A 25Gb/s Optical CDR + Driver Transmitter Using 14Gb/s VCSELs in 40nm-CMOS

A 25.78Gb/s optical transmitter with asymmetric equalizing driver and reference-less clock and data recovery (CDR) is presented in 40nm CMOS. A low power 3-tap asymmetric pulse-equalizer is proposed to extend bandwidth and compensate for the nonlinearity when using low-bandwidth 14Gb/s 850nm-VCSELs. A reference-less half-rate CDR with digital frequency-locked loop is integrated to recover the data and remove any input accumulated jitter. Measurement results demonstrate 25.78Gb/s optical link with error free operation (BER<10 ^-12 ) over 100m fiber. Significant improvements are observed in the optical eye-opening, which increases the horizontal and vertical opening up to 25% and 15% respectively compared to the traditional pre-emphasis. The transmitter dissipates 280mW from 1.2V and 3.3V dual supplies, in which the VCSEL driver with equalizer consumes 128mW.