Ultrasensitive Gas Detection with a Miniaturized All-Fiber Photoacoustic Spectrometer

Chinese researchers presented a novel miniaturized all-fiber photoacoustic spectrometer (FPAS) in a study that was published in the journal Advanced Photonics. This device is perfect for continuous intravascular gas analysis because it can analyze nanoliter-sized samples with millisecond response times and detect trace gases at the ppb level.

Miniaturized spectroscopy devices capable of detecting trace concentrations at the parts-per-billion (ppb) level are essential for applications such as industrial process management, environmental monitoring, and biomedical diagnostics. However, traditional bench-top spectroscopic devices are too large, complex, and impractical for use in confined spaces.

Conventional laser spectroscopy methods also pose limitations for minimally invasive applications like intravascular diagnostics. These systems rely on bulky components such as light sources, mirrors, detectors, and gas cells to measure light absorption or scattering, making them unsuitable for such scenarios.

We attempted to address the significant challenge of shrinking the current photoacoustic spectrometer into a microscale size while preserving its high sensing performance, particularly for intravascular diagnosis and lithium battery health monitoring that require minimal invasiveness.

Bai-Ou Guan, Study Corresponding Author and Professor, Jinan University

The proposed FPAS overcomes these challenges using photoacoustic spectroscopy (PAS), which detects sound waves produced by gas molecules when excited by modulated light. Unlike traditional PAS systems that rely on large microphones or resonant gas cells for acoustic sensitivity, the FPAS integrates key components into a single, compact design.

The FPAS features a laser-patterned elastic membrane integrated with a section of silica capillary at the tip of a single optical fiber, forming a microscale Fabry–Perot (F–P) cavity. The silica capillary acts as a sound-hard boundary, efficiently focusing and amplifying acoustic waves toward the flexible membrane. This design compensates for sensitivity loss due to miniaturization, resulting in a size-independent photoacoustic response.

Both the stimulation and detection of the photoacoustic signal are achieved through the same optical fiber, eliminating the need for bulky free-space optics. The FPAS system is extremely compact, with the F–P cavity measuring just 60 micrometers in length and 125 micrometers in diameter. Despite its small size, the FPAS achieves a detection limit as low as 9 ppb for acetylene gas, comparable to larger laboratory spectrometers. Its short cavity length enables ultrafast response times of 18 milliseconds—two to three orders of magnitude faster than conventional PAS systems.

The FPAS demonstrated versatility in various applications. Researchers used it to monitor dissolved CO₂ levels in rat blood vessels in vivo via insertion into a tail vein, detect fermentation in yeast solutions with sample volumes as small as 100 nanoliters, and track real-time CO₂ concentrations in flowing gas streams.

The spectrometer effectively measured CO₂ levels under hypoxic (low oxygen) and hypercapnic (high CO₂) conditions, highlighting its potential for real-time intravascular blood gas monitoring without the need for blood sample collection.

Jun Ma, Associate Professor, Jinan University

The optical fiber can easily connect to a low-cost distributed-feedback laser source and integrate with existing fiber-optic networks, offering a compact, flexible, and cost-effective spectroscopic solution.

This miniaturized spectrometer combines small size, high sensitivity, and low sample volume requirements, delivering laboratory-level precision in a microscale probe format. Potential applications include continuous intravascular blood gas monitoring, minimally invasive evaluation of lithium-ion battery health, and remote detection of explosive gas leaks in confined spaces.

Journal Reference:

Ma, J., et. al. (2024) Microscale fiber photoacoustic spectroscopy for in situ and real-time trace gas sensing. Advanced Photonics. doi.org/10.1117/1.AP.6.6.066008