Time-of-flight Measurements

Time-of-flight measurements are often used for the measurement of some distance, e.g. with a laser rangefinder, used e.g. in an airplane, possibly in the form of a scanning laser radar. Here, an apparatus sends out a short light pulse and measures the time until a reflected portion of the pulse is monitored. The distance is then calculated as half the measured round-trip time divided by the velocity of light. Due to that high velocity, the temporal accuracy must be very high – e.g. 1 ns for a spatial accuracy of 15 cm. Therefore, rather fast kinds of photodetectors are needed for such measurements.

A related method is the phase shift method for distance measurements. Here, a continuously modulated signal instead of separate light pulses is used. The higher the modulation frequency, the higher is the possible spatial accuracy of the measurements.

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.

For a much higher spatial resolution, ultrashort pulses can be used, e.g. from a femtosecond laser. However, fast photodetectors (usually photodiodes) are limited in resolution at least to the picosecond region. Femtosecond temporal resolutions require all-optical techniques such as measuring optical cross-correlations.

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 may 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.

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Encyclopedia articles:

• distance measurements with lasers
• laser rangefinders
• phase shift method for distance measurements