COHERENT OPTICAL TRANSMITTER AND FREQUENCY STABILIZING METHOD FOR COHERENT OPTICAL TRANSMITTER

摘要

PURPOSE:To keep a center wavelength of a signal light constant by applying a control current in response to a mark rate fluctuation of a digital modulation signal to an optical semiconductor element. CONSTITUTION:Since a mark rate detector 5 detects a rate of appearance of logic '1' in a transmission signal, a control output 8 proportional to a mark rate is outputted. The control output 8 is added to a bias current 7 and the result is fed to a semiconductor laser 1. Since the control output 8 compensates the fluctuation of a mean signal current attended with the mark rate fluctuation of a transmission signal 11, a current in response to the fluctuation is supplemented by a bias current as a reference of a half of the mark rate. That is, a pulse signal current 6 is supplemented by using the reduction of the mean signal current as a bias current if the mark rate is decreased to 1/4. Conversely when the mark rate is increased to 3/4, the increment of the mean signal current is reduced from the bias current 7. A change in a center wavelength of a signal light 20 is suppressed within 10MHz regardless of a change in the mark rate as the result of the control in this way.