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Time-of-flight Measurements

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Definition: distance measurements based on measuring the time of flight of a light pulse

German: Laufzeitmessungen

Categories: methods, optical metrology, light pulses

How to cite the article; suggest additional literature

Time-of-flight measurements are often used for the measurement of some distance, e.g. with a laser range finder, used e.g. in an airplane, possibly in the form of a scanning laser radar. Here, an apparatus sends out a short optical pulse and measures the time until a reflected portion of the pulse is monitored. The distance is then calculated using the velocity of light. Due to this high velocity, the temporal accuracy must be very high – e.g. 1 ns for a spatial accuracy of 15 cm.

The time-of-flight method is typically used for large distances such as hundreds of meters or many kilometers. Using advanced techniques (involving high-quality telescopes, highly sensitive photodetection, etc.), it is possible to measure e.g. the distance between the Earth and the Moon with an accuracy of a few centimeters, or to obtain a precise profile of a dam. Typical accuracies of simple devices for short distances are a few millimeters or centimeters.

As time-of-flight measurements are preferentially used for large distances, the beam quality of the laser source is crucial. In addition, a telescope can be used to obtain a large beam diameter and an accordingly increased Rayleigh length, i.e. a small beam divergence. The target can be equipped with a retroreflector in order to increase the amount of reflected light. The pulse duration used is usually between 100 ps and a few tens of nanoseconds, as achieved with a Q-switched laser. For large distances, high pulse energies are required. This can raise laser safety issues, particularly if the laser wavelength is not in the eye-safe region. For nanojoule to microjoule pulse energies (as required for moderate distances), it is possible to use a passively Q-switched microchip Er:Yb:glass laser, which can generate fairly short pulses (duration of the order of 1 ns) with pulse energies around 10 μJ in the eye-safe spectral region.

A related method is the phase shift method for distance measurements. Here, a continuously modulated signal instead of separated pulses is used.

Bibliography

[1]M.-C. Amann et al., “Laser ranging: a critical review of usual techniques for distance measurement”, Opt. Eng. 40 (1), 10 (2001)

(Suggest additional literature!)

See also: distance measurements with lasers, phase shift method for distance measurements
and other articles in the categories methods, optical metrology, light pulses

In the RP Photonics Buyer's Guide, 6 suppliers for equipment for time-of-flight measurements are listed.

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A Quiz Question

taken from the Photonics Quiz:

Assume that a periodic picosecond pulse train is generated with a gain-switched laser diode. Does the spectrum of the pulse train have the shape of a frequency comb, consisting of sharp lines?

(a) Yes, it does, because periodic pulse trains always have such a spectrum.

(b) No, it doesn't, because the pulses are not mutually coherent.

(c) This depends on the quality of the electronic driver of the laser diode.

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