Image Source – Wired
Advantages of different wavelength lasers
Self-driving vehicles which were a subject of science fiction even 20 years ago are poised to become reality. Companies like Google, Uber, Tesla and Toyota are all developing different levels of self-driving cars. Have you seen these autonomous vehicles on the streets yet? These cars are equipped with a lidar system on top. In the absence of a driver, a lidar system becomes the eyes of the vehicle continuously monitoring the environment to safely guide it to its destination. Most self- driving cars today use a combination of lidar, radar and cameras to create a 3D map of the surroundings.
Lidar stands for light detection and ranging. The technology has been around for many years and was primarily used in archaeology and geological mapping applications. Lidar uses laser pulses that bounce off objects and return to the sensor to measure distances and create a detailed 3D map. Lidars produce their own light sources making it ideal for day or night.
Originally, autonomous vehicles used 905nm laser diodes in their lidars because they were inexpensive. Silicon avalanche photodiodes which work with these lasers are also relatively affordable. Also, the 905nm lasers provide reasonably good atmospheric transmission. However, there are a few limitations to this system. Because this wavelength can damage the eye, pulse power must be limited. Which means they don’t function well in bad weather. The low pulse power also limits the range of the detection system to around 100m.
Another approach is to use a higher wavelength 1550nm laser. This wavelength is widely used in telecommunications and is not harmful to the eye. The higher pulse power allows a longer detection range of 200m and more efficient penetration in poor weather conditions. Lidars using 1550nm wavelengths are more expensive in general because they require costly Indium Gallium Arsenide (InGaAs) photodetectors and cannot use the cheaper silicon PDs.
Different wavelengths have different pros and cons in lidar applications. Selection of the right wavelength depends on various factors like cost, range and application. Optical component costs will continue to come down driven by volumes and the day that your groceries will be delivered by a self-driving car is not far away.