An optical fiber-based microheater is described. The fiber, a highly Yb-doped (23.4wt% Yb2O3) silicate glass, can produce thermal power densities in excess of 10 W/nL via optical pumping at 976nm. No evidence of luminescence is observed, indicating efficient conversion from optical to thermal energy. Demonstrated are two applications for this microheater. The first is an all-optical-fiber Pirani thermal vacuum gauge, which uses a dual-fiber configuration. The second is an all-optically-driven, all-optical-fiber, Mach-Zehnder-based modulator. The phase delay, introduced by inserting the microheater into one interferometer arm, is a function of its temperature and can be actively controlled by the pump power.
Progress towards the development of intrinsically low-nonlinearity and low-loss optical fibers for high-energy laser applications is outlined. Owing to the high optical intensities and relatively long interaction lengths in high-energy fiber lasers systems, a wide range of deleterious, power-limiting processes can be excited, all of which require careful management. Here, a materials-based approach is taken to enhance the power available from such systems. Specifically discussed in this context are: 1) decreasing the strength of Brillouin scattering, mainly through the reduction of glass photoelastic constant p12; 2) the reduction of the strength of Raman scattering by taking advantage of glass disorder and the judicious use low-Raman-gain additives; 3) elevation of the threshold for the onset of thermal mode instabilities through a reduction in the thermo-optic coefficient, dn/dT; and 4) all while not adversely impacting the relatively low value of n2 afforded by glasses comprised mainly of silica. The ultimate goal of this work is the development of active fibers with Brillouin gain coefficients reduced by 15 dB, Raman gain coefficients reduced by 3 dB, and dn/dT values reduced by 3 dB, with background losses well below 100 dB/km all in a single fiber. The current state of this effort will be discussed and the outlook for the development of such a fiber will be described.
This paper provides a road-map for the development of simple core/clad optical fibers whose enhanced performance - in particular, marked reductions in optical nonlinearities - is achieved materially and not through the more conventional present routes of geometrically complex fiber design. More specifically, the material properties that give rise to Brillouin, Raman, and Rayleigh scattering, transverse mode instabilities (TMI), and n2-mediated nonlinear effects are compiled and results on a wide range of optical fibers are discussed with a focus on trends in performance with glass composition. Further, optical power scaling estimations as well as binary and ternary property diagrams associated with Rayleigh scattering, the Brillouin gain coefficient (BGC) and the thermo-optic coefficient (dn/dT) are developed and employed to graphically represent general trends with composition along with compositional targets for a single intrinsically low nonlinearity, silica-based optical fiber that can achieve the powerscaling goals of future high energy fiber laser applications.
Lasers and amplifiers based on thulium-doped silica fibers require improved spectroscopic properties. In this context, one of the most promising approaches is based on the embedding of thulium ions in nanoparticles of tailored composition and structure. This paper presents various methods used to produce thulium-doped nanoparticles inside silica-based optical fibers. Effects of solution doping method during the elaboration of Modified Chemical Vapor Deposition preform and doping solution composition are studied. A comparison is made between the use of solutions containing LaF3:Tm3+ or YAG:Tm3+ nanoparticles and aluminum-lanthanum-thulium chlorides. Results show that for similar lanthanum content, lanthanum-thulium chlorides doping allows for similar enhancement of 3H4 level of Tm3+ than LaF3:Tm3+ doping. Also, effects of aluminum on 3H4 lifetime enhancement and inhibition of nanoparticle’s formation is discussed.
Holmium doped fiber lasers (HoDFL) are attractive candidates for high energy lasers used in directed energy applications because they operate at wavelengths that are safer to the eye. The common solution-doping technique for making HoDFs can result in the incorporation of hydroxyl (OH) impurity in the active fiber core. The HoDFL operational wavelength of ~2.0 µm is near the 2.2 µm combination absorption band of the OH fundamental mode and the SiO4 tetrahedron vibration, so the OH concentration must be <1 ppm to prevent degradation of the laser performance. We have routinely fabricated HoDF with [OH] < 0.5 ppm. We have developed an ultralow OH processing technique based on both atmospheric exclusion from the silica core soot preform and careful, extensive drying. We report a resonantly-pumped solution doped Ho3+ fiber laser with a slope efficiency of 74%, and an output power of 96W. We are also investigating nanoparticle (NP) doping, where the holmium ions are encaged in a nanoparticle host selected for properties such as low phonon energy, where they are shielded from the Silica lattice. By optimizing variables such as precursor concentrations, NP ripening times, and surfactant selection during synthesis we have been able to increase the Ho NP concentration levels in Silica fiber cores. This has also allowed us to increase concentrations of otherwise incompatible low phonon energy host materials into the cores of the fibers. Cores comprising Ho doped LaF3 and Lu2O3 nanoparticles exhibited slope efficiencies as high as 85% at 2.06 µm in a MOPA configuration.
A significant issue for holmium-doped fiber lasers (HoDFLs) operating near 2 μm is multiphonon quenching due to the high phonon energy ~1100 cm-1 of the silica host, which complicates power scaling due to reduced lifetimes and increased heating. Nanoparticle (NP) doping is a new technique where the structure surrounding the Ho ions is developed chemically prior to doping into the silica core. We have incorporated Ho3+ ions into various NPs, such as LaF3, Al2O3 and Lu2O3, to shield them from the silica glass matrix. Results indicate slightly longer lifetimes with Ho:LaF3 NPs and the possibility of further improvement with oxide NPs. We report the first of lasing in a Ho:Lu2O3 NP-doped fiber pumped at 1.95 μm and operating at 2.09 μm with a record slope efficiency of 85.2%.
We report on the spectroscopic characteristics and upconversion emission in Er3+ doped Yttria (Y2O3) transparent ceramics prepared by a modified two-step sintering method. The near-infrared (1.5 μm) emission properties were evaluated as a function of Er3+ concentration. Judd-Ofelt intensity parameters, radiative rates, branching ratios, and emission lifetimes were calculated and compared with results reported for Er3+ doped Y2O3 single crystal and nanocrystals. Following pumping at 1.532 μm, weak blue (~0.41 μm, 2H9/2→4I15/2), strong green (~0.56 μm, 2H11/2, 4S3/2→4I15/2) and red (~0.67 μm, 4F9/2→4I15/2) emission bands were observed as well as weak near-infrared emissions at 0.8 μm (4I9/2→4I15/2) and 0.85 μm (4S3/2→4I13/2) at room temperature. The upconversion emission properties under ~1.5 μm pumping were further investigated through pump power dependence and decay time studies. Sequential two-photon absorption leads to the 4I9/2 upconversion emission while energy transfer upconversion is responsible for the emission from the higher excited states 2H9/2, 2H11/2, 4S3/2, and 4F9/2. The enhanced red emission with increasing Er3+ concentration most likely occurred via the cross-relaxation process between (4F7/2 → 4F9/2) and (4I11/2 → 4F9/2) transitions, which increased the population of the 4F9/2 level.
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