We demonstrate an ultrasensitive refractive-index (RI) sensor utilizing the polarimetric interference of a rectangular silica microfiber. The measured sensitivity is as high as $18,987\mathrm{nm}/\mathrm{RIU}$ (refractive-index unit) around the RI of 1.33, which is 1 order of magnitude higher than that of the previously reported microfiber devices. Theoretical analysis reveals that such high sensitivity not only is originated from the RI-induced birefringence variation but also relies on the unique birefringence dispersion property for the rectangular microfiber. We predict that the sensitivity can be enhanced significantly when the group birefringence approaches zero.

A Mach-Zehnder interferometer (MZI) composed by a pair of long period gratings (LPGs) fabricated in silica microfiber for sensing applications is demonstrated. Each LPG is fabricated with a pulsed CO₂ laser by creating six periodical deformations along fiber length with only one scanning cycle. The length of the MZI can reach as short as 8.84 mm when the diameter of the microfiber is 9.5 μm. Compared with the ones fabricated in single-mode fibers, the present MZI is much shorter owing to the large effective-index difference between the fundamental and higher order modes. The microfiber MZI exhibits a sensitivity to surrounding refractive index (RI) of 2225 nm per refractive index unit and the temperature sensitivity of only 11.7 pm/°C. Theoretical analysis suggests that the performances of the MZI sensor can be improved by using thinner microfibers with a diameter down to 3.5 μm: The sensitivity can be greatly enhanced due to the stronger evanescent-field interaction and reduced dispersion factor; the transmission dips become narrower which benefits high-resolution measurement; the thinner fiber also allows further reduction in device length. The present device has great potential in biochemical and medical sensing due to the advantages including easy fabrication, excellent compactness and high sensitivity.

We demonstrate the inscription of fiber Bragg gratings by 193 nm ArF excimer laser in microfibers drawn from the standard single mode telecommunication fiber. Fiber Bragg gratings are directly inscribed in a series of microfibers with diameter ranged from tens of μm to 3.3 μm without hydrogen loading or other treatment to photosensitize the microfibers. Four reflection peaks are observed where three correspond to high order mode resonances. The resonance wavelength depends on the fiber diameter and it sharply blueshifts as the diameter is decreased below 10 μm. The gratings are characterized for their response to ambient refractive index. The higher order mode resonance exhibits higher sensitivity to refractive index.

We demonstrate the bending effect of microfiber on interference fringes in a compact taper-based modal interferometer and sensitivity for refractive index (RI) measurement. For the bend curvature ranging from 0 to 0.283 mm⁻¹, the measured RI sensitivity distinctively increases from 342.5 nm/RIU (refractive-index unit) to 1192.7nm/RIU around RI = 1.333 and from 3847.1 nm/RIU to 11006.0 nm/RIU around RI = 1.430, respectively. Theoretical analysis reveals that such enhancement is determined by the dispersion property of the intermodal index rather than other parameters, such as the variation of the straightforward evanescent field. The magnitude of sensitivity varies as a function of the microfiber bend curvature. Approaching a critical curvature (the intermodal-index dispersion factor approaches zero), the sensitivity is significantly enhanced, exhibiting great potential in RI sensing areas.

A relative humidity (RH) sensor based on a microfiber Sagnac loop interferometer is proposed and demonstrated. The sensor is formed by fusion splicing a high-birefringence elliptical microfiber into a fiber loop mirror without any humidity sensitive coating to the structure. Interferometric fringe with visibility of around 30dB can be achieved in the transmission spectrum due to phase difference between the two polarization modes travelling through the microfiber region. The proposed structure presents high sensitivity up to ∼201.25pm/%RH, within an RH range from 30%RH to 90%RH. Moreover, we realize the sensitivity enhancement (up to ∼422.2pm/%RH) by inserting a reference Panda fiber into the fiber loop. The sensitivity is two to five times of magnitude higher than the counterparts in the literature. The measured response time is around 60ms, which is much better than the previously-reported devices

We present here a detailed investigation into the sensitivity of the taper-based Mach-Zehnder interferometer as a function of external refractive index, with particular attention to the dispersion turning point (DTP) and possibilities for ultra-sensitive sensors. Our numerical simulation revealed that two DTPs exist with a decrease in the microfiber waist diameter; given this relationship, it is possible to obtain an ultra-sensitive operation. We then conducted experiments with fabricated devices with different waist diameters to achieve both positive and negative sensitivities at two DTPs. In particular, we achieved an ultrahigh refractive index sensitivity of approximately 95,832 nm/RIU at the second DTP when working with a diameter of 1.87 µm around the RI of air. These results show its potential for use in acoustic sensing and biochemical detection.