High Precision Estimation of Laser FM-Noise Using RF Quadrature Demodulation Techniques

We report on a technique that uses RF quadrature demodulation to measure laser FM-noise based on a delayed self-heterodyne setup. The measurement is self-referencing and the FM-noise can be measured with high precision over a sufficiently long duration that allows us to explore the limits imposed by the delay line in the well-known self-heterodyne laser phase noise measurement setup. We perform this measurement for two external cavity lasers with linewidths of tens of kilohertz and a fiber laser with a linewidth of 20 Hz. The scheme is checked and verified by finding good agreement between the FM-noise measured from beating the two external cavity lasers and the sum of the individual FM-noise of each laser.