Arbitrary energy-preserving control of the line spacing of an optical frequency comb over six orders of magnitude through self-imaging

Generation of GHz line-spacing tunable optical frequency combs using Talbot effects

In this paper, the generation of GHz line-spacing tunable optical frequency combs (OFCs) was demonstrated using an electro-optical (EO) Talbot laser and a phase modulator. In the EO Talbot laser, the frequency shifting was realized with a dual-parallel Mach-Zehnder modulator (DPMZM) working for carrier-suppressed single-sideband modulation. The PM was employed to achieve the spectral Talbot effect and compensate the phase introduced by the temporal Talbot effect in the laser loop. Arbitrary control of OFC line-spacing was realized using temporal and spectral Talbot effects. The principle of this OFC generator was theoretically modeled. In the experiments, the 2 GHz line spacing of an OFC was multiplied to be 4 GHz, 6 GHz, 8 GHz, and 10 GHz. The frequency spacing of the OFC can also be multiplied with a fractional factor of 3/4, 7/2, 8/5, and 10/7, which was confirmed by simulations.

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

Frequency shifting loops, consisting of a fiber optic ring cavity, a frequency modulator, and an amplifier to compensate for loss, enable high-speed frequency scanning with precise and easily controlled frequency steps. This platform is particularly attractive for applications in spectroscopy and optical ranging. However, amplified spontaneous emission noise accumulates due to the repeated amplification of light circulating in the cavity, limiting the frequency scanning range of existing frequency shifting loops (FSLs). Here, we introduce a cascaded approach which addresses this basic limitation. By cascading multiple FSLs in series with different frequency shifts we are able to dramatically increase the accessible scanning range. We present modeling showing the potential for this approach to enable scanning over ranges up to 1 THz-a tenfold increase compared with the state-of-the-art. Experimentally, we constructed a pair of cascaded FSLs capable of scanning a 200 GHz range with 100 MHz steps in 10 ms and used this platform to perform absorption spectroscopy measurements of an H¹³C¹⁴N cell. By increasing the operating bandwidth of FSLs, the cascaded approach introduced in this work could enable new applications requiring precise and high-speed frequency scanning.

Phase-sensitive distributed Rayleigh fiber sensing enabling the real-time monitoring of the refractive index with a sub-cm resolution by all-optical coherent pulse compression

We have developed a novel architecture enabling distributed acoustic sensing in a commercial single-mode fiber with a sub-cm spatial resolution and an interrogation rate of 20 kHz. More precisely, we report the capability of real-time and space-resolved monitoring of the distributed phase and of the refractive index variations along the sensing fiber. The system reported here is optimal in many aspects. While the use of broadband light waveforms enables a sub-cm spatial resolution, the waveforms are quasi CW, delaying the occurrence of non-linear effects. Coherent detection ensures direct access to the distributed phase and to the local variations of the refractive index. Moreover, an all-optical pulse compression feature enables to lower the detection bandwidth down to 10 MSa/s. Based on a bi-directional frequency shifting loop, the architecture makes use of a single CW laser, commercial telecom components, and low frequency electronics. It is expected to open new avenues in distributed acoustic sensing applications, where high spatial resolution and high interrogation rates are required.

All-optical coherent pulse compression for dynamic laser ranging using an acousto-optic dual comb

We demonstrate a new and simple dynamic laser ranging platform based on analog all-optical coherent pulse compression of modulated optical waveforms. The technique employs a bidirectional acousto-optic frequency shifting loop, which provides a dual-comb photonic signal with an optical bandwidth in the microwave range. This architecture simply involves a CW laser, standard telecom components and low frequency electronics, both for the dual-comb generation and for the detection. As a laser ranging system, it offers a range resolution of a few millimeters, set by a dual-comb spectral bandwidth of 24 GHz, and a precision of 20 µm for an integration time of 20 ms. The system is also shown to provide dynamic measurements at scanning rates in the acoustic range, including phase-sensitive measurements and Doppler shift velocimetry. In addition, we show that the application of perfect correlation phase sequences to the transmitted waveforms allows the ambiguity range to be extended by a factor of 10 up to ∼20 m. The system generates quasi-continuous waveforms with low peak power, which makes it possible to envision long-range telemetry or reflectometry requiring highly amplified signals.

Bidirectional frequency-shifting loop for dual-comb spectroscopy

We present a bidirectional recirculating frequency-shifting loop, seeded by a continuous-wave (cw) laser, to perform multi-heterodyne interferometry. This fiber-optic system generates two counter-propagating "acousto-optic" frequency combs with a controllable line spacing. Apart from its simple architecture, coherent averaging allows us to reach acquisition times up to the second scale without resorting to any active stabilization mechanism. We also show that the relative phase between the combs is quadratic and can be easily controlled by adjusting the parameters of the loop. The capability of our scheme to perform molecular spectroscopy is proven by dual-comb measurements of a transition of hydrogen cyanide in the near-infrared region (1550 nm).

Optimization of acousto-optic optical frequency combs

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.