LIDAR

The fundamental operation principle of time-of-flight LIDAR is based on the following two ideas:

• If laser pulses are sent in a certain direction in the form of a laser beam, and these pulses eventually hit some target which reflects or scatters light back towards the source, the detected time delay of that light can be used to determine the distance of the object (time-of-flight method).
• By doing many such measurements while scanning the direction of the beam, one can acquire information for 3D images (3D laser scanning). The image data can be collected with some kind of computer (e.g. realized with a microprocessor), which can then display them on a screen.

For time-of-flight LIDAR, one obviously requires a laser source emitting short pulses, often with pulse durations of the order of a few nanoseconds or sometimes <1 ns. The pulse energy must be high enough to provide sufficiently intense return signals; it may be limited by eye safety considerations (see below). Detailed spectral properties of the sent-out light are usually not important.

The used photodetector for such a LIDAR receiver usually must be highly sensitive and at the same time very fast. For example, one may use a Geiger-mode avalanche photodiode, or sometimes a whole focal plane array of such detectors, with which in principle one could record a whole two-dimensional image with one laser shot (flash LIDAR); however, one often needs to average such images over some substantial number of shots.

Coherent LIDAR is based on optical heterodyne detection. Here, the weak returned light is mixed with a “local oscillator” based on light from the laser used for sending out light. The heterodyne principle can greatly improve the sensitivity, so that relatively common photodiodes are sufficient for achieving a larger detection range than a direct detection LIDAR.

The sent-out light is frequently a single-frequency signal with a linear up-chirp during some time interval. Since the frequency difference between the returning light and the light emitted at the same time grows with the propagation time, a measurement of the beat note frequency allows one to measure the distance from which the reflection originated – without using short laser pulses. Of course, the instantaneous frequency of the laser light cannot grow forever, but needs to be regularly reset. A range ambiguity results which depends on the frequency sweep period. A low period of range ambiguity may be achieved with a slow sweep, which however may reduce the range accuracy. A coherent LIDAR the wires they alternatively use different sweep periods for obtaining the combination of best range accuracy with low ambiguity.

The recorded beat frequency is also modified by longitudinal motion of the target, or more precisely the relative velocity between LIDAR device and detected object. Devices which utilize the Doppler effect for velocity measurements are called coherent Doppler LIDAR.

Due to the principle of optical heterodyne detection, coherent LIDAR can be substantially more sensitive and can therefore offer a larger measurement range and/or enable the use of lower laser powers. Particularly when using semiconductor lasers, the much lower required peak power is advantageous. On the other hand, there is the requirement of carefully controlled single-frequency sweeps; in that respect, the demands on the laser are higher.

For coherent LIDAR, one usually uses a single detector in conjunction with a scanning apparatus for the transverse direction. Using a focal plane array for collecting 3D information more directly is difficult, because for each pixel one would require an AC amplifier and detection electronics, unless one would operate only one pixel at a time.

In addition, the following aspects may be relevant for applications:

• Light may be scattered not only by solid objects, but also for example by tiny particles in the atmosphere (Rayleigh or Mie scattering), or even in clean air at density fluctuations (Rayleigh scattering). In some cases, Raman scattering is used.
• The laser light may not only be scattered, but may induce fluorescence which can then be detected.
• In atmospheric LIDAR, one may utilize ground reflections for measuring light absorption in the atmosphere (see also the article on laser absorption spectroscopy). Differential absorption LIDAR (DIAL) utilizes differences in absorption at different wavelengths.

• It is possible to measure concentrations of substances and temperatures via Raman spectroscopy (Raman LIDAR).

In some cases, data acquired with LIDAR are complemented with data from other sources such as color cameras and radar.

LIDAR can be seen as a whole class of methods for remote sensing; see below for concrete applications.

LIDAR apparatuses can operate in different spectral regions between the infrared and the ultraviolet. The used optical wavelength can have various implications:

• Laser safety: the risk of affecting eyes heavily depends on the wavelength. There are devices based on eye-safe lasers, e.g. in the 1.5-μm spectral region, but it is more challenging to achieve a high laser performance in this region, and the choice of photodetectors is also more limited.
• Short wavelengths may be required for obtaining laser-induced fluorescence in certain substances or to cause Raman scattering. Also, in principle they allow for highest transverse spatial resolution.
• Long-distance transmission through the atmosphere works best in certain infrared spectral regions.

Some LIDAR apparatuses use laser light at different wavelengths – for example, 1064 nm, 532 nm and 355 nm, as obtained from a Q-switched YAG laser with frequency doubling and frequency tripling. They may then apply different kinds of detection:

• elastic backscattering at all three wavelengths
• Raman-shifted light from nitrogen (the most abundant substance in air) at 387 nm and 607 nm and water vapor (407 nm)
• parallel- and cross-polarized light components for measuring depolarization ratios

Because so different operation principles can be used, and this under very different circumstances (see below for typical applications), the technical details of LIDAR apparatuses vary substantially.

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