Tiny Fiber Photoacoustic Spectrometer Enables Fast, Minimally Invasive Analysis

A miniaturized, all-in-one optical fiber spectrometer from Jinan University can realize a parts-per-billion (ppb)-level detection limit for trace gas sensing, with no need for a resonant gas cell. The microscale, high-performance, all-in-one fiber photoacoustic spectrometer (FPAS) can analyze subnanoliter-sized samples with a response time in milliseconds (ms).

With its small footprint, high sensitivity, and low sample volume requirement, the FPAS is equipped to deliver laboratory-level precision in a microscale probe format. This could be beneficial for applications ranging from environmental monitoring and industrial process control to biomedical diagnostics.

The fiber photoacoustic spectrometer (FPAS) enables continuous intravascular gas monitoring. Courtesy of Jun Ma/Jinan University.

Existing bench-top spectroscopy systems are too large, complex, and impractical for narrow-space use. Moreover, traditional laser spectroscopy techniques, with their reliance on bulky components, are unsuitable for minimally invasive applications, like intravascular diagnosis.

“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,” professor Bai-Ou Guan said.

The FPAS consists of a single optical fiber, a silica capillary, and an elastic membrane. The key components of the spectrometer are a photoacoustic gas cell and an optical microphone, which are both incorporated into a single optical fiber tip.

The researchers integrated a laser-patterned elastic membrane into the fiber tip with a section of silica capillary to construct a microscale Fabry-Perot (F-P) cavity. The F-P cavity, with a diameter of 125 μm and a length of 60 μm, serves as both the optical microphone and the gas cell. The thin polymer membrane was used to build the fiber F-P microphone, and the silica capillary was used to build the gas cell.

The silica cavity acts as a sound-hard boundary, tightly confining and amplifying the locally generated acoustic waves. This local acoustic amplification compensates for the sensitivity loss caused by the reduction in the membrane diameter, and results in a size-independent photoacoustic response.

The F-P cavity shrinks the device footprint by more than 2 orders of magnitude compared with previous systems, and achieves a ppb-level detection limit without a bulky gas cell. The FPAS device delivers the pump and probe light beams for the excitation and detection of the photoacoustic signal through the same fiber, so there is no need to use free-space optics for light delivery.

With high-precision fiber optic interferometry to demodulate the membrane deflection of the microphone induced by the photoacoustic waves, the microscale FPAS demonstrates a detection limit of about 9 ppb for acetylene gas, which is almost as sensitive as a benchtop, lab-based system.

The miniaturized all-fiber photoacoustic spectrometer consists of a single optical fiber, a silica capillary, and an elastic membrane. The fiber's end facet and the membrane form a Fabry-Perot cavity. When gas molecules absorb pump light, they generate acoustic waves, which cause the membrane to vibrate. This vibration changes the intensity of reflected probe light, which is then analyzed to detect trace gas concentrations. Courtesy of J. Ma et al., doi: 10.1117/1.AP.6.6.066008.

The short cavity length of the device enables it to take ultrafast measurements. The FPAS exhibits response times as rapid as 18 ms, as well as a high spatial resolution of about 160 μm, an improvement of spatial and temporal resolution of more than 2 to 3 orders of magnitude compared to conventional photoacoustic spectroscopy systems.

The device needs only a subnanoliter sample volume, which reduces the required sample volume by 3 to 4 orders of magnitude compared to traditional systems. The small sample volume is suited to routine, real-time in situ trace gas measurement and could inspire new applications for 2D gas flow concentration mapping and in vivo intravascular blood gas monitoring.

The researchers used FPAS to monitor real-time carbon dioxide (CO₂) concentrations in flowing gas and to detect fermentation in yeast solutions. They tracked dissolved CO₂ levels in rat blood vessels in vivo by inserting the FPAS into the tail vein via a syringe.

“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,” professor Jun Ma said.

The optical fiber used to build FPAS can be connected to a low-cost distributed-feedback laser source and integrated with existing fiber optic networks, making the system a cost-effective, compact, flexible solution for spectroscopy.

In addition to continuous intravascular blood gas monitoring, potential applications for FPAS include minimally-invasive health assessment of lithium-ion batteries and remote detection of explosive gas leakage in extremely narrow spaces.

The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.6.6.066008).