Research On Analysis Methods For Diffraction Characteristics Of Varied-refractive-index Volume Holographic Grating And Fabricating Technologies

Volume holographic grating is becoming a more competitive dispersion opticalelement, due to its unique and desirable properties, such as high diffraction efficiency,high wavelength and angle selectivity, low noise, relatively low dependence onpolarization angle and temperature. Volume holographic grating has received muchmore attention in spectral analysis, astronomical observation, data storage and fiberoptic communication. However, practical gratings tend to deviate from the idealconditions during complicated technological processes, for instance, variable fringespace, non-uniform modulation distribution and so on, which lead to the technicalindexes deviated from the value of the design. Thus, constructing diffractionefficiency calculation method for non-uniform mixed modulation grating, andestablishing the model of varied-line-spacing grating caused by swelling or shrinking,are very necessary for analyzing diffraction characteristics of the non-ideal gratingand guiding the process of grating fabrication.Given these problems, the analysis methods for the diffraction characteristics of varied-refractive-index volume holographic grating, the fabricating technologies,and the application in PGP imaging spectroscopy are researched thoroughly in thispaper.First, a method for calculating diffraction efficiencies of attenuated grating andmixed phase-amplitude grating with a phase difference is presented. Coupled-waveequations about the amplitudes of internal diffraction waves are reconstructed, byassuming that the relative permittivity and absorption as a function of film thickness.Diffraction characteristics changing with the parameters during recording processare investigated in detail, including absorption constant, absorption modulation, andattenuation coefficient, phase difference between the phase and absorption grating,and the sub-regions. It is a useful and rigorous method for thick gratings analysis.Second, diffraction characteristics of reflection volume holographic grating atnon-uniform swelling and shrinking are analyzed, by introducing a function betweenswelling factor and film thickness. The expressions of average refractive index andthe modulation are derived to discuss the influences on angular and wavelengthselectivity. Accordingly, the efficiency curves are given not only when the swellingfactor is expressed as a constant, a linear or a nonlinear function, but also the case inwhich the swelling factors are unequal at the exposure area and the unexposed area.The method can solve the problem which the index varies along the depth.Third, a transmission grating with high diffraction efficiency at the visiblespectral coverage is developed. Parameters optimization, manufacturing flow,technological parameters and controlling method are given, the result is satisfiedwith design requirements, though testing the angle and wavelength selectivity.Fourth, as a specific application, integration design method of PGP is provided,and the software is developed with the functions of design, calculation and analysis.The influencing factors on diffraction characteristics of PGP are researched,including the structure parameters of prism and grating, the Bragg wavelengthshifting, and the index of the substrate. A PGP component with wide spectralcoverage from400nm to1000nm is designed, the diffraction efficiency is not less than50%. The method can realize PGP integration design and integration analysis.