Fully fibered microwave-optical sources at 1.5 μm are studied experimentally. It is shown that the beat note between two orthogonally polarized modes of a distributed-feedback fiber laser can be efficiently stabilized using an optical phaselocked loop that uses the pump-power induced-birefringence as actuator. Beat notes at 1 GHz and 10 GHz of Erbium doped fiber laser are successfully stabilized to a reference synthesizer, passing from the 3 kHz free-running linewidth to a stabilized sub-Hz linewidth, with a phase noise as low as -75 dBc/Hz at 100 Hz offset from the carrier. An ErbiumYtterbium co-doped fiber laser is also investigated and successfully stabilized. Such dual-frequency stabilized lasers could provide compact integrated components for RF and microwave photonics applications.
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.
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.