Contents Introduction
A precise measurement of the group-velocity dispersion (GVD) of optical fibres is of great importance in the system design and the operation of installed systems. So far, the phase-shift method has been widely used /1/, /2/, in which a sinusoidally intensity-modulated light is launched into the fibre and the group delay is detected as a function of the wavelength by the phase comparison between modulation components of the input and output lights. The disadvantage of the phase-shift method is, however, that the measurement error is easily caused by the drift of the group delay, which originates from the change in the fibre length due to environmental effects. One way to cope with this problem is to measure the group delay at wavelengths of interest and at a fixed reference wavelength alternately. The group-delay drift can then be compensated by measuring the relative group delay with respect to that at the reference wavelength. Even in this method, however, the group-delay drift faster than the time required for the wavelength switching can not be compensated.
