## 4 Experimental Results and Discussion  
As discussed in the previous section, mechanical stress causing the DoP reduction is enhanced by chip morphology and can be particularly high on waveguide edges; the result of COMSOL Multiphysics(r) simulations for the stress is shown in Figure 3b. The resulting polarization change in the optical field is equivalent to an internal loss producing an increase in the laser threshold current [25] and a decrease in the slope efficiency. See Figure 8, in which measurements of two sets of HPLD mounted with different die-bonding conditions are shown. Figure 6: Effect of the thickness of Cu layers in the carrier: (**a**) refractive index component changes (**b**) DoP as a function of the Cu layer thickness and the intrinsic refractive index anisotropy. Figure 7: Schematic of a HPLD mounted on a carrier embedding Cu layers. Two-sample \(t\) tests showed significant differences between the two sets, with \(p\)-value lower than 0.05 for all of the functional characteristics reported in Figure 8.  
The laser beam shape is also significantly affected by DoP reduction. Figure 9a shows the TE-polarized slow axis far field (FFSA) increase by about 9% and the TM-polarized FFSA decrease by about 2% for HPLD with different DoP from 0.97 to 0.85. The beam parameter product, which characterizes the beam quality and is defined as the product of half-beam divergence and half-beam width, is also significantly increased by DoP reduction. See Figure 9b. For all of the functional characteristics reported in Figure 9, two-sample \(t\)-tests showed significant differences with \(p\)-values lower than 0.05. Experimental setup for the DoP measurements and the polarization-resolved near-field measurements reported in this section are shown in Figure 10. Far-field measurements were performed on a standard goniometric radiometer setup. Figure 8: Measured effects of stress-induced DoP reduction on functional characteristics of two sets of HPLD mounted on carriers. Set A represents 10 high-stressed HLPD while set B represents 10 low-stressed HLPD. (**a**) Box plots of measured DoP; (**b**) box plots showing threshold current increase from B to A; (**c**) box plots showing slope efficiency decrease from B to A.  
Figure 9: Measurements of HPLD with different DoP. (**a**) Polarization-resolved slow axis far-field changes for 8 HPLD with DoP = 0.85 and 11 HPLD with DoP = 0.97; (**b**) slow-axis BPP changes for HPLD divided into three DoP categories (93 with DoP < 0.91, 124 with 0.91 \(\leq\) DoP \(\leq\) 0.95, and 64 with DoP > 0.95). Figure 11 shows the polarization-resolved near-field slow axis (NFSA) for 324 HPLD devices with different DoP, ranging from 0.86 to 0.98. In particular, the TM-polarized NFSA reported in Figure 11a shows sharp peaks at the waveguide edges [3; 26] when the DoP is high and spreads to the whole waveguide, about 200 \(\upmu\)m wide, when the DoP is decreasing. At the same time, the TE-polarized NFSA, reported in Figure 11b, shows an increase in optical filamentation with DOP decreasing. These effects, due to different mode excitation of the multimode lateral waveguide, are highlighted in the top color maps with the black shaded lines dividing regions of different behavior for high DoP (H) and low DoP (L), as a visual guide. The bottom images refer to single NFSA acquired for low-DoP and high-DoP devices, and clarify the NFSA shape discussed above. For mainly TE-polarized compressively strained-QW HPLD, the TM-polarized NFSA represents a distribution of the mechanical stress on the device and can be very useful in the development of high-DoP HPDL, by optimizing chip dielectrics, metals, shape,  
Figure 11: Measurement of 324 mounted HPLD: (**a**) TM-polarized NFSA; (**b**) TE-polarized NFSA. Figure 10: Measurement setup of (**a**) DoP; (**b**) slow axis near field. mounting process, and carrier structure.