Femtosecond laser fabricated fiber Bragg grating in microfiber for refractive index sensing

Abstract

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...

Microfiber Bragg grating for temperature and strain sensing applications

“…2(b) we can see that the resonant wavelength of FBG in a 35 m diameter fiber has a blue shift versus the FBG in an SMF fabricated with the same laser and phase mask. This is because a thinner fiber leads to a drop in the effective refractive index of the modes propagating in the fiber, thus a decrease in  B , which can be derived from the Bragg condition [10]: …”

“…Fiber sensors, based on the various fiber components, have been developed in many forms [4][5][6][7][8][9], and are exhibiting outstanding performances in the field of physical and bio-chemical sensing. Fiber Bragg grating (FBG) is among the most important optical components which has a wide variation of applications [10][11][12][13]. Here we fabricate an FBG in the microfibers with diameter down to several micrometers, and the temperature and strain responses of the microfiber FBG are investigated by the experiment.…”

Optical microfibers and nanofibers

“…Several fabrication techniques such as CO 2 laser irradiation [147], excimer laser irradiation [149,150], femtosecond laser exposure [151], etched corrugations [21], and focused ion beam milling [134-136, 152, 153], have been demonstrated to write gratings directly on as-drawn MNFs. Here we categorize typical MNF gratings into three types:…”

“…In 2005, relying on a wet chemical etch-erosion procedure, Liang et al fabricated an MNFBG out of a 6-μm-diameter silica MNF, and used the MNFBG for index measurement in liquids [21]. Later in 2010, using femtosecond laser pulse irradiation, Fang et al reported an MNFBG based on a 2-μm-diameter MNF, which was used for RI measurements with a maximum sensitivity of 231.4 nm/RIU at a RI value of 1.44 [151]. Around the same time, Zhang et al reported a MNFBG written in a photosensitive microfiber using KrF excimer laser, and demonstrated a sensitivity of 102 nm/RIU at a RI value of 1.378 in a 6-μm-diameter MNF [149].…”

Reconfigurable optical-force-drive chirp and delay line in micro- or nanofiber Bragg grating

“…Even optical nonlinear behavior -such as optical bistability and self-switching [12][13][14][15][16][17][18] is also difficult to observe under continuous-wave (CW) or quasi-CW excitation because the extremely high threshold power, which -without optimization -amounts to ~10 4 W. However, MNFs open new opportunities for research. With the emergence of low-loss MNFs, numerous techniques to inscribe gratings into MNFs have been successfully demonstrated over the past five years, for example, processing with an ultraviolet laser [19], a CO 2 laser [20], and a femtosecond laser [21]; and milling with a focused ion beam (FIB) [22]. With these gratings' very small cross-sectional area and their enhanced light intensity, the scale of radiation pressure is sufficient for creating a considerable longitudinal deformation in the structure, which thereby affects the optical characteristics of the grating.…”