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Diffractive Optical Elements (DOEs) utilize diffraction at sub-micron features to re-direct and control light. Volume phase holographic materials, such as photopolymer, are advantageous for use in fabricating optical elements because the diffraction efficiency can approach 100%, the whole element can be recorded in one exposure and high diffraction and slant angles are possible. Self-developing photopolymers also facilitate mass manufacture. Holographic gratings have been developed for numerous applications including spectroscopy, solar concentration, and monochrome LEDs, however, the inherent angular and wavelength selectivity of the volume phase hologram generally restricts applications to laser systems and sources with narrow spectral ranges. Multiplexing more than one grating into a single photopolymer layer can extend the range, however, unwanted additional gratings are frequently recorded.
In this paper, we discuss laminating multiple photopolymer HOEs together as a method for increasing the wavelength and angular working range of devices. This involves combining HOEs designed to produce the desired output beam for different angular and/or spectral input beams. Stacking of photopolymer layers has previously been demonstrated to increase the angular range of gratings and recently the authors produced a compound HOE with significantly broadened wavelength and angular selectivity curves by laminating two HOEs recorded sequentially at a single wavelength. However, such solutions are not easily translated to more complex elements such as lenses where the spatial frequency and grating slant angles are varying.
This paper discusses laminating together two photopolymer layers sensitized for different recording wavelengths for the purpose of holographically recording a compound-element volume-HOE lens for use with a broadband LED. The angular and wavelength selectivity are characterized and the challenges and advantages of the different approaches are discussed and compared.
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