Research On Fabrication Of Long Period Fiber Grating By Femtosecond Laser And Ites Characteristic

Fiber Bragg Gratings have small size,excellent resistance to electromagnetic interference,chemical corrosion resistance,and the ability to sense and sense environmental parameters in harsh environments,in aerospace,military,building health monitoring and energy Development has been put into use.The type I gratings obtained by the traditional method of writing optical fiber gratings by ultraviolet light cannot withstand higher temperatures and are easily erased at high temperatures,and the written gratings have strong photosensitivity and need to be sensitized after processing.The phase mask method for writing gratings is not flexible and costly.Gratings processed with femtosecond lasers can withstand higher temperatures without sensitization,which enhances the usability of fiber gratings.Long-period fiber gratings(LPFG)have a larger period length.Compared to Fiber Bragg grating(FBG),the coupling mode of light inside LPFG is the fundamental mode and the cladding mode of the same direction Coupling,radiating to the outside after continuing to transmit for a certain distance,resulting in the LPFG's spectral changes being more sensitive to external environmental parameters and capable of measuring more physical quantities.However,LPFG is sensitive to multiple parameters,and there is a serious crosssensitivity problem in single-parameter testing,resulting in inaccurate measurement results.Whether the cross-sensitivity problem can be solved is crucial for LPFG to be applied in a wider field.This article uses the 515 nm femtosecond laser to directly write LPFG on single-mode fiber,through formula derivation and experimental testing to explore the factors that affect the temperature sensitivity of LPFG,improve processing methods,and effectively reduce the crosstalk caused by the temperature during the test.The contents are as follows:First,the basic principle of femtosecond laser processing fiber grating and the coupled mode theory of the grating are described.Through analysis of the temperature sensitivity of LPFG,the factors that affect the temperature sensitivity of LPFG are derived: the effective refractive index difference between the core and the cladding The effective thermo-optic coefficient of the core.By adjusting the relative size of the refractive index modulation area in the fiber core,the temperature sensitivity of LPFG can be effectively changed.Secondly,by using a 515 nm femtosecond laser to write a long period fiber grating,the spectral characteristics of the LPFG written by the 515 nm femtosecond laser at different periods and different single pulse energies were studied,and its temperature characteristics,strain characteristics and refractive index The characteristics were explored.Using a single-pulse energy of 64 nJ and a repetition rate of 10 kHz femtosecond laser to write LPFG,through testing and found that its temperature sensitivity is less than 7 pm/?,and the refractive index sensitivity is within the range of 1.4065-1.4265 RIU,the sensitivity of the average refractive index response is 985.36 nm / RIU,too low temperature sensitivity can effectively reduce the effect of crosstalk caused by temperature.Since the temperature sensitivity of the LPFG is affected by the size of the refractive index modulation area,adjusting the size of the core refractive index modulation area will change the temperature sensitivity of the LPFG.Multiple scribe lines are used in the core to increase the size of the core refractive index modulation area.Among them,the LPFG temperature sensitivity written by the method of 2 dashes and 3 dashes reached 14.95pm/? and 15.94 pm /?,respectively,and the temperature sensitivity of LPFG written by the method of 5 dashes can reach 28pm/?,and It is verified that the size of the refractive index modulation area is directly proportional to the temperature sensitivity of LPFG.In order to better solve the problem of cross-sensitivity during testing,we have studied the longperiod grating of small period.Because of the simultaneous transmission spectrum and reflection spectrum,these two spectra can be used to test temperature and strain,The cross-sensitivity error between stress and temperature can be eliminated through the sensitivity matrix,which effectively solves the cross-sensitivity problem existing during testing and realizes dual-parameter sensingFinally,we propose and write a long-period grating with sensitivity at low temperature.The femtosecond laser direct writing method successfully prepared a long-period fiber grating with a period of 50 ?m on a single-mode fiber with a core diameter of 6?m.After testing,it was found that long-period fiber gratings processed with low laser energy in fine-core fibers have lower temperature sensitivity,while maintaining a larger extinction ratio and better spectral quality.The loss peak of this fine-core long-period fiber grating only drifts by 1.7nm in the temperature range of 20~700 ?.The grating also has a good response to refractive index changes.The environmental refractive index is in the range of 1.4065~1.4265,the sensitivity is up to 882.51nm/RIU,and the strain sensitivity is-2.2pm/??.This fine-core long-period fiber grating can better reduce the crosstalk caused by temperature in the refractive index and strain tests.It provides an effective reference for solving the cross-sensitivity problem caused by the simultaneous existence of various parameters during LPFG testing.