Analytic Calculation of Absorption Efficiency for Double-Clad Fibers Using Modal Method

The absorption efficiency for circular or offset double-clad optical fibers is investigated with modal analysis method. First, calculate the number of all the propagating modes approximately using WKB method and derive the analytic formulas for calculating the number of propagating modes that can't be absorbed by active core. Then the absorption efficiency can be obtained analytically. Comparison of modal method with the ray optics method is made and the condition of using both methods is also discussed.