# Optical Design for Laser Diode Scanner Headlamp with
## 4 Experimental Results and Discussion  
To verify the performance of the designed optical scanner system experimentally, we constructed an optical scanner system to detect the optical power at the phosphor plane, as shown in Figure 8. In this optical scanning system, the power of laser diode was always constant, and a difference in optical power at the phosphor plane occurred because of the difference in scanning speed due to the scanning mirror's driving characteristics. Therefore, to analyze the performance of the designed optical system, we focused on detecting the distribution of optical power due to the change in the scanning speed inside the phosphor plane. Previously studied laser scanning headlamps use MEMS mirrors to reduce the size of the optical system. However, in this experiment, the optical system was constructed using a two-axis galvanometer (GVS012, Thorlabs Inc., Newton, United States) for a simple setup. In addition, a photodiode (PD, SM05 PD2A, Thorlabs Inc., Newton, United States) with an effective detection area of 1 mm in diameter was selected to detect the optical power. A 450 nm beam, resized to 2 mm by a pair of expanding lenses and aperture, was reflected by a pair of galvanometers and focused by a designed prism and lens, as shown in Figure 8. The PD was placed at the phosphor plane of the system such that the optical power of the focused spots can be measured. As the phosphor plane was scanned by an optical spot, the light received by the PD was measured; hence, we can acquire the optical power of a \(\mathcal{O}\)1.0 mm area inside the plane where the PD was placed by multiplying the optical power and detection time. Furthermore, by moving the position of the PD and measuring the optical power of each position, the optical power distribution in the entire phosphor plane can be measured. Because the short axis (y-axis) was not affected by the prism, the optical power distribution by scanning along the short axis indicated a large amount of optical power at the edge, similar to that by a normal sinusoidal scanning mirror. In this experimental setup, the optical power was concentrated at the center of the phosphor plane for the long axis (x-axis) one-dimensional scan only. The experimental conditions were as follows: scanning angle of \(\pm\)12\({}^{\circ}\), scan frequency of 5 Hz considering the PD response time, and detection frequency of 160 kHz. Figure 7: Change in spot size on phosphor plane according to scanning angle. Figure 9 shows the PD signal measured on the phosphor plane at 2 mm intervals in the \(-8\) to 8 mm range using the designed scanning optical system. The \(x\)-axis in Figure 9 represents the scanning period, and the period of galvanometer scanning means that the range of \(-12^{\circ}\) to \(12^{\circ}\) was scanned with a sinusoidal curve. In Figure 9a, the signal measured by the PD is displayed as a graph in a different color according to the position where the optical power was measured. Subsequently, the beam spot position at the phosphor plane in the scanning period is shown in the same color bar on the graph in Figure 9b. For example, in Figure 9a, the voltage values measured at the PD around 0.25 T and 0.75 T are the signals measured when the PD is placed on the 0 mm of phosphor plane. The reason for the long duration of the signal measured by the red line in Figure 9a is that the scanning mirror is scanned slow around 0.25 and 0.75 T, and the duration of the beam spot staying on the PD is long. When the PD is placed in the center, the duration of the scanning beam spot was measured approximately 0.096 T, and this value may be slightly different due to the PD alignment error. The period within the phosphor plane region of 8 mm or less (scanning angle \(\geq 5^{\circ}\)) was approximately 0.086; therefore, when scanning period from 0 T to 0.086 T, the beam spot in the phosphor plane passed through \(-16\) to \(-8\) mm, as shown in Figure 9b. However, this area is not a valid phosphor plane, so it is marked as 0 in Figure 9a.