M. De Rosa

Frequency-comb-referenced singly-resonant OPO for sub-Doppler spectroscopy

We present a widely-tunable, singly-resonant optical parametric oscillator, emitting more than 1 W between 2.7 and 4.2 μm, which is phase locked to a self-referenced frequency comb. Both pump and signal frequencies are directly phase-locked to the frequency comb of a NIR-emitting fs mode-locked fibre laser, linked, in turn, to the caesium primary standard. We estimate for the idler frequency a fractional Allan deviation of ∼ 3 × 10⁻¹²τ⁻½ between 1 and 200 s. To test the spectroscopic performance of the OPO, we carried out saturation spectroscopy of several transitions belonging to the ν1 rovibrational band of CH₃I, resolving their electronic quadrupole hyperfine structure, estimating a linewidth better than 200 kHz FWHM for the idler, and determining the absolute frequency of the hyperfine components with a 50-kHz-uncertainty.

Walk-Off-Induced Modulation Instability, Temporal Pattern Formation, and Frequency Comb Generation in Cavity-Enhanced Second-Harmonic Generation.

We derive a time-domain mean-field equation to model the full temporal and spectral dynamics of light in singly resonant cavity-enhanced second-harmonic generation systems. We show that the temporal walk-off between the fundamental and the second-harmonic fields plays a decisive role under realistic conditions, giving rise to rich, previously unidentified nonlinear behavior. Through linear stability analysis and numerical simulations, we discover a new kind of quadratic modulation instability which leads to the formation of optical frequency combs and associated time-domain dissipative structures. Our numerical simulations show excellent agreement with recent experimental observations of frequency combs in quadratic nonlinear media [Phys. Rev. A 91, 063839 (2015)]. Thus, in addition to unveiling a new, experimentally accessible regime of nonlinear dynamics, our work enables predictive modeling of frequency comb generation in cavity-enhanced second-harmonic generation systems. We expect our findings to have wide impact on the study of temporal and spectral dynamics in a diverse range of dispersive, quadratically nonlinear resonators.

Sum-frequency generation of cw ultraviolet radiation in periodically poled LiTaO(3).

We present cw sum-frequency generation of UV radiation at 355 nm based on a high-power laser at 1064 nm and a two-stage quasi-phase-matching nonlinear interaction in periodically poled LiTaO3 crystals. In a first stage, second harmonic at 532 nm is generated. Then, the outcoming IR and green light interact in a second nonlinear crystal to generate about 7 mW of ultraviolet radiation at the sum frequency.

Experimental measurement of the dynamic photothermal effect in Fabry-Perot cavities for gravitational wave detectors

We report the experimental observation of the photothermal effect. The measurements are performed by modulating the laser power absorbed by the mirrors of two high-finesse Fabry-Perot cavities. The results are very well described by a recently proposed theoretical model [M. Cerdonio, L. Conti, A. Heidmann and M. Pinard, Phys. Rev. D 63 (2001) 082003], confirming the correctness of such calculations. Our observations and quantitative characterization of the photothermal effect demonstrate its critical importance for high sensitivity interferometric displacement measurements, as those necessary for gravitational wave detection.

Numerical solution of the Fokker-Planck equation. II : multidimensional case

Abstract In a recent paper [Phys. Lett. A 146 (1990) 378] a fast and reliable algorithm for the numerical solution of unidimensional Fokker-Planck-like equations has been proposed. However, in most of the systems currently under study the unidimensional Fokker-Planck description is derived as an approximated contraction of a multidimensional exact equation of the Fokker-Planck-type. The main purpose of this paper is to discuss the possibility of extending a consolidated fluid-dynamics algorithm to the analysis of these kinds of systems. Numerical stability and convergence of the method are discussed in detail. Some examples of applications of the numerical method are also given.

Status report and near future prospects for the gravitational wave detector AURIGA

We describe the experimental efforts to set up the second AURIGA run. Thanks to the upgraded capacitive readout, fully characterized and optimized in a dedicated facility, we predict an improvement in the detector sensitivity and bandwidth by at least one order of magnitude. In the second run, AURIGA will also benefit from newly designed cryogenic mechanical suspensions and the upgraded data acquisition and data analysis.

Frequency comb formation in doubly resonant second-harmonic generation

We theoretically study the generation of optical frequency combs and corresponding pulse trains in doubly resonant intracavity second-harmonic generation (SHG). We find that, despite the large temporal walk-off characteristic of realistic cavity systems, the nonlinear dynamics can be accurately and efficiently modeled using a pair of coupled mean-field equations. Through rigorous stability analysis of the systems steady-state continuous-wave solutions, we demonstrate that walk-off can give rise to an unexplored regime of temporal modulation instability. Numerical simulations performed in this regime reveal rich dynamical behaviors, including the emergence of temporal patterns that correspond to coherent optical frequency combs. We also demonstrate that the two coupled equations that govern the doubly resonant cavity behavior can, under typical conditions, be reduced to a single mean-field equation akin to that describing the dynamics of singly-resonant-cavity SHG [F. Leo et al., Phys. Rev. Lett. 116, 033901 (2016)]. This reduced approach allows us to derive a simple expression for the modulation instability gain, thus permitting us to acquire significant insight into the underlying physics. We anticipate that our work will have a wide impact on the study of frequency combs in emerging doubly resonant cavity SHG platforms, including quadratically nonlinear microresonators.

Room temperature gravitational wave bar detector with optomechanical readout

We present the full implementation of a room-temperature gravitational wave bar detector equipped with an optomechanical readout. The bar mechanical vibrations are read by a Fabry–Perot interferometer whose length changes are compared with a stable reference optical cavity by means of a resonant laser. The detector performance is completely characterized in terms of spectral sensitivity and statistical properties of the fluctuations in the system output signal. This kind of readout technique allows for wide-band detection sensitivity and we can accurately test the model of the coupled oscillators for thermal noise. Our results are very promising for cryogenic operation and represent an important step towards significant improvements in the performance of massive gravitational wave detectors.

SPECIAL issue: 22nd colloquium on high-resolution molecular spectroscopy HRMS Dijon 2011 (Part 2) A narrow-linewidth optical parametric oscillator for mid-infrared high-resolution spectroscopy

We present a narrow-linewidth, singly-resonant cw optical parametric oscillator, emitting more than 1 W in the 2.7–4.2 µm range. The OPO is pumped by a narrow linewidth (40 kHz) fibre-laser system and the signal frequency is locked to a high-finesse Fabry–Pérot cavity in order to increase the spectral resolution, thus obtaining a residual linewidth of 70 kHz for the signal. We tested the spectral performance of our OPO on several transitions in the ν1 rovibrational band of CH3I, measuring line intensities and showing sub-Doppler dip detection.

First room temperature operation of the AURIGA optical readout

In the frame of the AURIGA collaboration, a readout scheme based on an optical resonant cavity has been implemented on a room temperature resonant bar detector of gravitational waves. The bar equipped with the optical readout has been operating for a few weeks and we report here the first results.