The spirit of professional service - a tribute to Andrew Weiner.

James Leger pays tribute to former Optics Express Editor-in-Chief, Andrew Weiner.

Estimation of the 3D spatial location of non-line-of-sight objects using passive THz plenoptic measurements.

At THz frequencies, many building materials exhibit mirror-like reflectivity, greatly facilitating the 3D spatial location estimate of non-line-of-sight objects. Using a custom THz measurement setup that employs a high sensitivity room temperature THz sensor, we measure the spatial and angular components of the radiation from hidden objects scattered from rough walls. The three-dimensional location of a thermally elevated object can then be determined using this "light field" information together with a refocusing algorithm.

Two dimensional gradient-index beam shapers fabricated using ultrafast laser inscription.

In this paper gradient-index beam shapers are fabricated using the ultrafast laser inscription method. This method enables the fabrication of two-dimensional refractive index profiles inside silica glass, resulting in highly robust and compact beam shapers. The magnitude of this refractive index change can be tailored by adjusting the laser pulse energy, enabling arbitrary two-dimensional refractive index profiles to be manufactured.

The obligations of the scientific research community to society: an introduction to the report on methane production and mitigation.

The Editor-in-Chief of Optics Express introduces a new science and policy report prepared by the American Physical Society and Optica on "Monitoring Methane Emissions from Oil and Gas Operations."

Locating non-line-of-sight features and recovering imagery conveyed through parallax in passive plenoptic imaging.

We present a method for recovering the perspective image of a non-line-of-sight scene based on plenoptic observations of indirect photons scattered from a flat homogeneous surface. The plenoptic information is used to ascertain locations of non-line-of-sight features in a hidden scene. This latent knowledge is then used for the partial reconstruction of the scene by extracting the imagery perceived by the non-line-of-sight features from the scattered light field measured along the surface.

Photography-based real-time long-wave infrared scattering estimation technique.

The scattered light distribution of surfaces in the long-wave infrared (∼8-12µ) is measured using a small set of thermal camera images. This method can extract scatter patterns considerably faster than standard laboratory bidirectional reflectance distribution function measurements and is appropriate for passive homogeneous surfaces. Specifically, six images are used in this study, each taken with respect to a thermal light source at an angle ranging from 10° to 60° to the normal of the surface.

Non-line-of-sight object location estimation from scattered light using plenoptic data.

We investigate the use of plenoptic data for locating non-line-of-sight (NLOS) objects from a scattered light signature. Using Fourier analysis, the resolution limits of the depth and transversal location estimates are derived from fundamental considerations on scattering physics and measurement noise. Based on the refocusing algorithm developed in the computer vision field, we derive an alternative formulation of the projection slice theorem in a form directly connecting the light field and a full spatial frequency spectrum including both depth and transversal dimensions.

Numerical design of three-dimensional gradient refractive index structures for beam shaping.

A numerical design method is demonstrated for gradient refractive index (GRIN) beam shapers embedded in a medium. The three-dimensional refractive index profile Δn(x, y, z) gradually changes the spatial characteristics of a beam during propagation. Diffraction effects such as beam expansion are controlled and compensated by the refractive index profile, resulting in efficient field transformations with no coherent artifacts.

Light field reconstruction from scattered light using plenoptic data.

We investigate the utility of plenoptic data for extracting information from a scene where the light from objects in the scene is viewable only after scattering from a surface such as a wall. We derive the rigorous relationship between the object and the scattered light, which is cast in terms of a system of Fredholm integral equations of the first kind with the bidirectional reflectance distribution function (BRDF) of the scattering surface; further, the object information is reconstructed by solving the equations. Based on the Fourier transformation, we propose a simple BRDF model and analyze the reconstruction errors by introducing newly defined parameters reflecting the BRDF's characteristics, i.

Passive non-line-of-sight imaging using plenoptic information.

We present a methodology for recovering the perspective imagery of a non-line-of-sight scene based on plenoptic observations of indirect photons scattered from a homogeneous surface. Our framework segregates the visual contents observed along the scattering surface into angular and spatial components. Given the reflectance characteristics of the scatterer, we show that the former can be deduced from scattering measurements employing diversity in angle at individual surface points, whereas the latter can be deduced from captured images of the scatterer based on prior knowledge of occlusions within the scene.

Analysis and experimental demonstration of spatial mode selection in a coherently combined fiber laser.

Self-phasing due to spatial mode selection in a two-element passively coupled fiber laser is studied. We find that the addition of a second supermode in a coupled resonator results in a 90% increase in the average output power and nearly π/2 radians of passive phase adjustment versus applied phase errors between the gain elements. These results require a phase of zero (modulo 2π) between the beams in the external cavity.

Gain dependent self-phasing in a two-core coherently combined fiber laser.

The influence of the Kramers-Kronig phase is demonstrated in a coherently combined fiber laser where other passive phasing mechanisms such as wavelength tuning have been suppressed. A mathematical model is developed to predict the lasing supermode and is supported by experimental measurements of the gain, phase, and power. The results show that the difference in Kramers-Kronig phase arising from a difference in gain between the two arms partially compensates for an externally applied phase error.

Design and demonstration of a far-field imaging system with a one-dimensional form factor.

We present a far-field imaging system with a one-dimensional form factor based on coupling light into the side of an optical fiber. The point spread function of this threadlike camera is determined analytically and confirmed experimentally. Because the system is one-dimensional, high resolution is available in one spatial dimension.

Gradient-index design for mode conversion of diffracting beams.

Gradient-index (GRIN) media offer advantages over thin optical elements for beam shaping of strongly diffracting fields. A numerical GRIN design method is presented, where diffraction effects are considered in solving for the refractive index profile. The index profile is found by specifying a desired beam transformation throughout the GRIN volume and solving a series of phase retrieval problems.

Retrieval of refractive index fields in two-dimensional gradient-index elements from external deflectometry data.

In a previous work, we presented a numerical method for retrieving inhomogeneous refractive index fields in rectangular gradient-index elements from boundary positions and internal boundary slopes associated with a set of interrogating probe beams that transit the medium. The present work extends this method to external boundary beam slopes without knowledge of the refractive index along the surface of the optical element, requiring minimal additional information (outside of beam position and slope data) such as a single known index point inside the medium. The inverse problem is cast as a linear algebraic system describing the deflection of probe beams inside the optical material, and an iterative inversion algorithm is used to generate an index field that produces the boundary value data.

Optics Express review criteria; introducing the novelty and impact statement.

In an effort to maintain and improve the quality and importance of papers published, Optics Express has refined its review criteria and instituted a required novelty and impact statement.

Deflectometry for measuring inhomogeneous refractive index fields in two-dimensional gradient-index elements.

We present a numerical method for calculating inhomogeneous refractive index fields in rectangular gradient-index (GRIN) elements from measured boundary positions and slopes of a collection of rays that transit the medium. The inverse problem is reduced to a set of linear algebraic equations after approximating ray trajectories from the measured boundary values and is solved using a pseudo-inverse algorithm for sparse linear equations. The ray trajectories are subsequently corrected using an iterative ray trace procedure to ensure consistency in the solution.

Experimental measurements of the origin of self-phasing in passively coupled fiber lasers.

We have directly measured the intensity distribution, gain, and induced phase shift between two fiber lasers that are coherently combined by a Dammann grating. The induced phase shift between the lasers has been shown to approximately cancel out any applied phase error introduced into the cavity, allowing the combined resonator to operate at an efficient low-loss state. We show that the origin of this self-phasing stems from a redistribution of power between the two lasers.

Direct observation of Kramers-Kronig self-phasing in coherently combined fiber lasers.

A highly stable coherent beam-combining system has been designed to measure self-phasing in fiber lasers due to nonlinear effects. Whereas self-phasing in previous coherent combination experiments has been principally attributed to wavelength shifting, these wavelength effects have been efficiently suppressed in our experiment by using a dual-core fiber with closely balanced optical path lengths. The self-phasing from nonlinear effects could then be measured independently and directly by common-path interferometry with a probe laser.

Thin-sheet laser imaging microscopy for optical sectioning of thick tissues.

We report the development of a modular and optimized thin-sheet laser imaging microscope (TSLIM) for nondestructive optical sectioning of organisms and thick tissues such as the mouse cochlea, zebrafish brain/inner ear, and rat brain at a resolution that is comparable to wide-field fluorescence microscopy. TSLIM optically sections tissue using a thin sheet of light by inducing a plane of fluorescence in transparent or fixed and cleared tissues. Moving the specimen through the thinnest portion of the light sheet and stitching these image columns together results in optimal resolution and focus across the width of a large specimen.

Measurement of surface features beyond the diffraction limit with an imaging ellipsometer.

We report a sensitive surface and feature measurement technique that uses a novel imaging ellipsometer. Polarization signatures from unresolved subwavelength structures are utilized as a sensitive measure of linewidth. A focused beam rigorous coupled wave analysis method is developed to simulate the polarization effects from isolated subwavelength structures.

Properties of a phase-conjugate etalon mirror and its application to laser resonator spatial-mode control.

The concept of a phase-conjugate etalon mirror consisting of one flat and one aspheric surface is introduced. This new element can be used as an end mirror of a conventional resonator to promote spatial-mode selection and mode shaping. A phase-conjugate etalon designed for the fundamental Gaussian mode is experimentally implemented and tested with a single-mode He-Ne laser.

Microellipsometer with radial symmetry.

We report on a novel microellipsometer that uses a spatially filtered high-numerical-aperture (NA) lens for large-angle ellipsometric illumination and high spatial resolution. A radially symmetric ellipsometric signal is achieved with two half-wave plates to produce a pure polarization rotation and a birefringent cube as a radial analyzer. This radial symmetry offers a better signal-to-noise ratio compared with other microellipsometer techniques.

High-resolution imaging ellipsometer.

We report on a novel imaging ellipsometer using a high-numerical-aperture (NA) objective lens capable of measuring a two-dimensional ellipsometric signal with high resolution. Two-dimensional ellipsometric imaging is made possible by spatial filtering at the pupil plane of the objective. A Richards-Wolf vectorial diffraction model and geometrical optics model are developed to simulate the system.