We demonstrate for the first time a radiationresistant Erbium-Doped Fiber exhibiting performances that can fill the requirements of Erbium-Doped Fiber Amplifiers for space applications. This is based on an Aluminum co-doping atom reduction enabled by Nanoparticules Doping-Process. For this purpose, we developed several fibers containing very different erbium and aluminum concentrations, and tested them in the same optical amplifier configuration. This work allows to bring to the fore a highly radiation resistant Erbium-doped pure silica optical fiber exhibiting a low quenching level. This result is an important step as the EDFA is increasingly recognized as an enabling technology for the extensive use of photonic sub-systems in future satellites.
New generation systems are expected to include more intelligent amplifiers able to adapt to many conditions
including different gains, channel load, temperature, aging and transient events.<sup>1</sup> To face the challenge and
meet these new requirements, having an accurate control on the Er environment within the fiber core matrix
has never appeared to be so necessary and predominant as it is the case now. Unlike conventional solution
doping techniques where Erbium ions are randomly incorporated in the fiber core, our process makes use of
a soft chemical synthesis to initially produce Erbium-doped nanoparticles (NPs). Erbium ions are therefore
incorporated in the fiber core together with their local environment. So far, our investigations2 first showed that,
from the material point of view, quenching levels are intimately linked to the design of the NPs through their
chemical composition. Then, from the system perspective, we evidenced the higher power conversion efficiencies
exhibited by NP fibers when compared to their conventional counterparts in high power amplifier configurations.
In this paper, we address our most recent work focusing on the NP optimisation towards quenching-free Erbiumdoped
fibers with a particular focus on core-shell alumino-silicate NPs. Completing our first amplifier results
obtained in high power configurations, we also explore new NP fiber profiles that extend the range of their
applications. Gain and noise characteristics of typical WDM operating points serve as key indicators on the
benefits our NP doping process could provide.
In the last decade, there has been increased interest in photonic technology for new satellite applications. One
critical issue is the high sensitivity to radiative environments of the Erbium Doped Fiber (EDF). It leads to
a radiation-induced absorption (RIA) that is not due to erbium content but mainly to the aluminium that
ensures the erbium inclusion in glass. As the radiation induced losses grow as an exponential function of fiber
length, the principal way so far to reduce EDFA degradation has consisted in increasing erbium concentration
using conventional doping techniques. However, this is limited by the quenching effect, which impacts the fiber
length needed to reach high gain, but also by the Aluminium-induced RIA. It has been recently proposed an
original nanoparticle (NP) doping approach, which allows codopant content decrease with reduced quenching
impact, while keeping EDF amplifying performances. A radiation-resistant amplifier can thus be designed as a
"quenching-free", heavily-erbium-doped amplifier with low RIA.
We demonstrate for the first time an aluminium-free EDF, exhibiting low quenching and low RIA. Despite the
lack of aluminium, using silica NPs allows an erbium concentration close to the one of standard EDFs (200 ppm).
This fiber is compared to a 1400 ppm Erbium-doped optical fiber with a strong aluminium concentration.
Whereas the two fibers exhibit similar initial optical gain (15 dB under saturation conditions), the NP doped
Al-free EDF shows only 2 dB gain reduction after a 600 Gy gamma deposit, while the Al/Er EDF incurs more
than 10 dB gain degradation.
In 2009, we introduced a new doping concept involving Al<sub>2</sub>O<sub>3</sub>/rare-earth nanoparticles (NP) in a MCVD-compatible
process finding potential applications in Erbium-, Ytterbium- or Erbium-Ytterbium-doped fiber
amplifiers and lasers.<sup>1</sup> This approach, motivated by the need for increased efficiencies and improved attributes,
is characterized by the ability to control the rare-earth ion environment independently from the core composition.
The NP matrix can therefore be viewed as an optimized sub-micronic amplifying medium for the embedded rareearth
ion. The first experimental evidence to support this idea is reported in a comparative study with a standard
process<sup>2</sup> where homogeneous up-conversion (HUC) and pair-induced quenching (PIQ) levels are extracted from
Er<sup>3+</sup> unsaturable absorption measurements. NP-based fibers are found to mitigate the effects of the Er<sup>3+</sup> concentration increase seen in standard heavily-doped fibers. This conclusion is particularly clear when focusing
on the HUC coefficient evolution since, for a given type of NP, its level is independent from the Er<sup>3+</sup> concentration
in the doped zone. In this paper, we address our most recent work completing these preliminary results. First,
we investigate the quenching signature of a new NP design and its behavior when incorporated in different core
matrices. The interplay is further analysed by relating this set of measurements to practical EDFA performances.
Gain and noise characteristics of typical WDM amplifiers operating points serve as key benchmarking indicators
to identify the benefits of NP Erbium-doped fibers in the wide variety of EDFAs implementations.
Ever demanding network implementations brought new requirements to be addressed to offer cost effective and
power efficient solutions with smaller footprints. This general trend together with the constant need to improve
L-band optical amplification efficiency account for the renewed interest on highly doped Erbium fibers. Erbium
doped fiber amplifiers (EDFAs) performance degradation with Er<sup>3+</sup> concentration increase has extensively been
studied1 and is attributed to additional losses due to energy transfers between neighbouring ions. Experimental
observations have been interpreted by the homogeneous up-conversion (HUC) and pair-induced quenching (PIQ)
models, which account for pump power penalty and unsaturable absorption respectively. For a given Er3+ concentration,
studies have also showed that both fiber manufacturing process and core matrix composition have
a strong impact on quenching parameters. In 2009, we introduced a new doping concept involving Al<sub>2</sub>O<sub>3</sub>Er
nanoparticles (NP) in a MCVD-compatible process showing improved performances in terms of erbium homogeneity
along the fiber length for standard doping levels.<sup>2</sup> In this paper, we address our most recent work on
concentration quenching encountered in both standard and NP Erbium doped fibers.
For many years, fiber manufacturers have devoted research efforts to develop fibers with improved radiation
resistance, keeping the same advantages and basic properties as standard fibers. Today, both single-mode (SMF) and
multimode (MMF) RadHard (for Radiation-Hardened) fibers are available; some of them are MIL-49291 certified and
are already used, for example in military applications and at the Large Hadron Collider (LHC) in CERN or in certain
nuclear power plants. These RadHard fibers can be easily connected to standard optical networks for classical data
transfer or they can also be used for command control. Using some specific properties (Raman or Brillouin scattering,
Bragg gratings...), such fibers can also be used as distributed sensing (temperature or strain sensors, etc) in radiation
environments. At least, optical fibers can also be used for signal amplification, either in telecom networks, or in fiber
lasers. This last category of fibers is called active fibers, in opposition to passive fibers used for simple signal
transmission. Draka has also recently worked to improve the radiation-resistance of these active fibers, so that Draka can
now offer RadHard fibers for full optical systems.
Manufacture fiber amplifiers and lasers in a versatile and cost effective way while controlling rare-earths chemical environment becomes a real technology differentiator.
A MCVD compatible Nanoparticle Doping Process has been developed to master with a higher accuracy rare earth amplifier and ytterbium laser fibers. Improved doped erbium fibers with C-band gain shape were obtained with much less aluminum content and unprecedented low background attenuation losses. This process should better show its merits in high power regime paving the way to fiber amplifiers and lasers for low cost new performances.