Method and apparatus for chromatic dispersion measurements

A chromatic dispersion characteristic measuring method wherein an optical signal, obtained by modulating the optical output from a light source with a single-frequency signal, is applied to the input end of an optical transmission line to be measured, said modulated optical signal is received at the output end of said optical transmission line, the wavelength of said optical output is changed, and a phase difference between it and said single-frequency signal at said input end of said optical transmission line is measured to thereby measure the chromatic dispersion characteristic of said optical transmission line, said method comprising:

• a step of deriving said optical output from a fixed wavelength first optical signal and a tunable wavelength second optical signal and measuring a change in the light propagation time of said fixed wavelength optical signal through said optical transmission line by its elongation or shrinkage upon each measurement of a change in the light propagation time of said tunable wavelength second optical signal through said optical transmission line;
• a step of recording the times of measuring the changes in said light propagation times of said tunable wavelength second optical signal and said fixed wavelength optical signal, respectively;
• a step of accurately obtaining, by interpolation, the amount of change in said light propagation time of said second optical signal caused by the elongation or shrinkage of said optical transmission line at said measuring time of said second optical signal, from said measuring time of said first optical signal and the amount of change in said propagation time of said first optical signal;
• and
• a step of removing said amount of change in said propagation time by the elongation or shrinkage of said optical transmission line from said amount of change in said propagation time of said second optical signal, thereby eliminating a measurement error resulting from the elongation or shrinkage of said optical transmission line.