Cut-off wavelength and mode field radius determinations in monomode fibres by means of a new single measurement device

We present a new experimental set-up capable of providing measurements of both the cut-off wavelength (lambda(c)) and the mode field radius (w(0)) of monomode optical fibres. The work consists in an original implementation, by means of a single experimental device, of two efficient methods conceived previously in our laboratory: the azimuthal far-field analysis technique for lambda(c) and the analysis of the autocorrelation function of the far field for w(0). Two components (a mirror and a small screen pierced by a narrow slit) are fixed or rotated at the output of the fibre tested, to suitably filter the far field, according to the required parameter. This parameter is finally computed from the temporal variations of the power detected after the slit. The results obtained for both lambda(c) and w(0) are demonstrated to be precise and reliable. The measurements are self-referenced and can be achieved for any length of fibre as well as for fibres in cables or installed optical links.