We report the experimental demonstration of a polarized superfluorescent fiber source (SFS) made by inserting a low- loss polarizer at an optimum position along the erbium-doped fiber of a standards SFS. This source produces a strongly polarized output with nearly the same output power as a standard, unpolarized SFS. We show good agreement between the behavior of the source and numerical simulations.
We report a new theoretical study of the effects of anisotropic erbium dipoles on the polarization of Er-doped fiber lasers (EDFLs), as well as measurements of the polarization of EDFLs made of a polarization-maintaining fiber. Convenient closed-form expressions are presented for the gain of a signal polarized either parallel or perpendicular to the pump. These results are used to interpret the polarization behavior of our EDFLs, stressing the effects of pump orientation, fiber length, and cavity Q. We identify in particular operating conditions that produce a fiber laser with either a linearly polarized output or equal power in both polarizations.
11Erbium Doped Fiber Laser (EDFL) experimental slope efficiencies are used in conjunction with a paired ion model to estimate the percentage of paired ions in a fiber as a function of the erbium concentration. These percentages are compiled with other published results to derive a dependence of percentage of pairs on the aluminum co-dopant concentration. A mole ion concentration ratio of 20 aluminum to 1 erbium is found to greatly reduce pairing. Higher ratios are found to have minimal added benefit.
Simulations are used to examine the effects of Er3+ pairs on the characteristics of erbium-doped fiber amplifiers. As the percentage of pairs is increased, the most significant effect is that the small-signal gain steadily drops. On the other hand, the optimum fiber length (for maximum gain at a given pump power) and the saturation power (at optimum length and constant gain) vary minimally up to pair concentrations of 40%. The noise figure shows degradation with increasing pair concentrations that is due entirely to the reduction in gain, i.e., at constant gain a paired and unpaired amplifiers are predicted to have the same noise figure. The main effect of pairs is therefore to increase the amplifier pump power requirement.
Comparison of the output characteristics of different erbium doped fiber lasers show that the threshold pump power increases (by a factor of 4.6) and the conversion efficiency decreases (by a factor of 1.5) as the erbium concentration is increased from around 150 to 1040 mole ppm Er2O3. We propose that these two effects are mostly due to rapid interaction (perhaps upconversion) between a subset of paired ions. This work suggests that for Al-Ge- doped silica fibers, concentrations of 150 mole ppm or less should be used for optimum output power. A fiber with this concentration produced a low-threshold laser with a total power conversion efficiency of 90.4%.
To explain the sub-optimal performance of erbium-doped resonant fiber lasers and superfluorescent fiber sources observed experimentally, the effects of potential loss mechanisms are explored via computer simulations. Pump excited-state absorption (ESA) at 980 nm and 1.48 micrometers , and signal ESA are unable to explain the dependence of the observed effects on concentration. Cooperative upconversion among uniformly distributed erbium ions fails to explain the observed reduction in source slope efficiency with increasing concentration. On the other hand, rapid cross-relaxation between paired ions, which might form in high concentration fibers, can produce the observed dependences. Rate equations for paired ions are used to understand their saturation behavior and their effect on the slope and threshold of fiber sources. Methods to assess the fraction of paired ions are discussed. Measurements suggest that about 18% of the ions in an aluminum co-doped silica fiber with 5 X 1019 Er3+/cm3 are paired. The effects of paired ions on the gain of Er-doped fiber amplifiers are also briefly discussed.
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