A. Rocco

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.

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.

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.

Cavity-enhanced generation of 6 W cw second-harmonic power at 532 nm in periodically-poled MgO:LiTaO 3

We report on efficient cw high-power second harmonic generation in a periodically poled LiTaO3 crystal placed in a resonant enhancement cavity. We tested three configurations, differing in the coupling mirror reflectivity, and a maximum conversion efficiency of about 76%, corresponding to 6.1 W of green light with 8.0 W of fundamental power, was achieved. This is, to the best of our knowledge, the highest cw power ever reported using a periodically-poled crystal in an external cavity. We observed photo-thermal effect induced by photon absorption at the mirrors and in the crystal, which however does not affect stable operation of the cavity. A further effect arises for two out of the three configurations, at higher values of the input power, which degrades the performance of the locked cavity. We suggest this effect is due to the onset of competing nonlinearities in the same crystal.

Continuous in situ measurements of volcanic gases with a diode-laser-based spectrometer: CO2 and H2O concentration and soil degassing at Vulcano (Aeolian islands: Italy)

We report on a continuous-measurement campaign carried out in Vulcano (Aeolian islands, Sicily), devoted to the simultaneous monitoring of CO2 and H2O concentrations. The measurements were performed with an absorption spectrometer based on a semiconductor laser source emitting around a 2-μm wavelength. The emitted radiation was selectively absorbed by two molecular ro-vibrational transitions specific of the investigated species. Data for CO2 and H2O concentrations, and CO2 soil diffusive flux using an accumulation chamber configuration, were collected at several interesting sampling points on the island (Porto Levante beach- PLB, Fossa Grande Crater – FOG- and Valley of Palizzi, PAL). CO2/H2O values, measured on the ground, are very similar (around 0.019 (± 0.006)) and comparable to the previous discrete detected values of 0.213 (Fumarole F5-La Fossa crater rim) and 0.012 (Fumarole VFS – Baia Levante beach) obtaid during the 1977–1993 heating phase of the crater fumaroles.In this work much more homogeneous values are found in different points of the three sites investigated. The field work, although carried out in a limited time window (25th–28th August 2004), pointed out the new apparatus is suitable for continuous gas monitoring of the two species and their ratios, which are important geochemical indicators of volcanic activity, for which other reliable continuous monitoring systems are not yet available.

An interferometric demodulation method for visualizing and determining quasi-static strain by FBG sensors

In this paper, we propose an interferometric approach for visualizing and measuring the quasi-static strain experienced by fibre Bragg grating sensors. The method makes use of a simple bi-polished silicon sample acting like an etalon tuneable filter. The Bragg wavelength shift can be evaluated by analysing the overall interferometric signal achieved by tuning the etalon tuneable filter angularly. A fast Fourier transform method is applied for phase retrieval. The choice of the silicon sample is determined by the low-cost and well-developed silicon technology and fabrication, the easy design and tuneability of the spectral response, and the possibility of integrating on the same sample also the circuitry for electronic control. The principle of operation of this method is described and results obtained by employing such a configuration are reported.

Assessing Soil Respiration by Means of Near-Infrared Diode Laser Spectroscopy

High-resolution diode laser spectroscopy in the near-infrared region is applied to the accurate measurement of soil respiration. In particular, the use of a diode-laser-based spectrometer has allowed the implementation, for the first time, of a static accumulation method capable of measuring soil respiration from continuous measurements of CO2 concentrations, with minor perturbation on soil respiration as well as on CO2 transport and emission. The system has been tested in a laboratory experiment by detection of CO2 production from sandy matrices, inoculated with active soil microbes and supplied with different amounts of decomposable plant material. Respiration rates of all samples were then retrieved using a diffusion model. The results of the laboratory tests are in agreement with those expected on the basis of sample composition. Examples of operation with real soil samples are also reported. We discuss the possible field application of the system, in conjunction with closed static soil chambers.

A narrow-linewidth, frequency-stabilized OPO for sub-Doppler molecular spectroscopy around 3 μm

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.

Periodically-Poled Ferroelectric Crystals Based OPO—A Powerful Source for Precision Spectroscopy

Advances in material science and laser sources renewed the interest in optical parametric oscillators (OPOs). A major attraction of OPOs is the possibility to coherently generate new frequencies over extended tunability ranges. These characteristics are of great interest for spectroscopic applications, which require stable cw highly-coherent radiation sources in a wide range of frequencies, from visible-UV to infrared. We briefly present the operational principles of OPOs, with particular emphasis on the aspects related to spectroscopic applications. Then, we report on recent results achieved with a narrow-linewidth singly-resonant cw OPO, emitting in the MIR, used for high-resolution spectroscopy of molecular transitions.

Experimental study of face and edge-pumped ceramic slab DPSSLs in the 100-500W power range

We developed several prototypes of ceramic Nd:YAG slab lasers pumped by different laser diode stacks. Compactness, efficiency and ruggedness have been the principal design drivers. Nevertheless beam quality and insensitivity to diodes temperature have also been pursued. An edge-pumped device with less than 12% power variation over a 20°C temperature range has been demonstrated.