We numerically investigate the 2R-regeneration technique utilizing self-phase modulation and off-center filtering. Our numerical simulations take into account the incoherent nature of noise through its spectral representation. This approach allows to evaluate a Q-factor improvement of 2 dB for this regenerator. Furthermore, our study points out the role of both the input and the output filter of this regenerator. We show that the input filter must be suitably chosen in order to obtain the best Q-factor improvement. The output filter must also be suitably chosen in order to preserve the modulation format.
We theoretically and experimentally study the principle of phase-sensitive frequency conversion in a highly-nonlinear fiber using three pump waves. This mechanism, originally demonstrated with four continuous-wave pumps and a signal wave, is based on four-wave mixing and enables to convert the two quadrature components of the signal to different frequencies. In this work, we derive a set of two simple equations to describe this mechanism and find analytic solutions. We show that only three pumps are required, instead of four as originally proposed. We give simple relations to determine the initial conditions for the power levels and the phases of the pumps. To validate this approach, we perform an experimental demonstration of the three-pump scheme and find excellent agreement with the theory.
Blind adaptive source separation (BASS) based compensation for transmitter (Tx) IQ imbalance is presented for the first time in an MQAM optical coherent system. The proposed method is numerically investigated with 4QAM and 16QAM signals in the presence of Tx IQ imbalance up to 30. The robustness of the BASS method is studied after 200-km optical fiber transmission, in which the effects of chromatic dispersion (CD) and carrier frequency offset (CFO) are assumed to be dominant. It is also found that CFO, inherent to frequency difference between the transmitter and receiver lasers in optical coherent transmission, should be compensated before IQ imbalance compensation to achieve a better performance. The proposed method outperforms the Gram-Schmidt orthogonalization procedure (GSOP) in the presence of CD and CFO. We further validate experimentally the proposed method with 10-Gbaud optical 4QAM and 16QAM signals at 30 and 10 phase imbalance, respectively, with an emulated 200-km optical fiber transmission and 200-MHz CFO. More specifically, the optical signal-to-noise ratio (OSNR) penalty reduction of the BASS method compared to the GSOP method is 1 dB for 4QAM at a bit-error-ratio (BER) of 210 3 and 2 dB for 16QAM at a BER of 10 3. Moreover, instead of being a fully independent block and requiring statistical estimation as in GSOP, the BASS method can be integrated into an equalizer and operated at the sample rate, simplifying the operation and allowing parallel implementation.
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