Research Of Pre-twisted Optics Fiber Devices Fabricated By CO₂ Laser And Its Sensing Characteristics

Optical fiber sensors are widely used in real life because of the unique advantages as anti-electromagnetic interference,high sensitivity,small size,easy to integrate,anti-corrosion and so on.Torsion is important in large buildings such as bridges,houses and so on,and has a wide range of applications in the field of sensing for safety monitoring.In this paper,we have a systematic research on the torsion characteristics of optical devices fabricated by CO₂ laser micromachining method,and make some innovative achievements.The main work is summarized as follows:1.The refractive index modulation mechanism,the phase matching curve and the coupling mode theory of long-period fiber grating fabricated by CO₂ laser exposure is systematically analyzed theoretically.An experimental system of CO₂ laser exposure to fabricated LPFG was designed and build,including software control program,and several long-period fiber gratings are fabricated with different grating pitches.Besides,the spectra of different grating pitches and phase matching curve are analyzed experimentally.In addition,the torsion characteristics of the long-period fiber grating with different pitches and different coupling mode are analyzed in detail,which provides the basis for the comparison of the later experimental data.Further investigate in PT-LPFG about temperature and PDL are analyzed either,and the results show that the pre-twisting fiber will not affect the temperature characteristics,but can lower the PDL.2.An experimental rotate system of CO₂ laser exposure to fabricated PT-LPFG was designed and builds,including software control program and optical fiber rotators.The PT-LPFG is fabricated by CO₂ laser beam exposure while pre-twisting the fiber,thus making the shear strain freezed and formed a screw-type deformation.Compared with the LPFG,the grating period has the screw-type deformation and the helix is closely related to the pre-twist rotation.Several PT-LPFGs with different grating pitches,different per-twist rate,different helix?different pre-twist rotation?are fabricated in the above system,and the spectral characteristics and phase matching curves are analyzed.In addition,the torsion characteristics of the PT-LPFG with different pitches and different helix are analyzed in detail,which prove that the induced pre-twisted strain can increase the torsion sensitivity 5-10 times.3.An experimental rotate system of CO₂ laser exposure to fabricated PT-MZI was designed and builds,including software control program and optical fiber rotators.The PT-MZI is fabricated by CO₂ laser beam exposure while pre-twisting the fiber,thus making the shear strain freezed and formed a screw-type deformation in the two processing section.The interference and the wavelength shift are analyzed theoretically,and the spectra of PT-MZIs with different interference length and different pre-twist rate are analyzed experimentally.In addition,compared with the conventional MZI fabricated in the system without pre-twisting the fiber,the torsion characteristics of the PT-MZIs with different pre-twisted rate are analyzed in detail,which prove experimentally that the induced pre-twisted strain can change the MZI from torsion-independence to torsion-dependence that can measure the torsion rate and direction simultaneously.Besides,the torsion sensitivity can be effective enhanced by induced higher pre-twist rate within a certain range.Further investigate in PT-MZI about temperature is analyzed either,and the results show that the pre-twisting fiber will not affect the temperature characteristics.