Green Lasers in LiDAR Bathymetry Technology and Potential Improvements in Efficiency and Power

Introduction

A laser, also known as light amplification by stimulated emission of radiation, is a device that emits high powered light (1). Unlike LEDs, light used in common light sources, lasers can only create light of a single wavelength and light that travels in the same direction. This allows lasers to be used in a more practical setting with better precision and accuracy. The concept of a laser was first introduced by Albert Einstein in 1917, but was invented later by Theodore Maiman in 1960 (2). They were first applied in biology research, acting as optical tweezers to manipulate cells (3). Today, lasers can be applied to a wider variety of fields, such as the medical field with surgeries and cancer treatment and everyday technologies like computer display screens (4). The laser’s ability to emit a single wavelength makes it suitable for various applications. 

Difficulty in Creating a Laser

However, the preciseness also makes lasers harder to replicate. To produce light, electrons from atoms in optical materials absorb electrical current, exciting them to move to a higher energy state (Fig. 1). When they move back to their original energy state, they release energy in the form of light. For lasers to emit a single wavelength, the energized electrons must be sent through a material. This material will determine the wavelength and color of the light (1). Contamination or slight changes in the material will cause a significant difference in the final result of the laser. Therefore, creating a laser is a thorough and meticulous process. 

Figure 1  Electrons gaining energy from an outside source move them to a higher energy state. When they return to their original energy state, energy is released in the form of light.

Green Laser Production

Currently, research is being conducted on creating more efficient lasers, that is it requires less electrical power input for a high optical output. More specifically, green lasers are not as efficient compared to other lasers like red and blue. To solve this issue, researchers develop new green lasers and test out their efficiency. A green laser is produced by adding many layers of metal organic chemicals to Indium Gallium Nitride (InGaN), a semiconductor crystal (5). This compound is able to achieve wavelengths ranging from green to violet, depending on the ratio of indium to gallium. We use InGaN to create the semiconductor because of its ability to achieve the green wavelength and its wide bandgap (Fig. 2). To start, a recipe of various metal organic chemicals and the different timing of when the chemicals are added onto the layers is created. The InGaN crystal is placed into the reactor where the chemicals from the recipe are added as gasses layer by layer. The crystal is baked at high temperatures to solidify the chemicals into the layers and rotated for even spreading of the chemicals. This process is called the Metal Organic Chemical Vapor Deposition (MOCVD). It creates the semiconductor wafer, which is then taken to a clean room to etch and deposit more materials, such as gold as a conductor and silicon dioxide as an insulator. A conductor is needed for the electrons to pass through the semiconductor and the insulator ensures that the electrons from the top and the bottom of the wafer will not meet and create a short circuit. After the clean room, the laser is finished and can be taken for testing to measure the wavelength and efficiency.

Figure 2  Different materials used as the medium to pass electrons through and emit a single wavelength light source. Indium Gallium Nitride is used to create green lasers because GaN and InN can be combined to achieve the green wavelength. Other materials are not used because of InGaN’s wide bandgap, which allows for more powerful and efficient semiconductors.

Researchers are only able to achieve optical power of a lot less than 50 mW, but we generally want lasers to have power of about 100 mW. Green lasers are especially difficult to achieve high efficiency due to the difference in size of indium and gallium atoms (6). Their crystal structure becomes unstable, similar to how stacking different sized objects would be difficult. While green lasers can still be used in technologies, more efficient lasers will broaden and improve the applications we have today.

Lidar Technology

One such application of lasers that is rising in popularity is LiDAR, or light detection and ranging (7). LiDAR is operated on an aircraft and uses pulsed lasers to measure distances to the Earth, which will generate a 3D image of the landscape. There are two different types of LiDAR technology: topographic and bathymetry (7). Topographic LiDAR uses infrared lasers at 1064 nm to map out the surface of the Earth while bathymetry uses green lasers at 532 nm to map out the seafloor. The difference of wavelength allows the LiDAR technology to measure the land accurately in varying conditions as green lasers are better at penetrating water. We want to focus on bathymetry LiDAR technology since it has greater potential of improving and has important benefits to society. 

In Figure 3, the aircraft will emit two different wavelengths of lasers, near-infrared and green. The infrared laser will reflect off from the water surface back to the aircraft and into the optical receiver where it detects the amount of time it took to travel. The device can calculate the distance between the aircraft and the water and map out the surface. The green laser is optimal for penetrating through the surface of the water and reflects off the seafloor and back to the optical receiver. The water depth is calculated from the time difference between the infrared and green signals and the ocean floor can be mapped out (8). 

Figure 3  Green and near-infrared lasers are emitted from the aircraft. They are reflected off the seafloor and the water respectively. The time it takes for them to come back to the optical receiver is measured, and the difference is used to calculate the water depth.

Bathymetry LiDAR is used for underwater topography, scanning for any anomalies, detecting shipwrecks, finding gas leaks in pipelines, and more (9). By mapping the water depth of oceans, scientists are able to confirm any changes to the ocean floor and further investigate if they find anything. For example, if there is a missing shipwreck, we can use bathymetry LiDAR to scan the ocean floor and determine if the water depths are the same as before. This way, people can gain more information on what went wrong in the technology and most importantly, potentially save people quicker. Furthermore, LiDAR can have a major impact on preserving wildlife in the future. Currently, coral reefs face dangers due to climate change, but LiDAR can be used to accurately discriminate the structure of coral reefs (10). Therefore, once LiDAR detects a change, it can bring more attention to the increasing destruction of coral reefs, prompting people to take action. 

However, LiDAR technology does come with setbacks. Today, bathymetry LiDAR can only be used in shallow waters of up to 60 meters (8). This is caused by the lack of power in green lasers; they cannot penetrate waters at those depths. Additionally, weather conditions, aquatic vegetation, and clarity of water can affect the accuracy of measurements (11). Therefore, with further research into creating more powerful, efficient green lasers, LiDAR bathymetry will become more useful and versatile in exploring ocean floors. Having higher power will allow the laser to penetrate through larger depths and eliminate the limiting factors they have today. 

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[2] December 1958: Invention of the laser. (n.d.-b). https://www.aps.org/publications/apsnews/200312/history.cfm

[3] American Institute of Physics. (n.d.). Bright idea: the first lasers. https://history.aip.org/exhibits/laser/#

[4] ​​Azadgoli, B., & Baker, R. Y. (2016). Laser applications in surgery. Annals of Translational Medicine, 4(23), 452. https://doi.org/10.21037/atm.2016.11.51

[5] Kour, R., Arya, S., Verma, S., Singh, A., Mahajan, P., & Khosla, A. (2019). Review—Recent Advances and Challenges in Indium Gallium Nitride (InxGa1-xN) Materials for Solid State Lighting. ECS Journal of Solid State Science and Technology, 9(1), 015011. https://doi.org/10.1149/2.0292001jss

[6] Tan, A. K., Hamzah, N. A., Ahmad, M., Ng, S. S., & Hassan, Z. (2022). Recent advances and challenges in the MOCVD growth of indium gallium nitride: A brief review. Materials Science in Semiconductor Processing, 143, 106545. https://doi.org/10.1016/j.mssp.2022.106545

[7] What is lidar? (n.d.). https://oceanservice.noaa.gov/facts/lidar.html#

[8] Quadros, Nathan & Collier, P. & Fraser, C.. (2008). Integration of Bathymetric and Topographic LIDAR: A Preliminary Investigation. Remote Sensing and Spatial Information Sciences. 37.  

[9] Unmanned Systems Technology. (2022, May 11). Bathymetric LiDAR | Marine & Subsea LiDAR for Drones, USV & UUV. https://www.unmannedsystemstechnology.com/expo/marine-and-bathymetric-lidar/

[10] Foo, S. A., & Asner, G. P. (2019). Scaling Up Coral Reef Restoration Using Remote Sensing Technology. Frontiers in Marine Science, 6. https://doi.org/10.3389/fmars.2019.00079

[11] Szafarczyk A, Toś C. The Use of Green Laser in LiDAR Bathymetry: State of the Art and Recent Advancements. Sensors (Basel). 2022 Dec 27;23(1):292. doi: 10.3390/s23010292. PMID: 36616890; PMCID: PMC9824562.