Coherent stitching of light in femtosecond laser formed multi-layered volume gratings

Diffractive Optical Elements serve in complex light systems for enhancing control of beam shaping, pulse shaping, microscopy, optical tweezers and 3D imaging. Multi-layered diffractive elements such as photorefractive volume gratings and computer-generated holograms further offer phase and amplitude modulation, but have been challenging to fabricate and are limited by weak diffraction. To this end, femtosecond lasers have been attractive to pattern refractive index structures inside transparent materials, creating 3D optical circuits and diffractive optics with new types of form-birefringence. However, such flexible writing is burdened with limited resolution and low phase contrast from positive and negative index zones. In this paper, we show that the principle of cascading diffractive structures to accumulate stronger phase contrast breaks down for weak gratings, particularly when the phase elements shrink to the optical wavelength.