Erbium-ytterbium co-doped phospho-silicate double-clad fibers are used in many applications were powerful 1.5 μm sources are needed, such as telecommunication systems, LIDAR, medical lasers and much more. These fibers are typically pumped with diodes emitting at 915, 940 or 976nm to excite Ytterbium ions, which in turn transfer their energy to erbium ions through a phonon-assisted mechanism, thus leading to 1.5 μm emission. This energy transfer requires a large phosphorous content in the core of the fiber and therefore these fibers exhibit typically high numerical apertures. Properly optimized, the ytterbium to erbium ratio will minimize parasitic emission at 1 μm which provokes system failures through non-controlled spurious laser effects. We have recently observed, on such optimized fibers exhibiting 12 μm core diameter and 0.20 numerical aperture, that long term operation in CW mode in both amplifier and laser configuration, leads to a slow and irreversible decrease of the output power. This phenomenon starts at moderate signal power of just 7W and increases rapidly with the output power. This phenomenon is also observed in polarization maintaining version of the very same fibers. We have studied this phenomenon which resembles the well-known photodarkening effect in Ytterbium doped fibers. Our experiments show that all the commercially available fibers tested exhibit the same behavior. We will present a tentative explanation of the phenomenon and some solutions we implemented to drastically stabilize the output powers up to 20W enabling the use of such fibers in many industrials applications.
Large-Mode-Area (LMA) fibers are key elements in modern high power fiber lasers operating at 1 μm. LMA fibers are highly ytterbium-doped and require a fine control of the core refractive index (RI) close to the silica level. These low RI have been achieved with multi-component materials elaborated using a full-vapor phase Surface Plasma Chemical Vapor Deposition (SPCVD) process, enabling the fabrication of large core diameter preforms (up to 4 millimeters). Following the technology demonstration, presented in Photonics West 2017, with results on 10/130 (core-to-clad diameters (in μm) ratio) fibers, this paper aims to present updated results obtained for double-clad 11/130, 20/130 and 20/400 LMA fibers, with numerical apertures at, respectively, 0.08 and 0.065. The study is based on aluminosilicate core material co-doped either with fluorine or phosphorus to achieve optimal radial RI tailoring. The fiber produced exhibit low background losses (<20dB/km at 1100nm) and high power conversion efficiencies, up to 74% for output powers of 100W limited by our test setup. The Gaussian beam quality has been evaluated using the M<sup>2</sup> measurement. Photodarkening behavior will be discussed for both fluorine and phosphorus-doped aluminosilicate materials and particularly the use of cerium as co-dopant. The SPCVD technology can indeed be used for the production of Yb-doped LMA fibers. Current development is now focused on other rare-earth doped fibers.
The actual challenge for space researchers is to increase the free space telecommunications data speed transfer. One of the most promising solutions is the optical communication systems. This technology can be used for the inter-satellite and/or satellite-ground links, reaching the TB/s speed for data transfer in the case of Dense Wavelengths Division Multiplexing (DWDM) based technologies. However, to achieve such systems, two main issues need to be overcome: the first one is to validate that no unexpected radiation effect appears when the optical amplifier working in the DWDM configuration and the second one is to estimate the degradation of the Erbium/Ytterbium co-doped boost (High Power - HP) amplifier performances during the space mission lifetime. In this last case, the used high powers will result in a complex response of the amplifier due to photobleaching, photodarkening and thermal effects. In this work, we estimate the radiation effects on an Er/Yb co-doped boost amplifier operating in a Dense WDM configuration. Both radiation hardened and a conventional versions of EYDFA have been considered. The obtained results allow estimating the performances of our fibers under exposure in such amplification setup and also to validate its potential for use in an actual space mission. We demonstrate the good radiation resistance of Er/Yb co-doped 12 μm core diameter fibers reaching 20 W of output power for telecommunication applications. This core diameter provides a fewmode optical output signal (with low dispersion) and with enough power to ensure the signal propagation trough the atmosphere. This study is fundamental as several phenomena such as Photo/Thermal bleaching, photo-darkening… are in competition due to the high-power light density in the fiber core and the system radiation response cannot yet be predicted by actual simulation tools.
We investigated the radiation hardening of optical fiber amplifiers operating in space environments. Through a real-time analysis in active configuration, we evaluated the role of Ce in the improvement of the amplifier performance against ionizing radiations. Ce-codoping is an efficient hardening solution, acting both in the limitation of defects in the host glass matrix of RE-doped optical fibers and in the stabilization of lasing properties of the Er<sup>3+</sup>-ions. On the one hand, in the near-infrared region, radiation induced attenuation measurements show the absence of radiation induced P-related defect species in host glass matrix of the Ce-codoped active fibers; on the other hand, in the Ce-free fiber, the higher lifetime variation shows stronger local modifications around the Er<sup>3+</sup>-ions with the absence of Ce.
A radiation resistant optical fiber used in a broadband source is presented. Both ASE source and Fiber Optical Gyroscope (FOG) commonly used in space missions, suffer from failures and degradation after long term exposure to radiative environment. The aim of this article is to present the results of our investigation on fiber and ASE source architecture in order to design a Radiation Resistant Erbium Doped Fiber that offers long term stability of the gyroscope performances.
In these ICSO proceedings, we review recent advances from our group concerning the radiation hardening of optical fiber and fiber-based sensors for space applications and compare their benefits to state-of-the-art results. We focus on the various approaches we developed to enhance the radiation tolerance of two classes of optical fibers doped with rare-earths: the erbium (Er)-doped ones and the ytterbium/erbium (Er/Yb)-doped ones. As a first approach, we work at the component level, optimizing the fiber structure and composition to reduce their intrinsically high radiation sensitivities. For the Erbium-doped fibers, this has been achieved using a new structure for the fiber that is called Hole-Assisted Carbon Coated (HACC) optical fibers whereas for the Er/Ybdoped optical fibers, their hardening was successfully achieved adding to the fiber, the Cerium element, that prevents the formation of the radiation-induced point defects responsible for the radiation induced attenuation in the infrared part of the spectrum. These fibers are used as part of more complex systems like amplifiers (Erbium-doped Fiber Amplifier, EDFA or Yb-EDFA) or source (Erbium-doped Fiber Source, EDFS or Yb- EDFS), we discuss the impact of using radiation-hardened fibers on the system radiation vulnerability and demonstrate the resistance of these systems to radiation constraints associated with today and future space missions. Finally, we will discuss another radiation hardening approach build in our group and based on a hardening-by-system strategy in which the amplifier is optimized during its elaboration for its future mission considering the radiation effects and not in-lab.
Rare-earth doped optical fibers (REDF, Er or Er/Yb-doped) are a key component in optical laser sources (REDFS) and amplifiers (REDFA). The high performances of these fiber-based systems made them as promising solution part of gyroscopes, telecommunication systems… However, REDFs are very sensitive to space radiations, so their degradation limits their integration in long term space missions. To overcome this issue, several studies were carried out and some innovations at the component level were proposed by our group such as the Cerium co-doping or the hydrogen loading of the REDF. More recently we initiated an original coupled simulation/experiment approach to improve the REDFA performances under irradiation by acting at the system level and not only at the component itself. This procedure optimizes the amplifier properties (gain, noise figure) under irradiation through simulation. The optimization of the system is ensured using a PSO (Particle Swarm optimization) algorithm. Using some experimental inputs, such as the Radiation Induced Attenuation (RIA) measurements and the spectroscopic features of the fiber, we demonstrate its efficiency to reproduce the amplifier degradation when exposed to radiations in various experimental configurations. This was done by comparing the obtained simulation results to those of dedicated experiments performed on various REDFA architectures. Our results reveal a good agreement between simulations and experimental data (with <2% error). Finally, exploiting the validated codes, we optimized the REDFA design in order to get the best performances during the space mission and not on-ground only.
One key parameter in the race toward ever-higher power fiber lasers remains the rare earth doped optical core quality. Modern Large Mode Area (LMA) fibers require a fine radial control of the core refractive index (RI) close to the silica level. These low RI are achieved with multi-component materials that cannot be readily obtained using conventional solution doping based Modified Chemical Vapor Deposition (MCVD) technology. This paper presents a study of such optical material obtained through a full-vapor phase Surface Plasma Chemical Vapor Deposition (SPCVD). The SPCVD process generates straight glassy films on the inner surface of a thermally regulated synthetic silica tube under vacuum. The first part of the presented results points out the feasibility of ytterbium-doped aluminosilicate fibers by this process. In the second part we describe the challenge controlling the refractive index throughout the core diameter when using volatile fluorine to create efficient LMA fiber profiles. It has been demonstrated that it is possible to counter-act the loss of fluorine at the center of the core by adjusting the core composition locally. Our materials yielded, when used in optical fibers with numerical apertures ranging from 0.07 to 0.09, power conversion efficiency up to 76% and low background losses below 20 dB/km at 1100nm. Photodarkening has been measured to be similar to equivalent MCVD based fibers. The use of cerium as a co-dopant allowed for a complete mitigation of this laser lifetime detrimental effect. The SPCVD process enables high capacity preforms and is particularly versatile when it comes to radial tailoring of both rare earth doping level and RI. Large core diameter preforms - up to 4mm - were successfully produced.
We present a new class of Erbium-doped optical fibers: the Hole-Assisted Carbon-Coated, HACC fibers. Optical fibers
with this particular structure have been made by iXFiber on the basis of an appropriate choice of codopants in their core
and claddings. By using an additional pre-treatment with deuterium (D2) loading authorized by the HACC structure, we
highlight the efficiency of such components and demonstrated that this new type of fiber presents a strongly enhanced
radiation resistance compared to the other types of erbium-doped optical fibers studied in litterature. We also built an
Erbium-doped Fiber Amplifier (EDFA) with one of these HACC fibers and compared its radiation response to the one of
the same fiber composition but without the HACC structure and D<sub>2</sub> loading. We tested the performances of this EDFA
under Υ-rays and characterize its gain degradation up to doses of 315 krad. Before irradiation, the amplifier presents a
gain of about 31 dB that is comparable to the optical performances of amplifiers based on HACC fibers without the D<sub>2</sub>
pre-treatment and the HACC structure. During irradiation, our results demonstrate that the tested amplifier is nearly
unaffected by radiations. Its gain slowly decreases with the dose at a slope rate of about -2.2×10<sup>-3</sup> dB/krad. This strong
radiation resistance (enhancement of a factor of ×10 compared to the previous or conventional radiation tolerant EDFA)
will authorize the use of HACC doped fibers and amplifiers for various applications in space for missions associated
both with low or large irradiation doses.
Er/Yb doped fibers and amplifiers have been shown to be very radiation sensitive, limiting their integration in space. We
present an approach including successive hardening techniques to enhance their radiation tolerance. The efficiency of our
approach is demonstrated by comparing the radiation responses of optical amplifiers made with same lengths of different
rare-earth doped fibers and exposed to gamma-rays. Previous studies indicated that such amplifiers suffered significant
degradation for doses exceeding 10 krad. Applying our techniques significantly enhances the amplifier radiation
resistance, resulting in a very limited degradation up to 50 krad. Our optimization techniques concern the fiber
composition, some possible pre-treatments and the interest of simulation tools used to harden by design the amplifiers.
We showed that adding cerium inside the fiber phosphosilicate-based core strongly decreases the fiber radiation
sensitivity compared to the standard fiber. For both fibers, a pre-treatment with hydrogen permits to enhance again the
fiber resistance. Furthermore, simulations tools can also be used to improve the tolerance of the fiber amplifier by
helping identifying the best amplifier configuration for operation in the radiative environment.
Rare-earth doped optical fibers have been shown to be very sensitive to radiations, limiting the integration of fiber-based
systems in space missions. In this paper, we present the characterization of two amplifiers based on a set of prototype
active Erbium/Ytterbium codoped double clad fibers developed by Ixfiber SAS. One of these fibers has been codoped
with cerium inside its core to enhance its radiation tolerance whereas the other is a classical phosphosilicate Er/Yb fiber.
The two amplifiers based on these fibers have been exposed to γ-rays at a low dose rate (0.3 rad/s) and to doses up to
90 krad. Previous studies indicated that Er/Yb amplifiers using this type of fiber suffered significant degradation for
cumulated dose above 5-10 krad. We observed, on the contrary, that with our radiation hardened fiber, the degradation
of the fiber amplifier's output power can be limited to less than 30% after an exposure dose of ~90 krad.