Whether scanning, spinning, or flash technology – all LiDAR sensors have one thing in common: they detect their environment with the help of light. However, after this point, the similarities end, so much so that even the principle of measuring the distances using reflected light differs among the technologies. The two best-known principles are time-of-flight measurement (ToF) and frequency modulated continuous wave (FMCW).
Time-of-Flight: Distance measurement using laser pulses
The time-of-flight measurement measures the distance to objects by emitting laser pulses. Laser pulses mean that multiple collimated light pulses are emitted one after the other in short time intervals. These are reflected by objects and are picked up again by a detector. Using the time a laser pulse takes to be emitted, reflected, and recaptured and the speed of light, the distance to the reflective object is calculated.
This principle is the most common measuring method. It has high data reliability and can be implemented with inexpensive laser sources. However, ToF also has some limitations. For instance, to emit laser pulses, the light is collimated to a maximum as this achieves a higher range. The power with which the highly collimated pulses can be sent out is strictly limited due to eye safety regulations. There are ways to solve this problem; an intelligent design of LiDAR sensors, for example, with particularly large MEMS mirrors, such as the ones used in the Blickfeld Cube, can also achieve long ranges with the time-of-flight method. Hence this is still the preferred measurement method for many sensors.
Alternative to pulses: Frequency modulation
To avoid the challenge of pulses with high peaks of laser power, there is also the possibility of emitting a continuous laser beam. Here, frequency modulation is used to define the distance to objects. But what does frequency modulation mean? The emitted laser beam is modulated and “chirped” repeatedly, so the frequency of the signal is changed again and again. The laser beam now hits an object and is reflected. The reflection affects the frequency of the light compared to the frequency at the time of emission. When the reflected light returns to the detector, it is mixed with the emitted light and the difference in frequency is measured. This so-called intermediate frequency, which describes the difference between the two frequencies, is proportional to the distance and thus provides information about the object’s position. If this object moves towards the sensor, the Doppler effect can directly determine the object’s speed using the FMCW method.
Direct speed measurement for autonomous driving
With time-of-flight, the oncoming objects’ speed can also be measured, but not directly as with FMCW. ToF can only measure the distance between sensor and object using the recorded data points, therefore the velocity of objects has to be calculated from several measurements. The speed can be calculated by emitting several pulses from the change in distance between object and sensor in the individual measurements and the pulse frequency.
Coherent detector vs. coaxial sensor design
If the light is emitted coherently, as in frequency modulation, it must, of course, also be detected continuously. For this reason, FMCW uses coherent detectors, which are very sensitive since only the specifically emitted coherent light should be filtered and recorded. The increased sensitivity also means that the data is less affected by unwanted light influences, which is why FMCW exhibits a good signal-to-noise ratio. Thus, even weakly reflecting objects can be detected at greater distances. However, the time-of-flight measurement is not inferior in this regard. An intelligent LiDAR sensor design achieves the same effect; the coaxial design ensures that only light hitting the detectors at precisely the same angle as the outgoing light is captured again. In this way, sunlight and signals from other LiDAR sensors are filtered out.
Time-of-Flight still a more established measurement principle
So, is frequency modulation the better measuring principle? This cannot be said in general terms. Both methods have strengths and weaknesses. A significant advantage of time-of-flight over FMCW is, however, its development maturity. ToF has been successfully used in LiDAR sensors for many years, while FMCW is still in its infancy. The technology is complicated and currently still very costly, as more specific requirements are placed on the laser source than is the case for ToF. Especially in the LiDAR market, the question of industrialization is an important one since the LiDAR sensor as a whole is already subject to strong price pressure in the automotive market. FMCW modules must therefore become much cheaper than the current price of the laser detector modules for ToF. However, as development progresses, it will be possible to integrate FMCW on a single chip, which is very attractive in the long term. After all, size is something that all LiDAR manufacturers are trying to reduce in addition to the price.
