Time of Flight Measurements

Time-of-flight measurements are often used for 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 rather high – e.g. one nanosecond for a spatial accuracy of 15 cm.

The time-of-flight method is typically used for large distances like 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 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 reflector in order to increase the amount of reflected light. The used pulse duration is usually between 100 ps and a few tens of nanoseconds, as achieved with a Q-switched laser. For large distances, large 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 rather short Pulses (duration of the order of 1 ns) with pulse energies around 10 μJ in the eye-safe spectral region.

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