Microfluidic liquid samples are crucial in both biomedical and chemical engineering, however, their in-situ identification and testing remain a major challenge. Here, we developed a non-contact photoacoustic (PA) sensor for in-situ and real-time identification and concentration detection of substances in a microfluidic environment. The ultrasonic vibration of the microfluidic samples was generated by a free-space pulsed laser pump and detected by a non-contact laser Doppler vibrometer. The time- and frequency-domain responses of the PA signals were employed to achieve substance identification and concentration detection. Principal component analysis (PCA) and adaptive boosting (AdaBoost) algorithms were employed to extract the frequency-domain characteristics, and an identification accuracy of over 98% was achieved. Chemical samples down to the nanomolar (nM) level can be detected by analyzing either the time-domain or frequency-domain response. The proposed sensor with non-contact and fast detection provides a promising platform for in-situ and real-time monitoring of samples in microfluidic systems.

Fiber optofluidic laser (FOFL) integrates optical fiber microcavity and microfluidic channel and provides many unique advantages for sensing applications. FOFLs not only inherit the advantages of lasers such as high sensitivity, high signal-to-noise ratio, and narrow linewidth, but also hold the unique features of optical fiber, including ease of integration, high repeatability, and low cost. With the development of new fiber structures and fabrication technologies, FOFLs become an important branch of optical fiber sensors, especially for application in biochemical detection. In this paper, the recent progress on FOFL is reviewed. We focuse mainly on the optical fiber resonators, gain medium, and the emerging sensing applications. The prospects for FOFL are also discussed. We believe that the FOFL sensor provides a promising technology for biomedical analysis and environmental monitoring.

A fiber-optic humidity sensor has been fabricated by coating a moisture sensitive polymer film to the fiber Bragg grating (FBG). The Bragg wavelength of the polyimide-coated FBG changes while it is exposed to different humidity conditions due to the volume expansion of the polyimide coating. The characteristics of sensors, including sensitivity, temporal response, and hysteresis, were improved by controlling the coating thickness and the degree of imidization during the thermal curing process of the polyimide. In the relative humidity (RH) condition ranging from 11.3% RH to 97.3% RH, the sensitivity of the sensor was about 13.5 pm/% RH with measurement uncertainty of ±1.5% RH.

A novel optical microfiber asymmetric Fabry-Perot interferometric (MAFPI) sensor is developed for simultaneous measurement of force and temperature. The MAFPI structure is formed by a weak fiber Bragg grating (FBG), a section of the microfiber, and a cleaved fiber end surface. The narrowband beam reflected from the low-reflectivity FBG and the broadband beam from the Fresnel reflection interfere lead to its unique sensing performance. The force sensing is performed by detecting the bending-loss induced fringe contrast changes, while the Bragg wavelength shift is employed for temperature measurement. Sensitivities of 9.8pm/℃ and 0.025dB/μN were obtained experimentally for temperature and force measurements, respectively.

The fiber Bragg grating (FBG) strain sensor is used in the oil and gas derricks for the health monitoring. The system consisted of the FBG strain sensors, optical interrogator and monitoring software. 64 sensors were installed on a derrick for loading and unloading tests. The results showed that the monitoring system had the advantages of the high accuracy, good repeatability, large capacity and real-time monitoring ability.