Single-wavelength writing for free space WDM holograms at arbitrary wavelengths

Phenanthrenequinone doped poly(methyl methacrylate) (PQ:PMMA) is a well-known write-once read-many (WORM) holographic substrate polymer. Its WORM capacity makes it ideal for applications where stability and longevity are essential, such as in free-space communications and metrology. More specifically, wavelength division multiplexing (WDM) holograms that simultaneously operate at many arbitrary wavelength bands are useful for free space systems where each band may be optimized to deal with different properties of the medium. Holograms written in this substrate typically fall into the Bragg regime, allowing the grating to operate at nearly any optical wavelength, albeit at different angles. Yet, setups designed to write many overlapped gratings, each operating at a distinct wavelength, are often complex and require re-tuning for each target wavelength. WDM is achieved when multiple gratings are stacked: each designed to diffract efficiently at a distinct wavelength with a unique input angle, but a shared output angle. However, when designing holograms to multiplex radically distinct wavelengths (e.g., 780 nm and 1550 nm), one must consider the electrical susceptibility of PQ:PMMA in order to accurately predict the refractive index modulation that will result from a given exposure. In this work, we implement a two-level model of the electrical susceptibility into the WDM design process in order to better predict the refractive index modulation at any given wavelength. This allows the diffraction profiles to be optimized according to the application without requiring retuning of the setup.