Now at * Department of Photonics Engineering, Technical University of Denmark, 2800 Kongens Lyngby, Denmark, † BT Applied Research, Ipswich IP5 3RE, United Kingdom, # Infinera Pennsylvania, 7360 Winsdor Dr, Allentown, PA 18106,USA Impact of Chromatic Dispersion in Discrete Raman Amplifiers on Coherent Transmission Systems Pratim Hazarika1, Mahmood Abu-Romoh1, Mingming Tan1, Lukasz Krzczanowicz1,*, Tu T. Nguyen1, #, Md Asif Iqbal1, †, Ian Phillips1, Paul Harper1, Ming-Jun Li2 and Wladek Forysiak1 1Aston Institute of Photonic Technologies, Aston University, Birmingham B4 7ET, United Kingdom 2Corning Incorporated, 1 Riverfront Plaza, Corning, NY 14831, USA p.hazarika@aston.ac.uk Abstract: The impact of chromatic dispersion in discrete Raman amplifiers on coherent transmission systems was investigated. Systems with positive dispersion gain fibre show a SNR improvement of ~1.8 dB for 23-GBaud DP-QPSK signals transmitted over >1800km. 1. Introduction With the ever-growing demand of data, a need for wideband systems in conjunction with advance modulation formats is the foreseeable future for next generation optical fibre communication systems. In terms of amplifiers for such systems, the existing technology based on erbium doped fibre amplifiers (EDFA) suffices only for the C- and L-band of the optical spectrum [1]. For broader spectral support, Raman amplifiers are potential candidates for ultra- wide signal amplification, with recent literature showing an extensive amplification of ~ 150 nm bandwidth, and NF < 8.5 dB [2]. An important aspect in design of a discrete Raman amplifier (DRA) is the type of nonlinear fibre used for signal gain. A preferable fibre is the one with high Raman gain coefficient (gr). However, another parameter that can also potentially impair the amplifier performance is fibre chromatic dispersion (CD). The sign and value of dispersion can significantly affect the received signal to noise ratio (SNR) in long-haul transmission. Previous demonstrations have used Raman gain fibre with positive and negative dispersion in various DRA schemes [3, 4]. However, a comprehensive study and its impact on digital coherent systems has not been reported so far. In this paper, we studied the impact of positive and negative dispersion Raman gain fibre in DRA-assisted coherent transmission systems. For our test case we evaluated dispersion compensating fibre (DCF) and a new experimental fibre, referred to as Corning Raman fibre (CRF); the parameters of these fibres are stated in Table 1. Our experimental measurements show an improved SNR for positive dispersion CRF, with the improvement in SNR varying from ~0.6 dB over ~600 km transmission to ~1.8 dB over ~1800 km for 23 GBaud DP-QPSK signals, in reasonable agreement with theoretical predictions. 2. Theoretical modelling Schematic diagrams of the unit cell of transmission are shown in Figure 1. Our theoretical model is based on a piece-wise signal profile approximation for non-linear noise accumulation [5], where the accumulated non-linear power, after N transmission and amplification stages shown in Fig. 1 (a-b) can be represented by (Eq. 1). Fig. 1. a) Single span transmission with backward pumped discrete Raman amplifier b) Amplified links with log signal profile (1) Here, the two terms inside the modulus represent the transmission span single mode fibre (SMF) loss (CD= 16 ps/nm/km, 0.2 dB/km, = 1.3 /W/km) and DRA gain sections, respectively. and are the three non- linear components in the system, is the SMF length and is the DRA fibre length, is the Raman gain- Th1A.14 OFC 2021 © OSA 2021 Authorized licensed use limited to: ASTON UNIVERSITY. Downloaded on January 04,2022 at 10:20:54 UTC from IEEE Xplore. Restrictions apply.