Holographic diffuser

A method for the preparation of holographic diffusers applicable to light source with multiple wave lengths, comprising the following steps:

• preparing a substrate made of a plurality of units;
• determining a thickness of the holographic diffuser, a unit area of said holographic diffuser, and sizes of said units;
• calculating etching depths according to phase distribution matrices Φn of light having wavelengths λn passing through said diffuser by:
• (a) using a random number generator to generate initial etching depths do and amplitude values A1 for optical field function Ψ1=A1*exp(iφ1) in the spatial frequency domain;
• (b) calculating a phase distribution matrix φ1 in the spatial frequency domain from the initial etching depths to obtain an optical field function Ψ1=A1*exp(iφ1) in the spatial frequency domain;
• (c) performing an inverse Fourier transform on optical field function Ψ1 to obtain the optical field a1*exp(iΦ1) and the phase distribution matrix Φ1 in the spatial domain for a first wavelength λ1, to obtain adjusted etching depths;
• (d) using the phase distribution matrix Φ1 and the adjusted etching depths, to calculate a new phase distribution matrix Φ2 at a second wavelength λ2 in the spatial domain;
• (e) obtaining a second optical field in the spatial domain a1*exp (iΦ2), performing a Fourier transform on said second optical field to obtain an optical field function Ψ2=A2*exp(iφ2) for wavelength λ2 in the spatial frequency domain;
• (f) adjusting amplitude coefficients A2 of optical field function Ψ2 to permit a desired amplitude of light having wavelength λ2 to pass through said diffuser and therefore provide a desired mixing effect, thereby obtaining an adjusted amplitude A2′ and an amplitude adjusted optical field function Ψ2′;
• (g) performing an inverse Fourier transform of the amplitude-adjusted optical field function Ψ2′ for wavelength λ2 to obtain the optical field a2*exp(iΦ2′) and an adjusted phase distribution matrix Φ2′ in the spatial domain to obtain a new set of adjusted etching depths;
• (h) using the phase distribution matrix Φ2′ and the new set of adjusted etching depths, to calculate a new phase distribution matrix Φ3 at a third wavelength λ3 in the spatial domain;
• (i) obtaining a third optical field in the spatial domain a2*exp (iΦ3), performing a Fourier transform on said third optical field to obtain an optical field function Ψ3=A3*exp(iφ3) for the wavelength λ3 in the spatial frequency domain;
• (j) adjusting amplitude coefficients of A3 of optical field function Ψ3 to permit a desired amplitude of light having wavelength λ3 to pass through said diffuser and therefore provide a desired mixing effect, thereby obtaining an adjusted amplitude A3′ and an amplitude adjusted optical field function Ψ3′;
• (k) performing an inverse Fourier transform on amplitude-adjusted optical field function Ψ3′ for wavelength λ3 to obtain the optical field a3*exp(iΦ3′) in the spatial domain;
• (l) repeating steps (h)-(k) for wavelengths λ4 to λn where n is the number of wavelengths to be diffused by said diffuser;
• (m) repeating steps (h)-(k) for wavelengths λn to λ1 to form an iterative operation loop and obtain a set of final adjusted depths based on the final adjusted phase distribution matrix in the spatial domain;
• determining patterns of masks according to the final adjusted depths di so obtained;
• preparing masks according to patterns so determined;
• and etching said substrate with said masks.