Design Of Fluorotellurite Fiber With Near-zero-flatted Dispersion And Its Application In Mid-infrared Optical Frequency Comb

2⁵μm mid-infrared optical frequency combs have important applications in space and atmospheric communications,mid-infrared astronomy spectroscopy,high-speed molecular sensing and other fields,as well as remote sensing and ranging.In the fiber,the optical frequency comb is generated by injecting two or more continuous lights with a fixed frequency interval into the fiber.Using the cascade four-wave mixing effect,the comb-tooth interval can be adjusted from GHz to THz.At present,most fiber-based OFCs are produced by quartz-based highly non-linear optical fibers with near-zero flat dispersion.However,the quartz glass material is limited by its own transmission window,which makes it difficult to develop optical frequency combs in the mid-infrared band.In addition,the non-linear coefficient of quartz-based glass fiber is low,which limits the bandwidth of the OFC.In order to solve this problem,In response to the above problems,the author designed a fluorotellurite mid-infrared glass fiber with high non-linear coefficient and near-zero flattened dispersion,and this fiber is used as a nonlinear medium to successfully obtain broadband mid-infrared OFC with a wide-range tunable repetition frequencies in numerical simulation.The specific research results are as follows:（1）We designed a fluorotellurate fiber with near-zero flat dispersion produced by a mid-infrared optical frequency comb.The core material of the fiber is 70TeO₂-20BaF₂-10Y₂O₃（TBY）glass,and the cladding material is 33AlF₃-9BaF₂-17CaF₂-12YF₃-8SrF₂-11MgF₂-10TeO₂（ABCYSMT）glass.First,in the step structure fluorotellurate fiber,by changing the core diameter of the fiber,we obtained the dispersion curve with zero dispersion wavelength near 2μm.In order to obtain more flat dispersion characteristics at 2μm,a layer of high-refractive-index TBY glass ring structure is added to the cladding of the step structure fluorotellurate fiber,thereby introducing additional waveguide dispersion.By adjusting parameters such as the fiber core diameter and the thickness of the ring structure,and a 2μm band fluorotellurite glass fiber with nearly zero flat dispersion characteristics was obtained.Among them,when the wavelength is in the range of 1850²150 nm,the group velocity dispersion value of the fiber is-0.3 ps/km/nm⁰.32 ps/km/nm.This kind of optical fiber has the broadband mid-infrared transmission window,high nonlinear coefficient and near zero flat dispersion characteristics,which is beneficial to the generation of broadband mid-infrared optical frequency comb.（2）We studied the application of fluorotellurate microstructured fibers with near-zero flat dispersion in mid-infrared optical frequency combs.By using the symmetrical distribution Fourier transform method to solve the nonlinear Schr?dinger equation,the results of generating mid-infrared optical frequency combs based on the above-mentioned fluorotellurate fiber are studied.The ultra-short pulse laser with a repetition rate of 50 GHz is input into the above-mentioned all-solid-structure fluorotellurate glass optical fiber with near zero flat dispersion,then the broadband optical frequency comb with spectral range covering 1.1⁵μm was obtained in 2 m long fluorotellurate fiber.When the repetition frequency of the pump light is 10¹00 GHz,we can obtain a broadband mid-infrared optical frequency comb with adjustable repetition frequency of10¹00 GHz.The results show that the use of fluorotellurate fiber with near-zero flat dispersion as a nonlinear medium can produce broadband mid-infrared optical frequency combs with a wide range of tunable frequency intervals.