The invention relates to an optical loss technology for used in a wavelength division multiplexing passive network with a wavelength locked optical source. It provides a four-port optical path setting device to increase the light quantity injected into the light emitter so as to improve the wave-length locking performance, and besides, it can reduce the light loss in the optical path; by a light amplifier inserted into it, it also can compensate the transmission losses of light in optical path.