Design Of Holographic Optical Elements Research Articles

Modelling and Design of Holographic Optical Elements for Beam-Coupling Applications for a Range of Incident Beam Angles

Theoretical modelling has been used to calculate the holographic recording beam angles required in air (at any recording wavelength) to produce a Volume Holographic Optical Element (VHOE) for any defined input and output beam angles. The approach is used to facilitate the design and fabrication of diffractive coupling elements through a holographic process that avoids the use of coupling prisms during recording and will help in the design of recording arrangements that better suit the mass production of low-cost elements, especially those designed for non-normal incidence. In this study, the recording angles needed for a range of recording wavelengths were explored for VHOE couplers designed for input angles (in air) ranging from 0° to −55°. Then, in order to validate the model, holographic recording in Bayfol HX 200 photopolymer at 532 nm was used to fabricate photopolymer VHOE couplers for 633 nm light (−45° input angle in air). Bragg curves obtained experimentally for different probe wavelengths (403 nm, 532 nm and 633 nm) confirm the recording of the desired grating structures to a precision of ±1°, and coupling is demonstrated at 633 nm with a diffraction efficiency of up to 72%. Furthermore, the model is used to identify the origins of some weaker spurious gratings observed alongside the expected ones.

Prism-type holographic optical element design and verification for the blue-light small-form-factor optical pickup head

This paper presents the prism-type holographic optical element (PT-HOE) design for a small-form-factor (SFF) optical pickup head (OPH). The surface of the PT-HOE was simulated by three steps of optimization and generated by binary optics. Its grating pattern was fabricated on the inclined plane of a microprism by using the standard photolithography and specific dicing procedures. The optical characteristics of the device were verified. Based on the virtual image method, the SFF-OPH with the device was assembled and realized.

A Novel Pickup Head Design for DVD-Multi System

This paper introduces a novel holographic optical element (HOE) pattern design to generate focusing and tracking signal for using in digital versatile disc DVD-multi pickup heads. The digital verstile disk (DVD) multi drive will be able to read disks written in the DVD-ROM, DVD-video, DVD-audio, DVD-R/W, and DVD-R formats. DVD multi drive will also be able to write disks in the DVD-RAM and DVD-RW/R formats. The HOE design provides the focusing error signal and both differential phase detection (DPD) and pushpull tracking signals for DVD-ROM disks and rewritable land-groove disks. The HOE pattern is divided into two main regions. The inner region is for focus detection and the outer region is for DPD and Pushpull tracking detection. In this study, a rotary actuator is used to reduce the tilt angle of the objective lens in the DVD-multi pickup head. The finite element method of the actuator is studied to analyze the resonance of the structure and the magnetic field of the voice coil motor.

Design of Computer-Generated Hologram With Ring Focus for Nonmechanical Corneal Trephination With Er:YAG Laser in Penetrating Keratoplasty

Purpose: To calculate a beam-shaping optical element for homogeneous intensity distribution within a focal ring to be used in nonmechanical trephination with the Er:YAG laser in penetrating keratoplasty instead of a spot guiding device. Methods: The phase distribution behind a holographic optical element (HOE) kΨ( u) can be described by the addition of the hologram phase Φ H ( u) to the beam phase Φ E( u ): k Ψ( u ) = Φ H( u ) + Φ E( u ), k = 2 π/ λ , where u denotes the coordinates inside the hologram aperture, k an integer, and λ the laser wavelength. To avoid discontinuous wavefronts leading to speckle noise, a smooth phase function is necessary. After transforming the hologram aperture coordinates into the focal plane x in a focal distance f, Ψ can be retrieved from the slope equation: ▿ Ψ( u ) = x( u ) − u /f . Results: Creating a ring focus can be reduced to an essentially one-dimensional problem by separation of variables due to the symmetry condition. We calculated a computer-generated eight-level phase-only HOE with 4096 × 4096 pixels from a Gaussian-distributed 2.94 Er:YAG laser spot with a beam diameter of 10 mm and a focal distance of 100 mm. Thereby, a ring focus with an inner/outer radius of 7/8 mm can be created. To avoid Poisson's spot, the symmetry of the problem was broken by circular modulation of the phase leading to a spiral-like structure. The calculated efficiency of the HOE relating the energy within the ring to the total energy was 91%. Conclusion: With an HOE it is possible to redistribute the energy along the desired focal ring. The HOE design can be adapted to the intensity distribution of the impinging laser beam with its characteristic aperture shape. A circular homogeneous corneal trephination depth is possible, because the energy fluctuation from pulse to pulse does not locally affect the ablation process. A ring focus for the Er:YAG laser has the potential to render superfluous a manual beam control via micromanipulator and to allow a more rapid and more regular corneal trephination along aperture masks.

Holographic Optical Element with High Assembly Tolerance for the Optical Module of Digital Versatile Disc Pickup-Heads

The astigmatic focus detection method and differential phase detection (DPD) tracking method are now widely used in a digital versatile disk (DVD) pickup head. The DPD tracking method requires the returning spot to remain centered in a quad detector and therefore makes the optical alignment very critical. In this paper, a novel holographic optical element (HOE) pattern design to relax the alignment tolerance and achieve a stable DPD tracking servo property is described. Simulations show that the servo signals are insensitive to photo-detector (PD) misalignment when compared to that of a conventional HOE design.

Comparison of methods for determining the bias index of a dichromated gelatin hologram

Determination of the bias refractive index of a holographic emulsion before exposure and after development is an important factor in the design of holographic optical elements. Several experimental methods are discussed for determining the bias index of a volume hologram in dichromated gelatin, and the results for each technique are presented. It is shown experimentally that these measurement methods yield different results for the same hologram, and the cause of the differences is proposed to be a variation of the bias index with depth in the hologram. An index measurement technique is also presented that accounts for variation in the bias index and is shown to yield an accurate value for the bias index.

Experimental evaluation of shearing effects in volume holograms formed in bleached photographic emulsions

Transmission holograms recorded in Agfa 8E75 HD photographic emulsion and processed with an R-9 reversal bleach are experimentally analysed. Several important aspects related to the influence of recording materials and their processing on the design of holographic optical elements have been considered. The cases of the two-plane wave and that of the spherical object wave are analysed in detail. Emphasis is on the estimation of shear angles from their effects on hologram properties. The influence of exposure, spatial frequency and slant angle on shear angles is briefly analysed and discussed.

Analytical design of holographic optical elements for Fourier transform

A new method for designing holographic optical elements for Fourier transform is described. A phase function is assumed as a power series of coordinates of the optical element, and an analytical expression of wave-front aberration is derived. The coefficients in the phase function are explicitly determined such that the wave-front aberration may be kept small during variation of inputs. The location of the circle of least confusion and the balancing of the aberration over an entire spatial-frequency range are taken into consideration. By the simulation of spot diagrams it is confirmed that our designed phase function is optimum for a sufficiently wide spatial-frequency range.

Holographic optical elements — State-of-the-art review: Part I

A state-of-the-art review on holographic optical elements (HOE) is presented in two parts. In Part I a conceptual overview and an assessment of the current status on the design of HOE have been included. It is pointed out that HOE development based on the use of squeezed light, speckle, non-linear recording, comparative studies between optics and communication approaches, are some of the promising directions for future research in this vital area of photonics.

Design of holographic optical elements by using recursive techniques

In order to record holographic optical elements, recursive design techniques were developed for obtaining complex aspheric wave fronts from a relatively simple hologram. We present the mathematical formalism of this technique with a specific illustration of a holographic Fourier transform, designed with complex wave fronts that were derived recursively from two previous holograms. Both the calculated and the experimental results demonstrate a holographic lens that can handle input angles of up to ±12° with essentially diffraction-limited performance.