What Are the Applications of Hollow Core Fiber Optical Sensors

Applications for hollow core fiber (HCF) optical sensors include lithium-ion battery research and development (R&D), robotic arms, civil engineering infrastructure health monitoring, and biosensing. Some of the characteristics of HCF optical sensors include small size, non-conductivity, chemical stability and high dynamic range.

In activities such as lithium-ion R&D, HCF can be used to implement real-time Raman spectroscopy to provide a structural fingerprint to identify and track changes in the molecular structure of the electrolyte as the battery is charged and discharged. The method repeatedly extracts sub-microliter (sub-µL) samples of electrolyte from working Li-ion batteries. The sample is refilled into the cell and another sample is extracted. The process is continued as needed to track changes in electrolyte chemistry as the battery cycles.

The physical setup for HCF Raman analysis is shown in Figure 1. In the figure, a 785 nm continuous-wave laser is filtered and fed into the core of the single-ring HCF on the left side of (a) The left face of the HCF is embedded in a microfluidic chamber, which has a sapphire window as shown in the inset in (b) The HCF is connected to a syringe pump shown at the top of (a), which is used to control the sampling of the electrolyte. The right end of the HCF is embedded in the cell under study as shown in inset (d).The backscattered Raman signal in the HCF is reflected from the beam splitter and analyzed by the spectrometer. Other optical components include a bandpass filter (F1) and a trap filter (F2). The inset (c) shows a scanning electron microscope (SEM) image of the HCF with an outer diameter of 174 µm and a core diameter of 36 µm on the left, and the Raman signal detected by the spectrometer CCD camera on the right.

Figure 1. Implementation of the HCF Raman analysis of Li-in electrolyte during charging and discharging. 

Multispectral Biosensing

Common optical biosensors use a single resonance feature in the reflection-transmission-scattering spectrum and recognize resonance shifts caused by changes in the refractive index (RI) of the analyte and its concentration. This type of optical biosensor has the advantage of high sensitivity and many benefits. However, it cannot cost-effectively and rapidly provide detailed fingerprints of complex biological samples or enable real-time monitoring.

To address these needs, we have developed in-fiber multispectral optical sensing (IMOS) for real-time analysis of liquid biological samples. This new sensing technique relies on detecting spectral shifts of maxima and minima in the transmission spectrum as the analyte flows through the fiber. A broadband halogen lamp is used for the light source to provide the desired white light spectrum.

Instead of standard HCFs, hollow-core microstructured optical fibers (HC-MOFs) are used, which can sense liquid analytes by monitoring changes in transmission characteristics. HC-MOFs are also capable of carrying larger sample volumes for measuring light-analyte interactions, thereby increasing refractive index sensitivity (RIS).

Individual biological fluids and their concentrations can be accurately identified due to the resonance that occurs as a result of the interaction with the HC-MOF capillary wall, and the spectral position of the resonance is uniquely correlated with the biological analyte. Detections can be captured using the CCD camera within the spectrometer.

Measuring curvature and temperature

To simultaneously monitor curvature and temperature, a sensor based on hollow-core Bragg fiber (HCBF) has been developed, which is compact, inexpensive, sensitive, and electromagnetically immunity (Figure 2). Application areas are expected to include robotic arms, civil engineering infrastructure health monitoring, and aerospace composite structure monitoring.

Figure 2. Cross-section of the HCBF (a) and the refractive index distribution along the fiber radial direction. ne and nair are the effective RIs of the cladding, and the air core, respectively, and d is the thickness of the cladding.

The transmission spectrum produced by the four-layer Bragg structure reflects its cross-sensitivity to temperature and curvature. The four-layer Bragg structure forms a 2×2 matrix that can be detuned for temperature/curvature cross-sensitivity based on a specific resonant dip strength.

Summary

HCF optical sensors are a versatile technology. In addition to the basic HCF, HC-MOF and multilayer HC Bragg fibers have been developed for specific measurement functions in the field of biological and structural monitoring.

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