A resonant photoacoustic cell intended for laser-spectroscopy gas sensing is represented. This cell is a miniature imitation of a macro-scale banana-shaped cell developed previously. The parameters, which specify the cavity shape, are chosen so as not only to provide optimal cell operation at a selected acoustic resonance but also to reduce substantially the cell sizes. A miniaturized prototype cell (the volume of acoustic cavity of ~ 5 mm 3 ) adapted to the narrow diffraction-limited beam of near-infrared laser is produced and examined experimentally. The noise-associated measurement error and laser-initiated signals are studied as functions of modulation frequency. The background signal and the useful response to light absorption by the gas are analyzed in measurements of absorption for ammonia in nitrogen flow with the help of a pigtailed DFB laser diode oscillated near a wavelength of 1.53 µm. The performance of prototype operation at the second longitudinal acoustic resonance (the resonance frequency of ~ 32.9 kHz, Q-factor of ~ 16.3) is estimated. The noise-limited minimal detectable absorption normalized to laser-beam power and detection bandwidth is ∼ 8.07·10 -8 cm -1 W Hz -1/2 . The amplitude of the background signal is equivalent to an absorption coefficient of ~ 2.51·10 -5 cm -1 . Advantages and drawbacks of the cell prototype are discussed. Despite low absorption-sensing performance, the produced miniaturized cell prototype shows a good capability of gas-leak detection.

Abstract. A photoacoustic cell intended for laser detection of trace gases is represented. The cell is adapted so as to enhance the gas-detection performance and, simultaneously, to reduce the cell size. The cell design provides an efficient cancellation of the window background (a parasite response due to absorption of laser beam in the cell windows) and acoustic isolation from the environment for an acoustic resonance of the cell. The useful photoacoustic response from a detected gas, window background and noise are analyzed in demonstration experiments as functions of the modulation frequency for a prototype cell with the internal volume ∼ 0.5 cm 3 . The minimal detectable absorption for the prototype is estimated to be ∼ 1. The miniaturized spectroscopic hardware of trace-gas analysis is a promising field of application for the photoacoustic technique. The principle of the technique is based on measuring the amplitude and phase for the acoustic pressure oscillation (the so-called photoacoustic response) arising due to absorption of a modulated laser beam by molecules of gas inside a photoacoustic cell. The photoacoustic detection is realized with an enhanced sensitivity if the modulation frequency coincides with an acoustic resonance of the cell. Therefore, the modulation frequency should be correlated with the cell-resonator sizes: the

A gas sensor integrating a pulsed external-cavity quantum-cascade laser and a photoacoustic (PA) cell placed inside the laser cavity is described. The laser design allows a continuously tunable and mode-hop-free operation with an estimated instrument-limited linewidth of 0.063  cm^-1 in Littrow or Littman-Metcalf cavity configurations. Due to higher intracavity power levels, the PA signal for the cell located within the laser cavity is 80-320 times stronger than the signal for the same cell placed outside the cavity. The sensor shows a good capability to identify individual absorption lines of isotopic H₂O16, H₂O8, and H₂O17 species within a narrow spectral interval from 1814 to 1821  cm^-1 at natural isotope abundances.

A resonant photoacoustic cell capable of detecting the traces of gases at an amplitude-modulation regime is represented. The cell is designed so as to minimize the window background for the cell operation at a selected acoustic resonance. A compact prototype cell (the volume of acoustic cavity of ~ 0.2 cm 3 , total cell weight of 3.5 g) adapted to the narrow diffraction-limited beam of near-infrared laser is produced and examined experimentally. The noise-associated measurement error and laser-initiated signals are studied as functions of modulation frequency. The background signal and useful response to light absorption by the gas are analyzed in measurements of absorption for ammonia traces in nitrogen flow with the help of a pigtailed DFB laser diode operated near a wavelength of 1.53 µm. The performance of absorption detection and gas-leak sensing for the prototype operated at the second longitudinal acoustic resonance (the resonance frequency of ~ 4.38 kHz, Q-factor of ~ 13.9) is estimated. The noise-equivalent absorption normalized to laser-beam power and detection bandwidth is ~ 1.4410 -9 cm -1 W Hz -1/2 . The amplitude of the window-background signal is equivalent to an absorption coefficient of ~ 2.8210 -7 cm -1 .