Miniature Telescope Could Improve Lidar Performance, Advance Planar Optics

A University of Central Florida (UCF) team led by professor Shin-Tson Wu has developed a device made with cascaded liquid crystal (LC) flat optical elements, called a miniature planar telescope, that allows efficient, wide-angle, high-precision laser beam steering. The miniature planar telescope achieves steering angle magnification independent of the incident beam position. This angle magnification function cannot be achieved with a single-layer optical device, such as a grating or a refractive surface.

Although commercial-quality planar optical devices are available, research into planar optics has been focused on optical functionalities that can be fulfilled by a single-layer device. The UCF researchers thought that one way to extend the capabilities of planar optics would be to explore how cascaded planar optics, and the greater degree of design freedom they offer, could be applied to achieve more distinct functionalities.

At the same time, the UCF researchers wanted to preserve the many advantages of single-layer planar optics, such as efficiency, compactness, flexibility, and low cost.

Illustration of a planar telescope consisting of two layers of flat optics for achieving angle magnification. Both layers are assigned phase profiles following the sum of even order polynomials, and they are separated in space by the distance d. Courtesy of Ziqian He, Kun Yin, and Shin-Tson Wu.

The miniature planar telescope consists of two cascaded LC planar optical elements, each performing a predefined mathematical transformation. Both layers are assigned phase profiles following the sum of even order polynomials, and they are separated in space by a calculated distance referred to as d.

To test their system, the researchers fabricated different planar optical elements with pre-designed phase profiles, using all-solution processing, and assembled the elements into miniature planar telescopes with different magnification factors. Through ray-tracing simulations, they optimized the planar telescopes according to specific aperture size and incident angle range. They built two telescope modules with designed magnification factors of 1.67 (module 1) and 2.75 (module 2).

The researchers found that the magnification measurements closely matched the designed values. They further observed that, within the designed incident angle range, module 1 achieved an efficiency of greater than 89.8% and module 2 achieved an efficiency of greater than 84.6%. The researchers believe that the efficiency of the modules could be further improved by optimizing the fabrication process through error analysis.

The miniature planar telescope potentially could be used to expand the current steering range for nonmechanical beam steering. For example, for lidar applications with a working wavelength of 905 nm, a maximum output angle range of ±27° can be expected. Compared to a high-efficiency optical phase array with an incident field range of about ±5°, a magnification of 5.4 could be achieved. For a longer operating wavelength — λ=1550 nm, for example — the steering range could be expanded to about ±37°, corresponding to a magnification of 7.4.

The team also characterized the output beam profile to ensure the high quality of the telescope modules and their compatibility with high-end beam steerers. With the planar telescope, the beam steering angle range can be enlarged greatly without losing too much power.

The work of the UCF team shows the potential of LC polymer-based cascaded planar optical elements to enable lightweight, low-power, cost-effective optical components for practical applications. High efficiency, designable magnification factors, and excellent beam quality make the proposed planar telescope promising for applications requiring advanced laser beam steering technology.

The planar telescope could represent a milestone in the development of planar LC optics. The telescope demonstrates that cascaded LC planar optical elements can enable functions that cannot be enabled by single optical elements — potentially inspiring new and more elaborate cascaded planar optical designs for practical use.

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00576-9).