# High Brightness Diode Lasers Based on Beam Splitting and Polarization Combining
## 3 Results
### Bspc of BA Laser Bar  
To check the applicability of the proposed BSFC method, the light source was replaced by a laser bar combined by SBC [15]. The schematic diagram of a set-up is shown in Figure 4. SBC is a simple and effective way to realize power scaling and increase spatial brightness greatly [22; 23]. In SBC, a diode laser array (DA) was placed in an external cavity consisting of a transform lens (TL), a diffraction grating, and an output coupler (OC). After collimation in the fast and slow axis, the laser beams from the array focused on the grating by a transform lens. The OC fed the optical modes selected by the grating back to the laser array, and each emitter was forced to operate at a specific wavelength. The wavelength \(\lambda_{i}\) of each emitter can be expressed as:  
\[\lambda_{i}=d\cdot\left[sin(\theta_{\mathrm{litrow}})+sin\left(\theta_{ \mathrm{litrow}}+\frac{i\cdot p}{f}\right)\right] \tag{3}\]  
where \(d\) is the grating period; \(p\) represents the period of the emitters in the array; \(i\) is the number of the emitters along the SBC direction; and \(f\) is the focal length of the transform lens. Hence, the diode laser array was combined. After SBC, the brightness of the array can be further improved by BSFC.  
The laser array was a standard centimeter 980 nm BAL bar with 19 emitters. The lateral emitter width was 90 \(\upmu\)m, and the vertical emitter width was 1 \(\upmu\)m. The FAC and SAC are commercial products with a focal length of 0.286 mm and 3 mm for the collimation of fast-axis and slow-axis, respectively. The transformation lens has an effective focal length of 450 mm, the grating has a groove density of 1800 lines/mm, and the diffraction efficiency is around 90%. The OC with 10% reflectivity was used.  
Figure 3: The dependence of \(M^{2}\) factors and brightness B of the laser with and without BSFC on injection current.  
Figure 5 shows the power-current characteristics of the SBC with (black balls) and without (red balls) BSPC under CW operation. The power was measured by Ophir FL500A at the coolant temperature of 18 \({}^{\circ}\)C. As can be seen, the output power of the SBC without BSPC at 36 A was 19.65 W. With the same current injection, the output power of the SBC with BSPC was 18.26 W, with only approximately a 7% loss.  
Beam quality and brightness of the single emitter with and without BSPC were also measured. Figure 6 shows the current dependent \(M^{2}\) factors in the fast axis (\(M^{2}y\)) and slow axis (\(M^{2}x\)) of the SBC with and without BSPC from 10 A to 36 A. The \(M^{2}y\) of both of these two setups were very close in the range from 2.02 to 2.07 and increased slowly with the increase of current. The fast axis beam quality did not change due to the introduction of BSPC. The \(M^{2}x\) values of the two devices increased rapidly with the increase of current. The \(M^{2}x\) of the SBC without BSPC increased from 4.78 to 9.08, and the \(M^{2}y\) of the SBC with BSPC ranged from 2.35 to 4.75 with the same current injection. \(M^{2}x\) of the laser with BSPC showed a 48% improvement. A significant beam quality improvement was achieved at the expense of little power by BSPC on SBC.  
Figure 4: Schematic diagram of BSPC with SBC. DA: diode array. FAC: fast axis collimating lens. SAC: slow axis collimating lens. TL: transform lens. OC: output coupler.  
Figure 5: Optical power versus the injection current of the SBC with and without BSPC under continuous operation.  
Figure 6 also shows the brightness of the SBC with and without BSPC. It can be seen that the brightness B achieved by the SBC was about 108.4 MW cm\({}^{-2}\) sr\({}^{-1}\) at 36 A. The brightness increased to 195.8 MW cm\({}^{-2}\) sr\({}^{-1}\) by the same SBC with BSPC at this injected current, which corresponded to an improvement of 80.6%. The above results demonstrate that the proposed BSPC method was also suitable for the combined laser beam. The precondition is that the beam has the high purity of polarization.