Objective In the field of ultrashort pulse laser, green laser diodes (LDs) as the pump source of Ti: sapphire laser have received increasing attention. It has been reported that increasing the power of the green laser-pumping source can further improve the output power of Ti: sapphire lasers. However, compared with gallium arsenide-based materials in the near-infrared band, gallium nitride-based materials with a wider bandgap can radiate green light and are prone to produce defects in the growth process, which reduce the radiation efficiency and output power. The maximum power of a single LD commercially available is 1.5 W. Thus, it is necessary to use a beam combining technology to improve the output power. Presently, the incoherent beam combining methods, such as spatial or polarized combination, are mostly used to improve the output power of LD modules. However, it is found that the combined beam, in most cases, is a rectangular beam array. When such a beam is focused by a coupling lens, the angular filling factor (AFF) is low (AFF is defined in Section 2.1), resulting in wasting part of the angle space. If the beam filling can be performed in this region, the brightness of the fiber output can be improved further. The brightness of the pump source is also the key factor to increase the output power of the titanium sapphire laser. This study proposes a method for increasing the AFF to fully use the angle space by combining beams in a closest-packed structure. Based on the proposed method, a high-brightness fiber-coupled green LD module is demonstrated using ZEMAX.

Methods To solve the problem of low AFF and the inability of the numerical aperture of a fiber to be fully used when the beam in a rectangular array is coupled into the fiber, we propose to arrange the beams according to a closest-packed structure. Based on this arrangement, we design a high-brightness fiber-coupled module of green LDs. First, we divide 14 1.5-W green LDs into two groups of 7 each and placed them on the six vertices and centers of the regular hexagon to form a dense structure. Then, the beams emitted from green LDs are collimated, respectively, by the fast and slow axis collimators to form a series of parallel beams. Afterward, the beams at the vertices are moved toward the center beam through a custom-designed beam reducer. Thus, the dark area between the beams can be eliminated while the beam divergence angle is constant. Second, beams emitted from one of the two groups pass through a half-wave plate, and the polarization direction is rotated 90°, then two groups of beams with perpendicular polarization direction are combined into a beam through a polarization beam combiner. Finally, the beam is focused through an aspheric lens into a fiber with a core diameter of 105 μm and a Na of 0.15.

Results and Discussions After collimation, the output beam of LD is approximately circular symmetry (Fig. 3). The divergence angles in the fast and slow axes are 0.262 and 0.750 mrad, respectively (Table 1), which can meet the design requirements. Then, the combined beam passes through the beam reducer and eliminates the dark area between the beams. The combined beam diameter in the fast and slow axis is 6.88 and 7.83 mm, respectively (Fig. 7). When the beam is coupled into an optical fiber, the AFF is calculated to be 63.80%, which is higher than that of the rectangular beam (Eq (3)). The simulation results show that the power coupled out of the fiber is 19.13 W—corresponding to a brightness of 3.125 MW·cm ^-2·Sr ^-1—and the fiber-coupling efficiency is 93.75%. Notably, to describe the concept of the AFF clearly, we deliberately collimate the output of the LD into an approximate circular spot in the near field, and it result in an asymmetry of the spot focused on the facet of the fiber in Fig. 9. The asymmetry can be improved in practical applications by combining multiple beams into a circular beam, replacing the subspot in Fig. 7.

Conclusions In summary, we propose a design of an optical fiber coupling module based on the closest-packed structure of combined beam array. It can fully use the numerical aperture of fiber and improve beam brightness. The simulation shows that 14 1.5-W LDs can be coupled into an optical fiber with a numerical aperture of 0.15 and core diameter of 105 μm, and a total power of 19.13 W can be obtained with an output brightness of 3.125 MW·cm ^-2·Sr ^-1—corresponding to a fiber-coupling efficiency of 93.75%. In the future, experiments should be performed to provide a better pumping source for Ti: sapphire solid-state lasers.