With the rise of 5G, big data, and cloud computing, optical communication networks are under immense pressure due to exponential IP traffic growth. To meet the needs of these services and enhance the flexibility of bandwidth management in optical networks, ultra-dense wavelength division multiplexing (UDWDM) systems have been widely studied. However, in practical deployments, existing electrical switching methods involve multiple optoelectronic conversions, leading to high power consumption across the entire network. Furthermore, UDWDM systems necessitate communication transceivers with enhanced sensitivity for long-distance transmission, which has prompted the adoption of digital coherent optical communication technology. In this paper, we demonstrated a real-time UDWDM system with optical switches over 640-km fiber using low-complexity coherent transceivers. 10 x 10 Gb/s DP-QPSK signals spaced at 12.5 GHz are transmitted over a 640-km single-mode fiber. The receiver sensitivity achieved -47 dBm, under the BER threshold of 7% overhead hard-decision Forward Error Correction (HD-FEC). Furthermore, we demonstrated the feasibility of dynamic wavelength management
and allocation through wavelength selective switching in this system, which verified the potential for flexible optical switching in UDWDM systems.
In this paper, we propose a novel approach with extremely low complexity for digital signal processing (DSP) at the receiver end. The frequency offset compensation (FOC) and carrier phase recovery (CPR) are omitted due to the self-coherent system. Multiple-input multiple-output free (MIMO-free) operation is achieved by adaptive polarization controller (APC). Therefore, a self-coherent transmission system has been successfully demonstrated experimentally over an 81 km distance, of which the first kilometer uses 7-core fiber and the remaining 80km uses standard single mode fiber (SSMF), using an 18-GBaud dual-polarization quadrature phase shift keying (DP-QPSK) signal. The center of the 7-core fiber transmitted the local oscillator (LO), while its side core transmitted the QPSK signal. This allows rapid polarization tracking of remotely delivered LO and separately delivered signal. Simultaneously, space division multiplexing (SDM) technology was employed to transmit signals in multiple channels to achieve the purpose of capacity expansion. In the experiment, a simplified self-coherent QPSK transmission system with a capacity of 432Gbps (6 × 72 Gbps) was realized.
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