Our work on Space-Division Multiplexed Photonics is mainly supported by the 2016 Consolidator Grant InnoSpace awarded by the European Research Council, as well as two Spanish Projects. This research line strongly collaborates with the “Advanced Microwave Photonics” and “Optical Fibre Sensors & More Fun” research lines of the Photonics Research Labs.
Next-generation global telecommunication paradigms will require entirely new technologies to address the current limitations to massive capacity and connectivity. A full integration between optical fibre and wireless networks will be the key for the upcoming multigigabit-per-second 5G wireless communications and the era of Internet of Things. There is one revolutionary approach that has however been left untapped in finding innovative ways to increase the end user capacity and provide adaptive radiofrequency-photonic interfaces: exploiting space – the last available degree of freedom for optical multiplexing.
Space-Division multiplexing (SDM) has been recently touted as a solution for the capacity bottleneck in digital communications by establishing independent light paths in a single fiber. The idea underlying this research line is to develop a novel area of application for SDM in the context of fiber-wireless communications networks, not only for parallel distribution and multiple antenna connectivity, but also for radiofrequency signal processing.
1. Parallel distribution and multiple antenna connectivity
Centralized or Cloud Radio Access Networks (C-RAN) is an emergent architecture network that was proposed for 5G converged fiber-wireless access networks. One of the most relevant challenges that up-to-date C-RAN approaches for 5G communications have to face relates to the optical fiber availability, deployment and connectivity. In principle, at least 2M fibers are required to feed an M-sector cell only for single-antenna operation. If we target Multiple Input Multiple output (MIMO) antenna configurations, this figure will increase considerably. Therefore, we must consider some kind of optical multiplexing technique, beyond mere Wavelength Division Multiplexing (WDM), to cope with the constraint of not increasing the optical fiber count. The way to cope with this challenge is find in the extension of the concept of Space-Division Multiplexing in homogeneous multicore optical fibers to fiber-wireless communications. The SDM-based C-RAN architecture can be flexible enough to support both Digital and Analogue radio-over-fiber approaches, capacity upgrade by carrier aggregation and MIMO, CoMP operation, the distribution of control signals or a remote optical CW source, as well as Passive Optical Network integration.
2. Fiber distributed signal processing
SDM transmission media can act as distributed signal processing elements by exploiting their inherent parallelism to implement a broadband tuneable true time delay line for radiofrequency signals, . The true time delay is actually the basis of multiple Microwave Photonics functionalities, such as controlled signal distribution, microwave signal filtering, antenna beam-steering, optoelectronic oscillation and arbitrary waveform generation. This approach relies on the concept of “fiber-distributed signal processing”, where the SDM fiber provides not only radio access distribution but also broadband microwave photonics signal processing. In particular, we are currently investigating three different SDM technologies:
- Homogeneous multicore fibers with inscribed dispersive elements. Together with researchers from the “Optical Fibre Sensors & More Fun” research line, led by Prof. Salvador Sales, we have experimentally demonstrated – for the first time to our knowledge – the use of Spatial Division Multiplexing for Microwave Photonics signal processing. This entailed the development of the first fabrication method to inscribe high-quality gratings characterized by arbitrary frequency spectra and located in arbitrary longitudinal positions along the individual cores of a multicore fiber. This work was published in Scientific Reports, .
- Heterogeneous multicore fibers. We work on the design of trench-assisted heterogeneous multicore fibers that are composed of cores featuring individual spectral group delays and chromatic dispersion profiles. Besides fulfilling the requirements for true time delay line operation, the MCFs are optimized in terms of higher-order dispersion, crosstalk and bend sensitivity, [3,4].
- Few-mode fibres. The objective here is to tailor the dispersion behaviour of individual modes to achieve a constant differential group delay, while providing control over the losses and the intermodal crosstalk.
The future display of fiber-wireless 5G and mm-wave communications will benefit from the compactness and spatial parallelism of the developed SDM technologies leaving behind bulky, heavy, power-consuming and expensive replication architectures.