Design of ultrahigh brightness solar-pumped disk laser

doi:10.1364/AO.51.006382

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Design of ultrahigh brightness solar-pumped disk laser

Dawei Liang and Joana Almeida »View Author Affiliations

Applied Optics, Vol. 51, Issue 26, pp. 6382-6388 (2012)
http://dx.doi.org/10.1364/AO.51.006382

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▸ Article Outline
– Abstract
Introduction
Description of Ultrahigh Brightness Nd:YAG Disk Laser Pumping Scheme
Numerical Analysis of Laser Output Performances of Single-Crystal Nd:YAG Disks
Numerical Analysis of Laser Output Performances of Core-Doped Nd:YAG Ceramic Disks and its Comparison to Single-Crystal Nd:YAG Disks
Conclusions
– References and links

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Abstract

To significantly improve the solar-pumped laser beam brightness, a multi-Fresnel lens scheme is proposed for side-pumping either a single-crystal Nd:YAG or a core-doped ceramic ${Sm}^{3 +}$ Nd:YAG disk. Optimum laser system parameters are found through ZEMAX and LASCAD numerical analysis. An ultrahigh laser beam figure of merit $B$ of 53 W is numerically calculated, corresponding to a significant enhancement of more than 180 times over the previous record. $17.7 W / m^{2}$ collection efficiency is also numerically attained. The strong thermal effects that have hampered present-day rod-type solar-pumped lasers can also be largely alleviated.

1. Introduction

Solar-pumped lasers have gained an ever-increasing importance in recent years [1

D. Graham-Rowe, “Solar-powered lasers,” Nat. Photonics 4, 64–65 (2010). [CrossRef]

]. Compared to electrically powered lasers, solar laser is much simpler and more reliable due to the complete elimination of the electrical power generation and conditioning equipment. This technology has a large potential for many applications, e.g., high-temperature materials processing, renewable magnesium-hydrogen energy cycle, free space laser communications, space to earth power transmission, and so on. Highly efficient solar-pumped laser with ultrahigh brightness thus become essential to the success of these applications.

The first solar-pumped laser was reported by Young in 1966 [2

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

]. Since then, researchers have been exploiting parabolic mirrors and Fresnel lenses to attain enough concentrated solar radiation at focal point, and several pumping schemes have been proposed for enhancing solar laser output performance [3

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, 1051–1053 (1984). [CrossRef]

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

V. Krupkin, J. A. Kagan, and A. Yogev, “Nonimaging optics and solar laser pumping at the Weizmann Institute,” Proc. SPIE 2016, 50–60 (1993). [CrossRef]

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

T. Yabe, S. Uchida, K. Ikuta, K. Yoshida, C. Baasandash, M. S. Mohamed, Y. Sakurai, Y. Ogata, M. Tuji, Y. Mori, Y. Satoh, T. Ohkubo, M. Murahara, A. Ikesue, M. Nakatsuka, T. Saiki, S. Motokoshi, and C. Yamanaka, “Demonstrated fossil-fuel-free energy cycle using magnesium and laser,” Appl. Phys. Lett. 89, 261107 (2006). [CrossRef]

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, 261120 (2007). [CrossRef]

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

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

–11

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

]. The progress with Fresnel lenses and chromium codoped Cr:Nd:YAG ceramic laser medium [8

] has revitalized solar laser researches.

19.3 W / m^{2}

collection efficiency has been reported in 2011 [9

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

] by utilizing an economical Fresnel lens and the most widely used Nd:YAG single-crystal rod. The most recent solar-pumped laser with a liquid light-guide lens and a Nd:YAG rod has produced

30.0 W / m^{2}

collection efficiency in 2012 [10

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

Ultrahigh brightness renewable solar laser beams can be focused to heat magnesium oxide (MgO) to more than 4000 K and thus create pure magnesium [8

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

]. Magnesium can be easily stored and transported in the form of “pellets” and, when necessary, reacts with water to produce both hydrogen and thermal energy for fuel cell vehicle applications. Laser beam brightness 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 the beam quality factor

M^{2}

. The brightness figure of merit

B

is then defined [6

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

] as the ratio between laser power and the product of

M_{x}^{2}

and

M_{y}^{2}

. Most recently, a large improvement in solar-pumped laser beam brightness has been achieved by side-pumping of a thin Nd:YAG single-crystal rod. A record-high figure of merit

B

of 0.29 W has been registered [11

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

In rod geometries, however, the heat is removed on the circumferential surface of the cylinder, thereby generating a radial thermal gradient [12

M. Frede, R. Wilhelm, M. Brendel, C. Fallnich, F. Seifert, B. Willke, and K. Danzmann, “High power fundamental mode Nd:YAG laser with efficient birefringence compensation,” Opt. Express 12, 3581–3859 (2004). [CrossRef]

,13

W. Koechner, “Thermal lensing in Nd:YAG laser rod,” Appl. Opt. 9, 2548–2553 (1970). [CrossRef]

]. The change in temperature within a laser rod causes a thermal distortion of the laser beam. This imposes a limit to the extraction efficiency and energy scaling of lasers. The thin-disk laser concept, initially developed for diode-pumped laser systems [14

A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58, 365–372 (1994). [CrossRef]

,15

A. Giesen and J. Speiser, “Fifteen years of work on thin-disk lasers: results and scaling laws,” IEEE J. Sel. Top. Quantum Electron. 13, 598–609 (2007). [CrossRef]

], is one of the most suitable approaches when high power, high efficiency, and good beam quality are required simultaneously. Due to small volume-to-surface-area ratio, the gain medium can be cooled very efficiently. The direction of the heat flow is hence mainly parallel to the laser cavity axis, which in combination with short optical path length through the active medium, results in a reduction of the thermal lensing effect and thermally induced aberrations by orders of magnitude compared to typical high-power rod lasers. Valuable experiences have been gained by our research team in building a fiber optic pump beam shaping system for the high-power thin-disk laser [16

R. Pereira, B. Weichelt, D. Liang, P. Morais, H. Gouveia, M. Abdou-Ahmed, A. Voss, and T. Graf “Efficient pump beam shaping for high-power thin-disk laser systems,” Appl. Opt. 49, 5157–5162 (2010). [CrossRef]

]. We therefore propose a new concept of applying thin-disk laser technology to solar-pumped lasers in this paper. It can expand significantly the frontier of solar laser beam brightness, while alleviating the thermal management problems that have hampered solar-pumped lasers. Solar energy collection and concentration is achieved through the combination of plane mirrors and Fresnel lenses, symmetrically aligned around the Nd:YAG laser disk. The concentrated solar radiation from each Fresnel lens is then focused to the lateral face of the disk through a toroidal fused silica lens. Optimum pumping conditions and solar laser beam parameters are found through ZEMAX and LASCAD numerical analysis, respectively, for different Nd:YAG single-crystal disks. The solar laser performances of core-doped ceramic Nd:YAG disks with

{Sm}^{3 +}

-doped YAG cladding isstudied and compared to that of single-crystal disks. 53 W figure of merit

B

is numerically calculated, which surpasses the record-high figure of merit

B

for solar laser [11

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

] more than 180 times, and the strong thermal effects that have hampered the progress of rod-type solar-pumped laser can be drastically alleviated.

2. Description of Ultrahigh Brightness Nd:YAG Disk Laser Pumping Scheme

The solar-pumped disk laser scheme in Fig. 1 is composed of 12 Fresnel lenses (

F

) and 12 plane mirrors (

M

), radially mounted for side-pumping of the Nd:YAG disk.

Fig. 1. (a) Multi plane mirror—Fresnel lens scheme for solar-pumped disk laser. (b) Simplified side-view of the solar light collection and concentration from two symmetric plane mirrors—Fresnel lenses.

The Fresnel lenses are evenly distributed along the 0.8 m radius circumference around the center of the laser disk. The formation has 12-fold symmetry. To redirect the incoming solar radiation towards the Fresnel lenses, each plane mirror is mounted beside the Fresnel lens at a 45° angle, as shown in Fig. 1. Each Fresnel lens has

0.4 m \times 0.4 m

area and 0.8 m focal length. It is made of Polymethyl Methacrylate (PMMA) material, which is transparent at visible and near infrared wavelengths, but absorbs the infrared radiation beyond 2200 nm and cuts undesirable UV solar radiation below 350 nm. Solar tracking can be achieved by mounting the whole laser system onto a two-axis heliostat that follows the Sun continuously in direct tracking mode. Incoming solar radiation is focused to the laser disk, firstly through the Fresnel lenses and secondly through the toroidal fused silica lens, which further compresses the concentrated solar radiation from the focus of each Fresnel lens to the lateral face of the disk, as shown in Figs. 1 and 2.

Fig. 2. Further pump light compression to the Nd:YAG disk through the toroidal fused silica lens.

Fused silica is an ideal optical material for Nd:YAG laser pumping since it is transparent over the Nd:YAG absorption spectrum. It has a high softening point and is resistant to scratching and thermal shock. High optical quality (99.999%) fused silica torus lenses can be manufactured by optical machining and polishing [17

P. H. Bernardes and D. Liang, “Solid-state laser pumping by light-guides,” Appl. Opt. 45, 3811–3816 (2006). [CrossRef]

]. The back and lateral surfaces of the Nd:YAG laser disk are directly cooled by water, which also helps to eliminate some UV and IR radiation, which does not contribute to lasing. The cooled back surface of the disk is 1064 nm HR coated, while the front surface is 1064 nm AR coated. The laser resonant cavity is formed by both the 1064 nm HR mirror and the output coupler, as shown in Fig. 2. The optimized output coupler reflectivity, usually varying between 90% and 98%, can be achieved by LASCAD analysis. A circular plane reflector is also placed below the cooled surface of the disk for a more efficient pump light coupling to the disk.

3. Numerical Analysis of Laser Output Performances of Single-Crystal Nd:YAG Disks

ZEMAX nonsequential ray tracing is used to find the optimum pumping parameters and the mounting positions of all optical elements. The standard solar spectrum for one-and-a-half air mass (AM1.5) [18

ASTM Standard G159-98, “Standard Tables for References Solar Spectral Irradiance at Air Mass 1.5: Direct Normal and Hemispherical for a 37° Tilted Surface” (1998), withdrawn 2005, replaced with G173.

] is used as the reference data for consulting the spectral irradiance (

W / m^{2} / nm

) at each wavelength. The terrestrial solar irradiance of

950 W / m^{2}

is considered in ZEMAX software. The effective pump power of the light source takes into account the 16% overlap between the absorption spectrum of the Nd:YAG medium and the solar spectrum [19

Z. Bin, C. Zhao, J. He, and S. Yang, “The study of active medium for solar-pumped solid-state lasers,” Acta Opt. Sin. 27, 1797–1801 (2007).

]. The solar half-angle of 0.27° is also considered in the analysis. The absorption spectrum of Polymethyl Methacrylate (PMMA), fused silica, and water materials are included in ZEMAX numerical data to account for absorption losses. The above materials play an important role in preventing UV solarization and IR heating to the Nd:YAG medium.

Nd:YAG is the most widely used solid-state laser material and is a promising candidate in high-power thin-disk lasers. It has been demonstrated as the best material under solar pumping because of its superior characteristics on thermal conductivity, high quantum efficiency, and mechanical strength compared to other host materials [3

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, 1051–1053 (1984). [CrossRef]

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

V. Krupkin, J. A. Kagan, and A. Yogev, “Nonimaging optics and solar laser pumping at the Weizmann Institute,” Proc. SPIE 2016, 50–60 (1993). [CrossRef]

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

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

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

–11

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

]. For 1.1% Nd:YAG laser medium, 22 absorption peaks are defined in ZEMAX numerical data [9

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

,11

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

]. All the peak wavelengths and their respective absorption coefficients are added to the glass catalogue for Nd:YAG material in ZEMAX software. Solar irradiance values for the above-mentioned 22 peak absorption wavelengths could be consulted from the standard solar spectra for AM1.5 and saved as source wavelength data. In ray tracing, the laser disk is divided into a total of 18,000 zones. The path length in each zone is found. With this value and the effective absorption coefficient of 1.1% Nd:YAG material, the absorbed power within the laser medium can be calculated by summing up the absorbed pump radiation of all zones. The absorbed pump flux data from the ZEMAX analysis is then processed by LASCAD software to study the laser beam parameters and quantify the thermal effects applied in the active medium.

In LASCAD analysis, the optical resonator is comprised of two opposing parallel mirrors at right angles to the axis of the laser disk. One end mirror is high reflection coated (HR, 99.98%) and corresponds to the HR-coated surface of the disk. The output coupler is partial reflection coated (PR). The amount of feedback is determined by the reflectivity of the mirrors. Different disk heights give rise to different round-trip losses for LASCAD analysis. The round-trip resonant cavity loss is the sum of the absorption and scattering losses for laser emission wavelength within the laser medium and imperfect HR and AR losses. For

H = 5 mm

, for example, a round-trip loss of 0.90% is assumed. All the technical parameters and data used in both ZEMAX and LASCAD numerical analyses have already been confirmed by our previous experimental results [9

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

,11

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

]. For correct parameterization of the laser system, the laser output performances of different diameter (

D_{Nd : YAG}

) and height (

H

) disks are studied. For each case, the curvature radius

R

, ranging typically between 10 and 14 mm, of the toroidal lens is optimized to obtain the maximum absorbed pump power within the Nd:YAG medium.

Figs. 3 and 4 represent the behavior of both multimode laser power and brightness figure of merit

B

for single-crystal disks with different

D_{Nd : YAG}

and

H

. The resonator cavity length is optimized according to the different disks to attain the highest laser beam brightness. The radius of curvature of the output coupler (RoC) is assumed as

RoC = \infty

Fig. 3. Calculated multimode solar laser power of single-crystal disks with different

H

and

D_{Nd : YAG}

Fig. 4. Calculated brightness figure of merit

B

of single-crystal disks with different

H

and

D_{Nd : YAG}

Maximum multimode laser power of 29.6 W is numerically achieved by pumping the

H = 8 mm

disk with

D_{Nd : YAG} = 10 mm

15.4 W / m^{2}

collection efficiency is hence calculated. By reducing the disk height to 6 mm, multimode laser power of 28.7 W is calculated for the same disk diameter. For the

H = 5 mm

disk, multimode laser powers between 26 W and 28 W can also be achieved.

Brightness figure of merit

B

can be easily improved by pumping disks with small height and large diameter. High figure of merit

B

of 36 W is then numerically measured for the

D_{Nd : YAG} = 12 mm

H = 5 mm

Nd:YAG disk. This result is 124 times higher than the current brightness figure of merit

B

for solar-pumped laser with rod geometry [11

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

]. The lowest figure of merit

B

of 1.8 W is found for the Nd:YAG disk with

D_{Nd : YAG} = 8 mm

and

H = 8 mm

. This value is still 6.2 times higher than the record figure of merit

B

[11

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

4. Numerical Analysis of Laser Output Performances of Core-Doped Nd:YAG Ceramic Disks and its Comparison to Single-Crystal Nd:YAG Disks

Polycrystalline ceramic Nd:YAG laser material can act as host material and enables new possibilities in designing the laser medium with respect to dopant concentration as well as distribution, size, and geometry. A single-crystal undoped YAG cap and an

{Nd}^{+ 3}

doped active medium can be bonded together for slab geometries. However, it is not a viable concept to bond a cladding layer around a single-crystal rod. Using the ceramic technology instead, an undoped cladding around a doped core can be realized for the rod geometry. The core-doped ceramic mediums are laser active in the

{Nd}^{+ 3}

-doped core only, and bonded with the same host material, either undoped or doped, with a different element that effectively absorbs light at the signal wavelength. This technology has been applied in diode-pumped arrangements with rod geometries and has shown potential to provide better laser beam brightness when compared to conventional single-crystal rods [20

M. Ostermeyer and I. Brandenburg, “Simulation of the extraction of near diffraction limited Gaussian beams from side pumped core-doped ceramic Nd:YAG and conventional laser rods,” Opt. Express 13, 10145–10156 (2005). [CrossRef]

,21

A. Sträßer and M. Ostermeyer, “Improving the brightness of side pumped power amplifiers by using core-doped ceramic rods,” Opt. Express 14, 6687–6693 (2006). [CrossRef]

]. Since the medium cross-section is widened by the cladding, wider Gaussian intensity distributions can be accommodated in the laser active region without truncating its wings. This will lead to a more efficient use of the built-up inversion as the average intensity in the doped part of the laser medium becomes higher.

Among various absorber materials,

{Sm}^{3 +} : YAG

is found to be the best candidate to Nd:YAG laser, due to its spectroscopy properties. It is also effective in suppressing parasitic oscillations that occur at the disk edges which limit extraction efficiency and energy scaling of lasers [22

H. Yagi, J. F. Bisson, K. Ueda, and T. Yanagitani, “ $Y_{3} {Al}_{5} O_{12}$ ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers,” J. Lumin. 121, 88–94 (2006). [CrossRef]

R. Huß, R. Wilhelm, C. Kolleck, J. Neumann, and D. Kracht, “Suppression of parasitic oscillations in a core-doped ceramic Nd:YAG laser by Sm:YAG cladding,” Opt. Express 18, 13094–13101 (2010). [CrossRef]

–24

T. Denis, S. Hahn, S. Mebben, R. Wilhelm, C. Kolleck, J. Neumann, and D. Kracht, “Compact diode stack end pumped Nd:YAG amplifier using core doped ceramics,” Appl. Opt. 49, 811–816 (2010). [CrossRef]

]. For this reason, the influence of core-doped Nd:YAG disks with

{Sm}^{3 +} : YAG

cladding on laser performance is studied. The laser performance of the

H = 5 mm

Nd:YAG single-crystal disks, analyzed in section 3, is compared to that of core-doped Nd:YAG disks with same height and Nd:YAG diameter

D_{Nd : YAG}

, as represented in Fig. 5.

Fig. 5. Scheme of Nd:YAG single-crystal disk and core-doped ceramic Nd:YAG disk with the same

H

and

D_{Nd : YAG}

The absorption spectrum of 5.0% at

{Sm}^{3 +} : YAG

has five strong peaks in the NIR region at the central wavelengths of 1070, 1220, 1350, and 1460 nm. For the laser wavelength of 1064 nm, the absorption coefficient of

{Sm}^{3 +} : YAG

reaches

3.6 {cm}^{- 1}

, while low absorption coefficients of about

0.1 {cm}^{- 1}

are detected for the absorption peaks of Nd:YAG [22

]. Therefore, the remaining IR radiation can be strongly absorbed, reducing the thermal effects within the Nd:YAG medium. The absorption spectrum of

{Sm}^{3 +} : YAG

is added to the glass catalogue in ZEMAX software. The numerical simulations show that the absorption efficiency depends on the cladding diameter

D_{clad}

. An optimized

D_{clad}

is then found for each core diameter

D_{Nd : YAG}

The heat load within both the single-crystal and the core-doped Nd:YAG disks with the same

H = 5 mm

is given in Fig. 6, for three different disks

D_{Nd : YAG}

. Red means near maximum heat load for these plots, whereas blue means little or no heat generation. The absorbed pump power is also given in each case.

Fig. 6. Heat load of both

H = 5 mm

Nd:YAG single-crystal and core-doped disks with different

D_{Nd : YAG}

For both the single-crystal and the core-doped disks, the heat load inside the active medium is reduced with increased diameter, despite the increase of absorbed pump power, as observed in Fig. 6. This largely alleviates the thermal lensing effects, and the achievement of high-brightness laser beam becomes possible, as already demonstrated in Fig. 4. When the absorption profile is centrally peaked, the temperature on the axis increases further, resulting in stronger thermal lensing at the center, higher-order aberrations at the periphery, and larger stress in the laser medium compared with those of uniform excitation. At high average output power, even a uniform gain distribution in a water-cooled laser medium, as shown in the single-crystal Nd:YAG disk in Fig. 6, has been shown to induce a nonparabolic heat distribution as a result of the temperature dependence of the thermal conductivity [25

T. Brand, “Compact 170 W continuous-wave diode-pumped Nd:YAG rod laser with a cusp-shaped reflector,” Opt. Lett. 20, 1776–1778 (1995). [CrossRef]

]. This results in a radially dependent refractive power of the thermal lens, which has a maximum on the medium axis. Consequently, to achieve ultrahigh beam brightness, we should start with a low power deposition at the center of the medium, which can be easily achieved by the core-doped ceramic Nd:YAG disk, as shown in Fig. 6. Moreover, the absorbed pump power is also 118% more than that of single-crystal with the same Nd:YAG diameter. The influence of the core-doped disks on both multimode laser power and laser beam brightness is analyzed in Figs. 7 and 8, respectively.

Fig. 7. Multimode solar laser power of both core-doped and single-crystal disks with different

D_{Nd : YAG}

Fig. 8. Brightness figure of merit

B

of both core-doped and single-crystal disks with different

D_{Nd : YAG}

The multimode laser power is largely favored by the implementation of the core-doped Nd:YAG disks, as analyzed in Fig. 7. The highest multimode laser power of 34 W is numerically calculated for the

D_{Nd : YAG} = 9 mm

core-doped disk, corresponding to 124% improvement over that of the single-crystal disk with the same

D_{Nd : YAG}

. Collection efficiency of

17.7 W / m^{2}

is hence calculated. An optimized

D_{Clad}

of 16 mm is calculated in this case.

As shown in Fig. 8, solar laser beam brightness figure of merit is increased with the use of core-doped disks, especially for larger diameters. 53 W ultrahigh figure of merit

B

can be reached by pumping the

D_{Nd : YAG} = 12 mm

core-doped disk. This corresponds to an improvement of 147% over the figure of merit

B

of the single-crystal disk with same

D_{Nd : YAG}

and surpasses the record figure of merit

B

by more than 180 times. Therefore the introduction of core-doped ceramic technology to solar-pumped disk laser can be an effective choice in achieving more laser beam brightness.

5. Conclusions

For achieving the highest solar-pumped laser beam brightness, a multi-Fresnel lens scheme is proposed for side-pumping either the single-crystal Nd:YAG or the core-doped ceramic

{Sm}^{3 +}

Nd:YAG disk. Optimum laser system parameters are found through ZEMAX and LASCAD numerical analysis. The production of 34 W multimode solar laser power can be expected, corresponding to

17.7 W / m^{2}

collection efficiency. 53 W figure of merit

B

can be reached, surpassing by more than 180 times the record brightness figure of merit

B

for Nd:YAG solar laser with rod geometry. Significant reduction in thermal lensing effects is predicted. Laser beam brightness can still be improved by pumping small height and large diameter laser disks.

As shown in Table 1, maximum solar laser power and collection efficiency are given by [10

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

]. However, the large

M^{2}

factor of 135 degrades largely the beam brightness figure of merit to only 0.0066 W. For [11

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

M_{x}^{2} = 8.9

M_{y}^{2} = 9.6

are obtained. High laser beam figure of merit

B = 0.29 W

is achieved. With the proposed pumping scheme, LASCAD simulation leads to beam quality factors slightly less than unity. The very low pump power deposition at the center of the medium in Fig. 6 has resulted in a thermal lens with very large focal length. Near diffraction-limited laser beam can hence be extracted by a large

V

-shaped resonant cavity. Significant improvement in brightness figure of merit over the previous setups is therefore achieved. Even though, in recent years, a series of laser beam with

M^{2} < 1

has been realized experimentally [26

G. Zhou, D. Zhao, J. Xu, and S. Wang, “Semiconductor laser with beam quality factor $M 2 < 1$ ,” Opt. Commun. 187, 395–399 (2001). [CrossRef]

,27

S. Wang, D. Zhao, Z. Lu, G. Zhou, F. Huang, and J. Xu, “Demonstration for optical beam qualities of quantum well lasers,” Opt. Commun. 194, 425–428 (2001). [CrossRef]

], we still have some reservations regarding the LASCAD numerical calculation accuracy of the

M^{2}

factors from the unusual non-Gaussian absorbed pump flux profiles, as given in Fig. 6. Experimental research of the proposed scheme will be carried out by us to validate the results of numerical analysis in the next step.

Table 1. Solar Laser Performance of the Proposed Nd:YAG Disk Laser Scheme and its Comparison to the Previous Nd:YAG Solar Laser Setups

	Previous Setups with Rod Geometry		Proposed Scheme with Disk Geometry
	Ref. [10 T. H. Dinh, T. Ohkubo, T. Yabe, and H. Kuboyama, “120 watt continuous wave solar-pumped laser with a liquid light-guide lens and a Nd:YAG rod,” Opt. Lett. 37, 2670–2672 (2012). [CrossRef] ]	Ref. [11 J. Almeida, D. Liang, and E. Guillot, “Improvement in solar-pumped Nd:YAG laser beam brightness,” Opt. Laser Technol. 44, 2115–2119 (2012). [CrossRef] ]	Single-Crystal	Core-Doped
Laser Power
Laser output power ( $W$ )	120	24.7	26.4	31.0
Collection efficiency ( $W / m^{2}$ )	30	9.6	15.4	17.7
Laser beam quality
$M_{x}^{2} / M_{y}^{2}$	$135 / 135$	$8.9 / 9.6$	$0.86 / 0.86$	$0.77 / 0.76$
Figure of merit $B$ ( $W$ )	0.0066	0.29	35.7	53.0

With the proposed approach, solar-pumped solid-state lasers can benefit largely from the advantages of thin-disk laser geometry. The frontier of solar laser beam brightness can be significantly expanded, while the thermal management problems that have plagued present-day solar-pumped lasers can be drastically reduced. Highly efficient solar-pumped laser with ultrahigh brightness can thus become possible.

Acknowledgments

This research project (PTDC/FIS/103599/2008) was funded by the Science and Technology Foundation of the Portuguese Ministry of Science, Technology and Higher Education (FCT-MCTES).

References

1.	D. Graham-Rowe, “Solar-powered lasers,” Nat. Photonics 4, 64–65 (2010). [CrossRef]
2.	C. W. Young, “A sun-pumped cw one-watt laser,” Appl. Opt. 5, 993–997 (1966). [CrossRef]
3.	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, 1051–1053 (1984). [CrossRef]
4.	M. Weksler and J. Shwartz, “Solar-pumped solid-state lasers,” IEEE J. Quantum Electron. 24, 1222–1228 (1988). [CrossRef]
5.	V. Krupkin, J. A. Kagan, and A. Yogev, “Nonimaging optics and solar laser pumping at the Weizmann Institute,” Proc. SPIE 2016, 50–60 (1993). [CrossRef]
6.	M. Lando, J. Kagan, B. Linyekin, and V. Dobrusin, “A solar-pumped Nd:YAG laser in the high collection efficiency regime,” Opt. Commun. 222, 371–381 (2003). [CrossRef]
7.	T. Yabe, S. Uchida, K. Ikuta, K. Yoshida, C. Baasandash, M. S. Mohamed, Y. Sakurai, Y. Ogata, M. Tuji, Y. Mori, Y. Satoh, T. Ohkubo, M. Murahara, A. Ikesue, M. Nakatsuka, T. Saiki, S. Motokoshi, and C. Yamanaka, “Demonstrated fossil-fuel-free energy cycle using magnesium and laser,” Appl. Phys. Lett. 89, 261107 (2006). [CrossRef]
8.	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, 261120 (2007). [CrossRef]
9.	D. Liang and J. Almeida, “Highly efficient solar-pumped Nd:YAG laser,” Opt. Express 19, 26399–26405 (2011). [CrossRef]
10.	T. H. Dinh, T. Ohkubo, T. Yabe, and H. Kuboyama, “120 watt continuous wave solar-pumped laser with a liquid light-guide lens and a Nd:YAG rod,” Opt. Lett. 37, 2670–2672 (2012). [CrossRef]
11.	J. Almeida, D. Liang, and E. Guillot, “Improvement in solar-pumped Nd:YAG laser beam brightness,” Opt. Laser Technol. 44, 2115–2119 (2012). [CrossRef]
12.	M. Frede, R. Wilhelm, M. Brendel, C. Fallnich, F. Seifert, B. Willke, and K. Danzmann, “High power fundamental mode Nd:YAG laser with efficient birefringence compensation,” Opt. Express 12, 3581–3859 (2004). [CrossRef]
13.	W. Koechner, “Thermal lensing in Nd:YAG laser rod,” Appl. Opt. 9, 2548–2553 (1970). [CrossRef]
14.	A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58, 365–372 (1994). [CrossRef]
15.	A. Giesen and J. Speiser, “Fifteen years of work on thin-disk lasers: results and scaling laws,” IEEE J. Sel. Top. Quantum Electron. 13, 598–609 (2007). [CrossRef]
16.	R. Pereira, B. Weichelt, D. Liang, P. Morais, H. Gouveia, M. Abdou-Ahmed, A. Voss, and T. Graf “Efficient pump beam shaping for high-power thin-disk laser systems,” Appl. Opt. 49, 5157–5162 (2010). [CrossRef]
17.	P. H. Bernardes and D. Liang, “Solid-state laser pumping by light-guides,” Appl. Opt. 45, 3811–3816 (2006). [CrossRef]
18.	ASTM Standard G159-98, “Standard Tables for References Solar Spectral Irradiance at Air Mass 1.5: Direct Normal and Hemispherical for a 37° Tilted Surface” (1998), withdrawn 2005, replaced with G173.
19.	Z. Bin, C. Zhao, J. He, and S. Yang, “The study of active medium for solar-pumped solid-state lasers,” Acta Opt. Sin. 27, 1797–1801 (2007).
20.	M. Ostermeyer and I. Brandenburg, “Simulation of the extraction of near diffraction limited Gaussian beams from side pumped core-doped ceramic Nd:YAG and conventional laser rods,” Opt. Express 13, 10145–10156 (2005). [CrossRef]
21.	A. Sträßer and M. Ostermeyer, “Improving the brightness of side pumped power amplifiers by using core-doped ceramic rods,” Opt. Express 14, 6687–6693 (2006). [CrossRef]
22.	H. Yagi, J. F. Bisson, K. Ueda, and T. Yanagitani, “ $Y_{3} {Al}_{5} O_{12}$ ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers,” J. Lumin. 121, 88–94 (2006). [CrossRef]
23.	R. Huß, R. Wilhelm, C. Kolleck, J. Neumann, and D. Kracht, “Suppression of parasitic oscillations in a core-doped ceramic Nd:YAG laser by Sm:YAG cladding,” Opt. Express 18, 13094–13101 (2010). [CrossRef]
24.	T. Denis, S. Hahn, S. Mebben, R. Wilhelm, C. Kolleck, J. Neumann, and D. Kracht, “Compact diode stack end pumped Nd:YAG amplifier using core doped ceramics,” Appl. Opt. 49, 811–816 (2010). [CrossRef]
25.	T. Brand, “Compact 170 W continuous-wave diode-pumped Nd:YAG rod laser with a cusp-shaped reflector,” Opt. Lett. 20, 1776–1778 (1995). [CrossRef]
26.	G. Zhou, D. Zhao, J. Xu, and S. Wang, “Semiconductor laser with beam quality factor $M 2 < 1$ ,” Opt. Commun. 187, 395–399 (2001). [CrossRef]
27.	S. Wang, D. Zhao, Z. Lu, G. Zhou, F. Huang, and J. Xu, “Demonstration for optical beam qualities of quantum well lasers,” Opt. Commun. 194, 425–428 (2001). [CrossRef]

OCIS Codes
(140.0140) Lasers and laser optics : Lasers and laser optics
(140.3530) Lasers and laser optics : Lasers, neodymium
(140.3580) Lasers and laser optics : Lasers, solid-state
(140.6810) Lasers and laser optics : Thermal effects
(350.6050) Other areas of optics : Solar energy

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: July 16, 2012
Revised Manuscript: August 17, 2012
Manuscript Accepted: August 17, 2012
Published: September 10, 2012

Virtual Issues
September 14, 2012 Spotlight on Optics

Citation
Dawei Liang and Joana Almeida, "Design of ultrahigh brightness solar-pumped disk laser," Appl. Opt. 51, 6382-6388 (2012)
http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-51-26-6382

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References

D. Graham-Rowe, “Solar-powered lasers,” Nat. Photonics 4, 64–65 (2010). [CrossRef]
C. W. Young, “A sun-pumped cw one-watt laser,” Appl. Opt. 5, 993–997 (1966). [CrossRef]
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, 1051–1053 (1984). [CrossRef]
M. Weksler and J. Shwartz, “Solar-pumped solid-state lasers,” IEEE J. Quantum Electron. 24, 1222–1228 (1988). [CrossRef]
V. Krupkin, J. A. Kagan, and A. Yogev, “Nonimaging optics and solar laser pumping at the Weizmann Institute,” Proc. SPIE 2016, 50–60 (1993). [CrossRef]
M. Lando, J. Kagan, B. Linyekin, and V. Dobrusin, “A solar-pumped Nd:YAG laser in the high collection efficiency regime,” Opt. Commun. 222, 371–381 (2003). [CrossRef]
T. Yabe, S. Uchida, K. Ikuta, K. Yoshida, C. Baasandash, M. S. Mohamed, Y. Sakurai, Y. Ogata, M. Tuji, Y. Mori, Y. Satoh, T. Ohkubo, M. Murahara, A. Ikesue, M. Nakatsuka, T. Saiki, S. Motokoshi, and C. Yamanaka, “Demonstrated fossil-fuel-free energy cycle using magnesium and laser,” Appl. Phys. Lett. 89, 261107 (2006). [CrossRef]
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, 261120(2007). [CrossRef]
D. Liang and J. Almeida, “Highly efficient solar-pumped Nd:YAG laser,” Opt. Express 19, 26399–26405 (2011). [CrossRef]
T. H. Dinh, T. Ohkubo, T. Yabe, and H. Kuboyama, “120 watt continuous wave solar-pumped laser with a liquid light-guide lens and a Nd:YAG rod,” Opt. Lett. 37, 2670–2672 (2012). [CrossRef]
J. Almeida, D. Liang, and E. Guillot, “Improvement in solar-pumped Nd:YAG laser beam brightness,” Opt. Laser Technol. 44, 2115–2119 (2012). [CrossRef]
M. Frede, R. Wilhelm, M. Brendel, C. Fallnich, F. Seifert, B. Willke, and K. Danzmann, “High power fundamental mode Nd:YAG laser with efficient birefringence compensation,” Opt. Express 12, 3581–3859 (2004). [CrossRef]
W. Koechner, “Thermal lensing in Nd:YAG laser rod,” Appl. Opt. 9, 2548–2553 (1970). [CrossRef]
A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58, 365–372 (1994). [CrossRef]
A. Giesen and J. Speiser, “Fifteen years of work on thin-disk lasers: results and scaling laws,” IEEE J. Sel. Top. Quantum Electron. 13, 598–609 (2007). [CrossRef]
R. Pereira, B. Weichelt, D. Liang, P. Morais, H. Gouveia, M. Abdou-Ahmed, A. Voss, and T. Graf “Efficient pump beam shaping for high-power thin-disk laser systems,” Appl. Opt. 49, 5157–5162 (2010). [CrossRef]
P. H. Bernardes and D. Liang, “Solid-state laser pumping by light-guides,” Appl. Opt. 45, 3811–3816 (2006). [CrossRef]
ASTM Standard G159-98, “Standard Tables for References Solar Spectral Irradiance at Air Mass 1.5: Direct Normal and Hemispherical for a 37° Tilted Surface” (1998), withdrawn 2005, replaced with G173.
Z. Bin, C. Zhao, J. He, and S. Yang, “The study of active medium for solar-pumped solid-state lasers,” Acta Opt. Sin. 27, 1797–1801 (2007).
M. Ostermeyer and I. Brandenburg, “Simulation of the extraction of near diffraction limited Gaussian beams from side pumped core-doped ceramic Nd:YAG and conventional laser rods,” Opt. Express 13, 10145–10156 (2005). [CrossRef]
A. Sträßer and M. Ostermeyer, “Improving the brightness of side pumped power amplifiers by using core-doped ceramic rods,” Opt. Express 14, 6687–6693 (2006). [CrossRef]
H. Yagi, J. F. Bisson, K. Ueda, and T. Yanagitani, “Y3Al5O12 ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers,” J. Lumin. 121, 88–94(2006). [CrossRef]
R. Huß, R. Wilhelm, C. Kolleck, J. Neumann, and D. Kracht, “Suppression of parasitic oscillations in a core-doped ceramic Nd:YAG laser by Sm:YAG cladding,” Opt. Express 18, 13094–13101 (2010). [CrossRef]
T. Denis, S. Hahn, S. Mebben, R. Wilhelm, C. Kolleck, J. Neumann, and D. Kracht, “Compact diode stack end pumped Nd:YAG amplifier using core doped ceramics,” Appl. Opt. 49, 811–816 (2010). [CrossRef]
T. Brand, “Compact 170 W continuous-wave diode-pumped Nd:YAG rod laser with a cusp-shaped reflector,” Opt. Lett. 20, 1776–1778 (1995). [CrossRef]
G. Zhou, D. Zhao, J. Xu, and S. Wang, “Semiconductor laser with beam quality factor M2<1,” Opt. Commun. 187, 395–399 (2001). [CrossRef]
S. Wang, D. Zhao, Z. Lu, G. Zhou, F. Huang, and J. Xu, “Demonstration for optical beam qualities of quantum well lasers,” Opt. Commun. 194, 425–428 (2001). [CrossRef]

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