Limitation and improvement in the performance of recirculating delayed self-heterodyne method for high-resolution...

This paper presents a detailed analysis of the performance of a recirculating delayed self-heterodyne (R-DSH) method for high-resolution laser lineshape measurement. For increasing the delay time the R-DSH method utilizes circulation of light in a heterodyne ring interferometer (HRI) containing a frequency shifter, delay fiber, and fiber amplifier. It is shown both theoretically and experimentally that unwanted higher order frequency-shifted components induce distortion in the beat signal spectra, which significantly limits the maximum number of circulations. An effective technique is proposed and demonstrated for reducing the distortion by using optical filtering at the HRI output. Furthermore, a practical limit on the number of circulations is investigated by comparing the shape of observed beat signal spectra with theory. It is shown that the maximum delay is limited to about 180 km even with the use of the optical filtering technique.