Geometric-phase-based phase-knife mask for stellar nulling and coronagraphy.

Exoplanets can be detected very close to stars using single-mode cross-aperture nulling interferometry, a photonic technique that relies on the inability of an anti-symmetric stellar point-spread function to couple to the symmetric mode of a single-mode fiber. We prepared an asymmetric field distribution from a laboratory point source using a flat geometric-phase-based pupil-plane phase-knife mask comprised of a planar liquid crystal polymer layer with orthogonal optical axes on opposite sides of a linear pupil bisector. Our mask yielded an on-axis laboratory point-source rejection (i.e., an interferometric "null depth") of 2.2 × 10-5. Potential mask modifications to better reject starlight are described that incorporate additional phase regions to spatially broaden the rejection area, and additional layers to spectrally broaden the rejection. Also discussed is a topological correspondence between the spatial configurations of separated-aperture nullers, cross-aperture nullers and full-aperture phase coronagraphs.

Compact, lensless digital holographic microscope for remote microbiology.

In situ investigation of microbial life in extreme environments can be carried out with microscopes capable of imaging 3-dimensional volumes and tracking particle motion. Here we present a lensless digital holographic microscope approach that provides roughly 1.5 micron resolution in a compact, robust package suitable for remote deployment. High resolution is achieved by generating high numerical-aperture input beams with radial gradient-index rod lenses. The ability to detect and track prokaryotes was explored using bacterial strains of two different sizes. In the larger strain, a variety of motions were seen, while the smaller strain was used to demonstrate a detection capability down to micron scales.

Improved Tracking and Resolution of Bacteria in Holographic Microscopy Using Dye and Fluorescent Protein Labeling.

Digital holographic microscopy (DHM) is an emerging imaging technique that permits instantaneous capture of a relatively large sample volume. However, large volumes usually come at the expense of lower spatial resolution, and the technique has rarely been used with prokaryotic cells due to their small size and low contrast. In this paper we demonstrate the use of a Mach-Zehnder dual-beam instrument for imaging of labeled and unlabeled bacteria and microalgae. Spatial resolution of 0.3 µm is achieved, providing a sampling of several pixels across a typical prokaryotic cell. Both cellular motility and morphology are readily recorded. The use of dyes provides both amplitude and phase contrast improvement and is of use to identify cells in dense samples.

Robust, compact implementation of an off-axis digital holographic microscope.

Recent advances in digital technologies, such as high-speed computers and large-format digital imagers, have led to a burgeoning interest in the science and engineering of digital holographic microscopy (DHM). Here we report on a novel off-axis DHM, based on a twin-beam optical design, which avoids the limitations of prior systems, and provides many advantages, including compactness, intrinsic stability, robustness against misalignment, ease of use, and cost. These advantages are traded for a physically constrained sample volume, as well as a fixed fringe spacing. The first trade is not overly restrictive for most applications, and the latter provides for a pre-set assembly alignment that optimizes the spatial frequency sampling. Moreover, our new design supports use in both routine laboratory settings as well as extreme environments without any sacrifice in performance, enabling ready observation of microbial species in the field. The instrument design is presented in detail here, along with a demonstration of bacterial video imaging at sub-micrometer resolution at temperatures down to -15 °C.

A Mach-Zender digital holographic microscope with sub-micrometer resolution for imaging and tracking of marine micro-organisms.

Digital holographic microscopy is an ideal tool for investigation of microbial motility. However, most designs do not exhibit sufficient spatial resolution for imaging bacteria. In this study we present an off-axis Mach-Zehnder design of a holographic microscope with spatial resolution of better than 800 nm and the ability to resolve bacterial samples at varying densities over a 380 µm × 380 µm × 600 µm three-dimensional field of view. Larger organisms, such as protozoa, can be resolved in detail, including cilia and flagella. The instrument design and performance are presented, including images and tracks of bacterial and protozoal mixed samples and pure cultures of six selected species. Organisms as small as 1 µm (bacterial spores) and as large as 60 µm (Paramecium bursaria) may be resolved and tracked without changes in the instrument configuration. Finally, we present a dilution series investigating the maximum cell density that can be imaged, a type of analysis that has not been presented in previous holographic microscopy studies.

High performance testbed for four-beam infrared interferometric nulling and exoplanet detection.

Technology development for a space-based infrared nulling interferometer capable of earthlike exoplanet detection and characterization started in earnest in the last 10 years. At the Jet Propulsion Laboratory, the planet detection testbed was developed to demonstrate the principal components of the beam combiner train for a high performance four-beam nulling interferometer. Early in the development of the testbed, the importance of "instability noise" for nulling interferometer sensitivity was recognized, and the four-beam testbed would produce this noise, allowing investigation of methods for mitigating this noise source. The testbed contains the required features of a four-beam combiner for a space interferometer and performs at a level matching that needed for the space mission. This paper describes in detail the design, functions, and controls of the testbed.

Experimental evaluation of achromatic phase shifters for mid-infrared starlight suppression.

Phase shifters are a key component of nulling interferometry, one of the potential routes to enabling the measurement of faint exoplanet spectra. Here, three different achromatic phase shifters are evaluated experimentally in the mid-infrared, where such nulling interferometers may someday operate. The methods evaluated include the use of dispersive glasses, a through-focus field inversion, and field reversals on reflection from antisymmetric flat-mirror periscopes. All three approaches yielded deep, broadband, mid-infrared nulls, but the deepest broadband nulls were obtained with the periscope architecture. In the periscope system, average null depths of 4x10(-5) were obtained with a 25% bandwidth, and 2x10(-5) with a 20% bandwidth, at a central wavelength of 9.5 mum. The best short term nulls at 20% bandwidth were approximately 9x10(-6), in line with error budget predictions and the limits of the current generation of hardware.