A method for sending a signal, comprising: splitting a received digital signal into a first digital signal and a second digital signal; respectively converting the first digital signal and the second digital signal into an analog in-phase signal and an analog quadrature signal; respectively modulating the analog in-phase signal and the analog quadrature signal to two optical signals that are perpendicular to each other and are in a polarization state; after the two optical signals that are obtained through modulation, are perpendicular to each other, and are in a polarization state are combined into one optical signal, sending the optical signal to an optical network unit, ONU; and wherein the respectively converting the first digital signal and the second digital signal into an analog in-phase signal and an analog quadrature signal specifically comprises: separately processing the first digital signal and the second digital signal by using filters whose impulse responses are orthogonal, to respectively convert the first digital signal and the second digital signal into the digital in-phase signal and the digital quadrature signal, and respectively converting the digital in-phase signal and the digital quadrature signal into the analog in-phase signal and the analog quadrature signal by using a digital to analog converter; or respectively converting the first digital signal and the second digital signal into a first analog signal and a second analog signal by using a digital to analog converter, and separately processing the first analog signal and the second analog signal by using filters whose impulse responses are orthogonal, to respectively convert the first analog signal and the second analog signal into the analog in-phase signal and the analog quadrature signal splitting a received digital signal into a first digital signal and a second digital signal; respectively converting the first digital signal and the second digital signal into an analog in-phase signal and an analog quadrature signal; respectively modulating the analog in-phase signal and the analog quadrature signal to two optical signals that are perpendicular to each other and are in a polarization state; after the two optical signals that are obtained through modulation, are perpendicular to each other, and are in a polarization state are combined into one optical signal, sending the optical signal to an optical network unit, ONU; and wherein the respectively converting the first digital signal and the second digital signal into an analog in-phase signal and an analog quadrature signal specifically comprises: separately processing the first digital signal and the second digital signal by using filters whose impulse responses are orthogonal, to respectively convert the first digital signal and the second digital signal into the digital in-phase signal and the digital quadrature signal, and respectively converting the digital in-phase signal and the digital quadrature signal into the analog in-phase signal and the analog quadrature signal by using a digital to analog converter; or respectively converting the first digital signal and the second digital signal into a first analog signal and a second analog signal by using a digital to analog converter, and separately processing the first analog signal and the second analog signal by using filters whose impulse responses are orthogonal, to respectively convert the first analog signal and the second analog signal into the analog in-phase signal and the analog quadrature signal separately processing the first digital signal and the second digital signal by using filters whose impulse responses are orthogonal, to respectively convert the first digital signal and the second digital signal into the digital in-phase signal and the digital quadrature signal, and respectively converting the digital in-phase signal and the digital quadrature signal into the analog in-phase signal and the analog quadrature signal by using a digital to analog converter; or respectively converting the first digital signal and the second digital signal into a first analog signal and a second analog signal by using a digital to analog converter, and separately processing the first analog signal and the second analog signal by using filters whose impulse responses are orthogonal, to respectively convert the first analog signal and the second analog signal into the analog in-phase signal and the analog quadrature signal