Role of modulation bandwidth in narrow spectral linewidth semiconductor lasers under optical negative feedback

We investigated the FM noise characteristics of a distributed feedback laser under optical negative feedback from a Fabry-Perot optical filter using delayed self-homodyne coherent detection with a high-frequency range of 1 × 105-2 × 1010 Hz. The measured FM noise power spectral densities of the laser under the optical negative feedback strongly depend on the modulation responses of the laser. These results were reproduced in calculations based on a transfer function model. Our findings suggest that wide bandwidth semiconductor lasers are suitable for reducing FM noise in a wide frequency range and enhancing the feedback loop gain (FM noise reduction ratio) under a sufficient phase margin in optical negative feedback.