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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. [ABSTRACT FROM AUTHOR]

We report a detailed experimental and theoretical analysis of the influence of quartz tuning fork (QTF) dimensions on the main physical parameters controlling the QTF performance, namely, the quality factor Q , the resonance frequency, the fork stiffness, the spring constant, and the electrical resistance. Two different gold contact patterns were also compared. As a general trend, the QTF performance in terms of Q and electrical conductance values improves at increasing both the crystal thickness T and prong thickness w , while decreasing the prongs length L p . However, since the QTF resonance frequency f 0 is proportional to T / L p 2 , a trade-off should be found in order to keep f 0 < 40 kHz, i.e., well below the typical values of non-radiative relaxation rate of a targeted gas absorption lines. [ABSTRACT FROM AUTHOR]

We report on a quartz-enhanced photoacoustic sensor (QEPAS) based on a custom-made quartztuning fork (QTF) to operate in both the fundamental and the first overtone vibrational mode resonances. The QTF fundamental mode resonance falls at ~3kHz and the first overtone at ~18kHz. Electrical tests showed that the first overtone provides a higher quality factor and increased piezoelectric current peak values, with respect to the fundamental flexural mode. To evaluate the QTF acousto-electric energy conversion efficiency, we operated the QEPAS in the near-IR and selected water vapor as the target gas. The first overtone resonance provides a QEPAS signal-to-noise ratio ~5 times greater with respect to that measured for the fundamental mode. These results open the way to employing QTF overtone vibrational modes for QEPAS based trace gas sensing. [ABSTRACT FROM AUTHOR]

We report on a spectroscopic technique named intracavity quartz-enhanced photoacoustic spectroscopy (I-QEPAS) employed for sensitive trace-gas detection in the mid-infrared spectral region. It is based on a combination of QEPAS with a buildup optical cavity. The sensor includes a distributed feedback quantum cascade laser emitting at 4.33 μm. We achieved a laser optical power buildup factor of ~500, which corresponds to an intracavity laser power of ~0.75 W. CO2 has been selected as the target molecule for the I-QEPAS demonstration. We achieved a detection sensitivity of 300 parts per trillion for 4 s integration time, corresponding to a noise equivalent absorption coefficient of 1.4s10-8 cm-1 and a normalized noise-equivalent absorption of 3.2s10-10 W cm-1 Hz-1/2. [ABSTRACT FROM AUTHOR]

We report on the optical coupling between hollow core waveguides and external cavity mid-IR quantum cascade lasers (QCLs). Waveguides with 1000 μm bore size and lengths ranging from 2 to 14 cm, with metallic (Ag)/dielectric (AgI or polystyrene) circular cross-section internal coatings, have been employed. Our results show that the QCL mode is perfectly matched to the hybrid HE waveguide mode, demonstrating that the internal dielectric coating thickness is effective to suppress the higher losses TE-like modes. Optical losses down to 0.44 dB/m at 5.27 μm were measured in Ag/polystyrene-coated waveguide with an almost unitary coupling efficiency. [ABSTRACT FROM AUTHOR]

A quartz enhanced photo-acoustic sensor employing a single-mode quantum cascade laser emitting at 3.93 Terahertz (THz) is reported. A custom tuning fork with a 1 mm spatial separation between the prongs allows the focusing of the THz laser beam between them, while preventing the prongs illumination. A methanol transition with line-strength of 4.28 × 10-21 cm has been selected as target spectroscopic line. At a laser optical power of ∼ 40 μW, we reach a sensitivity of 7 parts per million in 4s integration time, corresponding to a 1σ normalized noise-equivalent absorption of 2 × 10-10 cm-1W/Hz½. [ABSTRACT FROM AUTHOR]

[Display omitted] • All fiber coupled QEPAS sensor without free space optics. • Mid-IR ICLs fiber coupled using indium fluoride glass fibers. • Plug and play connection to sensing module for extreme versatility. • Greenhouse gases detection (CH 4 , CO 2 , and NO) with multiple ICLs and a single ADM. • Sub-ppm detection limits for real time detection: suitable for on-field monitoring. Infrared laser-based gas sensors are mature to take the leap from laboratory prototype to outdoor operation. Considering the demand of high robustness and compactness, the reliability of the optical alignment in a sensor is the top priority. This paper proposes a solution designing an optical system embedded within a sealed metallic cylinder containing an aspheric micro-lens to couple a single-mode interband cascade laser with an indium fluoride glass fiber. The fiber output is plug & play connected to an acoustic detection module of a quartz-enhanced photoacoustic sensor (QEPAS) equipped with a fiber port, avoiding the use of any free-space optics, from the source to the detection module. To demonstrate the operability, three all-fiber-coupled QEPAS sensors were realized for detection of CH 4 , CO 2 and NO reaching sub-ppm ultimate detection limits with a signal integration time of 100 ms. [ABSTRACT FROM AUTHOR]

We report on the realization of a quartz-enhanced photoacoustic (QEPAS) sensor for measurement of HS gas traces. A distributed feedback diode laser working at 2.64 µm wavelength has been coupled to an acoustic detection module composed of a quartz tuning fork and a micro-resonator system, and the QEPAS signal has been optimized in terms of gas sample pressure and laser frequency modulation depth. The sensor shows a very good linearity with respect to the HS concentration. We performed an Allan-Werle variance analysis to investigate the sensor long-term stability, and we reached a detection limit of four parts per million for 1-s integration time and 500 parts per billion in 60-s integration time. The realized QEPAS sensor represents a good compromise between performance and handiness, in view of a fully portable device. [ABSTRACT FROM AUTHOR]