[0039] where ??n is a refractive index modulation, h is the height of the hologram (h1 and h2 in FIG. 11), ?? is a wavelength of light, and ?? is a diffraction angle (corresponding to the angle ????? of FIG. 1). [0040] The slope of the diffraction efficiencies ??s and ??p as a function of the wavelength ?? can be computed as: 2?????s?????=-sin???(2?????????? ???n??? ???h?????cos??? ?????)??2??????