200 Gb/s Short Reach Transmitters Based on Optical Frequency Combs

The exponential growth in demand for bandwidth and the increasingly dynamic nature of the traffic, requires novel low-cost, reduced channel spacing and spectrally efficient transceivers that can flexibly adapt to the network requirements [1]. This is particularly important for next generation data centre optical networks [2]. In addition, the transceivers used in such networks have to satisfy low power, small footprint and low-cost requirements [3]. One of the most promising transmitter technology, satisfying such stringent criteria, is a multichannel transceiver employing an optical frequency comb (OFC). OFCs have a potential to revolutionise the next-generation high-speed communications with their ability to produce frequency stable carriers with high-spectral purity [4]. The fixed frequency separation between individual comb tones alleviates the need for large spectral guard bands between the channels, thus enhancing the spectral efficiency [5]. However, OFCs suffer from low comb line power (CLP), which is further worsened by the insertion loss of a passive demultiplexer or optical filters, required to separate the individual comb tones. As a result, the application of OFCs as multicarrier transceivers requires the use of optical amplifiers, which in turn degrade the optical signal to noise ratio (OSNR) of the transmitted signal, as well as adding to the system cost, footprint and complexity. In this paper, the authors demonstrate the use of an active demultiplexer, based on an optical injection locking technique [6], [7], to demultiplex the individual comb tones. Furthermore, we show that the same device can also fulfil the role of an optical amplifier and a modulator. The schematic of the OFC-based multicarrier transmitter, employing the above-mentioned multifunctional active demultiplexer, is shown in Fig. 1(a). The OFC used in our demonstration is generated using an externally injected gain switched laser (EI-GSL) [4]. The optical spectrum of the OFC is shown as an inset in Fig. 1(a). The generated comb consists of 24 lines (within 20 dB from the highest tone i.e. -18 dBm) and an FSR of 12.5 GHz. The OFC is then split using a 3-dB coupler and injected into two standard semiconductor single mode lasers (denoted as Demx1 and Demux2). The wavelengths of the demultiplexers are thermally tuned to match the OFC comb tones that are to be filtered out. As a result, the demultiplexers are injection locked by the respective comb tones, inheriting their phase and frequency characteristics. To verify the number of comb tones that can be used for the data transmission, we demultiplex a pair of lines (separated by 37.5 GHz to avoid cross-channel interference) at the time. Demux1 is then directly modulated using a 6.625 GBaud M-QAM discrete multi-tone (DMT) signal. The two demultiplexed outputs are then combined together and transmitted through the fiber. At the receiver, the data signal (Demux1 output) is filtered, detected using a photodiode and processed offline. The authors will present a successful demultiplexing, data modulation and fiber transmission of 8 comb tones, spanning 262.5 GHz and carrying 12.5 Gb/s DMT (4-QAM) and 25 Gb/s DMT (16-QAM) signals. The results show that the proposed method could be used to achieve transmission of up to 100 Gb/s over 40 km (4-QAM) and up to 200 Gb/s (16-QAM) over 25 km of standard single-mode fibre. In addition, error free transmission is reported over the said distances, without the use of an external inline optical amplifier. In summary, the experimental demonstration of the multiple functionalities of the proposed active demultiplexer, which include filtering, amplification and data modulation, all using a single device is presented.