Effect of thermal stress on refractive index in clad fibers.

i.e., for a very thin fiber (a = λ) with n = 1.5 the index difference may be as large as 0.05, whereas in a macroscopic single-mode waveguide (2a = 100λ)with n = 1.5 the limiting Δn = 2 × 10 5 . In the manufacture of a cladded fiber, core and cladding are fused together at an elevated temperature. During the subsequent cooling to room temperature thermal stresses develop. Under these stresses, core and cladding will become optically anisotropic. If the thermal expansivity of the core is larger than that of the cladding, the principal indices of the core will be lowered and those of the cladding will be raised. For large changes the single-mode condition may be upset, and if n2 becomes larger than n1 even the total reflectivity will be lost. To find the magnitude of the effect we determine first the thermal stress configuration and evaluate subsequently the changes of the refractive indices for silicate glass. where 1 designates the inner and 2 the outer cylinder. Further B1 = 0 to keep the stress in the core finite. With ζ = b/a the solution becomes: