Feature: Sensors
Figure 1: A direct time-of-flight system measures the time taken for light to travel to a target and back
LiDAR types Te principle behind the most common type of LiDAR, a direct time- of-flight (dToF) system, is very simple: time is measured for a light pulse to travel to a target and back to the sensor. Since the speed of light is a known physical constant, it is a simple task to calculate the distance between the transmitter/detector and the reflective target. Tis technique generally uses a single, very-short pulse emitted by a
light source (most commonly a laser) that simultaneously activates an accurate timer. When the light pulse hits an object within range, it is reflected back to a highly-sensitive light sensor, typically co-located with the laser. Once the return pulse is detected, the timer is stopped and the time taken to travel to the object and back read. Knowing the elapsed time (t) between the pulse being sent and the
echo being received, it is a simple matter to calculate the distance (D) to the target object using the speed of light constant (c). Te dToF approach is fast and can measure multiple echoes, allowing
detection of several objects within the LiDAR’s field of view. It can be used with high precision in both long- and short-range applications (0.1m-300m). Tere’s also an indirect time-of-flight (iToF) LiDAR which uses
a continuous beam of light, also from a laser. Tis method does not measure the elapsed ToF directly but determines it from the phase difference between the transmitted and received waveforms. Tis technique is more appropriate for relatively short-range (< 10m) applications, especially indoors where light conditions are less challenging than outdoors where contrasts are greater. It is limited to detecting single objects, as it can only detect the strongest echo. Te third type of LiDAR is frequency-modulated continuous wave
(FMCW), used for short- and long-range applications. Tis technique uses a tunable laser to produce a continuous wave of light that is mixed with the reflected light at the detector. Tis mixing creates a beat frequency between the local and reflected waveforms, from which the object distance and directional velocity can be calculated. While FMCW can provide excellent ranging performance and
also capture directional velocity information, the system cost of such a LiDAR setup is quite high, because of the tunable lasers used with polarisation control and the reliance on short-wave infrared wavelengths, which require exotic semiconductors for the laser and the detector.
The ‘Great Wavelength Debate’ One of the most debated topics around LiDAR is which wavelength to use. IR is used in preference to visible light since there’s less background IR, allowing for better signal-to-noise ratio (SNR), making detection of the returned light easier. Within the IR spectrum there are multiple suitable wavelengths,
including in the near infrared (NIR) spectrum (850nm, 905nm, 940nm) and the short-wave infrared (SWIR) spectrum (1350nm, 1550nm). Deciding which of these to use is the crux of the ‘Great Wavelength Debate’. Te three most important criteria to consider are system
performance, availability of suitable components and overall system cost. Te detector is one of the most fundamental components of any
LiDAR system. Silicon-based (CMOS) detectors detect light with wavelengths in the range 400-1000nm, making them sensitive to visible and NIR light, but not to SWIR light. To detect SWIR light, III/V semiconductors such as InGaAs alloys are necessary, which are very expensive compared to silicon. Component availability is another consideration, especially
with the laser emitters. Edge emitting lasers (EELs) are being superseded by vertical cavity surface emitting lasers (VCSELs) that are easier to package into arrays and offer a stable wavelength over temperature. While VCSELs are currently less power-efficient and more expensive, as they become more widely adopted this is expected to improve. However, there are several suppliers for SWIR EELs, but
currently only one for SWIR VCSELs, while there are multiple vendors of NIR VCSELs. Terefore, choosing NIR promises greater security in the supply chain. Detection range is important, since this increases the reaction
time available, offering greater safety. However, overly powerful lasers can damage eyes, so IEC 60825 stipulates the maximum permissible exposure (MPE) for a 1ns laser pulse. While NIR must have a lower MPE, laser power can be increased if
the pulse width is shortened, and, with the use of sensitive detectors, ranges up to 300m are achievable. In good weather, the range of SWIR will exceed that of NIR, but SWIR is more adversely affected by moisture like rain or fog, so system performance based on NIR
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