Free space optical receiver and free space optical receiving method

A free space optical receiver, comprising:

• a light collecting unit configured to collect laser light having propagated through a free space transmission path;
• a multimode light generating unit configured to receive input of the laser light collected by the light collecting unit, excite multimode light, and output multimode propagation light including a plurality of propagation mode light beams with a number smaller than a number of multimode light beams possible to be excited;
• and
• a mode separating unit configured to separate the multimode propagation light into the plurality of propagation mode light beams and output the plurality of propagation mode light beams,
• wherein the multimode light generating unit includes
• a multimode light exciting unit configured to excite the multimode light including mode light beams with a first mode number and output the multimode propagation light including a plurality of propagation mode light beams with a second mode number smaller than the first mode number, and
• a multimode transmission medium having waveguide modes with a third mode number and configured to guide the multimode propagation light, the third mode number being equal to or more than the second mode number,
• wherein a fourth mode number is a number of modes that the mode separating unit is able to separate, and the fourth mode number is equal to or more than the third mode number,
• wherein the multimode light generating unit includes an optical amplifier optically connected to the multimode transmission medium, and
• the optical amplifier has a waveguide mode with a fifth mode number that is equal to or larger than the third mode number and equal to or smaller than the fourth mode number.