Scaling dispersion-compensated DWDM transmission toward Tb/s-class capacity: a 64-channel analysis at 25 and 40 Gb/s with NRZ, RZ, and PAM-4 modulation

Abstract

Direct-detection DWDM links combining dispersion compensating fiber (DCF) with chirped fiber Bragg gratings have been studied mainly at 10 Gb/s per channel and low channel counts. This paper examines how such compensation scales when rate and channel count increase together. A 64-channel, 50 GHz-grid system using DCF with a square-root chirped fiber Bragg grating (SR-CFBG) is analyzed at 25 and 40 Gb/s per channel (1.6 and 2.56 Tb/s aggregate) with NRZ, RZ, and PAM-4. An analytical Q-factor framework combines ASE-, ISI-, and receiver-limited contributions, and its OSNR model matches a 32-channel, 10 Gb/s reference within 0.08 dB over 100–2,000 km. With compensation fixed, the maximum FEC-compliant reach ( Q  ≥ 6; SMF distance only) falls from 2,433 km at 10 Gb/s to 405 km (25 Gb/s) and 160 km (40 Gb/s) for NRZ, whereas PAM-4 reaches 652 and 297 km. NRZ and RZ remain dispersion-limited, but PAM-4 becomes OSNR-limited: lowering launch power from −5 to −10 dBm shortens its reach by about half. Relative to the 10 Gb/s reference, the capacity–distance product is 0.83 and 0.53 times for NRZ but 1.34 and 0.98 times for PAM-4. Results are trend-level: the ISI term is calibrated at one operating point, and the PAM-4 advantage depends on the assumed rate scaling.