All-fiber SERS sensing with a depressed double cladding fiber probe embedded in a microfluidic chip.

A fiber surface-enhanced Raman scattering (SERS) probe based on the depressed double cladding fiber (DDCF) is demonstrated. Due to the depressed cladding, the optical field distribution of the DDCF is more approachable to the outer surface than that of a no core fiber without depressed cladding, improving the excitation efficiency for generating Raman scattered light. The cladding diameter of the DDCF is only 80 µm, which is smaller than the core diameter (105 µm) of the input multimode fiber (MMF). This small cladding diameter ensures a well collection coupling from the DDCF to the MMF. The impact of the DDCF length on the performance of the probe is investigated. Raman spectra of Rhodamine 6G (R6G) are measured using the DDCF-based SERS probe, which is embedded in a microfluidic chip. The all-fiber optical path configuration realizes an all-fiber Raman detection system that is stable, portable, and convenient to operate. Such results confirm the applicability of the DDCF-based fiber probe in the all-fiber SERS detection area and indicate that the proposed SERS probe renders a new scheme for optimal design of the fiber SERS probe structure.