These concepts show how to operate the network as more digital carriers are added to it. As more channels of the same level are added to the return network, the laser begins to see higher cumulative power at its input. As the input power increases, the maximum power input to the laser will be reached or exceeded, which affects the network. The simple response to this problem is to lower all carrier levels to a point that does not affect the laser. Unfortunately, the simple answer is a little too simple.
Digital signals have peak power points much like the peak power point of a television signal.
In the case of the television signal, the peak is the sync tip. A 64 QAM digital signal has a peak signal 9 to 10 dB higher than the RMS value. That peak point is represented in a 64 QAM signal at the point farthest away from the origin in the signal constellation. For a QPSK modulated signal, the peak can be 3 to 5 dB higher than the RMS value. The concern about peak power into the laser might not be a problem with one or two digital return carriers.