Abstract:We demonstrate a fiber in-line Fabry-Perot interferometer cavity sensor for refractive index measurement. The interferometer cavity is formed by drilling a micro-hole at the cleaved fiber end facet, followed by fusion splicing. A micro-channel is inscribed by femtosecond laser micromachining to vertically cross the cavity to allow liquid to flow in. The refractive index sensitivity obtained is ~994 nm/RIU (refractive index unit). Such a device is simple in configuration, easy for fabrication and reliable in operation due to extremely low temperature cross sensitivity of ~4.8 × 10

Fiber Bragg grating (FBG) is fabricated in the microfiber by the use of femtosecond laser pulse irradiation. Such a grating can be directly exposed to the surrounding medium without etching or thinning treatment of the fiber, thus possessing high refractive index (RI) sensitivity while maintaining superior reliability. The grating in the microfiber may have a number of propagation modes in its transmission spectrum, depending on the fiber diameter, and the higher order of mode has larger RI sensitivity. The RI sensitivity also depends on the fiber diameter and a smaller diameter corresponds to a large sensitivity. The maximum sensitivity obtained is ϳ231.4 nm per refractive index unit at the refractive index value of ϳ1.44 when the fiber diameter is ϳ2 m. The FBG fabricated in the microfiber has high potential in various types of optical fiber There has been increased research interest in optical microfibers/nanofibers in recent years [1-4] because of their many unique and interesting properties. An optical microfiber/nanofiber essentially consists of only fiber core, surrounded by air. When light travels along the fiber, it is tightly confined to the fiber core owing to the large refractive index (RI) contrast between the core and air, while a large fraction of the guided light can propagate outside the fiber as the evanescent wave, which makes it highly sensitive to the ambient medium. The small size of the microfiber/nanofiber also provides excellent flexibility and convenient configurability, allowing the easy manipulation of the microfiber-/nanofiber-based devices with a complex topology. Many microfiber-/nanofiber-based fiber devices have been developed, with important applications in the area such as RI sensing [5][6][7][8][9][10].Fiber Bragg grating (FBG) is one of the basic optical fiber components that have wide applications. However, FBG is intrinsically insensitive to the external RI change, as it is not directly exposed to the surrounding medium. Although such a difficulty can be alleviated by thinning or etching of the fiber after the FBG creation [11][12][13], the mechanical strength and durability of the sensing device are greatly reduced, which limits the applications of FBG-based RI sensors. In contrast, long period fiber grating (LPFG) is widely used as the RI sensor, owing to its intrinsically high sensitivity to the surrounding medium change in a large range [14]. However, the transmission dips of the LPFG are broad (typical of tens of nanometers) [12], resulting in a poor measurement accuracy. Meanwhile, the length of the LPFG is relatively large (typical of ϳ30 mm) [15], which limits its applications in accurate sensor devices.The FBG in microfiber can overcome the abovementioned difficulties because of its narrow bandwidth, small grating size, and good measurement accuracy. Moreover, the FBG can support the multiplexed system, showing significant advantages over the LPFG.A powerful tool for the FBG fabrication is a femtosecond laser, which allows the inscription of the FBG in almost any t...

We demonstrate a sub-micron silica diaphragm-based fiber-tip Fabry-Perot interferometer for pressure sensing applications. The thinnest silica diaphragm, with a thickness of ∼320  nm, has been achieved by use of an improved electrical arc discharge technique. Such a sub-micron silica diaphragm breaks the sensitivity limitation imposed by traditional all-silica Fabry-Perot interferometric pressure sensors and, as a result, a high pressure sensitivity of ∼1036  pm/MPa at 1550 nm and a low temperature cross-sensitivity of ∼960  Pa/°C are achieved when a silica diaphragm of ∼500  nm in thickness is used. Moreover, the all-silica spherical structure enhanced the mechanical strength of the micro-cavity sensor, making it suitable for high sensitivity pressure sensing in harsh environments.

We demonstrated a high-sensitivity strain sensor based on an in-fiber Fabry-Perot interferometer (FPI) with an air cavity, which was created by splicing together two sections of standard single-mode fibers. The sensitivity of this strain sensor was enhanced to 6.0  pm/με by improving the cavity length of the FPI by means of repeating arc discharges for reshaping the air cavity. Moreover, such a strain sensor has a very low temperature sensitivity of 1.1  pm/°C, which reduces the cross sensitivity between tensile strain and temperature.

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