In one embodiment, a LIDAR device of an autonomous driving vehicle (ADV) includes an array of light emitters to emit a number of light beams to sense a physical range associated with a target. The LIDAR device further includes a slope mirror having a slope surface and a flat surface supported by a rotatable platform. The rotatable platform is configured to rotate with respect to a vertical axis perpendicular to the flat surface. The light emitters are configured to project the light beams onto the slope surface of the slope mirror, which are deflected towards the target. The slope mirror rotates along with the rotatable platform while the array of light emitters remains steady. The LIDAR device further includes one or more light detectors to receive at least a portion of the light beams reflected from the target.