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Summary form only given. Shannon proved the remarkable information capacity theorem and derived the capacity boundary for additive-white-Gaussian-noise (AWGN) channel on all modulation schemes in AWGN channels. The capacities of AWGN channels employing different modulation formats have been well studied in the literature. However, these results cannot be directly applied to fiberoptic communication systems. Unlike the cases studied by Shannon, optical signals have the additional parameter of polarization. It has been proposed and experimentally shown that multiplexing the two orthogonal polarization modes for information transmission can increase the spectral efficiency. However, as far as we know, no work has been done to evaluate the fundamental capacity limit on fiberoptic communications systems by taking into account two polarization modes. In this paper we show that having two orthogonal polarization modes makes the fiberoptic channel have larger capacity than the AWGN channel. The fiberoptic channels studied here are channels dominated by linear amplified spontaneous emission noise (ASEN). We extend the capacity formulae for AWGN channels to ASEN channels by taking into account two polarization modes. Based on the evaluated capacities of ASEN channels, we discuss possible gains from different coding and modulation techniques.