Light amplification by evanescent wave coupling in a multimode fiber

Amplification of light propagating as bound modes of a multimode fiber through evanescent wave coupling is reported. Experiments have been performed by immersing sections of a stripped fiber 2-20 cm long in a Kiton red dye solution which is pumped by the evanescent fields of a cw Ar-ion laser also propagating as bound modes of the fiber. Gain coefficients as high as 0.02/cm have been observed by a 633-nm He-Ne probe beam when pumped by a 5-W laser. Numerical calculations obtained by performing a Taylor series expansion of the eigenvalue equation for a cylindrical waveguide with a small active component in the cladding region show that modes near cutoff can exhibit gains 60% or higher, approaching those assumed for the cladding region.