Improvement and analysis of a recirculating delayed self-heterodyne interferometer for laser linewidth measurement

• A simple modification to the passive RDSHI to separately control the reference beam and loop feedback power. • Equal power split for the reference beam confirmed best to enhance the detectable beat order number. • Influence of the loop coupler’s coefficient upon maximal beat notes studied experimentally. • Larger number of beats and better performance confirmed for the improved RDSHI scheme. This paper presents an improved scheme of a passive recirculating delayed self-heterodyne interferometer (RDSHI) for laser linewidth measurement. The proposed scheme includes two additional fiber couplers in a regular RDSHI to independently control the reference beam and the loop signal for best system performance. The analysis and experimental results have confirmed that an initial equal power split for the reference beam is best suited to enhance the maximal number of detectable beat notes. The output coupling coefficient of the fiber loop has also been experimentally studied for system optimization. The experimental results have confirmed that the proposed modification to a basic RDSHI can achieve more detectable beat notes (14 vs. 12) and finer linewidth resolution (∼14% improvement) than a basic RDSHI in our test setup of 2-km-long delay line.