An apparatus comprising: a polarization-sensitive coherent optical receiver (74 n) that includes an optical hybrid and a plurality of light detectors (161 1 -161 4) connected to receive light outputted by the optical hybrid in response to an optical data signal (130") applied thereto, the optical data signal having encoded thereon different respective data streams in each of first and second orthogonal polarizations thereof; and a digital signal processor configured to recover said respective data streams by processing digital samples (168 1 -168 4) representing electrical output signals (162 1 -162 4) generated by the light detectors in response to the received light; and wherein the digital signal processor comprises a power-regime monitor configured to: compute first and second sets of values representing a measure of transmission quality of the optical data signal in a sequence of symbol periods, the first and second sets corresponding to the first and second orthogonal polarizations, respectively; and determine (610/612) a launch-power adjustment for the optical data signal using statistical characteristics of the values from said first and second sets, wherein the power-regime monitor is configured to determine said statistical characteristics using third and fourth sets of values, the third set having smaller or higher values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods. a polarization-sensitive coherent optical receiver (74 n) that includes an optical hybrid and a plurality of light detectors (161 1 -161 4) connected to receive light outputted by the optical hybrid in response to an optical data signal (130") applied thereto, the optical data signal having encoded thereon different respective data streams in each of first and second orthogonal polarizations thereof; and a digital signal processor configured to recover said respective data streams by processing digital samples (168 1 -168 4) representing electrical output signals (162 1 -162 4) generated by the light detectors in response to the received light; and wherein the digital signal processor comprises a power-regime monitor configured to: compute first and second sets of values representing a measure of transmission quality of the optical data signal in a sequence of symbol periods, the first and second sets corresponding to the first and second orthogonal polarizations, respectively; and determine (610/612) a launch-power adjustment for the optical data signal using statistical characteristics of the values from said first and second sets, wherein the power-regime monitor is configured to determine said statistical characteristics using third and fourth sets of values, the third set having smaller or higher values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods. compute first and second sets of values representing a measure of transmission quality of the optical data signal in a sequence of symbol periods, the first and second sets corresponding to the first and second orthogonal polarizations, respectively; and determine (610/612) a launch-power adjustment for the optical data signal using statistical characteristics of the values from said first and second sets, wherein the power-regime monitor is configured to determine said statistical characteristics using third and fourth sets of values, the third set having smaller or higher values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods.