Study On Propagation Properties And Optimization Design Of Broadband Optical Filter

For large laser system,the beam quality can become worse because of the defects of optical components,thermal effect and nonlinear effect,which bring amplitude or phase modulation during the beam propagation.The quality of the beam is the main reason to restrict the development of high-power laser system.It's very necessary to use spatial filtering mechanism for improving the quality of laser beam.Mostly,the traditional spatial filters,such as 4F spatial filter,slit filter or the angular filter with volume Bragg grating,focus on the laser with monochromatic or quasi-monochromatic wavelength.With the development of ultra-fast laser technology,the laser pulse becomes shorter,as femtosecond pulse or even attosecond pulse,and the bandwidth of laser should be broadened.New methods of broadband optical spatial filters bring new requirements of broadband laser propagation.In our work,two types of broadband filters combined by binary diffraction gratings and volume Bragg gratings(VBGs)are investigated.The filtering properties are studied analyzed by using coupled-mode theory and Fourier transform method as follows:(1)Two types of three-pieces and four-pieces filtering systems are established and investigated under the design central wavelength of 1053 nm.The filtering properties of system efficiency and width are analyzed.The three-pieces filter with blazed gratings and VBG combined shows a bandwidth more than 200 nm with absolute diffraction efficiency up to 78.2%.The four-pieces filter with binary phase gratings and VBGs combined shows tunable bandwidth from 50 nm to 240 nm with absolute diffraction efficiency up to 65.6%.In addition,the near-field contrast,modulation and power spectral density,the cut-off frequency are also studied to characterize the filtering properties.The GUI(Graphical User Interface)of the optical filter is designed by Matlab software.(2)The design parameters tolerance of the two optical filters are discussed.The results show that the binary grating parameters mismatch affect the system efficiency more than the filtering performance.For binary grating with period of 10?m,cut-off frequency of 1.5 mm-1 and 3 mm-1,the tolerant changes of period are respectively 26 nm and 56 nm,as the diffraction efficiency declines less than 10%.For VBG with period of 5?m,thickness of 2.1 mm,refractive index of 1.496 with index modulation of 250 ppm,the thickness and index modulation mismatch tolerance are separately 0.43 mm and 51 ppm.(3)The beam divergence angle could be produced by the binary grating dispersion,which brings worse system filtering properties,such diffraction efficiency and angle selectivity.The relationship between the half-width of beam divergence angle and system parameters are also studied and analyzed.The beam divergence angle is mainly affected by the beam size for these two types of the filtering systems.For broadband optical filters combined with a blazed grating with center wavelength of 1053 nm and period of 5?m,and VBG with an angle selectivity of 1.50 mrad,1.0mrad,and 0.5mrad,the smallest beam sizes are separately 1.9 mm,2.8 mm,and 5.8mm.