Alamouti coding enabled polarization insensitive simplified self-homodyne coherent system for short-reach optical interconnects

Coherent technology inherent with more available degrees of freedom is deemed as a competitive solution for next-generation ultra-high-speed short-reach optical interconnects. However, the high system cost, elevated power consumption, and large footprint size pose formidable barriers to the application of conventional coherent systems in short-reach scenarios. Self-homodyne coherent detection exhibits its potential for short-reach applications due to the sharing of the signal carrier and the local oscillator (LO), which not only reduces the system cost but also eliminates the frequency offset. Nevertheless, the self-homodyne coherent technique suffers from the polarization fading issue due to the random polarization states of the remote LO. In this paper, an Alamouti coding enabled simplified self-homodyne coherent detection technique is proposed to solve the polarization fading issue, resulting in a polarization-insensitive receiver. Besides, to further reduce the computational complexity of the system, the digital subcarrier multiplexing (DSCM) technique is discussed to alleviate the complexity of chromatic dispersion compensation (CDC), which is another dominant power consumption module of the receiver-side digital signal processing. The performance of the proposed scheme is demonstrated with a 50Gbaud 4-subcarrier 16/32QAM DSCM signal. The results show that polarization-insensitive self-homodyne detection is achieved by the transmitter-side Alamouti coding technique, circumventing the sophisticated automatic polarization controller for the polarization state tracking of the remote LO. In addition, by applying the joint dispersion compensation and equalization (JDCE) method for the DSCM signal, the CD-induced penalty can be fully addressed by increasing 4 taps in the equalizer for an 80 km single-mode fiber transmission without an individual CDC module, reducing the computational complexity of the system significantly.