Such a transmitter is needed e.g. when signals of conventional services such as telephone, radio and analog cable TV are to be optically transmitted in conjunction with digital signals, e.g. QAM-signals, such as are provided for the transmission of CDV (Compressed Digital Video), by means of a laser or a laser diode.
Problems are created in that connection by so-called "clipping", a short-term disappearance of the optical carder due to dropping below the laser threshold current. Such dropping below the laser threshold current is caused by constructive interference of several analog modulated carder frequencies, which lead to an overmodulation of the laser toward the low current side. Although such overmodulation only takes place for an extremely short period of time, it requires restarting the laser, which takes considerably more time and results in a high bit error rate during the transmission of digital signals, e.g. QAM-signals.
This problem is described in an essay by K. Maeda et at, in Electronics Letters, Vol. 29, no. 7, page 640.