Laser
There are two main tecniques to measure the distance with laser beam. One way is to pulse the laser beam and measure the time between emitted and reflected pulse. Second way is to beam laser constantly and then shoot the camera with special filter to measure how much the dot of the laser has moved on obstacle. The latter is the cheaper method as it does not need that special processor that is capable of sampling at nanoscale timeframe.
Figure 1: Distance measurement with angle detection method