Multimode Fiber Grating Characteristics And The Superiority To Single-mode Fiber Grating

Fiber grating has characters of small size, light weight, and it can be integrated,anti-electromagnetic interference, cheap and so on. It has been widely used in sensortechnology, the field of dispersion compensation, wavelength division multiplexing. Atpresent, the most use of fiber Bragg grating is a single-mode grating, of course,single-mode fiber has its own advantages, but the single-mode fiber core is relativelysmall (generally less than10um), so in addition to single-mode fiber and laser diode,single-mode fiber is difficult to coupling with other optical devices or light. However,if the single-mode fiber exchange for multi-mode fiber will be able to compensate forthis defect.Multimode fiber grating not only has the characteristics of the single-mode fibergrating have, also has its own unique advantages.(1) Multimode fiber core is fairly thick (usually up to50um or62.5um), whichmakes it very easy to couple the light source and other optical devices, and lower thecost of the production of multi-mode fiber, so that the multimodefiber grating insteadof the single-mode fiber grating can reduce the cost of industrialization.(2) Multimode fiber grating reflection and transmission peaks have thecharacteristics of multi-peak structure, one can get more information from thereflection/transmission in the wind, so that you can take advantage of this multi-peakcharacteristics in sensing technology has been beset us in the cross sensitivity oftemperature and stress, can also solve the problem of demodulation of fiber gratingmatch the grating susceptible to external factors, these optical fiber sensor technologyis very beneficial.(3) Due to the large aperture of a multimode fiber grating, easy coupling, so it canwithstand a large recoverable strain.(4) Have a band wavelength tuning range. In this paper, we analyze the characteristics of multimode fiber Bragg grating andits superiority compared to single-mode fiber grating in three areas.1Sensing characteristics of multi-mode fiber gratingAs The single-mode fiber grating can only be used for sensing and measuring thechange of a single parameter (temperature, stress, pressure). When two or moreparameters change at the same time, single-mode fiber grating can not distinguish theimpact of various parameters on the center wavelength. Because there are many modesin the multi-mode fiber grating, the refractive index of the different modes are different,and temperature, stress, pressure sensitivity of different modes is also different.Therefore, the multimode fiber Bragg grating can achieve simultaneous measurementof temperature, stress, pressure. In this paper we analyze temperature, stress, pressuresensitivity of0-and17-order dominant mode. We calculated that the temperaturesensitivity of the zero-and17-order are11.55pm/0Cand11.11pm/0C, the pressuresensitivity of the zero-and17order are4.292pm/MPaand4.546pm/MPa. the strainsensitivity difference is very small, the gap can be ignored.Based on the above analysis, we simulate the transmission spectrum drift ofzero-and17order when temperature changes10, pressure changes1000Mpa. WeCalculate the drift of the zero order mode is4.0612nm,17order of the dominant modeof drift4.3243nm, the gap is about0.3nm, this gap is relatively small. So while whenthe temperature and pressure changes in a large amount of numbers, multi-mode fibergrating can be used to achieve the temperature and pressure measurements. Whenthese two quantities change small, it not suitable for the measurement of twoparameters.2Multimode fiber gratings for dispersion compensationFrom the fiber Bragg grating dispersion theory, we know that there have a largenegative dispersion in the position of δ=κin fiber Bragg grating. this negativedispersion just can be used to compensate for the normal dispersion introduced by theordinary optical fiber in WDM system. and we have come to the conclusion that fiberBragg grating suitable for dispersion compensation on transmitted light, not suitablefor reflected light.Single-mode fiber grating has only one transmission peak, so it can onlydispersion compensation one optical frequency of satisfy the dispersion relation.Can transmit more than one mode in a multimode fiber grating, so a multimode fibergrating can be used to achieve multi-channel dispersion compensation.This paper we analyzes the grating length L, the grating periodΛ, the refractiveindex changeδnimpact on multimode fiber Bragg grating dispersion. When thegrating length L is changed, both the dispersion maximum used for dispersioncompensation and wavelength corresponding to the maximum changed of each mode.when the grating length is too small, The positive and negative dispersion willsuperposition of two adjacent mode, it will lead to distortion in the process ofdispersion compensation, so the length of the fiber grating used for dispersioncompensation can not be too short. When the grating period change, the dispersionmaximum of each mode will not change, but the wavelength dispersion valuecorresponding to the dispersion maximum drift. With the increasing of the gratingperiod,wavelength drift to the direction of the long-wavelength. When the gratingcoupling coefficientκchange, the dispersion maximum value and wavelengthcorresponding to the dispersion maximum of each mode will change, but the change ofwavelength corresponding to the dispersion maximum is not large. Couplingcoefficient changes within an order of magnitude, the wavelength drift is no more than0.1nm. Coupling coefficient increases, the dispersion maximum of each mode alsoincreases.In the contion of Equality-power excitation, The maximum dispersion of eachmode is the same, the channel spacing of the adjacent modes are the same too. Withthe above parameters change, the variation of the dispersion maximum value of eachmode and the wavelength corresponding to the dispersion maximum is the same. Inunequal power excitation conditions, the maximum dispersion value of each modevaries with the different excitation conditions, but the interval between the channelunchanged.3Slow-light effect of multimode fiber Bragg gratingWe discussed the characteristics of a multimode fiber grating delay when gratinglength L, grating period Λ, the refractive index change δ nchange. When change thelength of the grating, both of the maximum delay and the wavelength corresponding tothe the maximum delay of each mode change, The longer the grating length, thegreater of delay maximum value, the pulsed light group velocity is slower the slow-light effect is more obviousily. the wavelength corresponding to the themaximum delay move to shorter wavelength direction with increasing grating length.The grating period changes only affect the wavelength corresponding to the themaximum delay,the maximum delay of each mode does not change with the change ofthe grating period. Refractive index change has an impact on the maximum delay valueand the wavelength corresponding to the the maximum delay. The greater theδn, thegreater the maximum delay value. the wavelength corresponding to the the maximumdelay, move to the long-wavelength direction with the increase of theδn. In thecontion of Equality-power excitation, The maximum delay of each mode is the same,the channel spacing of the adjacent modes are the same too. With the above parameterschange, the variation of the maximum delay value of each mode and the wavelengthcorresponding to the maximum delay is the same. In unequal power excitationconditions, the maximum delay of each mode varies with the different excitationconditions, but the interval between the channel unchanged.