Laser frequency noise measurement using an envelope-ratio method based on a delayed self-heterodyne interferometer

The delayed self-heterodyne interferometer (DSHI) is a simple and effective method of measuring the laser linewidth. However, conventional DSHIs generally cannot measure a full laser frequency noise power spectral density (PSD). In this article, we theoretically analyzed the PSD of the DSHI beat signal, and propose a new method to extract the PSD of the laser frequency noise directly from the PSD of the DSHI beat signal. Compared with the methods already established, our method does not need an additional frequency demodulation technique, and a full laser frequency noise PSD can be measured by using a short delay fiber. The experimental scheme is simple and can reach a high spectral resolution. The applicable conditions, resolution and influences of the experimental parameters of the method are also analyzed. An experiment is conducted to verify the proposed method. By using a 1.1 km delay fiber, the frequency noise PSD of an optical cavity-locked diode laser is evaluated for frequency range between 100 Hz and 1 MHz.