With the vitrification method, preforms of optical fibres doped with nanodiamonds, were created for the first time. The preforms were drawn into optical fibres. The effect of nanodiamonds integrated into the fabricated fibre core on the nonlinear refractive index of the fibre was investigated. Using an optical spectrum analyser, light spectrum before and after the optical fibre was measured for several levels of average beam power. Additionally, the Frequency Resolved Optical Gating technique was used to retrieve the light pulse characteristics. Obtained data and fibres characteristics were used for solving the Generalized Nonlinear Schrödinger Equation and simulating output spectra for different nonlinear refractive index values. Based on the simulations and the collected data, the value of the nonlinear refractive index of the optical fibre was estimated to be significantly lower than in reference pure silica
fibre. Nonlinear refractive index of nanodiamond-doped fibre turned out to be reduced by at least approximately 34%, compared to the reference SMF-28 fibre. The obtained results indicate, that nanodiamonds can be used in fibre optics to manipulate the nonlinear refractive index. Fibres doped with nanodiamonds are a very promising material, and further research in this field could push the boundaries of ultrafast optics.