The design of application-specific integrated circuit (ASIC) is at the core of modern ultra-high-speed transponders employing advanced digital signal processing (DSP) algorithms. This manuscript discusses the motivations for jointly utilizing transmission techniques such as probabilistic shaping and digital sub-carrier multiplexing in digital coherent optical transmissions systems. We firstly report the key-building blocks of high-speed modern DSP-based transponders working up to 800G per wave. Secondly, we show the benefits of these transmission methods in terms of system level performance. Finally, we report the first, to the best of our knowledge, long-haul experimental transmission - e.g., over 1000 km - with a real-time 7 nm DSP ASIC and digital coherent optics (DCO) capable of data rates up to 1.6 Tb/s using two waves (2x800G).

Short range optical data links are experiencing bandwidth limitations making it very challenging to cope with the growing data transmission capacity demands. Parallel optics appears as a valid short-term solution. It is, however, not a viable solution in the long-term because of its complex optical packaging. Therefore, increasing effort is now put into the possibility of exploiting higher order modulation formats with increased spectral efficiency and reduced optical transceiver complexity. As these type of links are based on intensity modulation and direct detection, modulation formats relying on optical coherent detection can not be straight forwardly employed. As an alternative and more viable solution, this paper proposes the use of carrierless amplitude phase (CAP) in a novel multiband approach (MultiCAP) that achieves record spectral efficiency, increases tolerance towards dispersion and bandwidth limitations, and reduces the complexity of the transceiver. We report on numerical simulations and experimental demonstrations with capacity beyond 100 Gb/s transmission using a single externally modulated laser. In addition, an extensive comparison with conventional CAP is also provided. The reported experiment uses MultiCAP to achieve 102.4 Gb/s transmission, corresponding to a data payload of 95.2 Gb/s error free transmission by using a 7% forward error correction code. The signal is successfully recovered after 15 km of standard single mode fiber in a system limited by a 3 dB bandwidth of 14 GHz.

The various topologies, traffic patterns and cost targets of optical networks have prevented the deployment of end-to-end solutions across multi-domains, and the optimization of the network as a whole. The consequent limitations in flexibility, scalability, and adaptability of optical networks will become increasingly important with new applications, such as 5G/6G. Coherent transceivers based on digital subcarrier multiplexing (DSCM) are proposed to address these current constraints. In particular, DSCM allows (i) the design of high-capacity point-to-point (P2P) and -multipoint (P2MP) optical networks; (ii) simplified aggregation with passive optics; and (iii) connections between low- and high-speed transceivers. Furthermore, DSCM-based networks reduce the number of opto-electro-opto stages, halve the number of bookended transceivers, and provide a better match for existing hub-and-spoke (H&S) traffic patterns in fast-growing and dynamic access/metro segments. A DSCM-based transceiver will pave the way for the deployment of next-generation flexible, adaptable, and scalable software-configurable optical networks. Key steps and elements to realize this solution are laid out, and promising applications outlined. The first real-time experimental results of coherent P2MP transceivers are presented.

We report on the development of a two-channel digital coherent optics (DCO) module, based on a monolithic InP photonic integrated circuit (PIC) transceiver and SiGe application-specific integrated circuit (ASIC), paired with a real-time 7 nm digital signal processing (DSP) ASIC. The high-performance coherent optical engine, which utilizes digital Nyquist subcarriers and probabilistic constellation shaping (PCS) techniques, enables long-haul and ultra-long-haul transmission distances over mixed fiber and amplifier types. This work discusses the performance of a DCO unit operating at multiple data rates over three practical real-world-like network distances. 800 Gb/s data transmission over a 1,000 km standard single mode fiber link was achieved using a 96 GBd, PCS-64QAM modulation format. Results of extended reaches of over 2,400 km and 5,000 km are also presented with data rates of 600 Gb/s and 400 Gb/s, respectively.

We present an O-band 400 Gbit/s optical client side Ethernet link with 40 km SSMF reach employing four LAN-WDM lanes, MultiCAP modulation and direct detection.

We demonstrate for the first time an intensity-modulated direct-detection link using four states of polarization. The four data-independent tributaries are each assigned distinct states of polarization to enable the receiver to separate the signals. Polarization rotation due to propagation over optical fiber is tracked and compensated with simple digital signal processing in Stokes space. Transmission below the forward error correction limit is shown for maximum net bitrates of 100 Gb/s (4 × 27 GBd) and 120 Gb/s (4 × 32 GBd) over 2-km standard single-mode fiber at a center wavelength of 1550 nm.