In numerous laser and optical amplifier engineering optimization problems, the prime design criteria are the device output power and the overall gain. The amount of pump power available is usually limited and fibre parameters such as core dimensions and numerical aperture are set for achieving minimum coupling losses. With this respect we developed optical amplifier design curves that could summarize in one plot existing relationships of the most pertinent design parameters. The model is based on . These diagrams show the required active medium length as a function of both Ytterbium and Erbium concentrations for achieving a given output power. Such a representation provides a quick, visual recognition of the optimal length, as can be observed in figure 1a). No unique optimal solution exists but instead several length-concentrations triplets. The generated “maps” allow a clear identification of the different operating regimes of the amplifier. For instance, it can be observed that at low Er concentrations no particular advantage is obtained if Yb concentration is increased. These conclusions are not quite obvious when using other representations. Regarding the selected concentration ranges, higher values are limited such that clustering effects be limited. Lower bounds are chosen in order to prevent obtaining excessively long devices. We used this approach for designing a core pumped fiber amplifier based on Yb3+-Er3+ co-doped phosphate glass which was then developed in house. The pump and signal powers were chosen to be Pp =166 mW and Ps = 3.16 mW (+5 dBm). The desired output power was Pout = 45 mW. Both Ytterbium and Erbium concentrations have been selected to be 2.5×1026 ion/m3, thus leading to a required length of 27 mm for the active medium, as indicated by a cross point in figure 1a). Single mode laser diodes working at 975.6 nm (pump) and 1535 nm (signal) were spliced to the inputs of a WDM combiner. Its output was butt-coupled to the active fiber. A photograph of the in-house fabricated amplifier section is shown in figure 1b). The optical output power at 1535 nm was 13.8 mW. The difference with the expected value can be partly related to the presence of green emission, attributed to an up-conversion process. In terms of gain, the amplifier provided 6.39 dB for an input signal of +5 dBm. The gain per unit length was thus 2.37 dB/cm, which is higher than or near to values reported in [2,3] (0.94 dB/cm and 3.3 dB/cm). In addition, a gain of 11 dB (with a -30dBm signal) was observed when the pump power was increased to Pp = 479 mW, which leads to a 4.07 dB/cm gain, a value that is comparable to [4,5] (4.2 dB/cm and 5.2 dB/cm). Later simulations showed that at this pump level a notable increase in output power can be obtained if 10 to 20 mm longer fibers are used.
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