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Holographic optical elements (HOEs) are versatile alternatives and complements to conventional refractive and reflective optics. With few exceptions, the manufacturing of HOEs based on holographic recording requires monochromatic light with a long coherence length and narrow linewidth, typically provided by lasers operating at a single longitudinal mode. The wavelength of the recording light is a key process parameter as it directly affects the spectral behavior and selectivity of the HOE at replay. Widely tunable continuous-wave optical-parametric-oscillator (OPO) devices are becoming increasingly popular in the field of applied holography, offering unparalleled wavelength coverage in the visible and near infrared spectral range in a single device with coherence and stability properties that match holography’s stringent requirements. The OPO’s wide tuning range translates into equally wide ranges of HOE replay wavelengths with little to no adjustment of the holographic recording geometry. This allows for drastic simplification of recording tools, as well straightforward adjustment of exposure recipes to compensate for any postprocessing shifts. In this paper, we present newly developed cw OPO systems operating at Watt-level output powers and their use in holographic recordings of uniformly slanted volume holographic gratings (VHGs). The tunability of the OPO system’s output wavelength is a crucial feature enabling the use of highly efficient scanned-beam exposure approaches. We demonstrate this by using a single recording geometry to produce an array of slanted VHGs whose notch spectral band centres were defined by adjusting only the recording wavelength supplied by the OPO systems. Importantly, most of those recording wavelengths did not correspond to commonly available laser wavelengths – they were only achievable thanks to the free tunability of the OPO sources. Captured in state-of-the art photopolymer material, the holographic filters themselves exhibit anomalous reflection-like behavior with high diffraction efficiency (DE) giving optical densities up to OD4, and linear angle-tuning around normal incidence. Collectively this array of filters covers the full visible spectrum. The capability presented here is straightforwardly extended to more complex VHGs like HOEs with optical power, wavelength-multiplexed waveguide-couplers and free-space holographic image combiners. |
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