We have developed a compact background and fluorescence free endoscopic Raman probe using shifted-excitation Raman difference spectroscopy (SERDS) with an optical fibre featuring a negative curvature excitation core and a coaxial ring of multimode collection cores. The probe consists of a single optical fibre with an outer diameter below 0.25 mm packaged in sub-millimetre tubing making it compatible with the working channels of standard endoscopic tools. The light in the fibre is guided in air and therefore interacts little with silica, enabling an almost background-free transmission of the excitation light and collection using a single optical fibre. In addition, we used the SERDS technique and a tunable 785 nm laser to separate the fluorescence and the Raman spectrum from highly fluorescence samples, demonstrating the suitability of the probe for biomedical applications.
We present a spectroscopic system and an optical fibre probe which enable the full exploitation of the temporal evolution and spectral information of a weak Raman signal against background fluorescence and intrinsic fibre Raman. The system consists of a single multimode fibre and a CMOS single-photon avalanche diode (SPAD) line sensor capable of resolving and histogramming the arrival times of photons for 256 pixels simultaneously, offering improved signal to background compared to a non-time resolved measurement modality. The capabilities of the system are tested for intrinsic Raman standards such as cyclohexane and for pH sensing with functionalised gold nanoshells exploiting surface enhanced Raman scattering (SERS). The nanoshells are functionalised with the pH responsive 4-mercaptobenzoic acid (MBA) enabling demonstration of wide range pH sensing with low excitation power (< 1 mW) and short acquisition times (10 s), achieving a measurement precision of ± 0.07 pH units.
In vivo fibre optic fluorescence-based sensing is the use of synthesised fluorophores which interrogate the local environment via variation in their fluorescence emission, addressed through an optic fibre. However, the emission intensity is influenced by intrinsic factors such as photobleaching, quantitative factors like concentration dependency and background signals from autofluorescence of tissue and the delivery optical fibre. Many of these problems can be addressed by using time-resolved spectroscopy which measures variations in the fluorescent lifetime. We present a versatile fibre-based time-resolved spectrograph based on a CMOS SPAD line sensor capable of acquiring time and spectral resolved fluorescent lifetime data in a single measurement exploiting the time-correlated single photon counting (TCSPC) technique. It is shown that these TCSPC histograms enable the differentiation between autofluorescence of tissue and synthesized fluorophores, as well as the removal of unwanted fibre background through post-processed time-gating. As a proof-of-principle application the pH- dependent changes in fluorescent lifetime of 5-carboxyuorescein (FAM) are measured.
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