Evolution of laser technology for automotive LiDAR, an industrial viewpoint
BPP is defined as the product of a laser beam’s divergence angle θ (half-angle) and the radius of the beam at its narrowest point r (the beam waist). Mathematically, it is expressed as
$$BPP=\frac\theta 2\times r=\frac\lambda \pi \, M^2$$
(1)
Where M 2 denotes the beam quality and λ is the wavelength. For the ideal Gaussian beam with M 2 = 1, BPP is at its minimum of λ/π. The product of BPPs along x and y is inversely proportional to the brightness of a laser when θ is small.
For a LiDAR system with sufficient sensor resolution, the spatial resolution limit is approximately equal to the size of the laser beam on the illuminated object after collimation, which can be expressed as
$$\Delta x=4\times \fracBPPD\times R$$
(2)
Where D is the diameter of the transmitting lens, and R is the distance to the target. Therefore, the smaller the BPP, the better the resolution for the same optics. Smaller BPP or M 2 allows the use of smaller lenses, facilitates longer ranges, and enhances resolution.
As shown in Fig. 3, the BPP for an EEL differs between its fast and slow axes16,27. The fast axis has a 3×–8× larger angle but is typically 10×–1000× smaller in diameter compared to the slow axis, resulting in a smaller BPP for the fast axis. Although multijunction EEL provides higher single-emitter power, as the number of junctions increases from 1 to 5, the BPP of the fast axis is significantly traded off, thereby limiting resolution at long distances.
Round dots are for single emitter VCSELs (1 J, 3 J, and 5 J) and AR-VCSELs (6 J) at various oxidation aperture diameters and driving currents. The 14 J OA 40 μm is the predicted value. Diamond dots are BPP along the fast and slow axis in EELs (1 J, 3 J, and 5 J).
VCSEL/AR-VCSEL’s circular aperture ensures a symmetrical BPP at the single emitter level. Their oxidation aperture (OA) size determines the radius of the beam. A larger OA allows higher power output while maintaining the driving current density but conversely increases both divergence and BPP. Therefore, the OA size must be picked carefully to balance power and BPP requirements. Although multijunction helps deliver sufficient power, traditional VCSELs struggle with BPP once the number of junctions reaches 5 or above and OA reaches over 20 μm. AR-VCSELs with exceptional BPP and M 2 enable longer distances and higher resolutions than traditional VCSELs. As shown in Fig. 3, a 6 J AR-VCSEL emitter with 40 μm OA has a lower BPP than a traditional 5 J VCSEL with 30 μm OA, but five times the power output. A 6 J AR-VCSEL emitter with 22 μm OA shows the same level of power but a quarter of the BPP compared to the 5 J VCSEL.
To echo Table 1’s range, we mark the BPP requirement to achieve the spatial resolution of 10 cm at 30 m, 100 m, 200 m, 300 m, and 400 m, assuming a collimation lens diameter of 5 cm. For example, 10 cm spatial resolution at 200 m is about 0.03° in angular resolution requiring a BPP of 6.25, which allows up to two columns of 40 μm AR-VCSEL emitters or up to six columns of 20 μm AR-VCSEL emitters to provide adequate power at the same time. The trend of progression from traditional VCSEL to AR-VCSELs, moving to the right and down, aligns with the anticipated trajectory for future long-range LiDAR lasers.
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