Holographic optical elements (HOEs) have the potential to enable more compact, versatile, and lightweight optical designs, but many challenges remain. Volume HOEs have the advantage of high diffraction efficiency, but they present both chromatic selectivity and chromatic dispersion, which impact their use with wide spectrum light sources. Single-color light emitting diode (LED) sources have a narrow spectrum that reduces these issues and this makes them better suited for use with volume HOEs. However, the LED source size must be taken into consideration for compact volume HOE-LED systems. To investigate the design limits for compact HOE-LED systems, a theoretical and experimental study was carried out on the effects of an extended source on the HOE output for different holographic lenses, with focal lengths from 25-100 mm. The lenses were recorded in a commercially available photopolymer [Bayfol HX200], and their diffraction efficiency was characterized across the lens aperture by measuring the Bragg angular selectivity curve at each location. Offset point sources were used to experimentally study the effects of a non-point source on the HOEs, and the system was modeled using Matlab and Zemax.
Holographic Optical Elements (HOEs) have the potential to enable more compact, versatile and lightweight optical designs, but many challenges remain. Volume HOE’s have the advantage of high diffraction efficiency but they present both chromatic selectivity and chromatic dispersion which impact on their use with wide spectrum light sources. Single-colour LED sources have a narrow spectrum that reduces these issues and this makes them better suited for use with volume HOEs. However, the LED source size must be taken into consideration for compact volume HOE-LED systems. To investigate the design limits for compact HOE-LED systems, a theoretical and experimental study was carried out on the effects of an extended source on the HOE output for different holographic lenses, with focal lengths from 2.5-10 cm. The lenses were recorded in Bayfol HX200 material and their diffraction efficiency was characterised across the lens aperture by measuring the Bragg angular selectivity curve at each location. Offset point sources were used to experimentally study the effects of a non-point source on the HOEs and the system was also modelled using Matlab and Zemax.
The fabrication of an analog holographic wavefront sensor, capable of detecting the low order defocus aberration, was achieved in an acrylamide-based photopolymer. While other implementations of holographic wavefront sensors have been carried out digitally, this process utilises a recording setup consisting only of conventional refractive elements so the cost and complexity of holographic optical element (HOE) production could be much reduced. A pair of diffraction spots, corresponding to a maximum and minimum amount of defocus, were spatially separated in the detector plane by multiplexing two HOEs with different carrier spatial frequencies. For each wavefront with a known aberration that was introduced during playback of the hologram, the resulting intensity ratio was measured in the expected pair of diffracted spots. A number of HOEs were produced with the diffraction efficiency of the multiplexed elements equalized, for a range of diffraction efficiency strengths, some as low as <5%. These HOEs were used to successfully classify four amounts of the defocus aberration through the observed intensity ratio.
Optical diffusers have uses in laser applications and machine vision. Typical fabrication at a commercial level requires master production and the stamping/copying of individual elements at scale. This expensive, indirect process inhibits custom diffusers at reasonable cost. Previously the authors published a novel, direct, single beam method of recording customizable and controllable volume holographic diffusers by manipulating laser speckle and recording the pattern in photopolymer. This method allows for beam-shaping to produce diffusion patterns of various sizes and shapes. In this work, the direct method of recording controllable holographic diffusers is refined to improve diffuser performance (i.e., a decrease in zero order strength) for a simple diffuser. This is achieved through optimising the recording conditions (exposure energy, power and layer thickness) for a given photopolymer formulation. Significant improvement in the diffuser efficiency is observed through the optimisation process for a particular speckle size, resulting in a five-fold decrease in the remaining zero order. Kogelnik Coupled Wave Theory (KCWT) is explored as a first step towards developing an appropriate model for the behaviour of holographic elements recorded with interference patterns formed through stochastic processes, such as speckle patterns.
Diffractive Optical Elements (DOEs) utilize diffraction at sub-micron features to re-direct and control light. Volume phase holographic materials, such as photopolymer, are advantageous for use in fabricating optical elements because the diffraction efficiency can approach 100%, the whole element can be recorded in one exposure and high diffraction and slant angles are possible. Self-developing photopolymers also facilitate mass manufacture. Holographic gratings have been developed for numerous applications including spectroscopy, solar concentration, and monochrome LEDs, however, the inherent angular and wavelength selectivity of the volume phase hologram generally restricts applications to laser systems and sources with narrow spectral ranges. Multiplexing more than one grating into a single photopolymer layer can extend the range, however, unwanted additional gratings are frequently recorded.
In this paper, we discuss laminating multiple photopolymer HOEs together as a method for increasing the wavelength and angular working range of devices. This involves combining HOEs designed to produce the desired output beam for different angular and/or spectral input beams. Stacking of photopolymer layers has previously been demonstrated to increase the angular range of gratings and recently the authors produced a compound HOE with significantly broadened wavelength and angular selectivity curves by laminating two HOEs recorded sequentially at a single wavelength. However, such solutions are not easily translated to more complex elements such as lenses where the spatial frequency and grating slant angles are varying.
This paper discusses laminating together two photopolymer layers sensitized for different recording wavelengths for the purpose of holographically recording a compound-element volume-HOE lens for use with a broadband LED. The angular and wavelength selectivity are characterized and the challenges and advantages of the different approaches are discussed and compared.
A volume cylindrical holographic lens is fabricated in a photopolymer material to obtain a simple, lightweight and inexpensive lens which can collimate a diverging light beam. For a collimated beam, it is necessary to have uniform intensity across the beam diameter and to achieve equal diffraction efficiency for different regions of the cylindrical holographic lens, two methods are discussed. In the first method, the diffraction efficiency is improved by modifying the recording geometry in order to operate at a range of spatial frequencies for which the photopolymer provides higher diffraction efficiency. In the second method, the recording has been carried out with varying laser power and exposure time while keeping the exposure energy constant, in order to improve the material’s performance at the lower spatial frequencies. The second approach increases the uniformity of diffraction efficiency across the Holographic optical elements (HOEs) even when low spatial frequency components are present. The results obtained provide cylindrical holographic lenses with overall higher and uniform diffraction efficiency. This type of lens has the potential to be used in combination with LED sources and different lighting applications.
Phase singularities have been shown to cause one of the major problems for adaptive optics (AO) systems which
attempt to correct for distortion caused by the atmosphere in line of sight free space optical communications over
mid-to-long range horizontal paths. Phase singularities occur at intensity nulls in the cross-section of the laser
beam at the receiver. When the light intensity drops to zero at these points the phase of the optical wavefront is
undefined. Phase singularities occur in pairs of opposite sign (or rotation) and are joined by a wave dislocation,
called a branch cut, with a corresponding 2π radian jump in the phase. It is this 2π jump which causes difficulties
for common AO techniques. To negate the effect of the phase singularities they must be detected and then taken
into account in the wavefront reconstruction. This is something not done by most of the zonal reconstruction
algorithms commonly used in atmospheric turbulence correction. An experimental set up has been built and is
used in the laboratory to examine the detection of phase singularities in atmospheric turbulence. This consists of
a turbulence generator using a spatial light modulator (SLM) to mimic the atmosphere and a Shack-Hartmann
wavefront sensor as the receiver. The branch point potential method for phase singularity detection is then
implemented in post processing to locate the position of the phase singularities. Phase singularity detection can
now be practiced under different conditions in a controlled manner. Some results of phase singularity detection
from this experimental setup are shown.
Branch points have been shown to cause problems for adaptive optics (AO) systems which attempt to correct for
atmospheric distortion over mid-to-long range horizontal paths. Where branch points (or singularities) occur, the
phase of the optical wavefront is undefined and cannot be reconstructed by conventional wavefront reconstruction
techniques. Branch points occur in pairs of opposite sign (or rotation) and are joined by wavefront dislocations
called branch cuts, which have a 2π jump in phase across them. The aim of the project is to construct a branch
point sensitive wavefront reconstructor using a Shack Hartmann wavefront sensor which can be used on a 3km
line-of-sight (LOS) free space optical (FSO) communications system currently being tested within our group.
The first step in our method is to detect the positions of singularities using the branch point potential method
first proposed by LeBigot and Wild. The most common zonal reconstruction method used (the least squares
reconstructor) is not sensitive to branch points and different methods are being investigated for this part of the
project. Results for the detection of singularities using the branch point potential method in simulations are
shown here. Some early results for the reconstruction of branch point affected wavefronts are also presented.
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