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Abstract
A novel scheme involving a three-dimensional (3D) coupling structure is used to build a laser diode (LD) module that utilizes the fiber angle space more completely. Compared with the traditional 2D configuration, the number of LD’s is increased by 4, obtaining an increased efficiency of over 90%. Another advantage is that some costly optical elements used in the traditional polarization combination method are not necessary.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The rapid development of high power semiconductor lasers has produced increased applications, such as materials processing, pumping solid-state lasers [1–3], and fiber lasers [4–6]. Other important examples include laser headlights [7], self-driving LIDAR system, and 3D-printing. However, the most compact kilo-watt laser fiber modules employ a polarization combination scheme [8–10] that uses a two-dimensional (2D) layout for the laser diodes (LD). For example, several LD bars are located appropriately to utilize the fiber’s angle space more completely [9]. Another method called “margin arrangement” to better use the space by analyzing the relation between fiber’s normalized frequency and the BPP (beam parameter product) of the emitter can also give high power and efficiency [10]. Some optical elements, including lenses, half-wave plate (HWP) and polarization beam splitters (PBS) are used to combine the LD beams into a fiber with a selected core diameter and numerical aperture (NA). Those two factors restrict the allowed LD power to be coupled [6]. As will be shown later, the 2D LD layout cannot fully utilize the fiber angular space; even when the physical space is fully used. Here we propose a three-dimensional (3D) coupling structure that can fully use both the fiber angular and physical space. The main feature of this novel 3D scheme is that the number of LDs (thus the total power) increases, while the efficiency can be maintained at the same level. More importantly, the polarization combination method is not needed, thus costly HWP and PBS can be saved.
2. Design and simulation
In order to compare the two schemes, a typical 2D LD module is employed as the reference. Figures 1 (a) and (b) show the top and side views, comprising 14 edge-emitting laser diodes. They are numbered as indicated in the figure for convenience of identification in the angular and physical spaces used later. As shown in Fig. 1(a), there are two rows of LDs in the x-z plane. Each LD is collimated by the fast-axis collimator (FAC) and slow-axis collimator (SAC) first. The collimated beams in the lower row (#1 to #7) are TM polarized (along x-axis). After being reflected by the dielectric mirror, they pass through the PBS and are focused by the focus lens on the fiber end. The collimated beams in the upper row (#8 to #14) are also TM polarized. However, after reflection by the dielectric mirror at the right, they first pass the HWP and the polarization is transformed into the TE mode. Thus those beams are reflected by the PBS and combined with the lower ones, and coupled into the fiber together. Of course the height of every LD beam is different in the two rows to avoid blocking the beams behind, just like a stairs as seen in Fig. 1(b). LD #1 is located in the lowest position and has shortest optical length, and LD #7 is located in the highest position and has the longest optical length. The height difference is 0.5 mm and LD #4 is at the center aligned with the focus lens center. This is how the 2D scheme and polarization combination method work. The simulation software is ZEMAX.
A typical high power edge-emitting LD is schematically shown in Fig. 2. The emitting area usually has a wide width (Wx) and narrow height (Wy). Due to the diffraction, the divergent angle (θx) along x is small (slow axis) and that along y is large (fast axis), as shown in the figure. That is why we need to use FAC and SAC to collimate the beam separately to maintain the beam quality. Table 1 illustrates the sources and elements specifications used in Figs. 1 and 2. LD is placed at the back focal length (BFL) of FAC and SAC, which are cylindrical lenses and usually made of high refraction index material, as shown in Table 1. All specifications listed in the Table are adapted from commercially available products [11].
Table 1. Details of the elements used in Fig. 1
After careful design and calculations, the whole 2D module was built for simulation. Figures 3(a) and (b) illustrate the irradiance and intensity distribution at the fiber entrance end respectively. The red circle indicates the physical space (±50μm) and angle space (±9.8°) of the fiber, as shown in the last row of Table 1. We find that most of the irradiance and intensity distributions are located inside of the circle and the collected power is 132 W. Since the total power emitted is 10*14 = 140 W (total 14 LDs and 10 W for each), the efficiency is 132/140 = 94.3%. Here the Fresnel loss from the mirrors or lenses is neglected, which can be reasonably accounted for by applying anti-reflection coating on those surfaces. From Fig. 3(a), it shows that a small part the flux falls outside of the red circle space, but all 7 angular intensities (LD #1 to #7) from the lower row in Fig. 1(a) lie inside the angle space circle, as seen in Fig. 3(b). The other 7 beams (LD #8 to #14) from the upper row have the same angle distribution since they are in the opposite positions and have the same heights as the lower row. Thus, this module is space-limited by the fiber’s core area. It is seen from Fig. 3(b), the angle distribution for LD #1 (or #8) is very clear and compact, but that for LD #7 (or #14) is more vague and diffused. That is because the optical path is the shortest for the former and it is the longest for the later. The longer the path distance is, the greater the spread for the collimated beam. When it finally hits the focusing lens, it uses a larger portion of the lens surface, resulting in more angular spread. More importantly, we can see that this 2D stair layout has used up almost all of the vertical angular space; and consequently, it is not possible to place other extra LDs before LD #1 (and #8) or behind #7 (and #14) to increase the power. Those extra LD beams after passing the focus lens will have slopes that are too steep to be coupled into the core’s acceptance angle and lay outside the red circle, as seen from Fig. 1(b) and Fig. 3(b). Note that from Fig. 3(b), plenty of angle spaces remain in the two horizontal sides, as indicated by the green arrows. Motivated by this observation, we propose a 3D LD layout that can utilize the angle space more completely. The basic principle is using same collimated LD beam in 2D structure, but the position and direction are carefully calculated and relocated to fully use the angle space. The total number of LDs can therefore be increased without using the polarization combination.
Figures 4(a) and 4(b) show the side-view and iso-angle view of the new 3D configuration. All optical elements have the same specifications, as shown in Table 1. Here the optical path length of each beam is set as 21 mm, which corresponds to LD #4 in Fig. 1. There are 3 layers and a total of 18 LDs. Each layer is indicated by L1, L2, and L3 respectively in Fig. 4. In order to use the entire angle space, the positions and directions of each LD and mirror must be carefully designed and calculated, as described below.
As shown in Fig. 5(a), Layer 1 is composed of an octagon mirror (length of a side is 1.5 mm with 0.45 mm height) and 1 LD at each side. So the LDs number in L1 is 8. The angle between adjacent beams is 45° and the distance between the mirror and the LD is about 20 mm. As shown in Fig. 5(b), Layer 2 is composed of a hexagon mirror (length of a side is 1.4 mm with 0.45 mm height) and 1 LD at each side. The angle between adjacent beams is 60°. So the LDs number in L2 is 6. The slope of each mirror side for the two mirrors in L1 and L2 is 45° to reflect the horizontal beams into vertical ones and hit the focus lens. Note that there is an opening (with 1.9 mm diameter) in the middle of the octagon and hexagon mirrors, which provides a gateway for the 4 LDs in Layer 3, as shown in Fig. 4 and Fig. 5. The distance between adjacent LD is 0.45 mm and total width is 1.8 mm, as indicated in Fig. 5(c). Thus, the total number of LD is 8 + 6+4 = 18.
Fig. 5. Configuration of each layer. (a) Layer 1 (top view). (b) Layer 2 (top view). (c) Layer 3 (side view).
Figure 6(a) and (b) illustrate the irradiance and intensity distribution of the 3D structure at the fiber entrance end respectively. We find that the irradiance is more cylindrically symmetric, compared with that of the 2D structure in Fig. 3(a). More importantly, as seen in Fig. 6(b), the angle space is fully utilized and the pattern clearly shows the three layers configuration; i.e. the outmost circle has 8 blocks coming from L1, the inner circle has 6 blocks coming from L2, and the middle has 4 blocks coming from L3. Each angle distribution block for every LD in Fig. 6(b) is also very solid, compared with those in the longest path (LD #7 or #14) in Fig. 3(b). That is because the path length is shorter (about 20 mm) for each LD in the 3D structure than those of longest path (about 30 mm) in the 2D structure, as seen in Fig. 1(b). Those solid angle blocks are also the key to successfully put all the LD beams inside the red circle angle space. The total power collected is 166 W. Since the total power emitted is 10*18 = 180 W (total 18 LDs and 10 W for each), the efficiency is 166/180 = 92.2%. Although this number (92.2%) is a little lower than the number (94.3%) in the 2D structure, it still reaches high efficiency requirement, over 90%. Another advantage of this 3D structure is that this scheme does not use the polarization combination method; thus the costly HWP and PBS can be saved.
Although this 3D structure offers some merits, the alignment issue is a challenging task. Thus the tolerance analysis is performed and the results are shown in Table 2. As the original coupling efficiency 92.2% is indicated in the first row to facilitate the comparison, we analyze the efficiency drop when the collimated laser beam is not aligned with the center of the mirror. Taking the left beam (blue one in Fig. 5(a)) in layer 1 as an example, the coordinate system in Fig. 2 is used. Thus the tolerance ±0.1mm in X direction means that the beam is moved to the right (or left) of the original position. Similarly, moving along Y direction means that the beam is moved to the up (or down) of the original position, and Z direction is moved forward or backward of it. As seen from Table 2, the efficiency drops to about 85% when the displacement along X and Y is ${\pm} 0.2$mm and ${\pm} 0.$1 mm respectively; thus it is more sensitive to the tolerance in Y direction. Also shown is the effect on efficiency moving along Z; it is found that the efficiency still remains high above 90%, even when the tolerance is as large as 5 mm. This is not surprised because the mirror is not so sensitive to a collimated beam when the beam moves backward or forward.
Table 2. Tolerance analysis
3. Conclusions
This work proposed a novel 3D scheme to increase laser diode module power. The traditional 2D LD module power is restricted by the fiber vertical acceptance angle space; thus the power cannot be increased further, even after employing the polarization combination method. In the present situation, there are total 14 LDs, with 132 W providing 94.3% efficiency. Our proposed 3D scheme uses 18 LDs with 166 W providing 92.2% efficiency. Using this scheme, the power increases by 34 W, although the efficiency is down by about 2%, due to slightly more irradiance lying outside of the core area. The main reason for more LDs being successfully used is that the fibers’ angle space is utilized more completely, compared with the 2D structure. Another advantage of this scheme is that the polarization combination method is not required; thus the HWP and PBS are not necessary and the cost can be reduced. We think this 3D configuration concept can be applied to other similar cases. When the ordinary 2D structures cannot give more power, this method is a good alternative to use, while maintaining high efficiency and saving costs.
Funding
Ministry of Science and Technology, Taiwan (110-2622-E-005-009).
Acknowledgments
P. Han thanks the National Chung Hsing University for supporting this work.
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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