Development of Holographic Phase Masks for Wavefront Shaping

This dissertation explores a new method for creating holographic phase masks (HPMs), which are phase transforming optical elements holographically recorded in photosensitive glass. This novel hologram recording method allows for the fast production of HPMs of any complexity, as opposed to the traditional multistep process, which includes the design and fabrication of a master phase mask operating in the UV region before the holographic recording step. We holographically recorded transmissive HPMs that are physically robust (they are recorded in a silicate glass volume), can handle tens of kilowatts of continuous wave (CW) laser power, are un-erasable, user defined, require no power to operate, can work over a wavelength band ranging from 350 to 2500 nm, and can modify the wavefront of narrow line or broad band coherent sources. The HPMs can be wavelength-tunable by angular adjustment over tens or even hundreds of nanometers. The HPMs incorporate the phase information in the bulk of a volume Bragg grating (VBG) resulting in only a single diffraction order and up to 100% diffraction efficiency. Recording in thick photosensitive medium also enables the multiplexing of HPMs in a single monolithic element. While these HPMs are physically overlapped in the space, they provide independent phase profiles, efficiencies, spectral and angular acceptances. Multiplexing HPMs allows splitting or combining of multiple beams while affecting their wavefronts individually. We also developed a new holographic phase mask of reflective-type. This device provides us the ability of recording RBGs with transversely shifted parts in the larger aperture which upon reconstruction will produce different phases to different parts of the diffracted beam. RBG\u27s diffraction spectrum possesses a very narrow bandwidth, and the holographic recording technique allows to multiplex multiple gratings into a single volume of PTR glass. If each of these Bragg wavelengths is assigned with a specific spatial mode, it can be achieved simultaneous spatial and spectral multiplexing. As a separate research topic, we look at how holographic optical elements (HOEs) can be used for improving the capabilities of the existing generation of head-up displays (HUDs), resulting in smaller, lighter units with a larger eye-box. Currently, surface relief gratings recorded in photosensitive polymers that are susceptible to the environmental conditions are used in HOE-based HUDs. This has an impact on their reliability and overall lifespan. We investigated a specific holographically recorded in the volume of photo-thermo-refractive glass transmissive gratings that generated multiple diffracted beams due to their operation in Raman-Nath regime. The Raman Nath gratings were successfully used to create an array of images because in augmented reality systems, this approach can be used to enhance the size of the exit pupil. These image splitting elements, due to the features of PTR glass, have a great resistance to temperature gradients, mechanical shocks, vibrations, and laser radiation

Comparative Assessment Of Five Algorithms To Control An Slm For Focusing Coherent Light Through Scattering Media

Light propagation in turbid media, such as biological tissues, experience scattering due to inhomogeneous distribution of refractive indices. Control of light scattering is important for focusing the light or imaging through scattering medium. By spatially shaping the wave-front of the incident beam, using spatial light modulator (SLM), the scattered light can be focused to a point. Iterative optimization is a popular way of obtaining the most optimum phase map on the SLM. In this study, we implement six optimization algorithms including continuous sequential, partitioning algorithm, transmission matrix estimation method, genetic algorithms, particle swarm optimization, and simulated annealing to obtain the optimum phase map of the SLM. The main characteristics of the algorithms such as convergence time, improvement ratio and performance are compared and discussed

Performance Modeling Of Terahertz (Thz) And Millimeter Waves (Mmw) Pupil Plane Imaging

Terahertz- (THz) and millimeter-wave sensors are becoming more important in industrial, security, medical, and defense applications. A major problem in these sensing areas is the resolution, sensitivity, and visual acuity of the imaging systems. There are different fundamental parameters in designing a system that have significant effects on the imaging performance. The performance of THz systems can be discussed in terms of two characteristics: sensitivity and spatial resolution. New approaches for design and manufacturing of THz imagers are a vital basis for developing future applications. Photonics solutions have been at the technological forefront in THz band applications. A single scan antenna does not provide reasonable resolution, sensitivity, and speed. An effective approach to imaging is placing a high-performance antenna in a two-dimensional antenna array to achieve higher radiation efficiency and higher resolution in the imaging systems. Here, we present the performance modeling of a pupil plane imaging system to find the resolution and sensitivity efficiency of the imaging system

Scene Reconstruction Via Two-Dimensional Complex Coherency Imaging

The location of 1D objects is determined by numerically back-propagating the experimentally measured spatial coherence function. A partially coherent light and dynamical double slits generated by digital light processor are used in the coherence measurements

Simulated Annealing Optimization In Wavefront Shaping Controlled Transmission

In this research, we present results of simulated annealing (SA), a heuristic optimization algorithm, for focusing light through a turbid medium. Performance of the algorithm on phase and amplitude modulations has been evaluated. A number of tips to tune the optimization parameters are provided. The effect of measurement noise on the performance of the SA algorithm is explored. Additionally, SA performance is compared with continuous sequential and briefly with other optimization algorithms

Incoherent Lensless Imaging Via Coherency Back-Propagation

The two-point complex coherence function constitutes a complete representation for scalar quasi-monochromatic optical fields. Exploiting dynamically reconfigurable slits implemented with a digital micromirror device, we report on measurements of the complex two-point coherence function for partially coherent light scattering from a “scene” composing one or two objects at different transverse and axial positions with respect to the source. Although the intensity shows no discernible shadows in the absence of a lens, numerically back-propagating the measured complex coherence function allows estimating the objects’ sizes and locations and, thus, the reconstruction of the scene subject to the effects of occlusion and shadowing

Scene Reconstruction Via Coherency Imaging

We implement numerical back-propagation of the experimentally obtained spatial complex coherence function to estimate both the axial and transverse positions of ID objects. The measurement of the coherence function of partially coherent light is performed using dynamical double slits implemented via digital micromirror device

Complex-Coherence Measurements For Lensless Object Positioning

We experimentally demonstrate complex-coherence function of incoherent light intercepted by a one-dimensional object in various configurations. By examining only the coherence function, we determine the axial and transverse position and size of the object

Coherence Measurements Of Scattered Incoherent Light For Lensless Identification Of An Object\u27S Location And Size

In the absence of a lens to form an image, incoherent or partially coherent light scattering o- an obstructive or reflective object forms a broad intensity distribution in the far field with only feeble spatial features. We show here that measuring the complex spatial coherence function can help in the identification of the size and location of a one-dimensional object placed in the path of a partially coherent light source. The complex coherence function is measured in the far field through wavefront sampling, which is performed via dynamically reconfigurable slits implemented on a digital micromirror device (DMD). The impact of an object - parameterized by size and location - that either intercepts or reflects incoherent light is studied. The experimental results show that measuring the spatial coherence function as a function of the separation between two slits located symmetrically around the optical axis can identify the object transverse location and angle subtended from the detection plane (the ratio of the object width to the axial distance from the detector). The measurements are in good agreement with numerical simulations of a forward model based on Fresnel propagators. The rapid refresh rate of DMDs may enable real-time operation of such a lensless coherency imaging scheme