56-Gb/s Optical Transmission Performance of an InP HBT PAM4 Driver Compensating for Nonlinearity of Extinction Curve of EAM

Abstract

This paper presents a four-level pulse amplitude modulation (PAM4) driver IC that can compensate for the nonlinearity of the extinction curve of an electroabsorption modulator (EAM). The driver was designed and fabricated by using our InP HBT technology ( $f_{t}\,= \,{\text{290 GHz}}$ , $f_{{\rm{max}}}\,= \,{\text{320 GHz}}$ ). To obtain a high extinction ratio, when an EAM is driven in the nonlinear region with PAM4 signals, eye patterns of PAM4 optical waveforms are unequally spaced. This driver can adjust the modulation voltage swing for each intensity level to achieve clear eye openings in the optical domain. The results show clear eye openings of the optical output waveform with compensation for the nonlinearity. To evaluate the effect of the compensation quantitatively, the bit error rate (BER) was measured. Minimum receiver sensitivities of –9 and –8 dBm were observed for the back-to-back (B to B) configuration and transmission over 20-km-long single-mode fiber with the compensation at a BER of $2\,\times \,10^{- 4}$ , while error-free operation was not observed at the BER without our proposed compensation technique. Furthermore, our PAM4 driver compensates for nonlinearity of various extinction curves of an EAM. For the B to B configuration, a minimum receiver sensitivity below –8 dBm was observed at the BER for each extinction curve.