Fabrication Of Novel Optical Fiber Grating Sensors And Study On Their Characteristics

Fiber grating refers to the grating by a certain means to periodically modulate the axial refractive index of the fiber core, as a new type of passive optical components, it can be produced into a variety of sensors, and it is widely used in hash environment sensing, construction sensing, chemical testing, biomedical and other fields. Fiber grating includes the fiber Bragg grating(FBG) and the long period fiber grating(LPFG). The period of FBG is generally less than 1 micrometer, while the period of LPFG is generally tens to hundreds of micrometers.The optical wave transmission mode of FBG is the coupling of the forward-propagating core mode and the backward-propagating core mode, forming Bragg resonance peaks in the reflective and transmissive spectrum. FBG is often used to measure temperature, strain and other physical quantities in the sensing field. In the LPFG, the forward-propagating core mode couples with the cladding mode. The light energy loss occurs in the external environment, resulting in forming a series of loss transmission resonance peaks in the spectrum. With respect to FBG, LPFG is not only capable of measuring temperature and strain, also very sensitive to the refractive index of the external environment, it is a very good refractive index sensors.The ultraviolet(UV) laser technology and the femtosecond laser micro-nanofabrication technology are the main methods of fabrication of FBG and LPFG. The UV laser technology for the fabrication of the fiber grating is very mature, people can produce FBG and LPFG with good performance and spectrums by this technology, but there are some drawbacks for the UV method. Not only before the preparation of the fiber grating there is a optical fiber sensitizing process, and the temperature stability of the fiber grating producing by the UV method is poor, the grating structure can be easily erased at temperature above 400℃. Therefore, FBG and LPFG prepared by this method can not be applied in high temperature conditions.Femtosecond laser has the ultrashort pulse duration and the high peak power(up to 1023 W/cm2), and it is widely used in three-dimensional processing of transparent materials. In recent years, the femtosecond laser technology for FBG and LPFG develops rapidly, compared to UV method, the advantage is that we can directly modulate the refractive index of the fiber core without the extra sensitive treatment for the fiber before the fabrication of the fiber grating and the temperature stability of these fiber gratings is excellent, the fiber gratings can be used in hash environment at high temperatures.In view of this, this paper proposes two novel types fiber grating structure: integrated structure of a FBG in parallel with a LPFG and micro arrays of FBG, which are fabricated by femtosecond laser direct writing technology, and we study the fabrication process, the principle and sensing applications of these two structures. The main contents of this paper are as follows:1. The study of the fabrication mechanism of the fiber grating by the femtosecond laser technology and the basic principles of FBG and LPFG. The transparent material such as quartz has a strong nonlinear absorption of the incident femtosecond laser. In order to produce an optical breakdown and material damage, in the nonlinear absorption the electrons must jump to the conduction band from the valence band to deposit enough laser energy in the material, and in this nonlinear absorption there are two non-linear excitation mechanisms: avalanche ionization and photoionization. If the material deposited enough energy by nonlinear absorption, it will produce defects and damage, which can be formed grating structure. Then we did simple theoretical analysis of FBG and LPFG, and mastered the basic principles.2. A new integrated sensor by a FBG parallel with a LPFG fabricated by femtosecond laser. The method of the FBG fabrication is the femtosecond laser point by point writing technology and the method of LPFG is laser scanning direct writing technology. FBG was inscribed in the center of the fiber core, the period of it is 1.07 μm, the length of it is 2.4 mm, and the wavelength of Bragg resonance peak is 1550 nm, which is the second order Bragg resonance peak. The LPG was written at the position deviated from the center of the core of about 1.8 μm, the period of it is 60 μm, and the length is also 2.4 mm. Then we test the temperature and refractive index sensitivities of this integrated sensor, the temperature sensitivities of FBG and LPFG are 12.98 pm / ℃ and 10.93 pm / ℃, while the sensitivities of refractive index are 0 nm/RIU and 196.46 nm/RIU of the loss peak in the vicinity of 1555 nm. Because the temperature and refractive index sensitivities of FBG and LPFG are different, according to the two-parameter matrix it can be used for simultaneously sensing of temperature and refractive index.3. A novel micro-channels Bragg grating structure. The micro-channels FBG was fabricated by refractive modulation of femtosecond laser and selective etching with HF solution. The fabrication steps: the first step is the refractive index modulation in the cross section of the fiber forming the periodic modulation region, this time FBG resonance peaks with a not very high reflectivity can be seen in the reflection spectrum. The second step is to put the fiber affected by the femtosecond laser into the 4% HF solution to etch. In the HF solution the etching speed of the region of modulation by fs laser was much faster than the other regions. The reflection spectrum changes gradually with the corrosion, so we can control the etching time and observe the reflection spectrum to control the fabrication of the micro-channels FBG. Finally the refractive index of the micro-channels FBG was tested and we found that it had a high refractive index sensitivity.