Theoretical And Experimental Study On Few-mode Long Period Fiber Gratings

Fiber grating is widely used in optical fiber communication system and optical fiber sensing field with its small size,light weight,simple structure,anti-electromagnetic interference and so on.As the arrival of the era of the Internet of things?IOT?,the fiber grating as the sensing element is widely distributed in various places,to detect changes in the surrounding environment,and provide reliable information for the data center,it will serve an important purpose all the time.The long period fiber grating,which is more sensitive to the external environment,plays an important role in health care,biological detection and production monitoring.With the development of spatial multiplexing technology,few-mode fiber reemerges in front of everyone,and the combination of the few-mode fiber and long period fiber grating will form a new optical sensor,which is called few-mode long period fiber grating?FM-LPFG?,it can be used as mode converter for the reason that few-mode fiber supports multiple modes in the core,and then it enriches the research content of long period fiber grating,bring more possibilities for the optical fiber sensing,showing a good development trend.Based on the coupling mechanism and sensing characteristics of the FM-LPFG,the following theoretical and experimental studies were carried out:?1?By the use of Matlab and Comsol software,a model of three-layer fiber waveguide was constructed.Theoretically we calculated the coupling coefficients and transmission spectrums between the core modes and the cladding modes,and then analyzed the influence of the parameters like grating period,refractive index modulation depth,cladding thickness and grating length on the transmission spectrums.?2?Furthermore,we simulated the phase matching curve,waveguide dispersion factor,and the sensing sensitivity of the temperature,strain and the refractive index of the surrounding environment about the FM-LPFG.A conclusion was draw that the sensitivity of the refractive index of the surrounding environment of the LP₀₂ mode FM-LPFG is two orders of magnitude higher than that of the single mode long period fiber grating?SM-LPFG?.?3?Experimentally we built the long period fiber grating experiment device based on high frequency CO₂ laser direct writing method,and successfully produced theFM-LPFG sample.In addition,we made the temperature sensing experiment with the sample and the sensitivity of the temperature reached-118.36pm??.?4?What's more,poly-dimethyl siloxane?PDMS?was coated on the FM-LPFG for enhancing the temperature sensitivity,and then super high sensitivity of-414pm??was got at low temperature range.These results indicate that the sensor based on the FM-LPFG is expected to make a breakthrough in fiber sensing and it has huge potential application value in the detection of the temperature,strain and the refractive index.