23, it immediately results that pumping has to be optical to excite some state at higher energy with respect to I13/2 to achieve a three level working. On the other hand, the wavelengths giving absorption from the ground state to an excited state beyond 113/2 are in the range typical of semiconductor lasers; thus, a suitable optical pump does exist. In particular, 800 and 980 nm are particularly interesting as pump wavelengths, existing powerful semiconductor lasers based on AlGaAs technology. Moreover, the split of the resonant level at 1531 nm in a small energy band allows also semiresonant pumping at 1480 nm via InGaAsP semiconductor lasers. These lasers are in general less powerful than AlGaAs one, but due to the pumping state, the pump should be more efficient. Among these three potential pumping frequencies, the 800 nm wavelength suffers the phenomenon of excited state absorption (ESA). This phenomenon consists in the absorp- tion of a photon at 800 nm by means of electrons already excited at the I13/2 state to jump to the S3/2 state (see Figure 4.23). From S3/2, the electrons then decays in a non-radiative The ESA imposes a limit on the population inversion due to the progressive deple- tion of the lasing state when the pump populates it; thus, 800nm is not a suitable pump wavelength. Two possible pump wavelengths remain: 1480 and 980nm. Using these wavelengths, different pumping schemes can be adopted for single- and double-stage amplifiers. In the single-stage case, the simpler schemes are Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices Simple copropagating pumping either at 1480 or at 980 nm Simple counter-propagating pumping either at 1480 or at 980nm Double pumping at the same wavelength (see Figure 4.24) Double counter-propagating pumping at different wavelengths Comparing 980 and 1480 nm in similar configuration, the 1480 nm pump does not exhibit the expected efficiency advantage since resonant pumping is never reached due to the broadening of the pumping level tha