Active Demultiplexer-enabled Directly Modulated DMT Transmission Using Optical Frequency Combs for Data Center Interconnects

In this paper, we experimentally demonstrate an optical frequency comb (OFC) based transmitter, employing directly modulated active demultiplexers, for data center interconnects. The results validate that the proposed transmitter has the potential to achieve aggregate data rates of 100 Gb/s (8 $ \times $ 12.5 Gb/s) and 200 Gb/s (8 $ \times $ 25 Gb/s) for systems employing 4- and 16- quadrature amplitude modulated (QAM) discrete multi-tone (DMT) modulation. An OFC based on an externally injected gain switched laser (EI-GSL) is used, providing excellent stability and flexibility in free spectral range (FSR). The OFC is followed by an injection locked active demultiplexer, which not only filters, but also amplifies individual comb tones, thus alleviating the need for an external optical amplifier to boost the low powered comb tones. Using the proposed configuration, we experimentally demonstrate a successful transmission of 12.5 Gb/s/ ${\boldsymbol{\lambda }}$ 4-QAM DMT and 25 Gb/s/ ${\boldsymbol{\lambda }}$ 16-QAM DMT signals over 40 km and 25 km of SSMF, respectively. In addition, we show that it is possible to filter and modulate comb lines that are 20 dB below the spectral peak, whilst achieving a BER below the hard decision (HD-) FEC limit of 3.8e-3. This gain flattening or comb expansion feature leads to a significant increase in the channel count, which in turn provides a reduction in the energy consumption and the footprint of the transmitter.