Distance measuring apparatus

A distance measuring apparatus comprising:

• a first unit comprising a first modulating means for polarization modulating at a first high frequency a first optical beam carrier radiation and a first transmitting means for transmitting the first optical carrier radiation over a path the length of which is to be measured;
• and a second unit comprising receiving means for receiving the polarization modulated first optical radiation after said first modulated light has traversed the path the length of which is to be measured, a second modulating means for polarization modulating at a second high frequency a second optical beam carrier radiation, means for supplying the received first optical radiation to said second modulating means so as to produce a low down-converted frequency corresponding to the difference between the first high frequency modulation and the second high frequency modulation, a further modulating means for modulating said second optical carrier radiation at the low down-converted frequency corresponding to the difference between the first high frequency modulation and second high frequency modulation to produce separate modulation of said second optical carrier radiation, and second transmitting means for transmitting the combined low frequency and high frequency modulated second optical carrier radiation to the first unit over the path the length of which is to be measured, said first unit further comprising means for supplying the received modulated second optical carrier radiation to said first modulating means so as to provide frequency down-conversion thereof, detecting means for detecting the polarization modulation phase of the second optical carrier after frequency down-conversion by said first modulating means and means for providing an indication of the length of the path based on said polarization modulation phase.