60 Gbps 4-PAM VCSEL-based Raman assisted hyper-scale data centre: In context of spectral efficiency and reach extension

Cost effective large-scale installation of hyper-scale data centres has attracted extensive research recently, due to the exponential bandwidth demand over extended reach. Low cost, power and spectral efficient techniques for increasing network capacity, as well as extending the transmission reach while maintaining duplex connectivity are still key challenges facing current big data networks such as hyper-scale data centre networks. In this work, a low cost energy efficient technique for maximizing carrier spectral efficiency and extending transmission reach is experimentally demonstrated. This is achieved through combined adoption of vertical cavity surface emitting lasers (VCSELs), four level pulse amplitude modulation (4-PAM), dense wavelength division multiplexing (DWDM) and Raman amplification. Three class 10 G VCSEL channels are directly modulated with a 20 Gbps 4-PAM data signal therefore maximizing carrier spectral efficiency by doubling the data rate per channel. The channels are then converged to a single fibre link through DWDM technique at 0.4 nm spacing, achieving a total aggregated data rate of 60 (3×20) Gbps. By exploiting the 8.6 dB flat gain of a forward Raman pump, a 77.12 km of standard single mode fibre (SMF) transmission is experimentally achieved. To the best of our knowledge, this is the longest error free fibre transmission distance reported using VCSEL-based 60 Gbps 4-PAM, without any forward error correction (FEC), equalization and pre-emphasis mechanisms. Results show that a real time uncorrected BER<5e−5 is achieved for all the multiplexed DWDM grid channels for 1550.71 nm - 1551.51 nm wavelength range over 77.12 km SMF. A minimum receiver sensitivity of −11.11 dBm to −10.90 dBm is attained for all the grid channels. However, a 77.12 km Raman assisted fibre transmission introduced a maximum transmission penalty of 4.78 dBm to 4.94 dBm for the considered grid channels. Forward Raman pumping offer a high flat gain over a wide spectra range (over 35.5 nm), while 4-PAM format significantly increases the aggregated data rate at different optical network units (ONUs), without expensive optics investment. We further present the design of a software defined digital signal processing (DSP) assisted receiver to efficiently recover the transmitted signal in real time as an alternative to costly receiver hardware.