Optical principles of laser emission and reception in LiDAR

1 Laser Radar

It is a radar system that detects the position, velocity, and other characteristic parameters of a target by emitting a laser beam. Its working principle is to emit detection signals (shock beams) to the target, and then compare the received signal (target echo) reflected from the target with the transmission signal. After appropriate processing, relevant information about the target can be obtained, such as distance, orientation, altitude, velocity, attitude, and even shape parameters, so as to detect, track, and recognize targets such as aircraft and missiles. It consists of a laser transmitter, an optical receiver, a turntable, and an information processing system. The laser converts electrical pulses into light pulses and emits them out. The light receiver then restores the light pulses reflected back from the target into electrical pulses and sends them to the display.


2. Characteristics of LiDAR

1: Lidar is developed from microwave radar

2: Lidar can present more details

Table 1 shows the comparison of LiDAR, Microwave Radar, and Camera.

The working principle of LiDAR is very similar to radar. Laser is used as the signal source, and the pulse laser emitted by the laser hits trees, roads, bridges, and buildings on the ground, causing scattering. A part of the light wave will be reflected onto the LiDAR receiver. According to the principle of laser ranging, the distance from LiDAR to the target point can be calculated. The pulse laser continuously scans the target object to obtain data of all target points on the target object. After imaging processing with this data, accurate three-dimensional images can be obtained.

Figure 1 Lidar Dot Matrix


2. Advantages of LiDAR

1) Higher resolution

Assuming the incoming pupil size of the receiving antenna is 10mm, the angular resolution corresponding to 905nm laser is 0.22mrad, and the angular resolution corresponding to 30GHz microwave is 2.44rad.

2) Smaller beam divergence angle

3) Greater Doppler frequency shift

Coherent detection has high speed resolution.

3D implementation method: single point scanning

4 3D implementation methods: array detection+coverage lighting


Representative: Jigsaw Plan

1) From 2000 to 2003, the US Department of Defense Advanced Research Projects Agency (DARPA) implemented the High Precision Laser 3D Imaging Radar Research Program (also known as the Jigsaw Program)

2) Detecting targets under ground camouflage: requiring data collection and transmission at an altitude of over 100 meters; The detection range on the ground can reach 20 square meters; The speed of the airplane is 25-35 meters per second and it operates all day.

3) MIT Lincoln Laboratory has developed a 32'32Si GM/APD focal plane detector, which integrates a high-speed digital CMOS timing circuit and has a single photon detection efficiency of over 20% at room temperature.

Figure 2 Jigsaw Non Scanning Focal Plane Laser Imaging Optical Head Developed by MIT Lincoln Laboratory

6. Working process of LiDAR

LiDAR working process:

1) Laser to target transmission (laser emission system);

2) Reflection and scattering of laser by the target;

3) The collection of scattered light by the receiver (receiving system).


7 Laser emission system

Figure 3 Lidar Optical Emission System

In the design and analysis of LiDAR, three typical beam shapes are often encountered:

Gaussian beam, Airy beam, Uniform beam (plane wave)

Figure 4 Laser beam waveform

7.1 Gaussian beam: solid-state laser, fiber laser, etc

In the far field, the divergence angle (1/e2) of a Gaussian beam can be approximated

Figure 4 Laser beam expansion and collimation system

As shown in Figure 4, the waist of the laser beam is 10 microns, and the divergence angle is 127 mrad. The laser beam is expanded and collimated to a waist of 2mm
The divergence angle is 0.32mrad.

8 semiconductor laser light sources

The fast axis divergence angle of the laser needs to reach 65 °, the slow axis divergence angle needs to reach 20 °, and the energy within the range of laser divergence angle ± 5 ° is greater than 50% of the total energy; General launch system divergence angle: smaller is better,<1mrad; At present, the emission beam of most 3D LiDARs is a collimated beam scanning.


Figure 5: A MEMS scanning radar

9 Target scattering

The scattering characteristics of targets are related to materials, roughness, etc; The scattering characteristics are usually described using the BRDF function; In general calculations, objects are often treated as Lambertian bodies.
Figure 6 Scattering Model

9.1 Target scattering model

BRDF: Bidirectional reflection distribution function; Scattering models for smooth surfaces: Abg model, Heavy model, rough surface scattering model: Phong model, polynomial model. Common simulation software: Tracepro Lighttools、ASAP、FRED
Figure 7 BRDF scattering model

10 receiving system

Lidar receiving antenna, also known as optical lens

Figure 8 Optical drift of array receiving lens and filter

11 Evaluation criteria for receiving lenses

11.1 Point target evaluation criteria

The detection of point targets is often non imaging systems because they do not provide morphological information. The design of its optical system mainly depends on the energy envelope diagram and point plot. The energy envelope diagram represents its energy concentration, while the point plot shows the details of energy distribution, including shape (roundness) and centroid of the image spot. MTF is not required, its evaluation is S/N, which should be at least greater than 3. In application, its evaluation is detection rate and false alarm rate. It only informs whether it is present or not, and cannot determine what it is.

Figure 7 Point Target Evaluation: Point Spread Function and Surrounding Circle Energy

11.2 Target evaluation

Facial targets refer to extended objects, where the image of the target fills the field of view (detector photosensitive surface). Ordinary cameras belong to this category and are imaging systems. There are requirements for its image quality, usually evaluated using MTF and S/N. The intensity (illuminance) of the image of the target on the detector is independent of distance.

MTF modulation transfer function

Contrast corresponding to spatial frequency
Space frequency: lp/mm

Nyquist frequency: The sampling frequency must be greater than twice the signal frequency, that is, each cycle (a pair of black and white stripes) corresponds to two pixels.

12 Power Budget

Solid angle:

Pixel receiving power:

E0 is the illuminance of the object; Ro is the reflectivity of the object; Ta is the air transmittance; TR is the transmittance of the receiving system; Tt is the transmittance of the transmission system.