# High Brightness Diode Lasers Based on Beam Splitting and Polarization Combining
## 3 Results  
### Bspc of Single BA Diode Laser  
A 980 nm BA diode laser with a ridge beam width of 180 \(\upmu\)m was used as the light source for Bspc, which was put on a water-cooled plate with a water temperature of 20 \({}^{\circ}\)C. Figure 2 shows the power curves of the laser before combining (black balls) and after Bspc (red balls) under CW operation. The power was measured by Ophir FL500A. As can be seen, the output power of the laser under free running was 6.78 W at 8 A, and the electro-optic conversion efficiency of the laser was 51.2%. The corresponding power for Bspc was 6.13 W at the same driven current, which was approximately 9.5% lower than that of the same laser at free running. The reason for the power reduction was because the polarization of the laser was not pure enough, resulting in the loss in PBC. In addition, the power loss was partly due to the insufficient reflectivity of \(M_{1}\), \(M_{2}\), and \(M_{3}\) and the precisely cut prism. The inset in Figure 2 shows the far-field spots before and after Bspc at 6 A. It can be seen that the spot shape of the laser with Bspc was asymmetric and exhibited a much narrower lateral far-field pattern, which enabled the improvement of beam quality. The inset is the far field of before and after Bspc. It can be seen that the shape after the Bspc's far field was asymmetric and like a shield. The reason for the asymmetric shape was that the original laser was separated from the middle of the beam. The power in the middle of the original beam was higher than that at the edge. The separated edge became blurred due to the diffraction effects of the triangle prism.  
To obtain the details of beam quality, the \(M^{2}\) factors were measured by using Thorlabs \(M^{2}\) measurement systems (M2 MS). Figure 3 shows the current dependent \(M^{2}\) factors in the fast axis (\(M^{2}y\)) and slow axis (\(M^{2}x\)) of the laser with and without Bspc from 3 A to 8 A. The \(M^{2}y\) of both devices were very close in the range from 1.1 to 1.28 and increased slowly with the increase of current. The small fluctuations in values were due to the measurement errors. Because the beam-waist splitting was along the slow axis, the improvement of beam quality happened only in slow axis and it was almost unchanged in the fast axis. In contrast, \(M^{2}x\) increased with the current. The reasons behind this might be the high order mode operation due to the increased driving current [20] or the effect of lateral carrier accumulation [21]. When the driving current was 8 A, \(M^{2}x\) was 18.45 under free running. After Bspc, it went down to 9.8, which corresponded to an improvement of 46%. Bspc achieved a significant beam quality improvement at little penalty of power. The \(M^{2}x\) of the laser with Bspc was not as good as half of \(M^{2}x\) with the same laser without Bspc, which was caused by the error of assembly and adjustment.  
Figure 2: Power characteristics of a single BA emitter with a ridge width of 180 \(\upmu\)m before and after Bspc. The insets (**a**,**b**) show, respectively, the beam spots before and after Bspc.  
Figure 3 also shows the brightness, \(B\), of the single emitter with and without BSFC, which is defined as  
\[B=\frac{P}{\lambda^{2}M_{x}^{2}M_{y}^{2}}, \tag{2}\]  
where \(P\) is the power; \(\lambda\) is the center wavelength. It can be seen that the brightness \(B\) achieved by the single emitter without BSFC was about 23.01 MW cm\({}^{-2}\) sr\({}^{-1}\) at 8 A. For the same laser with BSFC, \(B\) increased to 42.4 MW cm\({}^{-2}\) sr\({}^{-1}\) at this injection current, which showed 84.21% improvement due to a better beam quality in the slow axis.