How do distance measurement devices work on a laser?

Answer

Using the TOF (Time-Of-Flight) principle, we emit a series of pulses with the laser. The pulses are reflected on the object and bounce off there. Then the light returns to the transmitter. This transmitter knows when the pulse train has started and a receiver measures the phase difference (time between the start of a sent and received pulse) of the pulses. This phase difference is greater as the distance increases. Based on this, the distance can be calculated.

Light travels about 30cm per nanosecond. A light pulse sent towards an object at a distance of 30cm will therefore arrive 2 nanoseconds later (the light has to go back and forth) than the moment it left. It is therefore important to be able to measure very accurately. There are sensors that immediately calculate this phase difference and convert it into a voltage, proportional to the distance. In this way, fewer conversions are needed afterwards and the accuracy is increased (less noise). Usually different measurements are also taken and these measurements are averaged out.

It is important for this application that the light source is modulated (turning on and off), because the transition from on to off can be used as a starting point for a measurement.

Interferrometers can be used for very precise measurements (up to nanometer scale). The coherent light beam is split in two and then brought back together. One path is the reference path and the other path travels the distance we want to measure. Distance differences in both paths create a pattern (so-called interference pattern), from which the distance difference in both paths can be calculated.

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