Laser spectroscopy outperforms electrochemical and semiconductor gas sensors in selectivity and environmental survivability. However, the performance of the state-of-the-art laser sensors is still insufficient for many high precision applications. Here, we report mode-phase-difference photothermal spectroscopy with a dual-mode anti-resonant hollow-core optical fiber and demonstrate all-fiber gas (acetylene) detection down to ppt (parts-per-trillion) and <1% instability over a period of 3 hours. An anti-resonant hollow-core fiber could be designed to transmit light signals over a broad wavelength range from visible to infrared, covering molecular absorption lines of many important gases. This would enable multi-component gas detection with a single sensing element and pave the way for ultra-precision gas sensing for medical, environmental and industrial applications.

Gas evolution is fundamentally problematic in rechargeable batteries, and may lead to swelling, smoking, and device‐level failure. In laboratories, monitoring gas evolution can help understand dynamic chemical events inside battery cells, such as the formation of solid‐electrolyte interphases, structural change of electrodes, and electrolyte degradation reactions. However, gassing in commercial batteries, discrete or continuous, is not monitored due to a lack of compatible sensing technologies. Here we describe the working principles of four real‐time gas monitoring technologies for lithium‐ion batteries. Gassing mechanisms and reaction pathways of five major gaseous species, namely H₂, C₂H₄, CO, CO₂, and O₂, are comprehensively summarized. Since pertinent progress has been made on the optical fiber‐based sensing of strain, pressure, and temperature of various battery cells recently, special emphasis has been given to fiber‐based laser spectroscopy for gas detection. The technical details of the fiber‐enhanced photothermal spectroscopy are compared with the four gas sensing technologies, and the commercialization possibilities are discussed. Owing to its small size, flexibility, and robustness, fiber‐based sensing technology can be compatible with almost all kinds of battery cells, showcasing their great potential in various applications. It is envisioned that gas‐event monitoring of rechargeable cells can be unlocked soon by utilizing fiber‐based gas spectroscopy.

Gaseous molecules are inherent byproducts of (electro-)chemical reactions in lithium-ion battery cells during both formation cycles and long-term operation. While monitoring gas evolution can help understand battery chemistry and predict battery performance, the complex nature of gas dynamics makes conventional mass spectrometry approaches insufficient for real-time detection. Here, we present a radically different methodology for operando analysis of gas evolution in lithium-ion batteries using optical fiber photothermal spectroscopy. By placing an optical hollow-core fiber inside the battery cell, evolved gases can rapidly diffuse into the hollow core of the fiber, enabling photothermal spectroscopy which precisely and selectively quantifies their concentrations without altering the internal operation of the cell. This approach facilitates identification of individual gaseous species, thereby allowing for further clarification (electro-)chemical reaction pathways. Collectively, we show that the evolution paths of C₂H₄ and CO₂ are closely associated with the formation of the solid electrolyte interphase, the selection of electrolyte salts, and the inclusion of specific additives. Significantly, we confirm for the first time the spontaneous formation of CO₂, which occurs exclusively in the presence of LiPF₆ salt. Beyond the scope of batteries, the methodology presented here offers substantial potential for broader applications, particularly in characterizing electrocatalytic processes, providing unmatched precision, accuracy, and scalability compared to existing analytical techniques.

Photothermal interferometry (PTI) with hollow core fibers (HCFs) have enabled highly sensitive spectroscopic gas sensors in an all-fiber format. Here we report remarkable improvement in the limit of detection of HCF-PTI, in terms of noise equivalent concentration (NEC), by exploiting the optical-phase-modulation amplifying (OPMA) effect of an HCF resonating cavity. By locking the wavelength of a 1550 nm probe laser to the resonance of a 10-cm-long HCF Fabry-Pérot cavity with a finesse of ∼700, OPMA of more than two orders of magnitude is achieved, which enables ultra-sensitive gas detection with large dynamic range. With 1654 nm, 1532 nm, and 761 nm pump lasers, we demonstrate detection of methane, acetylene, and oxygen with noise-equivalent-concentration of 15 parts-per-trillion (ppt), 2.7 ppt, and 0.56 parts-per-million (ppm), respectively. Further improvement in NEC is possible by use of a higher finesse cavity with a longer length of HCF. Extension of the technique to other gases, other types of phase or dispersion modulation-based sensors, and other optical resonating cavities is straightforward.

We report a mid-infrared (MIR) photothermal laser spectroscopy system for multi-component gas sensing with a single broadband anti-resonant hollow-core fiber gas cell. Three MIR pump lasers are used as the excitation sources to generate photothermal phase modulation and a near-infrared probe laser is employed to detect the photothermal phase modulation by using a Fabry-Perot interferometer configuration. Simultaneous detection of carbon monoxide (CO), carbon dioxide (CO ₂ ), ethylene (C ₂ H ₄ ), and ethane (C ₂ H ₆ ) are experimentally demonstrated using time-division multiplexing. A noise-equivalent concentration (NEC) of 57 parts-per-billion (ppb) for CO, 300 ppb for CO ₂ , 430 ppb for C ₂ H ₄ and 73 ppb for C2H6 are achieved with 1 s lock-in time constant. Allan deviation analysis reveals that the NEC can be reduced to 35, 120, 310, and 49 ppb for CO, CO ₂ , C ₂ H ₄ and C ₂ H ₆ , respectively, with an averaging time of 280 s. The dynamic range of the system is measured to be more than 4 orders of magnitude for all of the four gas components.