Phase Shift Method for Distance Measurements | previous | next | feedback |
Laser range finders are often based on the phase shift method, a technique for measuring distances in the following way. A laser beam with sinusoidally modulated optical power is sent to a target. Some reflected light (from diffuse or specular reflections) is monitored, and the phase of the power modulation is compared with that of the sent light. The phase shift obtained is 2π times the time of flight times the modulation frequency. This shows that higher modulation frequencies can result in a higher spatial resolution.
As for an interferometer, there is some ambiguity regarding the measured distance, because with increasing distance the phase will vary periodically. However, the periodicity is much larger than in an interferometer, since the modulation frequency is much lower than the optical frequency. Also, the ambiguity can easily be removed, e.g. by measuring with two different modulation frequencies.
A special kind of power modulation is achieved by using a mode-locked laser. Advantages are the high modulation frequency (allowing for high accuracy) and (for passive mode locking) that no optical modulator is required.
Compared with interferometers, devices based on the phase shift technique are less accurate, but they allow unambiguous measurements over larger distances. Also, they are suitable for targets with diffuse reflection from a rough surface.
See also: distance measurements with lasers, interferometers
Categories: methods, metrology
