Patent ID: 7009693

Claim:
In an optical transmission system having first and second optical fiber lines, each of the first and second optical fiber lines transmitting a signal light in the opposite direction from the other, and a Coherent Optical Time Domain Reflectometry (C-OTDR) optical loop back circuit for optically connecting between the first and second optical fiber lines, a method for measuring a gain profile in the first optical fiber line, the method comprising: supplying the first optical fiber line with a plurality of laser lights, each of the plurality of laser lights having a different wavelength, and a probe pulse light having a measurement wavelength, the measurement wavelength being different from the wavelengths of the plurality of laser lights; measuring a power level of the probe pulse light at a predetermined position in the first optical fiber line from a return light of the probe pulse light entered through the C-OTDR optical loop back circuit and the second optical fiber line using C-OTDR; and performing the supplying and measuring steps while varying the measurement wavelength within a predetermined wavelength range.