It is disclosed a optical transmitter for an optical network unit of a multi-wavelength passive optical network comprising an optical line termination and a number of further optical network units for transmitting upstream signals to said optical line termination on multiple upstream channels. The optical transmitter is configured to generate and transmit an optical activation signal carrying activation information to be transmitted to said optical line termination on an upstream channel, the activation signal having an optical power lower than the optical power of each one of said upstream signals. The optical transmitter comprises an optical source, a first electric source configured to modulate the optical source with a first electric signal carrying the activation information; and a second electric source configured to directly modulate the optical source with a second electric signal so as to produce a frequency chirp on the optical activation signal.