A method for activating an ONU in a multi-wavelength PON. Other ONUs may be already active and transmit upstream signals to the OLT on various upstream channels. The new ONU transmits an activation signal superimposed to such upstream signals. Since the ONU may include an uncalibrated tunable transmitter, initially the activation signal wavelength might be different from that upon which the ONU shall be activated. The activation signal optical power is lower than the other upstream signals, so as not to impair their reception at the OLT. The OLT operates the ONUs already active on the upstream channel on which the new ONU shall be activated to suspend transmission for one or more time gaps, during which the OLT detects the activation signal. This improves activation signal detection conditions, thereby allowing to increase its bitrate. An acceptable duration of the activation procedure can then be achieved.