Optical hybrid

Coherent optical receiver comprising an optical local oscillator, polarization control means for matching/adjusting with respect to one another two signals, namely a received optical signal and a signal coming from the local oscillator, an optical hybrid having two input ports for receiving the two signals and a plurality of output ports for extracting coherent products of the two received signals, which coherent products differ in phase, and detection means for detecting said coherent products, said optical hybrid emitting a first and a second output signal via a first and a second output port of said plurality of output ports, respectively, said first and second output signals having a phase difference with respect to one another of 90.degree. wherein the detection means comprise only two photodetectors, a first one of said two photodetectors being coupled with the first output port and the second one of said two photodetectors being coupled with the second output port of the optical hybrid, and wherein a power transmission occurs from each of the two input ports of the optical hybrid to each of the first and second output ports to which the first and the second one of said two photodetectors are coupled, which power transmission is greater than 25%, wherein said optical hybrid includes a first and a second polarization controller on the first and second input port of said optical hybrid, respectively, for setting the polarization of the first and the second input signal, respectively, a polarization-independent power coupler for mixing the first and second input signals, the polarizations of which are set to form a mixed signal, and for distributing evenly according to power the mixed signal over the first and the second output ports, and a and a second polarizer on the first and the second output respectively, and wherein the first and second polarization controllers are set to a first and a second position, respectively, and the first and second polarizers are placed in a third and a fourth position, respectively, said first and second positions of the first and second polarization controllers, respectively, and the third and fourth positions of the first and second polarizers , respectively, being such that points Q1, Q2, Q3 and Q4, which in a Poincare representation represent said first, second, third and fourth positions, respectively, on a Poincare sphere fulfill the following two requirements:

• (a) angles of arc on the Poincare sphere which express distances on the Poincare sphere between the points Q1 and Q3, between the points Q1 and Q4, between the points Q2 and Q3 and between the points Q2 and Q4 all being smaller than 90.degree., and
• (b) a circle C1 on the Poincare sphere uniquely defined by the points Q1, Q2 and -Q3, and a circle C2 on the Poincare sphere uniquely defined by the points Q1, Q2 and -Q4 intersecting each other at angles of 90.degree. in their intersecting points Q1 and Q2, the points -Q3 and -Q4, respectively, being points on the Poincare sphere that are diametrically opposed to the points Q3 and Q4.