Photonics

An analytical heuristic model has been developed to describe the time-domain reflectance Raman signal in a two-layer diffusive medium. The model is based on the solution of two coupled diffusion equations at the excitation and emission wavelengths, respectively. The accuracy of the model is verified by comparison with the results of Monte Carlo simulations. This solver correctly accounts for the Raman emitters within the layers and the full wavelength dependence of the optical properties of the layers. The computational efficiency of the solver is compatible with the requirements of rapid reconstructions of Raman spectra from time-domain Raman measurements acquired for a large number of different wavelengths.

We report the experimental observation of dissipative solitons (DS) in an erbium-doped fiber laser mode-locked with ethylene glycol. Stable individual dissipative solitons with the fundamental repetition frequency of 35.17 MHz are obtained. The spectral profiles exhibit dynamic variation, adopting either a parabolic-like or an M-like shape at varying pump levels. Moreover, through increasing the pump power and/or appropriately regulating the polarization controller, multiple dissipative solitons, including soliton pairs and soliton triplet states have also been achieved. The generation mechanism is attributed to the phenomenon of peak clamping effect. This is the first demonstration of a dissipative soliton laser utilizing ethylene glycol as a saturable absorber. It is anticipated that this demonstration will spur further attention on organic liquid SAs and potentially extend significant influence for the application of ultrafast lasers.

The Faster-than-Nyquist (FTN) technology is widely used in optical wireless communication (OWC) systems to improve data rates and spectrum efficiency. However, it introduces inter-symbol interference (ISI), which can affect communication reliability. To address this issue, we propose a pre-equalization algorithm based on a deep neural network (DNN). The performance analysis primarily focuses on the bit-error-rate (BER) under a Gamma-Gamma atmospheric turbulence channel with varying acceleration factors. Simulation results show that our scheme effectively reduces the degradation in BER caused by ISI. Additionally, we observe an inverse relationship between the BER performance and the atmospheric refractive index constants as well as transmission distance, while a direct proportionality exists with respect to the filter roll-off factor and laser wavelength. Furthermore, comparing with conventional minimum mean square error (MMSE) and zero-forcing (ZF) algorithms highlights the superior performance of our proposal.