The present invention discloses an encoding apparatus, including: a polarization splitter-rotator PSR, a polarization rotation structure, and a modulator, where the PSR is configured to receive an input signal light, split the input signal light into two parts whose polarization modes are the same, and send the two parts to the polarization rotation structure and the modulator respectively; the polarization rotation structure has functions of rotating, by 180 degrees, a polarization direction of an optical signal entering the polarization rotation structure from one end, and keeping a polarization direction of an optical signal entering the polarization rotation structure from the other end unchanged; the modulator is configured to modulate a light input to the modulator; and the PSR is further configured to receive signal lights sent by the polarization rotation structure and the modulator, combine the two signal lights to send the output signal light.