Novel compact devices for wavelength division multiplexing and demultiplexing are presented. These devices are based on planar optics configurations. A method for designing and recording such planar devices is described. Experimental procedures and results for devices that can handle three closely separated wavelengths in the visible as well as near infrared radiation are presented.
Diffractive elements have several distinct advantages over conventional refractive elements. They are lighter and more compact and can be manufactured easily and quickly. These elements are usually used as optical elements, but they can also be used in other regions of the spectrum. An example of a diffractive element which is used for millimeter waves is a grating coupler (GC), which couples a guided wave out of a waveguide into free space. In most applications, the preferred direction of the output wave is normal to the waveguide plane. Unfortunately, as the imaging properties of diffractive elements depend strongly upon the wavelength of the readout wave, it is difficult to use GCs when the input source of the system is polychromatic or when it suffers from wavelength variations. In this paper, we present a method to design a double grating coupler (DGC), which couples a polychromatic wave out of a waveguide with only negligible angular dispersion. The aim of the design is to construct two grating functions in such a way that the chromatic dispersions of the two gratings will mutually compensate each other. In order to achieve this, we use an iterative method, whereby the main ides is to design a double grating which deflects two different wavelengths, with no dispersion, resulting in negligible dispersion within the band.
A novel compact planar configuration for correcting the asymmetric divergence of light emanating from diode lasers is presented. It is comprised of two holographic lenses that are recorded on one transparent substrate, where the light propagates form one lens to the other by means of total internal reflections. The design of the overall planar configuration is presented along with experimental result. The results reveal that it is possible to focus a collimated asymmetric beam to a circular spot.
Plastic optical elements are used extensively for high- volume, low-quality, commercial applications. The advantages of plastic over glass elements include low weight, ease of modeling, high shock resistance, and low price in mass production. The use of plastic optical elements in military systems is limited by stringent environmental demands, and by the high image quality which is usually required. The disadvantages of plastic from this point of view are softness, low chemical resistance, thermal defaces, and inhomogeneity. Display systems are located in the same environment as the user. In an enclosed space, such as a vehicle, the requirements for environmental conditions can be greatly eased. In addition, visual optical systems may incorporate a focusing adjustment which can be controlled by the viewer. In combination with a limited temperature range, this solves the problem of thermal defaces. The resolution required in display systems is generally far from the diffraction limit, which reduces the significance of inhomogeneity. In addition, the eye pupil is usually smaller than the lens diameters, so that inhomogeneity only produces a local effect in the field of view. It would thus appear that plastic optical elements are a practical option for display systems, despite relatively severe environmental conditions.In this paper, the application of these considerations will be illustrated by a practical optical design for an airborne military display system incorporating plastic optical elements. The points of image quality and environmental requirements will be addressed.
A compact planar configuration for performing optical correlation is described. It is comprised of two pairs of identical holographic lenses and a holographic filter. Each pair of lenses is recorded on a single substrate and together perform exact Fourier transformation. The light between the lenses propagates inside the substrate by means of total internal reflection. The design and recording considerations for each of the lenses along with experimental results for the overall planar correlator are presented.
In this paper, we present a method to design a focusing double grating coupler (FDGC), which couples a polychromatic wave out of a waveguide and focuses it with only a negligible angular dispersion. The aim of the design is to construct two grating functions in such a way that the chromatic dispersions of the two gratings will compensate each other mutually. To achieve that, we use an iterative method, where the main idea is to design a double grating which deflects and focuses with no dispersion two different wavelengths resulting in negligible dispersion inside the band. The FDGC described in this paper can be applicable to many devices. In particular, the FDGC provides an attractive solution to be used in various applications of integrated optics, as well as in optical parallel computing. In addition, it can be implemented within a wide variety of applications such as a CD player, optical mass storage devices or any other application that incorporates a solid state laser and a focusing lens.
A method for designing and fabricating optical systems based on planar holographic optics is presented. The planar optical system is usually composed of two (or more) holographic optical elements which are fabricated onto the same substrate. This paper compares and contrasts the two principal fabrication methods of holographic elements as Bragg volume holograms or as surface relief gratings. Various examples of planar holographic optical systems are presented, and their advantages over regular optical systems are illustrated.
A compact configuration for a real-time updatable optical correlator is considered. The correlator is designed to operate with incoherent illumination and efficient phase filters, which can be implemented with real-time, updatable, electronically addressed, liquid-crystal spatial light modulator. In order to obtain both a rigid and a compact correlator, a configuration which can be incorporated with planar optics is investigated. The overall correlator includes two pairs of diffractive lenses, each for performing a Fourier transformation, one spatial light modulator and input and output interface devices. The design consideration and fundamental limits of a compact updatable correlator using off-axis, diffractive lenses are presented, along with some experimental results.
A new design of a thermal imaging system which incorporates diffractive surfaces in order to enhance system performance is presented. The problem of the drastic vignetting, typical of Galilean telescopes, is solved by using a Keplerian telescope with a very small f-number (approximately 0.7). In addition, several other aspects of design, manufacturing and tolerancing are discussed.
A novel design for a linear beam steering module, based on diffractive elements, is presented in this paper. The steering module consists of two parallel gratings (with special chirped grating functions), where the input wave impinges on the first grating normal to the grating's plane. In the 0-state (no translation) the output wave emerges as a plane wave normal to the grating's plane. When the first grating is translated by an amount of (delta) x, the output wave is deviated by an angle of (Theta) , where the deviation ratio, (kappa) equals sin(Theta) /(delta) x, is a constant. This angular deviation can be easily converted to a linear scanning of a focused point, with the help of an appropriate focusing lens. In this paper, we use a novel design method that enables us to produce a beam steering device that does not suffer from any aberration. Specifically, the output wave emerges from the second grating as a plane wave, with diffraction limited performance, over the entire field of view. Moreover, the gratings can be fabricated with novel techniques that enable the manufacturing of the desired gratings in a mass-production line with relatively low costs. In addition, the system is small, compact, easy to work with and with a very large deviation relation. Hence, the desired translation can be accomplished using a small piezoelectric crystal that is attached to the first grating and there is no need to resort to expensive and complicated steering equipment.
A new design of a head up display (HUD) system, which incorporates a diffractive surface to enhance the performance of the system, is presented. The HUD is composed of three lenses, in comparison to a standard design of a similar conventional HUD which contains five spherical lenses. In addition to the advantage of compactness, the diffractive HUD has an improved performance over the entire field of view.
A symmetric substrate mode holographic doublet that performs Fourier transformation is presented. This doublet is composed of two identical quadratic off-axis holographic lenses which were recorded on the same glass substrate. The first lens couples the light from the input into the glass substrate. The second lens, located at the back focal plane of the first one, couples the light out of the glass substrate and corrects the phase of the emerging wavefront in order to obtain the desired Fourier transformation. In such a configuration the input and the output planes are adjacent to the lenses, so the overall alignment is very convenient.
In this work we propose a design procedure for producing an optical interconnection lens as a computer originated hologram. New methods for analyzing and designing holographic optical elements are developed. These are mainly based on the introduction of Taylor series expansion to solving the equations of holographic elements. Specifically, the Taylor expansion is exploited to solve the Bragg conditions after they are reformulated as a nonlinear partial differential equation. The expansion is also used for analyzing the propagation of a given wavefront in an optical system that can be comprised of several holographic and conventional elements. According to computer simulations, the spot size, calculated in accordance with geometrical optics, is several orders of magnitude less than the diffraction limited size, and the diffraction efficiency degradation is less than 0.01%.
Planar optical configurations combine holographic elements with a substrate mode planar structure where the light is trapped inside the structure by total internal reflections. As a result, usual free space propagation can be replaced by guided propagation in the planar structures on which holographic elements are recorded on either side. These configurations can thus be very compact and readily modularized so as to eliminate the, usually needed, extreme alignment accuracies between input light sources, holographic elements, and output detectors. The principles and examples of how such elements can be incorporated into display and data processing applications are presented.
This paper describes optical systems which compensate for the wavelength dispersion and distortion that arise in diffractive fan-out elements. Two approaches are investigated, a space variant and a space invariant. In the space variant approach, microlenses or diffractive optical elements were introduced in the system correcting the wavefronts. In the space invariant approach refractive and diffractive lenses compensates for the chromatic aberrations and shifts in beam direction that are caused by the fan-out element. Several designs for such compensating optical systems are presented, along with simulated results.
A method for designing and recording a holographic achromat, composed of two holographic optical elements (HOEs), is presented. The method is demonstrated with a doublet recorded at 488 nm for application at a central frequency of 820 nm. A near-diffraction-limited focus and high diffraction efficiency are achieved over a comparatively wide spectral range.
A method for designing a large scale holographic interconnects system is presented. The building-block of the interconnects system is composed of two identical holographic optical elements (HOEs), which are recorded on the same plate. To achieve both high diffraction efficiencies and diffraction limited performances, the holographic elements are recorded with predistorted wavefronts which are taken recursively from interim holograms. To provide optical interconnections for a large number of source-detector pairs on the same board, a method for recording simultaneously a large number of building blocks on the same recording plate, is presented. With this method, the HOEs are recorded by reconstructing simultaneously the `parents' holograms, which are pre-prepared on an intermediate plate, and using the output beams to record the final elements. A special recording geometry is suggested in order to prevent cross-talk between the various HOEs, and to assure high diffraction efficiencies and low aberrations for all the interconnects.
A method for designing and recording Fourier transform holographic lenses, in the presence of a recording-readout wavelength shift, using a substrate-mode holographic doublet, is presented. The system is composed of two holographic optical elements (HOEs) which were recorded on the same plate. The first one couples the various spatial frequencies of the input into plane waves which are trapped inside the plate by total internal reflection. The second HOE focuses the collimated waves into the Fourier plane. Since the chromatic dispersion of the first hologram can be corrected by the dispersion of the second hologram, this system is much less sensitive to source wavelength shifts. The method is illustrated with a compact system, recorded at 458 nm and reconstructed at 633 nm. Near diffraction limited performance and a comparatively low chromatic sensitivity over a wide field of view has been obtained.