Review of Femtosecond-Laser-Inscribed Fiber Bragg Gratings: Fabrication Technologies and Sensing Applications

Abstract

Fiber Bragg grating (FBG) is the most widely used optical fiber sensor due to its compact size, high sensitivity, and easiness for multiplexing. Conventional FBGs fabricated by using an ultraviolet (UV) laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures. Recently, the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra. The femtosecond laser could also be used for inscribing various FBGs on almost all fiber types, even fibers without any photosensitivity. Such femtosecond-laser-induced FBGs exhibit excellent thermal stability, which is suitable for sensing in harsh environment. In this review, we present the historical developments and recent advances in the fabrication technologies and sensing applications of femtosecond-laser-inscribed FBGs. Firstly, the mechanism of femtosecond-laser-induced material modification is introduced. And then, three different fabrication technologies, i.e., femtosecond laser phase mask technology, femtosecond laser holographic interferometry, and femtosecond laser direct writing technology, are discussed. Finally, the advances in high-temperature sensing applications and vector bending sensing applications of various femtosecond-laser-inscribed FBGs are summarized. Such femtosecond-laser-inscribed FBGs are promising in many industrial areas, such as aerospace vehicles, nuclear plants, oil and gas explorations, and advanced robotics in harsh environments.

Direct Bragg Grating Inscription in Single Mode Step-Index TOPAS/ZEONEX Polymer Optical Fiber Using 520 nm Femtosecond Pulses

“…Since the first FBG was fabricated on a silica optical fiber by Hill et al in 1978 [ 2 ], UV lasers and phase masks have been the most popular devices for FBG manufacturing. However, driven by the demands for novel FBGs able to operate in high temperature environments (400 °C to 1800 °C) or used as vector bending sensors, femtosecond lasers are widely becoming extremely useful because of ultra-short pulse width and extreme-high peak power, which can induce fiber core refractive index change in diverse transparent materials, such as silica, single crystal, glasses, etc., [ 3 ]. Compared to phase mask technology and holographic interferometry, direct writing technologies, such as point-by-point (PbP) and line-by-line (LbL) for FBG inscriptions, demonstrate unique advantages of high accuracy with reduced thermal effect, since refractive indices could be modified to the size of sub-microns inside transparent materials by focusing a femtosecond laser beam by a high NA objective with high magnification [ 4 , 5 , 6 , 7 ].…”

High-Temperature Monitoring in Central Receiver Concentrating Solar Power Plants with Femtosecond-Laser Inscribed FBG

“…The manufacturing of two commercial probes from FemtoFiberTec with 6-FsFBGs sensors each was considered (instead of only one probe with 12-FsFBGs sensors), since this choice will ease the future instrumentation process of the central receiver prototype and will make the sensor configuration more robust against possible breaks and failures. In the two probes, the FsFBGs were inscribed in the single mode fiber using the femtosecond point by point FBG manufacturing process using a laser beam from an 800 nm Titanium Sapphire laser in order to focus tightly into the fiber core [ 31 , 32 ]. FBGs inscribed by infrared femtosecond lasers are thermally stable at temperatures up to 900 °C [ 33 ].…”

A Review: Application and Implementation of Optic Fibre Sensors for Gas Detection

“…All three sensors have a commonality, which is that they have a comb-like, periodic structural variation in the refraction index within the fibre core that induces a coupling action between the core mode and other modes supported by the fibre. There are a number of different fabrication methods in use; for example, ultraviolet (UV) phase-mask inscription, UV point to point, direct-write femto-second laser inscription, and fusion-arc [52][53][54][55]. The mode coupling mechanism depends on the type of grating, physical geometry, and the material used in the fabrication process, and the core mode can be guided, lossy/leaky, and radiative [56,57].…”