Because of their applicability, numerous researchers have conducted studies on SPLs. Young reported the first continuous-wave Nd:YAG SPL emission with a power of 1 W in 1966 [
7]. To obtain appreciable laser output power, parabolic mirrors with large collection areas, for example, 78.5 m
2 [
8], 50.2 m
2 [
9], 38.5 m
2 [
10], and 6.75 m
2 [
11], were used to concentrate solar radiation on laser media in early studies. These primary concentrators with parabolic mirrors were very large and expensive, and the mechanical supporting structures of the laser cavities inside the parabolic mirrors produced shadows that reduced the effective solar energy collection area. In the last decade, the adoption of Fresnel lens designs has accelerated the development of low-cost solar lasers with reduced weight. As a consequence, many studies focusing on obtaining higher laser collection efficiency, which is defined as the laser output power per unit area of the solar collector, have been conducted in recent years. In 2007, Yabe et al. obtained an 18.7 W/m
2 laser collection efficiency by end pumping a 9-mm-diameter, 100-mm-long Cr:Nd:YAG ceramic rod using a 1.3-m
2 Fresnel lens [
12]. In 2011, a laser collection efficiency of 19.3 W/m
2 was achieved by Liang and Almeida using a 0.64-m
2 Fresnel lens to end-side pump a 4-mm-diameter, 25-mm-long Nd:YAG crystal rod [
13]. In 2012, Dinh et al. reported a 30-W/m
2 laser collection efficiency when end-side pumping a 6-mm-diameter, 100-mm-long Nd:YAG crystal using a 4-m
2 Fresnel lens [
14]. More recently, in 2018, Guan et al. achieved a 32.1-W/m
2 laser collection efficiency with the use of a 1.03-m
2 Fresnel lens for end-side pumping a 6-mm-diameter, 95-mm-long Nd:YAG–YAG-bonded crystal rod [
15]. However, because of the defects of Fresnel lenses such as dispersion, which limits the geometric concentration ratio, Fresnel lenses have not completely replaced parabolic mirrors in SPL designs. For example, in 2018, Liang et al. reported a laser collection efficiency of 32.5 W/m
2 when end pumping a 4.5-mm-diameter, 35-mm-long Cr:Nd:YAG ceramic rod using a heliostat-parabolic mirror with a 1.0-m
2 effective collection area, which represents the record for highest solar laser collection efficiency [
16]. In addition, sunlight is more focused when concentrated by parabolic mirrors rather than Fresnel lenses, because of the absence of dispersion. Thus, compared with SPLs with Fresnel lenses, SPLs with parabolic mirrors typically have a better brightness figure of merit [
17,
18], which is defined as the ratio between laser power and the product of the M
x2 and M
y2 factors [
11]. The use of a parabolic mirror in an SPL design also makes it compatible with side pumping to produce a TEM
00-mode output [
19,
20].
Either a Fresnel lens or a parabolic mirror is used as the primary concentrator in almost all SPLs. However, as mentioned above, a drawback of the former is dispersion, whereas when the latter is used, the laser cavity shadows the incoming solar light. Hence, neither option is the perfect choice for the primary concentrator of a SPL. Therefore, it is very important to develop new solar-energy collection and concentration systems. In recent years, many novel solar concentrators have been proposed, and numerically calculated laser performances for their use in SPLs have been reported [
21,
22,
23,
24,
25]. Among these, a modified ring-array concentrator (RAC) allowed some remarkable laser performances to be achieved. When a small Fresnel lens was added to the center of the RAC, laser collection efficiencies of 38.4 and 29.18 W/m
2 were obtained via end-side pumping and side pumping a Nd:YAG crystal, respectively [
22,
25].
A novel 1.5-m-diameter solar concentrator composed of a modified parabolic mirror and Fresnel lens mounted coaxially is proposed in this paper. To avoid shadowing in the laser cavity, a section of the parabolic mirror behind the focal point is removed. Thus, the total solar collection area, including the remaining annular parabolic mirror section and the Fresnel lens, spans 1.766 m2. As a compound structure, this solar concentrator combines the advantages of Fresnel lenses and parabolic mirrors. When compared with a parabolic mirror concentrator, the replaced part with a Fresnel lens produces a solar concentrator that is less heavy and shadowing does not occur. When compared with a typical Fresnel lens concentrator, the reduced diameter of the Fresnel lens in this compound system means that the influence of dispersion is reduced. Further, because the Fresnel lens and parabolic mirror pump the laser medium from the end and side, respectively, flexible adjustment of the solar power distribution in the laser medium is possible. It should also be mentioned that because the focus of the modified parabolic mirror is off-axis, serious heat accumulation in the laser medium is prevented.
A series of optimizations performed using the TracePro and ASLD simulation software packages ensured that, when combined with a novel dual-parabolic pump cavity, the solar radiation collected by this solar concentrator was uniformly and efficiently absorbed by a 5-mm-diameter, 22-mm-long Nd:YAG single-crystal laser rod. The continuous-wave solar laser output power was 74.6 W, and laser beam quality factors of M
x2 = 34.0 and M
y2 = 35.1 were obtained numerically; the corresponding laser collection efficiency was 42.2 W/m
2, and the beam brightness figure of merit was 0.063 W, i.e., 1.1 and 2.6 times greater than the previous records, respectively, for an end-side pumped SPL [
22]. The structure model, design principles, and optimization of the detailed parameters are explained in
Section 2. The laser performance, numerically calculated by ASLD software, is explained in
Section 3. A discussion of the results is presented in
Section 4.