Optical unbalanced QPSK for free-space laser links

For satellite or unmanned aerial vehicle links, the scheme of unbalanced quadrature phase shift keying (U-QPSK) has been widely utilized, where precise ranging and high-speed communication are performed in the in-phase and quadrature branches, respectively. Despite the maturity of microwave U-QPSK technology, the optical counterpart has not yet been investigated. With the rapid development of free-space laser link technologies in recent years, the optical U-QPSK scheme is proposed in this Letter to achieve flexible power allocation for ranging and communication. The feasibility of U-QPSK is demonstrated in carrier synchronization and baseband demodulation. By configuring a key parameter, optical QPSK and U-QPSK are unified in the signal processing algorithm, supporting fast switching between the two schemes according to the practical link budget in realistic scenarios. This research could promote the large-scale application of U-QPSK technology in the optical domain.