Laser range finding is one of the applications of light detection and ranging known as LIDAR. The principle of this technique is well known from the RADAR (Radio Wave Detection And Ranging). Instead of using radio waves, the LIDAR uses light as electromagnetic wave. Both techniques are based on the emission of a short pulse of electromagnetic radiation and the reception of back scattered signals from a target. The time t between the emission and reception of the pulse is measured and the distance d is calculated based on the velocity v of electromagnetic radiation. Using v‰ˆ3×108 m/sec for the speed of light, the time interval t for a distance d of 10 m will be 60 nsec. The pulse duration must be modified to match the required resolution and distance. The power of the laser diode is limited to 30 mW for safety reasons and therefore a corner cube reflector is used as a target which is positioned to the object of interest. The controller of the diode laser operates either in pulsed or continuous mode.

Principle of Operation

Short light pulses are generated by means of a diodelaser (A) which is driven by a control unit. The divergent emission of the laser diode is collimated (B) and directed to the polarising beam splitter cube (C). The polarisation of the emission of the diodelaser is oriented for maximum transmission. A small amount of laser light is passed to the start pulse detector (G). For a low divergence on large distances the laser beam is expanded by the telescope consisting of the elements (K) and (M). The back reflected light from the triple reflector target (T) enters the telescope and has a different polarisation state and will therefore mainly be reflected at the polarising beam splitter cube (C). With the quarter wave plate (Q) the polarisation state can be influenced in such a way that almost the entire back reflected light is passed to the fast Si PIN Photo detector. The scattered light is focused by means of the imaging lens (H) onto the sensitive area of the photo detector.