Distributed-feedback (DFB) Laser Coherence and Linewidth Broadening

摘要

The primary goal of the research activity presented in this report is to understand the range of applicability for different types of lasers to a variety of Electronic Warfare (EW) applications. This work provides a detailed investigation into laser coherence properties. In particular, the emphasis of this work was on linewidth broadening of commercial-of-the-shelf (COTS) distributed-feedback (DFB) semiconductor lasers, turning them into compatible sources for microwave photonic signal processing. Optical linewidth refers to the optical phase fluctuation of the lasing longitudinal modes. Laser devices having narrow linewidth are said to have a high degree of coherence. The investigation into laser coherence properties includes linewidth measurements, using a phase-modulated delayed self-heterodyning method, of multi-mode Fabry-Perot (FP) and single-mode DFB semiconductor lasers. Leveraging from these measurements, a combination of injection dithering and external phase modulation is proposed to broaden the linewidth of a COTS DFB laser device. Linewidth broadening from 10 MHz to over 200 MHz is possible without splitting the linewidth distribution, which is often associated with wavelength chirping in semiconductor lasers. The linewidth broadening from 10 MHz to over 200 MHz corresponds to a reduction of laser coherence length from 20 metres to less than 1 metre in optical fibres. The advantages and limitations of the linewidth broadening technique and its applicability to microwave photonic signal processing in EW systems are addressed in this report.