An optical fibre device comprising a splice between a first optical fibre and a second optical fibre , wherein the splice has a loss less than 0.5dB, and wherein the first optical fibre and the second optical fibre have a mismatch in core diameters , of at least 10μm, wherein the first optical fibre has a first refractive index profile that can be changed by heating to a second refractive index profile , the first optical fibre comprising at least one core , a cladding , at least one first dopant , at least one concealed dopant , and at least one mobile dopant , wherein prior to heating: • the first dopant is disposed in the core and is for providing the first refractive index profile ; • the concealed dopant and the mobile dopant are disposed in a doped region of the first optical fibre ; • the core has a refractive index that is greater than a refractive index of the cladding ; • one of the concealed dopant and the mobile dopant has a negative molar refractivity and the other one has a positive molar refractivity wherein molar refractivity is defined as the change in refractive index per mole percent of dopant; and • the mobile dopant is present in a concentration that would induce a refractive index change to the first refractive index profile if the concealed dopant were not present; the first optical fibre being such that prior to heating: • the concealed dopant is present in a concentration that provides an equal and opposite refractive index change to the refractive index change induced by the mobile dopant thus leaving the first refractive index profile induced by the first dopant unchanged; • the concentration of the first dopant in the core has a maximum value; • the concentration of the concealed dopant in the doped region is more than the maximum value; and • the mobile dopant has a diffusion constant greater than a diffusion constant of the concealed dopant so that heating of the first optical fibre causes the mobile dopant to diffuse from the doped region more quickly than the concealed dopant , thereby allowing the concealed dopant and the mobile dopant to change the first refractive index profile of the first optical fibre to the second refractive index profile and so change an optical guidance property of the first optical fibre .. • the first dopant is disposed in the core and is for providing the first refractive index profile ; • the concealed dopant and the mobile dopant are disposed in a doped region of the first optical fibre ; • the core has a refractive index that is greater than a refractive index of the cladding ; • one of the concealed dopant and the mobile dopant has a negative molar refractivity and the other one has a positive molar refractivity wherein molar refractivity is defined as the change in refractive index per mole percent of dopant; and • the mobile dopant is present in a concentration that would induce a refractive index change to the first refractive index profile if the concealed dopant were not present; • the concealed dopant is present in a concentration that provides an equal and opposite refractive index change to the refractive index change induced by the mobile dopant thus leaving the first refractive index profile induced by the first dopant unchanged; • the concentration of the first dopant in the core has a maximum value; • the concentration of the concealed dopant in the doped region is more than the maximum value; and • the mobile dopant has a diffusion constant greater than a diffusion constant of the concealed dopant so that heating of the first optical fibre causes the mobile dopant to diffuse from the doped region more quickly than the concealed dopant , thereby allowing the concealed dopant and the mobile dopant to change the first refractive index profile of the first optical fibre to the second refractive index profile and so change an optical guidance property of the first optical fibre .. <EOS>