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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 21 — Oct. 21, 2013
  • pp: 25107–25112
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Solar-Pumped TEM00 Mode Nd:YAG laser

Dawei Liang and Joana Almeida  »View Author Affiliations


Optics Express, Vol. 21, Issue 21, pp. 25107-25112 (2013)
http://dx.doi.org/10.1364/OE.21.025107


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Abstract

Here we show a significant advance in solar-pumped laser beam brightness by utilizing a 1.0 m diameter Fresnel lens and a 3 mm diameter Nd:YAG single-crystal rod. The incoming solar radiation is firstly focused by the Fresnel lens on a solar tracker. A large aspheric lens and a 2D-CPC concentrator are then combined to further compress the concentrated solar radiation along the thin laser rod within a V-shaped pumping cavity. 2.3 W cw TEM00 (M2 ≤ 1.1) solar laser power is finally produced, attaining 1.9 W laser beam brightness figure of merit, which is 6.6 times higher than the previous record. For multimode operation, 8.1 W cw laser power is produced, corresponding to 143% enhancement in collection efficiency.

© 2013 Optical Society of America

1. Introduction

The conversion of sunlight into laser light by direct solar pumping is of ever-increasing importance because broadband, temporally constant, sunlight is converted into laser light, which can be a source of narrowband, collimated, rapidly pulsed, radiation with the possibility of obtaining extremely high brightness and intensity. Nonlinear processes, such as harmonic generation, might be used to obtain broad wavelength coverage, including the ultraviolet wavelengths, where the solar flux is very weak. The direct excitation of large lasers by sunlight offers the prospect of a drastic reduction in the cost of coherent optical radiation for high average power applications. Up to now solar pumped lasers have not been viewed from this perspective. Among the potential applications of solar lasers are earth, ocean, and atmospheric sensing; detecting, illuminating, and tracking hard targets in space; deep space communications. Renewable solar lasers are also very promising for many terrestrial applications. The first solar-pumped solid-state laser was reported by Young in 1966 [1

1. C. G. Young, “A sun-pumped cw one-watt laser,” Appl. Opt. 5(6), 993–997 (1966). [CrossRef] [PubMed]

]. Since then, researchers have been improving constantly the solar laser performance [2

2. H. Arashi, Y. Oka, N. Sasahara, A. Kaimai, and M. Ishigame, “A solar-pumped cw 18 W Nd:YAG laser,” Jpn. J. Appl. Phys. 23(Part 1, No. 8), 1051–1053 (1984). [CrossRef]

10

10. D. Fang, X. H. Wang, F. Wang, and S. Liu, “Highly efficient side-pumped solar laser,” in 2012 International Conference on Optoelectronics and Microelectronics (IEEE, 2012), 95–98. [CrossRef]

]. The most-widely used Nd:YAG single-crystal material has been demonstrated as an excellent material under solar pumping because of its superior thermal conductivity, high quantum efficiency and mechanical strength characteristics compared to other host materials [6

6. D. Liang and J. Almeida, “Highly efficient solar-pumped Nd:YAG laser,” Opt. Express 19(27), 26399–26405 (2011). [CrossRef] [PubMed]

10

10. D. Fang, X. H. Wang, F. Wang, and S. Liu, “Highly efficient side-pumped solar laser,” in 2012 International Conference on Optoelectronics and Microelectronics (IEEE, 2012), 95–98. [CrossRef]

]. Significant solar laser collection efficiencies have been achieved by end-pumping small diameter Nd:YAG single-crystal rods [6

6. D. Liang and J. Almeida, “Highly efficient solar-pumped Nd:YAG laser,” Opt. Express 19(27), 26399–26405 (2011). [CrossRef] [PubMed]

]. The most recent Nd:YAG solar laser has produced the record collection efficiency of 30.0 W/m2 with a large diameter Nd:YAG rod [7

7. T. H. Dinh, T. Ohkubo, T. Yabe, and H. Kuboyama, “120 watt continuous wave solar-pumped laser with a liquid light-guide lens and an Nd:YAG rod,” Opt. Lett. 37(13), 2670–2672 (2012). [CrossRef] [PubMed]

]. However, brightness is one of the most important parameters of a laser beam. It is given by the laser power divided by the product of the beam spot area and its solid angle divergence. This product is proportional to the square of beam quality factor M2. Brightness figure of merit is thus defined [4

4. M. Lando, J. Kagan, B. Linyekin, and V. Dobrusin, “A solar-pumped Nd:YAG laser in the high collection efficiency regime,” Opt. Commun. 222(1-6), 371–381 (2003). [CrossRef]

] as the ratio between laser power and the product of Mx2 and My2. The highest brightness figure of merit of an end-pumped solar laser with Fresnel lens is 0.086 W [6

6. D. Liang and J. Almeida, “Highly efficient solar-pumped Nd:YAG laser,” Opt. Express 19(27), 26399–26405 (2011). [CrossRef] [PubMed]

]. Despite the successful production of 120 W cw solar laser power, very large beam quality factors, Mx2 = My2 = 137, have also been measured [7

7. T. H. Dinh, T. Ohkubo, T. Yabe, and H. Kuboyama, “120 watt continuous wave solar-pumped laser with a liquid light-guide lens and an Nd:YAG rod,” Opt. Lett. 37(13), 2670–2672 (2012). [CrossRef] [PubMed]

], resulting in only 0.0064 W brightness figure of merit. Although the most efficient laser systems have end-pumping approaches, side-pumping is an effective configuration for power scaling as it gives uniform absorption along the rod axis and spreads the absorbed power along the laser rod, reducing the associated thermal loading problems. Besides, the free access to both rod ends allows the optimization of more laser resonator parameters, improving largely laser beam quality. The record brightness figure of merit of 0.29 W has been achieved by side-pumping a 4 mm diameter, 30 mm length Nd:YAG rod, by a heliostat-parabolic mirror solar energy collection and concentration system [8

8. J. Almeida, D. Liang, and E. Guillot, “Improvement in solar-pumped Nd:YAG laser beam brightness,” Opt. Laser Technol. 44(7), 2115–2119 (2012). [CrossRef]

].

2. TEM00 mode Nd:YAG solar laser pumping approach

Fig. 1 (a) The solar-pumped TEM00 mode Nd:YAG laser system (b) The laser head.
As shown in Fig. 1, the solar-pumped TEM00 mode Nd:YAG laser system is composed of the first-stage 1.0 m diameter Fresnel lens, the laser head and its associated asymmetric optical resonator, all mounted on a two-axis solar tracker that follows automatically the Sun’s movement. The two-axis solar tracker is supplied by Shandong Huayi Sunlight Solar Energy Industry Co., Ltd. The Fresnel lens is supplied by Shandong Yuying Optical Instruments Co., Ltd. The Fresnel lens has 1.3 m focal length. The measured transmission efficiency is 76%. The concentrated solar power at the focal spot averaged over 2 min is 590 W for 890 W/m2 solar irradiance. The measured full width at half maximum is 13 mm at the minimum focal waist. Mechanical adjustments in X-Y-Z directions allow an easy alignment in the focal zone. Coarse angular adjustments are also very helpful for achieving the maximum laser output power.

The laser head is composed of the second-stage large fused silica aspheric lens, the third-stage 2D-CPC concentrator and the V-shaped pumping cavity within which the Nd:YAG rod is mounted. The large curved input face of the aspheric lens, the dimensions of the 2D-CPC concentrator is designed to compress the concentrated solar radiations from the focal zone onto the thin laser rod. To manufacture the large aspheric lens, an 85 mm diameter, 50 mm length fused silica rod of 99.999% optical purity from Beijing Kinglass Quartz Co., Ltd. is ground and polished to its final dimensions. The aspheric input face has 43 mm radius and r2 rear parameter of −0.0005. The third-stage 2D-CPC concentrator has 18 mm × 23 mm large rectangular input aperture, 8 mm × 23 mm narrow rectangular output aperture and is 10 mm in height, as shown in both Fig. 1(b) and Fig. 2.
Fig. 2 Detailed 3D view of the solar laser head.
The 2D-CPC is used to convert the rays from 18 mm × 23 mm large-aperture emitting into a small angle, 25°for example, to 8 mm × 23 mm small-aperture emitting into a large angle, 70° for example, thus the source étendue is preserved. This preservation implies that irradiance is larger at output aperture than at the entrance aperture, leading to a net concentration of the pump radiation [11

11. W. T. Welford and R. Winston, High Collection Nonimaging Optics (Academic Press, 1989).

,12

12. J. Almeida and D. Liang, “Design of a high-brightness solar-pumped laser by light-guides,” Opt. Commun. 285(24), 5327–5333 (2012). [CrossRef]

]. The V-shaped cavity is finally used to achieve an efficient multi-pass absorption of the highly concentrated pump radiation from the 8 mm × 23 mm output aperture. The inner walls of both the 2D-CPC hollow concentrator and the V-shaped pumping cavity are bonded with a protected silver-coated aluminum foil with 94% reflectivity. Distilled water with 6 Liter / min flow rate cools firstly the rod within the V-shaped cavity, then passes through the 2D-CPC concentrator and exits the laser head, as illustrated by Fig. 2. All the above optimized design parameters of the whole optical system are found by both non-sequential ray-tracing (ZEMAXTM) and laser cavity design and analysis (LASCADTM) codes.

3. Solar laser oscillation experiments

3.1 Multimode solar laser oscillation with a symmetric optical resonator

Brightness figure of merit=PLaser/Mx2My2
(3)

3.2 Fundamental mode solar laser oscillation with an asymmetric optical resonator

For the solar irradiance of 890 W/m2 in Lisbon area in August 2013, the 1.0 m diameter Fresnel lens collects about 550 W solar powers to its focal zone. The same 3 mm diameter, 30 mm length Nd:YAG rod is used to test its laser output performance of the asymmetric optical resonator, as shown in Fig. 1. It is comprised of two opposing concave-concave mirrors at right angles to the optical axis of the rod. The rear mirror is high reflection coated (HR, 99.8%), while the output mirror is partial reflection coated (PR, 94%). The output mirror is fixed at L2 = 100 mm from the center of the laser rod, while the HR rear mirror can be positioned at L1 = 100 mm to 675 mm from the center. The −5 m RoC rear mirror at L1 = 675 mm and the −5 m RoC output mirror at L2 = 100 mm combination provides the most favorable TEM00 solar laser beam profile, as shown in Fig. 4. Usually L2 is fixed at 100 mm, the laser output powers and beam profiles for different L1 (L1 = 100 mm, 200 mm, 300 mm, 400 mm, 500 mm, 600 mm, 625 mm, 650mm, 675 mm respectively) are firstly analyzed by LASCAD© software and then confirmed by detailed measurements during the first 20 days of August, 2013, as given by Fig. 3.
Fig. 3 Dependence of (a) laser power, (b) M2 factor and (c) brightness figure of merit on resonator length L1 is measured.

The laser output power decreases with the increase in L1. The M2 factor also decreases with the increase in L1. The beam brightness figure of merit increases nearly linearly with the increase in L1. On the one hand, 6.2 W solar laser power, M2 = 4.8 are obtained for L1 = 100 mm, resulting in 0.27 W brightness figure of merit; on the other hand, 2.3 W TEM00 (M2 ≤ 1.1) fundamental mode laser power is finally measured for L1 = 675 mm, corresponding to the highest brightness figure of merit of 1.9 W.

Plots of the measured near-field laser beam output pattern (40 mm from the output coupler) are shown in Fig. 4.
Fig. 4 Measured output laser 2D (a), 3D (b) beam profiles, 40 mm away from the output coupler.
A close examination of the TEM00 mode shows that it differs slightly in laser beam diameters at 1/e2 along both X and Y axis. The experimental TEM00 mode is slightly elliptical with axes 3% and 9% larger than the mode diameter of the empty resonator Gaussian mode. The ellipticity is caused by the slight change in absorbed pump distribution within the laser rod, caused by the slight misalignment between the laser head and the optimized focal position of the Fresnel lens. The measured average near-field laser beam diameter is 1.44 mm, as indicated by Fig. 4.

The far-field output pattern (6000 mm from the output coupler) represents also a good TEM00 Gaussian mode profile. The measured average laser beam diameter at 1/e2 width at the far-field is 8.15 mm. Since L = 5960 mm, M2 factor can be easily determined as 1.09 by using the Eq. (1)-(3). Considering both the solar tracking error and the laser head misalignment error during the laser emission, M2 ≤ 1.1 is finally considered as the final value of our solar laser beam quality factor.

4. Conclusions

Acknowledgments

These research projects were funded by the Science and Technology Foundation of Portuguese Ministry of Science (PTDC/FIS/103599/2008 and PTDC/FIS/122420/2010), Technology and Higher Education (FCT-MCTES).

References and links

1.

C. G. Young, “A sun-pumped cw one-watt laser,” Appl. Opt. 5(6), 993–997 (1966). [CrossRef] [PubMed]

2.

H. Arashi, Y. Oka, N. Sasahara, A. Kaimai, and M. Ishigame, “A solar-pumped cw 18 W Nd:YAG laser,” Jpn. J. Appl. Phys. 23(Part 1, No. 8), 1051–1053 (1984). [CrossRef]

3.

M. Weksler and J. Shwartz, “Solar-pumped solid-state lasers,” IEEE J. Quantum Electron. 24(6), 1222–1228 (1988). [CrossRef]

4.

M. Lando, J. Kagan, B. Linyekin, and V. Dobrusin, “A solar-pumped Nd:YAG laser in the high collection efficiency regime,” Opt. Commun. 222(1-6), 371–381 (2003). [CrossRef]

5.

T. Yabe, T. Ohkubo, S. Uchida, K. Yoshida, M. Nakatsuka, T. Funatsu, A. Mabuti, A. Oyama, K. Nakagawa, T. Oishi, K. Daito, B. Behgol, Y. Nakayama, M. Yoshida, S. Motokoshi, Y. Sato, and C. Baasandash, “High-efficiency and economical solar-energy-pumped laser with Fresnel lens and chromium co-doped laser medium,” Appl. Phys. Lett. 90(26), 261120 (2007). [CrossRef]

6.

D. Liang and J. Almeida, “Highly efficient solar-pumped Nd:YAG laser,” Opt. Express 19(27), 26399–26405 (2011). [CrossRef] [PubMed]

7.

T. H. Dinh, T. Ohkubo, T. Yabe, and H. Kuboyama, “120 watt continuous wave solar-pumped laser with a liquid light-guide lens and an Nd:YAG rod,” Opt. Lett. 37(13), 2670–2672 (2012). [CrossRef] [PubMed]

8.

J. Almeida, D. Liang, and E. Guillot, “Improvement in solar-pumped Nd:YAG laser beam brightness,” Opt. Laser Technol. 44(7), 2115–2119 (2012). [CrossRef]

9.

D. Liang and J. Almeida, “Design of ultrahigh brightness solar-pumped disk laser,” Appl. Opt. 51(26), 6382–6388 (2012). [CrossRef] [PubMed]

10.

D. Fang, X. H. Wang, F. Wang, and S. Liu, “Highly efficient side-pumped solar laser,” in 2012 International Conference on Optoelectronics and Microelectronics (IEEE, 2012), 95–98. [CrossRef]

11.

W. T. Welford and R. Winston, High Collection Nonimaging Optics (Academic Press, 1989).

12.

J. Almeida and D. Liang, “Design of a high-brightness solar-pumped laser by light-guides,” Opt. Commun. 285(24), 5327–5333 (2012). [CrossRef]

OCIS Codes
(140.0140) Lasers and laser optics : Lasers and laser optics
(140.3410) Lasers and laser optics : Laser resonators
(140.3530) Lasers and laser optics : Lasers, neodymium
(140.3570) Lasers and laser optics : Lasers, single-mode

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: August 22, 2013
Revised Manuscript: September 20, 2013
Manuscript Accepted: September 22, 2013
Published: October 14, 2013

Citation
Dawei Liang and Joana Almeida, "Solar-Pumped TEM00 Mode Nd:YAG laser," Opt. Express 21, 25107-25112 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-21-25107


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References

  1. C. G. Young, “A sun-pumped cw one-watt laser,” Appl. Opt.5(6), 993–997 (1966). [CrossRef] [PubMed]
  2. H. Arashi, Y. Oka, N. Sasahara, A. Kaimai, and M. Ishigame, “A solar-pumped cw 18 W Nd:YAG laser,” Jpn. J. Appl. Phys.23(Part 1, No. 8), 1051–1053 (1984). [CrossRef]
  3. M. Weksler and J. Shwartz, “Solar-pumped solid-state lasers,” IEEE J. Quantum Electron.24(6), 1222–1228 (1988). [CrossRef]
  4. M. Lando, J. Kagan, B. Linyekin, and V. Dobrusin, “A solar-pumped Nd:YAG laser in the high collection efficiency regime,” Opt. Commun.222(1-6), 371–381 (2003). [CrossRef]
  5. T. Yabe, T. Ohkubo, S. Uchida, K. Yoshida, M. Nakatsuka, T. Funatsu, A. Mabuti, A. Oyama, K. Nakagawa, T. Oishi, K. Daito, B. Behgol, Y. Nakayama, M. Yoshida, S. Motokoshi, Y. Sato, and C. Baasandash, “High-efficiency and economical solar-energy-pumped laser with Fresnel lens and chromium co-doped laser medium,” Appl. Phys. Lett.90(26), 261120 (2007). [CrossRef]
  6. D. Liang and J. Almeida, “Highly efficient solar-pumped Nd:YAG laser,” Opt. Express19(27), 26399–26405 (2011). [CrossRef] [PubMed]
  7. T. H. Dinh, T. Ohkubo, T. Yabe, and H. Kuboyama, “120 watt continuous wave solar-pumped laser with a liquid light-guide lens and an Nd:YAG rod,” Opt. Lett.37(13), 2670–2672 (2012). [CrossRef] [PubMed]
  8. J. Almeida, D. Liang, and E. Guillot, “Improvement in solar-pumped Nd:YAG laser beam brightness,” Opt. Laser Technol.44(7), 2115–2119 (2012). [CrossRef]
  9. D. Liang and J. Almeida, “Design of ultrahigh brightness solar-pumped disk laser,” Appl. Opt.51(26), 6382–6388 (2012). [CrossRef] [PubMed]
  10. D. Fang, X. H. Wang, F. Wang, and S. Liu, “Highly efficient side-pumped solar laser,” in 2012 International Conference on Optoelectronics and Microelectronics (IEEE, 2012), 95–98. [CrossRef]
  11. W. T. Welford and R. Winston, High Collection Nonimaging Optics (Academic Press, 1989).
  12. J. Almeida and D. Liang, “Design of a high-brightness solar-pumped laser by light-guides,” Opt. Commun.285(24), 5327–5333 (2012). [CrossRef]

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