Beyond 200-Gb/s O-band intensity modulation and direct detection optics with joint look-uptable- based predistortion and digital resolution enhancement for low-cost data center interconnects

We propose a joint look-up-table(LUT)-based nonlinear predistortion and digital resolution enhancement scheme to achieve high-speed and low-cost optical interconnects using low-resolution digital-to-analog converters(DACs).The LUT-based predistortion is employed to mitigate the patterndependent effect(PDE)of a semiconductor optical amplifier(SOA),while the digital resolution enhancer(DRE)is utilized to shape the quantization noise,lowering the requirement for the resolution of DAC.We experimentally demonstrate O-band intensity modulation and direct detection(IM/DD)transmission of 124-GBd 4∕6-level pulse-amplitude modulation ePAMT-4∕6 and 112-GBd PAM-8 signals over a 2-km standard single-mode fiber(SSMF)with 3∕3.5∕4-bit DACs.In the case of 40-km SSMF transmission with an SOAbased preamplifier,124-GBd on-off-keying(OOK)/PAM-3/PAM-4 signals are successfully transmitted with 1.5∕2∕3-bit DACs.To the best of our knowledge,we have achieved the highest net data rates of 235.3-Gb∕s PAM-4,289.7-Gb∕s PAM-6,and 294.7 Gb∕s PAM-8 signals over 2-km SSMF,as well as 117.6-Gb∕s OOK,173.8-Gb∕s PAM-3,and−231.8 Gb∕s PAM-4 signals over 40-km SSMF,employing low-resolution DACs.The experimental results reveal that the joint LUT-based predistortion and DRE effectively mitigate the PDE and improve the signal-to-quantization noise ratio by shaping the noise.The proposed scheme can provide a powerful solution for low-cost IM/DD optical interconnects beyond 200 Gb∕s.

Carrier-assisted differential detection

To overcome power fading induced by chromatic dispersion in optical fiber communications,optical field recovery is a promising solution for direct detection short-reach applications,such as fast-evolving data center interconnects(DCIs).To date,various direct detection schemes capable of optical field recovery have been proposed,including Kramers−Kronig(KK)and signal−signal beat interference(SSBI)iterative cancellation(IC)receivers.However,they are all restricted to the single sideband(SSB)modulation format,thus conspicuously losing half of the electrical spectral efficiency(SE)compared with double sideband(DSB)modulation.Additionally,SSB suffers from the noise folding issue,requiring a precise optical filter that complicates the receiver design.As such,it is highly desirable to investigate the field recovery of DSB signals via direct detection.In this paper,for the first time,we propose a novel receiver scheme called carrier-assisted differential detection(CADD)to realize optical field recovery of complex-valued DSB signals via direct detection.First,CADD doubles the electrical SE compared with the KK and SSBI IC receivers by adopting DSB modulation without sacrificing receiver sensitivities.Furthermore,by using direct detection without needing a precise receiver optical filter,CADD can employ cost-effective uncooled lasers as opposed to expensive temperature-controlled lasers in coherent systems.Our proposed receiver architecture opens a new class of direct detection schemes that are suitable for photonic integration analogous to homodyne receivers in coherent detection.