Those things are fun. The idea is that you modulate the outgoing beam with a RF carrier, and measure the phase shift of the incoming beam relative to the outgoing beam. Today you can do this directly, because electronics is fast enough to count in the gigahertz range. It used to have to be done by superheterodyning, like a radio receiver. You down-convert both the incoming and outgoing signals using a local oscillator, and get two new signals at a much lower frequency. This transformation has the useful property that the phase difference between the lower-frequency signals is the same as the phase difference at the higher frequency. But at the lower frequency, it's easier to measure the phase difference. Also, you get the selectivity of a superheterodyne receiver.

That's fine for a scanner, where you only need one copy of the electronics, but doing it for every pixel means something like a radio receiver behind each pixel. Still, you could probably do that now.

The SwissRanger [1], a short-range time of flight imaging camera, worked like that. It's been discontinued.

(I fooled around with this in the early 1990s, trying to build a laser scanner. Too many noise problems.)

[1] http://hptg.com/industrial/