Analysis Of Sampled FBGs In Polarization-maintaining Fiber And Design Of Their Application

All kinds of FBGs,due to their unique advantages such as compatibility, easy of use and low cost,have been designed and researched to perform filtering functions.Various sampled fiber Bragg gratings (sampled FBGs),due to their characteristic of multi-wavelength output, have attracted considerable research attention.In this paper,we propose a new type of sampled FBGs written in PMF. Due to the intrinsic stress-induced birefringence in PMF,the x- and ypolarization modes are split and each one has a different effective index. Different effective refractive indexes could lead to different Bragg central wavelength of gratings.In other words,the spectral response of the sampled FBGs written in PMF consists of two combs corresponding to orthogonal linear polarizations.The spectral interval between the combs could be changed by varying the fiber birefringence.To fully grasp the new type of gratings,reflection spectrum,phase,time delay and GVD are also analyzed,which could affect the gratings filtering capability.Due to great deal of side-lobe in the reflection spectrum of the gratings;the sampled FBGs in PMF not only have serious crosstalk of the sampled FBGs written in conventional SMF,but also have the crosstalk between the channels of two orthogonal linear polarizations. The reflection spectrum of sampled FBGs in PMF has to be optimized by apodization technology.It could increase Side Mode Suppression Ratio(SMSR).We also propose two kinds of multi-wavelength output schemes based on the novel gratings,which could be used as multi-wavelength polarization filter in the polarization division multiplex (PDM)system.Two kinds of multi-wavelength output schemesThe sampled FBGs in PMF could be used as basic device of filter, which could realize increasing the number of channels.We propose two kinds of schemes of flat multi-wavelength output based on the sampled FBGs in PMF.A.Expand bandwidth schemeFor gratings written in Hi-Bi fiber,the frequency spacing could be up to several hundred GHz,and the Bragg central wavelength intervalΔλ_B could be up to several nm.The difference of refractive index between two polarizations is up to 10^-3,which is useful for expanding bandwidth. When sampling rate SR is 0.23,refractive index differenceΔn_eff=0.0036,the flat bandwidth may be up to 8 nm,it is twice broader than the sampled FBGs written in conventional SMF.B.Narrow channel spacing schemeFor the Low-Bi fiber,the frequency spacing could be tens of GHz,and the Bragg central wavelength intervalΔλ_B could be sub-nm.By adjusting the parameters of sampled FBGs in PMF the channel spacing is about 0.4 nm,which could meet the ITU-T grid.It means a new method to realize flat multi-wavelength output by narrowing the channel spacing is proposed.Application schemesPolarization interleaving or multiplexing have been used in the quest to increase the spectral efficiency of DWDM optical transmission systems. PDM system could achieve the purpose of data double output through transmitting two independently modulated and orthogonally polarized optical signals corresponding to the same frequency over a fiber.The sampled FBGs in PMF could be used to realize multi-wavelength polarization filtering function,so we design two kinds of application schemes of polarization filtering in PDM system.ConclusionThe sampled FBGs in PMF are first proposed.The reflection spectrum of the sampled FBGs in PMF is analyzed by coupled mode equations. The gratings not only have the characteristic of conventional sampled FBGs,but also have the advantage of narrower channel spacing or broader bandwidth.Simulation results about the new type of sampled FBGs in PMF show that the new type of sampled FBGs could be used as a multi-wavelength polarization filter in PDM system.