Coherent beam combining of wavelength-division multiplexed telecom signals: toward high-power and high-bit-rate free-space optical telecommunications

This work presents a coherent beam combining system achieving constructive interferences of 8 arms seeded by 4 wavelength-division multiplexed (WDM) channels modulated at 10 Gbit/s each. The whole spectrum spans over 2.4 THz (19 nm) in the C-band. Phase-locking of one channel, combined with an accurate equalization process of the optical path differences (OPD) of the arms, allows us to maximize the combining efficiency of all the channels simultaneously. Each channel achieves a combining efficiency higher than 95%. Supported by an analytical description of the system, tolerancing rules underline the need for precise OPD equalization accuracy: a 5 µm OPD mismatch results in a 1% combining efficiency loss. Bit error rate and eye diagram analysis demonstrate the reliability of the system to transmit data without penalties. This marks a step toward the development of innovative WDM laser sources for high-power, high-bit-rate free-space optical telecommunications.