We experimentally investigate the performance of a spectrally efficient multi-carrier channel consisting of two or more optical carriers spaced around the baud rate, with each carrier modulated with polarization division multiplexed (PDM) quadrature phase shift keyed (QPSK) format. We first study the performance of a 100-Gb/s 2-carrier PDM-QPSK channel with each carrier modulated at 12.5 Gbaud as a function of various design parameters such as the time alignment between the modulated carriers, the frequency separation between the carriers, the oversampling factor at the receiver, and the bandwidth of the digital pre-filter used for carrier separation. While the measurements confirm the previously reported observations, they also reveal some interesting additional features. The coherent crosstalk between the modulated carriers is found to be minimized when these carriers are symbol aligned. Spacing the carriers at the baud rate, corresponding to the orthogonal frequency-division multiplexing (OFDM) condition, leads to a local maximum in performance only for some specific cases where large oversampling (>2 × ) is applied. It is found that 4 × oversampling, together with a constant modulus algorithm (CMA) based digital equalizer having multiple quarter-symbol (T/4) spaced taps, gives much better overall performance than 2 × oversampling with a CMA-based equalizer having T/2 spaced taps. In addition, using a T/4-delay-and-add filter (DAF) as a pre-filter for assist carrier separation is found to give better performance than the commonly used T/2-DAF. In addition, it is possible to set the carrier spacing to be as small as 80% of the baud rate while incurring negligible penalty at BER≈10−3. 3-carrier and 5-carrier PDM-QPSK channels at 12.5-Gbaud with frequency-locked carriers spaced at 12.5 GHz and 4 × oversampling are also studied, and shown to perform reasonably well with small relative penalties. Finally, increasing the baud rate of the 2-carrier PDM-QPSK to 25 Gbaud and 28 Gbaud is investigated. It is found that with a fixed sampling speed of 50 Gsamples/s, scaling from 12.5 Gbaud to 25 and 28 Gbaud causes excess crosstalk penalties of about 2.8 dB and 4.8 dB, respectively, indicating the need to increase the sampling speed and transmitter bandwidth in order to support these high-data-rate channels without excessive coherent crosstalk.
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