Abstract
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° up to 90°, respectively.
Highlights
Space-division multiplexing (SDM) provides a promising approach to overcome the capacity limit of conventional single-mode fiber networks [1]
The application of SDM fibers in microwave photonics lies in their ability to operate as optical sampled true-time delay lines (TTDLs), which are the building blocks of many signal processing functionalities based on discrete-time incoherent architectures, such as signal filtering, optical beam-steering in phased-array antennas and arbitrary waveform generation [20,21]
To verify the applicability of the designed TTDL in different microwave photonics (MWP) applications, we evaluate its performance over a 1-km few-mode fiber (FMF) link, in two representative scenarios: tunable microwave signal filtering and optical beamforming for phased array antennas
Summary
Space-division multiplexing (SDM) provides a promising approach to overcome the capacity limit of conventional single-mode fiber networks [1]. TTDLs provide a set of time-delayed samples of a radiofrequency (RF) signal, where the differential time delay between adjacent samples is constant Such delay lines have been implemented in singlemode fibers using different approaches, including dispersive fibers [22,23], fiber Bragg gratings (FBGs) [24] and the nonlinear stimulated Brillouin scattering effect [25]. 5 samples are provided by the modes LP01, LP11, LP21, LP31 and LP41 This feasible design is a promising platform for implementing reconfigurable RF signal processing functionalities, specially along short-reach (in the order of 1 km) radio-over-fiber links, such as those encountered in 5G radio access networks
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