We experimentally generate twin beams through cascaded quadratic processes in a nonlinear χ(2) crystal, leading to an internally pumped optical parametric oscillation, in a doubly resonant second-harmonic generation system. The exploration of the non-classical properties of these beams enabled the observation of up to 5 dB of noise reduction in their intensity difference below the standard quantum limit.
We report on the experimental realization of optical frequency comb (OFC) generation in a doubly-resonant cavity second harmonic generation (SHG) system. OFCs continue to attract significant interest, offering a wealth of potential applications beyond frequency metrology. Continuously-driven Kerr microresonators, whose nonlinear response is dominated by the third-order nonlinearity, have proven to be viable alternatives to comb sources based on femtosecond mode-locked lasers. Recently, OFCs have also been directly generated through second-order nonlinear interactions in cw-pumped resonators namely, a singly-resonant cavity SHG system and a nearly-degenerate optical parametric oscillator. Theoretical studies have also predicted OFCs in doubly-resonant cavity SHG systems with a much lower threshold with respect to the singly-resonant configurations. Here we report on the first observations of OFCs in such a doubly-resonant system. The experiment is based on a periodically poled lithium niobate crystal, placed in a traveling-wave optical cavity, pumped by a cw Nd:YAG laser emitting 0.5 W at 1064 nm. The cavity is resonant for frequencies around both the fundamental pump and its second harmonic at 532 nm, and an intracavity adjustable silica window is used to separately set the detunings of the pump and its second harmonic. Stable cavity locking to the pump laser is achieved via the Pound-Drever-Hall offset locking technique, thanks to a counterpropagating orthogonally polarized auxiliary beam. We measured a power threshold for comb formation as low as 5 mW, reduced by more than one order of magnitude with respect to singly-resonant configurations. The locking system permitted to explore frequency detunings up to several cavity linewidths, and to correspondingly observe a large variety of comb regimes, with different teeth spacing and spectral span, as well as the contribution of photothermal effect to the whole dynamics. In this regard, we developed an extended theoretical model that includes thermo-optical nonlinearities.
Nowadays, optical fiber links are key elements in optical metrology, spectroscopy, quantum communication and geodesy. In geodetic Very Long Baseline Interferometry, a local maser is responsible for providing time and frequency reference at radiotelescope. Here, we present our recent results on frequency dissemination using a coherent fiber link 1800 km long from Turin to Medicina and Matera, Italy. Metrological reference disseminated via fiber link improve the stability of about two order with respect to the local H-maser clocks. This kind of dissemination paves the way to VLBI observation using a remote clock reference on the Italian and European radio observatories.
We demonstrate optical frequency comb generation in a continuously pumped optical parametric oscillator, in the parametric region around half of the pump frequency. We also model the dynamics of such quadratic combs using a single time-domain mean-field equation, and obtain simulation results that are in good agreement with experimentally observed spectra. Moreover, we numerically investigate the coherence properties of simulated combs, showing the existence of correlated and phase-locked combs. Our work could pave the way for a new class of frequency comb sources, which may enable straightforward access to new spectral regions and stimulate novel applications of frequency combs.
We present a new generation of compact and rugged mid-infrared (MIR) difference-frequency coherent radiation sources referenced to fiber-based optical frequency comb synthesizers (OFCSs). By coupling the MIR radiation to high-finesse optical cavities, high-resolution and high-sensitivity spectroscopy is demonstrated for CH4 and CO2 around 3.3 and 4.5 μm respectively. Finally, the most effective detection schemes for space-craft trace-gas monitoring applications are singled out.
We present a widely-tunable, singly-resonant optical parametric oscillator, emitting more than 1 W in the region
between 2.7 and 4.2 μm. Two configurations have been studied in order to improve the frequency stability and
the linewidth of the OPO emission. First, we stabilized the signal frequency to a high-finesse Fabry-Perot cavity.
Then, we locked both pump and signal frequency to the frequency comb generated by a NIR fs mode-locked
fibre laser, linked to the caesium primary standard. With this last configuration we carried out saturation
spectroscopy of several transitions belonging to the ν1 rovibrational band of CH3I, resolving their electronic
quadrupole hyperfine structure, and determining the absolute frequency of the hyperfine components with a
50-kHz-uncertainty. An upper limit for the idler linewidth has been estimated as 200 kHz FWHM.
A novel generation of sensors of strain, temperature, absolute and relative molecular
concentration is reported. Such devices, based on 1-D photonic structures, rely on ultrastable laser sources,
referenced to a fiber-based optical frequency comb synthesizer (OFCS). In particular, recent advances in the
realization of two complementary laser sensors are presented. One is a spectroscopic facility which exploits
frequency mixing in a periodically-poled LiNbO3 crystal to generate highly coherent (a few hundred kHz
linewidth) infrared radiation tunable in the 2.9-3.5 micron wavelength range. Such radiation can be coupled
to high-finesse enhancement cavities to detect trace amounts of gases, including rare isotopes in natural
abundance. The other system, making use of fiber Bragg grating components, provides strain and
temperature sensing with extremely high sensitivities (about 100 fε, i.e. 10-13 ΔL/L). Due to the remoteness
guaranteed by the fiber coupling, these two systems can both be used in difficult environments and inserted
in a multi-parametric network for real-time and continuous monitoring of large areas. Prospects for
application in volcanic areas are also discussed.
We report the realization of a mid-infrared tunable coherent radiation source based on difference-frequency generation with an output power that achieves, for the first time, a few mW. The signal and pump beams coming from two widely tunable, cw high power rare-earth-doped amplifiers are mixed into a periodically poled LiNbO3 crystal to produce radiation in the 2.9-3.5 μm spectral interval. The low intensity noise and the good spatial profile of the idler beam are first verified. The suitability of this new spectrometer for high-sensitivity and high-resolution gas detection is then demonstrated by performing saturated absorption spectroscopy of methane. Finally, the future development of a portable, multicomponent gas sensor for environmental monitoring is discussed.
A novel broad-band telecom laser source is used to realize a lateral-shear scanning-wavelength interferometer for measuring the thickness of thin plates. We show that the wide tunability range allows to detect samples down to tens of microns with a relative uncertainty of less than 0.5% and a resolution of about 1 nm. A comparable accuracy in the thickness characterization of double-layer structures is also demonstrated. In turn, the wide tunability range needs the dispersion law of the materials to be taken into account in the model for correct thickness evaluation.
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