Solid-State Laser Intra-Cavity Photothermal Sensor (SLIPS) for Gas Detection with Parts-Per-Billion Sensitivity ^†

We report a novel, miniaturized gas sensor configuration with ppbv (parts-per-billion by volume) sensitivity, where detection of the gas sample concentration is realized inside a Nd:YVO₄/YVO₄/Air-Gap structure (2 × 2 × 14 mm³) of the double-beam, monolithic diode-pumped solid-state laser (DPSSL) resonator operating at 1064 nm. Both generated probe and reference beams are passed through an ultra-compact sensing volume (4 μL) of the air-gap section filled with gas molecules. Simultaneously, an auxiliary laser beam is targeted on the absorption line of a measured gas sample and focused on a 1064 nm probe beam only. Due to the absorption effect, excited gas molecules are heated locally, resulting in a negligible change in a gas refractive index (RI), which is inherent to the photothermal effect (PT). Hence, the PT-induced variations of the gas RI inside the laser resonator are modulating the optical path-length of the probe beam, which resulted in a significant optical frequency shift of the probe beam against the reference one. The optical frequency changes were measured by applying the heterodyne detection technique, where both 1064 nm beams were coupled onto the near-infrared (near-IR) high-speed photodiode (PD), resulting in a beat note signal readout down-converted into the radio-frequency (RF) domain. The RF mixer was used to shift the beat note in frequency accordingly to the frequency modulation (FM) demodulator range. The demodulator converts the beat note frequency changes into a proportional voltage signal. To provide better gas sensor properties, a typical wavelength modulation spectroscopy (WMS) technique was additionally used. The solid-state laser intra-cavity photothermal sensor (SLIPS) is a unique approach to gas spectroscopy, which provides tens of ppbv sensitivity, more than 5000 signal-to-noise (SNR) ratio, baseline-free measurements, miniature, versatile and non-complex sensor setup based on inexpensive DPSSL technology. The SLIPS has no limitation in terms of the excitation wavelength because only one near-IR detector for signal retrieval is needed.