Fiber grating is a kind of diffraction grating formed by the axial periodic modulation of the refractive index of the fiber core through a certain method, and it is a passive filter device. Because fiber grating has the advantages of small size, low splicing loss, full compatibility with optical fibers, and the ability to embed smart materials, and its resonance wavelength is more sensitive to changes in the external environment such as temperature, strain, refractive index, and concentration, it is used in optical fiber communication and The sensing field has been widely used.
Optical fibers are used in harsh environments. The main causes of damage include hydrogen damage to the fiber core and damage to the fiber structure caused by rays. In order to avoid the occurrence of the above situation, the structure and composition of the optical fiber need to be specially treated. Studies have shown that the hydrogen element reacts with the ions in the fiber core to destroy the structure of the fiber core, resulting in increased transmission loss; while the radiation transfers energy, causing the outer electrons of the atoms in the fiber core to undergo transitions, resulting in fiber performance. deterioration. In order to avoid the occurrence of the above situation and reduce the damage it brings, it is necessary to adopt optical fibers with special structures or materials. Studies have shown that the use of fluorine-doped cladding and pure silica core can greatly reduce hydrogen loss and radiation effects.
However, this will inevitably bring new problems, that is, the optical fiber grating prepared in the above-mentioned optical fiber core is used in the high temperature, high pressure, high corrosion environment in oil wells, nuclear power plants, and cosmic space, causing permanent damage and transmission. The loss increases. Because the traditional fiber grating preparation method is based on the excimer laser exposure method to prepare fiber grating, the fiber core used is required to have high photosensitivity, and the fiber core needs to be doped with germanium and boron elements. However, the pure silica core has no photosensitivity, and traditional methods cannot be used to prepare fiber gratings.
A new type of fiber grating preparation technology based on femtosecond laser solves the above problems well. The use of femtosecond laser technology makes use of the advantages of femtosecond laser with high instantaneous energy, non-thermal processing, and high processing accuracy. The system structure is shown in the figure above. Using 800nm femtosecond laser, after beam shaping, it is focused on the fiber core after being focused by a microscope objective lens. Since it is not possible to directly observe whether the focal position is located in the fiber core, it is necessary to judge the focal position by observing the back-facing CCD and observing the shape of the back-facing light spot. At the same time, by connecting a broadband light source and a spectrometer, it can monitor the changes in the spectrum during the preparation of the grating in real time and judge the preparation of the grating.
Fabrication of fiber gratings by femtosecond laser technology, compared with traditional methods, not only can fabricate gratings on non-photosensitive fibers, such as pure silica fiber, fluoride fiber, etc., but also has other advantages. First, the femtosecond laser preparation does not require the use of a phase template, so it can get rid of the limitation of the phase template. Theoretically, fiber gratings with any reflection wavelength can be prepared, for example, 2μm and 3μm fiber gratings can be prepared on fluoride fibers; secondly, 800nm femtoseconds The laser can penetrate the coating layer (acrylate, polyimide, etc.) of the optical fiber, therefore, there is no need to strip the coating layer of the optical fiber during the grid making process, which greatly improves the strength of the prepared fiber grating. More importantly, fiber gratings prepared by excimer cannot withstand high temperatures. When the temperature is higher than 150°C, fiber performance begins to degrade, while gratings prepared by femtosecond lasers can withstand temperatures up to 1000°C and can be used in high temperature environments.
Therefore, the emergence of femtosecond laser preparation of fiber grating technology has greatly solved the application of fiber grating sensing technology in a variety of harsh environments. Applied in the field of oil and gas engineering, fiber gratings must be resistant to hydrogen loss, and in many cases also need to withstand high temperatures of 300°C; in the field of fiber lasers, 2μm and 3μm fiber laser systems need to use fluoride fiber gratings; in nuclear power plants, In ray environments such as cosmic space, fiber gratings need to withstand high ray energy. In these special harsh environments, fiber gratings prepared by femtosecond lasers can meet all special requirements, greatly expanding the application fields of fiber grating sensing technology.
