The Variable Stripe Length (VSL) method has been revealed in the last years to be a very useful tool to measure the optical gain in thin film active media [1]. Basically the sample is optically pumped with a stripe-shaped beam of variable length at constant intensity, and the intensity of the edge-emitted Amplified Spontaneous Emission (ASE) is measured as a function of the excitation stripe length (Fig. 1a). A simple fit of an appropriate expression to the resulting data gives the optical gain as an output parameter.
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