Dual Time and Frequency Domain Optical Layer Digital Twin M. Devigili(1), M. Ruiz(1), N. Costa(2), A. Napoli(3), J. Pedro(2), and L. Velasco(1) (1) Universitat Politècnica de Catalunya (UPC), Barcelona, Spain (mariano.devigili@upc.edu) (2) Infinera Unipessoal Lda., Carnaxide, Portugal; (3) Infinera, Munich, Germany Abstract We demonstrate a digital twin for failure detection in optical networks. Artificial neural networks-based models for optical constellation analysis enable predicting the transmitted signal in the time domain whereas analytical models are usually used to estimate their spectral evolution.©2022 The Author(s) 1. Introduction Digital Twins (DT) can be defined as a combination of data, models, and algorithms that act as a bridge between real and virtual worlds. DT have been recently proposed to model optical communications [1]. Examples of applications of an optical layer DT are intelligent fault management and misconfiguration detection, where the real optical signal can be compared to the one generated by the DT to identify discrepancies and estimate their root cause. Note that the same objective can be achieved by applying different methods, like analytical models, simulation, or deep learning (see, e.g., [2]-[4]). The DT, however, provide a more complete characterization of the optical signal, which can be helpful to perform more complete analyses. For instance, employing methods that just estimate the channel’s OSNR or only work in the time domain may not be effective to detect a failure in an optical filter. The opposite is also true, by analyzing the received signal bandwidth, it may be difficult to detect excessive noise coming from an optical amplifier. In our previous work [4], we focused on the time domain and proposed the use of Artificial neural networks (ANN) to model the impairments arising in optical erbium doped fiber amplifiers (EDFAs), optical fibers and filters. Then, the total optical performance degradation in end-to-end lightpaths (E2E) could be modelled by concatenating such ANNs, from the transmitter to the receiver. In this paper, we extend the work presented in [4] by including also frequency domain models. The new – frequency – models proposed in this work can be concatenated to the time-domain ones to increase the accuracy of the E2E lightpath optical performance estimation. Illustrative use cases are analyzed to show the application of the proposed time and frequency domain digital twin (TF-DT) of the optical layer. 2. Optical Layer Digital Twin and Use Cases We assume reconfigurable optical add drop multiplexers (ROADM) following a route & select architecture equipped with optical spectrum analyzers (OSA). The optical signal to noise ratio (OSNR) and bandwidth of the desired signal can be derived from the measured optical spectrum. This monitoring – carried out in intermediate ROADMs – complements the more detailed ones performed at the coherent receiver. An illustrative network scenario with n ROADMs is represented in Fig. 1. Each ROADM consists of two wavelength selective switches (WSSs) and an EDFA (except the last one). The ROADMs are connected by optical links which may consist of several fiber spans and inline EDFAs. The proposed TF-DT is also illustrated, consisting of the concatenated ANNs for the time domain analysis of the transmitted data signal whereas analytical models are used for the frequency domain. The ANNs model the propagation of constellation points (CP)s in the ROADMs and into optical links, whereas analytical models are used to model the bandwidth inside the ROADMs. Measurements collected from the network can then be compared to the expected signals generated using the TF-DT. Let us illustrate the application of the TF-DT through the analysis of the use cases depicted in Fig. 2. In this case, the configuration a) corresponds to normal network operation, i.e., without any failure or misconfiguration; b) illustrates a misconfiguration where a channel is wrongly