S.A. Goorden

Superpixel-based spatial amplitude and phase modulation using a digital micromirror device

We present a superpixel method for full spatial phase and amplitude control of a light beam using a digital micromirror device (DMD) combined with a spatial filter. We combine square regions of nearby micromirrors into superpixels by low pass filtering in a Fourier plane of the DMD. At each superpixel we are able to independently modulate the phase and the amplitude of light, while retaining a high resolution and the very high speed of a DMD. The method achieves a measured fidelity F = 0.98 for a target field with fully independent phase and amplitude at a resolution of 8 × 8 pixels per diffraction limited spot. For the LG10 orbital angular momentum mode the calculated fidelity is F = 0.99993, using 768 × 768 DMD pixels. The superpixel method reduces the errors when compared to the state of the art Lee holography method for these test fields by 50% and 18%, with a comparable light efficiency of around 5%. Our control software is publicly available.

Programming balanced optical beam splitters in white paint

Wavefront shaping allows for ultimate control of light propagation in multiple-scattering media by adaptive manipulation of incident waves. We shine two separate wavefront-shaped beams on a layer of dry white paint to create two enhanced output spots of equal intensity. We experimentally confirm by interference measurements that the output spots are almost correlated like the two outputs of an ideal balanced beam splitter. The observed deviations from the phase behavior of an ideal beam splitter are analyzed with a transmission matrix model. Our experiments demonstrate that wavefront shaping in multiple-scattering media can be used to approximate the functionality of linear optical devices with multiple inputs and outputs.

Confocal microscopy through a multimode fiber using optical correlation.

We report on a method to obtain confocal imaging through multimode fibers using optical correlation. First, we measure the fibers transmission matrix in a calibration step. This allows us to create focused spots at one end of the fiber by shaping the wavefront sent into it from the opposite end. These spots are scanned over a sample, and the light returning from the sample via the fiber is optically correlated with the input pattern. We show that this achieves spatial selectivity in the detection. The technique is demonstrated on microbeads, a dried epithelial cell, and a cover glass.

Quantum pattern recognition

Summary form only given. In quantum cryptography, non-cloning properties of quantum states are exploited to achieve quantum security. At the same time, typically one optical mode is used, for instance that of a single-mode optical fiber, effectively reducing the problem to a one-dimensional system. Only recently have researchers begun to exploit higher dimensions for quantum cryptography [1]. High dimensional spaces appear naturally in imaging applications.We here experimentally investigate the question if it is possible to recognize an arbitrary pattern in a physical object using much fewer photons than the complexity of the pattern. Theoretically it has already been proposed to use a quantum computer for pattern recognition and that this can be achieved with very few photons [2]. To avoid using a quantum computer, we exploit a spatial light modulator, which can map a single incoming optical mode on thousands outgoing optical modes in a programmable way. In this way, a weak light pulse containing only a few or even a single photon can be distributed over the high number of pixels of a complex 2-dimensional image. The inverse is just as feasible: a known complex pattern can be mapped onto a single mode. We use this principle to analyze the light from a physical object when illuminated with, e.g., a plane wave. After reflection of or transmission through the object under investigation, the response can be projected onto a single mode. The signal in that mode can be analyzed with the standard repertoire of quantum optical measurements, determining, e.g., the field amplitude, the intensity or the number of photons.We use coherent pulses of light which have mean photon numbers much smaller than the thousands of modes of the complex image. We introduce the parameter S=K/n as the ratio of the number of modes, K, of the response field pattern and the number of photons, n, that is used for the illumination. This measurement is in the quantum domain if S>>1. We read out objects with S=4, well in the quantum regime. The method can be extended by preshaping the illumination light, with applications in authentication of physical objects [3,4].

Frequency width of open channels in multiple scattering media

We report optical measurements of the spectral width of open transmission channels in a three-dimensional diffusive medium. The light transmission through a sample is enhanced by efficiently coupling to open transmission channels using repeated digital optical phase conjugation. The spectral properties are investigated by enhancing the transmission, fixing the incident wavefront and scanning the wavelength of the laser. We measure the transmitted field to extract the field correlation function and the enhancement of the total transmission. We find that optimizing the total transmission leads to a significant increase in the frequency width of the field correlation function. Additionally we find that the enhanced transmission persists over an even larger frequency bandwidth. This result shows open channels in the diffusive regime are spectrally much wider than previous measurements in the localized regime suggest.

Secure communication with coded wavefronts

Communication between a sender and receiver can be made secure by encrypting the message using public or private shared keys. Quantum key distribution utilizes the unclonability of a quantum state to securely generate a key between the two parties [1]. However, without some way of authentication of either the sender or the receiver, a man-in-the-middle attack with an eavesdropper mimicking the receiver can break the security of the protocol.

High-resolution phase and amplitude modulation using a digital micromirror device

The ability to spatially control the phase and amplitude of light allows for many exciting applications. In adaptive optics, light fields are modulated to correct for aberrations in the atmosphere. It has recently been shown that by spatially modulating light it is possible to focus and image through and inside opaque materials.