Coherent optical communication algorithms and chips have served as the primary catalyst for the advancement of longhaul optical transmission systems over the past two decades. This paper examines the developmental trajectory of coherent optical algorithms and chips for longhaul optical transmission, providing a comprehensive analysis of key algorithms essential for engineering applications, including analysis of parallelization processing and loop bandwidth, reduced sampling rate and clock recovery, non-integer-oversampling equalization, and forward error correction. Additionally, it explores how implementation constraints of low-complexity and low-power chips affect high-performance algorithm design.

For future 1.6T and 3.2T longhaul optical transmission systems, maintaining QPSK mode as the single wavelength modulation format would require optical module baud rates exceeding 500 Gbaud/s, imposing substantial bandwidth and noise background requirements on optoelectronic components and oDSP chips for baseband signals. Furthermore, signal integrity in optical module routing connections under high baud rate conditions presents significant engineering challenges, with uncertain feasibility. Consequently, the aggregation of multiple optical carriers into super channel configurations emerges as a viable technical approach. However, multi-channel modes based on carrier aggregation limit spectral efficiency improvements. The wider channel spacing requirements reduce the channel count for 1.6T and 3.2T in single fibers, maintaining relatively constant total fiber capacity. Modern optical fiber cables contain hundreds or thousands of optical fibers. A more practical solution involves parallel aggregation of multiple optical fibers within multi-fiber cables to achieve next-generation ultra-large capacity longhaul transmission systems. This approach necessitates cost-effective multi-fiber shared optical amplification systems with fiber wavelength fusion capability and innovative coherent optical module architectures, signal processing algorithms, and efficient oDSP chips. These advancements facilitate reduced bit/km transmission costs, supporting the ongoing development of high-capacity longhaul optical transmission systems.

We propose a new spatial division multiplexing mode based on multi-fiber optical cables, as shown in Fig. 9. In a dual fiber parallel optical transmission system, the laser optical power of one wavelength is split and modulated by two 800G signals. The modulated data comes from two different 1.6T signals, and the remaining two 800G signals from the two 1.6T signals are modulated by the other wavelength split signal. This achieves laser sharing on both the transmitting and receiving sides, and the 1.6T signals are modulated on two adjacent wavelengths of the same fiber, ensuring controllable transmission delay. This architecture can evolve to a four-fiber mode, as shown in Fig. 10, where one laser is shared by four fibers while ensuring that the 3.2T signal is modulated on four adjacent wavelengths of the same fiber. For multiple wavelengths aggregated within the same fiber, considering non orthogonal multiplexing based on FTN shaping algorithm, as shown in Fig. 11, the channel spacing can be compressed to 800 GHz, achieving spectrum effectiveness (SE) of 4 bit/(s/Hz), corresponding to a 33% SE yield.

Coherent optical communication systems have experienced remarkable growth over the past two decades, advancing longhaul optical transmission rates from 40G QPSK to 800G QPSK and increasing single wave rates twentyfold. Coherent oDSP algorithms and ASIC chips represent the primary catalysts for this progress. The deceleration of Moore’s Law has imposed significant power constraints on chips, necessitating research focus on power-efficient algorithm design, including parallelism and loop bandwidth optimization, low sampling rate clock recovery and equalization, iterative error correction decoding, and Turbo equalization algorithms. The traditional approach of increasing baud rates under QPSK modulation for future 1.6T and 3.2T long-haul optical transmission systems has become unsustainable. This paper introduces a novel system solution, multi-fiber space division multiplexing, along with corresponding coherent optical modules, oDSP algorithms, and chip architectures, intended to support optical transmission advancement for the next two decades. This innovation faces considerable technical challenges, including multi-fiber shared optical amplifier design, new optical cross systems compatible with diverse fibers and wavelengths, cost-effective multi-fiber optical modules, and energy-efficient coherent oDSP-ASIC development. Successful longhaul optical transmission systems consistently achieve substantial reductions in bit/km transmission costs compared to previous generations while enabling continuous evolution of transmission rates and capacity upgrades.