Holographic beam shaping of partially coherent light

Computer-generated holography with ordinary display

We propose a method of computer-generated holography (CGH) using incoherent light emitted from a mobile phone screen. In this method, we suppose a cascade of holograms in which the first hologram is a color image displayed on the mobile phone screen. The hologram cascade is synthesized by solving an inverse problem with respect to the propagation of incoherent light. We demonstrate a three-dimensional color image reproduction using a two-layered hologram cascade composed of an iPhone and a spatial light modulator.

Zero-order free holographic optical tweezers

Holographic optical tweezers (HOTs) use spatial light modulators (SLM) to modulate light beams, thereby enabling the dynamic control of optical trap arrays with complex intensity and phase distributions. This has provided exciting new opportunities for cell sorting, microstructure machining, and studying single molecules. However, the pixelated structure of the SLM will inevitably bring up the unmodulated zero-order diffraction possessing an unacceptably large fraction of the incident light beam power. This is harmful to optical trapping because of the bright, highly localized nature of the errant beam. In this paper and to address this issue, we construct a cost-effective, zero-order free HOTs apparatus, thanks to a homemade asymmetric triangle reflector and a digital lens. As there is no zero-order diffraction, the instrument performs excellently in generating complex light fields and manipulating particles.

Designing diffractive optical elements for shaping partially coherent beams by proximity correction

We propose a new method to design diffractive optical elements (DOE) for shaping partially coherent beams. The diffraction patterns of a DOE under a certain partially coherent beam can be modeled by the convolution of the coherent diffraction pattern and the inherent degree of coherent function. Two basic types of diffraction anomalies induced by partially coherent beams are discussed, including line-end shortening and corner rounding. A proximity correction (PC) method similar to the optical proximity correction (OPC) technique in lithography is used to compensate for these anomalies. The designed DOE exhibits good performance in partially coherent beam shaping and noise suppression.

Incoherent computer-generated holography

We present a method for computer-generated holography (CGH) using spatially and temporally incoherent light. The proposed method synthesizes a hologram cascade by solving an inverse problem for the propagation of incoherent light. The spatial incoherence removes speckle noise in CGH, and the temporal incoherence simplifies the optical setup, including the light source. We demonstrate two- and three-dimensional color image reproductions by a two-layer grayscale hologram cascade with a chip-on-board white light-emitting diode.

Inverse design of gradient-index volume multimode converters

Graded-index optical elements are capable of shaping light precisely and in very specific ways. While classical freeform optics uses only a two-dimensional domain such as the surface of a lens, recent technological advances in laser manufacturing offer promising prospects for the realization of arbitrary three-dimensional graded-index volumes, i.e. transparent dielectric substrates with voxel-wise modified refractive index distributions. Such elements would be able to perform complex light transformations on compact scales. Here we present an algorithmic approach for computing 3D graded-index devices, which utilizes numerical beam propagation and error reduction based on gradient descent. We present solutions for millimeter-sized elements addressing important tasks in photonics: a mode sorter, a photonic lantern and a multimode intensity beam shaper. We further discuss suitable cost functions for all designs to be used in the algorithm. The 3D graded-index designs are spatially smooth and require a relatively small refractive index range in the order of 10^-2, which is within the reach of direct laser writing manufacturing processes such as two-photon polymerization.