1 and e 2 is the electric field amplitude in the second arm 16 of the interferometer n c u2212 1 from the terms in eqs 4 and 5 the relationship between the interferometer 39 s path imbalance and the intensity output u03b4l is the interferometer path imbalance m u03c6 t is the demodulated peak to peak phase amplitude rad and i t is the interferometer output intensity w thus for a known wavelength modulation u03b1 u03bb it is possible to demodulate the intensity change to obtain the amplitude of the phase excursion using eq 5 the path imbalance of the interferometer can then be deduced using eq 4 a dispersion measurement apparatus according to one embodiment of the present invention is shown schematically in fig2 this apparatus includes an interferometer that is similar to the prior art interferometer shown in fig1 but with one of the mirrors of the prior art device replaced with a chirped fibre bragg grating thus the interferometer of the apparatus shown in fig2 includes input fibre 30 coupler 32 first reflection arm 34 of length l second reflection arm 36 of length l u03b4l where u03b4l depends on the wavelength dependent location of the reflection point within arm 36 mirror 38 chirped fibre bragg grating 40 and output fibre 42 a probe laser beam of wavelength u03bb.