Optimization of acousto-optic optical frequency combs

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.

Programmable broadband optical field spectral shaping with megahertz resolution using a simple frequency shifting loop

“…In practice, typical values of P 0 range from a few µW to a few tens of µW. Stronger injection results in an exponentially decreasing comb envelope, due to gain saturation of the amplifier 41 . For P 0 = 1 µW, f s = 80 MHz, n sp = 2, G = 1/ T = 10, and N = 300, the SNR is about 35.…”

High-speed broadband absorption spectroscopy enabled by cascaded frequency shifting loops

“…While this simple approach has enabled high-speed frequency scanning with precisely controlled frequency steps, the total bandwidth and number of pulses generated in the FSL are limited by the accumulation of ASE 17 . The ASE introduced after each round-trip scales with the amplification required to compensate for loss and the bandwidth of the bandpass filter.…”

“…To optimize this scheme, we first simulated the ASE build-up following the model introduced in Ref. 17 . The model was used to calculate the signal-to-noise ratio (SNR) between the signal power in the n th pulse, , and the ASE power in the n th pulse, integrated across frequency bins of width : …”

Optical Frequency Combs Generated by Acousto-Optic Frequency-Shifting Loops

“…1(c), for instance, reveals that the slope of the comb critically depends on the product × . The data shown in this figure, reported and explained in [18], are retrieved from self-heterodyne measurements of an AO-OFC with a line spacing of 50 kHz. As a general design rule, using amplifiers with small NF and reducing the losses of the loop have a positive influence on the comb flatness.…”

“…In addition, the use of a single sideband EO shifter produces an unavoidable third harmonic, leading to cross-talk effects between the generated lines [17]. For that reason, when a relatively small line spacing is required, AO frequency shifters offer extinction ratios exceeding 50 dB, better performance in terms of efficiency and number of generated lines, and some remarkable features for applications in the time domain [18]. Indeed, these features have been exploited for the generation of optical/RF signals in laser ranging and sensing [13,14,19,20], as well as for the implementation of real-time signal processors [21,22].…”