An apparatus, comprising: a coherent optical data receiver including two optical hybrids (36, 38), two light detectors and a digital signal processor, each optical hybrid being configured to output mixtures of a corresponding one of the polarization components of a received data-modulated optical carrier with reference light, each light detector to output digital measurements of the mixtures at a sequence of times from a corresponding one of the optical hybrids; wherein the digital signal processor is configured to identify a sequence of data symbols of a constellation having parts transmitted on both polarization components of the data-modulated optical carrier in response to receipt of the digital measurements, the transmitted data-modulated optical carrier having about a same total light intensity in each optical modulation time slot thereof, each data symbol being defined by in-phase and quadrature-phase electric field coordinates of both polarization components; and wherein pairs of the in-phase and quadrature-phase electric field coordinates of each of the polarization components of the data symbols for each optical modulation time slot are located on a preselected set of two or more concentric circles about an origin, the constellation has four times an integer dimensions, wherein either each pair of the data symbols with pairs of in-phase and quadrature-phase electric field coordinates of the one of the polarization components at adjacent points on one of the circles has pairs of in-phase and quadrature-phase electric field coordinates of the other of the polarization components at or near diametrically opposite points on another of the circles, or each pair of data symbols with pairs of in-phase and quadrature-phase electric field coordinates of the one of the polarization components at a same point on one of the circles has pairs of in-phase and quadrature-phase electric field coordinates of the other of the polarization components at or near diametrically opposite points on another of the circles. a coherent optical data receiver including two optical hybrids (36, 38), two light detectors and a digital signal processor, each optical hybrid being configured to output mixtures of a corresponding one of the polarization components of a received data-modulated optical carrier with reference light, each light detector to output digital measurements of the mixtures at a sequence of times from a corresponding one of the optical hybrids; wherein the digital signal processor is configured to identify a sequence of data symbols of a constellation having parts transmitted on both polarization components of the data-modulated optical carrier in response to receipt of the digital measurements, the transmitted data-modulated optical carrier having about a same total light intensity in each optical modulation time slot thereof, each data symbol being defined by in-phase and quadrature-phase electric field coordinates of both polarization components; and wherein pairs of the in-phase and quadrature-phase electric field coordinates of each of the polarization components of the data symbols for each optical modulation time slot are located on a preselected set of two or more concentric circles about an origin, the constellation has four times an integer dimensions, wherein either each pair of the data symbols with pairs of in-phase and quadrature-phase electric field coordinates of the one of the polarization components at adjacent points on one of the circles has pairs of in-phase and quadrature-phase electric field coordinates of the other of the polarization components at or near diametrically opposite points on another of the circles, or each pair of data symbols with pairs of in-phase and quadrature-phase electric field coordinates of the one of the polarization components at a same point on one of the circles has pairs of in-phase and quadrature-phase electric field coordinates of the other of the polarization components at or near diametrically opposite points on another of the circles.