Model-aware Deep Learning Method for Raman Amplification in Few-Mode Fibers

One of the most promising solutions to overcome the capacity limit of current optical fiber links is space-division multiplexing, which allows the transmission on various cores of multi-core fibers or modes of few-mode fibers. In order to realize such systems, suitable optical fiber amplifiers must be designed. In single mode fibers, Raman amplification has shown significant advantages over doped fiber amplifiers due to its low-noise and spectral flexibility. For these reasons, its use in next-generation space-division multiplexing transmission systems is being studied extensively. In this work, we propose a deep learning method that uses automatic differentiation to embed a complete few-mode Raman amplification model in the training process of a neural network to identify the optimal pump wavelengths and power allocation scheme to design both flat and tilted gain profiles. Compared to other machine learning methods, the proposed technique allows to train the neural network on ideal gain profiles, removing the need to compute a dataset that accurately covers the space of Raman gains we are interested in. The ability to directly target a selected region of the space of possible gains allows the method to be easily generalized to any type of Raman gain profiles, while also being more robust when increasing the number of pumps, modes, and the amplification bandwidth. This approach is tested on a 70 km long 4-mode fiber transmitting over the C+L band with various numbers of Raman pumps in the counter-propagating scheme, targeting gain profiles with an average gain in the interval from 5 dB to 15 dB and total tilt in the interval from 1.425 dB to 1.425 dB. We achieve wavelengthand mode-dependent gain fluctuations lower than 0.04 dB and 0.02 dB per dB of gain, respectively

Analysis of modal coupling due to birefringence and ellipticity in strongly guiding ring-core OAM fibers

After briefly recalling the issue of OAM mode purity in strongly-guiding ring-core fibers, this paper provides a methodology to calculate the coupling strength between OAM mode groups due to fiber perturbations. The cases of stress birefringence and core ellipticity are theoretically and numerically investigated. It is found that both perturbations produce the same coupling pattern among mode groups, although with different intensities. The consequence is that birefringence causes the highest modal crosstalk because it strongly couples groups with a lower propagation-constant mismatch. The power coupling to parasitic TE and TM modes is also quantified for both perturbations and is found to be non-negligible. Approximate modal crosstalk formulas valid for weakly-guiding multi-core fibers, but whose parameters are adapted to the present case of strongly guiding OAM fibers, are found to provide a reasonable fit to numerical results. Finally, the effect that modal coupling has on OAM transmission is assessed in terms of SNR penalty

Main Lobe Control of a Beam Tilting Antenna Array Laid on a Deformable Surface

The projection method (PM) is a simple and low-cost pattern recovery technique that already proved its effectiveness in retrieving the radiation properties of different types of arrays that change shape in time. However, when dealing with deformable beam-tilting arrays, this method requires to compute new compensating phase shifts every time that the main lobe is steered, since these shifts depend on both the deformation geometry and the steering angle. This tight requirement causes additional signal processing and complicates the prediction of the array behavior, especially if the deformation geometry is not a priori known: this can be an issue since the PM is mainly used for simple and low-cost systems. In this letter, we propose a simplification of this technique for beam-tilting arrays that requires only basic signal processing. In fact the phase shifts that we use are the sum of two components: one can be directly extracted from strain sensor data that measure surface deformation and the other one can be precomputed according to basic antenna theory. The effectiveness of our approach has been tested on two antennas: a 4 × 4 array (trough full-wave simulations and measurements) and on an 8 × 8 array (trough full-wave simulations) placed on a doubly wedge-shaped surface with a beam tilt up to 40 degrees

Distributed optical fibre sensing for early detection of shallow landslides triggering

A distributed optical fibre sensing system is used to measure landslide-induced strains on an optical fibre buried in a\uc2\ua0large scale physical model of a slope. The fibre sensing cable is deployed at the predefined failure surface and interrogated by means of optical frequency domain reflectometry. The strain evolution is measured with centimetre spatial resolution until the occurrence of the slope failure. Standard legacy sensors measuring soil moisture and pore water pressure are installed at different depths and positions along the slope for comparison and validation. The evolution of the strain field is related to landslide dynamics with unprecedented resolution and insight. In fact, the results of the experiment clearly identify several phases within the evolution of the landslide and show that optical fibres can detect precursory signs of failure well before the collapse, paving the way for the development of more effective early warning systems

Distributed fiber optics strain sensors: from long to short distance

Developed for more than forty years, optical fibers have features that make them particularly attractive for making sensors. One of the strengths of these sensors is that they can measure different physical parameters in a distributed manner over a wide range of lengths (from a few cm up to tens of kilometers) with a spatial resolution ranging from millimeters to meters. In this article, we are particularly interested in distributed fiber sensors, mainly based on light scattering processes, for measuring strain variations. This review concerns both applications requiring long lengths of fiber in a geological context, as well as those using length less than one meter for the medical sector. While distributed fiber optics sensors have already shown their great potential for long-range applications, short-range applications are a niche sector emerging in the last few years

Microwave Photonic Notch Filter Based on Dynamic Brillouin Gratings Generated by PRBS Signals

A method to create a microwave notch filter through dynamic Brillouin gratings is proposed and numerically demonstrated. It exploits the thumbtack correlation peaks of pseudo random bit sequences