Mechanism And Application Of PTR Glass Exposure By Femtosecond Laser

Because of the specific characteristics of femotosecond(fs)pulse laser including ultrashort pulse,high peak power and other extreme physical conditions,the interaction between fs laser and dielectric materials can induce a series of special phenomena different from the traditional laser.These new effects and phenomena greatly enriched the understanding of the interaction between laser and materials and promoted the development of laser field.Meanwhile,the local refractive index of transparent materials can be selectively modulated by focusing fs laser,leading to the development of complex three-dimensional embedded optical path.Photo-thermo refractive(PTR)glass is a multifunctional substrate,which is utilized for diffraction optical devices,microfluidic devices and micro electromechanical system.In this thesis,high quality optical device was prepared by fs laser selective exposure and subsequent heat treatment(HT)process,which provided a new idea for the design and manufacture of integrated devices.Also,the application of PTR glass device was promoted.The main research contents and results are listed as following:The characteristics of interaction between pulsed laser and PTR glass were studied.The concentration and the size of the nano-crystals could be effectively regulated by controlling writing power,pulse duration and HT time.Microstructures of nano-crystals in laser exposure regions were observed by scanning electron microscopy(SEM).The results show that the distribution of nano-crystals depends on the laser power density distribution,but the size of the nano-crystals is found to be quasi laser-independent.With the increase of the writing power,the overall microstructure of the nano-crystals agglomeration evolves from sparse distributions to dense ones,and furtherly forms compact nano-crystal clusters.Besides,with the increase of the pulse durations,the distribution of nano-crystals becomes narrow and dense,and then it becomes sparse until disappears.Also,the size of nano-crystals is dependent on HT time,which decreases from 175 ± 50 to 100 ± 50 nm with the time of HT halved.These results demonstrate that the distribution and size of crystals could be controlled by writing laser and HT,respectively.In addition,nano-crystals in laser exposure regions were analyzed by wavelength dispersive spectrometer(WDS)and Raman spectroscopy,which confirmed Na F as the main component of the nano-crystal.The transmission volume Bragg gratings(TVBGs)with a period of 5 ?m were fabricated in PTR glass by fs laser Bessel beams.The influences of grating thickness and HT on the diffraction efficiency(DE)were investigated.The results show that the maximum DE increase 26.65% due to the increase of the grating thickness by using Bessel beams shaping technique.Moreover,owing to the increase of refractive index difference by HT,the maximum DE could increase 90.00%.The relationships between DE value and writing laser power were also tested.The results show that refractive index modulation of PTR glass is smaller at low laser power,which could affect the DE of grating.When the laser power is high,the self-focusing and self-defocusing phenomenon emerge in transmission direction producing discontinuous trace and deteriorating the quality of TVBGs.The highest DE value recorded with Bessel beam is 94.73% at writing power of 100 m W and testing wavelength of 532 nm.Based on the above experimental results and theoretical analyses,the diffraction properties of TVBGs in PTR glass can be tailored by controlling writing conditions and HT.The superior diffraction property of PTR TVBGs indicates that they can be attractive candidates for spatial filtering elements.Double line waveguide were successfully fabricated in bulk PTR glass using fs laser transverse writing.The waveguide geometrical structures and guiding properties were investigated before and after HT.The results show that double line separation,writing power and HT have significant effects on the guiding properties.The best single mode guiding performance is obtained at center wavelength of 800 nm,repetition rate of 50 KHz,pulse duration of 200 fs,writing pulse power of 50 m W and double line spacing of 30 ?m for PTR glass without HT.HT could eliminate the residual stress,also it could destroy the cross-section structure and affect guiding properties of double line waveguide.Besides,Bragg gratings with different periods were fabricated in PTR glass by controlling the pulse frequency and writing speed.Laser writing power and pulse duration to the modulation of the refractive index of traces were studied.Bragg grating waveguide were fabricated.The results show that Bragg grating waveguide with length of 6.5 mm owns a modulation of 3 d B prepared by writing parameters of repetition rate of 100 Hz,writing speed of 156.04 ?m/s,pulse duration of 3 ps,and writing pulse power of 150 m W.Tubular depressed cladding waveguide structures were successfully fabricated in bulk PTR glass by fs laser transverse and longitudinal writing.The results show that waveguide diameter,writing power and HT have significant effects on the guiding properties.Tubular depressed cladding waveguides with circular cross-sections possess good circular symmetry and guiding properties.Single mode guiding is observed when waveguide diameter is less than 30 ?m.Multimode fields can be obtained by controlling the waveguide diameter and writing power.The simulations agree well with the experiment results.Moreover,HT provides a facile route to eliminate uneven stress distribution,increase the effective refractive index variation and control the normalized frequency of the depressed cladding tubular waveguide.Such key parameters could be used to define the waveguide quality and determine modal control in the waveguide.Therefore,PTR glass can be an interesting material to fabricate depressed cladding tubular waveguides,which could transmit large mode area laser and long wavelength laser by designing and optimizing the structure of waveguides.