Recent advances in fiber optofluidic sensors
Gong Chaoyang, Zhang Chenlin, Gong Yuan, Rao Yunjiang     
Key Laboratory of Optical Fiber Sensing and Communications (Ministry of Education), University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, China

Overview: In this review, recent advances in optofluidic laser sensor and fiber optofluidic laser, as well as the passive fiber optofluidic sensors based on the optical force or photothermal effects are introduced.

Optofluidic laser (OFL) is an emerging technology that has been extensively investigated for biochemical detection. Due to the enhanced light-matter interaction, high sensitivity of OFL sensors have been demonstrated. We recently demonstrated a highly sensitive ion detection method using optofluidic laser based on Fabry-Perot cavity. A catalytic reaction that could be inhibited by the S2- ion was employed to produce a fluorescence gain material for optofluidic laser. The limit of detection by the OFL method was orders of magnitude lower than the fluorescence method.

Various types of microcavities including Fabry–Perot cavity, micro ring cavity and distributed feedback schemes have been investigated for optofluidic lasing. The lasing output is highly dependent on these microcavities. The mass productions with high repeatability are difficult for previous microcavities, making it hard to realize reproducible optofluidic laser. We introduced a novel fiber optofluidic laser with high reproducible microcavities. The optical fiber can be used as a ring resonator, providing optical feedback in the cross-section for lasing. Most importantly, thanks to the precise control of the fiber geometry by draw tower, the properties (including geometry, surface properties and thus Q-factor) of microcavities along the optical fiber are almost identical. The optical fiber can be mass produced with low cost and can be utilized to realize highly reproducible and disposable optofluidic laser.

Besides the fiber optofluidic laser, passive fiber optofluidic sensors based on the laser induced force and photo-thermal effects are introduced. The laser beam offers optical force at pico-Newton scale that is very sensitive to the ambient environments. By integrating the optical fiber with microfluidic chip, single microparticle can be trapped and high performance microfluidic flow rate detection was performed based on the force balance on the microparticle. Tunable optical manipulation of microparticle was also demonstrated.

Photo-thermal effect was also introduced by optical fiber into the microfluidic chip for sensing applications. Material with high absorption, including carbon nanotube or gold nanofilm, was coated on the fiber endface. Laser absorption near the fiber tip leads to a temperature rise. Thus microbubble was generated on the fiber tip based on the photo-thermal effect. By monitoring the generation and growth of microbubble, microfluidic parameters including flow rate, temperature, and concentration can be measured. The passive fiber optofluidic sensors have the advantages of flexible, easy to be integrated, multi-functional and reconfigurable.

Supported by National Natural Science Foundation of China (61575039) and the 111 Project (B14039)