Abstract:We report a trench-assisted heterogeneous multicore fiber optimized in terms of higher-order dispersion and crosstalk for radiofrequency true time delay operation. The analysis of the influence of the core refractive index profile on the dispersion slope and effective index reveals a tradeoff between the behavior of the crosstalk against fiber curvatures and the linearity of the propagation group delay. We investigate the optimization of the multicore fiber in the framework of this tradeoff and present a design that features a group delay relative error below 5% for an optical wavelength range up to 100 nm and a crosstalk level below -80 dB for bending radii larger than 103 mm. The performance of the true time delay line is validated in the context of microwave signal filtering and optical beamforming for phased array antennas. This work opens the way towards the development of compact fiber-integrated solutions that enable the implementation of a wide variety of distributed signal processing functionalities that will be key in future fiber-wireless communications networks and systems.

We present a novel procedure for designing a sampled discrete true-time delay line (TTDL) for Microwave Photonics applications based on a heterogeneous MCF. Both simple step-index (SI) and trench-assisted SI profiles are numerically evaluated in terms of physical dimensions and material dopant concentrations in order to individually tailor the group delay and chromatic dispersion of each core. The proposed TTDL features unique properties beyond the current state of the art in terms of record bandwidth, compactness, flexibility, and versatility. , and nonlinear effects in optical fibers [4]. Despite the aforementioned application potential, the widespread adoption of MWP is still limited by the noncompact, heavy, and power-consuming nature of its up-to-date systems, in both signal processing and radioover-fiber distribution scenarios. Integrated MWP has been proposed as a solution for the first scenario, but there is still a paramount need for a compact and efficient fiber-based technology able to support the required parallelization in distribution networks. This second scenario, which includes applications such as wireless access networks and fiber-to-the-home, usually resorts to the brute-force replication of a basic subsystem where the TTDL is built from discrete and bulky components. To solve these limitations and reduce the associated cost impact, we have proposed in [5] the extension of the concept of space-division multiplexing (SDM), currently restricted to high-capacity digital communications, to the area of MWP. More specifically, by exploitation of the inherent parallelism of heterogeneous multicore fibers (MCF), we envisioned the implementation of a sampled discrete TTDL featuring 2D (i.e., 2-dimensional) tunability.Most of the research activity on SDM has employed the so-called homogeneous multicore fiber, where N identical single-mode cores are confined inside a single cladding with an outer diameter ranging from 150 to 200 μm. In order to increase the core packing density, heterogeneous MCF were later proposed. These are composed of nonidentical cores arranged so that the intercore crosstalk becomes sufficiently small as the phase matching condition is prevented [6]. State-ofthe-art heterogeneous MCFs are composed of 2 or more different interleaved triangular lattices of homogeneous cores. Initial designs featured a crosstalk level below −30 dB at a length of 100 km when using 3 different core compositions in a standard 125-μm-diameter cladding [6]. High core densities were achieved when resorting to high values of the refractive index contrast (Δ 1.15%, 1.20%, and 1.25%) in a 19-core fiber characterized by a pitch Λ 23 μm and a core radius a 2.5 μm [6]. Kokubun and Watanabe [7] found that a double-cladding structure can keep nearly identical the propagation characteristics of the cores, while accommodating up to 9 different equivalent refractive indexes in a 19-core fiber. Further progress in crosstalk management has been achieved by designing heterogeneous trench-assisted core configurations, wh...

Abstract:We propose the use of both homogeneous and heterogeneous multicore fibers to implement multi-cavity optoelectronic oscillators. We present design equations and examples that show the potential for unique performance in terms of spectral selectivity, tunability and high-frequency operation. References and links 1.Technology focus on Microwave photonics, Nat. Photonics 5, 723-736 (2011 D. Barrera, I. Gasulla and S. Sales, "Multipoint two-dimensional curvature optical fiber sensor based on a non-twisted homogeneous four-core fiber," J. Lightwave Technol. (in press). 9.X.S. Yao and L. Maleki, "Converting light into spectrally pure microwave oscillation," Opt. Lett. 21, 483-485 (1996). 10. X.S. Yao and L. Maleki, "Optoelectronic microwave oscillator," J. Opt. Soc. Am. B 8, 1725-1735 (1996). 11. X.S. Yao and L. Maleki, "Opto-electronic oscillator for photonic systems," IEEE J. Quantum Electron. 32, 807-814 (1996) compound coupled structures for FDMA demultiplexing," J.

We report, for the first time to our knowledge, distributed radiofrequency signal processing built upon true time delay operation on a step-index few-mode fiber. Two 3sample configurations with different time delay properties are implemented over the same 60meter 4-LP-mode fiber link. The inscription of a long period grating at a specific fiber position converts part of the LP 01 mode into the LP 02 , permitting sample time delay engineering. Delay line performance is experimentally demonstrated when applied to radiofrequency signal filtering, example of fiber-distributed processing functionality exhibiting one order or magnitude gain in terms of compactness.

In this paper we provide the theoretical and experimental evaluation of fiber bending and twisting effects on the group delay performance of a homogeneous 7-core fiber. We have experimentally evaluated the differential group delay between the central and outer cores for different curvature radii and twisting conditions, demonstrating that fiber twisting counteracts the degradation introduced by the curvature itself. These findings are generally applicable to time-sensitive application areas such as radio-over-fiber distribution and microwave photonics signal processing in fiber-wireless access networks, as well as highcapacity long-haul digital communications where digital multiple-input multiple-output processing may be required.

We propose and experimentally demonstrate distributed microwave photonics signal processing over a few-mode fiber link by implementing 4-sample true time delay line operation. The inscription of a set of long period gratings at specific locations along the fewmode fiber allows the excitation of the higher-order modes while adjusting the individual sample group delays and amplitudes that are required for sampled true time delay line behavior. Since solely the injection of the fundamental mode at the few-mode fiber input is required, we render this signal processing system independent of any preceding fiber link that may be required in addition to distribute the signal. We experimentally validate the performance of the implemented true time delay line when applied to radiofrequency signal filtering.

We report, for the first time to our knowledge, the experimental demonstration of multi-cavity optoelectronic oscillators where the cavities are provided by the different cores of a multicore fiber. We implemented two multi-cavity architectures over a 20-m-long 7-core fiber link: unbalanced dual-cavity oscillation (the cavity lengths are a multiple of a reference value) and multi-cavity Vernier oscillation (the cavity lengths are slightly different). Since all the cavities are hosted under a single fiber cladding and are subject to the same environmental and mechanical conditions, optoelectronic oscillators built upon multicore fibers benefit from improved performance stability as compared to independent singlemode fiber cavities.

We propose, for the first time to our knowledge, tunable true time delay line operation for radiofrequency signals on a few-mode fiber link. In particular, the custom design of a 7-LP-mode ring-core few-mode fiber together with a set of 5 broadband long period gratings inscribed at the proper positions along the fiber allows 4-sample true time delay line tunability over a 20-nm optical wavelength range. We study the performance of the designed true time delay line in the context of reconfigurable microwave photonics signal processing by theoretically evaluating microwave signal filtering and optical beamforming networks for phased array antennas.