All-Fiber Raman Biosensor by Combining Reflection and Transmission Mode

A novel all-fiber on-line Raman cell based on metal (silver)-lined capillary is proposed, which could collect reflection and transmission Raman signal simultaneously. In our configuration, two optical fiber tips are inserted into both sides of the hollow-core metal-lined capillary to form a Raman cell. The other ends of fiber tips are closely bound together as a Sagnac loop and coupled with a Raman probe. Each fiber tip not only works in reflection mode, but also acts as the collection lens for the transmission mode of the other side's excitation. The hollow-core all-fiber structure could extend the interaction length between the samples and pump laser. Moreover, the Raman signal collection mechanism was significantly improved. With the combination of both Raman excitation modes, a Raman signal with two times of magnitude enhancement is observed in experiment and it agrees well with the theoretical analysis. We have also detected different concentrations (0.5-8 mg/mL) of antibiotic (Cefotaxime sodium, CTX), which proves that our proposed all-fiber and well-encapsulated Raman cell has tremendous potential for portable on-line rapid Raman biosensor.