An optical transmitter suitable for a time and wavelength division multiplexing system, comprising: a Falsy-Perot laser diode, FP-LD, for generating multiple-longitudinal mode light wave, wherein the FP-LD is also driven by an electrical signal, modulating the electrical signal to the multiple-longitudinal mode light wave, and outputting the modulated multiple-longitudinal mode light wave; an optical coupler, coupled with the FP-LD, wherein the optical coupler is used for feeding the modulated multiple-longitudinal mode light wave from the FP-LD to a fiber Bragg grating; the fiber Bragg grating is for filtering the received multiple-longitudinal mode light wave according to a predetermined parameter, and feeding back the optical signal generated after being filtered to the optical coupler; the optical coupler is for dividing the optical signal to a first branch optical signal and a second branch optical signal, wherein, the first branch optical signal is fed back to the FP-LD, and the second branch optical signal is fed to a detector; the detector is used for judging whether the wavelength and power of the second branch optical signal meet a predetermined condition; the FP-LD is configured to reflect the first branch optical signal to the optical coupler when the predetermined condition is not met, and the optical coupler, the fiber Bragg grating, the optical coupler and the detector in turn are configured to repeat respectively the operations of the feeding, the filtering and feedback, the dividing and the judging, when the predetermined condition is not met; when the predetermined condition is met, the output port of the second branch optical signal is an output port of the optical transmitter, wherein the detector is configured to output the second branch Optical signal to the output port of the optical transmitter when the predetermined condition is met, wherein the optical coupler cascaded with the fiber Bragg grating is directly integrated to the FP-LD thereby forming a feedback cavity of 10 mm length, wherein the fiber Bragg grating is with tunable wavelength; wherein the optical transmitter further includes a controller, the controller is used for receiving a control signal from an optical line terminal of a passive optical network, and adjusting the predetermined parameter of the fiber Bragg grating with tunable wavelength according to the control signal and wherein the controller is smaller than the FP-LD and integrated with the FP-LD into a common housing of the optical transmitter. a Falsy-Perot laser diode, FP-LD, for generating multiple-longitudinal mode light wave, wherein the FP-LD is also driven by an electrical signal, modulating the electrical signal to the multiple-longitudinal mode light wave, and outputting the modulated multiple-longitudinal mode light wave; an optical coupler, coupled with the FP-LD, wherein the optical coupler is used for feeding the modulated multiple-longitudinal mode light wave from the FP-LD to a fiber Bragg grating; the fiber Bragg grating is for filtering the received multiple-longitudinal mode light wave according to a predetermined parameter, and feeding back the optical signal generated after being filtered to the optical coupler; the optical coupler is for dividing the optical signal to a first branch optical signal and a second branch optical signal, wherein, the first branch optical signal is fed back to the FP-LD, and the second branch optical signal is fed to a detector; the detector is used for judging whether the wavelength and power of the second branch optical signal meet a predetermined condition; the FP-LD is configured to reflect the first branch optical signal to the optical coupler when the predetermined condition is not met, and the optical coupler, the fiber Bragg grating, the optical coupler and the detector in turn are configured to repeat respectively the operations of the feeding, the filtering and feedback, the dividing and the judging, when the predetermined condition is not met; when the predetermined condition is met, the output port of the second branch optical signal is an output port of the optical transmitter, wherein the detector is configured to output the second branch Optical signal to the output port of the optical transmitter when the predetermined condition is met, wherein the optical coupler cascaded with the fiber Bragg grating is directly integrated to the FP-LD thereby forming a feedback cavity of 10 mm length, wherein the fiber Bragg grating is with tunable wavelength; wherein the optical transmitter further includes a controller, the controller is used for receiving a control signal from an optical line terminal of a passive optical network, and adjusting the predetermined parameter of the fiber Bragg grating with tunable wavelength according to the control signal and wherein the controller is smaller than the FP-LD and integrated with the FP-LD into a common housing of the optical transmitter.