A novel sensor structure has been proposed and experimentally investigated for simultaneous strain and temperature measurement. The structure is fabricated by weak power modulation of CO2 laser exposure on tapered long period fiber grating (LPFG). Compared with the transmission spectrum of the tapered LPFG, two peaks appear in the transmission spectrum of the novel structure. These resonance peaks exhibit different sensitivity responses; thus, simultaneous measurement of strain and temperature is realized by monitoring the wavelength shift of the two peaks. Experiment results indicate that strain sensitivities of the two peaks are 1.82 pm/μE and 8.17 pm/μE, and temperature sensitivities are 47.9 pm/°C and 65 pm/°C, respectively.

In this Letter, we propose, to the best of our knowledge, the first in-fiber optofluidic Raman surface-enhanced Raman spectroscopy (SERS) sensor based on a microstructured hollow fiber (MHF) with a suspended core. Taking advantage of the unique internal structure, we immobilize silver nanoparticles with an SERS effect in the MHF by chemical bonding. The Raman signal of the microfluidic sample is excited by the excitation light in the suspended core through an evanescent field. Then the online SERS signal can be coupled back into the core and detected. To demonstrate the feasibility of the device, rhodamine 6G is chosen as the analyte, and high-quality SERS spectra are detected with the limit of detection of $1\times {10}^{-14}\mathrm{M}$. Furthermore, an online optofluidic test is conducted on ceftriaxone (${\mathrm{C}}_{18}{\mathrm{H}}_{18}{\mathrm{N}}_8{\mathrm{O}}_7{\mathrm{S}}_3$) to examine its capabilities in antibiotic sensing. The results show that the LOD of the samples is ${10}^{-6}\mathrm{M}$. Significantly, this Letter provides an integrated optofluidic in-fiber SERS sensor with a microchannel that can be integrated with chip devices without spatial optical coupling, which has a broad application in medicine and food safety, as well as various aspects of biological in-fiber sensing.

A fiber optic integrated modulation-depth-tunable modulator based on a type of hollow optical fiber with suspended core is proposed and investigated. We synthesized magnetic fluid containing superparamagnetic ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles and encapsulated it in the hollow optical fiber as the cladding layer of the suspended core by fusing the hollow optical fiber with the multimode optical fibers. The light with a wavelength of 632.8 nm is coupled in and out of the modulating element by a tapering technique. Experimental results show that the light attenuation in the system can be greatly influenced by only $2.0\times {10}^{-2}\mathrm{\mu L}$ of the magnetic fluid under different magnetic field strengths. The saturated modulation depth is 43% when the magnetic field strength is 489 Oe. The response time of the system is $<120\mathrm{ms}$. Significantly, this work presents information for the development of all-fiber modulators, including other integrated electro-optic devices, such as optical switch, optical fiber filter, and magnetic sensors utilizing the special structure of this hollow optical fiber with suspended core and superparamagnetic magnetic fluid.

Optical fiber sensors have gained significant attention in recent years owing to their remarkable advantages of remote operation and rapid response. The integration of optical fiber sensing with the microfluidics technology has paved the way for the establishment of optical fiber optofluidic sensing. Optical fiber optofluidic systems possess the advantages of the low invasiveness, compact structure, excellent biocompatibility, and the ability to handle small analyte volumes, rendering them particularly suitable for serving as chemical sensors and biosensors. In this paper, we present an in-depth overview of optical fiber optofluidic chemical sensors and biosensors. Firstly, we provide a comprehensive summary of the types of optical fibers commonly employed in optofluidic chemical and biosensing, elucidating their distinct attributes and performance characteristics. Subsequently, we introduce and thoroughly analyze several representative sensing mechanisms employed in optical fiber optofluidic systems and main performance parameters. Furthermore, this review delves into the modification and functionalization of optical fibers. Additionally, we showcase typical biosensing and chemical sensing applications to demonstrate the practicality and versatility of optical fiber optofluidic sensing. Finally, the conclusion and outlook are given.

We demonstrate a novel integrated optical fiber interferometer for in-fiber optofluidic detection. It is composed of a specially designed hollow optical fiber with a micro-channel and two cores. One core on the inner surface of the micro-channel is served as sensing arm and the other core in the annular cladding is served as reference arm. Fusion-and-tapering method is employed to couple light from a single mode fiber to the hollow optical fiber in this device. Sampling is realized by side opening a microhole on the surface of the hollow optical fiber. Under differential pressure between the end of the hollow fiber and the microhole, the liquids can form steady microflows in the micro-channel. Simultaneously, the interference spectrum of the interferometer device shifts with the variation of the concentration of the microfluid in the channel. The optofluidic in-fiber interferometer has a sensitivity of refractive index around 2508 nm/RIU for NaCl. For medicine concentration detection, its sensitivity is 0.076 nm/mmolL⁻¹ for ascorbic acid. Significantly, this work presents a compact microfluidic in-fiber interferometer with a micro-channel which can be integrated with chip devices without spatial optical coupling and without complex manufacturing procedure of the waveguide on the chips.