Measurement of the laser relative intensity noise

This article describes the measurement of the RIN power spectral density with the cross-spectrum method. Averaging the cross spectrum of two equal and independent channels that measure the same beam, the instrument background is not limited by the single-channel noise. Additionally, the measure can be validated on the basis of simple mathematical properties. This removes the need of a low-noise beam to validate the instrument background, which is a relevant experimental limitation of the (traditional) single-channel method.