Dispersion-tailored few-mode fiber design for tunable microwave photonic signal processing

[EN] We present a novel double-clad step-index few-mode fiber that operates as a five-sampled tunable true-time delay line. The unique feature of this design lies in its particular modal chromatic dispersion behavior, which varies in constant incremental steps among adjacent groups of modes. This property, which to the best of our knowledge has not been reported in any other few-mode fiber to date, is the key to tunable operation of radiofrequency signal processing functionalities implemented in few-mode fibers. The performance of the designed true-time delay line is theoretically evaluated for two different microwave photonics applications, namely tunable signal filtering and optical beamforming networks for phased array antennas. In the 35-nm optical wavelength tuning range of the C-band, the free spectral range of the microwave filter and the beam-pointing angle in the phased array antenna can be continuously tuned from 12.4 up to 57 GHz and 12.6 degrees up to 90 degrees, respectively.European Research Council (Consolidator Grant Project 724663); Ministerio de Ciencia, Innovacion y Universidades (Ramon y Cajal fellowship RYC-2014-16247).Nazemosadat-Arsanjani, SB.; Gasulla Mestre, I. (2020). Dispersion-tailored few-mode fiber design for tunable microwave photonic signal processing. Optics Express. 28(24):37015-37025. https://doi.org/10.1364/OE.412830S3701537025282

Tunable True-Time Delay Operation in A Dispersion-Engineered Few-Mode Fiber

[EN] We present a simple few-mode fiber design as a promising platform to implement tunable sampled true-time delay lines for radiofrequency signal processing. To the best of our knowledge, this is the first few-mode fiber ever reported featuring evenly spaced incremental dispersion values, which is an essential characteristic required for tunable operation of microwave photonics applications. The performance of the designed five-sample true-time delay line is theoretically validated in the context of microwave signal filtering, demonstrating free spectral range continuous tunability from 12.4 up to 57 GHz.This work is supported by the European Research Council (ERC) under Consolidator Grant Project 724663, and Ramon y Cajal fellowship RYC-2014-16247 for I. Gasulla.Nazemosadat-Arsanjani, SB.; Gasulla Mestre, I. (2020). Tunable True-Time Delay Operation in A Dispersion-Engineered Few-Mode Fiber. IEEE. 203-206. https://doi.org/10.23919/MWP48676.2020.9314392S20320

Mode-division multiplexing for microwave signal processing

[EN] We present an overview of different mode-division multiplexing fiber technologies engineered to provide tunable microwave signal processing, including signal filtering and optical beamforming for phased-array antennas. The exploitation of both the space and wavelength dimensions brings advantages in terms of increased compactness, flexibility and versatility.This research was supported by the ERC Consolidator Grant ERC-COG-2016 InnoSpace 724663 and the Spanish MINECO fellowship RYC-2014- 16247 for I. Gasulla.Nazemosadat-Arsanjani, SB.; Gasulla Mestre, I. (2021). Mode-division multiplexing for microwave signal processing. IEEE. 1-2. https://doi.org/10.1109/SUM48717.2021.95058021

Phased Array Antenna Beam-Steering in a Dispersion-Engineered Few-Mode Fiber

[EN] We present, for the first time to our knowledge, experimental demonstration of tunable optical beamforming for phased array antennas using a few-mode fiber. The double-clad step-index few-mode fiber is dispersion engineered such that it operates as a continuously tunable 5-sample true-time delay line, enabling continuous steering of the beam-pointing angle. Using this approach, we measure the radiation pattern from 5 elements of an in-house fabricated 8-element phased array antenna at the radiofrequency of 26 GHz and demonstrate continuous beam-steering over a 59$\circ$ range by sweeping the optical wavelength from 1543 nm up to 1560 nm. Such a few-mode fiber-based beamformer could be beneficial to next-generation fiber-wireless communications and radar systems, as it provides further versatility and capacity along with reduced size, weight and power consumption.This work was supported in part by the ERC Consolidator under Grant 724663, in part by Spanish Ministerio de Ciencia e Innovacion Project under Grant PID2020-118310RB-100, and in part by Generalitat Valenciana under Grants IDIFEDER/2018/031 and PROMETEO/2021/15.Nazemosadat, E.; Herranz Herruzo, JI.; Gasulla Mestre, I. (2023). Phased Array Antenna Beam-Steering in a Dispersion-Engineered Few-Mode Fiber. Journal of Lightwave Technology. 41(21):6651-6656. https://doi.org/10.1109/JLT.2023.329250966516656412

Elliptical-Core Highly Nonlinear Few-Mode Fiber Based OXC for WDM-MDM Networks

In order to realize an optical cross-connect (OXC) converting wavelengths and spatial modes into one-dimensional switching ports, we propose an active mode selective conversion without parasitic wavelength conversion, based on the intermodal four-wave mixing (FWM) arising in a few-mode fiber (FMF). First, we design a dispersion-engineered elliptical-core highly nonlinear FMF (e-HNL-FMF) with a graded refractive index (RI) profile, which can independently guide 3 linearly polarized (LP) spatial modes. Meanwhile, a high doping concentration of germanium in the core leads to relatively high intermodal nonlinear coefficients of 3.23 (W\ub7km)-1 between LP01 and LP11a modes and 3.14 (W\ub7km)-1 between LP01 and LP11b modes. Next, we propose an e-HNL-FMF based OXC scheme for wavelength division multiplexing-mode division multiplexing (WDM-MDM) networks. After optimizing both the e-HNL-FMF length and pump power, we can realize either active mode selective conversion over the designated wavelength-band or three-wavelength to three-mode superchannel conversion for 100 Gbaud 16-quadratic-amplitude modulation (16-QAM) signals over the C-band. Due to excellent characteristics of the e-HNL-FMF, both cost and configuration complexity of the OXC can be reduced, showing great potentials for all-optical signal processing in the future WDM-MDM networks

Design, fabrication, and characterization of a highly nonlinear few-mode fiber

We present the design, fabrication, and characterization of a highly nonlinear few-mode fiber (HNL-FMF) with an intermodal nonlinear coefficient of 2.8 (W \ub7 km)−1, which to the best of our knowledge is the highest reported to date. The graded-index circular core fiber supports two mode groups (MGs) with six eigenmodes and is highly doped with germanium. This breaks the mode degeneracy within the higher-order MG, leading to different group velocities among corresponding eigenmodes. Thus, the HNL-FMF can support multiple intermodal four-wave mixing processes between the two MGs at the same time. In a proof-of-concept experiment, we demonstrate simultaneous intermodal wavelength conversions among three eigenmodes of the HNL-FMF over the C band