The exponential growth in global data traffic, driven by the rapid development of the digital economy and diversified emerging services, presents unprecedented challenges and opportunities for optical transport networks (OTNs) as fundamental communication infrastructure. Traditional fixed-rate, fixed-grid optical transport technologies demonstrate limitations in spectrum utilization efficiency and service adaptation flexibility, rendering them inadequate for future differentiated service provisioning requirements. Flexible-rate grid-less optical transport technology has emerged as a solution, enabling next-generation high-efficiency, intelligent, and elastic optical transport networks through advancements in key technologies, including multi-format signal generation, flexible-rate signal reception, and multi-granularity elastic wavelength switching.

This paper systematically examines the core innovations of flexible-rate grid-less optical transport technology through an in-depth exploration of three dimensions: the transmitter, receiver, and switching node.

At the transmitter side, traditional optical networks utilize fixed modulation formats and coding rates, limiting their adaptability to flexible-rate requirements of diverse services. Flexible coded modulation technology addresses this limitation by enabling on-demand transmission of multi-format optical signals through dynamic adjustment of multi-dimensional parameters, including modulation order (e.g., QPSK, 16QAM, 64QAM), symbol rate, and forward error correction (FEC) coding rate. The core technologies comprise flexible coding and flexible modulation. Flexible coding dynamically adjusts coding parameters and strategies according to transmission conditions and service requirements, ensuring efficient and reliable optical signal transmission. Flexible modulation incorporates probabilistic constellation shaping (PCS) and time-domain hybrid modulation (TDHM). PCS optimizes power efficiency, enabling flexible rate adaptation without bandwidth increase, while TDHM achieves high-efficiency transmission and dynamic adaptation by combining different modulation signals in the time domain. These technologies enable optimal selection of modulation formats and coding schemes under varying transmission distances and channel conditions, establishing the foundation for dynamic and flexible optical signal generation.

At the receiver side, traditional optical transport equipment faces limitations due to single-rate reception, restricting effective processing of diverse modulation formats. Modulation format identification (MFI) and adaptive equalization technologies serve as essential components for enhancing receiver processing capability and adaptability. MFI technology precisely analyzes received optical signal characteristics to automatically identify modulation formats (e.g., QPSK, 16QAM, or 64QAM), providing vital information for subsequent signal processing. Adaptive equalization employs advanced digital signal processing (DSP) algorithms to dynamically compensate for signal impairments caused by chromatic dispersion, polarization mode dispersion, or nonlinear effects during transmission. Real-time adjustment of equalization parameters enables the receiver to optimize demodulation schemes based on signal characteristics and channel conditions, ensuring efficient signal recovery across diverse transmission scenarios. Additionally, deep learning-based MFI methods enhance identification accuracy and real-time performance, providing intelligent solutions for flexible-rate optical signal reception.

In terms of switching nodes, traditional fixed-grid wavelength-division multiplexing (WDM) systems exhibit limitations in bandwidth allocation and increasing spectrum fragmentation. Grid-less flexible optical switching technology overcomes these constraints through dynamic scheduling and reconfiguration of wavelength-level or sub-wavelength-level optical paths, eliminating conventional 50 GHz fixed channel spacing restrictions and enabling dynamic adjustment of channel bandwidth and center frequency. This technology utilizes advanced devices such as optical cross-connects (OXCs) and wavelength-selective switches (WSSs), combined with intelligent routing algorithms and resource virtualization techniques, substantially improving spectrum utilization and service adaptability. A software-defined networking (SDN)-based centralized control mechanism enhances switching node intelligence and automation, enabling real-time network traffic variation perception and dynamic spectrum resource allocation. Channel bandwidth can be expanded for high-throughput scenarios requiring high-order modulation signals, while narrower channels improve spectral efficiency for low-rate services. This on-demand flexibility minimizes resource wastage and strengthens network support for diversified services.

The optical transmission network increasingly aggregates and carries massive differentiated data generated from various application scenarios, driven by rapid emerging business development. Flexible rate grid-less optical transmission technology represents a crucial enabler for all-optical infrastructure. The convergence with cutting-edge technologies such as quantum communications and photonic neural networks may transform optical networks from “elastic adaptation” to “cognitive autonomy”. However, challenges persist in addressing energy efficiency degradation from dynamic modulation and coding strategy switching, real-time optimization of multi-rate adaptive equalization algorithms, and coordinated operation of high-dimensional optical switching nodes. Academia and industry must strengthen collaboration in device fabrication, algorithm architecture, and operational frameworks to advance optical communication networks toward intelligent all-optical networking.