High concentration erbium-doped silica fibers were developed for amplification of L-band optical signals. Power conversion efficiency as a function of erbium-doping concentration for various erbium-doped silica fibers was analyzed and characterized at a wavelength of 1585 nm and at a pump wavelength of 1480 nm by using a two-stage amplifier and a C-band ASE filter. The experimental results are consistent with the simulation results. High Al doping and optimization of NA and erbium-doped region can help reduce the pair-induced quenching effects and improve the efficiency.
In many practical wavelength-division multiplexed (WDM) systems, erbium-doped fiber amplifiers (EDFA) must maintain a flat gain spectrum over a broad bandwidth for a wide temperature range. When the flatness specification is tight, the temperature dependence of the erbium ion spectra is too great to allow flatness over a wide range of temperatures. To date, all observed temperature variation data has been for specific EDFA designs and no simple rule has been developed for extending the observations to a wide range of EDFAs. We show that the gain and absorption parameters of an erbium- doped fiber follow easily fit dependencies on temperature. Consequently, the change in an EDFA spectrum with temperature can be easily predicted by use of a computer model or sometimes by direct computations using the known dependencies. Physical insight into the meaning of the dependencies is presented and predictions for typical EDFAs are shown.
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.
The defect driven properties of erbium doped fiber amplifier devices are reviewed examining the role of the host dopants, aluminum, phosphorus and germanium, typically used in these applications. The effects of added loss on the performance of amplifiers are shown by way of modeling. Measurements of the gamma and UV radiation induced absorption are presented to show the generic host glass composition effects on radiation induced loss and their relative magnitude. In contrast to these `bad' defects, the UV induced effects in H2 impregnated fiber are discussed in their role as `good' defects, allowing the more efficient writing of UV induced gratings.
We present a simple rule for predicting the peak gain wavelength of an Er-doped fiber amplifier. For a given fiber type, the peak gain wavelength is determined solely by the operating gain per unit length. Using this rule coupled with a simple Er-doped amplifier model and measured modeling parameters, the gain peak is predicted for a particular Er-doped fiber. The result is verified by direct measurement in a fiber loop, using polarization scrambling to eliminate instabilities. We demonstrate that the gain peak does not vary with pump power, pump wavelength, or signal power as long as the gain per unit fiber length is fixed. The theory is then extended to roughly estimate the pump power, signal power, compression level, and other design parameters. Finally, the inclusion of the wavelength dependences of other components in an amplifier chain is also discussed and demonstrated.
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.
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.
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%.
Recent research has indicated that broadband fiber sources are a viable alternative to superluminescent diodes for fiber-optic gyro applications. The key issues in using such sources are possible source configurations, the effects of feedback, source power output, and the stability of such sources in the gyro system. Both Nd:silica and Er:silica sources are discussed with an emphasis on superfluorescent fiber sources pumped by laser diodes. In both media, 100 mW of pump power can produce more than 10 mW of source power with a spectral width in excess of 10 nm. Furthermore, such sources have consistently produced mean wavelength thermal coefficients of less than 10 ppm/C.
The characteristics of Er-doped superfluorescent fiber sources are simulated for pump wavelengths in the 980 nm and 1.48 ?m pump absorption bands. Because absorption near 980 nm occurs to a short-lived pump state, while absorption near 1.48 ?m occurs to the long-lived upper-laser state, sources pumped in these wavelength regions have different characteristics. Both pump bands are found to have optimal pump wavelengths for stability and power (976 nm and 1.475 ?m). While both pump bands are efficient, the 1.48 ?m pump band has a lower threshold pump power level and a higher slope efficiency. On the other hand, the 980 nm pump band produces broader spectra and permits the use of shorter fiber lengths. These and other source characteristics are discussed in detail.
The spectral thermal stability of broadband rare-earth-doped fiber sources makes them attractive for fiber sensor applications. We quantify the mean wavelength variation of both Nd- and Er-doped fiber sources operating as superfluorescent fiber lasers. Besides the intrinsic variation of such sources the effects of pump power and pump wavelength are also considered since both the power and wavelength of conventional laser diode pump sources are temperature sensitive. Other types of rare-earth-doped sources are also briefly considered.
A large signal model of three-level superfluorescent sources (SFS''s) is presented. It is found that due to the ground state signal absorption present in three-level systems theoretical backward signal quantum efficiencies well in excess of the limit of 0. 5 exhibited by four-level systems can be achieved. Feedback is also studied and it is shown that when used in high feedback applications SFS ''s may require the use of an optical isolator.
Stable, broadband, long-wavelength sources are required for accurate fiber sensors such as the fiberoptic gyroscope. The Er-doped superfluorescent fiber source and wavelength-swept fiber laser, which emit near 1 .55 m and can be pumped near 980 nm, are excellent candidates for this application. We discuss the design of such sources, their efficiency, pump source requirements, and the spectra they produce. The spectrum sensitivity to environmental factors such as temperature is also briefly discussed.