DIRECT-DETECTION OF OPTICAL DIGITAL TRANSMISSION BASED ON POLARIZATION SHIFT KEYING MODULATION

Receiver structures for the direct detection of binary and multilevel digital optical modulation schemes employing the modulation of the sta te of polarization of light, or POLarization Shift Keying (POLSK), are proposed and accurately analyzed, in the presence of optical amplifie r ASE noise and electrical receiver noise. A comprehensive set of resu lts shows that the performance of the binary system is approximately 3 dB better than IM/DD (on peak optical power), and stays so for a wide range of optical filter bandwidths. As a somewhat unexpected result, the multilevel schemes show a lower sensitivity to the use of a wide o ptical filter than the binary one. As a consequence, transmitting 3 b/ symbol on a cubic constellation on the Poincare sphere brings about vi rtually no penalty, whereas transmitting 2 b/symbol has a better perfo rmance than binary transmission, for even relatively low values of the optical filter bandwidth. A clear explanation of this phenomenon has been found. These results suggest that narrow-bandwidth and therefore low-dispersion, multilevel, POLSK transmission could be performed with no penalty. Together with the low excitation of nonlinear effects in the fiber that polarization modulation seems to ensure, these results make POLSK schemes look promising for very-long-haul transmission.