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Abstract
Herein, we report a significant improvement in solar-pumped laser collection efficiency based on end-side pumping a 6-mm-diameter 95-mm-length Ce:Nd:YAG/YAG grooved bonded crystal rod. A Fresnel lens, quartz cooling-water tube, and gold-plated conical cavity constituted the solar-energy collection and concentration system, which was designed to maximum pump light absorption and minimize thermal effects in the Ce:Nd:YAG laser medium. To the best of our knowledge, this is the first time that a Ce:Nd:YAG crystal has been pumped by a Fresnel-lens solar-energy collection and concentration system. The 0.69-m2 effective solar-collection area produced 26.93 W of continuous-wave laser power, corresponding to 6.33% slope efficiency. The collection efficiency (38.8 W/m2) was 1.21 times higher than the highest previously reported value for Fresnel-lens solar collection, and is a record for single-beam solar-pumped lasers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With fossil energy sources gradually being exhausted around the globe, and the increase in space exploration activities, solar energy applications are becoming increasingly important. Solar energy utilization should lead to a reduction in fossil fuel consumption and less environmental damage, and solar energy is also the only energy source for spacecraft. Solar-pumped lasers (SPLs) represent an avenue for solar energy usage. SPLs can directly convert incoherent, relatively weak solar radiation from the sun into coherent high-brightness laser radiation, without the requirement for electrically driven artificial pump light sources. Moreover, compared to three energy conversions in using a solar source to power a conventional electrically pumped laser, merely a single energy conversion is required for the transformation of solar light energy into SPL laser light, which means theoretically higher overall energy conversion efficiency should be achievable for SPLs. Thus, these lasers have significant potential in solar hydrogen generation [1–3] and space laser [4–6] applications.
Since the first solar pumped Nd:YAG laser, with 1-W cw power, was reported by Young et al. [7], several different gain media including gases [8,9], liquids [10], and solids [11,12] have been trialed in an effort to increase the efficiency of solar lasers. However, only solid active media such as yttrium aluminum garnet (Y3Al5O12, YAG) host materials have proved sufficiently promising—in particular, Nd:YAG crystals and Cr:Nd:YAG ceramics. Thanks to the use of these two materials as active media, and optimization of the optical concentration system, the laser collection efficiency (CE), defined as the ratio of the SPL output power to the solar collection area, has been gradually improved in recent years. In 2007, Yabe et al. obtained a CE of 18.7 W/m2 by end pumping a Cr:Nd:YAG ceramic rod using a 1.3-m2 Fresnel lens solar concentrator [13]. In 2011, a CE of 19.3 W/m2 was achieved by Liang and Almeida by using a 0.64-m2 Fresnel lens solar concentrator to end-side pump a Nd:YAG crystal rod [14], and in 2012, Dinh et al. reported a CE of 30 W/m2 when end-side pumping a Nd:YAG crystal rod using a 4-m2 Fresnel lens [15]. A CE of 31.5 W/m2 was achieved by Liang et al. in 2017 by pumping a Nd:YAG crystal rod using a parabolic mirror with an effective collection area of 1.18 m2 [16]. The highest CE for a Nd:YAG crystal system, 32.1 W/m2, was reported by Guan et al. in 2018. This was obtained using a 1.03-m2 Fresnel lens to end-side pump a Nd:YAG/YAG bonded rod [17]. By end-side pumping a Cr:Nd:YAG ceramic rod using a parabolic mirror with an effective collection area of 1.0 m2, Liang et al. were able to report a record CE for the Cr:Nd:YAG ceramic system of 32.5 W/m2 in the same year [18]. However, both Nd:YAG crystals and Cr:Nd:YAG ceramics have specific limitations. The narrow absorption spectrum of Nd:YAG means that only a relatively small amount of the solar spectrum can be utilized [19]. Although adding Cr3+ ions as sensitizers to Nd:YAG broadens its absorption spectrum [20], the improvement in performance that might have been expected for SPLs based on pumping Cr:Nd:YAG ceramic rods has not been realized because of the more serious thermal effects and higher absorption and scattering losses of this material [15,13]. Thus, the exploration of new solar laser materials is required.
Since the strong broadband absorptions centered at ∼339 nm and ∼460 nm and the efficient energy transfer from the Ce3+ to Nd3+ ions was confirmed [21–24], the Ce:Nd:YAG crystal has become the focus of significant expectations. Payziyev at al. used a simulation model to demonstrate that the solar-to-laser power efficiency of a Ce:Nd:YAG solar laser was double that of a Nd:YAG solar laser [25]. However, progress on solar pumped Ce:Nd:YAG crystal lasers was not initially rapid. One of the major constraints for the development of Ce:Nd:YAG SPLs was serious thermal effects. When co-doped with Ce3+ ions, two broad bands were added to the absorption spectrum of Ce:Nd:YAG, at shorter wavelengths with respect to the Nd:YAG absorption spectrum [26], leading to a larger Stokes shift of the Ce:Nd:YAG emission band with solar pumping. Therefore, Ce:Nd:YAG crystals with the same volume absorb more solar power, but have lower energy transition efficiency, which makes thermal effects consistently more pronounced for Ce:Nd:YAG under solar pumping compared to Nd:YAG. The first Ce:Nd:YAG solar laser was reported by Vistas at al. in 2020. Solar energy with a total power of 569 W was concentrated to end-side pump a Ce:Nd:YAG crystal rod, but only 6.0 W of cw output solar laser power was obtained, corresponding to a CE of 4.9 W/m2 [27]. Because of more severe thermal effects, the output power of the Ce:Nd:YAG laser was lower than that of the analogous Nd:YAG laser system. Moreover, crystal fracture occurred in Ce:Nd:YAG medium when the input solar power was increased. Subsequent studies on Ce:Nd:YAG laser media produced significant results by focusing on avoided localized heat accumulation, using parabolic mirror systems to concentrate the solar light. By side-pumped Ce:Nd:YAG, which generated a uniform absorption distribution along the Ce:Nd:YAG rod axis, and with 600 W of input solar power, Vistas at al. obtained 16.5 W of cw solar laser power and a CE of 23.6 W/m2; this was higher than that of an analogous system based on a Nd:YAG rod, and higher than the previous record CE for a side-pumped Nd:YAG SPL [28]. Garcia at al. designed a compact Ce:Nd:YAG solar laser head, and they obtained 11.2 W of cw solar laser power and a CE of 38.22 W/m2 with only 249 W of input solar pump power [29]. By dispersing the heat load to reduce thermal effects, Liang at al. achieved a total cw solar laser power of 16.5 W upon end-side pumping three Ce:Nd:YAG rods within a single conical pump cavity. This result corresponds to a CE of 41.25 W/m2, which is the current record for SPLs [30].
Fresnel lenses are cost-effective, light weight, and compact, and hence they are the most suitable option for the primary concentrators of solar pumped lasers in space applications. However, the strongly dispersed radiation distributed in the focal zone prevents efficient light concentration in a thin rod [14]. However, this “shortcoming” of Fresnel lenses may actually be advantageous for overcoming the serious thermal effects of end-side pumped Ce:Nd:YAG rods in SPLs. Because the absorption bands of Ce3+ and Nd3+ ions are in different spectral regions, the dispersion of the Fresnel lens results in spatial absorption distributions for the ions that span different spatial regions of the Ce:Nd:YAG rod, which may prevent heat accumulation in a single region of Ce:Nd:YAG medium.
Consequently, to exploit this advantage of Fresnel lenses, we designed a first Ce:Nd:YAG SPL with a Fresnel lens collection and concentration system. A quartz tube filled with cooling water was mounted inside a conical cavity and used to further concentrate the solar radiation from the focal zone of the Fresnel lens onto a 6-mm-diameter, 95-mm length of Ce:Nd:YAG/YAG grooved bonded crystal rod. The parameters of concentration system were optimized using TracePro and ASLD simulations. According to the numerical analysis, the Fresnel lens system was more suitable for the Ce:Nd:YAG medium than the parabolic mirror system. When 694 W of solar power was concentrated by a Fresnel lens with a collection area of 0.69 m2, the Ce:Nd:YAG emitted 26.93 W of cw solar laser power at 1064 nm, higher that emitted by a similar SPL with a Nd:YAG crystal medium. To the best of our knowledge, this was the first time that the performance of a single Ce:Nd:YAG crystal was demonstrated to be superior to that of a Nd:YAG crystal under the same solar end-side pumping condition. The obtained solar laser CE of 38.8 W/m2 was 1.21 times higher than the record value reported for a Fresnel lens concentration system, and it represents a new record for single-beam SPLs, being only slightly lower than the total CE of 41.25 W/m2 achieved by Liang at al [30]. with simultaneous emission from three Ce:Nd:YAG rods. In this study, we demonstrated a means of overcoming the limitations of SPLs based on Ce:Nd:YAG crystals. Furthermore, the utilization of a Fresnel lens ensures that our design is economically competitive.
2. Method
2.1 Solar energy collection and concentration system
As shown in Fig. 1, the solar energy collection and concentration system was composed of a Fresnel lens and solar tracking device. The Fresnel lens was mounted on a stable two-axis platform (Beijing Optical Century Instrument Co., Ltd) that automatically followed the solar rays. The Fresnel lens (NTKJ Co., Ltd., Itabashi-ku, Tokyo, Japan CF1200-B3), which was constructed from polymethyl methacrylate (PMMA), spanned an area of 1400 mm × 1050 mm and had a focal length of 1200 mm.
Fig. 1. Schematics of Fresnel lens solar energy collection and concentration system and front view of Fresnel lens.
In order to maximize the concentration efficiency, the four corners and edges of the Fresnel lens were masked such that a circular area of only 0.94 m in diameter at the center of the lens was utilized, and hence the effective solar collection area was 0.69 m2. Under typical solar irradiance conditions for Beijing of 1000 W/m2, with the 0.69 m2 collection area, about 694 W of solar power was focused into a concentrated pump-light spot with a near-Gaussian distribution characterized by a full width at half maximum (FWHM) of approximately 11.5 mm.
2.2 Solar laser head
Fig. 2(a) shows a schematic of the solar laser head, which consists of a quartz tube, conical cavity, grooved bonded crystal rod, and output coupler. All the parameters of the laser head were optimized using TracePro and ASLD before the laser was constructed. The laser rod was mounted inside the quartz tube, as shown in Fig. 2(a). When filled with water, the front part of this liquid light guide lens (LLGL) acted as a spherical lens. An external diameter of 12 mm was selected for the quartz tube to ensure the solar radiation within the FWHM of the focal spot of the Fresnel lens was directly coupled into the laser rod for end pumping. In addition, other parameters of the quartz tube—the 10-mm internal diameter, 6-mm radius of curvature of the front surface, and 80-mm length—were optimized using TracePro to obtain maximum absorbed solar power. Some of the solar rays that were not focused by the front face of laser rod were coupled by the conical cavity into the laser rod (side pumping). As shown in Fig. 2(a), the zigzag passage of the solar rays within the pump cavity ensured efficient multi-pass side pumping of the rod. The parameters of the conical cavity (36 mm input diameter, 12 mm output diameter, 80 mm length) were also optimized via simulation. Moreover, the copper surface of the conical cavity was polished and gold-plated to obtain high reflectivity in the cavity and durability.
Fig. 2. (a) Schematics of the laser head, composed of the quartz tube, conical cavity, and actively water-cooled Ce:Nd:YAG grooved and bonded crystal rod. (b) Photograph showing the laser rod consisted of 75 mm Ce:Nd:YAG and 20 mm YAG bonded, and was grooved on the sidewall. (c) Photograph showing the laser head mounted on the copper heat sink and X-Y-Z positioning system.
As shown in Fig. 2(b), a 6-mm-diameter Ce (0.05 at%):Nd (1.0 at%):YAG/YAG grooved bonded crystal rod (75 mm Ce:Nd:YAG, 20 mm bonded YAG) supplied by Chengdu, Dongjun Laser Co., Ltd., was used as the active medium. The bonded crystal structure was selected for increased SPL efficiency and reliability, and to reduce the absorption loss of the material at the clamping structure, as demonstrated in a previous study [17]. The grooved surface should increase the area of the interface between the laser medium and cooling water, which should diminish the thermal effects in the laser rod, which has been verified to be advantageous for SPLs [31]. The two end faces of laser rod were both coated for 1064 nm, with a high-reflection (HR) coating on the end face of the Ce:Nd:YAG part and an anti-reflection (AR) coating on the end face of YAG part. The output coupler behind the laser rod had a partial-reflection (PR) coating for 1064 nm, and the laser resonator was bounded by the PR-coated output coupler and the HR reflector. Aimed at maximizing the pump light transmission efficiency, the face of the Ce:Nd:YAG end of the rod was also AR-coated for the 400–595 nm, 735-825 nm and 855–885 nm wavelength bands (reflectivity, R < 5%).
As shown in Fig. 2(c), the conical pump cavity was mounted on a copper heat sink, and the space between the cavity and the heat sink was filled with aluminum foil. The cooling water was flowed through the quartz tube and into the heat sink at 5 L/min. After flowing out from the outlet of the heat sink, the heat from the rod, quartz tube, and conical cavity was removed by the coolant. A 6-mm gap between the far-end of the inner sphere of the quartz tube and the end face of the Ce:Nd:YAG part of the rod was more than sufficient for the passage of cooling water. Moreover, both the quartz material and the cooling water were useful minimizing UV solarization and IR heating of the rods. The output mirror was fixed in a mirror mount at a distance of 25 mm from end of the laser rod. The entire laser head was mounted on an X-Y-Z positioning system, ensuring that it was accurately optical aligned with the focal point of the output mirror.
3. Numerical analysis
3.1 Solar energy absorption and transfer in Ce:Nd:YAG medium
The AM1.5 solar irradiance spectrum [32] and absorption spectrum of Ce:Nd:YAG [26] are shown in Fig. 3. The broad absorption bands centered at 460 and 339 nm correspond to transitions of the Ce3+ ions from the 2F5/2 ground state to the 5d1 and 5d2 excited states, respectively [33]. The other absorption bands at wavelengths greater than 500 nm are characteristic absorptions from the 4I9/2 ground state of Nd3+ ions. The fluorescence of Ce3+ ions is characterized by a strong, broad green (500–720 nm) emission band assigned to radiative decay from the 5d excited levels to the 2F5/2 ground state [33], and this luminescence band overlaps two Nd3+ absorption bands at 515–540 nm and 565–595 nm.
Fig. 3. Standard solar emission spectrum (orange line), absorption spectrum of Ce:Nd:YAG (black line), fluorescence spectra of Ce3+ (green area), absorption spectrum of Ce3+ (blue area), and absorption spectrum of Nd3+ (red area).
The sensitization process and energy transfer mechanisms in Ce:Nd:YAG are summarized in the energy level diagram displayed in Fig. 4. When pump photons at wavelengths close to 339 nm and 460 nm are absorbed by Ce3+ ions, the energy is transferred from the Ce3+ to the Nd3+ ion via two pathways: radiative transfer [pathway (1) in Fig. 4] [24] and non-radiative transfer [pathway (2) in Fig. 4] [33]. Because of the overlap between the emission band of the Ce3+ ions and the Nd3 + absorption bands, photons produced by the radiative decay of the 5d excited levels to the 2F5/2 ground state of Ce3+ ions can be absorbed by Nd3+ ions, which are excited from the 4I9/2 ground level to the 4G7/2 and 4G5/2 excited levels [pathway (1)]. Another energy transfer mechanism is based on cooperative down-conversion, and it involves the transfer of energy from the pumped 5d levels of the Ce3+ ions to the 4F3/2 level of the Nd3+ ions via a non-radiative transfer pathway. Radiative transfer and non-radiative transfer have both been demonstrated to promote the emission of near-infrared radiation by Nd3+ ions, and the overall efficiency of energy transfer from the Ce3+ ions to the Nd3+ ions was calculated to be 76% [25].
Fig. 4. Energy level diagram illustrating the energy transfer mechanisms between Ce3+ ions and Nd3+ ions in the Ce:Nd:YAG active medium via radiative (1) and non-radiative (2) energy transfer pathways.
3.2 TracePro analysis
To calculate the absorbed solar pump power and determine the optimal parameters for the laser head, a simulation model of the solar pumped Ce:Nd:YAG laser with a Fresnel lens was built in TracePro (Lambda Research Corporation, Littleton, Massachusetts, USA). A similar solar pumped Ce:Nd:YAG laser, with a parabolic mirror instead of the Fresnel lens, was also simulated for comparison. To ensure the simulation was as realistic as possible, the standard solar spectrum (AM1.5) was used as a reference to distribute 1000 W/m2 of solar irradiance over the wavelengths of the solar spectrum [32], and a solar ray divergence angle of 0.53° was adopted. The light source was defined for a circular surface with diameter of 0.94 m, corresponding to 694 W of solar radiation. The Ce:Nd:YAG absorption spectrum was also defined in the simulation, and only additional two peaks were included, at approximately 339 and 460 nm, with the absorption spectrum of Nd:YAG [26]. Moreover, several parameters were also used, based on data provided in the TracePro software, including the absorption and dispersion spectra of polymethyl methacrylate (PMMA, the Fresnel lens material), silica, and water (materials of LLGL); in addition, R was set to 94% for the parabolic mirror and conical cavity.
After optimizing the parameters of the laser head, the distance between the laser head and primary concentrator was adjusted to maximize the laser medium absorption. Fig. 5 shows the focused solar rays incident on the laser head concentrated by the Fresnel lens [Fig. 5(a), (b)] or parabolic mirror [Fig. 5(c), (d)]. In Fig. 5, red rays represent the pump light wavelengths in the Nd3+ absorption bands, and purple rays represent the pump light wavelengths in the Ce3+ absorption bands. When solar rays are concentrated by the Fresnel lens, the majority of the pump light at wavelengths corresponding to the Nd3+ ions absorption bands is secondarily concentrated by the front of the LLGL and focused on the front end of laser rod, as shown in Fig. 5(a). Meanwhile, because of the dispersion of the Fresnel lens, pump light with wavelengths in the Ce3+ absorption band is focused before it is incident on the laser head; therefore, this pump light is mainly coupled by the conical cavity and absorbed by the middle and back of laser rod, as shown in Fig. 5(b). In contrast, when solar rays are concentrated by the parabolic mirror, all the pump light with wavelengths in the Nd3+ and Ce3+ absorption bands is both coupled by the front of the LLGL to obtain maximum absorption, as shown in Fig. 5(c), (d), and nearly all the pump light is focused on the front end of laser rod.
Fig. 5. Incidence of the solar pump rays on the laser head. (a) Solar pump rays with wavelengths in the Nd3+ absorption bands concentrated by the Fresnel lens. (b) Solar pump rays with wavelengths in the Ce3+ absorption bands concentrated by the Fresnel lens. (c) Solar pump rays with wavelengths in the Nd3+ absorption bands concentrated by the parabolic mirror. (d) Solar pump rays with wavelengths in the Ce3+ absorption bands concentrated by the parabolic mirror.
When pumped by a Fresnel lens, 96.25 W and 78.76 W of solar light was absorbed by the Nd3+ and Ce3+ ions, respectively, in a simulation based on 5 million solar rays in the wavelength range of 0.2–4 µm being traced. For comparison, the absorbed powers of the Nd3+ and Ce3+ ions were 98.80 W and 85.11 W, respectively, in the Ce:Nd:YAG system with a parabolic mirror. Thus, the absorption by Nd3+ ions was almost the same in these two systems, whereas the absorption by the Ce3+ ions was slightly lower with the Fresnel lens compared to when the parabolic mirror was used. Although the absorption efficiency of the laser rod was slightly lower, when the Fresnel lens was used part of the absorbed solar energy was transferred to the middle and back of laser rod to generate a more uniform absorption distribution along the length of the rod. The thermal effects and laser performances resulting from these different absorption distributions were analyzed using ASLD.
3.3 ASLD analysis
The absorbed and lost flux data generated by TracePro for the Ce:Nd:YAG part of the rod was converted into a cubic matrix containing 135000 zones and then processed by the ASLD laser simulation software package (ASLD GmbH, Erlangen, Germany) to determine the thermal effects and solar laser performances. For the Ce:Nd:YAG crystal, a stimulated emission cross-section of 2.8 × 10−19 cm2, fluorescence lifetime of 230 µs [34], and absorption and scattering loss of 0.0024 cm−1 were used in the ASLD analysis. For the Nd3+ ion absorption, a mean intensity-weighted absorbed solar pump wavelength of 660 nm [12] was used in the analysis. For the Ce3+ ion absorption, an overall efficiency for the energy transfer to the Nd3+ 4F3/2 level of 76% was utilized [25]. The two end faces of the laser rod were both coated for 1064 nm; a HR coating was used on the Ce:Nd:YAG end face and an AR coating was used on the YAG end face. In the ASLD program, the laser resonator was bounded by the HR-coated end face and a 1064-nm PR-coated concave mirror, which was positioned at a distance of 25 mm from the YAG end face. The reflectivity of concave mirror was varied in the range of 0.9–0.99 and the radius of curvature (RoC) in the range of −0.2 m to −2 m to achieve the highest laser output power for the Ce:Nd:YAG rod. It should be noted that because of cooling water flow, the boundary temperature of the laser rod was fixed at 300 K in ASLD. After running parameter study in ASLD, with an output coupler with a PR coating for 1064 nm (R = 0.97) and RoC of −1 m, the maximum multimode cw laser output power was numerically calculated to be 27.5 W. The laser beam quality factors Mx2 = 55.1 and My2 = 58.9 were also numerically obtained.
Numerical calculations based on ASLD analysis were used to obtain the heat load, temperature, and stress intensity distributions in the Ce:Nd:YAG laser rod section caused solely by Ce3+ or Nd3+ ion absorption, or by both absorptions [ Fig. 6(a)–(c)]. It is apparent that these distributions were dependent on the incidence of the solar rays as modeled using TracePro and shown in Fig. 5. Absorption by the Ce3+ ions occurred in the middle and at the front of the rod, and this produced a maximum heat load of 0.839 W/mm3, maximum temperature of 317.9 K, and maximum thermal stress of 31.82 N/mm2, when calculated separately. The Nd3+ absorption distribution was principally at the front end of the rod, and generated a maximum heat load of 1.334 W/mm3, maximum temperature of 345.9 K, and maximum thermal stress of 59.49 N/mm2 values, when calculated separately. Consequently, the solar radiation absorption in the Ce:Nd:YAG rod for the setup containing the Fresnel lens was distributed over the middle and front of the rod, and the maximum heat load was 1.766 W/mm3, maximum temperature was 364.3 K, and maximum thermal stress was 94.73 N/mm2.
Fig. 6. Heat load, temperature, and stress intensity distributions in the Ce:Nd:YAG part of the laser rod for the setup employing the Fresnel lens, as determined numerically using ASLD analysis: (a) Effects of absorption by Ce3+ ions alone. (b) Effects of absorption by Nd3+ ions alone. (c) Effects of absorption by both Ce3+ and Nd3+ ions.
For comparison, the absorbed and lost flux data from the simulation of the laser with the parabolic mirror collection and concentration system was used to perform an ASLD analysis of the thermal effects, as shown in Fig. 7 because the absorption by the Ce3+ and Nd3+ ions principally occurred in the front section of laser rod, the maximum heat load, temperature, and stress intensity distributions were all calculated to be significantly higher—2.767 W/mm3, 457.2 K, and 207.3 N/mm3, respectively. In particular, a maximum stress intensity of 207.3 N/mm3 would result in the laser rod fracturing, as its stress fracture limit is 200 N/mm2 [34], similar to the result reported by Vistas et al. [27]. Therefore, despite the fact that the total absorptions of 175.01 W and 183.91 W were similar, the setup incorporating the Fresnel lens experienced significantly lower thermal effects compared to the setup incorporating the parabolic mirror, owing to the dispersion of Fresnel lens. Moreover, the limiting input solar power corresponding to the fracture of the Ce:Nd:YAG rod was also calculated; for end-side pumping the Ce:Nd:YAG SPL using a Fresnel lens, this value was 1520 W, and for end-side pumping using a parabolic mirror it was 669 W. It is clear that when the Ce:Nd:YAG rod is end-side pumped, the system with the Fresnel-lens concentrator has the advantage of greater stability.
Fig. 7. Heat load, temperature, and stress intensity distributions in the Ce:Nd:YAG part of the laser rod in the setup employing the parabolic mirror, as determined numerically using ASLD analysis.
4. Experiment and results
Based on the numerically optimized design parameters and thermal effect calculation results obtained using TracePro and ASLD, a SPL prototype was constructed. The effective area of the Fresnel lens was 0.69 m2, and cardboard was used to form an aperture and cover the four corners and edges of the Fresnel lens, as shown in Fig. 1. The Ce:Nd:YAG SPL was tested in Beijing in October 2022, the day’s measured maximum solar irradiance was 1010 W/m2. For comparison, a 6-mm-diameter Nd (1.0 at. %):YAG/YAG grooved bonded crystal rod (75 mm of Nd:YAG, 20 mm of bonded YAG), supplied by Beijing JIEPU TREND Technology Co., Ltd was also tested. These two bonded laser rods had the same size and underwent the same surface processing and surface coating, and the only difference between them was the 0.05 at. % Ce3+ co-doping of the Ce:Nd:YAG rod.
Before the solar laser emission experiment, losses in these two laser rods were compared by setting up an optical measurement system with a 1064-nm polarized laser, 1/4 wave plate, and polarization beam splitter. The 1064-nm laser beam was incident on the AR-coated face of the laser rod, reflected by the HR coated face, underwent a round trip of the laser rod, and finally exited via the AR-coated face. After repeated measurements, it was found that with the same incident power, the laser power exiting from the Ce:Nd:YAG rod was 0.994 times that exiting from the Nd:YAG rod. This difference can be attributed to increased absorption and scattering losses caused by Ce3+ co-doping, and the loss coefficient was calculated to be 0.0004 cm−1 higher for the Ce:Nd:YAG rod.
To maximize the solar laser emission, output mirrors with a reflectivity of 0.97 and RoC of −1 m were mounted 25 mm away from the AR coated face of the laser rod, as shown in Fig. 8. During the experiments, the solar irradiance was varied from 990 to 1010 W/m2 and simultaneous measured by a sun-tracking pyrheliometer (NHZJ54, supplied by Wuhan Zhongke Nenghui Technology Development Co., Ltd). The laser output power was measured using a laser power meter (Thorlabs-S322C with Thorlabs-PM100D console).
By changing the position of the sunlight blind, different output laser powers were measured with the power meter, and the input solar power could be calculated from the effective collection area and measured solar irradiance. When the input solar power was increased to approximately 200 W, corresponding to a solar irradiance of 1000W/m2, lasing began to occur in the Ce:Nd:YAG medium. When the blind was fully opened, approximately 694 W of solar power was concentrated by the Fresnel lens, and a maximum output solar laser power of 26.93 W was obtained, as shown in Fig. 9, in good agreement with the numerical predictions. As a comparison, the maximum output power of the Nd:YAG rod was 17.87 W when the blind was fully opened and a solar irradiance of 1005 W/m2 (input power, 697W). Besides, about 260 W solar incidence was required for lasing to occur. The corresponding slope efficiencies for the Ce:Nd:YAG and Nd:YAG solar lasers were 6.33% and 4.70%, respectively. Therefore, the pumping efficiency of the Ce:Nd:YAG laser was 1.35 times greater than that of the Nd:YAG laser, slightly lower than the predictions of Payziyev et al. [25], which was attributed to the higher losses of the Ce:Nd:YAG rod, as measured before the experiment.
Fig. 9. Solar laser output power versus incident solar power for Ce:Nd:YAG and Nd:YAG grooved bonded crystal rods. A PR-coated output mirror (R = 97%, RoC = −1) was used and pumping occurred via a 0.69-m2 Fresnel lens.
The laser beam quality factors M2 were measured according to the ISO 11146-1 standard [35] using a Thorlabs BC106N-VIS camera. The measured solar laser beam widths along the beam caustic, measured after a laser beam attenuator and a focusing lens, are shown in Fig. 10; Mx2 = 57.3 and My2 = 61.9 were obtained, which are slightly higher than those predicted by the ASLD simulation.
Fig. 10. Measured beam diameters of the SPL at different distances from the output mirror with hyperbolic fits.
5. Discussion
As the most important index for characterizing SPL efficiency, the solar laser CE was compared for some significant previously reported SPL studies (Table 1). A Ce:Nd:YAG solar laser power of 26.93 W cw was obtained in our work, corresponding to a solar laser collection efficiency of 38.8 W/m2, which is 1.21 and 1.19 times higher than the previous record values for Nd:YAG [17] and Cr:Nd:YAG [18] lasers, respectively. The significant achievements of this study are discussed below.
Table 1. Comparison of our collection efficiency results with those of significant previously published solar laser studies
First, to the best of our knowledge, this study was the first in which a Fresnel lens was used to concentrate solar radiation to generate a Ce:Nd:YAG laser emission, and the CE obtained was 20.9% higher than the record for the use of a Fresnel lens as an SPL concentrator [17]. Second, compared to recently reported setups with Ce:Nd:YAG SPLs and parabolic mirrors, our design is competitive in terms of volume, weight, and cost. Third, in this study, the dispersion of the Fresnel lens was utilized to solve the problem of heat accumulation in the Ce:Nd:YAG crystal, and the CE was also the record value for a solar laser power emitted by single rod. Although, the CE was only slightly higher than the previous record value [29], our maximum output power was a factor of 2.4 greater. Moreover, upper limit for the input power was calculated to be 1520 W for our setup, hence it has the potential to generate greater output powers. Finally, the CE obtained in this study is higher than all the CEs reported, except for that obtained using the simultaneous-emission three-rod Ce:Nd:YAG laser, which is the current record value for all SPLs [30] shown in Table 1. However, single-beam lasers, such as the laser in this work, should have longer effective distances and simple structure which will be advantageous for various applications.
Considering that this study was the first to combine an Ce:Nd:YAG SPL with a Fresnel lens solar collection system, many aspects of the experimental design merit further optimization. For example, in this study, an effective collection area of only 0.69 m2 was used to collect 694 W of input solar power, which is far below the calculated limiting value of 1520 W. Thus, attempting to use a larger collection area is an obvious strategy for obtaining a higher output power, which will, hopefully, also correspond to a higher CE. However, as shown in Fig. 6(c), the thermal effects of the absorption in the middle of the Ce:Nd:YAG rod were very weak. Thus, the laser head, and in particular the conical cavity, needs to be specifically designed to focus the solar light such that the absorption distribution is centered in the middle of the laser rod, and subsequently it should be possible to reduce the length of the laser rod to achieve a higher output efficiency.
6. Conclusion
We designed a Ce:Nd:YAG SPL based on a Fresnel lens solar energy collection and concentration system and a solar laser head consisting of a quartz tube and conical cavity, within which a 6-mm-diameter, 75-mm-length of Ce:Nd:YAG crystal rod, with a 20-mm length of YAG bonded to its end, was efficiently pumped. Solar energy absorption and transfer in the Ce:Nd:YAG medium from Ce3+ ions to Nd3+ ions was investigated and modeled using the TracePro and ASLD simulation software packages.
The design parameters for the laser head and solar laser resonator were optimized using TracePro and ASLD, respectively. Compared with an analogous system, incorporating a parabolic mirror instead of a Fresnel lens with the same solar laser head, the thermal effects—which were always the most critical problems for Ce:Nd:YAG SPLs—were rigorously analyzed using ASLD. The simulation results demonstrated that, compared to the use of the parabolic mirror, the use of the Fresnel lens with the Ce:Nd:YAG laser rod resulted in reduced thermal effects and a lower risk of fracture; this was because of the dispersion of the pump light by the Fresnel lens such that different wavelengths were incident on different parts of the laser rod. In addition, for the Fresnel lens, a simulated solar laser performance characterized by an output power of 27.5 W and laser beam quality factors Mx2 of 55.1 and My2 of 58.9 was numerically calculated at 694 W of input solar power, and an upper limit for the input solar power of 1520 W was also predicted.
The performance of the Ce:Nd:YAG solar laser with the Fresnel lens collection and concentration system was analyzed and compared with that of the Nd:YAG solar laser constructed using same design. A cw output power of 26.93 W was obtained from the Ce:Nd:YAG laser, with a 0.69-m2 effective solar collection area, corresponding to a CE and slope efficiency of 38.8 W/m2 and 6.33%, respectively, 1.51 and 1.35 times those of the Nd:YAG laser. The experimentally measured output performance, i.e., the output power and laser beam quality factors (Mx2 = 57.3, My2 = 61.9), were very similar to those obtained by simulation. Furthermore, to the best of our knowledge, our CE of 38.8 W/m2 and slope efficiency were 1.21 and 1.17 times greater than the highest value reported for a Fresnel lens concentration system, and the index of CE represents a new record for single-beam SPLs. Considering this significant progress was obtained thanks to the first instance of a Fresnel lens being used with a Ce:Nd:YAG laser, this approach has potential to realize further improvements in SPL output efficiency based on increases in the input solar power and optimization of the laser head.
Funding
National Natural Science Foundation of China (61378020, 61775018).
Acknowledgments
The authors acknowledge the support of the National Natural Science Foundation of China (61378020) and (61775018).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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