The accessibility and affordability of optics for Light Detection and Ranging (LiDAR) and sensing have improved significantly over the last decade, so much so that even newer iPhone Pro models contain a mini LiDAR system.
LiDAR was first used in 1961 to track satellites by measuring the time it took for a laser signal to return. In 1971, a LiDAR altimeter was used to map the moon's surface, but the device was expensive, enormous and produced unimpressive results.
Recent improvements have cemented LiDAR as a key photonic technology with numerous applications ranging from creating maps and interpreting archeological sites to guiding autonomous vehicles.
This article discusses the theory behind LiDAR and the optics required for a successful, high-performance system.
What is LiDAR?
A LiDAR system is the photonic equivalent of radar. Light from a precisely directed, rapid-fire laser is reflected by an object or terrain, and the time of flight calculations are applied to the returning light to reveal the exact distance between two points. This information can then produce detailed 3D models or topographical maps.
LiDAR mapping is derived from time of flight calculations on laser light. Image Credit: Avantier Inc.
Lasers in LiDAR Systems
UV, vis, or NIR lasers can be incorporated into a LiDAR system. The choice depends on the objects being surveyed and the imaging environment.
Non-scientific applications typically require 600-1000 nm lasers, but care must be taken since these wavelengths can damage the human eye.
Er-doped fiber 1550 nm lasers are the favored option in many military applications, as they are relatively safe to the human eye and are not visible in night vision goggles.
These lasers are also used for topography mapping, measuring distance, and obstacle avoidance. However, they rely on InGaAs sensors and are, as a result, more expensive than lower-wavelength lasers.
LiDAR can generate topographic maps like these using eye-safe Er-doped fiber 1550 nm lasers. Image Credit: Avantier Inc.
For underwater and bathymetry applications, a laser with good transmission in pure water and limited backscattering from small seawater particles is required. For such environments, 534 nm frequency-doubled diode-pumped YA lasers are recommended, as they can penetrate water with minimal attenuation.
For airborne topographic mapping, 1064 nm diode-pumped YAG lasers are preferable.
Other laser parameters important to LiDAR setups include pulse repetition rate, laser power consumption, and beam divergence. Users must choose between flash LiDAR, in which the entire field of view is illuminated at once, or more conventional scanning LiDAR, which passes over the field of view point by point.
Control of Spectral Width Using Bandpass Filters
One challenge for long-range airborne LiDAR systems incorporating a 1064 nm laser is the high background noise levels due to Sun radiance.
Fitting the LiDAR receiver with a narrow bandpass filter can increase the signal-to-noise ratio. Due to harsh environmental conditions, a robust filter that can perform consistently and reliably is necessary. As a result, multilayer thin-film coatings that provide transmission narrowly matching the laser wavelength are often incorporated.
When narrow linewidth meter-oscillator power-amplifier (MOPA)-based pulsed lasers are used, the seeding laser pulse determines the central wavelength. Numerous diode conditions affect this, such as temperature stabilization, drive current, and pulse repetition rate. Such wavelength control is not simple, and a bandpass filter with a band wide enough to allow for any expected wavelength shifts is necessary.
Custom LiDAR Systems
Avantier is a company that focuses on custom optics tailor-made to meet the exact specifications of customers. In the field of LiDAR, there is no one size fits all and ready-made solutions often cause frustrations.
Working with optical engineers and designers to produce a custom-made system meeting the exact user requirements is recommended.
This information has been sourced, reviewed and adapted from materials provided by Avantier Inc.
For more information on this source, please visit Avantier Inc.