TECHNICAL
Holographic Optical Elements (HOEs)
A diffractive optical element is a new class of optics that operates on the principle of diffraction. Traditional optical elements use their shape to bend light. Diffractive optics work by breaking up incoming waves of light into a large number of waves, which recombine to form completely new waves. Diffractive optical elements are destined to re-define the role of optics in several areas.
DOEs can function as gratings, lenses, aspherics or any other type of optical element Large optical apertures, lightweight and lower cost are the main features of DOEs. They can offer unique optical properties that are not possible with conventional optical elements.
They can be fabricated in a wide range of materials viz. aluminum, silicon, silica, plastics, etc. providing the user greater flexibility in selecting the material for a particular application.
DOEs have enabled the development of some products that could not be produced with previous optical technologies.
Features of DOEs
The DOEs have following features:
DOEs are wavelength sensitive. The focal length and aberration characteristics of a diffractive element vary with wavelength.
Several different optical elements can share the same substrate without interfering with one another. Thus, a single DOE can be used as a lens, beam splitter and spectral filter simultaneously.
Diffractive elements are very light, as they are formed in thin films of a few um thickness only.
Because DOEs can generate unique optical functions that are not possible by conventional reflective or refractive optical elements, they provide greater flexibility in system configuration.
At least one surface of a conventional glass lens is curved, whereas for a diffractive lens there is no such requirement. A diffractive element can be fabricated on any arbitrary shape of the substrate.
They can be made to operate over a narrow wavelength band.
The fabrication and replication of DOEs are relatively easy and cheap because no precision shaping of a surface is required.
By using real-time recyclable recording media, any desired system function can be recorded and erased repeatedly.
Aberrations of DOEs
The DOEs exhibit chromatic and/or nonchromatic aberrations when there is a change in the reconstruction beam parameters and/or wavelength from the reference beam. The higher order aberrations become important for DOEs of large apertures for which alignment tolerances are very low. The condition that will eliminate all the aberrations simultaneously is to duplicate one of the construction beams in the reconstruction process. This is, however, difficult to maintain when DOEs are to be used as components in instruments.
Environmental effects
Environment plays an important role in the performance of systems involving conventional and diffractive optics. The effects of temperature changes, thermal gradients, stresses, shock etc. on glass optical elements have been fully understood. It is well known that stresses produced in thin optical films can cause deformation of the substrate The environmental effects on diffractive elements have received attention only recently. The stresses, thermal gradients and temperature changes can alter the behavior of DOEs significantly. Environment effects become very important for military, aerospace and other applications. For example, many military systems are required to operate over a temperature range of -40 to + 60 deg C.
Recording materials for DOEs
The fabrication of diffractive optical elements requires the basic facilities of hologram recording. Special attention is needed with regard to vibration isolation, quality of substrate and recording material. The full potentialities of DOEs depend on the availability of high quality recording materials to achieve high diffraction efficiency and image quality. The required characteristics of the recording materials are
(a) Peak diffraction efficiency above 90% at the working wavelength.
(b) High spectral sensitivity at the recording wavelength.
(c) High resolution capability.
(d) Resistance to environmental changes.
(e) Low scattering and absorption losses.
(f) Adequate thickness.
Photoresist is a good recording material for the surface relief holograms and dichromated gelatin and photopolymer for the volume holograms. The swelling property of DCG is very helpful in fabricating DOEs at any desired wavelength with narrow band or wide band operation. Very narrow band reflectivities can also be obtained.
Applications of DOEs
Holographic optical elements (Diffractive Optical Elements) can duplicate most of the functions provided by glass optics if optical system operates over narrow spectral bandwidth or requires chromatic dispersion. Some of the unique functions that DOEs provide are multiple function optics, unusual configuration, and narrow spectral response. These typical characteristics of DOEs provide advantages in beam combiners, head-mounted displays, spectral filters, compact rugged spectrometers, diode laser couplers, laser resonators, laser material processing, beam shaping optics, laser collimators, computer interconnects, solar concentrators, and wavelength division multiplexers/demultiplexers. Large aperture DOEs are useful for null optics, laser scan systems, laser beam delivery systems, and projection displays.
Spectral Filters
Beam Combiners
Head-up Display System for Aircrafts and Automobiles
Helmet (Head) Mounted Head-up Display
Night Vision Goggles
Center High Mounted Spot Light for automobile
Holographic optical elements in Optical Communication
Holographic optical elements in Compact Disks
Diffractive Laser Beam Attenuator
DOE based Fiber Optic Gyroscope
Diffractive Scanner
Diffractive-refractive Hybrid Components
Architectural Applications
Finger Print Sensor
Holographic optical elements for Beam Shaping
Holographic optical elements in Art
Holographic optical elements for Coherent Combination of Laser Beams
Holographic optical elements for Computer Interconnections
Holographic optical elements for Information Processing
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