Abstract

LIDAR is a device used for measuring the distance of an object using laser beams to create detailed 3-D images of the object. LIDAR has numerous applications, but one of its principle applications recently has been with autonomous vehicle where it is used to map the surroundings of the vehicle so that it can detect obstacles or differentiate between roads, other vehicles and passengers etc.
For a LIDAR to capture a complete 360° surrounding view of a vehicle, the sensor must be rotated around to detect images all around the vehicle. Current autonomous cars use spinning LIDAR sensors mounted on top of the vehicle. These sensors use mechanical motors to rotate the entire device, and have the disadvantage of being bulky, expensive, and inefficient. For this reason, non-mechanical methods of steering optical beams like Optical Phased Array (OPA) technology and Micro-electromechanical systems (MEMS) is being extensively researched.
This thesis aims at refining an alternative method of non-mechanical beam steering which uses focus tunable lenses. Focus tunable lenses have a variable focal length that can be controlled by applying appropriate electrical signals. By using two such lenses one after the other, the direction and focus of a laser beam can be controlled. The tunable lenses, along with other optical elements can be used to create a wide-angle scan. Past research on this method is limited, and the device size was too large for practical applications. This can be attributed to the long optical path lengths present between adjacent elements in the design, which is required for the beam scan angle to be as large as possible. So ultimately a tradeoff between device size and the scan angle exists. This work aims to explore this tradeoff and create a compact design which at the same time is capable of scanning over a large angle. Zemax software was used to model the elements, design the systems, and trace the rays to detect their exact position for different values of focal length of the tunable lenses.
The first design aimed at observing the effect of reducing the optical path length between the adjacent elements in the design. The design elements were placed close to each other to reduce the physical length (and consequently the optical path length) between them. The total length of the device was only 114 mm, but reducing the optical path resulted in a very low scan angle of 16°.
In the second design, instead of removing a big part of the optical path between the relay lens and the diffuser all together, it was replaced with two 90° prisms with their bases facing each other. With this arrangement, a total optical path of 224 mm was created within a physical length of 48mm. The focal length of the objective lenses placed after the diffuser were reduced from 50mm to 25mm. The results from the final design show a total beam scan angle of 52° for a device only 119mm in length.
The third design incorporated a third prism to further increase the optical path length to create a larger scan. The scan angle from this design was found to be 60°. The total size of the device however, increased due to the addition of a third prism.
Measurements were made of the RMS beam radius at different distances from the device, and the beam divergence was calculated to be 0.45°.