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Fundamental SNR Limits Imposed by ASE in Frequency-Shifting Loops

Recirculating frequency-shifting loops (FSLs) are a simple source of optical frequency combs with bandwidth compatible with microwave technologies. As such, they have demonstrated promising capabilities for telecommunications, remote sensing, and microwave photonics. In these systems, the coherent frequency comb is produced by recirculation of a single frequency laser in a fiber loop containing a frequency shifter. Due to the insertion of an optical amplifier in the loop to compensate for the losses, amplified spontaneous emission (ASE) is inevitably emitted and superimposes to the coherent output. In this paper, we quantify theoretically the contribution of the ASE background to the FSL output for different types of receivers used in FSL-based techniques: direct, self-heterodyne, and dual-comb detection. In particular, we focus on two important practical applications of FSL: coherent optical reflectometry and real-time Fourier transforms of radio-frequency signals. We provide for each of them numerical estimations of the signal-to-noise ratio and dynamic range. This work constitutes a compact framework for the general evaluation of techniques based on FSLs.