Frequency Stabilization of a Dye Laser to a Reference Cavity with Sub-Hertz Relative Linewidth

Abstract
We reduced the frequency fluctuation of a cw single frequency dye laser to the shot-noise limit at the Fourier frequency between 7 kHz and 20 kHz by the phase modulation technique. From the spectral density and the distribution of the instantaneous amplitude of the error signal, the relative linewidth of a dye laser to the reference cavity was estimated to be sub-Hz.