Research On The Techniques And Properties Of Surface Nanoscale Axial Photonics Microresonators With A Femtosecond Laser

Surface Nanoscale Axial Photonics(SNAP) technology,as an emerging nanophotonic technological platform that enables the fabrication of miniature whispering gallery mode resonant photonic circuits by modifying effective radius variations(ERVs)at the surface of an optical fiber with ultra-low loss,subangstrom precision.Researchers are struggling to pursue a flexible and precise technique to engineering the ERV distributions of SNAP microresonators,which helps promote its applications ranging from all-optical signal processors,optical sensing,optical computing,and communications to fundamental science.Compared with other fabrication techniques on SNAP devices,femtosecond laser inscription technology has the advantages of micrometer-sized focal volume,which is much suitable for introducing and engineering the ERVs along the fiber surface with high precision and excellent flexibility.However,the research on the physical mechanism of the interaction between femtosecond pulses and transparent materials is still incomplete.The related techniques are not deep and mature enough.In this thesis,the fabrication of SNAP structures with the femtosecond laser inscription technology is systematically researched.The influences of the femtosecond laser fabrication parameters and different processing methods on the performance of the fabricated SNAP microresonators are investigated and discussed,so as to realize the flexible control and accurate fabrication of SNAP devices with ultra-low loss and microscopic dimensions.The main contents of the thesis include:(1)based on the axially-oriented inscription inside the optical fiber with a femtosecond laser,we build the multi-dimensional fabrication parameters system and systematically investigate the relationships between the introduced ERV and the multidimensionally controllable fabrication parameters,such as the number of the axially inscribed lines,the spacing between the adjacent axially inscribed lines,the single pulse energy,the spacing between the adjacent inscribed layers,the number of inscribed layers,and the ratio of the repetition rate and the translating speed.Specifically,both the qualitative and quantitative processing principles are revealed.Meanwhile,we proposed the multidimensional fabrication method,which helps to increase the inscription space from one or two-dimension to three dimensions by the layer-to-layer inscription technique,thus effectively improve the controllable range of the ERV of SNAP microresonators.As a proofof-principle,by multi-dimensionally optimizing the fabrication parameters,we realize a SNAP microresonator with the characteristics of both small axial size(~50 ?m)and maximal ERV(>25 nm).The achieved ERV is almost 5 times larger than that achieved with the previous unoptimized method.The demonstrated multi-dimensional inscription scheme not only promotes the femtosecond laser inscription technology to be a flexible and precise approach on fabricating the complex SNAP devices with ultra-low loss and miniature sizes but also pays the way to improving the fabrication precision with the femtosecond laser post-processing technique.(2)The subangstrom precise correction of SNAP microresonators by the femtosecond laser postprocessing technique is demonstrated for the first time.In our experiments,the post-exposure is applied with lower pulse energy at the original exposure locations in the initial fabrication process.The results show that the ERV is nearly proportional to the pulse energy of the post-exposure process,which can be used to accurately correct the ERV errors.The repeatability is experimentally verified by accomplishing the correction on three SNAP microresonators with a precision of 0.75 ?.(3)Based on the axially-oriented inscription inside the optical fiber,we proposed a powerful approach for the fabrication of the SNAP structures with arbitrary profiles by a femtosecond laser.Our method is to accurately tailor the length of all inscribed lines and make the profile of the length distribution of the inscribed lines to match the profile of the required SNAP microresonator.In experiments,we demonstrate the design and fabrication of the SNAP structures with the parabolic,semi-parabolic,and bat profiles.The experimental results are well in agreement with the theoretical designs.The developed approach is expected to be universal for the fabrication of complex high Q-factor SNAP structures,which lays the groundwork for exploring the versatile performances of the SNAP devices.(4)A novel fabrication method of rectangular SNAP microresonators with high contrast,high Q-factor,and flexible control of axial size is developed,which consists of the inscription of series of lines along the cross-sections of an optical fiber with a femtosecond laser.The contrast of the fabricated resonators achieves ? ~ 1 nm/?m,which is a 50-fold improvement compared to methods developed previously.Besides,the introduction of ERV is several times larger than the maximum ERV demonstrated previously.Furthermore,for the first time,the group delay characteristics of rectangular SNAP resonators are investigated and verified.The experimental results are in good agreement with theoretical simulations.Overall,the developed approach makes a breakthrough of the important technical challenges in the SNAP platform for increasing the contrast of rectangular SNAP microresonators and allows us to reduce the axial scale of SNAP structures by an order of magnitude.It is meaningful for the future improvement of the capability of multifunctional SNAP devices.