Theoretical And Experimental Research On Sampled Fiber Gratings Used For Time-domain High-resolution Delayed Signal Processing

Fiber grating has many advantages such as good compatibility with fiber system,immune to electromagnetic interference,compact size,wavelength selectivity and so on,which is a key device for realizing reconfigurable and easy upgrade intelligent all-optical networks.Sampled fiber grating(SFG)which is realized using sample function to periodically modulate refractive index of fiber grating has multi-wavelength filtering characteristics,thus it is widely used in wavelength division multiplexing(WDM)system and multi-wavelength dispersion compensation.SFG with special time delay profile makes it possible to generate multi’wavelength channels with linear picosecond(ps)time delayIn this paper,Sine2 SFG is proposed to generate precise time delays in ps across many wavelength channels for phased array antenna system delay compensation.Fourier Transform Theory and Coupled-mode Equations are used to simulate spectral characteristics of SFG.We analyze and simulate for Sirne2 SFG,and show that 16 wavelenght channels cai be achieved with an average time delay step of 0.25ps with 4.2%linearity.However,fabrication of Sine2 SFG is difficult,as fast vary of refractive index change of Sine2 SFG in grating period down to several microns.As a result,we present that Gaussian SFG can realize short time delays over a number of wavelength channels with a required time delay profile.Modeling results show that the designed Gaussian SFG can provide 8 wavelength channels with an average delay step of 2.83ps with 7.4%linearity.Direct Gaussian laser beam inscription combined with phase mask plate technique is used to fabricate Gaussian SFG.For a Gaussian sampled fiber grating fabricated,an average time delay step of 4.49 ps with 10%linearity for 8 wavelength channels,and an average delay step of 2.5 ps with 8%linearity if for 4 channels,are realized.A physical model of cascaded equivalent Fabry-Perot cavities together with rectangular truncation of the inscription laser beam for the Gaussian sampled refractive index is suggested,and used to explain the grating spectra.A technique for time delay step tunability of the grating is presented,Average delay step can be tuned from 2.8 ps to 4.8 ps,and the tuning range is 2ps.We demonstrate that the sampled fiber gratings have capability to provide a large number of picosecond time delays with good linearity.This offers a simple technique for ps-high-resolution signal processing in time-domain and THz signal generation.