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Light moves very fast, so measuring short distances – 4 meters and less – precisely and accurately requires some very clever optical, semiconductor, electronic, and software tricks. This article describes how these tricks have been employed to achieve sub-centimeter accuracy at the speed of light. This should be of interest to potential users of these devices as well as those who are curious about how to capture and measure events at the picosecond level.

    Electromagnetic radiation, first as radio waves, and, more recently light, has been used for detection and ranging for nearly a century. With either form of radiation, the most common method is the same: transmit a short very intense pulse and measure the round-trip travel time until the echo of the transmitted pulse is received. The Time of Flight (ToF) is thus equal to twice the distance to the target divided by the speed of light. This sounds simple but is quite challenging in practice for three fundamental reasons. First, electromagnetic radiation is very fast – 3x108 m/s – so the round-trip time for a target 1m away is ~6ns and, if you want cm-level resolution, you have to be able to resolve events closer than 60ps apart. Second, the strength of the returning echo signal falls off roughly as 1/r4 and is therefore extremely weak. Finally, since the transmitter and receiver must be very close together so you can measure the ToF accurately, it is important that the very intense transmitted pulse not blind the very sensitive receiver either temporarily or permanently.
   As a result, there are several significant engineering challenges to overcome in order to make a practical light-based ranging system with sub-centimeter resolution. These challenges include: generating an outgoing pulse with picosecond rise time; detecting the return even if it occurs during the transmit pulse; accurately measuring the time of arrival of the leading edge of the return over several orders of magnitude of detected signal strength and background illumination; detecting returns from multiple targets; and, finally, producing this in a small package that consumes a minimum of power.
  This article will describe the many optical, semiconductor, electronic, and software innovations needed to implement a 64-sensor ToF array capable of sub-centimeter resolution in a low-power-consumption 6.4 x 3.0 x 1.75 mm SMD package. It’s a story of some very clever engineering that should be of interest to those curious about how these devices work as well as an introduction to some interesting devices to include in your next project.