Optical fiber interferometer with relaxed loop tolerance and QKD system using the same

An optical fiber interferometer comprising:

• a first optical fiber loop adapted to carry orthogonally polarized first and second optical signals in respective first and second arms of the first loop;
• a polarization-maintaining (PM) optical fiber section optically coupled at an input end to the first optical fiber loop and adapted to transmit the orthogonally polarized first and second optical pulses along respective fast and slow axes;
• a polarization-adjusting member arranged at or near an output end of the PM optical fiber section and adapted to receive the orthogonally polarized first and second optical signals transmitted by the PM optical fiber section and form co-polarized first and second optical signals;
• a second optical fiber loop optically coupled to the polarization-adjusting member and adapted to interfere the first and second co-polarized optical pulses, wherein the first and second optical fiber loops have an optical path length (OPL) difference between them;
• and
• wherein the PM optical fiber section has a length and orientation selected to compensate for the OPL difference.