1.Adopting four-wavelength polarization-multiplexing and PAM4 modulation schemes [25 Gb/s × 2 (PAM4 modulation) × 4 (four-wavelength) × 2 (polarization-multiplexing)] can remarkably multiply the aggregate transmission capacity to 400 Gb/s.
2.With doublet lenses between both sides, high BER performance (<10−9) and clear PAM4 eye diagrams are acquired.
3.Upon injection locking, VCSEL＇s 3-dB modulation bandwidth is considerably enhanced to 24.6 GHz (10.4 GHz → 24.6 GHz).
4.A pair of doublet lenses are placed on two sides to emit/receive laser light through the free-space in a two-way communication.
5.With TOBPF and PBS, the polarized wavelengths are filtered and de-multiplexed due to modulation at different wavelengths and polarization with orthogonal states.
6.An electrical equalizer is adopted to compensate for the frequency response and enhance the transmission performances.
7.Real-time BER is measured by a high-sensitivity error detector and the PAM4 three-eye sampling approach.

1.The total transmission capacity is greatly increased to 400 Gb/s by adopting four-wavelength polarization-multiplexing and PAM4 modulation schemes [25 Gb/s × 2 (PAM4 modulation) × 4 (four-wavelength) × 2 (polarization-multiplexing)].
2.With injection locking, VCSEL＇s 3-dB bandwidth is significantly enhanced to 24.6 GHz.
3.A set of doublet lenses are placed on two sides to emit/receive laser light through the free-space in a two-way communication.
4.With TOBPF and PBS, the polarized wavelengths are filtered and de-multiplexed due to modulation at different wavelengths and polarization with orthogonal states.
5.An electrical equalizer is used to enhance the transmission performance.
6.Real-time BER is measured by a high-sensitivity error detector and the PAM4 three-eye sampling approach.

As the transmission rate increases, the demand for related parts increase as well. Under such scenario, the transmission medium "copper out, light in" has become an inevitable trend. If 400-Gb/s ultra-high-speed bidirectional FSO communication for cloud server data transmission/switching is adopted in the cloud CO, many practical problems induced by optical cables will be resolved. With ultra-high-speed FSO communications, the optical communication and telecommunication industries will develop a new generation for high-transmission-rate applications. A 400-Gb/s ultra-high-speed bidirectional FSO communication for cloud server data transmission/switching enhances the enterprises to develop the broad market of optical communication and promotes the industrial international competitiveness.