Optical fiber sensors offer unprecedented features, the most unique of which is the ability of monitoring varia- tions of the observed physical field with spatial continuity along the fiber. These distributed optical fiber sensors are based on the scattering processes that originate from the interaction between light and matter. Among the three different scatter- ing processes that may take place in a fiber—namely Rayleigh, Raman and Brillouin scattering, this paper focuses on Rayleigh-based distributed optical fiber sensors. For a given optical frequency, Rayleigh-based sensors exploit the three main properties of light: intensity, phase and polarization. All these sensing mechanisms are reviewed, along with basic principles, main acquisition techniques and fields of application. Emphasis, however, will be put on polarization-based distributed optical fiber sensors. While they currently represent a niche, they offer promising unique features worth being considered in greater detail.

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Distributed Optical Fiber Sensing Based on Rayleigh Scattering

The ability to process optical signals without passing into the electrical domain has always attracted the attention of the research community. Processing photons by photons unfolds new scenarios, in principle allowing for unseen signal processing and computing capabilities. Optical computation can be seen as a large scientific field in which researchers operate, trying to find solutions to their specific needs by different approaches; although the challenges can be substantially different, they are typically addressed using knowledge and technological platforms that are shared across the whole field. This significant know-how can also benefit other scientific communities, providing lateral solutions to their problems, as well as leading to novel applications. The aim of this Roadmap is to provide a broad view of the state-of-the-art in this lively scientific research field and to discuss the advances required to tackle emerging challenges, thanks to contributions authored by experts affiliated to both academic institutions and high-tech industries. The Roadmap is organized so as to put side by side contributions on different aspects of optical processing, aiming to enhance the cross-contamination of ideas between scientists working in three different fields of photonics: optical gates and logical units, high bit-rate signal processing and optical quantum computing. The ultimate intent of this paper is to provide guidance for young scientists as well as providing research-funding institutions and stake holders with a comprehensive overview of perspectives and opportunities offered by this research field.

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Roadmap on all-optical processing

We achieved the lowest-ever transmission losses of 0.1419 dB/km at 1560 nm wavelength and 0.1424 dB/km at 1550 nm in a Ge-free silica-core optical fiber. It was an improvement by 4 mdB/km from the previous record realized in 2015. The Ge-free silica core included fluorine co-doping, which helps to reduce disorder in the microscopic glass network structure that causes Rayleigh scattering loss without a significant increase in waveguide imperfection loss. A two-layered polymer coating with an inner layer having lower elastic modulus than before also contributed to the ultralow loss without influence of microbending loss increase even with an enlarged effective area of 147 μm2. The present fiber with ultralow loss and a large effective area benefits an ultralong haul optical transmission system including transoceanic submarine cable systems. We estimate system performance based on the fiber figure of merit theory that the present fiber enables a 0.10 bit/s/Hz increase in spectral efficiency or 7% reduction in the number of repeaters, compared to the previous record-loss fiber.

The First 0.14-dB/km Loss Optical Fiber and its Impact on Submarine Transmission

Comparison of satellite reflectance algorithms for estimating chlorophyll-a in a temperate reservoir using coincident hyperspectral aircraft imagery and dense coincident surface observations

We demonstrate analytically a novel scheme to obtain polarization-independent operation in low-cost coherent OOK receivers, suitable for access networks. We move from a well-known phase-diversity receiver, exploiting a 3 × 3 coupler, three photodiodes, and basic analogue processing. If that receiver is modified to inject the local oscillator (or the signal) into two inputs with proper polarization states, we show, by both mathematical derivation and numerical simulations, that the resulting electrical signal can be polarization-independent; this is attained only if the frequency-detuning between the signal carrier and the local oscillator is high enough, so that intrinsic second order distortions can be spectrally isolated and suppressed by the low-pass filtering of the receiver.

Polarization-Independent Receivers for Low-Cost Coherent OOK Systems

We demonstrate a single-ended colorless coherent receiver using symmetric 3x3 couplers for optical hybrids. We show that the receiver can achieve colorless reception of fifty-five 112-Gb/s polarization-division-multiplexed quadrature-phase-shift-keyed (PDM-QPSK) channels with less than 1-dB penalty in the back-to-back operation. The receiver also works well in a long-haul wavelength-division-multiplexed (WDM) transmission system over 2560-km TrueWave® REACH fiber.

Colorless coherent receiver using 3x3 coupler hybrids and single-ended detection

We demonstrate, both experimentally and numerically, all-optical mode and wavelength conversion both within the C-band and between the C- and L-bands. This is achieved by exploiting phase-matched inter-modal four-wave-mixing processes among the spatial modes of a three-mode fiber. By increasing the number of spatial modes supported by the fiber and tailoring their dispersion profile, it is envisaged that broadband operation over widely separated wavelength bands can be achieved in a single multi-mode fiber using this method.

All-optical mode and wavelength converter based on parametric processes in a three-mode fiber

We experimentally demonstrate space division multiplexed data transmission using the LP01, LP11a and LP11b modes over a 1 km length of elliptical-core few mode fiber at 1550 nm using 10 Gb/s ON–OFF keying data per spatial channel. Space division multiplexed transmission without the use of any multiple input multiple output digital signal processing showed no power penalty relative to the single-mode or the back to back cases.

Elliptical Core Few Mode Fibers for Multiple-Input Multiple Output-Free Space Division Multiplexing Transmission

We report the longest reach fiber based distributed acoustic sensor (DAS) without inline amplification. A measurement range exceeding 112 km, and 125 km if the far end is optimized, is realized with a suitable signal to noise ratio (SNR) for real-world applications. This is achieved by a DAS system optimized for long distance measurements and a combination of different fiber types optimized either for low loss or for high backscatter coefficient.

Pushing the Reach of Fiber Distributed Acoustic Sensing to 125 km Without the Use of Amplification

We demonstrate unrepeatered transmission of 8x128Gb/s PDM-QPSK signals over a 515k-m fiber link. This ultra-long distance of 800 Gb/s unrepeatered transmission in a single fiber configuration is achieved by employing enabling techniques such as large-effective-area ultra-low-attenuation fibers, co-propagating and counter-propagating 2nd-order-pumped distributed Raman amplification, and remote optically pumped amplifier (ROPA). The ROPA itself is also counter-propagating 2nd-order Raman pumped. The designs and characteristics of the ROPA and 2nd-order pumped distributed Raman amplification are described, and optimization of the transmission performance of this ultra-long reach 800Gb/s unrepeatered transmission fiber link is discussed in this paper.

Tommy Geisler

Papers

Colorless coherent receiver using 3x3 coupler hybrids and single-ended detection

All-optical mode and wavelength converter based on parametric processes in a three-mode fiber

Elliptical Core Few Mode Fibers for Multiple-Input Multiple Output-Free Space Division Multiplexing Transmission

Pushing the Reach of Fiber Distributed Acoustic Sensing to 125 km Without the Use of Amplification

800Gb/s (8x128Gb/s) unrepeatered transmission over 515-km large-area ultra-low-loss fiber using 2nd-order Raman pumping.