Few-mode fibre-optic microwave photonic links

The fibre-optic microwave photonic link has become one of the basic building blocks for microwave photonics. Increasing the optical power at the receiver is the best way to improve all link performance metrics including gain, noise figure and dynamic range. Even though lasers can produce and photodetectors can receive optical powers on the order of a Watt or more, the power-handling capability of optical fibres is orders-of-magnitude lower. In this paper, we propose and demonstrate the use of few-mode fibres to bridge this power-handling gap, exploiting their unique features of small acousto-optic effective area, large effective areas of optical modes, as well as orthogonality and walk-off among spatial modes. Using specially designed few-mode fibres, we demonstrate order-of-magnitude improvements in link performances for single-channel and multiplexed transmission. This work represents the first step in few-mode microwave photonics. The spatial degrees of freedom can also offer other functionalities such as large, tunable delays based on modal dispersion and wavelength-independent lossless signal combining, which are indispensable in microwave photonics. The use of few-mode, large-core optical fibres can dramatically enhance the performance of microwave photonics communications. Fibre-optic microwave photonic links have become a basic building block for microwave photonics, but their performance is currently limited by the power-handling capability of optical fibres. Now, Guifang Li at the University of Central Florida in the USA and co-workers reduced nonlinear crosstalk in a wavelength-division multiplexed communications link by 30 decibels through employing a 20-kilometre-long few-mode fibre. Using a few-mode fibre allowed them to overcome the optical nonlinearity problems that tend to plague single-mode fibres when large optical signal powers are used. Experiments using single-wavelength channels also showed improved transmission. Future research will explore using few-mode fibres to perform functions such as tunable delay and wavelength-independent lossless signal combination.