| High temperature optical fiber sensors have an important research and application value in the fields of metal smelting,chemical industry,aerospace and oil and gas chemical industry.Especially,in the field of aircraft engine,high temperqture sensors need to withstand a high temperature of up to 1000 ?,even is 1800 ?.Single-crystal sapphire fibers have a high melting temperature of ?2053°C,and hence could be developed for fabricating various high-temperature sensors.To date,various structures based on sapphifre fibers,i.e.,blackbody radiance,Fabry-Perot interferometer and fiber Bragg gratings have been reported.Sapphire fiber Bragg gratings(SFBGs)are important high temperature sensors.However,sapphire fibers have no photosensitivity and no cladding.They are multi-mode fibers.Refractive index modulation can be induced in non-photosensitive fiber by use of femtosecond laser.In this paper,a femtosecond laser line-by-line technique is proposed to fabricate Bragg gratings in the sapphire fiber.SFBGs with different wavelengths can be inscribed using this method by simply changing the line spacing.Moreover,the index modulation area created by this method is much larger than that created by the point-by-point inscription method,and hence leads to a higher reflectivity of the SFBGs.We studied the specral properties of sapphire fiber Bragg gratings.Then we tested the high-temperature response of these sapphire fiber Bragg gratings.The main contents of this paper are as follows:1.The physical and chemical properties,the growth method and mechanism,the optical properties of single crystal sapphire,and theoretical analysis and simulation of sapphire fiber grating were introduced.The flat end face of sapphire fiber was obtained by means of mechanical polishing.The effects of three kinds of abrasive paper,i.e.,1,0.5 and 0.1 ?m,on the smoothness of sapphire fiber end face were investigated.The mode field conversion characteristics of graded index fiber were studied.It was found that 1/4 pitch graded index fiber could convert a single mode field with a diameter of 10.40 ?m to a single mode field with a diameter of 18.57 ?m.And its divergence angle decreased from 0.127 to 0.058 rad.The mode field matching degree of two modes with different field diameter can be improved based on 1/4 pitch graded refractive index fiber.But the coupling loss of single-mode fiber and sapphire fiber is not effectively reduced due to the high degree of multi-mode of sapphire fiber and the low ratio of single-mode energy.Therefore,the graded refractive index fiber is directly coupled with sapphire fiber.In comparison with the coupling of single-mode fiber and sapphire fiber,the coupling loss of graded refractive index fiber and sapphire fiber can reduce by 10 d B due to the multi-mode transmission characteristics of graded refractive index fiber.2.We study the fabrication of SFBGs by femtosecond laser line-by-line technique.The SFBGs fabricated with different fiber diameters of 60 ?m and 100 ?m have a fourth-order Bragg wavelength of 1547.17 nm and 1547.59 nm,a-3 d B bandwidth [i.e.,full-width-at-half-maximum(FWHM)] of 6.08 nm and 7.58 nm,a reflectivity of ?6.3% and 3.9%,and a signal-to-noise ratio(SNR)of 7.86 d B and 5.75 d B,respectively.Moreover,the SFBG with a diameter of 60 ?m has a higher reflectivity,a higher SNR,and a narrower bandwidth than the SFBG with a diameter of 100 ?m.The track length was increased from 5 to 50 ?m,and the SNR was increased from 2.24 d B to 7.86 d B.The SNR increased from 7.09 to 10.01 d B in the case in which the grating pitch quantity N increases from 1126 to 2252.We studied the influence of the position of SFBG in sapphire fiber on the reflection spectrum.SFBG fabricated in the middle of sapphire fiber has a higher SNR of 12.46 d B.SFBG fabricated in the 9 ?m from the middle has a SNR of 10.02 d B.And the reflection peak blue shifted from 1553.32 to 1550.59 nm.The SNR and-3d B band-width of SFBGs with different order were studied.The 6th order SFBGs have the lowest SNR of 6.75 d B and the narrowest-3 d B bandwidth of 2.42 nm.The influence of offset coupling on the reflection spectra of SFBGs was studied.By translating along the direction perpendicular to the tracks of SFBG,the-3 d B bandwidth of SFBG could be reduced from 2.64 nm to 1.39 nm,and the SNR was slightly reduced by 1.14 d B.A serial array consisting of five SFBGs with different reflection wavelengths was successfully constructed.3.Femtosecond laser plane-by-plane technique was studied to fabricate multi-layer SFBGs.The influence of the distance between femtosecond laser focus and the sapphire fiber top surface on track length was studied.The femtosecond laser with the same energy was used to inscribe the track length of 40 ?m into the 60 and 100 ?m diameter sapphire fiber.The track length of 40 ?m was intergrated and visibile in the depth range from 5 to 13 ?m in 60 ?m diameter sapphire fiber.And the track length of 40 ?m was intergrated and visibile in the depth range from 5 to 29 ?m in 100 ?m diameter sapphire fiber.The inscription sequence of the layers was studied.The SNR of SFBGs with the lower layer inscribed first was 1 d B higher than that with the upper layer.SFBGs with 2 layers and the layer distance of 5 ?m have a maximum SNR of 15.51 d B.The influence of offset coupling on the reflection spectra of SFBGs with double layers was studied.By translating along the direction perpendicular to the tracks of SFBG,the-3 d B bandwidth of SFBG could be reduced from 1.58 nm to 1.32 nm,and the SNR was slightly reduced by 0.5 d B.4.The high-temperature response of SFBGs has been texted.The temperature field distribution of furnace was detected by B type thermocouple sensor.The uniform temperarture zone was less than 20 cm below 1000 ?,and it increased to 20 cm above 1000 ?.The reflection spectrum of SFBG was the superposition of multiple reflection modes.Hence,the reflection spectrum had many spikes,which was not conducive to peak searching.The Adjacent averaging method and savitzky-golay method were used to smooth the reflection spectrum of SFBGs.It was concluded that savitzky-golay method could better preserve the characteristics of the height and width of reflection spectrum.The complete high-temperature response of the SFBG from the room temperature up to 1612 °C was tested.The temperature sensitivity of the SFBG increases at elevated temperatures,i.e.,23.4 pm/°C at 20°C,28.1 pm/°C at 749°C,and 36.5 pm/°C at 1612°C.The reflection peak of SFBG shrinked permanently at the temperature of 1785 ?.An SFBG with a reflectivity of 6.3%,a-3 d B bandwidth of 6.08 nm and a signalto-noise ratio(SNR)of 15.51 d B was obtained via optimization of fabrication parameters.The high-temperature response of these SFBGs was tested and the experimental results showed the SFBGs could withstand a high temperature of 1612°C.Moreover,a temperature sensitivity of 36.5 pm/°C was achieved in the high-temperature region.Additionally,a serial array consisting of five SFBGs at different wavelengths was successfully constructed. |
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