Metaoptics for aberration correction in microendoscopy.

Compact and minimally invasive scanning fiber endoscopy probes with micron-level resolution have great potential in detailed tissue interrogation and early disease diagnosis, which are key applications of confocal reflectance imaging at visible wavelengths. State-of-the-art imaging probes commonly employ refractive lens triplets or gradient refractive index (GRIN) lenses as the micro-objective. However, off-axis aberration emerges as a critical factor affecting resolution, especially at the extremities of the imaging field. In response to this challenge, we propose what we believe to be a novel design integrating a metasurface with the GRIN micro-objective to address optical aberrations during beam scan. The metasurface acts as a corrector element for optical aberrations in a fiber-scanning endoscope using the same fiber for excitation and collection. Modeling such hybrid refractive-metasurface designs requires the coupling of simulation techniques across macroscale and nanoscale optics, for which we used an Ansys simulation workflow platform. Operating at a wavelength of 644 nm, this metaoptical element serves as a thin and compact aberration correction surface, ensuring uniform resolution across the entire imaging field. Experimental results from our scanning fiber endoscopy system demonstrate a notable enhancement in optical performance both on-axis and off-axis, achieving a resolution of 3 µm at the center of the imaging field. Impressively, the resolution experiences only a modest degradation by a factor of 0.13 at the edge of the field of view compared to the center.

Transform-based phase retrieval techniques from a single off-axis interferogram.

Optical phase retrieval (OPR) methods are important because they are used to obtain the transverse phase profile information of a beam. Interference methods are extensively used to convert the phase information into an intensity pattern, which can then be processed further to retrieve the unknown phase. The most widely used interference method involves the interference of the unknown object beam and a known reference beam with an angle between them. There are several algorithms that retrieve the phase information from such a single off-axis interference pattern. For a particular application, the choice of an algorithm for OPR is very important. Therefore, it is necessary to choose between them, depending on the requirements. Three entirely different noniterative, transform-based algorithms, namely the Fourier transform (FT) method, the continuous wavelet transform (CWT) method, and the Hilbert transform (CWT) method, are explained in detail. A quantitative comparison is made using a combination of rms error and standard structural similarity measure. The advantages of using a standard unwrapping algorithm are also validated using the same combination of comparison metrics. We show that the HT method has a better response with object beam with higher spatial frequency content, but with the penalty of affected noise. The FT method and CWT method have better noise immunity, but have the limitation of the spatial frequency range of the object beam. The different constraints, advantages, and some practical limitations of the methods are discussed with the help of a quantitative phase imaging experiment of monodispersed polymethyl methacrylate beads.

All-digital Stokes polarimetry with a digital micromirror device.

Stokes polarimetry is widely used to extract the polarization structure of optical fields, typically from six measurements, although it can be extracted from only four. To measure the required intensities, most approaches are based on optical polarization components. In this work, we present an all-digital approach that enables a rapid measure of all four intensities without any moving components. Our method employs a polarization grating (PG) to simultaneously project the incoming mode into left- and right-circular polarized states, followed by a polarization-insensitive digital micromirror device (DMD), which digitally introduces a phase retardance for the acquisition of the remaining two polarization states. We demonstrate how this technique can be applied to measuring the SoP, vectorness, and intramodal phase of optical fields, without any moving components, and shows excellent agreement with theory, illustrating fast, real-time polarimetry.

MetaOptics: opensource software for designing metasurface optical element GDSII layouts.

Metasurfaces have recently emerged as a promising technology to realize flat and ultra-thin optical elements that can manipulate light at sub-wavelength scale. The typical design flow of a metasurface involves tedious Finite Difference Time Domain (FDTD) simulations followed by creation of a GDSII layout of the metasurface phase profile, the latter being essential for fabrication purposes. Both these steps can be time-consuming and involve the usage of expensive software. To make the design process more straightforward, we have developed an open-source software called MetaOptics built using Python for designing a generic metasurface optical element. MetaOptics uses the FDTD simulated phase response data of a set of meta-atoms and converts the phase profile of any given optical element into a metasurface GDSII layout. MetaOptics comes with in-built FDTD data for most commonly used wavelengths in the visible and infrared spectrum. It also has an option to upload user-specific dimension versus transmission phase data for any choice of wavelength. In this work we describe the software's framework and provide details to guide users to design a metasurface layout using MetaOptics.

A compact single channel interferometer to study vortex beam propagation through scattering layers.

We propose and demonstrate a single channel interferometer that can be used to study how vortex beams propagate through a scatterer. The interferometer consists of a multifunctional diffractive optical element (MDOE) synthesized by the spatial random multiplexing of a Fresnel zone plate and a spiral Fresnel zone plate with different focal lengths. The MDOE generates two co-propagating beams, such that only the beam carrying orbital angular momentum is modulated by an annular stack of thin scatterers located at the focal plane of the Fresnel zone plate, while the other beam passes through the centre of the annulus without any modulation. The interference pattern is recorded at the focal plane of the spiral Fresnel zone plate. The scattering of vortex beams through stacks consisting of different number of thin scatterers was studied using the proposed optical setup. Conflicting results have been reported earlier on whether higher or lower charge beams suffer more deterioration. The proposed interferometer provides a relatively simple and compact means of experimentally studying propagation of vortex beams through scattering medium.

Spatial Multiplexing Technique for Improving Dynamic Range of Speckle Correlation based Optical Lever.

Speckle correlation based optical levers (SC-OptLev) possess attractive characteristics suitable for sensing small changes in the angular orientations of surfaces. In this study, we propose and demonstrate a spatial multiplexing technique for improving the dynamic range of SC-OptLev. When the surface is in its initial position, a synthetic speckle intensity pattern, larger than the area of the image sensor is created by transversely shifting the image sensor and recording different sections of a larger speckle pattern. Then, the acquired images are stitched together by a computer program into one relatively large synthetic speckle pattern. Following the calibration stage, the synthetic speckle intensity pattern is used to sense changes in the surface's angular orientation. The surface is monitored in real-time by recording part of the speckle pattern which lies within the sensor area. Next, the recorded speckle pattern is cross-correlated with the synthetic speckle pattern in the computer. The resulting shift of the correlation peak indicates the angular orientations of the reflective surface under test. This spatial-multiplexing technique enables sensing changes in the angular orientation of the surface beyond the limit imposed by the physical size of the image sensor.

Implementation of a speckle-correlation-based optical lever with extended dynamic range.

A speckle-correlation-based optical lever (SC-OptLev) is constructed for the measurement of small changes in the orientation angle of a surface. The dynamic range of SC-OptLev is found to be twice that of a conventional OptLev for the same experimental configurations. Different filtering mechanisms are implemented, and the correlation results are compared. Two types of computer-automated SC-OptLevs, an open-source-based computing system with a low-cost image sensor and a commercial computing system, are presented with assistive computational modules.

Polarizing beam splitter integrated onto an optical fiber facet.

When light either leaves or enters an optical fiber, one often needs free-space optical components to manipulate the state of polarization or the light's phase profile. It is therefore desirable to integrate such components onto a fiber end facet. In this paper, we realize, for the first time, a polarizing beam splitter fabricated directly onto the end facet of a single-mode optical fiber. The element is composed of a refractive prism, intentionally slightly displaced from the core of the fiber, and an elevated and suspended sub-wavelength diffraction grating, the lamellae of which have an aspect ratio of about 5. This integrated micro-optical component is characterized experimentally at 1550 nm wavelength. We find that the two emerging output beams exhibit a degree of polarization of 81 percent and 82 percent for Transverse Magnetic (TM) and Transverse Electric (TE) polarization, respectively.

Simplified analysis of sub-wavelength triangular gratings by simplified modal method.

A phase-equivalence of a triangular grating and a "corresponding" blazed structure is proposed. This equivalence is used to simplify the analysis of the grating, which otherwise would require the repetitive application of the simplified modal method to each lamellar grating that constitutes the triangular grating. The concept is used to arrive at an equation for the phase introduced by the triangular grating. The proposed model is verified by finite element simulations. A method of fabricating a triangular grating in quartz is presented. The proposed theory, along with optical testing, can be used as a non-destructive means by which to estimate the height of the triangular grating during the dry etching process.

Direct patterning of vortex generators on a fiber tip using a focused ion beam.

The realization of spiral phase optical elements on the cleaved end of an optical fiber by focused ion beam milling is presented. A focused Ga+ ion beam with an acceleration voltage of 30 keV is used to etch continuous spiral phase plates and fork gratings directly on the tip of the fiber. The phase characteristics of the output beam generated by the fabricated structures measured via an interference experiment confirmed the presence of phase singularity in the output beam. The devices are expected to be promising candidates for all-fiber beam shaping and optical trapping applications.

Quasi-achromatic Fresnel zone lens with ring focus.

The phase of a standard Fresnel zone lens (FZL) is periodically modulated in the radial direction using the phase of a binary fraxicon. The resulting element (rf-FZL) focuses light into a ring. The ring is found to be quasi-achromatic, in that the diameter is wavelength independent but its location is not. The binary rf-FZL is fabricated using electron beam direct writing. Experimental results confirm the generation of a wavelength-independent ring pattern at the focus of the rf-FZL. An efficiency of 24% was obtained. The variation in radius of ring pattern was reduced from 61 µm to less than 10 nm for a corresponding wavelength variation from 532 to 633 nm.

Improvements in speckle tracking algorithms for vibrational analysis using optical coherence tomography.

We report on improvements in speckle tracking algorithms for analyzing subsurface vibrations using an optical coherence tomography system. Our technique uses axial deconvolution and linear interpolation of depth scans before using cross-correlation-based speckle tracking. We applied this method on M-mode images of a test sample and on wrist pulses to perform depth-resolved measurements of displacement, velocity, and frequencies of vibrations. Speckle pattern displacements and features spanning 0.5 pixels were tracked clearly. Extension of this technique to two dimensions can provide improved performance in the field of optical coherence elastography.

Multilayer porous silicon diffraction gratings operating in the infrared.

Transmission diffraction gratings operating at 1,565 nm based on multilayer porous silicon films are modeled, fabricated, and tested. Features down to 2 µm have been patterned into submicron-thick mesoporous films using standard photolithographic and dry etching techniques. After patterning of the top porous film, a second anodization can be performed, allowing an under-layer of highly uniform porosity and thickness to be achieved. High transmission greater than 40% is measured, and modeling results suggest that a change in diffraction efficiency of 1 dB for a 1% change in normalized refractive index can be achieved. Preliminary measurement of solvent vapor shows a large signal change from the grating sensor in agreement with models.

Design, fabrication, and evaluation of a multilevel spiral-phase Fresnel zone plate for optical trapping.

A compact optics configuration for the generation of donut beams for trapping atoms at the micrometer scale using a multilevel spiral-phase Fresnel zone plate (FZP) and a semiconductor laser is proposed. A FZP is designed and a multilevel spiral phase is integrated into it. A spiral-phase FZP with a radius of 1 mm and with more than 1300 half-period zones is designed with multiple angular levels for integer and fractional topological charges, and the device is fabricated using electron-beam lithography direct writing. The performance of the device is evaluated, and the generation of symmetric and asymmetric donut beams is successfully demonstrated.

Phase-shifted Fresnel axicon.

We propose an optical element: a phase-shifted Fresnel axicon for generation of multiple Bessel beams. By giving a binary phase modulation to the standard Fresnel axicon, the proposed element is generated. The phase profile of the binary phase modulation is engineered to generate two and three Bessel beams of equal intensities. This composite optical element is fabricated using electron beam direct writing. The performance of the fabricated device is evaluated using a semiconductor laser, and the generation of two and three Bessel beams is successfully demonstrated.

All-fiber interferometry: design and analysis.

This paper presents a design procedure for a fiber interferometer, the optical system, and its associated electronic control. Analog and digital circuits were optimized to achieve an inexpensive and compact system. The lock-in amplifier required for phase control was designed using a field programmable gate array that was also configured to carry out the required phase stepping. The interferometer was built into two stages. The first stage used only one wavelength to measure samples with step heights in the hundreds of nanometers, with improvements in accuracy through averaging of data. The next stage included two measurement wavelengths to extend the measurement range to a few hundreds of micrometers. The paper conducts a systematic study of sources of errors. A least squares method that minimizes measurement time but retains accuracy has been used to measure height.