Shanghai Institute Of Optics And Precision Machinery (SIPM) Makes Progress in Anti-resonant Hollow-core Fiber Raman Probe With Low Background Noise

Recently, a research team from the Special Glass and Fiber Research Center, Department of Advanced Laser and Optoelectronic Functional Materials, Shanghai Institute of Optics and Precision Machinery, Chinese Academy of Sciences (SIPM) has made progress in the field of Raman probes for low background noise by using two types of self-developed dual-cladding anti-resonant hollow- core fibers (AR-HCF), which have been designed and fabricated in-house. core fibers (AR-HCFs) and external optical modules to expand the functionality of the commercial Renishaw Invia confocal microscopy Raman spectrometer and add in-situ detection. The results are summarized as "In-situ background-free Raman probe using double-cladding anti The results were published in Biomedical Optics Express under the title "In-situ background-free Raman probe using double-cladding anti-resonant hollow-core fibers".
Conventional quartz solid-core optical fibers are widely used as probes for Raman detection because they are ideal medium for optical signals due to their low loss and wide transmission window. Its application, although it can be free from the limitations of sample shape, size, and position, its own quartz glass material interacts with the pump laser to generate a very strong background noise signal, which tends to mask the Raman spectral information of the sample to be tested. In past research reports, the mainstream solution is to use a multi-fiber probe, which uses different fibers to conduct the excitation light and collect the signal light. However, this solution also requires the addition of optical components such as filters at the distal end of the optical fiber, which not only reduces the collection efficiency of the signal, but also increases the size of the probe.
The researchers fabricated two different double-cladding AR-HCFs using the stack-and-draw method, the cross sections of which are shown in Figure 1. Both of them can confine the laser light to be conducted mainly in the hollow core, which greatly reduces the overlap of the light field with the quartz material of the fiber itself, thus greatly suppressing the quartz background noise. After performance testing, the two fiber probes can achieve about two orders of magnitude of quartz background noise suppression compared with the 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 large numerical aperture (numerical aperture, NA) of the outer cladding (the NA of the outer cladding is greater than 0.2, which is about ten times that of the fiber core). This work is characterized by the use of only one optical fiber as a probe for Raman detection, and by the use of a specially designed external optical path module to realize the probe in conjunction with a commercially available Renishaw Invia confocal micro-Raman spectrometer, as shown in Fig. 2. The module is connected to the original objective lens interface of the spectrometer, which can couple the internally emitted excitation light into the AR-HCFs, and can also transmit the Raman signal collected by the fiber-optic probe back to the spectrometer for detection and analysis. It can also expand its function of in-situ detection while playing the high detection accuracy characteristics of the instrument. The feasibility of the scheme is also verified by the detection of some solid and liquid samples using the probe, such as the in-situ detection of ABS plastic, as shown in Figure 3. The results are expected to have broader application prospects in environmental monitoring, biomedical and other fields.

Fig. 1 Electron microscope end-face photos of two anti-resonant hollow-core optical fibers are shown in (a) and (b), respectively, while (c) and (d) show photos of the two taken by backlighting of an optical microscope, respectively.

Fig. 2 Schematic diagram of the Raman sensing scheme optical path.

Figure 3 Figures (a) and (b) show the Raman spectral results of two kinds of anti-resonance hollow-core optical fibers used as probes to detect ABS plastics, respectively, where the orange curve is obtained from the sample measured by the probe, the blue curve is the background signal of the probe itself, and the yellow curve is the spectra obtained from the sample measured by Renishaw Invia confocal microscope Raman spectrometer directly.