146 CHAPTER 5 Gain changes of optical amplifiers at ROADM nodes 5.5 POSITIVE GAIN CHANGE OF GAIN SPECTRAL HOLE BURNING OBSERVED AT ROOM TEMPERATURE In a previous section, we explained the origin of the GSHB of the EDF by observing the gain spectral hole characteristics at 77 K. However, for practical systems it is important to understand the GSHB behavior at room temperature, which is nomi- nally the ambient temperature of most field deployed EDFAs. In this section, we explain the positive gain change of GSHB observed by applying the subtracting technique which is one of the unique behaviors of GSHB at room temperature. Figure 5.28 plots an example of a positive gain change of GSHB observed at room temperature by using the subtracting technique depicted in Figure 5.9. The wavelength and the input power of the saturation signal are 1540 nm and -10 dBm, respectively and the wavelength of the gain-locking is 1560 nm. In this figure, the positive gain change is observed at a wavelength between the main hole and the second hole. This positive gain change occurs because of the gain shift due to the gain-locking operation in the subtracting technique. When the gain at the gain-locking wave- length is affected by GSHB and decreases, the pump power is controlled to increase the gain of the gain-locking signal, then the gain shift corresponding to the compensation of the gain change at the gain-locking wavelength occurs. As a result, the positive gain change is observed at the wavelength at which the gain change of the gain shift is larger than the gain change of GSHB. At 77 K, the holes of the multihole structure are separated, as shown in Figure 5.25 and the additional gain shift due to the gain-locking operation can be avoided by choosing the gain-locking wavelength at the wavelength that is located between the holes. However, at room temperature, the gain change due to GSHB is broadly spread over the gain spectrum, and it is difficult to choose the gain-locking in which the GSHB gain change is sufficiently small. Therefore, at room tempera- ture, it is difficult to separate the original gain change of GSHB and the gain change 0.4 Positive gain change Gain change (dB) 0.2 0 -0.2 -0.4 -0.6 -0.8 1520 Saturation signal 1530 1540 Second hole Main hole Gain-lock signal 1550 1560 Wavelength (nm) FIGURE 5.28 Positive gain change observed by applying the subtracting technique at room temperature.