Polarization multiplexed free space optical communication system

An optical system comprising:

• an optical transmitter coupled to an optical signal transmission path;
• an optical receiver coupled to an optical signal reception path;
• and
• an external signal path extending between an external optical assembly and both the optical signal transmission path and the optical signal reception path;
• an optical polarization division multiplexer coupling the optical signal transmission path and the optical signal reception path to the external signal path, wherein the optical polarization division multiplexer is configured to couple optical signals having a first polarization between the external signal path and the optical signal transmission path and the optical polarization division multiplexer is configured to couple optical signals having a second polarization between the external signal path and the optical signal reception path, wherein the second polarization is orthogonal to the first polarization;
• a first non-reciprocal polarization rotator positioned along the external signal path between the optical polarization division multiplexer and the external optical assembly;
• and
• a quarter wave plate positioned along the external signal path between the non-reciprocal polarization rotator and the external optical assembly,
• wherein
• the optical transmitter is configured to emit a transmitted light signal having the first polarization in an outward direction along the external signal path towards the external optical assembly via the optical signal transmission path and the optical polarization division multiplexer;
• the optical receiver is configured to receive a received light signal having the second polarization having travelled from the external optical assembly along the external signal path in an inward direction towards the optical receiver via the optical signal transmission path and the optical polarization division multiplexer;
• the outward direction is opposite to the inward direction;
• and
• the first non-reciprocal polarization rotator and the quarter wave plate are arranged to convert any optical signal travelling in the outward direction with the first polarization to a first circular polarization having a first handedness and to convert any optical signal travelling in the inward direction along the external signal path with a second circular polarization back to the first polarization, wherein the second circular polarization has a second handedness orthogonal to the first handedness.