Abstract
Acousto-optic optical frequency combs can easily produce several hundreds of mutually coherent lines from a single laser, by successive frequency shifts in a loop containing an acousto-optic frequency shifter. They combine many advantages for multi-heterodyne interferometry and dual-comb spectroscopy. In this paper, we propose a model for an intuitive understanding of the performance of acousto-optic optical frequency combs in the steady state. Though relatively simple, the model qualitatively predicts the effect of various experimental parameters on the spectral characteristics of the comb and highlights the primordial role played by the saturation of the gain medium in the loop. The results are validated experimentally, offering a new insight in the performance and optimization of acousto-optic frequency combs.
Highlights
Optical frequency combs (OFCs) have led to numerous achievements in metrology, with dualcomb spectroscopy being one of the most paradigmatic applications [1,2,3,4]
The relative spacing between modes is set by the free spectral range (FSR) of the optical cavity, while the central frequency is fixed by the so-called carrier-to-envelope offset frequency
To provide a comprehensive picture of the characteristics of the AO-OFCS, in what follows, we briefly investigate the influence of various experimental parameters, such as the width of the tunable bandpass filter (TBPF), the comb FSR, the seed power (s0), the noise figure (N F), the small signal amplification factor of the amplifier (Gss), and the transmission coefficient of the loop (T)
Summary
Optical frequency combs (OFCs) have led to numerous achievements in metrology, with dualcomb spectroscopy being one of the most paradigmatic applications [1,2,3,4]. Efficient stabilization techniques have been proposed to control these two comb parameters, but they involve relatively complex and expensive locking systems [5] Despite their high performance, a major disadvantage of mode-locked OFCs is the fact that the FSR is fixed by the length of the cavity, and it is not adjustable. Our model is based on some simple yet valid hypotheses pertaining to AO-FSL as follows: (i) the system is considered to be in the steady state, (ii) the gain is the same for all the comb teeth, which is justified by the fact that the optical amplifiers considered here involve homogeneously broadened gain media, and (iii) the optical bandpass filter in the loop has an ideal flat-top transmission function (Fig. 1) With these assumptions, we can predict the typical exponential envelope of the AO-OFC, the power of the comb lines, and the spectral dependence of the ASE. Following the theoretical understanding of the AO-OFCs, the second part of the manuscript is dedicated to the experimental validation of the model by recording the AO-OFC under various experimental conditions
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