Study Of Polymer Micro-nano Photonic Devices Fabricated By Femtosecond Laser Direct Writing

Electronic technology has been developing at a very fast speed since its birth,leading the third industrial revolution in human history and bringing earth-shaking changes to human production and life.However,in recent years,with the further miniaturization of size and increase of integration density in integrated electronic circuit,the restriction of electronic thermal noise has been further magnified.Hence,the development of integrated electronic circuit has not accorded with the prediction of Moore's law,and the rate has gradually slowed down.Meanwhile,the integrated photonic circuit that analogous to integrated circuits has attracted a lot of attentions.The integrated photonic circuit,with photons as carrier,has taking advantages of higher transmission rate,more information load and lower loss.It is considered to be the direction of future innovation in science and technology.Therefore,the research on integrated photonic chips has broad prospects.Femtosecond laser direct writing technology has the advantages of high accuracy,high designability and true three-dimensional.It is irreplaceable in the fabrication of complex three-dimensional micro-nanostructures.Therefore,the purpose of this paper is to study the integrated photonic devices fabricated by femtosecond laser direct writing.Integrated photonic chip mainly includes integrated light sources,micro-optical components,optical waveguides,optical information processing devices and photodetectors.In this paper,we mainly focused on the fabrication and performance of whispering-gallery-mode(WGM)microcavity fabricated by femtosecond laser direct writing,which could be served as light source in photonic chip.Besides,we had studied the refractive index modulation in polymer induced by femtosecond laser,and fabricated tunable refractive index photonic devices included the quasi-microing microcavity,microlens and optical waveguides.In addition,we had explored the elementary work of polymer integrated photonic chips with femtosecond laser direct writing technology,which lays a foundation for the fabrication of polymer integrated photonic chips in the future.The main innovations of this paper are summarized as follows:1.Four kinds of Rhodamine B-doped photonic molecular micro-cavities with different coupling spacing were fabricated by femtosecond laser direct writing technology.The stable WGM laser oscillation mode was generated under 532 nm picosecond laser pumping.After studying and comparing the lasing spectra and threshold characteristics of four kinds of photonic molecular resonators with coupling spacing,it is concluded that the closer the coupling distance is,the stronger the micro-cavity coupling effect is,the stronger the suppression of laser main resonance mode and sub-resonance mode is,and the lower the threshold value correspondingly.After the intersection of two single disks,the more intersecting micro-cavities,the more severely the WGM mode is destroyed.Thirdly,as the suppression of the main and sub-resonant modes become weaker,the threshold become higher.To a certain extent,the output single-mode laser can be obtained.2.PEGDA hydrogel with humidity response was proposed as the material of micro-resonator.Rhodamine B was served as the laser gain medium and photoinitiator of two-photon polymerization.The hydrogel micro-disk resonator was fabricated by femtosecond laser direct writing technology.Hydrogel microcavities have good humidity response characteristics.When the ambient humidity increases,they can absorb water molecules and cause volume expansion,while the ambient humidity decreases,the volume shrinkage is caused by water loss.Under 532 nm picosecond laser pumping,a good output mode of WGM laser is achieved,and the Q value of the microcavity reaches a relatively high level of 2.8×10~3.Because the size of the hydrogel microcavity varies with the ambient humidity,the laser resonance wavelength of the hydrogel microcavity varies with the ambient humidity.The experimental results show that the resonance wavelength of the hydrogel microcavity is approximately linear red shift among the relative humidity of25%-65%,and the red shift of the wavelength is 1.72nm.The PEGDA hydrogel microcavity fabricated by femtosecond laser direct writing technology can achieve a relatively high sensitivity of 43.11 pm/%RH for environmental humidity sensing.3.The relationship between the change of refractive index of polymer induced by femtosecond laser and the scanning times of femtosecond laser was studied.The rough relationship between the change of refractive index and the scanning times of laser is obtained by measuring the focal length of microlenses with the same size and different scanning times.With the increase of laser scanning times,the change of refractive index of polymer induced by femtosecond laser increases until the maximum degree of polymer polymerization is reached.Based on the refractive index modification by femtosecond laser,we fabricated gradient refractive index(GRIN)microlenses with different sizes and quasi-micro-ring single-mode lasers using femtosecond oscillators.In addition,we have fabricated centimeter-scale optical waveguides and couplers using a large-range air-floated femtosecond processing system through multiple scans inside the polymer,and achieved good light transmission.This method can integrate any three-dimensional photonic devices with one step on integrated photonic chips.4.By studying the expansion ability of hydrogel microstructures with different laser scanning spacing,it is proved that the expansion/contraction ability of hydrogel microstructures is approximately proportional to the laser scanning spacing.Based on the experimental results,a hydrogel plano-convex microlens with different scanning spacing was designed,and the expansion ratio of the upper and lower layers was the same in water to obtain a single focus.But in air,the structure with large spacing between the upper and lower layers had a larger shrinkage ratio than that with small spacing between the lower layers,which resulted in the plano-convex lens showing two curved surfaces and obtaining two focal points.We used femtosecond laser to fabricate non-uniform hydrogel microlenses,which can intelligently transform single focus and double focus states in different environments,and can be applied to real-time dynamic imaging of moving objects.In summary,based on femtosecond laser direct writing technology,we had designed and fabricated active WGM microcavity lasers,microlenses,optical waveguides and other photonic devices.The integration of polymer photonic devices on a chip is attempted to provide a solution for the future fabrication of polymer integrated photonic chips.