The quest for high efficiency, compactness, and lightweight design in fiber-coupled laser diode modules is becoming increasingly prominent, particularly within the solid-state laser domain and other fields where high integration or portability is demanded. Given the considerations of cost and the complexities involved in mounting and tuning, laser diode stacks emerge as the preferred sources for high-power fiber-coupled laser diodes. Nonetheless, the pronounced disparity in beam quality across the fast and slow axes presents challenges in accomplishing efficient coupling. To bridge this quality gap, it is imperative to tailor the beam profile of the laser diode stack accordingly. Currently, geometric beam shaping is considered as the prevalent technique for modifying the output of laser diode stacks, enabling beam manipulation without altering the inherent output traits of the laser. This method primarily involves the cutting and reconfiguration of beams through the use of parallel plates or similar optical elements. However, this conventional approach leans on an extensive array of prisms, leading to cumbersome systems. A significant drawback is that optical components are limited to singular functions, lacking in integration. Thus, devising a cohesive beam-shaping strategy is vital for the advancement of integrated fiber-coupled laser diode systems.

To address the issues concerning the poor beam quality of the laser diode stack and the complexity of beam shaping elements, we propose a new cut-compression-rotation-rearrangement beam shaping method for the laser diode stack and design a stepped rotation rearrangement prism. First, to realize an efficient collimation effect, we collimate the beam by using fast and slow axis collimation lenses with effective focal lengths of 0.3 mm and 8 mm, respectively, resulting in a parallel beam. Subsequently, the designed stepped rotation rearrangement prism is utilized for cutting the beam in the direction of the slow axis in a staggered arrangement. The rotational rearrangement of the beam is realized through two total reflections of light to achieve a dense arrangement of light spots. Then, to effectively reduce the focused spot and obtain a higher energy density, by using beam expansion, a spot with a slow-axis beam width of 9 mm and a residual divergence angle of 4.65 mrad is produced. Finally, an aspherical lens with a focal length of 25 mm is selected for the purpose of focusing, facilitating the coupling of the spot into the target fiber.

The beam parameter products in the fast and slow axis directions after collimation are 12.62 mm·mrad and 17.46 mm·mrad, respectively (Fig. 1). However, the filling factor in the fast-axis direction is low, and there is a large dark area. Subsequently, the collimated beam passes through the stepped rotation rearrangement prism to eliminate the dark area (Fig. 3). The entire beam shaping process of cutting, compression, rotation, and rearrangement of the beam is accomplished by utilizing two total reflections of the beam, without the need for a prism stack. The consideration of aberration and material effects on the beam-shaping process is deemed unnecessary. At this time, the beam parameter products in the fast and slow axis directions are 5.39 mm·mrad and 10.48 mm·mrad, respectively, which satisfy the conditions for fiber coupling. To effectively reduce the focused spot and obtain a higher energy density, the shaped spot must be expanded. A 3× Galilean beam expansion system is constructed using a flat concave cylindrical lens having an effective focal length of -6.35 mm and a flat convex cylindrical lens having an effective focal length of 19 mm. Following this beam expansion system, the slow-axis beam width increases to 9 mm, which is equivalent to a 4.65 mrad residual divergence angle (Fig. 4). After the beam is expanded and focused, it can be coupled into a fiber with a numerical aperture of 0.22 and a core diameter of 200 µm (Fig. 5).

In summary, we propose a new cut-compression-rotation-rearrangement beam shaping method for the laser diode stack. The stepped rotation rearrangement prism beam shaping device, created using this method, only requires the use of multiple identical stepped compression rotation rearrangement prisms to realize the entire beam shaping process, in contrast to traditional beam shaping devices. This significantly simplifies the beam shaping process and minimizes the overall system size. According to the simulation results, a laser diode stack consisting of eight bars can be coupled into a fiber with a core diameter of 200 µm and numerical aperture of 0.22 by using a stepped rotation rearrangement prism. The resulting fiber has an output power of 455.4 W and a fiber coupling efficiency of 94.9%. The system measures a mere 20 mm×60 mm×15 mm, making it compact and highly suitable for solid-state laser pumping.