Generation of coherent and frequency-locked multi-carriers using cascaded phase modulators for 10 Tb/s optical transmission system

The generation of coherent and frequency-locked optical multi-carriers by using cascaded phase modulators and recirculating frequency shifter (RFS) based on an erbium doped fiber amplifier (EDFA) loop has been investigated theoretically and experimentally for a multi-terabit optical orthogonal frequency division multiplexing (OFDM) superchannel transmission. The phase and amplitude relationship between RF driving signals of two cascaded phase modulators and the impact of EDFA gain are analyzed. Experimental results are in good agreement with the theoretical analysis. The 112 coherent and frequency-locked subcarriers obtained after tilt filtering exhibit a tone-to-noise ratio of larger than 26 dB, a power fluctuation of less than 5 dB, and a frequency spacing of 25 GHz giving a 22.6 nm-wide optical OFDM signal spectrum. By using this method, we have successfully generated a 10-Tbit/s (112 × 100 Gbit/s) single optical OFDM polarization-division-multiplexing QPSK (PDM-QPSK) superchannel with an optical signal to noise ratio per OFDM subchannel greater than 20 dB at 0.1 nm bandwidth, and it has been transmitted over a 640-km SMF-28 and EDFA-only link with BER less than 2 × 10 ^{- 3}.