Fabrication Of Robust Fiber Gratings By Femtosecond Laser And Their Applications

Harsh surroundings are widely existed in industry, national defense, aviation area, this includes high temperature, pressure, strong radiation and electromagnetic interference, et al. How to realize precise, transient physical quantity sensing and measurement in harsh condition is a problem that blocks the way of science and industry.As a fresh troop in fiber sensing area, fiber gratings have the merits of passivity, anti‐electromagnetic interference, low insert loss, good wavelength selectivity and can realize long haul and distributed sensing that provide an opportunity for solving the above problems. The traditional fabrication of fiber Bragg gratings (FBG) is based on UV laser writing technology, the induced refractive index (10‐4‐10‐3) strongly depends on the photosensitivity of the fiber, which is readily erased when the temperature is more than400℃, limiting its sensing applications in hash environment, such as metallurgy, oil exploration, thermal power generation and nuclear energy. Therefore, the need for a high‐temperature resisting “extreme” FBG is urgent. The existed fabrication method includes high temperature re‐generated method and ultrafast laser machining method, the formed re‐generated gratings of the former have the defects of low reflectivity, bad spectrum characteristics and big loss that will increase the signal detecting difficulty and affects the resolution. The ultrafast laser micro‐machining technology have been proven to induce perpetual index changes (10‐3‐10‐2) in any transparent dielectrics (quartz, silicate glass, non‐linear crystal) that has nothing to do with the photosensitivity and chemical composition, which is ascribed to the high non‐linear effects of the high‐powered ultrafast pulse. This technology brings new vitality for the fabrication of high temperature resisting FBG.This paper is based on the method of femtosecond laser (fs) micro‐machining and aimed at fabricating high temperature resisting FBG to solve the harsh environment sensing problems. The study contents and achievements of the paper is listed as bellows:(1) Discussed the coupled mode theory of fiber gratings, analyzed the coupling characteristics among modes of uniform FBG and TFBG. Analyzed the factors that influence the grating reflectivity and bandwidth, which contain grating length, DC and AC index changes and AC coupling coefficients, then conducted the simulation to the reflection spectrum of FBG. Described the main mechanism of femtosecond laser induced index modulation to transparent dielectric medium, which is the high nonlinear effects. Described the phase mask and point‐by‐point writing technology and discussed the main problems of gratings writing: how to get pure double beam interference with the optimized phase mask and how to control the focal position precisely.(2) Fabricated the uniform high order FBG and non‐uniform high order TFBG by means of the high nonlinear effects of fs laser and high order phase mask(pm=3.33m), among which the high order TFBG was reported for the first time, they all have the high temperature stability up to1000℃. Realized the discerning of axial strain and temperature in harsh environment with single high order FBG. For high order TFBG, studied the sensing characteristics of the same order cladding mode (m‐LPvu) resonances corresponding to the peer Bragg diffraction; fabricated the reflective TFBG sensor by sputtering the gold reflective mirror, proposed the discerning of strain and high temperature (800℃) based on relative peak power‐wavelength demodulation technology.(3) Fabricated the high localized FBG based on the flexibility of fs laser PbP technology, which contains the low order and high order Bragg gratings structure, because the coupling of the high azimuth angle fiber mode caused by high localized index modulation, its characteristic spectrum showed wideband high order cladding mode resonance. Confirmed a kind of novel quasi‐chirped high order FBG that has multi‐reflective peaks. Multi‐reflective peaks distributed in the ultra‐wide frequency range of1200~1700nm in the form of quasi‐period or groups (series), which is suitable for the multi‐parameter sensing in harsh environment. Realized the multi‐wavelength fiber laser by using the9order quasi‐chirped FBG.(4) Inscribed high order SFBG in Single‐crystal sapphire fiber (250m) by means of fs laser combined to phase mask scanning technology. Proposed the direct fusing coupling method between the silica fiber and sapphire fiber, simulated the multi guiding mode stimulating characteristics and multi‐modes resonance spectra. Proved SFBG can be used in the sensing application in harsh environment at least as high as1600℃.This paper described the fabrication of the traditional uniform and non‐uniform FBG, at the same time, we devised and fabricated highly localized FBG with special structure and novel spectral characteristics taking advantage of the flexibility of PbP technology, which expanded the functionality of fiber gratings. Fs‐FBG can not only be used to sense in harsh environment, but also shows wide application prospect and commercial value in high power and multi‐wavelength fiber laser area.