Analysis of a single-mode evanescent-field-pumped dye amplifier

Amplification of a light signal propagating in a single-mode fiber of passive core and active cladding (dye solution) is investigated. The active cladding region is excited by the evanescent field of a pump mode. Calculations are based on exact field expressions (weakly guiding case) and rate equations for the dye under steady-state conditions. Assuming the 'slow growth approximation' to be valid, two coupled nonlinear differential equations are derived for the powers in pump and signal modes, respectively. The influence of various fiber and dye parameters on the gain is studied. It is concluded that CW single-pass amplification by factors of up to five seem realistically achievable.