Patent ID: 8180226

Claim:
An optical receiver which demodulates a DQPSK-modulated optical signal into a multilevel phase-modulated signal, comprising: a polarization branch unit that branches the DQPSK-modulated optical signal into two light waves A and B of which polarization surfaces are perpendicular to each other; a first branch rotation unit that further branches the branched light wave A into two light waves A 1 and A 2 and transmits the two light waves A 1 and A 2 through a +45° quarter wavelength plate; a second branch rotation unit that further branches the branched light wave B into two light waves B 1 and B 2 and transmits the two light waves B 1 and B 2 through a −45° quarter wavelength plate; a one-bit delay circuit unit that separates each of the four light waves A 1 , A 2 , B 1 , and B 2 transmitted through each of the branch rotation units into two light waves of which polarization surfaces are perpendicular to each other, generates a one-bit delay in one of the separated light waves and multiplexes both the separated light waves while the polarization surfaces of both the separated light waves are perpendicular to each other; a first rotation branch unit that passes each of the two light waves A 1 and B 2 from the one-bit delay circuit unit through a −45° quarter wavelength plate, further rotates their polarization surfaces by 22.5°, and branches each of the light waves A 1 and B 2 into two light waves A 11 and A 12 , and B 21 and B 22 , respectively, of which polarization surfaces are perpendicular to each other; a second rotation branch unit that passes each of the other two light waves A 2 and B 1 from the one-bit delay circuit unit through a +45° quarter wavelength plate, further rotates their polarization surfaces by 22.5°, and branches each of the light waves A 2 and B 1 into two light waves A 21 and A 22 , and B 11 and B 12 , respectively, of which polarization surfaces are perpendicular to each other; and a light wave multiplexing unit that transmits four light waves B 11 , B 12 , B 21 , and B 22 from the rotation branch units through a +45° half wavelength plate, and thereafter multiplexes eight light waves A 11 , A 12 , A 21 , A 22 , B 11 , B 12 , B 21 , and B 22 into particular combinations (A 11 , B 21 ), (A 12 , B 22 ), (A 21 , B 11 ) and (A 22 , B 12 ) while polarization surfaces are maintained.