A Study On The Short Delay Characteristics Of A Sampled Fiber Grating

Fiber grating as an important device has many advantages in optoelectronic field,such as wavelength selectivity,small size and weight,easy integration with fiber optic systems,anti-electromagnetic interference ability.These make it play an important role in all fields of the information system and make it become fundamental components for a reconfigurable,easy to upgrade the intelligent all-optical network.Sampled fiber Bragg grating(SFBG)can provide comb-like multi-channel reflection spectrum,resulting in important applications in multi-channel signal processing.In order to realize the time-domain ps high-resolution signal time delay,it can be seen from the Fourier transform that Sine2 SFBG can produce multi-channel linear short time delay,but it is difficult to achieve in fabrication.In this paper,the Gaussian SFBG fabricated by a Gaussian laser beam was innovative proposed to obtain an approximate linear multi-channel ps time delay step.We designed,simulated and fabricated many Gaussian SFBGs with linear ps short time delay and measured the time delay of Gaussian SFBG by using high-resolution time delay measurement system.The results of these studies show that the multi-channel linear ps short time delay can be obtained by Gaussian SFBGs.First of all,we introduced the related knowledge of optical waveguides and fiber gratings,focusing on various applications of SFBGs.We found that multi-channel short time delay characteristics for SFBG have not been studied,but the application is increasingly important.The relationship between the refractive index of fiber core in spatial domain and the reflection channel profile of SFBG in frequency domain was studied by Fourier transform theory.The reflection and transmission spectra of fiber Bragg grating(FBG)and rectangular SFBG are simulated by coupling model theory,and the principle of multi-channels SFBG is analyzed.And the relationship between the profile of the SFBG reflection spectrum and the profile of the time delay is obtained,that is,the linear reflection spectrum profile of SFBG and the linear time delay profile corresponds to each other under the condition of weak grating,which is also suitable for the reflectivity of the channel as high as 90%.Thus,we can determine the time delay spectrum shape according to the grating reflection spectrum shape.Then,we established the core refractive index change model of Gaussian SFBG and analyzed the effect of parameters such as sampling number s,Gaussian function shape factor S and fiber grating length L on the SFBG time delay profile.We designed two kinds of Gaussian SFBGs and compared them.The first group of SFBGs can select six linear channels with an average time delay of 5.95ps and the standard deviation is 0.54ps for the time delay step.The optimized SFBG can provide eight linear channels with an average time delay of 2.83 ps and the standard deviation is 0.21 ps for the time delay step.Thus,we theoretically proved that this Gaussian SFBG can realize the linear multi-channel ps time delay.Based on our theoretical results and designs,we used the phase mask lateral UV exposure writing method to fabricate several groups of Gaussian SFBGs which are 2cm long.Through simulation and analysis,we found that during the fabrication,when light blocking slit widen,SFBG reflection spectrum will generate asymmetry due to short wavelength effect and the degree of alignment between the slit and the Gaussian beam center,etc.Then,we optimized the core refractive index change model of Gaussian SFBG.The simulated reflection spectrum of the optimized SFBG is very consistent with the experiment result.Due to the limited utilization of the refractive index,the fabricated sampled fiber grating has weak multi-wavelength channels.We select a linear profile reflectivity channel on the reflectivity spectrum of SFBG,which is expected to realize a linear multi-channel ps short time delay.In order to evaluate the real time delay characteristics of the designed and manufactured SFBG,we designed and built a high-resolution ps short time delay measurement system with vector network analyzer.We specially wrote the computer control program of the related instrument to control the measurement process,used the program to obtain the measurement data,and took measures to improve the stability and resolution of the measurement system.The measurement system measured the phase of the electro-optical modulation signal and converted it into a time delay value technically and mechanically.Thus,we obtained a 4-channel time delay spectrum for the Gaussian SFBG and the average time delay step of the four channels is 9.67ps,but the linearity is not good.Then,we optimized the Gaussian SFBG design according key requirements such as linearity,multi-channel and re-created a kind of grating.Through the high-resolution measurement,we obtained the ideal multi-channel time delay spectrum,we selected four channels within the grating and found that the time delay step is about 2.5ps and the standard deviation is 0.2ps.Thus,we confirmed that our SFBGs can be used to achieve linear multi-channel ps time delays.The multi-channel signal design and arrangement is an important aspect of information technology in the time-domain.With the development of technology,time-domain ps time delay technology has more and more demands,because the time and frequency are reciprocal relationship,so linear multi-channel ps time delay device also provides a technical approach to generate a terahertz(THz)signal.Our Gaussian SFBG is an excellent linear multi-channel ps time delay device that will be used in high-resolution signal process in time domain such as multi-beam high-frequency phased array radar systems and THz signal generation.