Research on space-division multiplexing (SDM) came to prominence in early 2010 being primarily proposed as a means of multiplying the information-carrying capacity of optical fibers at the same time as increasing efficiency through resource sharing. Proposed SDM transmission systems range from parallel single-mode fibers with shared amplifier pump lasers to the full spatial integration of transceiver hardware, signal processing, and amplification around a fiber with over 100 spatial channels comprising multiple cores each carrying multiple modes. In this paper, we review progress in SDM research. We first outline the main classifications and features of novel SDM fibers such as multicore fibers (MCFs), multimode fibers, few-mode MCFs, and coupled-core MCFs. We review research achievements of each fiber type before discussing digital-signal processing, amplifier technology, and milestones of transmission and networking demonstrations. Finally, we draw comparisons between fiber types before discussing the current trends and speculate on future developments and applications beyond optical data transmission.

Space-division multiplexing for optical fiber communications

Overview of radiation induced point defects in silica-based optical fibers

We observe efficient supercontinuum generation that extends into the visible spectral range by pumping a low differential mode group delay graded index multimode fiber in the normal dispersion regime. For a 28.5 m long fiber, the generated spectrum spans more than two octaves, starting from below 450 nm and extending beyond 2400 nm. The main nonlinear mechanisms contributing to the visible spectrum generation are attributed to multipath four-wave mixing processes and periodic spatio-temporal breathing dynamics. Moreover, by exploiting the highly multimodal nature of this system, we demonstrate versatile generation of visible spectral peaks in shorter fiber spans by altering the launching conditions. A nonlinearly induced mode cleanup was also observed at the pump wavelength. Our results could pave the way for high brightness, high power, and compact, multi-octave continuum sources.

Visible supercontinuum generation in a graded index multimode fiber pumped at 1064 nm.

Over the past two decades, fiber laser technologies have matured to such an extent that they have captured a large portion of the commercial laser marketplace. Yet, there still is a seemingly unquenchable thirst for ever greater optical power to levels where certain deleterious light-matter interactions that limit continued power scaling become significant. In the past decade or so, the industry has focused mainly on waveguide engineering to overcome many of these hurdles. However, there is an emerging body of work emphasizing the enabling role of the material. In an effort to underpin these developments, this paper reviews the relevance of the material in high power fiber laser technologies. As the durable material-of-choice for the application, the discussion will mainly be limited to silicate host glasses. The discussion presented herein follows an outward path, starting with the trivalent rare earth ions and their spectroscopic properties. The ion then is placed into a host, whose impact on the spectroscopy is reviewed. Finally, adverse interactions between the laser lightwave and the host are discussed, and novel composition glass fiber design and fabrication methodologies are presented. With deference to the symbiosis required between material and waveguide engineering in active fiber development, this review will emphasize the former. Specifically, where appropriate, materials-based paths to the enhancement of laser performance will be underscored.

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Materials for optical fiber lasers: A review

In this topical review, the recent progress on radiation-hardened fiber-based technologies is detailed, focusing on examples for space applications. In the first part of the review, we introduce the operational principles of the various fiber-based technologies considered for use in radiation environments: passive optical fibers for data links, diagnostics, active optical fibers for amplifiers and laser sources as well as the different classes of point and distributed fiber sensors: gyroscopes, Bragg gratings, Rayleigh, Raman or Brillouin-based distributed sensors. Second, we describe the state of the art regarding our knowledge of radiation effects on the performance of these devices, from the microscopic effects observed in the amorphous silica glass used to design fiber cores and cladding, to the macroscopic response of fiber-based devices and systems. Third, we present the recent advances regarding the hardening (improvement of the radiation tolerance) of these technologies acting on the material, device or system levels. From the review, the potential of fiber-based technologies for operation in radiation environments is demonstrated and the future challenges to be overcome in the coming years are presented.

https://iopscience.iop.org/article/10.1088/2040-8986/aad271/pdf

Recent advances in radiation-hardened fiber-based technologies for space applications

Exposing optical silica fibers to radiative environments leads to an increase of fiber attenuation. This gamma-sensitivity of the fibers is strongly wavelength dependent. Many papers already mentioned the strong radiation-induced attenuation (RIA) in the UV and visible ranges, which is explained by radiation-induced defects absorbing in these spectral ranges. However, the origin of RIA at longer wavelengths (lambda > 1000 nm) is less clear. An exception is phosphorous-doped fibers for which P1 defects absorbing around 1700 nm have already been highlighted. For fibers with no phosphorus, the RIA at near-infrared (NIR) wavelengths is usually assumed to be small as it results from the UV-visible absorption tail, which decreases with increasing wavelength. In this paper, we study three prototype silica based optical fibers and show that the RIA does not monotically decrease with increasing wavelength, highlighting RIA-contributions having their origins at NIR-wavelengths. We show that these NIR-absorbing defects are generally the main contributor to RIA at telecommunication wavelengths (1310 nm and 1550 nm), the impact of UV-visible absorption tail being secondary only. The nature of defects involved in these NIR absorptions depends on fiber composition. For fibers with no phosphorous, we propose self trapped hole defects (STH) as origin.

Low-Dose Radiation-Induced Attenuation at InfraRed Wavelengths for P-Doped, Ge-Doped and Pure Silica-Core Optical Fibres

We demonstrate mode-multiplexed $90\times 90$ MIMO transmission over all nine mode groups of a 26.5-km graded-index multimode fiber span, achieving a record spectral efficiency of 202 bit/s/Hz.

High-Spectral-Efficiency Mode-Multiplexed Transmission Over Graded-Index Multimode Fiber

50-μm-diameter graded-index core multimode fibers can be adapted to mode-division-multiplexed transmissions that use multiple-input-multiple-output digital signal processing and selective mode multiplexing. We realize and characterize such fibers and compare them to low-differential-mode-group-delay few-mode fibers.

50 μm Multimode Fibers for Mode Division Multiplexing

We demonstrate combined wavelength- and mode-multiplexed transmission over a 125-km multimode single span composed of 10- and 15-mode fibers with a spectral efficiency of 29 b/s/Hz. A transmission capacity of 115.2 Tb/s is achieved over a distance of 87 km.

10-Mode mode-multiplexed transmission over 125-km single-span multimode fiber

We report the optimization of a few-mode fiber, adapted to multiple-input multiple-output mode division multiplexing, that supports 9 LP modes (15 spatial modes). The fiber is fabricated using standard multimode processes, which allows for tight process tolerances, yielding low differential mode group delays (<155 ps/km) with low attenuations (<0.22 dB/km), large effective areas (≥95 μm2) and low bend losses.

Frank Achten

Papers

Low-Dose Radiation-Induced Attenuation at InfraRed Wavelengths for P-Doped, Ge-Doped and Pure Silica-Core Optical Fibres

High-Spectral-Efficiency Mode-Multiplexed Transmission Over Graded-Index Multimode Fiber

50 μm Multimode Fibers for Mode Division Multiplexing

10-Mode mode-multiplexed transmission over 125-km single-span multimode fiber

Low-Differential-Mode-Group-Delay 9-LP-Mode Fiber