Digital Beam Steering for LiDAR: Technology Fundamentals FAQ

 

What is digital beam steering?

Digital beam steering technology for LiDAR is achieved using a stack of alternating liquid crystal cells and polarization gratings that can steer light at different angles at a specific wavelength.

 

How is LiDAR digital beam steering used in ADAS and autonomous driving?

Liquid crystals and light polarization are used for beam steering in LiDAR sensors. Beam steering refers to changing the direction of laser pulses in a LiDAR. Beam steering enhances LiDAR performance by sub-segmenting (dividing) the field of view into smaller sections and redirecting the emitter LiDAR and/or receiver detector towards that section. Importantly, since liquid crystals and light polarization are solid-state technology, it enables the development of light steering without moving parts and solid-state LiDARs, which are critical to automotive OEMs, off-road commercial and industrial vehicles as well as various other applications.

 

What are liquid crystal cells?

Liquid crystals are transparent cells that contain a small layer of liquid crystal with special coatings applied to the rear and front substrate. The coatings serve to create the boundary conditions and to apply the alternating square voltage field needed to change the LC material birefringence.

 

What is birefringence?

A material is said to be birefringent when its refraction index depends on the propagation direction and the polarization of the light. When light propagates through a birefringent material, its polarization changes. Also known as “double refraction,” birefringence refers to the phenomenon where an incident light beam on an anisotropic liquid is split into two beams. This happens because the anisotropic liquid has two refractive indices within its structure, which causes a phase shift in the incident light wave, thus leading to two different output directions (and path lengths).

 

What is polarization?

Polarization refers to the relation in both amplitude and phase between the orthogonal components of the electric field of a plane wave. The electric field of a plane wave can be described as the vector sum of two orthogonal components, typically horizontal and vertical components. The two components are characterized by their amplitudes and the relative phase between them. The most general case of polarization is elliptical. However, there exist two special cases where the vector component can instead oscillate in a single X or Y direction and, thus, create linearly polarized light (respectively, horizontally and vertically) or rotate around the Z direction, creating circularly polarized light (either clockwise, also called “right-hand side,” or anticlockwise, also called “left-hand side”).

 

What is phase transition?

When an object changes its state, such as an ice cube turning into water, this is called “phase transition.” Phase transition is not limited to an object changing state from solid to liquid; other examples include liquid to gas, gas to a liquid, liquid to a solid, etc. The key challenge in using liquid crystals for autonomous driving technologies is maintaining the liquid crystal temperature so that it doesn’t undergo a phase transition.

 

What is the difference between a polarizer and an analyzer?

The distinction between a polarizer and an analyzer is due to the purpose they serve. A polarizer and an analyzer act as filters, i.e., they allow only certain electromagnetic waves to pass through. While a polarizer is used to polarize light, an analyzer is used to determine if the light is polarized.

 

Does digital beam steering affect the quality of the light beam?

The beam is unaltered and the beam quality is excellent. There is a dilation from tile to tile (different angle settings) going from 7.5 degrees in the center tile to 10 degrees on the furthest edge. This ensures that the emission matches the received beam.

 

How fast can the digital steering change position?

The steering is done by controlling the state of the liquid crystal cells. There are fast and slow transitions depending if the state is stimulated or relaxed. The main configuration of LeddarSteer, providing 14 horizontal and 4 vertical angles, is optimized to have mostly fast transitions during a sweep since there are multiple combinations to control the same angle.
Fast transitions: 50 microseconds.
Relaxing transitions: 750 microseconds.

 

Is the beam steering done in steps or is it continuous?

Digital beam steering for LiDAR, such as the LeddarSteer solution, is done through steps.

 

How does a polarization grating work?

Light traveling through a polarization grating will be diffracted in a specified direction. This direction is dictated by the incoming light’s polarization, the period of the polarization grating and the wavelength of the incoming light. For a given setup, it is expected that the incoming light will be of a specified and constant wavelength. However, incoming light polarization can be manipulated from one polarization grating to the next. Thus, light that is right-hand circularly polarized will be diffracted at a different angle than left-hand circularly polarized light.

 

How can a stack of alternating liquid crystal cells and polarization gratings change light direction?

Considering that the liquid crystal material can dynamically change its birefringence through a change in the applied electric field, it is then possible to also dynamically change the incoming light polarization. In the LeddarSteer™, the liquid crystal cells act as a dynamic half waveplate. When this polarized light encounters a polarization grating, light is steered in one of two directions depending on the polarization of this light. Thus, repeating this stack of liquid crystal cells and polarization gratings increases the number of available steering angles by 2n, where n is the number of liquid crystal cells and polarization gratings in a stack.

 

What else than liquid crystal waveplates and polarization gratings are used in LeddarSteer digital beam steering?

Anti-reflective coating is also applied to the liquid crystal cells. There are also a total of three ITO (indium tin oxide) heaters in the liquid crystal stack assembly.

 

How does LeddarSteer beam steering scan the field of view? Does it go from left to right, up to down, or both?

Minimizing the transition time is critical in the LeddarSteer design. The transition time is dependent on a liquid crystal cell’s transition and, thus, the scanning sequence must be optimized to reduce the transition times. The scanning sequence that will favor fast transitions will most likely be random-access, and the data can then be rearranged post-acquisition.

 

What are some key benefits of using digital beam steering for LiDAR?

Some of the key benefits include:

  • Concentrated laser power on a small region of interest, leading to enhanced signal-to-noise ratio and enhanced range
  • Reduced size, cost and complexity of the LiDAR components
  • A smaller optical system is required in the LiDAR
  • Solid-state technology with no moving parts
  • Higher MTBF (mean time between failures)
  • Extended FoV elevation and azimuth; up to 120° per axis