A transmitter for an optical network unit, includes a 2.5 Gb/s direct modulation laser, for generating an uplink optical signal; wherein the direct modulation laser is driven by a modulation current and a bias current, and the bias current is configured to be greater than a threshold current of the direct modulation laser, and an amplitude of the modulation current is such configured that a difference between a frequency of 1 bit and a frequency of 0 bit of the uplink optical signal is a half of a transmission rate of the uplink optical signal.