Time–Frequency Packing Applied to Cost-Effective IM/DD Transmission Based on Directly Modulated VCSEL

Potential of directly modulated vertical cavity surface emitting lasers (VCSELs), widely employed for low-cost and energy-efficient intradata center transmissions, is here studied for transmissions up to access/metro scenarios. In particular, the manuscript refers to long-wavelength VCSELs for intensity modulation/direct detection (IM/DD) on-off keying transmission over standard single-mode fiber (SSMF). The intent of this paper primarily consists of demonstrating information rate and fiber reach enhancement in potentially uncooled environments or with relaxed thermal management requirements through the use of time-frequency packing (TFP) technique in a VCSEL-based transmission. TFP has been applied here to IM/DD transmission for the first time, although not in a multiple channel system. The digital signal processing unit implementation is clearly simplified with respect to coherent systems (no frequency synchronization and no phase error recovery), where TFP was firstly introduced, as it only takes into account the photo-detected signal intensity, regardless of the phase information. On the other hand, the signal processing is here more complex than in a traditional IM/DD system. Through the use of a commercial C-band 4 G VCSEL, net information rate up to 12.5 and 11.2 Gb/s has been measured for 25 and 45 km of SSMF, respectively, in an unamplified optical link and with power budget equal to 19 and 16 dB, respectively. The transmission is intrinsically polarization independent and wavelength independent and tolerance to temperature increase up to 60 °C has been analyzed together with energy consumption.