Coherent stitching of light in multilayered diffractive optical elements

Abstract

Diffractive optical elements serve an important function in many dynamic and static optical systems. Multilayered diffractive elements offer powerful opportunity to harness both phase and amplitude modulation for benefits in diffraction efficiency and beam shaping. However, multilayered combinations have been difficult to fabricate and provide only weak diffraction for phase gratings with low refractive index contrast. Femtosecond laser writing of finely-pitched multilayer volume gratings was optimized in bulk fused silica. We identify and quantify an optimum layer-to-layer separation according to Talbot self-imaging planes and present systematic experimental validation of this new approach to enhance otherwise weakly diffracting volume gratings.

High-performance geometric phase elements in silica glass

“…Also, the light wave propagating through the large thickness of the structure, i.e., from several microns to tens of microns, could be detuned, and an additional alignment of the geometry of stacked layers should be performed. 43 Roughly 90% of the transmitted light is projected to the ±1st order and 10% to the 0th and higher orders [ Fig. 3(d)].…”

Femtosecond laser writing of phase-tuned volume gratings for symmetry control in 3D photonic crystal holographic lithography

“…Two sets of linear phase gratings have been positioned orthogonally and illuminated with a laser to generate a 3D interference pattern in the proximity of the DOE as shown by the woodpile structure in the inset image. In order to build up strong first order diffraction, each X-or Ygrating set followed our prior design [22], with track layers periodically formed on an integer number of Talbot distances, g c , in the fused silica. When the offset, d, between the two orthogonal grating sets are also an integer or half integer number of Talbot distances, the combined interference pattern will follow BCT symmetry.…”

“…1) is considered for the orthogonal combination of identical linear phase grating stacks, where the X-and Y-oriented grating periods of x Λ and y Λ , respectively, are equal ( x Λ = y Λ = Λ ) and illuminated with coherent light of wavelength λ . Following [22], the optical coherent stitching condition requires an assembly of the grating layers on an integer multiple of Talbot distances given generally for a medium, i, of index, n i , by:…”

“…One typically desires a small lateral grating period together with a large diffraction angle such that Eq. 1 [22] for the case of 1 µm grating period, using optimal laser exposure conditions of 1 mm/s scan speed and 35 mW power. Coherent stacking of grating layers on Talbot planes was designed for 532 nm probing wavelength, applied on a double period spacing of 2 g c = 10.6 µm to accommodate the large axial modification zone of ~7 µm that would have otherwise coalesced the tracks if g c = 5.3 µm spacing was used.…”

“…A promising direction for enhancing the efficiency of low-contrast volume gratings relies on breaking apart the phase structure into multiple layered phase elements, an effect first observed in models by Nordin et al [13]. Ng et al took advantage of the flexibility of femtosecond laser writing and showed in both experiments and modeling that strong 1D diffraction can be built up coherently when multiple phase grating layers were assembled on Talbot self-imaging planes [22]. In this way, low contrast volume gratings can now be extended to highly efficient high-resolution applications, by-passing the restrictive Littrow configuration.…”