Design Methodology and Architectures to Reduce the Semiconductor Laser Phase Noise Using Electrical Feedforward Schemes

Analysis of feedforward linewidth reduction scheme for semiconductor lasers followed by measurements are presented in this paper. The design challenges for such a system, followed by improvements to the original scheme are explained and demonstrated using top-bench electrooptical setups. The experiments are carried out on a commercially available 1.55- m distributed feedback (DFB) laser. The measurement results show more than 40 times reduction in frequency noise power spectrum. Also the laser original full-width at half-maximum (FWHM) linewidth of 2.6 MHz is reduced to less than 140 KHz. The feedforward scheme does not have the limited noise cancellation bandwidth, instability, and speed issues that are common in feedback linewidth reduction systems. In this scheme, the ultimate achievable phase noise will be limited by the noise of electronic circuitry and laser intensity noise. Using the proposed feedforward approach, the frequency noise of semiconductor lasers can be reduced by 3-4 orders of magnitude in a monolithic approach using today's low-noise scaled transistors with THz gain-bandwidth product.