140 CHAPTER 5 Gain changes of optical amplifiers at ROADM nodes character of the second hole depth at 77 K is consistent with that observed at room temperature in the C-band wavelength region [13], which has been reported to become monotonically shallower as the wavelength of saturating signal became longer. This character of the second hole depth may suggest that the decrease of the transitions from the Stark manifolds within the 4 I 13/2 state to those within the 4 I 15/2 state that contribute to the emission corresponds to around 1530 nm. On the other hand, the second hole depth at 77 K showed the local peak around 1590 nm and started to decrease. At wavelengths longer than 1610 nm, the second hole depth stopped decreasing and increased again for three EDF samples. This character of the second hole at 77 K is now under consideration in connection with the real Stark energy structure of Er 3þ , where the assumption of equal energy gap (¼3) does not hold, in order to clarify the formation mechanism of the second hole in real EDFs. 5.3.4 Conclusion The saturating power, the wavelength, and Er 3þ ion concentration dependence of GSHB in silica-based EDFA at 77 K were reported. The depth of main and second hole decreased as the wavelength of the saturating signal became longer. The main