Ladar Frequency modulated Continuous Wave Laser Detection and Ranging John
What is LiDAR?
Light detection and ranging (LiDAR) techniques use lasers to create 3D (topographic) or gas concentration (atmospheric) imagery of the surveyed environment. Both uses for LiDAR can be performed using either pulses of laser light (pulsed lasers) or laser light that stays on all the time (continuous-wave lasers). Bridger Photonics makes use of continuous-wave LiDAR to measure both solid surfaces (hard targets) and gases (soft targets).
TOPOGRAPHIC LiDAR WITH CONTINUOUS-WAVE LASERS
Frequency-Modulated Continuous-Wave (FMCW) LiDAR
FMCW LiDAR uses the frequency (i.e. color) of the laser light to determine distance. This method enables the simultaneous measurement of distance and velocity, generating precise location data.
HOW IT WORKS
Transmit a continuous-wave laser to a portion of the object, where the laser's frequency is intentionally changed in time.
Measure the time it takes a certain laser frequency to travel to and return from that part of the object using interferometric techniques.
Calculate the distance to that part of the object using the constant speed of light.
Scan the laser beam over the object to determine the distance to all parts of the object and to create 3D imagery.
STRENGTHS
Allows for better distance resolution than direct-detect LiDAR, enabling the measurement of fine surface structure.
Insensitive to ambient light or other LiDARs.
Velocity sensitivity, enabling the ability to directly measure an object's speed.
WEAKNESSES
More complex than pulsed LiDAR.
Can confuse distance with motion or surface roughness.
ATMOSPHERIC LiDAR WITH CONTINUOUS-WAVE LASERS
Continuous-Wave Laser Absorption LiDAR
A continuous-wave laser can be changed or modulated to achieve the same effect as differential absorption. This laser absorption spectroscopy technique is known in various forms as wavelength modulation spectroscopy (WMS), frequency modulation spectroscopy (FMS), or tunable diode laser spectroscopy (TDLAS). Bridger mounts the LiDAR systems on aircraft, so the ground serves as a scattering point of laser light back to the LiDAR system.
HOW IT WORKS
Transmit a continuous-wave laser through the gas to the ground with a laser frequency that is known to be absorbed by the gas.
Modulate the laser's frequency in time.
Determine the concentration of the gas in the beam path by measuring the amount of modulated laser light returning from the ground.
Scan the laser beam to determine gas concentration for all areas and to create gas plume imagery.
STRENGTHS
Can enable compact, monolithic, and inexpensive laser sources that can be flown on light aircraft or drones.
Can enable fiber delivery for combining with other measurement (e.g. topographical LiDAR) lasers.
Does not require high peak power pulsed lasers for safer operation.
WEAKNESSES
Often requires topographical surfaces to scatter laser light back instead of atmospheric particles.
TOPOGRAPHIC LiDAR WITH PULSED LASERS
Direct-Detect LiDAR
The simplest and most well-known form of LiDAR, direct-detect LiDAR refers to the use of pulsed lasers to determine distance to topography (i.e. solid objects) for 3D imaging.
How It Works
Transmit a laser pulse to a portion of the object.
Measure the time it takes for the laser pulse to travel to and return from that part of the object.
Calculate the distance to that part of the object using the constant speed of light.
Scan the laser pulses over the object to determine the distance to all parts of the object and to create 3D imagery.
Strengths
Greater simplicity than continuous-wave LiDAR, as it is only sensitive to the amount of light returning from the object.
Insensitive to frequency and phase, which can be influenced by motion and roughness of the object.
Weaknesses
Affected by other light sources, which can confuse or degrade its performance.
Distance resolution limited to a handful of centimeters.
Require high-peak power laser pulses, which can cause damage.
ATMOSPHERIC LiDAR WITH PULSED LASERS
Pulsed Differential Absorption LiDAR (DIAL)
Using the fact that each gas absorbs a specific frequency of laser light, pulsed DIAL can determine the concentration of a gas by comparing the returning light from a laser pulse that is absorbed by a gas to a laser pulse that is not absorbed by the gas.
HOW IT WORKS
Transmit a first laser pulse to the gas with a laser frequency that is known to be absorbed by the gas.
Transmit a second laser pulse to the gas with a laser frequency that is known to not be absorbed by the gas.
Determine the concentration of the gas in the beam path by comparing the amount of light returning from the two laser pulses.
Scan the laser beam to determine gas concentration for all areas and to create imagery.
STRENGTHS
May be able to use scattering from atmospheric particles instead of from objects.
May be able to measure from greater distances.
WEAKNESSES
Often requires high-peak power laser pulses, which can cause damage.
Often requires large, heavy, complex, and expensive laser systems.
Limited available lasers at the desired frequency.
Source: https://www.bridgerphotonics.com/learn/what-is-lidar
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