Progress has been made in the development of anti resonant hollow core fiber Raman probes with low background noise at Shanghai Optics and Machinery Institute

Recently, the research team of the Special Glass and Fiber Research Center of the Advanced Laser and Optoelectronic Functional Materials Department of the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, aimed at the demand for in-situ detection of Raman signals, expanded the functions of the laboratory commercial Renishaw Invia confocal micro Raman spectrometer by using two independently designed and prepared anti resonant hollow core fibers (AR-HCFs) and external optical path modules, and added the in-situ detection function. The related achievements were published in Biomedical Optics Express under the title "In site background free Raman probe using double clamping anti resonant hollow core fibers".

Traditional quartz solid core optical fibers are widely used as probes for Raman detection due to their low loss and wide transmission window, making them ideal media for optical signals. When applied, although it can overcome the limitations of sample shape, size, and position, the interaction between the quartz glass material and the pump laser will generate a very strong background noise signal, which often masks the Raman spectral information of the test sample. In previous research reports, the mainstream solution was to use multiple fiber probes, utilizing different fibers to conduct excitation light and collect signal light. But this solution also requires adding optical components such as filters at the far end of the fiber optic, which not only reduces the efficiency of signal collection, but also increases the volume of the probe.

Researchers used the stack and draw method to manufacture two different double clad AR-HCFs, with cross-sections shown in Figure 1. They can mainly constrain the laser to conduct in the hollow core, greatly reducing the overlap between the optical field and the quartz material of the fiber itself, thereby greatly suppressing quartz background noise. After performance testing, the two fiber probes can achieve about two orders of magnitude of quartz background noise suppression compared to traditional solid core quartz fibers. Both AR-HCFs have been specially designed to achieve low loss in the visible and near-infrared bands, and have a larger numerical aperture (NA) in the outer layer (the NA of the outer layer is greater than 0.2, about ten times that of the fiber core). The characteristic of this work is to use only one fiber as the probe for Raman detection, and to combine the probe with the commercial Renishaw Invia confocal microscopy Raman spectrometer using a specially designed external optical path module, as shown in Figure 2. The module is connected to the original objective interface of the spectrometer, which can couple the excitation light emitted internally to AR-HCFs, and also transmit the Raman signal collected by the fiber optic probe back to the spectrometer for detection and analysis. While leveraging the high detection accuracy of the instrument, it can also expand its in-situ detection capabilities. The feasibility of the scheme was also verified by using probes to detect some solid and liquid samples, such as in-situ detection of ABS plastic, as shown in Figure 3. The research results are expected to have broader application prospects in fields such as environmental monitoring and biomedicine.

The electron microscope end face photos of two anti resonant hollow core optical fibers in Figure 1 are shown in (a) and (b), respectively, while (c) and (d) show photos of both taken by illuminating the back of the optical microscope.

Figure 2 Schematic diagram of Raman sensing scheme optical path.

Figures 3 (a) and (b) show the Raman spectra of two types of anti resonant hollow core fibers used as probes for detecting ABS plastic. The orange curve is obtained from the probe measuring the sample, the blue curve is the background signal of the probe itself, and the yellow curve is the spectrum directly measured by the Renishaw Invia confocal microscopy Raman spectrometer.

Source: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences