Closed-form Expression for the Power Profile in Wideband Systems with Inter-channel Stimulated Raman Scattering

Wideband systems experience significant inter-channel stimulated Raman scattering (ISRS) and channel-dependent losses. Due to the non-uniform attenuation profile, the combined effects of ISRS and fiber loss can only be accurately estimated using numerical methods. In this work, we present an approximate closed-form expression for the channels' power profile accounting for these combined effects. We validate the proposed expression against numerical solutions in the case of CLU transmission, showing high accuracy for both single-span and multi-span fiber-optic links. Additionally, we derive an inverse expression, formulated as a function of the output power, which can be utilized to target a desired optical signal-to-noise ratio (OSNR) profile through pre-emphasis of the launched channel powers.

SDM Optical Systems with MMSE Equalizers: Information Rates and Performance Monitoring

The information rate of coupled space-division multiplexing (SDM) transmission systems is impaired by the stochastic effects of mode-dependent gain (MDG) and mode-dependent loss (MDL), turning it into a random variable and reducing its average value. In systems operating with minimum mean squared error (MMSE) equalizers and no channel-state information (CSI), co-channel interference further reduces the instantaneous and average information rates. Analytical solutions for the average information rate in MDG- and MDL-impaired systems under strong coupling have been presented in early studies assuming ideal maximum-likelihood (ML) equalization. However, to the best of our knowledge, a solution encompassing co-channel interference under MMSE equalization has not been presented yet. In this work, we derive statistical models for the MMSE equalizer coefficients and develop analytical solutions for the post-filtering information rate. We also use these statistical models and analytical solutions to carry out MDG and signal-to-noise ratio (SNR) monitoring in coupled SDM systems. The derived analytical solutions and monitoring techniques are validated by Monte-Carlo simulations, exhibiting a suitable accuracy within practical operational values.