Patent ID: 7171123

Claim:
A method for decreasing and compensating the optical transmission loss at a wavelength-division multiplexed passive optical network characterized by using a 4-port optical path setting device for handling A-band optical signals and B-band optical signals, wherein numbers of wavelengths are included in each band respectively, characterized in that: an A-band optical signal inputted through # 1 port of said 4-port optical path setting device is outputted through # 3 port, an A-band optical signal inputted through # 3 port of said 4-port optical path setting device is outputted through # 4 port; and a B-band optical signal inputted through # 2 port of said 4-port optical path setting device is outputted through # 4 port and a B-band optical signal inputted through # 4 port of said 4-port optical path setting device is outputted through # 3 port.