Time-of-flight Measurements

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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 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.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 12 suppliers for time-of-flight measurement devices. Among them:

Analog Modules

AMI's series of rangefinder/processor assemblies comprise a high performance receiver with compact support electronics to provide corrected range data to a mother system. The serial range data output is calibrated using AMI’s patented technology to mitigate the effects of range errors from various sources including walk due to return signal amplitude variations. Exceptional sensitivity allows the use of low power lasers, or alternatively, long range operation. The modules provide high voltage bias, time programmed gain with noise tracking threshold, first, last, and strongest pulse selection and range gating. The compact layouts allow positioning at the optics for minimum overall system size. A variety of models are offered providing cost verses performance options. A non-ITAR module is also available.

RPMC Lasers

Serving North America, RPMC Lasers offers a wide range of compact pulsed laser sources for time-of-flight (ToF) measurement applications like range finding or LIDAR/bathymetry. These offerings include a wide variety of short pulsed DPSS lasers, CW and pulsed fiber lasers, laser diodes, and quantum cascade laser diodes. We provide standard and custom options that provide low SWaP, ruggedized, and high-performance output for many ToF-based applications, whether airborne or integrated into lightweight, handheld, and portable applications. Let RPMC help you find the right laser today!

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