Laser Diodes Enable Motion Sensing and Light-Based Distance Measurements (LiDAR)May 06, 2021 by ROHM Semiconductor
In this article, ROHM will discuss the role of laser diodes in motion sensing and LiDAR, as well as their properties and limitations.
White light comprises every color of the visible spectrum, with multiple colors having different frequencies and wavelengths. As a result, it's very challenging to beam this type of light onto a single point. LEDs commonly utilized for visual indication in electronic devices and equipment generate light containing electromagnetic waves of varying frequencies.
Laser diodes (LDs), on the other hand, produce "coherent light," which consists of a focused light beam of a specific frequency and wavelength. Their unique properties make them highly useful in today’s fast-changing world.
There are two seemingly at-odds needs for LDs that many designers have to seemingly choose between: most people need both improved sensing accuracy and longer detection distance from their LDs. The improvement of accuracy has been traditionally achieved by reducing the size of a laser beam’s spot; increased detection distance has been traditionally achieved by increasing the optical output of the laser. It is not easy, however, to realize these two competing needs at the same time.
What’s the Role of Laser Diodes in LiDAR?
LDs are semiconductor devices similar in function to LEDs, but capable of producing coherent laser light. LEDs generate light via electroluminescence — the process of passing an electric current through the device to create photons by creating excess electron and hole pairs. LDs, on the other hand, amplify visible light via stimulated emission of radiation.
Laser light has the following distinct properties:
- Coherence: Laser light can be termed coherent since the wavelength of the light waves emitted is in phase.
- High-power and Intensity: Laser is incredibly bright since it is emitted by continuous emissions with more power per unit surface area.
- Monochromaticity: Laser comprises light waves of a single wavelength.
- Directionality: Light emitted from laser diodes is highly directional, as it shows minimal divergence.
Laser diodes are designed by doping semiconductor materials like aluminum gallium arsenide to create n-type and p-type layers. Doping is the process of adding small amounts of impurities to pure semiconductors to improve conductivity.
LDs give off light when electric current applied to the device causes the holes and electrons in a semiconductor material to interact at the p-n junction, also known as stimulated emission. They can also accurately measure an object’s shape and distance by taking advantage of the laser beam’s linearity. This technology is known as Light Detection and Ranging (LiDAR).
The time of flight (ToF) method is the most used distance measurement method in LiDAR. In the ToF method as depicted in the image below, distance is calculated by measuring the time it takes for the light emitted from the light source to be reflected by the object and returned to the detector (flight time).
Figure 1. Conceptual diagram of the time of flight (ToF) method. Image from ROHM
A Broad Range of Applications for High Power Laser Diodes
Due to their linearity, coherence, pulse response characteristics, and monochromaticity, LDs are highly useful in a wide range of electronic devices for sensing and distance measurement. Key applications encompass industrial, consumer, and automotive, ranging from robotic vacuum cleaners and autonomous vehicles to automated control systems.
In many of these applications, as the need for both accuracy and distance increase, high-power laser diodes are a natural choice to suit the otherwise competing needs of the designer.
High-power laser diodes provide motion sensing and LiDAR capabilities for non-contact control of equipment, including HVAC systems used in commercial, industrial, and residential facilities.
Security and Surveillance
LDs can help detect the presence of intruders at factories, private facilities, construction sites, and more. They enable security and surveillance systems to capture images and video footage even in poor lighting conditions.
In commercial transport facilities, such as train stations, laser diodes utilize LiDAR to detect human presence at train platform doors, enabling automatic operation. Similarly, LDs can be used in advanced driver-assistance systems (ADAS) in modern cars for detecting variations in a driver's eyelids and facial features.
VR/AR and Gaming Systems
Laser diodes enable motion sensing in virtual reality/augmented reality systems and gaming consoles.
Robotic Vacuum Cleaners
LDs are used in robotic vacuum cleaners to deliver a laser beam for measuring the entire room’s shape before the operation to find an optimal cleaning path.
In 3D scanners for industrial and retail applications, laser diodes utilize LiDAR to obtain the coordinate data from the shapes of different objects.
Laser diodes deliver a narrow beam for precise measurements in laser rangefinders. These devices measure distance by calculating the phase difference between light emitted and reflected from an object, also known as the TOF method. Without the high-coherence light characteristics of semiconductor lasers, this application is not feasible.
Drones and UAVs
Laser diodes provide long-range LiDAR capabilities in military/commercial drones and UAVs for measuring the distance to the ground, 3D mapping, and automatic landing.
Figure 2. Applications of LiDAR technology using the TOF method. Image from ROHM
Automated Guided Vehicles
Laser diodes enable highly reliable sensory functions for navigation of automated guided vehicles (AGVs) utilized in a wide range of industries. Examples include portable robots used for transporting materials in factories, assembly plants, and warehouses.
Laser diodes help to enhance the efficiency of logistics by employing LiDAR to detect the shape and condition of objects in warehouses, allowing for more accurate inventory.
Laser diodes in self-driving vehicles utilize LiDAR for 3D representation of the surroundings and obstacle detection. LDs rated up to 125W can meet the high-power requirements of automotive applications with stable performance over a wide range of operating temperatures.
Limitations of Laser Diodes
The majority of laser diodes available today are manufactured using semiconductor materials and manufacturing processes that achieve typical lifetimes anywhere from 25,000 to 50,000 hours. However, designers note that reliability is highly dependent on operating temperature conditions. The long-term performance of laser diodes tends to degrade significantly when used at high temperatures. Nonetheless, today’s industrial applications require components that can perform reliably in high-temperature and pressure environments.
High Power LD Solutions from ROHM Semiconductor
ROHM is an industry-leading manufacturer of high-performance LD solutions for motion sensing and ranging via its optoelectronics division. ROHM's RLD series laser diodes utilize any of four methods for sensing; triangulation, Time of Flight (TOF), Flash TOF, and structured light. The detection range is 3m to 50m with wavelengths from 630-640nm and 800-950nm.
ROHM laser diodes offer a host of benefits, including higher power efficiency, a wide range of operating temperatures (-40°C to +85°C), lightweight, and small-footprint constructions. Its full range of LD solutions is available for sale on its website and via authorized distributors.
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