The all-in-one optical fiber spectrometer combines a compact microscale design with performance that matches traditional laboratory-based systems. These miniaturized spectroscopy systems, capable of detecting trace concentrations at parts-per-billion (ppb) levels, are crucial for fields such as environmental monitoring, industrial process control, and biomedical diagnostics.
However, conventional bench-top spectroscopy systems are often too large and complex for confined spaces. Traditional laser spectroscopy techniques rely on bulky components like light sources, mirrors, detectors, and gas cells, making them unsuitable for applications requiring compactness and precision, such as intravascular diagnostics.
In a groundbreaking study published in Advanced Photonics, researchers from China introduced a miniaturized all-fiber photoacoustic spectrometer (FPAS). This innovative system is capable of detecting trace gases at ppb levels and analyzing nanoliter-scale samples with millisecond response times, making it ideal for continuous intravascular gas analysis.
"We aimed to overcome the challenge of reducing the photoacoustic spectrometer's size while maintaining its high performance, particularly for minimally invasive applications such as intravascular diagnostics and lithium battery health monitoring," said Professor Bai-Ou Guan from Jinan University, the study's lead author.
Leveraging Photoacoustic Spectroscopy
Traditional laser spectroscopy systems, often based on open-path configurations, face challenges when scaled down, particularly in sensitivity. In contrast, the FPAS operates using photoacoustic spectroscopy (PAS), a technique that detects sound waves produced by gas molecules when excited by modulated light.
Rather than relying on bulky components like resonant gas cells or large microphones for amplification and sensitivity, the FPAS uses an integrated laser-patterned elastic membrane within an optical fiber tip. This microscale Fabry-Perot (F-P) cavity, created with a section of silica capillary, acts as a sound-hard boundary to amplify the acoustic waves generated by the gas molecules. This local amplification compensates for the reduced size of the membrane, ensuring that the photoacoustic response remains unaffected by the device's small footprint.
Additionally, both the excitation and detection light beams are delivered through the same optical fiber, eliminating the need for bulky free-space optics.
Compact Yet Powerful
Measuring just 60 micrometers in length and 125 micrometers in diameter, the FPAS is remarkably compact. Despite its size, it achieves a detection limit for acetylene gas as low as 9 ppb, comparable to much larger traditional laboratory systems. The short cavity length enables ultrafast measurements, with response times as quick as 18 milliseconds—2 to 3 orders of magnitude faster than conventional photoacoustic spectroscopy systems.
The FPAS has been successfully used for real-time monitoring of carbon dioxide (CO2) concentrations in flowing gas, detecting fermentation in yeast solutions with sample volumes as small as 100 nanoliters, and tracking dissolved CO2 levels in rat blood vessels by inserting the FPAS into the tail vein. "The spectrometer can monitor CO2 levels in hypoxic (low oxygen) and hypercapnic (high CO2) conditions, offering real-time intravascular blood gas monitoring without needing blood sample collection," said Associate Professor Jun Ma from Jinan University.
Furthermore, the optical fiber can be connected to a low-cost distributed-feedback laser source and integrated into existing fiber-optic networks, making this system a flexible, cost-effective solution for spectroscopy.
With its small size, high sensitivity, and low sample volume requirement, the miniaturized spectrometer provides laboratory-level precision in a microscale probe. Its potential applications include continuous intravascular blood gas monitoring, minimally invasive health assessment of lithium-ion batteries, and remote detection of explosive gas leaks in confined spaces.
