– for Efficient Multi-Band Network Architecture and Components for Petabit/s Elastic Networks

EMBRACE (for Efficient Multi-Band Network Architecture and Components for Petabit/s Elastic Networks) aimed to demonstrate the feasibility of a metropolitan/regional multi-span optical transmission system operating in the S+C+L-band between 1460 nm and 1620 nm with all the required multi-band components and devices, including the Wavelength Division Multiplex (WDM) transceivers, multi-band optical amplifiers and the associated band/wavelength (Multiplexer/Demultiplexer) MUX/DMUX. A study was also carried out on the impact of opening new wavelength windows on the operation of an all-optical multi-band transport network.

To reach these ambitious objectives, the CELTIC-NEXT EMBRACE project gathered a consortium of five French and Canadian partners, i.e. Orange (Leader), Ekinops, MPB Communications, EXAIL and IMT Atlantique. The project funded by BPI France, the Britanny Region, Lannion Trégor Communauté and the National Research of Canada started in October 2021 and finished in last December 2024 by a demo of the operation of the targeted S+C+L-band WDM transmission system. The demo was performed in front of the supporting (CELTIC Office, Pôle de Compétitivité Images & Réseaux, Pôle de Compétitivité ALPHA RLH) and funding organizations.

Multi-band optical transmission faces complex challenges, the most important of which is Stimulated Raman Scattering (SRS), which generates energy transfer from high-frequency (or low-wavelength) bands to low-frequency (or high-wavelength) bands. EMBRACE introduces an innovative optical amplification technology that combines distributed Raman amplification in the line fiber and discrete/lumped S-band Raman amplification in a specially designed nonlinear fiber (CRF) prototype to counteract inter-band energy transfer. SRS control in combination with periodic gain equalization (every N spans) is mandatory to allow the combination of multiple standard single-mode fiber (SSMF) spans and enable metropolitan/regional transmission applications. With this, the consortium was able to transmit 240 channels (considering the 50 GHz ITU grid) between 1480 and 1610-nm over 4 x 100 km standard single-mode fiber (SSMF) spans. Multi-band optical transmission is very useful to maximize the use of existing fiber optic infrastructures and to optimally fill loaned fibers in regions where Internet Service Providers (ISPs) do not own their fiber infrastructure. Multi-band technology is also an opportunity to make optical transport networks more flexible.

During the project, four multi-band distributed Raman amplifiers with multi-pump wavelengths were built and delivered for the final demo. Five lumped / discrete Raman amplifiers using multi-pump wavelengths were also manufactured to amplify the S-band at different points in the set-up. These amplifiers are particularly flexible in shaping the gain and have better performance in terms of noise figure compared to standard amplification technology for the S-band (i.e. Thulium-doped fiber amplification). Multi-band (S+C+L-band) and wavelength (O-, E-, S-, U-band) MUX/DMUX were also designed and built. Finally, a multi-band WDM transponder prototype able to operate between 1460-nm and 1640-nm was realized and used with success during the final demo. The final demo combined these various elements in a S+C+L-band transmission set-up of 4×100-km of SSMF with successful transmission of 240 channels at 33-Gbaud (on 50-GHz ITU grid). 600-Gbps channels were successfully transmitted in the C- and L-band by switching off 33-Gbaud dummy channels (to insert the 96-Gbaud channels) and 100-Gbps channels were propagated in the S-band.

After preliminary tests of some crucial components requested by the various elements of EMBRACE and the specification definition, EKINOPS looked for the possible integration of the requested functions inside the existing platforms or in new platforms. Following the Orange INNOV recommendations, EKINOPS developed the ad-hoc architecture of a multi-band WDM coherent Muxponder operating between 1460 and 1640-nm at 100/200-Gbps, as well as defined the design of a discrete/lumped Raman amplifiers operating on the S-band. Finally, EKINOPS manufactured one prototype of the 100/200-Gbps multi-band WDM coherent Muxponder (following the design previously defined) as well as an optical gain block per prototype for a total of 5 prototypes, with many iterations for an improved amplification performance.

One of the main goals of the distributed Raman amplification (DRA) at the end of each 100-km span was to compensate for the transmission penalty experienced by the short wavelength channels due to the SRS-induced power transfer to the longer wavelength channels. It was therefore important to determine the optimum combination of pump wavelengths and powers for the DRA pump sources. MPB Communications carried out simulations of the transmission of 192 channels running from 1480 to 1610-nm over a 100-km SSMF span. The iterative simulations began with launching a flat channel spectrum and then optimizing the wavelengths and powers of a multi-wavelength DRA pump source to obtain the flattest possible received channel spectrum. The next step involved inverting this received channel power profile and applying it as a pre-emphasis launch profile into the 100-km SSMF span instead of the flat profile. With the same composite signal launch power and the same Raman counter pump powers, the simulation confirmed that this strategy resulted in a flat received channel spectrum. The optimum pump wavelengths and powers were determined through simulations and confirmed experimentally. The required powers were provided by two interconnected modules built by MPB Communications. The 600-mW output of a VERSA2-N2-LDP-600-13XX was fed into a VERSA2-N2-LDP-850-14XX/14XX where it was combined with the required powers at the other wavelengths.

Hereafter are described some examples of the work performed by the various partners of EMBRACE.

Since the pump powers launched into the span for the DRA are well above the Hazard Level 1M limit, the fiber path integrity must be confirmed prior to the turn-on of the pumps and must be continuously monitored during system operation so that, if there is a fiber break or connector disconnect, the pumps will immediately be automatically shut down. To provide this vital fiber integrity monitoring (FIM) function, the 13XX-nm pump unit is equipped with an out-of-band OSC laser diode transmitter at 1624 nm and both units have an OSC receiver. The WDM signal channels are combined with the FIM OSC signal and then launched to co-propagate down the span, as shown in the figure below. Receipt of the OSC signal by the Raman pump units at the far end confirms the integrity of the incoming fiber and is a necessary condition for the high-power pumps to be turned on. Once turned on, any subsequent interruption of the received OSC signal will trigger an ALS.

Multiplexers / Demultiplexers (MUX/DEMUX) are developed for more than 25 years by EXAIL, Integrated Systems Activity. They are historically linked with common telecommunication systems; thus, their conventional design is focused on C- and L-band. The specificity of EXAIL demultiplexers lies in the highly customizable frequency grid on a rather high number of channels. Frequency spacing range can be tuned from 10-GHz up to 400-GHz for up to 48 channels. This agility is the result of a free-spaced fabrication process containing a diffraction grating providing the dispersive function of the MUX/DEMUX. Any frequency spacing of this range can be accessible by tuning the incidence angle on the diffraction grating and choosing its groove spacing. In EMBRACE, EXAIL designed the band/wavelength MUX/DMUX able to operate on each of the targeted amplification bands of EMBRACE (i.e. S-, C- and L-band). But, to address the demand of existing and future customers, EXAIL also performed the design of O-, and S-band components, covering a band going from 1260-nm to 1620-nm. EXAIL manufactured for the project some prototypes in the O- and S-band.

IMT Atlantique worked on the evaluation of network scenarios for the introduction of multi-band WDM transmission systems on the existing fiber cable infrastructure for the different network segments and transport applications, both in terms of technical and techno-economical aspects. The problem is quite prospective as new types of optical fibers offering much more capacity are likely to appear in the mid-long-term future and could be a game changer, especially for short and medium distances. In the short-medium term perspective, the maturity of EDFA-based C+L-band transmission systems is likely to be generalized in long-haul transport network. In the metro/regional area network (MAN) domain for distances up to 400 km, a third band (S or U) could be added later when corresponding transponders and amplifiers will be available. Finally, the MAN is by far the easiest place for the use of four or more amplification band but deployment is strongly dependent on the price of equipment with respect to conventional solutions such as additional fiber deployment. In parallel, EMBRACE funded a PhD work at IMT Atlantique on improving the planning tools for multi-band WDM optical networks by embedding QoT constraints, added to the existing spectral constraints, within the linear programming procedure used for the resolution of the routing and spectrum assignment problem. In particular, the OSNR constraint using the Gaussian Noise (GN) model has been linearized in order to be included in the ILP optimization code. This allows to dramatically reduce the number of possible solutions and selects only routes that are optically feasible by real WDM systems. Hence, this integrated QoT-aware ILP finds realistic optical channel routing solutions in much more reasonable computation times and is proven to be faster and more accurate than the combined method where the found routes are a posteriori assessed by a third party GN model calculation software. However, this latter method based on existing software remains a robust back-up for multi-band optical network planning.

VERSA2-N2-LDP-600-13XX (top) and VERSA2-N2-LDP-850-14XX/14XX (bottom)

Orange INNOV led the project since the preparation stage in 2019 up to the final demo in December 2024. Orange INNOV proposed the optical amplification strategy of EMBRACE (that is at the core of the project) with the combination of DRA in the SSMF spans and discrete/lumped optical amplifier per band at the end of fiber spans, to control as accurately as possible the accumulation of SRS between the various bands (span after span). One of the key ideas of EMBRACE was also to carry out multi-span WDM transmission on the S+C+L-bands to be compliant with the MAN topology. Orange INNOV proposed the overall system architecture of EMBRACE, designed in partnership with EKINOPS the discrete/lumped Raman amplifier operating in the S-band, performed all the numerical simulations to check the viability of the EMBRACE system and confirm the design of both the distributed and discrete/lumped Raman amplifiers used in the EMBRACE demo. Finally, with the support of all the partners, EMBRACE built between September 2024 and December 2024 the final demo that has been shown to all the supporting/funding organizations on last December 17th, 2024. INFINERA (now NOKIA) has kindly made available for EMBRACE two C- and L-band 600-Gbps WDM Muxponders which were used to assess the performance of the corresponding amplification band.

In summary, EMBRACE demonstrated the possible use of metro/regional legacy fiber infrastructure for multi-band WDM transmission by keeping under control the inter-band SRS. EMBRACE has experimentally demonstrated that WDM transmission in S+C+L bands is possible over a multi-span fiber infrastructure (at least four 100 km amplifier spans) while keeping the inter-band SRS under control. EMBRACE gives a reassuring signal to the optical communications industry that investing in components and devices addressing bands other than C- and L-bands is a future-proof position that can potentially generate new revenues in the years to come.

Further information

Project web site: www.celticnext.eu/project-embrace/