Multifunctional DOEs are elements that can simultaneously perform various
optical operations. The terms multifunctional and multiplexed diffractive optics come under the same category. Multiplexing refers primarily to spatial as well as wavelength multiplexing, where it is possible to encode several functions on the same element but extract one specific behavior at a time. In the case of phase multiplexing, however, rather than extracting one single behavior from the element independently, several functions can be combined and accessed simultaneously. Although, the above definition is used more commonly in computer-generated holograms (CGHs), we will use the same terminology to talk about diffractive optical elements. Multifunctionality due to tenability—achievable, for example, when the DOE is created with a SLM or using micro-electromechanical systems (MEMSs)—was not taken into account in this chapter. However, all of the multifunctional static DOEs described in this chapter can also be created using a SLM.
There are various types of phase-multiplexed DOEs. One well-known multiplexing technique is to use multiple DOEs or ROEs with different functions. Reports of beam shaping using multiple DOEs have been published. Bandpass filters with two DOEs fabricated as two layers have also been reported. In some cases, it is necessary to construct hybrid optical elements such as refractive–diffractive or reflective–diffractive combinations for different applications. While the above constructions are possible, there are many concerns that must be duly noted. In the case of multiple DOEs or ROEs, the optics configuration becomes bulky. The many optical components introduce problems during optical alignment and increase the overall weight of the system. In the case of hybrid DOEs, fabrication of the device is quite complex, as it involves fabricating high-resolution DOEs on curved surfaces.
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