  In a high-speed coherent optical transmission system, typically the signals
obtained at the receiver front-end are digitized using very high-speed ADCs and
then processed in the digital domain to remove optical channel impairments. In
this work, we show that these signals can instead be processed in the analog
domain itself, which can significantly reduce the power consumption as well as
the complexity of the receiver. The first all-analog adaptive equalizer for
receivers of coherent dual-polarization optical links has been presented with
its detailed architecture and measurement results. The proof-of-concept
equalizer uses the constant modulus algorithm for blind adaptation of its
weight coefficients to implement a 4x4 2-tap FIR filter in 130 nm SiGe BiCMOS
technology. Its functionality is evaluated experimentally for 40 Gb /s data
rate and 10 km standard single-mode fiber channel. This demonstration shows
that the use of all-analog processing for short-reach data-center interconnects
is feasible and is a much simpler solution than the use of the high-speed
ADC+DSP based approach. Moreover, when implemented in advanced CMOS or FinFET
technologies, the power consumption of the equalizer is expected to be
significantly lower than the DSP based implementations in similar process
technologies.
