High-speed framed-pulsewidth modulation transceiver for extended reach optical links

Abstract

This paper proposes a high-speed framed-pulsewidth modulation (FPWM) transceiver that reduces signal loss due to optical fiber dispersion as a method to improve frequency spectrum efficiency in optical communication. An additional 75% of coding gain is obtained through proposed time-domain modulation. It increases the minimum pulsewidth by 1.75 times compared to NRZ, lowering the Nyquist frequency to 4/7. The FPWM signaling not only reduces losses in the metal channel, but also suppresses the penalty due to chromatic dispersion in the optical fiber proportional to the square of bandwidth. Low power and latency modulator and de-modulator are implemented based on successive approximation and weighted sum algorithm each. Measurements under back-to-back condition using an externally modulated laser and avalanche photo diode show that the FPWM signal has 6dB better receiver sensitivity at a bit error rate of 2e-5 than PAM-4 at 26Gbps. FPWM outperforms NRZ by 8dB over 15km of single-mode fiber and PAM-4 by 4dB at 20km of SMF. Thus, proposed IC is possible to send a longer reach compared to the existing method under the same optic conditions. The full chip occupies 2.2x2.0mm2 with bi-directional two lanes and two PLLs. It consumes 262mW per lane from a 0.9V supply. The random jitter of the PLL measured at Tx clock pattern is 265fs, rms. The testchip prototype is fabricated in a 28-nm CMOS process and packaged in a flip-chip chip scale package.