Three-dimensional Refractive Index Profile Research Articles

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. The solution is found using phase retrieval and a paraxial scalar wave beam propagation model. An example design is shown in which small changes in refractive index (Δn < 10-3) are used to transform a beam over a device length of 10 mm.

Adaptive wavefront correction structured illumination holographic tomography

In this study, a novel adaptive wavefront correction (AWC) technique is implemented on a compactly developed structured illumination holographic tomography (SI-HT) system. We propose a mechanical movement-free compact scanning architecture for SI-HT systems with AWC, implemented by designing and displaying a series of computer-generated holograms (CGH) composed of blazed grating with phase Fresnel lens on a phase-only spatial light modulator (SLM). In the proposed SI-HT, the aberrations of the optical system are sensed by digital holography and are used to design the CGH-based AWC to compensate the phase aberrations of the tomographic imaging system. The proposed method was validated using a standard Siemens star target, its potential application was demonstrated using a live candida rugosa sample, and its label-free three-dimensional refractive index profile was generated at its subcellular level. The experimental results obtained reveal the ability of the proposed method to enhance the imaging performance in both lateral and axial directions.

Nonray-tracing determination of the 3D refractive index profile of polymeric fibres using single-frame computed tomography and digital holographic interferometric technique.

In this paper, we present a nonray-tracing technique for evaluating the three dimensional distribution of the refractive index values inside polymeric fibres. This technique, named 'single-frame computed-tomography (SFCT)', is applied to digital holography. A comparative study between the calculated optical phase values using ordinary tomography and SFCT is carried out, and a negligible deviation is detected. The proposed technique is used to determine the three-dimensional refractive index profile of isotactic Polypropylene fibres, IPP. The variation of the optical properties is measured before, during and after the formation of the necking phenomenon. In addition, SFCT technique is applied to the online determination of the change of the optical properties of IPP fibres. Holograms are given for illustration.

Reconstruction of the three-dimensional refractive index distribution by means of adaptive optics

The quality of the adaptive correction for atmospheric distortions is estimated in a system where the wavelength of the main laser source differs from that of the beacon. The possibility is considered of the reconstruction of the three-dimensional refractive index profile in different cross sections of the propagation path while the beam parameters are recorded only on the target plane. An algorithm is described that provides for the improvement of the beam control effectiveness in a “two-color” system with the use of the obtained information.

Measurement of refractive index profile of holey fiber using quantitative phase tomography

A method to determine three-dimensional refractive index profile of holey fiber using a combination of quantitative phase microscopy and tomographic reconstruction techniques is described. Results show that the method based on the transport-of-intensity equation for phase retrieval is straightforward to measure phase profile, nondestructive to the measured fiber, and valuable to determine three-dimensional refractive index profile of holey fiber.

Towards functional 3D T-ray imaging

We review the recent development of T-ray computed tomography, a terahertz imaging technique that allows the reconstruction of the three-dimensional refractive index profile of weakly scattering objects. Terahertz pulse imaging is used to obtain images of the target at multiple projection angles and the filtered backprojection algorithm enables the reconstruction of the object's frequency-dependent refractive index. The application of this technique to a biological bone sample and a plastic test structure is demonstrated. The structure of each target is accurately resolved and the frequency-dependent refractive index is determined. The frequency-dependent information may potentially be used to extract functional information from the target, to uniquely identify different materials or to diagnose medical conditions.

Three-dimensional refractive index profile of a GRIN optical waveguide using multiple beam interference fringes

Multiple beam Fizeau fringes across a GRIN fiber immersed in a silvered liquid wedge have been used to obtain a three-dimensional refractive index profile of optical fiber core. An image processing system has been used to analyze the fringe shape across the fiber cross-section. A simple mathematical approach has been applied utilizing the resulting experimental data to estimate the fiber three-dimensional refractive index profile. The tomographic back projection technique has been applied.