Abstract: An optical fiber is constructed by selecting a first material for a core of the optical fiber. The first material has a known coefficient of thermal expansion, .alpha..sub.1, and a known temperature dependent rate of change, dn/dT, of an index of refraction, n. A second material (cladding) has a predetermined coefficient of thermal expansion, .alpha..sub.2, and a radius, R.sub.2, about the axis of the fiber. The radius, R.sub.2, of the cladding is determined such that an effective coefficient of thermal expansion for the fiber is substantially equal to the negative of the product of the reciprocal of the index of refraction of the first material and the temperature dependent rate of change of the index of refraction of the first material, i.e. .alpha.=(-1/n)dn/dT.