Measurement of the modal reflectivity of an antireflection coating on a superluminescent diode

A method for measuring the modal reflectivity of the antireflection coating applied to a laser diode is described and demonstrated. It is based on measurements of the Fabry-Perot modulation depth of the resulting superluminescent diode (SLD) output spectrum at the threshold current of the original laser. A modal reflectivity of less than 2 \times 10^{-4} has been obtained.