M. Edgar

3D Computational Imaging with Single-Pixel Detectors

Cheap Pix Three-dimensional (3D) images can be captured by, for example, holographic imaging or stereoimaging techniques. To avoid using expensive optical components that are limited to specialized bands of wavelengths, Sun et al. (p. 844; see the Perspective by Faccio and Leach) projected pulses of randomly textured light onto an object. They were able to reconstruct an image of the 3D object by detecting the reflected light with several photodetectors without any need for lenses. The patterned light beams can thus in principle be substituted for light sources of any wavelength. A computational imaging method is used to reconstruct a three-dimensional scene, without the need for lenses. [Also see Perspective by Faccio and Leach] Computational imaging enables retrieval of the spatial information of an object with the use of single-pixel detectors. By projecting a series of known random patterns and measuring the backscattered intensity, it is possible to reconstruct a two-dimensional (2D) image. We used several single-pixel detectors in different locations to capture the 3D form of an object. From each detector we derived a 2D image that appeared to be illuminated from a different direction, even though only a single digital projector was used for illumination. From the shading of the images, the surface gradients could be derived and the 3D object reconstructed. We compare our result to that obtained from a stereophotogrammetric system using multiple cameras. Our simplified approach to 3D imaging can readily be extended to nonvisible wavebands.

Normalized ghost imaging

We present an experimental comparison between different iterative ghost imaging algorithms. Our experimental setup utilizes a spatial light modulator for generating known random light fields to illuminate a partially-transmissive object. We adapt the weighting factor used in the traditional ghost imaging algorithm to account for changes in the efficiency of the generated light field. We show that our normalized weighting algorithm can match the performance of differential ghost imaging.

Imaging high-dimensional spatial entanglement with a camera

The light produced by parametric down-conversion shows strong spatial entanglement that leads to violations of EPR criteria for separability. Historically, such studies have been performed by scanning a single-element, single-photon detector across a detection plane. Here we show that modern electron-multiplying charge-coupled device cameras can measure correlations in both position and momentum across a multi-pixel field of view. This capability allows us to observe entanglement of around 2,500 spatial states and demonstrate Einstein–Podolsky–Rosen type correlations by more than two orders of magnitude. More generally, our work shows that cameras can lead to important new capabilities in quantum optics and quantum information science.

Fast full-color computational imaging with single-pixel detectors.

Single-pixel detectors can be used as imaging devices by making use of structured illumination. These systems work by correlating a changing incident light field with signals measured on a photodiode to derive an image of an object. In this work we demonstrate a system that utilizes a digital light projector to illuminate a scene with approximately 1300 different light patterns every second and correlate these with the back scattered light measured by three spectrally-filtered single-pixel photodetectors to produce a full-color high-quality image in a few seconds of data acquisition. We utilize a differential light projection method to self normalize the measured signals, improving the reconstruction quality whilst making the system robust to external sources of noise. This technique can readily be extended for imaging applications at non-visible wavebands.

Simultaneous real-time visible and infrared video with single-pixel detectors.

Conventional cameras rely upon a pixelated sensor to provide spatial resolution. An alternative approach replaces the sensor with a pixelated transmission mask encoded with a series of binary patterns. Combining knowledge of the series of patterns and the associated filtered intensities, measured by single-pixel detectors, allows an image to be deduced through data inversion. In this work we extend the concept of a ‘single-pixel camera’ to provide continuous real-time video at 10 Hz , simultaneously in the visible and short-wave infrared, using an efficient computer algorithm. We demonstrate our camera for imaging through smoke, through a tinted screen, whilst performing compressive sampling and recovering high-resolution detail by arbitrarily controlling the pixel-binning of the masks. We anticipate real-time single-pixel video cameras to have considerable importance where pixelated sensors are limited, allowing for low-cost, non-visible imaging systems in applications such as night-vision, gas sensing and medical diagnostics.

An acoustic spanner and its associated rotational Doppler shift

Light carries a spin angular momentum associated with its polarization and an orbital angular momentum arising from its phase cross-section. Sound, being a longitudinal wave, carries no spin component but can carry an orbital component of angular momentum when endowed with an appropriate phase structure. Here, we use a circular array of loudspeakers driven at a common angular frequency ?s but with an azimuthally changing phase delay to create a sound wave with helical phase fronts described by exp?(i??). Such waves are predicted to have an orbital angular momentum to energy ratio of ?/?s. We confirm this angular momentum content by measuring its transfer to a suspended 60?cm diameter acoustic absorbing tile. The resulting torque on the tile (~6.1?10?6?Nm) is measured from observation of the motion for various torsional pendulums. Furthermore, we confirm the helical nature of the acoustic beam by observing the rotational Doppler shift, which results from a rotation between source and observer of angular velocity ?r. We measure Doppler shifted frequencies of ?s???r depending on the direction of relative rotation.

Single-pixel three-dimensional imaging with time-based depth resolution.

Time-of-flight three-dimensional imaging is an important tool for applications such as object recognition and remote sensing. Conventional time-of-flight three-dimensional imaging systems frequently use a raster scanned laser to measure the range of each pixel in the scene sequentially. Here we show a modified time-of-flight three-dimensional imaging system, which can use compressed sensing techniques to reduce acquisition times, whilst distributing the optical illumination over the full field of view. Our system is based on a single-pixel camera using short-pulsed structured illumination and a high-speed photodiode, and is capable of reconstructing 128 × 128-pixel resolution three-dimensional scenes to an accuracy of ∼3 mm at a range of ∼5 m. Furthermore, by using a compressive sampling strategy, we demonstrate continuous real-time three-dimensional video with a frame-rate up to 12 Hz. The simplicity of the system hardware could enable low-cost three-dimensional imaging devices for precision ranging at wavelengths beyond the visible spectrum.

Adaptive foveated single-pixel imaging with dynamic supersampling

The performance of a single-pixel computational video camera is enhanced by mimicking the compressive features of animal vision. In contrast to conventional multipixel cameras, single-pixel cameras capture images using a single detector that measures the correlations between the scene and a set of patterns. However, these systems typically exhibit low frame rates, because to fully sample a scene in this way requires at least the same number of correlation measurements as the number of pixels in the reconstructed image. To mitigate this, a range of compressive sensing techniques have been developed which use a priori knowledge to reconstruct images from an undersampled measurement set. Here, we take a different approach and adopt a strategy inspired by the foveated vision found in the animal kingdom—a framework that exploits the spatiotemporal redundancy of many dynamic scenes. In our system, a high-resolution foveal region tracks motion within the scene, yet unlike a simple zoom, every frame delivers new spatial information from across the entire field of view. This strategy rapidly records the detail of quickly changing features in the scene while simultaneously accumulating detail of more slowly evolving regions over several consecutive frames. This architecture provides video streams in which both the resolution and exposure time spatially vary and adapt dynamically in response to the evolution of the scene. The degree of local frame rate enhancement is scene-dependent, but here, we demonstrate a factor of 4, thereby helping to mitigate one of the main drawbacks of single-pixel imaging techniques. The methods described here complement existing compressive sensing approaches and may be applied to enhance computational imagers that rely on sequential correlation measurements.

Waveguide grating mirror in a fully suspended 10 meter Fabry-Perot cavity

We report on the first demonstration of a fully suspended 10 m Fabry-Perot cavity incorporating a waveguide grating as the coupling mirror. The cavity was kept on resonance by reading out the length fluctuations via the Pound-Drever-Hall method and employing feedback to the laser frequency. From the achieved finesse of 790 the grating reflectivity was determined to exceed 99.2% at the laser wavelength of 1064 nm, which is in good agreement with rigorous simulations. Our waveguide grating design was based on tantala and fused silica and included a ≈ 20 nm thin etch stop layer made of Al2O3 that allowed us to define the grating depth accurately and preserve the waveguide thickness during the fabrication process. Demonstrating stable operation of a waveguide grating featuring high reflectivity in a suspended low-noise cavity, our work paves the way for the potential application of waveguide gratings as mirrors in high-precision interferometry, for instance in future gravitational wave observatories.

Near video-rate linear Stokes imaging with single-pixel detectors

In this work we demonstrate a polarization sensitive computational imaging system based on a digital micro-mirror device (DMD) and several single-pixel photodetectors. By taking advantage of computational imaging techniques, the light measured by each single-pixel detector can reconstruct a 2D image for a specific linear polarization state. Using the rapid frame-rate of the DMD to continuously project a series of spatially orthogonal illumination patterns, near video-rate reconstructions can be achieved. In addition we extend this approach to provide full-colour images through a process of sequential colour selection (RGB). Taking the difference between photodetector signals from orthogonal linear polarization states, we obtain images corresponding to the linear Stokes parameters. We apply this rapid polarization sensitive imaging system to inert and biological material. Since the spatial information in the images reconstructed by this approach are determined by the projection system, rather than the detectors, the approach provides perfect pixel registration between the various polarization selective images and associated Stokes parameters. Furthermore, the use of single-pixel detectors and the large operational bandwidth afforded by DMDʼs means that the approach can readily be extended for imaging at wavelengths where detector arrays are unavailable or limited.