Marilena Giglio

Improved Tuning Fork for Terahertz Quartz-Enhanced Photoacoustic Spectroscopy

We report on a quartz-enhanced photoacoustic (QEPAS) sensor for methanol (CH3OH) detection employing a novel quartz tuning fork (QTF), specifically designed to enhance the QEPAS sensing performance in the terahertz (THz) spectral range. A discussion of the QTF properties in terms of resonance frequency, quality factor and acousto-electric transduction efficiency as a function of prong sizes and spacing between the QTF prongs is presented. The QTF was employed in a QEPAS sensor system using a 3.93 THz quantum cascade laser as the excitation source in resonance with a CH3OH rotational absorption line located at 131.054 cm−1. A minimum detection limit of 160 ppb in 30 s integration time, corresponding to a normalized noise equivalent absorption NNEA = 3.75 × 10−11 cm−1W/Hz½, was achieved, representing a nearly one-order-of-magnitude improvement with respect to previous reports.

Allan Deviation Plot as a Tool for Quartz-Enhanced Photoacoustic Sensors Noise Analysis

We report here on the use of the Allan deviation plot to analyze the long-term stability of a quartz-enhanced photoacoustic (QEPAS) gas sensor. The Allan plot provides information about the optimum averaging time for the QEPAS signal and allows the prediction of its ultimate detection limit. The Allan deviation can also be used to determine the main sources of noise coming from the individual components of the sensor. Quartz tuning fork thermal noise dominates for integration times up to 275 s, whereas at longer averaging times, the main contribution to the sensor noise originates from laser power instabilities.

Highly sensitive gas leak detector based on a quartz-enhanced photoacoustic SF6 sensor.

The implementation, performance validation, and testing of a gas-leak optical sensor based on mid-IR quartz-enhanced photoacoustic (QEPAS) spectroscopic technique is reported. A QEPAS sensor was integrated in a vacuum-sealed test station for mechatronic components. The laser source for the sensor is a quantum cascade laser emitting at 10.56 µm, resonant with a strong absorption band of sulfur hexafluoride (SF6), which was selected as a leak tracer. The minimum detectable concentration of the QEPAS sensor is 2.7 parts per billion with an integration time of 1 s, corresponding to a sensitivity of leak flows in the 10-9 mbar∙l/s range, comparable with state-of-the-art leak detection techniques.

Hollow core waveguide as mid-infrared laser modal beam filter

A novel method for mid-IR laser beam mode cleaning employing hollow core waveguide as a modal filter element is reported. The influence of the input laser beam quality on fiber optical losses and output beam profile using a hollow core waveguide with 200 μm-bore size was investigated. Our results demonstrate that even when using a laser with a poor spatial profile, there will exist a minimum fiber length that allows transmission of only the Gaussian-like fundamental waveguide mode from the fiber, filtering out all the higher order modes. This essentially single mode output is preserved also when the waveguide is bent to a radius of curvature of 7.5 cm, which demonstrates that laser mode filtering can be realized even if a curved light path is required.

Low-Loss Coupling of Quantum Cascade Lasers into Hollow-Core Waveguides with Single-Mode Output in the 3.7–7.6 μm Spectral Range

We demonstrated low-loss and single-mode laser beam delivery through hollow-core waveguides (HCWs) operating in the 3.7–7.6 μm spectral range. The employed HCWs have a circular cross section with a bore diameter of 200 μm and metallic/dielectric internal coatings deposited inside a glass capillary tube. The internal coatings have been produced to enhance the spectral response of the HCWs in the range 3.5–12 µm. We demonstrated Gaussian-like outputs throughout the 4.5–7.6 µm spectral range. A quasi single-mode output beam with only small beam distortions was achieved when the wavelength was reduced to 3.7 μm. With a 15-cm-long HCW and optimized coupling conditions, we measured coupling efficiencies of >88% and transmission losses of <1 dB in the investigated infrared spectral range.

Single-tube on beam quartz-enhanced photoacoustic spectrophones exploiting a custom quartz tuning fork operating in the overtone mode

We report here on the realization of a single-tube on-beam quartz-enhanced photoacoustic (QEPAS) spectroscopy sensor employing a custom-made quartz tuning fork (QTF) with a large prong spacing. The prongs of the QTF have been designed in order to provide a quality factor twice higher when the QTF operates in the first overtone flexural mode than in the fundamental mode. The influence of the microresonator tube on the main parameters characterizing the sensing performance of the QEPAS spectrophone, including the quality factor, the magnitude of the QEPAS signal and the associated background noise was investigated in detail.

Low-loss and single-mode tapered hollow-core waveguides optically coupled with interband and quantum cascade lasers

Abstract. We report single-mode midinfrared laser beam delivery through a 50-cm-long tapered hollow-core waveguide (HCW) having bore diameter linearly increasing from 200 to 260  μm. We performed theoretical calculations to identify the best HCW-laser coupling conditions in terms of optical losses and single-mode fiber output. To validate our modeling, we coupled the HCW with an interband cascade laser and four quantum cascade lasers with their emission wavelengths spanning 3.5 to 7.8  μm, using focusing lenses with different focal lengths. With the best coupling conditions, we achieved single-mode output in the investigated 3.5 to 7.8  μm spectral range, with minimum transmission losses of 1.27 dB at 6.2  μm.

Innovative quartz enhanced photoacoustic sensors for trace gas detection

A detailed analysis of the quality factor, the resonance frequency and the electrical resistance of custom quartz tuning forks (QTFs) having different geometrical parameters is reported. We implemented custom QTFs in a quartz enhanced photoacoustic sensor targeting water vapor detection and compared the fundamental and first overtone flexural modes gas sensor performances.

Compact and low-noise quartz-enhanced photoacoustic sensor for sub-ppm ethylene detection in atmosphere

We report on the development of a gas sensor system based on quartz-enhanced photoacoustic spectroscopy (QEPAS) for the detection of trace levels of ethylene using a quantum cascade laser operating at ~ 10.3 μm. To realize a compact sensor architecture, a dedicated acoustic detection module was designed and implemented. The module includes a QEPAS spectrophone, composed of a quartz tuning fork, a micro-resonator tube and a low-noise pre-amplifier chip for the signal readout. The volume of the ADM is ~30 cm3. A minimum detection limit of 30 part-per-billion in concentration was obtained with a data acquisition time of 10 s.

Tapered hollow-core fibers providing single-mode output in the 3.5um-7.8um spectral range

We report on an investigation of the optical performances of tapered hollow-core waveguides (HCWs) operating in the mid-IR spectral range. The HCW core diameter linearly increases over a fiber length of 50 cm. Five laser sources emitting in the range 3.5-7.8 μm were coupled with the HCWs by using a lens to focus the collimated beam at the waveguide entrance. We demonstrated single-mode circular-symmetric Gaussian-like beam profiles at HCW exit in all investigated spectral range, when operating with an HCW having core diameter varying from 200 μm to 260 μm, with slight variations in the measured propagation losses.