04 Aug LiDAR for Self-driving Cars and Beyond
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Light Detection and Ranging, aka LiDAR, is one of the laser applications that has been attracting more attention during the last few years, mostly due to autonomous driving vehicles and the increasing demand of the automotive industry. Whereas LiDAR was first implemented close after Theodore Maimans’ invention of the Laser in 1960, it was mostly used for non-civil applications and has been unknown to the broad public for a long time.
The basic principle consists in illuminating a target with a laser and receiving the reflected light back, which allows measuring the distance of the object and its velocity. LiDAR is used in many applications such as airborne topographic mapping, ground surveillance or autonomous driving cars, and this is just to mentioning a few.
Lidar can be sort in two main groups, depending on the detection scheme that is employed:
In time-of-flight (ToF) LiDAR (or direct detection), laser pulses are sent, which are reflected or scattered back towards the source and are collected by optics and directed to the detector. An electronic clock is started by the sent pulse and it stops when receiving the incoming signal. Multiple laser pulses are sent, and the average measured time is used to calculate the distance between the source and the object. In ToF LiDAR, the pulse duration determines the longitudinal measurement resolution. The shorter the pulse duration the better the resolution. However, using a too short pulses (in the order of few picoseconds) worsens the signal-to-noise ratio due to the higher bandwidth required by the receiver. Pulse durations in the order of few hundreds of picoseconds (or sub-nanosecond) seems is the best compromise for applications that require a high resolution, and pulse durations of few nanoseconds to few tens of nanoseconds to applications that require resolution in the order of ~10 centimeters.
In coherent LiDAR, the return signal beats against a sample of the emitted signal, which allows measuring the phase and frequency difference to obtain information about the distance of the object and its radial velocity, respectively. This detection scheme is known as optical heterodyne detection and can improve greatly the sensitivity of the signal detection (allowing the use of common photodetectors).
When taking a closer look to the laser source, we realize that selecting the ideal one for LiDAR depends greatly on the application, and on the type of LiDAR system employed. When using coherent LiDAR, there is not the need of using short laser pulses (CW to µs), as in comparison to ToF LiDAR (in the order of few ns or sub-ns). On top of this, the higher sensitivity of the detection method in coherent LiDAR permits the use of lower peak powers. The relaxation of these two requirements benefits the use of diode lasers.
The laser beam quality and divergence requirements are different depending on whether a single detector or an array of detectors are used. When using a single detector, the two- or three-dimensional image results from scanning the target area. Therefore, this method receives the name of scanning LiDAR. In this method, the lateral resolution depends on the laser beam quality and divergence. On the other hand, in flash LiDAR, a plane array of detectors is used, which allows recording a 2D image with a single laser shot that illuminates the area covered by the detectors. To obtain the 3D image, each pixel has an independent electronic clock to measure the ToF. The LiDAR application that better describes the discrepancy between flash LiDAR and scanning LiDAR is autonomous vehicle LiDAR, as commercial systems using these two technologies compete for the same market.
So, how should you choose the wavelength for your LiDAR application?
The answer seems not to be straight but depends on the application and interdependent parameters like the absorption of the medium, the overall cost of the system, and should you need an eye safety. For example, as opposed to airborne topographic LiDAR, which uses a wavelength of ~1 µm, bathymetric LiDAR systems (high resolution mapping of underwater depth of ocean and lake floors) uses a wavelength of ~532 nm, as it represents a good compromise between high transmission in water and limited backscattering from underwater particles
Table 1.Main advantages and disadvantages for 1550 nm and 905 nm laser sources for LiDAR.
|1550 nm||905 nm|
|Better for eye safety||Better transmission in atmosphere (lower water absorption)|
|Lower solar background noise||Silicon-based photodetector|
|Requires non-silicon photodetectors||Inexpensive laser diodes with high E-O efficiency|
In general, for most of the applications where the propagation medium is the air, the wavelength in use is either 1550 nm or 905 nm (or other wavelengths close to these values like 865 nm, 1064 nm, etc.). Lasers at 1550 nm are safer because water in the eye absorbs wavelengths in this region, preventing light from focusing on the retina. This in practice allows using lasers with higher output power extending the distance range by improving the signal-to-noise ratio of the detected signal. On top of this, the solar background is also lower at 1550 nm than at 905 nm, which also results in lower noise and less demanding filtering techniques. One of the main disadvantages of using 1550 nm laser sources is that they require the use of InGaAs photodetectors that are more expensive than silicon-photodetectors and have higher dark current.
On the contrary, 905 nm offers in comparison to 1550 nm a much better transmission through the atmosphere, specially under conditions of high humidity due to the fact that water absorption coefficient for 1550 nm is two orders of magnitude higher than at 905 nm. This allows LiDAR systems that uses 905 nm lasers performing better under conditions of rain and fog. However, the great advantage of using 905 nm lasers is that it allows using inexpensive silicon-based photodetectors. Additionally, laser diode sources at this wavelength have a much better electro-optical efficiency (~60%) and are offered at much lower price. Table 2 links some of the most typical LiDAR applications and their typical wavelength used.
Table 2. List of typical LiDAR applications and emission wavelengths used [ref].
|Seabed mapping / bathymetry||405 / 450 / 527 / 532|
|Autonomous driving car||865 / 905 / 15xx|
|Velocity profile of wind||1053 / 1064|
|Airborne topographic mapping||905 / 1064 / 15xx|
|Weapon guidance||905 / 1064|
|Ground surveillance||905 / 15xx|
|Forest canopy mapping LiDAR||905/ 1064 / 15xx|
|Monitor volcanic emission||355 / 532 /1064|
In Monocrom, we have been serving several different types of LiDAR laser sources, applying to different applications with different form factors, power levels and wavelengths, optimizing the laser source to the requirements of the application.
LiOM Series is Monocrom newly released product line of extremely short pulse, fiber-coupled lasers for light detecting and ranging. These lasers are used successfully in scanning, flash or iToF LiDAR applications, and can be suitable for many other applications as well. The laser module is equipped with electronics and a detachable fiber output , ready to be easily operated by providing the necessary DC voltage and TTL. The S9 version of our LiOM series is idle for scanning LiDAR thanks to its short pulse duration of 6.5 ns with repetition rates of up to 200 kHz. and pulse peak power of 200 W. The LiOM S10 serves gated imaging applications, which comes close to the more popular flash LiDAR. Eye-safety and homogeneous illumination of the “field of vision” are key requirements for lasers used for iToF. For that reason, this module is fitted with a square fiber. Designed to operate as a CW light source, it can be adjusted to different pulse durations with the appropriate driver electronics and optionally be delivered with fiber collimation optics.
Should you need more information, feel free to Contact us to find out more about our solutions.