology 41 (2009) (W/mm 3 ) The optical resonator is comprised of two opposing planeparallel mirrors at right angles to the optical axis of the rod. One mirror is high reflection coated (HR, 99.98%) and the other output coupler is partial reflection (PR) coated. The laser output power is optimized by varying the output coupler reflectivity from 82% to 97%, according to different rod diameter. The lengths of both the elliptical cavity and laser crystal vary accordingly, giving rise to different round-trip losses for LASCAD analysis. If the D ¼ 16 mm input aperture assembly is considered, for example, then 50 mm elliptical cavity length is chosen. A total of 70 mm length is considered suitable for both pumping and fixing the 2.5 mm diameter rod, 4.8% round-trip loss is assumed in this case. For maximizing the TEM 00 laser power, the resonant laser cavity length of 470 mm is now used in LASCAD analyses. Fig. 5 shows the strong dependency of output laser power on rod diameter. The 2.5 mm diameter rod, pumped through the D ¼ 16 mm aperture assembly, produces the highest multimode laser power of W and also considerable TEM 00 power of 3.74 W. For the 1 mrad tracking errors along both X- and Y-axis, the calculated multimode laser power is reduced to 10.8 W, corresponding to 4.6% reduction. The tracking error compensation capacity of the proposed approach is thus ensured. The proposed approach features the excellent capacity in pumping small-diameter rod. Minimizing the laser rod volume reduces cost, and reducing the diameter makes the rod more resistant to thermal stress. Also, with small rod diameter, highorder resonator modes are suppressed by large diffraction losses and TEM 00 power and beam quality are increased. For the 2.5 mm diameter rod, M 2 x ¼ 9.63, M 2 y ¼ 9.96 are calculated in LASCAD analysis, indicating a near-symmetric laser beam profile. TEM 00 laser power is gradually increased with the reduction of aperture D, from 22 to 14 mm, as verified in Fig. 5. If TEM 00 power is the major concern, then pumping the 1.5 mm 00 (W/mm 3 ) 12 Fig. 4. Pump absorption distributions of 2.5 mm diameter Nd:YAG rod. (Fig. 4a) 0 mrad tracking error, (Fig. 4b) 1 mrad tracking errors along both X- and Y-axis. the central cross-section C. The reduction is also compensated by the appearance of other strong flux/volume values along the rod. As the laser power depends upon the accumulated contributions of all the absorbed pump powers along the rod, the influence of tracking error on laser power is, therefore, largely attenuated. 3. Analysis of solar laser output powers 3.1. Solar laser output power and beam quality for the proposed pumping approach The input aperture D of the near-hexagonal input face, as indicated in Fig. 1, is the key design parameter for characterizing the assemblies of different dimensions. For the aperture D varying from 14 to 22 mm, the optimized spatial angle of 381 and the L/D ratio of 7.0 remain unaltered for all the scaled assemblies designed in AUTOCAD program. In ray-tracing analysis, the cylindrical laser medium is divided into a total of 12,000 zones. The path length in each intercepted zone is found. With this value and the effective absorption coefficient of 1.1% Nd:YAG material, the absorbed power within the cylindrical volume can be calculated by summing up the absorbed pump radiations of all zones. The ZEMAX TM non-sequential ray-tracing data is finally analyzed with LASCAD TM software to find the laser output powers. Solar laser output power (W) 10 D14 M-M D14 TEMoo 8 D16 M-M D16 TEMoo D18 M-M D18 TEMoo 6 D20 M-M D20 TEMoo D22 M-M D22 TEMoo Rod diameter (mm) Fig. 5. Output solar laser powers for different rod diameters and input apertures.

6 D. Liang, R. Pereira / Optics & Laser Technology 41 (2009) diameter rod through the D ¼ 14 mm aperture assembly provides the best solution, attaining the highest TEM 00 laser power of 4.8 W, corresponding to 60% of total multimode power of 8W. Low M 2 factors of M x 2 ¼ 3.76, M y 2 ¼ 3.82 are also calculated, indicating a superb laser beam profile Comparison of the laser powers from the proposed and the parallel-packed pumping approaches For correct comparison, the solar laser powers from the parallelpacked assembly 2D-EL-CYL cavity are re-calculated by assuming also the 470 mm resonant cavity length. The laser rods of different diameters, varying between 1 and 5 mm, incrementing at 0.5 mm interval, are re-analyzed in LASCAD analysis. The calculated laser output powers are shown in Fig. 6. Compared with the laser performance of the parallel-packed assembly 2D-EL-CYL cavity, the proposed approach provides a general improvement in both multimode and TEM 00 laserpowersfortherodsofalldiameters.whenthe 1.5 mm diameter Nd:YAG rod is pumped by the proposed approach, the highest TEM 00 laser power is achieved, corresponding, respectively, to 45% improvement in TEM 00 and 10% in multimode laser powers. Pumping the 2.5 mm diameter rod through the D ¼ 16 mm aperture assembly, on the other hand, generates the highest multimode power, corresponding, respectively, to 29% improvement in TEM 00 and 10.7% in multimode laser power. Due to the excellent capacity in pumping small-diameter rod, the M 2 factors of the laser beam from the sharp elliptical cavity are obviously reduced, when compared to that from the 2D-EL-CYL cavity, where only rod of slightly larger diameter can be efficiently pumped. 4. Experimental results 4.1. Test of the modified fused silica light guide assembly To validate the proposed pumping scheme, the modified fused silica light guide assembly with the D ¼ 16 mm input aperture Solar laser output power (W) Modified TEM 00 Parallel TEM 00 Parallel M-M Rod diameter (mm) Modified M-M Fig. 6. Multimode and TEM 00 laser power enhancement by the modified pumping approach. Laser powers from the parallel-packed assembly 2D-EL-CYL pumping scheme are also provided. was built. A total of 6.0 mm diameter fused silica rods were used as the raw material for manufacturing the curved light guide. The input side face of the central fused silica rod was firstly polished so as to form a hexagonal input column. No extra bending was necessary. The input side faces of the other six external fused silica rods were firstly polished, each on three sides. High-quality grinding and polishing were essential to ensure the maximum light transfer. The polished rods were then bent to their designated curvatures within large hydrogen flames of more than C. Pure graphite moulds were used to help the correct bending of the light guides [17]. When the input face of the assembly was positioned to the focal point of the primary parabolic mirror, the highly concentrated solar radiation of 1210 W at the focal spot was firstly collected by the nearhexagonal input face and then guided to the near-rectangular output end of the assembly. A total of 980 W solar power was measured, corresponding to the transmission efficiency of 81%, 2.6% less than the ZEMAX simulations. The output rim angle of 451 was measured, slightly larger than the 431 angle from the previous ZEMAX analysis. The imperfections in high temperature bending of the fused silica light guides were considered to be the main reason for the discrepancies. The output light distribution from the assembly matched well with the ZEMAX analysis. Strong output distribution variation from the assembly were also observed when the focal spot was not accurately aligned with the input face of the assembly Test of the focusing capacity of the sharp-elliptical cavity with intervening optics To prove the excellent focusing capacity of the sharp elliptical cavity with the intervening optics, a gold-plated elliptical pump cavity used in lamp-pumped laser was modified. It had a major semi-axis of a ¼ 18 mm, a minor semi-axis of b ¼ 15 mm and a cavity length of 50 mm, as shown in Fig. 7. Seven 6.1 mm diameter holes were drilled to allow the installation of the assembly output end. A Nd:YAG rod of 3 mm diameter and 70 mm length was cooled by water within a 9 mm diameter flow tube. To provide optimized absorption profile within the elliptical cavity, an intervening optics with 7 mm width, 2.9 mm thickness, 4.3 mm front surface curvature, 5.0 mm rear surface curvature and 50 mm length was firstly optimized in the ZEMAX program and then machined from a fused silica rod with 8.6 mm diameter and 50 mm length. The intervening optics was then installed 2.8 mm above the flow tube. The excellent focusing capacity was demonstrated by Fig. 7, where the sharp focusing image within the flow tube with cooling water, but without the rod, was clearly observed. The validity of the proposed pumping approach is, therefore, confirmed by the above experiments. Detailed results about the output solar laser power and the beam quality measurement are expected in the future. 5. Conclusions For maximizing TEM 00 solar laser power, the modified fused silica light guide assembly is proposed to pump the thin Nd:YAG laser rod within the sharp elliptical cavity. By reducing largely the transmission angles through the modified assembly, the pump radiations can be efficiently coupled into the rod. The superior radiation coupling capacity of the spatially packed assembly is combined with the excellent focusing properties of both the sharp-elliptical cavity and the intervening optics. Enhanced pump absorption distribution within the central core region of the laser rod is therefore obtained. Optimum pumping parameters are found through the ZEMAX TM non-sequential ray-tracing and

7 692 D. Liang, R. Pereira / Optics & Laser Technology 41 (2009) Elliptical pump cavity Light guide assembly output end Intervening optics improvements are valid for the rods of all diameters. Due to the small-diameter rod used, low M 2 factors laser beam is obtained. This improvement is very important for tight focusing, essential for reaching the high temperature for the future reduction of magnesium from magnesium oxide by solar-pumped laser. Highquality laser beam is also fundamental for space applications. A prototype fused silica light guide assembly was finally built to test its enhanced transmission efficiency. The lamp-pumped cavity was also modified to adapt the proposed pumping scheme. The excellent focusing capacity of the sharp elliptical cavity with the intervening optics was finally confirmed. The proposed pumping approach provides, in our opinion, an effective solution for improving both the beam quality and conversion efficiency of solar-pumped lasers. 3mm Nd:YAG rod References Sharp focusing image without rod 9mm flow tube Cooling water Fig. 7. Excellent focusing capacity of the adapted elliptical pump cavity with the intervening optics and the output end of the assembly. The sharp focusing image without the Nd:YAG rod is also included. LASCAD TM laser cavity analysis. Compared with the parallelpacked 2D-EL-CYL scheme, the superiority of the modified approach is apparent, as indicated in Fig. 6. The general efficiency [1] Young CG. Appl Opt 1966;5(6):993. [2] Weksler M, Shwartz J. IEEE J Quantum Electron 1988;24(6):1222. [3] Cooke D. Appl Opt 1992;31:7541. [4] Benmair R, Kagan J, Kalisky Y, Noter Y, Oron M, Shimony Y, et al. Opt Lett 1990;15:36. [5] Arashi H, Oka Y, Sasahara N, Kaimai A, Ishigame M. Jpn J Appl Phys 1984;23(8):1051. [6] Brauch U, Muckenschnabel J, Opower H, Wittner W. Space Power 1991;10:285. [7] Duchet M, Cabaret L, Laurens A, de Miscault JC. Space Power 1992;11:241. [8] Krupa T. Opt Photonics News 2002;13(1):9. [9] Lando M, Kagan J, Linyekin B, Dobrusin V. Opt Commun 2003;222:371. [10] Yabe T, Ohkubo T, Uchida S, Yoshida K, Nakatsuka M, Funatsu T, et al. Appl Phys Lett 2007;90: [11] Geraldes JP, Liang D. Sol Energy Mater Sol Cells 2008;92:836. [12] Liang D, Monteiro LF, Teixeira MR, Monteiro ML, Collares-Pereira M. Sol Energy Mater Sol Cells 1998;54:323. [13] Kamiryo K, Kano T, Matsuzawa H. Jpn J Appl Phys 1966;5(12):1217. [14] Schultz SB, Aagard RL. Appl Opt 1963;2(5):509. [15] Bowness C. Appl Opt 1965;4:103. [16] Koechner W. 4th ed. Solid-state laser engineering, vol. 1. Berlin, Germany: Springer Optical Sciences; [6, 378]. [17] Bernardes PH, Liang D. Appl Opt 2006;45(16):3811.